Polymers in Solar Energy Utilization 9780841207769, 9780841210417, 0-8412-0776-3

Table of contents :
Title Page......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Polymers in Solar Energy: Applications and Opportunities......Page 11
Applications......Page 12
Surface/Interface Properties of Polymers......Page 19
Photochemistry of Polymers......Page 21
Literature Cited......Page 24
Methods of Economic Analysis......Page 27
Consequences for Solar System Engineering......Page 31
Literature Cited......Page 32
3 Polymer Film and Laminate Technology for Low-Cost Solar Energy Collectors......Page 34
Printable Optical Selective Absorber Coating......Page 36
Insulation and Compression Substrate......Page 40
Application......Page 42
High Speed Mass Production......Page 43
Literature Cited......Page 45
4 Stability of Polymeric Materials in the Solar Collector Environment......Page 46
Thermal Aging in Air......Page 48
Experimental......Page 51
Materials Evaluated......Page 55
Hydrolytic Aging......Page 63
Class PS Elastomers......Page 70
Arrhenius Treatment of Data......Page 75
Caulking Compounds......Page 79
Thermal Aging Under Compression......Page 80
Conclusions......Page 85
Literature Cited......Page 86
Sources of Outgassing Products......Page 87
Experimental......Page 88
Results and Discussion......Page 90
Effects of Condensable Deposits on Relative Light Transmittance......Page 101
Conclusion......Page 103
Literature Cited......Page 104
6 Optical, Mechanical, and Environmental Testing of Solar Collector Plastic Films......Page 105
APPROACH......Page 109
PROGRAM RESULTS......Page 113
CONCLUSIONS......Page 119
7 Protective Coatings and Sealants for Solar Applications......Page 120
Experimental......Page 121
Back Surface Protection......Page 122
Edge Seal Protection......Page 123
Literature Cited......Page 127
Experimental Section......Page 129
Performance Ranking......Page 131
Chemical Failure: Polymer/Mirror Interface......Page 133
Chemical Failure: Mirror/Backing Interface......Page 140
Literature Cited......Page 145
9 Effect of Absorber Concentrations on IR Reflection-Absorbance of Polymer Films on Metallic Substrates......Page 146
Theoretical Basis for the IR-RA Experiment......Page 148
Determination of Optical Constants in the IR for Blsphenol-A Polycarbonate......Page 152
Experimental Apparatus and Procedures......Page 155
Results and Discussion......Page 157
Acknowledgment......Page 169
Literature Cited......Page 170
10 Adhesives Used in Reflector Modules of Troughs Effects of Environmental Stress......Page 171
Theory......Page 174
Experimental and Results......Page 175
Indirect Methods......Page 176
Ultrasonic Mapping......Page 178
Shear Strength......Page 180
Discussion and Conclusions......Page 186
Literature Cited......Page 187
APPENDIX I Information on Adhesives Used......Page 188
11 Salt-Gradient Solar Ponds and Their Liner Requirements......Page 189
Salt-gradient pond technology......Page 190
The pond liner and its installation......Page 192
Liner specifications......Page 193
Solar pond construction......Page 194
Conclusions......Page 195
Literature Cited......Page 196
12 Flexible Membrane Linings for Salt-Gradient Solar Ponds......Page 197
The Need for Pond Linings......Page 198
Candidate Materials for Impermeable Membranes......Page 200
Review of Installations Using Flexible Membrane Liners......Page 202
Test Data on Hypalon Performance in Salt Brines......Page 205
The Future for Flexible Membrane Liners in Salt Gradient Solar Ponds......Page 207
Literature Cited......Page 212
13 Plastic Pipe Requirements for Ground-Coupled Heat Pumps......Page 213
Fluid Compatibility......Page 214
Acknowledgments......Page 215
Literature Cited......Page 216
14 New Approach to the Prediction of Photooxidation of Plastics in Solar Applications......Page 217
Computer Model......Page 218
Mechanism of Photooxidation......Page 221
Preliminary Results......Page 223
Conclusions......Page 225
Literature Cited......Page 230
15 Effects of Photodegradation on the Sorption and Transport of Water in Polymers......Page 231
Moisture Sorption......Page 232
Moisture Diffusion and Permeation......Page 234
Effects of Temperature-Humidity Cycling......Page 238
Literature Cited......Page 242
16 UV Microscopy of Morphology and Oxidation in Polymers......Page 243
Additives in Crystalline Polymers......Page 245
Diffusion and Loss of Additives......Page 249
Localization of Thermal and Photo-oxidation......Page 252
Observations......Page 256
Methods......Page 259
Observations......Page 261
Literature Cited......Page 263
17 Novel Diagnostic Techniques for Early Detection of Photooxidation in Polymers......Page 264
4) Laser Photoacoustic Technique (LPAT)......Page 267
Results and Discussions......Page 268
Literature Cited......Page 272
18 Photodegradation of Poly(n-butyl acrylate)......Page 273
EXPERIMENTAL......Page 274
RESULTS AND DISCUSSION......Page 276
Literature Cited......Page 289
19 Photochemical Stability of UV-Screening Transparent Acrylic Copolymers of 2-(2-Hydroxy-5-vinylphenyl)-2H-benzotriazole......Page 291
Literature Cited......Page 303
20 Effects of Deformation on the Photodegradation of Low-Density Polyethylene Films......Page 305
Results and Discussion......Page 306
Literature Cited......Page 325
The LSC Concept: Collection and Concentration of Solar Energy......Page 327
Light Pipe Trapping of Luminescence: Geometrical Effects......Page 329
The Spectroscopy of Dyes in LSC's......Page 330
The Photon Transport Problem and Self-Absorption: LSC Gain and Efficiency......Page 332
Measurements of Self-Absorption and Performance......Page 335
Problems and Future Objectives......Page 346
Literature Cited......Page 348
22 Polymeric Encapsulation Materials for Low-Cost, Terrestrial, Photovoltaic Modules......Page 349
Polymeric Encapsulation Materials......Page 350
Encapsulation Engineering......Page 359
Low-Soiling Surface Coatings......Page 360
Literature Cited......Page 362
23 Encapsulant Material Requirements for Photovoltaic Modules......Page 363
Rigid Member......Page 365
Pottant......Page 367
Outer Cover/Insulator......Page 376
Adhesives, Primers, Surface Modifications......Page 378
Edge Sealants and Frames......Page 380
Literature Cited......Page 381
24 Encapsulant Degradation in Photovoltaic Modules......Page 382
Aging Tests and Results......Page 387
Discussion and Conclusions......Page 399
Literature Cited......Page 400
25 Vacuum Lamination of Photovoltaic Modules......Page 402
Equipment Development......Page 403
Materials Research......Page 406
Testing Methods......Page 411
Other Lamination-Related Efforts......Page 412
Acknowledgments......Page 413
Literature Cited......Page 414
26 Evaluation of Polyacrylonitrile as a Potential Organic Polymer Photovoltaic Material......Page 415
Feasibility of an Organic Polymer-Based Photovoltaic Device - Engineering Considerations......Page 416
Feasibility of an Organic Polymer-Based Photovoltaic Device - Photovoltaic Properties......Page 417
Properties of Pyrolyzed PAN......Page 418
Literature Cited......Page 428
27 Photovoltaic Properties of Organic Photoactive Particle Dispersions Polymeric Phthalocyanines......Page 430
EXPERIMENTAL......Page 431
MEASUREMENTS......Page 434
RESULTS AND DISCUSSION......Page 437
Literature Cited......Page 448
28 Photophysics of Films of Poly(2-vinylnaphthalene) Doped with Pyrene and 1,2,4,5-Tetracyanobenzene......Page 449
Experimental......Page 452
Results......Page 453
Summary and Discussion......Page 459
Literature Cited......Page 463
29 Photoelectrochemical Catalysis with Polymer Electrodes......Page 465
Operational Background......Page 467
Polymer Assisted Photoelectrochemistry......Page 474
Polymer Photoelectrochemistry......Page 482
Literature Cited......Page 483
A......Page 486
B......Page 487
C......Page 488
D......Page 491
E......Page 492
F......Page 494
G......Page 495
H......Page 496
K......Page 497
M......Page 498
O......Page 499
P......Page 500
R......Page 503
S......Page 504
T......Page 507
Z......Page 508

Citation preview

Polymers in Solar Energy Utilization

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Polymers in Solar Energy Utilization Charles G. Gebelein, EDITOR Youngstown State University

David J. Williams, EDITOR Xerox Corporation

Rudolph D. Deanin, EDITOR University of Lowell

Based on a symposium cosponsored by the ACS Divisions of Organic Coatings and Plastics Chemistry and Polymer Chemistry at the 183rd Meeting of the American Chemical Society, Las Vegas, Nevada, March 28-April 2, 1982

ACS

SYMPOSIUM

SERIES

220

AMERICAN CHEMICAL SOCIETY WASHINGTON, D.C. 1983 In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Library of Congress Cataloging in Publication Data Polymers in solar energy utilization. (ACS symposium series, ISSN 0097-6156; 220) Papers derived from a symposium sponsored by the American Chemical Society's Divisions of Organic Coatings and Plastics Chemistry and of Polymer Chemistry. Includes bibliographies and index. 1. Solar energy—Congresses polymerization—Congresses. I. Gebelein, Charles G., 1929. II. Williams, David J., 1943. III. Deanin, Rudolph D. IV. American Chemical Society. Division of Organic Coatings and Plastics Chemistry. V. American Chemical Society. Division of Polymer Chemistry. VI. Series. TJ810.P78 1983 ISBN 0-8412-0776-3

621.47'028

83-6367

Copyright © 1983 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article 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. 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 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 ACS 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. PRINTED IN THE UNITED STATES

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In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ACS Symposium Series M . Joa

Advisory Board David L. Allara

Robert Ory

Robert Baker

Geoffrey D. Parfitt

Donald D. Dollberg

Theodore Provder

Brian M . Harney

Charles N. Satterfield

W. Jeffrey Howe

Dennis Schuetzle

Herbert D. Kaesz

Davis L. Temple, Jr.

Marvin Margoshes

Charles S. Tuesday

Donald E. Moreland

C. Grant Willson

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

FOREWORD The A C S

SYMPOSIUM

SERIES

was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing IN

CHEMISTRY

SERIES

ADVANCES

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

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embrace both types of presentation.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

PREFACE THE

SUN ALWAYS SHINES, even on cloudy days, and pours forth an

enormous amount of energy onto the planet Earth. Obviously this solar energy is the cause of the basic meteorological phenomena that we observe daily, and the effect of solar storms and flares on electromagnetic communications systems is also well established. In the final analysis, solar energy is the ultimate source o fuels are residues of pas and growth were derived from the sun. M u c h of the current emphasis on renewable energy resources focuses on sources, such as wood, that store solar energy in some form. In addition, many of the proposed alternate energy sources (e.g., wind power) derive their basic energy from the action of solar energy on our planet. With the exception of nuclear energy, essentially all of our present energy sources are derived from the sun in one way or another. Even nuclear fusion is patterned after the basic mode of energy production in the sun. Energy is consumed in almost every facet of our lives ranging from food production to recreation, and the energy demand will increase as the results of technology reach out to other portions of the world. Where will the necessary energy come from? Better utilization of solar energy is considered by many to be the best potential source of this needed energy. It has been estimated that all the energy needs of the United States in the year 2000 could be met if we could only harness and utilize the total solar energy that falls on about 15,000 square miles. This land area is less than 0.5% of the area of the United States and is equivalent to about one-tenth the area of California or about one-half the area of Indiana, Maine, or South Carolina. Two major difficulties arise here, however: (1) the problem of how to harness or utilize this fairly diffuse and intermittent source of energy; and (2)

the problem of storing this energy and/or transmitting it to

another location where it is needed. Many approaches have been taken to solve both of these difficulties. In this book we will consider the use of polymeric materials in various methods of solar energy utilization. Although some overlap does occur, the 29 chapters in this book have been placed into three sections: General Solar Applications, Polymer Photodegradation in Solar Applications, and Photovoltaic and Related Applications.

xi

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Essentially, the 13 chapters in the first section consider the basic economics of solar energy, its collection by various devices, and the storage of the collected energy for short-term use. Chapters include a general survey of the various ways in which polymeric materials have been utilized in solar energy collection and of the problems as well as the potential of polymers in this application; basic economic considerations in solar heating systems with special emphasis on the role of plastics in cost reduction; descriptions of some solar collectors that utilize polymers with consideration of cost, effectiveness, and degradation; use of various polymeric materials for sealants; polymeric coatings on metal mirrors; and use of polymers in heat storage as liners in solar ponds in which water is the thermal storage medium, as well as the storage of thermal energy in the ground itself. The basic conclusio advantageously in solar Probably the most pressing problem cited is that of photodegradation, which is the central topic of the seven chapters in the second

section.

Chapters in this section include theoretical models to predict the utility of a polymer in solar applications by examining the formation of polar groups on the polymer backbone chains—which would make the polymer more water sensitive and less suitable for solar applications, especially photovoltaic applications; the use of U V and fluorescent microscopy to follow photooxidation in some polymers; the use of a laser photoacoustic technique to compare actual outdoor degradation studies with those obtained in an accelerated test; and photodegradation studies of some specific polymers. F o r example, poly(n-butyl acrylate) degradation and its implications for solar applications are reported and a fairly successful

at-

tempt to reduce photodegradation in poly(methyl methacrylate) by the copolymerization of a monomer that contains a UV-screening agent is described. The final chapter of this section considers the effect of uniaxial or biaxial deformation in polyethylene films on the photodegradation. The seven papers in the second section, along with several in the first section, clearly show that photodegradation is a potential problem in any solar utilization of polymers. In addition, they provide some insights into methods of predicting this effect and possible ways to alleviate this difficulty. Even though the efficiency of the processes may be lower than desired, the early chapters in this book, and other sources, clearly show that solar energy can be captured and utilized to some extent with our current technology. Enthusiasm in this area, however, must be tempered by the facts that storage of this captured energy is limited and that transmission of the solar-derived energy is even more problematic. Trapping the solar energy in some type of thermal storage system may suffice for short-term storage (e.g., for use at night or on rainy days) but cannot be used

xii In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

effectively for truly long-term applications. In addition, thermal energy has a very limited transmission or transportation range. More effective methods of energy storage and/or transmission would involve the conversion of the solar energy into some form of chemical energy (e.g., hydrogen) or into electricity. This problem is considered in the nine chapters of the third section. Most of the chapters in the third section are concerned with photovoltaic (PV) applications (conversion of light into electrical energy). Because of the diffuse nature of solar energy, the photovoltaic collection devices must be very large or else the light that strikes them must be concentrated. The first chapter in this section gives an overview of luminescent solar concentrators that can be used with the P V collectors. Most P V collectors or module voltaic cell element. Th

chapter

plastics as encapsulant or pottant materials in the P V modules. The majority of photovoltaic modules use silicon as the photovoltaic cell element, but other materials are, in principle, possible. The last four chapters consider the use of organic polymers (sometimes doped) as the cell element or in some related conducting property: acrylonitrile, some polymeric phthalocyanines; and polymers of 2-vinylnaphthalene that is doped with pyrene and 1,2,4,5-tetracyanobenzene.

T h e study on the last

group of polymers was initiated by the idea that they could be used to transfer solar energy to a reaction center and produce some type of chemical reaction. The final chapter carries this approach further in the consideration of polymeric electrodes that could be used to split water into oxygen and hydrogen. The latter could then be utilized as a source of storable, readily transportable chemical energy. In summary, the 29 chapters in this book point out the vast potential of solar energy utilization and note the advantages and problems of using polymers in this application. Polymers clearly offer advantages in cost, weight, and the variety of materials available but do suffer from varying degrees of photodegradation. This book points out several areas of needed research, but it also shows the promise of an energy-rich future in which solar energy can be used directly as thermal energy or indirectly as solarderived electricity or chemically stored energy (e.g., hydrogen derived via solar water splitting). This optimistic view is in marked contrast with many of the gloomy prognostications that abound in today's world, but such advances are well within the realm of possibility in the near future. We wish to express gratitude to the Divisions of Organic Coatings and Plastics Chemistry and of Polymer Chemistry who cosponsored the symposium from which this book is derived. Naturally, we thank all authors for the chapters that they contributed. In addition, we thank the xiii In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

many others who were involved in manuscript preparation and the review process. Finally, we wish to thank our families for their support while this book was being prepared. CHARLES G. G E B E L E I N Youngstown State University Department of Chemistry Youngstown, OH 44555 DAVID J. WILLIAMS Xerox Corporation Webster, NY 14580 RUDOLPH D . DEANIN University of Lowell Plastics Department Lowell, MA 01854 January 24, 1983

xiv

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1 Polymers in Solar Energy: Applications and Opportunities W. F. CARROLL California Institute of Technology, Jet Propulsion Laboratory, Pasadena,CA91109 PAUL SCHISSEL Solar Energy Research Institute Golden CO80401

Polymers have many potential applications in solar technologies that can help achieve total system cost-effectiveness. For this potential to be realized, three major parameters must be optimized: cost, performance, and durability. Optimization must be achieved despite operational stresses, some of which are unique to solar technologies. This paper identifies performance of optical elements as critical to solar system performance and summarizes the status of several optical elements: flat-plate collector glazings, mirror glazings, dome enclosures, photovoltaic encapsulation, luminescent solar concentrators, and Fresnel lenses. Research and development efforts are needed to realize the full potential of polymers to reduce life-cycle solar energy conversion costs. Problem areas which are identified are the interactions of a material with or its response to the total environment; photodegradation; permeability/ adhesion; surfaces and interfaces; thermomechanical behavior; dust adhesion; and abrasion resistance. Polymeric materials can play a key role in the future development of solar energy systems [1]. Polymers offer potentially lower costs, easier processing, lighter weight, and greater design flexibility than materials in current use. Polymeric materials are used in a l l solar technologies. In addition to such conventional applications as adhesives, coatings, moisture barriers, electrical and thermal insulation, and structural members, polymers are used as optical components in solar systems. Mirrors on parabolic troughs are made up of metallized fluoropolymers and acrylics. Commercial flat-plate collectors are glazed with fluoropolymers and ultraviolet-stabilized polyester/ glass fiber composites. Photovoltaic (PV) cell arrays are encap0097-6156/83/0220-0003$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4

POLYMERS

IN

SOLAR

ENERGY

UTILIZATION

s u l a t e d w i t h s i l i c o n e s and a c r y l i c s f o r p r o t e c t i o n from the weather. In laminated ( s a f e t y g l a s s ) m i r r o r s f o r c e n t r a l r e c e i v e r h e l i o s t a t systems, p o l y v i n y l b u t y r a l i s used as a l a m i n a t i n g and encapsulating agent. Cast and molded a c r y l i c F r e s n e l lenses are used to concentrate s u n l i g h t onto p h o t o v o l t a i c c e l l s and thermal receivers. The widespread use of polymers i s evident from the i n f o r m a t i o n d i s p l a y e d i n Table I , where a p p l i c a t i o n s of polymers are l i s t e d f o r each major s o l a r technology. I t i s p o s s i b l e to estimate the amount of polymer which might be used i n the o p t i c a l elements of s o l a r energy systems [ 1 ] . The estimate assumes a market p e n e t r a t i o n f o r s o l a r systems e q u i v a l e n t t o 0.4 Quads/year (U.S. 1980 energy use was about 85 Quads, 1 Quad - 1 0 Btu) s t a r t i n g i n the time p e r i o d 1985-1995. A s o l a r i n s o l a t i o n of 1 kW/m f o r ^bout 6 hours per day w i l l r e q u i r e an area p e n e t r a t i o n of 2 x 10° n r / y r (82 m i l e s / y r ) or about 5500 m e t r i c tons per year per m i l of t h i c k n e s s An average thickness of 10 m i l s would r e q u i r e the present market f o a b l y , c o n s i d e r a b l y l a r g e r amounts of polymer could be used i n nono p t i c a l s t r u c t u r e s but t h i s i s system s p e c i f i c and d i f f i c u l t to estimate. C a l c u l a t i o n s [1] a l s o suggest that the present low l e v e l of funding f o r R&D on polymers i m p l i e s an underestimation of the p o t e n t i a l which polymers have f o r a p p l i c a t i o n s to s o l a r systems• The use of polymers i n s o l a r equipment w i l l r e q u i r e major changes from past l a r g e - s c a l e applications, especially i n a c h i e v i n g s a t i s f a c t o r y performance under a l l combinations of s t r e s s . Cost, performance, and d u r a b i l i t y must be optimized. I f e a r l y c o s t - e f f e c t i v e commercialization of s o l a r energy i s to be r e a l i z e d , c r i t i c a l delays must be shortened. Service experience has t r a d i t i o n a l l y guided the e v o l u t i o n of systems toward an optimum design. The process can be hastened by a p p l y i n g a l l a v a i l a b l e understanding of the b a s i c behavior of m a t e r i a l s i n the i n i t i a l designs of equipment. The stalemate imposed by the l a c k of market and supply of s o l a r systems can be broken by governmentsupported development of technology that demonstrates the economic v i a b i l i t y of new or modified m a t e r i a l s . Such development would enable m a t e r i a l s s u p p l i e r s and manufacturers of s o l a r equipment to make knowledgeable business d e c i s i o n s and reduce development cost and time. 1 5

2

Applications The o p t i c a l elements of s o l a r systems are important a p p l i c a t i o n s f o r polymers. The use of polymers f o r o p t i c a l elements w i l l , however, impose s e v e r a l unusual m a t e r i a l requirements. F i v e examples of the current development of polymeric o p t i c a l elements are considered below. Problems such as d i r t accumulation and photodegradation, which are common to most o p t i c a l elements, are considered i n a l a t e r s e c t i o n . More conventional a p p l i c a t i o n s are then noted very b r i e f l y .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

x

x

x

Biological/chemical

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POLYMERS

IN

SOLAR

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Flat-Plate Collector Glazings* The cover g l a z i n g s protect the inner elements of the c o l l e c t o r from the environment and i n c r e a s e operating e f f i c i e n c y by reducing r e r a d i a t i o n and convection. Collectors with single glazings are l i m i t e d i n t h e i r o p e r a t i n g temperature; however, some recent work suggests that s i n g l y glazed c o l l e c t o r s can work i n the temperature ranges r e q u i r e d by desiccant and a b s o r p t i o n c o o l i n g [ 2 ] . Higher operati n g temperatures a r e obtained by u s i n g two g l a z i n g s . The outer g l a z i n g must withstand the environment while the inner g l a z i n g must be temperature r e s i s t a n t , t y p i c a l l y up to the s t a g n a t i o n temperature of the device. R e s u l t s on c o l l e c t o r g l a z i n g s have been reported [3] and environmental degradation studies of m a t e r i a l s f o r g l a z i n g s a r e i n progress [ 4 ] . None of the m a t e r i a l s are completely s a t i s f a c t o r y e i t h e r as an outer or inner g l a z i n g . The temperature requirement f o r the inner g l a z i n g e l i m i n a t e s most m a t e r i a l s other than fluorocarbon polymers and g l a s s Glass i s the most common outer g l a z i n and impact r e s i s t a n c e d u r a b i l i t y l i m i t s the a p p l i c a t i o n s of polymers. The transparent honeycomb concept i s an a l t e r n a t i v e t o t h e use of a second g l a z i n g [5] . The honeycomb i s attached t o o r i s an i n t e g r a l p a r t of the outer g l a z i n g f a c i n g the absorber p l a t e . The honeycomb improves c o l l e c t o r performance by suppressing conv e c t i o n and r a d i a t i o n heat l o s s e s w h i l e only s l i g h t l y reducing the incoming s o l a r energy. The e f f e c t i v e n e s s of the honeycomb f o r improving c o l l e c t o r performance i s approximately equivalent t o t h a t of an inner g l a z i n g . I n t e g r a l honeycombs formed from p o l y carbonate have good mechanical p r o p e r t i e s . Other m a t e r i a l s t e s t e d i n c l u d e p o l y e s t e r , fluorocarbon polymers, and polyimide [ 5 ] • Novel approaches t o c o l l e c t o r f a b r i c a t i o n use i n t e g r a l e x t r u s i o n s [6-8] or laminated t h i n f i l m s [ 2 ] . U n l i k e sheet-and-tube designs, some extruded i n t e g r a l u n i t s i n c l u d e the transparent g l a z i n g , h e a t - t r a n s f e r f l u i d pathways, and backing, a l l i n a conf i g u r a t i o n which could be r o l l e d out onto a r o o f t o p . Black f l u i d s can act as absorbers and be drained from the c o l l e c t o r t o prevent excessive s t a g n a t i o n temperatures. The designs vary i n d e t a i l but a common problem has been the i d e n t i f i c a t i o n of a polymer with acceptable environmental d u r a b i l i t y and low c o s t . An extruded polycarbonate c o l l e c t o r w i t h an i n t e g r a l o p t i c a l concentrator has been developed [ 8 ] . Other m a t e r i a l s that have been used i n designs i n c l u d e a c r y l i c and p o l y e t h e r s u l f o n e . Imaginative a p p l i c a t i o n s o f polymers t o f e n e s t r a t i o n can a l s o be used f o r f l a t - p l a t e c o l l e c t o r s . A transparent, coated p o l y meric g l a z i n g which transmits the s o l a r spectrum but r e t u r n s the i n f r a r e d r a d i a t i o n e f f e c t i v e l y increases the i n s u l a t i o n provided by the g l a z i n g , because the i n f r a r e d r a d i a t i o n generated i n s i d e the s t r u c t u r e i s r e t a i n e d . A polymeric f i l m which changes from transparent t o opaque when heated above a t r a n s i t i o n temperature a c t s as an automatic window shade which could help c o n t r o l s t a g n a t i o n temperatures [ 9 ] .

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Polymeric Glazings - Mirrors* The i n s t a l l e d p r i c e of h e l i o s t a t s i s estimated t o account f o r about h a l f of the t o t a l c a p i t a l cost o f a c e n t r a l - r e c e i v e r s o l a r thermal e l e c t r i c plant and a l a r g e r f r a c t i o n of the cost of systems f o r process heat product i o n [10]. M e t a l l i z e d t h i n polymeric f i l m s a r e one means to make l i g h t w e i g h t m i r r o r s that are l e s s expensive than current design. F l e x i b l e , l i g h t w e i g h t mirrors a l s o a l l o w l e s s expensive designs o f a u x i l i a r y equipment. Thin, f l e x i b l e f i l m s can be attached w i t h adhesives t o substrates w i t h s i n g l e o r compound curvature. E a r l i e r s t u d i e s of alurainized o r s i l v e r e d polymers have included a c r y l i c s , f l u o r i n a t e d polymers, polycarbonate, s i l i c o n e s , and p o l y e s t e r [11]. Tests a t Phoenix, A r i z o n a , showed n e g l i g i b l e degradation o f aluminum and s i l v e r m i r r o r s protected by a c r y l i c , T e f l o n , and glass during exposures exceeding two years, w h i l e s i m i l a r t e s t s a t other s i t e s r e s u l t e d i n severe degradation i n about one year [12]. I t was decided that the r e l i a b i l i t y of polymer-coated mirrors s t a t s a t the Barstow demonstratio environmental degradation s t u d i e s o f some o f these m a t e r i a l s a r e i n progress [4] and s e v e r a l m i r r o r c o n f i g u r a t i o n s , i n c l u d i n g aluminized a c r y l i c s , are being t e s t e d c u r r e n t l y a t s e l e c t e d l o c a t i o n s around the U. S. I n recent t e s t s conducted i n dry, r e l a t i v e l y benign c l i m a t e s , aluminized a c r y l i c s have performed w e l l f o r up t o f i v e years, polymeric g l a z i n g s that p r o t e c t s i l v e r surfaces f o r comparable time periods have not been i d e n t i f i e d . L o c a l i r r e g u l a r i t i e s ( s l o p e - e r r o r ) i n the shape of r e f l e c t o r s present a problem w i t h polymer-glazed m i r r o r s . A slope-error t o l e r a n c e as low as one m i l l i r a d i a n i s needed f o r some point-focus concentrators [14]. This tolerance has been met w i t h g l a s s m i r r o r s ; however, m e t a l l i z e d polymeric f i l m s have a poorer t o l e r ance. Dome Enclosures. An e n c l o s e d - h e l i o s t a t (dome) design e n v i sions l a r g e ( 3 0 - f t diameter) bubbles made of t h i n , a i r - s u p p o r t e d , transparent polymeric f i l m s as p r o t e c t i v e covers f o r m e t a l l i z e d polymeric m i r r o r s . Studies [10] i n d i c a t e that use of domeenclosed s o l a r concentrators may r e s u l t i n s i g n i f i c a n t cost reductions. The air-supported dome c o n f i g u r a t i o n i s capable of w i t h standing wind loads and can p r o t e c t l i g h t gauge p l a s t i c membrane h e l i o s t a t s and d r i v e mechanisms which lower c o s t s . The o r i g i n a l concept from the Boeing Engineering and Construction Company used i n t e g r a l domes w h i l e l a t e r designs by the General E l e c t r i c Company used segments assembled w i t h adhesives. A number of transparent polymers have been examined and t e s t e d f o r t h i s purpose; prototype domes have been f a b r i c a t e d from p o l y v i n y l f l u o r i d e which was l a t e r determined t o be too expensive and not s u f f i c i e n t l y s t a b l e . The dominant requirement f o r t h i s a p p l i c a t i o n i s good specular t r a n s m i s s i o n . Several p o l y e s t e r s

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were t e s t e d and had e x c e l l e n t i n i t i a l o p t i c a l transmittance. However, t h e i r environmental d u r a b i l i t y was too l i m i t e d . Energy l o s s e s due to absorption or s c a t t e r i n g decrease the system e f f i c i e n c y , reducing the cost advantages. In the case of enclosures f o r h e l i o s t a t s i n c e n t r a l r e c e i v e r systems, l o s s e s are m u l t i p l i e d because the s o l a r beam must pass through the dome t w i c e . The m a t e r i a l must be compatible w i t h c o s t - e f f e c t i v e dome f a b r i c a t i o n methods and must have s u i t a b l e mechanical p r o p e r t i e s and durab i l i t y to maintain operation under the combined e f f e c t s of wind, h a i l , temperature, s u n l i g h t , e t c . , f o r s e v e r a l years. Other m a t e r i a l s t e s t e d i n c l u d e p o l y v i n y l i d e n e f l u o r i d e , polycarbonate, and polypropylene. B i a x i a l l y oriented polyvinylidene fluoride i s p r a c t i c a l to manufacture commercially and i s s a i d to have good o p t i c a l p r o p e r t i e s and e x c e l l e n t w e a t h e r a b i l i t y [10]. Flat-Plate Photovoltaic (PV) Encapsulation.* Polymers can serve s e v e r a l f u n c t i o n s i n g l e polymer a p p l i c a t i o core of the encapsulation package I s the p o t t a n t , which embeds the s o l a r c e l l s and r e l a t e d e l e c t r i c a l conductors. The key r e q u i r e ments f o r a pottant are high transparency i n the range of s o l a r c e l l response, mechanical cushioning of the f r a g i l e s o l a r c e l l s from thermal and mechanical s t r e s s e s , e l e c t r i c a l i n s u l a t i o n to i s o l a t e module voltage, and c o s t - e f f e c t i v e m a t e r i a l and module f a b r i c a t i o n processes. Other encapsulation a p p l i c a t i o n s of polymers f o r s p e c i f i c designs i n c l u d e s o i l , u l t r a v i o l e t , and a b r a s i o n - r e s i s t a n t f r o n t covers. The cover can serve as a transparent s t r u c t u r a l superstate. Substrate support designs r e q u i r e a hard, durable f r o n t cover f i l m to p r o t e c t the r e l a t i v e l y s o f t pottant from mechanical damages and excess s o i l accumulation. A polymeric f r o n t cover must be low i n c o s t , h i g h l y transparent, and weather r e s i s t a n t to compete w i t h g l a s s . For a p p l i c a t i o n s out of the o p t i c a l path between the sun and the s o l a r c e l l s (adhesives, i n s u l a t i o n , edge s e a l s , gaskets) requirements f o r polymeric use i n encapsulation are the same as f o r other a p p l i c a t i o n s . Luminescent Solar Concentrators (LSCs). The LSC uses the p r i n c i p l e of l i g h t pipe t r a p p i n g , t r a n s m i s s i o n , and c o u p l i n g i n t o a p h o t o v o l t a i c c e l l (PV) to concentrate s o l a r r a d i a t i o n [16,17]. This use of a low-cost concentrator can reduce the area r e q u i r e ments of the more expensive PV c e l l s . The LSC has s e v e r a l important advantages. I t can be made from inexpensive m a t e r i a l s , can be nontracking and i t can concentrate the l i g h t input from e i t h e r d i r e c t or d i f f u s e i n s o l a t i o n . The LSC can act as a wavelength

*Encapsulation of p h o t o v o l t a i c s f o r concentrator systems depends on c o n c e n t r a t i o n r a t i o and other system s p e c i f i c parameter t e s t s , i s s u e s that are not discussed i n t h i s paper.

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matcher between the s o l a r r a d i a t i o n and the s p e c t r a l response o f the PV c e l l . A l s o , the s o l a r i n f r a r e d r a d i a t i o n and the r e s u l t a n t heat load are prevented from reaching the s o l a r c e l l . One planar c o n f i g u r a t i o n , shown i n Figure 1, uses a polymeric host (polymethylmethacrylate) i n t o which dye molecules are d i s persed randomly. Photons w i t h wavelengths i n the adsorption band of the dye enter the host, a r e absorbed by the dye, and ate reradiated i s o t r o p i c a l l y . Reradiated photons w i t h i n a c e r t a i n s o l i d angle are trapped by t o t a l i n t e r n a l r e f l e c t i o n and guided t o the edge of the p l a t e where s o l a r c e l l s are attached. The concent r a t i o n f a c t o r i s determined by the r a t i o of the areas of the face to an edge, by the f r a c t i o n of r e r a d i a t e d photons which i s trapped (75%), and other f a c t o r s r e l a t i n g to the dye. The trapping e f f i ciency i s p a r t l y determined by the r e f r a c t i v e index o f the polymer. A second planar c o n f i g u r a t i o n uses t h i n (25 urn) polymeric f i l m host ( c e l l u l o s e acetat (PMMA, g l a s s ) . When th acts as a g l a z i n g t o protect the t h i n f i l m . Some designs use s e v e r a l t h i n l a y e r s , each c o n t a i n i n g a dye matched t o d i f f e r e n t s o l a r wavelengths. The p h y s i c a l separation of the dyes can improve t h e i r d u r a b i l i t y . The t h i n f i l m approach means that more expensive m a t e r i a l s may be acceptable. F l u o r i n a t e d or deuterated dyes can improve o p t i c a l e f f i c i e n c y and h i g h l y concentrated dyes can a l t e r the mechanism and e f f i c i e n c y of energy t r a n s p o r t . System l i f e t i m e i s an important unknown f a c t o r p r i n c i p a l l y i n f l u enced by dye l i f e t i m e . Questions r e l a t i n g t o dye-host i n t e r a c t i o n s and the i n f l u e n c e of the host on system l i f e t i m e s a r e unanswered. Fresnel Lenses. F r e s n e l lens concentrators have been s t u d i e d f o r both thermal and p h o t o v o l t a i c systems. The economic v i a b i l i t y of t h e i r use depends on a l a r g e number of system-related f a c t o r s , i n c l u d i n g the performance, c o s t , and d u r a b i l i t y of the lenses. Performance requirements i n c l u d e minimum a b s o r p t i o n , s c a t t e r i n g , and surface r e f l e c t i o n . T o t a l cost depends on costs of m a t e r i a l s and f a b r i c a t i o n , or minimum thickness defined by mechanical requirements, and on a d d i t i o n a l m a t e r i a l required f o r o p t i c a l design. L i k e other s o l a r a p p l i c a t i o n s o f o p t i c a l polymers, durab i l i t y f o r extended periods i s r e q u i r e d . Other Applications. Polymers can a l s o be used as edge s e a l s i n g l a s s m i r r o r s , f i l m s f o r m i r r o r backings, adhesives, s t r u c t u r a l members, s o l a r pond l i n e r s , and energy storage systems. Glass m i r r o r s are more s t a b l e than m i r r o r s w i t h polymeric g l a z i n g s , but they are expensive, heavy, and probably not s t a b l e enough. The s t a t e of the a r t i s e x e m p l i f i e d by the developments of m i r r o r s f o r h e l i o s t a t a p p l i c a t i o n s [18]. The s t r u c t u r e c o n s i s t s o f s i l v e r e d g l a s s backed w i t h a p o l y i s o b u t y l e n e f i l m and mounted on an aluminum sheet-paper honeycomb s t r u c t u r e . The m i r r o r edges are sealed

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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w i t h p o l y i s o b u t y l e n e and s i l i c o n e , and the m i r r o r i s h e l d t o the supporting s t r u c t u r e using a neoprene phenolic adhesive. Other low-cost h e l i o s t a t m i r r o r modules have been designed and developed which use p l a s t i c to reduce weight and to accommodate high-volume production of complex forms by molding. Molded r i b , extruded panel, or sandwiched honeycomb s t r u c t u r e s a r e combined w i t h sprayed s i l v e r m e t a l l i z a t i o n , sprayed polymeric overcoats, o r laminated f i l m s [19]. Molded r e i n f o r c e d p l a s t i c s are a l s o used i n p a r a b o l i c trough module designs [20]• A survey of thermal energy storage p r o j e c t s i s a v a i l a b l e [21]• Research Opportunities Optical Elements. Problems which are common to many s o l a r r e l a t e d o p t i c a l elements i n c l u d e d i r t r e t e n t i o n , c l e a n i n g , surface a b r a s i o n , and photodegradation A common f e a t u r e of some of these problems i s that the d e l e t e r i o u Ultraviolet radiation, etc., can have a profound e f f e c t on performance by changing surface c h a r a c t e r i s t i c s . The l i f e t i m e s o f UV s t a b i l i z e r s can be l i m i t e d by exudation; p e r m e a b i l i t y can cause harmful r e a c t i o n s a t i n t e r f a c e s ; and mechanical p r o p e r t i e s can be i n f l u e n c e d by surface crazing. I n other a p p l i c a t i o n s mechanical behavior of the bulk polymer i s c r i t i c a l and v i r t u a l l y a l l a p p l i c a t i o n s r e q u i r e that the polymer system withstand m u l t i p l e environmental s t r e s s e s simultaneously. Surface/Interface Properties of Polymers Surface phenomena play a s i g n i f i c a n t r o l e i n the major problem areas associated w i t h polymers and, t h e r e f o r e , are b a s i c to most o f the s t u d i e s . For example, the l i f e t i m e s of UV s t a b i l i z e r s can be l i m i t e d by exudation and accumulation a t the s u r f a c e , p e r m e a b i l i t y can cause harmful r e a c t i o n s a t i n t e r f a c e s , adhesion Is an i n t e r f a c e phenomenon, and mechanical p r o p e r t i e s can be i n f l u e n c e d by surface c r a z i n g . Examples o f surface problems a f f e c t i n g m i r r o r s i n c l u d e abrasion, dust adhesion, and c l e a n i n g procedures. Surface i n t e r a c t i o n s a l s o occur during the production of polymers; the subsequent behavior o f a polymer can be c r i t i c a l l y dependent upon the m a t e r i a l against which i t i s formed. Surface measurement techniques w i l l form a general experimental b a s i s f o r work on s p e c i f i c a p p l i c a t i o n s . Experimental and a n a l y t i c a l studies are needed t o improve understanding of the chemistry, p h y s i c s , and morphology of s u r f a c e s . Study o f i n t e r faces between polymers and other m a t e r i a l s i s a l s o needed both f o r model i n t e r f a c e s and f o r candidate engineering m a t e r i a l i n t e r faces. Such s t u d i e s should c h a r a c t e r i z e the i n t e r f a c e s as o r i g i n a l l y f a b r i c a t e d and a f t e r changes caused by t y p i c a l e n v i r o n mental exposures. The accumulation of a i r b o r n e p a r t i c u l a t e s and aerosols on the

