Polymer Stabilization and Degradation 9780841209169, 9780841211117, 0-8412-0916-2

Table of contents :
Title Page......Page 1
Copyright......Page 2
ACS Symposium Series......Page 3
FOREWORD......Page 4
PdftkEmptyString......Page 0
PREFACE......Page 5
1 Introduction to Hindered Amine Stabilizers......Page 8
Early Free Radical Research......Page 9
Stable Free Radicals......Page 10
Stable Free Radicals in Polymer Stabilization......Page 12
Literature Cited......Page 17
Discovery......Page 18
Synthesis......Page 20
Chemistry of Nitroxyl Radicals......Page 25
Developments in Nitroxyl Radical Synthesis......Page 27
Application......Page 28
Polymer Stabilization......Page 29
Other Fields of Hindered Amine and Nitroxyl Radical Application......Page 36
Literature Cited......Page 39
Degradation and Stabilization......Page 43
Stable Nitroxyl Radicals......Page 44
Hindered Amine Compounds......Page 48
Selection of Practical Stabilizers......Page 56
Literature Cited......Page 59
4 A Decade of Hindered Amine Light Stabilizers......Page 61
Early Results......Page 63
Literature Cited......Page 74
5 Mechanistic Studies of Sterically Hindered Amines In the Photooxidation of Liquid Polypropylene Model Substances......Page 75
Results of Model Investigations on Isooctane......Page 77
Discussion......Page 84
b) Analytical procedures......Page 91
Literature Cited......Page 92
6 Reactions of Aminyl Radicals and Mechanisms of Amine Regeneration as Inhibitors of Oxidation......Page 93
Literature Cited......Page 96
7 Hindered Diazacycloalkanones as Ultraviolet Stabilizers and Antioxidants......Page 97
Acknowledgment......Page 103
Literature Cited......Page 104
SUMMARY......Page 105
RESULTS AND DISCUSSION......Page 106
REFERENCES......Page 113
Experimental......Page 114
Results and Discussion......Page 115
Literature Cited......Page 121
10 Electron Spin Resonance Determination of Nitroxide Kinetics in Acrylic/Melamine Coatings Relationship to Photodegradation and Photostabilization Kinetics......Page 123
Experimental......Page 124
Results and discussion......Page 125
Conclusion......Page 137
Literature Cited......Page 138
11 Stabilization of Polypropylene Multifilaments Utility of Oligomeric Hindered Amine Light Stabilizers......Page 140
Results and Discussion......Page 141
Acknowledgements......Page 149
Literature Cited......Page 150
12 Hexahydropyrimidines as Hindered Amine Light Stabilizers......Page 151
Experimental......Page 152
Results & Discussion......Page 153
Literature Cited......Page 157
13 Mechanisms of Aromatic Amine Antidegradants......Page 158
References......Page 172
14 Polymer-Bound Antioxidants......Page 174
Polymer-Bound Antioxidants......Page 177
Theoretical Implications of Polymer-Bound Antioxidants......Page 193
Literature Cited......Page 196
15 Polymerizable, Polymeric, and Polymer-Bound(Ultraviolet) Stabilizers......Page 198
Polymerizable and Bound Ultraviolet Stabilizers......Page 200
Polymerizable Antioxidants......Page 209
Literature Cited......Page 210
The Computer Model......Page 212
The Reaction Scheme......Page 214
Modelling Photooxidation......Page 221
Modelling Stabilization......Page 226
Literature Cited......Page 233
17 Investigation of Thermal Oxidation and Stabilization of High-Density Polyethylene......Page 236
Results and Discussion......Page 237
Literature Cited......Page 247
18 Bis- and Trisphosphites Having Dioxaphosphepin and Dioxaphosphocin Rings as Polyolefin-Processing Stabilizers......Page 248
Experimental Section......Page 249
Processing Stability of Polypropylene......Page 252
Hydrolytic Stability Test......Page 253
Comparative Effectiveness of Experimental Processing Stabilizers in Polyolefins......Page 254
Polyethylene......Page 255
Hydrolytic Stability of Experimental Processing Stabilizers......Page 256
Acknowledgments......Page 257
Literature Cited......Page 258
19 Formation of Anomalous Structures in Poly(vinyl chloride) and Their Influence on the Thermal Stability Effect of Polymerization Temperature and Pressure......Page 259
Experimental......Page 261
Results and Discussion......Page 264
Acknowledgments......Page 282
References......Page 283
20 Degradation of Poly(vinyl chloride) According to Non-Steady-State Kinetics......Page 285
Results and Discussion......Page 286
Literature Cited......Page 297
21 Effect of Antioxidants on the Thermooxidative Stabilities of Ultraviolet-Cured Coatings......Page 299
RESULTS......Page 300
DISCUSSION......Page 308
ACKNOWLEDGEMENT......Page 310
REFERENCES......Page 311
22 Photooxidation of Poly(phenylene oxide) Polymer......Page 312
Results and Discussion......Page 313
Literature Cited......Page 326
23 Photoaging of Polycarbonate: Effects of Selected Variables on Degradation Pathways......Page 328
EXPERIMENTAL......Page 329
RESULTS......Page 330
I. Films Without Added BPA......Page 333
DISCUSSION......Page 343
I. Effects of Variables......Page 344
II. Photolysis Products and Possible Mechanisms......Page 345
III. Scission Patterns......Page 348
Literature Cited......Page 349
24 Photooxidation and Photostabilization of Unsaturated Cross-linked Polyesters......Page 351
Results and Discussion......Page 352
Literature Cited......Page 356
25 Polypropylene Degradation by γ-Irradiation in Air......Page 357
Experimental......Page 358
Results......Page 359
Discussion......Page 365
Literature Cited......Page 368
26 Comparison of Chemiluminescence with Impact Strength for Monitoring Degradation of Irradiated Polypropylene......Page 370
Experimental......Page 371
Results......Page 372
Discussion......Page 378
Literature Cited......Page 381
27 Chemiluminescence in Thermal Oxidation of Polymers: Apparatus and Method......Page 383
Apparatus......Page 384
Method......Page 385
Initial Stages of Oxidation......Page 387
Advanced Stages of Oxidation......Page 390
Experimental Conditions......Page 391
Initial Stages of Oxidation......Page 392
Advanced Stages of Oxidation......Page 401
Conclusions......Page 404
Literature Cited......Page 405
28 Physical Techniques for Profiling Heterogeneous Polymer Degradation......Page 406
Oxygen Diffusion Effects......Page 407
Rapid Identification of Heterogeneous Degradation: Metallographic Polishing......Page 408
Degradation Profiling in Terms of Relative Hardness......Page 409
Effect of Heterogeneous Degradation on Macroscopic Property Changes: Viton......Page 411
Experimental......Page 414
Conclusions......Page 416
Literature Cited......Page 417
29 Synthesis of Biodegradable Polyethylene......Page 418
Biodegradable Polyamides......Page 419
Biodegradable Addition Polymers......Page 420
Determination of Biodegradability......Page 422
Experimental......Page 425
Literature Cited......Page 426
Author Index......Page 427
A......Page 428
C......Page 429
D......Page 430
E......Page 431
H......Page 432
M......Page 434
O......Page 435
P......Page 436
R......Page 438
S......Page 439
U......Page 440
Z......Page 441

Citation preview

ACS

SYMPOSIUM

SERIES

Polymer Stabilization and Degradation Peter P. Klemchuk, EDITOR CIBA-GEIGY

Corporation

Based on a symposium sponsored by the Division of Polymer Chemistry at the 187th Meeting of the American Chemical Society, St. Louis, Missouri, April 9-12, 1984

American Chemical Society, Washington, D.C. 1985

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

280

Library of Congress Cataloging in Publication Data Polymer stabilization and degradation. (ACS symposium series, ISSN 0097-6156; 280) "Based on a symposium sponsored by the Division of Polymer Chemistry at the 187th meeting of the American Chemical Society, St. Louis, Missouri, April 9-12, 1984." Bibliography: p. Includes indexes. 1. Polymers and polymerization—Deterioration— Congresses. 2. Stabilizing agents—Congresses. I. Klemchuk, Peter P., 1928II American Chemical Society. Division of Polyme III. American Chemical Society St. Louis, Mo.) IV. Series. QD380.P653 1985 ISBN 0-8412-0916-2

547.7

85-9011

Copyright © 1985 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, M A 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by A C S of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. P R I N T E D IN THE UNITED S T A T E S O F A M E R I C A

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ACS Symposium Series M . Joan Comstock, Series Editor Advisory Board Robert Baker

Robert Ory

U.S. Geological Survey Research Center

Martin L. Gorbaty Exxon Research and Engineering Co.

Roland F. Hirsch U.S. Department of Energy

Geoffrey D. Parfitt Carnegie-Mellon University

James C. Randall Phillips Petroleum Company

Herbert D. Kaesz

Charles N. Satterfield

University of California—Los Angeles

Massachusetts Institute of Technology

Rudolph J. Marcus Office of Naval Research

Vincent D. McGinniss Battelle Columbus Laboratories

Donald E. Moreland

W. D. Shults Oak Ridge National Laboratory

Charles S. Tuesday General Motors Research Laboratory

Douglas B. Walters

U S D A , Agricultural Research Service

National Institute of Environmental Health

W. H . Norton

C. Grant Willson

J. T. Baker Chemical Company

I B M Research Department

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

FOREWORD The A C S S Y M P O S I U M SERIES was founded in 1974 to provide a medium for publishing symposia quickly in book form. The format of the Series parallel IN CHEMISTRY SERIES except that, in order to save time, the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable, because symposia may embrace both types of presentation.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

PREFACE POLYMER

DEGRADATION A N D STABILIZATION have become increasingly of

interest to an international audience, and contributions to the body of knowledge have come from scientists worldwide. The symposium upon which this book is based was billed as international to encourage the participation of scientists from many countries, especially because it was the first Polymer Degradation and Stabilization Meeting of the Polymer Division of the American Chemical Society since March 1977

in New

Orleans, and also because stabilizers, had resulted fro The initial call for papers for this symposium asked for reports in the following areas, among others: new stabilizers, stabilizers of high permanence, new findings of degradation and stabilization mechanisms, stabilizing polymers for severe environments, the impact of new polymerization processes on polymer stability, the complexities contributed to stabilization by the solid state, and polymer thermooxidation at ambient conditions. The response was excellent. As a result, we were able to organize a comprehensive program for the symposium. We are grateful to the many contributors of papers to the symposium and especially to those individuals who submitted complete papers for inclusion in this volume. A major objective of the symposium and of this symposium volume was to attempt a review of the most important developments stabilization of the last decade.

The development

in polymer

of hindered amine

stabilizers was a major accomplishment in that decade and was truly an international achievement. Early work on stable free radicals in the United States, Germany, England, and the Soviet Union, among others, led in 1962 to the first synthesis of 2,2,6,6-tetramethyl-4-oxopiperidine-7V-oxyl, the Noxyl of triacetonamine. That first synthesis by Neiman, Rozantsev, and Mamedova of the Soviet Union triggered interest and excitement among scientists in many countries—Japan, France, England, the United States, Switzerland, Italy, and Canada—and ultimately led to the development of the hindered amine stabilizers. We had invited many of the scientists who were involved in the early history of hindered amine stabilizers to come to the symposium and to help us relive the hindered amine story. A l l could not come, but many did participate and helped us to review the story of the development of hindered amines and to gain an appreciation of that development. Three papers had been offered by scientists in the Soviet Union. There were to have been papers on " T h e Discovery of Hindered Amine Stabilizers" vii

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

by Rozantsev, on " T h e Reactions of Aminyl Radicals and Mechanism of Amine Regeneration as Inhibitors of Oxidation" by E . T Denisov, and on "Nitroxyl Radicals and Polymer Stabilization" by A . L . Buchachenko. None of the three authors was able to attend the symposium. However, Rozantsez and Denisov have submitted their papers for this symposium volume. This volume should serve as a good source for information on the status of current developments on polymer degradation and stabilization. It contains chapters on hindered amine light stabilizers from the major centers that have contributed to the development of this important class of polymer stabilizers. It contains contributions on stabilizers of increased permanence from the two major centers engaged in studying polymer stabilizers of reduced volatility and increased permanence—the laboratories of Scott at the University of Aston in Birmingham, England, and the laboratories of Vogl, formerly at the Universit Polytechnic Institute of New York in Brooklyn. Chapters on phosphites by Spivack and mechanisms of stabilization by aminic antioxidants by Pospisil highlight those important stabilizer classes. Chapters on the photodegradation and stabilization of poly(phenylene oxide) and polycarbonate by Pickett and Pryde, respectively, on the thermal degradation of polyvinyl chloride) by Hjertberg and the late Danforth, on the thermal oxidation and stabilization of polyethylene by Horng, on the thermal oxidation of U V cured coatings by Heyward, and on the photooxidation of polyester-styrene graft polymers by Ranby all provide state-of-the-art information on these important substrates. The chapter by Carlsson on y-irradiation of polypropylene provides information on a subject that has gained importance due to the advent of y-ray sterilization of plastic articles. Chapters on a computer model to study the degradation and stabilization of polyethylene by Guillet, on chemiluminescence in studying polymer degradation and stabilization by Zlatkevich and Mendenhall, and on techniques for studying heterogeneous degradation on polymers by Clough contribute significant information on these important and timely subjects. The book closes with a paper on the timely subject, biodegradable polyethylene, by Bailey. Many people have contributed to the preparation and publication of this volume. The efforts of the authors to prepare the manuscripts and share the findings of their work are acknowledged with appreciation. The editorial assistance of Bonnie B. Sandel of Olin Chemical C o . , the typing and proofreading assistance of Nancy Lovallo, Dianna DiPasquale, and D e n y Lounsbury, and the efforts of Suzanne Roethel and others at the American Chemical Society Books Department are all acknowledged with appreciation. The help of all these individuals and many others not named has been crucial to the success of this project. The A C S Petroleum Research Fund and the Polymer Division, Inc., of the American Chemical Society provided financial assistance for the symposium, which was used for the most part to help with the travel viii In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

expenses of symposium speakers from overseas. The Additives Department of the C I B A - G E I G Y Corp. also contributed to the success of the symposium by providing secretarial, typing, mailing, and telephone

support.

contributions of all these sources are gratefully acknowledged.

P E T E R P. K L E M C H U K

Additives Department C I B A - G E I G Y Corporation Ardsley, N Y 10502 December 7,

1984

ix

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

The

1 Introduction to Hindered Amine Stabilizers PETER P. K L E M C H U K CIBA-GEIGY Corporation, Ardsley, NY 10502

The hindered amine story is an interesting success story involving participation by scientists throughout the world. The success of hindered amines has partly been due to the free exchange of infor­ mation among scientists, so that the accomplishments of one group, studying stable free radicals, could be picked up by those in other groups, in other parts of the world, who were interested in stable free radicals as polymer stabilizers. The mechanisms of stabiliza­ tion by hindered amines were somewhat mysterious in the beginning since these compounds didn't f i t any of the known stabilizer types and known mechanisms. Since stable free radicals had been of inter­ est for their antioxidant activity as trappers of chain-propagating peroxy radicals the triacetoneamine-N-oxyl and derivatives thus were, at f i r s t , of interest as radical-trapping antioxidants. How­ ever, it wasn't long before the light stabilizing activity of that class of materials was recognized and not much longer before practi­ cal versions of hindered amine light stabilizers were synthesized, tested, and introduced for market development. Their high order of effectiveness as light stabilizers encouraged many scientists to investigate the mechanisms by which they functioned. We now know much about those mechanisms and can marvel at the effectiveness of hindered amines which apparently are dependent on what are very small concentrations of N-oxyls for their activity. There is much to learn and admire in the hindered amine story. Chemists can take pride in how effectively they have worked together across national boundaries to make hindered amine stabilizers an important product group for the stabilization of polymers. This introduction is a modest effort to review some of the early history of stable-free radicals including triacetoneamine-N-oxyl. This chapter was intended to serve primarily as an introduction to the hindered amine review which took place at the symposium and inten­ tionally avoids covering material which other participants were expected to present. It is a "light-touch" overview. 0097-6156/ 85/ 0280-0001 $06.00/ 0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

POLYMER STABILIZATION AND DEGRADATION

E a r l y Free R a d i c a l Research R a d i c a l p r o c e s s e s o c c u r i n many important c h e m i c a l r e a c t i o n s including: polymerization e l e c t r i c a l discharge reactions halogenation photochemical r e a c t i o n s combustion high e n e r g y - i n i t i a t e d reactions autooxidation heterogeneous c a t a l y s i s enzyme p r o c e s s e s The r a d i c a l n a t u r e o f t h e s e r e a c t i o n s was i d e n t i f i e d through mechani s t i c and k i n e t i c a n a l y s e s , but t h e l i f e t i m e s o f t h e f r e e r a d i c a l s i n v o l v e d a r e u s u a l l y t o o b r i e f t o p e r m i t them t o have been s t u d i e d directly. T h u s , s t a b l e f r e e r a d i c a l s o f f e r an o p p o r t u n i t y t o g a i n b a s i c i n f o r m a t i o n about f r e e r a d i c a l s and f r e e r a d i c a l r e a c t i o n s , in particular: 1.

2.

3.

e l u c i d a t i o n of the natur b e h a v i o r ; p r o p e r t i e s o f i t s m o l e c u l a r o r b i t a l s ; reasons f o r i t s stabil ity; e l u c i d a t i o n o f t h e k i n e t i c s and mechanisms o f r a d i c a l r e a c t i o n s w i t h s t a b l e r a d i c a l s which can be observed and measured directly; e l u c i d a t i o n of s t r u c t u r e s o f l i q u i d s and s o l i d s c o n t a i n i n g s t a b l e f r e e r a d i c a l s by t h e use o f EPR and ESR t e c h n i q u e s .

S t a b l e f r e e r a d i c a l s a r e o f p a r t i c u l a r importance t o t h o s e who a r e engaged i n polymer s t a b i l i z a t i o n because t h e y p l a y a key r o l e i n t h e i n h i b i t i o n of autooxidation r e a c t i o n s . The mechanisms o f a u t o o x i d a t i o n r e a c t i o n s were e l u c i d a t e d t h r o u g h t h e landmark r e s e a r c h c a r r i e d out at the B r i t i s h Rubber P r o d u c e r s ' R e s e a r c h A s s o c i a t i o n , where t h e k i n e t i c s o f a u t o o x i d a t i o n o f o l e f i n s were s t u d i e d i n t h e 1940's and e a r l y 1 9 5 0 ' s . Some o f t h e key r e s e a r c h e r s engaged i n t h a t work were L . Bateman, J . L . B o l l a n d , G. G e e , A . L . M o r r i s , P. Ten Have, among o t h e r s . They c o n t r i b u t e d enormously t o our u n d e r s t a n d i n g o f a u t o o x i d a t i o n r e a c t i o n s o f organic m a t e r i a l s . R e - r e a d i n g t h e i r papers produces a p p r e c i a t i o n o f t h e i r important work and emphasizes t h e debt we, i n polymer s t a b i l i z a t i o n work, owe them. That work e s t a b l i s h e d t h e f o l l o w i n g : 1. 2. 3. 4. 5. 6.

mechanisms and k i n e t i c s o f a u t o o x i d a t i o n r e a c t i o n s o f o r g a n i c materials; f r e e r a d i c a l n a t u r e of a u t o o x i d a t i o n ; chain r e a c t i o n nature of a u t o o x i d a t i o n ; h y d r o p e r o x i d e s as the main o x i d a t i o n p r o d u c t s ; p r o d u c t i o n o f a u t o o x i d a t i o n i n i t i a t i o n r a d i c a l s from h o m o l y s i s of h y d r o p e r o x i d e s ; i n h i b i t i o n o f a u t o o x i d a t i o n by h y d r o q u i n o n e ; t h e s t a b i l i t y o f r e s u l t a n t semiquinone r a d i c a l s t e r m i n a t e d o x i d a t i o n c h a i n s w i t h no c h a i n t r a n s f e r .

The work at t h e B r i t i s h Rubber P r o d u c e r s ' A s s o c i a t i o n and s u b sequent work by B i c k e l and Kooyman, Thomas and Tolman, among o t h e r s , l e d t o t h e a p p r e c i a t i o n t h a t a key r e q u i r e m e n t o f o x i d a t i o n i n h i b i -

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1. KLEMCHUK

Introduction to Hindered Amine Stabilizers

3

t o r s was t h e i r a b i l i t y t o form s t a b l e r a d i c a l i n t e r m e d i a t e s . These r a d i c a l i n t e r m e d i a t e s s h o u l d be s u f f i c i e n t l y s t a b l e so t h a t t h e y d o n ' t p a r t i c i p a t e i n hydrogen a b s t r a c t i o n from t h e s u b s t r a t e t h e y are s t a b i l i z i n g and t h e r e f o r e , not l e a d t o c h a i n t r a n s f e r . In Scheme I are shown the key s t e p s i n i n h i b i t i o n by t h e two main c l a s s e s o f o x i d a t i o n i n h i b i t o r s , h i n d e r e d phenols and s e c o n d a r y aromatic amines. Stable

Free

Radicals

The e f f e c t i v e n e s s o f compounds y i e l d i n g s t a b l e r a d i c a l i n t e r mediates i n the s t a b i l i z a t i o n of o r g a n i c m a t e r i a l s l e d to i n t e r e s t in stable free r a d i c a l s i n general. A r e v i e w o f the l i t e r a t u r e on s t a b l e f r e e r a d i c a l s i n d i c a t e s t h a t n i t r i c o x i d e and n i t r o g e n d i o x i d e have been known t o be s t a b l e f r e e r a d i c a l s f o r a v e r y l o n g time. In 1 8 4 5 , Fremy f i r s t d e s c r i b e d h i s s a l t which i s an i n o r g a n i c N-oxyl d e r i v a t i v e . The f i r s t r e p o r t o f an o r g a n i c N - o x y l was by W i e l a n d and O f f e n b a c h e r and p r o p e r t i e s o f d i p h e n y l - N - o x y l . D i p h e n y l p i c r y l h y d r a z y l was r e c o g n i z e d as a s t a b l e f r e e r a d i c a l i n 1 9 2 2 by G o l d s c h m i d t and Benn. S t r u c t u r e s o f o t h e r e a r l y s t a b l e f r e e r a d i c a l s may be seen i n F i g u r e 1. Those i n c l u d e s e v e r a l N - o x y l d e r i v a t i v e s as w e l l as g a l v i n o x y l , a s t a b l e - f r e e r a d i c a l d e r i v e d from a h i n d e r e d p h e n o l . These are o n l y a few o f the s t a b l e f r e e r a d i c a l s , e s p e c i a l l y N - o x y l r a d i c a l s , which have been d i s c o v e r e d . D r . Murayama o f the Sanyko Company i n c l u d e d many such s t r u c t u r e s i n a paper p u b l i s h e d i n t h e early 1970's ( J . ) . The f i r s t s y n t h e s i s of 2 , 2 , 6 , 6 - t e t r a m e t h y l - 4 - o x y p i p e r i d i n e - N o x y l was d e s c r i b e d i n a paper by M. B . Neiman, E . G. R o z a n t s e v , and Y . G . Mamedova 0 2 ) . The s y n t h e s i s was a c h i e v e d by t h e o x i d a t i o n o f t r i a c e t o n e a m i n e w i t h hydrogen p e r o x i d e i n the p r e s e n c e o f sodium tungstate. The key p r o p e r t i e s o f t h e t e t r a m e t h y l o x y p i p e r i d i n e - N o x y l , which s u b s e q u e n t l y l e d t o the whole f a m i l y o f h i n d e r e d amine s t a b i l i z e r s , were o u t s t a n d i n g t h e r m a l and c h e m i c a l s t a b i l i t y . D e t a i l s o f i t s p r o p e r t i e s are summarized i n F i g u r e 2 . The compound m e l t e d at 3 6 ° C It was s t a b l e t o m e l t i n g , r e c r y s t a l l i z a t i o n , and distillation. IJ^underwent r e a c t i o n s o f t h e c a r b o n y l f u n c t i o n a l i t y and had 6 . 1 x 1 0 spins/mole. The N - o x y l was reduced w i t h h y drogen on p a l l a d i u m c a t a l y s t t o the c o r r e s p o n d i n g h y d r o x y l a m i n e . The N - o x y l remained i n t a c t a f t e r r e a c t i o n s w i t h 1 ) h y d r o x y l a m i n e t o form t h e o x i m e , 2 ) s e m i c a r b a z i d e t o form the semicarbazone and 3) 2 , 4 - d i n i t r o p h e n y l h y d r a z i n e t o form the 2 , 4 - d i n i t r o p h e n y l h y d r a z o n e . The d i s c o v e r y o f the 2 , 2 , 6 , 6 - t e t r a m e t h y l - 4 - o x y l p i p e r i d i n e - N o x y l was an i m p o r t a n t m i l e s t o n e i n s t a b i l i z a t i o n t e c h n o l o g y but the t r i a c e t o n e a m i n e - N - o x y l was not u s e f u l by i t s e l f as a polymer s t a b i l i z e r because i t i s orange and would impart o b j e c t i o n a b l e c o l o r t o polymers and because i t has a r e l a t i v e l y low m o l e c u l a r weight and too h i g h a v a p o r p r e s s u r e f o r p r a c t i c a l u s e s . In o r d e r t o commerc i a l i z e a p r a c t i c a l s t a b i l i z e r based on t r i a c e t o n e a m i n e - N - o x y l , the f o l l o w i n g had t o be a c h i e v e d :

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

4

POLYMER STABILIZATION AND DEGRADATION

INH—^R0 H

+ IN*

2

IN* =

REQUISITES: 1 ) I N . MUST BE STABLE 2)

I N . MUST NOT P A R T I C I P A T E IN H-ABSTRACTION; (IN.

+ RH

NO CHAIN

TRANSFER.

INH + R.)

B I C K E L AND KOOYMAN, J . CHEM. S O C , 3 2 1 1 ( 1 9 5 3 ) , "

, ",

"

2215 (1956) "

, 2217 (1957)

THOMAS AND TOLMAN, J . AM. CHEM. S O C , 8 4 ' 2 9 3 0

(1962)

Scheme 1. Key requirement o f o x i d a t i o n i n h i b i t o r s : r a d i c a l intermediates. • N0

NO.

stable

O

FREMY ANN. CHIM. PHYS. 15, 108 (1845)

N0 ( C

H

)

6 5 2

N 0

WlELAND AND OFFENBACHER BER.,

2

*

0 N--O-N-N(C H ) * Ktr\ N0 2

6

5

2

N

47,2111 (1911)

2

DPPH GOLDSCMIDT BER.,

CH,

CH, -NO

H,CO

AND BENN,

5 5 , 628 (1322)

C.H?N^-C H 6

5

OCH MEYER AND REPPE

BANFIELD,KENYON

BER.,

J.

5 4 , 327 ( 1 9 2 1 )

CHEM.

1612

Soc.

(1926)

'NO-

HOFFMAN AND HENDERSON

GALVINOXYL COPPINGER, 77,

J . AM. CHEM. SOC

501 (1957)

Figure

1.

J.

AM. CHEM.

Soc,

83, 4671 (1961)

E a r l y si a b l e f r e e

redicals.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1. KLEMCHUK

5

Introduction to Hindered Amine Stabilizers

1.

a suitable derivative a. high s t a b i l i z i n g b. low c o l o r and low c. s t a b l e to polymer d. low v o l a t i l i t y ;

w i t h d e s i r e d p r o p e r t i e s had to be activity t e n d e n c y to d i s c o l o r i n polymers processing conditions

2.

an e c o n o m i c a l m a n u f a c t u r i n g p r o c e s s

had t o be

found

developed.

A l l t h e s e o b j e c t i v e s have now been a c h i e v e d and we a r e a b l e t o e n j o y t h e a v a i l a b i l i t y o f h i n d e r e d amine s t a b i l i z e r s w i t h o u t s t a n d i n g l i g h t s t a b i l i z i n g a n d , i n many i n s t a n c e s , t h e r m o s t a b i l i z i n g properties. The d i s c o v e r y by D r . Murayama and h i s c o - w o r k e r s at the Sankyo Company i n Japan t h a t the N - o x y l f u n c t i o n a l i t y was not c r i t i c a l t o the e f f e c t i v e n e s s o f t h e t r i a c e t o n e a m i n e - N - o x y l as a l i g h t s t a b i l i z e r but t h a t the N-H compounds o f t h a t c l a s s a l s o p o s s e s s e d i m p r e s s i v e l i g h t s t a b i l i z i n g a c t i v i t y , was a key f i n d i n g which l e d to the subsequent development o f polymer s t a b i l i z e r s which were c o l o r l e s s and had low d i s c o l o r a t i o S t a b l e F r e e R a d i c a l s i n Polymer S t a b i l i z a t i o n The e f f e c t i v e n e s s o f f r e e r a d i c a l s , such as N - o x y l s , h y d r a z y l s , and p h e n a z y l s , as s t a b i l i z e r s f o r polymers was d e s c r i b e d i n a p a t e n t i s s u e d i n 1952 ( 3 ) . V a r i o u s compounds o f t h e s e c l a s s e s were found t o be e f f e c t i v e as t h e r m a l and l i g h t s t a b i l i z e r s i n n y l o n f a b r i c , n y l o n f i l m , p o l y c h l o r o p r e n e , p o l y e t h y l e n e , and p o l y ( m e t h y l m e t h a c r y late). Some o f the s t r u c t u r e s d e s c r i b e d i n t h a t p a t e n t are shown i n F i g u r e 3. B r o w n l i e and I n g o l d (4) were among the f i r s t t o show the e f f e c t i v e n e s s o f N - o x y l s and h y d r o x y l a m i n e a n a l o g s i n s t a b i l i z i n g organic substances against a u t o o x i d a t i o n . The e f f e c t i v e n e s s o f N - o x y l s i n t r a p p i n g c a r b o n - f r e e r a d i c a l s and p e r o x y - f r e e r a d i c a l s was c l e a r l y demonstrated i n t h a t work. The N - o x y l s o f t h a t work, shown i n F i g u r e 4, were a l s o e f f e c t i v e i n i n h i b i t i n g the A I B N i n i t i a t e d p o l y m e r i z a t i o n of s t y r e n e . As shown i n F i g u r e 5, h i n d e r e d h y d r o x y l a m i n e s can f u n c t i o n as polymer s t a b i l i z e r s by the d o n a t i o n o f a hydrogen atom t o a peroxy r a d i c a l t o y i e l d an N - o x y l . The N - o x y l i s i n t u r n a b l e t o t r a p c a r b o n - f r e e r a d i c a l s and i n t h a t way t e r m i n a t e the o x i d a t i o n c y c l e at the f i r s t s t a g e , which i s the i d e a l p o i n t at which t o t e r m i n a t e c h a i n p r o p a g a t i n g f r e e r a d i c a l s - b e f o r e t h e y have an o p p o r t u n i t y t o r e a c t w i t h oxygen t o form peroxy r a d i c a l s . Non-hindered h y d r o x y l amine s , w i t h H-atoms on (X-carbon atoms, u s u a l l y g i v e r i s e t o n i t r o n e s as a r e s u l t o f hydrogen atom d o n a t i o n and do not t r a p c a r bon f r e e r a d i c a l s t o y i e l d alkoxyamino p r o d u c t s . The t h e r m a l s t a b i l i t y a s s o c i a t e d w i t h h y d r o x y l a m i n e s and the c o s t o f c o n v e r t i n g a t r i a c e t o n e a m i n e d e r i v a t i v e to a h y d r o x y l a m i n e d e r i v a t i v e a r e d i s advantages f o r the development o f h y d r o x y l a m i n e s as p r a c t i c a l comm e r c i a l polymer s t a b i l i z e r s . The e f f e c t i v e n e s s o f d i b e n z y l h y d r o x y l a m i n e as a i n h i b i t o r o f A I B N - i n i t i a t e d t e t r a l i n o x i d a t i o n can be seen from the d a t a i n F i g u r e 6. A comparison between d i b e n z y l h y d r o x y l a m i n e and b u t y l a t e d h y d r o x y t o l u e n e (BHT) can be made from t h o s e d a t a . The lower r a t e o f oxygen consumption i n the s o l u t i o n c o n t a i n i n g d i b e n z y l h y d r o x y l -

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6

POLYMER STABILIZATION AND DEGRADATION

O

M.P.

O

36°C.;

MELTING/ ZATION,

DISTILLATION,

REACTIONS SPIN

KEY

PROPERTY

- OUTSTANDING

S T A B L E TO

RECRYSTALLIOF C A R B O N Y U

PER MOLE:

6.1

>23

x 10'

STABILITY.

REACTIONS 1.

PD/H

2.

N-OXYL A.

2

REDUCTION INTACT

TO H Y D R O X Y L A M I N E

AFTER

REACTIONS

WITH:

HYDROXYLAMINE—>OXIM

B.

SEMICARBAZIDE—

C.

2,4-DlNITROPHENYLHYDRAZINE

^ 2,4~DlNITROPHENYL" HYDRAZONE

M. B. NEIMAN, E. G. ROZANTSEV, Y. G. MAMEDOVA, NATURE, 196,

472 ( 1 9 6 2 ) .

Figure 2 . F i r s t synthesis of 2 , 2 , 6 , 6 - t e t r a m e t h y l - 4 - o x o piperidine-N-oxyl.

COMPOUNDS EVALUATED:

POLYMERS TESTED

(THERMAL AND LIGHT S T A B I L I T Y ) :

NYLON FABRIC, NYLON F I L M ,

POLYCHLOROPRENE,

POLYETHYLENE, PMMA D. Me QUEEN, U . S . 2 , 6 1 9 , 4 7 9 ; APPLIED 1 2 / 2 8 / 5 0 ; ISSUED 1 1 / 2 5 / 5 2

F i g u r e 3. S t a b i l i z a t i o n o f polymers w i t h N - o x y l s , and p h e n a z y l s .

hydrazyls,

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

KLEMCHUK

Introduction to Hindered Amine Stabilizers

o

in

II I,

II,

III,

AND C O R R E S P O N D I N G

AUTOOXIDATION

OF

R.

AND R 0 . .

ALIPHATI

R.

+

0 —L->R0 .

R.

+

IN.^—>N0R

I,

II,

2

2

STYRENE.

HYDROXYLAMINES

AROMATIC

N-OXYLS

INHIBITED.THE REACTED

WITH

2

AND

III

BROWNLIE AND

— K

INHIBITED

INGOLD

10

AIBN-INITIATED

CAN.

J.

OF

FOR

I

2

STYRENE

CHEM.,

45,

POLYMERIZATION

2427

(1967).

F i g u r e 4. I n h i b i t i o n of a u t o x i d a t i o n of s t y r e n e by N - o x y l s and h y d r o x y l a m i n e s .

ADVANTAGES: GENERATE

N-OXYL

^NOH

+ R0 -

^NO.

+ R.

ON H Y D R O G E N > >N0.

2

>

EFFECTIVE

IN THERMAL

EFFECTIVE

IN

ABSTRACTION +

R0 H 2

>N0R OXIDATION

PHOTOOXIDATION

INHIBITION

INHIBITION

DISADVANTAGES: THERMAL COSTLY

Figure 5 .

STABILITY TO M A K E

N-HYDROXY-TRIACETONEAMINE

Hydroxylamines

as polymer

stabilizers.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7

8

POLYMER STABILIZATION AND DEGRADATION amine o v e r t h a t f o r BHT can be seen c l e a r l y from t h e d a t a p l o t t e d i n F i g u r e 6. C a l c u l a t i o n s o f o x i d a t i v e c h a i n l e n g t h - t h e number o f m o l e c u l e s o f oxygen consumed p e r f r e e r a d i c a l g e n e r a t e d - shows t h e h i g h e r e f f i c i e n c y o f d i b e n z y l h y d r o x y l a m i n e o v e r BHT as a r a d i c a l trapping i n h i b i t o r . Both d i b e n z y l h y d r o x y l a m i n e and BHT were found t o t r a p about two r a d i c a l s p e r i n h i b i t o r m o l e c u l e t o t h e end o f the induction period. Beyond t h e i n d u c t i o n p e r i o d both a d d i t i v e s were e s s e n t i a l l y w i t h o u t e f f e c t and t h e o x i d a t i o n proceeded as i f u n i n hibited . And s o , t h e work on mechanisms o f a u t o o x i d a t i o n at t h e B r i t i s h Rubber P r o d u c e r s ' A s s o c i a t i o n , t h e e a r l y work on t h e s y n t h e s i s and r e a c t i o n o f s t a b l e f r e e r a d i c a l s , the r e c o g n i t i o n o f t h e r j l e o f s t a b l e f r e e r a d i c a l s i n polymer s t a b i l i z a t i o n , t h e d i s c o v e r y o f s t a b l e t r i a c e t o n a m i n e - N - o x y l , and t h e s e a r c h f o r p r a c t i c a l c a n d i d a t e s f o r c o m m e r c i a l i z a t i o n , have l e d t o the development o f h i n d e r e d amine s t a b i l i z e r s , a new c l a s s o f polymer s t a b i l i z e r s . They a r e e f f e c t i v e i n many polymers a g a i n s t p h o t o d e g r a d a t i o n and a l s o a r e e f f e c t i v e against thermooxidation c u r r e n t c o m m e r c i a l l y a v a i l a b l e p r o d u c t s f o r polymer s t a b i l i z a t i o n may be seen i n F i g u r e 7. These compounds a r e e f f e c t i v e i n meeting the s t a b i l i z e r r e q u i r e m e n t s i n many commercial p o l y m e r s ; however, o t h e r s a r e under development t o s a t i s f y r e q u i r e m e n t s not b e i n g met by them.

TETRALIM,

2.50 M

INHIBITOR, AIBN,

2.0

CHLOROBENZENE

x 10'*

M

OXYGEN,

3.0 x 10~^ M

SOLUTION

1 ATM.

60°C

OXIDATIVE

CHAIN

RADICALS

LENGTH

INH.

INITIAL

END

TRAPPED IND. PER.

PER. I N H . M O L .

0.20

1.8

0.79

1.8

2.2

(

1

22.00

44.00

1

1

66.00

r

1

1

88.00

TIME

1

UO.00

1

C

6

H

1

132.00

5

C

1

H

2

)

2

N

0

T

154.00

H

BHT

/

1

1

176.00

1

1

198-00

1 ~

220-00

(MINUTES)

Figure 6 . I n h i b i t i o n of t e t r a l i n o x i d a t i o n by d i b e n z y l h y d r o x y l amine, A I B N - i n h i b i t e d .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1.

KLEMCHUK

Introduction to Hindered Amine Stabilizers

Chimassorb 944LD

Figure 7 .

Commercially a v a i l a b l e h i n d e r e d amine s t a b i l i z e r s .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

9

10

POLYMER STABILIZATION AND DEGRADATION

Literature Cited (1) (2) (3) (4)

K. Murayama, Yuki Gosei Kagaku Kyoka Shi, 29, 366 (1971). M. B. Neiman, E. G. Rozantsev, Y. G. Mamedova, Nature, 196, 472 (1962). D. McQueen, U. S. Patent 2,619,479. I. T. Brownlie and K. U. Ingold, Can., J . of Chem., 45, 2427 (1967).

RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2 Discovery, Chemistry, and Application of Hindered Amines 1

2

3

3

E. G. R O Z A N T S E V , E. SH. K A G A N , V. D. S H O L L E , V. B. IVANOV and V. A. SMIRNOV

2

1

2

3

Department of Biochemistry, M T I M M P Polytechnic Institute of Novocherkassk Institute of Chemical Physics of the Academy of Science of the USSR, Moscow, USSR

This chapter i s a comprehensive overview of the progress in the f i e l d of generation, chemistry, and application of nitroxyl radicals and their precursors, for example, hindered amines of the 2,2,6,6-tetramethylpiperidine series. Because of the impor­ tance of nitroxyl radicals to polymer stabi­ l i z a t i o n , this application i s discussed at length, while the others are touched upon briefly.

Discovery It is well known from chemical history that the discoveries of the first stable organic radicals, such as triphenylmethyl, diphenylpicrylhydrazyl, tri-tert-butylphenoxyl, and nitroxides are very significant contributions to theoretical chemistry. The relative stabilities of these radicals were attributed by chemists to the participation of an unpaired electron in conjugated 7r-electron systems. Classical stable radicals can thus be thought of as a superposition of many resonance structures with different localizations of an unpaired electron. The first stable radical obtained by Pilotti and Schwerin in 1901 in the pure state can be described by a variety of tautomeric and resonance structures as shown in Scheme 1. The development of the physical organic chemistry of stable radicals stimulated the investigation of relationships between their structures and reactivity. As a result, there developed the widely held concept that organic free radicals which do not possess a system of conjugated multiple bonds, such as 0097-6156/ 85/ 0280-0011 $07.50/ 0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12

POLYMER STABILIZATION AND DEGRADATION

CH

^

3

CH3 — C — CH3

CH

/^N ***

3

CH3—C—CH

3

0' (1)

(2)

(3)

cannot e x i s t under o r d i n a r y c o n d i t i o n s i n a c h e m i c a l l y pure s t a t e . Due t o t h e i r h i g h r e a c t i v i t y , the c o n c e n t r a t i o n s o f such r a d i c a l s d i m i n i s h v e r y r a p i d l y , and t h e i r l i f e t i m e s at room temperature do not o f t e n exceed f r a c t i o n s o f a s e c o n d . Y e t , p a r a d o x i c a l l y enough, i n t h e l a t e F i f t i e s and e a r l y S i x t i e s , the f i r s t f r e e r a d i c a l s i n a c h e m i c a l l y pure s t a t e were o b t a i n e d and though the t h e y were e x t r e m e l y s t a b l l y c o l o r e d c r y s t a l l i n e r a d i c a l s d i d not change t h e i r p h y s i c a l and c h e m i c a l c h a r a c t e r i s t i c s f o r months, even when s t o r e d i n a i r at room temperature. In t h e s e new r a d i c a l s , t h e u n p a i r e d e l e c t r o n does not p a r t i c i p a t e i n m u l t i p l e bond systems a n d , t h e r e f o r e , i t i s e s s e n t i a l l y l o c a l i z e d at the n i t r o g e n - o x y g e n bond ( 1 , 2 ) .

C0NH

2

A-

(A)

(5)

(6)

(7)

T h u s , i t was e x p e r i m e n t a l l y demonstrated t h a t a whole c l a s s o f s t a b l e f r e e r a d i c a l s does e x i s t , even though " t h e o r e t i c a l l y " t h e y are u n l i k e l y t o e x i s t under o r d i n a r y c o n d i t i o n s . From t h e S i x t i e s the i n t e r e s t i n the new r a d i c a l s by c h e m i s t s , p h y s i c i s t s and b i o l o g i s t s has been r a p i d l y i n c r e a s i n g . A f t e r t h e f i r s t p u b l i c a t i o n o f h i s p a p e r , one o f the a u t h o r s o f the p r e s e n t communication r e c e i v e d more t h a n a thousand i n q u i r i e s from many European c o u n t r i e s and A m e r i c a , and i n 1965 s e v e r a l independent r e s e a r c h groups and l a b o r a t o r i e s d e a l i n g w i t h d i f f e r e n t a s p e c t s o f the new r a d i c a l c h e m i s t r y a p p e a r e d . U n l i k e t h e c l a s s i c a l n i t r o x i d e s such as p o r p h y r e x i d e and o t h e r d e r i v a t i v e s o f q u a d r i v a l e n t n i t r o g e n , t h e new r a d i c a l s , c a l l e d n i t r o x y l s ( o r i m i n o x y l s ) c o u l d undergo r e a c t i o n s w i t h o u t involvement of the r a d i c a l s i t e . In 1961, E . G . Rozantsev and Y u . G . Mamedova were t h e f i r s t t o r e a l i z e t h e " n o n - r a d i c a l r e a c t i o n s o f f r e e r a d i c a l s " ( 3 , 4 ) ; e . g . , the formation of 2 , 2 , 6 , 6 - t e t r a m e t h y l - 4 o x o p i p e r i d i n e - l - o x y l oxime w i t h o u t c h e m i c a l involvement o f t h e f r e e

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

ROZANTSEV ET AL.

radical

Hindered Amines: Discovery, Chemistry, and Application 13

site: 0

N-OH

(8) T h i s and some o t h e r n o n - r a d i c a l r e a c t i o n s o f r a d i c a l s were t h e o r e t i c a l l y p r e d i c t e d by Neiman back i n t h e f i f t i e s . B u t , to o b t a i n t h e s t a r t i n g s t a b l e r a d i c a l s ( 5 , 6 ) , i t was n e c e s s a r y above a l l to o b t a i n v a r i o u s s t e r i c a l l development o f s t a b l e n i t r o x y hindered amines. It i s i n t e r e s t i n g t o note t h a t t h e f i r s t s t e r i c a l l y h i n d e r e d amine, c a l l e d " t r i a c e t o n a m i n e , was d e s c r i b e d as f a r back as 1874 b y two R u s s i a n c h e m i s t s , S o k o l o f f and L a c h i n o f f (7) and by a German c h e m i s t , H e i n t z ( 8 ) , whose works were p u b l i s h e d almost s i m u l t a n e o u s ly. The f i r s t s y n t h e s e s o f t r i a c e t o n a m i n e were based on t h e r e a c t i o n o f acetone w i t h ammonia. T h u s , t r i a c e t o n a m i n e became a c c e s s i b l e even though the y i e l d d i d not exceed 8%. At p r e s e n t , along w i t h a c e t o n e , some o t h e r s t a r t i n g m a t e r i a l s (9) such as d i acetone a l c o h o l , 2,2,4,4,6-pentamethyl-2,3,4,5-tetrahydropyrimidine and phorone a r e used f o r t h e s y n t h e s i s o f t r i a c e t o n a m i n e . I t i s not s u r p r i s i n g t h a t at the o u t s e t almost a l l h i n d e r e d amines n e c e s s a r y f o r t h e s y n t h e s i s o f v a r i o u s h i n d e r e d n i t r o x y l r a d i c a l s were o b t a i n e d from t r i a c e t o n a m i n e ( 1 0 - 1 2 ) . In 1962, u t i l i z a t i o n o f the new n i t r o x y l r a d i c a l s was suggested f o r i n h i b i t i o n o f o x i d a t i v e d e g r a d a t i o n o f t h e r m o p l a s t i c polymers (13). In 1965, M c C o n n e l l p u b l i s h e d h i s e a r l y work which l a i d t h e f o u n d a t i o n f o r a p p l i c a t i o n o f t h e new r a d i c a l s and t h e i r n o n - r a d i c a l r e a c t i o n s i n t h e s p i n - l a b e l method ( 1 4 ) . 1 1

Synthesis A c t u a l l y , t r i a c e t o n a m i n e s t i l l remains t h e o n l y s t a r t i n g compound f o r t h e s y n t h e s i s o f 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e d e r i v a t i v e s . The main methods o f i t s p r e p a r a t i o n a r e g i v e n i n Scheme 2. A summary o f methods o f s y n t h e s i s f o r t r i a c e t o n a m i n e and o t h e r h i n d e r e d p i p e r i d i n e s a r e summarized i n ( 1 5 ) . A h i g h l y e f f i c i e n t method o f p r o d u c i n g t r i a c e t o n a m i n e from acetone and ammonia t h r o u g h E q u a t i o n (4) was d e v e l o p e d i n the USSR (16). The c o s t o f t r i a c e t o n a m i n e produced on t h e b a s i s o f t h i s t e c h n o l o g y on a p i l o t p l a n t s c a l e i s a p p r o x i m a t e l y $10 p e r k i l o g r a m which i s s i g n i f i c a n t l y lower t h a n t h a t i n t h e A l d r i c h c a t a l o g (1982) o f c h e m i c a l s and i n t e r m e d i a t e s . By now a l a r g e number o f 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e d e r i v a t i v e s has been p r e p a r e d . These a r e m o s t l y compounds h a v i n g s u b s t i t u e n t s i n t h e f o u r - p o s i t i o n o f a p i p e r i d i n e r i n g , and t h e i r

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

N-H NH

2

N

'

?

NH

0 HN ^ 1 — . ^ N ^ XN . H I 0

7^N^NH

2

2

Scheme 1 . T a u t o m e r i c and resonance s t r u c t u r e s of a s t a b l e free r a d i c a l . 0 (I

+

CH3CCH3

CH

NH

3

^C-CH CCH « OH 0 2

CH, " L

3

CH

(1)

3

3

+

NH

3

+

CH3CCH0 » 0

3

+

1

(2)

CH3CCH3 » 0 If

3

.,C-CH CCH CH, I l« NH 0 2

3

2

CH3CCH3

+

NH

(3)

' CH ^

0

3

j ^ N

CH3CCH3

3

~ ^ N ^

0

H

(4)

(9)

0 NH,

Scheme 2 .

R e a c t i o n s f o r the s y n t h e s i s

(5)

of

triacetonamine.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application 15

s t r u c t u r e s show t h e i r c l o s e s t r u c t u r a l r e l a t i o n s h i p t o triacetonamine. A l t h o u g h t r i a c e t o n a m i n e has some s t r u c t u r a l p e c u l i a r i t i e s , i t undergoes many r e a c t i o n s t y p i c a l o f o r d i n a r y p i p e r i d i n e s (17) . D e t a i l e d i n f o r m a t i o n on t r i a c e t o n a m i n e c h e m i s t r y and s y n t h e s i s methods f o r 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e d e r i v a t i v e s can be found elsewhere ( 1 8 , 1 9 ) . The most important 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e d e r i v a t i v e s (Scheme 3) were s y n t h e s i z e d from t r i acetonamine (9) o r k e t o n e - r a d i c a l (5) i n one o r more s t e p s t h r o u g h t y p i c a l t r a n s f o r m a t i o n s o f the ketone c a r b o n y l g r o u p . The m a j o r i t y o f t h e w e l l known compounds i n t h e 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e s e r i e s was g e n e r a t e d from ( 1 0 , 11, 14). The most numerous ones a r e a l c o h o l e s t e r s ( 1 0 ) , amides and o t h e r amine d e r i v a t i v e s ( 1 1 ) . E l e c t r o c h e m i c a l methods f o r t h e s y n t h e s i s o f s e v e r a l i m p o r t a n t t r i a c e t o n a m i n e d e r i v a t i v e s have been e l a b o r a t e d r e c e n t l y ; e . g . , t h e e l e c t r o d e r e a c t i o n s shown b e l o w :

E l e c t r o c h e m i c a l methods f o r o b t a i n i n g t h e s i m p l e s t 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e d e r i v a t i v e s shown i n t h e scheme were a d v a n t a g e o u s l y used b y t h e a u t h o r s . E l e c t r o c h e m i c a l methods o f 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e p r e p a r a t i o n have u n d e n i a b l e advantages over t h e method based on t h e r e d u c t i o n o f t r i a c e t o n a m i n e by h y d r a z i n e at 150 - 200°C ( 2 5 ) . The r e a c t i o n s o f t h e a - m e t h y l e n e group ( p o s i t i o n s 3 and 5 ) , u n l i k e t h e r e a c t i o n s o f t h e c a r b o n y l g r o u p , a r e u n u s u a l and o c c u r under f o r c i n g c o n d i t i o n s . Some o f t h e s e r e a c t i o n s at p o s i t i o n 3 a r e g i v e n i n Scheme 4 . More d e t a i l e d i n f o r m a t i o n i s g i v e n i n ( 2 6 , 2 7 ) . R e a c t i o n s o f enamine (20) w i t h Mannich b a s e s , w i t h e s t e r s o f a, ) 8 - u n s a t u r a t e d a c i d s and w i t h a c r y l o n i t r i l e t a k e p l a c e under more s e v e r e c o n d i t i o n s as compared t o t h e s i m i l a r r e a c t i o n s o f c y c l o h e x a n e enamines. Enamines (20) do not undergo t h e r e a c t i o n s o f a c y l a t i o n and a l k y l a t i o n ; but t h e Mannich r e a c t i o n , t h a t o f b r o m i n a t i o n and t h e F a v o r s k i i rearrangement a r e u n e x p e c t e d l y e a s y . No o t h e r methods f o r i n t r o d u c i n g s u b s t i t u e n t s i n t o p o s i t i o n 3 o f t r i a c e t o n a m i n e , but those i n d i c a t e d i n Scheme 4, a r e known.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

Scheme 3 . Most i m p o r t a n t h i n d e r e d amine s t r u c t u r e s from t r i a c e t o n a m i n e .

derived

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18

POLYMER STABILIZATION AND DEGRADATION

3 - F o r m y l - 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e - l - o x y l o b t a i n e d by t h e r e a c t i o n g i v e n below c o u l d have been o f g r e a t importance f o r t h e synthesis chemistry of 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e derivatives. B u t , a f t e r i t s s y n t h e s i s had been r e p o r t e d , t h e r e i s no e v i d e n c e o f its u t i l i z a t i o n (28).

0

0

I. 0 Chemistry o f N i t r o x y l Radicals There i s no need t o g i v e h i n d e r e d n i t r o x y l r a d i c a l s as i t has been f u l l y d i s c u s s e d i n a number o f summaries i n c l u d i n g t h e papers w r i t t e n by t h e a u t h o r s o f the p r e s e n t communication (18,19 , 2 9 - 3 7 ) . T h i s paper i s aimed at g i v i n g some g e n e r a l c h a r a c t e r i s t i c s o f the c h e m i c a l p r o p e r t i e s o f n i t r o x y l r a d i c a l s and the methods f o r t h e i r s y n t h e s i s . N i t r o x y l r a d i c a l s have e x t r a o r d i n a r i l y h i g h s t a b i l i t y . N e v e r t h e l e s s , under some c o n d i t i o n s they behave as f r e e r a d i c a l s . H i n d e r e d n i t r o x y l r a d i c a l s recombine w i t h a c t i v e a l k y l r a d i c a l s to^givg hydroxylamine e t h e r s . The r a t e o f t h i s r e a c t i o n amounts t o 10 - 1 0 L / m o l . s e C . ; i . e . , n i t r o x y l s are e f f i c i e n t acceptors of a l k y l r a d i c a l s ; they a l s o react v i g o r o u s l y with h y d r o x y l r a d i c a l s (37-38). >N'-0

+

R-

>N-0

+

'OH

>N-0R

N

>N^ OH

The most c h a r a c t e r i s t i c r e a c t i o n o f h i n d e r e d n i t r o x y l s i s t h e i r r e d u c t i o n which r e s u l t s e i t h e r i n the c o r r e s p o n d i n g h y d r o x y l a m i n e o r amine. H y d r o x y l a m i n e s , as a r u l e , a r e r e a d i l y o x i d i z e d t o t h e c o r responding r a d i c a l s . O x i d a t i o n c a n be a c c o m p l i s h e d w i t h atmospheric oxygen i n t h e p r e s e n c e o f c a t a l y t i c amounts o f heavy m e t a l s a l t s ; e . g . , cupric s a l t s . PbC^ and K^[Fe(CN)^] c a n a l s o be used as o x i d i z i n g agents. This e a s i l y occuring t r a n s f o r m a t i o n - r a d i c a l —> h y d r o x y l a m i n e —> r a d i c a l - i s the b a s i s f o r s e v e r a l i m p o r t a n t s y n t h e s e s , i n p a r t i c u l a r , e l e c t r o c h e m i c a l s y n t h e s e s o f the n i t r o x y l r a d i c a l s (4) and (6) ( 3 9 , 4 0 ) :

OH 2e, 2 H

I OH

+

[0] N I • 0 (6)

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application

(4) Photoexcitation increase p h o t o c h e m i c a l r e a c t i o n o f the r a d i c a l (6) w i t h t o l u e n e y i e l d s q u a n t i t a t i v e l y t h e c o r r e s p o n d i n g h y d r o x y l a m i n e and i t s b e n z y l e t h e r (41). T h i s r e a c t i o n s i m u l a t e s t o a c e r t a i n e x t e n t the p r o c e s s o f polymer s t a b i l i z a t i o n by n i t r o x y l r a d i c a l s .

(5)

Though r e d u c t i o n o f t h e n i t r o x y l group o c c u r s q u i t e r e a d i l y , t h e r e are agents which a c h i e v e r e d u c t i o n o f o t h e r groups w i t h o u t i n v o l v i n g paramagnetic c e n t e r s . Examples o f such r e a c t i o n s are g i v e n below:

OH NaBH,

0*

•a

NaBELCN

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19

20

POLYMER STABILIZATION AND DEGRADATION

In a c i d medium, n i t r o x y l r a d i c a l s d i s p r o p o r t i o n a t e h y d r o x y l a m i n e s and oxoammonium s a l t s .

2

>N-0

+

2 HX

>>N^O*X"

to

+ >1*

produce

• x" ^DH

The mechanism o f t h i s r e a c t i o n i s c o n s i d e r e d i n (42) . At pH 1, t h e e q u i l i b r i u m o f t h i s r e a c t i o n i s s h i f t e d t o t h e r i g h t ; at pH 3, t o the l e f t . T h i s means t h a weaker t h a n t h e c o r r e s p o n d i n g a m i n e s . This p r o p e r t y f a c i l i t a t e s the s e p a r a t i o n o f amines from r a d i c a l s b y e x t r a c t i n g an amine w i t h d i l u t e h y d r o g e n c h l o r i d e (pH 2-3) . T h i s method o f r a d i c a l p u r i f i c a t i o n i s used f o r p r e p a r a t o r y p u r p o s e s . Oxoammonium s a l t s a r e s t r o n g o x i d i z i n g agents; i n f a c t , the o x i d i z i n g p r o p e r t i e s of n i t r o x y l r a d i c a l s i n a c i d medium a r e a s s o c i a t e d w i t h oxoammonium s a l t s . For example, i n a c i d medium, r a d i c a l s o x i d i z e a l c o h o l s t o t h e corresponding aldehydes.

S t r o n g o x i d a n t s such as c h l o r i n e and bromine o x i d i z e r a d i c a l s t o c o r r e s p o n d i n g oxoammonium s a l t s (30):

2

Developments

>N*-0

+

CI

2

i n Nitroxyl Radical

>

;>N=O

nitroxyl

CI

Synthesis

The u n u s u a l c h a r a c t e r i s t i c s o f t h e n i t r o x y l group d e t e r m i n e t h e methods o f n i t r o x y l r a d i c a l s y n t h e s i s . Here a r e t h e major ways o f their synthesis. 1.

O x i d a t i o n o f the corresponding hindered p i p e r i d i n e s . This s y n t h e s i s t e c h n i q u e c a n be used o n l y when t h e s t a r t i n g p i p e r i d i n e c o n t a i n s an H-atom on n i t r o g e n . This c o n d i t i o n r e s t r i c t s the a p p l i c a t i o n o f the method t o t h e most s i m p l e n i t r o x y l r a d i c a l s of the p i p e r i d i n e s e r i e s . The h y d r o g e n p e r o x i d e - s o d i u m t u n g s t a t e system i s most o f t e n used f o r p i p e r i d i n e o x i d a t i o n . The mechanism and k i n e t i c s o f t h i s r e a c t i o n a r e d e s c r i b e d i n (43). The t e c h n i q u e s o f h i n d e r e d p i p e r i d i n e s y n t h e s i s a r e constantly improving. T a k e , f o r example t h e method o f 2 , 2 , 6 , 6 -

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application

t e t r a m e t h y l - 4 - p i p e r i d i n o l o x i d a t i o n which makes i t p o s s i b l e t o o b t a i n the c o r r e s p o n d i n g r a d i c a l w i t h 98% y i e l d i n two h o u r s (44). T r i a c e t o n a m i n e and 2 , 2 , 6 , 6 - t e t r a m e t h y l - 4 - p i p e r i d i n o l are o x i d i z e d by the hydrogen p e r o x i d e - s o d i u m c a r b o n a t e system v e r y s e l e c t i v e l y , g i v i n g p r a c t i c a l l y a q u a n t i t a t i v e y i e l d (45). For amine o x i d a t i o n , the hydrogen p e r o x i d e - a c e t o n i t r i l e system i s o f t e n e f f e c t i v e enough ( 4 6 , 4 7 ) , w h i l e f o r h i n d e r e d p i p e r i d i n e o x i d a t i o n , p e r a c i d s can be a l s o u s e d . 2.

R e a c t i o n s which do not i n v o l v e the r a d i c a l s i t e . T h i s i s the most s i g n i f i c a n t c l a s s o f h i n d e r e d n i t r o x y l r e a c t i o n s . These r e a c t i o n s r e p r e s e n t a c h e m i c a l b a s i s f o r the s p i n - l a b e l method. The s i m p l e s t h i n d e r e d n i t r o x y l s , such as those i n d i c a t e d i n Scheme 3, a r e commonly used as the s t a r t i n g compounds. Some o f t h e s e r a d i c a l s are c o m m e r c i a l l y a v a i l a b l e i n s e v e r a l c o u n t r i e s . Many s e l e c t i v e r e a g e n t s are known at p r e s e n t which do not i n t e r act with a n i t r o x y l group you have o n l y t o choos u s i n g recommendations r e p o r t e d i n the s c i e n t i f i c l i t e r a t u r e and c a r r y out t h e r e a c t i o n .

3.

Hydroxylamine u t i l i z a t i o n . In t h o s e c a s e s where i t i s i m p o s s i b l e t o p r e s e r v e the r a d i c a l c e n t e r t h r o u g h a l l the s t a g e s o f s y n t h e s i s , i t can be c o n v e r t e d to the c o r r e s p o n d i n g h y d r o x y l amine. The l a t t e r as mentioned e a r l i e r , u n l i k e n i t r o x y l r a d i c a l s , i s s t a b l e i n a c i d i c medium and i s reduced at a h i g h e r potential. S e v e r a l examples o f h y d r o x y l a m i n e u t i l i z a t i o n f o r r a d i c a l s y n t h e s i s from the a u t h o r s ' e x p e r i e n c e a r e g i v e n on p. 18 and below ( 4 8 ) .

H 2 CI

NaOH

Application The i n v e s t i g a t i o n s u n d e r t a k e n w i t h the purpose o f s t u d y i n g the c h a r a c t e r i s t i c s o f r a d i c a l s r e s u l t e d i n wide p r a c t i c a l a p p l i c a t i o n o f the l a t t e r . N i t r o x y l r a d i c a l s are used i n b i o c h e m i s t r y , o r g a n i c

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION

c h e m i s t r y , h i g h m o l e c u l a r weight compound c h e m i s t r y , a n a l y t i c a l c h e m i s t r y and m e d i c i n e , and t h e range o f t h e i r a p p l i c a t i o n i s steadily increasing. H i n d e r e d n i t r o x y l s have found t h e i r w i d e s t a p p l i c a t i o n i n t h e s p i n - l a b e l method w i d e l y used i n b i o p h y s i c a l r e s e a r c h . As has a l r e a d y been m e n t i o n e d , t h i s method was d e v i s e d i n t h e c o u r s e o f n i t r o x y l c h e m i s t r y development when t h e r e a c t i o n s o f t h e r a d i c a l s o f t h i s c l a s s , not i n v o l v i n g a n i t r o x y l g r o u p , were d i s c o v e r e d . S e v e r a l monographs and many r e v i e w papers w i t h a t o t a l number o f more t h a n one thousand pages a r e devoted t o t h e s p i n - l a b e l method (35-39,49,50). These communications c o v e r a l l t h e a s p e c t s o f h i n d e r e d n i t r o x y l a p p l i c a t i o n i n t h i s method. T h e r e f o r e , the s p i n - l a b e l method i s not d i s c u s s e d h e r e at l e n g t h . Another important f i e l d o f the a p p l i c a t i o n o f n i t r o x y l r a d i c a l s and t h e i r p r e c u r s o r s , h i n d e r e d a m i n e s , i s t h e p r o t e c t i o n o f polymers and o t h e r o r g a n i c compounds a g a i n s t t h e f a c t o r s c a u s i n g t h e i r t h e r m o - and p h o t o - d e s t r u c t i o n . In t h e p r e s e n t work, an attempt has been made t o d e s c r i b e b r i e f l In c o n c l u s i o n , we s h a l l c o n s i d e r some o t h e r f i e l d s o f n i t r o x y l radical application. Polymer S t a b i l i z a t i o n " H i n d e r e d amine s t a b i l i z e r s have been the s i n g l e most important development o f t h e l a s t decade i n polymer s t a b i l i z a t i o n " P e t e r P . Klemchuk. The p o s s i b i l i t y o f s t a b i l i z i n g polymers w i t h h i n d e r e d n i t r o x y l r a d i c a l s i s based on t h e i r r e a c t i o n w i t h polymer r a d i c a l s which a r e engaged i n p r o p a g a t i n g polymer o x i d a t i o n . It was e s t a b l i s h e d t h a t hindered amines, the p r e c u r s o r s of n i t r o x y l r a d i c a l s , are a l s o e f f e c t i v e p h o t o s t a b i l i z e r s a n d , as s u c h , t h e y a r e more e f f e c t i v e t h a n most o f t h e common p h o t o s t a b i l i z e r s f o r p o l y m e r s . Complete i n f o r m a t i o n on s t r u c t u r e and a c t i o n mechanisms o f s t a b i l i z e r s based on h i n d e r e d amines c a n be found i n ( 3 8 , 5 2 - 5 6 ) . Here a r e p r e s e n t e d o n l y modern t r e n d s i n t h e c h e m i s t r y o f p h o t o - and t h e r m o s t a b i l i z e r s based on h i n d e r e d a m i n e s , t h e mechanisms o f t h e i r a c t i o n and t h e methods o f s e l e c t i n g s t a b i l i z i n g c o m p o s i t i o n s , i n c l u d i n g m i x t u r e s o f h i n d e r e d amines w i t h o t h e r c l a s s e s o f stabilizers. In Scheme 5 a r e g i v e n t h e s t r u c t u r e s o f some 2,2,6,6-tetramethylpiperidine d e r i v a t i v e s which a r e e f f e c t i v e p o l y mer s t a b i l i z e r s . The a n a l y s i s o f t h e s t r u c t u r e s o f t h e s e compounds a l l o w s t h e f o r m u l a t i o n o f t h e d e s i g n p r i n c i p l e s o f polymer s t a b i l i z e r s based on h i n d e r e d amines i n g e n e r a l and h i n d e r e d p i p e r i d i n e s i n p a r t i c u l a r (38,51) . Data on h i n d e r e d amine s t r u c t u r e s as polymer s t a b i l i z e r s l e a d t o the f o l l o w i n g c o n c l u s i o n s about the development o f r e s e a r c h i n t h i s field: 1.

Methods a r e b e i n g e l a b o r a t e d f o r s y n t h e s i z i n g m u l t i f u n c t i o n a l s t a b i l i z e r s c o m p r i s i n g i n one m o l e c u l e the fragments o f e i t h e r h i n d e r e d amine o r a n i t r o x y l r a d i c a l and UV a b s o r b e r , o r g a n i c p h o s p h i t e , hindered p h e n o l , t h i o e t h e r , metal d e r i v a t i v e , e s p e c i a l l y those of n i c k e l , e t c . The i n t r o d u c t i o n o f a UV a b s o r b e r fragment u s u a l l y r e s u l t s i n i n c r e a s e d e f f e c t i v e n e s s o f p h o t o p r o t e c t i v e a c t i o n because a

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application 23

d i f f e r e n t s y n e r g i s t i c mechanism o p e r a t e s between UV a b s o r b e r and a n t i o x i d a n t , and the UV a b s o r b e r fragment can quench the e x c i t e d s t a t e o f the n i t r o x y l r e s p o n s i b l e f o r p h o t o c h e m i c a l t r a n s f o r m a tion. I n t r o d u c t i o n o f an a d d i t i o n a l a n t i o x i d a n t group i s connected m a i n l y w i t h the n e c e s s i t y t o i n c r e a s e the s t a b i l i t y o f the m a t e r i a l i n the p r o c e s s o f i t s u t i l i z a t i o n . Among such s t a b i l i z e r s is Tinuvin 144, a C i b a - G e i g y s t a b i l i z e r ( 2 1 ) . At 1 1 0 ° - 1 2 0 ° C , i t performs comparably t o t h e r m o s t a b i l i z e r s such as Irganox 1010 (22).

2.

3.

Work i s c a r r i e d out t o o b t a i n compounds c o m p r i s i n g s e v e r a l h i n d e r e d amine or n i t r o x y l f r a g m e n t s . The mechanism by which the e f f e c t i v e n e s s o f p h o t o s t a b i l i z e r s w i t h two o r more h i n d e r e d amine fragments i s i n c r e a s e d i s not e s t a b l i s h e d . It was e m p i r i c a l l y o b s e r v e d t h a t n i t r o x y l e f f e c t i v e n e s s d u r i n g the i n h i b i t i o n o f p o l y p r o p y l e n e t h e r m o o x i d a t i o n i n c r e a s e s w i t h an i n c r e a s e i n the number o f paramagnetic c e n t e r s i n a m o l e c u l e . The e f f e c t i v e n e s s o f t h e r m o s t a b i l i z e r s o f the b i - r a d i c a l type i n c r e a s e s

when the d i s t a n c e between paramagnetic c e n t e r s d e c r e a s e s . This phenomenon may be due t o a cage r e a c t i o n o f the a l k y l r a d i c a l , which i s formed d u r i n g the i n t e r a c t i o n o f a polymer w i t h n i t r o x y l with another n i t r o x y l g r o u p . T h i s r e a c t i o n h i n d e r s the s i d e i n i t i a t i o n o f o x i d a t i o n by t h e n i t r o x y l r a d i c a l . There i s n ' t a s a t i s f a c t o r y e x p l a n a t i o n t o the i n c r e a s e i n p h o t o s t a b i l i z e r e f f e c t i v i t y w i t h the i n c r e a s e i n the number o f NH groups because mono- r a t h e r t h a n p o l y r a d i c a l s are formed d u r i n g p h o t o o x i d a t i o n of hindered amines. S t a b i l i z e r s o f h i g h c o m p a t i b i l i t y w i t h polymers are b e i n g s y n thesized. To improve t h e i r c o m p a t i b i l i t y w i t h p o l y m e r s , some fragments are i n t r o d u c e d i n t o the s t a b i l i z e r . F o r example, a l k y l s u b s t i t u e n t s p r o v i d e h i g h e f f e c t i v e n e s s to p h o t o s t a b i l i -

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

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Scheme 5 . S t r u c t u r e s o f h i n d e r e d amine s t a b i l i z e r s f o r polymers. Continued on next page.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application

Scheme 5 .

Continued.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

26

POLYMER STABILIZATION AND DEGRADATION

zers f o r p o l y o l e f i n s . Some c h a r a c t e r i s t i c s o f s t a b i l i z e r s s t r u c t u r e s appear t o c o n t r i b u t e t o t h e i r a c c u m u l a t i o n i n t h o s e s i t e s where p h o t o o x i d a t i o n o c c u r s . T h i s seems t o be p a r t i c u l a r l y a p p l i e d t o p o l y p r o p y l e n e which i s n o n u n i f o r m l y o x i d i z e d . 4. Methods o f h i n d e r e d amine s t a b i l i z e r s y n t h e s i s are b e i n g d e v e l oped w i t h t h e purpose o f r e d u c i n g c o s t and u s i n g a v a i l a b l e raw materials. The a c t i o n mechanism o f polymer p h o t o s t a b i l i z e r s based on h i n d e r e d amines a t t r a c t e d g r e a t a t t e n t i o n a f t e r numerous r e p o r t s on t h e i r h i g h e f f e c t i v e n e s s had a p p e a r e d . The main c o n c l u s i o n s from t h e s c i e n t i f i c l i t e r a t u r e on t h e mechanisms o f h i n d e r e d amine a c t i o n a r e offered below. More d e t a i l e d i n f o r m a t i o n c a n be found i n numerous r e v i e w s , t h e ones c o m p i l e d by t h e a u t h o r s o f t h e p r e s e n t a r t i c l e including (38,51-56). 1

The f o l l o w i n g methods a r e used at p r e s e n t t o p r o t e c t polymers against photooxidation (59): a. The use i n polymer c o m p o s i t i o n o f s u b s t a n c e s which at f i r s t absorb UV r a d i a t i o n form which i s not h a r m f u l t o t h e p o l y m e r . These compounds, c a l l e d UV a b s o r b e r s , p r o t e c t t h e polymer a g a i n s t UV r a d i a t i o n by d i r e c t a b s o r p t i o n . b. I n c o r p o r a t i o n i n t o polymer o f compounds which c a n d e a c t i v a t e photo e x c i t e d s t a t e s ( e i t h e r s i n g l e t o r t r i p l e t ) . c. I n c o r p o r a t i o n i n t o polymers o f s u b s t a n c e s c a p a b l e o f c a t a l y z i n g the n o n - r a d i c a l d e c o m p o s i t i o n o f h y d r o p e r o x i d e s . d. Make u s e o f compounds ( a n t i o x i d a n t s ) which a r e c a p a b l e o f trapping oxidation-propagating free r a d i c a l s thereby breaking the o x i d a t i o n c h a i n s . H i n d e r e d a m i n e s , n i t r o x y l r a d i c a l s , and t h e i r t r a n s f o r m a t i o n p r o d u c t s cannot a c t by t h e mechanisms o f UV l i g h t s c r e e n i n g o r q u e n c h i n g o f photo e x c i t e d s t a t e s . T h e i r a c t i o n i s due m a i n l y t o t h e i r involvement w i t h t h e o x i d a t i o n p r o p a g a t i n g c h e m i c a l r e a c t i o n s t a k i n g p l a c e d u r i n g polymer p h o t o o x i d a t i o n ; s o , t h e y a c t as r a d i c a l trapping antioxidants. But a number o f important f e a t u r e s e s s e n t i a l l y d i s t i n g u i s h them from common a r o m a t i c a n t i o x i d a n t s ( p h e n o l s and amines) which a r e poor polymer p h o t o s t a b i l i z e r s . S e v e r a l r e a c t i o n s may be i n v o l v e d i n p r o v i d i n g p r o t e c t i o n t o polymers a g a i n s t p h o t o d e c o m p o s i t i o n d e s t r u c t i o n . The c o n t r i b u t i o n of each o f t h e p r o c e s s e s depends on t h e n a t u r e o f t h e polymer and the o x i d a t i o n c o n d i t i o n s . The time d u r i n g which a h i n d e r e d amine i s t r a n s f o r m e d i n t o n i t r o x y l i n p o l y p r o p y l e n e i s s h o r t compared t o the i n d u c t i o n p e r i o d . T h e r e f o r e , p i p e r i d i n e s and c o r r e s p o n d i n g n i t r o x y l r a d i c a l s a r e almost e q u a l l y e f f e c t i v e f o r polymer s t a b i l ization. In r u b b e r , which i s more r a p i d l y o x i d i z e d t h a n p o l y p r o p y l e n e , n i t r o x y l r a d i c a l s a r e f a i r l y more e f f e c t i v e t h a n h i n d e r e d amines. Subsequent t r a n s f o r m a t i o n s o f n i t r o x y l r a d i c a l s , formed t h r o u g h R e a c t i o n s 6-8 a r e shown i n Scheme 6 ( 5 3 ) . The a n t i o x i d a n t a c t i o n o f n i t r o x y l r a d i c a l s i s due t o t h e i r a b i l i t y t o r e a c t w i t h a l k y l r a d i c a l s as i n R e a c t i o n 9 . Because o f t h e i r r e a c t i v i t y with a l k y l r a d i c a l s , n i t r o x y l s are b e t t e r photoo x i d a t i o n i n h i b i t o r s t h a n t h e i n h i b i t o r s o f o t h e r c l a s s e s . The r a t e c o n s t a n t f o r t{je r e a c t i o n o f n i t r o x y l s w i t h h y d r o x y l r a d i c a l ( E q u a t i o n 10) i s 10 L / m o l . s e c . T h i s r e a c t i o n c a n be o f p a r t i c u l a r importance f o r t h e i n h i b i t i o n o f polymer p h o t o o x i d a t i o n . R e a c t i o n s 11-13 i n i t i a t e polymer p h o t o o x i d a t i o n . However,

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

ROZANTSEV ET AL.

Hindered Amines: Discovery, Chemistry, and Application

t h e y do not have an a p p r e c i a b l e i n f l u e n c e on h i n d e r e d amine e f f i ­ c i e n c y because R e a c t i o n s 11 and 12 o c c u r o n l y at e l e v a t e d tempera­ t u r e s , and R e a c t i o n 13 o c c u r s o n l y under t h e a c t i o n o f l i g h t i n a narrow w a v e l e n g t h band, c o r r e s p o n d i n g t o n i t r o x y l r a d i c a l a b s o r p ­ t i o n , and t h i s i s t h e r e g i o n where n i t r o x y l s absorb l i g h t more p o o r l y t h a n p h e n o l s and a r o m a t i c amines. Moreover, h y d r o x y l a m i n e s formed from R e a c t i o n s 11-13 a r e e f f i c i e n t a n t i o x i d a n t s w h i l e t h e products of the p h o t o o x i d a t i o n of aromatic a n t i o x i d a n t s (phenols and amines) o f t e n f u n c t i o n as s e n s i t i z e r s f o r i n i t i a t i n g polymer photooxidat ion. R e a c t i o n 13 was s t u d i e d w i t h model compounds. R a d i c a l (6) undergoes t h e f o l l o w i n g p h o t o l y t i c r e a c t i o n :

50$ Under s i m i l a r c o n d i t i o n s , the five-membered n i t r o x y l (7) decomposes to e l i m i n a t e NO. Poor s t a b i l i t y o f five-membered n i t r o x y l s i s p r o b a b l y one o f the reasons why examples o f t h a t type a r e not o f f e r e d c o m m e r c i a l l y as polymer s t a b i l i z e r s . Hydroxylamines and t h e i r e t h e r s , formed d u r i n g the a f o r e s a i d R e a c t i o n s 9-13 a l s o i n h i b i t p h o t o o x i d a t i o n (Scheme 7 ) . Hydroxylamine s , u n l i k e n i t r o x y l r a d i c a l s , r e a c t w i t h p e r o x i d e r a d i c a l s (Reaction 11). The r a t e c o n s t a n t o f t h i s r e a c t i o n i s c l o s e to the c o r r e s p o n d i n g c o n s t a n t s e s t i m a t e d f o r most a c t i v e p h e n o l s . Hydroxylamine e t h e r s are p o o r e r i n h i b i t o r s t h a n commonly used phenols and a r o m a t i c amines ( r a t e c o n s t a n t f o r t h e i r r e a c t i o n w i t h Me2CNC00 r a d i c a l i n c h l o r o b e n z e n e at 65° i s 1-20 L / m o l . s e c . ) Under t h e r m a l c o n d i t i o n s , h y d r o x y l a m i n e e t h e r s c a n r e v e r s i b l y decompose ( R e a c t i o n 1 5 ) . The r a d i c a l s formed d i s p r o p o r t i o n a t e t o e l i m i n a t e o l e f i n s and y i e l d h y d r o x y l a m i n e ( R e a c t i o n 1 6 ) . In the presence of s u f f i c i e n t l y e f f e c t i v e acceptors of a l k y l r a d i c a l s ( e . g . , o x y g e n ) , the r e a c t i o n r a t e o f peroxy r a d i c a l f o r m a t i o n i s much h i g h e r t h a n t h a t o f h y d r o x y l a m i n e f o r m a t i o n . T h u s , i n the p r o c e s s o f polymer p h o t o o x i d a t i o n , n i t r o x y l r a d i c a l s r e g e n e r a t e and can b r e a k m u l t i p l e o x i d a t i v e c h a i n s . A n a l y s i s o f r e a c t i o n r a t e c o n s t a n t s i n model systems shows t h a t at room t e m p e r a t u r e , the main r e a c t i o n l e a d i n g t o r e g e n e r a t i o n of n i t r o x y l r a d i c a l s i s t h e i r i n t e r a c t i o n with peroxide r a d i c a l s , ( R e a c t i o n 11) and at e l e v a t e d t e m p e r a t u r e s (more t h a n 8 0 ° ) the main r e a c t i o n i s t h a t o f h y d r o x y l a m i n e e t h e r d e c o m p o s i t i o n ( R e a c t i o n 15) (53). It has been p r o p o s e d , m a i n l y i n the p a t e n t l i t e r a t u r e , to use h i n d e r e d amines i n m i x t u r e s w i t h o t h e r c l a s s e s o f s t a b i l i z e r s . A short survey of l i t e r a t u r e data along these l i n e s i s given i n (38). The m i x t u r e s c o n t a i n i n g h i n d e r e d amines and UV a b s o r b e r s 2-(hydroxy-

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27

POLYMER STABILIZATION AND DEGRADATION

>N-0

+

P-

>

>N -0

+

-OH ^

>N-OP

(6)

OH #

N

>N

(7) 0

>N-0

+

PH

N

>N-0

+

POOH

^

>N-OH

+

P0

>N10

+

PH

^

>N-OH

+

P.

^

Scheme 6 .

h i / >

(9)

2

(10)

T r a n s f o r m a t i o n s of n i t r o x y l

radicals.

>N-OH

+

P0 -

>

>N-0

+

POOH

>N-OH

+

POOH

>

>N-0

+

PO.

>N-OP

+

P'0

>

>N-0

+

P'OOH

+>C=CN-0P

+

P 0 .

>

>N-0

+

P'OOP

(14)

^

>K-0

+

P.

(15)

>

>N-0H

2

!

>N-OP >N-0

+

P-

2

2

cheme 7 . R e a c t i o n s of hydroxylamines n polymer s t a b i l i z a t i o n .

(11) +

H 0

(12)

2

+ >C=C


R-N-R' 0

0

C(CH ) 3

CN

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2

32

POLYMER STABILIZATION AND DEGRADATION

Complete information on application of the spin-trap method is given in summaries (70-72). Geophysics. Utilization of dynamic proton polarization phenomenon in nitroxyl radical solutions facilitated the improvement of spinprecision magnetometers by synchronizing the processes of nuclear polarization and precision measurement. Deuterated radical solutions serve as sensors in such instruments. Instruments of this type have already been produced in France. They find application in geophysical research, geological explorations and in searches for metallic objects under water. Oil Recovery. Nitroxyl radicals have found significant application in the oil production industry to control the rate of water encroachment of an oil pool. The method consists of the following: nitroxyl radicals are introduced into the water-injection well together with water, and water encroachment of the oil pool is followed by sampling and determining the nitroxyl radical content by EPR. Introduction of this method will permit increases in o i l recovery (73). Diamond Quality Assessment. The physical-chemical characteristics of nitroxyl radicals make it possible to evaluate crystal quality of diamonds, for example, with high sensitivity and reliability. It is possible to detect cracks on the crystal surface, to introduce quantitative criteria for estimation of the quality of diamonds, and to carry out defect differentiation (74). Antiwear. It has been shown that the addition of nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine series to lubricating compositions consisting of mineral oil and alkyl adipates (20%) considerably improves their lubricating properties (75). When utilizing the suggested compositions, the maximum load can be increased by 1.2-1.6 times for the same and sometimes even smaller friction factor. At the same time, the weight wearout rate is reduced by 1.2-2.0 times for bronze samples and by 2-8 times for steel samples. In conclusion, it is worth noting that nitroxyl radical applications have been limited heretofore by unjustifiably high prices. The prices of the simplest radicals in the Aldrich chemical catalog of 1982 were in the range of $1,000-$15,000 a kilogram. At present, they are being steadily reduced. In the authors opinion, the prices can be reduced even for small volumes of production at least by 1/50. This will lead to a wider investigation of their properties and, naturally, will extend the range of their application. 1

Literature Cited 1.

2. 3. 4.

Rozantsev, E. G.; Lebedev, O. L . ; Kararnovsky. S. N. "The Discovery of the Phenomenon of Stable Localized Free-Radical Center Formation," Otkrytie SSSR N 246 August 6, 1981. Otkrytiya SSSR, 18, Moscow. Rozantsev, E. G. "Free Iminoxyl Radicals," Khimiya, Moscow, 1970. Rozantsev, E. G. Izv. Akad. Nauk SSSR, Ser. Khim. 1963, 1669. Neiman, M. B.; Rozantsev, E. G.; Mamedova, Yu. G. Nature 1963, 200, 256.

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Hindered Amines: Discovery, Chemistry, and Application

USSR Patent 166 032, 1962. Rozantsev, E. G. Izv. Akad. Nauk SSSR, Ser. Khim. 1964, 2218. Sokoloff, N.; Latschinoff, P. Ber. 1874, 7, 1384. Heintz, W. Ann. 1874, 175, 133. Rozantsev, E. G. Ivanov, V. P. Khim.-Pharm. Zh. 1971, 47. Rozantsev, E. G. Izv. Akad. Nauk SSSR, Ser. Khim. 1964, 2187. Rozantsev, E. G.; Krinitzkaya, L. A. Tetrahedron 1965, 21, 491. Rozantsev, E. G.; Neiman, M. F. Tetrahedron 1964, 20, 131. USSR Patent 166 133, 1964. Ohnishi, S.; McConnell, H. M. J. Am. Chem. Soc. 1965, 87, 2293. Sosnovsky, G.; Konieczny, M. Z. Naturforsch. 1977, 32B, 328. Kedik, S. A.; Rozantsev. E. G.; Usvyatsov, A. A. Dokl. Akad. Nauk SSSR 1981, 257, N 6, 1382. Prostakov, N. S.; Gayvaronskaya, L. A. Usp. Khim. 1978, 47, 859. Rozantsev. E. G.; Kagan netic Models of Drugs and Biochemicals, Ed. Zdanov, R. I., C.R.C. Press Boca Raton, Florida 1985. Rozantsev, E. G.; Dagonneau, M.; Kagan, E. Sh.; Sholle, V. D.; Michailov, V. I. Synthesis 1984. Kagan, E. Sh.; Avrutskaya, I. A.; Kondrashov, S. V.; Novikov, V. T.; Fioshin, M. Ya.; Smirnov, V. A. Electrochemical Synthesis of 2,2,6,6-tetramethylpiperidine, Khimiya geterociclicheskih soedinenii 1984, 3, 358-359. Tomilov, A. P.; Smirnov, V. A.; Kagan, E. Sh. "Electrochemical Synthesis of Organic Compounds," Rostov University Ed. 1981, 59-61. USSR Patent 908.017, 1982. Surov, I. I.; Avrutskaya, I. A.; Fioshin, M. Ya. Electrochimia 1983, 19, 1561-1565. Tsarkova, T. G.; Avrutskaya, I. A.; Fioshin, M. Ya. Electrochimia 1984, 20, N3, 404-407. Rubtsov, M. V.; Baichikov, A. G. "Synthetic Pharmaceutical Chemicals, Meditsina," Moscow, 1971, 194. Rozantsev, E. G.; Dagonneau, M.; Kagan, E. Sh.; Mikhailov, V. I. J. Chem. Res. (S) 1979, 260; J. Chem. Res. 1979, 2901. Mikhailov, V. D. "Enamines in the Synthesis of Nitroxyl Radicals of the 2,2,6,6-tetramethylpiperidine Series," Cand. Dis. RGU, Rostov-on-the-Don, 1975. Briere, R.; Espil, J. C.; Ramasseul, R.; Rassat, A.; Rey, P. Tetrahedron Let. 1979, 941. Rozantsev, E. G. "Free Nitroxyl Radicals," Plenum Press, New York, London, 1970. Rosantsev, E. G.; Sholle, V. D. Organitscheskaya Khimia Svobodnych Radicalov, Khimia, Moscow 1979. Rozantsev, E. G.; Sholle, V. D. Synthesis 1971, 190-202. Rozantsev, E. G.; Sholle, V. D. Synthesis 1971, 401-414. "Spin-Labelling, Theory and Application," Ed. by L. Bezlinez, Academic Press, New York, London, 1976. Keana, I. F. W. Chem. Rev. 1978, 78, 37-63. Zhdanov, R. I. "Paramagnitnye Modeli Biologitscheski Aktivnych Soedinenii," Nauka, Moscow, 1981.