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o p t i c a l surfaces of s o l a r conversion equipment causes unwanted absorption and s c a t t e r i n g which has lowered operating e f f i c i e n c i e s more than 30%. Accumulation i s most serious f o r s o f t polymers (e.g., s i l i c o n e rubber) and l e a s t s e r i o u s f o r g l a s s . Data i n d i cate that hard polymers l i k e polymethylmethacrylate are n e a r l y as r e s i s t a n t as g l a s s . An understanding of adhesion mechanisms i s r e q u i r e d to develop c o s t - e f f e c t i v e c l e a n i n g methods and s o i l r e s i s t a n t polymeric m a t e r i a l s [22-24]. M o d i f i c a t i o n s of polymeric m a t e r i a l s , e i t h e r i n bulk or by surface treatment or c o a t i n g , may r e s u l t i n m a t e r i a l s that are " s e l f - c l e a n i n g " or that do not tend to hold s o i l , a l l o w i n g i t t o be removed e a s i l y by n a t u r a l forces such as wind and r a i n . Regular c l e a n i n g probably w i l l be r e q u i r e d to maintain high o p t i c a l performance of s o l a r systems, and c l e a n i n g w i l l be a major o p e r a t i o n a l cost f a c t o r . Automatic c l e a n i n g can be made c o s t e f f e c t i v e i f i t i s based on an understanding of s o i l adhesion mechanisms. P o s s i b l e problem surface by c l e a n i n g an c l e a n i n g agents to m a t e r i a l aging. The o p t i c a l f u n c t i o n of p o l y meric components can be s e r i o u s l y degraded by abrasion due to the c l e a n i n g process or by n a t u r a l causes. Considerations such as c o s t , mechanical c o m p a t i b i l i t y with supporting m a t e r i a l , or UV r e s i s t a n c e may preclude the use of i n h e r e n t l y a b r a s i o n - r e s i s t a n t materials. Since only a shallow l a y e r of r e s i s t a n t m a t e r i a l i s r e q u i r e d , adding a c o a t i n g o r using surface processes that produce a r e s i s t a n t " s k i n " can y i e l d the necessary r e s i s t a n c e . A c o a t i n g might have s e v e r a l uses, p r o v i d i n g abrasion r e s i s t a n c e , improving a n t i r e f l e c t i v e performance, screening UV r a d i a t i o n , or combining several functions. Adhesive f a i l u r e i s a problem i n s o l a r systems. In the past, polymers have been used to p r o t e c t the mechanical i n t e g r i t y of wood and metal s t r u c t u r e s i n severe outdoor environments and to p r o t e c t s e n s i t i v e e l e c t r o n i c components In r e l a t i v e l y benign enclosed environments. Polymers used i n s o l a r equipment w i l l have to p r o t e c t the o p t i c a l p r o p e r t i e s of r e f l e c t o r s , t h i n - f i l m e l e c t r i c a l conductors, and t h i n - f i l m p h o t o v o l t a i c s from the e f f e c t s of moisture and atmospheric p o l l u t a n t s i n severe outdoor environments w h i l e simultaneously maintaining o p t i c a l , mechanical, and chemical integrity. I n some systems, the prevention of mechanical f a i l u r e i s important; f r e q u e n t l y , adhesive f a i l u r e a t the metal/polymer i n t e r f a c e i s of p a r t i c u l a r concern because the ensuing c o r r o s i o n causes o p t i c a l f a i l u r e . Loss of adhesion may be caused by permeation problems. However, i n t e r n a l formation of v o l a t i l e species (outgassing) and primary bond f a i l u r e can a l s o c o n t r i b u t e t o l o s s of adhesion. A l l polymers are i n h e r e n t l y permeable, but t o widely varying degrees. Oxygen, moisture, a i r p o l l u t a n t s , e t c . , can penetrate polymer f i l m s and a t t a c k underlying r e f l e c t o r m e t a l i z a t i o n , cond u c t o r s , or other f u n c t i o n a l elements. Furthermore, these gases

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can modify the mechanical and o p t i c a l p r o p e r t i e s o f the polymer and a l t e r mechanical i n t e r f a c e s between l a y e r s . The p e r m e a b i l i t y of a polymer can be s e n s i t i v e t o how the m a t e r i a l i s processed and used and i s sometimes enhanced as the m a t e r i a l degrades. Although a fundamental understanding o f polymer permeability e x i s t s and experimental data are a v a i l a b l e , current information i s inadequate to model o r c o n t r o l the e f f e c t s of permeation of v a r i o u s species i n s o l a r equipment. Experimental and a n a l y t i c a l studies are needed t o consider modern t h e o r e t i c a l approaches (e.g., nonequilibrium thermodynamics, non-Fickian d i f f u s i o n ) with the g o a l of developing models f o r the transport o f H2O, O2, S0 , and other molecules i n polymers, and to develop and compile q u a n t i t a t i v e engineering data on transport through bulk m a t e r i a l and across and along i n t e r f a c e s s p e c i f i c a l l y f o r s o l a r a p p l i c a t i o n s [25, 26]. A l t e r n a t i v e l y , delamination may not be r e l a t e d d i r e c t l y t o permeation, but may b that are followed by models i n d i c a t e that the polymer/metal i n t e r f a c e morphology, and the changes i n the morphology with exposure t o the environment, play a key r o l e i n c o r r o s i o n r a t e s . These c h a r a c t e r i s t i c s may be even more important i n c o r r o s i o n c o n t r o l than e i t h e r the d i f f u s i o n of vapors through the polymer or the inherent c o r r o s i o n r e s i s t a n c e of the metal. X

Photochemistry of Polymers V i r t u a l l y a l l polymers d e t e r i o r a t e under exposure to outdoor weathering and s o l a r r a d i a t i o n , but a t g r e a t l y varying r a t e s . Polymers i n s o l a r equipment must maintain o p t i c a l , mechanical, and chemical i n t e g r i t y despite prolonged exposure to s o l a r u l t r a v i o l e t radiation. F o r most outdoor a p p l i c a t i o n s of polymers, s o l a r r a d i a t i o n exposure i s i n c i d e n t a l , but f o r many s o l a r a p p l i c a t i o n s , exposure t o s o l a r r a d i a t i o n i s d e l i b e r a t e l y maximized i n the equipment design. Transparency i s e s s e n t i a l f o r many o f t h e p o t e n t i a l l y most c o s t - e f f e c t i v e a p p l i c a t i o n s , and conventional approaches t o u l t r a v i o l e t p r o t e c t i o n such as opaque coatings and f i l l e r s are unacceptable. Photodegradation i n polymers begins w i t h the primary e x c i t e d s t a t e s produced by absorption of u l t r a v i o l e t photon energy by the polymer. These e x c i t e d s t a t e s undergo f a s t - r e a c t i o n sequences t o form chain r a d i c a l s which, i n t u r n , decay through chemical r e a c t i o n s w i t h i n the polymer o r with O2, R^O, e t c . These r e a c t i o n s can produce changes i n chemistry or molecular s i z e . The r e a c t i o n products can absorb a d d i t i o n a l photons, r e s u l t i n g i n f u r t h e r degradation by analogous processes. The cumulative changes may r e s u l t i n y e l l o w i n g ( l o s s o f transparency), change i n r e f r a c t i v e index, and d e t e r i o r a t i o n o f surface p r o p e r t i e s (e.g., crazing)• Changes i n the mechanical p r o p e r t i e s cause increased creep or c r a c k i n g , w h i l e changes i n the chemical p r o p e r t i e s r e s u l t i n increased permeability to R^O, S0 , e t c . , and subsequent corX

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r o s i v e i n t e r a c t i o n w i t h m e t a l l i c components i n contact w i t h the polymer. Polymers that are s e n s i t i v e to u l t r a v i o l e t r a d i a t i o n but that otherwise have d e s i r a b l e p r o p e r t i e s can be s t a b l i z e d by u l t r a v i o l e t screens, absorbers, quenchers, r a d i c a l scavengers, and antioxidants. U l t r a v i o l e t s t a b l i z e r s can be incorporated by simple a d d i t i o n or by chemical combination w i t h the polymer molecule. Long l i f e t i m e can best be a t t a i n e d by i m m o b i l i z i n g the a d d i t i v e as part of the molecular s t r u c t u r e . Using e x i s t i n g photochemistry, a n a l y t i c a l and experimental s t u d i e s are needed t o develop models of photochemical processes i n s o l a r - r e l a t e d polymers. Recent screening s t u d i e s u s i n g commercial a d d i t i v e s have been d i r e c t e d toward s o l a r a p p l i c a t i o n s . They can provide some input to the system s e l e c t i o n s [28-31]. Thermomechanical Behavior. Requirements f o r o p t i c a l performance impose unprecedente of polymers used i n h i g h - c o n c e n t r a t i o f o r mechanical c o m p a t i b i l i t y are a l s o s t r i c t f o r p h o t o v o l t a i c systems subjected to moisture and thermal s t r e s s e s . Moisture, temperature, and UV, s e p a r a t e l y and i n combination, can change the volume and thus the s t r e s s s t a t e of polymers. For example, temperature and humidity c y c l e s alone do not cause surface microcracks i n polycarbonate. However, i n the presence of UV r a d i a tion, such c y c l e s cause m i c r o c r a c k s , w h i l e UV alone does not [32]. An understanding of these r e l a t i o n s h i p s i s e s s e n t i a l t o permit r e l i a b l e design of equipment that uses polymers. These f a c t o r s , coupled w i t h need f o r r e l i a b l e design and low c o s t , n e c e s s i t a t e both a fundamental understanding of mechanical behavior and r e l i a b l e mechanical design d a t a . The r e l a t i o n s h i p s between process and environmental e f f e c t s to mechanical behavior have been developed f o r e l a s t o m e r i c polymers to the degree t h a t these m a t e r i a l s can be s e l e c t e d , and t h e i r long-term performance r e l i a b l y p r e d i c t e d , by a knowledge of some fundamental parameters determined from a few s t r a i g h t f o r w a r d experimental measurements. If the current l e v e l of understanding of v i s c o - e l a s t i c i t y of elastomers can be extended i n t o the range of g l a s s y polymers, then i t w i l l be p o s s i b l e to make comparable p r e d i c t i o n s of mechanical s t r e s s / t i m e / t e m p e r a t u r e / s t r a i n response and failure relationships. U l t i m a t e l y , e q u i v a l e n t understanding of g l a s s y polymers w i l l g r e a t l y reduce the need f o r c o s t l y e m p i r i c a l t e s t i n g each time a new a p p l i c a t i o n i s contemplated. Combined Environmental Effect. Any l i s t of s i g n i f i c a n t e f f e c t s of the environment on polymers i n s o l a r a p p l i c a t i o n s w i l l i n c l u d e UV degradation, weathering, p e r m e a b i l i t y , high-temperature performance, d e l a m i n a t i o n / f a t i g u e , dimensional s t a b i l i t y , and soiling/cleaning. These effects are not necessarily independent: polymers are expected to s u f f e r more s e r i o u s degradation during exposure to combined environmental s t r e s s e s

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E f f e c t i v e methods f o r p r e d i c t i n g and v e r i f y i n g the performance o f polymers under i n t e r a c t i v e e f f e c t s are r e q u i r e d , p a r t i c u l a r l y f o r those a p p l i c a t i o n s unique to s o l a r energy systems. The simultaneous and s e q u e n t i a l combinations of environmental s t r e s s e s that a l t e r the p r o p e r t i e s and a f f e c t the performance o f polymers should be i d e n t i f i e d e x p e r i m e n t a l l y . Assessment of the s t a b i l i t y o f polymers from fundamental r a t e data o r from e x p e r i mental engineering data r e q u i r e s an understanding of the i n t e r a c t i v e e f f e c t s o f environmental s t r e s s [11, 32]. Testing o f a l l combinations of the s t r e s s e s would r e q u i r e an unacceptable number of experiments. The number of combinations s t u d i e d can be l i m i t e d by r e c o g n i z i n g the unique p o s i t i o n o f o p t i c a l elements which may use transparent polymers and o f s t r u c t u r a l members; by f i r s t ranking the importance of i n d i v i d u a l s t r e s s e s ; and by using screening t e s t s to i d e n t i f y promising m a t e r i a l s [28-30]• Improved a n a l y t i c a l and t e s t methods are needed Accelerated t e s t s s t r e s s some parameter occurs, abbreviated t e s t f a i l u r e r a t e s from i n c i p i e n t degradation during short-term exposures at u s u a l s t r e s s l e v e l s . Methods are needed t o demonstrate c o r r e l a t i o n s between these t e s t r e s u l t s and r e a l - t i m e behavior. E a r l i e r s t u d i e s [33, 34] provide some b a s i s f o r t h i s work. Performance p r e d i c t i o n modeling (PPM) i s one method f o r e v a l u a t i n g m a t e r i a l s performance that has been defined and i s being a p p l i e d a t the Jet P r o p u l s i o n Laboratory. I n i t s simplest form, PPM can v e r i f y the s a t i s f a c t o r y performance of a p a r t i c u l a r m a t e r i a l used i n a s p e c i f i c design and subject to a defined set o f s t r e s s e s (e.g., temperature, thermal c y c l i n g , u l t r a v i o l e t r a d i a t i o n , mechanical l o a d s ) . Conversely, I t can d e f i n e l i m i t s o f s t r e s s e s f o r the m a t e r i a l s i n an a v a i l a b l e piece of hardware o r design. This approach has been used s u c c e s s f u l l y as part o f the demonstration of the f e a s i b i l i t y of using an u l t r a - t h i n polymer f i l m on a s o l a r s a i l f o r space p r o p u l s i o n [35], f o r a n a l y t i c a l assessment of an experimental f a c i l i t y t o study space r a d i a t i o n e f f e c t s [36], and f o r a n a l y t i c a l assessment and i d e n t i f i c a t i o n o f c r i t i c a l technologies f o r ceramic r e c e i v e r s [37]. The method i s c u r r e n t l y being a p p l i e d to p h o t o v o l t a i c encapsulation [38]. A second procedure, using the methods of thermodynamics a p p l i e d to i r r e v e r s i b l e processes, o f f e r s another new approach f o r understanding the f a i l u r e of m a t e r i a l s . For example, the e q u i l i b r i u m thermodynamics o f closed systems p r e d i c t s that a system w i l l evolve i n a manner that minimizes i t s energy (or maximizes i t s entropy). The thermodynamics of i r r e v e r s i b l e processes i n open systems p r e d i c t s that the system w i l l evolve i n a manner that minimizes the d i s s i p a t i o n o f energy under the c o n s t r a i n t that a balance of power i s maintained between the system and i t s environment. A p p l i c a t i o n o f these p r i n c i p l e s o f nonlinear i r r e v e r s i b l e thermodynamics has made p o s s i b l e a formal r e l a t i o n s h i p between thermodynamics, molecular and morphological s t r u c t u r a l parameters,

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than from a simple sum of e f f e c t s of i n d i v i d u a l s t r e s s e s , and t h e i r r a t e of change. E x p e r i m e n t a l l y , these p r i n c i p l e s emphasize dynamic measurements t h a t make p o s s i b l e the s e p a r a t i o n o f the d i s s i p a t i v e and the conservative components of energy i n c i d e n t upon the system. Dynamic mechanical a n a l y s i s has been an important area o f research f o r over 40 years. Computer-controlled experimentation now makes i t p o s s i b l e t o apply analogous techniques t o the measurement o f many other thermodynamic s t r e s s e s . One example c u r r e n t l y under i n v e s t i g a t i o n , dynamic photothermal spectroscopy, i s expected t o provide a new approach t o p r e d i c t i n g the long-term e f f e c t s of u l t r a v i o l e t r a d i a t i o n on m a t e r i a l s [39]. Acknowledgments

The authors wish t o express t h e i r g r a t i t u d e f o r the support of the M a t e r i a l s Researc A.W. Czanderna, and R.F a l s o a r e due t o members of the Energy and M a t e r i a l s Research S e c t i o n of the J e t P r o p u l s i o n Laboratory f o r t h e i r help i n preparing t h i s document. This document was prepared f o r the U.S. Department of Energy under Contract No. EG-77-C-01-4024. The J e t P r o p u l s i o n Laboratory i s a N a t i o n a l Aeronautics and Space A d m i n i s t r a t i o n f a c i l i t y , and the S o l a r Energy Research I n s t i t u t e i s a Department of Energy f a c i l i t y .

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RECEIVED February

9,1983

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2 Economics of Solar Heating Systems JOHN W. ANDREWS Brookhaven National Laboratory, Solar and Renewables Division, Upton, NY 11973

The development of solar space heating systems has required assessment of the allowable cost of such systems. Such assessments have most often utilized life-cycle costing with a 20-year period of analysis. The use of this method, especially in conjunction with high energy cost escalation rates, makes i t possible to justify theoretically almost any system cost. Thus, a Gresham's Law has come in play: in the competition for research doll a r s , costly, material-intensive, technically safe systems have crowded out more innovative but technically more risky approaches keyed to more realistic cost goals. Methods of Economic Analysis Three methods of analysis commonly used in evaluating r e s i dential solar systems are 1) simple payback; 2) positive cash flow; 3) l i f e cycle costing. Simple Payback is an answer to the question, "If I spend more now for a solar system than for a conventional alternative, how long w i l l i t take for my cumulative fuel savings to equal my extra i n i t i a l outlay?" The HVAC industry generally considers a three- to five-year payback to be necessary in order to justify new, more efficient products. The solar industry is accustomed to much longer periods, often 20 years or longer. It has been suggested that seven or eight years is a reasonable compromise which allows for an increased energy consciousness on the part of the public, but does not d i verge totally from current practice. If no allowance were made for the increasing cost of energy, this would mean that the i n cremental solar system cost, or difference between the f i r s t cost of the solar system and that of the conventional alternative, should be no more than eight times the f i r s t year's fuel 0097-6156/83/0220-0019$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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savings. When both f u e l c o s t e s c a l a t i o n and time value of money are taken i n t o account, t h i s l i m i t i n g r a t i o might be r a i s e d t o about 10. The required payback time i s a matter of judgement, and upon t h i s judgement can depend the e n t i r e d i r e c t i o n of a development program. Let us t h e r e f o r e look at some a d d i t i o n a l ways of addressing t h i s problem. P o s i t i v e Cash Flow i s based upon the i d e a t h a t people w i l l buy a s o l a r system i f t h e i r t o t a l payments f o r mortgage, maintenance, and energy are l e s s f o r the s o l a r system than f o r the conv e n t i o n a l one. Depending on the r a t e of i n t e r e s t , t h i s c r i t e r i o n could be compatible w i t h r e l a t i v e l y long payback p e r i o d s . In order to assess t h i s c r i t e r i o n , the c a p i t a l recovery f a c t o r s (CRF) f o r nine mortgages of terms 10, 20, and 30 years and i n t e r est r a t e s 6, 10, and 14% have been c a l c u l a t e d . The CRF i s the r a t i o of the annual mortgage payment ( i n t e r e s t plus p r i n c i p a l ) t o the face amount of the l o a n The loan amount i s taken as the incremental s o l a r syste the f i r s t cost of the s o l a one. This i s then modified by s u b t r a c t i n g the tax savings due to the d e d u c t i b i l i t y of the i n t e r e s t and by adding the incremental maintenance and miscellaneous c o s t s , where these are assumed to equal 2% of the incremental system c o s t . The net c a p i t a l r e covery f a c t o r represents the amount of f u e l savings r e q u i r e d , per d o l l a r of Incremental system c o s t , to achieve zero i n i t i a l cash flow r e l a t i v e to the competing conventional system. The i n v e r s e of the CRF i s the r a t i o of incremental s o l a r system cost to f i r s t year's f u e l s savings needed to achieve zero r e l a t i v e cash flow i n the f i r s t year. Values of t h i s Cost/Savings R a t i o are d i s p l a y e d i n Table I . A Cost/Savings R a t i o of 10 i s c o n s i s t e n t w i t h a 20year 10% loan and a marginal tax r a t e of 37%, or e l s e a 20-year 14% l o a n w i t h a 50% marginal tax r a t e . Table I Ratios of Incremental S o l a r System Cost to F i r s t Year F u e l Savings Consistent w i t h Zero R e l a t i v e Cash Flow f o r the F i r s t Year. Values Given f o r Marginal Tax Brackets of 30% and 50%. Term of Loan (years) 10

20

30

Interest Rate (%) 14 10 6 14 10 6 14 10 6

Cost/Savings R a t i o 30% 50% Marginal Tax Rate Marginal Tax Rate 5.9 6.5 7.2 7.8 9.3 11.2 8.3 10.4 13.3

7.0 7.5 7.9 9.9 11.5 13.0 10.8 13.2 15.9

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Economics of Solar Heating Systems

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L i f e - C y c l e C o s t i n g i s a method of t a k i n g i n t o account a l l of the v a r i o u s c o s t s and b e n e f i t s which occur i n the course of a c h i e v i n g an o b j e c t i v e over time. A cost which i s i n c u r r e d i n the f u t u r e i s not as great a l i a b i l i t y as the same cost i n c u r r e d now. This i s true even i n the absence o f i n f l a t i o n . Q u a l i t a t i v e l y i t i s human nature to defer pain as long as p o s s i b l e . Q u a n t i t a t i v e l y , by d e f e r r i n g a cost one can i n the meantime earn i n t e r e s t on the money set aside to pay the c o s t , and the longer one defers the payment, the more i n t e r e s t w i l l be earned. I t may a l s o happen, however, that the f u r t h e r i n the f u t u r e a cost may be d e f e r r e d , the greater i t w i l l be. I n an era of r i s i n g energy p r i c e s , the cost of a given amount of f u e l w i l l increase over time. A way i s needed to put on an equal f o o t i n g costs i n curred at d i f f e r e n t times. This i s accomplished by means of the present value f u n c t i o n (PVF) which i s defined (1) as the amount of money which must be set aside now a t an annual r a t e of r e t u r n d, to cover over pense which now costs on an annual r a t e e:

PVF

( d , e, N)

d-e N 1+d

1-

1+e 1+d

i f d^e (1) i f d=e

In e v a l u a t i n g the r e l a t i v e m e r i t s o f a s o l a r and a convent i o n a l HVAC system the present values o f the various costs (or b e n e f i t s ) are added (or subtracted) to o b t a i n a t o t a l present value (TPV) o f the l i f e - c y c l e costs of the s o l a r system and of the conventional system. The system having the lower TPV i s the more cost e f f e c t i v e under the given assumptions. For r e s i d e n t i a l systems the f o l l o w i n g costs and b e n e f i t s must be considered: 1) system f i r s t c o s t ; 2) maintenance and miscellaneous c o s t s ; 3) energy c o s t s ; and 4) tax b e n e f i t s due to d e d u c t i b i l i t y of i n t e r est payments. The s o l a r tax c r e d i t , because i t i s expected to e x p i r e soon, i s not considered. ( I t could be included i n the a n a l y s i s by s u b t r a c t i n g i t s value from the system f i r s t cost.) To i l l u s t r a t e what can be done w i t h l i f e - c y c l e c o s t i n g the PVF was c a l c u l a t e d f o r periods of a n a l y s i s of 5 to 25 years and f u e l cost e s c a l a t i o n r a t e s from 5% t o 30% w i t h a 10% discount r a t e . The values so c a l c u l a t e d are shown i n Table I I . The PVF i s approximately equal to the allowed c o s t - t o - s a v ings r a t i o under the s i m p l i f y i n g assumptions of equal d i s c o u n t , i n t e r e s t , and i n f l a t i o n r a t e s and o f c a n c e l i n g e f f e c t s of maintenance costs and of the b e n e f i t due to tax d e d u c t i b i l i t y of

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Table I I Values of the Present Value Function f o r Various Fuel E s c a l a t i o n Rates and Periods of A n a l y s i s 5 Fuel E s c a l a t i o n Minus Discount Rate -0.05 0 0.05 0.10 0.20

Present Value Function ( C a l c u l a t e d f o r d = 0.1)

Fuel Cost Escalation 0.05 0.10 0.15 0.20 0.3

Periods of A n a l y s i s (Years) 20 25 10 15

4.2 4.5 5.0 5.5

7.4 9.1 11.2 13.9

10.0 13.6 19.0 26.9

12.1 18.2 28.7 47.0

13.8 22.7 40.1 78.0

mortgage i n t e r e s t . * The point that i s i l l u s t r a t e d by Table I I i s that i t i s p o s s i b l e t h e o r e t i c a l l y to j u s t i f y very high c o s t - t o savings r a t i o s and hence very high s o l a r system costs by u s i n g high f u e l cost e s c a l a t i o n r a t e s and long periods of a n a l y s i s . The use of long system l i f e t i m e s such as 25 years and l a r g e f u e l e s c a l a t i o n r a t e s such as 30% r e s u l t s i n very high PVF's and appears to suggest that one should r a t i o n a l l y pay 320 times the f i r s t year's energy savings f o r a s o l a r system. I n a d d i t i o n to noting that these assumptions imply a 700-fold increase i n the p r i c e of f o s s i l f u e l s , one may observe as w e l l that t h i s apparent p r i c e freedom i s l i m i t e d by c e r t a i n a d d i t i o n a l c o n s t r a i n t s : 1. The period of a n a l y s i s should not exceed the system life. Indeed, i t should probably be enough s h o r t e r than the expected system l i f e that purchases w i l l be induced w i t h the exp e c t a t i o n of a p r o f i t . Since experience with s o l a r systems i s l i m i t e d , the use of system l i f e t i m e s as long as 20 years i s probably not warranted. Even 15 years may be too long. 2. The e x p e c t a t i o n of high f u e l cost e s c a l a t i o n r a t e s w i l l probably not be borne out as much f o r e l e c t r i c i t y as f o r f o s s i l f u e l s . Fuel costs are only a p o r t i o n of the t o t a l cost of e l e c t r i c power generation, and cheaper s o l i d f u e l s are i n any case expected to d i s p l a c e o i l i n e l e c t r i c power generation. 3. Insofar as f u e l costs do e s c a l a t e a t a r a p i d r a t e , t h i s w i l l l i k e l y lead to r e l a t i v e l y r a p i d i n n o v a t i o n i n s o l a r heating and other forms of energy c o n s e r v a t i o n . Thus a l o n g - l i f e h i g h *The l a t t e r two e f f e c t s are of the same order of magnitude and cancel e x a c t l y , f o r example, i f f i r s t - y e a r maintenance costs are 1.7% of system f i r s t c o s t , the consumer i s i n the 50% tax bracke t , and a 15-year period of a n a l y s i s i s used.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2.

23

Economics of Solar Heating Systems

ANDREWS

cost s o l a r system may be rendered obsolete w e l l before the end of i t s service l i f e . Recently there has been evidence of a change of heart i n £he s o l a r community concerning cost c r i t e r i a . At a meeting of t h e U.S. Department of Energy (DOE) s o l a r c o n t r a c t o r s h e l d i n September, 1981, the consensus moved away from 20-year l i f e c y c l e c o s t ing. For r e s i d e n t i a l systems, payback was seen as the most app r o p r i a t e c r i t e r i o n with a median recommended payback time of 6 years. For commercial systems, where the purchaser i s more s o p h i s t i c a t e d , l i f e - c y c l e c o s t i n g was seen as a p p r o p r i a t e , but w i t h a 10-year time horizon.(2^) A d d i t i o n a l l y , a recent DOE mark e t i n g study (3) I n d i c a t e d that a d e f i n i t e r e l a t i o n s h i p e x i s t s between payback and market p e n e t r a t i o n , w i t h p e n e t r a t i o n dropping below 20% f o r payback periods greater than 8 years. In l i n e w i t h the above d i s c u s s i o n , a Cost/Savings R a t i o of 10 was s e l e c t e d as an upper l i m i t c o n s i s t e n t with a f u e l cost e s c a l a t i o n r a t e of 5 under 10 years. Consequences f o r S o l a r System Engineering The amount of energy obtained annually from a s o l a r c o l l e c tor i n a well-designed system depends on the c o l l e c t o r performance c h a r a c t e r i s t i c s , the type of load to which the c o l l e c t o r i s matched, the s i z e of the c o l l e c t o r array r e l a t i v e to the l o a d , and the s o l a r a v a i l a b i l i t y and ambient temperatures experienced by the system. For a r e s i d e n t i a l space-and-water heating system using f l a t - p l a t e c o l l e c t o r s , however, 100,000 B t u / f t - y r (1.14 GJ/m -yr) represents a reasonable average over much of the U n i t ed States.(4) Current gas and o i l p r i c e s are t y p i c a l l y ^$0.50/ therm and ^$1.20/gallon r e s p e c t i v e l y . I f an average conversion e f f i c i e n c y o f 70% i s assumed, then the energy savings per u n i t area w i l l equal $ 0 . 7 1 / f t ($7.69/m ) against gas or $ 1 . 2 2 / f t ($13.18/m ) against o i l . A l l o w i n g f o r the l i k e l i h o o d that the p r i c e of gas w i l l r i s e t o near p a r i t y w i t h o i l , we chose $1.20/ f t ($13/m ) as the benchmark value of the energy savings, based on u n i t c o l l e c t o r area. The f a c t o r - o f - t e n r u l e then r e q u i r e s a system cost o f $12/ f t ($130/m ) of c o l l e c t o r . I s there any hope of meeting such a cost goal using current technology based on extruded metal and g l a s s c o l l e c t o r s ? The answer i s no. The m a t e r i a l s costs alone for such c o l l e c t o r s are $ 5 - $ 6 / f t ($55-$65/m ) .(5) By the time manufacturing, d i s t r i b u t i o n , and i n s t a l l a t i o n costs are i n c u r r e d , the p r i c e r i s e s t o $20-$25/ft ($220-$270/m ). The balance o f system ( s t o r a g e , p i p i n g , pumps, heat exchangers, v a l v e s , and cont r o l s ) c o n t r i b u t e s another $ 2 0 / f t ($220/m*). Thus the o v e r a l l system cost i s over budget by n e a r l y a f a c t o r of f o u r . I t i s c l e a r that both the c o l l e c t o r and the balance of s y s tem must experience d r a s t i c cost reductions before a c t i v e s o l a r space heating can be s a i d t o be c o s t - e f f e c t i v e . The approach 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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taken a t Brookhaven N a t i o n a l Laboratory (BNL) was to a l l o c a t e a budget o f $ 5 / f t ($55/m ) f o r the i n s t a l l e d cost of the c o l l e c t o r and the remainder of $ 7 / f t ($75/m ) f o r the balance of system. The design o b j e c t i v e s were then set t o : 1) reduce the m a t e r i a l s c o s t s of the c o l l e c t o r to ^ $ l / f t ( $ l l / m ) , and 2) design the c o l l e c t o r to be compatible w i t h balance-of-system cost savings. M a t e r i a l s cost r e d u c t i o n has been achieved through the use of t h i n - f i l m polymeric m a t e r i a l s i n both the absorber and g l a z i n g p o r t i o n s of the c o l l e c t o r . The f i l m s , attached to a l i g h t w e i g h t bent-metal frame, form a set of s t r e s s e d membranes that c o n t r i b ute to the o v e r a l l s t r e n g t h of the panel. In t h i s design water i s used as the h e a t - t r a n s f e r medium. T h i s water flows through the absorber at atmospheric pressure from the top to the bottom of the panel. The use of o r d i n a r y water without a n t i f r e e z e makes p o s s i b l e the e l i m i n a t i o n of a t l e a s t one heat exchanger and two pumps and can lead to an improvement i n system e f f i c i e n c y mode r e l a x e s design requirement w e l l as i n the c o l l e c t o r . A companion paper (6) d e s c r i b e s the Brookhaven t h i n - f i l m c o l l e c t o r i n greater d e t a i l . 2

2

2

2

2

2

Acknowledgment Work performed under the auspices o f the U.S. Department o f Energy under Contract No. DE-ACO2-76CH00016.

Literature Cited 1. 2.

3.

4.

Perino, A. M., A Methodology for Determining the Economic Feasibility of Residential or Commercial Energy Systems, SAND 78-0931, 1979, p.11. Active Solar Heating and Cooling Contractors' Review Meeting, U.S. Dept. of Energy, Roundtable Discussion on Solar Systems Evaluation, Washington, D.C., September, 1981, Proceedings in preparation. Lilian, G. L. and Johnston, P.E., A Market Assessment for Active Solar Heating and Cooling Products, Category B: A Survey of Decision Makers in the HVAC Market Place, OR/MS Dialogue, Inc., Final Report DO/CS/30209-T2, September 1980. For example, using the Balcolm-Hedstrom load-collector ratio method (Solar Engineering, January 1977, p.18) the median collectable solar energy was 99,000 Btu/ft -yr over a sample of 84 cities, for systems providing 50% of the building heating load from solar energy. All but 6 cities fell in the range 70,000 to 160,000 Btu/ft -yr. See also the editorial by Bruce Anderson in Solar Age, January 1978: " . . . there is no evidence, and little possibility, that any solar space-heating system will ever deliver more, annually, than 2

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Economics of Solar Heating Systems

25

100,000 Btus per square foot of aperture area in a climate where sunshine is possible 50 percent of the time." 5. "Cost Reduction Opportunities: Residential Solar Systems," Booz, Allen & Hamilton, Bethesda, MD, 1981 (Draft subject to revision). 6. Wilhelm, W. G., "The Use of Polymer Films and Laminate Technology for Low Cost Solar Energy Collectors," submitted to the Am. Chem. Soc. Polymers in Solar Energy Symposium, Las Vegas, NV, March 28- April 2, 1982. RECEIVED

November 22,1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

3 Polymer Film and Laminate Technology for Low-Cost Solar Energy Collectors WILLIAM G. WILHELM Brookhaven National Laboratory, Solar and Renewables Division, Upton, NY 11973

Solar energy collector panels using polymer film and laminate technology have been developed which demonstrate low cost and high thermal performance for residential and commercial applications. This device uses common water in the absorber/heat exchanger portion of the device which is constructed with polymer film adhesively laminated to the aluminum foil as the outer surfaces. Stressed polymer films are also used for the outer window and back surface of the panel forming a high strength structural composite. Rigid polymer foam complements the design by contributing insulation and structural definition. This design has resulted in very low weight (3.5Kg/m ), potentially very low manufacturing cost (~$11/m ), and high thermal performance. The development of polymer materials for this technology will be a key to early commercial success. 2

2

The potential for large reductions in capital cost for residential and commercial solar energy collectors can be realized with a design strategy that utilizes polymer films and high speed production equipment. Development work performed at Brookhaven National Laboratory (BNL) under U.S. Department of Energy (DOE) (1) contract has demonstrated that the concept can be applied to solar flat plate collector designs (Figure 1) that exhibit high thermal performance (Figure 2) for summer cooling (2) and winter space heating and hot water applications. Several collectors have been built and tested both at BNL and at the Florida Solar Energy Center with very encouraging results. The major contribution of this design towards high performance and low cost (3) 0097-6156/83/0220-0027$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Figure 1.

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BNL l i g h t w e i g h t polymer f i l m s o l a r c o l l e c t o r p a n e l .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Polymer Film and Laminate Technology

29

evolves from the use of high performance polymer f i l m s i n the window and absorber/heat exchanger p o r t i o n s of the s o l a r p a n e l . Present e f f o r t s i n the development of t h i s technology have i d e n t i f i e d polymer f i l m s which can meet the requirements of performance and c o s t . Of even g r e a t e r s i g n i f i c a n c e i s the p o t e n t i a l w i t h i n the i n d u s t r y f o r the c r e a t i o n of polymer f i l m s engineered s p e c i f i c a l l y f o r the design. S o l a r Absorber/Heat Exchanger P r e s e n t l y the absorber/heat exchanger i s the most unique c o n t r i b u t i o n i n the development because i t permits the use o f non-freeze-protected water to be used i n a vapor enclosed package ( F i g u r e 3 ) . In a d d i t i o n , i t allows the water to pass through the package a t atmospheric pressure w h i l e p e r m i t t i n g e f f e c t i v e heat exchange to the l i q u i d . This has been accomplished by the d e v e l opment of a channele bonded laminate with a here because i t t r a n s f e r s dimensional s t a b i l i t y t o the polymer while p r o v i d i n g good l a t e r a l heat t r a n s f e r f o r the v a r i o u s opera t i n g c o n d i t i o n s . The polymer f i l m l a y e r i n s u r e s good c o r r o s i o n p r o t e c t i o n w h i l e improving tear r e s i s t a n c e and o v e r a l l package i n t e g r i t y . The outer f o i l surfaces f u r t h e r c o n t r i b u t e by p r o v i d ing a back surface with low o p t i c a l e m i s s i v i t y f o r low heat l o s s and a s u i t a b l e f r o n t surface f o r d e p o s i t i o n of a s o l a r s e l e c t i v e c o a t i n g . This package demonstrates economy by r e q u i r i n g a m i n i mum of m a t e r i a l ( 90% r e d lead o x i d e Active ingredient, nickel dibutyl dithiocarbamate Fast e x t r u d i n g furnace b l a c k B l e n d o f waxes 2-mercapto-imidazoline

B. F. G o o d r i c h W i t c o C h e m i c a l Co. E a g l e - P i c h e r Co. DuPont

H y d r i n 200 Zinc stearate Red l e a d NBC

100 1,.0 5..0 1 ,0 ,

H y d r i n 200 (HM-13-SEC-3-2)

2-mercapto-imidazoline

Cabot T e c h n i c a l P r o c e s s i n g Co. DuPont

N550, FEE b l a c k TE-70 2-mercaptoImidazoline (NA-22)

E p i c h l o r o h y d r i n based e l a s t o m e r > 90% r e d l e a d o x i d e Active ingredient, nickel dibutyl dithiocarbamate Fast extruding furnace black

AO,.0 1,.0 1 ,5 ,

Supplier

30 min/175°C

Cure C o n d i t i o n s

H y d r i n LOO (HM 13-27-1)

B. F. G o o d r i c h E a g l e P i c h e r Co. DuPont

P a r t s by

H y d r i n 100 Red l e a d NBC

el:

Code

100 5,.0 1,.0

Compound and C u r t - Cond l L Lons

Continued

Reinforcement P l a s t i c i z e r , softener Mold l u b r i c a n t , p l a s t i c i z e r A u x i l i a r y cure m a t e r i a l Nonsulfur, vulcanizing, crossl i n k i n g agent

Accelerator Reinforcement

Primary a c t i v a t o r , p l a s t i c i z e r , softener Heat, l i g h t , w e a t h e r i n g , chemical stabilizer O3, w e a t h e r i n g , c r a c k i n g i n h i b i t o

Reinforcement Mold l u b r i c a n t , p l a s t i c i z e r Fast g e n e r a l purpose a c c e l e r a t o r

A c t i v a t o r , d u s t i n g agent Activator, vulcanizer O3, w e a t h e r i n g , c r a c k i n g i n h i b i t o

Reinforcement Lubricant, processing o i l Fast g e n e r a l purpose a c c e l e r a t o r

Activator, vulcanizer O3, w e a t h e r i n g , c r a c k i n g i n h i b i t o

Table I I . Chemical Formulations of Compounded E l a s t o m e r s —

52

POLYMERS

IN

SOLAR

ENERGY

UTILIZATION

P r o p e r t i e s measured i n the screening t e s t s i n c l u d e d compress i o n s e t a f t e r 70 h at 150°C, compression s e t a f t e r 166 h a t -10°C, u l t i m a t e e l o n g a t i o n , t e n s i l e s t r e n g t h and hardness and changes i n these three p r o p e r t i e s a f t e r aging 166 h at 150°C and v o l a t i l e s l o s t during the l a t t e r aging p e r i o d (Table I I I ) . Among the s i l i c o n e elastomers screened, compound H was e l i m i n a t e d from c o n s i d e r a t i o n f o r the more s t r i n g e n t t e s t i n g because i t s weight l o s s and drop i n t e n s i l e s t r e n g t h during aging were e x c e s s i v e . The remaining s i l i c o n e s performed very w e l l i n a l l c a t e g o r i e s . Nordel 3300-11 was the only EPDM rubber of the three formulations t e s t e d which e x h i b i t e d good r e t e n t i o n of p h y s i c a l p r o p e r t i e s on aging a t 150°C. The two f l u o r o c a r b o n elastomers screened appear t o be e s s e n t i a l l y i d e n t i c a l i n a l l r e s p e c t s . Their performance was e x c e l l e n t i n a l l c a t e g o r i e s except f o r t h e i r p r o c l i v i t y to develop c r y s t a l l i n i t y at low temperatures as evidenced by t h e i r high compression set values at -10°C. We s e l e c t e due p r i m a r i l y to i t s immediat e p i c h l o r o h y d r i n elastomers performed w e l l enough to merit f u r t h e r t e s t i n g , and, i n consequence, the e n t i r e category was e l i m i n a t e d from f u r t h e r c o n s i d e r a t i o n . The e t h y l e n e - a c r y l i c copolymer did not perform w e l l on the screening t e s t s , however, the supp l i e r , DuPont, i n d i c a t e d that the i n i t i a l batch provided had been under cured and that a second batch, which they l a t e r provided, would be f a r s u p e r i o r . This batch was placed i n the l i s t of compositions f o r more exhaustive t e s t i n g . The one c h l o r o s u l f o n ated polyethylene screened was e l i m i n a t e d because i t showed poor compression s e t a t both low and high temperatures; however, i t performed very w e l l i n other areas. The one p o l y a c r y l i c elastomer screened performed e x c e l l e n t l y i n a l l respects save f o r u l t i m a t e e l o n g a t i o n where i t f a i l e d by a s m a l l margin. I t was s e l e c t e d f o r f u r t h e r t e s t i n g . The bromobutyl elastomer performed poorly and was e l i m i n a t e d from f u r t h e r t e s t i n g . However, the b e t t e r b u t y l rubber S, even though i t f a i l e d by a s u b s t a n t i a l margin i n low temperature compression set and i n t o t a l v o l a t i l e s , was continued i n t o the f i n a l t e s t i n g because b u t y l rubbers have found extensive a p p l i c a t i o n i n the s o l a r c o l l e c t o r i n d u s t r y . Thus three s i l i c o n e s , one f l u o r o c a r b o n , one ethylene propylene terpolymer, one ethylene a c r y l i c , one p o l y a c r y l i c and one b u t y l rubber were s e l e c t e d f o r more extensive t e s t i n g . Their t e n s i l e p r o p e r t i e s a r e shown i n Table IV. Since our survey d i s c l o s e d only a very l i m i t e d number of v i a b l e candidates i n the C l a s s SC (caulks) category, comparable screening t e s t s were not performed i n t h i s area. S i x candidates were made a v a i l a b l e f o r the more extensive t e s t i n g . Three of these were s i l i c o n e s , one an a c r y l i c , one a b u t y l and one a c h l o r o s u l f o n a t e d polyethylene (Table I , Code A-F).

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

exposed

+

by r e l a t i v e l y

by s u b s t a n t i a l

fail

F~=

p a s s by s u b s t a n t i a l

F = fail

P =

margin

margin

margin

small

margin

small

t o 150°C/70 h

P = p a s s by r e l a t i v e l y

Materials

(Hardness g r a d e x 10) + 5 • Shore A d u r o m e t e r

N

M

Fluorocarbon

0

P

Q

EPDM

T

2

hardness

P+

2

Volatile Lost

Continued on next page.

P+

+

P

2

Tensile Strength Change

P+

Ultimate Elongat ion Change

F

2

Hardness Change

F~

Compr. S e t 1 6 6 h , 10°C

F"

P"

P

1

J

P

?

Compr. S e t 70 h , 150°CT

P

Ultimate Elongation

H

Hardness Grade

Class PS Elastomer Screening Tests

G

Silicone

C l a s s PS Material

Table I I I .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

r

i

l

Acrylic

U

Chlorosulfonated

L

Ethylene

P

7

+

margin

small margin

by r e l a t i v e l y

by s u b s t a n t i a l m a r g i n

F~- f a i l

p a s s by s u b s t a n t i a l m a r g i n

F - fail

P «

small

e x p o s e d t o 150°C/70 h

P - p a s s by r e l a t i v e l y

Materials

+

hardness

F"

p

F

p+

F"

F

F

F

p

o

+

+

P

F

+

"

F

p

F"

F"

P

P

Compr. S e t . Compr. S e t 70 h, 1 5 0 C ~ 166 h, 10°C

( H a r d n e s s g r a d e x 10) + 5 - S h o r e A d u r o m e t e r

P+

5

T

+

+

+

P

P

F

P

P

5-6

6

6-7

7-8

Polyethylene

+

+

Ultimate Elongation

S

Butyl

R

Bromobutyl

K

Polyacrylic

2

l

P

e

8

6

d

X

a

P+

r

7

G

Hardness

w

V

Epichlorohydrin

e

PS

t

a

a

Class

M

P

P

+

P

P

+

+

+

+

p+

P

F

P

P

2

Hardness Change

Table I I I . Class PS Elastomer Screening Tests —

P

P

F

Ultimate Elongation Change?

.Coatiaued

P

p

P

+

rensile Strength Change?

4.

MENDELSOHN

E TAL.

Stability of Polymeric Materials

55

Table IV I n i t i a l P h y s i c a l P r o p e r t i e s of Aged Sealants

Code

M a t e r i a l Category (Class)

Tensile Strength (lb/in2)

Ultimate Elongation 800 390 280

90 30 120 110

E F G I

Hypalon (SC) B u t y l (SC) S i l i c o n e (PS) S i l i c o n e (PS)

>880 1280

>750 700

120 220

J K L N

S i l i c o n e (PS) A c r y l i c (PS) E t h y l e n e - a c r y l i c (PS) Fluorocarbon (PS)

1060 1570 2080 1690

280 170 420 220

460 660 430 650

0 S

EPDM (PS) B u t y l (PS)

2010 1470

180 500

800 150

Elastomers s e l e c t e d f o r f u r t h e r aging s t u d i e s . V a l u e s f o r c a u l k i n g (SC) compounds were determined a f t e r 4-6 weeks aging under room c o n d i t i o n s and then baking f o r 24 h a t the f o l lowing temperatures: Silicones-225°C, a c r y l i c , Hypalon, b u t y l 150°C. Since caulks were postcured f o r 24 h a t the aging t e s t temperatures, t h e i r o r i g i n a l values w i l l d i f f e r f o r each t e s t . For example, on p o s t c u r i n g a t 125°C caulks A, B, and C d i s p l a y t e n s i l e strengths of 160, >60, and 340 and elongations of 210, 800, and 290 r e s p e c t i v e l y . The PS compounds had been postcured by t h e i r s u p p l i e r s and were tested as r e c e i v e d . ^Ultimate e l o n g a t i o n exceeded c a p a b i l i t i e s of t e s t apparatus, specimens d i d not break.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

56

POLYMERS

IN

SOLAR

ENERGY

UTILIZATION

H y d r o l y t i c Aging Resistance of elastomers to degradation i n hot and humid environments was studied by employing a c c e l e r a t e d aging t e s t s i n which t e n s i l e p r o p e r t i e s of specimens immersed continuously i n water at elevated temperatures were monitored (Table V; Examples of h y d r o l y t i c aging data f o r s e v e r a l m a t e r i a l s are shown i n Figures 3-6). I t must be emphasized that the t e s t c o n d i t i o n s were severe since i t i s q u i t e u n l i k e l y that 100% r e l a t i v e humidity w i l l be achieved f o r any s i g n i f i c a n t length of time i n s i d e a c o l l e c t o r which i s at an elevated temperature. Furthermore, the high s e r v i c e temperature w i l l have a d r y i n g e f f e c t on the polymer. Thus the r e s u l t s of these a c c e l e r a t e d t e s t s should be considered to be e s s e n t i a l l y of comparative value f o r the s p e c i f i c c o n d i t i o n s under which the t e s t s were performed. Since f i l l e r s and other a d d i t i v e s w i l l a f f e c t the pH of the water p e n e t r a t i n g the polymer matrix and may provide a r e a c t i o n s , the r e s u l t s o obtained i f the polymers were i n an e s s e n t i a l l y pure s t a t e . G e n e r a l l y during the very e a r l y stages of aging the s e a l ants e x h i b i t e d f u r t h e r c u r i n g as evidenced by concomitant increases i n t e n s i l e s t r e n g t h and e l o n g a t i o n above t h e i r o r i g i n a l v a l u e s . P h y s i c a l e f f e c t s of p e n e t r a t i o n by water and degradative e f f e c t s of h y d r o l y s i s became apparent a f t e r aging had progressed e s p e c i a l l y at the elevated temperatures. The degradative processes described f o r the v a r i o u s s e a l a n t s are based s o l e l y on observations of the changes of t e n s i l e prop e r t i e s of the aging specimens. The r i s k i n t h i s approach i s that one could be mislead by a simultaneous occurrence of d i f f e r e n t degradative phenomena that present opposing p h y s i c a l e f f e c t s . There i s concern that i n some cases t h i s may lead to erroneous conclusions that a m a t e r i a l i s s i g n i f i c a n t l y more s t a b l e than i t a c t u a l l y i s . However, the l i k e l i h o o d of a major i n t e r p r e t i v e e r r o r of t h i s nature i s low because of the r e l a t i v e l y long p e r i o d of aging i n which a given mode of degradation w i l l probably begin to dominate and thus t e n s i l e s t r e n g t h and e l o n g a t i o n are not l i k e l y to be a f f e c t e d to the same extent. In a d d i t i o n the r e l a t i v e e f f e c t s on the t e n s i l e p r o p e r t i e s w i l l d i f f e r at the v a r i o u s t e s t temperatures. Further work i n v o l v i n g compositional a n a l y s i s , and measurements of molecular weight and c r o s s - l i n k d e n s i t y of the exposed m a t e r i a l s would help s u b s t a n t i a t e the suggested natures of the degradation processes described below. The r e s u l t s of these processes are summarized i n Table V. S i l i c o n e Class PS, G, I , and J . The moderate d e c l i n e of both t e n s i l e s t r e n g t h and e l o n g a t i o n of composition J on immersion at 125°C i n d i c a t e s t h a t h y d r o l y s i s of the polymer i s t a k i n g place (Figure 3). A s l i g h t r i s e i n t e n s i l e s t r e n g t h f o r the 67 and 83°C aging temperatures, which peaked about midway during the aging p e r i o d , i n d i c a t e d that c r o s s - l i n k i n g or chain formation r e a c t i o n s

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

MENDELSOHN

57

Stability of Polymeric Materials

ET AL.

Table V R e t e n t i o n of T e n s i l e P r o p e r t i e s

Code

a

Material Category (Class)

A

S i l i c o n e (SC)

B

Silicone (SC)

C

S i l i c o n e (SC)

G

Silicone (PS)

I

b

A f t e r Immersion i n Water

Water Temp. °C

Days Aged

125 100 83 67

16 64 128 128

125 100

4 64

Percent R e t e n t i o n of O r i g i n a l Property Tensile Ultimate Strength Elong. 10 90 90 160

70 140 110 110

Too tacky' to measure

ROOH

+ Ketone + O H

0.5 χ ΙΟ"

1

—>

ROOH

+ Aldehyde + HOO

0.5 χ 10"

2

—>

ROOH

+

0.1

χ 10

3

—>

R0

0.3

χ 10

9

KET*

0.3

χ 10

KET*

0.3

χ 10"

+ Ketone •

2

2

ft

2

+ ROOH

+ SMROH •

+ Aldehyde OH

+ RH Ketone

+ Water

-5

hv

SMKetone

>

SMR0

2

+

SMRCO

>

SMR0

2

+ CO

KET*

>

Alkene + SMKetone

8 0 . 5 χ 10

2

>

Ketone +

0.1

χ 10

+ ROOH

>

Ketone + RO + OH

0.1

χ 10

KET*

>

Ketone

0.1

χ 10

0.6

χ 10

+

0

SMRCO

1

Ο

Λ

5

7 0 . 5 χ 10

KET*

KET* KET*

2

SMRCO

6 0 . 5 χ 10

1 0

2

>

8

1 0

5

°2

Continued on next page

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

220

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

Table I (continued) Reaction

1

Rate constant

->

ROOH

+ RH

->

SMROOH

SMROOH

->

SMRO + R H

->

0

+ Alkene

SMR0

2

SMRCO

+ O. 2

SMRCOOO

+ RH

SMRCOOOH ROOH

hv

0.1 χ 1 0

4

0.1 χ 10"

2

SMRO + OH

0.3 χ 10"

4

->

SMRCOOO

0.4 χ 1 0

->

SMRCOOOH +

->

SMRC0

hv

+ RO

rt

2

R0

2

+ OH

2

0.1 χ Ι Ο " 0.1 χ 10

RO + OH

0.3 χ 10

RO

-> ->

SMR0

0.1 χ 10

RO + R H

->

RO

2

+ Aldehyde

+ ROH

1 0

1

-3 -4 6

0.1 χ 10

6

0.1 χ 1 0

6

0.1 χ 1 0

5

il

SMRCO.

+ RH

->

Acid +

->

ROOR

R0

2

u

RO 2

+ RO 2

a -3 - [0 1 =10 M (constant); SMProduct = Product from chain cleavage. 2

- T h i s set of reactions and rate constant values is by no means final. We have used it only to demonstrate the approach and to test the use­ fulness of the method by adjusting a few of the important variables. Much more work is required to identify the most appropriate input data block.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

14.

GUILLET

ET AL.

Prediction of Photooxidation of Plastics

221

prediction has been possible in these c a s e s . However, we report here the first attempts to simulate photooxidation kinetics in the case of p o l y m e r s . Our modified program calculates by stepwise integration in r e a l time the varying concentrations of chemical species formed during photooxidation. T o validate our numerical procedure we employed the data base given for the cesium flare system and generated curves identical to those of Edelson(ll) for the same system (Figure 1 ) . The excellent agreement between predicted and actual rate curves shows that the p r o gram itself (irrespective of the data base) performs in a satisfactory manner. Mechanism of Photooxidation A s a starting point for polymer photooxidation we looked at a f o r mal linear low-molecular-weigh be similar to amorphous hig y polyethylen short-rang diffusion rates in reaction centres should approach that of viscous liquids In practice, many polymers w i l l show only chemical changes in the hydro carbon moiety since bond breakage w i l l commonly take place initially in the more labile C - H and C ~ C bonds. Initiation w i l l take place following U V absorption by ketone or hydroperoxide groups or even fortuitously by some C - H bond cleavage. The possibilities of energy transfer among different groups have also been included in the model. Propagation takes place via the formation of peroxy radicals followed by hydrogen atom abstraction from the backbone and repeated fast oxygen addition. Peroxy radical chain c a r r i e r s terminate by disproportionation to form alcohols and ketones. Further photolysis of ketone products leads to another autocatalytic chain. Initiation

RH

?

-> R -

+o

2

fast

->

R 0

2

-

Ketone

10 ROOH

-10

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

222

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

Ο

L o g time, s Figure 1. Our numerical solutions for concentration versus time p r o ­ files for the cesium fiare (cf. r é f . 10): ( Ο ) Ν · ( · ) Ο^, ( • ) C s , ( • ) C s 0 , ( Δ ) C s , e", ( Α ) Ο". +

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

14.

GUILLET E T AL.

223

Prediction of Photooxidation of Plastics Propagation

Peroxide chain + RH ~~

27

-> k = l(T

ROOH

ROOH

+

OH-

R

H k

„

1

->

>

9

0

+ R-

3

->

fast

RO

RO- + OH-

R

HOH

Ketone chain _ , Ketone

hv

^ >

(ρ — υ .δ k - 1θ8 Norrish I φ ~ 0.02 V SMR-

V 5

KET *

+ SMRCO-

Termination RO 2

η—>

· + RO · 2

RO

k~10

· + RO · 2

2

R O H + Ketone +

7

τ — > ROOR k~10

4

+

Χ

Ο

*0 2 0

Λ

2

ROOH ROo · + 2

' " Ketone Aldehyde

5 =-> k~10~ -10~ 2

ROOH

+ Other products

3

etc. P r e l i m i n a r y Results E a r l y computer modelling results have shown that the processes of photooxidation under the conditions of our scheme would involve a long induction period, of up to several y e a r s in the pure hydrocarbon, f o l ­ lowed by a fairly rapid deterioration (Figure 2). Initiation is effected

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

224

POLYMERS

Figure 2.

IN SOLAR

ENERGY

UTILIZATION

Photooxidation of linear alkane (RH) versus time.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

14.

GUILLET

ETAL.

Prediction of Photooxidation of Plastics

225

fortuitously in the program by assigning a low rate constant for R - H cleavage or using low initial concentrations (ca. 10" M ) for either k e ­ tone or hydroperoxide moieties. The principal products of photooxida­ tion are ketones, alcohols, water and alkenes, with smaller quantities of aldehydes, acids, carbon monoxide, etc. (Table Π ) . 5

The rates of formation of the major species show exponential be­ havior after the induction period i s over (Figure 3 ) . T h i s conforms to the rate behavior for polyethylene observed experimentally in a c c e l e r ­ ated tests (Figure 4 ) . A n increase in the light intensity (reflected in a chosen systematic increase in all the photochemical absorption rates in the program) shows a systematic change in the exponential formation of ketone products (Figure 5) while the induction p e r i o d is shortened ( F i g ­ ure 6 ) . Increase in termination rate shortens the kinetic chain length and reduces the formation of product ketone. In this case, the exponen­ tial formation of ketone remain lengthen the induction perio The autocatalytic process is initiated by a narrow concentration range of ketone initiator. A t low levels ( 10" M) the mutual termination of peroxy radicals is too fast to p e r m i t s i g ­ nificant Η - a b s t r a c t i o n from the substrate. These preliminary results are consistent with a l l of our experience to date on the photooxidation of ketone-containing polymers( 12). 7

3

Conclusions Much work remains to be done in refinement of the model to allow for the inclusion of substituent groups, the reactivity of secondary and tertiary C - H bonds, the significance of diffusion, the influences of tem­ perature cycling and dark reactions, and the impact of additives. In con­ clusion, we believe that these modelling studies which can simulate r e a l systems (given reliable input data), represent a novel approach to the general understanding of polymer photooxidation phenomena which should lead to a new understanding of the study of controlled lifetimes for p o l y ­ m e r s and for the development of procedures which would allow the p r e ­ diction of performance of plastics for solar applications.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

226

POLYMERS

IN SOLAR

ENERGY

UTILIZATION

Table II. Final Concentration A r r a y . Time of Photooxidation, 10 Y e a r s

Species label

Initial cone, M

Final c o n e . , M

—

0.25 χ 1 0 "

RH

5.0

3.8

ROOH

— —

0.77 χ 1 0 "

R0

2

ROH

10 "

Ketone

4

0.49 5

0.29 0.23

^2

7

1 2

10"

HOO HOOH Peroxy C O OH SMROH Aldehyde SMRCO

H 0 2

KET*

— — — — — — — —

0.21 χ 1 0 "

4

0.89 χ 1 0 "

3

1 6

0.10 χ 1 0 " 0.19 3

0.19 χ 10""

1 4

0.10 χ 1 0 " 0.40

1 5

0.88 χ 1 0 " 0.14 χ 1 0 "

1

0.14 χ 1 0 "

5

0.19 χ 1 0 "

1

SMRCOOOH

— — — — — — — — —

SMRC0

--

0.11 χ 1 0 "

—

0.15

SMKetone

SMR0

2

CO Alkene ROOR RO SMROOH SMRO SMRCOOO

Acid

2

0.16 χ 1 0

1

0.22 χ 1 0 "

3

1 4

0.48 χ 1 0 " 0.18 χ 1 0 "

3

0.14 χ 1 0 ~

1 3

0.11 χ 1 0 "

6

0.43 χ 1 0 "

4

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1 3

14.

GUILLET

Prediction of Photooxidation of Plastics

ET AL.

I ° 0

1

0.6

1 1.2

1

1

\ 1

1.8 2 . 4 3 . 0

227

1

Q

Time (x!0 s) Figure 3 . Variation of major product concentrations during photooxida­ tion: ( Δ ) alkene, (Q) R O H , ( • ) ketone.

Τ

0

20

40

60

Accelerometer, Figure 4.

80

100

hr

Photooxidation of polyethylene.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

228

POLYMERS

Time

Figure 5.

ENERGY

UTILIZATION

(xlO°s)

Effect of intensity on product formation during photooxidation: 8

(O) N x 5 , slope 5.5 χ 1 θ " , 1.1x10

8

(•)

g

slope

IN SOLAR

Ν χ 2, slope 2.4 χ 10~ ,

(Δ) Ν,

.

I

0.6

I

I

I

1—

1.2

1.8

2.4

3.0

8

Time(xl0 s)

Figure 6.

Photooxidation as a function of intensity of light:

(•)

(Δ) Ν χ 2, ( Ο ) Ν .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Ν χ 5,

14.

GUILLET

ET AL.

Prediction of Photooxidation of Plastics

229

Figure 7. Effect of termination rate on product formation during photo­ oxidation: ( Δ ) D , (O) D x 2 , ( • ) D x 5 .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

POLYMERS IN SOLAR ENERGY UTILIZATION

230

Acknovle dgment s T h i s r e s e a r c h is supported by a grant from the Jet Propulsion Laboratory,

Pasadena, California, and is part of a Solar Energy P r o ­

ject, administered through the U . S . National Aeronautics and Space A dministration.

Literature Cited 1.

2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12.

Carlsson, D. J.; Garton, Α . ; Wiles, D. M . "Developments in Polymer Stabilisation", vol. 1, G . Scott, Ed., Applied Science, Barking, 1980. Guillet, J. E . "Polymers and Ecological Problems", J. Guillet, Ed., Plenum, New York 1973 Scott, G . J. Polym We acknowledge usefu subjec Ingold of the National Research Council of Canada. Any omissions or errors are ours. Edelson, D.; Allara, D. L . Int. J. Chem. Kinet. 1980, 12, 605. Gear, C. W. Commun. ACM 1971, 14, 176. Allara, D. L.; Edelson, D. Int. J. Chem. Kinet. 1975, 7, 479. Sundaram, K . M . ; Froment, G . F . , Ind. Eng. Chem. Fundamen. 1978, 17(3), 174. Olson, D. B . ; Tanzawa, T . ; Gardiner, W. C . J r . Int. J. Chem. Kinet. 1979, 11, 23. Ebert, K . H . ; Ederer, H. J.; Isbarn, G . Angew. Chem. 1980, 19, 333. Edelson, D. J. Chem. Ed. 1975, 52, 642. Guillet, J. E. Pure Appl. Chem. 1980, 52, 285.

RECEIVED February

18, 1983

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15 Effects of Photodegradation on the Sorption and Transport of Water in Polymers C. E . ROGERS Case Western Reserve University, Department of Macromolecular Science, Cleveland,OH44106

The use of polymeri for solar energy systems must be assessed in terms of prospective changes in properties induced by long term exposure under conditions favoring photooxidative degradation. Changes in water sorption due to progressive changes in polymer composition by degradation can be estimated from knowledge of the dependence of water uptake on the functional group composition of the polymer. The effects of cyclic variations in temperature and humidity, combined with progressive photodegradation as a function of material thickness, can then be estimated to determine the probability of serious disruptions in material continuity and related properties. A model treatment has been developed to simulate the phenomena for purposes of design lifetime performance prediction. The use of polymeric m a t e r i a l s i n a p p l i c a t i o n s which i n v o l v e exposure t o s o l a r r a d i a t i o n , oxygen, and v a r y i n g temperatures i s common. Assessment of the s t a b i l i t y of polymeric m a t e r i a l s under such c o n d i t i o n s i s a primary design f a c t o r f o r the p r e d i c t i o n of l i f e t i m e and performance. Despite c o n s i d e r a b l e work by many i n v e s t i g a t o r s , the f e a s i b i l i t y and v a l i d i t y of p r e d i t i o n s of long term performance based on short term t e s t s g e n e r a l l y remains questionable at best. The interdependence of m a t e r i a l and environmental f a c t o r s , changing p r o g r e s s i v e l y w i t h exposure i n duced degradation, confounds any simple a n a l y s i s and p r e d i c t i v e model scheme. The s i t u a t i o n r e q u i r e s an enhanced understanding of the temporal and s p a t i a l v a r i a t i o n s i n polymer composition, s t r u c t u r e , and morphology due t o degradation coupled w i t h an understanding of the consequent changes i n p r o p e r t i e s i n c l u d i n g i n t e r a c t i o n s w i t h environmental agents such as water which a f f e c t t h e i r s o r p t i o n and t r a n s p o r t behavior. These c o n s i d e r a t i o n s are e s p e c i a l l y germane f o r polymeric m a t e r i a l s which are used as prot e c t i v e coatings or encapsulants f o r e l e c t r i c a l or e l e c t r o n i c 0097-6156/83/0220-023l$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

232

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devices i n which the m a t e r i a l i s subject to d e l a m i n a t i o n , c r a c k ing, or other f a i l u r e mode as a r e s u l t of the combined a c t i o n of degradation and moisture a t t a c k . In t h i s present case, we w i l l consider the nature of modes of s o r p t i o n and permeation of water i n polymers i n terms of degradation induced changes i n polymer composition. These changes can accentuate the e f f e c t s of temperature-humidity c y c l i n g on water s o r p t i o n and d i f f u s i o n to cause major changes i n propert i e s w i t h p r o g r e s s i v e degradation. Moisture S o r p t i o n The e q u i l i b r i u m sorbed moisture content i s judged to be a primary parameter a f f e c t i n g the s e l e c t i o n and design of p r o t e c t i v e m a t e r i a l s . Those polymers w i t h high sorbed water contents, or those which can be a n t i c i p a t e upon p h o t o o x i d a t i v e degradation m a t e r i a l s . Such m a t e r i a l s would have an adverse e f f e c t upon sub s t r a t e (e.g., s o l a r c e l l ) o p e r a t i o n a l c a p a b i l i t i e s and, i n a d d i t i o n , would not be expected t o maintain adequate adhesive bonding to the s u b s t r a t e . Dimensional changes i n the m a t e r i a l due to s w e l l i n g would be a major design c o m p l i c a t i o n . The magnitude of e q u i l i b r i u m water s o r p t i o n i n the i n i t i a l m a t e r i a l s can be assessed by d i r e c t measurement and/or by the app l i c a t i o n of e s t a b l i s h e d t h e o r i e s of polymer s o l u t i o n s . The change i n s o r p t i o n due t o the formation of p o l a r degradation products may be estimated by c o n s i d e r a t i o n of the s t o i c h i o m e t r y between sorbed water and p o l a r s u b s t i t u e n t groups e s t a b l i s h e d i n t h i s study by r e f e r e n c e t o l i t e r a t u r e data (1,2) as confirmed by experimental r e s u l t s . The data i n Table I of moles of sorbed water per mole of p o l a r groups shows that there are two c l a s s e s of behavior. There i s n e a r l y a one-to-one r e l a t i o n s h i p between water uptake and the molar c o n c e n t r a t i o n of the hydrogen-bonding donating groups: h y d r o x y l , c a r b o x y l , p e p t i d e , (and f r e e amide). There i s much l e s s s o r p t i o n on n i t r i l e , c a r b o n y l , e s t e r , and ether groups. Knowledge of the course of formation of o x i d a t i o n products a l l o w s an e s t i m a t i o n of the expected change i n water s o r p t i o n . One example (3) i s the formation of carbonyl and hydroxyl groups during the a c c e l e r a t e d weathering of polyethylene under UV exposure. A corresponding i n c r e a s e i n water s o r p t i o n occurs due to the l a t e r formation of h y d r o x y l groups as degradation products. The v a r i a t i o n s of s o r p t i o n mode w i t h time, c o n c e n t r a t i o n , tempe r a t u r e , and other experimental v a r i a b l e s has been shown (4) to lead to s i g n i f i c a n t changes i n polymer p r o p e r t i e s . A c a l c u l a t i o n of the p r e d i c t e d change i n e q u i l i b r i u m sorbed water content due to changes i n polymer s t r u c t u r e caused by photodegradation can be made on v a r i o u s l e v e l s of s o p h i s t i c a t i o n . A reasonably r i g o r o u s approach would need to consider e f f e c t s due to chemical group changes (e.g., formation of carbonyl w i t h c o r r e -

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

ROGERS

233

Sorption and Transport of Water

Table I :

Water-Polymer F u n c t i o n a l Group R a t i o s

F u n c t i o n a l Group (Donating) Hydroxyl Peptide Carboxylic Acid (Accepting) Nitrile Ketone Butyral A l k y l Ester Aryl Ester Methacrylate E s t e r Ether

Moles Water/Mole Group 0.93 1.1 0.78

0.22 0.20 0.13 0.15 0.15 0.11 0.06

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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sponding l o s s of the precursor r e a c t a n t group), l o s s of v o l a t i l e products w i t h concurrent decrease i n r e s i d u a l sample mass, and s t r u c t u r a l / m o r p h o l o g i c a l changes, e s p e c i a l l y v o i d and c r o s s l i n k formation and p e r t u r b a t i o n of c r y s t a l l i n e r e g i o n s . In t h i s present case, we w i l l neglect any e f f e c t s on sorp­ t i o n due to gross changes i n s t r u c t u r a l / m o r p h o l o g i c a l aspects of the polymer. The occurrence of such changes due to photodegrada­ t i o n would be expected to s e r i o u s l y a f f e c t other polymer proper­ t i e s of consequence f o r the a p p l i c a t i o n such as adhesion and impact s t r e n g t h . T h i s , of i t s e l f , would s u f f i c e to e l i m i n a t e the m a t e r i a l from c o n s i d e r a t i o n f o r the a p p l i c a t i o n . The e f f e c t on s o r p t i o n of p r o g r e s s i v e changes i n chemical group composition can be expressed q u i t e simply by a weighted solubility coefficient: S

=

... are f o r degradation products) and are the corresponding characteristic solubilities. Values of may be estimated by c o n s i d e r a t i o n of s o l u b i l i t y parameter concepts coupled w i t h polymer s o l u t i o n t h e o r i e s such as the simple Flory-Huggins e x p r e s s i o n . A more d i r e c t approach would u t i l i z e the experimental data given i n Table I or c o r r e ­ sponding experimental data f o r s o r p t i o n i n polymers w i t h composi­ t i o n groups c h a r a c t e r i s t i c of the o r i g i n a l polymer and the p r i ­ mary degradation products. A r e p r e s e n t a t i v e set of data ( 5 ) , given i n Table I I , can be used to estimate the change i n water s o r p t i o n i n polyethylene at constant temperature and r e l a t i v e humidity as the c o n c e n t r a t i o n of hydroxyl groups i n c r e a s e s w i t h p r o g r e s s i v e photodegradation, as shown i n F i g u r e 1. Determination of the dependence of φ^ on exposure time would then s u f f i c e f o r a p r e d i c t i o n of the change i n water s o r p t i o n w i t h degradation. The e f f e c t s of changes i n temperature and i n r e l a t i v e humidity a l s o can be p r e d i c t e d by i n c o r p o r a t i n g the f u n c t i o n a l dependence of s o l u b i l i t y on those parameters, as d i s ­ cussed below, i n t o the e x p r e s s i o n above. This method can be ex­ pected t o hold w i t h reasonable v a l i d i t y up to moderate extents of degradation beyond which other changes i n p h y s i c a l p r o p e r t i e s would become overwhelming c o n s i d e r a t i o n s . 0

Moisture D i f f u s i o n and

Permeation

P r e d i c t i o n of moisture d i f f u s i o n parameters i n polymers undergoing p h o t o o x i d a t i v e degradation i s considered to be of somewhat l e s s e r s i g n i f i c a n c e than s o r p t i o n p r e d i c t i o n s f o r three reasons. F i r s t , i t seems probable that the occurrence of anoma­ lous s o r p t i o n - d i f f u s i o n behavior may be expected i n most polymer

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

ROGERS

Sorption and Transport of Water

Table I I : Water Transport 25°C 60% R.H. Polymer

Ρ χ 10

Polyethylene Poly(vinyl) alcohol)

9

235

Parameters

D(c=o) χ 1 0

9

230

9.6

1.25

9

S = P/D 0.039

7.68

Units : :P

ccSTPcm sec" /cmHgcnf" Ι­

D

cm sec

S

ccSTP/cmHgcm

2

Ο

_

1

i

3

F i g u r e 1. P r e d i c t e d change i n r e l a t i v e water t r a n s p o r t parameters v i t h . p r o g r e s s i v e conversion o f -CH CH2- t o -CH CH0H- (volume f r a c t i o n φΐ) due t o degradation. 2

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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m a t e r i a l s as they undergo p r o g r e s s i v e degradation (3). The nature and magnitude of such e f f e c t s , r e l a t e d t o s t r u c t u r a l r e ­ l a x a t i o n s and/or s p e c i f i c s o r p t i o n - i m m o b i l i z a t i o n modes, w i l l de­ pend upon the polymeric m a t e r i a l and the exposure c o n d i t i o n s . Second, the p r o b a b i l i t y of the formation of c r a z e s , c r a c k s , and other gross defect s t r u c t u r e s to i n c l u d e v o i d i n g and delamina­ t i o n , w i l l a f f e c t the o v e r a l l moisture t r a n s p o r t t o u n d e r l y i n g substrates much more profoundly than b u l k d i f f u s i o n , per se. T h i r d , i t can be a n t i c i p a t e d that i n the absence of a p p r e c i a b l e degradation causing e i t h e r of the above two circumstances, the long term exposure t o the environment f o r any polymer of reason­ able a p p l i c a t i o n t h i c k n e s s w i l l r e s u l t i n s t a t i c sorbed moisture content at the polymer-substrate interphase w i t h i n a time which i s short r e l a t i v e t o the d e s i r e d exposure l i f e t i m e . The value of the nominal d i f f u s i o n parameter i s necessary f o r p r e d i c t i o n of the e q u i l i b r i u m s o r p t i o n l e v e l under c y c l i c temperature-humidity c o n d i t i o n s , as discusse An estimate of th d a t i o n can be combined w i t h the estimate of s o l u b i l i t y , as given above t o o b t a i n an estimate of the change i n the d i f f u s i o n coef­ f i c i e n t . One method f o r the p r e d i c t i o n of p e r m e a b i l i t y constants which has been shown t o be reasonably s u c c e s s f u l i s the Permachor method as developed by Salame (6,7). For the case of water permeation ( 7 ) , the expression i s 7

Ρ = (2.5 χ 10" )exp(-60.5n/RT) 2

where P_ i s the p e r m e a b i l i t y constant i n u n i t s of gms cm/sec cm cmHg, R i s the gas law constant, Τ i s the temperature, and Π i s the polymer's "Permachour". The value of Π i s c a l c u l a t e d by ad­ ding i n d i v i d u a l segmental values f o r the atoms and groupings i n the polymer repeat u n i t i n much the same way (and w i t h some correspondence i n t h e o r e t i c a l b a s i s ) as the c a l c u l a t i o n of s o l u ­ b i l i t y parameters (6) based on group c o n t r i b u t i o n s . A Table of values of Π f o r water p e r m e a b i l i t y has been e s t a b l i s h e d ( 7 ) . T h i s e m p i r i c a l treatment, when compared w i t h the standard Arrhen­ ius expression f o r temperature dependence of permeation, s t a t e s that the p r e e x p o n e n t i a l term i s e f f e c t i v e l y a constant f o r water i n a l l polymers (2.5 χ 1 0 ~ ) . The "Permachour" c o r r e l a t e s the apparent a c t i v a t i o n energy f o r permeation as Ep = 60.5Π. T h i s t a c i t t h e o r e t i c a l r e l a t i o n s h i p i s not completeTy s a t i s f a c t o r y , i t can be circumvented or r e v i s e d t o d e r i v e more r i g o r o u s expres­ s i o n s ( 8 ) , but the ease of a p p l i c a t i o n and e s t a b l i s h e d success of the present form can j u s t i f y i t s use i n t h i s present case. An e f f e c t i v e value of Π can be c a l c u l a t e d f o r a degrading polymer by a weighting expression s i m i l a r t o the one f o r the solubility coefficient: 7

Π = φ Π 0

0

+

φ ΊΙ ι

ι

+ φ ΙΪ + ... + φ - ^ + 2

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

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237

where a r e the mole f r a c t i o n s of c o m p o s i t i o n a l groups and are the c h a r a c t e r i s t i c Permachour v a l u e s . As an example, r e f e r r i n g t o the Table of Π-values (7) we note the f o l l o w i n g f o r the groups i n d i c a t e d : -CH CH - ( d e n s i t y 0.92)

Π = 101

-CHoC-

Π = 100

2

2

0

Π = -43

-CH CH9

OH The minor d i f f e r e n c e i n Π between the p o l y e t h y l e n e repeat u n i t and carbonyl group repeat u n i t and the major d i f f e r e n c e f o r the h y d r o x y l group repea t i v e s o r p t i o n behavio h a v i o r (e.g., Table I I and a l l r e f e r e n c e s ) . Thus, knowledge of the dependence of φ^ f o r hydroxyl degradation product on expo­ sure time should lead t o p r e d i c t i o n of permeation behavior. For d i r e c t water contact a t 23°C the e x p r e s s i o n f o r Ρ r e ­ duces t o : Ρ = (2.5 χ 10 )exp(-0.102n) _7

For the case i n example, when only one degradation product i s of consequence i n a f f e c t i n g permeation, the expression f o r the r e l a t i v e change i n permeation i s : P / I ^ = εχρ[(0.102)(φ )(Π 1

ο

- Π )] χ

=βχρ[(0.102)(φ )(144)] 1

The r e s u l t of t h i s c a l c u l a t i o n and the r e l a t i v e change i n D from the r e l a t i o n s h i p D = P/S i s i l l u s t r a t e d i n F i g u r e 1. We note a n o n l i n e a r i n c r e a s e i n V_ w i t h i n c r e a s i n g h y d r o x y l content and a concurrent very pronounced decrease i n D. T h i s type of behavior i s t o be expected i n c o n s i d e r a t i o n of the i n c r e a s e i n cohesive energy d e n s i t y of the polymer, w i t h i n c r e a s i n g h y d r o x y l content, which a c t s t o r e s t r i c t polymer segmental c h a i n motion thereby reducing the ease of d i f f u s i o n of s m a l l molecules through the polymer m a t r i x . The r e l a t i v e decrease i n D i s more than o f f s e t by l a r g e i n c r e a s e i n s o l u b i l i t y . The product of D and S leads t o a moderate i n c r e a s e i n P^ w i t h i n c r e a s i n g φ.. These p r e d i c ­ t i o n s a r e g e n e r a l l y confirmed by experiment (3). I t must be r e a l i z e d t h a t there i s another f a c t o r t h a t should be considered f o r a reasonable p r e d i c t i o n , namely, the s p a t i a l dependence of degradation products through the polymer

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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sample t h i c k n e s s . Such a d i s t r i b u t i o n can be a n t i c i p a t e d as the r e s u l t both of l i g h t a t t e n u a t i o n and oxygen d i f f u s i o n cont r o l of the p h o t o o x i d a t i v e degradation process. The d e s o r p t i o n r a t e and gradient of v o l a t i l e degradation products a l s o may be a f a c t o r a f f e c t i n g water t r a n s p o r t behavior ( t r a n s i e n t s o r p t i o n s i t e s and p l a s t i c i z a t i o n e f f e c t s ) . The e f f e c t s of a gradient i n polymer composition on t r a n s p o r t has been e s t a b l i s h e d (9-11) and could be i n c o r p o r a t e d i n t o the p r e d i c t i v e scheme given here. The simplest procedure i s to consider the sample as e f f e c t i v e l y being two f i l m s i n s e r i e s (3,12) w i t h the top f i l m of t h i c k n e s s undergoing degradation and the bottom f i l m of t h i c k n e s s £ remaining undegraded; + ^ t o t a l f i l m thickness. The net permeation, !P , i s then given by the standard r e l a t i o n ship: 2

=

=

t r i e

T

w i t h ?_ and as the p e r m e a b i l i t i e s of the two f i l m l a y e r s . In absence of t r a n s p o r t data on f i l m s of measured and we do not make a d i r e c t comparison of p r e d i c t i o n and experimental data. E f f e c t s of Temperature-Humidity

Cycling

A very important c o n s i d e r a t i o n i n the s e l e c t i o n and design of p r o t e c t i v e m a t e r i a l s i s the magnitude of sorbed moisture cont e n t as a f u n c t i o n of encapsulant t h i c k n e s s . T h i s moisture content i s expected to vary as a f u n c t i o n of d i s t a n c e i n t o the c o a t i n g as the c o a t i n g i s exposed to c y c l i c temperature and humid i t y c o n d i t i o n s a n t i c i p a t e d i n exposure environments. The consequent changes i n a n i s o t r o p i c s w e l l i n g as a f u n c t i o n of d i s tance and time could l e a d to the onset or enhancement of v a r i o u s modes of unfavorable deformation or other p h y s i c a l m o d i f i c a t i o n s i n the encapsulant m a t e r i a l . A p r e d i c t i o n of the course of s o r p t i o n - d e s o r p t i o n phenomena i n encapsulant m a t e r i a l s r e l a t e d t o c y c l i c v a r i a t i o n s i n the amb i e n t environment during p h o t o o x i d a t i v e degradation must cons i d e r a number of c o n t r i b u t i n g f a c t o r s and the i n t e r p l a y between those f a c t o r s . The "normal" dependence of d i f f u s i o n and sorpt i o n parameters can be expressed by the f o l l o w i n g simple r e l a tionships: Temperature dependence D = D

Q

S = S

Q

exp(-E /RT) D

exp(-AH /RT) s

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

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Sorption and Transport of Water

Concentration dependence D = D(o)exp(Ac) £ D(o) (1 + Ac) S = S(o)exp(Bp) £ S ( o ) ( l + Bp) Degradation dependence D = D ( t = o ) [ l - F (exposure time)] S = S(t=o)[l + F

T

(exposure time)]

The e s t i m a t i o n a l s o should consider the e f f e c t s o f : R a d i a t i o n a t t e n t u a t i o n changes due to d e n s i t y other changes.

color

and

Leaching of r e a c t i o n products by l i q u i d water c o n t a c t . Loss of l e a c h a b l e and v o l a t i l e products l e a d i n g t o m a t e r i a l shrinkage. C r o s s l i n k i n g (and other changes i n polymer molecular weight and i t s d i s t r i b u t i o n ) . Swelling-induced polymer c h a i n o r i e n t a t i o n normal to the c o a t i n g surface which decreases d i f f u s i o n r a t e s and m o d i f i e s sorption equilibrium. Cooperative degradation r e a c t i o n s i n v o l v i n g sorbed water and/or s w e l l i n g induced s t r e s s e s and/or deformations. The r e s t r a i n t s on i s o t r o p i c s w e l l i n g due t o c o a t i n g adhes i o n t o the s u b s t r a t e . The number of f a c t o r s t o be considered f o r a q u a n t i t a t i v e p r e d i c t i o n of the e f f e c t s of temperature-humidity c y c l i n g r e q u i res a mathematical model w i t h s u f f i c i e n t parameters t o d e s c r i b e most of the above v a r i a b l e s . T h i s n e c e s s i t a t e s the use of computer analog or other computative methods. For the present case, we w i l l only d e s c r i b e a q u a l i t a t i v e model r e p r e s e n t a t i o n based on knowledge of the e f f e c t s of general "normal" dependenc i e s of s o r p t i o n and d i f f u s i o n parameters on the major a n t i c i pated f a c t o r s . A more complete a n a l y s i s w i l l be considered pending the accumulation of knowledge regarding the dependence of other parameters and f a c t o r s i n s e l e c t e d degrading m a t e r i a l systems. A b a s i s f o r judgment as t o the dependence o f s o r p t i o n and t r a n s p o r t on the course o f degradation are d e r i v e d from t h e data and trands observed i n t h i s study, from the data o f K i m b a l l

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and Munir (13), and from o t h e r , l e s s s p e c i f i c experimental and l i t e r a t u r e sources. The simple p r e d i c t i v e method assumes the s o l u t i o n of equat i o n s f o r sorbed c o n c e n t r a t i o n as a f u n c t i o n of t h i c k n e s s and time, c ( x , t ) , as given i n standard monographs on d i f f u s i o n (14). For a model p r e d i c t i o n i t i s necessary t o choose a r e p r e s e n t a t i v e s e t of temperature and humidity c y c l e s . The choice made f o r present purposes i s a uniform s i n u s o i d a l temperature c y c l e and the c o n d i t i o n of constant water content. T h i s leads t o a temperature-related v a r i a t i o n i n r e l a t i v e humidity i n which the r e l a t i v e humidity i n c r e a s e s w i t h decreasing temperature and v i c e versa. The q u a l i t a t i v e p r e d i c t i o n then considers the e f f e c t s of passing from low t o medium t o high t o medium t o low t o medium (etc.) temperatures t o e s t a b l i s h a q u a s i - e q u i l i b r i u m sorbed content a t the c o a t i n g - s u b s t r a t e i n t e r f a c e w i t h a f l u c t u a t i n g s o r bed c o n c e n t r a t i o n i n sorbed c o n c e n t r a t i o n undergoin face regions w i t h a gradual b u i l d u p i n the i n t e r i o r regions of the c o a t i n g t o achieve eventual e q u i l i b r i u m i n terms of a " k i n e t i c " temperature-humidity averaged v a l u e . The range of surface s o r p t i o n and d e s o r p t i o n and the change i n i n t e r f a c e s o r p t i o n t o be expected changes when the s o r p t i o n and d i f f u s i o n parameters are constants (D(o), S ( o ) ) , when they e x h i b i t concentration-dependence ( 5 ) ( D ( c ) , S ( p ) ) , when the d i f f u s i o n parameter decreases and the s o r p t i o n parameter i n c r e a s e s as a r e s u l t of degradation (D(o), S ( o ) ) , degrade), and the e f f e c t of degradation w i t h a concentration-dependence of s o r p t i o n and d i f f u s i o n ( D ( c ) , S ( p ) , degrade). The l a t t e r case, a decrease i n D w i t h i n c r e a s i n g concentration-dependence coupled w i t h an i n c r e a s e i n S magnitude and concentration-dependence as a f u n c t i o n of p r o g r e s s i v e degradative i s deemed to be a very probable e x p e c t a t i o n f o r many encapsulant m a t e r i a l s . T h i s can r e s u l t i n the formation of a surface l a y e r which undergoes wide v a r i a t i o n s i n s w e l l i n g under c y c l i c ambient c o n d i t i o n s . The r e s u l t of such v a r i a t i o n s i n a n i s o t r o p i c swell i n g can e a s i l y enhance v a r i o u s f a i l u r e modes. T h i s i s c o n s i dered t o be a s i g n i f i c a n t f a c t o r f o r s e l e c t i o n , design, and pred i c t i o n of encapsulant m a t e r i a l performance. T y p i c a l values u s e f u l f o r p r e d i c t i v e purposes are given i n the l i t e r a t u r e f o r water d i f f u s i o n , i t s apparent a c t i v a t i o n energy, and sorbed c o n c e n t r a t i o n as a f u n c t i o n of r e l a t i v e vapor p r e s s u r e . The data i n Table I I I i n c l u d e values c a l c u l a t e d f o r d i f f u s i o n parameters a t 5°C and 30°C as r e p r e s e n t a t i v e v a l u e s f o r nominal temperature extremes f o r an environmental exposure c y c l e . I t i s seen that v a l u e s may e a s i l y vary from polymer t o polymer by four or more orders of magnitude. The v a l u e s of (Dt/£ ) f o r 30°C and 5°C are those c a l c u l a t e d f o r a c o a t i n g 1 cm t h i c k exposed t o an average c y c l e time of 12 hours. Comparison of those values w i t h standard p l o t s of r e l a t i v e uptake r a t i o s (14) shows that p o l y s i l i c o n e e a s i l y achieves 2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

15.