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36.

Forrester, A. K.; Hay, G. M.; Thomson, R. H. "Organic Chemistry of Stable Free Radical," Academic Press, London, 1967, 405. 37. Buchachenko, A. L . ; Vasserman, A. M. "Stabilnye Radicaly," Khimia, Moscow, 1973. 38. Dagonneau, M.; Ivanov, V. B.; Rozantsev, E. G.; Sholle, V. D.; Kagan, E. Sh. "Sterically Hindered Amines," J . M. S.-Rev. Macromol. Chem. Phys. 1982-1983, c. 22(2), 169-202. 39. Kagan, E. Sh.; Kondrashov, S. V.; Selivanov, V. N. in " A l l Union Conference on Electrochemistry" June 21-25, 1982, Moscow, 1982, 208. 40. Bogdanova, N. N.; Surov, I. I.; Avrutskaya, I. A.; Fioshin, M. Ya. Electrochimia 1983, 19, 1286. 41. Keana, I. F. W.; Dinerstein, R. I.; Baitis, F. J . Org. Chem. 1970, 36, 209-211. 42. Golubev, V. A.; Zhdanov, R. I.; Gida, V. M.; Rozantsev, E. G. Izv. Akad. Nauk SSSR Ser. Khim. 1971, 853-855. 43. Sen, V. D.; Golubev SSSR, Ser. Khim. 1982, 61-72. 44. Sosnovsky, G.; Konieczny, M. Z. Naturforshc. 1976, Teil B., 31, 1376-1378. 45. Levina, T. M.; Rozantsev, E. G. A. S. Chegolya. Dokl. Akad. Nauk SSSR 1981, 261, 109-110. 46. USSR Patent 391 137, 1973. 47. Raucman, E. I.; Rosen, G. M.; Abon-Donia, M. B. Synth. Comm. 1975, 5, 409-413. 48. Kagan, E. Sh.; Mikhailov, V. I.; Sholle, V. D.; Rozynov, B. V.; Rozantsev, E. G. Izv. Akad. Nauk SSSR, Ser. Khim. 1977. 49. Likhtenshtein, G. I. "Spin-Labelling Methods in Molecular Biology," Nauka, Moscow, 1977. 50. Kuznetsov, L. N. "The Spin-Probe Method," Nauka, Moscow, 1976. 51. Toda, T.; Kurumada, T. Sankyo Kenkyosho Nempo "Research and Development of Hindered Amine Light Stabilizers," Ann. Rep. Sankyo Res. Lab. 1983, 35, c. 1-37. 52. Shlyapintokh, V. Ya. "Photochemical Transformations and Polymer Stabilization," Khimia, Moscow, 1979. 53. Shlyapintokh, V. Ya.; Ivanov, V. B. "Developments in Polymer Stab.," Ed. G. Scott, Appl. Sci. Publ. London, 1982, 5, 41. 54. Allen, N. S. "Developments in Polym. Photochemistry," Ed. N. S. Allen. Appl. Sci. Publ., London, 1981, 239. 55. Carlsson, D. J . ; et. a l . "Developments in Polymer Stabilization," Ed. G. Scott, Appl. Sci. Publ., London, 1979, 1, 219. 56. Gugumus, F . , "Developments in Polymer Stabilization," Ed. G. Scott, Appl. Sci. Publ., London, 1979, 1, 261. 57. Shlyapintokh, V. Ya.; Bystritskaya, E. B.; Shapiro, A. B.; Smirnov, V. A.; Rozantsev, E. G.; Izv. Akad. Nauk SSSR, Ser. Khim. 1973, 1915. 58. Shlyapintokh, V. Ya.; Ivanov, V. B.; Hvostach, D. M.; Shapiro, A. B.; Rozantsev, E. G. Dokl. Akad. Nauk SSSR, 1975, 225, 1132.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2.

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59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75.

Hindered Amines: Discovery, Chemistry, and Application 35

Randy, B.; Rabek, J. F. "Photodegradation, Photooxidation and Photostabilization of Polymers," Wiley-Interscience, New York, 1975. Allen, N. S.; McKeller, I. F.; Wilson, D. Chem. Ind. 1978, 887-889. Rozantsev, E. G.; Krinitskaya, L. A.; Troitskaya, L. S. Khim. Prom. 1964, 20, 180. Vakula, V. A.; Pritykin, L. H. "Foundation of Physical Chemistry of Polymer Adhesion," Khimia, Moscow, 1984. USSR Patent 600 162, 1978. Pritykin, L. M.; Genel, L. S.; Gerenrot, V. G.; Shapiro, A. B.; Bakula, V. L. Plastics 1981, 36-39. USSR Patent 732 405, 1980. Danjushina, G. A.; Kagan, E. Sh.; Smirnov, V. A.; Pritykin, L. M. Plasticheskie Massy 1982, 83. Pritykin, L. M. Biofisika 1976, 21, 1059. Pelevina, I. I.; chnye Faktory Reaktsi pevticheskie Vozdeistvija, , , , All-Union Conference on Nitroxyl Radicals, Chernogolovka, May 12-14, 1982, Tezisy Dokladov. Pedersen, I. A.; Torssel, K. Acta. Chem. Scand. 1971, 25, 3151-62. Yansen, E. G. Acc. Chem. Res. 1971, 4, 31-40. Lagercrantz, C. J. Phys. Chem. 1971, 75, 3466. Bukin, I. I.; Rozantsev, E. G. et. al. Neftyanoe Khozyaistvo 1978, 46. Bukin, I. I.; Kagarmanov, N. F. et al. Sverkhtverdye Materialy 1983, 27, 23. USSR Patent 802 358, 1981.

RECEIVED January 31, 1985

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3 Progress in the Light Stabilization of Polymers T O S H I M A S A T O D A , T O M O Y U K I K U R U M A D A , and KEISUKE M U R A Y A M A Chemical Research Laboratories, Research Institute, Sankyo Co., Ltd., 2-58, Hiromachi 1-chome, Shinagawa-ku, Tokyo 140, Japan

Since their i n i t i a l production by our laboratories about ten years ago (HALS) have become established as excellent light stabilizers of polymers. This review deals with their synthesis, evaluation and development. Hindered Amine Light Stabilizers (HALS) are a new class of highly efficient stabilizers protecting polyolefins and other polymers against light-induced deterioration. They were i n i t i a l l y developed into commercial products in our laboratories. In this review we describe the details of how they were synthesized, evaluated and developed. In order to find a new potential stabilizer, our initial efforts were directed toward synthesizing new stable nitroxyl radicals and evaluating their stabilizing activity. Although these stable radicals were good light stabilizers, they could not be used commercially since they contributed color to the polymers. Finally, intensive studies led to the discovery that hindered amine compounds derived from 2,2,6,6-tetramethyl-4-oxopiperidine may be converted into the corresponding stable nitroxyl radical and are excellent light stabilizers for polymers. Derivatives of 2,2,6,6-tetramethylpiperidine were synthesized and tested, and as a result, esters of 4-hydroxy2,2,6,6-tetramethylpiperidine were selected as practical light stabilizers, particularly for polyolefins. Degradation and Stabilization Oxidation reactions are generally believed to involve a free radical chain reaction as proposed by Bolland and Gee Q). The main steps of this reaction are: (1) R. + 0

2

R00- + RH

>R00> R00H + R-

(2) (3)

0097-6156/85/0280-0037$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

38

POLYMER STABILIZATION AND DEGRADATION ROOH

heat or l i f i h t ^

2 ROOH

>R0-

R0- + RH

+

(A)

# Q H

+ ROO- + H 0 2

>ROH + R.

HO- + RH 2 ROO

R Q
HOH + R. ^

Product ketones, alcohols, e t c ,

(5) (6) (7)

(8)

Radicals formed during i n i t i a t i o n react with oxygen leading to chain reactions. G e n e r a l l y , R e a c t i o n 2 i s f a s t e r than Reaction 3. The decomposition of hydroperoxides by heat or UV l i g h t (Equation 4) causes f o r m a t i o n of alkoxy and hydroxy r a d i c a l s l e a d i n g to c h a i n branching. UV absorbers can absorb t h i s harmful energy, s l o w i n g i n i t i a t i o n reactions, while antioxidants (InH) act to interfere with propagation according to the following reaction: ROO- + InH

> ROOH + In-

(9)

F i n a l l y , peroxide decomposers can decompose hydroperoxides i n a nonr a d i c a l process to interfere with Reaction 4. T y p i c a l examples of the s t a b i l i z e r s g e n e r a l l y used to prevent the above c h a i n r e a c t i o n a r e : (a) UV absorbers — 2 ( 2 - h y d r o x y - 3 tert-buty 1-5-methylpheny 1 ) - 5 - c h i or oben zo t r i a zo le an d 2-h yd ro xy - 4 octoxybenzophenone; (b) Antioxidants — 3 , 5 - d i - t e r t - b u t y l - 4 - h y d r o x y toluene and octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate; (c) P e r o x i d e decomposers — d i l a u r y l thiodipropionate. In a d d i t i o n quenchers such as the o r g a n i c n i c k e l complex, N i ( I I ) b i s ( d i i s o p r o p y l d i t h i o c a r b a m a t e ) are used for the deactivation of the e x c i t e d s t a t e s of the chromophoric groups r e s p o n s i b l e for l i g h t initiation. Since p o l y o l e f i n s r e a d i l y undergo o x i d a t i v e d e t e r i o r a t i o n on exposure to UV l i g h t , c o n v e n t i o n a l s t a b i l i z e r s were found to be unsatisfactory for long-term outdoor applications. Therefore, more effective s t a b i l i z e r s for polyolefins were desired. Stable Nitroxyl Radicals Since UV absorbers cannot completely prevent the i n i t i a t i o n reactions above, we f e l t t h a t better a n t i o x i d a n t s were necessary to o b t a i n improved l o n g - t e r m p h o t o - o x i d a t i v e s t a b i l i t y of p o l y o l e f i n s . We reasoned that stable r a d i c a l s which scavenge r a d i c a l s formed i n the o x i d a t i v e process might compensate for the i m p e r f e c t i o n s of the absorbers. On the basis of the assumption that the d i a r y l n i t r o x y l r a d i c a l s , rather than t h e i r parent d i a r y l a m i n e s , are the a c t i v e a n t i f a t i g u e s p e c i e s i n rubber p r o t e c t i o n , we f i r s t attempted to obtain new stable n i t r o x y l r a d i c a l s . The f i r s t radicals we looked at, the alkylaminotropone nitroxy r a d i c a l s (1), were unstable (2). However, s i n c e Neiman e t a l . (_3) had reported that 2,2,6,6-tetramethyl-4-oxopiperidine-l-oxyl (2) was extremely stable, we turned to n i t r o x y l r a d i c a l s derived from f u l l y hindered amines. In t h i s section we w i l l describe the synthesis of new stable n i t r o x y l r a d i c a l s and their evaluation i n polypropylene.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

TODA ET AL.

39

Light Stabilization of Polymers

2,2,6,6-Te t r amet hy 1 - 4 - s u b s t i t u t e d p i p e r i d i n e - 1 - o x y I s . Neiman's stable n i t r o x y l r a d i c a l (2) was prepared i n our laboratory and tested f o r i t s s t a b i l i z i n g a c t i v i t y on polymers. Its light s t a b i l i z i n g effect was superior to that of conventional UV absorbers; however, i t was unstable a t e l e v a t e d temperatures. Heating c r y s t a l s o f (2) a t 105-110°C for 3 hours under nitrogen gave l-hydroxy-2,2,6,6-tetramethyl-4-oxopiperidine (3) and a trace of 2,6-dimethyl-2,5-heptadien-4one (4). On the other hand, when c r y s t a l s of (2) were a l l o w e d to stand at room temperatur th y l -3 - ( 2 , 2 , 6 , 6 - te t r a m was o b t a i n e d as a p r e c i p i t a t e from the l i q u e f i e d substance. We believe the decomposition of (2) proceeds as shown i n Chart 1 (4). The heat s t a b i l i t y of a d d i t i v e s i s an important f a c t o r s i n c e polymer processing i s done at high temperatures (150-280°C). Therefore we considered ways t o o b t a i n more t h e r m a l l y s t a b l e n i t r o x y l radicals. If the hydrogen alpha to the keto group of (2) i s involved i n i t s decomposition as shown i n Chart 1, analogous compounds having no keto group should be more stable than (2). Indeed, after heating under the conditions described above, both 2,2,6,6-tetramethylpiperid i n e - l - o x y l (6) and 4 - h y d r o x y - 2 , 2 , 6 , 6 - t e t r a m e t h y l p i p e r i d i n e - l - o x y l (7) were almost completely recovered. OH

Additional examples of stable r a d i c a l s derived i n our laboratory from 4 - k e t o p i p e r i d i n e - l - o x y l (2) are shown i n Chart 2. Substituted 4-oxoimidazolidine-l-oxyls. We found that the hydrogen peroxide o x i d a t i o n of 2,5-bis(spiro-l-cyclohexyl)-4-oxoimidazolidine (17) (9) prepared by self-condensation of 1-amino-l-cyanocyclohexane (16) yielded the corresponding n i t r o x y l r a d i c a l (27) (10).

(16)

(O)

(27)

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

40

POLYMER STABILIZATION AND DEGRADATION

When the new stable r a d i c a l (27) showed excellent l i g h t s t a b i l i zing a c t i v i t y f o r p o l y p r o p y l e n e , syntheses of analogous compounds were planned. From a self-condensation reaction only two different substituents may be introduced on the imidazolidine r i n g . Mechanist i c s t u d i e s of t h i s r e a c t i o n u s i n g the N - l a b e l e d compound (16a) showed that the nitrogen of the n i t r i l e group appears only i n the 3p o s i t i o n of the i m i d a z o l i d i n e r i n g (11). The s e l f - c o n d e n s a t i o n of (16a) yielded the corresponding 4-oxoimidazolidine(3N) (17a), as confirmed by i t s mass spectrum (molecular ion at m/e 223.171 c o r r e sponding to C13H92O NN) and the presence i n esr of the corresponding n i t r o x y l r a d i c a l (27a) t r i p l e t . The r e a c t i o n of (16a) with c y c l o hexanone also yielded (17a). The reaction i s considered co proceed as shown i n Chart 3 (11). T h e r e f o r e , i t i s p o s s i b l e to obtain compounds w i t h four d i f ferent substituents at the 2- and 5-positions of the 4-oxoimidazolidine ring by reacting the appropriate carbonyl compound with an alpha aminonitrile formed i n a The o x i d a t i o n of these presence of c a t a l y t i c amounts of sodium tungstate i n acetic acid gave new stable n i t r o x y l radicals (12) shown i n Chart 4. Light S t a b i l i z i n g A c t i v i t y of Nitroxyl Radicals. The polymer s t a b i l i z i n g a c t i v i t y of the stable radicals was evaluated as follows: (i) The s t a b i l i z e r (0.25 weight %) was i n c o r p o r a t e d i n t o polypropylene powder by wet b l e n d i n g with e t h a n o l . The r e s u l t i n g mixture was preheated to 215°C for 2 minutes, and then compression-molded, i . e . , at 215°C for 0.5 minutes to y i e l d 0.5 mm s h e e t s . As a c o n t r o l , unstabilized polypropylene sheet was prepared i n a s i m i l a r manner, ( i i ) For testing of l i g h t s t a b i l i t y each test specimen was exposed to l i g h t i n a Fade Meter (carbon arc lamp) at a black panel temperature of 63±3°C and examined p e r i o d i c a l l y by bending test to determine the time to embrittlement. ( i i i ) For testing of thermal s t a b i l i t y each t e s t specimen was p l a c e d i n a f o r c e d a i r c i r c u l a t i o n oven a t 150°C and the embrittlement time was measured by bending t e s t s . Results are summarized i n Table I. As shown i n Table I, these s t a b l e r a d i c a l s showed s t r i k i n g l y higher l i g h t s t a b i l i z a t i o n a c t i v i t y i n polypropylene than that of the UV absorber tested. We f e l t that their a c t i v i t y was related to their r a d i c a l scavenging a b i l i t y . T h i s hypothesis i s supported by the observation that the coupled products (32) and (33) were obtained by the r e a c t i o n of the n i t r o x y l r a d i c a l s (2) and (27), r e s p e c t i v e l y , with a C - r a d i c a l d e r i v e d from AIBN (10). The r a d i c a l scavenging a b i l i t y of the stable n i t r o x y l radicals i s now w e l l known to play a major role i n the mechanism of l i g h t s t a b i l i z a t i o n by hindered amine compounds (13). Unfortunately, the stable n i t r o x y l r a d i c a l s caused yellowing i n polymers. The yellowing may be due to the weak absorption of v i s i b l e l i g h t by the n i t r o x y l radicals themselves or to formation of colored r e a c t i o n products i n the polymers. We observed t h a t y e l l o w i n g occured when n i t r o x y l r a d i c a l s were used i n combination with a phen o l i c a n t i o x i d a n t such as 3 , 5 - d i - t e r t - b u t y l - 4 - h y d r o x y t o l u e n e (34). T h i s may be due to the r e a c t i o n of (2) and (34) which gives a h i g h l y colored product (35) as shown i n Chart 5.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

TODA ET AL.

o

u

• > •N 0c 6«

41

Light Stabilization of Polymers

>

> N0' c OH

(2)

+

6

(3)

2)

o

+ OH

[

( ) 1.

Chart

2.

D e c o m p o s i t i o n of

O. _l

(2) NH NH 2

2

NaCN |(NH4) C0

R2CH2CN

R1NH2

NO*]

5

^R

2

3

3

• R3 k

NC-C-R2

8 -n-Butyl K I

V

J
T x I H

R

2

R4

R

Ri 27 28 29 30 31

Cyclohexyl Me-, MeMe-, Et MeMe2-Me-Cyclohexyl

Chart 4. New s t a b l e n i t r o x y l r a d i c a l s i m i d a z o l i d i n e s by hydrogen p e r o x i d e .

H

3

2

Cyclohexyl H, n-Propyl MeMeMe-, hButyl, MeEtCyclohexyl H, n-Undecyl H, Ph Cyclohexy 2-Me-Cyclohexy

17 Cyclohexyl 18 Cyclohexyl 19 Me-, Me20 Me-, Me21 Me-, Me2 2 MeMe23 Cyclohexyl 24 Cyclohexyl 25 MePh2 6 2-Me-Cyclohexyl

R R / ^ N ^ R4 I 0»

2

4

R3

2

N H

RnV

Rs H Q

2

° \ 3 "

C

H

2

~*

#

H

O

"

-

4

Cyclohexyl Me-, MeMeEtCyclohexyl 2-Me-Cyclohexyl

from t h e o x i d a t i o n o f

^ ^ "

C

H

2

"

C

H

2

^ ^ J

^ > 2

CH-CHP(

>=0

+( ) 3

(35) Chart 5 .

R e a c t i o n s of 2 and 34.

Light

2 " x CI

(38)

(36)

(28)

(42)

>0^< +

>0< CI

(37)

Light

# c |

^ *0N" < I O. ~~

(39)

(PhCH ) Hg 2

2

^ jC^X:

C

I CH Ph 2

L i g h t

( P h C H 2 ) 2 H

(40)

C h a r t 6. R a d i c a l s g e n e r a t e d by p h o t o l y s i s c h l o r a m i n e s 36 and 3 7 .

(6)

9 * N' H (41)

of s o l u t i o n s o f N-

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

TODA ET AL.

Light Stabilization of Polymers

45

Synthesis and S t a b i l i z i n g _ A c t i v i t y of Hindered Amines. As mentioned previously, the hindered piperidine compounds showed excellent l i g h t s t a b i l i z i n g a c t i v i t y i n polypropylene. In order to find more e f f i cient compounds, various derivatives of 2,2,6,6-tetramethyl-4-oxopip e r i d i n e (42) were s y n t h e s i z e d and t e s t e d . In t h i s s e c t i o n we describe some t y p i c a l examples from the great number of derivatives prepared i n our laboratory. In analogy to the Bucherer reaction, we carried out the reaction of 4-amino-4-cyano-2,2,6,6-tetramethylpiper i d i n e (46) with phenyl isocyanate. Heating the r e s u l t i n g ureido compound i n benzene gave the imino-hydantoin (47), while heating i n aqueous hydrochloric acid gave 3-phenylhydantoin (48). S i m i l a r l y , r e a c t i o n of 4 - c y a n o - 4 hydroxy-2,2,6,6-tetramethylpiperidine (49) with p - t o l y l i s o c y a n a t e gave the imino-oxazolidone (50) and the oxazolidinedione (51). Compounds having various substituents at the 3 - p o s i t i o n were prepared (Chart 8). The l i g h t - s t a b i l i z i n g a c t i v i t y of these compounds and the 4-oxoimidazolidines described e a r l i e r are l i s t e d i n Table II (22,23). Table I I .

L i g h t - s t a b i l i z i n g A c t i v i t y of Hindered Amines

Compound m.p. ( b . p . ) Number °C 42 (105/18mmHg) 47 177 52 196 53 157 54 115 48 143 55 187 56 166 57 96 50 152 58 95 51 156 174 59 60 107 17 220 170 19 23 (170/0.0005mmHg) 25 131 UV absorber None

Embrittlement (hours) 320 220 300 400 800 860 660 660 800 60 80 60 200 40 180 120 380 600 80 40

a) Tested by the method described for Table I . Syntheses of hydantoin compounds from 1 , 3 , 5 - t r i a z a - 7 , 7 , 9 , 9 tetramethylspiro[4.5]decane-2,4-dione (61) were also planned, since derivatives of t h i s type showed high a c t i v i t y compared to o x a z o l i dones and 4 - o x o i m i d a z o l i d i n e s . F u r t h e r , three d i f f e r e n t types of

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

46

POLYMER STABILIZATION AND DEGRADATION

H2O I O II CH3CCH3+NH3

, f

CaCl2

CH3COCH3

CaCl2 or NH4CI ^ N j
H

H

(43)

CH3

H

(42)

O/f CH3CCH3

\ ^

Chart 7 . ammonia.

i O

1H

2

O

*

(44)

42

(45)

< >

Compounds o b t a i n e d from the r e a c t i o n of acetone and

N-R RNHCONH CN

H N^CN 2

RNCO *N' H (46)

NH

47 -Ph 52 -Cyclohexyl 53 -Et 5 4 -Octadecyl

3

HO^CN

48 55 56 57

R: -Ph -Cyclohexyl "Et -Octadecyl

51 59 60

-p-Tolyl -Cyclohexyl -Octadecyl

RNHCOO^ CH RNCO

A

H (49) 50 58

Chart 8.

R: -p-Tolyl -Octadecyl

Compounds having v a r i o u s s u b s t i t u e n t s a t the 3 - p o s i t i o n .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

3.

TODA ET AL.

47

Light Stabilization of Polymers

p i p e r i d i n e a c e t a l s were s y n t h e s i z e d (Chart 9), and t h e i r s t a b i l i z i n g a c t i v i t i e s are shown i n Table III (24).

Table III.

L i g h t - s t a b i l i z i n g A c t i v i t y of

Compound m.p. ( b . p . ) Number °C 63 125 64 53 65 139 66 (128/2mmHg) 67 (194/3mmHg) 68 137 69 20 70 21 UV absorber None

light-

Piperidine-spiroacetals

Embrittlement Time" (hours) 160 1540 800 460 600 220 80 40

a) Tested by the method described for Table I .

Catalytic hydrogenation of (42) led to 4-hydroxy-2,2,6,6-tetramethylpiperidine (71) which was derivatized by known methods as shown i n Chart 10. Since i t was expected that bifunctional compounds would exhibit more s t a b i l i z i n g a c t i v i t y than monof u n c t i o n a l ones, p o l year boxy l i e acid p i p e r i d i n o l esters were synthesized. Their s t a b i l i z i n g a c t i v i t i e s are summarized i n Table IV. The l i g h t - s t a b i l i z a t i o n a c t i v i t y of several esters was tested by an improved t e s t method (25). T h i s method was c a r r i e d out as f o l lows: (i) Preparation of the test specimen. A mixture of 100 parts of polypropylene powder, 0.2 parts of the antioxidant, octadecyl 3(3,5-di-tert-butyl-4-hydroxyphenyl)propanoate, and 0.25 parts of the s t a b i l i z e r was needed for 10 minutes at 200°C i n a Brabender p l a s t i Corder to give a homogeneous m a t e r i a l . T h i s m a t e r i a l was then pressed to a thickness of 2-3 mm i n a laboratory press. A portion of t h i s sheet was pressed for 6 minutes at 260°C i n a h y d r a u l i c press and then immediately placed i n cold water, yielding a 0.5 mm sheet. Following the same procedure, a 0.1 mm f i l m was obtained from the 0.5 mm s h e e t . T h i s f i l m was cut i n t o t e s t pieces 50x120 mm. P o l y p r o pylene films without any s t a b i l i z e r s were prepared as above and used as c o n t r o l s , ( i i ) T e s t i n g of l i g h t - s t a b i l i t y . The t e s t specimens were aged as d e s c r i b e d for Table I above. The exposed f i l m s were subjected to tension tests at regular i n t e r v a l s , and the times r e c o r ded when the test pieces contracted to 50% of their o r i g i n a l extension (Half L i f e Time, HLT).

American Chemical Society Library 1155 16th St., N.W. In Polymer Stabilization and 0.C. Degradation; Klemchuk, P.; Washington, 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

48

POLYMER STABILIZATION AND DEGRADATION

NaCN/(NH4) C0 2

3

>

H

xT

'

V,

HN

N

o

~ T

V

0

v

RX D

H X ~ \ ,N-pO

* "OCT > r - A -

NH

o

61

M

e

C

(

C

H

2

°

H

)

^

62

HN^YY" \—

/

6

2

N

-

R

R-Octyl

HN^YY* \ — ' 0-^\—OCOR

«« ...»

HOCH(CH OH) * 2

^ C O O ^

O-/V_oH

)H/°n

H (42)

/

y »

p , «

— OCOR

65

6 6 R: -Me 67 : - P h

68

69 R: -Me 70 :-CH2Ph

C(CH20H)4

Chart 9. S y n t h e s i s of three d i f f e r e n t types of p i p e r i d i n e and h y d a n t o i n compounds.

acetals

Tun

^ N ^ H

I

RCOOMe

RNCO 0CONHR

r^S J

L H

OCOR

7 2 -Et 73 -Cyclohexyl

J L ^ N ^ H

MeOCORCOOMe

OCO-R-COO R: R: 74 -Me -Me r^S /\ 7 7 --CH2( C -Heptadecyl.J 75 -Heptadecyl L J L 7 8-I H ) -(CH2) -Ph 76 -Ph >^N^ 7980 0P Phenylene H H 8 2

4

[PPH] >NX]

"

2

0

)

A c c o r d i n g t o p r e d i c t i o n s made by C a r l s s o n (2) appreciable s t a b i l i z a t i o n a l r e a d y o c c u r s even i n the case o f r e l a t i v e l y low PPOO- /NX interaction. ( F o r h i g h e r v a l u e s o f v c o r r e s p o n d i n g l y lower s t a b i l i z e r r e a c t i v i t y would be needed t o a c h i e v e the same s t a b i l i z i n g effect.) The c o n d i t i o n f o r s t a b i l i z a t i o n by PPOO-/ NX i n t e r a c t i o n i s , a c c o r d i n g to e q u a t i o n (1) s i m p l y g i v e n by m

k

N X

[ >NX] [PPOO-] »

As l o n g as t h i s r e l a t i o n h o l d s , oxidation is greatly reduced. Curve c : complete

2k [PP00r

2

the k i n e t i c c h a i n l e n g t h o f

I n h i b i t e d p h o t o o x i d a t i o n by ^NX/PPOOperacid radical destruction k

]

photo-

i n t e r a c t i o n and

[PPH]

NX i n t e r a c t i o n . T h e r e f o r e , p a r t i a l p e r a c i d r a d i c a l scavenging would g i v e an e f f e c t somewhere i n between c u r v e s b and c . As a l r e a d y m e n t i o n e d , the e s t i m a t i o n p r e s e n t e d h e r e a p p l i e s to the r a t h e r u n r e a l i s t i c a s s u m p t i o n , t h a t each HOO- r a d i c a l l e a v i n g a p a i r o f r e c o m b i n i n g p r i m a r y peroxy r a d i c a l s would be a b l e to i n i t i a t e a propagating o x i d a t i v e c h a i n . In r e a l i t y , t h i s might

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

84

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

dlPPOQ]

J(t)-2k [PP00'f - k N X ] [ p P 0 0 l = 0

a

t

NX

(steady s t a t e

dt

d[PP00H] dt

e

(1)

approximation]

(2)

k [PPH](PP0O], D

H

where k : propagation const, j ^ ppQQp

k^: termination const.| k :rate const, of *IX with PPOO' NX

J(t) .rate of initiation of propagating macroperoxiradical chains. J(t) o>(PPOOH] = e

where J

J

J « «

O

D

O

.

•

J «

D

,

(3)

H O O - ^f-lPPOOH] = rate of chain initiation tt

RCOOO 3

ation by RCOOO".

with 0 ^ p^ 1 p=1 refers to no interaction of peracid radicals with ^NX. p=0 refers to complete scavenging of peracid radicals by^NX. Scheme IV. K i n e t i c s f o r i n i t i a t i o n of p r o p a g a t i n g peroxy r a d i c a l c h a i n s by HOO*.

F i g u r e 6. Model c a l c u l a t i o n a c c o r d i n g t o Scheme I I I ( f o r f u r t h e r e x p l a n a t i o n see t e x t ) .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5.

FELDER

Mechanistic Studies of Sterically Hindered Amines

85

not be t h e c a s e , s i n c e f r e e l y m o b i l e HOO- r a d i c a l s a r e expected t o d i s a p p e a r q u i t e q u i c k l y by r e c o m b i n a t i o n w i t h o t h e r s p e c i e s before c h a i n i n i t i a t i o n can take p l a c e . A c c o r d i n g l y , c h a i n i n i t i a t i o n by p e r a c i d r a d i c a l s might be o f much more importance t h a n assumed i n our model c a l c u l a t i o n . T h e r e f o r e , p e r a c i d r a d i c a l s c a v e n g i n g by TMP d e r i v a t i v e s i s expected t o be an i m p o r t a n t a d d i t i o n a l s t a b i l i z i n g process i n p o l y p r o p y l e n e . Experimental a) R a d i c a l c o u n t i n g and p h o t o o x i d a t i o n experiments I r r a d i a t i o n s were c a r r i e d out on an o p t i c a l bench i n q u a r t z c e l l s of 1 cm path l e n g t h u s i n g l i g h t from a 200 W h i g h p r e s s u r e Hg lamp. The l i g h t was f i l t e r e d by a 315 nm i n t e r f e r e n c e f i l t e r and r e n d e r e d p a r a l l e l by use o f a q u a r t z l e n s . In o r d e r t o p r o t e c t t h e i n t e r f e r e n c e f i l t e r from damage caused by l o n g time exposure t o s h o r t wave l e n g t h r a d i a t i o n , a pyrex p l a t e ( c u t o f f t-BuOH + R*) was measured by >N0- consumption ( >N0- + R- —»>N0R) under complete absence o f oxygen ( N f l u s h i n g b e f o r e and d u r i n g ?

irradiation). The >N0- used ( i n i t i a l c o n c e n t r a t i o n : 2.10 M) was N i t r o x i d e I d e s c r i b e d i n (_1). >N0- consumption was measured s p e c t r o s c o p i c a l l y ( \ m a x . = 470 nm, e ^ 10(Mcm) ). Photooxid a t i o n s were c a r r i e d out e i t h e r i n 10 ml q u a r t z c e l l s ( i n c a s e o f h i g h r a d i c a l i n i t i a t i o n ) o r i n a s p e c i a l c e l l o f a p p r o x i m a t e l y 100 ml and 1 cm path l e n g t h ( i n c a s e o f low i n i t i a t i o n ) . The c e l l s u r f a c e was i n a l l c a s e s u n i f o r m l y i l l u m i n a t e d . The r a t e o f r a d i c a l p r o d u c t i o n I was c o n t r o l l e d by u n i f o r m a t t e n u a t i o n o f l i g h t i n t e n s i t y , u s i n g a c o m b i n a t i o n o f c a l i b r a t e d copper g r i d s p l a c e d i n the space between t h e l e n s and the r e a c t i o n c e l l . The TMP d e r i v a t i v e s used i n p h o t o o x i d a t i o n s t u d i e s were amine I I i n (_1) and a >N-octyl derivative. N

Q

b) A n a l y t i c a l procedures T o t a l c o n c e n t r a t i o n o f h y d r o p e r o x i d e s , p e r o x i d e s and p e r a c i d s : The i o d o m e t r i c method d e s c r i b e d by C a r l s s o n & W i l e s (16) was a p p l i e d . Note: t e r t i a r y BuOOBu i s not measured under t h e s e c o n d i t i o n s ; o t h e r p e r o x i d e s , f o r example R C H ^ O C ^ R a r e however t i t r a t a b l e . D e t e r m i n a t i o n o f " a c t . _ 0 " i n t h e p r e s e n c e o f >N0- : S i n c e >NOa l s o c o n v e r t s I t o 1^ , c o r r e c t i o n s f o r t h i s c o n t r i b u t i o n have been made by gas c h r o m a t o g r a p h i c e s t i m a t i o n o f ( > N 0 - ) . (Standard used: N i t r o x i d e d e r i v e d from amine I ) . T i t r a t i o n of peracids. P e r a c i d s have been determined by making use 2

of t h e q u a n t i t a t i v e and s e l e c t i v e r a c t i o n o f p e r a c i d s w i t h o l e f i n s [epoxidation (10)]: To a g i v e n volume o f irradiated i s o o c t a n e an e q u a l volume o f 7 - t e t r a d e c e n e s o l u t i o n ( 2 . 1 0 M) was added. A f t e r 15 hours s t a n d i n g i n t h e d a r k , t h e r e m a i n i n g h y d r o p e r o x i d e s and p e r o x i d e s were t i t r a t e d (C ., = C_ . .. - C . . ). peracids total remaining r

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

86

P O L Y M E R STABILIZATION

A N D DEGRADATION

c) Distribution of >NO-isooctyl isomers formed by trapping of isooctyl radicals with >N0- . Irradiation was carried out as for radical counting; the 100 ml —2 cell_Yas used, and full light intensity was applied ( l « 10 M sec. ). The distribution of isomers was measured by gas chromatography using a Varian 2740 FID instrument (OV 101, Poropak columns). Identification: Separation of isomers by preparative thin layer 13 chromatography; identification by C-NMR in the spectroscopic laboratory of our Analytical Department. Acknowledgement The author wishes to thank Dr. G. Rist for recording and interpreting C-NMR spectra. He is very grateful to Dr. H. Gysling for very valuable encouragemen management of CIBA-GEIGY Limited in Basle for permission to report the results of this investigation. Literature Cited 1. B. Felder, R. Schumacher and F. Sitek, ACS Symposium on Photodegradation and Photostabilization of Coatings, ACS Symposium Series 151, 65 (1980). 2. D.J. Carlsson, K.H. Chan and D.M. Wiles, ACS Symposium on Photodegradation and Photostabilization of Coatings, ACS Symposium Series 151, 51 (1980). 3. D. Bellus, H. Lind and J.F. Wyatt, J . Chem. Soc. Chem. Commun. 1972, 1199. 4. B. Felder and R. Schumacher, Angew. Makromol. Chem. 31, 35 (1973). 5. J.B. Shilov and E.T. Denisov, Vysokomol. Soed. 16A 2313 (1974). 6. D.J. Carlsson, D.W. Grattan and D.M. Wiles, Coatings and Plastic Preprints 39, 628 (1978). 7. D.W. Grattan, D.J. Carlsson and D.M. Wiles, Polym. Degrad. Stab. 1, 69 (1979). 8. D.J. Carlsson, K.H. Chan, A. Garton and D.M. Wiles, Pure Appl. Chem. 52., 389 (1980). 9. A. Garton, D.J. Carlsson and D.M. Wiles, Die Makromol. Chem. 52, 389 (1980). 10. Houben Weyl, Bd. II, Analytische Methoden, 308 (1953). 11. C. Walling and B.B. Jacknow, J . Amer. Chem. Soc. 82, 6108 (1960). 12. E. Niki and Y. Kamiya, J . Org. Chem. 38 Nr. 7, 1403 (1973). 13. B. Felder, R. Schumacher and F. Sitek, Helv. Chim. Acta 63, 132 (1980). 14. D.J. Carlsson, K.H. Chan, J . Durmis, and D.M. Wiles, J. Polymer Sci., Pol. Chem. Ed., 20, 575 (1982). 15. E. Niki and Y. Kamiya, Bull. Soc. Japan, 38, 3226 (1975). 16. D.J. Carlsson and D.M. Wiles, Macromolecules 2, 597 (1969). RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

6 Reactions of Aminyl Radicals and Mechanisms of Amine Regeneration as Inhibitors of Oxidation E. T. DENISOV Institute of Chemical Physics, USSR Academy of Sciences, 142 432 Chernogolovka, USSR

Diphenylaminyl radicals, In•, produced in tetraphenylhydrazine decomposition diphenyl amine and oligomeric semidienes. Diphenyl amine is formed in the reaction of In• with labile dimers, AmAm, with iminoquinone structure. The rate constant of Am•recombination measured by flash photolysis technique (FTP) was found to be 1.8x10 l.mole s (cyclohexane, 293°K). The combination of In• andRO •was studied by FPT using monitoring at two different wavelengths. It runs with the rate constant of 6x10 l.mole s (cyclohexane, 293°K) with formation of iminoquinone as the main product. RadicalsAm •react with cumylhydroperoxide very fast (k = 1.1x10 l.mole s , cyclohexane, 293°K, FTP). The mechanism of this reaction is complicated at [ROOH] > 5xl0 mole 1-1 by parallel consecutive reaction with proton transfer and InH+• formation and electron transfer from ROO to InH+•. Quinone imine retards the oxidation of n-heptadecane (393°K) with regeneration of 4-hydroxydiphenylamine in the presence of hydroperoxide. 7

-1

-1

2

8

-1

-1

5

-1

-1

5

-3

Each aromatic amine molecule, InH, terminates many free radical chains in autooxidation of alcohols and amines due to the ability of oxyperoxy and aminoperoxy radicals to oxidize InH as well as to reduce In to InH CO. However, the coefficient of inhibition, f > 2, can be very often observed in oxidizing hydrocarbons too (_2) . Therefore, some reduction of aminyl radicals to InH proceeds in oxidizing hydrocarbons. To ellucidate the ways of such reduction we have studied the products and kinetics of the reactions of diphenylaminyl radical In*. Recombination of Aminyl Radicals. A convenient source of In* is tetraphenylhydrazine, Inln. Diphenylamine and oligomeric semidienes were found to be the products of Inln decomposition at T > 400°K (.3*4) . The kinetics of their formation were studied by Varlamov C5). He found that InH and o-semidienes were primary stable products of 0097-6156/85/0280-0087$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

88

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

I n l n d e c o m p o s i t i o n and p - s e m i d i e n e s were formed a f t e r any p e r i o d o f induction. InH was proposed t o be formed i n t h e f o l l o w i n g sequence of r e a c t i o n s (4^,5^):

In-

+ In'

-

-

In-C H =N-C H 6

5

6

—

5

InH + (

C

6

H

5

)

2

N C

6 4^ 6 5 H

C

H

As t h e r e s u l t o f such r e a c t i o n pathways, o l i g o m e r i c s e m i d i e n e s were formed. The y i e l d o f InH was found t o be 80% on decomposed I n l n (CCl^, 348°K). Such r e a c t i o n s may be t r e a t e d as r e g e n e r a t i o n o f InH from In* . The k i n e t i c s o f I n l n d e c o m p o s i t i o n a r e f i r s t o r d e r ( 6 ) ; t h e i n t r o d u c t i o n of a free r a d i c a l acceptor i n c r e a s e s the rate_^onst^ant. F o r example, I n l n decomposes w i t h t h e r a t e c o n s t a n t 6.3x10 s in C C l ^ at 348°K and i n t h e j j r e s e n c e o f N - p h e n y l - 2 - n a p h t h y l a m i n e t h e r a t e c o n s t a n t i s 14.8x10 s . This suggests that the decomposition o f I n l n i n c l u d e s an e q u i l i b r i u m s t a g e and may be d e s c r i b e d by t h e f o l l o w i n g scheme: Inln

From t h e d a t a mentioned k

2

/

k

-L

=

° '

7

4

~

2In

—

Products

above,

one can e s t i m a t e

the r a t i o

,

The k i n e t i c s o f In* r e c o m b i n a t i o n were s t u d i e d by FPT i n c y c l o hexane s o l u t i o n (_7). The c o n c e n t r a t i o n o f In* was measured s p e c t r o p h o t o m e t r i c a ^ l y at X_= 77(j) nm. The e x t i n c t i o n c o e f f i c i e n t was found t o be 3.9x10 l.mole cm . I n . r a d i c a l s were found t o d i s a p p e a r i n a bimo^ecular r ^ a c | i o n with the o v e r a l l r a t e constant of 1.8x10 l.mole s , which i s t h e sum o f r a t e c o n s t a n t s on Inr e c o m b i n a t i o n i n J o _ J n I n and q u i n i n e i m i n e ^ f o y m a t i o n e q u a l t o 0.75x10 l.mole s and 1.05x10 l.mole s , respectively (cyclohexane, 293°K). R e a c t i o n o f In* With R O ' ^ The k i n e t i c s o f In- r e a c t i o n w i t h RO^ from c y c l o h e x a n e were a l s o s t u d i e d by FPT (1). D i - t - b u t y l p e r o x i d e was d e composed p h o t o c h e m i c a l l y , and t h e r e a c t i o n o f TCH„)^C0* r a d i c a l s w i t h cyclohexane produced c y c l o h e x y l r a d i c a l s . The l a t t e r were t r a n s formed i n t o p e r o x y r a d i c a l s R0£ a f t e r t h e a d d i t i o n o f o x y g e n . R a d i c a l s In* were formed by t h e p h o t o l y s i s o f I n l n . The k i n e t i c s o f RO' and In* d i s a p p e a r a n c e were m o n i t o r e d s i m u l t a n e o u s l y at X = 270 and 770 nm, r e s p e c t i v e l y . Under t h e e x p e r i m e n t a l c o n d i t i o n s when [R0 ] » [In* ] , In* d i s a p p e a r e d m a i n l y by t h e r e a c t i o n w i t h R0 » Because o f t h e slow r e c o m b i n a t i o n o f RO^* t h e r e i s a n e a r l y c o n s t a n t R0 concentration ( ~ 6 x l 0 mole.l ). The r a t e c o n s t a n t o f t h e r e a c t i o n w i t h ^ I n ^ w i t h ROl was e s t i m a t e d t o be e q u a l t o the 6x10 l.mole s i n t h e t e m p e r a t u r e range o f 2 8 3 - 3 0 3 ° K i n c y c l o hexane. The d e c o m p o s i t i o n o f I n l n i n t h e p r e s e n c e o f c u m y l h y d r o p e r o x i d e y i e l d e d d i p h e n y l a m i n e , quinone i m i n e , cumyl a l c o h o l and acetophenone ( 8 ) . D i p h e n y l n i t r o x i d e was d e t e c t e d by EPR t e c h n i q u e s . The comparison o f t h e c o n c e n t r a t i o n s o f decomposed ROOH and InH and ROH formed shows t h a t ROOH i s n o t t h e o n l y donor o f h y d r o g e n atoms but a p a r t o f InH and ROH was formed i n t h e r e a c t i o n s o f In* and R0" with l a b i l e intermediate products. The most p r o b a b l e p r o d u c t s a r e 2

q

2

2

l a b i l e quinone i m i n e s ° 2 6 5 6 4 ^ 6 5^2 6 5 6 5* w i t h I n and Ro* forming InH and ROH. Quinone imine a p p a r e n t l y R

C

H

C

H

a

n

d

C

H

N C

H

N C

H

w

h

i

c

h

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

r

e

a

is

c

t

6.

DENISOV

89

Aminyl Radical Reactions and Amine Regeneration

formed from RO„C^H^NC H,_. So RO^ r e a c t s benzene rriinn gg o or r aminyl aminyl r a d i c a l : R0

+ In

2

RO! + In* 2

w i t h a n i t r o g e n atom and

—-

RO* + InO*

—

R0 C,H,NC.H, 2 o D b D o

The Mechanisms o f A m i n y l R a d i c a l R e a c t i o n w i t h ROOH. The k i n e t i c s o f In* r e a c t i o n w i t h c u m y l h y d r o p e r o x i d e were s t u d i e d i n c y c l o h e x a n e at 393°K u s i n g F P T . The c o n c e n t r a t i o n o f In* was measured s p e c t r o p h o t o m e t r i c a l l y at X = 812 nm. Aminyl r a d i c a l d i s a p p e a r e d i n t h e p r e s e n c e of ROOH i n two r e a c t i o n s , namely In* + In* and In* + ROOH. r e a c t i o n r a t e c o n s t a n t o f In* w i t h ROOH at [ROOH] = 1 . 4 - 5 . 1 x 1 0 was c a l c u l a t e d v i a computer from experimental_£in|tic curves o f In* d i s appearance and found t o be 1.1x10 l.mole s . A new a b s o r p t i o n was observed a t _ ^ m a x = 6^0 nm i n experiments w i t h [ROOH] > 10 mole.l . which may be formed by In*

+ HOOR

»

InH* + R0~ +

The k i n e t i c s o f In* and IriR' consumption were measured at X = 712 nm. It was found t h a t I n ' + InH* d i s a p p e a r i n t h e p r e s e n c e o f ROOH w i t h f i r s t o r d e r k i n e t i c s , t h e observed r a t e c o n s t a n t k k [ROOH] A l n D ^ ^ / t decreases with increasing hydroperoxide concentration. The f o r m a t i o n o f I n H . and dependence o f k [ROOH] may be e x p l a i n e d by t h e f o l l o w i n g k i n e t i c scheme: +

g

=

+

Q b s

In*

+ HOOR

InH

In*

+ HOOR

InH* + R 0

2

InH

2

+ InH* + R 0

o

+

o

n

R0'

+ R0

2

The treatment of t h e e x p e r i m e n t a l (lata according t o t h i s scheme^gives the f o l l o w i n g v a l u e s : k = 1.1x10 l.mole s , K = 42 l . m o l e , k^ = 1.0x10 s~" ( 2 9 3 ° K , c y c l o h e x a n e ) . When ROOH c o n c e n t r a t i o n i s h i g h enough ( 0 . 1 m o l e . l o r more) t h e r e a c t i o n runs v i a p r o t o n a t i o n of In* and e l e c t r o n t r a n s f e r . Quinoneimine as an I n h i b i t o r o f O x i d a t i o n R e a c t i o n s . Quinoneimine, C H NC H 0 , was found t o r e t a r d o x i d a t i o n f o r a l o n g p e r i o d o f t i m e . If quinoneimine i s introduced i n t o o x i d i z i n g n-heptadecane (393°K) containing hydroperoxide, i t retards oxidation. The h i g h e r t h e hydroperoxide c o n c e n t r a t i o n the stronger the r e t a r d i n g a c t i o n of quinoneimine. Aminophenol, HOC^H^NC^H , was found i n s m a l l c o n c e n t r a t i o n among t h e r e a c t i o n p r o d u c t s . T h i s means t h a t q u i n o n e i m i n e i s reduced t o aminophenol i n o x i d i z i n g h y d r o c a r b o n i n t h e p r e s e n c e o f hydroperoxide. S i n c e t h e aminophenol i s o x i d i z e d by peroxy r a d i c a l s to quinoneimine, the opposing c y c l i c o x i d a t i o n - r e d u c t i o n r e a c t i o n s proceed i n o x i d i z i n g h y d r o c a r b o n w i t h p a r t i c i p a t i o n o f R0« and ROOH.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

P O L Y M E R STABILIZATION

90

A N D DEGRADATION

Hence, the extra chain termination (f > 2) when aromatic amines are used as inhibitors of oxidation may be the result of the following reactions: In* + In-

> InC H NC H 6

5

6

5

RO: + InC^H NC^H > ROOH + InCJH. NC.H,, 2 6565 6465 In' + RO! > R0 C.H,.NC.H_ 2 2oD o 5 In + R0 C,H_NC H •> InH + RO* + 0C,H. NC H. 26565 6465 ROOH + 0C H,NC^H 5> . . . > HOC.H. NHC.H_ 6465 6 4 6 5 All the reactions mentioned above make the inhibiting coefficient f much more than 2, when amines are used as inhibitors. At present we do in such reactions or the stages of quinoneimine reduction to aminophenol . Literature Cited c

c

o

o

c

£

1. 2. 3. 4. 5. 6. 7. 8.

c

C

c

Denisov, E.T. in "Developments in Polymer Stabilization 3"; Scott, G., Ed.; Appl. Sci. Publ. LTD: London, 1980; pp. 1-20. Berger, H.; Bolsman T.A.B.M.; Brower, D.M. in "Developments in Polymer Stabilization 6"; Scott, G., Ed.; Appl. Sci. Publ. LTD: London, 1983; pp. 1-27. Musso, H. Chem. Ber. 1959, 92, 2881. Welzel, P. Chem. Ber. 1970, 103, 1318. Varlamov, V.T. Izv. Akad. Nauk SSSR, ser.khim. 1982, 1481. Varlamov, V.T. Izv. Akad. Nauk SSSR, ser.khim. 1982, 1629. Varlamov, V.T. Safiullin, R.L.; Denisov, E.T. Khimicheskaya Fizika 1983, 408. Varlamov, V.T.; Denisov, E.T. Kinetika i Kataliz 1983, 24, 547.

RECEIVED December 7,1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7 Hindered Diazacycloalkanones as Ultraviolet Stabilizers and Antioxidants J. T. LAI, P. N. SON, and E. JENNINGS The BFGoodrich Company, Brecksville, OH 44141

The properties of hindered diazacycloalkanones were evaluated as light stabilizers and antioxidants for polypropylene. A totally hindered piperazinone was found to be excellent light stabilizer with many other desirable properties as a polymer additive. Partially hindered decahydroquinoxalinone derivatives were found to be surprisingly good antioxidants. The relationship between structure parameters and activity was investigated. Since Hindered Amine Light Stabilizers (HALS) were first introduced about a decade ago, their commercial applications have proliferated to the stage where they may well dominate the polymer light stabilizers market in the near future. Among the many commercial and experimental HALS, only the BFGoodrich Company offers one that does not bear a 2,2,6,6-tetramethylpiperidine (1) structure. In this presentation, we would like to report our work on the chemistry and properties of our developmental HALS 1,1 -(1,2-ethanediyl) bis(3,3,5,5-tetramethyl-2-piperazinone) (2), and the interesting antioxidant (AO) and light-stabilizing properties of some substituted decahydroquinoxalinone s (3) . 1

1

, 0

0

.

HN N - CH CH - N NH 2

H

'

2

1

R* | Cj^-N^R HR

Dr. Layer has given one plausible explanation as to how 3 performs as an AO. The structural parameters of 3 will also be probed to locate the origins of its activity. 2

0097-£156/85/0280-0091$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

92

POLYMER STABILIZATION A N D

DEGRADATION

1,1 -(1,2-Ethanediyl)bis(3,3,5,5-tetramethyl-2-piperazinone) (2) ( p i p e r a z i n e ) was p r e p a r e d from a n o v e l s y n t h e s i s u s i n g e a s i l y a v a i l a b l e raw m a t e r i a l s : 1

3

HC 3

CH J N0

HC H H CH yCH NCH CH NCH -/ H C* CH

3

3

+

(CH 0) 2

+ H2NCH2CH2NH2 — >

2

X

2

2

3

2

1

2

3

3

N0

N0

2

2

4

N

+

H

2

—

H C H H CH \-CH NCH CH NCH -4— C H 3

•

H

5 + CHC1

NaOH

0 3

J N a U n

+

HoC

>

3

2

3

NH

C

2

2

2

NH

2

2

HN ' ^ N - C H C H - N

CHQ

2

H C 3

CH

NH

2

H

3

3

3

C

C H

3

piperazine 2 P i p e r a z i n e HALS 2 i s a c o l o r l e s s c r y s t a l l i n e s o l i d which m e l t s at 134-6°C. I t i s a p o w e r f u l UV s t a b i l i z e r f o r p o l y o l e f i n s as i l l u s t r a t e d i n the outdoor aging data i n i s o t a c t i c polypropylene tapes.

2x100 m i l S l i t Tapes - A r i z o n a Aged F a i l u r e = Months t o 50% Loss o f T e n s i l e

0.1 p h r HALS +

AO-1 AO-2 A0-3 A0-4

Piperazine

Piperidine

9 10 12 8

5 6 10 3

Polymeric Piperidine

A O ' s a r e added a t 0 . 1 p h r . P i p e r i d i n e : Bis(2,2,6,6-tetramethyl4 - p i p e r i d i n y l ) decanedioate; Polymeric p i p e r i d i n e : Poly[[6[(l,l,3,3-tetramethylbutyl)amino]-l,3,5-trizaine-2,4-diyl(2,2,6,6tetramethyl-4-piperidinyl)imino-1,6-hexanediyl-(2,2,6,6-tetramethyl4-piperidinyl)imino] A0-1: l,3,5-Tris(3,5-di-t-butyl-4-hydroxyphenyl)methyl-l,3,5triazine-2,4,6-(lH,3H,5H)-trione. AO-2: Tris 2-[p-(3,5-di-t-butyl-4-hydroxyphenyl)propionoxy]ethyl isocyanurate. A0-3: Octadecyl 3,5-di-t-butyl-4-hydroxybenzenepropionate. AO-4: Pentaerythritol tetrakis-[0-(4-hydroxy-3,5-di-t-butylphenyl) propionate].

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.

LAI ET AL.

0.25

93

Hindered Diazacycloalkanones

phr HALS +

AOA0A0AO-

Piperazine

Piperidine

Polymeric Piperidine

15 17 21 14

13 15 19 15

13 15

Piperazine 2 i s also highly synergistic with commercial UV absorbers, as shown i n 20 m i l - t h i c k compression-molded polypropylene samples aged i n a xenon Weather-Ometer.

• = 0.25 80-

phr piperazine

0.125 phr benzophenone benzophenone: 2-hydroxy4-octyloxybenzophenone

IOOO 2000 3000 4000 HOURS in XENON WEATHER-OMETER

0.25 phr piperazine = 0.25 phr benzotriazole = 0.125 phr piperazine + 0.125 phr benzotriazole benzotriazole: 2-(2-hydroxyl-3,5-di-t-butylphenyl)-5chloro-2H-benzotriazole A

0

1000 2000 3000 4000 HOURS in XENON WEATHER-OMETER Piperazine HALS 2 has a p o s i t i v e effect on the processing s t a b i l i t y of polypropylene when used i n conjunction with a phenolic antioxidant:

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

94

16

X - Q

12

00

— CO

.

J — CO

UJ

02 05 0.10 0.50

E x t r u s i o n Passes 2 a l s o has a v e r y lene f i l m s :

favorable

water

at

piperazine piperazine piperazine piperazine

270°C

carryover behavior

i n polypropy-

F i l m Speed

Additive

1. 2. 3.

phr phr phr phr

(ft/min)

134 78.5 150.5

Benzophenone P i p e r i d i n e HALS P i p e r a z i n e HALS

PP c o n t a i n i n g 0.1 p h r a d d i t i v e was e x t r u d e d a t 260°C i n t o a 5 m i l f i l m t h a t was quenched i n a 3 7 . 8 ° C water b a t h . Tachometer measurements o f the f i l m speed a t the p o i n t o f water c a r r y o v e r was recorded. The h i g h e r the number, the f a s t e r t h e p r o d u c t i o n would b e , which i s p r e f e r r e d .

I n the second p a r t o f t h i s p r e s e n t a t i o n , we would l i k e t o i n t r o d u c e a n o t h e r f a m i l y o f amine s t a b i l i z e r s . Namely, d e c a h y d r o q u i n o x a l i n e s 3 (R,R =alkyl) w h i c h not o n l y show l i g h t - s t a b i l i z i n g a c t i v i t y , b u t also antioxidation properties. f

C^^N-^p R

C x ^ N ^ - iC H

H

H H

R

CH CH

3

3

3 i s p r e s e n t as a m i x t u r e o f c i s - and t r a n s - i s o m e r s . Cis-3,3dimethyl-decahydroquinoxalin-2-one (6) i s a known compound while 3

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7.

95

Hindered Diazacycloalkanones

LAI ET AL.

a l l trans-3 are n o v e l . 3 (R =H) c a n be s y n t h e s i z e d from two d i f f e r e n t routes^*: a) from t h e c o n d e n s a t i o n o f 1 , 2 - d i a n r i n o c y c l o h e x a n e and a ketone c y a n o h y d r i n i n h o t w a t e r , o r b) from t h e r e a c t i o n among DCH, k e t o n e , c h l o r o f o r m , and b a s e . !

/ ^ N H

a

2

HO

+

CN

J

fY^f

^

H

NH

+ CHC1

NH

0 II

2

3

+ R

2

NaOH "*" ^

R

3 (R'=H)

J 1

R

C i s - and t r a n s - 3 a r e e a s i l y s e p a r a b l e by an a c e t o n e wash, s i n c e t h e t r a n s - i s o m e r i s i n s o l u b l e i n almost a l l s o l v e n t s w h i l e t h e cis-isomer i s easily soluble o r as a m i x t u r e , was t h e n a l k y l a t e d w i t h v a r i o u s a l k y l a t i n g agents t o g i v e 3 (R* = a l k y l , a l k y l e n e , e t c . ) . R

H

f

R

As a p a r t i a l l y h i n d e r e d amine, 3 showed light-stabilizing activity lower t h a n t h a t o f p i p e r a z i n e o r p i p e r i d i n e HALS, b u t comparable t o commercial benzophenones. L i k e HALS, t h e y a r e s t r o n g l y s y n e r g i s t i c w i t h benzophenones and b e n z o t r i a z o l e s i n p o l y o l e f i n s . L i g h t - S t a b i l i z i n g A c t i v i t y of 3 Formulations

1. 2. 3. 4. 5.

a

(phr)

3 (0.15) Benzophenone ( 0 . 1 5 ) B e n z o t r i a z o l e (0.15) 3 ( 0 . 0 7 5 ) + Benzophenone ( 0 . 0 7 5 ) 3 (0.075) + B e n z o t r i a z o l e (0.075)

H r s . to F a i l u r e

b

1250 1260 2100 2540 2830

20 m i l PP p l a q u e s c o n t a i n i n g 0.1 p h r A 0 - 2 50% t e n s i l e l o s s from a g i n g i n xenon Weather-Ometer®

More s u p r i s i n g t o us i s t h e a n t i o x i d a t i o n a c t i v i t y o f 3 i n polypropylene. Not o n l y s t r o n g p r i m a r y a n t i o x i d a n t s by t h e m s e l v e s , they are h i g h l y s y n e r g i s t i c w i t h other p h e n o l i c A O ' s .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

96

POLYMER STABILIZATION AND DEGRADATION Oven-aging of 3 i n Polypropylene at 125°C Days 10 m i l 20 m i l

Formulations (phr) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

AO-1 (0.1) A0-1 (0.25) AO-2 (0.1) AO-2 (0.25) 3 a (0.1) 3a,(0.25) 3b (0.1) 3b (0.25) A0-1 (0.125) + DSTDP (0.125) A0-1 (0.125) + DSTDP (0.25) A0-1 (0.125) + 3a (0.125) AO-1 (0.125 a

D

C

3a:

R = -CH -o- + Y* >N0Y + Y00-

»

*N0*

> >N0Y *> )N0- + Y00Y

(16) (17) (18)

Through these reactions the HALS additive and i t byproducts reduce the steady-state concentration of Y- and Y00* thus reducing k i n e t i c chain length. The longer the i n i t i a l chain length, the larger i s the effect of adding HALS.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

GERLOCK ET AL.

Nitroxide Kinetics in Acrylic I Melamine Coatings

1.50

0

.25

.50

.75

1.0 1/2

(LIGHT INTENSITY)

Figure 4. K versus the square root of the l i g h t i n t e n s i t y f o r coating £. The dewpoints were 50 C ( Q ) and -40 C (["1).

0

200

400

600

800

1000

EXPOSURE TIME (HRS.)

Figure 5. Extent of photodegradation versus exposure time f o r s t a b i l i z e d and unstabilized coating £ ( ) and polypropylene ( ) showing the effect of hindered amine and UV absorbers on the photodegradation k i n e t i c s .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

130

One method to follow the s t a b i l i z a t i o n k i n e t i c s of HALS additives i s to follow the nitroxide concentration as a function of time of exposure. As shown i n Figure j i , the nitroxide concentration i n HALS-I doped coatings r i s e s more or l e s s rapidly to a maximum and then slowly decays. Despite s i g n i f i c a n t differences i n photooxidation chemistry, s i m i l i a r behavior i s observed i n HALS doped polypropylene samples (31) though the maximum nitroxide concentration observed i s roughly 10 times lower than observed for the acrylic/melamine coatings. In the i n i t i a l part of the exposure, up to about 30% of the maximum nitroxide l e v e l reached i n the coating, the increase i n nitroxide with time i s found to be roughly l i n e a r (Figure X ) . I t i s also found that there i s some nitroxide formation during cure of these coatings suggesting that there i s a small amount of free r a d i c a l thermal degradation during cure. The amount of nitroxide formation during cure seems to be greatest f o r the l e a s t durable coatings. From the slopes i n Figure £, the i n i t i a l rate of formatio (d[>N0*]/dt) measured. At long times has been found to f i t th g expression DNO'Kt) = [ > N 0 - ]

max

exp( - K

d e c a y

(t - t

m a x

))

(19)

The nitroxide k i n e t i c s of HALS doped coatings are a complex function of formation, decay, and regeneration and to date no a n a l y t i c a l expression has been derived which completely predicts the nitroxide behavior. I t i s possible to describe the k i n e t i c behavior through parameters such as the maximum nitroxide concentration ( [ / N 0 * ] ) , the time to maximum ( t ) , and the above mentioned nitroxide formation and decay rates. Values of these parameters are given i n Table I. Except f o r Coatings D and H, a l l values of t / N 0 ] are around 150 x 10"° moles/g and no obvious trends with coating composition are observed. Values of K^ecay independent of coating composition. Higher rates of nitroxide formation and shorter values of t generally correspond to coatings that have higher rates or gloss l o s s though there are exceptions. For example, coatings £ and £ have the same value of t^max but have s i g n i f i c a n t l y d i f f e r e n t gloss l o s s rates. Also the i n i t i a l rate of nitroxide formation i n Coating Z i s lower than i n Coating J£ even though the rate of gloss l o s s i s greater i n Coating £. The time to maximum nitroxide and the i n i t i a l rate of formation of nitroxide are slowest i n Coatings J) and ft even though the rate of gloss l o s s i n Coating ft i s lower than either ft or ft. m a x

m a x

#

m a x

a r e

8 1 8 0

n

e

a

r

l

v

x

The exact mechanisms f o r the conversion of hindered amine to nitroxide are not known i n these coatings. According to the treatment above, the i n i t i a l rate of formation of nitroxide should be proportional to [Y00*]. From the scheme of equation 5, the o v e r a l l rate of photooxidation should also be proportional to [Y00*]. Even though there appears to be a q u a l i t a t i v e correlation between the i n i t i a l nitroxide formation rate and the rate of degradation, the i n i t i a l formation rate i s hardly proportional to either K or K . Apparently nitroxide formation from HALS-1 i s more complex than indicated by the simple scheme above. Compared to the excellent c o r r e l a t i o n found between photodegradation rates and p h o t o i n i t i a t i o n rates i n these coatings, no single parameter i n the e

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

GERLOCK ET AL.

Nitroxide Kinetics in Acrylic I Melamine Coatings

5

10

20

50

100 200

500

1000

EXPOSURE TIME (HOURS) Figure 6. Nitroxide concentration versus exposure time i n acrylic/melamine coatings A-ik Coatings were doped with 1? by weight HALS-1 and exposed i n the modified weather chamber at a dewpoint of 25 C.

0

2

4

6

8

10

12

14

16

18

EXPOSURE TIME (HOURS)

Figure 7. I n i t i a l buildup of nitroxide on exposure of HALS-1 doped coatings. Samples and exposure conditions are given i n Figure

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

132

POLYMER STABILIZATION AND DEGRADATION

HALS-1 based nitroxide k i n e t i c s has been found which correlates with photodegradation rate. Despite the lack of d i r e c t correlation between the hindered amine based nitroxide k i n e t i c s and the photodegradation rates of the acrylic/melamine coatings, the hindered amine based nitroxide k i n e t i c s have been found to be very sensitive to exposure conditions (38). In p a r t i c u l a r , i t has been found that humidity increases the rate for formation of nitroxide from HALS-1 doped coatings (38) even though i t does not a f f e c t the p h o t o i n i t i a t i o n rate (42). I t has been suggested that hydrolysis i s primarily responsible for t h i s e f f e c t (38). During hydrolysis of acrylic/melamine coatings, formaldehyde i s released i n t o the coating. Formaldehyde i s e a s i l y oxidised to a peracid which can increase the rate of photooxidation at constant i n i t i a t i o n rate as observed i n Figure 3.* Peracids are also known to be able to rapidly convert hindered amines to nitroxide (55). The r o l k i n e t i c s i s currently bein acrylic/melamine coating acrylic/urethan coating no formaldehyde on cure or degradation. I t should also be noted that HALS-1, i n addiiton to reducing the photooxidation rate, also reduces the rate of hydrolysis observed i n acrylic/melamine coatings. The high rate of p h o t o i n i t i a t i o n of radicals i n acrylic/melamine coatings together with the long l i f e t i m e of nitroxide i n the HALS-1 doped coatings confirms the key role that nitroxide regeneration (reaction 18) plays i n the effectiveness of HALS additives. Without nitroxide regeneration, the hindered amine would be rapidly consumed and the nitroxide would be completely converted to amino ether. The i n i t i a l rate of formation of nitroxide i n the HALS-1 doped coatings i s much smaller than W^ indicating that either few of the r a d i c a l s formed convert hindered amine to nitroxide or that some of the nitroxide immediately reacts with a l k y l radicals to form amino ethers. Preliminary measurements of the l o s s of HALS-1 i t s e l f from the coating during exposure (43) suggests that both processes are o c c u r r i n g . The i n i t i a l r a t e o f l o s s of HALS-1 from coating G i s smaller that W but i s greater than the i n i t i a l formation rate of nitroxide. The rate of conversion of HALS-1 to nitroxide does not seem large enough r e l a t i v e to W to greatly reduce the photodegradation rate. The average concentration of nitroxide i s large enough i n most of the coatings to reduce the photodegradation rate. However, i t should be noted that the addition of HALS-1 to coating H reduces the photodegradation rate even though the maximum nitroxide concentration i s only 20 x 10" moles/g. In polypropylene i t has also been suggested that the nitroxide concentration observed i s i n s u f f i c i e n t to account for the degree of s t a b i l i z a t i o n observed (21). To account for the apparent high e f f i c i e n c y of HALS based additives i n t h i s polymer i t has been suggested that HALS p r e f e r e n t i a l l y associates with hydroperoxides, the known major chromophore i n polypropylene (30). I t i s not known at t h i s point whether or not s i m i l i a r associations exist i n the very polar acrylic/melamine coatings studied here. i

A

8

The measurements of the concentration of HALS-1 i n the coating

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

10.

GERLOCK ET AL.

Nitroxide Kinetics in Acrylic/Melamine Coatings

133

as a function of exposure time show that the hindered amine i s e s s e n t i a l l y consumed long before the nitroxide concentration has decreased s i g n i f i c a n t l y from i t s maximum. This means that for most of the exposure time s t a b i l i z a t i o n by HALS-X involves reactions 17 and 18 but not 16. The length of time that the additive i s e f f e c t i v e thus depends not on the l i f e t i m e of the HALS i t s e l f but rather depends on the value of . The value of K d does not depend greatly on coating composition. I t i s sensitive to exposure conditions, however. I t has been found that K i s proportional to l i g h t i n t e n s i t y . The mechanism whereby nitroxiae or amino ethers are l o s t from the coating i s unknown at present. Understanding the factors that control Kdecay substantial improvements i n effectiveness of HALS aaditives. This behavior may also explain the synergism that i s often observed between HALS additives and UV absorbers. UV absorbers reduce the p h o t o i n i t i a t i o n rate by reducing the l i g h t i n t e n s i t y . Reducing the p h o t o i n i t i a t i o n rate increases the k i n e t i c chain lengt HALS additive. Reducin of K thus increasing perio photostabilizer i s e f f e c t i v e . e c a y

d e

c

o

u

l

d

l

e

a

d

t

o

d

Conclusion The k i n e t i c s of nitroxide formation and decay have been studied by ESR i n acrylic/melamine coatings doped with persistent nitroxides or hindered amine l i g h t s t a b i l i z e r s . These k i n e t i c s together with measurements of the rates of chemical and physical changes o c c u r r i n g i n the coating have been used to study the photodegradation and photostabilization chemistry of these coatings. I t has been found that photodegradation rate as measured by gloss l o s s of pigmented coatings i s constant with time but depends strongly on the nature of the a c r y l i c copolymer used i n the coating. Measurement of the the nitroxide decay rate i n coatings doped with a hindered amine based nitroxide provide a convenient and rapid means to determine the p h o t o i n i t i a t i o n rate i n these coatings. I t has been found that the photodegradation rate i s simply proportional to the square root of the p h o t o i n i t i a t i o n rate. Comparison of infrared spectroscopic measures of the photooxidation rate with the p h o t o i n i t i a t i o n rate y i e l d r e l a t i v e l y short k i n e t i c chain lengths (5 - 15). The data also suggest that hydroperoxide chain branching i s unimportant i n acrylic/melamine coatings. Use of the nitroxide based assay of p h o t o i n i t i a t i o n rate i s predicted to correlate with long term measurements of d u r a b i l i t y as long as the photooxidation rates are roughly constant over the service l i f e of the material and the k i n e t i c chain lengths are reasonably short (< 30). The addition of hindered amine l i g h t s t a b i l i z e r s were found to reduce the rate of photodegradation i n these coatings. Comparision of the p h o t o i n i t i a t i o n rate and various nitroxide k i n e t i c parameters i n the hindered amine doped coatings confirmed the importance of nitroxide r e c y c l i n g to the s t a b i l i z a t i o n of these coatings. The exact mechanism of conversion of hindered amine to nitroxide and of nitroxide regeneration are not yet known i n these coatings. Further measurements of the k i n e t i c s of HALS doped coatings as a function of exposure conditions are i n progress.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

134

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

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In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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135

31. Carlsson, D. J.; Chan, K. H.; Wiles, D. M. J. Polym. Sci. Polym. Lett. 1981, 19, 549. 32. Carlsson, D. J.; Chan, K. H.; Durmis, J.; Wiles, D. M. J. Polym.Sci.Polym.Chem. Ed. 1982, 20, 575. 33. Kurumada, T.; Ohsawa, H.; Fujita, T.; Toda, T. J. Polym. Sci. Polym. Chem. Ed. 1984, 22, 277. 34. Krejcar, E.; Kolar, O. Prog. Org. Coat. 1972, 1, 249. 35. Morimoto, K.; Lida, T. ibid 1973, 2, 35. 36. Killgoar, P. C., Jr.; van Oene, H. In "Ultraviolet Light Induced Reactions of Polymers"; Labana, S. S. Ed.; ACS Symposium Series No. 25 American Chemical Society: Washington D.C., 1976; p. 407. 37. Bauer, D. R. J. Appl. Polym. Sci. 1982, 27, 3651. 38. Gerlock, J. L.; van Oene, H.; Bauer, D. R. Euro. Polym. J. 1983, 19, 11. 39. English, A. D.; Spinelli, H. J. In "Characterization of Highly Crosslinked Polymers" Labana S S. Dickie Eds.; ACS Symposiu Washington DC, 1984; p 40. Bauer, D. R.; Briggs, L. M. ibid., p. 271. 41. Gerlock, J. L.; Bauer, D. R.; Briggs, L. M. ibid., p. 285. 42. Gerlock, J. L.; Bauer, D. R.; Briggs, L. M.; Dickie, R. A. J. Coat. Techno. Submitted. 43. Gerlock, J. L.; Bauer, D. R.; Briggs, L. M. Polymer Preprints 1984, 25, 30. 44. Gerlock, J. L. Anal. Chem. 1983, 54, 1529. 45. Gerlock, J. L.; Bauer, D. R. J. Polym. Sci. Polym. Lett. Ed. 1984, 22, 477. 46. Chattha, M. S.; Cassatta, J. C. Coat. Technol. 1983, 55. (700), 39. 47. Beckman, P.; Spizzichino, A. "The Scattering of Electromagnetic Waves from Rough Surfaces"; Pergamon Press Ltd.: Oxford, 1963; Chapter 5. 48. Simpson, L. A. Progr. Org. Coat. 1978,6,1. 49. Berner, G.; Kreibach, U. T. In "Sixth International Conference in Organic Coatings Science and Technology"; Parfitt, G. D.; Patsis, A. V. Eds.; Technomic Pub.: Westport, CT, 1982; p. 334. 50. Kaiser, W. D. Korrosion (Dresden) 1976,7,33. 51. Finzel, W. A. J. Coat. Techno. 1980, 52 (660), 55. 52. Bogatryeva A. I.; Buchachenko, A. L. Kinetics and Catalysis 1971, 12, 1226. 53. Keana, J. F. W.; Dinerstein, R.; Baitis, F. J. Org. Chem. 1971,36,209. 54. Shlypintokh, V. Y.; Ivanov V. B. In "Developments in Polymer Stabilization -5"; Scott, G. Ed.; Applied Sci.: London, 1982; p. 41. 55. Rozantzev, E. G. "Free Nitroxyl Radicals"; Plenum Press: New York, 1970; Chap. 9. RECEIVED October 26, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11 Stabilization of Polypropylene Multifilaments Utility of Oligomeric Hindered Amine Light Stabilizers ROBERT J . T U C K E R and PETER V. SUSI Polymer Products Division, American Cyanamid Company, Bridgewater, NJ 08807

The light stabilizatio places special requirements on the stabilization system due to the high surface to volume ratio, the severe processing conditions, and the post spinning treatments. Earlier systems provided only moderate performance lifetimes thus limiting the markets for PP fibers. The development of oligomeric hindered amine light stabilizers (HALS) has led to PP fibers with greatly increased service lifetimes, thus opening up new applications for this fiber. Some of the factors leading to the enhanced performance obtainable with certain newer oligomeric HALS are described and evaluated. Also, the importance of processing conditions and interactions of HALS with other additives on the light stability of the resulting fiber are discussed. Because of polypropylene s unusual properties, such as light weight and ease of fabrication, it has been used to make a variety of fabrics and is expected to be the growth fiber of of the eighties (1,2). Adding impetus to this expectation was the development of hindered amine light stabilizers (HALS) (3)_ that enable polypropylene (PP) fibers to penetrate new markets. The processes for producing these fibers range from conventional melt spinning for continuous filament and staple through heavy denier monofilaments produced by extrusion into water, with additional large volumes produced from film by slitting or splitting. Even direct production of fabrics from polymer by spun-bonded processes is possible with polypropylene. The deniers of fibers produced by the varied techniques cover a wide range varying from micro deniers produced by melt blowing processes to the heavy deniers used in carpet backings, sacks, bags and rope or cordage (2). Essential to the continued success for this polymer in the fiber market is the ongoing effort to continually enhance the stability of the products produced especially towards oxygen, heat and light. 1

0097-6156/85/0280-0137$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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While the e a r l y HALS gave c o n s i d e r a b l y g r e a t e r l i g h t s t a b i l i t y t o PP f i b e r s than c l a s s i c a l l i g h t s t a b i l i z e r s , they s t i l l f a i l e d to p e r f o r m w e l l i n low d e n i e r f i b e r s . These p l a c e s p e c i a l r e q u i r e m e n t s on the s t a b i l i z e r system due t o t h e h i g h s u r f a c e a r e a , t h e s e v e r e p r o c e s s i n g c o n d i t i o n s u s e d , and t h e use o f v a r i o u s c o l o r a n t s and post spinning treatments such as t e n t e r i n g , l a u n d e r i n g , and d r y cleaning. More r e c e n t l y , o l i g o m e r i c type HALS have been found to provide the b e s t b a l a n c e o f p r o p e r t i e s f o r most a p p l i c a t i o n s . The products a t t a i n their improved structural properties without a substantial reduction i n h i n d e r e d amine c o n t e n t , thus r e t a i n i n g a high s p e c i f i c a c t i v i t y . Data a r e p r e s e n t e d showing the s u p e r i o r performance o f c e r t a i n o l i g o m e r i c HALS i n PP m u l t i f i l a m e n t s . Experimental As used h e r e i n , y a r n d e n i e r i s t h e number o f grams p e r 9000 m e t e r s . Y a r n t e n a c i t y i s the t e n s i l e s t r e s s expressed as force per unit l i n e a r d e n s i t y o f the u n s t r a i n e (gf/den.). The m a j o r i t y o f t e s t s were c a r r i e d out on m u l t i f i l a m e n t y a r n s p r e p a r e d from H e r c u l e s PRO-FAX 6401 p o l y p r o p y l e n e powder. To the base polymer was added 0.05% c a l c i u m s t e a r a t e and 0.1% 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate (processing stabilizer). Additives were d r y b l e n d e d i n t o the powder and t h e r e s u l t i n g b l e n d s e x t r u d e d a t 227°C and p e l l e t i z e d . The p e l l e t s were spun i n t o y a r n s , u s i n g a NRM e x t r u d e r a t 280°C w i t h a 30 h o l e d i e , and the y a r n s were t h e n drawn a t a 6:1 r a t i o i n two s t a g e s and g i v e n a 2Z t w i s t . The y a r n s (240/30 d e n i e r ) were woven i n t o t e s t s t r i p s (40 y a r n s p e r i n c h ) and used f o r exposure studies. Accelerated light stability studies were carried out i n an A t l a s Xenon A r c Weather-Ometer (WOM) w i t h a 6500 Watt b u r n e r . Operating conditions were 30% r e l a t i v e humidity, 44°C ambient temperature and a b l a c k p a n e l temperature o f 65 + 3 ° C . F o r the GM-WOM t e s t , an A t l a s twin globe e n c l o s e d c a r b o n a r c u n i t was used w i t h a 3.8 hour l i g h t c y c l e and 1 hour water m i s t c y c l e . The ambient temperature was 72°C with a black panel temperature of 89° + 3°C d u r i n g t h e l i g h t c y c l e . T e n t e r i n g was s i m u l a t e d by h e a t i n g the y a r n s a t 120°C f o r 20 minutes i n a c i r c u l a t i n g a i r oven. In t h e l a u n d e r i n g t e s t , yarns were machine washed with d e t e r g e n t and d r i e d t h r e e t i m e s . F o r the d r y c l e a n i n g t e s t s , y a r n s were c o m m e r c i a l l y d r y c l e a n e d t h r e e t i m e s . In a l l s t u d i e s , the f a i l u r e p o i n t was a 50% l o s s i n o r i g i n a l breaking strength of the y a r n s as measured by I n s t r o n t e n s i l e p r o p e r t y measurements. HALS s t r u c t u r e s a r e shown i n F i g u r e 1 . Results

and D i s c u s s i o n

Most h i n d e r e d amine light stabilizers have evolved from the discovery (4) that compounds containing a 2,2,6,6t e t r a m e t h y l p i p e r i d i n e moiety can stabilize polymers against photodegradation and t h i s m o i e t y has been i n c o r p o r a t e d i n t o HALS o f v a r i o u s types ( F i g u r e 1 ) . Much has been p u b l i s h e d on the mechanism of a c t i o n o f HALS and t h e l i t e r a t u r e i n t h i s a r e a has r e c e n t l y been c r i t i c a l l y reviewed ( 5 , 6 ) . W h i l e the complete mechanism of action has not been fully elucidated, the h i g h performance o f HALS i s g e n e r a l l y a t t r i b u t e d t o the a b i l i t y o f t h e i r o x i d a t i o n products to

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11. TUCKER AND SUSI

Stabilization of Polypropylene Multifilaments

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

139

POLYMER STABILIZATION AND DEGRADATION

140

act as r a d i c a l scavengers i n a c y c l i c s e l f - p e r p e t u a t i n g shown i n E q u a t i o n s 1 through 3 ( 7 , 8 ) . VH / V o -

oxidation +

R.

^ ^ >

/

V o /

(1)

N-0-R

(2)

N

f a s h i o n as

/

V-O-R + /

ROO-

*

V o - +

ROOR

(3)

•

R e c e n t l y , most e f f o r t s i n the HALS a r e a have centered around obtaining the b e s t s t a b i l i z a t i o n a t the lowest c o s t i n a v a r i e t y o f demanding a p p l i c a t i o n s such as p o l y p r o p y l e n e m u l t i f i l a m e n t s . While the stabilizing activity o f HALS i s c e n t e r e d around t h e h i n d e r e d p i p e r i d i n e n i t r o g e n , the r e s t o f t h e m o l e c u l e s t i l l has an i n f l u e n c e on o v e r a l l p e r f o r m a n c e . It i s f e l t that HALS c o n c e n t r a t e i n the amorphous area o f p o l y o l e f i n s where d e g r a d a t i o n i s more l i k e l y t o o c c u r due t o i n c r e a s e d oxygen d i f f u s i o n and a l a c k of crystalline order. Some c h e m i c a l s t r u c t u r e more f a v o r a b l e than o t h e r s i n a l l o w i n g closer association o f the HALS w i t h potential damage s i t e s . T h i s " c o m p a t i b i l i t y " c a n be a very important HALS a t t r i b u t e and an i m p o r t a n t factor i n the e f f e c t i v e n e s s o f a HALS over the l i f e o f a polymer s u b s t r a t e . The stabilization of polypropylene yarns is a demanding a p p l i c a t i o n because o f the e x t r e m e l y high surface area (9) and intimate exposure of the f i l a m e n t s t o oxygen and l i g h t throughout t h e i r very thin cross section. Table I shows the e f f e c t of thickness on the t h e r m a l s t a b i l i t y o f p o l y p r o p y l e n e c o n t a i n i n g , as the a n t i o x i d a n t s y s t e m , 0.1% tetrakis[methylene(3,5-di-tert-butyl4-hydroxyhydrocinnamate)]methane and 0.3% distearylthiodipropionate. At 1 5 0 ° C , an 8 d e n i e r p e r f i l a m e n t y a r n f a i l s much sooner than a 0.4 m i l t h i c k f i l m which i n t u r n i s l e s s s t a b l e than a 4 . 0 m i l f i l m d e m o n s t r a t i n g the d r a m a t i c e f f e c t o f sample thickness on thermal s t a b i l i t y .

Table I .

PP Thermal S t a b i l i t y v e r s u s

Sample T h i c k n e s s 4.0 m i l F i l m 0.4 m i l F i l m 240/30 D e n i e r Y a r n (8 d p f )

Thickness

Hours t o E m b r i t t l e m e n t at 150°C >500 200 25

In Table II, the e f f e c t of f i l a m e n t d i a m e t e r on t h e l i g h t s t a b i l i t y of polypropylene yarn, containing 0.1% o c t a d e c y l 3,5di-tert-butyl-4-hydroxyhydrocinnamate as the the a n t i o x i d a n t , i s shown. F a i l u r e was the time t o 50% o r i g i n a l b r e a k i n g s t r e n g t h . The lower d e n i e r f i b e r showed a g r e a t l y reduced l i f e t i m e due t o i t s much s m a l l e r c r o s s s e c t i o n , and thus increased exposure to l i g h t and oxygen.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11. TUCKER AND SUSI Table I I . D e n i e r per Filament 146 8

Stabilization of Polypropylene Multifilaments PP L i g h t S t a b i l i t y v e r s u s

F i l a m e n t Diameter 6.0 m i l 1.4 m i l s

Thickness

Hours t o F a i l u r e Xenon WOM 750 200

The s e v e r e processing conditions used t o produce low d e n i e r f i b e r s r e q u i r e a s t a b i l i z e r w i t h v e r y good t h e r m a l s t a b i l i t y and low volatility. V o l a t i l i t y o f the s t a b i l i z e r can a l s o be an important f a c t o r i n the e f f e c t i v e n e s s o f a HALS under e n d - u s e c o n d i t i o n s . The volatility of several commercial HALS, as measured by thermogravimetric a n a l y s i s (TGA), is shown i n Table III. The oligomeric HALS (3-6) have the best thermogravimetric p r o f i l e , showing low p r o d u c t l o s s a t h i g h t e m p e r a t u r e s , and s h o u l d survive polymer processing better than HALS 1 and 2, as w e l l as remain i n the h i g h s u r f a c e a r e a f i b e r s over t i m e . Table I I I .