ROGERS

Sorption and Transport of Water

Table I I I :

Polymer

C a l c u l a t e d D i f f u s i o n Parameters f o r Water i n Polymer Coatings

E (kcal)

Silicone Rubber 3

D(30°C) D(5°C) (Dt/£ ) (Dt/* )

7.0 χ 10" 4.5 χ 10" 3.0 1.9

D

2

30

2

5

2

l(Dt/Z

= 0.1)

3 0

241

5.5 cm

Polyethylene

Nylon

14 5 5

6

2.3 χ 10" 4.6 χ 10~ 0.01 2 χ ΙΟ"

7

e

5

1 χ 10~

9

4 χ 10"

5

8

3

0.3 cm

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0.02 cm

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s o r p t i o n and desorption e q u i l i b r i u m throughout the sample. Polyethylene sorbs t o about 10% of e q u i l i b r i u m and then desorbs to about 5% of nominal e q u i l i b r i u m . S o r p t i o n i n Nylon i s r e s t r a i n e d t o surface l a y e r s . A c a l c u l a t i o n can be made of the f i l m t h i c k n e s s r e q u i r e d to achieve an advance of the sorbed moisture only t o the subs t r a t e - c o a t i n g i n t e r f a c e ( i n one i n i t i a l s o r p t i o n c y c l e process) which corresponds t o the c o n d i t i o n that Dt/£ = 0.1 a t 30°C f o r D values as given i n Table I I I and f o r t equal t o 12 hours. That c o n d i t i o n would r e q u i r e a s i l i c o n e rubber f i l m of 5.5 cm t h i c k n e s s , and a Nylon f i l m of only 0.021 cm t h i c k n e s s . These values r e f l e c t the dependence of s o r p t i o n r a t e on a concentration-independent d i f f u s i o n c o e f f i c i e n t . As s t a t e d p r e v i o u s l y , more r e f i n e d p r e d i c t i o n s r e q u i r e both a more extensive data base and a more s o p h i s t i c a t e d (computer) mathematical model Such a procedure i s considered w i t h i n the range of f e a s i b i l i t y 2

Acknowle dgment s Support of t h i s research program by a c o n t r a c t from the J e t P r o p u l s i o n Laboratory as part of the Low-Cost Solar Array P r o j e c t ( F l a t - P l a t e S o l a r Array P r o j e c t ) i s g r a t e f u l l y acknowledged. We thank Drs. A. Gupta andJ.Moacanin f o r t h e i r d i s cussions and cooperation. Literature Cited 1. J. Crank and G. S. Park, eds., Diffusion in Polymers, Academic Press, New York, 1968. 2. A. D. McLaren and J. W. Rowan, J. Polym. S c i . , 289(1951). 3. A. Dudek and C. E. Rogers, to be published. 4. A. Sfirakis and C. E. Rogers, Polym. Engr. S c i . , 20, 294 (1980). 5. C. E. Rogers and D. Machin, "The Concentration Dependence of Diffusion Coefficients in Polymer-Penetrant Systems", CRC Critical Rev. in Macromol. S c i . , April 1972, pp. 245-313. 6. M. Salame and J. Pinsky, Mod. Pack., 36, 153 (1962). 7. M. Salame, 67th AlChE Meeting, Feb. 1970. 8. C.E. Rogers, "Prediction of Polymer Permeability", CSL Conference on Chemical Defense Research, November 1981, to be published. 9. S. Sternberg and C. E. Rogers, J. Appl. Polym. S c i . , 12, 1017 (1968). 10. A. Peterlin, J. Appl. Polym. S c i . , 15, 3127 (1971). 11. J. H. Petropoulos, J. Polym. Sci. Phys. Ed., 12, 35 (1974). 12. D. Benachour and C. E. Rogers, ACS Symp. Ser., 151, 263 (1981). 13. W. H. Kimball and Z. A. Munir, Polym. Engr. S c i . , 18, 230 (1978). 14. J. Crank, The Mathematics of Diffusion, 2nd Ed., Clarendon Press, Oxford, 1975. RECEIVED

December 27,1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

16 UV Microscopy of Morphology and Oxidation in Polymers P. D. C A L V E R T , N. C. BILLINGHAM, J. B. KNIGHT, and A . UZUNER University of Sussex, School of Chemistry and Molecular Sciences, Brighton, England

Ultraviolet been applied to a variety of polymer systems to investigate changes of morphology and composition on the scale of 0.25 μm upwards. Studies are briefly described on the behaviour of stabilisers in polypropylene, diffusion of additives in poly­ mers, spherulite morphology, polyolefin oxida­ tion, inhomogeneities in epoxy resins and polymer blends. The ultraviolet microscope, operating in the region 230 nm to 280 nm, was developed by Kohler (1) with the intention of taking advantage of the increased resolving power theoretically associated with shorter wavelengths. The increased resolution actually yielded l i t t l e new information but the microscope did show unexpected contrast effects in biological samples which were completely transparent in visible light. It was later shown that these effects are due to the strong absorption of uv by nucleic acids and this observation quickly led to extensive use of uv microscopy to study the distribution of nucleic acids within cells. Techniques were developed for making quantitative and spectro­ scopic analyses of the species present and for working with living cells by allowing only brief exposures to the damaging uv light. The development of the electron microscope has meant that there is l i t t l e advantage in using uv light to obtain increased resolving power, as compared to the enormous increase allowed by electron illumination. Rather, most uses have been to make quali­ tative or quantitative concentration observations on systems where one component is strongly uv absorbing. In principle, similar measurements could be made with a wide range of coloured substances using a normal visible light microscope. However, in all forms of light microscopy the depth of focus is limited, particularly as the magnification is increased. The result is that very thin samples are required for successful light micro­ scopy so that only absorbing species with high extinction 0097-6156/83/0220-0243$06.25/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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c o e f f i c i e n t s w i l l y i e l d acceptable c o n t r a s t . The main advantages of uv i l l u m i n a t i o n are thus the greater range o f absorbing compounds with a high e x t i n c t i o n c o e f f i c i e n t and the number o f uv absorbing substances which are of i n t e r e s t i n t h e i r own r i g h t . A f u r t h e r advantage o f the uv microscope i s t h a t i t can be used t o e x c i t e and observe f l u o r e s c i n g compounds. These can o f f e r greater s e n s i t i v i t y as fluorescence i s observed against a dark background, but the range o f s u i t a b l e substances i s more l i m i t e d and q u a n t i t a t i v e a n a l y s i s presents more problems. Most commercially important s y n t h e t i c polymers have no strong uv absorption i n the e a s i l y a c c e s s i b l e range from 250 nm t o 400 nm. Hence u s e f u l a p p l i c a t i o n o f the uv microscope w i l l depend on there being added uv absorbing molecules o r attached side groups whose concentration v a r i e s w i t h i n the polymer. Since the only systems which o b v i o u s l y f a l l i n t o t h i s category are polymers cont a i n i n g uv s t a b i l i s e r s t h i s has u n t i l r e c e n t l y been the only a p p l i c a t i o n i n polymer s c i e n c e t i a l range o f a p p l i c a t i o n uv absorbers and f l u o r e s c e r s can be s e l e c t i v e l y bound t o s p e c i f i c chemical e n t i t i e s i n the polymer o r w i l l p r e f e r e n t i a l l y i n t e r a c t with o r d i s s o l v e i n p a r t s o f the s t r u c t u r e . In t h i s way these molecules can be used as s t a i n s and probes o f the morphology o f the polymer on the s c a l e from 0.25 ym upwards, i n a manner very s i m i l a r t o that i n which the b i o l o g i s t uses s t a i n s t o develop c o n t r a s t i n t i s s u e specimens. Further, i n so f a r as uv absorbers resemble other small molecules o f i n t e r e s t , such as drugs and p e s t i c i d e s , they can be used t o study the t r a n s p o r t o f such molecules i n polymers. Outside o f polymers uv microscopy has p r i n c i p a l l y been used by b i o l o g i s t s to study the d i s t r i b u t i o n o f n u c l e i c acids i n whole cells. T h i s work has been reviewed by Freed ( 2 ) . The technique has a l s o been used t o measure l i g n i n concentrations i n wood (3) and p l a n t s and t o study c o a l , o i l and peat. Kam and co-workers were able to measure lysozyme concentrations i n s o l u t i o n around growing c r y s t a l s o f t h i s enzyme and so to determine the importance of d i f f u s i o n i n l i m i t i n g the c r y s t a l growth ( 4 ) . Sodium and potassium are s l i g h t l y transparent i n the uv and uv microscopy has been used to observe segregation during s o l i d i f i c a t i o n o f Na-K alloys (5). Each o f these a p p l i c a t i o n s i s s p e c i f i c to a p a r t i c u l a r problem so that uv microscopy should be seen as a technique with some s p e c i a l i s e d a p p l i c a t i o n s r a t h e r than being g e n e r a l l y u s e f u l . Also each new a p p l i c a t i o n r e q u i r e s the development o f new methods of sample p r e p a r a t i o n and t h i s can be time consuming. In t h i s paper we describe the apparatus and review those areas o f polymer science where we have found t h a t t h i s method gives valuable new information. We have r e c e n t l y completed a d e t a i l e d review o f uv microscopy (6) and t h e r e f o r e concentrate on recent advances.

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Apparatus Figure 1 shows, s c h e m a t i c a l l y , the equipment which we use f o r uv microscopy o f polymers. I t comprises a normal Z e i s s U n i v e r s a l microscope f i t t e d w i t h uv-transparent quartz o p t i c s , f r o n t surface m i r r o r beam switches, a 150w Xenon arc lamp and a uvs e n s i t i v e TV viewing system. A waveform monitor d i s p l a y s the i n t e n s i t y d i s t r i b u t i o n on any s i n g l e l i n e o f the TV image so a l l o w i n g determination of concentrations o f uv absorbing s p e c i e s . Wavelength s e l e c t i o n i s by i n t e r f e r e n c e , l i q u i d o r c o l o u r g l a s s f i l t e r s . A 10 cm c e l l f i l l e d w i t h d i s t i l l e d water acts as a heat f i l t e r . Images can be d i r e c t l y recorded on normal photographic f i l m and analysed by microdensitometry. Concentration measurements may be made a f t e r c a l i b r a t i o n w i t h samples of known t h i c k n e s s and uniform absorber c o n c e n t r a t i o n . Other sources o f c o n t r a s t such as d i f f r a c t i o n e f f e c t s can be detected by comparing u measurements can a l s o b taken to a v o i d s a t u r a t i o n which occurs at high concentrations where most o f the incoming uv i s absorbed. At some cost to the u n i f o r m i t y of the f i e l d o f i l l u m i n a t i o n i t i s p o s s i b l e t o use the microscope i n c o n j u n c t i o n w i t h a hot stage ( M e t t l e r FP2) and d i r e c t l y r e c o r d c o n c e n t r a t i o n changes d u r i n g c r y s t a l growth and m e l t i n g . A d d i t i v e s i n C r y s t a l l i n e Polymers Commercial c r y s t a l l i n e polymers contain a v a r i e t y o f i m p u r i t y species and a d d i t i v e s , most o f which are excluded from the c r y s t a l l i n e regions as the s p h e r u l i t e s grow. L i g h t s t a b i l i s e r s such as s u b s t i t u t e d benzophenones and b e n z o t r i a z o l e s are f r e q u e n t l y added t o p o l y o l e f i n s i n concentrations o f 0.1 t o 0.5%. Since these absorb s t r o n g l y around 320 nm w h i l e the polymer i s t r a n s parent down t o 200 nm t h i s system i s i d e a l f o r uv microscopy. Curson (7) and Frank and Lehner (8) have looked at polypropylene c o n t a i n i n g uv absorbers and showed t h a t the a d d i t i v e was concent r a t e d c l o s e t o the s p h e r u l i t e boundaries. Ryan s t u d i e d the r e j e c t i o n process during c r y s t a l l i s a t i o n (9). A wave o f r e j e c t e d a d d i t i v e b u i l d s up ahead of the growing s p h e r u l i t e as shown i n Figures 2 and 3. The shape o f t h i s wave depends on the d i f f u s i o n c o e f f i c i e n t o f the a d d i t i v e and the growth r a t e o f the s p h e r u l i t e and has been compared w i t h computer s i m u l a t i o n s o f the r e j e c t i o n . A d d i t i v e c o n c e n t r a t i o n gradients are induced by the c r y s t a l l i s a t i o n process but would be expected t o r e l a x on subsequent annealing. I n f a c t the g r a d i e n t s do r e l a x t o some extent but a s t a b l e gradient of uv a b s o r p t i o n remains w i t h the s p h e r u l i t e centre l e s s absorbing than the boundary (10). T h i s cannot represent a c o n c e n t r a t i o n change o f the a d d i t i v e w i t h i n the amorphous regions o f the polymer so i t must be due t o a v a r i a t i o n i n the

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

246

POLYMERS

IN

SOLAR

ENERGY

WAVEFORM MONITOR

UTILIZATION

PICTURE MONITOR

UV VIDICON

35 mm CAMERA

OBJECTIVE

SAMPLE I

STAGE

CONDENSER

Σ TUNGSTEN LAMP

XENON ARC

BEAM SWITCH

PRIMARY FILTERS

F i g u r e 1.

HEAT FILTER

B l o c k diagram o f the UV microscope.

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F i g u r e 2. Polypropylene f i l m c o n t a i n i n g 0.1% U v i t e x OB, viewed i n fluorescence during c r y s t a l l i z a t i o n a t 130 C.

American Chemical Society Library '1155 16th St. N . W. Washington. 0. C. 20036

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135

130

Ζ

125

0

1mm

Figure 3. A d d i t i v e d i s t r i b u t i o n s f o r polypropylene s p h e r u l i t e s c o n t a i n i n g 0.5% Uvitex OB c r y s t a l l i z i n g at 125, 130, and 135 °C. Waveform monitor t r a c e o f UV image.

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amorphous content o f the s p h e r u l i t e . Thus the c r y s t a l l i n i t y o f a s p h e r u l i t e must decrease from the centre t o the boundary and can be measured along a radius i n annealed samples by uv microscopy. This p r i n c i p l e can be widened t o say t h a t uv absorbing s t a i n s can be used t o probe f o r l o c a l density v a r i a t i o n s w i t h i n polymers provided t h a t these d e n s i t y v a r i a t i o n s l e a d t o s o l u b i l i t y changes f o r the s t a i n i n the polymer. A t a c t i c Impurities

i n C r y s t a l l i n e Polymer

The most l i k e l y cause o f the c r y s t a l l i n i t y v a r i a t i o n s i n polypropylene s p h e r u l i t e s i s the accumulation o f r e j e c t e d a t a c t i c and low molecular weight i m p u r i t i e s . This view i s supported by the observation t h a t adding i n c r e a s i n g amounts o f a t a c t i c m a t e r i a l to polypropylene p u r i f i e d by octane e x t r a c t i o n , leads t o changes i n the c r y s t a l l i n i t y d i s t r i b u t i o n (Figure h). A t a c t i c polypropylen seen d i r e c t l y . We t h e r e f o r r e a c t i n g a t a c t i c polymer with N,N-dimethylamino sulphonyl azide (dansyl azide) which c o v a l e n t l y bonds the f l u o r e s c e n t dansyl groups t o the polymer a t l e v e l s o f about 1 per hundred monomer u n i t s . By i n c o r p o r a t i n g t h i s m a t e r i a l i n t o i s o t a c t i c polypropylene we can f o l l o w the r e j e c t i o n o f the a t a c t i c m a t e r i a l using the uv microscope. We can reverse the r e l a t i o n s h i p and s t a i n the i s o t a c t i c m a t e r i a l i n which case we see b r i g h t f l u o r e s c i n g s p h e r u l i t e centres with darker a t a c t i c - r i c h boundaries as shown i n Figure 5. We are now a n a l y s i n g the r e j e c t i o n behaviour o f the a t a c t i c m a t e r i a l along s i m i l a r l i n e s t o those used f o r the uv absorbing a d d i t i v e s t o determine the m o b i l i t y o f the a t a c t i c f r a c t i o n s w i t h i n the polymer. D i f f u s i o n and Loss o f A d d i t i v e s The u s e f u l l i f e t i m e o f a p o l y o l e f i n corresponds e s s e n t i a l l y t o the end o f the o x i d a t i o n i n d u c t i o n time. In s t a b i l i s e d m a t e r i a l s t h i s may be determined e i t h e r by the time a t which the s t a b i l i s e r has been chemically consumed o r t h a t a t which i t has been l o s t from the sample by evaporation o r e x t r a c t i o n . We have developed a model t o describe these a d d i t i v e l o s s processes but i t requires a knowledge o f the d i f f u s i o n r a t e s and s o l u b i l i t i e s of a d d i t i v e s i n polymer (11, 12). The most common method o f measuring d i f f u s i o n i n polymers uses r a d i o l a b e l l e d a d d i t i v e . T y p i c a l l y a t h i n f i l m o f the l a b e l l e d a d d i t i v e i s put on the surface o f the polymer and the increase i n a c t i v i t y a t the opposite surface i s monitored as the a d d i t i v e d i f f u s e s i n t o the polymer. We have used uv microscopy to f o l l o w the d i f f u s i o n o f a d d i t i v e s and o f a t a c t i c polypropylene i n t o polypropylene. Samples are exposed t o a saturated s o l u t i o n o f the a d d i t i v e i n a non-swelling solvent such as g l y c e r o l then

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Figure U. D i s t r i b u t i o n of U v i t e x OB i n samples c r y s t a l l i z e d and annealed at 125 °C. Observed at room temperature.

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a

b

Figure 5· F l u o r e s c e n t l y l a b e l l e d polypropylene c o n t a i n i n g h0% by -weight o f u n l a b e l l e d a t a c t i c polymer, c r y s t a l l i z e d and annealed at ihO C. (a) Fluorescence mode; and (b) v i s i b l e t r a n s m i s s i o n . Dark boundaries i n fluorescence p i c t u r e are a t a c t i c r i c h .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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s e c t i o n e d and the a d d i t i v e p r o f i l e w i t h i n the polymer i s deter­ mined from the uv monitor d i s p l a y . A f t e r c a l i b r a t i o n the d i f f u ­ s i o n r a t e and the s a t u r a t i o n s o l u b i l i t y can be d i r e c t l y measured. This has the advantage over other methods t h a t i t i s f a r f a s t e r since the d i f f u s i o n distance need only be about 50 ym. Thus under e q u i v a l e n t c o n d i t i o n s a d i f f u s i o n c o e f f i c i e n t can be determined i n 2 hours which took 2 days by the t r a c e r method. The microscope requirements are t h a t the i l l u m i n a t i o n must e i t h e r be narrow i n wavelength or correspond roughly with the shape of the absorption maximum of the a d d i t i v e as shown i n Figure 6. Otherwise the Lambert-Beer law may not h o l d . Figure Τ shows a s e c t i o n o f a f i l m which has taken up a d d i t i v e and the corresponding waveform monitor t r a c e . Figure 8 shows the f i t t e d and observed d i f f u s i o n p r o f i l e s f o r such a sample. Table I gives data f o r the d i f f u s i o n o f octoxybenzophenone i n polypropylene compared w i t h the data of other workers. L o c a l i z a t i o n o f Therma Most s t u d i e s o f o x i d a t i o n i n polymers i m p l i c i t l y assume t h a t the process i s homogeneous and l i t t l e or no reference i s made t o p o s s i b l e n o n - u n i f o r m i t i e s . However i n s o l i d polymers there are a number of reasons why o x i d a t i o n may be uneven and the b r i t t l e c r a c k i n g , which i s the main undesirable consequence of degrada­ t i o n , i s l o c a l i s e d by i t s very nature. I t i s known t h a t o x i d a t i o n i s l o c a l i s e d to sample surfaces i n c o n d i t i o n s where the k i n e t i c s are so f a s t t h a t oxygen d i f f u s i o n i s l i m i t i n g (12) o r where the polymer i s i n contact w i t h a c a t a l y s t such as copper (13). A f t e r e l i m i n a t i n g e f f e c t s of t h i s s o r t there i s s t i l l reason t o t h i n k t h a t o x i d a t i o n may be l o c a l i s e d . I t i s w e l l e s t a b l i s h e d t h a t i m p u r i t i e s i n polymers can c a t a l y s e o x i d a t i o n and r e j e c t i o n pro­ cesses of the k i n d described here are expected t o l e a d to concen­ t r a t i o n o f these c a t a l y t i c i m p u r i t i e s i n s p h e r u l i t e boundaries, where they may cause l o c a l i z e d degradation. As f o l l o w e d by oxygen uptake or development o f carbonyl groups, o x i d a t i o n i n p o l y o l e f i n s i s an a u t o a c c e l e r a t i v e process, due t o the a b i l i t y of the hydroperoxides i n i t i a l l y produced t o decompose w i t h the generation of new f r e e r a d i c a l s , a decomposi­ t i o n which i s c a t a l y s e d by both heat and s o l a r uv. The f a c t t h a t i t i s a u t o a c c e l e r a t i n g means t h a t i t may a l s o become l o c a l i s e d . Whether t h i s happens w i l l depend on the branching r a t e of the chain r e a c t i o n and the d i f f u s i o n r a t e of the o x i d a t i o n products w i t h i n the polymer. I f one area of a sample becomes more o x i d i z e d than the surrounding r e g i o n s , o x i d a t i o n can then proceed more r a p i d l y there and the o x i d a t i o n products d i f f u s i n g outwards w i l l spread the "disease" t o the surroundings. There are a number o f pieces of evidence which would support the i d e a t h a t o x i d a t i o n i s l o c a l i s e d . Thermally o x i d i s i n g samples o f t e n do show brown spots of degradation which grow w i t h time. I f samples are o x i d i s e d t o embrittlement the r e s u l t a n t molecular

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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\ \

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Wave Length (nm) F i g u r e 6. A b s o r p t i o n s p e c t r a o f d i f f u s i n g a d d i t i v e s compared w i t h illuminating spectra. ( ), A b s o r p t i o n spectrum; and ( ), i l l u m i n a t i o n spectrum.

F i g u r e 7. UV image o f absorber d i f f u s i n g i n t o a f i l m , w i t h wave­ form monitor t r a c e o f i n t e n s i t y .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Figure 8. Concentration d i s t r i b u t i o n f o r U v i t e x OB d i f f u s e d i n t o polypropylene at 125 °C f o r 20 min. (·), Experimental p o i n t s ; and ( ), f i t t e d curve f o r D = 6.5 ym. s"^-.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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TABLE I D i f f u s i o n C o e f f i c i e n t s o f 2-Hydroxy-4-Octoxybenzophenone (UV 531) i n I s o t a c t i c Polypropylene D, cm

Temperature °C

1

sec

Data o f Johnson and Westlake

Our Data

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b

Cicchetti, et a l .

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30

1.76

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40

4.48

χ 10

44

1.29

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1 0

1.7

50

3.88

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1 0

3.39

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60

1.23

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9

1.02

χ 10"

75

6.47

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9

90

1.98

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8

4.7

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χ 10

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9

-9

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2.99

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Activation Energy 25.09

23.6

20.8

kcal/mole

Measured at 44-75°C

f

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Measured a t 80-110°C, Eur.Poly. J . (1967) 3 , 4 7 3 .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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weight decrease i s not s u f f i c i e n t t o e x p l a i n e i t h e r the b r i t t l e ness o r the f a c t t h a t remoulding r e s u l t s i n the r e g a i n o f much o f the toughness o f the polymer ( i h ) . With t h i s i n mind we a p p l i e d uv microscopy t o search f o r l o c a l i s a t i o n o f o x i d a t i o n i n polypropylene. Methods The carbonyl absorption a t around 280 nm can be used t o see o x i d a t i o n i n polypropylene d i r e c t l y b u t t h i s i s o n l y p o s s i b l e i n h e a v i l y o x i d i s e d samples which are a l s o too b r i t t l e t o be sect i o n e d f o r microscopy. I t i s p o s s i b l e i n t h i s way t o look o n l y a t samples which have been d i r e c t l y c r y s t a l l i s e d from the melt as thin films. In order t o work with samples with more normal o x i d a t i o n l e v e l s we must s t a i n the oxygen c o n t a i n i n g groups t o make them uv absorbing. In p r i n c i p l e i t should be p o s s i b l e t o separ a t e l y s t a i n carbonyl o on 2 , 4 - d i n i t r o p h e n y l h y d r a z i n groups. Dansyl hydrazine was a l s o t r i e d as a f l u o r e s c e n t s t a i n but was l i m i t e d i n i t s a b i l i t y t o penetrate the sample so t h a t only the surface was s t a i n e d . The DNPH s t a i n i n g was done by immersing 10 ym s e c t i o n s i n 1% DNPH i n i s o p r o p a n o l - 5 % cone HC1 a t 6 0 ° C f o r 24 hours ( 1 5 ) . The s e c t i o n was then e x t r a c t e d with f r e s h i s o p r o p a n o l a t 6 0 ° C f o r 24 hours t o remove excess DNPH. I r and uv spectroscopy o f t h i s s t a i n i n g r e a c t i o n i n 100 ym f i l m s showed t h a t there was no f u r t h e r hydrazone formation a f t e r t h i s time with about 66% o f the carbonyl groups having r e a c t e d with DNPH. The remainder are probably e s t e r s and a c i d s . During the r e a c t i o n the hydroperoxides appear to be converted t o carbonyl groups. Quite a number o f other species are present and s i d e r e a c t i o n s do occur. However the most important l i m i t a t i o n o f t h i s method i s t h a t a t a c t i c and low molec u l a r weight s p e c i e s are e x t r a c t e d from the f i l m by t h e i s o p r o p a nol. Up t o 50% o f the c a r b o n y l content i s removed from h e a v i l y o x i d i s e d polypropylene by prolonged e x t r a c t i o n with isopropanol at 6 0 ° C . Methanol i s a b e t t e r s o l v e n t i n t h i s r e s p e c t but i s l e s s s a t i s f a c t o r y f o r the s t a i n i n g r e a c t i o n due t o more r a p i d a c e t a l o r k e t a l formation. No s t a i n i n g c o u l d be seen from aqueous a c i d s o l u t i o n s o f DNPH. The n e t e f f e c t o f the s t a i n i n g process i s t h a t the e x t i n c t i o n c o e f f i c i e n t a t 352 nm o f s t a i n e d o x i d i s e d p o l y propylene i s 2 0 - f o l d greater than t h a t a t 280 nm f o r the unstained m a t e r i a l , a 2 0 - f o l d s e n s i t i v i t y gain. Oxidation can be observed at about 1/4 o f the i n d u c t i o n time i n u n s t a b i l i s e d m a t e r i a l . In our samples the i n d u c t i o n times f o r thermal o x i d a t i o n were about 4 hours a t 120°C o r 12 hours a t 1 0 0 ° C . Observations From our s t u d i e s o f a d d i t i v e r e j e c t i o n we b e l i e v e d t h a t s i m i l a r r e j e c t i o n o f i m p u r i t i e s , i n c l u d i n g o x i d a t i o n products, during

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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c r y s t a l l i s a t i o n would l e a d t o high subsequent o x i d a t i o n r a t e s a t s p h e r u l i t e boundaries* S t a i n i n g o f samples which were c r y s t a l l i s e d and then thermally o x i d i s e d revealed l o c a l i s e d centres o f o x i d a t i o n but ones which bore no r e l a t i o n s h i p t o the s p h e r u l i t e morphology (Figure 9 ) . W i t h i n no s p h e r u l i t e was t h e r e any gradient i n UV absorption between the center and t h e boundary. I f these o x i d i z e d s e c t i o n s were f l e x e d t h e cracks r a n apparently at random. S i m i l a r samples which were c r y s t a l l i s e d and o x i d i s e d as t h i n f i l m s tended t o crack along s p h e r u l i t e r a d i i . I f the sample was f i r s t o x i d i s e d t o about the i n d u c t i o n time and then c r y s t a l l i s e d there was again no evident r e l a t i o n s h i p between o x i d a t i o n and morphology. However the s t a i n i n g process had c l e a r l y e x t r a c t e d m a t e r i a l from the s p h e r u l i t e boundaries l e a v i n g them as channels i n the f i l m . I f the unstained s e c t i o n s were f l e x e d they f r e q u e n t l y cracked along the s p h e r u l i t e boundaries. We conclude from t h i s t h a t c r y s t a l l i s a t i o n o f p r e - o x i d i s e d polymer does l e a d t o segregatio s o l u b l e m a t e r i a l t o th At low l e v e l s o f o x i d a t i o n where the isopropanol e x t r a c t a b l e mat e r i a l i s only a small f r a c t i o n o f the t o t a l carbonyl content, t h i s segregation does not l e a d t o increased o x i d a t i o n r a t e s l o c a l l y . At higher o x i d a t i o n l e v e l s much o f the o x i d i s e d m a t e r i a l i s e x t r a c t e d so t h a t we cannot f o l l o w the process. B i l l i n g h a m and Manke (16) have i n v e s t i g a t e d the e f f e c t s o f o x i d a t i o n on the i s o t a c t i c and a t a c t i c components o f polypropylene by e x t r a c t i o n , GPC and s t a i n i n g . They found t h a t a t low l e v e l s o f o x i d a t i o n the 2-3% o f polymer which was e x t r a c t e d by b o i l i n g heptane was r e s p o n s i b l e f o r 30% o f the o x i d a t i o n as measured by carbonyl content. This d i f f e r e n c e would not be expected on the b a s i s o f the stereochemical d i f f e r e n c e between a t a c t i c and i s o t a c t i c polymer o r on the b a s i s o f the ready access o f oxygen t o the amorphous regions c o n t a i n i n g the a t a c t i c m a t e r i a l . I t i s most l i k e l y t o stem from a greater d e n s i t y o f chain defects such as unsaturation i n the a t a c t i c polymer. Although there i s no strong morphological e f f e c t on o x i d a t i o n k i n e t i c s there i s a high degree o f l o c a l i s a t i o n o f o x i d a t i o n as seen i n Figure 9· I n s e c t i o n s across compression molded f i l m s t h i s i s seen as a "wood-grain" p a r a l l e l t o the s u r f a c e . By studyi n g f i l m s made by compression moulding under very low pressures i t can be seen t h a t t h i s e f f e c t a r i s e s from v a r i a t i o n s between the o r i g i n a l granules o f powdered polymer, f i g u r e (10), such t h a t a t low o x i d a t i o n l e v e l s a few granules are h e a v i l y o x i d i s e d while the r e s t are unaffected. Those p a r t i c l e s which were h e a v i l y o x i d i s e d a l s o contained c l u s t e r s o f sub-microscopic dots observable i n v i s i b l e l i g h t microscopy. S i m i l a r c l u s t e r s o f dots could a l s o be seen i n some i n d i v i d u a l powder p a r t i c l e s when molten a t 170OC, suggesting t h a t the dots are i n f u s i b l e p a r t i c l e s r a t h e r than voids or bubbles. Up t o 10% o f p a r t i c l e s o f unprocessed commercial diluent-phase polypropylene contained these dots. Scanning e l e c t r o n microscopy w i t h EDAX, c a r r i e d out by Mr A.Cobbold o f I C I

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Figure 9· S e c t i o n o f polypropylene viewed a t 350 nm a f t e r o x i d a t i o n and s t a i n i n g with DNPH. Bar i s 50 ym.

Figure 10. S e c t i o n o f l i g h t l y pressed polypropylene partially o x i d i z e d and s t a i n e d w i t h DNPH. Viewed at 350 nm. Bar i s 100 ym.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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i d e n t i f i e d these dots as p a r t i c l e s o f l e s s than 0.5 ym c o n t a i n i n g T i , A l and CI. Thus they are almost c e r t a i n l y c a t a l y s t r e s i d u e s . D i f f e r e n t diluent-phase polypropylenes showed s i m i l a r behaviour but the p r o p o r t i o n o f a f f e c t e d powder p a r t i c l e s v a r i e s . C l e a r l y these l o c a l v a r i a t i o n s would tend t o disappear i n the processing of the polymer t o produce s t a b i l i s e d p e l l e t s . V a r i a t i o n s i n o x i d a t i o n r a t e s of i n d i v i d u a l p a r t i c l e s were a l s o found w i t h gas-phase polymerized propylenes (Figure l l ) , i n t h i s case the c a t a l y s t residues could not be seen. Figure 12 shows a s t a i n e d s e c t i o n o f an o x i d i z e d p e l l e t o f gas-phase polypropylene cut normal t o the e x t r u s i o n d i r e c t i o n . The o x i d a t i o n appears t o be spreading out from centres of high o x i d a t i o n i n t o the r e l a t i v e l y unoxidised surroundings. This suggests t h a t o x i d a t i o n i s enhanced by the d i f f u s i o n of r e a c t i o n intermediates from h e a v i l y o x i d i s e d regions, analogous t o a bad apple spreading r o t through a b a r r e l . Most of our s t u d i e photo-oxidation s i m i l a r e l a t e d t o the s p h e r u l i t i c morphology. The c a t a l y s t residue e f f e c t was not s p e c i f i c a l l y i n v e s t i g a t e d but i s expected t o be the same. Inhomogeneities i n Thermosetting Resins Most thermosetting r e s i n s have a h i g h l y exothermic c u r i n g r e a c t i o n which i s consequently a u t o a c c e l e r a t i v e . I t seems p o s s i b l e t h a t l o c a l hot-spots, i n c u r i n g more r a p i d l y than the surroundings, c o u l d deplete the region o f hardener such t h a t the f i n a l f u l l y - c u r e d r e s i n would vary l o c a l l y i n degree of c r o s s l i n k i n g . Accordingly we set out t o apply uv microscopy t o search f o r t h i s and other sources of inhomogeneity i n epoxy r e s i n s . There are a number of p o s s i b l e approaches. The simplest i s t o use a d e n s i t y marker, a compound which does not r e a c t w i t h the r e s i n but w i l l be d i s t r i b u t e d i n such a way as t o r e f l e c t i t s l o c a l s o l u b i l i t y w i t h i n the s t r u c t u r e . This s o l u b i l i t y should be higher i n the l e s s c r o s s - l i n k e d regions. The marker can e i t h e r be d i f f u s e d i n t o the cured r e s i n or blended i n before c u r i n g . A second approach i s to use a r e a c t i v e s t a i n which attaches t o unreacted r e s i n or c a t a l y s t remaining a f t e r c u r i n g . In view o f the i n t e r e s t i n water d i f f u s i o n i n r e s i n s i t would a l s o be v a l u a b l e t o have a reagent which could mark the ingress of water. Methods The r e s i n system was DGEBA (Epikote 828) cured w i t h a molar e q u i v a l e n t of t r i e t h y l e n e t e t r a m i n e (TETA). As non-reactive s t a i n s we used a v a r i e t y of uv 'absorbing and f l u o r e s c i n g compounds i n c l u d i n g the benzophenones and o p t i c a l b r i g h t e n e r s used i n our experiments on polypropylene. The most s u c c e s s f u l r e a c t i v e s t a i n was d i n i t r o f l u o r o b e n z e n e (DNFB, Sanger's)

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Figure 11. Sample as i n Figure 10 except t h a t the polymer was from a gas-phase process. Bar i s 100 μ m.

Figure 12. S e c t i o n from an extruded p e l l e t o f gas-phase p o l y p r o ­ pylene a f t e r o x i d a t i o n and s t a i n i n g . Bar i s 100 urn.

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reagent) which i s e x t e n s i v e l y used i n p r o t e i n chemistry. T h i s r e a c t s w i t h primary and secondary amines (17) and so s t a i n s unreacted groups on ΤΕΤA. The main problem i s t o o b t a i n good s e c t i o n s o f f u l l y cured r e s i n . The epoxy can be microtomed e a s i l y when p a r t i a l l y cured but a f t e r f u l l c u r i n g i t becomes too b r i t t l e . A c c o r d i n g l y t o study cured samples the r e s i n was allowed t o p a r t i a l l y cure a t room temperature, s e c t i o n e d and the s e c t i o n s cured a t 1200C f o r 2 hours mounted i n a s p e c i a l h o l d e r t o keep them f l a t . The DNFB s t a i n i n g was performed by immersing the s e c t i o n s i n a 2% s o l u t i o n o f DNFB i n b o i l i n g ethanol f o r two hours. The unreacted s t a i n was removed by b o i l i n g the s e c t i o n i n dimethylformamide f o r 48 hours. Observations Non-reactive s t a i n no s i g n o f l o c a l c o n c e n t r a t i o the epoxy r e s i n before c u r i n g these s t a i n s d i d tend t o form s t r e a k s and t o concentrate i n haloes around bubbles which form during c u r i n g . The r e a c t i v e s t a i n , DNFB, showed l a r g e bands o f unreacted amine t o be present i n the cured r e s i n , F i g u r e 13. The v i s i b l e l i g h t micrographs show l i t t l e c o n t r a s t except f o r d i f f r a c t i o n e f f e c t s around bubbles and f a i n t p a r a l l e l l i n e s due t o corruga­ t i o n s i n the s e c t i o n s . These samples were mixed thoroughly by hand s t i r r i n g the amine i n t o the r e s i n w i t h a g l a s s rod f o r 5 minutes. Similar zones o f unreacted amine were found when the mixing was c a r r i e d out at 60oc. i n order t o achieve u n i f o r m i t y i t was necessary t o mix the components i n a h i g h shear m i x e r - e m u l s i f i e r f o r 15 minutes or t o blend them as a 50% s o l u t i o n i n dichloromethane which was then evaporated. In these cases the s e c t i o n s were uniform except f o r a h a l o o f s t a i n e d amine around the bubbles. The a b s o r p t i o n i n t e n s i t i e s i n these s e c t i o n s were too great t o permit determina­ t i o n o f the c o n c e n t r a t i o n o f unreacted amine but we expect t o o b t a i n q u a n t i t a t i v e i n f o r m a t i o n by u s i n g narrow band i l l u m i n a t i o n i n the blue r e g i o n where the a b s o r p t i o n i s t a i l i n g o f f . We have a l s o observed low l e v e l s o f cure c l o s e t o the surface of r e s i n cured i n 1 cm tubes i n a water bath at 60°C. T h i s r e f l e c t s the f a c t t h a t the perimeter remains at 60°C w h i l e the centre i s heated and f u r t h e r cured by the exotherm. P o s t - c u r i n g removes t h i s d i f f e r e n c e . Figure ik shows the d i f f u s i o n o f d i n i t r o p h e n o l i n aqueous s o l u t i o n i n t o r e s i n f i l l e d w i t h 5% s h o r t g l a s s f i b r e s . The s t a i n c l e a r l y f o l l o w s the water up the f i b r e - r e s i n i n t e r f a c e . Thus well-mixed, f u l l y cured epoxy r e s i n s appear uniform on a s c a l e o f 1 ym upwards. However, very vigorous mixing o f the com­ ponents i s needed t o achieve t h i s u n i f o r m i t y and i n many normal circumstances cured epoxy r e s i n s w i l l be inhomogeneous.