R e l a t i v e V o l a t i l i t y o f HALS by

TGA

Temperature ( ° C ) a t X% Weight L o s s T TlO T20 267 236 251 305 291 275 339 318 329 380 331 351 325 301 277 371 401 344 5

HALS HALS HALS HALS HALS HALS a

1 2 3 4 5 6

Heating

Rate 10°C/minute i n A i r

Another i m p o r t a n t a t t r i b u t e o f a HALS is its effect on the thermo-oxidative stability of polypropylene m u l t i f i l a m e n t s . This p r o p e r t y i s i m p o r t a n t i n c e r t a i n e n d - u s e a p p l i c a t i o n s where e l e v a t e d temperature o v e r a p e r i o d o f time is experienced, such as in automobile rear shelf fabrics. The e x c e l l e n t performance o f the o l i g o m e r i c HALS 4 and 6, as determined by 120°C oven a g i n g , i s shown i n F i g u r e 2. The s u p e r i o r a c t i v i t y o f t h e s e p r o d u c t s may be due not o n l y t o t h e i r low v o l a t i l i t y but a l s o t o the p r e s e n c e o f a triazine moiety i n the s t r u c t u r e , which appears t o have a p o s i t i v e e f f e c t on the t h e r m o - o x i d a t i v e s t a b i l i t y o f p o l y p r o p y l e n e . The GM-WOM i s a high temperature (72 ° C ) , high humidity a c c e l e r a t e d w e a t h e r i n g t e s t , s p e c i f i e d by G e n e r a l M o t o r s , f o r f i b e r s and p l a s t i c s for automotive i n t e r i o r a p p l i c a t i o n s . In t h i s u n i t , the o l i g o m e r i c HALS 4 and 6 gave the b e s t p e r f o r m a n c e , a l t h o u g h HALS 2 was a l s o v e r y e f f e c t i v e ( F i g u r e 3 ) . O l i g o m e r i c HALS g e n e r a l l y o u t p e r f o r m o t h e r t y p e s when a t h e r m a l t r e a t m e n t , such as a t e n t e r i n g o r a l a t e x i n g o p e r a t i o n , i s performed on the y a r n s . As shown i n F i g u r e 4, in simulated tentered yarns (heated at 120° C for 20 m i n u t e s ) , the o l i g o m e r i c HALS 3,4 and 6 showed the b e s t performance i n the Xenon WOM. Another i m p o r t a n t c o n s i d e r a t i o n i n s e l e c t i n g a l i g h t s t a b i l i z e r system f o r p o l y p r o p y l e n e y a r n s i s the r e s i s t a n c e to activity loss after l a u n d e r i n g or dry c l e a n i n g . In F i g u r e 5 d a t a a r e p r e s e n t e d

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION

80

r

HALS-1

HALS-2

HALS-3

HALS-4

HALS-5

HALS-6

0.3% HALS, 0.1% AO, 0.05% CaSt F i g u r e 2. Thermal s t a b i l i z i n g a c t i v i t y i n 240/30 p o l y p r o p y l e n e y a r n ( y a r n t e n a c i t i e s 5.2 ± 4%).

HALS-1

HALS-2

HALS-3

HALS-4

HALS-5

denier

HALS-6

0.25% HALS, 0.1% AO, 0.05% CaSt F i g u r e 3. L i g h t s t a b i l i z i n g a c t i v i t y i n 240/30 p o l y p r o p y l e n e y a r n ( y a r n t e n a c i t i e s 4.7 ± 4%).

denier

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11.

Stabilization of Polypropylene Multifilaments

TUCKER AND SUSI

HALS-1

HALS-2

HALS-3

HALS-4

HALS-5

HALS-6

0.25% HALS, 0.1% AO, 0.05% CaSt

Figure 4. Light s t a b i l i z i n g a c t i v i t y i n 240/30 denier polypropylene yarn (simulated tentering).

2000

r

2

3

4

Laundered

6

2

3

4

6

Dry Cleaned

0.25% HALS, 0.1% AO, 0.05% CaSt

Figure 5. Laundering and dry cleaning effects on 240/30 denier polypropylene yarns.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION

showing t h e e f f e c t o f t h e s e o p e r a t i o n s on y a r n s containing various HALS as measured by Xenon-WOM e x p o s u r e . While HALS 2 showed good a c t i v i t y a f t e r l a u n d e r i n g , i t l o s t most o f i t s a c t i v i t y after the dry cleaning treatment. The o l i g o m e r i c HALS 6 outperformed a l l the o t h e r HALS e v a l u a t e d , d e m o n s t r a t i n g good resistance to extraction from the yarns by a h o t aqueous d e t e r g e n t s o l u t i o n o r by o r g a n i c s o l v e n t s used i n d r y c l e a n i n g . The r e s u l t s of a concentration versus light stabilizing activity study with two o l i g o m e r i c HALS (4 and 6) a r e shown i n F i g u r e 6. In t h e GM-WOM u n i t , HALS 6 shows a b e t t e r activity r e s p o n s e t o i n c r e a s i n g c o n c e n t r a t i o n t h a n does HALS 4. A t t h e lower c o n c e n t r a t i o n l e v e l s o f 0.15% t o 0.3%, HALS 6 shows an almost l i n e a r a c t i v i t y increase. The d i f f e r e n c e s o b s e r v e d a r e p r o b a b l y r e l a t e d t o subtle structural differences resulting in altered polymer compatibility. T h i s e f f e c t w i t h HALS 4 has a l s o been seen by o t h e r workers ( 1 0 ) . Another area o f concern i n s t a b i l i z i n g polypropylene f i b e r s i s the development o f c o l o HALS a r e c o l o r l e s s and impart l i t t l e o r no c o l o r on p r o d u c t i o n , " g a s y e l l o w i n g " o f f i b e r s i n use c a n be a c o n c e r n . In T a b l e I V , d a t a a r e shown on the gas y e l l o w i n g resistance of s e v e r a l HALS a t 0.5% concentration i n n a t u r a l polypropylene multifilament. The y a r n s were exposed using a m o d i f i e d AATCC 23-1972 t e s t f o r 1 c y c l e and then e v a l u a t e d u s i n g a gray scale comparator with a 5.0 rating i n d i c a t i n g no change, and a 1.0 r a t i n g d e n o t i n g s e v e r e c o l o r change. With 0.1% p h e n o l i c a n t i o x i d a n t p r e s e n t , HALS 2 and HALS 6 showed a b a r e l y p e r c e p t i b l e c o l o r development, w h i l e HALS 4 d i s c o l o r e d to a greater extent. I n the absence o f t h e p h e n o l i c a n t i o x i d a n t , HALS 4 s t i l l showed a n o t i c e a b l e d i s c o l o r a t i o n , w h i l e the y a r n s containing HALS 6 showed no c o l o r development, i n d i c a t i n g t h a t d i s c o l o r a t i o n i n the presence o f t h e p h e n o l i c a n t i o x i d a n t was due t o the l a t t e r and not t o HALS 6 i t s e l f . Table IV.

HALS HALS HALS HALS HALS

2 4 6 4 6

Gas Y e l l o w i n g R e s i s t a n c e

% Phenolic A . O . 0.1 0.1 0.1 0 0

f O . 5 % HALS M o d i f i e d AATCC 23-1972 t e s t - 1 c y c l e ; change; 1.0 = s e v e r e c o l o r change b

i n 240/30 D e n i e r PP Y a r n s Rating 4.5 4.0 4.5 4.0 5.0

a

1

5.0 - no c o l o r .

P o l y p r o p y l e n e f i b e r s a r e o f t e n pigmented and t h e pigments used can i n f l u e n c e t h e l i g h t s t a b i l i t y o f t h e system ( 1 1 ) . Some improve s t a b i l i t y , some a r e n e u t r a l , w h i l e o t h e r s are deterimenal due t o their prodegradative tendencies or to pigment-stabilizer interactions (12). A comparison o f two o l i g o m e r i c HALS (4 and 6) i n b l u e and r e d pigmented y a r n s i s shown i n T a b l e V . As can be seen, even structurally similar products show d i f f e r e n t relative s t a b i l i z a t i o n e f f e c t i v e n e s s with d i f f e r e n t pigments.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

TUCKER AND SUSI

Stabilization of Polypropylene Multifilaments

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

146

T a b l e V.

S t a b i l i z a t i o n o f Pigmented PP M u l t i f l a m e n t

Stabilizer HALS 4

Hours t o F a i l u r e GM WOM Blue Red 390 620 --370 775 565 855 — 375 1230

% Concentration 0.2 0.4 0.5 0.8 0.2 0.4 0.5 0.8

HALS 6

a

%00/34 D e n i e r b l u e y a r n ; 300/70 d e n i e r r e d y a r n

^_

F i n a l l y , t h e importanc i s i l l u s t r a t e d i n Table but t h e s p i n pack temperatur preparatio yarn was i n c r e s e d . Both t h e o l i g o m e r i c HALS 4 and 6 gave y a r n s with improved l i g h t s t a b i l i t y when p r o c e s s e d a t the h i g h e r temperature. Table VI.

E f f e c t o f P r o c e s s i n g C o n d i t i o n s on Yarn

Stabilizer HALS 4 HALS 6

240/30 D e n i e r y a r n s ST = s i m u l a t e d

Processing Temperature 265~*C 280 °C 265 °C 280 °C (0.25% HALS); NT

Stability

3

Hours t o F a i l u r e Xenon WOM ST GM WOM NT 390 785 675 980 1878 1772 475 840 710 1067 2440 1950 non-tentered;

Summary In polypropylene m u l t i f i l a m e n t s , o l i g o m e r i c h i n d e r e d amine l i g h t s t a b i l i z e r s have been found to offer superior light stabilizing activity. Properties such as good thermal stability, low v o l a t i l i t y , c o m p a t i b i l i t y , and e x t r a c t i o n r e s i s t a n c e have been shown t o be important f a c t o r s f a v o r i n g t h e use o f o l i g o m e r i c HALS i n polypropylene fiber applications. Differences i n molecular s t r u c t u r e among t h e o l i g o m e r i c HALS a r e p r o b a b l y r e s p o n s i b l e f o r t h e b e s t o v e r a l l s t a b i l i z i n g a c t i v i t y o b s e r v e d w i t h HALS 4 and 6. Acknowledgements The a u t h o r s w i s h t o acknowledge the c o n t r i b u t i o n made t o t h e s e studies by co-workers and thank the management o f American Cyanamid Company f o r p e r m i s s i o n t o p u b l i s h t h e work and M i s s J . C. W i l l i a m s f o r t y p i n g the manuscript.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

11.

TUCKER AND SUSI

Stabilization of Polypropylene Multifilaments

Literature Cited 1. Blore, J. H. Knitting Times November 6 1978, p. 24. 2. Polypropylene Fiber Symposium, New York, N.Y., 1981 3. Patel, A. R.; Usilton, J. J. In "Stabilization and Degradation of Polymers"; Allara, D. L.; Hawkins, W. L., Eds.; ADVANCES IN CHEMISTRY SERIES No. 169, American Chemical Society: Washington, D. C., 1978; p. 116. 4. Sankyo Co. Ltd. British Patent 1 196 224, 1970. 5. Sedlar, J.; Marchal, J.; Petruj, J. Polymer Photochemistry 1982, 2, 175. 6. Dagonneau, M. et al. Rev. Macromol. Chem. Phys. 1982, C22 (2), 169. 7. Carlsson, D. J.; Grattan, D. W.; Wiles, D. M. Organic Coatings and Plastics Chemistry 1980, 39, 628. 8. Durmis, J. et al. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 549. 9. Carlsson, D. J.; Wiles Macromol. Chem. 1976, C14, 65. 10. Gugumus, F.; Linhart, H. Chemicke Vlakna 1982, 32, 94. 11. Klemchuk, P. P. Polymer Photochemistry 1963, 3, 1. 12. Uzlmeier, C. SPE Journal 1970, 26, 69. RECEIVED October 26, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12 Hexahydropyrimidines as Hindered Amine Light Stabilizers C. E. R A M E Y and C. J . ROSTE Chemical Division, Ferro Corporation, Bedford, O H 44146

Novel hindered amine light stabilizers (HALS) derived from 2,2,4,4,6-pentamethylhexahydropyrimidine were shown to exhibit excellent performance in exposed polypropylene films. Test data for other HALS compounds prepared from the previously described 2,2,5,5-tetramethyl-4-imidazolidinone and 4,4-dimethyloxazolidine ring systems are provided for comparison. Inferior performance was generally observed for those additives which would be expected to form low molecular weight nitroxide radicals upon oxidation. A relatively large extention of film lifetime was produced by formulations containing a combination of HALS and commercial hydroxybenzoate stabilizer. This effect was not evident when HALS compounds containing an intramolecualr hydroxybenzoate group were tested. Hindered amine light stabilizers are at least partially converted to their corresponding nitroxides during the processing (1) and exposure of stabilized polymers. Allen (2) showed that the hydroperoxides present in thermally oxidized polypropylene were capable of effecting this transformation under compression molding conditions. Hindered amine nitroxides have been recognized as key intermediates in the stabilization mechanism of HALS compounds Q , 3_-5). Although the parent hindered amines may contribute to photostabilization (6^), in our development work the stability and nature of the nitroxide radicals resulting from HALS oxidation were assumed to be critical to viable stabilizer activity. At the outset of our HALS program a number of chemically stable nitroxide free radicals had been identified from work in spin labelling (7^), but only derivatives of tetramethylpiperidine had appeared in the additive marketplace.

0097-6156/85/0280-0149$06.00/0 §> 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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We wanted to i n v e s t i g a t e how the e f f e c t i v e n e s s of a g i v e n l i g h t s t a b i l i z e r might depend upon the p a r t i c u l a r h e t e r o c y c l i c amine used in i t s synthesis. The p o l a r i t y , v o l a t i l i t y and c o m p a t i b i l i t y of the o r i g i n a l a d d i t i v e were a l s o known to be i m p o r t a n t f a c t o r s i n d e t e r m i n i n g s t a b i l i z e r performance ( 8 ) . Three h e t e r o c y c l i c r i n g systems were chosen p r i m a r i l y f o r t h e i r ease of s y n t h e s i s . The compounds 2,2,4,4,6-pentamethylhexahydropyrimidine (HHP) ( 9 , 1 0 ) and 2 , 2 , 5 , 5 , t e t r a m e t h y l - 4 - i m i d a z o l i d i n o n e (IMZ) ( 1 1 - 1 3 ) were chosen as s t a r t i n g m a t e r i a l s to e v a l u a t e the e f f e c t of d e r i v a t i v i z a t i o n and s u b s t i t u t i o n on the a c t i v i t y of the r e s u l t i n g l i g h t s t a b i l i z e r s . A few 4 , 4 , - d i m e t h y l o x a z o l i d i n e s were made from 2 - a m i n o - 2 - m e t h y l - l - p r o p a n o l and ketones to examine t h i s s t r u c t u r a l l y l i m i t e d r i n g system (see R e s u l t s and D i s c u s s i o n ) .

0

HHP

IMZ

4,4-Dimethyloxazolidine

Experimental The a d d i t i v e s were t e s t e d i n p o l y p r o p y l e n e ( P r o f a x 6501, Hercules) f i l m s c o n t a i n i n g 0.1% G o o d r i t e 3114 ( p h e n o l i c a n t i o x i d a n t of B . F . G o o d r i c h C o . ) and .05% c a l c i u m s t e a r a t e . The c a n d i d a t e a d d i t i v e s were added at 0.25% or 0.5% f i n a l c o n c e n t r a t i o n s as methylene c h l o r i d e s o l u t i o n s (100 ml methylene c h l o r i d e / l O O g p o l y p r o p y l e n e ) and the m i x t u r e s t i r r e d 15-20 m i n s . w h i l e the s o l v e n t was a l l o w e d to e v a p o r a t e . The r e s u l t a n t powder was then d r i e d and e x t r u d e d i n t o 3/32 i n c h s t r a n d , which was cut i n t o p e l l e t s . A f t e r d r y i n g , the p e l l e t s were e x t r u d e d i n t o a broad ( 8 " ) band. A 1/4" s e c t i o n was s l i t from the band and o r i e n t e d by drawing at 175°F at a 7:1 draw r a t i o . The d i m e n s i o n s o f the o r i e n t e d f i l m are about 1 x 80 m i l s . The o r i e n t e d f i l m specimens were mounted on aluminum frames and exposed on an A t l a s Weather-Ometer, Model 65WR. An 18 minute s p r a y c y c l e t o g e t h e r w i t h an 102 minute c y c l e at 55% r e l a t i v e h u m i d i t y and a p p r o x i m a t e l y 65°C was u s e d . At r e g u l a r i n t e r v a l s , the t e s t specimens were removed from exposure and t h e i r t e n s i l e s t r e n g t h measured on an I n s t r o n Model 1102. A d e c r e a s e i n t e n s i l e s t r e n g t h , e x p r e s s e d as t e n a c i t y , over the t e n s i l e s t r e n g t h of the same f o r m u l a t i o n before e x p o s u r e , i s a measure of the d e t e r i o r a t i o n of the p h y s i c a l p r o p e r t i e s of the p o l y m e r . " F a i l u r e " i n t h i s t e s t i s d e f i n e d as a l o s s of 50% or more of the i n i t i a l sample t e n a c i t y . P h o t o m i c r o g r a p h s to observe a d d i t i v e c o m p a t i b i l i t y i n the o r i e n t ed f i l m were t a k e n at 400X u s i n g a t r a n s m i s s i o n l i g h t m i c r o s c o p e . Blooming was measured at 70°C u s i n g u n o r i e n t e d f i l m samples 7 m i l i n thickness. Thermal o x i d a t i v e s t a b i l i t y was measured i n a c i r c u l a t i n g a i r oven at 140°C on p o l y p r o p y l e n e f i l m s 5" x 1" x . 0 2 5 " .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12. RAMEY AND ROSTEK Results

Hexahydropyrimidines

151

& Discussion

The compounds i n T a b l e I r e p r e s e n t some e a r l y t r i a l s a r i s i n g from s c r e e n i n g IMZ d e r i v a t i v e s and o x a z o l i d i n e s . The lower a c t i v i t y shown by compounds (1) and (2) i s c o n s i s t e n t w i t h t h e o x a z o l i d i n e group b e i n g l o s t as a low m o l e c u l a r weight fragment a f t e r n i t r o x i d e f o r m a t i o n o c c u r s . T h i s i l l u s t r a t e s a drawback o f t h e o x a z o l i d i n e compounds compared t o o t h e r s y s t e m s , t h a t o n l y t h e amine n i t r o g e n and h i n d e r i n g group ( d e r i v e d from t h e s t a r t i n g k e t o n e ) a r e a v a i l a b l e t o b u i l d t h e m o l e c u l a r weight o f t h e s t a b i l i z e r . A s i m i l a r effect i s apparently shown by compound (5) w h i c h i s b r i d g e d t h r o u g h t h e h i n d e r e d amine n i t r o g e n s , a l l o w i n g t h e i m i d a z o l i d i n o n e group form low m o l e c u l a r weight n i t r o x i d e r a d i c a l s . Compound (3) i s a low m o l e c u l a r weight IMZ d e r i v a t i v e which showed a l o n g f i l m l i f e t i m e a t 0.5% c o n c e n t r a t i o n . However, t h i s a c t i v i t y was n o t r e f l e c t e d i n t h e h i g h e r m o l e c u l a r weight a n a l o g (4) which was t e s t e d a t a lower c o n c e n t r a t i o n . Here the methyleneamino grou o f t h e IMZ group due t o h i g h t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) weight l o s s o f t h e e t h y l e n e d i amine d e r i v e d model compound ( 1 6 ) . I n T a b l e I I a r e shown some i n t e r m e d i a t e c a n d i d a t e s where b e t t e r a c t i v i t y i s b e g i n n i n g t o become a p p a r e n t . The s u b s t i t u t i o n of t h e hydroxybenzoate group i n compound (8) does not seem t o make much i m provement i n t h e a c t i v i t y o f t h e l o n g s i d e - c h a i n IMZ d e r i v a t i v e s . The polymer l i f e t i m e of t h e HHP based a d d i t i v e (9) was e n c o u r a g i n g , as the e a r l i e r f a i l u r e of d i i s o c y a n a t e adduct (10) was a t t r i b u t e d t o i t s lower c o m p a t i b i l i t y i n t h e p o l y p r o p y l e n e f i l m . In T a b l e I I I , t h e p o s i t i v e i n t e r a c t i o n between IMZ s t e a r a t e (6) and t h e hydroxybenzoate s t a b i l i z e r UV-Chek AM-340 i s shown. This degree of l i f e t i m e enhancement was not a c h i e v e d i n any of t h e HALS compounds s y n t h e s i z e d i n which t h e hydroxybenzoate group was p r e s e n t as an i n t r a m o l e c u l a r s u b s t i t u e n t . The p r o x i m i t y o f t h e h i n d e r e d p h e n o l i c group may i n some way a f f e c t g e n e r a t i o n o f t h e n i t r o x i d e r a d i c a l s from t h e amine n i t r o g e n .

The s u b s t i t u t i o n o f a hydroxybenzoate group f o r a s t e a r a t e group seemed t o have a d e t r i m e n t a l r e s u l t i n comparing compounds (14) and (15) i n T a b l e I V . I n c o r p o r a t i o n o f a more c o n v e n t i o n a l p h e n o l i c a n t i o x i d a n t group i n (13) may have s u p p r e s s e d a c t i v i t y even more. However, t h e e f f e c t o f p h y s i c a l f a c t o r s on t h e s t a b i l i z i n g a c t i v i t y of t h e s e a d d i t i v e s s h o u l d n o t be u n d e r e s t i m a t e d . The IMZ h y d r o x y benzoate compound (12) has good a c t i v i t y p o s s i b l y due t o i t s p h y s i c a l properties. A l s o i n T a b l e I V , b i s HHP sebacamide (11) shows commerc i a l l y viable stabilizing capabilities. The d i f u n c t i o n a l type o f s t r u c t u r e o f compound (11) and T i n u v i n 770 ( b i s ( 2 , 2 , 6 , 6 - t e t r a m e t h y l 4 - p i p e r i d i n y l ) d e c a n e d i o a t e ) may be a f a c t o r i n o p t i m i z i n g t h e a c t i v i t y of these a d d i t i v e s .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION

T a b l e I. Cone % 0.25 0.25 0.5 0.25 0.5

Initial

Additive (1) (2) (3) (4) (5)

Compatibility:

Compounds

Compatibility G G G G G

w/o L i f e t i m e 850 hours 900 2820 890 1320

G = Good, F = F a i r

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12. RAMEY AND ROSTEK

Hexahydropyrimidines

Table I I , Cone % 0.25 0.25 0.25 0.25 0.25

Additive (6) (7) (8) (9) (10)

Intermediate

Compounds

Compatibility GG G G F

w/o L i f e t i m e 1950 hours 1970 1980 2210 1790

(10)

T a b l e I I L Hydroxybenzoate-HALS Cone % 0.25 0.25 0.25 0.25 0.25 0.25

Compound Chimassorb 994* T i n u v i n 622* IMZ (6) AM-340 AM-340 + 0.25 IMZ (6) T i n u v i n 770*

Interaction w/o L i f e t i m e 1970 hours 1750 1950 1350 2550 2460

*A11 a r e commercial HALS o f C i b a - G e i g y C o r p o r a t i o n UV-Chek AM-340 = Hydroxybenzoate s t a b i l i z e r of F e r r o C o r p o r a t i o n

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

154

POLYMER STABILIZATION AND DEGRADATION Table IV.

Cone % 0.25 0.25 0.25 0.25 0.25

Additive (11) (12) (13) (14) (15)

Final

Compounds

Compatibility G G G G G

w/o L i f e t i m e 2920 hours 2410 1450 2750 1950

(16)

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

12.

RAMEY AND ROSTEK

Hexahydropyrimidines

155

Acknowledgments The authors wish to acknowledge the work of Dr. Goutam Gupta, who was the first to prepare HHP sebacamide and who also prepared the oxazolidines. We wish to thank, also, Dr. Ronald E. Thompson, who prepared the higher IMZ compounds, and Walter J . Wawro, Sr., who worked out the preparation of HHP by catalytic reduction. Literature Cited 1.

Bagheri, R; Chakraborty, K.B.; Scott, G. Polym. Degrad. and Stab. 1982, 4, 1.

2.

Allen, N.S. Polym. Degrad. and Stab. 1980, 2, 179.

3.

Grattan, D.W.; Reddoch, A.H.; Carlsson, D . J . ; Wiles, D.M. J. Polym. S c i . , Polym. Lett. Ed. 1978, 16, 143.

4.

Hodgeman, D.K.C. J . Polym. S c i . , Polym. Chem. Ed. 1980, 18, 533.

5.

Carlsson, D.J.; Chan, K.H.; Wiles, D.M. J . Polym. S c i . , Polym. Lett. Ed. 1981, 19, 549.

6.

Carlsson, D . J . ; Chan, K.H.; Dumas, J.; Wiles, D.M. J.Polm. S c i . , Polym. Chem. Ed. 1982, 20, 575.

7.

Keana, J.D.W. Chem. Reviews 1978, 78(1), 37.

8.

Bailey, D; Vogl, O. J . Macromol. Sci-Rev. Macromol. Chem. 1976, C14(2), 267.

9.

Gupta, G.; Ramey, C.E. U.S. Patent 4, 404, 302, 1983.

10.

Matter, E. Helv. Chim. Acta

1947, 30, 1114-23.

11.

Ramey, C.E.; Rostek, C.J.

12.

Murayama, K.; Morimura, S.; Toda, T.; Akagi, S; Kurumada, T.; Watanabe, I. Ger. Offen. 1, 817, 703, 1969.

13.

Ferrini, P.; Marxer, A. Helv. Chim Acta 1963, Vol. XLVI, Fasc IV, 1207-1212.

U.S. Patent 4, 448, 969, 1984.

RECEIVED October 26, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

13 Mechanisms of Aromatic Amine Antidegradants J A N POSPISIL Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia

Nitroxides and benzoquinonediimine aromatic amines an consequence of amine involvement in antioxidant and/or antiozonant processes. Their participation in antioxidant regenerative mechanisms is suggested. Features of phenylenediamine involvement in antiozonant processes are discussed in relation to contemporary theories.

Secondary aromatic amines are effective antioxidants in the protection of saturated hydrocarbon polymers (polyolefins) against autooxidation. Their role in the stabilization of unsaturated hydrocarbon polymers (rubbers) is more complex: depending on their structure, they impart protection against autooxidation, metal catalyzed oxidation, flex-cracking, and ozonation. The understanding of antioxidant, antiflex-cracking and antiozonant processes together with involved mechanistic relations are of both scientific and economic interest. Rubber stabilizers are generally very reactive organic compounds. Research of their reactivity under the influence of deteriogens involved in rubber weathering indicates stabilizer transformation and formation of some different classes of products. Data on their structure and reactivity under the influence of various deteriogens and/or reactive intermediates arising in oxidized and ozonized rubber are of importance in the elucidation of the individual pathways of amine protection mechanisms. Because of the extreme reactivity of many of amine transformation products observed even during their analyses, all mechanistic conclusions have to be made very carefully: the specific property of a product may be incorrectly ascribed to another compound formed by consecutive transformations instead of to the originally arising structure. A specific role may be played by the acidity of some deteriogens or impurities and processing additives. Antioxidant Properties. Participation of secondary amines in autooxidation processes involves, in particular, reactions with free0097-6156/ 85/ 0280-0157$06.00/ 0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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r a d i c a l s and h y d r o p e r o x i d e s generated i n the a u t o o x i d i z e d s u b s t r a t e and w i t h ground and s i n g l e t s t a t e m o l e c u l a r oxugen (X) • I n t e r m e d i a t e f o r m a t i o n o f N-centered r a d i c a l s ( a m i n y l s ) i s o p e r a t i v e i n these r e a c t i o n s . Aminyls may r e a c t i n mesomeric i m i n o c y c l o h e x a d i e n y l forms ( C - c e n t e r e d r a d i c a l s ) and are t r a n s f o r m e d i n t o p r o d u c t s c r e a t e d v i a r e c o m b i n a t i o n , c o u p l i n g , d i s p r o p o r t i o n a t i o n , and o x i d a t i o n p r o c e s s e s . I t i s o f importance t o d i s t i n g u i s h between the r e a c t i v i t y o f secondary monoamines ( o n l y d i a r y l a m i n e s are c o n s i d e r e d as e f f e c t i v e a n t i o x i d a n t s ) and t h a t o f b i f u n c t i o n a l N j N ' - d i s u b s t i t u t e d 1,4phenylenediamine (PD). Two p r i n c i p a l types o f p r o d u c t s a r e formed important from the p o i n t o f v i e w o f a n t i o x i d a n t mechanisms: N i t r o x i d e s ( c h a r a c t e r i s t i c o f monoamines) and benzoquinone d i i m i n e s (BQDI, c h a r a c t e r i s t i c o f PD). Involvement o f N i t r o x i d e s . D i a r y l n i t r o x i d e s a r e formed w i t h h i g h e f f i c i e n c y from d i a r y l a m i n e s (2) . Free e l e c t r o n d e l o c a l i z a t i o n i s c h a r a c t e r i s t i c o f them. reactivity i n dimerization r e a c t i o n s Q ) • The r e a c t i v i t y diarylnitroxide highe t h a t o f n i t r o x i d e s d e r i v e d from s t e r i c a l l y h i n d e r e d p i p e r i d i n e s (HALSes) (J3). I t may be, t h e r e f o r e , m i s l e a d i n g t o t r a n s f e r g e n e r a l l y the r e s u l t s o f m e c h a n i s t i c s t u d i e s o b t a i n e d w i t h the l a t t e r i n t o t h e a r o m a t i c s e r i e s . The a b i l i t y o f d i a r y l n i t r o x i d e s t o o x i d i z e phenols or t h i o l s o r t o a b s t r a c t hydrogen atom from C-H bonds i n c r e a s e s t h e c o m p l e x i t y o f p r o c e s s e s t a k i n g p l a c e i n amine s t a b i l i z e d rubber v u l c a n i z a t e s . The r e a c t i v i t y w i t h a s t e r i c a l l y h i n d e r e d phenoxyl was c o n f i r m e d by the i s o l a t i o n o f a 2 , 5 - c y c l o h e x a d i e n e - l - o n y l d e r i v a t i v e (4). The r e c o m b i n a t i o n w i t h r a d i c a l s R» and RO^ accounts f o r d i a r y l n i t r o x i d e a n t i o x i d a n t a c t i v i t y C5), which i s however weaker than t h a t o f t h e c o r r e s p o n d i n g amine. The r e a c t i o n w i t h R* r e s u l t s i n 0 - a l k y l - N , N - d i a r y l h y d r o x y l a m i n e f o r m a t i o n . T h i s compound i s c o n s i d e r e d t o be i n v o l v e d i n an important m e c h a n i s t i c pathway: i t s t h e r m o l y s i s c r e a t e s the c o r r e s p o n d i n g N-hydroxylamine and an o l e f i n . The former i s an a n t i o x i d a n t s p e c i e s r e g e n e r a t i n g d i a r y l n i t r o x i d e i n r e a c t i o n s w i t h RO* and ROOH ( A ) . A r e g e n e r a t i v e c y c l i c a l process was proposed C D tScheme 1, R = p h e n y l ) . T h i s p r o c e s s i s v e r y important i n the HALS s t a b i l i z a t i o n mechanism. I t s h o u l d , however, be c o n s i d e r e d o n l y as a m i n o r i t y pathway i n d i a r y l a m i n e s j u s t because o f the lower s t a b i l i t y o f t h e c o r r e s p o n d i n g d i a r y l n i t r o x i d e s . This r e s u l t s i n the p a r t i c i p a t i o n o f the l a t t e r i n s i d e r e a c t i o n s l e a d i n g t o a n t i o x i d a n t i n e f f e c t i v e s p e c i e s , e.g. benzoquinone (BQ) and n i t r o b e n z e n e (7). Transformation of d i a r y l n i t r o x i d e i n t o a m i x t u r e o f d i a r y l a m i n e and N - a r y l - 1 , 4 benzoquinone monoimine-N-oxide (4_, 8) seems t h e r e f o r e t o be a more p r o b a b l e pathway r e g e n e r a t i n g p a r t l y amine a n t i o x i d a n t than t h e h y d r o x y l a m i n e / n i t r o x i d e c y c l i c a l process (Scheme 2 ) . The l a t t e r pathway i s v e r y vague i n the 1,4-PD s e r i e s . No n i t r o x i d e c o r r e s p o n d i n g t o N , N ' - d i p h e n y l - l , 4 - P D (DPPD) was d e t e c t e d i n the o x i d i z e d s u b s t r a t e by Adamic and co-workers ( 2 ) . I t has been e x p l a i n e d as a consequence o f the q u i c k o x i d a t i o n o f the i n i t i a l l y formed monoaminyl i n t o BQDI (4) w i t h o u t n i t r o x i d e f o r m a t i o n . An ESR s i g n a l c h a r a c t e r i s t i c of n i t r o x i d e corresponding t o N-isopropyl-N p h e n y l - 1 , 4 - p h e n y l e n e d i a m i n e (IPPD) was d e t e c t e d i n an oxygen d e f i c i e n t system (9), i . e . under rubber f a t i g u i n g c o n d i t i o n s . I t i s t h e r e f o r e p o s s i b l e t h a t t h e m o n o n i t r o x i d e i s an i n t e r m e d i a t e formed 1

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

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i n PD under t h e s e c o n d i t i o n s - a l t h o u g h i n a v e r y low c o n c e n t r a t i o n o n l y - and i s immediately o z i d i z e d t o d i n i t r o x i d e . The l a t t e r e x i s t s i n the more s t a b l e mesomeric d i n i t j o n e 2 f o r m , not i n v o l v e d i n the r e g e n e r a t i v e p r o c e s s (Scheme 3: R , R are a l k y l s or a r y l s ) . The importance o f the c y c l i c a l r e g e n e r a t i o n o f n i t r o x i d e i n the a r o m a t i c s e r i e s seems t o be t h e r e f o r e q u e s t i o n a b l e : N,N -dis u b s t i t u t e d PD, most p r o b a b l y not i n v o l v e d i n t h i s c y c l e , are g e n e r a l l y more e f f i c i e n t c h a i n - b r e a k i n g a n t i o x i d a n t s t h a n both d i p h e n y l a m i n e and N-phenyl-1-naphthylamine, p o t e n t i a l l y p a r t l y i n v o l v e d i n the n i t o x i d e c y c l e . I t may, t h e r e f o r e , be supposed t h a t the h i g h a n t i o x i d a n t a c t i v i t y o f PD more p r o b a b l y accounts f o r the p o s i t i v e c o o p e r a t i v e e f f e c t s o f PD w i t h i t s p r i n c i p a l o x i d a t i v e t r a n s f o r m a t i o n p r o d u c t , BQDI. f

Involvement of B e n z o q u i n o n e d i i m i n e s . BQDI's are e a s i l y formed from PD by v a r i o u s o x i d i z i n g agents (_1) and were d e t e c t e d i n P D - s t a b i l i z e d h y d r o c a r b o n s d u r i n g the oxidation (12). PD and i n f l u e n c e d by the a c i d i t y o f the medium and o x i d a t i o n p o t e n t i a l o f the p a r t i c i p a t i n g compounds i n the r e a c t i o n m i x t u r e i s e s t a b l i s h e d . S i m u l t a n e o u s l y , the c h e m i c a l s t a b i l i t y o f BQDI, i n f l u e n c e d by the N , N ' - s u b s t i t u t i o n , i s r e f l e c t e d i n the f i n a l product composition. Important d i f f e r e n c e s i n the r e a c t i v i t y o f BQDI s u b s t i t u t e d by N - s e c . a l k y l s and N - a r y l s have been o b s e r v e d . The h y d r o l y s i s p r o ceeds more e a s i l y on t h e C = N - s e c . a l k y l bond than on the C=N-aryl bond ( 1 3 ) . N - I s o p r o p y l - N - p h e n y l - l , 4 - B Q D I (IP-BQDI) i s h y d r o l y z e d i n the p r e s e n c e o f o r g a n i c c a r b o x y l i c a c i d s i n t o N-phenyl-1,4-benzoquinone monoimine (BQMI) ( 1 3 , 1 4 ) . D e c o m p o s i t i o n o f IP-BQDI t a k e s p l a c e a l s o i n the absence o f w a t e r : a p p r o x i m a t e l y one h a l f o f IP-BQDI i s r e duced t o t h e c o r r e s p o n d i n g IPPD w h i l e the o t h e r h a l f y i e l d s a comp l i c a t e d mixture of products (14). Because o f the h i g h e r h y d r o l y t i c s t a b i l i t y o f N,N'-diphenyl-l,4-BQDI (DP-BQDI), a s e r i e s o f c h a r a c t e r i s t i c s t r a n s f o r m a t i o n p r o d u c t s was i s o l a t e d and pathways o f t h e i r f o r m a t i o n , i n v o l v i n g h y d r o l y t i c , c o n d e n s a t i o n , and redox r e a c t i o n s , were e s t a b i l i s h e d ( 1 3 ) . These p r o c e s s e s r e s u l t i n the f o r m a t i o n o f BQMI, BQDI o f the Bandrowski base type ( I , I I ) and n i t r o g e n h e t e r o c y c l i c compounds, d e r i v a t i v e s o f phenazine ( I I I ) and f l o u r i n d i n e ( I V ) . 2 - S u b s t i t u t e d and 2 , 5 - d i s u b s t i t u t e d DPPD, c o r r e s p o n d i n g t o the i s o l a t e d Bandrowski BQDI I and I I are formed o n l y as i n t e r m e d i a t e s . Because o f t h e i r low redox p o t e n t i a l s , a l l PD d e r i v a t i v e s formed ( w i t h the o n l y e x c e p t i o n o f DPPD) are p r e s e n t i n the m i x t u r e e x c l u s i v e l y i n t h e i r o x i d i z e d forms. The a n t i o x i d a n t p r o p e r t y on N , N ' - d i s u b s t i t u t e d 1,4-DBDI has been e v i d e n c e d i n p o l y u n s a t u r a t e d h y d r o c a r b o n s , i . e . , i n s q u a l e n e (11) and v u l c a n i z e d NR ( 1 5 ) . BQDIs are o f comparable a n t i - f a t i g u e and a n t i - a b r a s i o n e f f i c i e n c y w i t h the c o r r e s p o n d i n g PD and are s l i g h t l y i n f e r i o r a n t i o z o n a n t s i n t h e NR v u l c a n i z a t e . In o x i d i z e d and p h o t o - o x i d i z e d t e t r a l i n o r c y l c o h e x a n e , t h e y have o n l y a c o n c e n t r a t i o n dependent r e t a r d a t i o n e f f e c t (11). Both the e f f i c i e n t s c a v e n g i n g o f R * r a d i c a l s and r e g e n e r a t i o n o f c o r r e s p o n d i n g PD may be r e s p o n s i b l e f o r the a n t i f a t i g u e and a n t i o x i d a n t p r o p e r t i e s o f BQDI ( 1 3 - 1 5 ) . The l a t t e r , formed v i a s c a v e n g i n g o f R0 ' o r 0„ i n o x i d i z e d s u b s t r a t e s t a b i l i z e d w i t h PD, c o n t r i b u t e s thus t o the i n t e r g r a l l y observed e f f i c i e n c y a s c r i b e d t o PD. An e f f i c i e n t f

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In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

13.

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In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

161

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a n t i o x i d a n t c o o p e r a t i v e system i s formed i n the PD/BQDI m i x t u r e (16) as a r e s u l t o f a l t e r n a t i v e s c a v e n g i n g o f both the p r o p a g a t i n g r a d i c a l s R0£ and R! T h i s c o o p e r a t i o n may be e x p r e s s e d i n p a r t i c u l a r i n oxygen d e f i c i e n t f a t i g u i n g p r o c e s s e s . The r e g e n e r a t i o n o f N j N ' - d i s u b s t i t u t e d PD from t h e c o r r e s p o n d i n g BQDI i s dependent on r e a c t i o n c o n d i t i o n s and r e a c t a n t s . No c o n v e r s i o n o f BQDI i n t o PD was observed d u r i n g the l o w - t e m p e r a t u r e o x i d a t i o n ( 6 5 ° C ) o f v a r i o u s h y d r o c a r b o n s ( 1 1 ) , perhaps because o f t h e s t e a d i l y h i g h r a t e o f R0« g e n e r a t i o n . About 50% o f BQDI was c o n v e r t e d i n t o PD i n a weakly a c i d medium (14) , and 60-75% i n raw NR o r NR compounded w i t h s u l p h u r , N - c y c l o h e x y l - 2 - b e n z o t h i a z o l e s u l p h e n a m i d e , HAF b l a c k e . t . c . a f t e r treatment at t h e v u l c a n i z a t i o n temperature ( 1 4 0 ° C ) (15) . The r e d u c t i o n o f BQDI i n t o PD o b s e r v e d i n NR seems t o be due t o t h e r m a l p r o c e s s e s . It i s not i n f l u e n c e d by v u l c a n i z a t i o n ingredients. In a n a l o g y t o BQDIs, N - a r y l - 4 - B Q M I s were c o n v e r t e d i n t o t h e c o r r e s p o n d i n g N - a r y l - 4 - a m i n o p h e n y l s w i t h a y i e l d o f 30-39%. The r e m a i n i n g p a r t o f BQDI o r BQMI r e s p e c t i v e l y was not r e c o v e r e d from NR by e x t r a c t i o n an c h a r a c t e r o f t h e p o l y m e r i c s p e c i e s and the mode o f t h e l i n k a g e i n t o NR were not e s t a b l i s h e d . It i s i m p o r t a n t t h a t both p r o d u c t s , i . e . , r e g e n e r a t e d PD and r u b b e r - b o u n d s p e c i e s , p o s s e s s a n t i o x i d a n t properties. N , N ' - D i s u b s t i t u t e d PD r e a c t w i t h t h e same c h a i n - b r e a k i n g mechanism as p h e n o l s ( 1 7 ) . A m i x t u r e o f a m i n i c and p h e n o l i c R0£ scavenger i s a b l e t o be i n v o l v e d i n homosynergism (_1_) • The g e n e r a l mechanism o f t h e l a t t e r accounts f o r a r e g e n e r a t i o n o f the more e f f i c i e n t c h a i n - b r e a k e r ( i . e . , amine) v i a hydrogen t r a n s f e r from the l e s s e f f i c i e n t one ( i . e . , p h e n o l ) t o the p r i m a r i l y formed a m i n y l . An e q u i l i b r i u m between 0- and N - c e n t e r e d r a d i c a l i s s u g g e s t e d . Comp l i c a t i o n i n t h i s s i m p l e mechanism i s caused by t h e p a r t i c i p a t i o n o f the r e s p e c t i v e r a d i c a l s i n c o u p l i n g , d i s p r o p o r t i o n a t i o n , o x i d a t i o n and r e c o m b i n a t i o n r e a c t i o n s . It i s connected w i t h t h e f o r m a t i o n o f p r o d u c t s d i f f e r e n t from t h o s e i n the o r i g i n a l m i x t u r e and not i n v o l ved i n t h e r e g e n e r a t i o n c y c l e . A stepwise d e p l e t i o n of antioxidant a c t i v e s p e c i e s depedent on both the amine and phenol s t r u c t u r e s s h o u l d t h e r e f o r e be c o n s i d e r e d . PD a p p l i e d as a component i n the a m i n e / p h e n o l h o m o s y n e r g i s t i c system i s c o n v e r t e d d u r i n g R02 s c a v e n g i n g i n t o t h e c o r r e s p o n d i n g BQDI. To o b t a i n more i n f o r m a t i o n about t h e r e g e n e r a t i o n o f PD from BQDI i n t h e p r e s e n c e o f p h e n o l , a p r o d u c t s t u d y has been performed i n benzene s o l u t i o n i n a weak a c i d medium u s i n g a m i x t u r e o f DP-BQDI or IP-BQDI w i t h 2 , 6 - d i t e r t - b u t y l p h e o n o l ( 1 4 ) . The r e a c t i v i t y o f BQDI plays a s p e c i f i c r o l e : DP-BQDI was c o n v e r t e d i n t o DPPD and t e t r a t e r t - b u t y l b i p h e n y l d i o l was f o r m e d . An a d d i t i o n a l r e a c t i o n was o b s e r v e d w i t h IP-BQDI as a consequence o f t h e p r e s e n c e o f the r e a c t i v e C=N-isopropyl moiety. In t h e absence o f oxygen ( i . e . , under c o n d i t i o n s s i m u l a t i n g f a t i q u i n g o f a h y d r o c a r b o n p o l y m e r ) , IPPD was c r e a t e d i n about 75% y i e l d , i . e . , the PD r e g e n e r a t i o n was enhanced by 50% i n c o m p a r i s o n w i t h t h e p h e n o l - f r e e p r o c e s s . About 15% o f IP-BQDI was c o n v e r t e d i n t o 2,6-ditert.butyl-4-(4-phenylamino-4-phenylimino)#

2.5- cyclohexadiene-l-one (imino-CHD). We suggest t h a t a r e a c t i o n o f 2 . 6 - d i t e r t . b u t y l p h e n o x y l and W u r s t e r ' s c a t i o n r a d i c a l combined w i t h i s o p r o p y l group e l i m i n a t i o n p a r t i c i p a t e s i n the imino-CHD f o r m a t i o n . 5,5,3 ,5'-Tetratert.butyl-4,4'-biphenyldiol was formed 2 , 6 - d i t e r t . butylphenol v i a the r e s p e c t i v e phenoxyl. 2 , 6 - D i t e r t . b u t y l - 1 , 4 - B Q and f

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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163

3 , 5 , 3 * , 5 ' - t e r t r a t e r t . b u t y l - 4 , 4 ' - d i p h e n o q u i n o n e f o r m , i f oxygen i s p r e s e n t i n the r e a c t i o n m i x t u r e (Scheme 4, R- i s P r o p y l ) . The p r o d u c t s t u d y r e v e a l e d the r e g e n e r a t i o n o f the s t r o n g PD a n t i o x i d a n t as a r e s u l t o f the c o o p e r a t i o n o f the weak a n t i o x i d a n t , 2 , 6 - d i t e r t . b u t y l p h e n o l , w i t h BQDI o r i g i n a t i n g i n the f i r s t s t e p o f the a n t i o x i d a n t r e g e n e r a t i v e c y c l e from PD. Another s t r o n g a n t i o x i d a n t , t e t r a t e r t . b u t y l d i p h e n y l d i o l , i s simultaneously generated i n the p r o c e s s . 2 , 4 - D i a l k y l p h e n o l s do not c o n t r i b u t e t o the PD regeneration. The imino-CHD c r e a t e d i n the r e g e n e r a t i o n c y c l e a l s o p o s s e s s e s antioxidant properties (18). It i s t r a n s f o r m e d d u r i n g the s t a b i l i z a t i o n o f o x i d i z e d squalene i n t o an aminyl a b l e t o d i m e r i z e . The dimer i s decomposed q u i c k l y i n o x i d i z e d squalene and i s reduced i n t o the s t a r t i n g i m i n o - C H D . At the same t i m e , however, the s t a b i l i z i n g e f f e c t of imino-CHD i n s q u a l e n e i s s t e p w i s e l o s t without the t i t l e compound being d e s t r o y e d . It seems t h a t the squalene a u t o o x i d a t i o n i s a c c e l e r a t e d because o f the e f f e c t o f some i n t e r m e d i a t e s formed from squalene d u r i n g r e g e n e r a t i o r e g e n e r a t i o n s h o u l d be t h e r e f o r e c o n s i d e r e d as an u n d e s i r a b l e o n e . F a c t o r s i n Ozone Weathering of Rubbers and S t a b i l i z e r s . Ozone i s a s p e c i f i c atmospheric p o l l u t a n t c h a r a c t e r i s t i c o f urban and some i n d u s t r i a l areas o f the t r o p o s p h e r e . Its c o n c e n t r a t i o n i s very variable. Because o f both the p h o t o c h e m i c a l c h a r a c t e r o f the o r i g i n o f ozone and i t s h i g h r e a c t i v i t y w i t h o r g a n i c atmospheric p o l l u t a n t s as w e l l as some o r g a n i c m a t e r i a l s on the E a r t h ' s s u r f a c e , i t s n i g h t c o n c e n t r a t i o n drops t o z e r o . Only ground s t a t e m o l e c u l a r oxygen a t t a c k s r u b b e r i n t h i s p e r i o d o f the d a y , but m e c h a n i c a l l y i n i t i a t e d p r o c e s s e s and r e a c t i v e o z o n a t i o n i n t e r m e d i a t e s and p r o d u c t s remain involved. Due t o t h e h i g h r e a c t i v i t y of ozone w i t h u n s a t u r a t e d h y d r o c a r b o n s m o i e t i e s , s u r f a c e c r a c k i n g o f s t r e s s e d or f l e x e d NR, BR, NBR, and SBR v u l c a n i z a t e s a r i s e s . Rubber goods d e s i g n e d f o r outdoor a p p l i c a t i o n s must t h e r e f o r e be s t a b i l i z e d a g a i n s t both 0^ and 0^ attacks. A n t i o x i d a n t p r o t e c t i o n mechanisms have been d i s c u s s e d i n d e t a i l (_1) . D i s c u s s i o n s d e a l i n g w i t h a n t i o z o n a n t mechanism i n v o l v e some c o n t r a d i c t o r y e x p e r i m e n t a l o b s e r v a t i o n s . Any approach t o the f o r m u l a t i o n o f an a n t i o z o n a n t mechanism should r e f l e c t a l l p o s s i b l e i n t e r a c t i o n s of a s t a b i l i z e r molecule w i t h ozone and a c t i v e s p e c i e s formed v i a r u b b e r o z o n a t i o n . The l a t t e r i n v o l v e s an i o n i c mechanism, and a v a r i e t y o f a c t i v e oxygen c o n t a i n i n g products having s t r u c t u r e s of ozonides, polymeric o z o n i d e s , and p e r o x i d e s i s formed ( 1 9 ) . A p a r t i a l o c c u r r e n c e of f r e e r a d i c a l p r o c e s s e s d u r i n g h y d r o c a r b o n o z o n a t i o n may be a s c r i b e d t o the s i m u l t a n e o u s a t t a c k o f the 0 l 0^ m i x t u r e . Under r e a l i s t i c o u t door s e r v i c e c o n d i t i o n s , r u b b e r w e a t h e r i n g i s caused not o n l y by ozonation. A u t o o x i d a t i o n and f l e x - c r a c k i n g are e q u i v a l e n t d e t e r i o r a t i o n p r o c e s s e s ( s u r p a s s i n g o z o n a t i o n i n the s e r v i c e p e r i o d i n the dark). M o r e o v e r , s i n g l e t oxygen has t o be c o n s i d e r e d as another d e t e r i o g e n , although present i n trace concentration o n l y . Its o c c u r r e n c e i n the o z o n a t i o n o f polymer C-H bonds has been r e p o r t e d (20). T h u s , a m i x t u r e o f r a d i c a l , i o n i c and m o l e c u l a r s p e c i e s a r i s i n g i n s i m u l t a n e o u s l y p r o c e e d i n g o z o n a t i o n , a u t o o x i d a t i o n and f a t i g u i n g o f v u l c a n i z a t e s under dynamic c o n d i t i o n s c r e a t e s a f a i r l y complicated r e a c t i v e system. Any o v e r s i m p l i f i c a t i o n i n the r a t i n g a

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o f p a r t i c u l a r w e a t h e r i n g f a c t o r s s e p a r a t e l y from t h e o t h e r ones may be a s o u r c e o f s e r i o u s m i s u n d e r s t a n d i n g s i n t h e i n t e r p r e t a t i o n o f t h e i n t e g r a l r o l e o f r u b b e r o x i d a t i o n p r o c e s s e s and t h e i r involvement i n s t a b i l i z e r mechanisms and t r a n s f o r m a t i o n . Many experiments have t o be done i n s i m p l i f i e d model systems i n which some important f a c t o r s are n e g l e c t e d . T h i s sometimes d i s t o r t s t h e i n t e r p r e t a t i o n v e r y gravely. Rubber o x i d a t i o n p r o d u c t s a b l e t o undergo redox and/or c o n d e n s a t i o n r e a c t i o n s must be c o n s i d e r e d i n p a r t i c u l a r as r e a c t i v e partners with antiozonant s p e c i e s . Analogies i n the r e a c t i v i t y with low m o l e c u l a r weight o r g a n i c s a r e m o s t l y c o n s i d e r e d . L i m i t s c o n t r o l l i n g polymer-analogous r e a c t i o n s cannot however, be o m i t t e d i n p a r t i c u l a r r e s t r i c t i o n s o f t h e r e a c t i v i t y by p h y s i c a l e n v i r o n m e n t a l f a c t o r s i n the s o l i d m a t r i x . A n t i o z o n a n t P r o p e r t i e s . Aromatic secondary diamines a r e t h e o n l y c l a s s o f organic chemical growth o f v u l c a n i z a t e s unde the same time from both view. The p r e s e n c e o f a s e c o n d a r y a r o m a t i c amine m o i e t y i t s e l f i n a molecule i s not a s u f f i c i e n t c o n d i t i o n t o a t t a i n antiozonants efficiency. ( E . g . , secondary monomaines a r e o n l y a n t i o x i d a n t s and f l e x - c r a c k i n h i b i t o r s without a p p r e c i a b l e a n t i o z o n a n t a c t i v i t y . On the o t h e r hand, a l l N , N ' - d i s u b s t i t u t e d PD a n t i o z o n a n t s a r e a l s o e f f i c i e n t a n t o x i d a n t s and most o f them a l s o a c t as f l e x - c r a c k i n h i b i t o r s ( J O . Both t h e s e s t a b i l i z a t i o n a c t i v i t i e s have t o be c o n s i d e r e d i n t h e complex a n t i o z o n a n t mechanism, t o g e t h e r w i t h some metal d e a c t i v a t i n g a c t i v i t y . An e x t e n s i v e s c r e e n i n g o f s t r u c t u r e - a c t i v i t y r e l a t i o n s r e v e a l e d (_1_) t h e o u t s t a n d i n g p r o p e r t i e s o f N , N ' - d i s u b s t i t u t e d PD. I t i s g e n e r a l l y a c c e p t e d t h a t t h e p r e s e n c e o f N - s e c . a l k y l s accounts f o r b e t t e r a n t i o z o n a n t p r o t e c t i o n than t h a t o f N-prim. and N - t e r t . a l k y l s or N - a r y l s ( 2 1 ) . T h i s may be one o f t h e c l u e s t o d e c i p h e r t h e c h e m i c a l pathways o f t h e a n t i o z o n a n t mechanism. The f i n a l e f f e c t i s moreover f u l l y dependent on t h e c o m p o s i t i o n o f t h e v u l c a n i z a t e . The s t r u c t u r e o f c o m m e r c i a l l y used a n t i o z o n a n t s i s an optimum compromise o f e f f i c i e n c y , p h y s i c a l p r o p e r t i e s and t o x i c i t y . N, N ' - D i s e c . a l k y l 1-4-PD a r e used i n t h e U.S.A., N - s e c . a l k y l - N - a r y l - l , 4 - P D a r e p r e f e r r e d i n Europe. N , N - D i a r y l d e r i v a t i e s a r e not a p p l i e d as a n t i ozonants i n NR, BR, IR, o r SBR. One o f t h e reasons may be t h e i r low s o l u b i l i t y i n rubber v u l c a n i z a t e s (22). I t does not a l l o w them t o r e a c h a c o n c e n t r a t i o n l e v e l i n t h e r u b b e r b u l k which i s a b l e t o a c t as a l o n g - t e r m o p e r a t i v e s t o r e o f a s t a b i l i z e r ready t o s u p p l y t h e r u b b e r s u r f a c e s l o w l y but c o n t i n u o u s l y w i t h a c t i v e compounds by m i g r a t i o n and t o m a i n t a i n t h e p r o t e c t i v e e f f e c t w i t h o u t i n e f f i c i e n t q u i c k blooming o f an i n c o m p a t i b l e PD. A c h e m i c a l r e a s o n a c c o u n t i n g f o r t h e m i n o r i t y a n t i o z o n a n t r o l e o f N , N ' - d i a r y l PD i s d i s c u s s e d later. f

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A n t i o z o n a n t Mechanism. No s i m p l e model approach t o t h e e x p l a n a t i o n o f t h e a n t i o z o n a n t a c t i v i t y o f PD i s a p p l i c a b l e . Most i d e a s were i n f l u e n c e d by t h e f a c t t h a t t h e rubber o z o n a t i o n i s a s u r f a c e p r o c e s s , not e x c e e d i n g a t h i c k n e s s o f about 40 m o l e c u l a r d i a m e t e r s (23). The r e a c t i o n o f ozone w i t h an a n t i o z o n a n t i n t h e v u l c a n i z a t e s u r f a c e l a y e r and r e p l e n i s h m e n t o f t h e consumed s t a b i l i z e r by means

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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o f the m i g r a t i o n o f the f r e s h one t o the r u b b e r s u r f a c e from the rubber b u l k was t h e r e f o r e c o n s i d e r e d as the most p r o b a b l e a n t i o z o n a n t a c t i v i t y e x p l a n a t i o n i n the e a r l i e s t t h e o r i e s . It has been a d m i t t e d s u c c e s s i v e l y t h a t more p r o c e s s e s or r e a c t a n t s may be i n v o l v e d . A h i g h s u r f a c e c o n c e n t r a t i o n of PD has been r e p o r t e d a l s o i n modern m e c h a n i s t i c o z o n a t i o n s t u d i e s (%4) as a n e c e s s a r y c o n d i t i o n t o achieve antiozonant e f f i c i e n c y . But even t h i s c o n d i t i o n cannot be the s o l e one v a l i d as i t may be e x t r a p o l a t e d from some e x p e r i m e n t a l proofs. The h i g h s u r f a c e a n t i o z o n a n t c o n c e n t r a t i o n i s a l s o d i f f i c u l t to be m a i n t a i n e d d u r i n g the l o n g s e r v i c e time o f v u l c a n i z a t e s : antiozonants a r e d e p l e t e d on the s u r f a c e not o n l y be c h e m i c a l ( i . e . , o x i d a t i o n / o z o n a t i o n r e a c t i o n s ) but a l s o by p h y s i c a l p r o c e s s e s ( v o l a t i l i z a t i o n , leaching, abrasion). Only one p a r t o f the c h e m i c a l d e p l e t i o n p r o c e s s e s c o n t r i b u t e s t o the a n t i o z o n a n t e f f i c i e n c y . Moreo v e r , t h e d e c i s i v e r o l e o f a n t i o z o n a n t m i g r a t i o n has been made d o u b t f u l by the r e s u l t s o f S c o t t (25) o b t a i n e d w i t h the polymer bound 4-(mercaptoacetamido)diphenylamine (V). The h i g h r u b b e r p r o t e c t i o n was a c h i e v e d even i n e x t r a c t e m i g r a t i o n o f s t r u c t u r a l l y bound a n t i o z o n a n t s p e c i e s . D i s h a r m o n i e s i n the C o n c e p t i o n o f the D i r e c t O ^ / A n t i o z o n a n t R e a c t i o n Importance. Four a n t i o z o n a n t t h e o r i e s have been f o r m u l a t e d w i t h i n the l a s t 25 y e a r s . Ozone s c a v e n g i n g t h e o r y s u g g e s t s a p r e f e r e n t i a l d i r e c t r e a c t i o n o f an a n t i o z o n a n t w i t h ozone on the r u b b e r s u r f a c e as a d e c i s i v e process (26-27). As the a n t i o z o n a n t i s d e p l e t e d v i a d i r e c t o z o n a t i o n on the s u r f a c e , f r e s h a n t i o z o n a n t d i f f u s e s r a p i d l y from the r u b b e r b u l k t o r e e s t a b l i s h the e q u i l i b r i u m s u r f a c e c o n c e n tration. At a comparable a d d i t i v e c o n c e n t r a t i o n and m i g r a t i o n r a t e , the a n t i o z o n a n t e f f i c i e n c y o f an a d d i t i v e s h o u l d be t h e r e f o r e dependent on i t s o z o n a t i o n r a t e and the v u l c a n i z a t e w i l l be p r o t e c t e d u n t i l the a n t i o z o n a n t i s d e p l e t e d below t h e lowest c r i t i c a l c o n c e n tration. From t h i s p o i n t o f v i e w , the o z o n a t i o n r a t e seems t o be a more i m p o r t a n t f a c t o r t h a n the t o t a l amount o f ozone scavenged by one mole o f an a n t i o z o n a n t ( t h i s l a t t e r phenomenon may be c a l l e d ozonation f a c t o r ) . R e l a t i o n s between a n t i o z o n a n t e f f i c i e n c y i n v u l c a n i z a t e and a n t i o z o n a n t o z o n a t i o n r a t e o r a n t i o z o n a n t s u r f a c e c o n c e n t r a t i o n have been indeed r e p o r t e d i n some papers and an a p p r e c i a b l e h i g h e r o z o n a t i o n r a t e o f PD i n comparison w i t h r u b b e r u n s a t u r a t i o n , a p r e f e r e n t i a l consumption o f an a n t i o z o n a n t i n model o l e f i n s o l u t i o n or i n r u b b e r were o b s e r v e d . The r u b b e r s u r f a c e was not a t t a c k e d by ozone u n t i l the a n t i o z o n a n t was almost c o m p l e t e l y consumed (28). Theory o f p r o t e c t i v e f i l m f o r m a t i o n supposes c r e a t i o n o f a r u b b e r s u r f a c e f i l m from PD o x i d a t i o n a n d / o r o z o n a t i o n p r o d u c t s . Ozone a t t a c k on the o x i d a t i v e l y v i o l a t e d r u b b e r s u r f a c e i s thus p r e v e n t e d ( 2 1 , 27, 29, 3 0 ) . C r e a t i o n o f the s u r f a c e l a y e r was c o n firmed using m i c r o s c o p y . The t h e o r y i s i n agreement w i t h the o b s e r v a t i o n t h a t the i n i t i a l r a p i d ozone consumption i s s t a b i l i z e d r u b b e r drops and may be renewed a f t e r m e c h a n i c a l break o f the formed f i l m (27). There i s a c h e m i c a l p r o o f o f the t h e o r y . Ozonation p r o d u c t s o f N j N ' - b i s d - m e t h y l h e p t y D - l ^ - P D (DOPPD) form a s u r f a c e f i l m on o z o n i z e d and DOPPD doped v u l c a n i z e d NR (31) and c a r b o n - b l a c k loaded NR (24, 3 2 ) . In a d d i t i o n t o u n r e a c t e d DOPPD, many o f the l o w m o l e c u l a r weight compounds o b s e r v e d i n the f i l m were found a l s o i n the o z o n a t i o n o f pure DOPPD ( 2 4 ) ; the o n l y d i f f e r e n c e was a

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r e s t r i c t e d f o r m a t i o n o f Bandrowski bases i n the s u r f a c e f i l m . No r u b b e r o z o n a t i o n p r o d u c t s were found u s i n g ATR-IR i n the degraded s u r f a c e l a y e r o f s t a b i l i z e d NR ( 3 1 , 3 2 ) , a l t h o u g h o z o n i d e s and c a r b o n y l compounds were i d e n t i f i e d on the s u r f a c e o f degraded but u n s t a b i l i z e d v u l c a n i z e d unloaded NR (31) , c l a y (31) or c a r b o n b l a c k loaded NR ( 3 2 ) . The d i f f u s i o n o f DOPPD t o the r u b b e r s u r f a c e i s supposed t o be s u f f i c i e n t l y r a p i d t o account f o r the o b s e r v e d a n t i ozonant e f f e c t (33) . The r e p o r t e x p e r i m e n t a l d a t a have been i n t e r p r e t e d as a d u a l s c a v e n g e r - p r o t e c t i v e f i l m f o r m a t i o n t h e o r y o f a n t i o z o n a n t mechanism o f N , N ' - d i s u b s t i t u t e d 1 , 4 - P D (24, 32). There e x i s t numbers o f e x p e r i m e n t a l o b s e r v a t i o n s and c o n t r a d i c t o r y r e s u l t s making vague the u n i v o c a l v a l i d i t y o f both the s i m p l e ozone scavenger and the p r o t e c t i v e f i l m t h e o r i e s . Products formed v i a o z o n a t i o n o f N - ( 1 , 3 - d i m e t h y l b u t y l ) - N - p h e n y l - 1 , 4 - P D (HPPD) and DOPPD are acetone s o l u b l e . But a n o t h e r p a r t o f the p r o d u c t s formed i n PD doped NR and e s t i m a t e d by L o r e n z and Parks (29) to about 23-37% are " u n e x t r a c t a b l e n i t r o g e n compounds" and s h o u l d be b e l i e v e d t o be polymer bound m o i e t i e ozonized rubber. There a r e o t h e r e x p e r i m e n t a l o b j e c t i o n s t o o : the d i f f u s i o n r a t e o f PD t o the s u r f a c e i s not q u i c k enough t o ensure a h i g h a n t i o z o n a n t c o n c e n t r a t i o n to be a t t a i n e d w i t h i n a s h o r t time p e r i o d a f t e r the p r o d u c t i o n o f r u b b e r goods (34) . Antiozonant e f f i c i e n c y i s not u n l i m i t e d l y p r o p o r t i o n a l to the a n t i o z o n a n t c o n c e n t r a t i o n (30). The r a t e c o n s t a n t s o f d i r e c t amine o z o n a t i o n do not v a r y so e x p r e s s i v e l y as t h e i r a n t i o z o n a n t e f f i c i e n c i e s (35) and no r e a s o n a b l e d i r e c t r e l a t i o n between t h e s e two phenomena c o u l d be found. M o r e o v e r , an e a s y r e a c t i v i t y w i t h 0^ i s not a s u f f i c i e n t c o n d i t i o n f o r a compound t o be an a n t i o z o n a n t as may be e x e m p l i f i e d on v a r i o u s r e a c t i v e o r g a n i c compounds h a v i n g no a n t i o z o n a n t p r o p e r ties. There i s a l s o no c l e a n - c u t d i f f e r e n c e between the o v e r a l l ozone consumption (ozone e q u i v a l e n t s ) by v a r i o u s N , N - d i s u b s i t u t e d 1,4-PD ( 3 6 ) . T h e r e f o r e , a comparison o f the a n t i o z o n a n t a c t i v i t y o f a p a r t i c u l a r compound i n a v u l c a n i z a t e w i t h the compound r e a c t i v i t y w i t h ozone i s a vague m e c h a n i s t i c e x p l a n a t i o n and i s o f m i n o r v a l u e from the p o i n t o f v i e w o f g e n e r a l i z a t i o n o f t h e p r o c e s s . The c h e m i s t r y o f PD o z o n a t i o n i s more i m p o r t a n t . f

f

In s p i t e of many o b j e c t i o n s , ozone scavenger and p r o t e c t i v e f i l m t h e o r i e s cannot be n e g l e c t e d because o f s e r i o u s e x p e r i m e n t a l e v i dence. They s h o u l d be c o n s i d e r e d as an i m p o r t a n t p a r t o f the o v e r a l l a n t i o z o n a n t mechanism. T h e i r r o l e p r e v a i l s i n rubber s o l u t i o n or i n v e r y t h i n rubber f i l m s . It i s v e r y p r o b a b l e t h a t a p a r t o f an a n t i ozonant i s w a s t e f u l l y d e p l e t e d j u s t because o f d i r e c t o z o n a t i o n . Involvement o f the O z o n i z e d Rubber M o i e t i e s i n A n t i o z o n a n t Mechanism. The r u b b e r c h a i n r e l i n k i n g t h e o r y (30) i s c o n s i s t e n t i n p a r t w i t h the s e l f - h e a l i n g f i l m formation theory (37): a r e a c t i o n between an a n t i ozonant o r some o f i t s t r a n s f o r m a t i o n p r o d u c t s and o z o n i z e d e l a s t o m e r is considered. E i t h e r s c i s s i o n of ozonized rubber i s prevented i n t h i s way or s e v e r e d p a r t s o f the r u b b e r c h a i n are recombined ( i . e . , relinked). A " s e l f - h e a l i n g " f i l m r e s i s t a n t t o o z o n a t i o n i s formed on the rubber s u r f a c e . Such a f i l m formed by the c o n t r i b u t i o n o f n o n v o l a t i l e and f l e x i b l e fragments o f the rubber m a t r i x s h o u l d be more

p e r s i s t e n t than any f i l m suggested i n the p r o t e c t i v e f i l m t h e o r y . C r e a t i o n o f an u n e x t r a c t a b l e polymer bound p a r t o f o r i g i n a l l y added PD has been r e p o r t e d (29) and c o n s i d e r e d as a p i e c e o f e v i d e n c e o f

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the r e a c t i v i t y o f PD w i t h o z o n i z e d r u b b e r . To e x p l a i n the mode o f f o r m a t i o n o f a l i n k between a n t i o z o n a n t o r i t s t r a n s f o r m a t i o n p r o d u c t s and the r u b b e r n e t w o r k , r e a c t i o n s o f a n t i o z o n a n t w i t h r u b b e r z w i t t e r i o n s , o z o n i d e s , and a l d e h y d e s have been c o n s i d e r e d . E . g . , the f o r m a t i o n o f an a d d i t i o n p r o d u c t from z w i t t e r i o n (37) or from o z o n i d e (29) and PD a c c o u n t i n g f o r the polymer bound m o i e t y was suggested (Scheme 5 ) . A n i t r o x i d e and W u r s t e r ' s c a t i o n - r a d i c a l were r e p o r t e d to be i n t e r m e d i a t e s i n the r e a c t i o n between IPPD and a low m o l e c u l a r weight ozonide (38). Some e x p e r i m e n t a l p r o o f b r i n g i n g s e r i o u s o b j e c t i o n s a g a i n s t r e l i n k i n g t h e o r y must a l s o be c o n s i d e r e d : the r e a c t i o n o f r u b b e r o z o n i d e s w i t h IPPD and DPPD has been r e p o r t e d t o be v e r y slow ( s l o w e r t h a n w i t h ozone) ( 2 8 , 29, 38, 3 9 ) . No p r o d u c t s a r i s i n g from the i n t e r a c t i o n between N , N - d i s u b s t i t u t e d PD and v u l c a n i z e d NR d u r i n g o z o n a t i o n were e v i d e n c e d i n (24, 31, 3 2 ) . The l o w - r a t e r e a c t i v i t y o f PD w i t h o z o n i d e s i n comparison w i t h t h a t w i t h ozone i s r a t h e r opposed t o the c h a i n - l i n k i n g t h e o r y . Both r e a c t i o n s s h o u l d , however, be c o n s i d e r e d a l s importance: Og i s p r e s e n t i n the t r o p o s p h e r e o n l y d u r i n g s u n s h i n e hours. On the c o n t r a r y , the a c c u m u l a t i o n o f o z o n i d e s (and o f o t h e r o z o n a t i o n p r o d u c t s ) i n the r u b b e r s u r f a c e i n c r e a s e s s t e p w i s e and p e r m a n e n t l y and the oxygenated s p e c i e s r e a c t w i t h PD a l s o i n the d a r k period. The importance o f the r e a c t i o n w i t h o z o n i d e i s thus augmented. M o r e o v e r , the r e a c t i o n o f PD w i t h an o z o n i d e may be a p p r e c i a b l y a c c e l e r a t e d i n a weak a c i d m e d i a , i . e . , by the p r e s e n c e of a l i p h a t i c c a r b o x y l i c a c i d s (39). Ozonide i s reduced i n t o a m i x t u r e o f a l d e h y d e s i n t h i s p r o c e s s (Scheme 6 ) . T h i s may be one o f the pathways l e a d i n g t o polymer bound s p e c i e s . f

U s i n g e x t r a p o l a t i o n from o r g a n i c c h e m i s t r y mechanisms, l i n k and network f o r m a t i o n v i a r e a c t i o n between PD and a l d e h y d i c ( 2 9 , 30, 33) o r z w i t t e r i o n i c (37) and groups formed i n o z o n i z e d r u b b e r o r i t s fragments w i t h a l d e h y d e s formed by r e d u c t i o n o f r u b b e r g o z o n i d e s (39) or w i t h a l d e h y d e s a r i s i n g from the o z o n a t i o n o f N - s e c . a l k y l s i n PD m o l e c u l e (24, 40) seems t o be o p e r a t i v e . A l l t h e s e model c o n c e p t i o n s i n d i c a t e p o s s i b i l i t i e s of rubber c h a i n r e - l i n k i n g or extension or network f o r m a t i o n i n o z o n i z e d r u b b e r . Whatever the mode o f l i n k a g e f o r m a t i o n may b e , i t has been supposed t h a t t h i s c h e m i c a l t r a n s f o r m a t i o n causes r e l a x a t i o n o f s t r e s s e d r u b b e r s u r f a c e and i n c r e a s e s the c r i t i c a l energy n e c e s s a r y f o r the s u r f a c e ozone c r a c k s f o r m a t i o n (41). At the same t i m e , a c h e m i c a l l y m o d i f i e d , r e l a x e d , f l e x i b l e r u b b e r s u r f a c e l a y e r l e s s s e n s i t i v e t o the f u r t h e r 0^ a t t a c k i s created. T h i s c h e m i c a l s u r f a c e r e s t r u c t u r i n g needs some time and may be one o f the causes o f d i f f e r e n c e s between a n t i o z o n a n t e f f i c i e n c y o b s e r v e d i n n a t u r a l and a c c e l e r a t e d o z o n a t i o n t e s t s . At the same t i m e , i t may e x p l a i n t h e f a i l u r e i n m o n i t o r i n g r u b b e r / a n t i o z o n a n t i n t e r a c t i o n s i n short ozonation experiments. P r o d u c t s S t u d i e s i n A n t i o z o n a n t Mechanisms. Experimental proof of the i n d i v i d u a l a n t i o z o n a n t t h e o r i e s based on p r o d u c t s t u d i e s are scarce. Using a v e r y s o p h i s t i c a t e d instrumental a n a l y t i c a l approach, the c o m p o s i t i o n o f a v e r y c o m p l i c a t e d m i x t u r e o f o z o n a t i o n p r o d u c t s o f two t e c h n i c a l l y i m p o r t a n t a n t i o z o n a n t s , i . e . , DOPPD and HPPD has been r e v e a l e d and the r e a c t i v i t y pathways w i t h ozone have been e s t a b l i s h e d (24, 40). I n f l u e n c e o f the c h a r a c t e r o f N - s u b s t i t u e n t s on the o z o n a t i o n mechanism has been e v i d e n c e d . Some i m p o r t a n t

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m e c h a n i s t i c d i f f e r e n c e s between N ,N - d i s e c - a I k y 1 - s u b s t i t u t e d PD and between t h o s e N - s e c . a l k y l - N p h e n y l s u b s t i t u t e d are thus c l a r i f i e d . Two p r i n c i p a l pathways govern the o z o n a t i o n o f DOPPD ( 2 4 ) : ( i ) amine o x i d e pathway; ( i i ) N - s e c . a l k y l ( i . e . s i d e c h a i n ) o x i d a t i o n pathway. The t h i r d m e c h a n i s t i c f e a t u r e i s a m i n o r i t y n i t r o x i d e r a d i c a l pathway l e a d i n g t o a s t a b l e d i n i t r o n e . In the HPPD o z o n a t i o n , the f r e e r a d i c a l c h e m i s t r y seems t o be more important t h a n i n DOPPD, due to the s t a b i l i z i n g e f f e c t p r o v i d e d by the d i a r y l a m i n e m o i e t y ( 4 0 ) . The n i t r o x i d e pathway i s t h e r e f o r e of r e l e v a n t importance i n HPPH o z o n a t i o n t o amine o x i d e and s i d e c h a i n o x i d a t i o n pathways. Because o f the i n f l u e n c e o f N - s u b s t i t u e n t e f f e c t s the o z o n a t i o n o f HPPH o c c u r s o n l y on the a l i p h a t i c s i d e o f the m o l e c u l e and a n i t r o n e i s the most abundant o z o n a t i o n p r o d u c t . The f o r m a t i o n o f a d i n i t r o n e - i n c o n t r a s t t o the o z o n a t i o n o f DOPPD - i s i n h i b i t e d most p r o b a b l y j u s t by the s t a b i l i z i n g e f f e c t o f the Nphenyl g r o u p . The same s t r u c t u r a l m o i e t y s t a b i l i z e s a r o m a t i c n i t r o and n i t r o s o compounds forme Bandrowski bases were d e t e c t e a u t h o r s (40) f a v o r the N - a l k y l s i d e r e a c t i v i t y i n HPPD a l s o i n the i n t e r p r e t a t i o n o f the f o r m a t i o n o f N-phenyl-N'-acyl-l,4-PD, i.e. in the c o n d e n s a t i o n w i t h a l i p h a t i c a l d e h y d e s . 1

1

F o r m a t i o n o f an aminyl r a d i c a l on the a r o m a t i c s i d e o f HPPD i s a r e s u l t o f the NH-bond o x i d a t i o n . The N-N c o u p l i n g o f a m i n y l s accounts f o r the f o r m a t i o n o f i n t e r e s t i n g d i m e r s , e.g. VI, a b l e t o be o x i d i z e d i n t o n i t r o x i d e and n i t r o n e . F o r m a t i o n o f a C c e n t e r e d r a d i c a l , mesomeric t o the o r i g i n a l l y formed a m i n y l , s h o u l d be e x p e c t e d . Thus, a l s o o t h e r o l i g o m e r i c p r o d u c t s may be formed v i a C-N and C-C couplings. By e x t r a p o l a t i n g d a t a o f L a t t i m e r and coworkers (24, 4 0 ) , i t may be a n t i c i p a t e d t h a t the o z o n a t i o n o f DPPD proceeds most p r o b a b l y v i a n i t r o x i d e and aminyl f o r m a t i o n pathways. D i n i t r o n e and N-N o r N-C and C-C c o u p l i n g p r o d u c t s s h o u l d t h e r e f o r e be c o n s i d e r e d as o z o n a t i o n p r o d u c t s formed v i a f r e e r a d i c a l i n t e r m e d i a t e s . The r e a c t i v i t y o f DPPD w i t h aldehydes w i l l be v e r y l i m i t e d . A l l these m e c h a n i s t i c f e a t u r e s t o g e t h e r w i t h the low o z o n a t i o n r a t e o f DPPD i n comparison w i t h N , N ' - d i s e c . a l k y l PD (26) may be r e s p o n s i b l e f o r the g e n e r a l l y r e p o r t e d poor a n t i o z o n a n t e f f i c i e n c y o f N , N ' - d i a r y l PD i n comparison w i t h N - s e c . a l k y l - N ' - a r y l and N , N - d i s e c . a l k y l PD. There i s a common m e c h a n i s t i c f e a t u r e i n the d i r e c t o z o n a t i o n o f v a r i o u s N , N ' - d i s u b s t i t u t e d 1,4-PD: F o r m a t i o n or a c c u m u l a t i o n o f any s i m p l e BQDI, BQMI o r BQ d e r i v a t i v e s was not observed (12, 24, 4 0 ) , a l t h o u g h BQDI themselves were proven t o be ozone r e s i s t a n t ~ T l 8 T 7 But i t has been e v i d e n c e d i n experiments aimed t o c l a r i f y a s p e c t s o f the c h a i n - r e l i n k i n g t h e o r y t h a t IPPD and DPPD a r e o x i d i z e d by 1hexadecane o z o n i d e i n the p r e s e n c e o f a l i p h a t i c c a r b o x y l i c a c i d s (39) t o c o r r e s p o n d i n g BQDI ( a l i p h a t i c a c i d s which c a t a l y z e the r e a c t i o n may a r i s e e i t h e r d u r i n g o x i d a t i o n o f rubber or are used as rubber compounding i n g r e d i e n t s ) . The b a s i c i t y o f PD and s t a b i l i t y o f BQDI p l a y an important r o l e . N i t r o x i d e or Wurster's c a t i o n - r a d i c a l may be i n v o l v e d i n the f o r m a t i o n o f BQDI (38, 3 9 ) . The l a t t e r i s t r a n s f o r m e d i n the weakly a c i d i c medium used f o r the r e a c t i o n o f PD w i t h o z o n i d e i n t o a r a t h e r c o m p l i c a t e d m i x t u r e o f p r o d u c t s mentioned e a r l i e r . f

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POLYMER STABILIZATION AND DEGRADATION

Scheme 5. Mechanism o s w i t t e r i o n or from ozonide and PD a c c o u n t i n g f o r the polymer-bound moiety.

NHR

- l

NHR R C00H 1

»C 0 —

1

1 4

H 9CH0 + CH 0 + 2

2

0 I

NHR

V Scheme 6.

R e a c t i o n o f PD w i t h an o z o n i d e .

^

N

-

^

N

H

R

VI

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Combinations of Individual Mechanisms. It is extremely difficult to obtain an undoubted proof of the superiority of one particular antiozonant stabilization mechanism and its unequivocal validity in complicated rubber vulcanizate systems exposed to a complex environmental attack. Using a product study concerned with BQDI chemistry, the possibility of mechanistic relations between the participation of PD in antiozonant, antioxidant and/or antiflex-crack processes during rubber weathering has been suggested: the corresponding BQDI are generally formed from PD during the stabilization of hydrocarbons against thermal oxidation as well as photo-oxidation (11, 12) and by the reaction with ozonides (39). More complicated BQDI of Bandrowski base type are formed in the PD ozonation (24, 40) and in the weak acid catalyzed decomposition of primarily formed BQDI (13). The latter have therefore to be considered as one of the clue compounds in the rubber stabilization against weathering. They are supposed to be more efficient scavengers of R* radicals than the corresponding PD derivatives (16) and contribute therefore to antiflex-cracking activity. Participation ozone and rubber oxidation/ozonation products in the presence of other additives and/or acid impurities may account for the simultaneous and/or consecutive course of reactions resulting in a scavengerprotective film-chain relinking ternary combined mechanism. The predominance of any of these three particular mechanisms is substrate and environmental conditions dependent. Rubber stabilization efficiency of amines results not only from an interaction of stabilizers with chemical deteriogens. The importance of physical factors (e.g. diffusion, migration, solubility, volatility or leaching of stabilizers), of physical or chemical consequences of the interaction of stabilizers with fillers (carbon black in particular) or catalytic impurities of various origin, and of environmental effects (acid rain in particular) is mostly underrated or even ignored in mechanistic discussions, although only a properly estimated influence of all factors and of their relative importance is an efficient approach targetting the explanation of rubber stabilization mechanisms. Unfortunately, many experimental data are s t i l l lacking and it is moreover uncertain if they will be available in the near future. References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Pospisil, J., "Developments in Polymer Stabilization", Scott, G. (Ed.), Vol. 7, Chapter 1, Applied Science Publishers, Ltd., Barking, 1984. Adamie, K., Bowman, D. F . , Ingold, K. U., J . Amer. Oil Chemists' Soc. (1970), 47, 109. Rozantsev, E. G., Sholle, V. D., Synthesis (1971), 190 and 401. Forrester, A. R., Hay, J . M., Thomson, R. H., "Organic Chemistry of Stable Free Radicals", Academic Press, London, 1968. Ingold, K. U., Advan. Chem. Ser. (1968), 75, 296. Cholvad, V., Stasko, A., Tkac, A., Butchatchenko, A. L . , Malik, M., Collect. Czech. Chem. Commun. (1981) 46, 823. Bolsman, T. A. B. M., Blok, A. P., Frijns, J . H. G., Rec. Trav. Chim. Pays Bas (1978) 97, 310 and 313. Nelsen, J . F . , "Free Radicals", Kochi, J . K. (Ed.), Vol. 2, p. 257, J . Wiley & Sons, New York, 1973. Katbab, A. A., Scott, G., Eur. Polym. J . (1981) 17, 559.

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10. Lorenz, O., Parks, C. R., Rubber Chem. Technol. (1961) 34, 816. 11. Rotschova, J., Pospisil, J., Collect. Czech. Chem. Commun. (1982) 47, 2501. 12. Rotschova, J., Pospisil, J., J. Chromatogr. (1981) 211, 299. 13. Taimr, L., Pospisil, J., Angew. Makromol. Chem. (1980) 92, 53. 14. Taimr, L., Pospisil, J., Polym. Degradation Stab. (1984) 8, 23. 15. Cain, M. E., Gelling, I. R., Knight, G. T., Lewis, P. M., Rubber Ind. (1975) 9 (6), 216. 16. Mazaletskaya, L. I., Karpukhina, G. V., Maizus, Z. K., Izv. Akad. Nauk. USSR, otd. khim. (1981), p. 1988. 17. Pospisil, J., "Developments in Polymer Stabilization", Scott, G. (Ed.), Vol. 1, Chapter 1, Applied Science Publishers, Ltd., Barking, 1979. 18. Taimr, L., Pospisil, J., Polym. Degradation Stab. (1984) 8, 67. 19. Bailey, P. S., "Ozonation in Organic Chemistry", Vols. 1 and 2, Academic Press, New York, 1978 and 1982. 20. Murray, R. W., Kaplan Chem (1968) 90 537 21. Layer, R. W., Rubbe 22. Dibbo, A., Gummi, Asbest, (1965) , 23. Razumovskij, S. D., Podmasterov, V. V., Zaikov, G. E., Plaste Kaut. (1983) 30, 625. 24. Lattimer, R. P., Hooser, E. R., Diem, H. E., Layer, R. W., Rhee, C. K., Rubber Chem. Technol. (1980) 53, 1170. 25. Katbab, A. A., Scott, G., Polym. Degradation Stab. (1981) 3, 221. 26. Cox, N. L., Rubber Chem. Technol. (1959) 32, 364. 27. Ericsson, E. R., Berntsen, R. A., Hill, E. L., Kusy, P., Rubber Chem. Technol. (1959) 32, 1062. 29. Lorenz, O., Parks, C. R., Rubber Chem. Technol. (1963) 36, 194 and 201. 30. Braden, M., Gent, N. A., J. Appl. Polym. Sci. (1962) 6, 449. 31. Andries, J. C., Ross, D. B., Diem, H. E., Rubber Chem. Technol. (1975) 48, 41. 32. Andries, J. C., Rhee, C. K., Smith, R. W., Ross, D. B., Diem, H. E., Rubber Chem. Technol. (1979) 52, 823. 33. Lake, G. J., Rubber Chem. Technol. (1970) 43, 1230. 34. Braden, M., J. Appl. Polym. Sci. (1962) 6, S6. 35. Parfemov, V. M., Rakovski, S., Shopov, D., Popov, A. A., Zsikov, G. E., Izv. Khim. (1979) 11 (1), 180. 36. Braden, M., Gent, N. A., Rubber Chem. Technol. (1962) 35, 200. 37. Loan, N. D., Murray, R. W., Story, P. R., J. Inst. Rubber Ind. (1968) 2, 73. 38. Pobedimskij, D. G., Razumovskij, S. D., Izv. Akad. Nauk USSR, otd. khim. (1970), p. 602. 39. Taimr, L. Pospisil, J., Angew. Makromol. Chem. (1982) 102, 1. 40. Lattimer, R. P., Hooser, E. R., Layer, R. W., Rhee, C. K., Rubber Chem. Technol. (1983) 56, 431. 41. Andrews, E. H., Braden, M., J. Polym. Sci. (1961) 55, 787. RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14 Polymer-Bound Antioxidants G E R A L D SCOTT Department of Chemistry, University of Aston in Birmingham, Birmingham B4 7ET, England

The reasons for th antioxidants i s important approaches to the chemical attachment of antioxidants and stabilizers to polymer molecules are b r i e f l y reviewed. It i s concluded that the modification of rubbers after manufacture with chemically reactive antioxidants offers the most promising procedure for producing concentrates of polymer-bound antioxidants that can be used as conventional additives. Unexpected advantages of bound antioxidants have been observed due to the selective protection of the most oxidatively sensitive regions of the polymer i n rubber-modified polymer blends.