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Figure 13. UV p i c t u r e o f epoxy r e s i n cured w i t h TETA hardener at 50 °C. Dark s t r e a k s are DNFB s t a i n i n g o f unreacted amine groups. Bar i s 100 ym.

Figure ik. UV p i c t u r e o f cured TETA/epoxy c o n t a i n i n g a s m a l l number o f glass f i b e r s . Sample was immersed i n an aqueous s o l u t i o n o f d i n i t r o p h e n o l f o r 2k h r s . before examination. Bar i s 300 ym.

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Conclusions Uv microscopy has wide a p p l i c a b i l i t y t o the study o f the degradation and r e a c t i o n s o f s o l i d polymers. However each new use does r e q u i r e a s u b s t a n t i a l e f f o r t f o r the development o f s u i t a b l e techniques. There are p i t f a l l s i n i n t e r p r e t i n g the r e s u l t s and i t i s p r e f e r a b l e t o combine uv, v i s i b l e and f l u o r e s cent microscopy t o a v o i d m i s i n t e r p r e t a t i o n s . Acknowle dgment s We wish t o acknowledge the work o f past and present members o f our research group i n developing the techniques d e s c r i b e d here and f o r a l l o w i n g us t o quote t h e i r unpublished r e s u l t s . Thanks are due p a r t i c u l a r l y t o T.G. Ryan, J.B. Knight and A. Uzuner. We a l s o thank the Science Research C o u n c i l f o r the award o f a grant to allow the purchase o Mr A. Cobbold o f ICI Petrochemical t h e i r help with e l e c t r o n microscopy.

Literature Cited 1. Kohler A.Z.; Wiss Microskopie, (1904), 21, 129, 275. 2. Freed, J.J.; in 'Physical Techniques in Biological Research' 2nd Ed, Vol.IIIc, Ed. A.W. Pollister, Acad Press, New York (1969). 3. Wood, J.R.; Goring, D.A.I, J. Microscopy (1979) 100, 105. 4. Kam, Z . ; Shore H.B., Feher G, J. Mol. Biol (1978), 123, 539. 5. Forty, A . J . ; and Woodruff, D.P.; Tech Metals Res, (1968), 2 97. 6. Billingham, N.C. and Calvert, P.D.; Dev Polymer Character, (1982) 3 Ch.6. 7. Curson, A.D.; Proc. Roy. Microsc. Soc., (1972), 7, 96. 8. Frank, H.P. and Lehner, H.; J. Polymer Sci. Symp. (1970), 31, 193. 9. Calvert, P.D. and Ryan, T.G.; Polymer, (1978), 19, 611. 10. Calvert, P.D. and Ryan, T.G.; Polymer, (1982), 23, 877. 11. Calvert, P.D. and Billingham, N.C.; J. Appl. Polymer Sci, (1979), 24, 357. 12. Billingham, N.C.; and Calvert, P.D.; Dev. Polymer Degradation, (1980), 3 Ch.5. 13. Allara, D.L., White, C.W, A.C.S. Adv Chem. Ser, (1978), 169, 273. 14. Adams, J.H.; J. Polymer Sci. Chem, (1970), 8, 1077. 15. Kato, K . J . ; Appl. Polymer Sci, (1974) 18, 2449, 3087; (1975) 19, 951; (1976) 20, 2451; (1977) 21, 2735. 16. Manke, A.S.; D.Phil Thesis, University of Sussex, 1981. 17. McIntire, F.C.; Clements; L.M, Sproull M, Anal Chem, (1953), 25, 1757. RECEIVED November 22, 1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

17 Novel Diagnostic Techniques for Early Detection of Photooxidation in Polymers RANTY H. LIANG, DANIEL R. COULTER, CATHY DAO, and A M I T A V A GUPTA California Institute of Technology, Jet Propulsion Laboratory, Pasadena,CA91109

A Laser Photoacoustic Technique (LPAT) has been developed t oxidation in Ethylen Methylacrylat copolyme (ΕΜΑ). LPAT has been used to demonstrate that the Controlled Environmental Reactor (CER), an acceler­ ated testing chamber that was developed at JPL, is a valid accelerated simulator of the real-time outdoor photooxidation with respect to the rate of formation of the hydroxyl functional group. Polymers for solar energy applications have to meet rigorous goals in terms of material and fabrication costs. In addition, solar applications require that these low-cost polymers meet stringent performance criteria over long periods (> 20 years) of outdoor deployment. Therefore, development of reliable models for life prediction of polymeric components is essential, if projec­ tions of life cycle cost of solar energy conversion devices are to be meaningful. These models are based on understanding of degra­ dation mechanisms of the materials. Validation of the models, however, is often difficult and time consuming, and as a result, has been attempted almost exclusively on results of accelerated tests of material specimens. In order for accelerated test data to be valid it must be derived from experiments in which test con­ ditions preserve the basic mechanisms of degradation. However, development of valid accelerated test methodology is by far the most controversial aspect in validating theoretical models. Ideally the best way to validate accelerated testing metho­ dology would be to compare accelerated data with those obtained under real-time outdoor exposure. Typical real-time outdoor test­ ing data, however, show that observable changes in material pro­ perties (degradations) have induction periods, thus making valida­ tion of accelerated test procedures by this means a very time con­ suming process. Furthermore, primary degradation mechanisms, which may, in general, be elucidated only during the very early stages of aging, may be masked as a result of the inability to de­ tect degradation during the induction period. 0097-6156/83/0220-0265$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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At JPL, a t h r e e - f o l d approach i s taken i n developing l i f e time p r e d i c t i o n models of polymeric m a t e r i a l s . I t i n v o l v e s : 1)

Determination als.

2)

Development of a c c e l e r a t e d t e s t i n g hardwares and procedures which d u p l i c a t e degradation observed under outdoor conditions.

3)

Development of s e n s i t i v e d i a g n o s t i c techniques which can be used to detect e a r l y r e a l - t i m e outdoor degradation.

of degradation mechanisms of polymeric m a t e r i -

Recently, we have demonstrated that the Laser Photoacoustic Technique (LPAT) can be used to monitor i n c i p i e n t photooxidation i n polymers aged outdoors We have a l s o used t h i s technique to show that the C o n t r o l l e ated t e s t i n g chamber tha v a l i d a c c e l e r a t e d simulator of the r e a l - t i m e outdoor photooxidat i o n with respect to the rate of formation of the hydroxyl funct i o n a l group. In L P A T ( l ) , a sample i s placed i n s i d e a s p e c i a l l y designed c e l l containing a s e n s i t i v e microphone. The sample i s then i l l u minated with chopped l a s e r r a i d a t i o n as i l l u s t r a t e d i n Figure 1. L i g h t absorbed by the sample i s converted i n part i n t o heat by n o n - r a d i a t i v e d e - e x c i t a t i o n processes w i t h i n the sample. The r e s u l t i n g p e r i o d i c heat flow from the sample to the surroundings creates pressure f l u c t u a t i o n s i n the c e l l . These pressure f l u c t u a t i o n s are then detected by the microphone as a s i g n a l which i s phase coherent at the chopping frequency. The magnitude of the r e s u l t i n g photoacoustic s i g n a l i s d i r e c t l y r e l a t e d to the amount of l i g h t absorbed by the sample. Since only the absorbed l i g h t i s converted to sound, l i g h t s c a t t e r i n g , which i s a very serious problem when d e a l i n g with many s o l i d m a t e r i a l s by conventional spectroscopy, presents no d i f f i c u l t i e s i n LPAT. Moreover, LPAT i s extremely s e n s i t i v e i n d e t e c t i n g small amounts of absorption as compared to conventional absorption spectroscopy. For instance, Tarn and P a t e l i ) demonstrated that l a s e r photoacoustic spectroscopy can be used to a c c u r a t e l y measure extremely weak absorption of water i n the v i s i b l e region, and we have obtained s e n s i t i v i t y improvement of 2 orders of magnitude by LPAT as compared to FT-IR f o r small absorbances i n polymers. 2

The C o n t r o l l e d Environmental Reactor (CER) was developed because s e v e r a l key design requirements which c o n t r o l the closeness of c o r r e l a t i o n of a c c e l e r a t e d t e s t data with data obtained i n f i e l d are not met by a v a i l a b l e commercial weatherometers. This may e x p l a i n why c o r r e l a t i o n between weatherometer and f i e l d data i s o f t e n so poor. D e t a i l s of design and o p e r a t i o n a l procedures of the CER have been reported before(^) i w i l l be described only b r i e f l y i n t h i s paper. a

n

(

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OPTICAL WINDOW

SAMPLE

1

MODULATED LASER LIGHT

Ρ

il —1 M I M = MICROPHONE

LOCK - IN AMPLIFIER Figure 1.

Schematic diagram o f photoacoustic set up.

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Experimental 1)

C o n t r o l l e d Environmental Reactor (CER)

A 550 watt Conrad/Hanovia medium pressure A.C. mercury lamp surrounded by a 1 cm t h i c k pyrex water j a c k e t , f o r c o o l i n g and i n ­ f r a r e d absorption, provided the i r r a d i a n c e , w h i l e an e l e c t r i c a l heater and fan system was used to adjust the sample temperature. The lamp was allowed to operate from i t s standard power supply w i t h no attempt at r e g u l a t i o n of the i r r a d i a n c e . Lamp voltage and current were the only parameters monitored to c h a r a c t e r i z e i t s performance. The exposure r e g i o n was a perforated aluminum c y l i n ­ der 34 cm i n diameter and 23 cm high. A 5 cm t h i c k thermal blanket surrounded the assembly and a l ­ lowed c o n t r o l of the temperature of a t y p i c a l sample between 30 to 60°C. A water nozzle wa A s t r i p chart s i n g l e channel recorder w i t h an adjustable set p o i n t , using voltage from one of the copper constantan thermo­ couples i n s t a l l e d on a t e s t module was used as an on-off tempera­ ture c o n t r o l l e r . Gross adjustment of the temperature was achieved by blocking the fan i n l e t and a i r o u t l e t . Fine tuning was accom­ p l i s h e d by a d j u s t i n g the heater c u r r e n t . A c y c l i c a l v a r i a t i o n of ± 1°C was achieved using the various adjustments. 2)

Actinometry

The 200 to 400 nm s p e c t r a l i r r a d i a n c e i n s i d e the CER was measured using a Gamma S c i e n t i f i c Spectroradiometer. A selenium p h o t o v o l t a i c c e l l and Corning 7-45 u l t r a v i o l e t f i l t e r was used to monitor the UV i r r a d i a n c e . A 1 χ 2 cm s i l i c o n s o l a r c e l l was used to measure the near-IR i r r a d i a n c e . CER photon f l u x was a l s o c a l i ­ brated by using 0-nitrobenzaldehyde (0-NBA) as an a c t i n o m e t e r ( ^ ) . Outdoor photon f l u x was measured by d i s p e r s i n g 0-NBA i n t h i n f i l m s (25 μπι) of polymethyl methacrylate. These f i l m s were then exposed at the outdoor s i t e behind a n e u t r a l density f i l t e r and were ex­ amined on a weekly b a s i s . Outdoor weekly UV photon f l u x was c a l ­ c u l a t e d based on the conversion rate of the 0-NBA. 3)

Sample P r e p a r a t i o n

Samples of E t h y l e n e / M e t h y l a c r y l a t e copolymer (EMA) were ob­ t a i n e d from Gulf O i l Company (TD938) and r e p r e c i p i t a t e d from hot cyclohexane. The p u r i f i e d samples were then compression molded i n t o t h i n f i l m s (25-50 μπι). FT-IR s p e c t r a were recorded on a D i g i l a b FT-IR Spectrophotometer Model FTS 15. 4)

Laser Photoacoustic Technique (LPAT)

LPAT was c a r r i e d out by using a hydrogen f l u o r i d e (HF) che­ m i c a l l a s e r operating at 2.83 μπι as the e x c i t a t i o n source and a

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condensor microphone as the d e t e c t o r . Calcium f l o r i d e which does not absorb 2.83 μιη l i g h t was used as the window m a t e r i a l . Samples of EMA were aged i n s i d e the CER as w e l l as under r e a l - t i m e outdoor exposure at Pasadena, CA. Photoacoustic s i g n a l s from the aged EMA f i l m s were compared w i t h those obtained from a c o n t r o l EMA f i l m . The d i f f e r e n c e i n the microphone s i g n a l s i s assumed to be due to the formation of hydroxyl groups as a r e s u l t of aging. Since photooxidation r e q u i r e s the access of oxygen, formation of the m a j o r i t y of hydroxyl group i s expected to take place on the polymer surface and thus polymer f i l m thickness was not taken i n t o c o n s i d e r a t i o n i n c a l c u l a t i n g hydroxyl c o n c e n t r a t i o n . However, the background photoacoustic s i g n a l between c o n t r o l and aged EMA f i l m s which a r i s e s from absorption throughout the bulk was c a l i ­ brated with respect to t h e i r t h i c k n e s s . C a l i b r a t i o n was a l s o c a r ­ r i e d out to c o r r e l a t e the microphone s i g n a l to the change i n ab­ sorbance. L i n e a r i t y of the amount of l i g h t absorbed was a l s o v e r i f i e d i n a separate e x p e r i ­ ment i n which a CO2 l a s e r operating at 10.6 μπι was used as the ex­ c i t a t i o n source to i l l u m i n a t e t h i n f i l m s of PMMA. Figure 2 and 3 i l l u s t r a t e that the photoacoustic s i g n a l increases l i n e a r l y w i t h respect to both l a s e r power and percentage of l i g h t aborbed up to - 300 m i l l i w a t t s . R e s u l t s and

Discussions

EMA l i k e many other polymers, undergoes photooxidation when i t i s exposed to s o l a r u l t r a - v i o l e t l i g h t i n the presence of oxy­ g e n i c ) . This i s evidenced by the formation of hydroxyl and hydrop e r o x y l groups as detected by FT-IR. Figure 4 i l l u s t r a t e s the FTIR absorption spectra of EMA before and a f t e r 200 hours of a c c e l ­ erated aging i n s i d e the CER. An increase i n absorbance at 3530 cm~l i s unmistakable. F i g u r e 5 shows the FT-IR absorption s i g n a l of EMA at 3530 cm~l as a f u n c t i o n of a c c e l e r a t e d aging time i n s i d e the CER. Equivalent outdoor aging time c a l i b r a t e d by actinometry i s a l s o shown i n Figure 5. The f a c t that a f t e r 10 hours of a c c e l ­ erated aging (equivalent to 65 days of outdoor exposure), the -OH and -00H absorption peaks b a r e l y begin to appear i n the FT-IR spectrum leads to the c o n c l u s i o n that techniques other than FT-IR are needed, i n order to monitor e a r l y photooxidation. Figure 3 i s a p l o t of photoacoustic s i g n a l from LPAT as a f u n c t i o n of percent of l i g h t absorbed. The slope of the p l o t y i e l d s the s e n s i t i v i t y of LPAT which i s 2.5 χ 10" % absorption/ μν. This can be t r a n s l a t e d i n t o a d e t e c t i o n l i m i t of 10~5 absorbance/μν, at l e a s t 2 orders of magnitude b e t t e r than FT-IR. 3

Figure 6 i s a l o g - l o g p l o t of hydroxyl concentration i n EMA as a f u n c t i o n of a c c e l e r a t e d and r e a l - t i m e aging. Both FT-IR and LPAT data are i l l u s t r a t e d i n Figure 6. Whereas FT-IR could not detect any hydroxyl or hydroperoxyl formation w i t h l e s s than 10

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25 h

LASER POWER (watt) OPTICAL DENSITY OF PMMA AT 1 0 . 6 μ » 0.3

Figure 2. Photoacoustic s i g n a l o f PMMA. as a f u n c t i o n o f C 0 l a s e r power.

30

% ABSORPTION C0 LASER POWER IS KEPT CONSTANT AT 500 mw 2

lftv - 2.5 χ 10" % OF ABSORPTION 3

Figure 3. Photoacoustic s i g n a l o f PMMA as a f u n c t i o n o f percentage a b s o r p t i o n .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

17.

LIANG ET A L .

Diagnostic Techniques of Photooxidation

I I II

M i l l

Ί — ι — r CONTROL

350 h

400036003200280024002000180016001400 12001000 800 600 400 WAVE NUMBER

I 1 I 1 IM i l l

1

1

1

350 300 8" 250 χ

1

1

1 1

AFTER 200 hrs OF ACCELERATED AGING

y 2oo

Iο 150 œ .00

ι JL

ι ι ι ι I 1 111 1 1 4000 360032002800 2400 200018001600140012001000 800 600 400 1 1111 1 WAVE NUMBER

F i g u r e k.

FTIR s p e c t r a o f e t h y l e n e methyl a c r y l a t e (EMA)

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

271

272

POLYMERS

1.0

I

0.5 0.4 0.3 0.2

~

IN SOLAR

I

I

I

ENERGY

UTILIZATION

I I

• FTIR SIGNAL + PHOTOACOUSTIC SIGNAL (ACCELERATED TESTING)

*

© PHOTOACOUSTIC SIGNAL (REAL TIME TESTING)

/ /

0.1

- 2000 _

0.05 < Ο

Σ7;

> 1000 ^

Ε °

- 500

°0.02

OU

fcT-K

85% PMMA BLEND 1700 hrs/CONTROL

0.5

280

Figure

1.

1020 hrs/CONTROL

300

320

340 nm

360

A b s o r b a n c e o f Copolymer I F i l m s Irradiation Period. a) A b s o r b a n c e Data Recorded on b) A b s o r b a n c e Data Recorded on o f Aged F i l m s ; S\ = I^K/^e00

380

400

as a F u n c t i o n Films. Solutions ( text). s e e

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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W-Screening Transparent Acrylic Copolymers

297

CONTROL SAMPLE

4000

3000

2000

1200

WAVENUMBERS F i g u r e 2.

800

FT-IR S p e c t r o s c o p i c A n a l y s i s F i l m s of t h e Copolymer I. a) b)

800 700 600

3000

4000 3600 3400

b

3 o f Aged and

Control

T r a n s m i s s i o n FT-IR S p e c t r a , ATR FT-IR D i f f e r e n c e S p e c t r a .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

298

POLYMERS IN SOLAR ENERGY UTILIZATION

200 TIME, hr

Figure

3.

300

IR A b s o r b a n c e I n c r e a s e a t 3580 c m - i a s a F u n c t i o n Time on Copolymer F i l m s from ATR FT-IR S p e c t r a l Data.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

of

19.

GUPTA ET AL.

Figure

4.

UV'-Screening Transparent Acrylic Copolymers

299

ATR FT-IR D i f f e r e n c e S p e c t r a f o r F i l m s of B l e n d s of t h e Copolymer ( I ) and PMMA ( 1 5 : 8 5 by W e i g h t ) ; C o n t r o l i s a F i l m o f t h e B l e n d M a i n t a i n e d i n Dark.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

300

POLYMERS IN SOLAR ENERGY UTILIZATION

- —

BINDING ENERGY (eV)

SAMPLE: PURE COPOLYMER (I)

—

BINDING ENERGY (eV)

SAMPLE: BLEND OF I AND PMMA (15:85)

Figure

5.

ESCA

Data

on F i l m s

of t h e Copolymer

(I)

and t h e

Blend.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

19.

GUPTA ET AL.

UV-Screening Transparent Acrylic Copolymers

301

d ρ Y s = Y s + YS

d Ys =

when

(TÏ

(W /2)l a

D

=

(TÏ

(Yi

b 1/2 (Yi ).

). 1

(r,

)

b (Yi

k

Here s u b s c r i p t i d e n o t e s

Table

I.

D2

(W /2)k a

where

)l/2

1 1/2

H2O and s u b s c r i p t k d e n o t e s PPG.

C a l c u l a t e d Surface Tension Irradiation Period.

PERIOD OF IRRADIATION (hr)

V a l u e s as a F u n c t i o n

WATER (/ • 72.8 dynes/cm ) L

of

PG-E-200 (η_ 43.5 dynes/cm ) 3

EXPOSED SIDE

DARK SIDE

EXPOSED SIDE

DARK SIDE

67.2

68.2

36.0

35.6

93.5

69.3

69.5

38.9

278

90.4

66.7

70.3

35.5

419.5

89.2

72.2

69.4

35.0

The o b s e r v e d p h o t o o x i d a t i v e c r o s s l i n k i n g p r o c e s s was j u d g e d t o be a c o n s e q u e n c e of i n t r o d u c t i o n o f t e r t i a r y hydrogen atoms on c o p o l y m e r ! z a t i o n of v i n y l d e r i v a t i o n s of u l t r a v i o l e t a b s o r b i n g chromophores. Hence, a propenyl d e r i v a t i v e of the 2h y d r o x y l - p h e n y l b e n z o t r i a z o l e n u c l e u s was s y n t h e s i z e d , as shown i n Scheme 2. D e t a i l s o f t h e s y n t h e s i s o f t h i s compound w i l l be r e ­ ported, subsequently. The a b s o r p t i o n s p e c t r u m o f t h e p r o p e n y l d e ­ rivative*. P h o t o d e g r a d a t i o n r a t e measurements on t h i s m a t e r i a l are in progress. Copolymer w i t h methyl

methacrylate

i s shown i n F i g u r e 6.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

302

POLYMERS IN SOLAR ENERGY UTILIZATION

OH

ce

OR

C=0 I CH

OH

CH.-C-OH 3 ι CH

3

CH - C = C H 3 ^ ?

3

la, b R,=H,Ac

I la,b R = H,Ac

Scheme

11 la,b R = H, C H

2

4.00

3.00 LU

1

_

o

2

g

0

3

2.001-

1.00

o 300 350 WAVELENGTH (nm)

Figure

6.

A b s o r p t i o n Spectrum o f 2 [ ( 2 - h y d r o x y 5-propenyl) p h e n y l ] 2 H - b e n z o t r i a z o l e i n Methylene C h l o r i d e at 30°C.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

19.

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JJV-Screening Transparent Acrylic Copolymers

303

The a b s o r p t i o n s p e c t r a of a model compound has been r e p o r t e d i n s e v e r a l d i f f e r e n t s o l v e n t s a t room t e m p e r a t u r e s and a l s o as a f u n c t i o n of t e m p e r a t u r e down t o U K . These s p e c t r o s c o p i c m e a s u r e ments i n d i c a t e t h a t t h e r e i s an e q u i l i b r i u m between two or more c o n f o r m e r s i n t h e ground s t a t e . Two c o n f o r m e r s a b s o r b i n g a t 302 nm and 340 nm may be s t a b i l i z e d by a c o m b i n a t i o n o f i n t r a - and i n t e r m o l e c u l a r hydrogen b o n d i n g , as shown i n F i g u r e 7 a . Preliminary C - 1 3 nmr s p e c t r a l d a t a i n d i c a t e t h a t t h e degree of a r o m a t i c i t y i s q u i t e s o l v e n t dependent. The complex d i s t r i b u t i o n of chromophore m o l e c u l e s i n t h e ground s t a t e make i t d i f f i c u l t t o propose a s t r a i g h t f o r w a r d i n t e r p r e t a t i o n o f e m i s s i o n and e x c i t e d s t a t e decay d a t a o b t a i n e d from ground s t a t e a b s o r b a n c e r e c o v e r y r a t e and f l u o r e s c e n c e decay r a t e m e a s u r e m e n t s . Some o f t h e s e measurements w i l l be r e p o r t e d . The mechanism o f p h o t o d e g r a d a t i o n o f t h e c o p o l y m e r i s p r e sumed t o i n v o l v e an e l e c t r o n i c energy t r a n s f e r p r o c e s s from t h e b e n z o t r i a z o l e chromophor backbone, e . g . , hydroperox t i a r y hydrogen atoms as shown i n Scheme 3 . T h i s mechanism i s n e c e s s a r i l y confined to the s u r f a c e , s i n c e i t r e q u i r e s p e n e t r a t i o n o f oxygen and a c t i n i c r a d i a t i o n ( 3 0 0 - 4 0 0 nm). H y d r o x y l groups and s i m u l t a n e o u s c r o s s l i n k i n g and c h a i n s c i s s i o n a r e t h e p r i n c i p a l p r o d u c t s of p h o t o o x i d a t i o n . P h o t o o x i d a t i o n causes a decrease i n s u r f a c e energy o f t h e f i l t e r s , a somewhat u n e x p e c t e d r e s u l t . The decrease i n s u r f a c e energy should decrease the s o i l i n g c h a r a c t e r of f r o n t covers of p h o t o v o l t a i c modules. The r a t e of energy t r a n s f e r from t h e b e n z o t r i a z o l e c h r o m o phore t o t h e h y d r o p e r o x y groups i s c o n t r o l l e d by t h e l i f e t i m e of t h e e x c i t e d s t a t e , as l o n g as i t i s h i g h e r t h a n 1.5 ev a p p r o x i mately. D e t a i l s o f decay mechanisms of t h e e x c i t e d s t a t e s w i l l be published l a t e r . Here we w i l l note t h a t t h e p r i n c i p a l f e a t u r e of t h e d e a c t i v a t i o n mechanism i n v o l v e s an i n t r a m o l e c u l a r p r o t o n t r a n s f e r p r o c e s s w h i c h may o c c u r b e f o r e v i b r a t i o n a l e q u i l i b r a t i o n of the v e r t i c a l e x c i t e d s t a t e i s completed. The f l u o r e s c e n c e has a b l u e ( \ x = 405 nm) and a red ( \ x = 585 nm) component, w i t h t h e b l u e component o n l y b e i n g p r e s e n t a t room t e m p e r a t u r e i n d i l u t e s o l u t i o n , and a t low t e m p e r a t u r e s i n p o l a r m a t r i c e s . The red component i s p r e s e n t i n e m i s s i o n a t room t e m p e r a t u r e from p o l y c r y s t a l l i n e powders and a t low t e m p e r a t u r e s i n h y d r o c a r b o n m a t r i c e s . I t may be p o s t u l a t e d t h a t t h e b l u e component a r i s e s from a v i b r a t i o n a l ^ e x c i t e d 0 - p r o t o n a t e d s p e c i e s , w h i l e t h e red component a r i s e s from a p r o t o n t r a n s f e r r e d z w i t t e r i o n i c e x c i t e d s t a t e . P h o s p h o r e s c e n c e i s d e t e c t e d from t h e model compound ( I I ) i n p o l a r m a t r i c e s a t 77K. T a b l e II g i v e s some e x c i t e d s t a t e l i f e t i m e d a t a on t h e c o p o l y m e r and model s y s t e m s . P h o t o o x i d a t i o n of t h e c o p o l y m e r may be i n h i b i t e d e i t h e r by r e d u c i n g a c c e s s o f oxygen o r by r e d u c i n g t h e number o f t e r t i a r y h y d r o g e n atoms on t h e main c h a i n . In t h e b l e n d o f t h e c o p o l y m e r w i t h PMMA, t h e pendant chromophores a r e e x c l u d e d from t h e s u r f a c e , as shown by ESCA m e a s u r e m e n t s . F o r m a t i o n o f e x c i t e d s t a t e s of ma

ma

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

304

POLYMERS IN SOLAR ENERGY UTILIZATION

Figure

7.

P r o p o s e d Mechanism of E l e c t r o n i c Energy D e a c t i v a t i o n i n the Orthohydroxybenzotriazole Nuclei.

CH - f CH, — 2

00H

3

C -h- C H - t - C — η 2 0

C00CH

C H -h2 m 0

Φ

3

00H C —

η : m>10

00H CH —

— -

2

Φ

—

C —

CH

2

—

Φ

ι OH — CHAIN SCISSION A N D

CROSSLINKING

C — I

0· CH — 2

é + 9

+H0 -

C L

Scheme

2

C —

CH — 2

+ OH

CH u n

2

3

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

19.

GUPTA ET AL.

W-Screening Transparent Acrylic Copolymers

305

b e n z o t r i a z o l e groups and t h e e l e c t r o n i c energy t r a n s f e r p r o c e s s t h e r e f o r e takes p l a c e i n s i d e t h e bulk of the f i l m . The c o n s e q u e n t d e c r e a s e i n p h o t o o x i d a t i o n r a t e t e n d s t o s u p p o r t t h e energy t r a n s -

Table

II.

Fluorescence Derivatives.

Lifetimes

SOLVENT

MOLECULE

of the Orthohydroxybenzotriazole

TEMPERATURE

WAVELENGTH

LIFETIME

TOTAL FIT

14 ± 3 ps

30°C

TOTAL FIT

52 ± 4 ps

METHYLENE CHLORIDE

30°C

TOTAL FIT

19 ± 5 ps

COPOLYMER

- D0-

30°C

TOTAL FIT

1 5 ± 4 ps

R =CH

3

EPA

R =CH

3

2 METHYL PENTANE

77K

420 nm

2.2 ± 1.0 ns

- D0-

77K

600 nm

1.4 + 0.7 ns

R =CH

3

METHYLCYCLOHEXANE

30°C

ETHANOL

R = CH~

f e r mechanism and r u l e out d i r e c t e x c i t a t i o n o f h y d r o p e r o x y groups as an i n i t i a t i o n s t e p . In c o n c l u s i o n , we have i n v e s t i g a t e d t h e mechanism o f s e n s i t i z e d p h o t o o x i d a t i o n of u l t r a v i o l e t absorbing c l e a r a c r y l i c f i l m s c o n t a i n i n g pendant u l t r a v i o l e t a b s o r b e r g r o u p s . The main c o n c l u s i o n s of the m e c h a n i s t i c study i n d i c a t e d t h a t propenyl derivatives o f u l t r a v i o l e t c h r o m o p h o r e s , c o p o l y m e r ! z a t i o n o f w h i c h would l e a d t o development o f methyl groups on t h e backbone would be more a p p r o p r i a t e candidates f o r outdoor a p p l i c a t i o n s r e q u i r i n g long s e r vice l i f e . S y n t h e s i s o f t h e f i r s t such comonomer has been r e ported here.

Ackncwle dgment s The r e s e a r c h d e s c r i b e d i n t h i s paper was p e r f o r m e d by t h e J e t P r o p u l s i o n L a b o r a t o r y , C a l i f o r n i a I n s t i t u t e o f T e c h n o l o g y and was s p o n s o r e d by t h e F l a t - P l a t e S o l a r A r r a y P r o j e c t , Department o f Energy.

Literature Cited 1. 2.

D. Bailey and O. Vogl, J. Macromol. Sci. Reviews, C14(2), 267 (1976). S. Tocker, Makromol. Chem. 101, 23 (1967).

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

306

POLYMERS IN SOLAR ENERGY UTILIZATION

3. O. Vogl and S. Yoshida, Rev. Roum. de Chimie, 25(7), 1123 (1980). 4. S. Yoshida and O. Vogl, Polymer Preprints, ACS Divison of Polymer Chemistry, 21(1), 203 (1980). 5. W. Pradellok, O. Vogl and A. Gupta, J. Polym. Sci, Polym. Chem. Ed., 19, 3307 (1981). RECEIVED April 19, 1983.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20 Effects of Deformation on the Photodegradation of Low-Density Polyethylene Films DJAFER BENACHOUR and C. E. ROGERS Case Western Reserve University, Department of Macromolecular Science, Cleveland,OH44106

The effects of uniaxia meric material's resistanc be significant in many practical applications. In this study it was found that both uniaxial and biaxial elongation of low density polyethylene film enhances the photodegradation rate at 40°C. The enhancement process for uniaxial deformation has been shown to be closely related to the mechanism of deformation and the morphological changes induced upon elongation. The necking development region, where original material structure is most disrupted, showed the largest enhancement. Highly oriented material is less sensitive to photodegradation. The experimental evidence suggests that the increase in degradation rate may be attributed primarily to strain effects (morphological changes) with some contribution from stress per se (stored energy). Biaxial stretching was found to result in greater degradation, probably because of a larger decrease in film thickness and more constraint applied. A comparison of the nature of uniaxial and biaxial deformations gives some further insight into the drastic effects of photooxidative degradation on mechanical properties. Cyclic deformation (fatigue) involves a competition (dependent on deformation frequency, amplitude, and the number of cycles) between the formation of fatigue damages (microcracks, etc.) which promote degradation and orientation of structure which reduces the degradation process. Photooxidation and deformation of polyethylene (PE) have been intensively investigated, and mechanisms for each process have been suggested which are, more or less, well accepted (1-7). However, how photooxidation can be affected by deformation (type, extent ...) has not been given much attention until recently (811), In a previous paper, we reported data on the effects of 0097-6156/83/0220-0307$06.50/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

POLYMERS IN SOLAR ENERGY UTILIZATION

308

u n i a x i a l s t r e t c h i n g on the photodegradation behavior of low dens i t y p o l y e t h y l e n e (LDPE) f i l m s (11). The present paper i s a cont i n u a t i o n of the mentioned work, and deals w i t h the e f f e c t s of c y c l i c u n i a x i a l e l o n g a t i o n and b i a x i a l s t r e t c h i n g . The data are explained i n terms of the p h o t o o x i d a t i o n mechanism and the r e s u l t a n t degradation products and the type of deformation a p p l i e d and i t s e f f e c t s on the m a t e r i a l ( o r i e n t a t i o n , damages, morphol o g i c a l changes ...)· We a l s o give a b r i e f account o f the dependence of f a t i g u e l i f e (Np) on the p h o t o o x i d a t i o n extent of LDPE f ilms. Experimental Low d e n s i t y polyethylene f i l m s were used. The m a t e r i a l c h a r a c t e r i s t i c s are l i s t e d i n Table I . The samples were exposed Table I Thickness (mils)

1.25

Density (g/cc)

0.914

Molecular Weight M^

60.000

Melt Index (g/10 min) C r y s t a l l i n i t y (%) (estimated by IR and x-ray)

1.6

50

i n a Q-UV weatherometer (Q-Panel Co.) a t 40°C and ambient atmosphere. The o x i d a t i o n extent was f o l l o w e d by measuring the c a r bonyl absorbance a t 1716 cm~l u s i n g F o u r i e r Transform I n f r a r e d Spectroscopy ( D i g i l a b FTS-14). U n i a x i a l deformations were done u s i n g a s p e c i a l l y designed s t r e t c h e r (11) which was made t o f i t i n the FTIR sample h o l d e r , thus a l l o w i n g s p e c t r a to be taken w h i l e f i l m s are kept elongated. C y c l i c s t r e t c h i n g and f a t i g u e t e s t s were performed on an I n s t r o n T e n s i l e machine. The s t r a i n - c o n t r o l mode was used f o r f a t i q u e . A T. M. Long Company b i a x i a l f i l m s t r e t c h e r was used i n the constant r a t e of deformation mode f o r concurrent and s e q u e n t i a l b i a x i a l deformations. A l l s t r e t c h i n g s and mechanical t e s t i n g s were c a r r i e d out a t room temperature. R e s u l t s and D i s c u s s i o n E f f e c t s of Photodegradation on Fatigue L i f e : The p h o t o o x i d a t i o n behavior of the m a t e r i a l used i n t h i s study i s i l l u s t r a t e d i n F i g . 1 where the extent of o x i d a t i o n i s p l o t t e d as a f u n c t i o n of UV exposure time. N o t i c e the e x p o n e n t i a l shape of the curve. S i m i l a r behavior has been observed by other workers (1,3) f o r PE,

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

BENACHOUR AND ROGERS

Low-Density Polyethylene Films

6 UV

8

10

EXPOSURE

309

15 ( DAYS )

Figure 1. Photodegradation of LDPE at 40°C as a f u n c t i o n of exposure time i n the QUV apparatus.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

POLYMERS IN SOLAR ENERGY UTILIZATION

310

as w e l l as polypropylene, f i l m s and i s due t o the a u t o c a t a l y t i c nature of the photooxidation process i n such m a t e r i a l s . Despite many s t u d i e s o f the e f f e c t s of photooxidation on mechanical p r o p e r t i e s (such as Young s modulus, t e n s i l e s t r e n g t h , u l t i m a t e e l o n g a t i o n , e t c . . . . ) , there i s very l i t t l e information about these e f f e c t s on f a t i g u e l i f e . For that reason, we s t u d i e d the f a t i g u e l i f e of LDPE f i l m s as a f u n c t i o n of UV expo­ sure. The r e s u l t s are shown i n F i g . 2 where logNp (Np being the number of c y c l e s sustained before f a i l u r e ) i s p l o t t e d vs. time of UV exposure. I n t h i s case the s t r a i n amplitude i s 8% and the frequency (ω) i s 10 cycles/min. The l i n e a r r e l a t i o n s h i p which i s observed can be described by an equation such as: 1

logNp = a + b t

(1)

where

a = fatigu considered f a t i g u e t e s t c o n d i t i o n s , a i s given by the i n t e r c e p t of the p l o t , b = constant, depending on f a t i g u e t e s t c o n d i ­ t i o n s and m a t e r i a l c h a r a c t e r i s t i c s ; b i s given by the slope o f the curve, t = time of UV exposure. S i m i l a r behavior was observed f o r two other frequencies and the values of a and b, as a f u n c t i o n of ω, are l i s t e d i n Table I I . A l l frequencies used were lower than 2 Hz i n order to m i n i Table I I : Values of a and b as a f u n c t i o n of frequency (Equation (1)) Frequency: ω (cycles/min)

a

N-p a t t = 0

b

5

5.40

255 χ 1 0

10

5.00

100 χ 10

3

- 0.263

50

4.50

32 χ 1 0

3

- 0.253

3

- 0.283

mize any thermal e f f e c t s . The c o n s t a n t - s t r a i n mode was chosen to prevent sample f a i l u r e by creep. I t appears that a decreases as ω increases while b decreases a l s o but l e s s s t e e p l y (as a matter of f a c t , we can consider t h a t , w i t h i n the e r r o r margin of ± 10%, b remains constant.) The r e l a t i o n s h i p between f a t i g u e l i f e , o x i d a t i o n extent and UV exposure i s i l l u s t r a t e d i n F i g . 3 where both l o g N and l o g [carbonyl] are p l o t t e d vs. UV exposure ( f o r ω = 10 c y c l e s . ) I t can be seen that as the time of UV exposure i n c r e a s e s , i . e . , as [carbonyl] i n c r e a s e s , Np decreases sharply. F i g . 4 shows that logNp depends i n a l i n e a r f a s h i o n on l o g [ c a r b o n y l ] . Such a deF

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Low-Density Polyethylene Films

311

u.

ο

2

4 UV

Figure

6

8

EXPOSURE

10

15 (DAYS)

2. Dependence of f a t i g u e l i f e (Nf, the number of c y c l e s t o f a i l u r e ) on UV exposure time f o r LDPE. (Δε = 8%, ω = 10 cycles/min).