Factors Determining the Effectiveness of Antioxidants. It was for many years a source of puzzlement to polymer chemists that the rating of antioxidants i n polymers and in model hydrocarbons appeared to be very different. The widely used antioxidant BHT (I) i s one of the most " e f f i c i e n t " antioxidants known for l i q u i d hydrocarbons as measured by oxygen absorption but i s v i r t u a l l y ineffective i n rubbers

of plastics i n an a i r oven heat-aging test, which i s normally carried out with continual displacement of a i r over the surface of the sample. The bisphenol I I i s much more effective than BHT i n a heat aging test i n rubber but i s not very effective i n polypropylene i n a similar test at somewhat higher temperature. A systematic study of the effect of structure variation i n the homologous series ( i l l ) i n 0097-6T56/85/0280-0173S07.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

174

POLYMER STABILIZATION AND DEGRADATION

OH

III CH CH C00R 2

2

which the normal a l k y l c h a i n , R, was v a r i e d from CHg t o C^gH^y threw some l i g h t on t h i s problem Q ) . Table 1 shows t h a t i n d e c a l i n a l l the a n t i o x i d a n t s have a v e r y s i m i l a r m o l a r a n t i o x i d a n t a c t i v i t y * as measured by oxygen a b s o r p t i o n ( D ) , but t h e y are a l l c o n s i d e r a b l y l e s s e f f i c i e n t t h a n BHT at the same m o l a r c o n c e n t r a t i o n . In p o l y p r o p y l e n e i n an a i r oven t e s t at 140°C (PP ) , on the o t h e r hand, the lower members o f the s e r i e s are c o m p l e t e l y i n e f f e c t i v e , as i s BHT. The h i g h e s t member o f the s e r i e s , the commercial a n t i o x i d a n t IRGANOX 1076 ( i l l R i8 37^ effectiv unde these c o n d i t i o n s . C

c

H

i s

h

i

h

l

v

1/2 V o l a t i l i t y and M i g r a t i o n R a t e . A study of the h a l f - l i v e s (T ) of the a n t i o x i d a n t s i n the polymer (see T a b l e 1) at the same temperature

suggests a reason f o r the l a c k of c o r r e l a t i o n between the two s e t s of results. In an oxygen a b s o r p t i o n t e s t , v o l a t i l i z a t i o n cannot o c c u r , and the r e s u l t i s a t r u e measure of the i n t r i n s i c a c t i v i t y of the antioxidant molecule. In an a i r oven t e s t , on the o t h e r hand, p h y s i c a l l o s s of the a n t i o x i d a n t by m i g r a t i o n and v o l a t i l i z a t i o n from the s u r f a c e must d o m i n a n t l y i n f l u e n c e the t e s t r e s u l t s . Billingham and h i s coworkers (2) have shown t h a t these two p h y s i c a l parameters d e t e r m i n e the r a t e o f l o s s of a n t i o x i d a n t s from p o l y m e r s . Increase i n m o l e c u l a r mass g e n e r a l l y d e c r e a s e s m o l e c u l a r m o b i l i t y as w e l l as v o l a t i l i t y , and which f a c t o r dominates depends on the t h i c k n e s s o f the sample (2) . Solubility. T a b l e 1 a l s o compared the b e h a v i o r o f the same s e r i e s o f a n t i o x i d a n t s by oxygen a b s o r p t i o n i n p o l y p r o p y l e n e f i l m and i n decalin. In PP a n t i o x i d a n t a c t i v i t y i s o p t i m a l at O^H^S* ^ commercial p r o d u c t , IRGANOX 1076, i s l e s s than h a l f as e f f e c t i v e . The d i f f e r e n t o r d e r o f a c t i v i t y i n the polymer from t h a t the model s u b s t r a t e r e f l e c t s the d i f f e r e n t s o l u b i l i t i e s o f the a d d i t i v e s i n the polymer. A* c o r r e l a t i o n i s t h e r e f o r e observed between PP and S, the s o l u b i l i t y of the a d d i t i v e s i n hexane at 25°C. S o l u b i l i t y i n the polymer has been shown t o be o f d o m i n a t i n g importance i n the case o f phot©antioxidants (see T a b l e 2) ( 3 - 5 ) . n e

A n t i o x i d a n t i n Polymers S u b j e c t e d t o A g g r e s s i v e E n v i r o n m e n t s . The methods used t o e v a l u a t e the e f f e c t i v e n e s s o f a n t i o x i d a n t s d e s c r i b e d above are a c c e l e r a t e d t e s t s and none o f them can be assumed t o a d e q u a t e l y r e p r e s e n t the c o n d i t i o n s t o which most polymers w i l l be s u b j e c t e d i n normal use. However, as polymers move i n t o more and more demanding e n g i n e e r i n g a p p l i c a t i o n s , they have t o w i t h s t a n d i n ­ c r e a s i n g l y a g g r e s s i v e environments ( 6 ) . For example, whereas at one

* The h i g h e r members o f the s e r i e s are l e s s e f f e c t i v e on a weight basis.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Polymer-Bound Antioxidants

14. SCOTT

175

T a b l e 1. A n t i o x i d a n t E f f e c t i v e n e s s o f t h e H i n d e r e d P h e n o l S e r i e s ( I I I ) a t the Same M o l a r C o n c e n t r a t i o n (2 x 10~ M) a t 140 °C Q

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Of

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INDUCTION PERIOD

S

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8

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0

2

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3

H

6 13

292

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32

25

95

2

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oo

23

312

2

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20

200

165

150

140

2

C

12 25

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446

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H

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660.0

BHT(I)

220

0.1

18 37

T

; Antioxidant h a l f - l i f e (h) i n PP i n N stream at 140°C. 2

S D

100

; S o l u b i l i t y i n hexane at 25°C, g/lOOg. c

PP

; i n d e c a l i n by oxygen absorption at 140°C, hr. S

; i n PP f i l m by oxygen absorption at 140°C, hr.

PP° ; i n PP f i l m i n a moving a i r stream at 140°C, by carbonyl measurement, hr

T a b l e 2. M o l a r A n t i o x i d a n t E f f e c t i v e n e s s o f t h e Z i n c D i a l k y l D i t h i o c a r b a m a t e Homologous S e r i e s as UV S t a b i l i z e r s P o l y p r o p y l e n e (3 x 10" mol/100 g)

(RJCSS) Zn ?

EMBRITTLEMENT TIME, hr

UV ABSORBANCE,* 285 nm

R CH

145

0.11

H

170

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193

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330

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C

2 5

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UV Absorbance i s d i r e c t l y proportional to the concentration of the a d d i t i v e i n the polymer.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

176

time i t was not c o n s i d e r e d n e c e s s a r y t o t e s t r u b b e r s at temperatures above 70°C, some r u b b e r s now, when used i n engine s e a l s , g a s k e t s and h o s e s , have t o w i t h s t a n d s e r v i c e temperatures i n excess o f 100°C, o f t e n i n the p r e s e n c e of l u b r i c a t i n g o i l s , i n c r i t i c a l components whose f a i l u r e would not o n l y be v e r y c o s t l y but might a l s o be h i g h l y dangerous. T i r e s r e p r e s e n t a n o t h e r a r e a of modern t e c h n o l o g y i n which o p e r a t i n g temperatures have s t e a d i l y r i s e n . I t i s not uncommon f o r t e m p e r a t u r e s i n t h e s h o u l d e r s of heavy d u t y t r u c k t i r e s t o r i s e of o v e r 100°C under h i g h speed motorway c o n d i t i o n s (1_) . S i m i l a r e f f e c t s are a l s o o b s e r v e d i n the t i r e s of f u l l y l a d e n w i d e - b o d i e d a i r c r a f t even at r e l a t i v e l y modest t a x y i n g speeds ( J B ) . Both the above s i t u a t i o n s l e a d t o r a p i d l o s s o f a n t i o x i d a n t s t.o the s u r r o u n d i n g environment; i n s e a l s and hoses by s i m p l e s o l v e n t e x t r a c t i o n and i n t i r e s by m e c h a n i c a l e x t r u s i o n , w i t h subsequent l o s s at the s u r f a c e by water l e a c h i n g ( 9 ) . Other areas where p h y s i c a l l o s s of a d d i t i v e s from polymers i s i n c r e a s i n g i n importanc m e d i c a l uses of p o l y m e r s toward the s i t u a t i o n wher y irrespectiv t h e i r t o x i c i t y , may be p r o h i b i t i v e above a c e r t a i n l e v e l . It i s , t h e r e f o r e , no l o n g e r an academic e x e r c i s e t o r e t a i n a n t i o x i d a n t s and s t a b i l i z e r s i n polymers m e r e l y t o s a t i s f y u n r e a l i s t i c a c c e l e r a t e d tests. I t i s a p r a c t i c a l n e c e s s i t y t o improve the s u b s t a n t i v i t y o f a n t i o x i d a n t s and s t a b i l i z e r s f o r a v a r i e t y o f q u i t e d i f f e r e n t p r a c t i c a l reasons. The above arguments suggest t h a t an i d e a l a n t i o x i d a n t , i n a d d i t i o n t o h a v i n g a h i g h i n t r i n s i c a c t i v i t y , s h o u l d a l s o s a t i s f y the following physical c r i t e r i a . 1. 2.

I t s h o u l d be c o m p l e t e l y s o l u b l e i n the polymer at the c o n c e n t r a t i o n used. I t s h o u l d be c o m p l e t e l y n o n - m i g r a t i n g from the polymer w h i l e s t i l l s a t i s f y i n g requirement I.

P u b l i s h e d work s u g g e s t s t h a t t h e r e may be some mutual c o n t r a d i c t i o n i n t h e above r e q u i r e m e n t s . For example, i t i s b e l i e v e d t h a t a n t i o z o n a n t s f o r r u b b e r s fjunction by m i g r a t i n g t o the s u r f a c e of the r u b b e r where t h e y r e a c t w i t h ozone and are s l o w l y c o n v e r t e d t o ozone impermeable f i l m s (9^, 10) . A second and more g e n e r a l problem i s t h a t when the s i z e o f an a n t i o x i d a n t m o l e c u l e i s i n c r e a s e d i n o r d e r t o reduce v o l a t i l i t y , i t becomes i n c r e a s i n g l y i n s o l u b l e i n the host polymer. The u l t i m a t e example o f t h i s i s a h i g h l y p o l y m e r i c a n t i o x i d a n t ; even minor d i f f e r e n c e s i n t h e repeat u n i t r e s u l t i n a two-phase system. Many p o l y m e r i c a n t i o x i d a n t s have been made, but l i t t l e s u c c e s s has been a c h i e v e d i n d e v e l o p i n g h i g h l y e f f e c t i v e a n t i o x i d a n t s w i t h s u f f i c i e n t l y h i g h m o l e c u l a r mass t o be r e s i s t a n t t o solvent leaching (11). Polymer-Bound

Antioxidants

In p r i n c i p l e , an a n t i o x i d a n t which forms p a r t o f the h o s t should approach the i d e a l o u t l i n e d above s i n c e i t s h o u l d 1.

be m o l e c u l a r l y soluble;

dispersed

i n the polymer and

hence

polymer

completely

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14.

2. 3.

SCOTT

Polymer-Bound Antioxidants

177

not be l o s t t o t h e s u r f a c e ; and be c o m p l e t e l y n o n - v o l a t i l e .

However, i f m i g r a t i o n i s important t o t h e f u n c t i o n o f an a n t i o x i d a n t , t h e n polymer-bound a n t i o x i d a n t s s h o u l d n o t be e f f e c t i v e . In t h e f o l l o w i n g s e c t i o n s , d a t a w i l l p r o v i d e a t l e a s t a p a r t i a l answer t o t h e l a s t p o i n t .

Approaches t o Polymer-Bound A n t i o x i d a n t s . Three b a s i c methods o f p r o d u c i n g bound a n t i o x i d a n t s have been i n v e s t i g a t e d . These a r e 1. 2. 3.

c o p o l y m e r i z a t i o n o f v i n y l a n t i o x i d a n t s w i t h v i n y l monomers d u r i n g polymer s y n t h e s i s ; r e a c t i o n o f c o n v e n t i o n a l a n t i o x i d a n t s w i t h f u n c t i o n a l groups re-formed i n t h e polymer and d i r e c t reaction of antioxidant normal p o l y m e r s .

Polymer M o d i f i c a t i o n by C o p o l y m e r i z a t i o n D u r i n g M a n u f a c t u r e . Since the e a r l y 1960s, a v a r i e t y of c o p o l y m e r i z a b l e monomers have been d e s c r i b e d ( 1 1 ) , but o n l y one s u c c e s s f u l p r o d u c t has been r e p o r t e d . T h i s r e s u l t s from r e s e a r c h at Goodyear (12^ 13) and a commercial grade of n i t r i l e r u b b e r (Chemigum HR 665) i s now produced c o n t a i n i n g a s m a l l p r o p o r t i o n (< 2%) o f a c o p o l y m e r i z e d monomer, IV.

There i s no doubt t h a t t h i s grade o f n i t r i l e r u b b e r i s much more r e s i s t a n t t o h o t l u b r i c a t i n g o i l s and h i g h temperatures t h a n i s c o n v e n t i o n a l l y s t a b i l i z e d n i t r i l e r u b b e r ( s e e T a b l e 3 ) . The main problem i n t h i s approach i s t h e c o s t o f p r o d u c i n g a s p e c i a l i z e d rubber i n r e l a t i v e l y s m a l l q u a n t i t y . Some s u c c e s s has been r e p o r t e d i n p r o d u c i n g UV s t a b l e polymers by c o p o l y m e r i z i n g v i n y l UV s t a b i l i z e r s w i t h o t h e r monomers (11) and e v i d e n c e from t h e l i t e r a t u r e suggests t h a t many groups a r e c o n c e n t r a t i n g on t h i s a s p e c t ( 1 5 ) .

R e a c t i o n s o f A n t i o x i d a n t s w i t h Preformed F u n c t i o n a l Groups. The r e a c t i v e c h l o r i n e i n e p i c h l o r o h y d r i n polymers i s a s p e c i f i c though t y p i c a l example o f t h i s approach (11) . A more g e n e r a l r e a c t i o n i s the e p o x i d a t i o n o f t h e d o u b l e bonds i n r u b b e r s and subsequent r e a c t i o n o f t h e epoxide group w i t h an amine a n t i o x i d a n t ( r e a c t i o n 1) (16).

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

178

-CH.

The p r o d u c t formed sec-alkylaminodiphenylamin rubber a n t i d e g r a d a n t s . No commercial p r o d u c t s based on t h i s p r i n c i p l e have so f a r been r e p o r t e d , p o s s i b l y because o f the c o s t o f e p o x i d a t i o n , n o r m a l l y c a r r i e d out i n s o l u t i o n . R e a c t i o n s o f P o l y m e r - R e a c t i v e A n t i o x i d a n t s . Three d i f f e r e n t m o d i f i c a t i o n s o f t h i s approach have been r e p o r t e d . 1. 2. 3.

Normal c h e m i c a l r e a c t i o n i n s o l u t i o n o r i n l a t i c e s (17, 18). R e a c t i o n by post treatment o f polymer a r t i f a c t s (19-21). R e a c t i o n d u r i n g p r o c e s s i n g o r f a b r i c a t i o n p r o c e d u r e s ~ T l 8 ^ 20-25).

R e a c t i o n s o f R e a c t i v e A n t i o x i d a n t s w i t h Polymers by Normal Chemical Procedures. G r a f t i n g o f v i n y l a n t i o x i d a n t s ; e.g., V I , i n t o r u b b e r s has been used t o produce m o d i f i e d rubber l a t i c e s (26) . Even s i m p l e

p h e n o l i c a n t i o x i d a n t s such as I can a l i m i t e d e x t e n t i n the p r e s e n c e o f A more s i g n i f i c a n t development adduct c o n c e n t r a t e s can be a c h i e v e d p r o c e s s , r e a c t i o n 2 (11, 17, 2 8 ) .

R -£H=CH-

+

ASH

be made t o r e a c t w i t h polymers t o r a d i c a l generators (27). because q u i t e h i g h y i e l d s o f i s the Kharasch-Mayo t h i o l adduct

R a d i c a l generator

-CVLCHSA

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

(2)

14.

SCOTT

179

Polymer-Bound Antioxidants

A. v a r i e t y o f a n t i o x i d a n t s has been r e a c t e d w i t h e l a s t o m e r s i n the l a t e x i n t h e p r e s e n c e o f t y p i c a l redox r a d i c a l g e n e r a t o r s ( 2 7 ) . The mechanism o f t h i s p r o c e s s has been c o n s i d e r e d i n d e t a i l elsewhere ( 1 1 , 17, 27, 28) and w i l l not be d i s c u s s e d f u r t h e r h e r e . The r e s u l t i n g p r o d u c t s are much more a g e - r e s i s t a n t t h a n c o n v e n t i o n a l r u b b e r a n t i o x i d a n t s under a i r oven a g i n g c o n d i t i o n s (see F i g u r e 1 ) , and i n the p r e s e n c e o f aqueous d e t e r g e n t s and d r y c l e a n i n g s o l v e n t s (27). F i g u r e 2 compares t h e l o s s o f BHBM-B* w i t h a commercial n o n - s t a i n i n g a n t i o x i d a n t , WSP ( I I ) i n NR under l e a c h i n g c o n d i t i o n s . It i s e v i d e n t t h a t a f t e r removal o f the non-bound m a t e r i a l , no f u r t h e r r e d u c t i o n i n antioxidant occurs with e i t h e r leachant. The same p r o c e d u r e has been used t o produce c o n c e n t r a t e s o f t h i o l adducts i n n i t r i l e r u b b e r ( 2 3 ) . In a d d i t i o n t o BHBM ( V I I ) , the a r y l a m i n e a n t i d e g r a d a n t MADA ( V I I I ) has a l s o been g r a f t e d t o NBR l a t i c e s . T a b l e 4 shows t h a t t h e

coagulum p r o d u c t s c o n t a i n s u b s t a n t i a l y i e l d s o f adduct c o n c e n t r a t e s . When t h e s e a r e d i l u t e d i n u n s t a b i l i z e d r u b b e r s as normal a d d i t i v e s , t h e y g i v e i n t e r p e n e t r a t i n g networks when v u l c a n i z e d i n t h e c o n v e n t i o n a l way ( 2 3 ) . The l a t e x c o n c e n t r a t e s can a l s o be d i l u t e d t o normal a n t i o x i d a n t c o n c e n t r a t i o n s i n u n s t a b i l i z e d n i t r i l e r u b b e r l a t e x f o l l o w e d by normal c o a g u l a t i o n and v u l c a n i z a t i o n ( 2 3 ) . Some t y p i c a l r e s u l t s * o b t a i n e d by the l a t e x c o n c e n t r a t e ( m a s t e r b a t c h ) p r o cedure a r e d e s c r i b e d i n T a b l e 5, b e f o r e and a f t e r s o l v e n t e x t r a c t i o n . The c o p o l y m e r i z e d a n t i o x i d a n t (HR 665) and the t h i o l adduct a n t i o x i d a n t adducts are much more e f f e c t i v e t h a n the best a v a i l a b l e o l i g o m e r i c a n t i o x i d a n t , F l e c t o l H. N i t r i l e r u b b e r i s the p r e f e r r e d e l a s t o m e r f o r use i n s e a l s and g a s k e t s i n c o n t a c t w i t h hot h y d r o c a r b o n o i l s because o f i t s r e s i s tance t o s w e l l i n g (6^). However, c o n v e n t i o n a l a n t i o x i d a n t s are r e a d i l y removed by l e a c h i n g d u r i n g use and the polymer r a p i d l y becomes s u b j e c t t o o x i d a t i v e d e t e r i o r a t i o n . This leads to increase i n modulus and h a r d n e s s and t o l o s s o f u s e f u l p r o p e r t i e s . T h i s i s an obvious a p p l i c a t i o n f o r polymer-bound a n t i o x i d a n t s . F i g u r e 3 shows the e f f e c t o f a 20% l a t e x c o n c e n t r a t e o f MADA-B when d i l u t e d t o 2% i n NBR i n a c y c l i c a l o i l e x t r a c t i o n ( 1 5 0 ° C ) / o v e n a g i n g ( 1 5 0 ° C ) t e s t (27). It i s e v i d e n t t h a t p r i o r e x t r a c t i o n has l i t t l e e f f e c t on the performance o f MADA-B, which i s v e r y much s u p e r i o r i n t h i s t e s t t o e i t h e r t h e c o n v e n t i o n a l a n t i o x i d a n t , F l e c t o l H , o r the c o p o l y m e r i z e d a n t i o x i d a n t , Chemigum HR 665. The a d d i t i v e used as a coagulum c o n c e n t r a t e (LMC) was a l s o more e f f e c t i v e t h a n the l a t e x c o n c e n t r a t e (LML). R u b b e r - m o d i f i e d p l a s t i c s ( e . g , A B S ) , which a r e a v a i l a b l e i n l a t e x f o r m , have a l s o been m o d i f i e d w i t h t h i o a n t i o x i d a n t s and UV stabilizers. The 2-hydrobenzophennone (IX) r e a c t s to g i v e a 30% c o n c e n t r a t e o f which 80% o f the UV s t a b i l i z e r used becomes c h e m i c a l l y a t t a c h e d t o the polymer ( 2 4 ) . The c o n c e n t r a t e s can be used e i t h e r as

* The s u f f i x - B i n d i c a t e s to the p o l y m e r .

that

the a d d i t i v e i s

substantially

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

bound

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

180

T a b l e 3. Comparison of N i t r i l e Rubbers C o n t a i n i n g a C o p o l y m e r i z e d A n t i t o x i d a n t (HR 665) and a C o n v e n t i o n a l A d d i t i v e ( O c t y l a t e d Diphenylamine, OD)

TIME TO ABSORB 1% 0

FORMULATION

HR 665

9

AT 100°C, HRS.

OD

UNVULCANISED (U)

676

250

UNVULCANI5ED (E)

620

10

VULCANISED

(U)

290

185

VULCANISED

(E

Contains copolymerized

\^J)y-

N H >

NHC0C=CH (IV) o

U, u n e x t r a c t e d ; E, e x t r a c t e d

F i g u r e 1. Comparison of bound a n t i o x i d a n t (BHBM) w i t h a c o n v e n t i o n a l b i s p h e n o l i n NR ( a i r oven t e s t at 100 ° C ) .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14. SCOTT

Polymer-Bound Antioxidants

181

Time (h)

Figure 2 . Residual a n t i o x i d a n t remaining during solvent (pet e t h e r / t o l u e n e a t 2 0 ° C ) and detergent ( 1 % Tide a t 1 0 0 C) l e a c h i n g o f a t y p i c a l b i s p h e n o l (WSP) and a polymer-bound phenol (BHBM).

Table 4. E f f e c t o f A n t i o x i d a n t C o n c e n t r a t i o n Y i e l d o f Adduct

ADDED ANTIOXIDANT CONCENTRATION g/100g NBR

i n NBR L a t t i c e s on t h e

PERCENTAGE BOUND (g/lOOg of antioxidant added) BHBM

1

37

MADA 0

5

-

13

10

58

40

20

60

52

30

63

»

40

62

*

50

62

L a t i c e s coagulated

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

182

Table 5. E f f e c t i v e n e s s of Bound A n t i o x i d a n t s from 20% C o n c e n t r a t e , Reduced t o 2 g/100 g i n NBR A f t e r V u l c a n i z a t i o n VULCANIZATE

TIME TO IX OXYGEN ABSORPTION AT 150°C UNEXTRACTED EXTRACTED h

5

2

38

6

45

41

BHBM-B

33

29

MADA-B

49

48

CONTROL FLECTOL H

C

CHEMIGUM HR 665

d

a High a c c e l e r a t o r / l o w i t h MeOH (2 Flectol H i

c

d

Commercial polymer containing a copolymerised antioxidant.

30 Control

(no antiox) Flectol

•H

a

H

Chgmigum 20

CO CO

w z

Q OS < 55 35 10

CYCLES F i g u r e 3. I n c r e a s e i n hardness o f n i t r i l e c o n t a i n i n g rubber a n t i o x i d a n t s i n a c y c l i c a l hot o i l (150 C)/hot a i r (150 C) t e s t . LML, l a t e x c o n c e n t r a t e (20%) d i l u t e d t o 2%; LMCE, l a t e x coagulum ( 2 0 % ) , u n e x t r a c t e d b e f o r e i n c o r p o r a t i o n i n t o polymer; LMCU, l a t e x coagulum ( 2 0 % ) , u n e x t r a c t e d before i n c o r p o r a t i o n i n t o polymer a t 2%.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14.

SCOTT

Polymer-Bound Antioxidants

183

l a t e x a d d i t i v e s o r as s o l i d polymer a d d i t i v e s d u r i n g p r o c e s s i n g t o g i v e a s u p e r i o r a n t i o x i d a n t e f f e c t compared w i t h t h e commercial UV s t a b i l i z e r C y a s o r b UV 531 (X) which c o n t a i n s t h e same f u n c t i o n a l group; see T a b l e 6. I t i s e v i d e n t t h a t even a f t e r e x h a u s t i v e e x t r a c t i o n , EBHPT ( I X ) i s c o n s i d e r a b l y more e f f e c t i v e than t h e c o n v e n t i o n a l s t a b i l i z e r which has n o t been e x t r a c t e d . HO

HO

0CH CH 0C0CH SH 2

2

OC H

2

8

1 ?

IX

P o s t - F a b r i c a t i o n Treatment of f u n c t i o n a l a n t i o x i d a n t e i t h e r by i n c o r p o r a t i n g t h e r e a c t i v e a n t i o x i d a n t d u r i n g p r o c e s s i n g (20) o r by s u b s e q u e n t l y s u r f a c e t r e a t i n g t h e a r t i f a c t (19) . The former i s more c o n v e n i e n t , and F i g u r e 4 shows t h a t 68% o f BHBM ( V I ) can be combined w i t h p o l y p r o p y l e n e f i l m w i t h i n two h o u r s o f UV exposure. The r e a c t i o n i n v o l v e d i s a U V - i n i t i a t e d t h i o l a d d i t i o n p r o c e s s i n v o l v i n g t h e t e r t i a r y hydrogens i n PP (19). Table 7 compared t h e e f f e c t i v e n e s s o f BHBM-B made by t h e above p r o c e s s i n p o l y p r o p y l e n e and i t s lower m o l e c u l a r mass analogues ( X I ) added normally during processing ( 1 1 ) .

OH

XI

CH SR 2

T h i s i l l u s t r a t e s a g a i n t h e importance o f n o n - v o l a t i l i t y , and t h e r e i s e v i d e n c e (11) t h a t BHBM may i t s e l f be p a r t i a l l y c o n v e r t e d t o l e s s v o l a t i l e a n t i o x i d a n t s d u r i n g normal p r o c e s s i n g . R e a c t i o n s o f A n t i o x i d a n t s w i t h Polymers D u r i n g P r o c e s s i n g . One o f the e a r l i e s t polymer-bound a n t i o x i d a n t s was o b t a i n e d by r e a c t i o n o f n i t r o s o a n t i o x i d a n t s (ANO) w i t h r u b b e r s d u r i n g v u l c a n i z a t i o n ( 2 9 ) . The c h e m i s t r y o f t h i s p r o c e s s i s complex, but i t s d i s c o v e r e r s proposed an "ene" r e a c t i o n w i t h t h e u n s a t u r a t i o n i n t h e polymer.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

184

T a b l e 6. Comparison of EBHPT_B, Added as a 30% C o n c e n t r a t e to ABS, w i t h ^ C o n v e n t i o n a l UV S t a b i l i z e r at the Same C o n c e n t r a t i o n (3.o3 x 10 mol/100 g)

UV STABILIZER

EMBRITTLEMENT TIME, hr

CONTROL, NO ADDITIVE

25

CYASORB UV 531 (U)

35

EBHPT-B (U)

90

EBHPT-B (E)

7

U, unextracted

o

10

20

30 Irradiation time, h

40

F i g u r e 4. B i n d i n g of BHBM to p o l y p r o p y l e n e i n n i t r o g e n and i n a i r by UV i r r a d i a t i o n . P h e n o l i c OH measured by IR b e f o r e (U) and a f t e r (E) e x t r a c t i o n .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14.

SCOTT

185

Polymer-Bound Antioxidants

1 3 -CH=CCHCH -

H

? 3

-CH C=CHCH 2

+ ANO

2

0

z

j

NOH i

CH, 1 3 -CH=CC N-*0

+

I

A

A

(3)

1

+ CH, j 3 -CH=CCHCH 1 2 NH 0

1

A T h i s p r o c e s s was not d e v e l o p e d c o m m e r c i a l l y , but c l o s e l y r e l a t e d c h e m i s t r y i s used i n a n o v e (30, 31). A n o t h e r polymer r e a c t i v e a n t i o x i d a n t which can be combined w i t h r u b b e r d u r i n g v u l c a n i z a t i o n i n v o l v e s the 1,3 a d d i t i o n r e a c t i o n o f n i t r o n e s t o the d o u b l e bond i n r u b b e r s , r e a c t i o n 4 (11).

0

CH,

-CH CH \^ / CH,-C-CH / \ 0 CHR" \ / N R» 0

f

-CH C=CHCH -

R'N=CHR"

z

0

(4)

3

The a n t i o x i d a n t f u n c t i o n may be i n e i t h e r R o r R " . Again, commercial development has not o c c u r r e d due t o an u n d e s i r a b l e b a l a n c e of o t h e r t e c h n o l o g i c a l p r o p e r t i e s d u r i n g v u l c a n i z a t i o n . It has more r e c e n t l y been shown t h a t t h e r e are advantages i n s e p a r a t i n g the c h e m i c a l r e a c t i o n o f a n t i o x i d a n t s w i t h s o l i d polymers from the v u l c a n i z a t i o n p r o c e s s . It has l o n g been r e c o g n i z e d (32) t h a t m e c h a n o - s c i s s i o n o f polymer c h a i n s d u r i n g p r o c e s s i n g o r polymers g i v e s r i s e t o f r e e r a d i c a l s which can i n i t i a t e r a d i c a l c h a i n r e a c t i o n s (32) . B l o c k copolymers can be s y n t h e s i z e d i n t h i s way, r e a c t i o n 5; 1

CH, CH, I -C=CHCH CH C=CHj

3

3

2

S b nhee a r

2

»

CH, -C=CHCH 1

3

2

(R-) R

-

CH, + CH =CC00CH

The same p r o c e s s

•

can be used t o g r a f t

polymer c h a i n s d u r i n g p r o c e s s i n g o f

CH 'CH^CHi

+

CR') CH, CH R(CH CH) CH CH*

vinyl

elastomer

(5)

antioxidants

or to

f o r m a t i o n o f polymer adducts w i t h t h i o l a n t i o x i d a n t s

initiate (21,

22,

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

to the 24).

186

POLYMER STABILIZATION AND DEGRADATION

T a b l e 8 shows t h a t over 50% b i n d i n g o f MADA ( V I I I ) c a n be a c h i e v e d i n both NR and SBR by t h e mechanochemical p r o c e d u r e (21) and t h a t t h i s i n c r e a s e s d u r i n g v u l c a n i z a t i o n t o about 70%. Other s u l f u r c o n t a i n i n g a n t i o x i d a n t s , V I I and IX, c a n be bound t o s i m i l a r l e v e l s i n both p o l y m e r s . F i g u r e 5 shows t h e e f f e c t o f e x t r a c t i o n on MADA-B i n NR i n comparison w i t h a c o n v e n t i o n a l t h e r m a l a n t i o x i d a n t , WSP ( I I ) , at 100°C. S i m i l a r r e s u l t s f o r SBR a r e l i s t e d i n T a b l e 9 b e f o r e and a f t e r s o l v e n t e x t r a c t i o n . The h i n d e r e d p h e n o l i c a n t i o x i d a n t , BHBM-B i s i n g e n e r a l somewhat l e s s e f f e c t i v e t h a n MADA-B, but i s n e v e r t h e l e s s much l e s s a f f e c t e d by e x t r a c t i o n t h a n a r e c o n v e n t i o n a l add i t i v e s . BHBM-B and MADA-B c o n c e n t r a t e s , made by t h e mechanochemical procedure, are h i g h l y e f f e c t i v e i n preventing hardening o f n i t r i l e rubber i n t h e c y c l i c a l h o t o i l / h o t a i r t e s t r e f e r r e d t o e a r l i e r . F i g u r e 6 shows t h a t MADA-8 i s a g a i n much s u p e r i o r t o t h e o l i g o m e r i c antioxidant F l e c t o l H i (35). A summary o f t h commercial a n t i o x i d a n t s i s g i v e n i n T a b l e 10. Rubber-modified p l a s t i c s a l s o react r e a d i l y with t h i o l a n t i o x i d a n t s m e c h a n o c h e m i c a l l y t o g i v e h i g h y i e l d s o f bound a n t i o x i dants ( 1 8 ) . An i n t e r e s t i n g f e a t u r e o f t h i s method o f p r e p a r a t i o n i n ABS i s t h e i n c r e a s e i n e f f e c t i v e n e s s t h a t o c c u r s w i t h i n c r e a s e i n c o n c e n t r a t i o n ( s e e F i g u r e 7 ) . Moreover, t h e e f f e c t i v e n e s s o f t h e m e c h a n o c h e m i c a l l y formed adducts i s h i g h e r t h a n t h e c o r r e s p o n d i n g latex adducts. T a b l e 11 compares t h e e f f e c t i v e n e s s o f a s y n e r g i s t i c UV s t a b i l i z e r (BHBM-B + EBHPT-B) w i t h some commercial s t a b i l i z i n g systems f o r ABS added c o n v e n t i o n a l l y . The e x c e p t i o n a l a c t i v i t y o f the polymer-bound system i s b e l i e v e d t o be due t o t h e f a c t t h a t i t i s c o n f i n e d t o t h e rubber phase o f t h e p o l y b l e n d ( 1 8 ) , which i s known t o be more s e n s i t i v e t h a n t h e t h e r m o p l a s t i c phase t o t h e e f f e c t s o f both heat and l i g h t ( 3 6 ) . This f i n d i n g , i f confirmed i n other multiphase systems, c o u l d be o f c o n s i d e r a b l e importance f o r t h e s t a b i l i z a t i o n o f h e t e r o g e n e o u s polymer b l e n d s . Ethylene-propylene-diene t e r p o l y m e r s (EPDM) a r e being used i n c r e a s i n g l y as impact m o d i f i e r s and s o l i d phase d i s p e r s a n t s (37) as w e l l as b e i n g important e l a s t o m e r s i n t h e i r own r i g h t . A l t h o u g h EPDM i s r e l a t i v e l y s t a b l e i n comparison w i t h the p o l y d i e n e e l a s t o m e r s , the u n s a t u r a t i o n i n t h e polymer can be used t o improve i t s t h e r m a l and UV s t a b i l i t y even f u r t h e r ( 3 8 ) . T a b l e 12 shows t h a t a s u b s t a n t i a l l e v e l of b i n d i n g o f both EBHPT and MADA c a n be a c h i e v e d i n EPDM by t h e mechanochemical p r o c e d u r e . The r e s u l t i n g a n t i o x i d a n t - m o d i f i e d EPDM, when added t o normal EPDM, gave an i m p a c t - m o d i f i e r f o r PP w i t h c o n s i d e r a b l y improved heat r e s i s t a n c e ( s e e T a b l e 13) ( 3 9 ) . MADA-B i s also a reasonably e f f e c t i v e photoantioxidant with s i m i l a r a c t i v i t y t o EBHPT-B ( s e e T a b l e 1 4 ) . B e f o r e s o l v e n t e x t r a c t i o n , both a r e l e s s e f f e c t i v e t h a n a c o n v e n t i o n a l UV s t a b i l i z e r (UV 531), but they a r e more e f f e c t i v e a f t e r e x t r a c t i o n . A l t h o u g h t h e major t h e r m o p l a s t i c s ( P E , PP, PVC, and PS) do not f o r m a l l y c o n t a i n u n s a t u r a t i o n , t h e y do undergo m e c h a n o m o d i f i c a t i o n w i t h r e a c t i v e a n t i o x i d a n t s (40 - 4 2 ) . T h i s p r o c e s s i s p a r t i c u l a r l y e f f i c i e n t i n t h e case o f PVC due t o t h e u n s a t u r a t i o n produced d u r i n g p r o c e s s i n g (41 - 4 3 ) . T h i s i s shown t y p i c a l l y f o r BHBM i n F i g u r e 8 i n t h e presence o f a t y p i c a l melt s t a b i l i z e r , d i b u t y l t i n maleate (DBTM, X I I ) .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14.

187

Polymer-Bound Antioxidants

SCOTT

T a b l e 7. A n t i o x i d a n t A c t i v i t y o f Homologous P h g n o l i c S u l p h i d e s ( X I ) i n an A i r Oven a t 140 C ( C o n c e n t r a t i o n 6 x 10~ mol/100 g) OH tBu. J \ tBu

C H

2

S R

INDUCTION PERIOD TO THERMAL OXIDATION, h r 21.0

C H 2

5.0 5

6.0 C

5 11

H

C

8 17

C

12 25

C

18 37

7.5 H

H

H

PP (BHBM-B)

T a b l e 8. a t 70 °C

Mechanochemical

81.0

B i n d i n g o f MADA t o Rubbers

(10 g/100 g)

EXTENT OF BINDING,% OF THEORETICAL RUBBER BEFORE VULCANISATION AFTER VULCANISATION NR

50

68

SBR

58

74

T a b l e 9. A n t i o x i d a n t A c t i v i t y 100 °C (2 g/100 g)

o f 10% MADA C o n c e n t r a t e i n SBR a t

PROCESSING AND POST-

ACTUAL ANTIOXIDANT

INDUCTION

TIME TO

TREATMENT PROCEDURE

CONCENTRATION,g/100g

PERIOD,hr

1% 0 ABS,hr

U,V (U)

2.0

30

E,V (U)

1.16

29

143

U,V (E)

1.48

45

189

E,V (E)

1.16

42

107

-

1

CONTROL (U) CONTROL (E) WSP (U)

2.0

WSP (E)

-

0

2

196

9.5 7.0

32

132

4

13

U, unextracted before v u l c a n i s a t i o n , E, extracted before vulcanisation V, vulcanised, (U),vulcanisate unextracted, (E), vulcanisate extracted.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

188

0

1

2

3

CYCLES F i g u r e 6. Decay i n e l o n g a t i o n a t break ( E ) o f n i t r i l e rubbers c o n t a i n i n g a n t i o x i d a n t s (27o) i n a c y c l i c a l hot o i l (150 C ) / hot a i r (150 C) t e s t . MCMU, mechanochemical c o n c e n t r a t e , u n e x t r a c t e d b e f o r e use; MCME, mechanochemical c o n c e n t r a t e , s o l v e n t e x t r a c t e d b e f o r e use. fi

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14. SCOTT

189

Polymer-Bound Antioxidants

Table 10. Changes i n M e c h a n i c a l P r o p e r t i e s of NBR V u l c a n i s a t e s C o n t a i n i n g Bound A n t i o x i d a n t s i n a C y c l i c a l O i l / A i r Oven Test a t 150 °C. NUMBER OF CYCLES TO PROPERTY CHANGE

ANTIOXIDANT

H

M

10

T S

50

50

E b

50

2

1

3

3

3

1

1

1

1

1

2

2

2

BHBM-B*

1

MADA-B*

3

FLECTOL H CHEMIGUM HR 665

2

CONTROL (NO ANTIOX *Diluted from 20% masterbatch concentrate. H , increase i n hardness by 10 points. 1Q

M , 50% increase i n modulus. 5Q

Ts

5 Q I

50% decrease i n t e n s i l e strength.

E b , 50% decrease i n elongation at break. 5 n

0

20

40

60 IRRADIATION

80 TIMK.

100

120

h

F i g u r e 7. Loss o f impact s t r e n g t h ( I ) o f ABS d u r i n g UV exposure. L, l a t e x c o n c e n t r a t e ( m a s t e r b a t c h ) ; U, u n e x t r a c t e d ; P, mechanochemical c o n c e n t r a t e . Numbers on curves a r e t h e concentrations of photoantioxidants i n the concentrates.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

190

T a b l e 11. Comparison o f a Bound S y n e r g i s t i c UV S t a b i l i z e r C o n v e n t i o n a l A d d i t i v e s a t 1 g/100 g ABS

ADDITIVE

INDUCTION

EMBRITTLEMENT

PERIOD,hr

TIME.hr

BHBM-B + EBHPT-B (U)

80

380

BHBM-B + EBHPT-B (E)

50

220

UV 531 (U)

10

40

9

34

25

85

BHT (U)

with

DLTP (U) BHT + UV 531 (U) BHT + UV 531 + DLTP (U)

Table

12.

Y i e l d s o f Polymer-Bound A n t i o x i d a n t s i n EPOM

ANTIOXIDANT CONCENTRATION REACTED IN POLYMER (g/100g)

EXTENT OF BINDINGS OF THEORETICAL #

2

+

EBHPT

MADA

65

75

5

70

82

10

77

87

* Reacted at 100°C / 15min.

+

Reacted at 150°C / 15 min.

T a b l e 13. I n d u c t i o n P e r i o d s ( I P ) and Time t o 50% Loss o f Impact S t r e n g t h ( I s ) f o r PP/EPDM (75/25) B l e n d s i n an A i r Oven a t 140 °C

ANTIOXIDANT

CONCENTRATION (10

NONE

+

IP,hr

I s

50»

h r

mol/lOOg)

-

MADA-B NiDBD

4

0

5

3*

50

105

3

50

93

From 2% masterbatch concentrate,

+

Nickel d i b u t y l dithiocarbamate

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

14.

SCOTT

191

Polymer-Bound Antioxidants

T a b l e 14. E f f e c t i v e n e s s as UV S t a b i l i z e r s of EBHP-B and MADA-B i n PP/EPDM (75:25) B l e n d s B e f o r e and A f t e r S o l v e n t E x t r a c t i o n (from 10% C o n c e n t r a t e s )

SIABIL1ZER

EMBRITTLEMENT TIME,hr

CONCENTRATION g/lOug

UNEXTRACTED EXTRACTED

-

180

-

EBHPT-B

0.16

330

310

MAUA-B

0.16

330

315

NONE

UV 531

B H B M (initial) B H B M (5.78) unext o • o—

D B T M (4.48) + B H B M (1.3) unext —T3

n

B

5 —

DBTM ( 4 . 4 8 ) * B H B M ( l . 3 ) e x t

J

I

I

L 10 15 Processing time, min

20

25

F i g u r e 8. R e t e n t i o n o f BHBM i n PVC d u r i n g p r o c e s s i n g at 170 C i n the p r e s e n c e and absence of d i b u t y l t i n maleate (DJjTM). Numbers on t h e c u r v e s a r e a d d i t i v e c o n c e n t r a t i o n s (10 mol/100 g ) .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

192

.0 o-c^ B

u

2

S

"

n x

o-c
=CH

2

2

!:H, 2H5V

2H5P

F i g u r e 2. S y n t h e s i s of 2 - ( 2 - h y d r o x y p h e n y l ) - 2 H - b e n z o t r i a z o l e s w i t h p o l y m e r i z a b l e v i n y l or i s o p r o p e n y l groups.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

15. VOGL ET AL.

205

Polymerizable Ultraviolet Stabilizers

obtained. One of the intermediates of t h i s synthesis, 2A5A, was also a u s e f u l intermediate for the preparation of 2 ( 2 - h y d r o x y - 5 - i s o p r o penyl phenyl)2H-benzotriazole (2H5P). 2A5A was allowed to react with methyl Grignard reagent to give the t e r t i a r y a l c o h o l . Dehydration of t h i s compound occurred very r e a d i l y even by vacuum sublimation; hyd r o l y s i s gave 2H5P*

R'

H

(4)

R : H,CH R' C H , C00CH 3

6

5

R"' H , CH Structure

3

S

k.

Attempts to prepare 2(2-hydroxy-3-vinylphenyl)2H-benzotriazole or 2( 2-hydroxy-^-vinvlphenyl)2H-benzotriazole f a i l e d . When 3 - e t h y l phenol or ^-ethylphenol were allowed to condense with o-nitrobenzenediazonium s a l t s , the condensation did not occur i n the 2 p o s i t i o n but rather i n the k p o s i t i o n . As a consequence, after r i n g c l o s u r e , b r o mination and dehydrobromination, 2(^-hydroxy-2-vinylphenyl)2H-benzot r i a z o l e (^H2V) and 2(4-hydroxy-3-vinylphenyl)2H-benzotriazole (te^V) were obtained ( 5 0 ) instead of the desired 2H^V and 2 H 3 V . The synthesis of 2(2-hydroxy-5-methylphenyl)2R-kf-vinyl-benzot r i a z o l e was accomplished ( j l ) by a sequence of reactions s i m i l a r to those which gave 2H5V. The s t a r t i n g material for t h i s synthesis was, however, not o - n i t r o a n i l i n e but ^ - e t h y l - o - n i t r o a n i l i n e . After diazot i a t i o n , the diazonium compounds were allowed to react with p - c r e s o l ; the condensation product gave 2(2-hydroxy-5-methylphenyl)2H-^-ethylbenzotriazole after reductive c y c l i z a t i o n . This compound was a c e t y l ated, brominated, dehydrobrominated, and hydrolyzed to 2(2-hydroxy-5methylphenyl) 2H-4 — v i n y l - b e n z o t r i a z o l e . Polymerization, Copolymerization, and Grafting. In t h i s work we have prepared v i n y l and isopropenyl derivatives of 2(2-hydroxyphenyl)2Hbenzotriazoles: 2H5V, 2 H 5 P , and 2(2-hydroxy-5-methylphenyl)2H-4 — v i n y l - b e n z o t r i a z o l e ; 2H5V and 2(2-hydroxy- 5-methylphenyl)2H- i--vinylbenzotriazole were homopolymerized and copolymerized with styrene, i

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

206

POLYMER STABILIZATION AND DEGRADATION

m e t h y l m e t h a c r y l a t e , and i n some c a s e s n - b u t y l a c r y l a t e ( S t r u c t u r e 4 ) . E v e n when the p o l y m e r i z a t i o n s were c a r r i e d out t o h i g h c o n v e r s i o n , copolymers were o b t a i n e d whose copolymer c o m p o s i t i o n s were s i m i l a r t o t h a t o f t h e i n i t i a l comonomer f e e d . The p o l y m e r i z a t i o n s had t o be c a r r i e d out w i t h complete e x c l u s i o n o f oxygen, o t h e r w i s e t h e p h e n o l i c h y d r o x y l group became an i n h i b i t o r o f the p o l y m e r i z a t i o n s i n c e oxygen r a d i c a l s are r e a d i l y terminated by phenols. 2H5P, an o/-methylstyrene d e r i v a t i v e , seems t o have a low c e i l i n g temperature and c o n s e q u e n t l y d i d not homopolymerize b u t underwent cop o l y m e r i z a t i o n w i t h s t y r e n e , m e t h y l m e t h a c r y l a t e , and n - b u t y l a c r y l ate. Based on t h e h o m o p o l y m e r i z a t i o n a t t e m p t s , i t appears t h a t 2H5P i s p r e s e n t as i s o l a t e d monomer u n i t s i n t h e s e copolymers. The cop o l y m e r i z a t i o n p a r a m e t e r s o f 2H5V and 2H5P w i t h s t y r e n e , m e t h y l metha c r y l a t e , and n - b u t y l a c r y l a t e have a l s o b e e n d e t e r m i n e d . The r e s u l t s are shown i n F i g u r e 3» The c o p o l y m e r i z a t i o n experiments were done t o 5$ c o n v e r s i o n s . 2H5V was u s e d as g r a f t i n e t h y l e n e / p r o p y l e n e copolymer p o l y ( m e t h y l a c r y l a t e ) an c a s e s , g r a f t i n g was a c h i e v e d i n c h l o r o b e n z e n e w i t h d i t e r t i a r y b u t y l p e r o x i d e as t h e i n i t i a t o r . Up t o now, we have not succeeded i n g r a f t i n g 2H5P under t h e same c o n d i t i o n s onto t h e s e polymers. 2H5V and m e t h y l - 5 - v i n y l s a l i c y l a t e were a l s o g r a f t e d onto c i s l,k-poly(butadiene) and 1 , 2 - p o l y ( b u t a d i e n e ) . S o l u t i o n g r a f t i n g has been a c c o m p l i s h e d when t h e g r a f t i n g r e a c t i o n was c a r r i e d out i n low polymer c o n c e n t r a t i o n . G r a f t i n g e f f i c i e n c y f o r t h e base polymers as w e l l as f o r t h e monomer has been e s t a b l i s h e d ( 3 3 * 3 ^ ) • 2H5V and 2H5P have a l s o been u s e d as monomers f o r i n c o r p o r a t i o n i n t o p o l y m e r i z i n g m i x t u r e s o f u n s a t u r a t e d p o l y e s t e r s and s t y r e n e . E x p e r i m e n t s were c a r r i e d out w i t h o l i g o m e r i c p o l y e s t e r s made from m a l e i c a n h y d r i d e , p h t h a l i c a n h y d r i d e , 1 , 3 - d i h y d r o x y p r o p a n e ( 7 0 wt. $) and s t y r e n e ( 3 0 wt. $) ( 3 5 . ) . Under t h e n o r m a l r e a c t i o n c o n d i t i o n s ( b e n z o y l p e r o x i d e as t h e i n i t i a t o r ) , t h e two p o l y m e r i z a b l e b e n z o t r i a z o l e d e r i v a t i v e s were e s s e n t i a l l y q u a n t i t a t i v e l y i n c o r p o r a t e d when t h e f e e d r a t i o was between 0 . 5 and 5 mol I n i t i a l a c c e l e r a t e d t e s t s have shown copolymers o f 2H5V and 2H5P t o be e f f e c t i v e as u l t r a v i o l e t s t a b i l i z e r s , p a r t i c u l a r l y f o r a c r y l i c p o l y m e r s , and t o p r o t e c t t h e polymers f o r more t h a n 2 0 y e a r s normal outdoor exposure t o t h e environment. 1

Mu.lt i c h r o m o p h o r i c 2 ( 2 - H y d r o x y p h e n y l ) 2 H - B e n z o t r i a z o l e Ultraviolet Stabilizers. Up t o now, we have d i s c u s s e d p r i m a r i l y t h e p r e p a r a t i o n of polymerizable 2(2-hydroxyphenyl)2H-benzotriazoles. I n our most r e c e n t work, we became a l s o i n t e r e s t e d i n d e v e l o p i n g e n t i r e l y new u l t r a v i o l e t - a b s o r b i n g systems b a s e d on 2 ( 2 - h y d r o x y p h e n y l ) 2 H - b e n z o t r i a z o l e s ; t h e main emphasis was on t h e p r e p a r a t i o n o f i n u l t i b e n z o t r i a z o l i z e d phenols or polyphenols. Of p a r t i c u l a r i n t e r e s t was t h e p r e p a r a t i o n o f 2 ( 2 - h y d r o x y p h e n y l ) 2 H - b e n z o t r i a z o l e s w i t h more t h a n one p h e n o l i c h y d r o x y l group i n t h e benzene r i n g . By c o n d e n s i n g o - n i t r o b e n z e n e d i a z o n i u m s a l t s w i t h r e s o r c i n o l o r p h l o r o g l u c i n o l under c a r e f u l l y c o n t r o l l e d c o n d i t i o n s , m o n o b e n z o t r i a z o l i z e d p r o d u c t s were obt a i n e d ; t h e n o n h i n d e r e d p h e n o l i c groups i n t h e k- p o s i t i o n were now a v a i l a b l e f o r r e a c t i o n w i t h a c r y l o y l c h l o r i d e or m e t h a c r y l o y l c h l o r ide (8).

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

15.

VOGL ET AL.

Polymerizable Ultraviolet Stabilizers

F i g u r e 3. C o p o l y m e r i z a t i o n o f 2H5V and 2H5P. I n i t i a t o r : Temperature: 60 C. C o n v e r s i o n : " 5 % . (A) S t y r e n e . (B) Methyl methacrylate. (C) n - B u t y l a c r y l a t e .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

207

AIBN.

POLYMER STABILIZATION AND DEGRADATION

208

We have e s t a b l i s h e d t h a t n o r m a l c o n d e n s a t i o n o f o - n i t r o b e n z e n e d i a z o n i u m c h l o r i d e w i t h r e s o r c i n o l i n s t r o n g l y b a s i c media gave d i b e n z o t r i a z o l i z a t i o n ( a f t e r r e d u c t i v e c o u p l i n g o f t h e azo-compounds). B e n z o t r i a z o l i z a t i o n o c c u r r e d i n t h e 2 and k p o s i t i o n s o f r e s o r c i n o l (36) . With p h l o r o g l u c i n o l , the r e a c t i o n ( a l s o i n b a s i c s o l u t i o n ) l e a d s t o d i - and i f done w i t h e x c e s s d i a z o n i u m c h l o r i d e even t o t r i benzotriazolization. I t i s o b v i o u s t h a t t h e s e compounds are o f i n t e r e s t because o f t h e i r v e r y b r o a d u l t r a v i o l e t a b s o r p t i o n s p e c t r a and t h e i r e x t r e m e l y h i g h e x t i n c t i o n c o e f f i c i e n t s , w h i l e a t t h e same time h a v i n g a s h a r p c u t o f f a t 3 7 0 nm. U l t r a v i o l e t - a b s o r b i n g compounds have a l s o been p r e p a r e d h a v i n g t h e 2 ( 2 - h y d r o x y p h e n y l ) 2 H - b e n z o t r i a z o l e and t h e 2-hydroxybenzophenone ( o r acetophenone) chromophor i n t h e same m o l e c u l e , by b e n z o t r i a z o l i z a t i o n o f 2,^-dihydroxybenzophenone or 2 , ^ - d i h y d r o x y a c e t o p h e n o n e . In e i t h e r c a s e , the d i b e n z o t r i a z o l i z e d p r o d u c t s were o b t a i n e d : 2{2,^hydroxy-5-acetyl(benzoyl)phenyl]l,3-2H-dibenzotriazole. These compounds are v e r y p o w e r f u 3 7 0 nm a t v e r y h i g h e x t i n c t i o (37)

(see a l s o

38,39).

Polymerizable Antioxidants I n t h e l a s t few y e a r s , i t has become f u l l y a p p r e c i a t e d t h a t p o l y m e r i c a n t i o x i d a n t s a r e e f f e c t i v e i n r e t a r d i n g t h e t h e r m a l and a u t o o x i d a tion. Such polymer-bound s t a b i l i z e r s are s i m i l a r i n e f f i c i e n c y t o t h e low m o l e c u l a r w e i g h t s t a b i l i z e r i n c o r p o r a t e d i n t o t h e polymer by blending. The polymer-bound s t a b i l i z e r s h o u l d have a f l e x i b l e s p a c e r between t h e p o i n t o f attachment t o t h e polymer and t h e f u n c t i o n a l group o f t h e p h e n o l i c a n t i o x i d a n t s . A n t i o x i d a n t s b a s e d on 2 , 6 - d i t e r t i a r y b u t y l - ^ - v i n y l p h e n o l o r 2 , 6 d i t e r t i a r y b u t y l - ^ - i s o p r o p e n y l p h e n o l a r e t h e o n l y monomeric s t a b i l i z e r s t h a t have been s y n t h e s i z e d and s t u d i e d . We have d e v e l o p e d e f f i c i e n t s y n t h e t i c methods f o r t h e p r e p a r a t i o n o f such compounds and have p o l y m e r i z e d them w i t h s t y r e n e or m e t h y l m e t h a c r y l a t e i n s o l u t i o n o r i n b u l k w i t h AIBN as t h e i n i t i a t o r . More i m p o r t a n t l y , we have d e v e l o p e d a good e m u l s i o n p o l y m e r i z a t i o n o f 2 , 6 - d i t e r t i a r y b u t y l - ^ - v i n y l p h e n o l and 2 , 6 - d i t e r t i a r y b u t y l - ^ - i s o p r o p e n y l p h e n o l w i t h b u t a d i e n e o r i s o p r e n e . The copolymers o f good m o l e c u l a r w e i g h t s had comonomer c o n t e n t s between 6 mol $ and 2 0 mol $ o f t h e v i n y l o r i s o p r o p e n y l monomer. The polymers were e f f e c t i v e a t a 0 . 1 weight p e r c e n t l e v e l i n r e t a r d i n g a u t o o x i d a t i o n o f p o l y b u t a d i e n e and p o l y i s o prene. C a t a l y t i c h y d r o g e n a t i o n w i t h c o b a l t / a l u m i n u m c a t a l y s t gave p o l y e t h y l e n e copolymers ( f r o m t h e h y d r o g e n a t i o n o f b u t a d i e n e copolymers) o r e t h y l e n e / p r o p y l e n e copolymers ( f r o m i s o p r e n e copolymers) c o n t a i n i n g 2 , 6 - d i t e r t i a r y b u t y l - ^ - v i n y l p h e n o l or 2 , 6 - d i t e r t i a r y b u t y l - i | - - i s o p r o p e n y l p h e n o l i n t h e polymer. These polymers have been u s e d as p o l y m e r i c a n t i o x i d a n t s and a r e e f f e c t i v e i n r e t a r d i n g a u t o o x i d a t i o n o f p o l y o l e f i n s (^-0). We have shown t h a t p o l y m e r i c s t a b i l i z e r s , p o l y m e r i c u l t r a v i o l e t s t a b i l i z e r s , and p o l y m e r i c a n t i o x i d a n t s a r e e f f e c t i v e i n r e t a r d i n g p h o t o o x i d a t i o n and t h e r m a l o x i d a t i o n i n p o l y m e r s . Polymeric s t a b i l i z e r s are not v o l a t i l e , not e x t r a c t a b l e o r l e a c h a b l e , and do not p r o vide the u n d e s i r a b l e c h a r a c t e r i s t i c s of t o x i c i t y , p o t e n t i a l c a r c i n o -

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

15.

VOGLETAL.

Polymerizable Ultraviolet Stabilizers

209

genicity and allergenicity, and other potential side effects of low molecular weight compounds. Acknowledgment s We appreciate the contributions of A. Gupta, W. Dickstein, W. Bassett Jr., P. Grosso, and C. P. Lillya. We are also indebted to Mrs. E. Cary and L. S. Corley for their assistance in preparing this manuscript. Literature Cited 1.

Bailey, D.; Vogl, O. J. Macromol. Sci., Reviews 1976, C14(2), 267. 2. Bailey, D.; Tirrell, D.; Vogl, 0. J. Macromol. Sci., Chem. 1978, A12, 661. 3. Tirrell, D.; Bailey Donaruma, L. G.; Vogl 1978; p. 77. 4. Hawkins, W. L. "Polymer Stabilization"; Wiley: New York, 1972. 5. Vogl, O.; Muggee,J.;Bansleben, D. Polymer J. (Japan) 1980, 12, 677. 6. Vogl, O.; Loeffler, P.; Bansleben, D.; Muggee, J. In "Coordination Polymerization"; Price, E. C.; Vandenberg, E. J., Eds.; POLYMERIC SCIENCE AND TECHNOLOGY, Plenum Press; New York and London, 1983, Vol. 19, p. 95. 7. Tocker, S. Makromol. Chem. 1967, 101, 23. 8. Mannens, M. G.; Hove, J.J.;Aarschot, W. J.; Priem, J. J. U.S. Patent 3 813 255, 1974; Germ. Patent 2 128 005, 1971; Chem. Abstr. 1972, 76, 119908m. 9. Balaban, L.; Borkovec,J.;Rysary, D. Czech. Patent 108 792, 1963; Chem. Abstr. 1964, 61, 3267n. 10. Belousa, J.; Janousek, Z.; Knoflickova, H. Chem. Zvesti 1974, 28, 673. 11. Stevenson, D.; Beeber, A.; Gaudiana, R.; Vogl, O. J. Macromol. Sci., Chem. 1977, A l l , 779. 12. Vogl, O. . Pure and Applied Chem. 1979, 51, 2409. 13. Vogl, O.; Yoshida, S. "Preprints, Plen. Lect., Ann. Meeting, Soc. Pol. Sci."; Kyoto, 198O, 29(4), 648. 14. Vogl, O.; Yoshida, S. Rev. Roum. de Chim. 198O, 7, 1123. 15. Bailey, D.; Tirrell, D.; Vogl, O. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 2725. 16. Iwasaki, H.; Tirrell, D.; Vogl, 0. J. Polym. Sci., Polym. Chem. Ed. 1980, 18, 2755. 17. Tirrell, D.; Vogl, 0. Makromol. Chem. 1980, 181, 2097. 18. Tirrell, D.; Bailey, D.; Pinazzi, C.; Vogl, 0. Macromolecules 1978, 11, 213. 19. Tirrell, D.; Bailey, D.; Vogl, 0. Polymer Preprints, ACS Division of Polymer Chemistry 1977, 18(1), 542. 20. Sumida, Y.; Yoshida, S.; Vogl, 0. Polymer Preprints, ACS Division of Polymer Chemistry 1980, 21(1), 201. 21. Sumida, Y.; Vogl, 0. Polymer J. (Japan) 1981, 13, 521. 22. Yoshida, S., unpublished results.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

210

23. Elbs, K.; Keiper, W. J. Prakt. Chem. [2] 1905, 67, 580. 24. Elbs, K. J. Prakt. Chem. 1924, 108, 20925. Hardy, W. B. In "Developments in Polymer Photochemistry"; Allen, N. S., Ed.; Applied Science Publishers Ltd.: London, 1981; Chap. 8. 26. Milionis, J. P.; Hardy, W. B.; Baitinger, W. F. U.S. Patent 3 159 646, 1964. 27. Yoshida, S.; Vogl, 0. Polymer Preprints, ACS Division of Polymer Chemistry 198O, 21(1), 203. 28. Yoshida, S.; Vogl, 0. Makromol. Chem. 1982, 183, 259. 29. Nir, Z.; Vogl, O.; Gupta, A. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 2737. 30. Yoshida, S.; Lillya, C. P.; Vogl, 0. Monatsh. Chem. 1982, 113, 603. 31. Yoshida, S.; Lillya, C. P.; Vogl, 0. J. Polym. Chem., Polym. Chem. Ed. 1982, 20, 2215. 32. Pradellok, W.; Gupta Chem. Ed. 1981, 19 33. Kitayama, M.; Vogl, 0. J. Macromol. Sci., Chem. 1983, 19(3), 875. 34. Kitayama, M.; Vogl, 0. Polymer J. (Japan) 1982, 14, 537. 35. Borsig, E.; Ranby, B.; Vogl, 0., unpublished results. 36. L i , S.J.;Gupta, A.; Vogl, 0. Monatsh. Chem. 1983, 114, 93. 37. L i , S.J.;Gupta, A.; Vogl, 0. J. Polym. Sci., Polym. Chem. Ed., in press. 38. Eastman Kodak Co. U.S. Patent 4 256 626, 1981. 39. Eastman Kodak Co. U.S. Patent 4 271 307, 1981. 40. Grosso, P., Ph.D. Thesis, University of Massachusetts, Amherst, 1983. RECEIVED December 17, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16 Computer Modeling Studies of Polymer Photooxidation and Stabilization A. C. S O M E R S A L L and J. E. G U I L L E T Department of Chemistry, University of Toronto, Toronto, Canada M5S 1A1

A computer model ha istic concentration versu formed during photooxidation of hydrocarbon polymers using as input data a set of elementary reactions with corresponding rate constants and initial conditions. Simulation of different mechanisms for stabilization of clear, amorphous linear polyethylene as a prototype suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which can trap peroxy radicals and also decompose hydroperoxides. The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. The Computer Model A complex chemical mechanism can be expressed as a system of NR elementary (fundamental) chemical reactions comprising NS different 0097-6156/85/0280-0211S07.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

212

POLYMER STABILIZATION AND DEGRADATION

chemical species, {Y^}, i = l , N S . The resulting mechanistic scheme takes the concise form given by v.. Y . = 0

;

j =1 , NR

i=l

(1)

in which the stoichiometric coefficients, vy , are positive for product and negative for reactant species. Furthermore, each reaction j (j = l , NR) is characterized by a rate constant k j . From the molecularity of the elementary reactions comprising the reaction scheme it is then possible to write explicit expressions for the concentration-time d e r i v a tives of all chemical species, Y j , as shown by Equation 2. In this (2) equation the product II includes only those I - values for reactant species (vjj negative). The time behavior of a derived reaction scheme can be obtained by integration of the resulting system of ordinary differential equations ( O D E ) , given a specified set of initial conditions. This yields explicit concentration-time data for all species. Generally speaking, the system of ODE is nonlinear, necessitating numerical solution. Furthermore, the rates of the individual reactions in the usual kinetic scheme commonly vary by many orders of magnitude, giving rise to the so-called stiff system of O D E . Attempts to employ classical integration algorithms require limiting the integration step size so as to keep pace with the fastest transients in the system. This has the undesirable consequence that extremely small step sizes are taken and exorbitant amounts of computing time are required to complete the problem. Clearly, for the usual chemical kinetics p r o b lem, integration algorithms especially designed to tackle a stiff system of ODE must be employed. For this work, the original Gear routine (1) and its various modifications (2) have been employed in the simulation package. Essentially, the algorithms are multi-step predictorcorrector methods utilizing the backward differencing formulation as applied by Gear with automatic error-controlled, order-step size selection. To validate our numerical procedure, the data base given for the cesium flare system (which is becoming a standard in the literature) was u s e d , and the curves generated were identical to those of Edelson (3) for the same system. T h e excellent agreement between predicted and actual rate curves showed that the program itself (irrespective of the data base) performs in a satisfactory manner. A similar computational modelling approach has been shown to be useful, for example, in studying the mechanism of low-temperature o x i dation of alkanes (4), pyrolysis of alkanes (5-7), other gas-phase r e actions (8), the formation of photochemical smog (9,10), and peroxide decomposition (11), among others. It is not uncommon to begin with all possible species and by permutation and combination derive a complete set of reactions, and then eliminate a subset by chemical

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16.

SOMERSALL AND GUILLET

Computer Modeling Studies

213

intuition or by sensitivity analysis. Polymer photooxidation with over 40 different species involved is too complex a process for such an a p proach. O u r model is built up sequentially from the basic chemistry of the system and we presume that it approaches the real process in its essential elements. The Reaction Scheme A s a starting point for this computational approach to the photooxidative process in polymeric materials, we have examined the simplest prototype: neat, amorphous, linear polyethylene above its glass t r a n sition temperature. In practice, polyethylene is partially crystalline, and contains truly linear olefins, vinylidene groups, ketones and h y droperoxides in addition to the short side chains. Much insight has already been gained into the photooxidation process by conventional experimentation on such polymers (12,13), yet several important questions still remain. Several good reviews have appeared recently (14-16). A mechanistic scheme has been devised which includes most, if not all of the apparent non-trivial elementary processes. T h e 56 relevant reactions and their corresponding rates are listed in Table I and are shown schematically in Scheme I. Initiation. Three types of initiation have been considered in the model: the photolytic cleavage of ketones and/or hydroperoxide and some general fortuitous generation of alkyl radicals (such as thermal C - H homolysis, ultrasonics, mechanical treatment, ionizing radiation, e t c . ) . More often the Norrish type I cleavage of ketones has been used as standard, assuming initial ketone concentrations of 10"^M ketone. The Norrish type II process is v e r y important for chain scission but no participation is presumed in the initiation step for the suspected b i radical intermediate. Although " p u r e " saturated polyolefins should not be expected to absorb beyond 2000 the absorption m the far U V tail in the atmospheric sunlight spectrum around 3000 A has commonly been attributed to the low concentrations of ketone and/or h y d r o p e r oxide groups introduced in the commercial polymers during processing. No consideration has been given to other proposed initiation processes such as absorption by dyes, pigments, catalyst residues, oxygen a d ducts (18), charge-transfer or polynuclear aromatics absorbed from polluted urban atmospheres (19). The possibility of energy transfer from excited ketones leading to photosensitized decomposition of h y d r o peroxide (20) has also been included. The extinction coefficient for the hydroperoxide is very low but the quantum yield of cleavage is very near to u n i t y . The quantum yield for ketones is low for carbonyl incorporated in the backbone of the polymer chain, but is much higher when at a chain end or branch (21). The radicals generated are p r e sumed to have free access to oxygen, and no special cage effect in the bulk polymer has been assumed. A l l of the rates for the initiation r e actions follow directly from the absorption spectra and known photochemical quantum yields. T h e standard light intensity employed refers to the average solir intensity which is one-third of the typical peak value of 0.15 E m " h incident on a 1 mm film. 2

_ 1

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

214

Table I.

Data Set: Photooxidation Reaction Scheme and Activation Parameters

Reaction m a t r i x 1.

Ketone — >

2.

KET*

3.

SMRCO

4. 5. 6.

2

0.70

x 10

0.80

x 10

8. 9.

-> S M R 0

SMKET*

17

15

+ CO

2

Ref.

37

0.56

x 10

Ref.

36

0.70

x 10

8.5

Ref.

38

ROOH — >

2.0

Ref.

36

1U

17.0

Ref.

39

1U

17.0

Ref.

39

2.0

8

SMKET*

> SMR0

SMKET* — >

0

0

Alkene + SMKetone

SMKetone — >

R0

9

+ SMRCO

2

9

0

+ CH CO 3

0.32 7.

Remarks

KET*

>SMR0

KET* —>

E kcal/mol

A

x 10

13

Alkene + Acetone 0.56

x 10

0.13

x

9

RO + OH

+ RH — >

ROOH + R 0

10

9

0

10

10.

11. 12.

SMR0

0

^

+ RH

SMROOH — > SMRO + RH

0.10 x 10 > SMROOH + R 0 ^ m 0

0.10

x 10

0.13

x 10"

SMRO + OH -> SMROH + R 0

9

0

0

in

13.

RO + RH

-> ROH + R 0

0.16

x 10

1U

6.2

Ref.

39

0.16

x 10

1U

6.2

Ref.

39

9

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16.

Computer Modeling Studies

SOMERSALL AND GUILLET

Table

I,

14.

RO

Continued

—>

SMR0

+ Aldehyde

2

0.32 15.

K E T * + ROOH

x 10

S M K E T * + ROOH

x 10

SMRCO + 0

10

x 10

10

2

SMRCO + RH — >

R0

x 10"

39

11.6

Ref.

40

11.6

Ref.

40

SMRCOOO + RH

4

9.6

Ref.

41

+ Aldehyde

9

^

19.

Ref.

> SMRCOOO 0.80

18.

17.4

> SMKetone + RO + OH 0.25

17.

16

> Ketone + RO + OH 0.25

16.

215

in

> 10

0.10 20.

SMRCOOOH — > SMRCOO

> SMR0

+ C0

2

x 10"

cf.

36,

37

0

9

2

0.10 22.

17.0

1U

SMRCOO + OH 0.13

21.

x 10

SMRCOO + RH — >

Acid + R 0

OH + RH

0.10 + Water

x 10

6.6

15

Ref.

43

0

10

23.

-> R 0

0

^

24.

C H C O + RH

> R0

Q

0

x 10

0.10 x 10 + CH CHO

17.0 10 1U

CH CO + 0 Q

6

—>

0

1

0.10 CH COOO

Q

Q

6

CH COOOH —> Q

0.10

CH COO 3

+

RH

7.3

Ref.

42

9.6

Ref.

41

x 10

10

1U

9

17.0

cf.

36,

37

C H C O O + OH Q

0.13 28.

44

i n

0.89 x 1 0 > CH COOOH + R 0

6

27.

Ref.

Q

6

C H C O O O + RH

0.5

37

10

x 10

iU

26.

36,

Q

1

25.

cf.

x 10

> CH COOH + R 0 3

0.10

0

y

x 10 * 1

9

6.6 Continued

Ref.

43

on next

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

page

216

POLYMER STABILIZATION AND DEGRADATION

Table I.

29. 30. 31.

Continued

KET*

-> Ketone

SMKET* KET* + 0

0.10 x 10

9

0

0.10 x 10

9

0

> SMKetone —>

2

Ketone + S 0

2

0.89 x 1 0 32.

SMKET* + 0

> SMKetone + S 0

2

1 4

9.6

Ref. 41

1 4

9.6

Ref. 41

11.6

Ref. 39

15.3

Ref. 39

9

15.0

Ref 45

9

2.1

Ref. 38

8.9

Ref. 46

2

0.89 x 1 0 33.

R 0 + R0 2

—>

2

ROH + Ketone + S O 2 0.25 x 1 0

34.

R 0 + ROH ^

1 0

> ROO

9

m 0.10 x 10

35.

HOO + RH

> HOOH + R 0

2

0.32 x 10 36.

HOO + R 0

> ROOH + S 0

2

2

0.32 x 10 37.

R 0 + Ketone

> ROOH + Peroxy CO

2

0.13 x 10

5

38.

Peroxy CO + RH

> PER OOH + R 0

39.

0.10 x 1 0 PER OOH — > PER O + OH

0.13 x 1 0 " 40.

PER O + R 0

2

—>

R0

2

+ ROOH — >

R0

0

+ SMROH

^

0 Ref. 40

11.6

Ref. 39

> ROOH + Aldehyde + HOO 1n 0.10 x 10 15.3 > ROOH + SMRCO

Ref. 39

ROOH + Ketone + OH 8

1

43.

RO

+ Aldehyde

c f . 36, 37

11.6

1 0

0.25 x ID 42.

9

17.0

DIKetone + ROOH 0.25 x 1 0

41.

2

1 0

0.25 x 1 0

i U

11.6

Ref. 40

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16.

Computer Modeling Studies

SOMERSALL AND GUILLET

Table

44. 45. 46. 47. 48. 49. 50. 51. 52.

53. 54.

I.

Continued

R0

S 0

2

2

S0 R0

+ R0

—>

2

2

2

56.

Ref.

47

9

11.6

Ref.

39

x 10

9

11.6

Ref.

39

x 10

8

5.2

Ref.

39

x 10

0.20

x 10

14

0.16

x 10

0.16 0.16

16.0

0

5

> ROOH

> Ketone + Heat

ROOH + QD

0.80

x 10

13

9.5

Ref.

48

0.80

x 10

13

9.5

Ref.

48

0.63

x 10

15

35

Ref.

49

0.63

x 10

15

35

cf.

52

x 10

15

35

cf.

52

0.63

x 10

15

35

cf.

52

0.63

x 10

15

35

cf.

52

> PRODS R O ' + OH *

SMROOH — >

SMRO + OH

SMRCOOOH — > CHgCOOOH — > PER OOH — >

- SMProduct

10.0

0.63

12

> ROO

KET* + Q l

ROOH — >

39

x 10

> Branch

+ Alkene

+ QH

Ref.

0.38

> ROOH

+ Alkene

2

S0

°2

+ Alkene

2

SMR0 R0

-> ROOR +

2

SMRCOO + OH 0.63

55.

217

C H C O O + OH 3

PER O + OH

= product from chain cleavage, SO

=

O

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

Initiation

+

RH

2

Q

-> RO

fast hv -I

KETONE

-> KET*

"

0

R

' *

R

>

1

ROOH*

V e r

f a S t

y ROOH + Other products k - i o

1

- 10 "

3

Scheme I . Polyethylene photooxidaton scheme.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16.

SOMERSALL AND GUILLET

Computer Modeling Studies

219

Propagation. The key radical in the propagation sequence is the p e r oxy radical formed by the addition of oxygen to alkyl radicals. Molecular oxygen 0 2 ( E g ) found in nature is paramagnetic, with two parallel-spin electrons, giving it the characteristics of a biradical. A s a consequence the oxygen molecule combines very rapidly with free radicals to form the peroxy radicals (22). The solubility of oxygen in amorphous polyethylene is low (10"^M) but practically constant due to the steady diffusion of oxygen from the atmosphere to replenish what is consumed during oxidation over long periods of time. T h u s we have assumed that all carbon-centered radicals will inevitably add oxygen to form peroxy radicals so we have incorporated this step directly in those reactions involving alkyl radical products, to simplify the computation (23) . 3

The key propagation step is the abstracton of a hydrogen atom by the peroxy radical to generate a new peroxy radical and a h y d r o p e r oxide group. Alkoxy radicals and hydroxy radicals formed by cleavage of the hydroperoxide wil further peroxy radicals. These reactions are well studied in solution (24) and the same rates have been assumed in our model. Other r a d i cals formed in Norrish type II chain scission and subsequent reactions can also abstract hydrogen atoms to generate peroxy radicals. R e arrangement of alkoxy radicals in the $-scission process leads to aldehydes, whereas thermal cleavage of intermediates leads to small molecule fragments and products such as carbon dioxide and formaldehyde. Acids are formed in the model via the addition of oxygen to acyl r a d i cals, followed by hydrogen atom abstraction, then cleavage of the r e sulting peroxy acid and a further H-atom abstraction. Esters would develop from the condensation of acid with the alcohols formed by H atom abstraction of alkoxy radicals. In linear amorphous polyethylene, all the C - H bonds are presumed to be secondary and on the backbone of the polymer chains, with n e glect of the few chain ends. The rates of hydrogen abstraction by various radical species are taken from similar processes in solution, but subsequent processes are modified to allow for the higher internal viscosity of the medium. Termination. Just as peroxy radicals are key to the propagation sequence , so the bimolecular recombination of these radicals is the major termination process in the unstabilized polymer. The existence of an intermediate tetroxide has been established in solution (2j>). Several factors influence the competitive pathways of subsequent decomposition to form alcohols, ketone and singlet oxygen or to form alkoxy radicals which can couple before separation from the reaction center to form a peroxide. This latter process is a route to crosslinking in the case of polymeric peroxy radicals. The effect of steric control, viscosity and temperature have been studied in solution. However, in the solid phase the rates of bimolecular processes which require the mutual diffusion of the reactant groups will be limited by the diffusion process. A s a standard, we have assumed a value close to that determined from o x y gen absorption (26) and by ESR spectra (27) for oxidized polypropylene films.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

220

POLYMER STABILIZATION AND DEGRADATION

For those bimolecular reactions that do not require mutual diffusion such as H-atom abstraction, in which one reactant is a virtual solvent, the rates are similar to solution values. The peroxy radical may react with many other species and in some cases the rate constants are even lower than the diffusion rate and hence the value is not limited in that way. Modelling Photooxidation Photooxidation of amorphous polyethylene. Figure 1 shows the general behavior predicted for exposure to different intensities of UV l i g h t . Assuming initiation by 10"^ M ketone groups and constant ambient oxygen pressure, the model shows that the C - H bonds become oxidized slowly at first and then more rapidly later o n . A 5% C - H bond (1% monomer units) oxidation level has been used to assign a point of failure which is within the range one would anticipate for marked change in mechanical properties. Under typical conditions, the time to failure (if) of unstabilized polyethylen perate climates and shorter in regions of high solar intensity. Product formation and other observations are consistent with the authors' e x perimental knowledge of polyethylene weathering (28). Table II summarizes the concentration of all chemical species in the model at the chosen time of failure. The major products are ketones, h y d r o p e r oxides, alcohols and water. The only anomaly is the relatively high concentration of hydroperoxide which the authors were unable to eliminate in their model predictions and which may point to the inadequacy of experimental methods for monitoring - O O H groups in oxidized films. The time to failure was plotted as a function of the intensity of light to find the relationship that T J « \~i one would expect for photochemical initiation producing two radical species. 9

a

s

Also, one finds that the behavior is almost unaffected by both the initiator type and concentration (Figure 2). This is not surprising for an autocatalytic process since the result is dominated by relative rates of propagation and termination. Two interesting conclusions can be drawn. F i r s t , there has been much controversy in the literature about the relative importance of the various possible mechanisms for initiation (ketone groups, h y d r o p e r oxide, catalyst residues, singlet oxygen), but the controversy is practically irrelevant if initiation does not much influence the course, rate or extent of photooxidation. Second, in terms of stabilization, minute amounts of photoinitiation will lead to practical failure so that the purification of polymers to minimize the initiating residue is not a practical alternative for stabilization. Both points underline the value of this approach to the understanding of the complex photooxidation process. Figure 3 shows the dependence of the time to failure on the rate of propagation, i . e . , the rate of hydrogen abstraction from the polymer backbone. The log-log plot is linear with a negative slope less than u n i t y . The rate of abstraction of t C - H as in branched polyethylene and ethylene copolymers would only be a factor of two to three higher than for C - H in linear polyethylene. The time to failure in these S

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16.

Computer Modeling Studies

SOMERSALL AND GUILLET

1

CM

i

r

221

1-

i

o

—

—i°2__ fa/lure

•o c 0 -Q

1 i

0.8

o

6 c o 0.4

o E

\\ \\

\\ (f

(Q)\

\

2

Time,

Figure 1. (c) h v / 3 ; hv x 10 .

months

Photooxidation of unstabilized P E : (a) hv/10; (b) h v / 2 ; (d) hv average daylight; (e) hv x 5; (f) hv x 10; (g)

2

-4 log

-2

0

[initiator]

Figure 2. Photooxidation of unstabilized P E : tion of initiator type and concentration.

time to failure as a func-

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

222

POLYMER STABILIZATION AND DEGRADATION Table II.

Concentration of A l l Species, Initially and at Failure (5% C - H Oxidation) Concentration

Species

Initial

1.

Ketone

0.1 x 10

2.

KET*

3.

SMR0

4.

SMRCO

5.

CO

6.

Alkene

7.

SMKetone

8. 9.

Final (5% C - H oxidation) 0.39

X

10

—

0.26

X

lO"

—

0.20

X

l 2

ROOH + Q

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16. SOMERSALL AND GUILLET

Computer Modeling Studies

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

231

232

POLYMER STABILIZATION AND DEGRADATION

Hydroperoxide decomposition. Yet another reaction (number 48) was added to the basic mechanism scheme to allow for the stabilization of amorphous linear polyethylene by the harmless removal of the hydroperoxide by an additive (QD). Reactions of the products (alcohol, •k>2) were not considered further. Also included in Figure 8 is the effect of this type of stabilizer on the time to failure (5% oxidation) as a function of its concentration. The mechanism is effective as low as 10~ M (about 10" wt-%) and the increasing effectiveness at higher concentrations could reflect the autocatalytic participation of the hydroperoxide which normally decomposes to produce two destructive radicals per molecule. 4

3

The model suggests that optimum photoprotection for clear polyethylene films would be obtained with a stabilizer which effectively scavenges peroxy radicals and also decomposes hydroperoxide. This agrees with the conclusions put forward earlier by Carlsson et al. (32). Conclusions In principle, the computational approach to the kinetics of the complex photooxidation process can give meaningful insight into the effects of outdoor weathering of hydrocarbon polymers. For clear amorphous linear polyethylene, the model suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which could trap peroxy radicals. An additive which decomposes hydroperoxides would also be effective but would require higher concentrations. The useful lifetime of unstabilized polyethylene is predicted to vary from a few months in hot weather (100°F) to almost two years in cool weather (45°F), which correlates well with experimental results and general experience. Acknowledgments This work was performed as part of a research contract for the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the U . S . Department of Energy through an agreement with the National Aeronautics and Space Administration. The computer program was adapted for use here by Dr. J . W. Gordon, now at Canadian General Electric Company, Toronto. The authors wish to acknowledge the invaluable advice of Dr. K. U . Ingold, National Research Council of Canada, Ottawa, Dr. R. R. Hiatt, Brock University, St. Catharines, Ontario, Dr. J . R. MacCallum, St. Andrews University, Scotland, and Dr. A . Gupta, Jet Propulsion Laboratory, Pasadena, California. Literature Cited 1. (a) Gear, C. W. ACM 1971, 14, 176; (b) Hindmarsh, A . C. "GEAR Ordinary Differential Equation Solver," Lawrence Livermore Laboratories, Rep. UCID-30001-R3, 1974. 2. (a) Stabler, R. N . ; Chescick, J . HAVCHM, Haverford College, 1977; (b) Carver, M. B . ; Hanley, D. V . ; Chaplin, K. R. MAKSIMACHEMIST, Atomic Energy of Canada CRN2, 1979; (c) Whitten, G. Z. CHEMK, S. A . I., San Rafael, Calif., 1979. 3. Edelson, D. J . Chem. Ed. 1975, 52, 642.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

16.