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

312

POLYMERS IN SOLAR ENERGY UTILIZATION

I 0

ι 2

ι 4 UV

Figure 3,

ι 6

1 8

EXPOSURE

1 10

1

15

1 20

(DAYS)

Fatigue l i f e and carbonyl content of LDPE as a f u n c t i o n of UV exposure (Δε = 8 % , ω = 10 cycles/min.)·

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

BENACHOUR AND ROGERS

Low-Density Polyethylene Films

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

313

314

POLYMERS IN SOLAR ENERGY UTILIZATION

N

F

= A[carbonyl]

B

(2)

where A and Β a r e c o n s t a n t s , depending on m a t e r i a l c h a r a c t e r i ­ s t i c s and UV exposure and f a t i g u e t e s t c o n d i t i o n s . A and Β were c a l c u l a t e d (from the i n t e r c e p t s and s l o p e s , r e s p e c t i v e l y , of p l o t s s i m i l a r t o F i g . 4.) f o r a l l three frequencies used; the values are l i s t e d i n Table I I I . I t appears that w h i l e Β remains Table I I I : Values of A and Β as a f u n c t i o n of frequency (Equation (2)) Frequency: ω (cycles/min) 5

A

Β

6.25

- 3.52

10

6.0

50

5.50

- 3.47

Note: A w i l l be the f a t i g u e l i f e of LDPE f i l m s c o n t a i n i n g 1% of carbonyl absorbance a t 1716 cm"! (most commercial f i l m s show such amounts of chromophores, probably r e s u l t i n g from o x i d a t i o n during processing.) more o r l e s s constant ( t h e r e f o r e , Β can be assumed not t o depend on frequency) A decreases as ω i n c r e a s e s . This i s a t t r i b u t e d mostly t o the higher deformation speed w i t h higher frequency ( s i m i l a r t o s t r e s s - s t r a i n experiments where the higher the de­ formation r a t e , the sooner the sample f a i l u r e . ) The r e l a t i o n s h i p between f a t i g u e l i f e and carbonyl content can be e x p l a i n e d as f o l l o w s : according t o the photooxidation mechanism of PE, carbonyl groups r e s u l t mainly from a N o r r i s h type I I r e a c t i o n , i . e . , f o r each carbonyl formation, there i s a s c i s s i o n of a segment of a molecule c h a i n . Such s c i s s i o n c r e ­ ates a defect i n the s t r u c t u r e which can grow and propagate i n t o a microcrack under a p p l i c a t i o n o f a l o a d . Under c y c l i c l o a d i n g , i t i s understandable that the number of c y c l e s the sample can s u s t a i n w i l l be d i r e c t l y r e l a t e d t o the number of defects (such as microcracks, microvoids . . . ) , as i s c l e a r l y described by equa­ t i o n (2). More work i s needed t o see i f equation (2) holds f o r d i f ­ f e r e n t s t r a i n amplitudes and d i f f e r e n t m a t e r i a l c h a r a c t e r i s t i c s ( d i f f e r e n t c r y s t a l l i n i t i e s , molecular weights . . . ) . E f f e c t s of U n i a x i a l E l o n g a t i o n : LDPE f i l m s were f i r s t s t r e t c h e d t o d i f f e r e n t elongations (deformation speed: 1 i n c h / min.), then photooxidized w h i l e being kept s t r e t c h e d . The extent of o x i d a t i o n , a t a given UV exposure (10 days a t 40°C), as a

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

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315

f u n c t i o n of draw r a t i o i s i l l u s t r a t e d i n F i g . 5. (The i n t e r p r e ­ t a t i o n of the data has been r e p o r t e d , i n more d e t a i l , i n our pre­ vious paper (11), and only a b r i e f summary i s given here.) There i s enhancement of the degradation process due t o the deformation. The enhancement process i s r e l a t e d t o the deformation mechanism; the l a r g e r the d i s r u p t u r e of the f i l m s t r u c t u r e (necking r e g i o n development) the greater the enhancement. The o r i e n t a t i o n e f ­ f e c t s , which takes place f o r λ > 4, tend t o reduce the enhance­ ment . The r e d u c t i o n of degradation enhancement due to o r i e n t a t i o n i s b e t t e r seen when samples are s t r e t c h e d and then the time to f a i l , under UV r a d i a t i o n , i s recorded. The r e s u l t s are shown i n F i g . 6 where one should n o t i c e the break i n s c a l e f o r the r e f e r ­ ence (non-oxidized) sample. There i s a d r a s t i c decrease i n f a i l ­ ure time (F.T.) f o r low draw r a t i o s 1 < λ < 1.7. This can be a t t r i b u t e d t o s t o r e d e l a s t i c energy which makes the chemical bonds more r e a c t i v e towar increases and the polyme ented, F.T. i n c r e a s e s s t e e p l y before reaching a p l a t e a u once the o r i e n t a t i o n process i s more or l e s s completed. I f we consider that p h o t o o x i d a t i o n i s oxygen d i f f u s i o n c o n t r o l l e d (1-5), the o r ­ i e n t a t i o n e f f e c t i s to decrease such d i f f u s i o n by making the s t r u c t u r e much more compact so that the degradation w i l l be r e ­ duced. In o r i e n t e d samples, o x i d a t i o n i s much more concentrated i n the surface l a y e r , thus decreasing the formation of microcracks w i t h i n the bulk of the sample which i n c r e a s e s i t s a b i l i t y t o r e ­ s i s t f a i l u r e under UV r a d i a t i o n . E f f e c t s of C y c l i c U n i a x i a l S t r e t c h i n g : LDPE f i l m s were f a t i g u e d (Δε = 20%, ω = 10 c y c l e s / m i n . , f o r 10 c y c l e s ) before being put i n the weatherometer i n "the r e l a x e d s t a t e " , i . e . , w i t h f r e e ends. T h e i r photooxidation behavior i s compared to that of nonf a t i g u e d (reference) samples i n F i g . 7. The f a t i g u e d samples show more degradation which i s a t t r i b u t e d to damages r e s u l t i n g from c y c l i n g ( f a t i g u e damages: m i c r o c r a c k s , m i c r o v o i d s , microcrazes . . . ) . Such damages were c l e a r l y observed by o p t i c a l microscopy (12) and are known t o enhance the s u s c e p t i b i l i t y of LDPE f i l m s t o photodegradation. As Ν i n c r e a s e s , the f a t i g u e dam­ ages w i l l i n c r e a s e r e s u l t i n g i n more and more degradation as shown i n F i g . 8. An increase i n s t r a i n amplitude (Δε) a l s o i n c r e a s e s p l a s t i c deformation (non-recoverable elongation) as i l l u s t r a t e d i n F i g . 9. The number of c y c l e s i s given i n a l o g a r i t h m i c s c a l e , and the p l a s t i c deformation (P.D.) i s given i n percent: P.D. = (£ - l ) I £ , where £ and l are the sample lengths a f t e r and before f a t i ­ gue, r e s p e c t i v e l y . Such p l a s t i c deformation i s accompanied by an o r i e n t a t i o n e f f e c t which, as we have seen, tends to lower degradation. F i g s . 10 and 11 show t h a t samples f a t i g u e d f o r 10^ c y c l e s degrade l e s s pendence can be described by the f o l l o w i n g equation: Q

Q

Q

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

316

POLYMERS IN SOLAR ENERGY UTILIZATION

CO of

Ζ Ο

ϋ Q Χ Ο

2

3 DRAW

4

5

RATIO

Figure 5. Dependence of o x i d a t i o n extent ( r e l a t i v e s c a l e ) of LDPE a f t e r UV exposure f o r 10 days a t 40°C on u n i a x i a l e l o n g a t i o n (draw r a t i o ) .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

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Low-Density Polyethylene Films

317

"r 300

ο

φ

en Ο 1

ο

250

200

O

h

ο

ΗLU h-

O

o

o

150

100

-Ι­ 2

3 DRAW

4

5

RATIO

Figure 6. F a i l u r e time under UV exposure v s . u n i a x i a l draw r a t i o of LDPE.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

318

POLYMERS IN SOLAR ENERGY UTILIZATION

60

τ*

FATIGUED

•

10

4

CYCLES

U = 20 V;

R E F E R E N C E

50

I

5 Ο

40

ΙΟ

m


2

+ Q< >+ ... =

IN

SOLAR E N E R G Y

UTILIZATION

n(l-P)d-r) 1 - η [ Γ Ρ + r ( l - P)]

( 4 )

For s i m p l i c i t y , we w i l l assume that the edge mounted s o l a r c e l l has the same quantum e f f i c i e n c y f o r a l l i n c i d e n t l i g h t above i t s bandgap energy. Let the a i r mass one (AMI) e f f i c i e n c y of the c e l l f a c i n g the sun d i r e c t l y be Π ^ · I f I i s the t o t a l s o l a r f l u x of s u n l i g h t absorbed by the LSC^ then the e f f i c i e n c y of the LSCc e l l combination ( e l e c t r i c a l power o u t / s u n l i g h t power i n c i d e n t ) i s approximated by: e

E f f

*

=

n

cell

*

Q

*

S

/

I

(

5

)

S i m i l a r l y the l i g h t a m p l i f i c a t i o n or f l u x g a i n of the p l a t e i s the ideal flux gain, G , absorbed, S/I, t i m i | w i l l be transported to the c e l l s , Q: O T n

G_ = G . Q . S/I flux geom Ί

x

(6)

In a r r i v i n g a t Equations (5) and ( 6 ) , we have ignored s c a t t e r i n g and a b s o r p t i o n by the m a t r i x m a t e r i a l , r e f l e c t i o n a t the L S C - c e l l i n t e r f a c e , and v a r i a t i o n s i n the output of the c e l l s due t o the d i f f e r e n t spectrum and i n t e n s i t y of the i n c i d e n t l i g h t . These e f f e c t s are discussed elsewhere (1, 5 ) . Suppose a p l a s t i c p l a t e w i t h a geometric g a i n of 12 i s impregnated w i t h rhodamine-6g and has 12% AMI c e l l s mounted on i t s perimeter. As we s h a l l see l a t e r , t y p i c a l v a l u e s f o r the f r a c t i o n of the s o l a r spectrum absorbed i s S/I = 30% w i t h a c o l l e c t i o n e f f i c i e n c y of Q = 50%. This r e s u l t s i n an o v e r a l l e f f i c i e n c y of 2% w i t h a f l u x g a i n of 2. The en­ hanced v a l u e of S/I cannot be obtained i n s i n g l e dye LSC; m u l t i p l e dye LSC s serve t h i s purpose q u i t e e f f i c i e n t l y as shown by Swartz et a l (3) and Batchelder e t a l ( 5 ) . That the p l a t e j u s t mentioned should have absorbed 30% of the u s e f u l i n c i d e n t s u n l i g h t i s c a l c u l a t e d e a s i l y enough by c o n v o l u t ing the s o l a r spectrum w i t h the a b s o r p t i o n spectrum across the t h i c k n e s s of the p l a t e . To increase the f r a c t i o n of l i g h t absorb­ ed and insure an e f f i c i e n t cascade from one dye t o another, sev­ e r a l dyes are chosen i n a sequence from the h i g h e s t t o lowest en­ ergy of t h e i r a b s o r p t i o n bands, such that the luminescence spec­ trum of one dye overlaps s t r o n g l y w i t h the a b s o r p t i o n of the dye f o l l o w i n g i t . I f dyes chosen i n t h i s manner are mixed together i n an LSC p l a t e , l i g h t absorbed by one dye w i l l be emitted and ab­ sorbed by the next one, and t h i s process i s repeated u n t i l the photon reaches the dye w i t h the lowest energy a b s o r p t i o n . (The energy of e x c i t a t i o n l o s t i n cascading from one dye t o the next i s d i s s i p a t e d i n the p l a t e . ) The r e s u l t i s an LSC w i t h a very broad band a b s o r p t i o n of s o l a r energy, and w i t h an emission energy that 1

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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339

Luminescent Solar Concentrators

f u n c t i o n of the product of the c o n c e n t r a t i o n times the pathlength. can be made c l o s e to the band gap (Figure 4 ) . Measurements of S e l f - A b s o r p t i o n and Performance The p r i n c i p l e f a c t o r reducing the o v e r a l l e f f i c i e n c y of pre­ sent LSC's i s s e l f - a b s o r p t i o n of luminescence due to the non-zero overlap of the a b s o r p t i o n and emission s p e c t r a . In the past three years, one of our major research e f f o r t s on LSC*s has focussed on measuring and understanding these s e l f - a b s o r p t i o n p r o b a b i l i t i e s . Four techniques have proven u s e f u l i n measuring s e l f - a b s o r p t i o n : d i r e c t measurement of the steady s t a t e a b s o r p t i o n and emission spectrum, o b s e r v a t i o n of the s p e c t r a l s h i f t of the emission w i t h i n c r e a s i n g sample pathlength, time-resolved measurements from picosecond pulsed e x c i t a t i o n , and p o l a r i z a t i o n a n i s o t r o p y measure­ ments. Before h i g h l i g h t i n g these d i f f e r e n t methods, we should mention the approximation d i th theor (1 5) developed f o explaining self-absorptio a b s o r p t i o n model, whic e x p l a i n majo perfor mance, we assume the presence of a simple r e a b s o r p t i o n event, and we develop a r e a b s o r p t i o n p r o b a b i l i t y that i s averaged over the e n t i r e luminescence band. In References 1_ and the d e t a i l s of the model and the c o n d i t i o n s f o r i t s v a l i d i t y are given. The agreement between theory and experiments i s s a t i s f a c t o r y , and we use the s e l f - a b s o r p t i o n p r o b a b i l i t y r e s u l t only i n an average sense f o r performance e v a l u a t i o n . Steady-State S p e c t r a l Convolution. The steady s t a t e absorp­ t i o n and emission s p e c t r a of d i l u t e dye samples can be measured using standard s p e c t r o s c o p i c techniques. Once the e x t i n c t i o n co­ e f f i c i e n t , ε(ν), and the normalized luminescence spectrum, f (ν), are known f o r a p a r t i c u l a r dye, the s e l f - a b s o r p t i o n p r o b a b i l i t y r over a pathlength L i n the sample c o n t a i n i n g the dye at a concen­ t r a t i o n C i s given by r = I

dv f(v) 1 - i ( f

L

C

e

^]

(7)

ο An i n t e r e s t i n g outcome of Equation 7 i s that i t produces an aver­ age pathlength f o r a sample w i t h a known s e l f - a b s o r p t i o n r a t e . For example, a n a l y t i c techniques (1) e x i s t f o r summing the output l i g h t f o r a p a r t i c u l a r i n f i n i t e r i b b o n geometry of LSC, i n c l u d i n g s e l f - a b s o r p t i o n e f f e c t s . This a n a l y t i c c o l l e c t i o n e f f i c i e n c y can i n t u r n be converted to the a p p r o p r i a t e s e l f - a b s o r p t i o n p r o b a b i l i ­ t i e s u s i n g Equation 4, and from there i n t o the average pathlength traversed by the c o l l e c t i o n r a d i a t i o n . The r e s u l t of t h i s manipu­ l a t i o n i s that the average pathlength of sample traversed by c o l ­ l e c t e d luminescence i n an i n f i n i t e r i b b o n LSC i s approximately equal to the width of the r i b b o n , f o r c o n c e n t r a t i o n s and p l a t e thicknesses such that the peak o p t i c a l d e n s i t y through the t h i c k ­ ness of the p l a t e i s approximately equal to one. S e l f - a b s o r p t i o n i n s i d e the c r i t i c a l cones i s u s u a l l y n e g l i g i b l e i n such cases.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

340

P O L Y M E R S IN S O L A R E N E R G Y

UTILIZATION

EXCITATION SPECTRA SHOWINO ENERGY TRANSFER IN MULTIPLE DYE METHANOL SOLUTIONS

ONE DYE AT ,LOW CONC. 3 DYES AT j HIGH CONC. 3 DYES AT LOW CONC 13,000 WAVENUMBERS F i g u r e Ua. Three e x c i t a t i o n s p e c t r a o f methanol dye s o l u ­ t i o n s w i t h emission d e t e c t i o n a t 6k00 A. The three s p e c t r a correspond t o t h e same s o l u t i o n s used i n Figure Ub.

EMISSION SPECTRA SHOWING ENERGY TRANSFER IN MULTIPLE DYE METHANOL SOLUTIONS.

ONE DYE AT j LOW CONC. 3 DYES AT j HIGH CONC. 3 DYES AT , . ,. _ r π — , , , , 1 LOW CONC. 11.000 16.000 21.000 WAVENUMBERS F i g u r e Ub. Three emission s p e c t r a o f methanol dye s o l u ­ t i o n s r e s u l t i n g from h^OO % e x c i t a t i o n . The t o p spectrum i s from a micromolar oxazine-720 s o l u t i o n s , and t h e lower s p e c t r a are hundred micromolar and micromolar c o n c e n t r a t i o n s , r e s p e c t i v e l y , o f coumarin-5^0, rhodamine-61+0, and oxazine-720.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

21.

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BATCHELDER

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341

The important p o i n t here i s that f o r a given L, knowledge of ε and f produce r . Pathlength Dependent S p e c t r a l S h i f t , Equation 7 i m p l i e s that as the luminescence passes through more LSC m a t e r i a l i t w i l l get s h i f t e d ( i n t o the red) s i n c e most of the overlap of the a b s o r p t i o n and emission bands occurs at the high energy end of the emission spectrum. This e f f e c t can be measured d i r e c t l y , as i s shown i n F i g u r e 5. A rod of LSC m a t e r i a l was e x c i t e d w i t h a focussed spot of l i g h t from a mercury lamp (or a l a s e r ) , and s p e c t r a were taken of the emission from the end of the rod as a f u n c t i o n of the d i s ­ tance of the e x c i t a t i o n from the end of the rod. The organic l a s e r dye used was rhodamine-575, and the sample pathlengths f o r each spectrum i n order of the most to l e a s t intense were 0.3, 1.0, 3.0, 10.0, and 30.0 cm. The spectra have been c o r r e c t e d only f o r the system response of the d e t e c t i o n (monochromator and the PMT), so that the amplitude o t i e s emerging from the en F i g u r e 6 shows the r e s u l t s of an a n a l y t i c c a l c u l a t i o n of the spectrum s h i f t f o r the same system measured i n F i g u r e 5. The c a l ­ c u l a t i o n i n v o l v e s an average of the Beer-Lambert law over a l l p o s s i b l e pathlengths and wavenumbers of the emission (5). This technique suggests what i s probably the simplest way to determine the p r o b a b i l i t y of s e l f - a b s o r p t i o n e m p i r i c a l l y f o r a p a r t i c u l a r p l a t e . The peak p o s i t i o n of the luminescence spectrum can be c a l c u l a t e d f o r a v a r i e t y of s e l f - a b s o r p t i o n r a t e s u s i n g the above formalism. T h e r e a f t e r , the s e l f - a b s o r p t i o n r a t e f o r a par­ t i c u l a r device can be found (crudely) by a moderate r e s o l u t i o n measurement of the peak p o s i t i o n of the luminescence spectrum. Time-Resolved Emission. When an LSC i s e x c i t e d w i t h a pulsed l i g h t source whose d u r a t i o n i s much l e s s than a nanosecond, the l i g h t output from the sample w i l l decay e x p o n e n t i a l l y w i t h time.If the c o n c e n t r a t i o n of the dye i n the sample i s i n c r e a s e d , t h i s ex­ p o n e n t i a l decay takes a longer time. We have shown 0 5 ) that the s e l f - a b s o r p t i o n r a t e i s r e l a t e d to the r a t i o between the measured l i f e t i m e of the p l a t e , τ, and the l i f e t i m e measured i n the l i m i t of low c o n c e n t r a t i o n , T Q , i n the f o l l o w i n g wayi r = [1 - τ /τ]/η(1 - Ρ) 0

(.8)

Equation 8 i s t r u e i n the l i m i t that the t r a n s i t time f o r l i g h t i n the p l a t e i s short compared to the l i f e t i m e , and that s e l f a b s o r p t i o n i n the c r i t i c a l cones i s n e g l i g i b l e ( i . e . , r of the c r i t i c a l cone, r , i s z e r o ) . Thus Equation 8, which can be e a s i l y r e w r i t t e n to i n c l u d e r , i s not v a l i d f o r l a r g e v a l u e s of P. Note that when τ - το, r 0, and when (1 - Ρ) i n c r e a s e s both r and τ w i l l change. The s m a l l e s t e r r o r bars i n F i g u r e 7 show the mea­ sured s e l f - a b s o r p t i o n r a t e s f o r rhodamine-575 i n methanol as a

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

342

POLYMERS

τ

IN

1

1.32

SOLAR E N E R G Y

UTILIZATION

Γ

1.72

2.12

Figure 5· Emission s p e c t r a from a 92 micromolar methanol s o l u t i o n o f rhodamine-575. The sample pathlengths, i n order o f most t o l e a s t i n t e n s e , were 0 . 3 , 1 . 0 , 3 . 0 , 1 0 . 0 , and 30.0 cm.

ι

» » •

15,000

^

•

17,000

19,000

21,000

ι I 23,000

NRVENUMBERS Figure 6. A n a l y t i c r e s u l t t i o n o f sample pathlength. emission s p e c t r a were each gaussians. These are used experiment shown i n F i g u r e 1000 cm" .

f o r emission s p e c t r a as a func­ Experimental a b s o r p t i o n and f i t t e d as the sum o f two w i t h our model t o i m i t a t e t h e 5· The Stokes s h i f t used i s

1

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ZEWAIL AND BATCHELDER

8.00

ι

1

Luminescent Solar Concentrators

1

1

"

343

r

CONC. X PATHLENGTH

Figure 7. S e l f - a b s o r p t i o n p r o b a b i l i t i e s f o r rhodamine-575. This shows a j u x t a p o s i t i o n o f p r e d i c t e d s e l f - a b s o r p t i o n p r o b a b i l i t i e s f o r three measurement methods : s p e c t r a l over­ l a p convolution ( s o l i d ) , emission d e p o l a r i z a t i o n (boxes), and time-resolved s p e c t r a ( b a r s ) . The second two techniques are p l o t t e d assuming the quantum e f f i c i e n c y o f luminescence i s one.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

344

POLYMERS

IN

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Due t o the high accuracy p o s s i b l e w i t h photon counting measure­ ments of the l i f e t i m e , t h i s technique y i e l d s the most accurate measurement of any technique we have studied of the product nr, the quantum e f f i c i e n c y of luminescence and the s e l f - a b s o r p t i o n p r o b a b i l i t y , f o r low r a t e s of s e l f - a b s o r p t i o n . P o l a r i z a t i o n Anisotropy. Since the dye molecules which ab­ sorb and emit l i g h t i n the LSC behave as d i p o l e antennas, they have a tendency to emit l i g h t which i s p o l a r i z e d i n the same d i r e c ­ t i o n as a p o l a r i z e d source. For example, suppose t h a t a randomly o r i e n t e d sample of dye molecules l o c a t e d a t the o r i g i n i s e x c i t e d by l i g h t p o l a r i z e d i n the ζ d i r e c t i o n . I t has been shown that luminescence i n the χ d i r e c t i o n w i l l have three times as much i n ­ t e n s i t y i n the ζ p o l a r i z a t i o n component as i n the y component (11). I f t h i s luminescence i s subsequently s e l f - a b s o r b e d , the r e s u l t i n g luminescence would be l e s s p o l a r i z e d . The degree of p o l a r i z a t i o n can be measured by the reduce i n t e n s i t y (z component ponent) d i v i d e d by the sum of the p a r a l l e l plus twice the perpen­ dicular intensities:

Ξ

RA - ϋ

I

+

A

9

21^

The reduced a n i s o t r o p y of f i r s t generation emission i s t h e r e f o r e (3-1)/(3+2*1) = 2/5. We have shown (5) that the reduced a n i s o ­ tropy of the i t h generation of emission i s ( 2 / 5 ) 2 i - l . For example, the reduced a n i s o t r o p y of very high generations i s n e a r l y zero ( u n p o l a r i z e d ) . Due to e f f e c t s such as r o t a t i o n of the dye molecule or the a b s o r p t i o n and emission d i p o l e s not being c o l l i near, the f i r s t generation reduced a n i s o t r o p y i s u s u a l l y l e s s than 2/5 even a t very low c o n c e n t r a t i o n s and pathlengths. Let RAo(Ao) be the highest measured reduced a n i s o t r o p y of a p a r t i c u l a r dye and surrounding m a t e r i a l , and RA (A ) be the measured reduced a n i s o ­ tropy a t the c o n c e n t r a t i o n and pathlength of i n t e r e s t . The s e l f a b s o r p t i o n p r o b a b i l i t y can be shown to be (5) A r = [1 - f H / n d - P) d - A^ A ) 0

(10)

As before we have assumed that s e l f - a b s o r p t i o n i n the c r i t i c a l cones i s n e g l i g i b l e . A l s o , we have taken an average of the r e ­ duced anisotropy over a l l generations. Thus, Equation 10 cannot be a p p l i e d i n the l i m i t of A -> 0 s i n c e Q w i l l be going to zero. Note that when A = A , r becomes zero as expected. The e r r o r boxes i n F i g u r e f show the measured s e l f - a b s o r p t i o n r a t e s f o r rhodamine-575 as computed u s i n g t h i s technique. The s o l i d l i n e i n F i g u r e 7 i s found by i n s e r t i n g the measured a b s o r p t i o n and emis­ s i o n s p e c t r a of rhodamine-575 i n Equation 7. There i s good Q

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

21.

ZEWAIL AND BATCHELDER

Luminescent Solar Concentrators

345

agreement among t h e three techniques, and very good agreement between the p o l a r i z a t i o n and l i f e t i m e r e l a t e d measurements. The d i s p a r i t y between the s p e c t r a l c o n v o l u t i o n technique and the other two i s i n the v a r i a t i o n of the average pathlength of sample t r a v ­ ersed by luminescence as the c o n c e n t r a t i o n i s v a r i e d . CODEs f o r E f f i c i e n c y and Performance E v a l u a t i o n . I t i s very u s e f u l t o g i v e p r o s p e c t i v e luminescent m a t e r i a l s a " f i g u r e of m e r i t " which measures the m a t e r i a l ' s s e l f - a b s o r p t i o n c h a r a c t e r i s ­ t i c s (hence Q,..., etc.) i n an LSC. We have defined (5) the c r i t ­ i c a l o p t i c a l d e n s i t y (CODE) to be the product of the o p t i c a l pathl e n g t h L times__the dye c o n c e n t r a t i o n C times the peak e x t i n c t i o n c o e f f i c i e n t ε(ν ) such t h a t the luminescence from the dye has a 50% chance of being self-absorbed over that o p t i c a l pathlength: CODE Ξ L · C · ε(ν )

1/2

= fdv '

f(V)

lcf

L C e ( V

>

0

T y p i c a l CODEs f o r organic l a s e r dyes are between 10 and 40. Most coumarin dyes are about 100, w h i l e some dyes w i t h unusually l a r g e Stokes s h i f t s , l i k e DCM, have CODEs above 200. C l e a r l y the Stokes s h i f t comes i n t o play d i r e c t l y ! The CODE of a dye can be used d i r e c t l y t o determine the maxi­ mum f l u x g a i n that can be achieved i n an LSC using that dye. Most LSCs use PMMA as a m a t r i x m a t e r i a l , which has an index of r e f r a c ­ t i o n of 1.49. Most of the l a s e r dyes of i n t e r e s t have a quantum e f f i c i e n c y of about 90%. I f we ignore s e l f - a b s o r p t i o n i n the c r i ­ t i c a l cones, we f i n d from Equation 4 that a s e l f - a b s o r p t i o n proba­ b i l i t y of 50% produces a c o l l e c t i o n e f f i c i e n c y of about 50%. We next observe that i f we a r e t r y i n g to minimize s e l f - a b s o r p t i o n , we should have the dye c o n c e n t r a t i o n i n the p l a t e as low as poss­ i b l e . The lowest c o n c e n t r a t i o n of the dye which w i l l s t i l l pro­ duce reasonably good a b s o r p t i o n of the i n c i d e n t s u n l i g h t w i l l pro­ duce a maximum peak o p t i c a l d e n s i t y across the thickness of the p l a t e of one. I f the dye c o n c e n t r a t i o n i n the LSC i s adjusted such that t h i s i s the case, then the CODE becomes the number of p l a t e thicknesses through which the luminescence can pass before being 50% self-absorbed. F i n a l l y we note that s i n c e the average pathlength of c o l l e c t e d emission i n the sample i s on the order of the sample width, and s i n c e the geometric gain of a square p l a t e i s given by the width d i v i d e d by the t h i c k n e s s of the p l a t e ( i f three s i d e s are mirrored) then the CODE i s the geometric g a i n of such an LSC such that i t w i l l produce a c o l l e c t i o n e f f i c i e n c y of about 50%. I f an LSC i s designed such that i t s peak o p t i c a l d e n s i t y across the t h i c k n e s s of the p l a t e i s equal t o one and i t s geometric g a i n i s equal to the CODE of the dye used, then the o v e r a l l

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

346

POLYMERS

e f f i c i e n c y of the p l a t e (Equation Eff.

IN

SOLAR E N E R G Y

5) can be approximated as

= η • S/2I 'cell

and the l i g h t a m p l i f i c a t i o n of the p l a t e (Equation becomes G

flux

UTILIZATION

(12) 6) s i m i l a r l y

= CODE · S/2I

(13)

This technique i s e s p e c i a l l y u s e f u l i n p r o j e c t i n g the performance of high gain LSCs. Prototype E f f i c i e n c y Measurements. A v a r i e t y o f LSC devices have been tested by ourselves and others. Table I i s intended to be a r e p r e s e n t a t i v e l i s t of t y p i c a l performance parameters Again, the geometric g a i n i s th the a c t i v e area of the the short c i r c u i t current increased when attached t o the p l a t e , as opposed t o f a c i n g the sun d i r e c t l y . The c e l l e f f i c i e n c y i s the measured or assumed AMI e f f i c i e n c y of the s o l a r c e l l s used (which i n a l l cased were s i l i c o n ) . The c o l l e c t o r e f f i c i e n c y i s the t o t a l e l e c t r i c a l power out d i v i d e d by the t o t a l s u n l i g h t power i n c i d e n t on the p l a t e . In Table I devices Β and D were b u i l t and tested by OwensI l l i n o i s (8), the r e s t were b u i l t a t Caltech. The dyes were con­ tained i n t h i n p l a s t i c f i l m s attached to the surface of a c l e a r substrate i n the 01 case. Measurements were made under a c t u a l i n s o l a t i o n , w i t h the p l a t e edges roughened and blackened where c e l l s were not mounted. These p l a t e s have achieved the highest e f f i c i e n c i e s , but t h e i r small geometric gains make them somewhat i n e f f e c t i v e as concentrators. For example, c e l l s mounted on LSC device D w i l l have only a 70% increase i n output over an equal area of c e l l s f a c i n g the sun d i r e c t l y . Device C was a meter square l i q u i d c e l l which we b u i l t . The f l u x g a i n was measured under a c t u a l i n s o l a t i o n . We c a l c u l a t e that the two dye combination used absorbed 30% of an AMO spectrum i n a two pass (an LSC p l a t e followed by a backing m i r r o r ) geometry. Device Ε has the highest f l u x g a i n of any LSC reported. It is a s i n g l e dye PMMA p l a t e c o n t a i n i n g DCM, a dye w i t h a r e l a t i v e l y smaller r a t e f o r s e l f - a b s o r p t i o n (the CODE f o r DCM i s about 200 but depends on the solvent or matrix used). Thermodynamics of LSC's. Several years ago, we i n i t i a t e d a program t o c a l c u l a t e the u l t i m a t e gain expected f o r an LSC (12). We s t a r t e d by c o n s i d e r i n g the a p p l i c a t i o n of the Winston-Rabl (13) idea t o LSC-type systems. But, because the energy of the i n c i d e n t s o l a r l i g h t i s not preserved i n an LSC, we used a d e t a i l e d balance c a l c u l a t i o n which gives the i n t e n s i t y of the p l a t e output as a f u n c t i o n of s i z e , i g n o r i n g s e l f - a b s o r p t i o n e f f e c t s .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

- A

3J

*3

3

s? cr ç?

If §

Pits

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

3

1

1

D

Ε

F PMMA/DM SO

PMMA

thin film

ethylene g l y c o l

3

C

PMMA

Matrix

thin film

2

No.Dyes

Β

A

Device

92

68

11

36

11

23

Geometrical Gain

4.4

5.1

1.7

3.8

1.3

2.1

Flux Gain

18%

18%

21%

18%

21%

18%

Assumed Cell Eff.

0.9%

1.3%

3.2%

1.9%

2.5%

1.9%

Collector Eff.

Table I . Prototype performances. Devices Β and D were made and t e s t e d by Owens I l l i n o i s . The r e s t were made and t e s t e d a t C a l t e c h . The c o l l e c t o r e f f i c i e n c y i s the assumed AMI c e l l e f f i c i e n c y times the f l u x g a i n d i v i d e d by the g e o m e t r i c a l g a i n , and corresponds t o the e l e c t r i c a l power output per s o l a r power i n p u t .

2

s

1

S'

υ w

Η ο χ m

w >

α

>

î

m

Ν

348

POLYMERS

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UTILIZATION

In F i g u r e 8, we show the thermodynamic g a i n f o r an i d e a l i z e d LSC. We computed the g a i n f o r a plan geometry by s u b s t i t u t i n g a b l a c k body c a v i t y f o r the edge mounted c e l l s , and then computing the temperature of the c a v i t y by the d e t a i l e d balance c a l c u l a t i o n . The s t r a i g h t diagonal l i n e i n d i c a t e s the operation of concentrators using geometric o p t i c s such as m i r r o r s and lenses. We considered an LSC absorbing i n the r e g i o n from 3500A to 8000Â, e m i t t i n g a t 8300A, and a geometric g a i n ranging from 1 to 1 0 . Under the cond i t i o n s of the model there i s a considerable buildup of energy i n the p l a t e - c a v i t y system. For example, an i d e a l i z e d LSC w i t h no g a i n a t a l l (G = 1) would cause the c a v i t y t o r i s e t o a temperature of 1000°ê?° A s i m i l a r b l a c k body f a c i n g the sun d i r e c t l y would r i s e to about 400°K. I t i s d i f f i c u l t t o e x p l i c i t l y i n c l u d e e f f e c t s such as s e l f - a b s o r p t i o n that a r e seen i n more r e a l i s t i c dye s p e c t r a , so that the r e s u l t s i n t h e i r present form are o v e r l y o p t i m i s t i c i n the p r e d i c t e d l i g h t output. I n what f o l l o w s we use the CODE method to o b t a i s u l t s w i t h that obtaine The thermodynamic l i m i t t o the performance of an LSC can be obtained by c o n s i d e r i n g the i n c i d e n t s u n l i g h t and trapped l i g h t i n the p l a t e as two systems of photon gases i n e q u i l i b r i u m . This system was f i r s t s t u d i e d by Kennard (14) and Ross (15). Recently Y a b l o n o v i t c h (16) has a p p l i e d these r e s u l t s t o the LSC and obtained (from a g e n e r a l i z a t i o n of the b r i g h t n e s s theorem of o p t i c s to i n e l a s t i c p r o c e s s e s ; o p t i c a l elements which change the l i g h t energy): 5

m

_ G

V

2

hc(v - ν )
ΙΟ**, p r e f e r a b l y > 10? p s i ) t h e r i g i d e n t i t y s h o u l d be l o w c o s t (< $ 1 . 0 0 / f t , p r e f e r a b l y < $ 0 . 5 0 / f t ) and o f minimum weight I t must be weather r e s i s t a n t (> 20 y e a r s ) expansion c o e f f i c i e n t t h a c e l l material. This i s i n order not t o put e x c e s s i v e ( f a t i g u i n g l e v e l ) m e c h a n i c a l s t r e s s on the c e l l s u r f a c e e l e c t r i c a l c o n t a c t s from the f o r c e s o f d i f f e r e n t i a l thermal expansion over t h e d a i l y thermal c y c l i n g o f a PV a r r a y . The thermal expansion c o e f f i c i e n t of c r y s t a l l i n e s i l i c o n i s q u i t e low a t 3 x 10"^ °C~ . Optical Weight and P e r m e a b i l i t y R e q u i r e m e n t s . I f the s t r u c t u r a l member i s the f r o n t cover o f t h e module ( s u p e r s t r a t e c o n f i g u r a t i o n ) , i t must be o p t i c a l l y c l e a r Ο 9 0 Î t r a n s m i s s i o n ) through the s o l a r spectrum o f i m p o r t a n c e t o a b s o r p t i o n by t h e s o l a r c e l l ( 0 . 4 - 1 . 1 m i c r o n s ) . I t must a l s o be r e l a t i v e l y hard (> 90 shore A durometer), s o i l r e p e l l e n t , p r e f e r a b l y UV absorbing below 0.36-0.37 microns, and non-permeable t o oxygen, water vapor, and atmospheric p o l l u t a n t s . I f t h e r i g i d member i s t h e back cover, i t may be opaque. C a n d i d a t e s , Cost. Tempered g l a s s i s t o date the best known material f o r a r i g i d f r o n t cover f o r s i l i c o n c e l l s . I t costs ~ $ 0 . 7 5 - $ 1 . 2 5 / f t f o r t h e l o w - i r o n c o n t e n t g l a s s i d e a l f o r PV a p p l i c a t i o n s . F o r a r i g i d back c o n f i g u r a t i o n , s u r f a c e p a s s i v a t e d s t e e l p r e s e n t l y a p p e a r s t o be t h e optimum choice f o r use w i t h s i l i c o n c e l l s , considering i t s thermal expansion c o e f f i c i e n t , w e a t h e r r e s i s t a n c e , a n d c o s t f o r t h e w e i g h t and s t i f f n e s s required. I t can range from $ 0 . 3 0 / f t f o r z i n c g a l v a n i z e d t o $3·00 for porcelainized. P a s s i v a t e d s t e e l i s a l s o non-permeable. Glass r e i n f o r c e d concrete, sealed hardwood, and aluminum are a l s o l o w - c o s t p o s s i b i l i t i e s , b u t each has s i g n i f i c a n t disadvantages compared t o s t e e l i n t h e a r e a s o f w e i g h t o r t h e r m a l e x p a n s i o n coefficients. Weather and C o r r o s i o n R e s i s t a n c e Requirements. There are many commercial ways t o " r u s t - p r o o f " s t e e l t o make i t weather resistant. They range from p a i n t , which i s u s u a l l y the cheapest but l e a s t e f f e c t i v e method, t o chromeplating, which i s one o f the most e x p e n s i v e and e f f e c t i v e methods. I n between, i n both cost 2

2

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and performance, are p o r c e l a i n enameling and v a r i o u s l o w m e l t i n g m e t a l d i p c o a t i n g s such as t i n , z i n c , aluminum, o r combinations thereof. P o r c e l a i n i z i n g i s on the expensive side (over $ 1 . 0 0 / f t , even i n l a r g e volume) b u t i s one o f the o l d e s t , most e f f e c t i v e ways t o p r o t e c t the s t e e l . Zinc g a l v a n i z i n g i s t h e most common lower cost method. M o s t o r d i n a r y " r u s t p r o o f " s t e e l i s n o t adequate as a substrate f o r solar c e l l s . The p r o t e c t i v e l a y e r must s t o p n o t o n l y g r o s s r u s t i n g , which would cause t o t a l delamination, but i t must a l s o h a l t even the s l i g h t e s t progress o f c o r r o s i o n o f e i t h e r the s t e e l o r any a c t i v e c o a t i n g metals on i t once the piece i s laminated t o the s o l a r c e l l s . T h i s i s because e v e n s l i g h t c o r r o s i o n g e n e r a t e s s m a l l amounts o f hydrogen long before the c o r r o s i o n l a y e r b u i l d s up s u f f i c i e n t l y t o cause adhesive f a i l u r e . I f hydrogen i s generated f a s t enough and i f the pottant l a y e r i s r e l a t i v e l y s o f t , t h e gas c o l l e c t s as bubbles behind t h e impermeable c e l l s o r g l a s s t r e s s p o i n t s on the c e l l a l s o reduce the d i e l e c t r i c standoff of the i n t e r v e n i n g e l e c t r i c a l l y i n s u l a t i n g l a y e r s and become peel s t r e s s p o i n t s f o r delamination propagation w i t h thermal c y c l i n g . Thus, no c o r r o s i o n can be t o l e r a t e d . P o r c e l a i n i z i n g i s t h e b e s t method f o r e l i m i n a t i n g s t e e l c o r r o s i o n . I t can be used only on r i g i d s u r f a c e s , however, s i n c e i t cracks w i t h very l i t t l e f l e x i n g . I t s e l e c t r i c a l p r o p e r t i e s a r e i n t r i n s i c a l l y very good but are f r e q u e n t l y degraded by c r a c k s , d i r t and p i n h o l e s . I t must a l s o be p r o p e r l y f i r e d s i n c e f i r i n g temperatures a f f e c t i t s s u r f a c e c h a r a c t e r i s t i c s f o r b o n d i n g t o s o l a r c e l l p o t t i n g polymers. I n s u f f i c i e n t l y f i r e d f i l m s are b a s i c enough t o r a p i d l y h y d r o l y z e t h e v i n y l a c e t a t e e s t e r g r o u p s i n e t h y l e n e / v i n y l acetate. Adhesion i s l o s t very q u i c k l y , w i t h r a p i d generation o f a c e t i c a c i d a t the i n t e r f a c e . P a i n t over z i n c g a l v a n i z i n g has been found t o b l i s t e r e a s i l y when exposed t o high humidity (100$ a t elevated temperatures up t o 100°C). P l a s t i c f i l m l a m i n a t e d t o z i n c g a l v a n i z i n g does not b l i s t e r as e a s i l y , but o u t g a s s i n g s t i l l occurs r e a d i l y . B l i s t e r i n g s t a r t s a t t h e c e l l o r module edges and moves i n . E v e n t u a l l y enough "white r u s t " develops t o produce d e l a m i n a t i o n . The q u a l i t y o r t y p e o f z i n c g a l v a n i z i n g such as d e g r e e o f p a s s i v a t i o n (chromate coatings, etc.) o r g r a i n s i z e t o a f f e c t the z i n c c o r r o s i o n r a t e s , w h i c h a r e q u i t e r a p i d i n even the best cases. Zinc/aluminum a l l o y s and aluminum coatings s t i l l corrode, but not as much as z i n c alone on s t e e l . Thin F i l m C e l l s . Future, l o w e r c o s t s o l a r c e l l m a t e r i a l s w i l l l i k e l y be more f l e x i b l e t h a n c r y s t a l l i n e s i l i c o n and t h e r e f o r e may not r e q u i r e a r i g i d member i n t h e module l a y up. They w i l l s t i l l need e l e c t r i c a l i s o l a t i o n and p r o t e c t i o n from abrasion and c o r r o s i o n , however, and w i l l thus s t i l l need p o t t a n t or p r o b a b l y t h i n n e r a d h e s i v e l a y e r s as w e l l as o u t e r covers/insulators. 2