SOMERSALL AND GUILLET

Computer Modeling Studies

233

4. Allara, D. L . ; Edelson, D . ; Irwin, K. C. Int. J . Chem. Kinet. 1972, 4, 345. 5. Allara, D. L . ; Edelson, D. Int. J . Chem. Kinet. 1975, 7, 479. 6. Olson, D. B . ; Tanzawa, T . ; Gardiner, W. C. Int. J . Chem. Kinet. 1979, 11, 23. 7. Sundaram, K. M . ; Froment, G. F. Ind. Eng. Chem. Fundam. 1978. 17, 174. 8. Ebert, K. H . ; Ederer, H. J.; Isbarn, G. Angew. Chem., Int. Ed. Engl. 1980, 19, 333. 9. Farrow, L . ; Edelson, D. Int. J . Chem. Kinet. 1974, 6, 787. 10. Carter, W. P. L . ; Lloyd, A . C.; Sprung, J . L ; Pitts, J . N. Int. J . Chem. Kinet. 1979, 11, 45. 11. Hiatt, R.; Nair, V . G. K. Can. J . Chem. 1979, 57, 450. 12. Adams, J . H. J . Polym. Sci., Part A 1970, 8, 1279. 13. Heacock, J . F . ; Mallory, F . B . ; Gay, F . P. J . Polym. Sci., Part A 1968, 6, 2921. 14. Garton, A . ; Carlsson, D. J.; Wiles, D . M . In "Developments in Polymer Photochemistry" Barking, 1980, vol. 1, pp. 93-123. 15. Shlyapintokh, V . Ya. In "Developments in Polymer Photochemistry"; Allen, N. S., E d . ; Applied Science, Barking, 1980; vol. 2, p. 215. 16. Hardy, W. B. In "Developments in Polymer Photochemistry"; Allen, N. S., E d . ; Applied Science, Barking, 1980; vol. 3, p. 187. 17. Cicchetti, O . ; Gratini, F . Eur. Polym. J . 1972, 8, 561. 18. Stenberg, V . I.; Sneeringer, P. V . ; Niu, C.; Kulevsky, N. Photochem. Photobiol. 1972, 16, 81. 19. Aspler, J.; Carlsson, D. J.; Wiles, D. M. ADVANCES IN CHEMISTRY SERIES no. 169, American Chemical Society: Washington, D. C , 1976, p. 5. 20. Ng, H. C.; Guillet, J . E . Macromolecules 1978, 11, 937. 21. Sitek, F . ; Guillet, J . E . ; Heskins, M. J . Polym. Sci., Polym. Symp. 1976, 57, 343. 22. Walling, C. "Free Radicals in Solution"; Wiley: New York, 1957. 23. In earlier computations we allowed for reactions of allyl radicals without oxygen and found extremely low radical concentrations and little or no photooxidation after very many iterative steps. 24. Mill, T . ; Hendry, D. G. Comp. Chem. Kinet. 1980, 16, 1. 25. Howard, J . A . ; Ingold, K. U. J . Am. Chem. Soc. 1968, 90, 1056. 26. Mayo, F . R. Macromolecules 1978, 11, 942. 27. Garton, A . ; Carlsson, D. J.; Wiles, D. M. Macromolecules 1979, 12, 1071. 28. Guillet, J . E . Pure Appl. Chem. 1980, 52, 285. 29. Wilson, J . E . Ind. Eng. Chem. 1955, 47, 2201. 30. Blum, G. W.; Shelton, J . R.; Winn, H. Ind. Eng. Chem. 1951, 43, 464. 31. Jellinek, H. H. G . ; Lipovac, S. N. Macromolecules 1970, 3, 231, 237. 32. Carlsson, D. J.; Garton, A . ; Wiles, D . M . In "Developments in Polymer Stabilisation"; Scott, G . , E d . ; Applied Science: Barking, 1979; vol. 1, pp. 219-259. 33. Heskins, M . ; Guillet, J . E . Macromolecules 1968, 1, 97. 34. Heller, J . H . ; Blattman, H. R. Pure Appl. Chem. 1973, 36, 141. 35. Gupta, A., personal communication.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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36. Hartley, G . ; Guillet, J . E . Macromolecules 1968, 1, 165. 37. Watkins, K. W.; Thompson, W. W. Int. J . Chem. Kinet. 1973, 5, 791. 38. Golemba, F . ; Guillet, J . E . Macromolecules 1972, 5, 63. 39. Mill, T . ; Hendry, D. G. Comp. Chem. Kinet. 1980, 1, 16. 40. Mayo, F. R. Macromolecules 1978, 11, 942. 41. Rabek, J . F. Comp. Chem. Kinet. 1978, 14, 463. 42. Hendry, D. G . ; Mill, T . ; Piszkiewics, L . ; Howard, J . A . ; Eigenmann, H. K. J . Phys. Chem., Ref. Data 1974, 3, 937. 43. Braun, W.; Rajbenbach, L . ; Eirich, F. R. J . Phys. Chem. 1962, 66, 1591. 44. Wilson, W. F. J . Phys. Chem., Ref. Data 1972, 1, 570. 45. Ingold, K. U . , personal communication. 46. Denisov, E . G . Comp. Chem. Kinet. 1980, 16, 169. 47. Hoch, E . Tetrahedron 1968, 24, 6295. 48. Braun, J . - M . ; Guillet, J . E . J . Polym. Sci., Polym. Lett. Ed. 1976, 14, 257. 49. Brandrup, J.; Immergut New York, 1975; 2nd ed., pp. 11-12 ff. RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

17 Investigation of Thermal Oxidation and Stabilization of High-Density Polyethylene PAUL-LI H O R N G and P E T E R P. K L E M C H U K Additives Department, Plastics and Additives Division, CIBA-GEIGY Corporation, Ardsley, NY 10502

A commercial HDPE hindered phenolic thermal oxidation at 140°, 100° and 40°C by oxygen uptake. The oxidative induction times of the unstabilized samples were found to f i t into a linear apparent Arrhenius relationship. The calculated activation energy for thermooxidative degradation of the HDPE agrees with literature data. Ultimate elongation, carbonyl formation and molecular weight distribution were found to change little before the induction time was reached. The degree of chain breaking, calculated from molecular weight data, shows an average of about one scission per molecule caused the polymer to lose its elongation property totally. Stabilization provided by a phenolic antioxidant demonstrated a relatively long induction time; e.g., 4700 versus 35 hours at 100°C. Within the induction time, chain scissioning and elongation were nearly unaffected. After the induction time, chain scissioning became uninhibited and was manifested by loss of elongation. Mechanisms of chain scissioning and stabilization are discussed. Polyolefins are sensitive to heat- and light-induced oxidative degradation. Studies in the past on thermal oxidative stability of high density polyethylene (HDPE) have generated information on how HDPE is oxidized under thermal stress (1-4). Alkyl and peroxy radicals, hydroperoxides, beta-scission after hydroperoxide decomposition to carbonyl and an alkyl radical end group are recognized as the major elements in the general oxidation pathway. Stabilization through interruption of the degradation cycle has resulted in the development of effective stabilizer systems for the many uses of this polymer. Recently we have been studying both the molecular weight changes and the physical property changes in HDPE as a function of oxidation. Unstabilized and stabilized HDPE were evaluated by oxygen uptake at 0097-6156/85/0280-0235$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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140°, 100° and 40°C. T h i s paper p r e s e n t s t h e r e s u l t s o f our f i n d i n g s which p r o v i d e some i n s i g h t s i n t o t h e r e l a t i o n s h i p s among degree o f o x i d a t i o n , m o l e c u l a r weight changes, p h y s i c a l p r o p e r t y r e t e n t i o n , and s t a b i l i z a t i o n o f t h e polymer. Experimental The polymer used was A l a t h o n 5496, DuPont HDPE. The s t a b i l i z e r used was tetrakis[methylene-(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane, r e f e r r e d t o as A01. Powdered samples o f HDPE, s t a b i l i z e d and u n s t a b i l i z e d , were p r e p a r e d and s u b j e c t e d t o o x i d a t i o n i n a c l o s e d system w i t h oxygen. Oxygen uptake was m o n i t o r e d p e r i o d i c a l l y at g i v e n t e m p e r a t u r e s . The i n d u c t i o n p e r i o d was p i c k e d from t h e curve where t h e onset o f a u t o c a t a l y t i c o x i d a t i o n took p l a c e . E l o n g a t i o n d a t a o f 5-mil f i l m s were g e n e r a t e d on an I n s t r o n T e n s i l e T e s t e r a c c o r d i n g t o ASTM D882 a t a p u l l i n g r a t e o f 50mm/min. M o l e c u l a r weights o f polyme t u r e g e l p e r m e a t i o n chromatograph d a r d s , except t h e r e s u l t g polystyrene standards. C h a i n s c i s s i o n was c a l c u l a t e d as [Mn(unoxidized)/Mn(oxidized)]-l. C a r b o n y l absorbances were determined by I n f r a r e d S p e c t r o s c o p y a t 1710 cm" . 1

R e s u l t s and D i s c u s s i o n O x i d a t i o n Curves a t 140°C, 100°C and 40°C. Samples o f HDPE, u n s t a b i l i z e d o r s t a b i l i z e d w i t h 0.1% o f A01, were o x i d i z e d i n a c l o s e d system w i t h oxygen. The o x i d a t i o n curves a t 1 4 0 ° , 100° and 40°C a r e shown i n F i g u r e s 1-3. These d a t a i n d i c a t e t h e e f f e c t i v e n e s s o f A01 i n p r e v e n t i n g oxygen consumption a t b o t h h i g h and low t e m p e r a t u r e s . Once t h e i n d u c t i o n p e r i o d was passed a t 140°C and 100°C, t h e oxygen consumption r a t e s were v i r t u a l l y t h e same f o r the u n s t a b i l i z e d and s t a b i l i z e d samples. The u n s t a b i l i z e d HDPE consumed oxygen a t s i g n i f i c a n t r a t e s , even a t 40°C, w i t h the i n d u c t i o n p e r i o d l a s t i n g about two y e a r s . The need t o s t a b i l i z e polymers f o r use a t a l l temperat u r e s i s e v i d e n t from t h e s e d a t a . C o r r e l a t i o n o f I n d u c t i o n Times a t 1 4 0 ° , 100° and 40°C. The i n d u c t i o n times o b t a i n e d f o r t h e u n s t a b i l i z e d HDPE a t t h r e e temperatures a r e t a b u l a t e d i n T a b l e I and a l s o p l o t t e d a g a i n s t r e c i p r o c a l a b s o l u t e temperature i n an apparent A r r h e n i u s r e l a t i o n s h i p i n F i g u r e 4.

Table

I.

Stabilizer None 0.1% A01 *95

O x i d a t i o n o f HDPE by Oxygen Uptake. I n f l u e n c e o f Temperature on O x i d a t i v e I n d u c t i o n Time.

I n d u c t i o n Time a t Temperature, Hours 100°C 40°C 140°C 35 15,960* 1 4,700 39,480+** 120

weeks; **235+ weeks (no i n d i c a t i o n o f

degradation)

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

17.

High-Density Polyethylene

HORNG AND KLEMCHUK

E

0.1% A 0 1

1 0 - 1 Unstabilized H D P E

D

9 E

I 50

- I — 150

100

- 1 — 200

Hours

F i g u r e 1. Oxygen Uptake o f S t a b i l i z e d and U n s t a b i l i z e d 140°C and 1 Atmosphere

a>

8 -

HDPE a t

0.1% A 0 1

Unstabilized H D P E

Q.

E

CO co

o)

6—

=> 49 E 2-

1000

2000

3000 Hours

F i g u r e 2. Oxygen Uptake o f S t a b i l i z e d and U n s t a b i l i z e d 100°C and 1 Atmosphere

HDPE a t

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

238

10yrs -105.

1/T (°K), Shown as Degrees Centigrade

F i g u r e 4. A r r h e n i u s P l o t o f S t a b i l i z e d and U n s t a b i l i z e d HDPE D u r i n g O x i d a t i o n , Oxygen Uptake a t 1 Atmosphere and Three Temperatures

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

17.

HORNG AND KLEMCHUK

239

High-Density Polyethylene

It i s i n t e r e s t i n g to note that they f a l l i n t o a l i n e a r r e l a t i o n s h i p w i t h a c a l c u l a t e d a c t i v a t i o n energy o f 23.7 K c a l / m o l e . Literature d a t a showed v a l u e s o f 21-26 K c a l / m o l e . (5,6) T h i s i s one o f t h e few i n s t a n c e s t h a t o x i d a t i v e i n d u c t i o n times from 140°C t o near ambient temperature have been a v a i l a b l e . N o t a b l y , t h e temperature range i n c l u d e s t h e m e l t i n g p o i n t o f t h i s HDPE polymer so t h e range from 140°C to 40°C i n c l u d e s a phase change. In t h i s i n s t a n c e , we d i d not f i n d a discrepancy i n p l o t t i n g above-melting experimental r e s u l t s along with those o b t a i n e d a t much lower t e m p e r a t u r e s . Our f i n d i n g on t h e l i n e a r i t y o f the A r r h e n i u s r e l a t i o n s h i p s u g g e s t s t h a t e x t r a p o l a t i o n o v e r a moderate temperature range i s warranted a t l e a s t f o r u n s t a b i l i z e d HDPE, p r o v i d e d s e v e r a l d a t a p o i n t s a r e a v a i l a b l e . P r o p e r t y C o r r e l a t i o n D u r i n g O x i d a t i o n a t 100°C and 40°C. A s e r i e s of u n s t a b i l i z e d and 0.1% A O l - s t a b i l i z e d HDPE samples were o x i d i z e d at 100°C and removed p e r i o d i c a l l y f o r e v a l u a t i o n . A monotonic change i n the r e t e n t i o n o f e l o n g a t i o i n F i g u r e 5. C a r b o n y l f o r m a t i o c o r r e l a t e w i t h oxygen uptak relationship o f e l o n g a t i o n was found t o c o r r e l a t e w i t h 0.25 c a r b o n y l absorbance o f 5-mil f i l m and z e r o e l o n g a t i o n was found t o c o r r e s p o n d t o 0.5 c a r b o n y l absorbance. O x i d a t i o n t o 6.5 ml-0 /g-HDPE o r 2.1 mmole Oj mmole HDPE d e s t r o y e d e l o n g a t i o n c o m p l e t e l y f o r b o t h u n s t a b i l i z e d and s t a b i l i z e d HDPE. However, t h e time i n v o l v e d f o r such a c a t a s t r o p h i c change was d r a m a t i c a l l y p r o l o n g e d from t h e u n s t a b i l i z e d t o A O l s t a b i l i z e d HDPE; 75 v s . 4700 hours ( F i g u r e 6 ) . At 40°C, o x i d a t i o n took p l a c e a t a r e l a t i v e l y low, but measurable rate. Samples s t a b i l i z e d w i t h 0.1% A01 showed no l o s s i n e l o n g a t i o n r e t e n t i o n i n o v e r f o u r y e a r s as compared t o t h e u n s t a b i l i z e d sample which showed a c a t a s t r o p h i c d e c r e a s e i n e l o n g a t i o n a f t e r t h e 95-week i n d u c t i o n p e r i o d ( T a b l e I I ) .

Table I I .

Change i n E l o n g a t i o n o f HDPE O x i d i z e d a t 40°C

Sample Unstabilized

Stabilized 0.1% A01

with

Time (wks) 0 105 115 120

Oxygen Uptake (ml/g) 0 0.25 0.53 1.0

0 105 150 235

0 0.10 0.12 0.1

Elongation (%) 750 200 165 55 720 690 800 650

The m o l e c u l a r weight d i s t r i b u t i o n change o f t h e u n s t a b i l i z e d HDPE was a l s o m o n i t o r e d and i s p l o t t e d i n F i g u r e 7. At 105 weeks, oxygen consumption o f t h e u n s t a b i l i z e d HDPE was about 0.25 ml/g and t h e sample showed a s l i g h t r e d u c t i o n i n t h e h i g h m o l e c u l a r weight f r a c tion. C o n t i n u i n g o x i d a t i o n showed e v i d e n c e o f more c h a i n s c i s s i o n i n g and l o w e r i n g o f average m o l e c u l a r w e i g h t s . Again, A O l - s t a b i l i z e d

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER

STABILIZATION A N D

DEGRADATION

ml-0 /g-HDPE 2

F i g u r e 5. C o r r e l a t i o n o f P e r c e n t R e t e n t i o n o f E l o n g a t i o n and C a r b o n y l Absorbance t o t h e Degree o f O x i d a t i o n , Oxygen Uptake o f U n s t a b i l i z e d HDPE a t 100°C and 1 Atmosphere

F i g u r e 6. Comparison o f E l o n g a t i o n R e t e n t i o n o f S t a b i l i z e d and U n s t a b i l i z e d HDPE, Oxygen Uptake a t 100°C and 1 Atmosphere

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

17.

High-Density Polyethylene

HORNG AND KLEMCHUK

241

HDPE a f t e r f o u r y e a r s a t t h i s temperature e x h i b i t e d no change i n b o t h e l o n g a t i o n r e t e n t i o n and m o l e c u l a r weight r e t e n t i o n . C h a i n S c i s s i o n i n g and i t s E f f e c t on M o l e c u l a r Weight. Table I I I shows t h e m o l e c u l a r weight d a t a o f t h e u n s t a b i l i z e d HDPE when o x i d i z e d a t 100°C t o d i f f e r e n t oxygen uptake l e v e l s .

Table I I I .

M o l e c u l a r Weight Data and C h a i n S c i s s i o n o f U n s t a b i l i z e d HDPE O x i d i z e d a t 100°C

Oxygen Uptake (ml-0 / -HDPE) 2

Mw

g

0 0.12 2.3 3.8 9.2 11.1 19.1 25.7 S = Average

Mn

151,000 137,000

8,270 6,450

13,80 10,400 8,460 7,080

3,70 3,200 2,670 2,390

Mw/Mn 18.1 21.1

3.2 3.2 3.0

S 0 0.3

1.6 2.1 2.5

Scisssions p e r M o l e c u l e = [Mn(0)/Mn(t)] -1

Weight average m o l e c u l a r weight (Mw) was found t o be more p r o n o u n c e d l y a f f e c t e d a t t h e e a r l y s t a g e o f o x i d a t i o n , w h i l e number average m o l e c u l a r weight (Mn) was a f f e c t e d t o a l e s s e r degree. Based on t h e a v a i l a b l e d a t a , t h e g r e a t e s t change i n m o l e c u l a r weight took p l a c e between 0.12 and 2.3 ml 0 /g HDPE oxygen u p t a k e . In t h a t i n t e r v a l , Mw was reduced 80% bu? Mn o n l y about 15%. C l o s e r examinat i o n o f t h e m o l e c u l a r weight d i s t r i b u t i o n (MWD) c u r v e s ( F i g u r e 8) i n d i c a t e s t h i s was t h e i n t e r v a l where t h e l o s s o f t h e h i g h m o l e c u l a r weight f r a c t i o n was g r e a t e s t and so was t h e f o r m a t i o n o f lower m o l e c u l a r weight s p e c i e s . S t a t i s t i c a l l y speaking, longer chains have g r e a t e r p r o b a b i l i t y f o r o x i d a t i v e a t t a c k and c h a i n r u p t u r e t h a n do s h o r t e r c h a i n s . The m a t h e m a t i c a l moment o f Mw r e f l e c t s a h e a v i e r c o n t r i b u t i o n from h i g h e r m o l e c u l a r weight s p e c i e s t h a n Mn. Theref o r e , t h e s t a t i s t i c a l l y g r e a t e r s c i s s i o n i n g o f l o n g e r c h a i n s had a g r e a t e r impact on changes i n Mw t h a n on Mn. F i g u r e 8 and T a b l e I f u r t h e r i n d i c a t e when samples were o x i d i z e d from 0.12 ml 0^/g HDPE (35 h o u r s ) t o 2.3 ml 0 / HDPE oxygen consumption (40 h o u r s ) , t h e y had passed t h r o u g h t h e i n d u c t i o n p e r i o d and e n t e r e d i n t o t h e a u t o c a t a l y t i c region of oxidation. An e x a m i n a t i o n o f t h e s e q u e n t i a l change o f t h e MWD c u r v e s o f b o t h u n s t a b i l i z e d and s t a b i l i z e d samples ( F i g u r e s 8 and 9) shows o x i d a t i o n up t o about 0.1 ml-0 /g-HDPE had l i t t l e e f f e c t on average m o l e c u l a r weight as compared t o t h e o r i g i n a l . However, i t took about 4300 hours f o r t h e s t a b i l i z e d sample t o r e a c h t h a t p o i n t as opposed t o m e r e l y 35 hours f o r t h e u n s t a b i l i z e d sample. When o x i d a t i o n c o n t i n u e d p a s t t h e i n d u c t i o n p e r i o d , a s i g n i f i c a n t change i n m o l e c u l a r weight became e v i d e n t as a r e s u l t o f c h a i n s c i s s i o n i n g . O x i d a t i o n t h e r e a f t e r was a u t o c a t a l y t i c , and t h e m o l e c u l a r weight r e d u c t i o n was c a t a s t r o p h i c w i t h i n a s h o r t p e r i o d o f t i m e . g

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

Degree of Oxidation ml-Q

•

'

•

oooooo

2X10

6

10

10

6

10

5

2

g-HDPE

Wks

S

0

0

0

o.25

105

0.1

10

4

1 80

3

Molecular Weight

F i g u r e 7. M o l e c u l a r Weight D i s t r i b u t i o n o f O x i d i z e d and U n o x i d i z e d HDPE, Oxygen Uptake o f U n s t a b i l i z e d HDPE a t 40°C and 1 Atmosphere

Degree of Oxidation ml - Q

2x10

6

10

6

10

5

10

4

2

g-HDPE

Hrs

S

0 0.12 2.3 9.2 25.7

0 35 40 60 70

0 0.3 0.5 1.2 2.5

10

3

1 80

Molecular Weight

F i g u r e 8. M o l e c u l a r Weight D i s t r i b u t i o n o f O x i d i z e d and U n o x i d i z e d HDPE, Oxygen Uptake o f U n s t a b i l i z e d HDPE a t 100°C and 1 Atmosphere

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

17.

HORNG AND

KLEMCHUK

243

High-Density Polyethylene

Average c h a i n s c i s s i o n s p e r m o l e c u l e as c a l c u l a t e d from Mn changes a r e p l o t t e d a g a i n s t oxygen uptake i n F i g u r e 10. T h i s g r a p h demonstrates by l i n e a r r e g r e s s i o n t h a t t h e c h a i n s c i s s i o n r a t e o f HDPE was s i m i l a r f o r b o t h t h e s t a b i l i z e d and u n s t a b i l i z e d samples when compared by degree o f o x i d a t i o n i n terms o f oxygen u p t a k e . A g a i n , t h e s t a b i l i z e d samples showed a l o n g i n d u c t i o n p e r i o d d u r i n g which c h a i n s c i s s i o n i n g was q u i t e i n s i g n i f i c a n t . A f t e r the induction p e r i o d , c h a i n s c i s s i o n i n g c o n t i n u e s w i t h i n c r e a s i n g oxygen consumption i n a linear fashion. The e m b r i t t l e m e n t p o i n t , z e r o e l o n g a t i o n , was found t o c o i n c i d e w i t h o n l y 1 and 1.2 average s c i s s i o n s p e r molec u l e f o r s t a b i l i z e d and u n s t a b i l i z e d HDPE, r e s p e c t i v e l y . C h a i n S c i s s i o n i n g and i t s E f f e c t on E l o n g a t i o n . Since a l i n e a r r e l a t i o n s h i p was found between oxygen uptake and c h a i n s c i s s i o n r a t e ( F i g u r e 10) and a l s o between oxygen uptake and r e t e n t i o n o f e l o n g a t i o n ( F i g u r e 5 ) , i t i s o b v i o u s t h a t the c h a i n s c i s s i o n r a t e and r e t e n t i o n of elongation ca ship. Chain s c i s s i o n i n m e c h a n i c a l p r o p e r t i e s , suc elongation per i o d , e l o n g a t i o n d e c r e a s e d r a p i d l y w i t h b o t h time and c h a i n s c i s s i o n ing (Figure 6 ) . O x i d a t i o n o f s e m i c r y s t a l l i n e polymers i s g e n e r a l l y c o n s i d e r e d t o o c c u r w i t h i n t h e amorphous r e g i o n which c a n be t r e a t e d as a boundary phase o f t h e n e i g h b o r i n g c r y s t a l l i n e r e g i o n s . P e t e r l i n ' s model (_7) o f t e n s i l e d e f o r m a t i o n e x p l a i n e d the c o n t r i b u t i o n o f t i e m o l e c u l e s i n the amorphous r e g i o n t o t h e n e c k i n g e l o n g a t i o n o f a s e m i - c r y s t a l l i n e polymer. S i n c e o x i d a t i o n t a k e s p l a c e m o s t l y i n amorphous r e g i o n s , t i e m o l e c u l e s which connect c r y s t a l l i t e s through amorphous r e g i o n s may be s c i s s i o n e d i n t h e o x i d a t i o n p r o c e s s r e s u l t i n g i n a d e c r e a s e o f e l o n g a t i o n and o t h e r p h y s i c a l p r o p e r t i e s . At l a t e r s t a g e s o f o x i d a t i o n when many c h a i n s i n t h e amorphous phase and a l s o a t t h e c r y s t a l l i n e boundary a r e r u p t u r e d , samples e x h i b i t b r i t t l e n e s s upon e x t e r n a l stress. Mechanism o f C h a i n S c i s s i o n i n g and S t a b i l i z a t i o n . A review o f the average number o f c h a i n s c i s s i o n s as a f u n c t i o n o f oxygen consumption shows t h e number o f oxygen m o l e c u l e s consumed p e r c h a i n s c i s s i o n i n c r e a s e d w i t h i n c r e a s i n g oxygen consumption. Beyond t h e i n d u c t i o n p e r i o d , t h e mmoles o f oxygen p e r c h a i n s c i s s i o n i n c r e a s e d r a p i d l y w i t h time; t h e d a t a a r e summarized i n T a b l e IV.

T a b l e IV.

Time (hours) 0 35 40 50 60 66 70

C a l c u l a t e d Oxygen M o l e c u l e s Consumed p e r C h a i n S c i s s i o n D u r i n g O x i d a t i o n o f U n s t a b i l i z e d HDPE a t 100°C Oxygen Uptake A B 0 0 0.12 0.04 2.3 0.7 3.8 1.2 9.2 3.1 19.1 6.3 25.7 8.5

S 0 0.3 0.5 0.8 1.2 2.1 2.5

Oxygen M o l e c u l e s Consumed per Chain S c i s s i o n 0.2 1.6 1.6 2.5 3.0 3.5

A = ml 0 /g HDPE; B = mmole 0 /mmole HDPE.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

244

POLYMER STABILIZATION AND DEGRADATION

Degree of Oxidation ml-Q

2

g-HDPE

Hrs

S

Molecular Weight

F i g u r e 9. M o l e c u l a r Weight D i s t r i b u t i o n o f O x i d i z e d and U n o x i d i z e d HDPE, Oxygen Uptake o f 0.1% A O l - S t a b i l i z e d HDPE a t 100°C and 1 Atmosphere

— stabilized with . 1 % A O 1 .--unstabilized

Oxygen Uptake, m l - 0 / g - H D P E 2

F i g u r e 10. Comparison o f C h a i n S c i s s i o n Rates i n S t a b i l i z e d and U n s t a b i l i z e d HDPE, O x i d a t i o n by Oxygen Uptake a t 100°C and 1 Atmosphere

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

17. HORNG AND KLEMCHUK

High-Density Polyethylene

245

T h i s r e s u l t i s i n agreement w i t h those r e p o r t e d by M. I r i n g , e t a l (2) f o r p o l y p r o p y l e n e and p o l y e t h y l e n e . The oxygen consumption d a t a i n d i c a t e t h e o x i d a t i o n o f p o l y e t h y l e n e c o n s i s t s o f a complex group o f r e a c t i o n s beyond t h e i n d u c t i o n p e r i o d w i t h no s i n g l e , simple r e l a t i o n s h i p between t h e number o f oxygen m o l e c u l e s consumed and t h e number o f c h a i n s c i s s i o n s . Using t h e d a t a o f C h i e n (8), we c a l c u l a t e d t h e h a l f - l i f e o f p o l y e t h y l e n e h y d r o p e r o x i d e t o be 6.4 hours a t 100°C. Thus, s i n c e our s h o r t e s t i n d u c t i o n p e r i o d a t 100°C was over 35 h o u r s , i t i s r e a s o n a b l e t o p o s t u l a t e t h e s c i s s i o n i n g o f p o l y e t h y l e n e r e s u l t s from unimolecular homolytic d i s s o c i a t i o n of polyethylene hydroperoxide, a major o x i d a t i o n p r o d u c t : #

ROOH »

» R 0 + HO* > R'CHO + R"

R 0

#

Chain s c i s s i o n i n g o f p o l y e t h y l e n a t e r m i n a l aldehyde and Both p r o d u c t s w i l l r e a c oxygen aldehyd o x i d i z e d r e a d i l y to the p e r a c i d v i a a c h a i n r e a c t i o n : 0 R'CHO + R 0 > R'CO*---—> R'CO -----> R'CO + R* 00* OOH #

2

The p o l y e t h y l e n e r a d i c a l would be expected t o r e a c t r e a d i l y w i t h oxygen and c o n t r i b u t e t o o x i d a t i v e p r o p a g a t i o n as another peroxy radical: RH

R"

#

+ 0

2

> R»0

# 2

> R"0 H 2

+

R

#

Since t h i s r e s u l t a n t hydroperoxide i s a terminal hydroperoxide, i t s decomposition w i l l not r e s u l t i n c h a i n s c i s s i o n i n g . Assuming each r e a c t i o n t o be 100% e f f i c i e n t , i . e . , h y d r o p e r o x i d e decomposition t o a l k o x y l r a d i c a l ; a l k o x y l r a d i c a l c h a i n s c i s s i o n t o aldehyde and s h o r t e r p o l y e t h y l e n e r a d i c a l ; aldehyde o x i d a t i o n t o p e r a c i d ; and oxygen r e a c t i o n w i t h t h e p r i m a r y p o l y e t h y l e n e r a d i c a l t o y i e l d a peroxy r a d i c a l ; t h e maximum number o f oxygen m o l e c u l e s a s s o c i a t e d w i t h each c h a i n s c i s s i o n would be t h r e e . Not s u p r i s i n g l y , the o x i d a t i o n p r o c e s s i s t o o complex t o have shown a s i n g l e r e l a t i o n s h i p between oxygen m o l e c u l e s consumed and t h e number o f c h a i n scissions. A t t h e time o f e m b r i t t l e m e n t , about 2 m o l e c u l e s o f oxygen had been consumed p e r c h a i n s c i s s i o n and i n c r e a s e d beyond t h i s p o i n t . The i n c r e a s e i n t h e number o f oxygen m o l e c u l e s p e r c h a i n s c i s s i o n beyond t h e i n d u c t i o n p e r i o d i s n o t s u r p r i s i n g s i n c e i n t h e u l t i m a t e o x i d a t i o n o f p o l y e t h y l e n e t o c a r b o n d i o x i d e and w a t e r , 1.5 m o l e c u l e s o f oxygen would be r e q u i r e d f o r each methylene group. T h i s means a polymer w i t h an i n i t i a l number average m o l e c u l a r weight o f 8,270 would r e q u i r e 886 m o l e c u l e s o f oxygen f o r complete c o n v e r s i o n t o c a r b o n d i o x i d e and water. The a d d i t i o n o f t h e p h e n o l i c a n t i o x i d a n t A01 t o t h e polymer demonstrated a l o n g i n d u c t i o n p e r i o d ( T a b l e I ) d u r i n g which c h a i n s c i s s i o n i n g and p h y s i c a l p r o p e r t y changes were n e g l i g i b l e . The e q u a l l y complex s t a b i l i z a t i o n c h e m i s t r y (9) f o r p h e n o l i c a n t i o x i d a n t s can be summarized by t h e t r a p p i n g o f p e r o x y and a l k o x y r a d i c a l s and a l s o by t h e d e c o m p o s i t i o n o f h y d r o p e r o x i d e w i t h phenols and i t s

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

246

transformation products. Figure 7 at 40°C oxidation and Figure 9 at 100°C oxidation showed the MWD was almost unchanged during the induction period of the stabilized polymer. The oxidative chain reaction leading to chain scissioning as discussed above, is interrupted at the expense of A01 which is nearly consumed at the end of the induction period. Beyond the induction period, oxidation continues uninhibitedly similar to the unstabilized undergoing oxidation at the beginning. Conclusions 1.

During the induction period, the stabilized and unstabilized samples underwent little chain scissioning and loss of elongation. When the induction period was passed, both stabilized and unstabilized samples exhibited considerable chain scissioning and loss of elongation during this uninhibited period of oxidation. 2. The time difference i th inductio period fo AOl-stabilized and unstabilized HDP and 4700 versus 35 respectively 3. At 40°C the oxidation rate was slow but measurable for unstabilized HDPE. The stabilization provided by AOl for more than four years was evident from the oxygen uptake, elongation and molecular weight data. 4. The Arrhenius plot of induction period vs. temperatures ranging from 140°C to 40°C for unstabilized HDPE suggests extrapolation is permissable provided several data points are available. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Chan, M., Proc. Int. Wire Cable Symp. 23rd, 1974, pp. 34-41. Iring, M.; Laszlo-Hedvig, S.; Barabas, K.; Kelen, T.; Tudos, F., Eur. Polym. J., 1978, 14, 439-442. Holmstrom A., in "Durability of Macromolecular Materials"; Eby, R. K., Ed.; ACS Symp. Ser. No. 95, ACS: Washington, D.C., 1979; Chap. 4. Holmstrom, A.; Sorvik, E. M., J. Polym. Sci. Polym. Chem. Ed., 1978, 16, 2555-86. Hawkins, W. L.; Matreyek, W.; Winslow, E. H., J. Polym Sci., 1959, 41, 1-11. Grieveson, B. M.; Howard, R. N.; Wright, B., in "Thermal Degradation of Polymers"; Society of Chemical Industry: London, 1961; pp. 413-20. Peterlin, A., Int. J. Polym. Mater., 1980, 8, 285-301. Chien, J. C. W., J. Polym. Sci., 1968, 6, 375-379. Pospisil, J. in "Development in Polymer Stabilization - I"; Scott, G. Ed.; Applied Science; London, 1979; Chap. 1.

RECEIVED October 26, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18 Bis- and Trisphosphites Having Dioxaphosphepin and Dioxaphosphocin Rings as Polyolefin-Processing Stabilizers J O H N D. SPIVACK, S T E P H E N D. PASTOR, A M B E L A L PATEL, and L E A N D E R P. STEINHUEBEL CIBA-GEIGY Corporation, Ardsley, NY 10502

In a previous paper, the effectiveness of certain hindered mono phosphites and phosphonites as processing stabilizers for polyolefins(1) was discussed. The present paper describes several new classes of hindered phenyl bis- and tris-phosphites having dibenzo[d,f][1,3,2]dioxaphosphepin and dibenzo[d,g][1,3,2]dioxaphosphocin rings. The latter compounds exhibit superior effectiveness as processing stabilizers together with greater resistance to discoloration and hydrolysis. The di- and tri-alkanolamine esters are of particular interest because of their even greater resistance to hydrolysis at 50°C for extended time periods previously achievable only in the case of certain di-hindered phenyl phosphonites. Processing stabilizers are a special class of antioxidants used to inhibit polymer degradation during the processing steps subsequent to polymerization such as extrusion, injection molding, spinning, etc. These steps are carried out at relatively high temperatures (220-320°C) in the presence of some entrapped oxygen and the resulting radical species. Attempts to counteract degradation by the use of 2,6-di-tertbutyl-4-methylphenol (BHT) and various organophosphorus compounds such as tris-nonylphenyl phosphite (_1) , 3 ,9-dioctadecyloxy-2 ,4,8,10tetraoxa-3,9-diphospha[5.5]-spiroundecane (2) and the 3,9 bis(2,4di-tert-butylphenyl) analog (3) have only been partially successful. BHT is volatile at high temperatures and contributes to discoloration during processing. 1, J2, and can undergo hydrolysis and loss of processing stabilizer effectiveness i f stored improperly. Products of hydrolysis can lead to problems with extrusion and spinning as well as contamination of extrudates with hydrolysis products. 0097-6156/85/0280-0247$06.00/0 © 1985 American Chemical Society

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

248

O-CHo C H 9

1 9

•-0--P

CHo-0

RO-P

P-OR

3

2a

2

R = n-C

1 8

H

2 7

A g e n e r a l l y a c c e p t e d mechanism f o r t h e o x i d a t i o n o f p o l y o l e f i n s , RH, i n t h e p r e s e n c e o f a n t i o x i d a n t s o f t h e hydrogen-atom donor t y p e , AH, i n v o l v e s i n i t i a t i o n , p r o p a g a t i o n and t e r m i n a t i o n s t e p s (2). In t h e i n i t i a t i o n s t e p , t h e p o l y o l e f i n , RH, i s c o n v e r t e d t o the p o l y m e r i c r a d i c a l , R», w h i l e t h e p o l y m e r i c peroxy r a d i c a l , ROO*, i s formed i n t h oxygen. I n t h e absenc r a d i c a l i s c o n v e r t e d t o t h e h y d r o p e r o x i d e , ROOH, b y hydrogenatom a b s t r a c t i o n from t h e polymer c h a i n g i v i n g r i s e t o another p o l y m e r i c r a d i c a l , R» , and peroxy r a d i c a l ROO*. P r o p a g a t i o n v i a c h a i n t r a n s f e r i s a l s o promoted b y h o m o l y s i s o f ROOH t o R0» and »0H i n t h e absence o r d e p l e t i o n o f t h e a n t i o x i d a n t AH. C h a i n t r a n s f e r w i l l take p l a c e t o an i n s i g n i f i c a n t degree i f A* i s a s t a b l e f r e e r a d i c a l such as i s p r o v i d e d b y a h i n d e r e d phenol as i s shown i n F i g u r e 1. A s i m p l i f i e d v e r s i o n o f t h e r e a c t i o n sequence i n t h e p r e s e n c e o f h i n d e r e d phenols (AH) and a t e r t i a r y phosphorus ( i l l ) compound i s shown i n F i g u r e 1.

I t i s thus apparent t h a t s e l e c t e d a n t i o x i d a n t s , AH, i n t h e p r e s e n t case h i n d e r e d p h e n o l i c s , and h y d r o p e r o x i d e decomposers, such as PR ^, a c t s y n e r g i s t i c a l l y t o i n h i b i t r a d i c a l i n i t i a t e d polymerchain oxidations. The two c l a s s e s o f h i n d e r e d s u b s t i t u t e d b i s - and t r i s - phosp h i t e s having e i t h e r (a) dibenzo[d,f][1,3,2]dioxaphosphepin or (b) d i b e n z o [ d , g ] [ 1 , 2 , 3 ] d i o x a p h o s p h o c i n r i n g s were s e l e c t e d f o r s t u d y as p r o c e s s i n g s t a b i l i z e r s because t h e i r b i c y c l i c s t r u c t u r e s p r o mised s t a b i l i z e r s o f i n c r e a s e d thermal and h y d r o l y t i c s t a b i l i t y compared t o t h e a c y c l i c h i n d e r e d p h e n y l p h o s p h o n i t e s p r e v i o u s l y studied ( 1 ) . 11

The s p e c i f i c compounds s u b m i t t e d shown i n F i g u r e 2.

f o r comparative

studies are

Experimental Section E v a l u a t i o n o f E x p e r i m e n t a l Organophosphorus E s t e r s i n P o l y o l e f i n s . The compounds were t e s t e d i n t h e f o l l o w i n g p o l y o l e f i n s as l i s t e d below, t h e r e s u l t s b e i n g shown i n t h e T a b l e s i n p a r e n t h e s i s : P o l y p r o p y l e n e ( T a b l e I , compounds 4^ t o ^9 i n c l u s i v e ) . High M o l e c u l e r Weight High D e n s i t y P o l y e t h y l e n e .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18.

SPIVACK ET AL.

Bis- and Trisphosphites

249

Initiation Initiation

> R* l i g h t , heat metal c a t a l y s t s , e t c .

RH

->

ROO*

fast

RH ROO-

->

ROOH + R-

Chain T r a n s f e r

ROOH

->

RO» + -OH

Propagation

ROO-

->

Molecular Products

. . Termination

ROOH

R

1

QOR (Chain ITransfer

RO*

+

0=PR" .

ROH

+

0=PR" ..Termination

3

However: RO \ ROOH

+

PR"

3

/

PR"
6. and _9 a r e about e q u a l l y e f f e c t i v e when t e s t e d s i m i l a r l y w i t h AO-1.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

SPIVACK ET AL.

255

Bis- and Trisphosphites

L i n e a r - L o w D e n s i t y P o l y e t h y l e n e (LLDPE) and E t h y l e n e P r o p y l e n e Diene (EPDM) S t a b i l i z a t i o n . Prevention of d i s c o l o r a t ion o f LLDPE and EPDM are prime o b j e c t i v e s i n a d d i t i o n t o t h e r m a l stabilization. While t h e r m a l s t a b i l i z a t i o n i s r e a d i l y a c h i e v e d by the use o f p h e n o l i c a n t i o x i d a n t s , p r e v e n t i o n o f d i s c o l o r a t i o n r e q u i r e s a c o - a d d i t i v e such as a p h o s p h i t e . TABLE I I I PREVENTION OF DISCOLORATION L-LDPE at 90"C 0.02% AO-1 + Lba 0.02% C o - A d d i t i v e Initial AO-1 a l o n e 0.60 2 -2.70 £ -2.80 £ -3.70 Note:

(a) (b)

OF Lba A f t e r 6 Weeks 4.50 0.70 0.60 0.00

Dow grad Lba C o l o r .

I t i s apparent from the d a t a o f T a b l e I I I t h a t w h i l e a l l t h r e e compounds ^2* * J$> are e f f e c t i v e i n p r e v e n t i n g d i s c o l o r a t i o n i n i t i a l l y , the most e f f e c t i v e i n c o u n t e r a c t i n g the c o l o r c o n t r i b u t e d by AO-1 i s compound J3. S i m i l a r l y , compounds 2, and are most p r o f i c i e n t i n p r e v e n t i n g y e l l o w i n g o f EPDM plaques c o n t a i n i n g AO-2 kept at 90°C at 100% R.H. f o r 20 days ( T a b l e I V ) . a n c

TABLE IV PREVENTION OF DISCOLORATION OF EPDM AT 9 0 C and 100% R.H. Gardner C o l o r Antioxidant phr Initial 13 Days Blank 3 4 AO-2 0.05 3 5 AO-2 + J. 0.05 + 0.05 3 5 AO-2 + 2 0.05 + 0.05 3 3 AO-2 + £ 0.05 + 0.05 3 5 AO-2 + 8 0.05 + 0.05 3 4 Note: (1) v e r y y e l l o w W

Hydrolytic

S t a b i l i t y of Experimental

Processing

20

Days 4

5 5 4, 6 4 K

n

'

Stabilizers

Compounds j4 and J>, two o f the e x p e r i m e n t a l p r o c e s s i n g s t a b i l i z e r s l i s t e d i n F i g u r e 2, remained e s s e n t i a l l y unchanged when exposed neat t o an a i r atmosphere o f 80% r e l a t i v e h u m i d i t y at room tempera t u r e f o r 50 d a y s . T h i s i s i n c o n t r a s t t o some o f the commercial compounds such as 1, 2, and 3 which h y d r o l y z e r a p i d l y under the same c o n d i t i o n s ( T a b l e V ) . H y d r o l y s i s t e s t s at 50°C/80% r e l a t i v e h u m i d i t y show t h a t compounds 4 t o 9 i n c l u s i v e a r e much more r e s i s t a n t t o h y d r o l y s i s t h a n _3 ( T a b l e V I ) . Of p a r t i c u l a r i n t e r e s t i n t h i s r e s p e c t i s the d r a m a t i c i n c r e a s e i n h y d r o l y t i c s t a b i l i t y o f compounds ^6 and _9 a l l o f which have t e r t i a r y amino f u n c t i o n s i n t h e i r phosphite s t r u c t u r e s capable of p r e f e r e n t i a l l y n e u t r a l i z i n g

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

256

p r o t o n s from a c i d i c m o i e t i e s . The p r o t o n a t i o n o f t r i v a l e n t phosphorous e s t e r s , which i s a p r e c u r s o r t o h y d r o l y s i s , i s a v o i d e d . Such p r o t o n a t i o n has been i m p l i c a t e d i n t h e c l e a v a g e o f t r i a l k y l p h o s p h i t e s ( 8 ) , m o n o c y c l i c p h o s p h i t e e s t e r s (9) as w e l l as phosphonite esters ( 1 0 ) .

TABLE V HYDROLYTIC STABILITY AT ROOM TEMPERATURE ( c a 25°C) - 80% RELATIVE HUMIDITY HYDROLYSIS TIME % DAYS 100

i

4

(D

1

>4r

1 8 2 Note: General

u

>62 >62 (1) S l i g h t i n c i p i e n t h y d r o l y s i s

Conclusions

I t i s apparent t h a t o f t h e compounds s t u d i e d h e r e i n t h e h i n d e r e d s u b s t i t u t e d b i s - and t r i s - p h o s p h i t e s h a v i n g d i b e n z o [ d , f ] [ 1 , 3 , 2 ] d i o x a p h o s p h e p i n r i n g s , a r e t h e most e f f e c t i v e compounds i n t h e s p e c i f i c p o l y o l e f i n substrates tested. Of p a r t i c u l a r i n t e r e s t i n t h i s r e g a r d i s t h e t r i e t h a n o l a m i n e t r i s - p h o s p h i t e compound J5. Acknowledgment s The a s s i s t a n c e o f M e s s r s . Thomas M. C h u c t a and P e t e r W. Stewart i s g r a t e f u l l y acknowledged f o r t h e i r h e l p i n p r o v i d i n g some o f the e v a l u a t i o n d a t a p r e s e n t e d h e r e i n . The a u t h o r s w i s h t o thank V i c t o r i a R i v e r a and T e r e s a S c h a e f f e r f o r p r e p a r a t i o n o f t h e manuscript.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

18. SPIVACK ET AL.

Bis- and Trisphosphites

257

Literature Cited 1.

Previous Paper, Spivack, J. D.; Patel, A.; and Steinhuebel, L. P."Phosphorus Chemistry" - ACS Symposium 171 1981, pp. 351-354. 2. For Example, Chapter 12, Howard, J. A., in "Free Radicals Vol. II", Kochi, Jay K.; Ed. John Wiley, N.Y. 1973 3. Spivack, J. D., U.S. 4,233,207, 1980; Chem. Abstr., 94, 6670y. 4. (a) Spivack, J. D., U.S. 4,288,391, 1981; Chem. Abstr., 96, 7586m. (b) U.S. 4,351,759, 1982; Chem. Abstr. 97, 199111t. (c) Odorisio, P. A.; Pastor, S. D.; Spivack, J.D.; Bini, D.; Rodebaugh, R. K. Phosphorus and Sulfur, 1984, 19, 285 5. (a) Spivack, J. D.; Dexter M.; Pastor S D. U.S 4,318,845 1982; Chem. Abstr. (b) Odorisio, P. A. and Sulfur 1984, 19, 1. 6. ASTM METHOD 1238 Condition L 7. ASTM METHOD D1925-70T 8. Hudson, H. R.; Roberts, J.C., J. Chem. Soc. Perkin II, 1974, 1575-1580. 9. Weiss, R.; Vande Griend, L.J.;Verkade, J.G, J. Org. Chem. 1979, 44, 1860-1863. 10. Hudson, H.R.; Kow, A.; and Roberts, J.C. Phosphorus and Sulfur 1984, 19, 375-378. RECEIVED January 31, 1985

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19 Formation of Anomalous Structures in Poly(vinyl chloride) and Their Influence on the Thermal Stability Effect of Polymerization Temperature and Pressure T H O M A S H J E R T B E R G and ERLING M. SORVIK The Polymer Group, Department of Polymer Technology, Chalmers University of Technology, S-412 96 Goteborg, Sweden

Vinyl chloride was polymerize conditions in the temperature range of 45-80°C. The ratio between the monomer pressure (P) and the saturation pressure (P ) varied from 0.53 to 0.97. The rate of dehydrochlorination decreased with decreasing values of P/P and showed a minimum at 55°C. To explain this behavior, the content of structures with thermally labile chlorine was determined: internal double bonds by ozonolysis and the branching structure by C-NMR after reduction with tributyltin hydride. The content of tertiary chlorine was determined as the total content of ethyl, butyl and long chain branches. The anomalous structures are formed by different inter- and intramolecular transfer reactions and generally, their content increased with decreasing monomer concentration and increasing temperature in accordance with the proposed mechanisms. Instead, the content of internal double bonds decreased with increasing temperature. This is suggested to be due to a decreased tendency of chlorine atoms to selectively attack methylene groups at increased temperatures. If the content of labile chlorine is calculated as the sum of tertiary and internal allylic chlorine, a good relation is obtained between rate of dehydrochlorination and labile chlorine. Extrapolation to zero content of labile chlorine supports the existence of random dehydrochlorination. In all samples, the content of tertiary chlorine is considerably higher than the content of internal allylic chlorine. It is obvious that tertiary chlorine contributes most to the instability of PVC, while the contribution from internal allylic chlorine is of the same order as that of random dehydrochlorination. o

Q

The low thermal stability of poly(vinyl chloride) (PVC) has been ascribed to the pressure of low amounts of anomalous structures with NOTE: This chapter is part 4 in a series. 0097-6156/85/0280-O259$07.50/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

260

more l a b i l e c a r b o n - c h l o r i n e bonds than t h o s e i n the nominal s t r u c t u r e (1-3). There has been c o n s i d e r a b l e c o n t r o v e r s y c o n c e r n i n g the exact n a t u r e and c o n c e n t r a t i o n o f t h e s e anomalous s t r u c t u r e s i n the p o l y mer. However, t h e advancements a c h i e v e d w i t h d i f f e r e n t a n a l y t i c a l t e c h n i q u e s d u r i n g t h e l a s t y e a r s have l e d t o a s u b s t a n t i a l i n c r e a s e i n o u r knowledge o f d i f f e r e n t anomalous s t r u c t u r e s i n PVC (see e . g . r e f s . 4-9). In our work we have used the p o l y m e r i z a t i o n o f v i n y l c h l o r i d e at p r e s s u r e s (P) below t h e s a t u r a t i o n v a l u e ( P ) as a way t o produce polymers, sufosaturation PVC (U-PVC), w i t h i n c r e a s e d amounts o f d e f e c t s . T h i s system i s a l s o a model f o r the l a t e r s t a g e s i n a c o n v e n t i o n a l b a t c h p o l y m e r i z a t i o n o f v i n y l c h l o r i d e , i . e . a f t e r the p r e s s u r e d r o p . With d e c r e a s i n g r e l a t i v e monomer p r e s s u r e , P/P , t h e t h e r m a l s t a b i l i t y o f PVC d e t e r i o r a t e s s t r o n g l y (6-8. l 8 ) . In a s e r i e s o f i n v e s t i g a t i o n s we have determined d i f f e r e n t s t r u c t u r e s i n s e v e r a l U-PVC samples and, as a r e f e r e n c e , i n a s e r i e s o f f r a c t i o n s o f a commercial s u s p e n s i o n PV Using H-NMR we c o u l end groups i n o r d i n a r y PV ( 6 ) y r e d u c t i v e l y d e h a l o g e n a t e d PVC, Bovey e t a l . (4.5) had e a r l i e r shown t h a t 3 i s the d o m i n a t i n g s h o r t c h a i n s t r u c t u r e on PVC. Q

~CH -CH=CH-CH 2

Cl 1

2

~CH -CH-CH 2

2

Cl Cl 2

~

CH-CH-CH '—' 2

C l CH C1 2

3

These f i n d i n g s as w e l l as o t h e r i n v e s t i g a t i o n s (see e.g. r e f s . p 12-14) have r e s u l t e d i n a c o m p l e t e l y new v i e w o f the mechanism f o r c h a i n t r a n s f e r t o monomer: an o c c a s i o n a l head-to-head a d d i t i o n i s f o l l o w e d by 1 , 2 - C l - m i g r a t i o n and subsequent e l i m i n a t i o n o f a c h l o r i n e atom r e s u l t s i n s t r u c t u r e 1 w h e r e a f t e r 2 i s formed by a d d i t i o n o f t h e c h l o r i n e atom t o a monomer m o l e c u l e . I f propagation occurs before the e l i m i n a t i o n o f c h l o r i n e , a c h l o r o m e t h y l branch, 3, i s formed. However, a comparison o f PVC w i t h d i f f e r e n t amounts o f 1 and 2 i n d i c a t e d t h a t n e i t h e r 1 n o r 2 has a major i n f l u e n c e on the t h e r m a l s t a b i l i t y o f PVC. In a r e c e n t paper, v a n den Heuvel and Weber (JLi) showed t h a ^ t h e s e s t r u c t u r e s a r e s t a b l e a t 180°C. The C-NMR s p e c t r a o f U-PVC reduced w i t h t r i b u t y l t i n h y d r i d e (Bu^SnH) showed t h a t PVC c o n t a i n s b u t y l and l o n g c h a i n branches t o an i n c r e a s i n g e x t e n t a t d e c r e a s i n g v a l u e s o f P/P (Jj . F o r samples o b t a i n e d a t P/P = 0 . 6 c o n t e n t s o f about 3-4 and°l-1.5 p e r 1000 monomer u n i t s (lOOO VC), r e s p e c t i v e l y , were d e t e r m i n e d . In t h e f r a c t i o n s o f o r d i n a r y PVC v a l u e s between 0.5 and 1 p e r 1000 VC were found f o r the combined c o n t e n t o f the two branch s t r u c t u r e s . R e d u c t i o n s w i t h t r i b u t y l t i n d e u t e r i d e (Bu^SnD) showed t h a t t h e m i c r o s t r u c t u r e o f the b u t y l branches i s 2 , 4 - d i c h l o r o b u t y l w i t h a c h l o r i n e a t t a c h e d t o the branch c a r b o n , i . e . a t e r t i a r y c h l o r i n e . It was a l s o shown t h a t the major p a r t o f the l o n g c h a i n branches (LCB) a l s o c o n t a i n t e r t i a r y c h l o r i n e but the p r e s e n c e o f hydrogen a t t h e LCB branch c a r b o n c o u l d not be e x c l u d e d . T

By f o l l o w i n g the changes i n M due t o o x i d a t i v e c l e a v a g e o f a l l d o u b l e bonds, we c o u l d a l s o show U-PVC c o n t a i n s i n c r e a s e d amounts o f i n t e r n a l d o u b l e bonds (jD. In the S-PVC f r a c t i o n s o n l y

tEat

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HJERTBERG AND SORVIK 0.05-0.3 i n t e r n a l double

Stabilization of Polyvinyl chloride) bonds p e r

1000

261

VC were found w h i l e

the

c o n t e n t had i n c r e a s e d t o about 1 i n samples o b t a i n e d at P/P - 0.6. F o r m a t i o n o f b u t y l branches t a k e s p l a c e by a b a c k - b i t i n g mechanism v i a a six-membered r i n g , w h i l e t r a n s f e r t o polymer from m a c r o r a d i c a l s i s a r e a s o n a b l e s o u r c e t o LCB w i t h t e r t i a r y c h l o r i n e (7, 9 ) . We have suggested an a l t e r n a t i v e mechanism which a l s o e x p l a i n s t h e f o r m a t i o n o f i n t e r n a l double bonds and LCB w i t h t e r t i a r y hydrogen ( 7 , 8 ) . T h i s mechanism i s based on t r a n s f e r t o polymer from c h l o r i n e atoms produced i n t h e mechanism f o r t r a n s f e r t o monomer: Q

~-CH -CH-CH -CH~ o

Cl

2

-\ ci

~

2

VC

VC

0

\

CH

/

Cl

\

-Cl'

H

CH -C-CH 2

CH-CH-CH Cl

Cl

Cl

Cl-

Cl

/ CH -C-CH

+

0

0 Z

~

—

CH-C -

CH ~

i

l

Cl 2

^

l

—

CH=CH-CH

CH C l V 2 n

1

c

l

The r a t e o f d e h y d r o c h l o r i n a t i o n i n n i t r o g e n at 190°C showed l i n e a r r e l a t i o n s t o t h e c o n t e n t o f both t e r t i a r y c h l o r i n e and i n t e r n a l d o u b l e bonds ( c o r r e l a t i o n and c o e f f i c i e n t s 0.97 and 0.88, respectively). With r e f e r e n c e t o t h e b e t t e r c o r r e l a t i o n o b t a i n e d w i t h t e r t i a r y c h l o r i n e and t o t h e f a c t t h a t i t s c o n c e n t r a t i o n i s r o u g h l y 5 times h i g h e r t h a n t h a t o f i n t e r n a l d o u b l e bonds, we c o n s i d e r e d t e r t i a r y c h l o r i n e t o be t h e most i m p o r t a n t l a b i l e s t r u c t u r e i n PVC ( 7 . 8 ) . The r e s u l t s d i s c u s s e d so f a r were o b t a i n e d w i t h polymers p r e p a r e d at 55°C. We have now extended t h i s i n v e s t i g a t i o n t o U-PVC prepared at d i f f e r e n t temperatures. We r e p o r t h e r e i n the t h e r m a l s t a b i l i t y o f U-PVC o b t a i n e d i n the temperature range 45-80°C and w i t h the r e l a t i v e monomer p r e s s u r e P/P r a n g i n g from 0.61 t o 0.97. The s t r u c t u r a l c h a r a c t e r i z a t i o n i n c l u d e s : m o l e c u l a r weight by G P C / v i s c o m e t r y , branches by C-NMR measurements on samples reduced w i t h Bu^SnH and i n t e r n a l double bonds by o z o n o l y s i s . Experimental Polymers. The samples o f s u b s a t u r a t i o n PVC were o b t a i n e d by the p o l y m e r i z a t i o n t e c h n i q u e d e s c r i b e d e a r l i e r (10. 16). To m a i n t a i n a c o n s t a n t monomer p r e s s u r e , v i n y l c h l o r i d e was c o n t i n u o u s l y charged as

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

262

vapor from a s t o r a g e v e s s e l kept at lower temperature than t h e reactor. D i s t i l l e d water (2 1) was used as suspending medium, ammonium p e r o x y d i s u l p h a t e (0.5 g) as i n i t i a t o r and u n c o a g u l a t e d e m u l s i o n PVC l a t e x as seed (100 g, d r y c o n t e n t 25%, p r i m a r y p a r t i c l e d i a m e t e r 0.03 ^m). V i n y l c h l o r i d e o f p o l y m e r i z a t i o n grade was k i n d l y s u p p l i e d by KemaNord AB, Sweden. To ensure t h a t the p o l y m e r i z a t i o n s were not i n f l u e n c e d by d i f f u s i o n , a s u f f i c i e n t a g i t a t i o n was used ( i m p e l l e r , 1500 rpm). F u r t h e r m o r e , t h e p o l y m e r i z a t i o n s were stopped b e f o r e any agglomera t i o n of the primary p a r t i c l e s o c c u r r e d . Insufficient agitation and p a r t i c l e a g g l o m e r a t i o n would g i v e a d i f f u s i o n c o n t r o l l e d system w i t h d e c r e a s e d monomer c o n c e n t r a t i o n i n the polymer g e l and c o r r e s pondingly h i g h e r content of d e f e c t s (17). The U-PVC samples s t u d i e d e a r l i e r (6-8. 11) were o b t a i n e d under c o n d i t i o n s which p a r t l y r e sulted i n d i f f u s i o n control. The p o l y m e r i z a t i o n temperatures and p r e s s u r e s are g i v e n i n T a b l e 1 t o g e t h e r w i t h the m o l e c u l a r weight data. M o l e c u l a r Weight D i s t r i b u t i o n . Gel chromatography (GPC) and v i s c o m e t r y were used f o r d e t e r m i n a t i o n o f m o l e c u l a r weight d i s t r i b u t i o n (MWD). D e t a i l s o f t h e GPC a n a l y s i s and v i s c o m e t r y measurements have been g i v e n e a r l i e r (18) . I n t r i n s i c v i s c o s i t y was determined i n t e t r a h y d r o f u r a n at 25°C w i t h an Ubbelohde v i s c o m e t e r . No c o r r e c t i o n f o r k i n e t i c energy l o s s e s was n e c e s s a r y . A Waters A s s o c i a t e s GPC Model 200 o p e r a t i n g at 25°C w i t h t e t r a h y d r o f u r a n as s o l v e n t was used The column c o m b i n a t i o n o f f ^ v e S t y r a g e l columns w i t h p e r m e a b i l i t i e s r a n g i n g from 10 t o 10 A, g i v i n g good s e p a r a t i o n i n t h e m o l e c u l a r weight range o f i n t e r e s t . To c a l c u l a t e MWD and m o l e c u l a r weight averages the computer program d e v i s e d by D r o t t and Mendelson (19) was used, assuming t r i f u n c t i o n a l l o n g c h a i n branch points. The program c o r r e c t s the MWD and LCB and g i v e s a measure o f the c o n t e n t o f LCB. The c a l i b r a t i o n f o r l i n e a r PVC was o b t a i n e d v i a the u n i v e r s a l c a l i b r a t i o n c u r v e as d e s c r i b e d e a r l i e r ( 1 8 ) .

consisted

Thermal S t a b i l i t y . Thermal s t a b i l i t y was measured by dehydrochlorination. E x p e r i m e n t a l d e t a i l s have been g i v e n e a r l i e r (20). Bulk samples (100 mg) were t r e a t e d at 190°C i n pure n i t r o g e n atmosphere and HC1 was measured by conductometry. The r a t e o f d e h y d r o c h l o r i n a t i o n i s expresed as e v o l v e d HC1 ( i n p e r c e n t o f the t h e o r e t i c a l amount) per m i n u t e . The l i n e a r p a r t o f t h e c o n v e r s i o n c u r v e between 0.1 and 0.3% was used. D e t e r m i n a t i o n o f Branches. R e d u c t i v e d e c h l o r i n a t i o n s were performed w i t h Bu~SnH as r e d u c i n g agent. We have m o d i f i d the o r i g i n a l twos t e p method g i v e n by S t a r n e s e t . a l . (21) t o a o n e - s t e p method ( 2 2 ) . To a v o i d p r e c i p i t a t i o n , a m i x t u r e o f t e t r a h y d r o f u r a n and x y l e n e i s used as s o l v e n t . At h i g h c o n v e r s i o n , t h e c o n c e n t r a t i o n o f x y l e n e and t h e temperature i s i n c r e a s e d . By t h i s p r o c e d u r e , a c h l o r i n e c o n t e n t l e s s t h a n 0.1% i s o b t a i n e d i n 6 h . The e x p e r i m e n t a l d e t a i l s are as p r e v i o u s l y given^CZ^) . Proton-decoupled C-NMR s p e c t r a were o b t a i n e d w i t h a V a r i a n XL-200 s p e c t r o m e t e r equipped w i t h a " H i g h - s e n s probe. Free i n d u c t i o n decays w i t h s p e c t r a windows o f 8000 Hz were s t o r e d i n 16K computer l o c a t i o n s w i t h 32 b i t s w o r d l e n g t h s . The a c q u i s i t i o n time was 1 s, the t i p a n g l e 60° and t h e p u l s e i n t e r v a l 10 s. The reduced 11

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

Stabilization of Polyvinyl chloride)

HJERTBERG AND SORVIK

Table

1.

Molecular weight

263

data

fi

a

Sample

Pol.temp.

P/P

Q

M

n

•io"

3

°C A1

45

0.97

59.8

161

2.69

12

A2

0.92

50.4

161

3.19

16

A3

0.85

46.8

153

3.27

21

A4

0.76

32.0

117

3.66

22

A5

0.70

26.5

3.71

30

98.2

A6

B1

0.97

48.6

117

2.41

12

B2

55

0.92

45.0

118

2.63

11

B3

0.85

41 .0

110

2.68

13

B4

0.76

37.7

106

2.81

18

B5

0.70

33.1

101

3.05

19

B6

0.61

29.3

94.7

3.23

23

B7

0.53

25.1

91.9

3.66

41

93.9

2.53

20

C1

65

0.97

37.1

C2

0.92

35.2

93.4

2.65

22

C3

0.85

34.1

92.4

2.71

25

C4

0,76

31.6

90.0

2.88

23

C5

0.70

30.3

96.7

3.19

35

C6

0.61

25.2

79.5

3.15

40

D1

80

0.97

26.7

66.9

2.51

30

D2

0.92

25.3

65.0

2.57

38

D3

0.85

26.0

76.6

2.95

45

D4

0.76

20.9

66.3

3.17

44

D5

0.70

20.2

64.2

3.18

51

D6

0.61

19.2

67.0

3.49

53

of long

chain branches

a)

X i s t h e number weight u n i t .

per molecular

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

264

samples were o b s e r v e d at 115°C as 10-15% (w/v) s o l u t i o n s i n 1,2,4t r i c h l o r o b e n z e n e w i t h 20% benzene-d^ t o p r o v i d e t h e d e u t e r i u m l o c k . The number o f scans accumulated was 8-15000. The F o u r i e r t r a n s f o r m a t i o n s were performed w i t h f l o a t i n g p o i n t a r i t h m e t i c . As r e f e r ence, t h e main m e t h y l e n e peak was used; 30.0 ppm v e r s u s TMS. D e t e r m i n a t i o n o f I n t e r n a l Double Bonds. The number o f i n t e r n a l double bonds was determined by f o l l o w i n g t h e changes i n M^ due t o o x i d a t i v e c l e a v a g e by ozone o f a l l d o u b l e bonds. The o z o n o l y s i s was performed a c c o r d i n g t o M i c h e l e t . a l . (23) and t h e e x p e r i m e n t a l d e t a i l s have been g i v e n e a r l i e r (8). The r e a c t i o n was c a r r i e d out at -20°C i n t e t r a c h l o r o e t h a n e s o l u t i o n w i t h a s m a l l amount o f methanol added i n o r d e r t o f a c i l i t a t e the c l e a v a g e o f t h e formed ozonoide. The r e a c t i o n time was 2 h and t h e polymer was r e c o v e r e d by p r e c i p i t a t i o n i n methanol and was d r i e d i n vacuum f o r 24 h . The c o n t e n t o f i n t e r n a l double from t h e number averag o x i d a t i v e treatment: C=C.

mt

bonds (

c

=

c

i

n

t

)

VC = ( l / E -1/M ) • 62 n n,o

/1000

w

a

s

calculated

500

where M i s t h e o r i g i n a l number average m o l e c u l a r weight and M the numBer average m o l e c u l a r weight a f t e r o z o n a t i o n .

R

R e s u l t s and D i s c u s s i o n The r a t e o f d e h y d r o c h l o r i n a t i o n o f most commercial PVC samples f a l l w i t h i n a r a t h e r narrow i n t e r v a l . A t y p i c a l scatter of s t a b i l i t y d a t a can be found i n r e f . 24 where the r e s u l t s from 11 commercial samples a r e g i v e n . The r a t e o f d e h y d r o c h l o r i n a t i o n at 190°C i n n i t r o g e n v a r i e d i n t h e range o f 1-2.5 • 10 % per m i n u t e . I f the l a b i l e s t r u c t u r e s s h o u l d be t h e major r e a s o n t o the i n s t a b i l i t y o f PVC, t h e c o n t e n t o f such s t r u c t u r e s s h o u l d a l s o v a r y w i t h i n a c o r r e s p o n d i n g l y narrow i n t e r v a l . There a r e two main reasons f o r t h e s i m i l a r i t y i n d e h y d r o c h l o r i n a t i o n b e h a v i o r among d i f f e r e n t PVC samples. First, v i n y l c h l o r i d e i s most o f t e n p o l y m e r i z e d i n a r a t h e r narrow temperat u r e i n t e r v a l , 40-75°C, because o f t h e h i g h tendency t o c h a i n t r a n s f e r t o monomer which determines t h e m o l e c u l a r weight ( 2 5 ) . Second, t h e monomer c o n c e n t r a t i o n at t h e r e a c t i o n s i t e i s c o n s t a n t d u r i n g t h e major p a r t o f t h e p o l y m e r i z a t i o n because PVC i s i n s o l u b l e i n v i n y l c h l o r i d e and the p r o p a g a t i o n m a i n l y o c c u r s i n the p o l y m e r i c phase (26) . At 50°C, t h e g e l c o n t a i n s 30 g v i n y l c h l o r i d e per 100 g PVC ( 2 7 . 28) as l o n g as a s e p a r a t e l i q u i d monomer e x i s t s . In p r a c t i c e , t h e " p r e s s u r e d r o p " o c c u r s at about 70% c o n v e r s i o n and t h e r e a f t e r t h e monomer c o n c e n t r a t i o n d e c r e a s e s i n t h e g e l . However, o n l y a m i n o r p a r t o f the batch i s formed under s u b s a t u r a t i o n conditions. The c o n t e n t o f t e r t i a r y and i n t e r n a l a l l y l i c c h l o r i n e i s low i n normal PVC: about 1 ( 7 9) and 0.1-0.3 ( 8 24. 29. 30) per 1000 VC, r e s p e c t i v e l y . At t h e s e c o n c e n t r a t i o n l e v e l s the a c c u r a c y o f t h e a n a l y t i c a l t e c h n i q u e s i s not t o o good. In c o m b i n a t i o n w i t h the e x p e c t e d narrow c o n c e n t r a t i o n range, c o r r e l a t i o n s between t h e r m a l s t a b i l i t y and l a b i l e s t r u c t u r e s are u n c e r t a i n when o r d i n a r y PVC i s u s e d . f

T

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

HJERTBERG AND SORVIK

Stabilization of Polyvinyl chloride)

265

In a s u b s a t u r a t i o n p o l y m e r i z a t i o n , the monomer c o n c e n t r a t i o n can be kept c o n s t a n t at any g i v e n l e v e l below t h e s a t u r a t i o n l e v e l . We have, t h e r e f o r e , used t h i s k i n d o f p o l y m e r i z a t i o n i n o r d e r t o o b t a i n polymers w i t h d e c r e a s e d s t a b i l i t y . The d e g r a d a t i o n r a t e (190°C, N«) o f the U-PVC samples used i n our p r e v i o u s i n v e s t i g a t i o n s (6-8) c o v e r e d a much wider i n t e r v a l than o r d i n a r y PVC.; i . e . 1.7-5.5 • 10 % per m i n u t e . The i d e n t i f i c a t i o n was t h e r e f o r e considerably simplified. For the f i r s t t i m e , a d e f i n i t e r e l a t i o n c o u l d be proposed between the r a t e o f d e h y d r o c h l o r i n a t i o n and the most f r e q u e n t l a b i l e s t r u c t u r e s , t e r t i a r y and i n t e r n a l a l l y l i c c h l o r i n e . The r e s u l t s from t h e d e g r a d a t i o n experiments i n t h e p r e s e n t i n v e s t i g a t i o n are g i v e n i n T a b l e 2. The jjate o f d e h y d r o c h l o r i n a t i o n c o v e r s a s t i l l wider i n t e r v a l ; 2-11 « 10 % per m i n u t e . However, i t i s not p o s s i b l e t o d i r e c t l y compare t h e s e v a l u e s w i t h those d i s cussed above as another d e g r a d a t i o n equipment was u s e d . In a g r e e ment w i t h the e x p e r i e n c e g a i n e d w i t h i n the IUPAC "Working P a r t y on PVC D e f e c t s " (31) the sam v a l u e s may d i f f e r . The equipment i s 40-50% h i g h e r . Even i f the d e g r a d a t i o n had been performed w i t h the same equipment, i t would have been d i f f i c u l t t o compare the polymers p r e p a r e d at 55°C i n t h i s i n v e s t i g a t i o n w i t h those s t u d i e d e a r l i e r . The morphology o f the l a t t e r showed t h a t a g g l o m e r a t i o n o f the p r i m a r y p a r t i c l e s had o c c u r r e d ( 1 0 ) . The p o l y m e r i z a t i o n a f t e r the. agglomerat i o n i s d i f f u s i o n - c o n t r o l l e d , which d e c r e a s e s the monomer c o n c e n t r a t i o n (17). For comparison, a sample was p r e p a r e d under the same c o n d i t i o n s used t o o b t a i n sample B l except t h a t the p o l y m e r i z a t i o n was c o n t i n u e d an a d d i t i o n a l 15% a f t e r t h e ^ a g g l o m e r a t i o n p o i n t . The r a t e o f dehydrochlorination was 3.4 • 10 % per minute compared t o 2.2 • 10 % f o r sample B l . An exact comparison between the e a r l i e r polymers and the p r e s e n t 55°C samples i s t h e r e f o r e not p o s s i b l e . However, on a r e l a t i v e b a s i s the two s e r i e s are s i m i l a r . F i g u r e 1 shows the i n f l u e n c e o f P/P on the r a t e o f d e h y d r o c h l o r i n a t i o n at d i f f e r e n t p o l y m e r i z a t i o n t e m p e r a t u r e s . At a l l t e m p e r a t u r e s , t h e thermal s t a b i l i t y i s s t e a d i l y d e c r e a s i n g w i t h d e c r e a s i n g monomer p r e s s u r e i n accordance w i t h our e a r l i e r f i n d i n g s (JJL). In the range o f 55-80°C, an i n c r e a s e i n p o l y m e r i z a t i o n tempera t u r e l e a d s t o d e c r e a s e d s t a b i l i t y at a l l l e v e l s o f P/P . In a r e c e n t paper, Hamielec e t . a l . (32) suggested t h a t the temperature s h o u l d be i n c r e a s e d at the l a t e r s t a g e s o f h i g h c o n v e r s i o n batch p o l y m e r i z a t i o n s o f v i n y l c h l o r i d e i n o r d e r t o a v o i d a d e c r e a s e i n the thermal s t a b i l i t y a f t e r the p r e s s u r e d r o p . Based on f r e e volume i d e a s t h e y argued t h a t the p r o p a g a t i o n r e a c t i o n would be r e l a t i v e l y more f a v o r e d than s i d e r e a c t i o n s l e a d i n g t o l a b i l e d e f e c t s . The p r e s e n t e x p e r i m e n t a l f i n d i n g s , as i l l u s t r a t e d i n F i g u r e 1, c l e a r l y contradict that suggestion. Apart from the h i g h e s t v a l u e o f P/P used, the r a t e o f d e h y d r o c h l o r i n a t i o n o f the polymers p r e p a r e d at 45°C i s h i g h e r t h a n f o r the 55°C polymers. A p p a r e n t l y , t h e r e i s an optimum i n p o l y m e r i z a t i o n temperature w i t h r e s p e c t t o the t h e r m a l s t a b i l i t y . As i l l u s t r a t e d i n F i g u r e 2, t h i s i s a c c e n t u a t e d at lower r e l a t i v e p r e s sures. For o r d i n a r y batch p o l y m e r i z e d PVC t h i s e f f e c t s h o u l d not be too l a r g e as the major p a r t o f the p o l y m e r i z a t i o n t a k e s p l a c e at P/P_ = 1. However, we have not shown t h a t the r a t e o f o . . . d e h y d r o c h l o r i n a t i o n i s doubled i f t h e c o n v e r s i o n i s pushed up from Q

Q

Q

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

266

POLYMER STABILIZATION AND DEGRADATION

T a b l e 2.

Thermal s t a b i l i t y

Pol.

data

deHCl

temp. °C

10

2

a)

1%/min.) P/P

0.76

0.70

0.61

3.50

5.43

6.75

7.85

3.26

4.28

5.41

6.38

6.25

7.85

8.50

0.97

0.92

0.85

45

2.08

2.92

55

2.21

2.44 4.68

Q

65

3.65

80

3.90

a)

.

dt

rate

10.9

o f d e h y d r o c h l o r i n a t i o n a t 190°C i n n i t r o g e n .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

0 .53

7 .89

19. H JERTBERG AND SORVIK

Stabilization of Polyvinyl chloride)

267

75% t o 95%. F o r such h i g h c o n v e r s i o n m a t e r i a l s , a maximum i n t h e r m a l s t a b i l i t y s h o u l d be found f o r a p o l y m e r i z a t i o n t e m p e r a t u r e around 55°C. F o r t u n a t e l y , t h i s temperature l e v e l c o i n c i d e s with the t e m p e r a t u r e s most o f t e n used i n commercial p r o d u c t i o n o f PVC. The maximum o b s e r v e d f o r t h e t h e r m a l s t a b i l i t y c a n p e r se be e x p l a i n e d i f t h e r e a r e two v a r i a b l e s w i t h o p p o s i t e i n f l u e n c e on t h e formation o f l a b i l e d e f e c t s . As d i s c u s s e d i n t h e I n t r o d u c t i o n , such s t r u c t u r e s a r e formed a f t e r i n t e r - and i n t r a m o l e c u l a r t r a n s f e r t o polymer. Reasonably, the r e l a t i v e frequency o f these t r a n s f e r r e a c t i o n s s h o u l d i n c r e a s e w i t h d e c r e a s i n g monomer c o n c e n t r a t i o n and i n c r e a s i n g p o l y m e r i z a t i o n temperature. Instead o f the experimental v a r i a b l e P/P , t h e monomer c o n c e n t r a t i o n at t h e r e a c t i o n s i t e s h o u l d be c o n s i d e r e 8 . A c c o r d i n g t o Berens ( 2 7 ) , t h e c o n c e n t r a t i o n o f monomer i n t h e g e l i s o n l y i n f l u e n c e d by t h e r e l a t i v e p r e s s u r e i n t h e P/P -range o f interest. However, by u s i n g polymers p r e p a r e d a t 11, 50 and 90°C, N i l s o n e t . a l . (28) c o u l f u n c t i o n o f both P/P an i n t e r p o l a t i n g t h e i r S a t a , i t c a n be e s t i m a t e d t h a t an i n c r e a s e i n p o l y m e r i z a t i o n t e m p e r a t u r e from 45 t o 80°C s h o u l d i n c r e a s e t h e monomer c o n c e n t r a t i o n w i t h about 25% at s a t u r a t e d c o n d i t i o n s . This w i l l tend t o c o u n t e r a c t t h e e x p e c t e d f a s t e r f o r m a t i o n o f l a b i l e s t r u c t u r e s due t o t h e t e m p e r a t u r e a l o n e which q u a l i t a t i v e l y i s i n a c c o r d a n c e w i t h t h e o b s e r v a t i o n o f an optimum p o l y m e r i z a t i o n temperature with r e s p e c t t o t h e thermal s t a b i l i t y . I t i s a l s o p o s s i b l e t o u s e t h e d a t a g i v e n i n r e f . 28 t o c a l c u l a t e t h e monomer c o n c e n t r a t i o n at a l l P/P - l e v e l s . In the p r e s e n t c a l c u l a t i o n s , no c o r r e c t i o n s have been made f o r t h e p a r t i c l e s i z e o r t h e s u r f a c e t e n s i o n , due t o t h e p r e s e n c e o f e m u l s i f i e r (1 g ammoniumlaurate p e r 1 H^O). I f t h e r a t e o f d e h y d r o c h l o r i n a t i o n i s p l o t t e d against the concentration instead o f P / P Figure 3 i s obtained. Compared t o F i g u r e 1, t h e 45°C s e r i e s now becomes almost i d e n t i c a l w i t h t h e polymers a t 55°C, e x c e p t at t h e lowest monomer concentration. The r e l a t i o n s between t h e r a t e o f d e h y d r o c h l o r i n a t i o n and p o l y m e r i z a t i o n t e m p e r a t u r e a t c o n s t a n t monomer c o n c e n t r a t i o n w i l l t h e r e f o r e show a p l a t e a u below 55°C, s e e F i g u r e 4. A minimum i s o n l y i n d i c a t e d f o r monomer c o n c e n t r a t i o n s below about 10 g VC p e r 100 g PVC. At t e m p e r a t u r e s above 55°C, t h e t h e r m a l s t a b i l i t y d e c r e a s e s with i n c r e a s i n g p o l y m e r i z a t i o n temperature. I f t h e l a b i l e s t r u c t u r e s a r e t h e main r e a s o n t o t h e low t h e r m a l s t a b i l i t y o f PVC, F i g u r e s 3 and 4 s h o u l d a l s o r e f l e c t t h e c o n c e n t r a t i o n of the defects. In o u r p r e v i o u s work ( 7 . 8 ) . we showed t h a t t h e r a t e o f d e h y d r o c h l o r i n a t i o n c o u l d be r e l a t e d t o t h e amounts o f t e r t i a r y and i n t e r n a l a l l y l i c c h l o r i n e . However, i t i s a l s o l i k e l y t h a t random d e h y d r o c h l o r i n a t i o n w i l l c o n t r i b u t e t o a c e r t a i n e x t e n t (2 3. 33. 3 4 ) . A c c o r d i n g t o o u r e s t i m a t i o n , random d e h y d r o c h l o r i n a t i o n c o u l d account f o r 10-15% o f t h e i n i t i a t i o n d u r i n g d e g r a d a t i o n o f o r d i n a r y PVC . I t has been s u g g e s t e d t h a t t h e s t e r e o - s t r u c t u r e s h o u l d i n f l u e n c e d e h y d r o c h l o r i n a t i o n from o r d i n a r y monomer u n i t s ( 3 3 . 35-37). The p r e s e n t samples a l s o c o v e r a change i n p o l y m e r i z a t i o n t e m p e r a t u r e , 45-80°C. A comparison between t h e c o n t e n t o f l a b i l e s t r u c t u r e s and t h e r m a l s t a b i l i t y might t h e r e f o r e r e v e a l an e v e n t u a l influence of the t a c t i c i t y . The amounts o f i n t e r n a l d o u b l e bonds a r e g i v e n i n T a b l e 3. I n agreement w i t h o u r p r e v i o u s r e s u l t s (J^) a d e c r e a s e i n P/P r e s u l t s i n q J

T

Q

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

F i g u r e 3 . The r a t e of d e h y d r o c h l o r i n a t i o n as a f u n c t i o n of monomer c o n c e n t r a t i o n and p o l y m e r i z a t i o n temperature.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Stabilization of Polyvinyl chloride)

19. HJERTBERG AND SORVIK

269

10

~i

40

i

i

1

50

60

70

Pol. temp.

,

r

80

( °C)

F i g u r e 4. The i n f l u e n c e of p o l y m e r i z a t i o n temperature on the d e g r a d a t i o n r a t e a t f o u r l e v e l s o f monomer c o n c e n t r a t i o n .

Table

3.

Content

of internal

Pol.

double

C= C,

bonds

/1000

VC

temp. 0.97

0.92

0.85

0.76

0.70

0.61

45

0.28

_

0.58

0.54

0.60

0.79

°C

P/P

Q

55

0.14

0.33

0.52

0.40

0.55

0.66

65

0.23

0.25

0.32

0.44

0.63

0.55

80

0.08

0.14

0.18

0.39

-

0.49

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

0 .53

0 .78

POLYMER

270

STABILIZATION A N D D E G R A D A T I O N

higher values. In F i g u r e 5, t h e c o n t e n t o f i n t e r n a l d o u b l e bonds i s i n s t e a d p l o t t e d a g a i n s t t h e monomer c o n c e n t r a t i o n . I t i s obvious t h a t t h e monomer c o n c e n t r a t i o n i s t h e most important parameter, but the p o l y m e r i z a t i o n temperature may a l s o i n f l u e n c e t h e f r e q u e n c y o f t h i s s t r u c t u r e . Admittedly, there i s a rather large s c a t t e r i n the d a t a , but a t i n c r e a s i n g p o l y m e r i z a t i o n t e m p e r a t u r e t h e c o n t e n t o f i n t e r n a l d o u b l e bonds tends t o d e c r e a s e . T h i s can be seen from t h e s o l i d and broken l i n e s which r e p r e s e n t t h e extreme v a l u e s o f t h e p o l y m e r i z a t i o n t e m p e r a t u r e , 45 and 80°C r e s p e c t i v e l y . We have s u g g e s t e d t h a t i n t e r n a l d o u b l e bonds a r e formed a f t e r t r a n s f e r t o polymer from c h l o r i n e atoms ( 7 . 8 ) . B e s i d e s u n s a t u r a t i o n , t h e subsequent r e a c t i o n s may a l s o r e s u l t i n l o n g c h a i n branches:

CHC1-CH -CHC1~ 2

CH -CC1-CH ' 2

2

VC

(3)

Cl

H I

I

r~ C H - C - C H ~ 2

~CHC1-C-CHC1~

2

CH

CH

0

?

— CH=CH-CH — ' Cl

0

?

The c h l o r i n e atom i s formed i n t h e r e a c t i o n scheme l e a d i n g t o c h a i n t r a n s f e r t o monomer ( 6 , 9. 13. 1 5 ) . N o r m a l l y , i t r e a c t s w i t h a monomer m o l e c u l e i n i t i a t i n g a new polymer c h a i n :

Cl-

+

C H = CH 2

Cl

(

6

)

>

C H - CH 2

Cl

Cl

At d e c r e a s i n g monomer c o n c e n t r a t i o n , r e a c t i o n s 1 and 2 s h o u l d t h e r e f o r e be more f a v o r e d . T h i s c a n e.g. be shown by t h e r a t i o between 1 , 2 - d i c h l o r o alkane end groups and i n t e r n a l d o u b l e bonds. I t can be assumed t h a t each macromolecule c o n t a i n s 1 end group o f t h i s type ( 6 . 1 5 ) . F o r t h e 65°C s e r i e s , t h e m o l e c u l a r weight d a t a i n T a b l e 1 i n d i c a t e s t h a t t h i s r a t i o d e c r e a s e s from 8.4 at P / P t o 4.5 at P / P = 0.61. F u r t h e r m o r e , t h e r a t e o f f o r m a t i o n o f c h l o r i n e atoms w i l l i n c r e a s e at d e c r e a s i n g monomer c o n c e n t r a t i o n ( s e e below). The observed i n c r e a s e i n t h e c o n t e n t o f i n t e r n a l d o u b l e bonds i s t h e r e f o r e easy t o u n d e r s t a n d . I t i s more d i f f i c u l t t o e x p l a i n t h e e f f e c t o f t h e p o l y m e r i z a t i o n temperature. As t h e r e i s no mass t r a n s f e r i n r e a c t i o n 6 i n c o n t r a s t q

Q

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

271

Stabilization of Polyvinyl chloride)

HJERTBERG AND SORVIK

t o r e a c t i o n s 1 and 2, t h e former s h o u l d have lower a c t i v a t i o n energy. A c c o r d i n g l y , t h e r e l a t i v e f r e q u e n c y o f r e a c t i o n 6 compared t o 1 and 2 should i n c r e a s e with d e c r e a s i n g temperature. The r a t i o d i s c u s s e d above d e c r e a s e d t o 3.7 f o r t h e sample p r e p a r e d a t 45°C and P / P = 0.97, i n d i c a t i n g a change i n t h e o p p o s i t e d i r e c t i o n . However, t h e i n f l u e n c e o f t h e temperature on the r e l a t i o n between r e a c t i o n s 1 and 2 must be c o n s i d e r e d . Experiments w i t h c h l o r i n a t i o n o f PVC have i n d i c a t e d t h a t t h e methylene groups are p r e f e r a b l y a t t a c k e d ( 3 8 ) . The s e l e c t i v i t y s h o u l d , however, d e c r e a s e w i t h i n c r e a s i n g temperature which might e x p l a i n t h e d e c r e a s e i n t h e c o n t e n t o f i n t e r n a l double bonds w i t h i n c r e a s i n g p o l y m e r i z a t i o n t e m p e r a t u r e . Typical C-NMR s p e c t r a o f reduced samples, p o l y m e r i z e d a t P/P e q u a l t o 0.97 and 0.70, a r e shown i n F i g u r e 6. The spectrum o f t h e sample o b t a i n e d a t P/P = 0.97 i s v e r y s i m i l a r t o t h o s e o b t a i n e d w i t h o r d i n a r y PVC. B e s i d e s the main peak a t 30.0 ppm from u n d i s t u r b e d CH^-carbons, t h e r e a r e some s m a l l e r peaks m a i n l y o r i g i n a t i n g from branches and normal a l k a n has been d i s c u s s e d i n d e t a i the peaks from t h e s e s t r u c t u r e s and t h e i r c h e m i c a l s h i f t s a r e summarized i n F i g u r e 7 and T a b l e 4. As shown e a r l i e r , branches w i t h one c a r b o n atom are the most f r e q u e n t s h o r t c h a i n branch ( 4 5. 7 9. 13). In t h e polymers o b t a i n e d a t low v a l u e s o f P/P i t i s easy t o o b s e r v e b u t y l branches, a l t h o u g h t h i s s t r u c t u r e c a n be seen a l s o i n polymers o b t a i n e d c l o s e to the s a t u r a t i o n pressure ( 7 2 2 ) . I t i s a l s o p o s s i b l e t o detect l o n g c h a i n branches i n agreement w i t h o u r e a r l i e r r e s u l t s ( 7 . 2 2 ) . The b e t t e r s i g n a l - t o - n o i s e r a t i o o b t a i n e d i n t h e p r e s e n t work (due t o the " H i g h - s e n s " probe) has a l l o w e d us t o d e t e c t t h e p r e s e n c e o f e t h y l branches. The e x i s t e n c e o f t h e l a t t e r t y p e o f branches has e a r l i e r been proven by S t a r n e s and h i s coworkers ( 9 . 3 9 ) . The i n f l u e n c e o f the p o l y m e r i z a t i o n c o n d i t i o n s on t h e c o n t e n t o f d i f f e r e n t branches w i l l be d i s c u s s e d below. We have a l s o been a b l e t o d e t e c t s e v e r a l o t h e r s t r u c t u r e s . One o f them i s i s o l a t e d c h l o r i n e r e s i d u e s due t o i m p e r f e c t r e d u c t i o n . The c o n t e n t i n t h e p r e s e n t m a t e r i a l v a r i e s between not o b s e r v a b l e t o about 1 C l p e r 1000 VC. In agreement w i t h the r e s u l t s p u b l i s h e d by S t a r n e s and h i s coworkers ( 9 . 40) we have a l s o found n o n a l l y l i c p r i m a r y h a l o g e n , i . e . CH C1- groups i n l o n g c h a i n ends and m e t h y l and b u t y l branches. For t h e polymers s t u d i e d h e r e i t i s e s p e c i a l l y c h l o r o m e t h y l branches which c a n be o b s e r v e d , a l t h o u g h b u t y l branches w i t h c h l o r i n e r e s i d u e s were o b s e r v e d i n some polymers as w e l l . When n e c e s s a r y , t h e c o n t e n t o f branches ( s e e below) has been c o r r e c t e d w i t h r e s p e c t t o t h e presence o f t h e s e c h l o r i n a t e d s t r u c t u r e s . S t a r n e s e t . a l . have shown t h a t u n s a t u r a t e d s t r u c t u r e s i n t h e s t a r t i n g PVC may form c y c l i c p r o d u c t s d u r i n g r e d u c t i o n w i t h Bu^SH. The most f r e q u e n t u n s a t u r a t e d end groups (1) i s thus c o n v e r t e d t o a l - ( e t h y l ) - 2 - ( l o n g a l k y l ) c y c l o p e n t a n e group (41) w h i l e i n t e r n a l double bonds a r e c o n v e r t e d i n t o i n t e r n a l l , 2 - d i ( l o n g a l k y l ) c y c l o p e n t a n e moities (42). These s t r u c t u r e s have a l s o been o b s e r v e d i n t h e p r e s e n t m a t e r i a l . The most important o b s e r v a t i o n i s t h a t t h e i n t e r n a l c y c l o p e n t a n e group can most e a s i l y be seen i n polymers o b t a i n e d a t low v a l u e s o f P/P . T h i s i s an agreement w i t h t h e measurements o f i n t e r n a l double bonds, T a b l e 3. These d e t a i l s i n Q

Q

T

T

r

2

Q

C-NMR s p e c t r a o f reduced a s e p a r a t e paper ( 4 3 ) . branching

PVC w i l l

be more c o m p l e t e l y d e a l t w i t h i n

In the f o l l o w i n g d i s c u s s i o n , i t i s t h e

s t r u c t u r e which i s o f i n t e r e s t .