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Pottant The p o t t a n t i s t h e s o f t , e l a s t o m e r i c , vibration-damping m a t e r i a l that immediately surrounds both s i d e s o f f r a g i l e s o l a r c e l l w a f e r s and t h e i r e l e c t r i c a l contacts and i n t e r c o n n e c t s . I t p r o t e c t s t h e c e l l s from s t r e s s e s due t o t h e r m a l expansion d i f f e r e n c e s and e x t e r n a l impact. I t i s o l a t e s them e l e c t r i c a l l y and helps p r o t e c t t h e i r m e t a l l i c contacts and i n t e r c o n n e c t s f r o m corrosion. O p t i c a l Requirements, Cost. The p o t t a n t must be h i g h l y transparent (> 90$ from 0.4 t o 1.1 μ ) , s e r v i n g a s an o p t i c a l c o u p l i n g medium t o provide maximum l i g h t t r a n s m i s s i o n t o the c e l l surface. Because i t i s used i n a f a i r l y t h i c k l a y e r f o r b r i t t l e c e l l s (10-20 m i l s on each c e l l s i d e ) , the pottant must be very inexpensive ( < $ 0 . 3 0 / f t , p r e f e r a b l y < $ 0 . 2 0 / f t ) . At 30 m i l s t o t a l , t h i s t r a n s l a t e s t o between $1.00 and $2.00/lb i n c l u d i n g compounding and f a b r i c a t i n i n h e r e n t l y weather r e s i s t a n s e v e r a l moderately s t a b l e m a t e r i a l s which can be upgraded w i t h s t a b i l i z e r s a r e i n t h i s range. Mechanical Requirements. The pottant m a t e r i a l should have a r e l a t i v e l y low modulus (< 2000 p s i a t 2 5 ° C ) . The maximum t o l e r a b l e m o d u l u s d e p e n d s on t h e d i f f e r e n c e i n e x p a n s i o n c o e f f i c i e n t s o f t h e c e l l s and t h e r i g i d member a n d o n t h e t h i c k n e s s o f t h e l a y e r between them. R e l a t i v e l y high modulus rubbers could be used but would r e q u i r e i n o r d i n a t e l y t h i c k and thus expensive l a y e r s t o damp out the expansion d i f f e r e n c e s . For example, w i t h an 1 / 8 - i n . - t h i c k g l a s s s u p e r s t r a t e and s i l i c o n c e l l s , which w i l l take 5000 p s i maximum l i n e a r s t r e s s , a pottant o f 1000 p s i modulus needs t o be a minimum o f o n l y 1 .5 m i l s each s i d e f o r a 1:1 s a f e t y f a c t o r i n s e r v i c e ; with a 2500 p s i modulus the minimum i s - 3.5 m i l s , e t c . A s a f e t y f a c t o r higher than 1:1 i s highly desirable. F a b r i c a t i o n technique i s a l s o a f a c t o r . For example, whether the c e l l s t r i n g s are pressed a g a i n s t the pottant while i t i s s t i l l c o l d and must cushion the c e l l s under pressure, or whether i t i s squeezed only w h i l e i t i s molten, or not a t a l l , can determine the minimum t h i c k n e s s t o l e r a n c e s f o r module f a b r i c a t i o n . The minimum usable pottant thickness can a l s o be l i m i t e d by the green s t r e n g t h of the m a t e r i a l i t s e l f i f i t i s f a b r i c a t e d as a c a s t sheet. If i t i s e x t r u s i o n - c o a t e d on t h e s u p p o r t and c o v e r m a t e r i a l s , t h e pottant can be l e s s tough and t h e r e f o r e t h i n n e r , but must be f r e e o f p i n h o l e s and o t h e r f l a w s t o p r e v e n t e l e c t r i c a l leakage. E x p e r i e n c e suggests a minimum thickness o f 10-15 m i l s t o achieve the n e c e s s a r y freedom f r o m f l a w s f o r s u f f i c i e n t electrical i n s u l a t i o n p r o p e r t i e s as w e l l as ease o f handling. The p o t t i n g m a t e r i a l must have a g l a s s t r a n s i t i o n temperature b e l o w t h e l o w e s t t e m p e r a t u r e e x t r e m e t h e PV module m i g h t experience, which i s — 4 0 ° C . The m a t e r i a l must remain rubbery i n o r d e r t o damp i m p a c t s a n d v i b r a t i o n o f t h e f r a g i l e c e l l s . 2

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S i m i l a r l y , i t must e x h i b i t no s i g n i f i c a n t mechanical creep at the upper o p e r a t i n g t e m p e r a t u r e extreme o f 90°C i n o r d e r f o r the layup, c e l l p o s i t i o n s , e t c . to r e m a i n i n t a c t when t i l t e d a t an angle f a c i n g the sun. The pottant must e x h i b i t strong, m o i s t u r e - r e s i s t a n t adhesion (>10 l b / i n . peel strength) to a l l the s u r f a c e s i t must bond t o , over a 20-year l i f e t i m e . The 10 l b / i n . peel strength may decrease to 5 l b / i n . while the p o t t a n t or a d h e s i v e s a r e s a t u r a t e d w i t h water i n a non-hermetic design as long as i t recovers to w i t h i n 10$ o f the o r i g i n a l v a l u e when r e d r i e d a t up t o the n o m i n a l o p e r a t i n g c e l l temperature of the p a r t i c u l a r design. Moisturer e s i s t a n t adhesion i s tested mostly by exposure to 100°C/100$ RH. Ten months a t these c o n d i t i o n s would equal 20 years at 70°C, 50$ RH, but u s u a l l y the f i r s t week or two w i l l separate the good bonds from the poor ones. Exposure to b o i l i n g water f o r a few hours or overnight i s a l s o a good i n i t i a l screening technique. I n a vacuum l a m i n a t i o viscosity/temperatur better. The l a y e r s need t o be dry and n o n - t a c k y d u r i n g t h e i n i t i a l evacuation step so as not to trap a i r between them. At the same time, the pottant must then melt to as f l u i d a s t a t e as p o s s i b l e i n o r d e r t o e f f e c t i v e l y p e n e t r a t e and wet a l l t h e i r r e g u l a r i t i e s o f the c e l l c i r c u i t . Block or g r a f t thermoplastic elastomers with r e l a t i v e l y low m o l e c u l a r w e i g h t amorphous segments o f a weather r e s i s t a n t saturated backbone have the p o t e n t i a l of being s u p e r i o r p o l y m e r s for potting solar cells. The c r o s s l i n k f o r m i n g c r y s t a l l i n e segments make r e l a t i v e l y s o f t , low m o l e c u l a r w e i g h t , r u b b e r y polymers handle w e l l . They e x h i b i t h i g h cohesive strength or toughness and low s u r f a c e tack when the c r y s t a l l i n e domains a r e s o l i d i f i e d (see Figure 3 ) . Candidates, Free r a d i c a l polymerized vinyl or a c r y l i c / e t h y l e n e copolymers made i n h i g h p r e s s u r e p o l y e t h y l e n e r e a c t o r s have been shown by E. Cuddihy to be block polymers of pure c r y s t a l l i n e homopolyethylene and amorphous high v i n y l acetate (- 70 weight $) or methyl a c r y l a t e - c o - e t h y l e n e segments. When the c r y s t a l l i t e s are submicron i n s i z e as i n DuPont's Elvax 150, they do not s i g n i f i c a n t l y s c a t t e r the i n c o m i n g l i g h t . A number of l a b o r a t o r i e s have shown, however, t h a t even when some l i g h t s c a t t e r i n g o c c u r s , i t does not n e c e s s a r i l y decrease s o l a r c e l l output. For example, i t was found that e n c a p s u l a t i n g a 3 - m i l t h i c k non-woven g l a s s mat i n 15 m i l s o f e t h y l e n e / v i n y l acetate (EVA) drops the t r a n s m i s s i o n by 60$. The same c o m p o s i t e when fused to the f r o n t of a textured ( a n t i r e f l e c t i v e treated) s i l i c o n s o l a r c e l l does not drop the output a t a l l unless d i s c o l o r i n g from d e g r a d a t i o n t a k e s p l a c e . Aromatic c r y s t a l l i n e segments such as polystyrene are undesirable even i n i t i a l l y because the degradation products are l i g h t absorbing. A G u l f O i l ethylene/methyl a c r y l a t e rubber of 20 weight $ EMA which i s not n e a r l y as t r a n s p a r e n t as E l v a x 150 EVA i s b e i n g 1

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Advantages •

Reversible cure by simple

time • Steep viscosity vs temperature curve •

High ceiling temperature for processing

•

No outgassing from decomposing peroxides

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Low (room temp) tack

Unknowns

Figure 3 ·

•

Availability of materials of proven weather resistance

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UV sensitivity of aromatic crystalline blocks

Structures o f Thermoplastic Elastomers*

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e v a l u a t e d by a number of l a b o r a t o r i e s , and appears t o o f f e r some a t t r a c t i v e p r o p e r t i e s . I t i s more thermally s t a b l e and does n o t appear t o need a s much c r o s s l i n k i n g t o prevent creep. It i s considerably l e s s transparent, however, p a r t i c u l a r l y when uncured. Lower m e l t i n d e x E l v a x e s a r e a l s o p o s s i b i l i t i e s f o r a noncuring t h e r m o p l a s t i c elastomer p o t t a n t . P l a s t i c i z e d p o l y v i n y l b u t y r a l (PVB) i s easy t o p r o c e s s because i t i s t h e r m o p l a s t i c r a t h e r than thermosetting, but has the disadvantage o f c o n t a i n i n g p l a s t i c i z e r (see below). There are some other advantages o f t h e r m o p l a s t i c e l a s t o m e r s as p o t t i n g m a t e r i a l s f o r s o l a r c e l l s besides t h e steep melt v i s c o s i t y curve. They c r o s s l i n k by c o o l i n g so t h a t l o n g c u r e t i m e s a r e u n n e c e s s a r y and t h e c u r e i s r e v e r s i b l e t o help t h e recovery o f f l a w e d p a n e l s . They have no c e i l i n g f a b r i c a t i o n t e m p e r a t u r e above which the cure w i l l s e t o f f nd/or outgassing can occur. Thus t h e r m o p l a s t i c elastomers a l l o w more l a t i t u d e than i s a v a i l a b l e w i t h a thermosettin a d j u s t v i s c o s i t y f o r optimu P l a s t i c i z e d PVB, used e x t e n s i v e l y i n laminated s a f e t y g l a s s , has been s t u d i e d . I t i s expensive and e a s i l y degraded when n o t h e r m e t i c a l l y sealed.3 This can be compensated f o r by s e a l i n g i t between a g l a s s f r o n t and metal f o i l back cover i n a s u p e r s t r a t e design module. E l e c t r i c a l Requirements, The p o t t a n t s h o u l d c o n t a i n no p l a s t i c i z e r because p l a s t i c i z e r can reduce the volume r e s i s t i v i t y of a polymer d r a s t i c a l l y . I t reduces t h e r e s i s t i v i t y of PVB by 5 orders o f magnitude i n some f o r m u l a t i o n s . PVB w i t h 40$ d i e s t e r p l a s t i c i z e r measures only 1 0 ohm-cm i n laminated form a t room temperature whereas i t measures 1 0 ^ ohm-cm w i t h the p l a s t i c i z e r d r i v e n o u t . Volume r e s i s t i v i t i e s o f 1 0 ohm-cm or l e s s w i l l conduct s m a l l amounts o f c u r r e n t f a i r l y r e a d i l y , a l b e i t s l o w l y . ( F o r example, a r e s o l v e d 5 l i n e pair/mm charge image has been observed t o b l u r w i t h i n t h e f i r s t few seconds when placed on the s u r f a c e o f a f i l m o r immersed i n a l i a u i d o f 1 0 - 1 0 ohm-cm resistivity. The same image on o r i n 1 0 ^ ohm-cm m a t e r i a l w i l l not b l u r f o r s e v e r a l hours. On a 1 0 ^ " ^ ohm-cm m a t e r i a l an image w i l l l a s t unblurred from weeks t o months.) Because i o n i c i m p u r i t y m o b i l i t y determines r e s i s t i v i t y i n a polymer r a t h e r than e l e c t r o n m o b i l i t y as i n metals, higher module o p e r a t i n g t e m p e r a t u r e s drop r a t h e r t h a n r a i s e the r e s i s t i v i t y because o f the v i s c o s i t y drop w i t h t e m p e r a t u r e . The v i s c o s i t y d r o p i n p l a s t i c i z e d PVB w i t h t e m p e r a t u r e i s e x t r e m e l y steep. Indeed, a f t e r o n l y a day o f d r y oven a g i n g a t 150°C, an open p l a s t i c i z e d PVB f i l m i s b r i t t l e from t o t a l l o s s o f the p l a s t i c i z e r and already s i g n i f i c a n t l y d i s c o l o r e d from o x i d a t i v e d e g r a d a t i o n . The volume r e s i s t i v i t y , however, r i s e s t o 1 0 ^ ohm-cm from t h e o r i g i n a l 1 0 ohm-cm by the removal of p l a s t i c i z e r . As an added d i f f i c u l t y , the p l a s t i c i z e r s s o l v a t i o n e f f e c t i n PVB a p p e a r s t o e n h a n c e t h e p o l y m e r v i s c o s i t y d r o p w i t h temperature. U n f o r t u n a t e l y , t h i s r a i s e s t h e leakage current o f a 1

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module a t 1.5 kV by over an order o f magnitude with an o p e r a t i n g t e m p e r a t u r e r i s e of only 25°C when an u n p l a s t i c i z e d , high volume r e s i s t i v i t y b a r r i e r l a y e r i s not present between the c i r c u i t and any grounded m e t a l s u r f a c e s i n c l o s e proximity (see Figure 4 ) . 25°C i s t h e normal r i s e f o r a module between d a r k and f u l l y i l l u m i n a t e d a t a f u l l sun f l u x o f 100 mW/cm . The volume r e s i s t i v i t y o f an u n p l a s t i c i z e d p o t t a n t m a t e r i a l such as EVA i s 1 0 ^ ohm-cm. The module current leakage a t 1.5 kV with EVA i s an order o f magnitude lower than with p l a s t i c i z e d PVB a t 25-30°C and t h e r e a p p e a r s t o be no r i s e i n leakage a t ~ 5060°C. (See F i g u r e 4 and Table I.) S i m i l a r l y , the c u r r e n t leakage o f modules c o n t a i n i n g p l a s t i c i z e d PVB c a n be b l o c k e d by t h e i n s e r t i o n of an a d d i t i o n a l h i g h volume r e s i s t i v i t y l a y e r such a s p o l y e t h y l e n e t e r e p h t h a l a t e f i l m as d i s c u s s e d below, which i s r e s i s t a n t to the s o l v a t i o n e f f e c t of d i e s t e r p l a s t i c i z e r s . 2

1

Table I· Volume Volume r e s i s t i v i t y value Material

Measured Initial

PVB* EVA PVB (Tedlar) PET (Mylar) *Plasticized **Most l i k e l y PVB

1 1

10 10 10 *** 10 ? 1 4

1i

Literature

Dry Oven Aged (150°C) 10 10 5 x 10

1 6

1 4

1 4

1

reduced

when " e f f e c t i v e l y " laminated

101* 1018

to p l a s t i c i z e d

The p o t t a n t s h o u l d have a d i e l e c t r i c s t r e n g t h o f a t l e a s t 400-500 v o l t s / m i l , w h i c h i s t y p i c a l f o r u n o r i e n t e d amorphous polymers. S i n c e t h e p o t t a n t i s d e s i g n e d t o flow, however, i t cannot be r e l i e d upon t o provide s u f f i c i e n t d i e l e c t r i c s t a n d o f f by itself. I t w i l l tend t o move o u t o f t h e a r e a s where i t i s mechanically s t r e s s e d (squeezed), u n f o r t u n a t e l y , those areas are u s u a l l y a l s o t h e a r e a s o f highest e l e c t r i c a l s t r e s s s i n c e f i e l d l i n e s a r e the densest around i r r e g u l a r i t i e s i n the geometry o f the c i r c u i t m e t a l , e.g., i n t e r c o n n e c t r i b b o n or wire kinks, excess s o l d e r beads, e t c . A n o n - s o f t e n i n g , h i g h volume r e s i s t i v i t y i n s u l a t o r l a y e r i s thus needed t o guarantee c i r c u i t i s o l a t i o n . Chemical Requirements. The pottant must be s t a b l e ; that i s , chemically r e s i s t a n t to o x i d a t i o n and h y d r o l y s i s unless p r o t e c t e d i n a hermetic package, t o r e d u c t i o n by metals, and t o o u t g a s s i n g o f d i s s o l v e d g a s e s o r l i q u i d s o r d e c o m p o s i t i o n products under normal o p e r a t i n g c o n d i t i o n s o f -40°C to +90°C f o r 20 y e a r s . The need f o r chemical s t a b i l i t y i s e s p e c i a l l y s t r i n g e n t when a lower cost non-hermetic design i s used. Even when a hermetic package i s

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

376

POLYMERS

Figure 4 .

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Module Leakage Current vs.Temperature.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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377

u s e d , however, some s t a b i l i t y i s r e q u i r e d because t h e s m a l l amounts of d i s s o l v e d oxygen and moisture w h i c h w i l l be f o u n d i n any a m o r p h o u s , r u b b e r y m a t e r i a l c a n s t i l l cause n o t i c e a b l e d i s c o l o r a t i o n of a p o t t a n t , p a r t i c u l a r l y when c a t a l y z e d by c e r t a i n metal o x i d e s i f present i n the metallized c e l l contacts.3 Degradation i n a hermetic package can a l s o occur from the extreme heat t h a t can develop under "hot spot" c o n d i t i o n s which can occur i n i s o l a t e d areas of a module when e l e c t r i c a l mismatching of c e l l output i n a s e r i e s s t r i n g without diodes has occurred such as may be i n i t i a t e d by shadowing of c e l l s . T h e i d e a l m a t e r i a l would t o l e r a t e a t l e a s t i n t e r m i t t e n t e x c u r s i o n s up t o the m e l t i n g p o i n t of s o l d e r (~ 1°0°C) without d i s c o l o r i n g , e m b r i t t l i n g or r e v e r t i n g , outgassing, or breaking down e l e c t r i c a l l y . O x i d a t i v e breakdown of polymers can f o l l o w one o r more paths of change i n p h y s i c a l and chemical p r o p e r t i e s . O x i d a t i v e a t t a c k can be c a t a l y z e d by h e a t UV l i g h t c e r t a i n metals or metal o x i d e s , sometimes m o i s t u r e f u n c t i o n a l weaknesses oxygen r e g a r d l e s s o f the s p e c i f i c mechanisms by which the energy i s a c t u a l l y absorbed. The r e s u l t i s u s u a l l y e i t h e r embrittlement, c h a i n s c i s s i o n , h y d r o l y s i s , or combinations o f these. Embrittlement comes from e x t e n s i v e c r o s s l i n k i n g . C h a i n s c i s s i o n or r e v e r s i o n r e s u l t s i n e x t e n s i v e molecular weight l o s s and both c h a i n s c i s s i o n and h y d r o l y s i s r e s u l t i n t h e f o r m a t i o n o f h y d r o p h i l i c end g r o u p s . When both embrittlement and r e v e r s i o n occur s i m u l t a n e o u s l y , t h e r e s u l t i s t h a t t h e o v e r a l l p h y s i c a l p r o p e r t i e s , which depend mostly on molecular weight, o f t e n remain u n c h a n g e d f o r some t i m e u n t i l o n e m e c h a n i s m dominates. Embrittlement w i l l a l l o w i n c r e a s e d mechanical s t r e s s t o reach the s o l a r c e l l s , and i n c r e a s e s t h e p o s s i b i l i t i e s o f i n t e r f a c e d e l a m i n a t i o n from s t r e s s concentrations. Reversion allows d i s t o r t i o n or creeping of the l a y e r s , l e s s mechanical p r o t e c t i o n of c e l l s , and e a s i e r bubble formation from outgassing. The c h e m i c a l c h a r a c t e r o f a polymer o f t e n c h a n g e s more r a p i d l y w i t h o x i d a t i v e breakdown than the mechanical p r o p e r t i e s . H y d r o p h i l i c group formation changes the m o i s t u r e a b s o r p t i o n and p e r m e a b i l i t y o f t h e m a t e r i a l , a problem when p r o t e c t i n g metals from c o r r o s i o n as i n a PV module. H y d r o p h i l i c group f o r m a t i o n r e d u c e s t h e m o i s t u r e r e s i s t a n c e o f adhesive bonds, changing the a c i d - b a s e c h a r a c t e r i s t i c s o f t h e b o n d e d i n t e r f a c e s . The d e v e l o p m e n t o f c o n j u g a t e d c a r b o n - c a r b o n and c a r b o n - o x y g e n unsaturated color c e n t e r s changes the o p t i c a l a b s o r p t i o n p r o p e r t i e s o f the m a t e r i a l , reducing the amount o f l i g h t r e a c h i n g the s o l a r c e l l s and t h u s r e d u c i n g t h e i r e l e c t r i c a l o u t p u t . Reversion o r h y d r o l y s i s can a l s o generate v o l a t i l e s p e c i e s o f very low molecular weight, which can evolve from the polymer, l e a v i n g v o i d s which degrade v o l t a g e s t a n d o f f , reduce the o p t i c a l c o u p l i n g of l i g h t t o the c e l l s and create s t r e s s p o i n t s f o r c e l l f r a c t u r e . The e x a c t amount o f d i s s o l v e d v o l a t i l e s p e c i e s t h a t can be t o l e r a t e d i n a n e n c a p s u l a t e d PV system depends o n t h e v a p o r

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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p r e s s u r e of the p a r t i c u l a r d i s s o l v e d s p e c i e s i n the p a r t i c u l a r pottant medium and on the v i s c o s i t y of t h a t p o t t a n t a t module o p e r a t i n g temperatures. The i m p o r t a n c e o f m i n i m i z i n g such d i s s o l v e d v o l a t i l e s was d i s c o v e r e d by ARCO S o l a r i n o u t d o o r t e s t i n g o f t h e f i r s t s u b s t r a t e d e s i g n r o o f t o p module. The encapsulated c e l l s began b u l g i n g up and c r a c k i n g as the a r r a y r e a c h e d summer o p e r a t i n g temperatures f o r the f i r s t time. The p o t t a n t was a s t a b i l i z e d t r a n s p a r e n t e t h y l e n e / v i n y l a c e t a t e pottant based on Elvax 150.7 The outer c o v e r / i n s u l a t o r l a y e r was a f l e x i b l e a c r y l i c copolymer f i l m . The b u l g i n g c e l l s r e s u l t e d from a c o m b i n a t i o n o f poor adhesion between unprimed s u r f a c e s and outgassing of the v o l a t i l e d e c o m p o s i t i o n p r o d u c t s o f the l a r g e amount of peroxide used to c r o s s l i n k the EVA. The a c r y l i c copolymer used melts under normal l a m i n a t i o n c o n d i t i o n s so the edges became buried and a peel was d i f f i c u l t to s t a r t . Although other i n v e s t i g a t o r s ? t & have reported that a c r y l i c f i l m and EV to be very weak (~ 1-2 l b / i n . amount o f 2 , 5 - d i m e t h y l - 2 , 5 - b i s - ( t - b u t y 1 p e r o x y ) hexane c r o s s l i n k i n g agent ( L u p e r s o l 101 by Pennwalt) was used by the designers to i n s u r e the m a t e r i a l would s u f f i c i e n t l y c r o s s l i n k even when heated very slowly. Lupersol 101 forms s i g n i f i c a n t amounts o f a c e t o n e and t - b u t a n o l when i t decomposes i n a d d i t i o n t o methane, ethane, and e t h y l e n e (see F i g u r e 5 ) . Acetone and t b u t a n o l a r e not e f f e c t i v e l y removed from EVA during most vacuum p r o c e s s i n g and r e l i q u i f y u p o n c o o l i n g t h e m o d u l e t o room temperature. When the modules i n the outdoor array began to reach summer operating temperatures of 75-80°C, the v a p o r i z i n g t r a p p e d l i q u i d s began to b u i l d up pressure behind the c e l l s from the l a r g e volume i n c r e a s e of v a p o r i z a t i o n . When the vapor p r e s s u r e became g r e a t e r t h a n the combined adhesive strength of the bonded l a y e r s and the f l e x u r a l strength of the c e l l s , delamination began, w i t h the c e l l s b u l g i n g and c r a c k i n g . The problem was solved by a c o m b i n a t i o n o f a l t e r i n g the c u r i n g s y s t e m and r a i s i n g the adhesion. The p o t t a n t must be c h e m i c a l l y i n e r t i n that i t must not r e a c t with the metals or other s u r f a c e s i t bonds t o . Related to t h i s i s the need f o r i t to e x h i b i t l i t t l e or no water absorption to c o r r o d e m e t a l s bonded t o i t or t o r e d u c e i t s volume resistivity. The l a r g e f r a c t i o n of homopolyethylene i n the EVAs we are studying make them q u i t e i n e r t and low i n water absorption. As e x p l a i n e d above, the pottant should c o n t a i n l i t t l e or no p l a s t i c i z e r s i n c e i t can generate e l e c t r i c a l problems. The l a s t chemical requirement f o r the pottant i s t h a t i t s melt e q u i l i b r i u m contact angle with a l l the surfaces to which i t bonds be as low as p o s s i b l e below 90°C. This speeds p r o c e s s i n g as w e l l as maximizing adhesion and minimizing the c o l l e c t i o n of water and oxygen a t the i n t e r f a c e s t o r e d u c e m e t a l c o r r o s i o n and metal oxide c a t a l y z e d polymer changes to form c o l o r centers. T h i n F i l m Systems, As p r e v i o u s l y mentioned, the same types

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

23.

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CH

I

w

I

H C — C — Ο ­

Ι

CH

CH

cm

3

I 3

379

Encapsulant Material Requirements

«(CH ), 2

3

I

-ο

Ι

3

CH

3

CH-

CH

CH,

3

Δ

CH H C»

I

Ο

3

II • OH + H C

3

3

.c

" f

CH

Η Η >CH + C H + C = C + H C — Η Η 3

4

3

3

b.p. 80°C Figure 5.

b.p. 60°C

Gases

Decomposition o f L u p e r s o l 101.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

C H , etc. 3

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of m a t e r i a l s w i t h the same o p t i c a l , e l e c t r i c a l , chemical, and some of the same mechanical requirements ( p a r t i c u l a r l y bond strengths) would be needed f o r t h i n f i l m c e l l e n c a p s u l a t i o n . A l l the p r e v i o u s l y mentioned candidates would q u a l i f y but could probably be used i n t h i n n e r l a y e r s . Because l e s s c r i t i c a l m e c h a n i c a l requirements enable the use o f t h i n n e r l a y e r s , the f i e l d opens t o i n c l u d e more expensive p o s s i b i l i t i e s such a s s i l i c o n e s . Liquid systems such as 100? s o l i d s c a s t i n g m a t e r i a l s o r s o l u t i o n a p p l i e d coatings (by spray, d i p , brush, r o l l e r , e t c . ) a l s o become more p r a c t i c a l f o r a r a p i d throughput f a c t o r y . C a s t i n g l i q u i d s a r e a p o s s i b i l i t y even now but are more complicated t o use w i t h vacuum p r o c e s s i n g than a r e sheets. Outer Cover/Insulator The o u t e r c o v e r i s t h e t o u g h , h a r d , s o i l - r e s i s t a n t , i n h e r e n t l y weather r e s i s t a n pottant l a y e r s from th I t augments the p o t t a n t i n e l e c t r i c a l l y i s o l a t i n g the c i r c u i t and p r e f e r a b l y a c t s a s a UV s c r e e n i f used on t h e f r o n t . It is u s u a l l y a f l e x i b l e o r conforming p l a s t i c and may be a f i l m o r a solution-applied coating. O p t i c a l , C h e m i c a l , and Cost R e q u i r e m e n t s . I f t h e outer c o v e r / i n s u l a t o r i s the f r o n t cover on a r i g i d - b a c k module, i t must have the same o p t i c a l and chemical p r o p e r t i e s as the p o t t a n t ; t h a t i s , high t r a n s m i s s i o n , good o p t i c a l c o u p l i n g , and inherent weather resistance. Being i n h e r e n t l y weather r e s i s t a n t means meeting a l l the c r i t e r i a p r e v i o u s l y d e s c r i b e d under c h e m i c a l stability requirements f o r p o t t a n t s without need o f added s t a b i l i z e r s . The only known i n h e r e n t l y w e a t h e r r e s i s t a n t o r g a n i c m a t e r i a l s a r e a c r y l i c s , s i l i c o n e s , and f l u o r o c a r b o n s , ranging from $3-5/lb f o r a c r y l i c s , t o $8-10/lb f o r s i l i c o n e s , t o $10-20/lb f o r fluorocarbons. However, i f t h e cover m a t e r i a l i s s u f f i c i e n t l y tough and f l e x i b l e , i t can be q u i t e t h i n (1-4 m i l s ) and c a n be made from a more expensive polymer than the p o t t a n t . I t can cost up t o $10-15/lb and s t i l l be economical. I f t h i n p l a s t i c i s used a s a f r o n t c o v e r i n a s u b s t r a t e design module, i t w i l l not provide a hermetic s e a l no matter how o r i e n t e d t h e m i c r o s t r u c t u r e i s . The o x y g e n a n d w a t e r p e r m e a b i l i t i e s may be low but f i n i t e . The main q u e s t i o n , y e t t o be f u l l y answered, i s whether a non-hermetic package can l a s t f o r 20 years. S i n g l e - c r y s t a l s i l i c o n s o l a r c e l l s themselves are known t o be f a i r l y i n e r t t o t h e e f f e c t s o f heat, l i g h t , oxygen, and water. A c c e l e r a t e d c o r r o s i o n t e s t s a r e i n p r o g r e s s on m i n i c i r c u i t s t o determine the s t a b i l i t y o f the c e l l contacts and i n t e r c o n n e c t systems ( c o p p e r r i b b o n s , s o l d e r , e t c . ) . The s t a b i l i t y o f f u t u r e c e l l s t h e m s e l v e s w i l l be t h e r e m a i n i n g q u e s t i o n t o be answered f o r f u t u r e PV modules. The back cover, may, o f course, be opaque. For stand-alone, glass-front arrays i t i s preferably white i n c o l o r . This i s

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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because l i g h t s c a t t e r e d by t h a t white surface i s r e f r a c t e d i n the g l a s s and enhances the t o t a l module output by about 5%· There i s l i t t l e or no enhancement from a white background, however, w i t h a t h i n p l a s t i c f r o n t and a r c h i t e c t s have objected to the p o l k a d o t appearance i n r o o f t o p a p p l i c a t i o n s . Thus, the r o o f t o p module design has a b l a c k - c o l o r e d back p l a s t i c l a y e r w h i c h matches the c e l l s w e l l . I t does not i n c r e a s e the module temperature over a white back f i l m by more than a few degrees. The back cover must be i n h e r e n t l y weather r e s i s t a n t as must the f r o n t but i t s u l t r a v i o l e t s t a b i l i t y can be more e f f e c t i v e l y enhanced w i t h l i g h t absorbing pigments as w e l l as transparent UV stabilizers. Thin p l a s t i c a l o n e on the back cannot p r o v i d e a hermetic s e a l , but because i t can be opaque, a metal f o i l such as s t e e l o r aluminum may be used i n the back c o v e r c o m p o s i t e t o e f f e c t a hermetic s e a l except at the edges, when g l a s s i s used as the f r o n t cover. A minimum t h r e e - l a y e r laminate i s r e q u i r e d f o r a back c o v e r c o m p o s i t e softening e l e c t r i c a l insulato f o i l and i s p r o t e c t e d from oxygen. The metal f o i l l a y e r i s the hermetic b a r r i e r and the t h i r d l a y e r i s the t r u e outer cover where weather r e s i s t a n c e i s more s t r i n g e n t l y r e q u i r e d . Mechanical Requirements. The outer cover p l a s t i c f i l m must be tough and f l e x i b l e t o r e s i s t a b r a s i o n and gouging, both i n manufacture and i n the f i e l d . I f i t i s the f r o n t cover, i t must a l s o be r e l a t i v e l y s o i l r e s i s t a n t . E i t h e r f r o n t or back covers must form r e l i a b l e , m o i s t u r e - r e s i s t a n t bonds to the pottant and to the f o i l i n the case of a back cover. Moisture r e s i s t a n t adhesion i s e s p e c i a l l y important a t the edges where moisture can penetrate even a "hermetic" d e s i g n . L a s t l y , the c o v e r l a y e r s must be dimensionally stable (non-shrinking or y i e l d i n g ) t o thermal c y c l i n g s t r e s s e s of manufacture and f i e l d o p e r a t i n g c o n d i t i o n s . E l e c t r i c a l Requirements, The o u t e r c o v e r must be made o f h i g h v o l u m e r e s i s t i v i t y m a t e r i a l t h a t does not s o f t e n a t l a m i n a t i o n temperatures i n o r d e r t o c o n t r o l e l e c t r i c a l c u r r e n t leakage through i t . Outer covers can be a p p l i e d as o r i e n t e d f i l m s by l a m i n a t i o n , a s l i q u i d o r powder c o a t i n g s by e l e c t r o s t a t i c spray, d i p , brush or r o l l e r , or by e x t r u s i o n . Oriented f i l m s have by f a r the b e s t e l e c t r i c a l p r o p e r t i e s i n terms o f d i e l e c t r i c s t r e n g t h ( v o l t a g e s t a n d o f f per u n i t t h i c k n e s s ) . Only a c r y l i c s could r e a l l y be considered economical i n l i q u i d or powder coatings b e c a u s e o f t h e g r e a t e r t h i c k n e s s e s r e q u i r e d w i t h o u t the o r i e n t a t i o n of a blown f i l m . Oriented f i l m s are a l s o mechanically t h e t o u g h e s t f i l m s and have the lowest gas p e r m e a b i l i t y of most p l a s t i c s because o f the i n c r e a s e d d e n s i t y a n d i n d u c e d c r y s t a l l i n i t y of t h e i r s t r u c t u r e . A f a i r l y l a r g e margin i n terms of d i e l e c t r i c s t a n d o f f i s r e q u i r e d between t h e t e s t o r u s e v o l t a g e s and the p a r a l l e l p l a t e d i e l e c t r i c s t r e n g t h values of the i n s u l a t i n g l a y e r ( s ) because, a s p r e v i o u s l y m e n t i o n e d , t h e i r r e g u l a r i t i e s o r geometry o f the c i r c u i t g i v e r i s e t o u n p r e d i c t a b l e f i e l d concentrations f o r leakage or breakdown. w

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

w

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Candidates. The only commercially a v a i l a b l e o r i e n t e d f i l m s known a t t h i s time which f i t the weather r e s i s t a n c e r e q u i r e m e n t s are p o l y v i n y l i d e n e fluoride ( P V F 2 ) , polyvinyl fluoride (Tedlar), polymethyl m e t h a c r y l a t e (PMMA), and p o l y b u t y l a c r y l a t e / m e t h y l m e t h a c r y l a t e copolymer (PBA/MMA). P V F 2 i s c u r r e n t l y expensive. PBA/MMA i s inexpensive but i n c l e a r form does n o t appear t o be s u f f i c i e n t l y o x i d a t i v e l y s t a b l e f o r our purposes. I t i s a l s o too water s e n s i t i v e and t o o e a s i l y s o f t e n e d i n many l a m i n a t i n g processes. PMMA a p p e a r s t o be somewhat more chemically s t a b l e than PBA/MMA and i s a l s o r e l a t i v e l y inexpensive, but has the same d i m e n s i o n a l s t a b i l i t y problems a t 150°C, t h e normal p o t t a n t processing temperature. Both a c r y l i c s maintain e x c e l l e n t o p t i c a l c l a r i t y on heat aging, however. T e d l a r i s m o d e r a t e i n c o s t a n d h a s known l o n g - t e r m performance o u t - o f - d o o r s . I t h a s e x c e l l e n t toughness, good weather r e s i s t a n c e , and m o d e r a t e l y good e l e c t r i c a l and o p t i c a l performance. I t s therma adequate (2-6$ shrinkag higher than optimum, but can be used as a t h i n f i l m , e s p e c i a l l y when c o u p l e d with l e s s expensive polyethylene t e r e p h t h a l a t e f i l m f o r b e t t e r e l e c t r i c a l p r o p e r t i e s a t a lower c o s t . Thin F i l m Systems, An i d e a l l o w - c o s t system c o u l d be continuously processed i n t o r o l l s of a r r a y s . These r o l l s would consist of a clear, f l e x i b l e , e l e c t r i c a l l y insulating plastic f r o n t cover, a t h i n l a y e r o f pottant or adhesive on e i t h e r s i d e o f t h e f l e x i b l e t h i n f i l m PV c i r c u i t , and an opaque, f l e x i b l e , e l e c t r i c a l l y i n s u l a t i n g p l a s t i c back c o v e r . These r o l l s c o u l d t h e n s i m p l y be u n r o l l e d on t h e r o o f , n a i l e d i n t o p l a c e , and connected t o the household c i r c u i t r y . A l l components would have t o be f l e x i b l e . Figure 6 i l l u s t r a t e s the p o s s i b l e components o f an a l l f l e x i b l e system — both hermetic and non-hermetic depending on f u t u r e c e l l requirements.

Adhesives, Primers, Surface M o d i f i c a t i o n s Good adhesion peel strengths a t an i n t e r f a c e r e s u l t from a c o m b i n a t i o n o f s e v e r a l i n t e r a c t i n g phenomena. The i n t e r f a c i a l f o r c e s are a combination o f d i s p e r s i o n (van der Waals). p o l a r , and acid-base i n t e r a c t i o n f o r c e s across the interface.9-13 They w i l l determine long-term r e l i a b i l i t y . Besides the i n t e r f a c i a l f o r c e s i n v o l v e d , t h e r h e o l o g y o f t h e m a t e r i a l s a t the i n t e r f a c e has a l a r g e i f not dominant i n f l u e n c e i n determining the p e e l s t r e n g t h o r d e l a m i n a t i o n t e n d e n c i e s o f a p a r t i c u l a r bond. This i s p a r t i c u l a r l y true because o f the t h e r m a l c y c l i n g s t r e s s e s a PV array i s subjected t o . The goal i s t o have cohesive r a t h e r than adhesive f a i l u r e a t the i n t e r f a c e . I f the rheology i s such that s t r e s s c o n c e n t r a t i o n s o c c u r , t h e cohesive f a i l u r e can be q u i t e low. This i s p a r t i c u l a r l y true i f a bubble i s t r a p p e d i n a PV module. E x p a n s i o n and c o n t r a c t i o n o f trapped or e v o l v i n g gases during thermal c y c l i n g generate h i g h l y c o n c e n t r a t e d s t r e s s e s a t

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Figure 6 .

Thin Film F l e x i b l e Module.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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t h e b u b b l e p e r i m e t e r by n a t u r e o f t h e geometry. At l e a s t one component of any module i n t e r f a c e should be f l e x i b l e , a s i s t h e s o f t p o t t a n t , t o spread these f o r c e s as much as p o s s i b l e . I f p r o p e r l y f u n c t i o n a l adhesives or c o u p l i n g a g e n t s a n d / o r o t h e r s u r f a c e m o d i f i c a t i o n s a r e used where p o s s i b l e (such as changing the a c i d i c or b a s i c character of one or both s u r f a c e s a t an i n t e r f a c e ) , the r e s u l t should always be a s t a b l e bond. Edge Sealants and Frames An edge s e a l a n t i s needed i f the module i s t o have an added frame. I t i s p a r t i c u l a r l y important i f one i s making a module w i t h two impermeable outside l a y e r s , e.g., g l a s s and metal f o i l , f o r a " h e r m e t i c a l l y * sealed package. The s e a l s h o u l d be as gas and moisture t i g h t as p o s s i b l e a t the module edges i f the pottant l a y e r s being sealed between t h e impermeable l a y e r s a r e a t a l l o x i d a t i v e l y or h y d r o l y t i c a l l as i s PVB. EVA does no mechanical cushion. Edge S e a l a n t s . M e c h a n i c a l l y , the edge s e a l i n g m a t e r i a l f o r a module which contains a h y g r o s c o p i c p o t t a n t must have m o i s t u r e r e s i s t a n t adhesion o f Σ 10 l b / i n . or cohesive f a i l u r e t o a l l the s u r f a c e s i t touches: frame, g l a s s , p o t t a n t , and f o i l c o v e r s . I t must be v e r y f l e x i b l e ( 5 x 1 0 p s i ) and r e l a t i v e l y l o w t h e r m a l e x p a n s i o n c o e f f i c i e n t as c l o s e a s p o s s i b l e t o g l a s s so the channel w i l l not have t o be too deep and can m a i n t a i n a maximum p a c k i n g d e n s i t y o f exposed c e l l s . The m a t e r i a l must be weather r e s i s t a n t (20 years) and bondable. I t should be r e l a t i v e l y low cost ( p r e f e r a b l y < 0 . 2 5 / f t o f m o d u l e ) . Geometry, t h i c k n e s s , f a b r i c a t i o n technique, e t c . , which s t r o n g l y e f f e c t the economics, w i l l v a r y w i t h t h e m a t e r i a l s . The b e s t c a n d i d a t e s t o date a r e extruded o r r o l l - f o r m e d metals w i t h p a s s i v a t e d s u r f a c e s such as anodized aluminum or g a l v a n i z e d s t e e l . Highly f i l l e d , molded, or extruded, weatherable p l a s t i c s a r e a l s o a p o s s i b i l i t y b u t an o p t i m u m c a n d i d a t e h a s n o t y e t b e e n identified. 5

B

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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385

Accelerated Testing O v e r a l l , t h e g o a l i s chemical s t a b i l i t y o f the pottant and a l l other organic m a t e r i a l s i n the array t o the extent that they w i l l undergo no more than a 20? change i n o p t i c a l , e l e c t r i c a l , or mechanical p r o p e r t i e s ( i n c l u d i n g bond strengths) over 20 years of o u t d o o r w e a t h e r i n g and module o p e r a t i o n . T h i s i s d i f f i c u l t to determine when most of the m a t e r i a l s have n o t even e x i s t e d t h a t l o n g , l e t a l o n e been exposed outdoors i n a p h o t o v o l t a i c module. The near-term goal with each design change i s t o make s o m e t h i n g b e t t e r i n performance as w e l l as lower i n cost than what was being used. Thus, most t e s t s today compare m a t e r i a l s r e l a t i v e t o each o t h e r i n a c c e l e r a t e d c o n d i t i o n s that i n some cases a r e probably e x c e s s i v e l y severe. F o r example, t h e 150°C d r y oven and 100°C/100? RH exposure t e s t s are used mostly as screening t o o l s .