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

272

F i g u r e 5.

The content

o f i n t e r n a l double bonds (C=C.

) per

int

1000 VC as a f u n c t i o n o f monomer c o n c e n t r a t i o n and p o l y m e r i z a t i o n temperature. The symbols a r e the same as those used i n F i g u r e s 1-4^ ( ) r e p r e s e n t s the 45 C s e r i e s ; ( ) r e p r e s e n t s the 80 C s e r i e s .

Table

1 3

4.

C

shifts

for saturated

expressed

i n ppm

C

Structure CH

0

chain

versus

a

r

b

TMS

o

n

a

ends

and

branches

^

^

2

3

4

32.17

29.58

bi

a

LE

14.08

22.88

Me

19.94

-

33.21

37.51

27.44

Et

11.07

26.83

39.69

34.06

27.22

Bu

14.13

23.39

38.09

34.52

27.25

38.12

34.52

27.25

29.50

L

a)

s e e r e f . 8.

b)

according

to Figure

34.13

7.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Stabilization of Polyvinyl chloride)

HJERTBERG AND SORVIK

a

Pol. P/P

Mle-a

temp.

0

273

8 0 °C

= 0.97

•-P Me-br L/Bu/Et

L/Bu-br

B

Et-a T 50

1 40

L j

2

Bu-CH ^

1

3

>«^>»i^i^

1 30

*****

T-

1 20

Pol.

temp.

p/p

o

10

6 5 °C

= 0.70

L/Bu/Et-p LE-CH Bu-2 Et-br

2

LE- |

B u

"

C H

3

3 j

j

Et-a ~l 50

40

30

20

T

13 F i g u r e 6. C-NMR s p e c t r a of reduced U-PVC samples. F o r peak assignments, see F i g u r e 7 and T a b l e 4. ( a ) P o l y m e r i z a t i o n (b) P o l y m e r i z a t i o n temperature temperature 80 C, P/P = 0.97. 65 C, P/P = 0.70. ° o

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

10

POLYMER STABILIZATION AND DEGRADATION

Long end , L E

Methyl branch P

- c - c - c - c - CH 4 3 2

3

~

, Me

a br a P c - c - c - c - c 1

CH

Ethyl branch P

a

b

r

,

Et

a

P

- c - c - c - c - c C

0

a

, Bu

br

a

p

- c - c - c - c - c C 4

2

I

CH

Butyl branch

3

1

3

C

3

C

2

I

I

CH Long

branch

3

, L

P a br a p ~ c - c - c - c - c I

C

a

c

p

F i g u r e 7.

S t r u c t u r e s i d e n t i f i e d i n F i g u r e 6.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

HJERTBERG AND SORVIK

275

Stabilization of Polyvinyl chloride)

As shown e a r l i e r , the s h o r t c h a i n branches have the f o l l o w i n g s t r u c t u r e s i n the o r i g i n a l PVC: c h l o r o m e t h y l (4, 5 ) , 2 - c h l o r o e t h y l ( 9 . 39) and 2 , 4 - d i c h l o r o b u t y l (7. 9. 4 4 ) . The two l a t t e r have c h l o r i n e connected t o the branch c a r b o n . The same i s v a l i d f o r the main p a r t of the l o n g c h a i n branches ( 7 . 9. 4 4 ) . However, the p r e s e n c e of t e r t i a r y hydrogen i n c o n n e c t i o n w i t h l o n g c h a i n branches cannot be e x c l u d e d . A c c o r d i n g t o the C-NMR spectrum of a d e u t e r i d e reduced U-PVC p o l y m e r i z e d at 55°C and P/P = 0.59 up t o o n e - t h i r d o f the l o n g c h a i n branch p o i n t s c o u l d c o n t a i n t e r t i a r y hydrogen (J). The c o n t e n t of the d i f f e r e n t branches i s g i v e n i n T a b l e 5. The r e l a t i o n between t h e m e t h y l branch f r e q u e n c y and the p o l y m e r i z a t i o n c o n d i t i o n s i s somewhat d i f f e r e n t compared t o t h a t o f the o t h e r s . The c o n t e n t o f t h i s branch s t r u c t u r e d e c r e a s e s w i t h d e c r e a s i n g p o l y m e r i z a t i o n t e m p e r a t u r e and monomer c o n c e n t r a t i o n , F i g u r e 8. T h i s i s what c o u l d be expected from the mechanism f o r c h a i n t r a n s f e r t o monomer:

CH

CH -CH 2

=CH

Cl VC

VC

- C H - C H - C H - -CH 1 Cl Cl 2

-CH -CH-CH-CH Z , , z Cl Cl 0

0

'CH -CH-CH-CH 0

Z

1

\

cl

Cl

0

Z

VC

-cl-

CH-CH-CH ' 1 1 z C l CH C1

'CH -CH=CH-CH z 1 z Cl 0

Cl-

0

+

2

C H = CH 2 1 Cl n

2

>

CH -CH 1 z 1 Cl Cl 0

>

The changes w i t h the p o l y m e r i z a t i o n temperature r e f l e c t the b a l a n c e between the two r e a c t i o n s at l e v e l A. As e x p e c t e d , a d e c r e a s e i n p o l y m e r i z a t i o n temperature l e a d s t o l e s s head-to-head additions. A change i n the monomer c o n c e n t r a t i o n s h o u l d , however, not i n f l u e n c e t h i s b a l a n c e . I n s t e a d , the r e a s o n f o r the c o n t e n t o f c h l o r o m e t h y l branches w i t h d e c r e a s i n g monomer c o n c e n t r a t i o n can be found at l e v e l C. I t i s o b v i o u s t h a t t h a t balance between t h e s e r e a c t i o n s s h o u l d change i n t h e observed manner, i . e . l e s s p r o p a g a t i o n . The d e c r e a s i n g m o l e c u l a r weight w i t h d e c r e a s i n g P/P i s another i n d i c a t i o n of t h i s e f f e c t . ° The c o n t e n t o f the o t h e r branches, e t h y l , b u t y l and l o n g c h a i n

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

276

Table

Pol,

5.

Content

o f branches

i n the reduced

Branch

_

.

/

i

n

PVC

samples

n n

B r a n c h e s / 1 0 0 0 VC temp. °C

45

type P/P

Me

Q

0.97

0.92

3.S

0.85

0.76

0.70

0.61

0.53

3.5

3.7

Et Bu L

55

65

80

0.6 *0.1

1.2

2.2

0.1

0.3

Me

4.2

4.2

4.1

4.0

4.0

3.8

3.9

Et

0.2

0.2

0.3

0.4

0.4

0.5

0.5

Bu

0.7

1.1

1.2

1.6

1.7

1.9

2.2

L

0.2

0.2

0.3

0.4

0.6

0.6

0.8

Me

4.6

4.5

4.5

4.4

4.2

Et

0.2

0.4

0.35

0.5

0.5

Bu

0.8

1 .6

1 .8

2.0

2.3

L

0.3

0.5

0.5

0.6

0.7

Me

4.8

4.7

4.7

4.6

4.5

Et

0.3

0.4

0.4

0.5

0.6

Bu

1.4

1.5

1.7

2.1

2.4

L

0.3

0.5

0.6

0.6

0.8

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

HJERTBERG AND SORVIK

Stabilization of Polyvinyl chloride)

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

278

POLYMER STABILIZATION AND DEGRADATION

b r a n c h e s , i n c r e a s e s w i t h i n c r e a s i n g p o l y m e r i z a t i o n temperature as w e l l as w i t h d e c r e a s i n g monomer c o n c e n t r a t i o n , s e e F i g u r e 9. F o r e t h y l (JL) and b u t y l ( 7 . 9. 44) branches t h i s i s i n agreement w i t h t h e b a c k - b i t i n g mechanism suggested f o r t h e i r f o r m a t i o n . The same b e h a v i o r s h o u l d a l s o be e x p e c t e d f o r t h e f o r m a t i o n o f l o n g c h a i n branches by t r a n s f e r t o polymer from m a c r o r a d i c a l s . However, we have a l s o s u g g e s t e d t h a t l o n g c h a i n branches c a n be formed a f t e r t r a n s f e r to polymer from c h l o r i n e atoms (7« 8 ) . N a t u r a l l y , the formation of l o n g c h a i n branches v i a r e a c t i o n s 1 and 3 s h o u l d i n c r e a s e w i t h d e c r e a s i n g monomer c o n c e n t r a t i o n , s i m i l a r t o t h e b e h a v i o r v a l i d f o r the f o r m a t i o n o f i n t e r n a l d o u b l e bonds (see a b o v e ) . On t h e o t h e r hand, t h e b a l a n c e between r e a c t i o n 4 and 5 i n d i c a t e s t h a t t h e r a t i o between l o n g c h a i n branches w i t h t e r t i a r y hydrogen and i n t e r n a l d o u b l e bonds s h o u l d d e c r e a s e w i t h d e c r e a s i n g monomer c o n c e n t r a t i o n . O b v i o u s l y , t h e t o t a l e f f e c t l e a d s t o an i n c r e a s e d f o r m a t i o n o f l o n g c h a i n branches at low v a l u e s o f P/P . The d i s c u s s i o n abou temperature i n d i c a t e d tha 2 at i n c r e a s i n g temperature Accordingly, g branch p o i n t s w i t h t e r t j j r y c h l o r i n e s h o u l d i n c r e a s e as w e l l . To prove t h i s s u g g e s t i o n , C-NMR s p e c t r a o f polymers reduced w i t h Bu^SnD must be o b t a i n e d . In t h e p r e s e n t i n v e s t i g a t i o n , we have assumed t h a t a l l l o n g c h a i n branch p o i n t s a r e a s s o c i a t e d w i t h t e r t i a r y c h l o r i n e . The c o n t e n t o f t e r t i a r y c h l o r i n e c a n t h e r e f o r e be t a k e n as t h e sum o f e t h y l , b u t y l and l o n g c h a i n b r a n c h e s . E a r l i e r , t e r t i a r y c h l o r i n e was g e n e r a l l y c o n s i d e r e d t o be l e s s r e a c t i v e t h a n i n t e r n a l d o u b l e bonds. T h i s was based on experiments w i t h low m o l e c u l a r weight model s u b s t a n c e s , see e.g. r e f . 45. Howe v e r , u s i n g copolymers between v i n y l c h l o r i d e and 2 - c h l o r o - p r o p e n e , Berens (46) s t a t e d t h a t t h e p r e s e n c e o f 1-2 t e r t i a r y c h l o r i n e p e r 1000 VC p e r se would account f o r t h e t h e r m a l l a b i l i t y o b s e r v e d i n o r d i n a r y PVC. F u r t h e r m o r e , o u r p r e v i o u s i n v e s t i g a t i o n i n d i c a t e d t h a t the t h e r m a l r e a c t i v i t y o f i n t e r n a l a l l y l i c c h l o r i n e i s o f t h e same o r d e r as t h a t o f t e r t i a r y c h l o r i n e ( & ) . We, t h e r e f o r e , c o n s i d e r i t j u s t i f i a b l e t o use t h e t o t a l c o n t e n t o f l a b i l e c h l o r i n e atoms. As shown i n F i g u r e 10, t h e r e i s a v e r y good r e l a t i o n between t h e r a t e o f d e h y d r o c h l o r i n a t i o n and t h e amount o f l a b i l e c h l o r i n e o b t a i n e d i n t h i s way. F o r comparison, i t c a n be mentioned t h a t t h e d e g r a d a t i o n r a t e o f commercial samples i s found i n t h e i n t e r v a l 1.5-3.5*10 % per m i n u t e . In a l l samples t h e amount o f t e r t i a r y i s c o n s i d e r a b l y h i g h e r than that o f i n t e r n a l a l l y l i c c h l o r i n e . This i s a l s o v a l i d f o r o r d i n a r y PVC where t y p i c a l v a l u e s a r e : 0-0.5 i n t e r n a l a l l y l i c and 1-2 t e r t i a r y c h l o r i n e p e r 1000 VC. We have, t h e r e f o r e , s u g g e s t e d t h a t t h e l a t t e r s t r u c t u r e i s the most important l a b i l e s t r u c t u r e i n PVC ( 8 ) . In a r e c e n t p a p e r , Ivan e t . a l . (47) have i n s t e a d c l a i m e d i n t e r n a l a l l y l i c c h l o r i n e t o be most i m p o r t a n t . F o r a commercial sample, t h e c o n t e n t o f t h i s s t r u c t u r e was g i v e n t o 0.1 p e r 10^0 VC_^ and t h e r a t e c o n s t a n t o f t h e d e h y d r o c h l o r i n a t i o n t o about 10 m i n They d i d n o t measure t h e amount o f t e r t i a r y c h l o r i n e but s u g g e s t e d the p r e s e n c e o f an u n i d e n t i f i e d l a b i l e s t r u c t u r e . I t s r a t e c o n s t a n t of d e g r a d a t i o n s h o u l d be o n l y somewhat l e s s than t h a t o f i n t e r n a l a l l y l i c c h l o r i n e . F u r t h e r m o r e , t h e c o n t e n t o f t h e unknown s t r u c t u r e was c l a i m e d t o be about f o u r times h i g h e r t h a n t h e concent o f

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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HJERTBERG AND SORVIK

20

30

g VC / 100 g PVC

n 10

1

1 20

1

r 30

g VC / 100 g PVC

F i g u r e 9. The i n f l u e n c e of p o l y m e r i z a t i o n c o n d i t i o n s on the c o n t e n t of branches. (a) E t h y l and long c h a i n branches. ( b ) B u t y l branches.

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i n t e r n a l a l l y l i c c h l o r i n e . I t i s t h e r e f o r e p l a u s i b l e t o assume the u n i d e n t i f i e d s t r u c t u r e t o be t e r t i a r y c h l o r i n e and t o c o n c l u d e t h a t t h i s d e f e c t i s the most important f o r the d e g r a d a t i o n o f PVC. In a s e r i e s o f p a p e r s , M i n s k e r et a l . (see, e.g., r e f . 48) have c l a i m e d t h a t a l l a c t u a l l a b i l e s t r u c t u r e s h o u l d be k e t o a l l y l i c (^(C=O)-CH=CH-CHC1^0. The f o r m a t i o n o f t h i s group i s suggested t o o c c u r by o x i d a t i o n o f a l l y l i c methylene groups d u r i n g the p r o d u c t i o n and s t o r a g e of PVC. As an a l t e r n a t i v e , S v e t l y e t . a l . (49-51) have suggested t h a t t h i s s t r u c t u r e s h o u l d e x e r t i t s adverse e f f e c t by catalysis. As d i s c u s s e d i n the p r e c e d i n g paper i n t h i s s e r i e s (j8) H-NMR s p e c t r a o f U-PVC samples w i t h a h i g h c o n t e n t o f i n t e r n a l double bonds d i d not show any e v i d e n c e o f k e t o a l l y l i c g r o u p s . F u r t h e r m o r e , u s i n g an a p p r o p r i a t e model compound, S t a r n e s e t . a l . have r e c e n t l y shown t h a t t h i s s t r u c t u r e , i n f a c t , i s r e l a t i v e l y t h e r m a l l y s t a b l e (52) and t h a t i t does not have any c a t a l y t i c i n f l u e n c e on the d e h y d r o c h l o r i n a t i o n ( 5 3 ) . I f F i g u r e 10 i s i n s p e c t e d i detail it b that the 55°C s e r i e s tends t at a g i v e n l e v e l o f l a b i l i f t h e c o n t e n t o f l a b i l e c h l o r i n e i s r e l a t e d t o the p o l y m e r i z a t i o n conditions. The c o n t e n t o f the s t r u c t u r e at d i f f e r e n t l e v e l s o f monomer c o n c e n t r a t i o n can be o b t a i n e d from F i g u r e s 5, 8 and 9. The i n f l u e n c e o f the p o l y m e r i z a t i o n temperature i s g i v e n f o r f o u r d i f f e r e n t monomer c o n c e n t r a t i o n s i n F i g u r e 11. O b v i o u s l y , the d e f e c t c o n c e n t r a t i o n i n c r e a s e s at a l l monomer l e v e l s . This behavior i s d i f f e r e n t from t h a t o f the d e g r a d a t i o n r a t e g i v e n i n F i g u r e 4. A comparison between t h e two f i g u r e s i n d i c a t e s t h a t the d e g r a d a t i o n r a t e o f t h e 45°C s e r i e s i s too h i g h i n r e l a t i o n t o i t s content of labile chlorine. At p r e s e n t , we c o n s i d e r two p o s s i b l e e x p l a n a t i o n s o f t h i s difference. F i r s t , the d a t a g i v e n i n F i g u r e s 5, 8 and 9 i n d i c a t e s t h a t the r e l a t i o n between t e r t i a r y and i n t e r n a l a l l y l i c c h l o r i n e decreases with decreasing temperature. In the 80°C s e r i e s , t h i s r a t i o i s 7 or h i g h e r w h i l e i t i s o n l y 2 t o 3.5 i n the 45°C s e r i e s . I f the l a b i l i t y o f i n t e r n a l a l l y l i c c h l o r i n e i s i n f a c t h i g h e r than t h a t o f t e r t i a r y c h l o r i n e , the low s t a b i l i t y o f the 45°C s e r i e s can be e x p l a i n e d q u a l i t a t i v e l y . A s t a t i s t i c a l e v a l u a t i o n of the d a t a g i v e n h e r e might r e v e a l a d i f f e r e n c e i n r e a c t i v i t y . T h i s w i l l be d i s c u s s e d i n another paper. I t must a l s o be remembered t h a t the r a t e of d e h y d r o c h l o r i n a t i o n i s i n f l u e n c e d by the i n i t i a t i o n as w e l l as by the l e n g t h o f the p o l y e n e sequence. I t i s known t h a t the p o l y e n e sequence l e n g t h d e c r e a s e s w i t h i n c r e a s i n g s y n d i o t a c t i c i t y ( 2 . 36, 54. 5 5 ) . i . e . w i t h decreasing p o l y m e r i z a t i o n temperature. T h i s e f f e c t w i l l a l s o tend t o d e c r e a s e t h e o v e r a l l s t a b i l i t y o f the 45°C s e r i e s i n r e l a t i o n t o the amount o f l a b i l e c h l o r i n e . D e t e r m i n a t i o n o f the p o l y e n e sequence l e n g t h at low d e g r e e s o f d e g r a d a t i o n s h o u l d show i f t h i s f a c t o r i s o f importance f o r t h e temperature r a n g e , 45-80°C, used i n t h i s investigation. A n o t h e r o b s e r v a t i o n made i n F i g u r e 10 i s t h a t the r e l a t i o n between d e g r a d a t i o n r a t e and c o n t e n t o f l a b i l e c h l o r i n e i s nonlinear. T h i s may be an e f f e c t o f h i g h e r s t a t i o n a r y c o n c e n t r a t i o n s o f HC1 i n samples w i t h a h i g h r a t e o f d e h y d r o c h l o r i n a t i o n . T h i s would g i v e a t o o h i g h v a l u e o f d e g r a d a t i o n r a t e due t o H C l - c a t a l y s i s (see e.g. r e f . 56). I t i s e.g. known t h a t the p r e s e n c e o f HC1 lengthens

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

19.

Stabilization of Polyvinyl chloride)

HJERTBERG AND SORVIK

Pol. temp.,

c E

10.

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A

45

O

55

•

65

•

80

281

°C •

•

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•

/ /

°

%/ o 0 1

o o

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F i g u r e 10. The r e l a t i o n between r a t e o f d e h y d r o c h l o r i n a t i o n and the c o n t e n t o f l a b i l e c h l o r i n e per 1000 VC. g VC/l00g PVC 10 5_

15 20

2 o

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o 0) n CO

1 40

1 50 Pol.

1 60 temp.,

1 70

I 80

°C

F i g u r e 11. The i n f l u e n c e o f the p o l y m e r i z a t i o n c o n d i t i o n s on the c o n t e n t of l a b i l e c h l o r i n e p e r 1000 VC.

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the p o l y e n e sequence l e n g t h (56. 5 7 ) . F u r t h e r m o r e , we have s u g g e s t e d t h a t t h a t p r i m a r y e f f e c t o f HC1 i s t o c a t a l y z e random d e h y d r o c h l o r i n a t i o n (56) . D e g r a d a t i o n i n s o l u t i o n i n s t e a d o f i n the s o l i d s t a t e would g i v e a b e t t e r p o s s i b i l i t y t o a v o i d H C l - c a t a l y s i s (54) even i n low s t a b i l i t y samples. A c c o r d i n g t o e a r l i e r o p i n i o n , i n i t i a t i o n s h o u l d not o c c u r at the normal P V C - u n i t s below 200°C. T h i s c o n c l u s i o n was m a i n l y based on experiments w i t h low m o l e c u l a r weight model s u b s t a n c e s ( s e e e.g. r e f . 45). The e x t r a p o l a t i o n t o z e r o c o n t e n t o f l a b i l e c h l o r i n e i n F i g u r e 10 g i v e s an i n d i c a t i o n o f the importance o f random dehydrochlorinat ion. U s i n g the d a t a p u b l i s h e d by Ivan e t . a l . (^7) and our own c o n c l u s i o n s , the r e l a t i v e r a t e c o n s t a n t s f o r i n i t i a t i o n from i n t e r n a l a l l y l i c , t e r t i a r y and normal s e c o n d a r y c h l o r i n e can a p p r o x i m a t e l y be g i v e n as 1, 1, and 2 • 10 , r e s p e c t i v e l y . On the o t h e r hand, t o p i c a l r e l a t i v e c o n c e n t r a t i o n s o f t h e s e s t r u c t u r e s are 2, 10 and 10 , r e s p e c t i v e l y , i n an o r d i n a r t h i s shows t h a t the t o t a t i o n i s o f the same o r d e r as t h a t from i n t e r n a l a l l y l i c c h l o r i n e . In agreement w i t h our e a r l i e r c o n c l u s i o n (3. 8) the d o m i n a t i n g i n f l u e n c e of i n i t i a t i o n from t e r t i a r y i s o b v i o u s . Conclusions The minimum i n d e g r a d a t i o n r a t e found f o r s u b s a t u r a t i o n PVC o b t a i n e d around 55°C becomes l e s s o b v i o u s i f the monomer c o n c e n t r a t i o n at the r e a c t i o n s i t e i s used as v a r i a b l e i n s t e a d o f the r e l a t i v e monomer pressure, / P « The o b s e r v e d b e h a v i o r i s m a i n l y due t o the i n f l u e n c e o f the p o l y m e r i z a t i o n c o n d i t i o n s on the f o r m a t i o n o f t h e r m a l l y l a b i l e c h l o r i n e , i . e . t e r t i a r y c h l o r i n e and i n t e r n a l a l l y l i c c h l o r i n e . T e r t i a r y c h l o r i n e i s a s s o c i a t e d w i t h e t h y l , b u t y l and l o n g c h a i n branches. The l a b i l e s t r u c t u r e s are formed a f t e r d i f f e r e n t i n t e r and i n t r a m o l e c u l a r t r a n s f e r r e a c t i o n s . G e n e r a l l y , t h e c o n t e n t i n c r e a s e s w i t h d e c r e a s i n g monomer c o n c e n t r a t i o n and i n c r e a s i n g temperature i n a c c o r d a n c e w i t h t h e proposed mechanisms. The c o n t e n t of i n t e r n a l d o u b l e bonds i n s t e a d d e c r e a s e s w i t h i n c r e a s i n g temperatures. p

0

The c o n t e n t o f l a b i l e c h l o r i n e can be c a l c u l a t e d as the sum o f t e r t i a r y and i n t e r n a l a l l y l i c c h l o r i n e . The r e l a t i o n between t h i s measure and the r a t e o f d e h y d r o c h l o r i n a t i o n i s v e r y good. E x t r a p o l a t i o n -to z e r o c o n t e n t i n d i c a t e s the p r e s e n c e o f random dehydrochlorination. The t o t a l c o n t r i b u t i o n from t h i s type o f i n i t i a t i o n i s of, the same o r d e r as t h a t from i n t e r n a l a l l y l i c c h l o r i n e . However, t e r t i a r y c h l o r i n e must be c o n s i d e r e d as the most important l a b i l e s t r u c t u r e i n PVC. Acknowledgment s F i n a n c i a l support from the Swedish Board f o r T e c h n i c a l Development i s g r a t e f u l l y acknowledged. The a u t h o r s a l s o which t o e x p r e s s t h e i r thanks t o Anne Wendel f o r t h o r o u g h e x p e r i m e n t a l work.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20 Degradation of Poly(vinyl chloride) According to Non-Steady-State Kinetics J O S E P H D. D A N F O R T H

1

Grinnell College, Grinnell, IA 50112

Poly(vinyl chloride by a chain mechanis steady-state kinetics (NSSK). Degradations are initiated at a very few sites within a chain, and the subsequent zip reaction which accounts for substantially a l l of the evolved hydrogen chloride is confined to a single chain. The initiation reaction is temperamental. It is influenced by surfaces, impurities, and hydrogen chloride pressure. The number of initiations that result in a zip chain influences the average length of a zip chain, which in turn establishes the time at which the maximum degradation rate is attained. Although the uncertainty of the initiation reaction can significantly alter degradation patterns and make reproducibility under presumably identical conditions difficult to achieve, the NSSK model gives excellent agreement of observed and calculated data over the entire range of a degradation. The degradation of PVC has been described in terms of non-steadystate kinetics (NSSK) which were developed on the basis of the zipper mechanism (1,2). Observed degradation patterns were altered significantly by the presence of hydrogen chloride, inert surfaces, and intentionally added impurities (3). These degradation patterns were reproduced by NSSK using the best fit values of three parameters (previously named less descriptively) : k-^, the fraction of chains starting degradation per sec; k , the rate of unzipping of degrading chains expressed as the fraction of a chain unzipping per sec; and kx, a chain terminating constant that was based on certain assumptions about the way that impurities prevented the initiation of degradation chains. The initiation and zip constants were primarily responsible for degradation patterns. The terminating constant, kp, reduced the values of a, the fraction dehydrochlorinated, as a function of time. Its primary function was to obtain best f i t parameters when degradations did not go to completion. A l l degradation reactions for a l l samples were acceleratory. That is, the evolution z

1

Deceased. 0097-6156/85/0280-0285$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

286

POLYMER STABILIZATION AND DEGRADATION

of hydrogen c h l o r i d e was slow a t t h e b e g i n n i n g , i n c r e a s e d t h r o u g h a maximum and t h e n d e c r e a s e d . The i n i t i a t i o n r e a c t i o n a c c o u n t e d f o r o n l y n e g l i g i b l e amounts o f hydrogen c h l o r i d e . The z i p r e a c t i o n a c c o u n t e d f o r g r e a t e r t h a n 99% o f t h e e v o l v e d hydrogen c h l o r i d e ( 3 ) . A l p h a - t i m e c u r v e s c a l c u l a t e d from b e s t f i t v a l u e s o f t h e parameters c o u l d be superimposed upon t h e d a t a c u r v e s . E x c e l l e n t agreement o f t h e o r y and d a t a was a l s o o b t a i n e d i n t h e more c r i t i c a l comparison i n which d e g r a d a t i o n r a t e s were p l o t t e d as a f u n c t i o n o f t i m e . I n d i v i d u a l d e g r a d a t i o n c u r v e s f o r n i n e samples o f PVC from t h r e e d i f f e r e n t s u p p l i e r s have been o b s e r v e d a t many temperatures i n t h e range 210-240°C. Each d e g r a d a t i o n was e f f e c t i v e l y r e p r o d u c e d by t h e p r o p e r assignment o f b e s t f i t parameters, b u t t h e d u p l i c a t i o n o f d e g r a d a t i o n c u r v e s under presumably i d e n t i c a l r u n c o n d i t i o n s was uncertain. D u p l i c a t i o n was o f t e n a t t a i n e d f o r r u n s made i n sequence, but a r u n made a t a n o t h e r time under presumably i d e n t i c a l c o n d i t i o n s would o c c a s i o n a l l y g i v e an a l t e r e d d e g r a d a t i o n p a t t e r n and s i g n i f i cantly d i f f e r e n t value I t i s now a p p a r e n t c h a r a c t e r i s t i c of zipper k i n e t i c s . Even though r e p r o d u c i b i l i t y i s a r e c o g n i z e d problem, t h e v a l u e s o f t h e parameters k i and k have t h e o r e t i c a l and p r a c t i c a l s i g n i f i c a n c e and c a n be d i r e c t l y r e l a t e d to t h e p r o c e s s e s t h a t o c c u r i n any s p e c i f i c d e g r a d a t i o n . K i n e t i c models o f polymer d e g r a d a t i o n based on c h a i n p r o c e s s e s have been s u g g e s t e d ( 4 - 8 ) . These models and o t h e r models f o r p o l y mer d e g r a d a t i o n have n o r m a l l y i n v o k e d t h e s t e a d y - s t a t e h y p o t h e s i s (9). B a r r o n and Boucher (10) d e s c r i b e a k i n e t i c model f o r z i p k i n e t i c s b u t assume an apparent f i r s t o r d e r b e h a v i o r and do n o t a c c o u n t e f f e c t i v e l y f o r t h e a c c e l e r a t o r y phase o f t h e d e g r a d a t i o n . MacCallum has q u e s t i o n e d t h e v a l i d i t y o f t h e s t e a d y - s t a t e h y p o t h e s i s (11) b u t d i d n o t d e v e l o p a p r a c t i c a l k i n e t i c model f o r NSSK. I t i s the non-steady-state f e a t u r e that i s r e s p o n s i b l e f o r the s u c c e s s f u l a p p l i c a t i o n o f t h i s model t o t h e z i p d e g r a d a t i o n o f PVC. z

Experimental The a p p a r a t u s w h i c h measures p r e c i s e l y t h e r a t e s o f gas e v o l u t i o n a s a f u n c t i o n o f time has been d e s c r i b e d (2, 12-14). PVC samples from G o o d r i c h and S c i e n t i f i c Polymer P r o d u c t s have been c h a r a c t e r i z e d by the name o f t h e s u p p l i e r and t h e average degree o f p o l y m e r i z a t i o n . C u r v e f i t t i n g t e c h n i q u e s and t h e computer g e n e r a t i o n of d e g r a d a t i o n c u r v e s f o r s i n g l e and mixed parameters have been d e s c r i b e d (15). The e q u a t i o n s r e p r e s e n t i n g NSSK when i n i t i a t i o n i s f i r s t o r d e r have been d e r i v e d f o r c h a i n t e r m i n a t i o n t h a t i s f i r s t o r d e r i n i m p u r i t i e s (16). Other i n i t i a t i o n and z i p o r d e r s have been e v a l u a t e d . A l l nons t e a d y - s t a t e models appear t o be markedly s u p e r i o r t o models t h a t i n v o k e t h e s t e a d y - s t a t e h y p o t h e s i s , b u t none i s a p p r e c i a b l y b e t t e r than t h e s i m p l e assumption o f f i r s t o r d e r i n i t i a t i o n and z e r o o r d e r zip. The d e r i v a t i o n and e v a l u a t i o n o f o t h e r n o n - s t e a d y - s t a t e models w i l l be t h e s u b j e c t o f a l a t e r p u b l i c a t i o n . R e s u l t s and D i s c u s s i o n The

equations

representing the behavior of a c c e l e r a t o r y degradations

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20.

Degradation of Polyvinyl chloride)

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w h i c h have been p r e v i o u s l y d e s c r i b e d ( 1 , 2 , 1 6 ) a r e summarized i n Table I. Though complex i n a p p e a r a n c e , n o n - s t e a d y - s t a t e equations a r e based on v e r y s i m p l e assumptions about i n i t i a t i o n and p r o p a g a tion. W i t h computer f a c i l i t i e s a v a i l a b l e i t i s no l o n g e r n e c e s s a r y to i n v o k e s t e a d y - s t a t e assumptions i n o r d e r t o s o l v e t h e p r o b l e m s . The i d e a l e q u a t i o n r e p r e s e n t s e x p e c t e d b e h a v i o r s when t h e r e i s no c h a i n t e r m i n a t i o n by s u r f a c e s and i m p u r i t i e s . When c h a i n t e r m i n a t i o n o c c u r s d u r i n g the i n i t i a l s t a t e s o f a d e g r a d a t i o n , o n l y a p o r t i o n o f the c h a i n s t h a t a r e t h e r m a l l y i n i t i a t e d a c t u a l l y p r o d u c e s a zip reaction. R e a c t i o n o f an i n i t i a t e d c h a i n w i t h s u r f a c e o r i m p u r i t i e s p r e v e n t s the z i p r e a c t i o n f o r t h a t i n i t i a t i o n . The n o n i d e a l e q u a t i o n p r o p e r l y a c c o u n t s f o r i n i t i a t i o n s t h a t do not r e s u l t in a zip reaction.

Table I.

Equations for Non-Steady-State

Kinetics

Ideal Equatio A c c e l e r a t o r y Phase,

t = o to t =

^/^

a = k t

exp

k /k.

+

z

(k /k.) z

D e c e l e r a t o r y Phase, (Wk.)

a = 1 +

(-k.t)

t = l/k

exp

(-

-

z

to t =

z

k.t)

z

[(1

-

0 0

exp

(k /k )] ±

z

Equation with Chain Termination A c c e l e r a t o r y Phase, a = k t

+

k /k.

t = o to t =

exp

z

D e c e l e r a t o r y Phase,

(-

k.t

t = 1/k

1/k z (k k /k.)

+

z

to t =

T

exp

(-k.t)

0 0

z a = (k /k.) z

exp

(-

-

(k^ly^k.)

exp

-

(k k /2k.)

(1 -

z

T

k.t)

(-

(1 -

2k.t)

exp

(-

exp

(1 -

(k./k ))

exp

z

(k./k )) z

+ 1

k./k )) z

The a v s . time c u r v e s o f F i g u r e 1 have been g e n e r a t e d from a s s i g n e d v a l u e s o f k ^ , k , and k to i l l u s t r a t e how t h e v a l u e s o f the parameters i n f l u e n c e d e g r a d a t i o n p a t t e r n s . For a z i p r a t e , k = 0.0005 s e c " " * , t h e i n f l e c t i o n p o i n t o f the r e s u l t i n g a - t c u r v e s w i l l f a l l at 2000 sec because the maximum r a t e w i l l appear a t t = l/k . A t c o n s t a n t k the d e g r a d a t i o n p a t t e r n s a r e s i g n i f i c a n t l y a l t e r e d by the v a l u e s o f k^ and t o a l e s s e r e x t e n t by t h e v a l u e s assigned to kp. F i g u r e 2 p r e s e n t s g e n e r a t e d r a t e d a t a as Aa/100 s e c v s . t . The m i d d l e c u r v e o f F i g u r e 2 r e p r e s e n t s t h e r a t e d a t a f o r t h e c a s e where k . = k = 0.0005 and K j , = 0. The maximum r a t e appears a t 2000 s e c , wfeich i s the i n f l e c t i o n p o i n t o f the c o r r e s p o n d i n g a - t c u r v e o f z

T

z

z

z

z

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

288

POLYMER STABILIZATION AND DEGRADATION

1000

2000

3000

SECONDS Figure 1. Calculated alpha-time curves for assigned values of sec - 1 . sec"" ; and k . ( A ) ki = 0.0005, kp - 0; 0.00010, (A) i = 0.0005, k = 0.0005, k = 0.2; (•) k 0.0005, 0; i - 0.0001, k = 0.0010, kr = 0.

ki,

1

k

?

z

±

x

k

z

2000 SECONDS

Figure 2. Calculated rate curves for changing k±, sec and k , sec . (0) k = 0.0005, k = 0.0010; ( Q ) k = k = 0.0005; (A) i 0.0001, k = 0.0010. 1

- 1

±

k

=

z

±

z

z

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

z

20.

Degradation of Polyvinyl chloride)

DANFORTH

289

F i g u r e 1. The two upper c u r v e s o f F i g u r e 2 r e p r e s e n t d e g r a d a t i o n b e h a v i o r s a t c o n s t a n t i n i t i a t i o n ( k i = 0.0005 s e c " ) f o r k z v a l u e s of 0.001 sec"" (upper c u r v e ) and 0.0005 s e c " ( m i d d l e c u r v e ) . The lower r a t e c u r v e has t h e same kg v a l u e as the upper c u r v e b u t t h e v a l u e o f k i has been reduced t e n - f o l d t o 0.0001 s e c " . These gene r a t e d r a t e c u r v e s i l l u s t r a t e t h e d r a s t i c changes i n d e g r a d a t i o n p a t t e r n s t h a t c a n be e x p e c t e d from changes i n t h e i n i t i a t i o n and z i p constants. A t v a l u e s w i t h i n t h e range n o r m a l l y e n c o u n t e r e d t h e c h a i n t e r m i n a t i n g c o n s t a n t l o w e r s somewhat t h e i n i t i a l p o r t i o n s o f a d e g r a d a t i o n c u r v e b u t does n o t s i g n i f i c a n t l y change d e g r a d a t i o n p a t t e r n s a t l o n g e r times and h i g h e r c o n v e r s i o n s . 1

1

1

1

I n a d d i t i o n t o t h e t h r e e p a r a m e t e r s , k i , k , and kp, t h e r e i s a time s h i f t , t . The time s h i f t i s n e c e s s a r y t o a c c o u n t f o r i n d u c t i o n periods. When c h a i n s a r e immediately t e r m i n a t e d d u r i n g an i n d u c t i o n p e r i o d , the k i n e t i c model based on z i p k i n e t i c s o b v i o u s l y does n o t apply. The time s h i f t g i v e s an i n i t i a l t i m e t h a t depends on t h e behavior of the degradatio almost n e g l i g i b l e . It of c u r v e f i t t i n g b u t does n o t a l t e r t h e g e n e r a l shape o f t h e d e g r a d a t i o n curve. I t has been shown i n e a r l i e r work t h a t z i p k i n e t i c s g i v e b e s t f i t p a r a m e t e r s t h a t w i l l r e p r o d u c e a - t and r a t e - t i m e c u r v e s o v e r t h e e n t i r e range o f a d e g r a d a t i o n (3,16). The q u a n t i t a t i v e s i g n i f i c a n c e of t h e i n i t i a t i o n and z i p c o n s t a n t s was i m p l i e d b u t n o t emphasized because d i f f e r e n t d e g r a d a t i o n p a t t e r n s and c o r r e s p o n d i n g l y d i f f e r e n t parameter v a l u e s were o f t e n o b t a i n e d f o r t h e same sample under p r e sumably t h e same c o n d i t i o n s . I t i s now a p p a r e n t t h a t d i f f i c u l t r e p r o d u c i b i l i t y i s an e x p e c t e d c h a r a c t e r i s t i c of z i p k i n e t i c s . The n a t u r e o f z i p k i n e t i c s t h a t makes r e p r o d u c t i o n o f d a t a d i f f i c u l t w i l l be d e s c r i b e d and t h e quant i t a t i v e c h a r a c t e r i z a t i o n o f sample b e h a v i o r i n terms o f i n i t i a t i o n and z i p parameters w i l l be demonstrated. I t i s u s e f u l f o r the b e t t e r u n d e r s t a n d i n g o f z i p k i n e t i c s t o i l l u s t r a t e how a p p r o x i m a t e d e g r a d a t i o n d a t a c o r r e s p o n d i n g t o o b s e r v e d d a t a c a n be c a l c u l a t e d f o r two h y p o t h e t i c a l samples. E a c h i s 100 mg; t h e i r c h a i n l e n g t h s a r e 500 and 1000 v i n y l c h l o r i d e u n i t s . The term, " a p p r o x i m a t e , i s a p p r o p r i a t e because o f t h e way Aa/A more c l o s e l y approaches d a / d t , and t h e term, " a p p r o x i m a t e " , would be u n n e c e s s a r y for the c a l c u l a t e d data. These c a l c u l a t e d d a t a r e p r e s e n t an i d e a l s i t u a t i o n i n w h i c h t h e r e i s no m i x t u r e o f c h a i n l e n g t h s and i n which no premature t e r m i n a t i o n o f z i p c h a i n s o c c u r s . A c t u a l samples w i l l always have some v a r i a t i o n s i n c h a i n l e n g t h , and s t a r t i n g c h a r a c t e r i s t i c s may a l s o show v a r i a t i o n s due t o premature c h a i n t e r m i n a t i o n . A l t h o u g h i t has been shown t h a t s i n g l e v a l u e d p a r a m e t e r s g i v e good r e p r e s e n t a t i o n o f d a t a g e n e r a t e d from samples i n w h i c h c h a i n l e n g t h s and s t a r t i n g c h a r a c t e r i s t i c s have been p u r p o s e l y mixed (3,15), t h e term " a p p r o x i m a t e " w i l l always be a p p r o p r i a t e when s i n g l e v a l u e d p a r a m e t e r s a r e used t o r e p r e s e n t a c t u a l d e g r a d a t i o n i n w h i c h t h e r e must be c h a i n s o f d i f f e r e n t l e n g t h s . z

g

11

C a l c u l a t i o n o f D e g r a d a t i o n Data f o r H y p o t h e t i c a l Samples o f D i f f e r e n t Degrees o f P o l y m e r i z a t i o n . One hundred m i l l i g r a m s o f PVC, r e g a r d l e s s o f c h a i n l e n g t h , w i l l c o n t a i n 0.100 • DP moles hydrogen MW

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

290

c h l o r i d e where DP i s the v i n y l c h l o r i d e u n i t s per c h a i n , and MW i s the number a v e r a g e m o l e c u l a r w e i g h t . I f t h e weight o f t h e sample i s doubled the r a t e of hydrogen c h l o r i d e e v o l u t i o n e x p r e s s e d as moles hydrogen c h l o r i d e • sec"" w i l l d o u b l e , but the r a t e e x p r e s s e d as Aasec"" w i l l n o t change w i t h the sample s i z e . The r a t e e x p r e s s e d as moles HC1 • sec"" w i l l depend upon the number o f c h a i n s t h a t a r e unz i p p i n g and the i n t r i n s i c r a t e o f u n z i p p i n g e x p r e s s e d as moles HC1 • chain • sec" . I n the z i p p e r mechanism t h e i n t r i n s i c r a t e o f unz i p p i n g i s assumed t o be t h e same f o r a l l samples r e g a r d l e s s o f c h a i n l e n g t h . However k , t h e f r a c t i o n o f a c h a i n u n z i p p i n g p e r s e c , w i l l depend upon the c h a i n l e n g t h . D u r i n g the a c c e l e r a t o r y phase o f a d e g r a d a t i o n the moles hydrogen c h l o r i d e e v o l v e d p e r s e c a t any time w i l l be the moles o f d e g r a d i n g c h a i n s m u l t i p l i e d by the i n t r i n s i c rate. The moles of d e g r a d i n g c h a i n s d u r i n g a c c e l e r a t i o n can be e s t i mated a t any time as the c u m u l a t i v e sum o f c h a i n s t h a t have s t a r t e d during preceding i n t e r v a l s . 1

1

- 1

-1

z

The c h a i n s t h a t s t a r t h e sample w e i g h t , t h e degre i n i t i a t i o n c o n s t a n t , k^, and the l e n g t h o f the i n t e r v a l . Thus, 0.10 | samples w i l l c o n t a i n i n i t i a l l y 1.60 • I O " (DP 1000) and 3.2 x 10~ (DP 500) moles o f polymer c h a i n s . I f k^ i s a s s i g n e d t h e r e a s o n a b l e v a l u e , 0.0005 s e c " , t h e moles o f polymer c h a i n s u n z i p p i n g a f t e r 100 s e c w i l l be (moles o f i n i t i a l polymer) • 0.0005 sec"" • 100 s e c . The moles o f polymer c h a i n s s t a r t i n g d u r i n g the f i r s t 100 s e c can be s u b t r a c t e d from t h e number of polymer c h a i n s p r e s e n t i n i t i a l l y t o g i v e the number o f u n s t a r t e d c h a i n s p r e s e n t a t the s t a r t of the i n t e r v a l , 100-200 s e c . The number o f c h a i n s s t a r t i n g d u r i n g t h e 100-200 s e c i n t e r v a l can t h e n be c a l c u l a t e d . D u r i n g the a c c e l e r a t i n g p e r i o d the c u m u l a t i v e sum o f t h e average number of c h a i n s s t a r t i n g p e r i n t e r v a l m u l t i p l i e d by the i n t r i n s i c ^ i p r a t e w i l l g i v e t h e moles HC1 p e r i n t e r v a l . When t = l / k the f i r s t s t a r t e d c h a i n s a r e c o m p l e t e l y decomposed. A t t = l / k + 1 0 0 the number of p r o d u c i n g c h a i n s c a l c u l a t e d as t h e c u m u l a t i v e sum must be c o r r e c t e d by s u b t r a c t i n g t h e c h a i n s t h a t have t e r m i n a t e d . The s u b t r a c t e d number w i l l be the a v e r a g e number o f c h a i n s p r o d u c i n g from 0-100 s e c , and so on f o r subsequent i n t e r v a l s . The a v e r a g e number o f p r o d u c i n g c h a i n s m u l t i p l i e d by the i n t r i n s i c r a t e (DP • k ) g i v e s the r a t e o f hydrogen c h l o r i d e e v o l u t i o n as moles HC1 p e r i n t e r v a l . This value d i v i d e d by the moles h y d r o g e n c h l o r i d e i n the o r i g i n a l samples g i v e s Aa/interval. The c a l c u l a t e d moles hydrogen c h l o r i d e e v o l v e d p e r i n t e r v a l can be e n t e r e d i n the same way t h a t o b s e r v e d peak a r e a s of an a c t u a l r u n a r e e n t e r e d and the b e s t f i t v a l u e s of t h e parameters obtained. The b e s t f i t v a l u e s of the parameters from the c a l c u l a t e d d a t a , as would be e x p e c t e d , a r e e s s e n t i a l l y the same as t h o s e v a l u e s t h a t were used i n making the c a l c u l a t i o n s . The i d e a l e q u a t i o n shown i n T a b l e 1 d e s c r i b e s more s i m p l y the k i n e t i c b e h a v i o r t h a t has been e s t i m a t e d by the p r e c e d i n g s t e p w i s e c a l c u l a t i o n s . The s t e p w i s e c a l c u l a t i o n i s s i m i l a r t o the way r u n d a t a a r e o b t a i n e d s i n c e i n a r u n the peak a r e a r e c o r d e d r e p r e s e n t s A a / i n t e r v a l . 6

6

1

1

z

z

z

T h e r e i s one c r i t i c a l p o i n t t h a t r e q u i r e s a d d i t i o n a l comment. When the i n t r i n s i c r a t e i s a s s i g n e d a v a l u e , DP • k , the unwarranted assumption t h a t a z i p c h a i n l e n g t h i s the same as t h e l e n g t h o f a polymer c h a i n has been made. T h i s a s s u m p t i o n would be t r u e i f t h e r e were one i n i t i a t i o n s i t e p e r polymer c h a i n . I f t h e r e were two i n i z

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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t i a t i o n s i t e s p e r polymer c h a i n t h e z i p c h a i n would be o n e - h a l f as l o n g but the f r a c t i o n u n z i p p i n g per sec would be d o u b l e d . The i n t r i n s i c z i p r a t e as i n i t i a l l y assumed f o r z i p k i n e t i c s would not depend upon c h a i n l e n g t h . However, the number of s t a r t s p e r c h a i n w i l l c e r t a i n l y i n f l u e n c e b o t h k^ and k . The s t a r t s p e r c h a i n must be a s m a l l number i f the z i p p e r mechanism i s t o a p p l y . A reasonable model f o r PVC would assume t h a t each sample c o n t a i n e d an a v e r a g e number o f p o t e n t i a l s t a r t i n g p o s i t i o n s p e r c h a i n c h a r a c t e r i s t i c of t h a t sample. The a c t u a l number of s t a r t s t h a t l e d t o a z i p r e a c t i o n would depend upon the i m p u r i t i e s p r e s e n t and the number o f s u r f a c e c o n t a c t s t h a t were c h a i n t e r m i n a t i n g . The p a r t i a l p r e s s u r e of h y d r o gen c h l o r i d e , which has been shown t o i n f l u e n c e the i n i t i a l r e a c t i o n , would a l s o i n f l u e n c e not o n l y the r a t e of i n i t i a t i o n but a l s o the f r a c t i o n of p o t e n t i a l s t a r t i n g p o s i t i o n s t h a t a c t u a l l y i n i t i a t e s a z i p r e a c t i o n . S i n c e the b u i l d u p of hydrogen c h l o r i d e a t the b e g i n n i n g of a d e g r a d a t i o n depends on z i p c h a i n s a l r e a d y p r e s e n t and weak l i n k a g e s t h a t s l o w l y decompos hydrogen c h l o r i d e c o n t r i b u t e same way t h a t c h a i n t e r m i n a t i n g i m p u r i t i e Thus, i n i t i a t i o n comprises o n l y an i n s i g n i f i c a n t p a r t o f d e g r a d a t i o n but i n f l u e n c e s s i g n i f i c a n t l y the number of c h a i n s t h a t w i l l be p r o d u c i n g a t any time and the avergage c h a i n l e n g t h t h a t t h o s e c h a i n s w i l l have. The a v e r a g e c h a i n l e n g t h i s d i r e c t l y r e l a t e d t o the time a t w h i c h the maximum r a t e i s a t t a i n e d ( t a x - l / k ) so t h o s e substances or run c o n d i t i o n s that a l t e r s t a r t i n g c h a r a c t e r i s t i c s w i l l a l s o a l t e r somewhat the v a l u e of k . Changes i n k^ and k , as i l l u s t r a t e d i n F i g u r e s 1 and 2, s i g n i f i c a n t l y i n f l u e n c e d e g r a d a t i o n p a t terns. Thus, the p r e s e n c e of Chromsorb o r a p i e c e of a p a p e r c l i p , p r e h e a t i n g a sample, the sample s i z e w h i c h i n f l u e n c e s the r a t i o of sample t o s u r f a c e c o n t a c t s , the p a r t i a l p r e s s u r e of hydrogen c h l o r i d e , the r a t e a t w h i c h hydrogen c h l o r i d e forms i n the sample, and c h a i n t e r m i n a t i n g i m p u r i t i e s a l r e a d y p r e s e n t (3) w i l l a l t e r the i n i t i a t i o n c h a r a c t e r i s t i c s and t h e s e i n t u r n w i l l i n f l u e n c e the a v e r a g e c h a i n length. z

m

z

z

z

S i n c e i t has a l r e a d y been e s t a b l i s h e d t h a t PVC d a t a from the b e g i n n i n g t o t h e end o f a t h e r m a l d e g r a d a t i o n can be r e p r o d u c e d from NSSK, (2,3,16) t h e r e seems t o be no r e a s o n t o b u r d e n t h e l i t e r a t u r e w i t h the many d e g r a d a t i o n c u r v e s and b e s t f i t p a r a m e t e r s t h a t have been o b t a i n e d f o r o v e r 600 i n d i v i d u a l r u n s u s i n g n i n e d i f f e r e n t samples and a v a r i e t y of r u n c o n d i t i o n s . I t does seem a p p r o p r i a t e t o i l l u s t r a t e how d a t a can be r e p r o d u c e d under v e r y c a r e f u l l y c o n t r o l l e d c o n d i t i o n s and t o show how o c c a s i o n a l samples d e v i a t e from t h e i r expected behavior. I t w i l l a l s o be s u g g e s t e d t h a t f o r e i g h t of the n i n e samples s t u d i e d the c h a i n l e n g t h of the polymer has an o v e r r i d i n g i n f l u e n c e on d e g r a d a t i o n p a t t e r n s . The b e h a v i o r of one sample t h a t d i d not f o l l o w t h e d e g r a d a t i o n p a t t e r n t h a t was e x p e c t e d on the b a s i s o f i t s c h a i n l e n g t h w i l l be c o n s i d e r e d . The i n f l u e n c e of p r e h e a t i n g on d e g r a d a t i o n p a t t e r n s w i l l a l s o be d e s c r i b e d . The C h a r a c t e r i z a t i o n of PVC Samples i n Terms of T h e i r K i n e t i c P a r a m e t e r s . Under c a r e f u l l y c o n t r o l l e d c o n d i t i o n s runs t h a t a r e made i n sequence may g i v e r e p r o d u c i b l e d a t a and i l l u s t r a t e t h a t d i f f i c u l t i e s o f r e p r o d u c i b i l i t y a r e n o t caused by s h o r t c o m i n g s o f the t e c h n i q u e s and a p p a r a t u s used i n the measurements. The sample G o o d r i c h

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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POLYMER STABILIZATION AND DEGRADATION

684 has been chosen t o i l l u s t r a t e t h e r e p r o d u c i b i l i t y o f d a t a and a l s o t o i l l u s t r a t e t h a t o c c a s i o n a l runs do not d u p l i c a t e the e x p e c t e d d e g r a d a t i o n p a t t e r n . F i g u r e 3 shows a - t c u r v e s f o r i d e n t i c a l runs a t 225°C u s i n g samples o f G o o d r i c h 684 i n t h e 70-80 mg range. Three samples d e m o n s t r a t e r e p r o d u c i b i l i t y . The o t h e r sample i s s i g n i f i cantly different. Rate c u r v e s f o r t h e s e samples a r e r e p r o d u c e d i n F i g u r e 4. T a b l e I I g i v e s t h e v a l u e s o f t h e b e s t f i t parameters o b t a i n e d by m i n i m i z i n g t h e d i f f e r e n c e s between t h e o b s e r v e d and c a l c u l a t e d v a l u e s of a o v e r t h e e n t i r e range of t h e d e g r a d a t i o n . In t h e s e runs t h e sample was removed s h o r t l y a f t e r t h e d e g r a d a t i o n r e a c h e d t h e d e c e l e r a t o r y phase.

Table I I .

B e s t F i t Parameters f o r G o o d r i c h

i 1 1 (sec-i-lO^) 1.37 2.24 2.15 2.13

684

a t 225°C

k

Run 2D46A 2D25A 2D24A 2D49A

0.65 0.82 0.83 0.76

0.36 0.78 0.59 0.48

0.73 0.97 0.94 1.17

189 131 197 211

U s i n g t h e n o n - i d e a l e q u a t i o n o f T a b l e I and t h e b e s t f i t v a l u e s o f t h e parameters, c a l c u l a t e d a - t and r a t e d a t a a g r e e w i t h . o b s e r v e d d a t a , and the c a l c u l a t e d time o f t h e maximum r a t e f a l l s v e r y c l o s e t o t h e time of t h e o b s e r v e d maximum r a t e . Each s e t o f d e g r a d a t i o n d a t a i s e f f e c t i v e l y r e p r e s e n t e d by t h e b e s t f i t parameters, y e t t h e r e i s a s i g n i f i c a n t v a r i a t i o n o f parameter v a l u e s f o r r u n 2D46A which was presumably r u n under c o n d i t i o n s i d e n t i c a l w i t h t h e o t h e r r u n s . Run 2D46A d i s p l a y s the c h a r a c t e r i s t i c s t h a t a r e always e n c o u n t e r e d i n runs t h a t misbehave. The v a l u e s o f and k a r e lower than t h o s e o f t h e "good" runs w h i c h i n v a r i a b l y show more r a p i d a c c e l e r a t i o n . The j u s t i f i c a t i o n f o r d i f f i c u l t r e p r o d u c i b i l i t y and d e c r e a s e i n k^ and k f o r runs t h a t misbehave i s of c o u r s e i n h e r e n t i n t h e mechanism. The i n i t i a t i o n r e a c t i o n r e p r e s e n t s o n l y an i n s i g n i f i c a n t p a r t of t h e o v e r a l l reaction. I t i s extremely s e n s i t i v e to i m p u r i t i e s , to s u r f a c e c o n t a c t s , and t o t h e p a r t i a l p r e s s u r e of hydrogen c h l o r i d e . The d a t a o f F i g u r e s 1 and 2 i l l u s t r a t e t h e d r a m a t i c changes i n d e g r a d a t i o n p a t t e r n s t h a t a r e t o be e x p e c t e d f o r changes i n t h e f r a c t i o n o f c h a i n s s t a r t i n g p e r s e c even i f t h e c h a i n l e n g t h remained unchanged. However, when t h e f r a c t i o n i n i t i a t e d p e r sec i s d e c r e a s e d the f r a c t i o n o f a c t u a l s t a r t s p e r c h a i n i s a l s o somewhat d e c r e a s e d . Even though t h e p o t e n t i a l s t a r t s p e r c h a i n a r e i n i t i a l l y i d e n t i c a l f o r a g i v e n sample, the a c t u a l number of z i p c h a i n s s t a r t e d p e r polymer c h a i n i s l e s s than n o r m a l under poor s t a r t i n g c o n d i t i o n s . The fewer the number s t a r t s p e r c h a i n the l o n g e r w i l l be t h e average l e n g t h o f a z i p c h a i n , and t h i s i s r e f l e c t e d i n a lower v a l u e of k and a l o n g e r time t o a c h i e v e t h e maximum r a t e . z

z

z

B e f o r e the i m p l i c a t i o n s o f t h e z i p p e r mechanism i n terms o f r e p r o d u c i b l e d a t a were f u l l y a p p r e c i a t e d , e f f o r t s t o a t t a i n r e p r o d u c i b i l i t y commensurate w i t h t h e q u a l i t y of the a p p a r a t u s and the method o f o p e r a t i o n l e d t o hundreds of d e g r a d a t i o n r u n s . Some of t h e s e runs

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

20.

DANFORTH

Degradation of Polyvinyl chloride)

293

1000 SECONDS F i g u r e 3. F r a c t i o n degraded v s time f o r i d e n t i c a l r u n c o n d i t i o n s G o o d r i c h 684 a t 225°C. ( A ) 2D46A, ( Q ) 2D25A, ( A ) 2D24A, (Q) 2D49A.

1000 SECONDS F i g u r e 4. R a t e - t i m e c u r v e s f o r G o o d r i c h 684 a t 225°C, ( A ) (Q) 2D25A, ( A ) 2D24A, ( Q ) 2D49 A.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

2D46A,

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294

gave good i n i t i a l a c c e l e r a t i o n and were i n t u i t i v e l y c o n s i d e r e d t o be "good r u n s " . O t h e r s d i d n o t " t a k e o f f " and a l t h o u g h t h e y gave b e s t f i t parameters t h a t would d u p l i c a t e the d a t a f o r t h a t r u n , the p a r a meter v a l u e s d i f f e r e d somewhat from t h o s e o b t a i n e d f o r o t h e r runs under presumably t h e same c o n d i t i o n s . From many hundreds of runs on a v a r i e t y of samples t h e r e a r e a number of d e g r a d a t i o n b e h a v i o r s t h a t can be q u a n t i t a t i v e l y e x p r e s s e d i n terms of parameter v a l u e s . From t h e v e r y l a r g e amounts of d a t a a v a i l a b l e some of t h e sample c h a r a c t e r i s t i c s and r u n v a r i a b l e s t h a t a r e r e f l e c t e d i n the v a l u e s of the parameters w i l l be d e s c r i b e d . Changes i n t h e Number of P o t e n t i a l S t a r t s Per C h a i n Can I n f l u e n c e t h e D e g r a d a t i o n P a t t e r n . The sample SP 917 has p r e v i o u s l y been shown t o a c c e l e r a t e more r a p i d l y and t o a t t a i n i t s maximum r a t e i n l e s s time t h a n SP 634 and SP 1381 ( 3 ) . Samples SP 634 and SP 1381 f o l l o w a p p r o x i m a t e l y the p a t t e r n of a l l G o o d r i c h samples i n w h i c h a t comparable conditions long chai l o n g e r time t o a c h i e v e t h e i r e p r e s e n t a t i v e p l o t s o f Aa/60 sec as a f u n c t i o n o f time f o r SP 634, SP 917 and SP 1381. A l l runs were made by t h e d i r e c t i n t r o d u c t i o n o f t h e sample i n t o the r e a c t i o n chamber a f t e r t h e temperature of t h e r u n had been a t t a i n e d . B e s t f i t v a l u e s o f t h e p a r a m e t e r s f o r each o f the t h r e e runs and t h e i n t e r p o l a t e d v a l u e s e x p e c t e d f o r SP 917 on t h e b a s i s of the l e n g t h of t h e c h a i n a r e r e c o r d e d i n T a b l e I I I .

T a b l e I I I . B e s t F i t V a l u e s o f Parameters f o r t h e o f SP 634, SP 917 and SP 1381 a t 225°C Comparable C o n d i t i o n s

k

Sample SP 634 SP 917 SP 1381 Interp. 917

Run C110 C109 C108 _

i - l (sec «10 ) 1.12 1.73 0.58 3 J

Degradations

k

i - l 3 fsec -10 ) 0.68 0.85 0.32 J

s sec 171 46 164 r

0.6 1.0 1.0 _

0.52

0.88

t

A,% d i f per p o i n t

0.53 0.39 0.44

_

_

The parameter v a l u e s o f SP 634 and SP 1381 a r e i n t h e range o f v a l u e s e x p e c t e d from d e g r a d a t i o n s t u d i e s on f i v e G o o d r i c h samples i n w h i c h the time t o a c h i e v e t h e maximum r a t e ( l / k ) i n c r e a s e d w i t h i n c r e a s i n g c h a i n l e n g t h o f t h e sample. Sample SP 917 i s c o m p l e t e l y out o f l i n e , g i v i n g k^, 1.73 x 10~ s e c " and k , 0.85 x 10" sec vs e x p e c t e d v a l u e s o f 0.88 x 10"" s e c " and 0.52 x 10~ sec" , respectively. On t h e b a s i s o f t h e p r e v i o u s d i s c u s s i o n s o f mechanism, t h e anomalous b e h a v i o r o f SP 917 can be u n d e r s t o o d i f t h a t sample, e i t h e r i n p r e p a r a t i o n o r i n subsequent h a n d l i n g , had s i g n i f i c a n t l y more p o t e n t i a l s t a r t i n g p o s i t i o n s p e r c h a i n . The f r a c t i o n s t a r t i n g p e r sec and t h e number of a c t u a l z i p s p e r c h a i n a r e s i g n i f i c a n t l y g r e a t e r than would be e x p e c t e d f o r t h a t c h a i n l e n g t h . A l t h o u g h no t e c h n i c a l i n f o r m a t i o n i s a v a i l a b l e concerning the s t a b i l i t y characz

3

1

3

- 1

z

3

1

3

1

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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295

Degradation of Polyvinyl chloride)

t e r i s t i c s o f SP 917, i t would be e x p e c t e d t h a t t h e sample would be l e s s s t a b l e t h a n t h e o t h e r samples t h a t have been s t u d i e d . Thermal P r e t r e a t m e n t I n f l u e n c e s t h e S t a r t i n g C h a r a c t e r i s t i c s and t h e Degradation Pattern. Even though a l l PVC samples e x c e p t SP 917 demonstrate t h a t t h e r e i s a u n i f o r m dependence o f t h e d e g r a d a t i o n p a t t e r n on t h e c h a i n l e n g t h o f t h e sample, t h e r e i s a s i g n i f i c a n t i n f l u e n c e o f t h e t h e r m a l p r e t r e a t m e n t o f t h e sample on t h e d e g r a d a tion pattern. F i g u r e 6 shows a c o m p a r i s o n o f d e g r a d a t i o n p a t t e r n s f o r SP 1381 a f t e r t h e r m a l p r e t r e a t m e n t s a t 235°C o f 0, 2, and 3 minutes. Thermal p r e t r e a t m e n t seems d e f i n i t e l y t o i n c r e a s e t h e number of p o t e n t i a l s t a r t s p e r c h a i n and t h e number o f z i p c h a i n s a c t u a l l y started. Because t h e number o f z i p c h a i n s s t a r t e d i s i n c r e a s e d , t h e a v e r a g e l e n g t h o f a z i p c h a i n i s r e d u c e d , so k , t h e f r a c t i o n o f a chain unzipping per sec, i s increased. z

The b e h a v i o r o f samples a f t e r p r e h e a t i n g a t 235°C i l l u s t r a t e s the problem t h a t one e n c o u n t e r a r e compared. D u r i n g t h sample i s p r e h e a t e d a t t h e r u n t e m p e r a t u r e . As has been shown, t h e time, and presumably t h e t e m p e r a t u r e a t which t h e p r e h e a t i n g o c c u r s , i n f l u e n c e t h e d e g r a d a t i o n p a t t e r n o f t h e sample. Thus, even though r u n s a t d i f f e r e n t t e m p e r a t u r e s g i v e d e g r a d a t i o n p a t t e r n s t h a t c a n be p r e c i s e l y a c c o u n t e d f o r by t h e b e s t f i t parameters o f NSSK, t h e r e i s no a s s u r a n c e t h a t t h e p a r a m e t e r s a t t h e two d i f f e r e n t t e m p e r a t u r e s a r e d i r e c t l y comparable. T h i s problem was r e c o g n i z e d and has been more f u l l y d i s c u s s e d i n a p u b l i c a t i o n t h a t d e s c r i b e s t h e s p e c i a l p r e c a u t i o n s t h a t a r e neces 300 urn) i r r a d i a t e d a t t h e h i g h e r dose r a t e ( 1 . 3 5 Mrad h " ) i n a i r , a weak macro-alkyl s i g n a l was v i s i b l e under the dominant p e r o x y l signal immediately a f t e r i r r a d i a t i o n , but c o n v e r t e d q u i t e q u i c k l y ( u n d e t e c t a b l e a f t e r - 10 m i n u t e s a t 6 0 ° C o r - 2 hours a t 2 3 ° C ) t o p e r o x y l radicals. P h o t o - o x i d a t i o n o f t h e a d d i t i v e - f r e e PP f i l m gave s i m i l a r o x i dation product y i e l d s and e m b r i t t l e m e n t as exposure t o the 1.35 Mrad.h-i y - c e l l (Table 1 ) . Y

1

Post-y-Degradation. During storage of a p r e - i r r a d i a t e d f i l m , o x i dation continues steadily as can be seen from the IR changes i n F i g u r e 1. The p e r o x y zero (Figure 2 ) . The decay o f p e r o x y l r a d i c a l s under a v a r i e t y o f s t o r a g e c o n d i t i o n s i s shown i n F i g u r e s 3 and 4. R a d i c a l s decayed somewhat f a s t e r i n a t a c t i c PP as compared to the i s o t a t i c film. F i l m immersion i n hexane caused the p e r o x y l s i g n a l t o decay quite q u i c k l y b u t immersion i n a hexane s o l u t i o n o f the r a d i c a l - s c a v e n g i n g n i t r o s o compound c a u s e d t h e f a s t e s t d e c a y a t 2 3 ° C . The r a t e o f peroxyl r a d i c a l decay showed a pronounced temperature dependence, b e i n g e x t r e m e l y r a p i d a t 6 0 ° C y e t e s s e n t i a l l y z e r o a t - 5 ° C (not shown). Some p e r o x y l r a d i c a l decay data a r e combined w i t h d a t a f o r the a c c u m u l a t i o n o f -OH s p e c i e s i n F i g u r e 4 f o r 23°C s t o r a g e . Changes i n t h e c a r b o n y l r e g i o n absorbance and i n e l o n g a t i o n a t break f o r the same samples s t o r e d a t 60°C a r e c o l l e c t e d i n F i g u r e s 5 and 6 f o r a d d i t i v e f r e e and a d d i t i v e c o n t a i n i n g samples. That the p r o t e c t i v e e f f e c t o f AOX, StNO* and StNCH continues i n the p o s t - y p e r i o d i s clearly visible, w i t h AOX the most e f f e c t i v e but a l s o seriously discoloured (yellow). These s t u d i e s were made a t 60°C t o reduce t h e p r o t r a c t e d l i f e t i m e a t room t e m p e r a t u r e f o r t h e s t a b i l i z e d s a m p l e s . As compared t o 2 3 ° C , t h e 60°C a g i n g c a u s e d about a x 10 a c c e l e r a t i o n i n t h e r a t e o f o x i d a t i v e d e t e r i o r a t i o n o f the u n s t a b i l i z e d f i l m as measured both by changes i n t h e IR and e l o n g a t i o n a t b r e a k . IR data f o r the -OH a b s o r p t i o n s a t 3400 c n r f e l l i n t o two d i s t i n c t p a t terns. In t h e a d d i t i v e - f r e e samples and w i t h DSTDP, the 3400 c m " a b s o r p t i o n i n c r e a s e d r a p i d l y and was always n u m e r i c a l l y about t w i c e the 1715 c n r a b s o r p t i o n . F o r t h e S t N C H , StNO- and AOX c o n t a i n i n g samples, t h e low i n i t i a l 3400 c m " absorption (Table 1) either s t a y e d r o u g h l y c o n s t a n t , o r even d e c r e a s e d s l i g h t l y up t o 1000 hours at 60°C. I t i s a l s o worth m e n t i o n i n g t h a t the c a r b o n y l y i e l d ( F i g u r e 5) appears to bear l i t t l e r e l a t i o n s h i p t o t h e e l o n g a t i o n a t break f o r the s t a b i l i z e d films (Figure 6 ) . A t 60°C t h e p r e - p h o t o o x i d i z e d f i l m was a l s o e x t r e m e l y u n s t a b l e as shown by t h e growth o f carbonyl species (at 1715 c m " ) and the drop i n e l o n g a t i o n a t break ( F i g u r e s 5 and 6 ) . 3

1

1

1

3

1

1

The o x i d a t i o n o f a s e r i e s o f a d d i t i v e f r e e samples i s shown i n F i g u r e 7. The p o s t - U V - i r r a d i a t e d and p o s t - y - i r r a d i a t e d , b u t n i t r o s o treated, films show very similar oxidation rates. However, y -

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

364

50 Figure 2 Films a l l Film A l l other stored in

GQUSS

E . S . R . Spectr y-irradiate i r r a d i a t e d under vacuum. curves r e f e r to f i l m i r r a d i a t e d i n a i r at a i r f o r the shown times a t 6 0 ° C .

200

HOURS

400 DECAY

35°C

then

80C

Figure 3 P e r o x y l R a d i c a l Decay i n PP F i l m A l l f i l m s y - i r r a d i a t e d i n a i r a t 0.14 M r a d - h " t o 2.8 Mrad. d o s e . S p e c t r a measured a t p e a k - t o - p e a k maximum f o r 0.030 g s a m p l e s . I s o t a c t i c PP f i l m , s p e c t r o m e t e r g a i n 2 x 1 0 : • F i l m stored at 23°C i n a i r . • F i l m s t o r e d a t 60°C i n a i r . A F i l m immersed i n hexane immediately a f t e r i r r a d i a t i o n . • F i l m immersed i n 2-methyl-2-nitrosopropane/hexane s o l u t i o n immediately a f t e r i r r a d i a t i o n . A t a c t i c PP f i l m , s p e c t r o m e t e r g a i n 5 x I O : O Film stored in a i r at 60°C. 1

2

3

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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365

Figure 4

Comparison o f P e r o x y l Decay and O x i d a t i o n Growth After y-Irradiation F i l m i r r a d i a t e d i n a i r a t 0.14 M r a d - h " to 2.8 M r a d . t o t a l d o s e . 1

Figure 5 Film Oxidation After y - I r r a d i a t i o n F i l m s y - i r r a d i a t e d i n a i r t o 2.0 M r a d . dose a t 1.35 M r a d - h " , then aged i n a i r a t 6 0 ° C . • No a d d i t i v e . A AOX (2 x 10"2 mol-kg-i). • StNCH (9 x 1 0 - 3 m o L k g - i ) , • StNO- (2 x 10-2 mol-kg-i). F i l m p r e - p h o t o - o x i d i z e d i n a i r (Xe a r c , 44 h ) , then aged a t 60°C:- O 1

3

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366

P O L Y M E R STABILIZATION

T

1

1

1

Figure 6 Film Embrittlement A f t e r Samples and a e t a n s as i n F i g u r e 5.

A N D

DEGRADATION

r

y-Irradiation

800

HOURS AT

23°C

Figure 7 F i l m O x i d a t i o n A f t e r I r r a d i a t i o n Exposures y -1 r r a d i a t e d H l m s M r a d . , a t 0.14 M r a d - h - M : — C T R a d i c a l s d e s t r o y e d by a b r i e f immersion o f the f i l m i n 2 - m e t h y l 2 - n i t r o s o p r o p a n e s o l u t i o n . A H y d r o p e r o x i d e s d e s t r o y e d by a b r i e f (15 h) exposure o f the f i l m t o gaseous S 0 . P h o t o - o x i d i z e d f i l m (95 h, Xe a r c exposure i n a i r ) : # 2

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25. CARLSSON ET AL.

Polypropylene Degradation by y-Irradiation

i r r a d i a t e d f i l m which had been exposed t o e i t h e r gaseous S 0 (not shown) immediately a f t e r i r r a d i a t i o n showed n e g l i g i b l e oxidation.

367

2

or SF further 4

Discussion Polymer (RH) o x i d a t i o n , i r r e s p e c t i v e of the p r e c i s e i n i t i a t i o n p r o c e s s , can be e x p r e s s e d l a r g e l y i n terms of the c l a s s i c a l , thermal o x i d a t i o n scheme ( r e a c t i o n s 1-3) (9). In the case of PP, t e r t . R. + 0 R 0 - + RH 2 R0 -

R0 ROOH + R. some n o n - r a d i c a l

2

(1) (2) (3)

2

9

o

products

peroxyl radicals are dominant and their termination is quite complete (reacton 4) {]). Only p e r o x i d e formation is truely a t e r m i n a t i o n s t e p , whereas 3 - s c i s s i o n t o form a ketone i s e x p e c t e d t o CH 1

2 ~CH -C~ 9

2

1

CH

q 3

+CH -C~ + 0 9

2

1

+ ~CH -C

9

9

2

2

1

CH -C-0-0-C-CH, I l CH CH Q

0

0

PP CH

C H

scission

0

^CH -C~ + 2

/

~CH -C~

~CH

2

CH^ 3

c

+

* *

CH

9

0

OH

CH, I C ~ I H (4)

be an i m p o r t a n t backbone s c i s s i o n p r o c e s s . In a y - i n i t i a t e d , t h e r mal o x i d a t i o n , f r e e r a d i c a l s w i l l be g e n e r a t e d t h r o u g h o u t the sample c r o s s - s e c t i o n a f t e r a c o m p l e x , r a p i d cascade of r e a c t i o n s i n v o l v i n g i o n s and e x c i t e d s t a t e s ( r e a c t i o n 5 ) . Even a t the h i g h e r degrees of o x i d a t i o n (~ 0.2 m o l ^ k g ) y - i n t e r a c t i o n w i t h o x i d a t i o n p r o d u c t s i s much l e s s l i k e l y than C-H bond s c i s s i o n based on the sheer c o n c e n - 1

RH

—wv*R. ^ R . 1

+ H« + R.

(5a) (5b)

t r a t i o n of C-H s i t e s . We have f o l l o w e d the i n c r e a s e of -OH and )C=0 s p e c i e s by IR f o r f i l m s i r r a d i a t e d p r o g r e s s i v e l y up to 5 Mrad t o t a l dose. The b u i l d u p o f both p r o d u c t s was a c c u r a t e l y l i n e a r w i t h dose up t o t h i s p o i n t , c o n s i s t e n t w i t h the complete i n s e n s i t i v i t y of the o x i d a t i o n p r o d u c t s t o the y - i r r a d i a t i o n p r o c e s s . Thus i n c o n t r a s t t o p h o t o - i n i t i a t e d o r t h e r m a l l y i n i t i a t e d o x i d a t i o n i n which h y d r o p e r o x i d e s c i s s i o n must o c c u r to generate the first r a d i c a l s , a n d , through the i n t e r m e d i a c y of the m a c r o - a l k o x y l radic a l s , some c h a i n s c i s s i o n , c h a i n s c i s s i o n d u r i n g y - i r r a d i a t i o n o n l y o c c u r s as the r e s u l t of the i r r a d i a t i o n i t s e l f ( r e a c t i o n 5b) and the termination process (reaction 4). The c o m p l e x i t y of the c a r b o n y l envelope ( F i g u r e 1) indicates t h a t r e a c t i o n s o t h e r than 3 - s c i s s i o n of t e r t . - a l k o x y l r a d i c a l s c o n t r i b u t e to t h i s a b s o r p t i o n . O t h e r sources i n c l u d e t e r m i n a t i o n of

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

P O L Y M E R STABILIZATION A N D

368

DEGRADATION

primary o r secondary p e r o x y l r a d i c a l s and f u r t h e r o x i d a t i o n of u n s t a b l e p r o d u c t s such as a l d e h y d e s . The s m a l l e r e f f e c t on e l o n g a t i o n a t break o f the 2.5 Mrad. dose a t the f a s t e r dose r a t e ( T a b l e 1) i s not e x p e c t e d from r e a c t i o n 1 to 5. The h i g h e r dose r a t e s h o u l d g e n e r a t e more r a d i c a l s , l e a d to a h i g h e r r a t e of peroxyl selft e r m i n a t i o n and so generate more c h a i n s c i s s i o n and s c i s s i o n p r o ducts (carbonyl s p e c i e s ) but l e s s o x i d a t i o n p r o d u c t ( - 0 0 H ) . The d i s c r e p a n c i e s w i t h t h i s scheme may r e s u l t from o t h e r t e r m i n a t i o n r e a c t i o n s (such as R* + R or R* + R0 * e t c . ) becoming i m p o r t a n t a t t h e h i g h e r dose r a t e as a r e s u l t of some 0 d e p l e t i o n i n more h i g h l y o x i d i z e d domains. 2

2

The s t a b i l i z a t i o n e f f e c t d u r i n g i r r a d i a t i o n of the p i p e r i d y l compounds and the phenol most l i k e l y r e s u l t from the s c a v e n g i n g o f p e r o x y l r a d i c a l s but the phenol i s c o n v e r t e d t o y e l l o w quinone p r o ducts (10). P i p e r i d y l compounds are much l e s s e f f i c i e n t scavengers o f p e r o x y l r a d i c a l s , but can a l s o compete w i t h oxygen f o r the macroa l k y l s ( r e a c t i o n 1) ( 1 1 ) . None o f the s t a b i l i z e r systems c o m p l e t e l y s u p p r e s s e d the y - i n i t i a t e view of the h i g h number of randomly d i s p e r s e d i n i t i a t i o n p r o c e s s e s from y - i r r a d i a t i o n . The a c c e l e r a t i o n o f e m b r i t t l e m e n t by the t h i o e s t e r (DSTDP), which i s n o r m a l l y b e l i e v e d t o s t a b i l i z e v i a a h y d r o p e r o x i d e d e c o m p o s i t i o n mechanism r a t h e r than r a d i c a l s c a v e n g i n g , i s unexpected but may r e s u l t from the r a p i d o x i d a t i o n of the thio a d d i t i v e to unstable i n t e r m e d i a t e s . Horng and Klemchuk have a l s o r e p o r t e d t h a t DSTDP i s i n e f f e c t i v e a g a i n s t y - d e t e r i o r a t i o n ( 6 h The y - i r r a d i a t i o n of samples i n a i r o c c u r s i n the presence of some ozone formed by the r a d i a l y s i s of the a i r i t s e l f . (The smell o f ozone i s always d e t e c t a b l e when the Gamma c e l l i s o p e n e d . ) Ozone can a t t a c k s a t u r a t e d p o l y o l e f i n s to g e n e r a t e r a d i c a l s and i n i t i a t e o x i d a t i v e chains (12). The small but r e p r o d u c i b l e r e t a r d a t i o n of t h e o x i d a t i o n o f the PP f i l m s i n t i m a t e l y wrapped i n a n a t u r a l r u b b e r f i l m i s then c o n s i s t e n t w i t h an 0 component to the o x i d a t i o n , the u n s a t u r a t e d rubber s h i e l d i n g the PP f i l m to some e x t e n t from 0 a t t a c k by means of the r a p i d C L - u n s a t u r a t i o n r e a c t i o n . As w e l l as f r e e r a d i c a l g e n e r a t i o n , the 0 - P P r e a c t i o n may a l s o l e a d to u n s t a b l e o z o n i d e and p e r o x i d e i n t e r m e d i a t e s . The pos t - y o x i d a t i o n i s markedly r e t a r d e d by the t h r e e a d d i t i v e s ( F i g u r e s 5 and 6 ) . The p h e n o l i c a d d i t i v e i s o u t s t a n d i n g but i t s performance i s marred by the y e l l o w i n g o f the f i l m as the phenol i s c o n v e r t e d to quinone p r o d u c t s . Horng and Klemchuk have found t h a t p h e n o l i c , p h o s p h i t e and a secondary h i n d e r e d amine were e s s e n t i a l l y unchanged by a 2.5 M r a d . y - d o s e , and so a v a i l a b l e t o suppress the p o s t - y d e t e r i o r a t i o n ( 6 h The p r o t r a c t e d thermal o x i d a t i o n which f o l l o w s the end of y - i r r a d i a t i o n c o u l d stem from two s o u r c e s : 1) M a c r o - a l k y l r a d i c a l s t r a p p e d i n the c r y s t a l l i n e domains. 2) Decomposition of unstable o x i d a t i o n p r o d u c t s . The former mechanism has f r e q u e n t l y been o b s e r v e d , but u s u a l l y o n l y f o r samples exposed t o e l e c t r o n beam o r y - i r r a d i a t i o n a t 77°K under vacuum f o l l o w e d by a n n e a l i n g a t ambient, then 0 admission (2-4). Under our experimenal c o n d i t i o n s our data appear t o l a r g e l y r e f u t e the o c c u r r e n c e o f p r o c e s s 1 . The d e t e c t i o n s o l e l y o f p e r o x y l r a d i c a l s i n t h i n f i l m s and the r a p i d post-y c o n v e r s i o n o f the macro3

3

3

2

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a l k y l r a d i c a l s i m p l i e s t h a t a l l of the d e t e c t e d r a d i c a l s a r e i n 0 a c c e s s i b l e (amorphous o r d e f e c t i v e c r y s t a l l i n e ) r e g i o n s . A macroa l k y l r a d i c a l / h y d r o g e n atom p a i r o r m a c r o - a l k y l p a i r g e n e r a t e d i n the c r y s t a l l i n e domains ( r e a c t i o n 5) p r o b a b l y recombines q u i c k l y i n the r i g i d domain. A l t e r n a t i v e l y , H» may hydrogen a b s t r a c t c l o s e t o the geminate m a c r o - a l k y l r a d i c a l and r a p i d c o m b i n a t i o n of the two m a c r o - a l k y l s then takes p l a c e . F u r t h e r m o r e , the p e r o x y l r a d i c a l s i n the y - i r r a d i a t e d a t a c t i c sample ( c o m p l e t e l y n o n - c r y s t a l l i n e ) ( F i g u r e 3) decayed i n a s i m i l a r f a s h i o n to those i n the s e m i - c r y s t a l l i n e isotatic film. In both the a t a c t i c and i s o t a c t i c samples, PP0 « decay appears to f i t a good second o r d e r r e l a t i o n s h i p as e x p e c t e d from r e a c t i o n 4. However, the r e l a t i v e second o r d e r r a t e c o n s t a n t f o r p e r o x y l decay i s about a f a c t o r of 6 f a s t e r i n the a t a c t i c p o l y mer as compared t o the i s o t a c t i c f i l m . F u r t h e r m o r e , from an ongoing study of the e f f e c t of f i l m morphology on PP0 * decay we do f i n d a slower decay as m o r p h o l o g i c a l p e r f e c t i o n i n c r e a s e s . T h i s may r e s u l t from a p r o g r e s s i v e i n c r e a s e i n r e s t r i c t i o n of peroxyl m o b i l i t y w i t h i n the amorphous domains weight a l k a n e a n d , even more d r a m a t i c a l l y , a s o l u t i o n of a r a d i c a l t r a p can a c c e l e r a t e the PP0 * decay i m p l i e s t h a t the r a d i c a l s i t e s are i n the amorphous domains. The e f f e c t of hexane appears to be analogous to the e f f e c t of a " m o b i l i z e r " (an a l k a n e o i l ) reported p r e v i o u s l y (2). From the e . s . r . o f f i l m s t r e a t e d w i t h the n i t r o s o s o l u t i o n s , t h e r e was an i n d i c a t i o n t h a t a new r a d i c a l s p e c i e s was formed, but was always weak i n comparison w i t h the r e s i d u a l p e r o x y l signal. N i t r o s o compounds are known to scavenge peroxyl r a d i c a l s , but the peroxy N - o x y l p r o d u c t i s not s t a b l e a t ambient temperature and decays through a s e r i e s of u n s t a b l e i n t e r m e d i a t e s ( r e a c t i o n 6) 2

2

2

2

(14). R0 * 2

+4-NO

•4-N-0-0-R—^—• 0.

non r a d i c a l

products (6)

Of the p r o d u c t s from y - i n i t i a t e d thermal o x i d a t i o n , p e r o x i d e s , h y d r o p e r o x i d e s and o z o n i d e s are the prime c a n d i d a t e s to decompose q u i c k l y enough a t room temperature to i n i t i a t e the p o s t - y o x i d a t i o n . From F i g u r e 4, the r a t e of post-y o x i d a t i o n slows and becomes a p p r o x i m a t e l y c o n s t a n t a f t e r 5-600 h a t 2 3 ° C , which c o r r e s p o n d s t o the p o i n t a t which the peroxyl s i g n a l has e x t e n s i v e l y d e c a y e d . It i s t e m p t i n g t o a s s o c i a t e the f a s t e r , p o s t - y o x i d a t i o n stage to a c o m b i n a t i o n of the r e s i d u a l p e r o x y l r a d i c a l e f f e c t and o x i d a t i o n p r o d u c t e f f e c t but the subsequent, steady o x i d a t i o n r e g i o n s o l e l y t o o x i d a t i o n product decomposition. However, from F i g u r e 7, p o s t - y o x i d a t i o n i s stopped c o m p l e t e l y by S0 ( o r SF^) t r e a t m e n t of the f i l m . Both treatments d e s t r o y -OOH groups (8) y e t were found t o cause n e g l i g i b l e change i n the p e r o x y l s i g n a l l e v e l . This result c l e a r l y p o i n t s to the r e s i d u a l p e r o x y l l e v e l c o n t r i b u t i n g l i t t l e to the post-y o x i d a t i o n . When the p e r o x y l p o p u l a t i o n was q u i c k l y d e s t r o y e d by 2 - m e t h y l 2-nitrosopropane treatment, post-y o x i d a t i o n s t i l l occurred ( F i g u r e 7). In a d d i t i o n , f i l m which had been p r e - o x i d i z e d by x e n o n - a r c p h o t o - i n i t i a t e d o x i d a t i o n showed an i d e n t i c a l , s l o w , thermal o x i d a t i o n d u r i n g s t o r a g e , as w e l l as a r a p i d e m b r i t t l e m e n t i n the a c c e l e r a t e d aging at 60°C (Figure 6). The p h o t o - o x i d a t i o n o f PP i s w e l l 2

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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documented to produce hydroperoxide and lesser amounts of peroxides, but only low concentrations of peroxyl radicals. The thermal instability of peroxidic groups in several polymers has, in fact, been invoked previously as a source of oxidation initiation at moderate temperatures. Clough and Gillen suggested that thermal decomposition of -OOH sites promoted the degradation of polyethylene and polyvinyl chloride) during exposure to low dose rates of y-irradiation in a nuclear power station (15). Citovicky et al and Kishore have also invoked peroxidation of^PP by ozone or y-exposure as a source of initiation of thermal degradation (16-17). Conclusions The post-y oxidation appears to result predominantly from the slow decomposition of hydroperoxides and/or some other unstable peroxidic product. The more rapid, early rate of oxidation (Figure 4) might include a component from an extremely unstable oxidation product (ozonide, peroxide, etc.) not present after photo-oxidation and destroyed by the nitros characterization of the products from y-initiated oxidation, but is made difficult by the problems of quantifying the peroxide species. Degradation both during y-irradiation and during post-y storage can be prevented by phenolic and piperidyl stabilizers. The latter were less effective than the phenol, but did not cause discolouration. Other hindered amines with a higher radical scavenging efficiency would be extremely valuable. Acknowledgment Issued as NRCC 23466. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Geymer, D.O., "The Radiation Chemistry of Macromolecules", Dole, M. E d . , Academic Press, New York, 1973, vol. 2, p.4. Dole, M., Adv. Rad. Chem. 1974, 4, 307. Dunn, T.S. and Williams, J . L . , J . Ind. Irrad. Technol. 1983, 1, 33. Bohm, G.G.A., J . Polym. S c i . 1967 A2, 5, 639. Tsuji, K., Adv. Polym. S c i . 1973, 12, 131. Horng, P. and Klemchuk, P, Plast. Eng. 1984, (April), 35. Decker, C. and Mayo, F.R., J . Polym. S c i . , Polym. Chem. Ed. 1973 11, 2847. Carlsson, D.J. and Wiles, D.M., Macromolecules 1969, 2, 597. Garton, A . , Carlsson, D.J. and Wiles, D.M., Dev. Polym. Photochem., 1980, 1, 93. Carlsson, D.J. and Wiles, D.M., J . Macromol. S c i . , Rev. Macromol. Chem., 1976. C14, 155. Wiles, D.M., Tovborg Jensen, J . P . and Carlsson, D . J . , Pure Appl. Chem., 1983, 55, 1651. Krisyuk, B . E . , Popov, A.A. Griva, A.P. and Denisov, E . T . , Dokladi Akad. Nank S.S.S.R. 1983, 269, 400. Vieth, W. and Wuerth, W.F., J . Appl. Polym. S c i . , 1969, 13, 685. Chatgilialoglu, C . , Howard, J . A . and Ingold, K.U., J . Amer. Chem. Soc., 1982, 47, 4361. Clough, R.L. and Gillen, K.T., J . Polym. S c i . , Polym. Chem. E d . , 1981, 19, 2041. In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25. CARLSSON ET AL.

Polypropylene Degradation by y-irradiation

16. Citovicky, P., Mikulasova, D. and Chrastova, V., Euro. Polym. J . 1976, 12, 627. 17. Kishore, K, J . Macromol. S c i . , Chem., 1983, A19, 937. RECEIVED October 26, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

371

26 Comparison of Chemiluminescence with Impact Strength for Monitoring Degradation of Irradiated Polypropylene 1

1

2

G. D. M E N D E N H A L L , H . K. A G A R W A L , J. M . C O O K E , and T. S. DZIEMIANOWICZ 1

2

2

Department of Chemistry and Chemical Engineering, Michigan Technological University, Houghton, MI 49931 Plastics Technical Center, Himon

The loss of impact strength of polypropylene was followed from sheets stored in air at 25°C and 60°C after irradiation with electron beams. A marked difference in efficacy of phenolic and thioetherbased stabilizers at the two temperatures was found, with the thioether active alone at 60°C but only synergistically at 25°C. This difference was also reflected qualitatively in differences in chemiluminescence emission from the samples. Irradiation of polymers with y- or electron beams is an attractive alternative to chemical sterilization because of i t s speed, ease of control, and the absence of residue. Radiation treatment of polypropylene, however, also initiates chemical changes which lead ultimately to embrittlement. These changes in physical properties may not become apparent until some time after the treatment. The ability of antioxidants to prevent radiation damage does not always follow the trends observed in thermal oxidation, which has stimulated efforts to develop new stabilizers or optimized combinations of existing ones. The testing of polymers stabilized against radiation damage raises the familiar questions (a) whether the use of elevated temperatures for accelerated aging is valid, and (b) whether new analytical techniques under actual use conditions can give the same information in comparable time. In this study we measured chemiluminescence of polypropylene stabilized with different combinations of antioxidants and irradiated to different extents, and made correlations with conventional impact strength measurements of the same materials. Chemiluminescence has been used by a number of workers to characterize the thermal oxidation of polypropylene (1_,2). This study allowed an opportunity to use fiber-optics for transmitting chemiluminescence from the heated sample to the detector, which promises to simplify greatly the apparatus required for the technique. 0097-6156/85/0280-0373$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

374

The l i g h t e m i s s i o n from a u t o x i d i z i n g o r g a n i c m a t e r i a l s can a r i s e from s e l f - r e a c t i o n o f p r i m a r y o r secondary a l k y l p e r o x y l o r alkoxyl radicals. In the p r e s e n t c a s e , the predominent s i t e o f p e r o x y l r a d i c a l a t t a c k w i t h h y d r o p e r o x i d e f o r m a t i o n i n PP i n v o l v e s the t e r t i a r y c e n t e r s , so t h a t the c h e m i l u m i n e s c e n c e p r o b a b l y a r i s e s (1) from p r i m a r y o r m e t h y l p e r o x y l r a d i c a l s formed from 3 - s c i s s i o n , eg. S i n c e the l i g h t e m i t t e d by the o x i d i z i n g polymer i s e x t r e m e l y f a i n t , i t i s however p o s s i b l e t h a t a minor r e a c t i o n pathway w i t h a r e l a t i v e l y h i g h quantum y i e l d c o u l d be the predominent s o u r c e o f excited states. The mechanism i n Scheme 1 makes p o l y p r o p y l e n e an a t t r a c t i v e s u b s t r a t e f o r s t u d y , because i t i m p l i e s a p r o p o r t i o n a l i t y between r a t e s o f l i g h t e m i s s i o n and s c i s s i o n o f polymer c h a i n s . Experimental

Himont p o l y p r o p y l e n f l o w r a t e o f 12 dg m c o n t a i n i n g 0.0100% o f a p h e n o l i c p r o c e s s i n g s t a b i l i z e r , was combined w i t h c a l c i u m s t e a r a t e , i n h i b i t o r s as r e q u i r e d , and e x t r u d e d t o g i v e 9" x 0.040" sheet w i t h the f o l l o w i n g compositions: 0.1%

Ca s t e a r a t e

Neat

x

Phenol Thio Comb

x x x

a. b.

0.030% G o o d r i t e 3114*

1.0%

DLTDP

b

x x

x x

1,3,5-Tris-(4-hydroxy-3,5-di-tert-butylbenzyl)cyanuric acid. Di-n-dodecyl 2,2 -thiodipropionate. 1

I r r a d i a t i o n procedure. The s h e e t s were i r r a d i a t e d w i t h a Van de G r a a f e l e c t r o n a c c e l e r a t o r (High V o l t a g e E n g i n e e r i n g , Model AK) w i t h an e l e c t r o n energy o f 2Mev and a dose r a t e o f about 0.3MR s A n y l o n m a t r i x r a d i o c h r o m a t i c f i l m ( F a r West T e c h n o l o g y ) was used f o r dosimetry. C a l c u l a t i o n s i n d i c a t e d t h a t the dosage a t the lower s u r f a c e o f t h e sheet was about 20% h i g h e r than a t the upper surface. Impact measurement. The impact t e s t s were conducted i n the u s u a l manner on s i n g l e s h e e t s w i t h a Gardner L a b o r a t o r y Impact T e s t e r (Model IG-1120) w i t h a 0.625" d i a m e t e r punch hammer. At l e a s t t e n drops were performed i n the c e n t e r 50% o f the s h e e t s and w i t h p o i n t s o f impact a t l e a s t 1" a p a r t . The f a i l u r e c r i t e r i o n was a b r i t t l e (not d u c t i l e o r t e a r ) b r e a k . The v a l u e s r e p o r t e d i n T a b l e I a r e 50% p r o b a b i l i t i e s (energy a t w h i c h 50% o f f a i l u r e s a r e brittle). The s t a n d a r d d e v i a t i o n o f the v a l u e s i s about 10%. Chemiluminescence. The PP s h e e t s were s t o r e d a t ambient temperature i n t h e d a r k . Chemiluminescence e m i s s i o n from the i n i t i a l b a t c h o f samples was measured 4, 19, 38, 52, 65, 80, and

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

94

26.

MENDENHALL ET AL.

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375

days a f t e r i r r a d i a t i o n from c i r c u l a r p i e c e s ( 3 / 8 " d i a m . , w t . 60.0 71.5 mg) o f PP f r e s h l y c u t from a sheet w i t h a c o r k b o r e r . The p i e c e s were weighed to ±0.1mg and p l a c e d i n a sample h o l d e r shown i n F i g u r e 1. The s t e e l - j a c k e t e d f i b e r - o p t i c c a b l e ( D o l a n - J e n n e r I n c . , h i g h - t e m p e r a t u r e v a r i a n t Model BXT424, 1/4" f i b e r b u n d l e ) was screwed i n t o the top o f the sample h o l d e r , w h i c h was then p l a c e d i n a 1 x 6" copper tube w h i c h had been immersed i n an e t h y l e n e g l y c o l b a t h a t 150.0 ± 0 . 1 ° C ( P o l y Temp Model 8 0 ) . A minor p r o b l e m w i t h the c a b l e was the c o a t i n g o f the ends o f the g l a s s f i b e r s w i t h d a r k m a t e r i a l w h i c h had to be sanded o f f a f t e r s e v e r a l months ( t h i s d e p o s i t p r o b a b l y a r o s e from d i f f e r e n t m a t e r i a l s c o n c u r r e n t l y under study). The o t h e r end o f the f i b e r - o p t i c c a b l e was connected t o an e n d - o n p h o t o m u l t i p l i e r system d e s c r i b e d elsewhere ( 3 ) . The d a t a were n o r m a l i z e d t o the average sample w e i g h t o f 70.0 mg. For p l o t t i n g , the t e x t f i l e s f o r i n d i v i d u a l e x p e r i m e n t s were l o a d e d i n t o a S p e r r y mainframe computer and p l o t t e d w i t h m o d i f i e d commercial s o f t w a r e (TELAGRAF; I s s c o G r a p h i c s , I n c . ) . Control e x p e r i m e n t s showed t h a j u s t b e f o r e e x a m i n a t i o n a t 150°C d i d not change the shape o f the chemiluminescence c u r v e . S e v e r a l samples were weighed b e f o r e and a f t e r the h i g h t e m p e r a t u r e e x a m i n a t i o n , and no s i g n i f i c a n t w e i g h t d i f f e r e n c e s were f o u n d . Chemiluminescence a t ambient t e m p e r a t u r e i n t h i s s t u d y was o b t a i n e d from c i r c u l a r samples 1.0" i n d i a m e t e r (average w t . 0 . 5 g ) , i r r a d i a t e d t o 5MR o n l y , w h i c h were p l a c e d i n the sample w e l l o f an a p p a r a t u s w i t h automated c o u n t i n g f u n c t i o n ( T u r n e r D e s i g n s , I n c . Model 20 L u m i n o m e t e r ) . The l i g h t e m i s s i o n was measured f o r s e v e r a l time p e r i o d s o f 120 seconds e a c h . Average v a l u e s and s t a n d a r d d e v i a t i o n s were t h e n o b t a i n e d w i t h p o c k e t c a l c u l a t o r s . Care was t a k e n i n the c h e m i l u m i n e s c e n c e work t o keep the samples c l e a n , unexposed t o room l i g h t s f o r more than a few m i n u t e s , and they were h a n d l e d g e n t l y w i t h g l o v e s o r t w e e z e r s . Oven a g i n g . P l a q u e s were p h y s i c a l l y s e p a r a t e d from each o t h e r d u r i n g a c c e l e r a t e d a g i n g on a r a c k i n an oven m a i n t a i n e d a t 60±1 C with forced a i r c i r c u l a t i o n . Results Chemiluminescence ( 1 5 0 ° C , 50 min) from i r r a d i a t e d samples a f t e r 4 and 65 days a r e p r e s e n t e d i n F i g u r e s 2 - 3 . The i r r a d i a t i o n d o s e , s t a b i l i z e r s , and ambient s t o r a g e t i m e s b e f o r e e x a m i n a t i o n a r e i n d i c a t e d on each f i g u r e . Smoothed, 3D p l o t s d e r i v e d from complete s e t s o f a g i n g d a t a f o r i n d i v i d u a l c o m p o s i t i o n s and dosages appear i n Figures 4-6. Note the c o n t o u r l i n e s on the xy p l a n e o f each f i g u r e , and t h a t the z a x i s has been expanded i n some f i g u r e s to r e v e a l d i f f e r e n c e s between the more h i g h l y s t a b i l i z e d s a m p l e s . In s p i t e o f l o s s e s o f l i g h t t h r o u g h the c a b l e and the absence o f any f o c u s s i n g l e n s e s , the chemiluminescence i n t e n s i t y was f u l l y sufficient for precise monitoring. The c u r v e s show a r i s e i n i n t e n s i t y from z e r o time whose magnitude r e f l e c t s the amount o f p e r o x i d i c i n i t i a t o r s p r e s e n t , and the a b i l i t y o f added s t a b i l i z e r s t o p r e v e n t the c h e m i l u m i n e s c e n t r e a c t i o n s . In g e n e r a l , the l i g h t e m i s s i o n at 150°C i n c r e a s e d w i t h i n c r e a s i n g number o f days from

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

Photomultiplier^. tube/ housing

Interface and power supplies >

(—Fiber-optic

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Apple II

Steel sample holder (thread 1/2" 20)

Sample

F i g u r e 1. Schematic o f a p p a r a t u s f o r measurement o f chemilum­ i n e s c e n c e a t 150°C

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

MENDENHALL ET AL.

ROH

+o»

+

Degradation of Irradiated Polypropylene

Pv

*

ROO.

P v v

CH0

CH 002

i

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Scheme 1 .

20

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40

60

80

100

80

100

80

100

0.5 Minutes

20

40

60

0.5 Minutes 5000 65 D a y s No MR

4000 3000 2000 1000 0 20

40

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0.5 Minutes F i g u r e 2. R e p r e s e n t a t i v e p l o t s o f c h e m i l u m i n e s c e n c e , i n c o u n t s p e r 30-second i n t e r v a l s , v s . time a t 150°C.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

378

POLYMER STABILIZATION AND DEGRADATION 25000-1

20

40

60

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3000

2000

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0.5 Minutes F i g u r e 3. R e p r e s e n t a t i v e p l o t s o f c h e m i l u m i n e s c e n c e , i n c o u n t s p e r 30-second i n t e r v a l s , v s . time a t 150°C.

F i g u r e 4. 3 D - P l o t s o f c h e m i l u m i n e s c e n c e v s . time a t 150°C, v s . a g i n g time a t 2 5 ° C

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

26. MENDENHALL ET AL.

Degradation of Irradiated Polypropylene

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379

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the time o f i r r a d i a t i o n , and o t h e r w i s e e q u i v a l e n t samples showed more l i g h t w i t h h i g h e r r a d i a t i o n dose. The u n i r r a d i a t e d samples, as e x p e c t e d , showed l i t t l e change on s t a n d i n g . There was no pronounced d i f f e r e n c e whether o r not a sample was wrapped i n aluminum f o i l d u r i n g i r r a d i a t i o n , e x c e p t i n one c a s e , where the f o i l - w r a p p e d , sample dosed w i t h 5MR showed a chemiluminescence c u r v e e s s e n t i a l l y i d e n t i c a l t o the unwrapped sample w h i c h r e c e i v e d a dose o f 3MR. The s t a b i l i t y o r d e r deduced from the r e l a t i v e i n t e n s i t i e s i n the c h e m i l u m i n e s c e n c e c u r v e s a t 150°C ( F i g u r e s 2-6) i s comb >^ t h i o >> p h e n o l > n e a t , the former two showing l i t t l e d i f f e r e n c e between i r r a d i a t e d and unexposed samples. The o r d e r i n g i s i n good agreement w i t h the r a n k i n g d e r i v e d from the impact s t r e n g t h measurements ( T a b l e I and F i g u r e 7) o f the samples s t o r e d a t 60°C, Table I. Aging

Before

T,°C

Gardner impact

strengths (in-lb) of

polypropylene

Time, day

25

1 7 18 32

-22.5 11.4 4.6 2.0

22.0 23.0 14.0 3.5

22.6 25.4 7.8 2.0

22.2 25.0 21.8 19.8

60

3 7 18 32

3.0 2.0 2.0 2.0

10.3 5.0 2.0 3.7

23.8 20.2 21.0 20.6

21.5 24.2 19.4 19.4

26.2

27.8

31.0

26.6

irradiation

but not a t 2 5 ° C When the l a t t e r d a t a became a v a i l a b l e , i t was h y p o t h e s i z e d t h a t b e t t e r agreement between c h e m i l u m i n e s c e n c e and impact d a t a might be o b t a i n e d i f the l i g h t e m i s s i o n a l s o c o u l d be measured a t ambient t e m p e r a t u r e . E x a m i n a t i o n o f the neat and i r r a d i a t e d (5MR) n e a t samples i n hand 110 days a f t e r i r r a d i a t i o n , w i t h t h e commercial a p p a r a t u s gave v a l u e s o f 0.4 ± 0.2 and 2.7 ± 0.3 c o u n t s / m i n u t e , r e s p e c t i v e l y , a t room temperature. A l t h o u g h the e m i s s i o n r a t e s were a t the t h r e s h h o l d o f d e t e c t a b i l i t y , the r e s u l t and h i g h p r e c i s i o n were s u f f i c i e n t l y e n c o u r a g i n g t h a t a second b a t c h o f p o l y p r o p y l e n e samples was i r r a d i a t e d , and the c h e m i l u m i n e s c e n c e was measured r e p e a t e d l y from the same samples d u r i n g ambient s t o r a g e f o r s e v e r a l weeks. The r e s u l t s a r e g i v e n i n T a b l e I I and F i g u r e 7, i n w h i c h the v a l u e s r e f l e c t the average o f ten 2-minute c o u n t i n g i n t e r v a l s . As we had hoped, t h i s a p p r o a c h r e s t o r e d a q u a l i t a t i v e agreement between the two methods. The r a n k i n g w i t h l i g h t e m i s s i o n a f t e r s i x days c o r r e s p o n d s t o the o r d e r from impact s t r e n g t h a f t e r about a month. The p l o t s o f chemilumi n e s c e n c e i n F i g u r e 7 i n c o r p o r a t e s i n g l e p o i n t s o b t a i n e d o v e r weeks of i n t e r m i t t a n t o b s e r v a t i o n and do not resemble t h e ones o b t a i n e d d u r i n g much s h o r t e r e x a m i n a t i o n times a t 150°C.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

26. MENDENHALL ET AL.

Degradation of Irradiated Polypropylene

381

Table I I . Chemiluminescence ( c o u n t s / 2 m i n . ) a t ambient t e m p e r a t u r e from p o l y p r o p y l e n e i r r a d i a t e d a t day 0 w i t h a dose o f 5MR. Day

Neat

6 7 11 12 14 16 21

11.7 10.8 5.8 4.0 2.8 2.0 2.2

± ± ± ± ± ± ±

Phenol 2.0 3.8 3.0 0.7 1.9 1.2 0.8

8.7 6.2 1.5 1.2 0.6 0.8 0.6

± ± ± ± ± ± ±

1.4 3.8 0.5 1.3 0.5 0.8 0.5

Comb

Thio 11.8 11.2 5.3 4.4 4.6 4.0 4.0

± ± ± ± ± ± ±

1.2 1.6 0.5 0.6 0.5 0.4 1.4

1.2 1.7 1.7 1.0 0.5 0.5 0.5

± ± ± ± ± ± ±

0.8 1.3 0.6 1.0 0.5 0.5 0.5

Discussion At f i r s t g l a n c e , the d a t a seem a l i t t l e i n c o n s i s t e n t because the c h e m i l u m i n e s c e n c e was measured (at 25°C o r 150°C) from samples w h i c h had s t o o d a t ambien case. A p p a r e n t l y the i r r a d i a t i o n and a m b i e n t - t e m p e r a t u r e s t o r a g e l e f t the t h i o e t h e r e s t e r e s s e n t i a l l y unchanged, so t h a t upon h e a t i n g to 150°C a f t e r v a r i o u s s t o r a g e p e r i o d s , n e a r l y the f u l l complement o f a n t i o x i d a n t became a v a i l a b l e to r e t a r d a u t o x i d a t i o n . The r e s u l t i n g chemiluminescence data c o r r e l a t e t h e r e f o r e w i t h aging r e s u l t s at 6 0 ° C , a t w h i c h t e m p e r a t u r e the t h i o e t h e r e s t e r i s an a c t i v e a n t i o x idant . The r e v e r s a l i n the o r d e r o f e f f e c t i v e n e s s o f the s t a b i l i z e r s toward impact s t r e n g t h on a g i n g at 60°C v s 25°C i s u n u s u a l because the temperature d i f f e r e n c e i s a r e l a t i v e l y modest o n e . The s t a b i l i z i n g e f f e c t s and s y n e r g i s m o f t h i o compounds w i t h p h e n o l s a t h i g h t e m p e r a t u r e s has been i n v e s t i g a t e d p r e v i o u s l y i n some d e t a i l ( 4 - 6 ) . The i n i t i a l , p r o g r e s s i v e d e c l i n e o f c h e m i l u m i n e s c e n c e a t ambient t e m p e r a t u r e ( T a b l e I I and F i g u r e 7) i s c o n s i s t e n t w i t h a d e c r e a s e i n i n i t i a t i n g s p e c i e s , such as t r a p p e d r a d i c a l s o r l a b i l e p e r o x i d e s , w h i c h a r e p r o d u c e d i n the i r r a d i a t i o n p r o c e s s i n amounts above the s t e a d y - s t a t e c o n c e n t r a t i o n s . The l i g h t e m i s s i o n from the "thio sample exceeded t h a t from the " n e a t " sample a f t e r two weeks, suggesting a s u l f i d e prooxidant e f f e c t (7). The s h o r t b u r s t o f l i g h t i n the t h i o e t h e r - s t a b i l i z e d runs a t 150°C ( F i g u r e 6) may s i m i l a r l y a r i s e from i n d u c e d d e c o m p o s i t i o n o f p e r o x i d i c s p e c i e s by the t h i o e t h e r to produce r a d i c a l s , even though r e a c t i o n s o f p e r o x i d i c compounds w i t h s u l f i d e s a r e g e n e r a l l y thought t o be predomi n e n t l y n o n - r a d i c a l i n nature (8). Any d e t a i l e d m e c h a n i s t i c i n t e r p r e t a t i o n o f the d a t a i s r a t h e r l i m i t e d by the p r e s e n c e o f p r o c e s s i n g s t a b i l i z e r i n a l l s a m p l e s , and by the unknown d e t a i l s o f the l i g h t - e m i t t i n g r e a c t i o n s . I n h i b i t o r s reduce o x i d a t i v e c h e m i l u m i n e s c e n c e by r e d u c i n g the r a t e o f l i g h t - p r o d u c i n g p e r o x y l s e l f - t e r m i n a t i o n s (Scheme 1 ) , by q u e n c h i n g e l e c t r o n i c a l l y e x c i t e d s t a t e s (9) o r s i m p l y by a b s o r p t i o n of emitted l i g h t . The second f a c t o r may be much l e s s i m p o r t a n t i n p o l y m e r s because o f lowered r a t e s d i f f u s i o n ( 1 0 ) . The r a t e o f d i f f u s i o n o f 2 , 4 - d i h y d r o x y b e n z o p h e n o n e i n p o l y p r o p y l e n e has been -11 2 -1 measured (11) as 5.5 x 10 cm s at 4 4 ° C . I f we assume t h a t 1 1

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

0

10

20

30

40

Days After Irradiation

0

10

20

30

40

Days After Irradiation

5MR 25°C

X

\

\\

Thio

PhC e no omNb \ g

0

10

20

30

40

Days After Irradiation F i g u r e 7. Top and M i d d l e : L o s s o f impact s t r e n g t h o f p o l y p r o p y l e n e , i r r a d i a t e d t o 5MR, v s . s t o r a g e time a t 25 and 60°C. Bottom: Ambient chemiluminescence from i r r a d i a t e d (5MR) p o l y p r o p y l e n e samples v s . s t o r a g e time a t 25°C.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

26.

383

Degradation of Irradiated Polypropylene

MENDENHALL ET AL.

the c o n s t a n t f o r t h e s t a b i l i z e r s i n our systems i s o f s i m i l a r o r d e r of magnitude, we can c a l c u l a t e from the Smoluchowski e q u a t i o n (12,13) the c o r r e s p o n d i n g d i f f u s i o n - l i m i t e d r a t e c o n s t a n t f o r 4 -1 -1 s e l f - e n c o u n t e r , k,_,-- = 4 x 10 M s . For a second-order dirt quenching

p r o c e s s k^ = k ^ f f [P*][Q]» where [Q] i s the quencher,

can then v e r y r o u g h l y e s t i m a t e the pseudo f i r s t - o r d e r [Q] from the i n i t i a l and 0.02

c o n c e n t r a t i o n s o f our a d d i t i v e s . 4 4 1 * 3 x 10 = 6 s and

t h i o e t h e r these are 2 x 1 0 M =400 s

i f we

we

term k ^ f f For

phenol 4 *

2 x 10

c o r r e c t the above d i f f u s i o n - l i m i t e d

rate

c o n s t a n t f o r d i f f e r e n c e i n approximate m o l e c u l a r d i a m e t e r s . For m o l e c u l a r oxygen i n p o l y p r o p y l e n e we s i m i l a r l y c a l c u l a t e k 7 -1 -1 dirt (25°C) = 8 x 1 0 M s starting with a recently published e q u a t i o n ( 1 3 ) , and f o f

=

s

T

n

e

f

s

^ d i f f ^2^ appear t o be c o m p e t i t i v e w i t h the e x c i t e d s t a t e l i f e t i m e s o f most s i m p l e a l i p h a t i c c a r b o n y l s , and the e s t i m a t e s a l s o suggest t h a t quenching by oxygen would predominate even i f t h e y were. Singlet s t a t e l i f e t i m e s a r e r a r e l y l o n g e r than a few nanoseconds, while,. a l i p h a t i c t r i p l e t c a r b o n y l decay c o n s t a n t s a r e u s u a l l y about 10 s ( 1 5 ) . The t r i p l e t l i f e t i m e o f e x c i t e d c a r b o n y l s i n s o l i d _g e t h y l e n e - c a r b o n monoxide copolymer has been e s t i m a t e d as o n l y 10 s ( 1 6 ) . R e c e n t l y a l o w e r i n g o f quenching r a t e s toward e x c i t e d s t a t e s i n d i s s o l v e d macromolecules has a l s o been demonstrated ( 1 7 ) . The r o l e o f p h y s i c a l quenching o f e x c i t e d s t a t e s i n polymer p h o t o o x i d a t i o n i s a c l o s e l y r e l a t e d q u e s t i o n . A l t h o u g h we acknowledge t h a t many d e t a i l s such as sample homogeneity a r e n o t known i n our system, W i l e s and C a r l s s o n c o n c l u d e d i n an e a r l i e r s t u d y t h a t p h y s i c a l quenching by p h o t o p r o t e c t i v e a g e n t s i n commercial ( s o l i d ) polymers was o f l e s s e r importance than o t h e r modes o f p r o t e c t i o n ( 1 8 ) . To the e x t e n t t h a t t h i s g e n e r a l i z a t i o n i s t r u e , the c h e m i l u m i n e s c e n c e i n t e n s i t i e s i n s o l i d s w i l l n o t i n g e n e r a l be reduced by p h y s i c a l q u e n c h i n g . The above c o n s i d e r a t i o n s b e a r on the ambient c h e m i l u m i n e s c e n c e from p o l y p r o p y l e n e , a l t h o u g h a t 150°C one would s t i l l e x p e c t t h a t the f l u o r e s c e n c e e m i s s i o n from s i n g l e t c a r b o n y l s would be r e l a t i v e l y u n a f f e c t e d by quenching from s t a b i l i z e r s i n our samples. One can a l s o i n f e r t h a t p h y s i c a l quenching a l o n e i s not s u f f i c i e n t t o e x p l a i n the d e c r e a s e i n l i g h t from the p o l y p r o p y l e n e sample w i t h b o t h p h e n o l and t h i o e t h e r ( T a b l e I I , comb), s i n c e t h e i n i t i a l r e d u c t i o n i n i n t e n s i t y i s more than can be a c c o u n t e d f o r by the combined e f f e c t s o f the a d d i t i v e s s e p a r a t e l y . The d i f f e r e n c e s i n c h e m i l u m i n e s c e n c e i n t e n s i t i e s o f the samples a t 25°C a r e d i s c e r n i b l e a t a much e a r l i e r p o i n t than the d i f f e r e n c e s i n impact s t r e n g t h s . T h i s o b s e r v a t i o n i s r e a s o n a b l e , s i n c e the former t e c h n i q u e measures a dynamic p r o c e s s a t the m o l e c u l a r l e v e l , w h i l e the l a t t e r responds t o the c u m u l a t i v e damage to the sample up t o the time o f t e s t i n g . The d a t a a r e not c o m p l e t e l y i n a c c o r d w i t h t h i s g e n e r a l i z a t i o n , s i n c e the l i g h t

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

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emission from the neat and thio samples (Figure 7) i s identical up to about two weeks after irradiation, whereas the impact strengths (Figure 6) of these two samples are quite different after only one week. Although the comparison involved separate batches of irradiated polymers, the impact strengths showed good reproducibility in routine studies. Conclusions 1. Chemiluminescence at ambient temperature and at 150°C of several irradiated (electron-beam) polypropylene formulations can be qualitatively correlated with loss of impact strength. The correlation with chemiluminescence i s preserved at high vs. low temperatures, even though the ranking changes. 2. The thioether ester (DLTDP) shows a very strong synergistic effect with the phenolic stabilizer at room temperature, and i s active alon 60° fo protectio fro los f impact strength of the 3. The thioether temperatur protec polypropylene from loss of physical properties after electron-beam treatment, although irradiated samples that are subsequently heated to 150°C are apparently protected from thermal oxidation. 4. Fiber-optics is a convenient technique to isolate a heated sample from cooled photon-detecting equipment. 5. The differences of the relative efficacy of the stabilizers at 25°C vs 60°C emphasizes the importance of monitoring changes in physical properties after irradiation under use conditions. 6. Irradiation of samples of polypropylene sealed in aluminum f o i l makes l i t t l e difference in their subsequent properties compared with unsealed samples under the same conditions. Although the number of samples was rather limited, and the results tend to raise rather than settle mechanistic questions, the study indicates the possible u t i l i t y of chemiluminescence for nondestructive evaluation of radiation-treated polyolefins. Acknowledgement s The authors are grateful to Himont, Inc. for a fellowship to H.K.A., support from 3M Co. for hardware and software development, and to Mr. George Turner of Turner Designs, Inc., for loan of a Luminometer. The assistance of Mr. Burt Blanch with the Gardner impact tests is gratefully acknowledged. Literature Cited 1. 2. 3.

George, G.A., Develop. Poly. Degrad., 1981, 3, 173 and references therein. Flaherty, K.F., M.S. Thesis, Michigan Technological University, Houghton, Michigan, 1982. Ogle, CA.; Martin, S.W.; Dziobak, M.P.; Urban, M.P.; Mendenhall, G.D., J. Org. Chem., 1983, 48, 3728.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

MENDENHALL ET AL.

4.

5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Degradation of Irradiated Polypropylene

De Paolo P.A.; Smith, N.P.; In "Stabilization of Polymers and Stabilizers Processes"; Gould, R.F., Ed.; ADVANCES IN CHEMINSTRY SERIES No. 85, American Chemical Society: Washington, D.C., 1968, p. 202. Ray, W.C.; Isenhart, K., Poly. Sci. and Eng., 1975, 15, 703 de Jonge, C.R.H.I.; Giezen, E.A.; van der Maeden, F.P.B.; Huysmans, W.G.B.; de Klein, W.J., Mijs, W.J. In "Stabilization and Degradation of Polymers"; Allara, D.L.; Hawkins, W.L., Eds.; ADVANCES IN CHEMISTRY SERIES No. 169, American Chemical Society: Washington, D.C., 1978; p. 399. Scott, G., Communication at the time this paper was presented Swern, D., "Organic Peroxides"; Wiley-Interscience: New York, 1971; Vol. II, p.73 and references therein. Vasil'ev, R.F., Prog. React. Kin., 1967, 4, 305. Pratte, J.F.; Webber, S.E., Macromolecules, 1983, 16, 1188. Westlake J.F.; Johnson, M., J. Appl. Poly. Sci., 1975, 19, 319. Smoluchowski, M.V. Frost, A.A.; Pearson, R.G., "Kinetics and Mechanism"; John Wiley and Sons: New York, 1961; 2nd Ed., p. 271. Kiryushkin, S.G.; Gromov, B.A., Vysokomol. Soedin., Ser. A., 1972, 14, 1746. Wilson, T.; Halpern, A.M., J. Amer. Chem. Soc., 1980, 102, 7279. Heskins, M.; Guillet, J.E., Macromolecules, 1970, 31, 3224. Scaiano, J.C.; L i s s i , E.A.; Stewart, L.C.; J. Amer. Chem. Soc., 1984, 106, 1539. Wiles, D.M.; Carlsson, D.J., Poly. Degr. and Stab. 1980-81, 3, 61.

RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27 Chemiluminescence in Thermal Oxidation of Polymers: Apparatus and Method L. Z L A T K E V I C H Pola Company, Skokie, IL 60077

A chemiluminescence multi-sample apparatus and method are described for determining polymer s t a b i l i t y by measuring the intens i t y of the l i g h t emitted during thermal oxidation. The chemiluminescence technique is shown to provide essential advantages over the other methods for studying thermal oxidative s t a b i l i t y of polymers (DSC, oxygen uptake, oven aging). Depending on the nature of a material analyzed the chemiluminescence experiments are performed either under O atmosphere at a constant temperature or under N atmosphere at a constant heating rate. In the former case applicable to polypropylene (PP) and a e r y l o n i t r i l e butadiene-styrene copolymers (ABS) parameters such as induction time and oxidation rate can be evaluated. In the l a t t e r case applicable to nylon the extent of oxidation in a certain temperature region can be evaluated by measuring the area under the intensity of l i g h t - temperature curve. Along with providing a great deal of knowledge on thermal oxidative s t a b i l i t y , the chemiluminescence approach gives the additional information concerning polymer quality. The appearance of the low temperature pulses on the chemiluminescence curve observed before the onset of autocatalytic oxidation i s associated with the history and processing of the sample and with the natural aging of the polymer. 2

2

It i s often desirable for polymer producers, end-use manufacturers, additive suppliers, academicians, and others 0097-6156/85/0280-0387$07.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

P O L Y M E R STABILIZATION A N D

388

DEGRADATION

to e s t a b l i s h q u a l i t y c o n t r o l t e s t s concerning antioxidant concentration or oxidative s t a b i l i t y . Numerous t e c h n i q u e s have been developed over the years to study the o x i d a t i v e s t a b i l i t y of polymers. Among v a r i o u s m e t h o d s , c h e m i l u m i n e s c e n c e accompanying the thermal o x i d a t i o n has been r e f e r r e d t o by a number o f a u t h o r s ( 1 - 9 ) . I t was pointed out t h a t t h e i n t e n s i t y o f e m i t t e d l i g h t c o u l d be a c o n venient c r i t e r i o n f o r the estimation of thermal oxidative s t a b i l i t y of polymers. The r e l a t i o n s h i p I

t

= C

[ROOH ]

(1)

t

has been p r o p o s e d where I i s time dependent light i n t e n s i t y , C i s a constant and [ROOH] i s hydroperoxide concentration ( 3 ) . In s p i t e of the fact that the f i r s t publications concerning the p o s s i b i l i t y of using the chemiluminescence techniqu polymer thermal o x i d a t i v y e a r s ago a n d , a f t e y separat finding f i e l d were p u b l i s h e d , neither a standard method nor a commercial i n s t r u m e n t o f t h i s k i n d has so f a r been offered. There a r e s e v e r a l reasons e x p l a i n i n g this discrepancy: 1. Chemiluminescence technique i s s t i l l l a r g e l y a matter of d i s c o v e r i n g c o n d i t i o n s under which the l i g h t emission r e l a t e s to the properties of i n t e r e s t . 2. A l t h o u g h s e v e r a l methods f o r t h e e v a l u a t i o n of o x i d a t i o n i n i t i a t i o n , p r o p a g a t i o n and t e r m i n a t i o n rate constants and a c t i v a t i o n e n e r g i e s o f these p r o c e s s e s have been proposed (8,9), they d i d n o t p r o v i d e the ground f o r samples c o m p a r i s o n on r o u t i n e b a s i s and thus were o f limiting practical value. 3. T h e c h e m i l u m i n e s c e n c e t e s t may r e q u i r e many h o u r s , e s p e c i a l l y when p e r f o r m e d a t r e l a t i v e l y l o w t e m p e r a t u r e s and a p p l i e d t o t h e a n a l y s i s o f h i g h l y s t a b i l i z e d polymer systems. Thus, the p r o d u c t i v i t y of the instruments used was l o w a n d c o u l d n o t s a t i s f y t h e d e m a n d s . I t i s t h e r e f o r e d e s i r a b l e t o have a method f o r t h e e v a l u a t i o n of chemiluminescence r e s u l t s which will provide information on i n d u c t i o n t i m e , o x i d a t i o n rate, and e x t e n t of oxidation, relevant to the thermal oxidative s t a b i l i t y of materials. I t i s also advantageous t o have an i n s t r u m e n t w h i c h w o u l d be a b l e t o a n a l y z e numerous polymer samples simultaneously. The object of t h i s paper i s to present a new c h e m i l u m i n e s c e n c e i n s t r u m e n t and method h a v i n g t h e above mentioned features. The a p p a r a t u s and method d e s c r i b e d a r e p a t e n t e d and patent pending i n several countries. t

t

Apparatus The apparatus developed ( F i g . 1 ) , comprises a dark chamber 1 w i t h a s l i d i n g s t a g e 2 w h i c h h o l d s numerous

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27. ZLATKEVICH

Chemiluminescence in Polymer Oxidation

389

i n d i v i d u a l t e s t c e l l s 3. The t e s t c e l l s a r e maintained on a m e t a l s u p p o r t p l a t e 4 w h i c h p r o v i d e s e v e n t e m p e r a t u r e d i s t r i b u t i o n to the c e l l s . A heater 5 i s placed under the metal p l a t e . The l o w e r p a r t o f t h e d a r k c h a m b e r i s s e p a r a t e d f r o m i t s u p p e r p a r t by a m e t a l p l a t e 6 w i t h a number o f h o l e s e q u a l t o t h e number o f t e s t cells. E a c h h o l e i n t h e s e p a r a t i n g p l a t e i s c o v e r e d by a g l a s s window. When t h e s l i d i n g s t a g e i s i n " i n position, each of the h o l e s i n the s e p a r a t i n g p l a t e i s s t r i c t l y a b o v e one o f t h e t e s t c e l l s . The l i g h t e m i t t e d by the samples p l a c e d i n the t e s t c e l l s i s s e q u e n t i a l l y m e a s u r e d by a r o t a t i n g p h o t o m u l t i p l i e r 7 p l a c e d i n t h e upper p a r t of the dark chamber. The r o t a t i o n o f the p h o t o m u l t i p l i e r i s p r o v i d e d b y a n e l e c t r i c m o t o r 8. The e l e c t r o n i c p a r t of the apparatus 9 c o n s i s t s of a photometer, a temperature programmer/controller, a digital data processing board The t e m p e r a t u r thermocouple locate suppor plat instrument g i v e s the o p p o r t u n i t y of measuring the i n t e n s i t y o f e m i t t e d l i g h t v s . t e m p e r a t u r e f r o m room t e m p e r a t u r e up t o 3 0 0 ° C . I z o t h e r m a l as w e l l as v a r i o u s h e a t i n g r a t e e x p e r i m e n t s c a n be c a r r i e d o u t w i t h the p r e c i s i o n of the t e m p e r a t u r e c o n t r o l of -1°C. In o r d e r to a v o i d the p h o t o m u l t i p l i e r o v e r h e a t i n g d u r i n g the experiments constant o u t s i d e a i r c i r c u l a t i o n i s p r o v i d e d by a f a n p l a c e d i n t h e u p p e r p a r t o f t h e dark chamber. L i g h t e m i t t e d by t h e s a m p l e s i s r e g i s t e r e d by a g e n e r a l purpose side-on p h o t o m u l t i p l i e r tube (Hamamatsu 1P28, the s p e c t r a l response f r o m 185 t o 700nm, t h e p e a k s e n s i t i v i t y a t 450 n m ) , a n d r e c o r d e d independ e n t l y f o r e a c h o f e i g h t c e l l s by a m u l t i c h a n n e l recorder (Hewlett Packard 7418A) The t e s t c e l l s ( F i g . 2 ) , h a v e a c o n s t r u c t i o n c o n t r i b u t i n g t o t h e w a r m up o f t h e g a s b e f o r e reaching the t e s t samples. Each c e l l contains metal shavings which have a l a r g e s u r f a c e area f o r heat exchange. The gas f l o w t o e a c h s a m p l e i s e v e n l y d i s t r i b u t e d by a m a n i f o l d with i n d i v i d u a l flow adjustment. Each t e s t c e l l i s c o v e r e d by a g l a s s c o v e r t o p r e v e n t c r o s s contamination. G l a s s covers a l s o r e s t r i c t r e a c t i o n volume of each cell and p r o m o t e f a s t r e p l a c e m e n t o f one g a s by another. S a m p l e s i n a p o w d e r f o r m as w e l l as p l a q u e s can be analyzed. In the l a t t e r case a s p r i n g r i n g i s put on the top of the sample to a s s u r e good c o n t a c t between the s a m p l e and t h e cuvette. 1 1

Method I t was e s t a b l i s h e d b y B o l l a n d a n d Gee that organic hydrop e r o x i d e s a p p e a r as one o f t h e f i r s t p r o d u c t s o f o x i d a t i o n (10, 11). Subsequent o x i d a t i o n of the polymer i s a u t o c a t a l y s e d by t h e d e c o m p o s i t i o n of hydroperoxides which produce f r e e r a d i c a l c h a i n c a r r i e r s f o r the c h a i n reaction.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

390

F i g u r e 1. The d i a g r a m o f t h e m u l t i - s a m p l e c h e m i l u m i n escence apparatus. The numbers a r e i d e n t i f i e d i n t h e text.

Sample

Glass Cover

Metal Shavings

F i g u r e 2. The d i a g r a m o f t h e c e l l chemiluminescence apparatus.

used

i nthe

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27.

ZLATKEVICH The

391

Chemiluminescence in Polymer Oxidation

following

Initiation

reaction

2R00H

Propagation

scheme

has been

+

R + H 0

K' » R0&

R' + 0 Kl» R0£ R0g + R H - ^ * ~ R 0 0 H

#

offered: (2)

2

+

(3) (4)

+

(5) (6) (7)

2

R* + R' V R-R R* + ROJ-^^ROOR ROi + ROy&^-ROOR

Terminat ion

K

w h e r e ROOR a n d RR a r e h y d r o p e r o x i d e a n d p o l y m e r , respectively; R a n d ROJ a r e f r e e r a d i c a l s . (The R shown on t h e r i g h t s i d e o f e q u a t i o n (2) i s assumed t o r e s u l t f r o m e i t h e r a c h a i n t r a n s f e r s t e p o f RO" w i t h RH o r b y s e l f - d i s m u t a t i o n o f RO* t o R* (12.) . #

Initial

Stages

-

of

Oxidatio

Under m i l d c o n d i t i o n s o f o x i d a t i o n the c h a i n l e n g t h s a r e l o n g a n d t h e amount o f o x y g e n p a r t i c i p a t i n g i n t h e r e a c t i o n i s a p p r o x i m a t e l y e q u a l t o t h e amount o f h y d r o peroxides formed. U n d e r t h i s c o n d i t i o n t h e amount o f h y d r o p e r o x i d e s which decompose to i n i t i a t e further o x i d a t i o n i s v e r y s m a l l and n e g l e c t o f t h e r e a c t i o n (2) in writing the expression for oxidation rate i s v a l i d . At h i g h oxygen p r e s s u r e s , s t e p s (5) and (6) c a n be n e g l e c t e d and t h e s o l u t i o n of t h e e q u a t i o n s (2) (7) using the steady state approximation i s d[(M dt At to

d[R00H] dt

=

low oxygen give

(K, / K ) ^ [ R 0 0 H ] 6

pressures,

nmsi .

- _ a i a a .

steps

K2

(K)/K

F o r l o n g c h a i n l e n g t h s , many are formed p e r f r e e r a d i c a l f o r e t e r m i n a t i o n o c c u r s and rate constant K e s s e n t i a l l y of p r o p a g a t i o n , i . e . i n e q . K

-

K (K, / K 3

6

) ^

K

j

(6)

^

and (7)

are

neglected

,

[R00H]

[08

(9)

molecules of hydroperoxide i n i t i a t i n g the reaction b e hence v a r i a t i o n s of o v e r - a l l reflects changes i n the r a t e (8)

and i n e q .

(9)

K -

L e t u s c o n s i d e r t h e two c a s e s p r e s e n t e d (9) s e p a r a t e l y a n d t r y t o e s t i m a t e w h a t l u m i n e s c e n c e r e s p o n s e one s h o u l d expect High oxygen p r e s s u r e . can be r e w r i t t e n :

(8)

[RH]

Isothermal

K (K, / K ) % « K 2

v

2

by e q s . (8) and kind of chemif o r each o f them.

conditions.

E q . (8)

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

392

^

- ^3 [A]

(10)

[B]

where [A] i s p o l y m e r and [B] i s h y d r o p e r o x i d e concentration, respectively. K 3 i s the oxidation rate constant. (10) p r e s e n t s t h e a u t o c a t a l y t i c r e a c t i o n w i t h regard to t h e s u b s t r a t e and h y d r o p e r o x i d e and as i t i s t y p i c a l for a u t o c a t a l y t i c r e a c t i o n s , i n d u c t i o n and a c c e l e r a t i o n p e r i o d s s h o u l d be e x p e c t e d . Designating the increase i n [B] d u r i n g t h e o x i d a t i o n a s X = [ B ] - [ B ] a n d n o t i n g t h a t the i n c r e a s e i n [B] i s e q u a l t o t h e d e c r e a s e i n [A], ( [ B ] - [ B ] = [ A ] - [A]) , 0

0

0

dx dt

=

K

3

( [ A ] -x)

([B]

0

where [ A ] and [B]o a r e t h e i n i t i a l peroxide concentrations Integration of

+ x)

(ID

polymer

c

i.(u3.

0

and hydro-

1.1.)«-en(f£^{ft'

•

(12)

Since the chemiluminescence emission i n t e n s i t y i s proport i o n a l to hydroperoxide c o n c e n t r a t i o n ( s e e eq. ( 1 ) It When

-

C

([B]

-

[B] )

the chemiluminescence Imax

= C

[A]

= CX

0

intensity

[A]

Substituting eq. ( 1 5 )

0

t =

t h e maximum (14)

Q

3

reaches

c

Two c a s e s s h o u l d b e c o n s i d e r e d : (1) [B] f Imax \

oxygen p r e s s u r e . be r e w r i t t e n djBl dt

(18) 0

) t,

0

of the quadratic

equation

_ A

Imax/

Constant

Imax/

\Im Imax

heating

rate.

Eq. (9)

_ K

[B]

2

•0

(19)

[0 ] 2

where [ 0 ] i s oxygen c o n c e n t r a t i o n and K i s the oxidation rate constant. D e s i g n a t i n g t h e i n c r e a s e i n [B] d u r i n g t h e o x i d a t i o n as X=[B] - [ B ] and n o t i n g t h a t t h e i n c r e a s e i n [B] i s equal to the decrease i n [ 0 ], ([B] - [ B ] = [ 0 ] [0 ]), 2

2

0

2

X)

e

([B]

0

+

2

o

2

(20)

X)

Taking into account eq. ( 1 3 ) a n d i n t r o d u c i n g t h e c o n s t a n t h e a t i n g r a t e T=To + oL t a n d t h e A r r h e n i u s t y p e equation for t h e change of K with temperature K = Ko e x p ( - E / R T ) eq. (20) can be r e w r i t t e n f o r the i n i t i a l s t a g e s of the reaction ( [ 0 ] • X, [ B ] ^>X) 2

2

2

dl dT

^

[0 ] 2

o

0

[B]

G

0

exp

(-E/RT)

(21)

Thus, one s h o u l d e x p e c t t h e e x p o n e n t i a l i n c r e a s e o f t h e chemiluminescence i n t e n s i t y with the temperature. Since the extent of o x i d a t i o n i n a c e r t a i n temperature region (T - T, ) i s p r o p o r t i o n a l t o t h e a m o u n t o f h y d r o p e r o x i d e s f o r m e d , i t c a n be e x p r e s s e d a s J l d T and e v a l u a t e d 2

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

394 by m e a s u r i n g t h e a r e a temperature curve. Advanced

Stages

of

under

the

intensity

of

light-

Oxidation

Under r e l a t i v e l y s e v e r e c o n d i t i o n s of o x i d a t i o n ( h i g h temperatures, long time i n t e r v a l s , presence of m e t a l l i c a c t i v a t o r s and l i g h t ) t h e d e c o m p o s i t i o n of the hydrop e r o x i d e s becomes a p p r e c i a b l e , and t h e r a t e of o x i d a t i o n c a n no l o n g e r b e e q u a t e d to the r a t e of hydroperoxide f o r m a t i o n a s r e p r e s e n t e d b y t h e f i r s t two t e r m s i n equation (8). In t h i s case the d i s a p p e a r a n c e of hydrop e r o x i d e s b y e q . ( 2 ) s h o u l d be i n c l u d e d i n w r i t i n g the e q u a t i o n f o r the r a t e of change of h y d r o p e r o x i d e conc e n t r a t i o n w i t h time at h i g h oxygen p r e s s u r e : A [^ dt

0 Q H

]

=

K. **

(K,/K*

T h e a d v a n c e d s t a g e s o f o x i d a t i o n m u s t be m a r k e d b y appreciable disappearance o f s u b s t r a t e and t h i s factor may be i n t r o d u c e d i n t o t h e a b o v e e q u a t i o n i f one assumes at a f i r s t a p p r o x i m a t i o n that the u n o x i d i z e d s u b s t r a t e p r e s e n t a t any g i v e n t i m e i s e q u a l t o t h a t p r e s e n t initially l e s s the c o n c e n t r a t i o n of hydroperoxides f o r m e d , i . e . [RH] = [ R H ] - [ROOH] ( 1 4 ) . T h i s a s s u m p t i o n leads to the f o l l o w i n g e q u a t i o n : 0

d [

^Q

Eq.

Q H ]

(23)

[ROOH]

-

K (K,

/K

3

can

be

f c

)

3 5

[RH]

3

S

i n t e g r a t e d to

=

/

I

[

R

Q

° ? l n n

I

T

1

\

, i

[R00H]

6

give

(24)

[ROOHU \l

1

A -at K

/

o

w h e r e [ROOH]©© i s the s t e a d y s t a t e v a l u e of (the c o n c e n t r a t i o n approached at long times and [R00H] i s the i n i t i a l c o n c e n t r a t i o n of peroxides ;

hydroperoxide a s t-^-°°) hydro-

o

5

a = K (K, / K * ) * a / [ K j (K, / K ) % + 5

6

[RH] K,] = 0

2

[ R 0 0 H ] - [ K ( K , /K ) 5+K|][ROOH] (23)

0

= K [RH] (K/K+K, )

C

,

[R00H]oo [RH]o

=

As i t f o l l o w s f r o m e q . ( 2 4 ) , t h e c o n c e n t r a t i o n o f hydroperoxides approaches a steady s t a t e value which i s a maximum v a l u e i f [ R 0 0 H ] < [ROOHJoo a n d a m i n i m u m v a l u e if [R00H] > [ROOH]co o

o

Three

situations

can

exist:

(1) [ROOHlo < TROPHIC R e p l a c i n g [ R H ] , [ R 0 0 H ] , [ROOH] a n d [B] , [B] and (K/K+K, ) [A]o 0

o

[ R O O H ] ^ by

[A]

0

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

c

,

Chemiluminescence in Polymer Oxidation

27. ZLATKEVICH

(K/K+K, )

[B] =

1

(_

J

Since

I

When (16) (2)

[Ajo

(

| " | _ (K/K+K, )

= C

t

[B]

J

395

= C [A]„

a n d Imax

- ln[

[Aloj

(K}K19K|)

[

a

]

o

g -K

[A]

I

t

0

= C[B],

Io = C [ B ]

0

, Imax

tn[£ a i : ; ; i ] ° K [ A , . [ROOHlo

>

It

= C[B],

it

i u

)

t

-

[ B ] o

]

+ K

^

'

(

2

6

)

into eq.

= C ( K / K + K , ) [A]©

(27)

a

a x

(3)

5

(K/K+K,)

[A] ^ [ B ] a n d K ^> K| e q . ( 2 6 ) t r a n s f o r m s f o r i n i t i a l stages of oxidation [ROOHlo < [ROOHloo 0

0

2

[ROOHloo Io = C

[ B ] o ,Imin

[

Experimental

Results

Experimental

Conditions

A

]

.

- C (K/K+K,) [A]o

t

(28)

and D i s c u s s i o n

A l l m a t e r i a l s a n a l y z e d w e r e g r o u n d t o 40 m e s h p a r t i c l e s i z e , t h e s t a n d a r d amount o f p o w d e r ( 0 . 1 g) was p o u r e d i n t o the metal c u v e t t e ( 1 " diameter) and c a r e f u l l y spread to a u n i f o r m t h i c k n e s s . The c u v e t t e s were p l a c e d i n t h e separate test c e l l s of thechemiluminescence apparatus, and c o v e r e d by g l a s s c o v e r s a t room t e m p e r a t u r e under n i t r o g e n atmosphere. Two d i f f e r e n t p r o c e d u r e s h a v e b e e n utilized: (1) I s o t h e r m a l i n o x y g e n a t m o s p h e r e . H e a t i n g under n i t r o g e n f r o m room t e m p e r a t u r e up t o a c h o s e n temperature f o l l o w e d by r e p l a c e m e n t o f n i t r o g e n by oxygen and s t a r t of t h e i s o t h e r m a l e x p e r i m e n t . P o l y p r o p y l e n e s (PP) a n d a e r y l o n i t r i l e - b u t a d i e n e - s t y r e n e (ABS) c o p o l y m e r s have been e v a l u a t e d under these c o n d i t i o n s . Isothermal experiments have been c a r r i e d o u t a l s o w i t h n y l o n samples, (2) C o n s t a n t H e a t i n g R a t e i n N i t r o g e n A t m o s p h e r e . Heati n g u n d e r n i t r o g e n f r o m room t e m p e r a t u r e up t o 3 0 0 ° C w i t h c o n s t a n t h e a t i n g r a t e (10 degrees/min.). Constant heating r a t e experiments have been performed w i t h n y l o n

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

P O L Y M E R STABILIZATION A N D

396

DEGRADATION

samples. I n a l l c a s e s two s a m p l e s o f t h e same m a t e r i a l w e r e s t u d i e d and t h e a v e r a g e r e s u l t s o f t h e two measurements taken. The r e p r o d u c i b i l i t y o f t h e experimental d a t a was f o u n d t o be g o o d ( 15%) Initial

Stages

of

Oxidation

I t was f o u n d t h a t l i g h t i s e m i t t e d b y PP a n d ABS only u n d e r an o x y g e n a t m o s p h e r e . I n t h e c a s e o f PP, switching f r o m n i t r o g e n t o o x y g e n a t m o s p h e r e was not accompanied by a b u r s t o f l i g h t a n d i t r e q u i r e d some t i m e b e f o r e the i n t e n s i t y of c h e m i l u m i n e s c e n c e s t a r t e d to i n c r e a s e steadily. A t y p i c a l l i g h t i n t e n s i t y - v e r s u s - t i m e curve f o r the c h e m i l u m i n e s c e n c e p r o d u c e d b y a u t o x i d a t i o n o f PP c o n s i s t s of f o u r r e g i o n s ( F i g . 3). T h e r e i s an i n d u c t i o n p e r i o d d u r i n g w h i c h t h e r e i s p r a c t i c a l l y no l i g h t e m i t t e d b y the sample, o x i d a t i o n i hydroperoxides i s slow i t e d a very short i n d u c t i o n p e r i o d , whereas f o r s t a b i l i z e d m a t e r i a l t h i s p e r i o d was longer. Following t h e i n d u c t i o n p e r i o d , t h e r e i s an a u t o c a t a l y t i c s t a g e in which the h y d r o p e r o x i d e s c a t a l y z e f u r t h e r o x i d a t i o n and the i n t e n s i t y of e m i t t e d light increases quite rapidly. The i n d u c t i o n and a c c e l e r a t i o n p e r i o d s a r e n o t separate phenomena, but p a r t s of a t y p i c a l a u t o c a t a l y t i c r e a c t i o n . The l i g h t i n t e n s i t y next reaches the h i g h e s t l e v e l (peak hydroperoxide concentration). F i n a l l y , there i s a period of l i g h t decay (a d e c e l e r a t i o n of the r a t e of o x i d a t i o n ) . The d e c r e a s e i n o x i d a t i o n r a t e a f t e r p a s s i n g t h e maximum has been o b s e r v e d p r e v i o u s l y ( 1 4 ) . Possible explanations f o r the r a t e drop c o u l d i n v o l v e a d e c r e a s e i n the perm e a b i l i t y of the o u t e r s u r f a c e of o x i d i z e d sample to oxygen or the f o r m a t i o n of r e a c t i o n p r o d u c t s which tend t o i n h i b i t t h e o x i d a t i o n r e a c t i o n e i t h e r by interaction w i t h c h a i n c a r r i e r s o r by n o n r a d i c a l i n d u c e d decomposit i o n of h y d r o p e r o x i d e s . For our p u r p o s e s the f i r s t three r e g i o n s are of most i m p o r t a n c e s i n c e they r e p r e s e n t the autocatalytic process. Fig. 4 presents t h e p l o t a c c o r d i n g t o eq. (16) of the chemiluminescence r e s u l t s o b t a i n e d f o r PP s a m p l e s A a n d B. The experimental r e s u l t s are w e l l approximated by a s t r a i g h t l i n e f o r each of the samples s t u d i e d from w h i c h Ch([A] / [B ] ) and K [ A ] v a l u e s c a n be evaluated. R e s u l t s f o r PP s a m p l e s A a n d B t o g e t h e r w i t h t h e results obtained f o r two o t h e r PP s a m p l e s a r e s h o w n i n T a b l e I. Chemiluminescence data are presented together with the c o n v e n t i o n a l oven aging t e s t r e s u l t s . One can conclude that both techniques g i v e c o r r e l a t i v e r e s u l t s : long oven l i v e s correspond t o l o n g i n d u c t i o n t i m e s and low oxida0

0

0

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

ZLATKEVICH

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397

30+

150°C 0 atmosphere 2

6

12

18

t (hours)

F i g u r e 3. The c h e m i l u m i n e s c e n c e c u r v e s o f u n s t a b i lized (A) and s t a b i l i z e d (B) p o l y p r o p y l e n e samples,

F i g u r e 4. stabilized samples.

P l o t of In [ I / ( I m a x - I t ) ] v s . t f o r un(A) and s t a b i l i z e d (B) p o l y p r o p y l e n e t

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

POLYMER STABILIZATION AND DEGRADATION

398 Table

Sample A

B C D

I.

The e v a l u a t i o n o f p o l y p r o p y l e n e t h e r m a l t i v e s t a b i l i t y by t h e c h e m i l u m i n e s c e n c e oven a g i n g methods at 150°C Chemiluminescence Analysis I n d u c t i o n time Oxidation rate (relative units)(relative units) 1.9 0.05 12.2 0.006 3.3 0.02 5.2 0.02

oxidaand

Oven l i f e (days) 2 106 12 74

tion rates. I t h a s t o be e m p h a s i z e d that the time r e q u i r e d f o r c h e m i l u m i n e s c e n c e a n a l y s i s was 2 h o u r s f o r s a m p l e A a n d 25 h o u r s f o r s a m p l e B, c o m p a r e d t o 48 h o u r s a n d 106 d a y s , r e s p e c t i v e l y test. The o t h e r i m p o r t a n escence technique i t a t i v e i n f o r m a t i o n ( i n d u c t i o n t i m e and o x i d a t i o n rate), whereas the f a i l u r e p o i n t i n the oven a g i n g t e s t i s d e f i n e d as t h e f i r s t o b s e r v a t i o n o f p o w d e r y d i s i n t e g r a t i o n o r b r i t t l e n e s s and t h u s i s e s s e n t i a l l y qualitative. I n o r d e r t o e s t i m a t e t h e a c t i v a t i o n e n e r g y ( E ) o f PP a u t o x i d a t i o n i n the s o l i d s t a t e , the a n a l y s i s of sample A was p e r f o r m e d a t f i v e d i f f e r e n t t e m p e r a t u r e s a n d t h e v a l u e s o f K [ A ] , tf\ ( [ A ] o / [B ]o) a n d T x o b t a i n e d ( T a b l e I I ) 0

m a

Table

II.

Parameters

Temp. (°C) 150 140 130 120 110

en([A] / [ B ] ) K [ A ] - 10* (relative units)(relative units) 1.97 5 3.17 2. 9 4.13 1. 7 4.71 0. 85 5.12 0. 43 G

0

of a u t o x i d a t i o n Sample A 0

for polypropylene

Imax (relative units) 30 18.9 8.2 3.7 1.9

Two d i f f e r e n t a p p r o a c h e s i n e v a l u a t i n g E h a v e b e e n used: the c o n v e n t i o n a l p l o t o f 6n(K[A]o) vs.. l / T a n d t h e method o r i g i n a l l y d e v e l o p e d f o r i s o t h e r m a l solid-state d e c o m p o s i t i o n r e a c t i o n s s t u d i e d b y DTA ( 1 5 ) w h e r e i t was suggested that the slope of 6h(hmax) v s . l / T p l o t yields the a c t i v a t i o n e n e r g y ( h m a x i s t h e maximum p e a k h e i g h t o f the i s o t h e r m a l DTA t r a c e ) . The a c t i v a t i o n e n e r g y i n t h e 110-150°C t e m p e r a t u r e i n t e r v a l was 19.4 a n d 23.3 kcal/mole, respectively. Although the second v a l u e p r e c i s e l y c o i n c i d e s w i t h t h e a c t i v a t i o n e n e r g y o f PP o x y l u m i n e s c e n c e r e p o r t e d b y M.P. S c h a r d and C A . Russell (_4) , w h e r e a s t h e f i r s t v a l u e i s s l i g h t l y l o w e r , we be-

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

27. ZLATKEVICH

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l i e v e t h a t the d i r e c t method of a c t i v a t i o n energy e v a l u a t i o n p r o v i d e d by t h e l o g a r i t h m o f r e a c t i o n r a t e v s . r e c i p r o c a l t e m p e r a t u r e p l o t i s more r e l i a b l e . I t has been r e p o r t e d t h a t t h e i n d u c t i o n p e r i o d f o r l i n s e e d o i l (_16) a n d p o l y b u t a d i e n e ( 1 4 ) oxidation d e c r e a s e s l o g a r i t h m i c a l l y as t h e t e m p e r a t u r e i s r a i s e d . T h e a t t e m p t t o u s e t h e s a m e a p p r o a c h f o r PP oxidation failed. Both lt\ ([ B ]o / [ A ] ) v a l u e and t h e s l o p e o f t h e curve plotted i n 6n([ B ] / [ A ] ) - T coordinates monotonic a l l y i n c r e a s e w i t h the temperature showing that at l e a s t at h i g h t e m p e r a t u r e s the i n d u c t i o n time d e c r e a s e s w i t h t e m p e r a t u r e f a s t e r t h a n i s p r e d i c t e d b y Cn([B]o / [ A ] ) vs. T linearity. S i m i l a r l y t o P P , ABS d o e s n o t e m i t l i g h t under n i t r o g e n atmosphere. The o n l y d i f f e r e n c e i n t h e c h a r a c ter o f c h e m i l u m i n e s c e n c e v s . t i m e c u r v e s b e t w e e n PP a n d ABS i s the i n i t i a l burs ABS s a m p l e s w h e n t h to o x y g e n . The i n i t i a ligh y i m m e d i a t e l y a f t e r the i n t r o d u c t i o n of oxygen has been o b s e r v e d p r e v i o u s l y and a t t r i b u t e d t o t h e p r e s e n c e o f e a s i l y o x i d i z a b l e c e n t e r s i n a polymer at the b e g i n n i n g of t h e c h e m i l u m i n e s c e n c e e x p e r i m e n t , when a c c u m u l a t e d h y d r o p e r o x i d e s a r e n o t as y e t p r e s e n t ( 1 7 ) . The e x p e r i m e n t a l c h e m i l u m i n e s c e n c e r e s u l t s o b t a i n e d for two ABS s a m p l e s a t 1 5 0 ° C a n d 1 9 0 ° C a r e p r e s e n t e d i n Fig. 5. The c h e m i l u m i n e s c e n c e e x p e r i m e n t s a t 190°C h a v e b e e n p e r f o r m e d i n o r d e r t o be a b l e t o c o m p a r e t h e d a t a w i t h t h e DSC a n d o x y g e n u p t a k e r e s u l t s s i n c e t h e s e n s i t i v i t y o f t h e DSC a n d o x y g e n u p t a k e t e c h n i q u e s i s n o t s u f f i c i e n t enough f o r t h e i r a p p l i c a t i o n at 150°C. Besides e x h i b i t i n g the i n i t i a l b u r s t of e m i s s i o n , the c h a r a c t e r of t h e c h e m i l u m i n e s c e n c e v s . t i m e c u r v e s f o r t h e ABS s a m p l e s was s i m i l a r t o t h a t f o r PP a n d h a s b e e n treated by a p p l y i n g e g . ( 1 6 ) f o r t h e 1 5 Q ° C e x p e r i m e n t . The c h e m i l u m i n e s c e n c e d a t a t o g e t h e r w i t h t h e DSC a n d oxygen u p t a k e r e s u l t s a r e shown i n T a b l e I I I . 0

0

0

0

Table

Sample

A B C D E

III.

T h e e v a l u a t i o n o f ABS thermal oxidative i l i t y b y t h e c h e m i l u m i n e s c e n c e , DSC and oxygen uptake methods

DSC Oxyg en u p t a k e 190 °C 190 ° C I n d u c t i o n Time (min) V.Short 34 13 11 13

V.Short 43 11 5 10

stab-

Chemiluminescence 150°C 190°C Induet i o n O x i d a t i o n R a t e t m a x ^ i n ./I Time (relative units) (min) 0. 0.014 7 5.3 0. 0.012 35 10. 7 0. 9 3.8 0.017 6 0. 0.020 3.8 0. 4.2 0.020 8

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

max 46 17 24 26 21

400

POLYMER STABILIZATION AND DEGRADATION

S i n c e t h e c h e m i l u m i n e s c e n c e e x p e r i m e n t a t 1 9 0 ° C was c o m pleted within several minutes, the k i n e t i c approach a c c o r d i n g t o eq. ( 1 6 ) was n o t u s e d . Instead the time to r e a c h t h e maximum i n t e n s i t y ( t max) a n d t h e r a t i o o f t h e i n i t i a l b u r s t o f e m i s s i o n ( I i ) t o t h e maximum intensity ( I m a x ) w e r e m e a s u r e d ( T a b l e I I I ) . As i t f o l l o w s from the T a b l e I I I , t h e r e i s b a s i c a l l y a good c o r r e l a t i o n b e t w e e n t h e i n d u c t i o n t i m e v a l u e s o b t a i n e d b y t h e DSC a n d o x y g e n u p t a k e m e t h o d s a n d t h e t i m e t o r e a c h t h e maximum i n t e n s i t y ( t h e chemiluminescence method). The o n l y e x c e p t i o n was s a m p l e A. The i n d u c t i o n time f o r t h i s s a m p l e was e v a l u a t e d a s " v e r y s h o r t " b y t h e DSC a n d o x y g e n u p t a k e t e c h n i q u e s a n d was s m a l l e r t h a n f o r t h e o t h e r samples. On t h e o t h e r h a n d , t max was s i m i l a r f o r t h e s a m p l e s A , C, D a n d E w h e n e s t i m a t e d b y t h e c h e m i l u m i n escence method. There i s a b e t t e r c o r r e l a t i o n between the DSC, o x y g e n u p t a k e a n d c h e m i l u m i n e s c e n c e results w h e n i n s t e a d o f t ma f i r s t approximation the amount o f e a s i l y o x i d i z a b l e s i t e s on p o l y m e r surface. T h u s , i t seems t h a t " v e r y s h o r t " i n d u c t i o n t i m e o b t a i n e d b y t h e DSC a n d o x y g e n u p t a k e m e t h o d s f o r s a m p l e A i n t h i s p a r t i c u l a r c a s e i s r e a l l y n o t t h e i n d u c t i o n t i m e o f an a u t o c a t a l y t i c process but rather i s associated with the c o n t e n t o f u n s t a b l e p r o d u c t s w h i c h , h o w e v e r , do n o t autocatalyze oxidation. F u r t h e r i n d i c a t i o n o f t h i s was o b t a i n e d by t h e c h e m i l u m i n e s c e n c e e x p e r i m e n t s p e r f o r m e d at 150°C ( T a b l e I I I ) . At t h i s temperature sample A e x h i b i t e d l o n g e r i n d u c t i o n time and s m a l l e r oxidation r a t e t h a n t h e s a m p l e s C, D, a n d E , a l t h o u g h t h e s a m p l e B r e m a i n e d t h e most s t a b l e . I t s h o u l d be e m p h a s i z e d that, a s i t was i n d i c a t e d a b o v e , t h e i n d u c t i o n a n d a c c e l e r a t i o n p e r i o d s a r e n o t s e p a r a t e phenomena b u t p a r t s o f a t y p i c a l autocatalytic process. Thus b o t h t h e s e p a r a m e t e r s s h o u l d be c o n s i d e r e d t o g e t h e r a n d t h e u t i l i z a t i o n o f t h e i n d u c t i o n t i m e o n l y b y t h e DSC a n d o x y g e n u p t a k e m e t h o d s may n o t b e a p p r o p r i a t e . n

I t i s k n o w n t h a t ABS e x h i b i t s d r a m a t i c l o s s o f i m p a c t r e s i s t a n c e when a g e d i n an a i r o v e n e v e n a t 1 3 0 ° C . T h e DSC a n d o x y g e n u p t a k e m e t h o d s a r e p r o b a b l y u s e f u l i n estimating the high temperature performance but not n e c e s s a r i l y d i r e c t l y a p p l i c a b l e to p r e d i c t l i f e times at s e r v i c e t e m p e r a t u r e s (1_8) . Thus t h e a b i l i t y o f chemi l u m i n e s c e n c e t o b e a p p l i e d f o r e v a l u a t i o n o f ABS t h e r m a l o x i d a t i v e s t a b i l i t y a t 150°C and even lower t e m p e r a t u r e s seem t o be i m p o r t a n t . In c o n t r a s t t o PP a n d A B S , n y l o n e m i t s w e a k l i g h t e v e n when h e a t e d i n n i t r o g e n , a l t h o u g h t h e l e v e l o f l i g h t e m i t t e d b y t h i s p o l y m e r u n d e r n i t r o g e n i s 1-2 o r d e r s o f magnitude smaller than the chemiluminescence i n t e n s i t y of PP a n d ABS u n d e r o x y g e n . S i n c e n y l o n glows under n i t r o g e n , t h e e x p e r i m e n t s were p e r f o r m e d under this atmosphere at constant heating rate. Both u n s t a b i l i z e d and s t a b i l i z e d n y l o n samples e x h i b i t v e r y weak, p r a c t i c a l l y c o n s t a n t l i g h t e m i s s i o n i n

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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401

the temperature i n t e r v a l 40-150°C ( F i g . 6). At h i g h e r temperatures, the l i g h t i n t e n s i t y i n c r e a s e s exponentially. When t h e m e l t i n g t e m p e r a t u r e i s r e a c h e d , t h e r e i s a s h a r p decrease i n the l i g h t e m i s s i o n . The a r e a u n d e r t h e l i g h t i n t e n s i t y v s . t e m p e r a t u r e c u r v e was l a r g e r f o r t h e u n s t a b i l i z e d sample i n d i c a t i n g t h a t the v a l u e l / / l d T can be t a k e n as a m e a s u r e o f t h e d e g r e e o f o x i d a t i v e stability. A t t h e same t i m e a l o w a n d s t e a d y l e v e l o f l i g h t e m i s s i o n i n the 40-150°C t e m p e r a t u r e r e g i o n s h o w s t h a t up to 150°C n y l o n a u t o - o x i d a t i o n i s not significant. S i n c e a c e r t a i n ( p r o b a b l y v e r y low) c o n c e n t r a t i o n of oxygen i s n e c e s s a r y f o r the r e a c t i o n r e s p o n s i b l e f o r the e m i s s i o n of l i g h t , the s o u r c e of oxygen i n the system must be e s t a b l i s h e d . In t h i s regard the r e s u l t s o b t a i n e d b y L . M a t i s o v a - R y c h l a e t . a l . (_7) a r e o f i n t e r e s t . It was s h o w n t h a t p r e o x i d i z e d p o l y p r o p y l e n e e x h i b i t s c h e m i l u m i n e s c e n c e whe pure p o l y p r o p y l e n e glow i n p o l a r i t y s h o u l d promot oxyge adsorptio assumed t h a t d u r i n g s t o r a g e o f a p o l y m e r an e q u i l i b r i u m ^ 2

r\

physically

U2

adsorbed

»

f\

chemically adsorbed

i s s e t up b e t w e e n g a s e o u s o x y g e n a n d o x y g e n p h y s i c a l l y o r c h e m i c a l l y a d s o r b e d on t h e s u r f a c e o f t h e p o l y m e r . Oxygen i n i t s a d s o r b e d form can i n t e r a c t w i t h h y d r o p e r oxides d i r e c t l y i n a b i m o l e c u l a r r e a c t i o n a c c o r d i n g to equation (19). The a b i l i t y o f t h e s u r f a c e t o c h e m i s o r b o x y g e n d e p e n d s e s s e n t i a l l y on t h e n a t u r e o f a p o l y m e r , i . e . i t s h o u l d d i s p l a y a p o l a r e f f e c t when t h e e l e c t r o n a f f i n i t y o f o x y g e n r e s u l t s i n t h e f o r m a t i o n o f a n O2" r a d i c a l - i o n i n the p r e s e n c e of s u i t a b l e e l e c t r o n donors

e +

0

2

=:

o" 2

This e q u i l i b r i u m i s s h i f t e d to the l e f t s i d e w i t h i n c r e a s i n g t e m p e r a t u r e and t h u s c a n a l s o be a s o u r c e o f o x y g e n when t h e r m a l t r e a t m e n t i s p e r f o r m e d u n d e r inert atmosphere. That r a i s e s the q u e s t i o n c o n c e r n i n g the p o s s i b i l i t y to deal w i t h p u r e l y thermal d e g r a d a t i o n f o r polymers with e l e c t r o n supplying groups. Further experiments with both u n s t a b i l i z e d and s t a b i l i z e d n y l o n s a m p l e s showed t h a t any k i n d of a d d i t i o n a l heat treatment i s accompanied by a r i s e i n t h e e m i t t e d l i g h t i n t e n s i t y e s p e c i a l l y a t low t e m p e r a t u r e s . T h e e x a m p l e o f t h i s k i n d i s s h o w n on F i g . 7 f o r stabilized nylon. B o t h a n n e a l e d and q u e n c h e d samples w e r e h e a t e d u n d e r n i t r o g e n f r o m r o o m t e m p e r a t u r e up t o 2 5 0 ° C and h e l d a t t h i s t e m p e r a t u r e f o r one h o u r . Then t h e a n n e a l e d s a m p l e was s l o w l y c o o l e d t o room t e m p e r a t u r e when s t i l l u n d e r n i t r o g e n w h e r e a s t h e q u e n c h e d sample was q u i c k l y immersed i n i c e water. A s i s s h o w n i n F i g . 7, b o t h s a m p l e s e x h i b i t e d an i n t e n s e maximum a r o u n d 8 0 ° C on

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F i g u r e 5. The c h e m i l u m i n e s c e n c e c u r v e s o f two a c r y l o n i t r i l e - b u t a d i e n e - s t y r e n e c o p o l y m e r samples A and B o b t a i n e d a t 150 a n d 190°C.

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F i g u r e 6. The c h e m i l u m i n e s c e n c e c u r v e s o f i z e d (A) and s t a b i l i z e d (B) n y l o n s a m p l e s .

unstabil­

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the chemiluminescence c u r v e a l t h o u g h t h i s e f f e c t i s more pronounced f o r the annealed sample. A t t h e same t i m e t h e q u e n c h e d and a n n e a l e d s a m p l e s showed s i m i l a r i n c r e a s e i n the l i g h t i n t e n s i t y of the e x p o n e n t i a l h i g h temperature p o r t i o n o f t h e c u r v e (150-220°C). These r e s u l t s are i n a g r e e m e n t w i t h t h e d a t a o b t a i n e d b y G. A. G e o r g e (19) w h e r e i t was shown t h a t t h e i n t e n s i t y o f t h e initial i n c r e a s e i n l i g h t e m i s s i o n a t 100°C a f t e r a d m i s s i o n o f o x y g e n d e p e n d s on t h e p r e v i o u s t i m e o f h e a t i n g t h e n y l o n sample i n n i t r o g e n . T h u s i t c a n be c o n c l u d e d t h a t n y l o n undergoes o x i d a t i o n w h i c h i s most p r o b a b l y p r o v i d e d by p h y s i c a l l y a n d / o r c h e m i c a l l y a d s o r b e d o x y g e n e v e n when exposed to h i g h temperatures under n i t r o g e n . The a c t i v a t i o n e n e r g i e s e v a l u a t e d u s i n g the method w i d e l y a p p l i e d i n l u m i n e s c e n c e e x p e r i m e n t s , t h e so c a l l e d method of i n i t i a l r i s e s (2_0) was 37 k c a l / m o l e a t t e m p e r a t u r e s of 5 0 - 8 0 ° C a n d 18 k c a l / m o l The l a t t e r v a l u e c o r r e l a t e e n e r g y o f 15.4 kcal/mol reporte nylo oxylumin e s c e n c e ( 4 ) , w h e r e a s t h e v a l u e o f 37 k c a l / m o l e i s c l o s e t o 42 k c a l / m o l e o b t a i n e d f o r t h e l o w temperature chemiluminescence o b s e r v e d f o r p r e o x i d i z e d PP h e a t e d under n i t r o g e n (j6) . A l a r g e a c t i v a t i o n energy of the r e a c t i o n r e s p o n s i b l e f o r the appearance of the chemiluminescence maximum a r o u n d 80°C t o g e t h e r w i t h t h e f a c t t h a t i t t a k e s p l a c e a t r e l a t i v e l y low t e m p e r a t u r e s c e r t a i n l y indicates that this i s a neighboring group-associated reaction where the h y d r o p e r o x i d e c l u s t e r s p r o v i d e a v e r y h i g h v a l u e of the p r e - e x p o n e n t i a l f a c t o r . Neighboring hydrop e r o x i d e s h a v e been shown t o decompose w i t h g r e a t e r e a s e than the i s o l a t e d h y d r o p e r o x i d e s r e s p o n s i b l e f o r the autoc a t a l y t i c o x i d a t i o n (2V) . T h u s , t h e low temperature chemiluminescence and i t s i n t e n s i t y seem t o be associated w i t h t h e h i s t o r y and p r o c e s s i n g o f t h e s a m p l e and w i t h the n a t u r a l a g i n g of the polymer. I t s h o u l d be u n d e r l i n e d t h a t t h e chemiluminescence r e s p o n s e d e p e n d s n o t o n l y on t h e s a m p l e t h e r m a l t r e a t m e n t b u t a l s o o n t h e t i m e i t was k e p t a t room temperature a f t e r a g i n g p r i o r t o the a n a l y s i s ( F i g . 8). The i n t e n s i t y o f t h e e m i t t e d l i g h t i n the low temperature r e g i o n i n c r e a s e s w i t h the time of exposure to ambient c o n d i t i o n s f o r both samples a g e d a t 120 a n d 160°C. At the same t i m e t h e h i g h t e m p e r a t u r e p a r t o f t h e chemiluminescence curve remains p r a c t i c a l l y unchanged (Fig. 9). S u c h a b e h a v i o r i s u n d e r s t a n d a b l e i f one bears i n mind t h a t t h e s o l u b i l i t y and c h e m i s o r p t i o n o f o x y g e n in a polymer decreases with i n c r e a s i n g temperature and that c h e m i s o r p t i o n i s a r e l a t i v e l y slow p r o c e s s . Thus i t t a k e s some t i m e t o r e a c h a n e q u i l i b r i u m l e v e l o f chemisorbed o x y g e n a t room temperature. The s t a b l e l e v e l o f t h e l i g h t e m i s s i o n a t h i g h temperatures independent of the time of sample exposure to ambient conditions indicates that p h y s i c a l l y adsorbed oxygen p a r t i c i p a t e s i n the r e a c t i o n with isolated hydroperoxides. The low c o n c e n t r a t i o n o f o x y g e n

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F i g u r e 7. The c h e m i l u m i n e s c e n c e c u r v e s (a) and q u e n c h e d (b) s t a b i l i z e d nylon.

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F i g u r e 8. The c h e m i l u m i n e s c e n c e c u r v e s o f stabilized n y l o n s a m p l e a g e d i n t h e a i r o v e n a t 120 a n d 1 6 0 ° C f o r 16 h o u r s a n d t h e n e x p o s e d t o a m b i e n t c o n d i t i o n s f o r various time.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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s u f f i c i e n t to promote the r e a c t i o n w i t h i s o l a t e d hydrop e r o x i d e s may a l s o e x p l a i n the chemiluminescence response at h i g h temperatures. The s e n s i t i v i t y o f t h e c h e m i l u m i n e s c e n c e technique to t h e c h a n g e s i n t h e c o n c e n t r a t i o n o f c h e m i s o r p e d o x y g e n c a n be p r o b a b l y u t i l i z e d i n k i n e t i c s t u d i e s o f oxygen a d s o r p t i o n i n polymers. Advanced Stages

of O x i d a t i o n

At low t e m p e r a t u r e s the i s o t h e r m a l chemiluminescence c u r v e f o r n y l o n p r o d u c e d u n d e r oxygen a t m o s p h e r e has a sigmoidal s h a p e (J3) a n d c a n be e v a l u a t e d b y utilizing the approach developed f o r the i n i t i a l s t a g e s of oxidation. However, at r e l a t i v e l y h i g h temperature nylon o x i d a t i o n does not e x h i b i t a n o t i c e a b l e i n d u c t i o n p e r i o d . When t h e i n i t i a l c o n c e n t r a t i o polymer does not e s s e n t i a l l e i t h e r a n i n c r e a s e ( [ R 0 0 H ] < [ROOHjoa ) o r a decrease ([R00H] > [R00R]oo ) i n t h e l i g h t e m i s s i o n w i t h time c a n be e x p e c t e d . F i g . 10 p r e s e n t s t h e r e s u l t s obtained for u n s t a b i l i z e d and s t a b i l i z e d n y l o n s a t t h r e e d i f f e r e n t temperatures. A l l samples e x h i b i t a b u r s t of e m i s s i o n when o x y g e n i s i n t r o d u c e d i n t o t h e s y s t e m ( z e r o t i m e ) . Then the l i g h t e m i s s i o n from u n s t a b i l i z e d n y l o n i n c r e a s e s a t 150 a n d 1 7 0 ° C a n d d e c r e a s e s a t 1 9 0 ° C . The e q u i l i b r i u m l e v e l of chemiluminescence f o r s t a b i l i z e d n y l o n i s r e a c h e d v e r y f a s t a t 150 a n d 1 7 0 ° C , w h e r e a s a t 1 9 0 ° C s i m i l a r l y to u n s t a b i l i z e d m a t e r i a l there i s a decay i n light emission. S e v e r a l problems i n e v a l u a t i n g advanced stages of o x i d a t i o n by c h e m i l u m i n e s c e n c e s h o u l d be noted: 1. C o m p a r a t i v e e v a l u a t i o n of d i f f e r e n t samples can be made o n l y i f t h e y e x h i b i t r e l a t i v e l y p r o l o n g e d g r o w t h o r decay of l i g h t e m i s s i o n b e f o r e r e a c h i n g the e q u i l i b r i u m (for e x a m p l e , u n s t a b i l i z e d and s t a b i l i z e d n y l o n s c a n n o t b e c o m p a r e d a t 150 a n d 1 7 0 ° C b e c a u s e t h e e q u i l i b r i u m f o r the s t a b i l i z e d sample i s reached at these temperatures practically instantly). 2. I n some c a s e s i t i s d i f f i c u l t t o e s t a b l i s h a r e l i a b l e e q u i l i b r i u m l e v e l of l i g h t e m i s s i o n s i n c e the l i g h t growth or d e c a y may c o n t i n u e over a l o n g p e r i o d of time. This m i g h t be a s e r i o u s o b s t a c l e b e c a u s e t h e e v a l u a t i o n a c c o r d i n g t o eqs. (27) and ( 2 8 ) i s s e n s i t i v e t o Imax and Imin v a l u e s . 3. A t t h e b e s t o n l y K (K, /K ) [A] and K (K, / K ) *5 [ A ] / [ K (K, / K ) ^ + K,] v a l u e s can be o b t a i n e d and t h u s c o m p l e t e e l u c i d a t i o n o f t h e o x i d a t i o n process at i t s advanced stages (independent e v a l u a t i o n of K|, K3 a n d ) c a n n o t be accomplished. When t h e g r o w t h a n d d e c a y t o t h e s t e a d y s t a t e i s p l o t t e d a c c o r d i n g t o e g s . (27) and (28), a poor f i t i s observed f o r u n s t a b i l i z e d n y l o n a t 150 a n d 1 9 0 ° C . A b e t t e r f i t i s obtained f o r s t a b i l i z e d n y l o n at 190°C. T h i s i s shown i n F i g . 11. The f a i l u r e t o e x p r e s s the o

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F i g u r e 9. The d e p e n d e n c e o f t h e i n t e n s i t y o f e m i t t e d l i g h t a t 80°C (a) and 230°C (b) v s . t h e time of exposure to ambient conditions f o r s t a b i l i z e d n y l o n a g e d i n t h e a i r o v e n a t 1 2 0 ° C f o r 16 h o u r s .

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F i g u r e 10. The g r o w t h and d e c a y o f l i g h t e m i s s i o n f o r unstabilized ( a , b , c ) and s t a b i l i z e d ( d , e, f ) n y l o n s a m p l e s a t 150 ( a , d ) , 170 ( b , e) a n d 1 9 0 ° C ( c , f ) a f t e r h e a t i n g i n n i t r o g e n and t h e n a d m i t t i n g oxygen at zero time.

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F i g u r e 11. A n a l y s i s o f t h e c h e m i l u m i n e s c e n c e e m i s s i o n g r o w t h and d e c a y a c c o r d i n g t o e g s . (27) and (28). a- u n s t a b i l i z e d n y l o n ( 1 5 0 ° C ) b - unstabilized nylon (190°C), c - s t a b i l i z e d n y l o n (190°C) >

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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e x p e r i m e n t a l r e s u l t s by e q u a t i o n s (27) and (28) i n d i c a t e s t h a t a d v a n c e d s t a g e s o f o x i d a t i o n a r e c o m p l i c a t e d by processes not taken into account. Some o f t h e c o m p l i c a t i o n s a r e : ( 1 ) t h e p r o d u c t s o f o x i d a t i o n may p l a y a n i m p o r t a n t r o l e as i n h i b i t o r s o r a c t i v a t o r s o f t h e o x i d a t i o n ; ( 2 ) i m p u r i t i e s may b e v e r y i m p o r t a n t a s i n h i b i t o r s o r a c t i v a t o r s ; and (3) a s i d e r e a c t i o n o r t h e chain c a r r y i n g r a d i c a l s themselves may c a u s e d e s t r u c t i o n of hydroperoxides. Thus i t c a n be c o n c l u d e d t h a t t h e a p p l i c a t i o n o f chemiluminescence f o r the e v a l u a t i o n of advanced stages of o x i d a t i o n i s r e s t r i c t e d by b o t h t h e a b i l i t y o f t h e t e c h n i q u e and t h e complex n a t u r e o f t h e p r o c e s s . Iti s d o u b t f u l , however, whether the r a t e c o n s t a n t s measured i n t h e r e g i o n where c h a i n l e n g t h s a r e c l o s e t o u n i t y and the h y d r o p e r o x i d e concentration i s reaching i t s limiting value are of s i g n i f i c a n c reflected i n a material' during the i n d u c t i o period the e v a l u a t i o n of the u s e f u l l i f e t i m e of a polymer should be b a s e d o n t h e d e t e r m i n a t i o n o f t h e p a r a m e t e r s o f o x i d a t i o n i n t h e s e two r e g i o n s . Conclus ions The c h e m i l u m i n e s c e n c e a p p a r a t u s and method d e s c r i b e d p r o vide a s e n s i t i v e technique f o r thermal o x i d a t i v e s t a b i l i t y e v a l u a t i o n o f m a t e r i a l s w i t h many important advantages: 1) The c h e m i l u m i n e s c e n c e a n a l y s i s r e q u i r e s a v e r y s m a l l amount o f m a t e r i a l . 2) The d a t a a r e i n s t a n t l y and p e r m a n e n t l y recorded. 3) For i n i t i a l stages of o x i d a t i o n the chemiluminescence experiment p e r m i t s t h e e v a l u a t i o n o f i n d u c t i o n time and o x i d a t i o n r a t e v a l u e s when p e r f o r m e d at a constant temperature and under oxygen atmosphere, and t h e e x t e n t of o x i d a t i o n i n a c e r t a i n temperature r e g i o n when p e r formed a t a c o n s t a n t h e a t i n g r a t e and under n i t r o g e n atmosphere. 4) T h e c h e m i l u m i n e s c e n c e m e t h o d i s much f a s t e r a n d l e s s t e d i o u s than the a i r oven aging test. 5) I n c o n t r a s t t o t h e DSC a n d o x y g e n u p t a k e m e t h o d s , t h e high s e n s i t i v i t y of chemiluminescence enables testing to be d o n e a t r e l a t i v e l y l o w t e m p e r a t u r e s closely associated with actual temperatures the m a t e r i a l i s exposed to i n the field. 6) T h e c h e m i l u m i n e s c e n c e a p p a r a t u s furnishes the opportuni t y t o a n a l y z e numerous samples s i m u l t a n e o u s l y and does not r e q u i r e the a t t e n t i o n of the o p e r a t o r a f t e r the experiment i s s e t up. 7) The m u l t i - s a m p l e chemiluminescence apparatus i s low i n c o s t and up-keep e x p e n s e s , s i m p l e i n o p e r a t i o n , l i g h t weight and f l e x i b l e i n u s e . Along with p r o v i d i n g a great d e a l of knowledge concerning the thermal o x i d a t i v e s t a b i l i t y , the chemilumin-

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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escence approach gives additional information related to polymer quality. The appearance of the low temperature pulses on the chemiluminescence curve observed before the onset of the autocatalytic process i s associated with the history and processing of the sample and with the natural aging of the polymer. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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RECEIVED December 7, 1984

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

28 Physical Techniques for Profiling Heterogeneous Polymer Degradation R. L . C L O U G H and K. T. G I L L E N Sandia National Laboratories, Albuquerque, N M 87185

Three general technique degradation in polymer ample of heterogeneou degradatio havior is illustrated. Heterogeneous degradation frequently occurs in materials subjected to high dose rate irradiation in air and results in oxidation only near surfaces. Optical examination of cross-sectioned,metallographically-polished samples provides qualitative information on oxidation depth. Quantitative profiles of heterogeneous material property changes are provided by relative hardness measurements across cross-sectioned surfaces using e i ther of two experimental apparatuses. Quantitative profiles of oxidation are obtained using density gradient columns. Viton is found to become embrittled when irradiated at high dose rate but becomes soft when irradiated at low dose rate; this i s shown to result from differences in oxidative penetration depth at different dose rates. Degradation in polymeric materials frequently takes place in a heterogeneous manner, such that the nature of the degradation in the i n terior of a sample may be very different from that near the edges . Some examples where degradation can be strongly heterogeneous are: 1) photochemical aging, where light is absorbed near the surface, 2) thermal aging, where plasticizer might be volatilized from near the surface or 3) chemical aging, where some corrosive gas or liquid is diffusing into a material. However, for polymers aged in air, probably the most common cause of heterogeneous effects is oxygen diffusion-limited degradation. This mechanism is relevant in many different environments where oxidation may be the predominant degradation mechanism. Examples include high-energy radiation, UV light, elevated temperature and mechanical stress. A long-standing goal in polymer science has been the development of accelerated aging tests for predicting polymer degradation rates in long-term applications. Design of meaningful accelerated aging tests must begin with replication of degradation mechanisms and modes which occur in the environment to be simulated (5-8). Hetero0097-6156/85/0280-0411$06.00/0 © 1985 American Chemical Society In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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geneous oxidative degradation i s frequently an impediment to this goal, due to the fact that at the high stress levels c h a r a c t e r i s t i c of accelerated t e s t s , the oxidation rate may be s u f f i c i e n t l y high that oxygen diffusion becomes a dominant rate-determining step. Thus in studying polymer degradation and p a r t i c u l a r l y i n attempting a c c e l erated aging simulations, the a b i l i t y to identify and characterize heterogeneous effects i s of fundamental importance. In this report we describe three techniques which we have developed and used for investigating heterogeneous oxidative degradation i n i r r a d i a t e d polymers. We believe these techniques should be widely applicable to heterogeneous degradation studies involving a variety of environmental c o n d i t i o n s . We also present an i l l u s t r a t i o n of the dramatic effects that d i f f e r i n g oxidation depths can exert over mechanical property changes. Oxygen Diffusion Effects If an air-saturated polymer sample i s placed i n a radiation environment, homogeneous oxidatio l y high oxidation rates, the i n i t i a l l y dissolved oxygen may be used up faster than i t i s replenished from the atmosphere. This gives r i s e to heterogeneous degradation with the oxidation rate i n i n t e r i o r regions of the material decreasing to zero (or to some fixed rate lower than that near the surfaces). For materials exposed to h i g h energy radiation conditions which result i n heterogeneous degradation, i t i s possible to estimate an absorbed dose by which strongly heterogeneous degradation i s already taking place. This can be accomplished by calculating the amount of oxygen which w i l l be dissolved i n a p o l y mer at equilibrium with the surrounding atmosphere, and then dividing by the amount of oxygen consumed (by chemical reaction) per rad of radiation absorbed by the polymer. We have obtained the following general expression for the equivalent dose, R, i n rads, required to use up a l l the oxygen i n i t i a l l y dissolved in a given polymer: R -

S • P —G(-0 )

• A • (1.6

x 10-12)

(1)

2

where S i s oxygen s o l u b i l i t y i n the polymer i n mol/g'bar, P i s oxygen pressure ( i n bars) i n the atmosphere surrounding the polymer sample, G(-02) i s the oxygen consumption y i e l d ( i n molecules per 100 eV absorbed energy), and A i s Avogadro's number. The numerical constant serves to convert eV to rads, and has the units of rad»g/100 eV. For the majority of common polymeric materials, we have calculated that the t r a n s i t i o n from homogeneous to strongly heterogeneous degradation w i l l occur at quite low doses—generally less than a few tenths of a megarad. For v i r t u a l l y a l l polymers, measurable degradation occurs only after substantially higher doses. Thus, where oxidation inhomogeneities occur i n high-energy radiation environments, the degradation can t y p i c a l l y be treated as coming e n t i r e l y from a heterogeneous mechanism. If the oxygen permeation constant and the rate of oxygen consumption for the material remain r e l a t i v e l y constant as a function of t o t a l absorbed dose, a steady state in heterogeneous oxidation w i l l be approached. If the rate of oxygen consumption by reaction with free radicals generated by the radiation exceeds the rate of supply of oxygen from the edges of the polymer sample, d i s t i n c t

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Techniques for Profiling Polymer Degradation

r e g i o n s of o x i d i z e d and n o n o x i d i z e d polymer can r e s u l t . In comparing polymer samples i r r a d i a t e d a t d i f f e r e n t dose r a t e s , those exposed a t the highest dose rate may degrade heterogeneously, becoming o x i d i z e d o n l y near the s u r f a c e s . At s u c c e s s i v e l y lower dose r a t e s , o x i d a t i o n depth w i l l become p r o g r e s s i v e l y larger. Eventually, at s u f f i c i e n t l y low dose r a t e s , the oxygen can f u l l y penetrate the sample, g i v i n g r i s e t o o x i d a t i o n which i s homogeneous throughout the material. R a p i d I d e n t i f i c a t i o n of Heterogeneous D e g r a d a t i o n : Metallographic Polishing The f i r s t t e c h n i q u e i s a q u a l i t a t i v e or semiquantitative method for rapid identification of oxidative inhomogeneities. By this method, samples a r e mounted i n epoxy and a c r o s s - s e c t i o n a l s u r f a c e p o l i s h e d using standard metallographic techniques (10). I n the degraded m a t e r i a l have undergone changes du cizer loss, etc. Materia have d i f f e r e n t p h y s i c a l p r o p e r t i e s , and so take on d i f f e r e n t l u s t e r s upon p o l i s h i n g . Oxidized and nonoxidized regions have d i f f e r e n t r e f l e c t i v i t i e s , r e s u l t i n g i n v i s u a l bands when examined under an o p t i c a l microscope. We r e f e r to the w i d t h s of such bands as the depth o f oxidation, although strictly speaking, a step function change from o x i d i z e d to n o n o x i d i z e d r e g i o n s r a r e l y o c c u r s , as w i l l be c l e a r from some of the examples g i v e n below. The photographs shown i n F i g . 1 were o b t a i n e d on a s e r i e s of c l a y - f i l l e d e t h y l e n e - p r o p y l e n e rubber (EPR) m a t e r i a l s which were i r r a d i a t e d and then s u b j e c t e d to c r o s s - s e c t i o n a l p o l i s h i n g . The occ u r r e n c e of h e t e r o g e n e o u s o x i d a t i o n i s c l e a r l y i l l u s t r a t e d i n photo B f o r a sample i r r a d i a t e d at h i g h dose r a t e (6.7 x 10 rad/h). Photo C r e p r e s e n t s a sample i r r a d i a t e d to s i m i l a r t o t a l dose, but a t lower dose r a t e (1.1 x 10~* r a d / h ) ; h e r e , oxygen permeates t h r o u g h out the sample g i v i n g r i s e to e s s e n t i a l l y homogeneous oxidation. Photo A r e p r e s e n t s an u n i r r a d i a t e d sample. Photo D r e p r e s e n t s a sample i r r a d i a t e d a t h i g h dose r a t e (1.1 x IQr r a d / h ) but i n the absence of oxygen. The two samples (B and C) which were i r r a d i a t e d at d i f f e r e n t dose r a t e s but t o s i m i l a r t o t a l doses showed s i g n i f i c a n t d i f f e r e n c e s i n u l t i m a t e t e n s i l e s t r e n g t h ; t h i s stems from d i f f e r e n c e s i n the ext e n t of o x i d a t i o n a t the d i f f e r e n t dose r a t e s . Sample B had a t e n s i l e s t r e n g t h t h a t was 85% (+7%) o f the t e n s i l e s t r e n g t h o f unaged m a t e r i a l , w h i l e C e x h i b i t e d a t e n s i l e s t r e n g t h o f 53% (+7%) compared w i t h unaged. As might be e x p e c t e d , the o p t i c a l r i n g s became p r o g r e s s i v e l y f a i n t e r on g o i n g t o s u c c e s s i v e l y lower d o s e s ; r i n g s were not v i s i b l e a t doses below about 50 Mrad. However, the s i z e of the r i n g s i n t h i s m a t e r i a l d i d not change s i g n i f i c a n t l y as a f u n c t i o n of dose a t c o n s t a n t dose r a t e . T h i s o b s e r v a t i o n i n d i c a t e s t h a t oxygen p e r m e a t i o n and consumption r a t e s i n t h i s m a t e r i a l a r e not s i g n i f i c a n t l y dependent on dose. The v i s u a l r i n g s seen i n cross-sectioned, polished samples c o r r e s p o n d t o a r e a s h a v i n g l a r g e d i f f e r e n c e s i n the e x t e n t of o x i d a t i o n , and a r e u s e f u l f o r i d e n t i f i c a t i o n of h e t e r o g e n e o u s d e g r a d a t i o n and f o r a q u a l i t a t i v e or s e m i q u a n t i t a t i v e d e t e r m i n a t i o n of the d e p t h of o x i d a t i o n . However, the exact shape of the d e g r a d a t i o n p r o f i l e 5

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w i l l depend on t h e u n d e r l y i n g oxidation kinetics. Equations f o r c a l c u l a t i n g g r a d i e n t s i n systems h a v i n g s i m u l t a n e o u s d i f f u s i o n and c h e m i c a l r e a c t i o n have been d e s c r i b e d (11-14). Using f r e e r a d i c a l r e a c t i o n k i n e t i c s , o x i d a t i o n p r o f i l e s r a n g i n g from g e n t l e " p a r a b o l i c " shapes t o s t e p - f u n c t i o n t r a n s i t i o n s c a n be o b t a i n e d ( 1 4 ) . The shapes depend on t h e r e l a t i v e r a t e s o f t e r m i n a t i o n and p r o p a g a t i o n steps. Thus o x i d a t i v e rings visible from c r o s s - s e c t i o n a l p o l i s h i n g may r e p r e s e n t s i t u a t i o n s r a n g i n g from sharp t r a n s i t i o n s between o x i d i z e d and n o n o x i d i z e d r e g i o n s , t o more g r a d u a l t r a n s i t i o n s between h e a v i l y o x i d i z e d r e g i o n s and r e g i o n s h a v i n g e i t h e r no o x i d a t i o n o r l i g h t oxidation. The term o x i d a t i o n depth has q u a n t i t a t i v e meaning i n t h e former l i m i t , but has a somewhat more q u a l i t a t i v e meaning i n t h e latter. D e g r a d a t i o n P r o f i l i n g i n Terms o f R e l a t i v e Hardness More q u a n t i t a t i v e p r o f i l e t a i n e d by measuring change cross-sectional surfaces o f degraded samples. Relative hardness p r o f i l e s a r e o b t a i n e d u s i n g one of two types o f i n s t r u m e n t a t i o n . For r e l a t i v e l y hard p l a s t i c s , a commercial Knoop Hardness T e s t e r g i v e s good r e s u l t s . T h i s a p p a r a t u s employs a t h i n , convex diamond b l a d e which i s p r e s s e d i n t o t h e sample under c o n s t a n t weight f o r a s e t p e r i o d of time. The s o f t e r the m a t e r i a l , t h e deeper t h e b l a d e p e n e t r a t e s i n t o t h e sample. Data i s o b t a i n e d by m e a s u r i n g t h e l e n g t h o f t h e i m p r e s s i o n l e f t by t h e b l a d e . F o r t h i s e x p e r i m e n t , t h e sample i s p l a c e d on a c a l i b r a t e d t r a n s l a t i o n a l m i c r o s c o p e s t a g e , and measurements made a t r e g u l a r i n t e r v a l s . We can r e a d i l y o b t a i n 20 t o 30 d a t a p o i n t s o v e r a d i s t a n c e o f 1 mm. By o b t a i n i n g measurements o v e r a c r o s s - s e c t i o n o f a h e t e r o g e n e o u s l y degraded m a t e r i a l , a u s e f u l p r o f i l e o f t h e h e t e r o g e n e i t y i n m a t e r i a l p r o p e r t y changes i s o b t a i n e d (10). M a t e r i a l h a r d n e s s i s r e l a t e d t o modulus: i n c r e a s e d p e n e t r a t i o n c o r r e s p o n d s t o d e c r e a s e d modulus. ( I n f a c t , modulus c a n be c a l c u l a t e d from such p e n e t r a t i o n e x p e r i m e n t s , dependent upon t i p geometry and experimental procedure.) ( 1 5 ) . F i g u r e 2 g i v e s an example of a p r o f i l e of changes i n r e l a t i v e h a r d n e s s f o r a c l e a r p o l y p r o p y l e n e m a t e r i a l exposed t o UV l i g h t i n a Rayonet chamber. The data i n d i c a t e t h a t t h e p o l y p r o p y l e n e becomes p r o g r e s s i v e l y h a r d e r near t h e edges w i t h UV exposure, y i e l d i n g a b r o a d profile. The m a t e r i a l i n t h e i n t e r i o r o f t h e sample has undergone e s s e n t i a l l y no change i n r e l a t i v e h a r d n e s s . F o r s o f t e r , r u b b e r y polymers, a d i f f e r e n t e x p e r i m e n t a l p r o c e d u r e g i v e s b e t t e r r e s u l t s . F o r t h e s e e x p e r i m e n t s we have measured d i r e c t l y the p e n e t r a t i o n d i s t a n c e of a t i n y weighted probe i n t o t h e c r o s s s e c t i o n e d s u r f a c e o f polymer samples. F o r t h i s purpose we have made use o f a P e r k i n - E l m e r Thermomechanical A n a l y z e r equipped w i t h a t i p m o d i f i e d t o be s m a l l enough t o p r o v i d e measurements o f t h e d e s i r e d resolution. (We a r e c u r r e n t l y u s i n g a c o n i c a l diamond phonograph needle having a t i p angle of 60°.) F i g u r e 3 shows p r o f i l e s i n d i c a t i n g changes i n r e l a t i v e h a r d n e s s on c r o s s - s e c t i o n e d samples o f t h e EPR m a t e r i a l o f F i g . 1. F o r u n i r r a diated material, the p r o f i l e i s e s s e n t i a l l y flat (x's). F o r the samples i r r a d i a t e d a t 6.7 x 10^ r a d / h , a d i s t i n c t flat-bottomed, U-shaped p r o f i l e i s seen ( c i r c l e s ) . The boundary p o s i t i o n between o p t i c a l bands (photo B, F i g u r e 1) c o r r e s p o n d s t o t h e steep p a r t of

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

CLOUGH AND GILLEN

Techniques for Profiling Polymer Degradation

F i g u r e 1. C r o s s - s e c t i o n e d , p o l i s h e d samples o f gamma i r r a d i a t e d EPR. A: U n i r r a d i a t e d m a t e r i a l . B: 6.7 x l t V r a d / h ( i n a i r ) to 165 Mrad. C: 1.1 x 1 0 r a d / h ( i n a i r ) t o 175 Mrad. D: 1.1 x 1 0 r a d / h ( i n vacuum) t o 253 Mrad. A l l i r r a d i a t i o n s c a r r i e d o u t a t 70°C. Sample t h i c k n e s s - 3.15 mm. 5

6

F i g u r e 2. Hardness p r o f i l e f o r p o l y p r o p y l e n e exposed t o UV l i g h t i n a Rayonet. • - 6 day exposure A - 3 day exposure, X = unexposed m a t e r i a l . A Knoop Hardness T e s t e r was employed.

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the p r o f i l e ( s l i g h t l y l e s s than 20 p e r c e n t of the way i n on both sides). The i r r a d i a t e d m a t e r i a l has become s i g n i f i c a n t l y harder throughout ( i . e . , i n c r e a s e d modulus) w i t h the l a r g e s t i n c r e a s e o c c u r r i n g a t the i n t e r i o r p o r t i o n where oxygen i s a b s e n t . F o r a sample i r r a d i a t e d a t a lower dose r a t e (1.1 x 10^ r a d / h ) the p r o f i l e approaches a homogeneous c o n d i t i o n , showing o n l y a s l i g h t , s h a l l o w curvature (squares). D e g r a d a t i o n P r o f i l i n g i n Terms of

Density

I n a n o t h e r t e c h n i q u e , the d e n s i t i e s of p i e c e s of degraded samples a r e p r o f i l e d u s i n g a s a l t g r a d i e n t ' column ( 7 ) . This technique provides a p r o f i l e on oxygen uptake i n the s a m p l e — i n f o r m a t i o n which i s comp l e m e n t a r y t o t h a t p r o v i d e d by the t e c h n i q u e s based on changes i n mechanical p r o p e r t i e s . O x i d a t i o n of samples n o r m a l l y l e a d s to i n c r e a s e s i n sample d e n s i t y . F i g u r e 4 shows d e n s i t 1) t h a t were i r r a d i a t e d S t r o n g l y heterogeneous o x i d a t i o n i s a g a i n I n d i c a t e d f o r the h i g h - d o s e r a t e sample, i n c o n t r a s t to n e a r l y homogeneous o x i d a t i o n f o r the lowd o s e - r a t e sample. The r e s u l t s c o r r e l a t e w e l l w i t h r e s u l t s o b t a i n e d by o p t i c a l e x a m i n a t i o n of p o l i s h e d samples and by r e l a t i v e h a r d n e s s measurements. Figure 5 shows another example of density gradient results o b t a i n e d w i t h an EPR i n s u l a t i o n m a t e r i a l w h i c h had been e x t r u d e d o n t o a copper conductor. For the d e g r a d a t i o n s t u d i e s , the i n s u l a t i o n had been s t r i p p e d from the c o n d u c t o r and i r r a d i a t e d i n a i r as a h o l l o w t u b e . At h i g h dose r a t e s , a U-shaped g r a d i e n t c h a r a c t e r i s t i c o f oxygen d i f f u s i o n was o b t a i n e d ( d a t a not shown). However, a t low dose r a t e s , a p r o f i l e showing d r a m a t i c a l l y enhanced o x i d a t i o n o n l y near the i n t e r i o r surface ( i . e . , the s u r f a c e which had been a d j a c e n t t o the c o p p e r ) was f o u n d . The p r o f i l i n g r e s u l t s l e d t o the c o n c l u s i o n t h a t oxidation catalyzed by copper i o n s was a predominant d e g r a d a t i o n mechanism i n t h i s m a t e r i a l ( 7 ) . S u p p o r t i n g e v i d e n c e was o b t a i n e d by d e m o n s t r a t i n g a l a r g e copper c o n c e n t r a t i o n i n the r e g i o n near the i n t e r i o r s u r f a c e by means of e m i s s i o n s p e c t r o s c o p y . T h i s example i l l u s t r a t e s a c a s e of heterogeneous d e g r a d a t i o n w h i c h r e s u l t e d from a mechanism d i f f e r e n t from oxygen d i f f u s i o n . Effect Viton

o f Heterogeneous D e g r a d a t i o n on M a c r o s c o p i c P r o p e r t y

Changes:

The d e g r a d a t i o n b e h a v i o r of a V i t o n m a t e r i a l p r o v i d e s a good example o f the e f f e c t s of heterogeneous o x i d a t i o n . Figure 6 provides a p l o t of m e c h a n i c a l p r o p e r t y changes i n V i t o n as a f u n c t i o n of absorbed dose I n a i r a t t h r e e d i f f e r e n t dose r a t e s . The d a t a show t h a t a t h i g h dose r a t e (5.5 x 10^ r a d / h , open s q u a r e s ) the e l o n g a t i o n drops markedl y w h i l e the t e n s i l e s t r e n g t h undergoes a more modest d e c r e a s e . At low dose r a t e (1.3 x 10^ r a d / h , c i r c l e s ) the e l o n g a t i o n changes v e r y l i t t l e , whereas the t e n s i l e s t r e n g t h d e c r e a s e s s h a r p l y . Data o b t a i n e d a t an i n t e r m e d i a t e dose r a t e (9.2 x 10^ r a d / h , diamonds) show i n t e r mediate b e h a v i o r . In terms of v i s u a l e x a m i n a t i o n , samples i r r a d i a t e d a t the h i g h dose r a t e a r e found to become p r o g r e s s i v e l y h a r d e r and e v e n t u a l l y so e m b r i t t l e d t h a t they break r e a d i l y when f l e x e d l i g h t l y by hand. Samples i r r a d i a t e d a t the low dose r a t e degrade i n j u s t the

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Techniques for Profiling Polymer Degradation

28. CLOUGH AND GILLEN

0.15

« X

A

1 1 X X X x

Y

1 * » X

K

>

E E

§ o-io

c 4)

a.

0.05

0.00 Edg Position On Sample F i g u r e 3. P r o f i l e s o f r e l a t i v e h a r d n e s s i n terms o f p e n e t r a t i o n d i s t a n c e o f a w e i g h t e d probe i n t o c r o s s - s e c t i o n e d samples o f i r r a d i a t e d EPR. X unirradiated material, O 6.7 x 1 0 r a d / h t o 165 Mrad. • - 1.1 x 1 0 r a d / h t o 175 Mrad. Probe l o a d was 5 g. s

5

5

1.16

1.11 Edge

Edge

Center Position On Sample

F i g u r e 4. D e n s i t y p r o f i l e s f o r i r r a d i a t e d EPR samples. Solid l i n e symbols a r e f o r 165 Mrad a t 6.7 x 1 0 rad/h. Dotted l i n e symbols a r e f o r 175 Mrad a t 1.1 x 1 0 r a d / h . The arrow i n d i c a t e s the d e n s i t y of u n i r r a d i a t e d m a t e r i a l , which g i v e s a f l a t profile. 5

5

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418 1.40

1.30 Edge

Center

Edge

F i g u r e 5. D e n s i t y p r o f i l e f o r an EPR c a b l e i n s u l a t i o n m a t e r i a l which had been s t r i p p e d from the copper c o n d u c t o r and i r r a d i a t e d as a h o l l o w tube. At low dose r a t e (1.6 x 10^ r a d / h ) t h e mater i a l shows enhanced o x i d a t i o n near the i n t e r i o r s u r f a c e ( s o l i d l i n e symbols). The d o t t e d l i n e symbols show t h e f l a t p r o f i l e of u n i r r a d i a t e d m a t e r i a l .

c 0

I 0

0

CH -C-0-(CH ) -(CH -CH ) -CH -C-0-(CH )4-(CH -CH ) 2

2

4

2

2

x

2

2

2

2

y

In o r d e r to ensure t h a t the copolymers were r e a s o n a b l y homogeneous i n view of the f a c t t h a t the 2 - m e t h y l e n e - l , 3 - d i o x e p a n e (I) i s o n l y s l i g h t l y more r e a c t i v e than e t h y l e n e , a l l c o n v e r s i o n s were h e l d to below 2% as i n d i c a t e d i n T a b l e I . A s e r i e s of copolymers c o n t a i n i n g from 2.1 to 10.4 mol-% o A l l o f these copolymers were about 5,000 m o l e c u l a r w e i g h t and had m e l t i n g p o i n t s v a r y i n g from 1 0 0 - 1 0 5 ° C f o r the copolymer c o n t a i n i n g 2.1 mole-% of I to 8 4 - 8 8 ° C f o r the copolymer c o n t a i n i n g 10.4 mole-%. D e t e r m i n a t i o n of

Biodegradability

The r a t e of b i o d e g r a d a b i l i t y was determined by a method based on the d e t e r m i n a t i o n of the carbon d i o x i d e produced by the m i c r o b i a l metabolism of the polymer samples. Our s c r e e n i n g t e s t , which was the method developed by Kramer and E n n i s ( 5 , £ ) , c o n s i s t e d o f f e e d i n g a s t a n d a r d amount of h y d r o l y z e d c a s e i n and the polymer to a c u l t u r e c o n t a i n i n g a mixed m i c r o - f l o r a from s o i l or known m i c r o o r g a n i s m s i n a m o d i f i e d Warburg a p p a r a t u s . The amount of C 0 l i b e r a t e d was m o n i t o r e d by a F i s h e r - H a m i l t o n gas p a r t i t i o n e r . The i n c r e a s e i n the amount of carbon d i o x i d e i n p o l y m e r - c o n t a i n i n g c u l t u r e s over t h a t of the c o n t r o l was used as a measure o f the r a t e o f b i o d e g r a d a t i o n on the time s c a l e u s e d . S t a t i s t i c a l a n a l y s i s i n v o l v e d a n a l y s i s of v a r i a n c e and the s t a t i s t i c a l d i f f e r e n c e was d e t e r m i n e d w i t h a D u n c a n ' s t e s t a t a 5% l e v e l . The r e s u l t s are l i s t e d i n T a b l e II and a r e p l o t t e d i n F i g u r e 1. I t i s o b v i o u s t h a t the copolymers c o n t a i n i n g a t l e a s t 6.7 mol-% of the e s t e r - c o n t a i n i n g u n i t s are h i g h l y d e g r a d a b l e p r o d u c i n g C 0 a t a r a t e 108 to 118% of the h y d r o l y z e d casein c o n t r o l . The copolymers c o n t a i n i n g the lower amount o f the e s t e r groups b i o d e g r a d e a t q u i t e low r a t e s but much g r e a t e r than polyethylene. I n o c u l a t i o n o f a sandy loam to n u t r i e n t b r o t h and i n c u b a t i n g f o r 4 days p r o v i d e d the mixed s o i l m i c r o - f l o r a s o u r c e f o r 1 ml i n o c u l a t i o n o f the a r t i f i c i a l medium. Thus 20 mg o f t e s t m a t e r i a l was added to 20 ml o f the medium i n 50 ml f l a s k s , which c o n t a i n e d 5 g o f a p a n c r e a t i c d i g e s t o f c a s e i n p l u s 1 g o f d e x t r o s e per l i t e r . A f t e r s t e r i l i z a t i o n a t 121° C f o r 15 m i n . , the samples were i n o c u l a t e d w i t h 1 ml o f the s o i l m i c r o - f l o r a s o u r c e and the headspace was f l u s h e d w i t h oxygen. C o n t r o l s c o n s i s t e d o f samples t e s t e d i n d e n t i c a l l y but c o n t a i n i n g no t e s t m a t e r i a l . 2

2

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

5.00 10.00 15.00 19.00 22.00

2.1 4.8 6.7 9.3 10.4

Amount of I (mol-%) In In Feed Copolymer

Table

0.80 1.6 1.2 1.5 1.7 100-105 95-99 90-95 89-96 84-88

Copolymer (mp, °C)

0.13 0.14 0.122 0.16 0.144

Intrinsic* Viscosity (dl/g)

(I)

83.90 81.90 80.61 79.25 78.46

13.84 13.35 13.04 12.71 12.51

83.78 81.85 80.55 78.98 78.18

14.00 13.62 13.38 13.03 12.80

Analysis Calculated Found

Copolymerization of Ethylene with 2-Methylene-l,3-dioxepane

Conversion (weight-%)

I.

BAILEY AND GAPUD

Synthesis of Biodegradable Polyethylene

DAYS Figure

1.

ethylene

Cumulative headspace and

of

C0

2

from copolymer

2-methylene-l,3-dioxepane.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

of

430

P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

Table

II.

B i o d e g r a d a t i o n o f Copolymers o f E t h y l e n e and 2-Methylene-l,3-dioxepane

Amount o f I i n Copolymer (mol-%) 2.1 4.8 6.7 9.3 10.4

E x p r e s s e d as p e r c e n t o Significantly different b

(I)

Cumulative C 0

2

a

after

7 days

20 days

103 107 116 116 121

98 103 108 113 118

b

b

b

from the c o n t r o l .

Thus i t has been demonstrated t h a t the i n t r o d u c t i o n o f e s t e r groups i n t o the backbone of p o l y e t h y l e n e w i l l r e n d e r the copolymer highly biodegradable. The copolymers a r e s u f f i c i e n t l y high m e l t i n g to make them u s e f u l f o r a wide v a r i e t y o f a p p l i c a t i o n s i n the form o f i n j e c t i o n molded p l a s t i c s , e x t r u d e d f i l m s , and melt spun f i b e r s . For example, t h e copolymer c o n t a i n i n g 9.3% e s t e r - c o n t a i n i n g u n i t s and 90.7% e t h y l e n e u n i t s has a m e l t i n g p o i n t o f 8 9 - 9 6 ° C and has many o f t h e c h a r a c t e r i s t i c s o f h i g h p r e s s u r e p o l y e t h y l e n e . At the same t i m e , t h i s copolymer b i o d e g r a d e s a t a r a t h e r f a s t r a t e .

Experimental Copolymerization of Ethylene with 2-Methylene-l,3-dioxepane (I)— T y p i c a l c o p o l y m e r i z a t i o n s were c a r r i e d o u t as f o l l o w s : To a 10.125 X 1 . 5 - i n c h s t e e l p r e s s u r e v e s s e l (300-ml c a p a c i t y ) was added 50 ml o f a s o l u t i o n o f 1.1 g ( 1 . 0 mol-%) o f d i - t e r t - b u t y l p e r o x i d e and 5.0 g (0.043 mol) o f 2 - m e t h y l e n e - l , 3 - d i o x e p a n e , bp 4 9 - 5 0 ° C (20 mm), i n p u r i f i e d c y c l o h e x a n e to g i v e 50 ml o f the r e a c t i o n m i x t u r e . The s e a l e d v e s s e l was f l u s h e d twice w i t h e t h y l e n e gas (99.9% pure) and f i n a l l y f i l l e d w i t h e t h y l e n e to an e q u i l i b r i u m p r e s s u r e o f 1000 psi. I f one n e g l e c t s the amount o f e t h y l e n e t h a t d i s s o l v e s i n the o r g a n i c l a y e r , the amount o f e t h y l e n e gas w i t h a volume of 250 ml a t 1000 p s i was c a l c u l a t e d to be 21 g (0.75 m o l e ) . The c o p o l y m e r i z a t i o n was a l l o w e d to proceed to low c o n v e r s i o n s ( l e s s than 2%) a t 120°C f o r 30 m i n u t e s . A f t e r the r e a c t i o n v e s s e l was q u i c k l y c o o l e d i n Dry I c e , i t was opened and methanol was added to i t to f a c i l i t a t e the removal o f the p r o d u c t as a white p r e c i p i t a t e . A f t e r the s o l i d was c o l l e c t e d by f i l t r a t i o n with s u c t i o n and washed w i t h m e t h a n o l , the polymer was p u r i f i e d f u r t h e r by d i s s o l u t i o n i n h o t c h l o r o f o r m and a d d i t i o n o f the r e s u l t i n g s o l u t i o n i n t o the n o n s o l v e n t m e t h a n o l . The polymer was c o l l e c t e d by f i l t r a t i o n and d r i e d i n vacuo a t 40°C f o r 24 hours to g i v e a white powder.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

29.

BAILEY AND GAPUD

Synthesis of Biodegradable Polyethylene

431

Acknowledgments The authors are grateful to the Frasch Foundation, the Polymer Progam of the National Science Foundation, and the Goodyear Tire and Rubber Company for partial support of this work. Literature Cited 1.

Potts, J . E . ; Clendinning, R. A.; Ackart, W. B.; Niegisch, W. D. In "Polymers and Ecological Problems"; Guillet, J., Ed.; Plenum Press: New York, 1973, p. 61. 2. Shelton, J . R.; Agostini, D. E . ; Lando, J . B. Am. Chem. Soc., Div. Polymer Chem., Preprints 1971, 12(2), 483. 3. Frazza, E. J.; Schmitt, E. E. J . Biomed. Mater. Res. Symp. 1970, 1, 43. 4. Bailey, W. J.; Okamoto, Y.;Kuo, W. C.; Narita T. In "Proc. Third International Kaplan, A. M., Eds.; Applied Science Publishers: Essex, England, 1976, p. 765. 5. Ennis, D.; Kramer, A. Lebensm. -Wiss. u. Technol. 1974, 7(4), 214. 6. Ennis, D.; Kramer, A. J . Food Science 1975, 40, 181. 7. Bailey, W. J . Preprints, IUPAC Post-congress Symposium, Biomedical Materials, Kyoto, Japan, September 13, 1977, p. 10. 8. Albertson, A. - C . ; Ranby, B. In "Proc. Third International Biodegradation Symposium"; Sharpley, J . M.; Kaplan, A. M., Eds. Applied Science Publishers: Essex, England, 1976, p. 743. 9. Corbin, D. G., cited by Henman, T. J . Proc. Third International Conference on Advances in the Stabilization of Polymers, Lucerne, Switzerland, June 1981, p. 116. 10. Bailey, W. J.; Chen, P. Y.; Chiao, W. - B . ; Endo, T.; Sidney, L.; Yamamoto, N.; Yamazaki, N.; Yonezawa, K. In "Contemporary Topics in Polymer Science"; Shen, M., Ed.; Plenum Publishing Company: New York, 1979, Vol. 3, p. 29. 11. Bailey, W. J.; Ni, Z.; Wu, S. -R. J . Polym. S c i . , Polym. Chem. Ed. 1982, 20, 3021. 12. Bailey, W. J.; Gapud, B. Am. Chem. Soc., Div. Polym. Chem., Preprints 1984, 26(1), 58. RECEIVED February 21, 1985

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

A u t h o r Index

Layer, R. W., 99 Lucki, J . , 353 Ludwick, A. G., 299 Mendenhall, G. D., 373 Muller, H. K., 55 Murayama, K., 37 Pastor, S. D., 247 P a t e l , A., 247 P i c k e t t , J . E., 313 P o s p i s i l , J., 157

Agarwal, H. K., 373 Albertsson, A. C , 197 B a i l e y , W. J., 423 B a l i n t , G., 109 Bauer, D. R., 119 Briggs, L. M., 119 Carlsson, D. J., 359 Chan, M. G., 299 Clough, R. L., 411 Cooke, J . M., 373 Danforth, J. D. Denisov, E. T., Dobbin, C. J . B., 359 Dziemianowicz, T. S., 373 Felder, B. N., 69 Gapud, B., 423 Gerlock, J . L., 119 G i l l e n , K. T., 411 G u i l l e t , J. E., 211 Heyward, I. P., 299 Hjertberg, T., 259 Horng, P.-L., 235 Ivanov, V. B., 11 Janovic, Z., 197 Jennings, E., 91 Jensen, J . P. T., 359 J i a n , S. Z., 353 Kagan, E. Sh., 11 Kelen, T., 109 Klemchuk, P. P., 1, 235 Kurumada, T., 37 L a i , J. T., 91, 99 Lattimer, R. P., 99

Ramey, C. E., 149 Ranby, B., 353 Rockenbauer, A., 109 Rosantsev, E. G., 11 Rostek, C. J . , 149 Scott, G., 173 Sholle, V. D., 11 Smirnov, V. A., 11 Somersall, A. C , 211 Son, P. N., 91 Sorvik, E. M., 259 Spivack, J. D., 247 Steinhuebel, L. P., 247 Susi, P. V., 137 Toda, T., 37 Tucker, R. J., 137 Tudos, F., 109 Vogl, 0., 197 Westfahl, J. C , 99 Wiles, D. M., 359 Z l a t k e v i c h , L., 387

Subject I n d e x

UV spectra of epoxy d i a c r y l a t e r e s i n , 300,302f,303 Acrylate coating s o l u t i o n studies e f f e c t of s t a b i l i z e r on p h o t o i n i t i a t o r e f f i c i e n c y during cure, 303,305t photolysis of s t a b i l i z e r s , 303,304t photoproduct d i s t r i b u t i o n , 303,306 s o l u b i l i t y parameters, 301t,303

Acrylate coating oxidation studies changes i n p h y s i c a l properties, 303 oxygen absorption data f o r u n s t a b i l i z e d r e s i n s , 300,302f s t r e s s - s t r a i n p r o f i l e s at epoxy d i a c r y l a t e r e s i n , 303,304f temperature at onset of oxidation, 300,303t

433 In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

A u t h o r Index

Layer, R. W., 99 Lucki, J . , 353 Ludwick, A. G., 299 Mendenhall, G. D., 373 Muller, H. K., 55 Murayama, K., 37 Pastor, S. D., 247 P a t e l , A., 247 P i c k e t t , J . E., 313 P o s p i s i l , J., 157

Agarwal, H. K., 373 Albertsson, A. C , 197 B a i l e y , W. J., 423 B a l i n t , G., 109 Bauer, D. R., 119 Briggs, L. M., 119 Carlsson, D. J., 359 Chan, M. G., 299 Clough, R. L., 411 Cooke, J . M., 373 Danforth, J. D. Denisov, E. T., Dobbin, C. J . B., 359 Dziemianowicz, T. S., 373 Felder, B. N., 69 Gapud, B., 423 Gerlock, J . L., 119 G i l l e n , K. T., 411 G u i l l e t , J. E., 211 Heyward, I. P., 299 Hjertberg, T., 259 Horng, P.-L., 235 Ivanov, V. B., 11 Janovic, Z., 197 Jennings, E., 91 Jensen, J . P. T., 359 J i a n , S. Z., 353 Kagan, E. Sh., 11 Kelen, T., 109 Klemchuk, P. P., 1, 235 Kurumada, T., 37 L a i , J. T., 91, 99 Lattimer, R. P., 99

Ramey, C. E., 149 Ranby, B., 353 Rockenbauer, A., 109 Rosantsev, E. G., 11 Rostek, C. J . , 149 Scott, G., 173 Sholle, V. D., 11 Smirnov, V. A., 11 Somersall, A. C , 211 Son, P. N., 91 Sorvik, E. M., 259 Spivack, J. D., 247 Steinhuebel, L. P., 247 Susi, P. V., 137 Toda, T., 37 Tucker, R. J., 137 Tudos, F., 109 Vogl, 0., 197 Westfahl, J. C , 99 Wiles, D. M., 359 Z l a t k e v i c h , L., 387

Subject I n d e x

UV spectra of epoxy d i a c r y l a t e r e s i n , 300,302f,303 Acrylate coating s o l u t i o n studies e f f e c t of s t a b i l i z e r on p h o t o i n i t i a t o r e f f i c i e n c y during cure, 303,305t photolysis of s t a b i l i z e r s , 303,304t photoproduct d i s t r i b u t i o n , 303,306 s o l u b i l i t y parameters, 301t,303

Acrylate coating oxidation studies changes i n p h y s i c a l properties, 303 oxygen absorption data f o r u n s t a b i l i z e d r e s i n s , 300,302f s t r e s s - s t r a i n p r o f i l e s at epoxy d i a c r y l a t e r e s i n , 303,304f temperature at onset of oxidation, 300,303t

433 In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

434

POLYMER STABILIZATION A N D DEGRADATION

Acrylonitrile-butadiene-styrene thermal oxidative stability, evaluation by chemiluminescence, DSC, and oxygen uptake, 399t,400 Addition polymers, resistance to biodegradation, 425 Advanced stages of thermal oxidation application of chemiluminescence for evaluation, 405 concentration of hydroperoxides, 394,395 high oxygen pressure, 394 problems in evaluating chemiluminescence, 405 Alkylaminotropone nitroxy radicals stability, 38 structure, 39 Aminyl radicals kinetics of reaction wit kinetics of recombination mechanisms of reaction with ROOH, 89 reaction with R02», 88 recombination, 87,88 source, 87 Antioxidants ideal physical criteria, 176 loss to surrounding environment, 176 reactions with preformed functional groups, 177,178 Antioxidant effectiveness determining factors, 173 polymers subjected to aggressive environments, 174,176 Antioxidant mechanisms involvement of benzoquinonediimines, 160 involvement of nitroxides, 158,160 Antiozonant mechanisms combinations, 171 dimers formed from N-N couplings of aminyls, 169,170 product studies, 168,169 Antiozonant theories ozone scavenging theory, 165 protective film formation theory, 165,167 reactions of antiozonant with rubber zwitterions, ozonides, and aldehydes, 168,170 reaction of phenylenediamine with an ozonide, 168,170 rubber chain relinking theory, 167 self-healing film formation theory, 167 Aromatic amine molecule, autoxidation inhibition, 87 Autoxidation reactions, mechanisms, 2

B Benzoquinonediimines antioxidant property, 160,162 decomposition, 160,161 hydrolysis, 160,161 imino-CHD formation, 12,163,166 transformation products, 160,161 [(4-Benzoyl-3-hydroxylphenyl)oxyJ ethyl 2-mercaptoethanoate, structure, 183 Biodegradable addition polymers examples, 426 preparation, 426 Biodegradable polyamides examples, 424 synthesis, 424,425 Biodegradable polyesters synthesis by free radical mechanism, 424 3,9-Bis(2,4-di-tert-butylphenyl) analogue, structure, 248 3,9-Bis(octadecyloxy)-2,4,8,10-tetraoxaS^-phosphaiS.S] spiroundecane, structure, 248 Bisphenol effectiveness as antioxidant, 173 structure, 173

C Chain scissioning calculated oxygen molecules consumed per chain scission, 243t chain scission rates, 243,244f molecular weight data and chain scission, 241t molecular weight distribution of stabilized HDPE, 24l,244f molecular weight distribution of unstabilized HDPE, 24l,242f Changes in 220-300-nm region of polycarbonate films methanol extracts of films exposed to long wavelengths, 337,340f UV data for alcohol-soluble portions of degraded polycarbonate, 337,341t UV-vis spectra of thin films, 337,339f

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

INDEX

435

Chemiluminescence estimation of thermal oxidative stability of polymers, 388 multisample chemiluminescence apparatus, 388 reasons for lack of standard method or instrument, 388 Chemiluminescence intensity curves of acrylonitrile-butadienestyrene copolymers, 399,402f light intensity versus time curve, 396,397f nylon under nitrogen, 400,401 source of oxygen, 401 Chemiluminescence of irradiated polypropylene samples ambient chemiluminescence versus time, 380,38lt,382f inhibition, 377,380,383 intensity, 375,380,381,38 representative plots, 375,377f,378f three-dimensional plots versus time, 375,378f,379f,380 Chemiluminescence measurement experimental details, 374,375 schematic of apparatus, 375,376f Chemiluminescence method constant heating rate in nitrogen atmosphere, 395,396 discussion, 389 isothermal in oxygen atmosphere, 395 Chemiluminescence technique, advantages, 398 Coating durability, testing, 119,120 Coating kinetic data, 120,121,123t Coating photodegradation absorption of C-H stretching band versus exposure time, 122,124f area of C=0 band relative to that of C-H band, 122,124f,125 gloss loss and chemistry, 121,122 gloss loss-resin loss relationship, 122 photooxidation rate, 122,125 rate of change of surface roughness, 122,125 reduced surface roughness versus exposure time, 122,123f Computer model of photooxidation and stabilization cesium flare system data base, 212 chemical mechanism, 211,212 concentration-time expressions, 212 Computer modeling photooxidation, effect of diffusion on chemical degradation, 224 Computer modeling reaction scheme propagation, 219 termination, 219,220

Computer modeling stabilization energy transfer, 229 hydroperoxide decomposition, 230f,232 lifetimes of peroxy radicals i n the dark,229,231f mechanisms, 225,228 peroxy radical trapping, 229 stabilization of polyethylene, 229,230f stabilization of polyethylene by radical trapping, 229,230f UV shielding, 225,227f,229 Computer modeling studies effect of temperature, 225 photooxidation of amorphous polyethylene, 220,221f,222 polyethylene photooxidation reaction scheme, 213,214-17t,218 biodegradation, 425 properties, 425 synthesis, 425 Cured polyesters, mechanisms of photooxidation, 354,355 Cured polyester photooxidation C i and 0-| ESCA spectra at different UV irradiation times, 355,356f IR spectra, 358 kinetics, 355f,356f,357f,358 kinetics of C=0 band formation, 355f 0-| /Ci peak intensity ratio as a function of UV irradiation time, 356,357f ratio of total C-| areas for C bonded to H, 356,357f Cyasorb UV 531, structure, 183 s

s

s

s

s

D Decahydroquinoxalines antioxidation activity in polypropylene, 95,96 light stabilizing activity, 95 similar compounds with no antioxidant activity, 96 similar compounds with similar activity, 96,97 structure, 94 synthesis, 95 Decahydroquinoxalinones, structure, 91 Density profiles EPR cable insulation material, 4l6,4l8f irradiated EPR samples, 4l6,4l7f

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

436

POLYMER STABILIZATION A N D

Density profiles—Continued measurements, 421 oxygen uptake, 416 salt gradient column, 416 Desired properties of polymer stabilizers, 5 2.2- Dialkyldecahydroquinoline, primary nitroxyl radical, 104, 105f 3.3- Dialkyldecahydroquinoxalin-2-ones computed ESR spectra of nitroxyl radicals, 100,101f ESR spectra, 100,101f ESR spectra during stepwise oxidation, 100,102f irradiation of polypropylene in a Weather-Ometer, 104f possible mechanism for antioxidant and light stabilizing activity, 104,106 stability of primary nitroxy radical, 100,103f Diaryl nitroxides formation, 158 mesomeric dinitrone form, 160,161 reactivity, 158 regenerative cyclical process, 158,159 side reaction leading to antioxidant ineffective species, 158,159 transformation into a mixture of diarylamine and N-aryl-1,4-benzoquinonemonoimine N-oxide, 158-60 Dibenzylhydroxylamine comparison to butylated hydroxytoluene, 5,8 effectiveness as AIBN-initiated tetralin oxidation, 5,8f 2,6-Di-tert-butyl-4-methylphenol effectiveness as antioxidant, 173 structure, 173 3.4- Di-tert-butyl-4-methylphenolic sulfide analogues, structure, 183 2,6-Di-tert-butyl(4-phenylamino-4phenylimino)-2,5-cyclohexadien-1one (imino-CHD) antioxidant properties, 163 formation, 162,163,166 Diffusion effects on biomolecular reactions polymer moiety-polymer moiety reactions, 224 small molecule-polymer moiety reactions, 224 small molecule-polymer solvent reactions, 224 small molecule-small molecule reactions, 224

DEGRADATION

2,4-Dihydroxy-4 -vinylbenzophenone, synthesis, 202 cis-3,3-D imet hy1de cahydroquinoxa1in2-one, structure, 94 4,4-Dimethyloxazolidine, structure, 150 2,4-Dimethylpentane, investigations, 78 1,6-Dioxa-7-(dibutylstanna)-2,5-oxocyclohept-3-ene, structure, 192 N,N -Disubstituted phenylenediamine reaction mechanism, 161 regeneration, 162,163 1

f

E Effects of variables on polycarbonate minimum wavelength, 345 presence of oxygen, 345,346 relative humidity, 346 Electron-transfer mechanism action of quenchers, 321,324 1,4- or 1,2-addition across aromatic ring, 325,326 another possible mechanism, 325-27 cyanoanthracene-sensitized photooxidations, 319,322 effect of electron-transfer quenchers on oxygen uptake, 321,323f EPR spectra of authentic trapped 0 - « , 321,322f EPR spectra of polymer-generated 02--, 321,323f formation of 0 " , 321 isolation of Ar-02, 325 nitrone-trapping experiment, 321,322 poly(phenylene oxide) transient spectra, 321,324f,325 scheme, 319-21 transient lifetimes, 325t 1,1•-(1,2-Ethanediyl)bis(3,3,5,5tetramethyl-2-piperazinone) effectiveness as UV stabilizer, 92,93 properties, 92 structure, 91 synergism with commercial UV absorbers, 93 synergism with a phenolic antioxidant, 93,94 synthesis, 92 water carry-over behavior i n PP films, 94 Ethyl 4-vinyl-a-cyano-B-phenylcinnamate, synthesis, 202 2

2

#

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

INDEX

437

Ethylene and 2-methylene-1,3dioxepane copolymers cumulative headspace of C02, 427,429f rate of biodegradability, 427,430t Ethylene and 2-methylene-1,3dioxepane copolymerization data, 427,428t experimental details, 430 reaction, 426,427 Ethylene and 2-methylene-1,3dioxepane copolymers cumulative headspace of CO2, 427,429f rate of biodegradability, 427,430t Ethylene-pro pylene-di ene prevention of discoloration, 255t stabilization, 255 Ethylene-propylene-diene rubber prevention of discoloration

F 3-Formyl-2,2,6,6-tetramethylpiperidine-1-oxyl, synthesis, 18 Free radicals, occurrence of processes, 2

G Gel permeation chromatography of polycarbonate f i l m s effects of added BPA on molecular weight d i s t r i b u t i o n , 337,342f molecular weight changes from GPC s t u d i e s , 337,343t,344

H Heterogeneous degradation, effect on macroscopic property changes, 416 Heterogeneous oxidation effects change in ultimate tensile properties for Viton material, 416,418f,419 differences in degradation behavior as a function of dose rate, 419 oxidation depth, 419t

Heterogeneous polymer degradation examples, 411 oxygen diffusion limited degradation, 411 High-density polyethylene (HDPE) oxidation, 235,236 processing stability, 251 254t High-density polyethylene oxidation Arrhenius plot of induction time versus reciprocal absolute temperature, 236,238f,239 chain scissioning and i t s effect on elongation, 243 chain scissioning and i t s effect on molecular weight, 241,243 change in elongation, 239t correlation of elongation retention elongation retention of HDPE, 239,240f mechanism of chain scissioning and stabilization, 243,245,246 molecular weight distribution of HDPE, 239,24l,242f oxidative induction times, 236t,239 oxygen uptake at 40 o c , 236,238f oxygen uptake at 100 o c , 236,237f oxygen uptake at 140 o c , 236,237f property correlation, 239 Hindered amines light stabilizing activity, 45t mechanism of action, 310 nitroxyl radical precursors, 42-44 reactions at the 3- and 5-methylene groups, 15,17 structures of polymer stabilizers, 22,24,25 synthesis, 45,46 use in mixtures with other stabilizers, 27,29 Hindered amine light stabilizers (HALS) activity loss after laundering and dry cleaning, I4l,l43f,l44 application, 62,67 commercial stabilizers, structures, 50,51 compatability, 140 concentration-light stabilizing activity study in PP yarns, I44,l45f conversion to nitroxides, 149 developmental compounds sampled in 1973, 57,58f effect of processing conditions on yarn stability, I46t

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

438

POLYMER STABILIZATION A N D DEGRADATION

Hindered amine light stabilizers (HALS) —Continued effect on thermooxidative stability of PP multifilaments, 141 failure of metallic paint, 62,64f gas yellowing resistance in PP yarns, I44t impact strength of ABS after light exposure, 62,66f influence of China clay on light stability of LDPE blown film, 67t influence of f i l l e r s on light stability of LDPE blown film, 67t influence of pigments on light stability, I44,l46t inhibition of light-induced discoloration of SANS, 62,65 light exposure of HDPE tapes, 62, 65t light stability of HDPE plaques, 62,66f light stability of PP tapes, 57,59,60f light stability of treated paint, 62,64f light stability on 2-mm PP plaques, 59,60f light stabilizing activity of commercial stabilizers, 50,52f light stabilizing activity in PP yarn, I4l,l42f light stabilizing activity in simulated tentered PP yarn, I4l,l43f mechanism of action, 138,140 mechanisms for conversion to nitroxide, 130,132 molecular combination of a s t e r i cally hindered phenol and HALS, 59,6lf outdoor of PP-stretched tape, 57,58f patent applications, 55,56f performance loss on thermotreatment, 59 performance of a molecular hindered phenol-HALS combination, 59,61t polymers, 59 selection, 50 structural variants, 55,56f structures, 121,138,139f structures of commercially available products, 8,9f synthesis, 37 thermal stabilizing activity in PP yarn, I4l,l42f transition from N-oxyls, 57,58f Hindered amine light stabilizer additives effectiveness, 132

Hindered amine light stabilizer additives—Continued key role of nitroxide regeneration, 132 Hindered amine light stabilizer based nitroxide kinetics and photostabilization extent of photodegradation versus time of exposure, 128,129f i n i t i a l nitroxide buildup, 128 interference with propagation of free radicals, 128 nitroxide concentration versus exposure time, 130,122f sensitivity of exposure conditions, 132 Hindered amine light stabilizer compound-antioxidant interaction induction period of oxygen absorption, 113,H4f maximal rate of oxygen formation, 113,1l4f reactions, 116 time dependence of concentration, 113,115f Hindered amine nitroxides, key intermediates in HALS stabilization mechanism, 149 Hindered phenol series effect of structural variation on antioxidant activity, 174,175t Irganox 1076, 174 molar antioxidant effectiveness as UV stabilizers, 174,175t solubility, 174,175t structure, 174 volatility and migration rate, 174,175t Hydrolytic stability test of processing stabilizers procedure at 50 °C/80% relative humidity, 253 procedure at room temperature/80$ relative humidity, 252 2-(2-Hydroxy-5-isopropenylphenyl)-2Hbenzotriazole copolymerization, 206,207f synthesis, 205 Hydroxylamine effectiveness as polymer stabilizers, 5,7f inhibition of styrene autoxidation, 5,7f 2-(2-Hydroxy-5-methylphenyl)-4-vinyl2H-benzotriazole, synthesis, 205 4-Hydroxy-2,2,6,6-tetramethylpiperidine, derivatives, 47,48

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

INDEX

439

2-(2-Hydroxyvinylphenyl)-2Hbenzotriazoles copolymerization, 206,207f g r a f t i n g , 206 s t r u c t u r e , 203 synthesis, 203,205 2-(2-Hydroxy-3-vinylphenyl)-2Hbenzotriazole, synthesis, 205

I Iminoxyls—See Nitroxyls Impact measurement, 374 Initial stages of thermal oxidation autocatalytic stage, 396 high oxygen pressures, 391 induction period, 396 intensity of chemiluminescence light decay, 396 low oxygen pressures, 391 oxidation rate, 396 peak hydroperoxide concentration, 396 plot of ln [l /(Imax - It)] versus t, 396,397f propagation rate, 391 Irganox 1010, structure, 109,111T Irradiated polycarbonate gel formation, 331,333t physical changes, 331,333t solubility, 331,333t yellowing, 331,333t Irradiated polycarbonate films analysis, 331 embrittlement, 331,333t Irradiated polypropylene chemiluminescence, 374,375,380 impact strengths, 380t,382f,383,384 stability, 380 Irradiation procedure, 374 Isooctane distribution of radicals, 73,75 photooxidation, 71 photooxidation initiated by t-BuO» radicals, 71,72f primary radical derivatives, structure, 75 structure, 71 Isooctane photooxidation consumption of N-H and formation of N0-, 78,79f discussion, 78,80,82-84 f i t between kinetic model and experimental data, 76,77f peracid formation versus irradiation time, 73,74f proposed model, 75,76,78,79 t

Isooctane photooxidation products characterization, 73 relative i n i t i a l rate of formation of non-peracid products, 73,74f

Kinetic characterization of poly(vinyl chloride) samples best f i t parameters, 292t fraction degraded versus time, 292,293f rate-time curves, 292,293f reproducibility of data, 291,292,294

L Light emission, source, 374 Light s t a b i l i z i n g a c t i v i t y , t e s t method, 47 Linear low-density polyethylene, resistance to d i s c o l o r a t i o n , 252 Low-density polyethylene (LDPE) prevention of d i s c o l o r a t i o n , 255t s t a b i l i z a t i o n , 255

M Macroalkyl r a d i c a l s trapped i n the c r y s t a l l i n e domains, 368,369 4-(Mercaptoacetamido)diphenylamine, s t r u c t u r e , 165,166 Metallographic p o l i s h i n g c r o s s - s e c t i o n a l , polished samples of Y - i r r a d i a t e d EPR, 4 l 3 , 4 l 5 f degradation p r o f i l e , 413,414 d e s c r i p t i o n , 413 i d e n t i f i c a t i o n of heterogeneous oxidation, 413 o p t i c a l r i n g s , 413 oxidation depth, 414 t e n s i l e strength, 413 v i s u a l rings, 413 Methyl v i n y l s a l i c y l a t e s copolymerization, 202,203 g r a f t i n g , 206 synthesis, 201,202 2-Methylene-1,3-dioxepane, synthesis, 426

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

440

POLYMER STABILIZATION A N D DEGRADATION

Multichromophoric 2-(2-hydroxyphenyl)2H-benzotriazole UV s t a b i l i z e r s , synthesis, 206,208 Multisample chemiluminescence apparatus d e s c r i p t i o n , 388,389 diagram, 388,390f diagram of t e s t c e l l , 389,390f

N i t r o x i d e , s t r u c t u r e , 121 N i t r o x i d e decay k i n e t i c s estimation of chain length, 127 gloss l o s s rates versus square root of p h o t o i n i t i a t i o n rate i n i t i a l rate o f consumption measurement of r a t e of primary p h o t o i n i t i a t i o n , 126,127 N i t r o x y l s , r e a c t i o n s , 12,13 Nitroxyl radicals antioxidant a c t i o n , 26 a p p l i c a t i o n s , 22,29-32 chemistry, 18,20 d i s p r o p o r t i o n a t i o n , 20 electrochemical syntheses, 18,19 examples, 39 hydroxylamine u t i l i z a t i o n , 18,21 l i g h t s t a b i l i z i n g a c t i v i t y , 40 photooxidation i n h i b i t i o n , 26,27 polymer s t a b i l i z a t i o n , 22 polymer yellowing, 42,44 p u r i f i c a t i o n , 20 r a d i c a l scavenging a b i l i t y , 42 reactions, 18 reduction-oxidation, 18-20 synthesis, 20,21,31 Non-steady-state k i n e t i c s a versus time curves, 287,288f c a l c u l a t i o n o f degradation data, 289-91 equations, 287t generated r a t e data as Aa/100 s versus time, 287,288f,289 mechanistic i m p l i c a t i o n s , 295,297 model f o r p o l y ( v i n y l c h l o r i d e ) , 291 s t a r t s per chain, 291 time s h i f t , 289 z i p r a t e , 291 zipper mechanism, 290 Number o f s t a r t s per chain versus degradation pattern best f i t values of parameters, 294t rate-time curves, 294,296f Nylon a n a l y s i s of chemiluminescence emission growth and decay, 405,407f,408

Nylon—Continued chemiluminescence curves of annealed and quenched s t a b i l i z e d nylon, 401,403,404f chemiluminescence curves of s t a b i l i z e d nylon aged and exposed, 403,404f chemiluminescence curves of u n s t a b i l i z e d and s t a b i l i z e d nylon, 401,402f dependence of emitted l i g h t i n t e n s i t y versus time o f exposure, 403,406f growth and decay curves of l i g h t emission, 405,406f l i g h t i n t e n s i t y , 401 o x i d a t i v e s t a b i l i t y , 401

O p t i c a l f i b e r s , manufacture, 299 Oven aging, 375 Oxidation depth, o p t i c a l determinations, 419 Oxidation reactions free r a d i c a l chain r e a c t i o n , 37,38 i n h i b i t i o n by s t a b i l i z e r s , 39 Oxygen d i f f u s i o n e f f e c t s equivalent dose, 412 heterogeneous mechanism f o r polymer degradation, 412 oxidation depth, 413 regions of o x i d i z e d and nonoxidized polymer, 413 steady s t a t e i n heterogeneous oxidation, 412 Oxygen d i f f u s i o n l i m i t e d degradation, examples, 411 N-Oxyls e f f e c t i v e n e s s , 5,7f i n h i b i t i o n of styrene autoxidation, 5,7f N-Oxyl r a d i c a l s formed from a secondary amine ESR spectra, 110,112f nature, 110 N-Oxyl r a d i c a l s formed from a t e r t i a r y amine ESR spectrum, 110,112f nature, 110 Ozone, p r o p e r t i e s , 163 Ozone weathering o f rubbers and stabilizers antiozant p r o p e r t i e s , 164 antiozonant mechanism, 163-65 antiozonant theory, 165 f a c t o r s , 163,164

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

441

INDEX

P P a r i t i a l l y hindered amines, o x i d a t i o n to n i t r o x y l r a d i c a l s , 100 2,2,4,4,6-Pentamethylhexahydropyrimidine, s t r u c t u r e , 150 Peracids, r e a c t i o n with o l e f i n s , 73 Peroxy r a d i c a l s , i n i t i a t i o n k i n e t i c s , 83,84 Photodegradation chemistry, 120 d e f i n i t i o n , 119 Photooxidation k i n e t i c s rate of photooxidation, 126 r a t e of primary p h o t o i n i t i a t i o n , 125,126 r e s i n erosin rate versus square root of l i g h t i n t e n s i t y , 126,128,126 scheme of f r e e r a d i c a l reactions, 125,126 Photoyellowing and photo-Fries products of polycarbonate f i l m s anomalous peak i n f i l m s exposed to only longest wavelength l i g h t , 337,338f appearance of photo-Fries products, 334,337 e f f e c t s of humid atmospheres and p r i o r h y d r o l y s i s , 334 e f f e c t of wavelength i n dry atmospheres, 334 f i l m s containing trans-4hydroxystilbene, 337,338f f i l m s exposed to humid atmosphere and p r i o r h y d r o l y s i s , 334,336f f i l m s exposed to long wavelengths, 334,335f f i l m s exposed to nitrogen versus films containning model compounds, 334,336f f i l m s exposed co short wavelengths, 334,335f Physical properties of f i l m s , 308t Piperidine-phenols reactions, 49,51 s t a b i l i z i n g a c t i v i t y , 49t Piperidine-spiroacetals l i g h t s t a b i l i z i n g a c t i v i t y , 45,47t syntheses, 45,48 Piperidinols, light stabilizing a c t i v i t y , 47, 49t Polycarbonate engineering uses, 329 source, 330 Polycarbonate f i l m s with added bisphenol A a l c o h o l - s o l u b l e e x t r a c t s , 344 GPC traces, 342f,344

Polycarbonate films with added bisphenol A—Continued photolyses, 344 UV-vis spectra, 336f,344 Polycarbonate photoaging See a l s o Polycarbonate photodegradation schematic of apparatus, 330,332f v a r i a b l e s , 330 Polycarbonate photodegradation See a l s o Polycarbonate photoaging chemical process, 329,330 e f f e c t s of v a r i a b l e s , 344 Polycarbonate p h o t o l y s i s products and p o s s i b l e mechanism e f f e c t s of oxygen or nitrogen atmospheres 346,349

lack of evidence of photo-Fries products, 349 r e a c t i o n s , 346-49 Polycarbonate s c i s s i o n patterns, non-random behavior, 349,350 Poly(2,6-dimethyl-1,4-phenylene oxide), photodegradation, 313 Polyethylene, biodegradation, 426 Polymer-bound antioxidants antioxidants and antiozonant a c t i v i t y of MADA-B, 192,194t approaches, 177 i d e a l , 176 t h e o r e t i c a l i m p l i c a t i o n s , 192 Polymer degradation, rate-determining, step, 412 Polymer i r r a d i a t i o n , changes i n physic a l properties, 373 Polymer m o d i f i c a t i o n by copolymerization conventionally s t a b i l i z e d rubbers versus copolymerized rubber, 177,180t s t r u c u t r e of a copolymerized monomer, 177 Polymer o x i d a t i o n , c l a s s i c a l , thermal o x i d a t i o n scheme, 367 Polymer photooxidation i n i t i a t i o n , 26,27 r o l e of p h y s i c a l quenching o f e x c i t e d states, 383 Polymer-reactive a n t i o x i d a n t s , reactions, 178 Polymer-reactive antioxidant reactions 1,3-addition r e a c t i o n of nitrones to rubbers, 183 r e a c t i o n s of n i t r o s o antioxidants with rubbers, 183,185

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

442

POLYMER STABILIZATION A N D

Polymer-reactive antioxidant reactions by normal chemical procedures age resistance comparison of bound antioxidant with a conventional bisphenol,

179,l80f

arylamine antidegradant MADA s t r u c t u r e , 179 comparison o f EBHPT-B with conven­ t i o n a l UV s t a b i l i z e r , I 8 3 , l 8 4 t e f f e c t of antioxidant concentration on adduct y i e l d ,

179,181t

effectiveness of bound antioxidants after vulcanization, 1 7 9 , l 8 2 t increase i n hardness of n i t r i l e containing rubber antioxidants,

179,l82f

Kharasch-Mayo t h i o l adduct process, 178,179 r e s i d u a l antioxidant remainin during leaching,

PP s t a b i l i z a t i o n ,

192,194f

e f f e c t of BHBM-B and DBTM on unsaturation i n PVC, 1 9 2 , 1 9 3 f e f f e c t s of BHBM-B and MADA compared with those of commercial 186,189t

e f f e c t of e x t r a c t i o n on MADA-B i n comparison with a conventional thermal antioxidant, 1 8 6 , 1 8 8 f e f f e c t of solvent e x t r a c t i o n on photooxidant a c t i v i t y , 1 9 2 , 1 9 3 t effectiveness of EPHPT-B and MADA-B as UV s t a b i l i z e r s ,

186,191t

effectiveness of a s y n e r g i s t i c UV s t a b i l i z e r with conventional additives,

186,190t

elongation decay at break of n i t r i l e rubbers containing antioxidants,

I86,l88f

induction period and time t o 50% l o s s of impact strength,

Polymer s t a b i l i z a t i o n b i r a d i c a l t h e r m o s t a b i l i z e r s , 23 development of research, 22,23,26 nitroxyl radical transformations, 26,28 photooxidation p r o t e c t i o n , 26 reactions of hydroxylamines and t h e i r e s t e r s , 27,28 Polymer s t a b i l i z e r s examples, 38 i d e a l properties, 198,199 Polymer thermal o x i d a t i o n , r e a c t i o n scheme, 389,391 Polymeric hindered amine l i g h t stabilizers antioxidant a c t i v i t y , 59, 63t e f f e c t of thermal treatment on

179,

Polymer-reactive antioxidant reactions during processing antioxidant a c t i v i t y o f MADA con­ centrate i n SBR, I 8 6 , l 8 7 t comparison o f bound antioxidant and conventional heat s t a b i l i z e r s i n

antioxidants,

DEGRADATION

186,190t

l o s s of impact strength o f ABS during UV exposure, I 8 6 , l 8 9 f

mechanochemical binding o f MADA t o rubbers, I 8 6 , l 8 7 t mechanoscission of polymer chains, 185 r e t e n t i o n o f BHBM i n PVC during processing, 1 8 6 , 1 9 1 f

synthesis of block copolymers, 185 y i e l d s of polymer-bound antioxidants i n EPDM, 1 8 6 , 1 9 0 t

performance i n PP m u l t i f i l a m e n t s , 59,63t s t r u c t u r e s , 59,61f Polymerizable antioxidants e f f e c t i v e n e s s , 208 properties, 208,209 synthesis, 208 Polymerizable 2-(2-hydroxyphenyl)-2Hbenzotriazole UV s t a b i l i z e r s copolymerization, 206,207f g r a f t i n g , 206 polymerization, 205,206 synthesis with polymerizable v i n y l or isopropyl groups, 203,204f Polymerizable u l t r a v i o l e t a b s o r b e r s — See Polymerizable u l t r a v i o l e t stabilizers Polymerizable u l t r a v i o l e t s t a b i l i z e r s approaches to production, 199,201 examples, 199,200f miscellaneous types, 201-3 P o l y o l e f i n s , photooxidation scheme, 70-72 Polyolefin oxidation i n h i b i t i o n , 248,249f mechanism, 248 P o l y o l e f i n processing s t a b i l i z e r s comparative e f f e c t i v e n e s s , 253 d e s c r i p t i o n , 247 h y d r o l y t i c s t a b i l i t y , 255,256t P o l y o l e f i n r a d i a t i o n degradation, stages, 359 Polyolefin testing evaluation, 248,251 processing s t a b i l i z e r s and antioxidants, 248,250f Poly(phenylene oxide) photooxidation attenuated t o t a l r e f l e c t a n c e IR spectroscopy o f f i l m , 3l4,315f

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

INDEX

443

PolyCphenylene oxide) photooxidation— Continued carbonyl formation i n peroxide-dope f i l m , 317,3l8f chain s c i s s i o n , 314 changes i n molecular weight upon photooxidation, 317t d i r e c t p h o t o l y t i c cleavage, 317,318 electron-transfer mechanism, 313,319-21 evidence against the free r a d i c a l mechanism, 314,317 hydroxperoxide-mediated f r e e r a d i c a l o x i d a t i o n of methyl groups, 313,315 i n i t i a t i o n by peroxides, 317 l o s s of i n t r i n s i c v i s c o s i t y upon photooxidation, 3 l 4 , 3 l 6 f l o s s of methyl groups upo photooxidation, 314t l o s s of molecular weight upon photooxidation, 3 l 4 , 3 l 6 f o x i d a t i v e cleavage, 317 oxygen uptake of poly(phenylene oxide) and model compounds, 317,319,320f photooxidation of model compounds, 317,319 thermal oxidation of model compounds, 319t Polypropylene (PP) l i g h t s t a b i l i t y versus thickness, 140,14U thermal s t a b i l i t y versus thickness, I40t oxidation mechanism, 374,377 processing s t a b i l i t y , 251 253t,254 Polypropylene autoxidation a c t i v a t i o n energy, 398,399 induction time, 399 l i g h t i n t e n s i t y , 398 parameters, 398t Polypropylene f i b e r s , production processes, 137 Polypropylene Y - i r r a d i a t i o n e f f e c t s , 36l,362t elongation, 26l,362t extent of o x i d a t i o n and embrittlement, 361,362t increases i n bonds, 361 Polypropylene photooxidation model c a l c u l a t i o n , 83,84f proposed model, 80-83,85 Polypropylene thermal o x i d a t i v e s t a b i l i t y , e v a l u a t i o n by chemiluminescence and oven aging methods, 398t Poly(vinyl chloride) anomalous s t r u c t u r e s , 259,260 content of t e r t i a r y and i n t e r n a l a l l y l i c c h l o r i n e , 264

Poly(vinyl chloride)—Continued degradation behavior, 297 degradation according to non-steady-state k i n e t i c s , 285 dominating short-chain structure, 260 formation of butyl branches, 261 mechanism f o r chain t r a n s f e r to monomer, 260 most frequent end groups, 260 rate of dehydrochlorination, 264 r a t e of dehydrochlorination i n nitrogen, 261 P o s t f a b r i c a t i o n polymer treatment antioxidant a c t i v i t y of homologous phenolic s u l f i d e s , I83,l87f binding of BHBN to PP by UV

Post-Y-irradiation deterioration theories decomposition of unstable o x i d a t i o n products, 368-70 l o n g - l i v e d macroradical mechanism, 360 Post-Y-oxidation, source, 369 P r e i r r a d i a t e d polypropylene f i l m oxidation during storage, 362f,363 peroxyl r a d i c a l decay during storage, 362,364f,365f post-Y-degradation, 363 Protracted thermal oxidation, sources, 368

Q

Quinonimine i n h i b i t i o n of oxidation reactions, 89,90 reduction, 89,90

R

Rapid heterogeneous degradation identification density p r o f i l e s , 416 metallographic p o l i s h i n g , 413 r e l a t i v e hardness p r o f i l e s , 414 R e l a t i v e hardness p r o f i l e s d e s c r i p t i o n , 414 i r r a d i a t e d V i t o n m a t e r i a l , 419 Knoop hardness t e s t e r , 414 measurements, 421 penetration distance of a weighted

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

444

POLYMER STABILIZATION A N D

Relative hardness profiles—Continued probe into cross-sectioned samples of irradiated EPR, 4l4,4l6,4l7f Perkin-Elmer thermomechanical analyzer, 414 PP exposed to UV light, 4l4,4l5f Rubber stabilizers, properties, 157

S

DEGRADATION

Styrene derivatives, synthesis, 201,202 Subsaturation poly(vinyl chloride) branching structure, 275 C-NMR spectra, 260 3c-NMR spectra of reduced samples, 271,273f ^ c shifts for reduced samples, 271,272f content of branches, 275,276t content of internal double bonds, 267,269t,270 content of internal double bonds versus monomer concentration and temperature, 270,271,272f degradation rate, 265 determination of branches, 262,264 determination of internal double 13

1

1

Secondary amines, reactions, 310 Secondary aromatic amines antioxidant properties, 157,15 stabilization of unsaturate bonds, 270 hydrocarbon polymers, 157 influence of polymerization Smoluchowski equation, 383 conditions on contents of Spiro(cyclohexane-1,3*-1'-benzyl branches, 278,279f decahydroquinoxalin-2'-one), influence of polymerization FD-MS oxidation studies, 104,105t conditions on labile chlorine Stabilized acrylate coating cure content, 280,28lf analysis influence of polymerization decrease in IR intensity for conditions on methyl branch unstabilized resin, 308,309f content, 275,277f effect of stabilizers on cure, 306 measurement of thermal effect of stabilizers on stability, 262 postcure reactions, 308 mechanism for chain transfer to IR spectra of vinyl unsaturation monomer, 275 absorptions, 306,308,309f molecular weight data, 262,263t Stabilized acrylate coating film molecular weight distribution studies determination, 262 antioxidant activity, 306 monomer concentration as a function isothermal DSC of P/Po and temperature, 267 measurements, 306,307f optimum polymerization temperature oxygen absorption data, 306,307f with respect to thermal temperature at onset of stability, 265,267,268f oxidation, 306t polymerization, 261,262 Stable free radicals rate of dehydrochlorination as a effectiveness as polymer function of relative monomer stabilizers, 5 pressure and polymerization examples, 3 temperature, 265,266f importance, 2 rate of dehydrochlorination as a stability, 11 function of monomer structures, 3,4f,11,12,14 concentration and structures of polymer stabilizers, 5,6f temperature, 267,268f,269f structure-reactivity relation between rate of dehydrochlorination and labile chlorine relationships, 11,12 content, 278,280,28lf,282 Stable radical intermediates, key structures of reduced requirement of oxidation samples, 271,274f inhibitors, 3,4 thermal stability data, 265,266t Sterically hindered amines See also 2,2,6,6-Tetramethylpiperidine Substituted 4-oxoimidazolidine-1-oxyls self-condensation, 40,41 derivatives synthesis, 39-41,44 triacetonamine, 13

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

INDEX

445

T

Temperature e f f e c t s on polymer lifetimes Arrhenius plot of oxidation rate, 225,227f time to f a i l u r e as a function of temperature, 225,226f T e r t i a r y amines, reactions, 310 T e r t i a r y chlorine importance i n p o l y ( v i n y l c h l o r i d e ) , 278,280,282 r e a c t i v i t y i n subsaturation p o l y ( v i n y l c h l o r i d e ) , 278 2,2,5,5-Tetramethyl-4-imidazolidinone, structure, 150 2.2.5.5- Tetramethyl-4-imadazolidinon d e r i v a t i v e s and oxazolidine hydroxybenzoate-HALS i n t e r a c t i o n , 151,153t structure and a c t i v i t y of f i n a l compounds, 151,154t structure and a c t i v i t y of i n i t i a l compounds, 151,152t s t r u c t u r e and a c t i v i t y of i n t e r mediate compounds, 151,153t 2.2.6.6- Tetramethyl-4-oxopiperidine, synthesis, 43,45,46 2,2,6,6-Tetramethyl-4-oxopiperidineN-oxyl decomposition, 39,41 heat s t a b i l i t y , 38,39 properties, 3,6f stable r a d i c a l d e r i v a t i v e s , 39,41 synthesis, 3,6f 2,2,6,6-Tetramethylpiperidine, structure, 91 2,2,6,6-Tetramethylpiperidine derivatives See a l s o S t e r i c a l l y hindered amines electrochemical synthesis, 15 properties, 69,70 reactions, 82 s t r u c t u r e , 13,15,16,69 synthesis, 15 Tetramethylpiperidine-N-oxyl r a d i c a l concentration change during i r r a d i a t i o n , 113 structure, 110,111f 2,2,6,6-Tetramethyl-4-substituted piperidine-1-oxyls, properties, 39 Tetraphenylhydrazine, decomposition, 88 Thermal oxidation at high oxygen pressure a u t o c a t a l y t i c r e a c t i o n , 392 chemiluminescence emission i n t e n s i t y , 392 induction time, 392,393 oxidation rate, 392

Thermal oxidation at low oxygen pressure, chemiluminescence i n t e n s i t y , 393 Thermal oxidation scheme a c c e l e r a t i o n of embrittlement, 368 chain s c i s s i o n during Y - i r r a d i a t i o n , 367 Y - i n i t i a t e d , thermal oxidation, 367 Y - i r r a d i a t i o n of samples i n a i r , 368 peroxide formation, 367 peroxyl r a d i c a l s , 367 p h o t o i n i t i a t e d or thermally i n i t i a t e d oxidation, 367 post-Y-oxidation, 368 r o l e of ozone, 368 s t a b i l i z a t i o n e f f e c t during

pretreatmen degrada t i o n pattern, rate-time curves a f t e r preheating, 295,296f Thermooxidative s t a b i l i z e r s , s o l u b i l i t y parameters, 300,301t Thermoplastics, mechanomodification with r e a c t i v e antioxidants, 186 Tinuvin 144, s t r u c t u r e , 23 Tinuvin 622, structure, 110,111f Tinuvin 770 concentration change during i r r a d i a t i o n , 113 structure, 110,111f Triacetonamine, synthesis, 13-15 Triacetonamine d e r i v a t i v e s , synthesis scheme, 55,56f Triacetonamine-N-oxyl, use as l i g h t s t a b i l i z e r , 57 Tris(nonylphenyl) phosphite, structure, 248

U

U l t r a v i o l e t - c u r e d acrylates a p p l i c a t i o n and curing, 299 properties, 299,300 usage, 299 Ultraviolet f i l t e r s , spectral c h a r a c t e r i s t i c s , 331,332t Unsaturated polyesters a p p l i c a t i o n s , 353 structure, 353

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

446

POLYMER STABILIZATION A N D DEGRADATION

U n s t a b i l i z e d polyethylene photooxidation concentration o f a l l chemical species a t time o f f a i l u r e , 220,222t,223t time t o f a i l u r e as a function of i n i t i a t o r type and concentration, 220,221f time to f a i l u r e as a function of propagation r a t e constant, 220,223f,224 time t o f a i l u r e as a function of rate o f bimolecular r a d i c a l termination, 224,226f U n s t a b i l i z e d polypropylene f i l m d i f f e r e n c e IR spectra f o r Y - i r r a d i a t e d f i l m , 36l,362f ESR spectra o f Y - i r r a d i a t e d f i l m , 363,364f f i l m embrittlement a f t e Y - i r r a d i a t i o n , 363,366f f i l m oxidation a f t e r Y - i r r a d i a t i o n , 363,365f f i l m oxidation a f t e r i r r a d i a t i o n exposures, 363,366f,367 immediate e f f e c t s of Y and UV r a d i a t i o n , 36l,362t photooxidation, 363t, 363 Unstable product decomposition, 368-70

V

V i n y l antioxidants, structure, 178

W Weathering, d e f i n i t i o n , 198

Y

Yarn denier

definition

138

Z

Zip k i n e t i c s , nature, 289 Zipper mechanism, basis for nonsteady-state k i n e t i c s , 285,286

Indexing and production by Deborah H. Steiner Jacket design by Pamela Lewis Elements typeset by Hot Type Ltd.,

Washington, D.C.

Printed and bound by Maple Press Co., York, Pa.

In Polymer Stabilization and Degradation; Klemchuk, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.