Literature Cited 1. Cuddihy, E. "LSA Progress Report 18 and Proceedings of the 18th Project Integration Meeting," Jet Propulsion Laboratory, in press. 2. Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants, 13th Quarterly Progress Report for May 12, 1978-August 12, 1979, DOE/JPL/954527-12, Springborn Laboratories, Inc., Jan. 1980. 3. Megerle, C.; Lewis, K. Encapsulant Degradation in Photovoltaic Modules, this symposium. 4. Arnett, J . C . ; Gonzalez, C. C. "Fifteenth IEEE Photovoltaic Specialists Conference -- 1981," p. 1099. 5. Ross, R. G . , J r . "Fifteenth IEEE Photovoltaic Specialists Conference -- 1981," p. 1157. 6. Gonzalez, C . ; Weaver, R. "Fourteenth IEEE Photovoltaic Specialists Conference -- 1980," p. 528. 7. "Investigation of Test Methods, Material Properties, and Processes for Solar Cell Encapsulants, Annual Report," DOE/JPL - #954527-79-10, Springborn Laboratories, Inc., June 1979. 8. Pluddemann 9. Drago, R. S.; Vogel G. C.; Needham, T. E. J. Am. Chem. Soc. 1971, 93, 6014. 10. Drago, R. S.; Parr, L. B.; Chamberlain, C. S. J. Am. Chem. Soc. 1977, 99, 3203. 11. Fowkes, F. M.; "Donor-Acceptor Interactions at Interfaces," J. Adhesion 1972, 4, 155. 12. Fowkes, F. M.; Maruchi, S. Coatings and Plastics Preprints 1977, 37, 605. 13. Fowkes, F. M.; Mostafa, M. A. Ind. Eng. Chem., Prod. R&D 1978, 17, 3. RECEIVED November 22,1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

24 Encapsulant Degradation in Photovoltaic Modules K. J. LEWIS and C. A. M E G E R L E ARCO Solar, Inc., Research and Development, Woodland Hills, CA 91367

The aging behavio for photovoltaic surface were studied i n the field and v i a accelerated aging. Two pottant polymers and two outer cover/insulator films were tested for resistance to degradation. Test methods included dry oven aging, humidity chamber aging, field aging and accelerated outdoor weathering. Evidence of degradation included discoloration, embrittling and other changes in mechanical properties, development of opacity, changes in electrical resistivity and the appearance of polymer oxidation products observed by ESCA and multiple internal reflection IR spectroscopy. Encapsulants in a photovoltaic (PV) module provide electrical insulation and protect the metallized c e l l contacts and interconnect system against corrosion over a 20-year lifetime outof-doors. The typical environmental stresses and possible resulting failures in exposed PV modules are listed in Table I. In the case of brittle cells such as single or polycrystalline silicon, encapsulants must also provide mechanical protection for the fragile wafers and interconnect ribbons or wires. Figure 1 shows the typical layup for a plastic front design. The functions and performance requirements for the various components are described in detail in the accompanying paper in this symposium proceedings entitled "Encapsulant Material Requirements for Photovoltaic Modules" ( 1_) . The layers most vulnerable to degradation are the pottant and the flexible outer cover/insulator, the latter being particularly susceptible in the substrate design where it is on the module front and thus must be transparent. These two layers are the most easily degraded because the quantities required and the cost

0097-615 6/83/0220-03 87$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Table I·

Humiditv - Cell Metallization Delamination - Encapsulant Delamination

ultraviolet

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P r i n c i p a l Damaging Environments

Thermal C v c l i n g - Interconnect Fatigue - Encapsulant Delamination - Solar C e l l Cracking

-

IN

Optical Material Degradation Encapsulant Delamination

S t r u c t u r a l Loading - C e l l Interconnect Fatigue S t r u c t u r a l Fatigue H a i l Impact - O p t i c a l Cover Breakage - C e l l Cracking

Voltage Stress - I n s u l a t i o n Breakdown - C e l l Corrosion (Ion M i g r a t i o n )

O p t i c a l Surface

Soiling

c o n s t r a i n t s necessary f o r low cost s o l a r arrays d i c t a t e that they be upgraded, good performance m a t e r i a l s . The pottant i s the v i b r a t i o n damping, elastomeric m a t e r i a l that immediately surrounds both sides of the f r a g i l e s o l a r c e l l wafers and t h e i r e l e c t r i c a l contacts and i n t e r c o n n e c t s . I t must be s o f t , transparent, e l e c t r i c a l l y i n s u l a t i n g , weather r e s i s t a n t , chemically i n e r t and form strong and s t a b l e adhesive bonds t o the surfaces i t touches. I t p r o t e c t s the c e l l s from stresses due t o t h e r m a l e x p a n s i o n d i f f e r e n c e s and e x t e r n a l impact and i s o l a t e s them e l e c t r i c a l l y . The pottant a l s o h e l p s p r o t e c t t h e c i r c u i t m e t a l l i c contacts and interconnects from the c o r r o s i v e e f f e c t s of moisture, s a l t , smog, e t c . The o u t e r c o v e r / i n s u l a t o r must be a t o u g h , s o i l r e s i s t a n t , weather r e s i s t a n t and e l e c t r i c a l l y i n s u l a t i n g l a y e r . I t may be a f l e x i b l e o r c o n f o r m i n g p l a s t i c f i l m o r a c o a t i n g a p p l i e d from s o l u t i o n . As a f r o n t l a y e r , i t i s d e s i r a b l e that i t act as a UV s c r e e n f o r t h e p o t t a n t w h i l e i t must a t the same time be >90% transparent t o wavelengths from 0.4 t o 1.1 microns. I t must form s t a b l e bonds t o the pottant and t o other module m a t e r i a l s t o which i t seals. Most o r g a n i c m a t e r i a l s c o n t a i n s i t e s where r a d i c a l s can form more o r l e s s e a s i l y , d e p e n d i n g u p o n s t r u c t u r e . Only p e r f l u o r i n a t e d m o l e c u l e s a r e t o t a l l y f r e e of such s i t e s since r a d i c a l formation usually involves a b s t r a c t i o n of hydrogen radicals. B o t h t h e p o l y m e r and a b s t r a c t e d h y d r o g e n r a d i c a l s become s t a b i l i z e d by the i n t e r v e n t i o n of oxygen. D e g r a d a t i o n i n the form o f c r o s s l i n k i n g , c h a i n s c i s s i o n or both f o l l o w s . The

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Encapsulant Degradation

Figure 1.

Module Cross S e c t i o n ,

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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b a s i c mechanism of t h i s process, i d e n t i f i e d over 35 years ago Bolland and Gee ( 2 ) , i s as f o l l o w s : r ^ ( r a t e of (1)

initiation)

Initiation

k (2),—fc-R- + 0

by

^

free radicals

0

fc-R0 '

2

2

propagation (3)

RÛ2* +

(4)

R02* + R02*

ROOH +

RH 2k

3'

t

^-products - termination at O2 s a t u r a t i o n

Fluorocarbon polymer degradation because th with an energy on the order of 116 kcal/mole, compared to carbonh y d r o g e n b o n d e n e r g i e s o f 91-98 kcal/mole (3., 4, 5.). F l u o r o c a r b o n s a r e , however, e x t r e m e l y e x p e n s i v e b e c a u s e t h e monomers are more complicated to synthesize and more dangerous to handle. The polymers c o s t on the o r d e r of $10-20 per pound compared with the most widely used hydrocarbon polymers which can be $1-5 per pound. S i l i c o n e polymers u s u a l l y have an a l l O-Si-O-Si-O backbone and thus do not undergo hydrogen r a d i c a l a b s t r a c t i o n i n a p o s i t i o n where i t can cause s i g n i f i c a n t c r o s s l i n k i n g or chain s c i s s i o n . The S i - 0 bond can be h y d r o l y z e d , a l t h o u g h not e a s i l y . When s i l i c o n e s do degrade, they become more h y d r o p h i l i c , allowing more moisture to reach c i r c u i t metals. They can a l s o h a r d e n i f the r u b b e r y s i d e c h a i n s a r e l o s t , thus l o s i n g t h e i r s t r e s s damping c h a r a c t e r i s t i c s . They a l s o cost about $10 per pound. The s i t e s f o r r a d i c a l f o r m a t i o n on a c r y l i c s are e i t h e r deactivated by the carbonyl group as i n ordinary a c r y l i c e s t e r s , or t o t a l l y blocked as with the m e t h a c r y l i c e s t e r s . This i s what makes them stable compared to other saturated hydrocarbon backbone m a t e r i a l s l i k e p o l y e t h y l e n e w h e r e t h e r e a r e no e l e c t r o n withdrawing groups to s t a b i l i z e against r a d i c a l f o r m a t i o n . The d e g r e e o f s t a b i l i t y depends on the s t r e n g t h of the e l e c t r o n withdrawing e f f e c t . Most a c r y l i c s a l s o e x h i b i t some h y d r o p h i l i c character, even when they have very f a t t y s i d e chains because they too can hydrolyze, however s l i g h t l y . The l e a s t s t a b l e m a t e r i a l s have u n s a t u r a t i o n i n t h e i r backbones which can be d i r e c t l y and e a s i l y attacked by oxygen and p e r o x y r a d i c a l s t o degrade a c c o r d i n g t o the mechanisms j u s t described. The l e s s s t a b l e , l e s s e x p e n s i v e m a t e r i a l s w i t h s a t u r a t e d backbones such as p o l y o l e f i n s , and e s p e c i a l l y those containing e l e c t r o n withdrawing groups such as e s t e r s , h a l o g e n o t h e r than f l u o r i n e , amides, urethane groups, e t c . to s t a b i l i z e

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

24.

LEWIS AND

MEGERLE

Encapsulant Degradation

391

them, can be d r a m a t i c a l l y u p g r a d e d w i t h a n t i o x i d a n t s a n d p h o t o s t a b i l i z e r s t o make them p o t e n t i a l l y acceptable f o r use i n photovoltaics. There a r e f i v e c l a s s i f i c a t i o n s o f o x i d a t i o n i n h i b i t o r s . These are based on d i f f e r e n c e s i n the mechanism by w h i c h they f u n c t i o n to i n t e r f e r e w i t h one or more of the r e a c t i o n s described i n the p r e v i o u s e q u a t i o n s t o p r e v e n t o r d e l a y c a t a s t r o p h i c degradation by o x i d a t i o n ( 6 ) . These c l a s s i f i c a t i o n s are: 1.

2.

3.

4.

5.

Metal d e a c t i v a t o r s , which form i n a c t i v e c h e l a t e s o r i n s o l u b l e r e a c t i o n products w i t h t r a n s i t i o n metals o r i g i n a l l y present i n a form t h a t p r o m o t e s t h e d e c o m p o s i t i o n o f p e r o x i d e s to f r e e r a d i c a l s . Examples are e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d , s a l i c y l a l d e h y d e diamine condensation products or m e t a l a l k y l dithiocarbamates such as of n i c k e l or z i n c Light absorbers a b s o r b i n g th o t h e r w i s e i n i t i a t e o x i d a t i o n , e i t h e r by decomposing peroxides or by s e n s i t i z i n g the o x i d i z a b l e m a t e r i a l t o oxygen a t t a c k . The absorbed energy must be disposed of by processes that do not produce a c t i v a t e s s i t e s o r f r e e radicals. Examples are 2-hydroxybenzophenones, 2-(2'hydroxyphenyl)benzotriazoles, c e r t a i n s a l i c y l a t e e s t e r s or c e r t a i n organonickel or chromium compounds. Peroxide decomposers, w h i c h promote the c o n v e r s i o n o f p e r o x i d e s t o non-free r a d i c a l products, presumably by a p o l a r mechanism. Examples a r e d i a l k y l a r y l p h o s p h i t e s , d i a l k y l t h i o d i p r o p i o n a t e s or long c h a i n alkylmercaptans. Free r a d i c a l c h a i n s t o p p e r s o r " r a d i c a l t r a p s , " w h i c h i n t e r a c t w i t h c h a i n - p r o p a g a t i n g RU2* r a d i c a l s to form i n a c t i v e products. This i s u s u a l l y accomplished by i t s donation of an Η· r a d i c a l to terminate an a c t i v e polymer r a d i c a l , i t s e l f forming a more s t a b l e one ( u s u a l l y by resonance) which w i l l not r e r e a c t w i t h the polymer (e.g., w i t h the help of s t e r i c hindrance) and w i l l e v e n t u a l l y r e l a x i t s energy through t h e r m a l i z a t i o n , fluorescence o r other innocuous means. Examples are s t e r i c a l l y hindered phenols or secondary arylamines. I n h i b i t o r regenerators. which r e a c t w i t h intermediates o r p r o d u c t s formed i n the c h a i n - s t o p p i n g ( t e r m i n a t i o n ) r e a c t i o n so as to regenerate the o r i g i n a l i n h i b i t o r o r f o r m a n o t h e r p r o d u c t c a p a b l e o f f u n c t i o n i n g as an antioxidant. Examples are d i a l k y l p h o s p h o n a t e s w i t h hindered phenols or diphenoquinones w i t h t h i o l s .

Since the cannot provide the c i r c u i t , s t a b i l i t y , and

available, inherently weather-resistant m a t e r i a l s a t o t a l l y moisture- and oxygen-free environment f o r whether o r not they t h e m s e l v e s r e q u i r e i t f o r because s t a b i l i z e r s can so d r a m a t i c a l l y improve the

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

392

POLYMERS

IN

SOLAR E N E R G Y

UTILIZATION

performance of much lower cost m a t e r i a l s , the lower cost m a t e r i a l s are p r e f e r r e d over s i l i c o n e s and most fluorocarbons f o r use i n low cost t e r r e s t r i a l s o l a r a r r a y s . I t i s l i k e l y that a c r y l i c s i n the $1-5 per pound range w i l l u l t i m a t e l y be the optimum m a t e r i a l f o r b o t h t h e p o t t a n t and t h e o u t e r c o v e r / i n s u l a t o r . Their a v a i l a b i l i t y i n the current market, however, i n forms s u i t a b l e f o r PV a p p l i c a t i o n i s c u r r e n t l y l i m i t e d and, f o r t h e most p a r t , unproven. The two p o t t a n t m a t e r i a l s s t u d i e d i n t h i s r e p o r t a r e p l a s t i c i z e d p o l y v i n y l b u t y r a l (plPVB) w h i c h i s e a s i l y a v a i l a b l e and used i n s a f e t y g l a s s , and a h i g h l y s t a b i l i z e d , p e r o x i d e c r o s s l i n k e d e t h y l e n e / v i n y 1 a c e t a t e (EVA) copolymer c o n t a i n i n g about 33 w e i g h t % v i n y l acetate (7.). The outer c o v e r / i n s u l a t o r m a t e r i a l s studied i n c l u d e p o l y v i n y l f l u o r i d e (PVF) and a b u t y l a c r y l a t e / m e t h y l m e t h a c r y l a t e g r a f t copolymer (BAgMMA); both are blown f i l m s . Aging Tests and Result Mechanical and O p t i c a l . C l e a r 1 5 - m i l - t h i c k f i l m s of EVA and plPVB, 4-mil PVF and 3-mil BAgMMA were exposed i n a c i r c u l a t i n g a i r oven a t about 150°C f o r periods of 0 t o 26 days. This type of t e s t i s used e x t e n s i v e l y t h r o u g h o u t t h e polymer i n d u s t r y as a screening t o o l f o r comparing the o x i d a t i v e s t a b i l i t i e s of polymers and compound formulas. A summary of the o p t i c a l t r a n s m i s s i o n changes as a r e s u l t o f the oven aging can be seen i n Figure 2. EVA e x h i b i t e d very l i t t l e y e l l o w i n g i n t h e 26 d a y s . M e c h a n i c a l l y , i t s t e n s i l e s t r e n g t h , e l o n g a t i o n a t break and permanent set decreased considerably w i t h the a g i n g . A t t h e same t i m e , however, i t s t e a r s t r e n g t h and e l a s t i c moduli a t 10% and 100% e l o n g a t i o n s , which are w i t h i n t h e regions of concern f o r PV use, remained r e l a t i v e l y constant (Table II). The plPVB d a r k e n e d r a p i d l y d u r i n g t h e o v e n e x p o s u r e , becoming too b r i t t l e t o permit t e n s i l e t e s t i n g . The BAgMMA showed no measurable y e l l o w i n g throughout the f u l l 26 days o f aging, but the l a r g e increase i n UV t r a n s m i s s i o n during the f i r s t 7 t o 10 days s u g g e s t s t h a t i t l o s e s much o f i t s UV absorber during t h i s period (Figure 3 ) . Further aging then r e s u l t e d i n a decrease i n UV t r a n s m i s s i o n . The BAgMMA f i l m was becoming n o t i c e a b l y b r i t t l e a t the 10-day sample and was q u i t e b r i t t l e a f t e r 18 days. PVF showed v e r y l i t t l e y e l l o w i n g (

r

s

S

w

ο r

ο

*d

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

BURGER A N D CUDDIHY

411

Vacuum Lamination

smaller configurations. The most e x p e n s i v e p a r t o f a l a r g e l a m i n a t o r i s t h e chamber t h a t must w i t h s t a n d t h e a t m o s p h e r i c p r e s s u r e l o a d . A t p r e s e n t t h e l o w e s t - c o s t vacuum chamber t h a t has been c o n s i d e r e d u s e s s t a n d a r d h e m i s p h e r i c p r e s s u r e v e s s e l - e n d c a p s c o s t i n g a b o u t $600 e a c h ( i n c l u d i n g a 2 - i n . - w i d e f l a n g e ) . A l e a d a l l o y c o u n t e r w e i g h t w i l l p e r m i t t h e o p e r a t o r t o r a p i d l y and s a f e l y l o a d and u n l o a d t h e l a m i n a t o r . S u p p o r t and i n s u l a t i o n o f t h e p l a t e n was a l s o a p r o b l e m , and t h e u s e o f m a r b l e s seems t o be an i n e x p e n s i v e c h o i c e b e c a u s e a b o u t $700 w o r t h o f m a r b l e s w o u l d p r o v i d e s u p p o r t and t h e r m a l i s o l a t i o n , and has t h e added a d v a n t a g e s o f e a s y t r a n s p o r t and m o d i f i c a t i o n o f t h e chamber. M a r b l e s a l s o r e d u c e t h e volume o f t h e vacuum chamber and t h e r e b y r e d u c e pump-down t i m e and e n e r g y . C o n t r o l o f t h e l a m i n a t o r c y c l e w i l l be v e r y f l e x i b l e due t o u s e o f f o u r i n d e p e n d e n t l y a d j u s t a b l e t i m e r s and a 24 s t e p i n d u s t r i a l c o n t r o l l e r w i t h 10 i n d i v i d u a l s w i t c h m o d u l e s . Materials

Research

D e s i g n o f a PV module t h a t w i l l w i t h s t a n d 2 0 y e a r s o f e x p o s u r e t o a v a r i e t y o f t e r r e s t r i a l e n v i r o n m e n t s c r e a t e s many problems. An FSA c o s t a l l o c a t i o n o f $14/m f o r e n c a p s u l a t i o n , s u p e r s t r a t e o r s u b s t r a t e and e d g e - s e a l / g a s k e t p l a c e s an a d d i t i o n a l b u r d e n on t h e e n c a p s u l a t i o n m a t e r i a l s , b e c a u s e t h e g l a s s s u p e r s t r a t e a l o n e has a p r o j e c t e d c o s t o f a b o u t $10/m2. D e t a i l s and b a c k g r o u n d on e a r l y m a t e r i a l s r e s e a r c h e f f o r t s have been p u b l i s h e d ( 3 - 7 ) . A d e t a i l e d discussion of present e n c a p s u l a t i o n m a t e r i a l s w i l l be p u b l i s h e d soon {8). This report c o v e r s t h e a p p l i c a t i o n t e s t i n g o f d e v e l o p e d m a t e r i a l s and o t h e r r e q u i r e m e n t s f o r s u c c e s s f u l vacuum l a m i n a t i o n . 2

Ethylene Vinyl Acetate System. The f i r s t new l a m i n a t i o n m a t e r i a l d e v e l o p e d by FSA was compounded by S p r i n g b o r n L a b o r a t o r i e s I n c . ( S L I ) f r o m an e t h y l e n e v i n y l a c e t a t e (EVA) f e e d s t o c k a v a i l a b l e f r o m Du P o n t . Compared w i t h P V B , EVA c o s t s a b o u t o n e - t h i r d a s much, has much l o w e r v i s c o s i t y a t p r o c e s s t e m p e r a t u r e , and has no h u m i d i t y - c o n t r o l r e q u i r e m e n t d u r i n g p r o c e s s i n g . E a r l y l a m i n a t o r e x p e r i e n c e uncovered problems w i t h c u r i n g and a d h e s i o n o f EVA. The o r i g i n a l m a t e r i a l f r o m S L I a l s o w o u l d block (adhere t o i t s e l f ) . Subsequent m a t e r i a l d e l i v e r e d from S L I a n d Du Pont d i d n o t b l o c k . The Du Pont m a t e r i a l had one w a f f l e d s u r f a c e , w h i c h enhanced a i r removal d u r i n g vacuum pumpdown T h e r e i s more t h a n one c o r r e c t c u r e c y c l e f o r EVA. Like most p o l y m e r s w i t h p e r o x i d e p r o m o t e r s , i t i s good p r a c t i c e t o r a i s e t h e b o n d - l i n e t e m p e r a t u r e r a p i d l y t o a v o i d p e r o x i d e decomp o s i t i o n b e f o r e an a d e q u a t e c u r e has been o b t a i n e d . One c y c l e t h a t has been p r o v e n u s e s two s t e p s , one a t 100°C f o r e v a c u a t i o n and a d h e s i o n , t h e o t h e r a t 150°C f o r l o n g - t e r m oven c u r e . This c y c l e p r o v i d e s a h i g h t h r o u g h p u t w i t h o n l y one l a m i n a t o r .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

412

POLYMERS

IN

SOLAR E N E R G Y

UTILIZATION

A d e s c r i p t i o n of t h e c u r e c y c l e used f o r m a t e r i a l s t e s t i n g at our l a b o r a t o r y i s : E v a c u a t i o n f o r 5 m i n , t h e n 25 mi η o f c u r e w i t h t h e t o p chamber b l e d t o a t m o s p h e r e . During the 25-min c u r e , t h e f i r s t 8 - 1 0 min i s r e q u i r e d t o r a i s e t h e e n c a p s u l a n t t e m p e r a t u r e t o 1 6 0 ° C , where i t i s m a i n t a i n e d t o t h e end o f t h e cycle. Modules a r e t h e n removed w i t h o u t b e i n g c o o l e d . Modules f a b r i c a t e d w i t h t h i s c u r e c y c l e show even and c o m p l e t e c u r i n g and no b u b b l e s . A d h e s i o n i s a more d i f f i c u l t p r o b l e m . T h e r e a r e many c h e m i ­ c a l l y d i f f e r e n t i n t e r f a c e s i n a laminant stack: glass-EVA, E V A - s o l a r c e l l s u r f a c e ( o x i d i z e d s i l i c o n o r some a n t i r e f l e c t i o n (AR) c o a t i n g ) , cell back s u r f a c e m e t a l l i z a t i o n - E V A , EVA-back s h e e t , and EVA-bus b a r s ( c o p p e r o r t i n n e d c o p p e r ) . Each o f t h e s e i n t e r f a c e s i s i m p o r t a n t , b e c a u s e m e c h a n i c a l l y good a d h e s i v e bonds w i l l o f t e n f a i l by d e l a m i n a t i o n a f t e r e x p o s u r e t o h u m i d i t y i n the f i e l d . Water v a p o r there i s a non-chemicall c a u s e f a i l u r e by d i s p l a c e m e n t . T a b l e s I, I I , and I I I p r o v i d e d e t a i l s o f some o f t h e r e s e a r c h e f f o r t s i n a d h e s i o n . A m a t e r i a l s u r v e y was made u s i n g EVA and e t h y l e n e m e t h y l a c r y l a t e (EMA) e n c a p s u l a n t s w i t h K o r a d 6 3 0 0 0 , S c o t c h p a r 20CP and T e d l a r 200BS 30WH as back s h e e t s . T a b l e I shows t h e d e t a i l e d r e s u l t s of ths survey. P r i m e d and u n p r i m e d s u r f a c e s and a new Du Pont a d h e s i v e , 6 8 0 4 0 , were i n v e s t i g a t e d . T h i s s u r v e y showed good g l a s s b o n d i n g w i t h SLI P r i m e r Al 1 8 6 1 - 1 , (Dow C o r n i n g s i l a n e Z - 6 0 3 0 , 9 p a r t s ; N, N - d i m e t h y l - b e n z y l ami n e , 1 p a r t ; L u p e r s o l 101, 0.1 p a r t , and methanol 8 9 . 9 p a r t s ) . The o n l y back s h e e t t h a t a d h e r e d t o EVA was T e d l a r w i t h A d h e s i v e 6 8 0 4 0 . Earlier t e s t s showed good m e c h a n i c a l b o n d i n g t o u n t r e a t e d T e d l a r b u t poor humidity performance. B e c a u s e t h e g l a s s - t o - E V A i n t e r f a c e b o n d i n g p r o b l e m seemed t o be s o l v e d when a l l s a m p l e s e x h i b i t e d a d h e r e n t and p e r s i s t e n t b o n d s , t h e f o c u s o f t h e e f f o r t was s h i f t e d t o b a c k - s h e e t a d h e s i o n . An a d d i t i o n a l s e r i e s o f t e s t ( s e e T a b l e I I ) c o n f i r m e d t h e good r e s u l t s o f T e d l a r w i t h t h e Du P o n t a d h e s i v e 6 8 0 4 0 . K o r a d 63000 may be a u s e f u l m a t e r i a l , b u t c u r e t e m p e r a t u r e s d u r i n g l a m i n a t i o n c a u s e d some d e g r a d a t i o n . A d d i t i o n a l t e s t s on t h i s a c r y l i c s h e e t may be r u n . A p o l y e s t e r f i l m , S c o t c h p a r 2 0 C P , was i n t e r e s t i n g , b e c a u s e i t w o u l d be l e s s e x p e n s i v e t h a n a p o l y v i n y l f l u o r i d e f i l m , s u c h as T e d l a r . T h i s t e s t s e r i e s showed t h a t a new p r i m e r o r a d h e s i v e was needed f o r t h e S c o t c h p a r f i l m . F o r t u n a t e l y , E. P. Plueddemann o f Dow C o r n i n g C o r p . had a l r e a d y developed a primer f o r p o l y e s t e r f i l m s . The p r i m e r c o n s i s t s o f A m e r i c a n Cyanamid Cymel 3 0 3 , 90 p a r t s ; Dow C o r n i n g s i l a n e Z - 6 0 4 0 , 10 p a r t s ; and m e t h a n o l , 300 p a r t s . Peel t e s t s of S c o t c h p a r bonded t o EVA u s i n g t h i s p r i m e r were e x c e l l e n t . Unfor­ t u n a t e l y , t h i s system d i d not perform w e l l i n t h e 7-day c o l d w a t e r soak t e s t as shown i n T a b l e I I I .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. A l l 861-1 P r i m e r None A l l 861-1 P r i m e r None A l l 861-1 P r i m e r 68040

A l l 861 -1 None A l l 861 -1 None A l l 861 -1 None

K o r a d 63000

S c o t c h p a r 20CP

S c o t c h p a r 20CP

S c o t c h p a r 20CP

S c o t c h p a r 20CP

T e d l a r 200BS

EMA

EVA

EVA

EMA

EMA

EVA

EVA

EMA

EMA

4

5

6

7

8

9

10

11

12

30WH

T e d l a r 200BS

30WH A l l 861 - 1

None

T e d l a r 200BS

30WH

A l l 861 - 1

T e d l a r 200BS

A l l 861-1 Primer

68040 and

68040

A l l 861-1 P r i m e r

68040 and

Not

None

None

K o r a d 63000

EMA

3

30WH

10,000

Al 1861-1 P r i m e r

A l l 861 -1

K o r a d 63000

EVA tested

tested

tested

tested

tested

7,000

Not

5,100

Not

7,000

Not

4,300

Not

2,900

tested

2

Not

None

None

K o r a d 63000

EVA

EncapsulantGlass P e e l , g

1

Back-Sheet Ahesive

Glass Primer

Back Sheet

Encapsulant

M a t e r i a l Survey

Sample No.

T a b l e I.

but

4,400;

peel

peel

peel

broke

couldn't

Adherent;

couldn't

Adherent;

couldn't

g

brittle

broke Adherent;

3,200;

110

590-680

55

Adherent,

0

370

185

Back S h e e t Encasulant P e e l ,

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 68040

A l l 861 -1

EMA

A-10

EVA

EVA

A-l 3

EMA

A-8

A-9

EVA

EVA

A-7

EVA

Gel

EMA

A-6

A-l 2

Gel

EVA

A-l Ί

K o r a d 63000

EMA

A-4

A-5

Combination

test

Combination

K o r a d 63000

S c o t c h p a r 20CP

Gel

EVA

Tedlar/68040/

EVA

Tedlar/68040/

Test

Test

30WH

T e d l a r 200BS

30WH

T e d l a r 200BS

K o r a d 63000

EVA

A-3

A l l 861 -1

A l l 861 -1

None

A l l 861 -1

A l l 861 -1

None

None

A l l 861 -1

Al 1861 -1

1359

1359

(68040)

(68040)

68040

68040

68040

68040

A l l 861 -1

S c o t c h p a r 20CP

EMA

A-2

A l l 861 -1

68040

S c o t c h p a r 20CP

Sheet A l l 861 -1

Adhesion Back-Sheet Adhesive

Back-Sheet Glass Primer

Back

EVA

Encap­ sulant

A-l

Sample No.

Table I I .

Adherent,

0

couldn't

Adherent;

couldn't

Adherent;

couldn't

Adherent;

couldn't

Adherent;

Adherent,

Adherent,

0

0

Peel

but b r i t t l e

peel

peel

peel

peel

but b r i t t l e

but b r i t t l e

Back-Sheet Encapsulant

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

b

a

No.

10

parts;

Peels Sample g i v e n away Peeled a f t e r cure Peeled a f t e r cure Adherent Adherent Adherent Adherent Adherent Adherent Adherent Adherent Adherent Peels Brittle Peels Peels Peels Peels a f t e r cure Peels Peels

Results]!

Du Pont Z - 6 0 4 0 ,

S c o t c h p a r 20CP S c o t c h p a r 20CP K o r a d 63000 K o r a d 63000 T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS T e d l a r 200BS S c o t c h p a r 20CP Acrylar Acrylar Acrylar Acrylar T e d l a r 100BG 30UT T e d l a r 200BS T e d l a r 200BS

Cymel Cymel Cymel Cymel 68040 68040 68040 68040 68040 68040 68040 68040 68040 Cymel Cymel A - l 1861-1 Cymel A - l 1861-1 68040 68040 68040

EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EVA EMA EVA EVA EMA EMA EMA None None

Sheet

Back

7-Day Water Soak T e s t

PrimerfL

of

Encapsulant

Results

P r i m e r c o n s i s t s o f Cymel 303 ( A m e r i c a n C y a n a m i d ) , 90 p a r t s ; m e t h a n o l ; 300 p a r t s . A f t e r 7-day soak u n l e s s o t h e r w i s e n o t e d .

C-l C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 C-10 C-ll C-l 2 C-13 E-l E-2 E-3 E-4 E-5 E-7 E-8 E-9

Coupon

Table III.

416

POLYMERS

IN

SOLAR E N E R G Y

UTILIZATION

A n o t h e r p r i m e r s y s t e m was s u g g e s t e d . T h i s p r i m e r was made f r o m Monsanto Resimene 7 4 0 , 2 3 . 5 p a r t s ; D o w - C o r n i n g S i l a n e Z - 6 0 4 0 , 1 . 2 5 p a r t s ; and a n h y d r o u s i s o p r o p a n o l , 75 p a r t s . The i n i t i a l p e e l t e s t s were e x c e l l e n t and r e s i s t a n c e t o 7-day c o l d w a t e r soak was f a i r t o g o o d . F u r t h e r d i s c u s s i o n w i t h Plueddemann l e d t o t h e a d d i t i o n o f 1.25 p a r t s o f D o w - C o r n i n g S i l a n e Z - 6 0 3 0 t o p r o v i d e d o u b l e bond f o r i m p r o v e d a d h e s i o n t o t h e EVA. Again, i n i t i a l p e e l t e s t s were e x c e l l e n t . Water soak r e s i s t a n c e was good t o v e r y good on t h o s e s a m p l e s where t h e p r i m e r had been d i l u t e d 10 t o 1 w i t h a n h y d r o u s i s o p r o p a n o l . Additional tests a r e p l a n n e d w i t h t h e p o s s i b l e m o d i f i c a t i o n o f a s m a l l amount o f L u p e r s o l 101 t o i m p r o v e t h e EVA bond a g a i n . EVA does not bond w e l l t o a l l m e t a l s . Copper i s p a r t i c u l a r l y d i f f i c u l t , which poses a problem: copper i s the best c a n d i date f o r low-cost p h o t o v o l t a i c c e l l m e t a l l i z a t i o n . Some p r i m e r t e s t s were run u s i n g two p r i m e r s . The f i r s t was z i n c c h r o m a t e p o w d e r , 10 p a r t s ; D o w - C o r n i n m e t h y l b e n z y l ami n e , 0 . 1 p a r t t h e z i n c chromate i s opaque and d i f f i c u l t t o keep i n s u s p e n s i o n , the second primer o m i t t e d i t . The p r i m e r w i t h o u t t h e z i n c c h r o m a t e gave e x c e l l e n t i n i t i a l adhesion i f thoroughly wiped a f t e r a p p l i c a t i o n . Additional t e s t s were made a f t e r 10 t o 1 d i l u t i o n w i t h m e t h a n o l . Again w i p i n g was r e q u i r e d and i n i t i a l p e e l t e s t s were e x c e l l e n t . A d h e s i o n a f t e r w a t e r soak was e x c e l l e n t . T e s t s were a l s o made on c o p p e r w h i c h had been f u s i o n s o l d e r p l a t e d , z i n c p l a t e d , or n i c k e l p l a t e d . None o f t h e s e showed good i n i t i a l adhesion. Ethylene Methyl A c r y l a t e System. EMA, new e n c a p s u l a n t a d h e s i v e , i s under development. P r e l i m i n a r y work showed e x c e l l e n t a d h e s i o n o f EMA t o g l a s s when t h e g l a s s i s p r i m e d w i t h Al 1 8 6 1 - 1 . Long-term soaking i n c o l d water reduced the a d h e s i o n . Additional work and samples a r e n e e d e d . T e s t s w i t h T e d l a r and A d h e s i v e 68040 showed a d h e s i o n t o EMA. However, a d h e s i o n a f t e r c o l d - w a t e r soak was p o o r . T h i s problem i s being i n v e s t i g a t e d . The Cymel p r i m e r t h a t was used t o bond EVA t o a p o l y e s t e r was a l s o t r i e d i n b o n d i n g EMA t o a p o l y e s t e r . T h i s system a l s o degraded a f t e r l o n g - t e r m s o a k i n g i n c o l d w a t e r . K o r a d 63000 has a d h e r e d t o EMA, but t h e r e s u l t i n g back s h e e t was b r i t t l e . EMA a d h e s i o n t o c o p p e r w h i c h has been p r i m e d as above was e x c e l l e n t b o t h i n i t i a l l y and a f t e r t h e 7-day w a t e r soak t e s t . Testing

Methods

A g e l t e s t , recommended by S L I , was made on EVA coupon s a m p l e s p r o d u c e d when t h e o r i g i n a l f o u r modules were made. B e c a u s e u n c r o s s l i n k e d EVA i s s o l u b l e i n t o l u e n e , w e i g h e d s a m p l e s

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

BURGER AND CUDDIHY

Vacuum Lamination

417

were p l a c e d i n 60°C t o l u e n e f o r 2 h , and t h e r e s u l t a n t s o l u t i o n and sample was p o u r e d t h r o u g h w e i g h e d f i l t e r p a p e r . After f i l ­ t r a t i o n , t h e samples were d r i e d i n a 90°C c i r c u l a t i n g - a i r oven f o r 5 h . The p e r c e n t a g e o f EVA r e m a i n i n g i s a measure o f t h e degree o f g e l a t i o n or c r o s s l i n k i n g d u r i n g c u r e . SLI s p e c i f i e s a n o m i n a l 80% g e l w i t h 65% as t h e l o w e r l i m i t . The g e l t e s t on t h e s a m p l e s p r o d u c e d above showed b e t t e r t h a n 95% g e l a t i o n . P e e l - t e s t s a m p l e s were p r e p a r e d by c u t t i n g t h r o u g h t h e l a y e r t o be t e s t e d u s i n g a 0 . 2 5 - i n . - w i d e t e m p l a t e . The d e s i r e d l a y e r was t h e n p e e l e d back by c u t t i n g when n e c e s s a r y . Peel s t r e n g t h was measured u s i n g a U n i t e k M i c r o p u l l I , Model 6 - 0 9 2 . Several a d h e r e n t s a m p l e s had a c o h e s i v e s t r e n g t h above t h e 5 - 1 b l i m i t o f t h e t e s t e q u i p m e n t , s o t h e s e s a m p l e s were t e s t e d u s i n g a c a l i ­ brated spring s c a l e . In c a s e s where t h e a d h e s i o n i s e x c e l l e n t i t was n o t e d t h a t a p e e l - t e s t sample c o u l d n o t a l w a y s be p r e p a r e d . These s i t u a t i o n s a r e n o t e d i n t h e a t t a c h e d t a b l e s . Performing a peel t e s as o n l y a good s c r e e n i n selection. Plueddemann recommends a 7 - d a y soak i n r o o m - t e m p e r a ­ t u r e w a t e r a s an a d d i t i o n a l t e s t , w i t h f i n a l p e e l t e s t s demon­ s t r a t i n g cohesive f a i l u r e r a t h e r than adhesive f a i l u r e (Reference 9). A l l o f t h e l a m i n a n t s made a t J P L have been s u b j e c t e d t o t h e 7-day room-temperature water-soak t e s t . T a b l e I I I summarizes t h e r e s u l t s o f 7-day r o o m - t e m p e r a t u r e w a t e r - s o a k t e s t s . Other

Lamination-Related

Efforts

The f i r s t l a m i n a t i o n e f f o r t s were m e c h a n i c a l l y s u c c e s s f u l b u t v i s u a l l y u n s u c c e s s f u l . Many b u b b l e s and v o i d s were f o u n d t h a t were r e l a t e d t o s o l d e r j o i n t s . A n o t h e r v i s u a l p r o b l e m was c e l l m i s a l i g n m e n t and p o o r p l a c e m e n t . Both o f t h e s e problem a r e a s were n o t c a u s e d by t h e l a m i n a t i o n m a t e r i a l s b u t w o u l d have a p r o f o u n d e f f e c t on t h e m a r k e t a b i l i t y and f i e l d s e r v i c e l i f e of the f i n a l laminated product. S o l d e r - F l u x Removal. Removal o f s o l d e r i n g f l u x r e s i d u e s i s an e s t a b l i s h e d p r o c e s s i n t h e p r i n t e d - c i r c u i t - b o a r d and e l e c ­ tronic-assembly industries. The q u a l i t y o f t h e l a m i n a t i o n p r o ­ c e s s i s dependent upon c h e m i c a l b o n d i n g o f a l l s u r f a c e s w i t h i n t h e l a m i n a n t , so v e r y c l e a n c e l l - s t r i n g a s s e m b l i e s a r e r e q u i r e d . P r o p e r removal o f f l u x r e s i d u e s r e q u i r e s s o l v e n t s t h a t c a n remove b o t h p o l a r and n o n - p o l a r s o l u b l e c o n t a m i n a n t s , s o u s e o f p r o p r i ­ e t a r y f l u x - r e m o v a l s o l v e n t s was i n d i c a t e d . B e c a u s e c e l l i n t e r ­ c o n n e c t s p r o v i d e f l u x t r a p s ( e x p e c i a l l y t h e M o t o r o l a I n c . and ARCO S o l a r c o m b i n a t i o n b u s - b a r - i n t e r c o n n e c t d e s i g n s ) , i t was d e c i d e d t o t r y u l t r a s o n i c c l e a n i n g f o l l o w e d by v a p o r d e g r e a s i n g . S i x c e l l - s t r i n g a s s e m b l i e s f o r m i n i m o d u l e s and f o u r a s s e m b l i e s f o r 1 χ 4 - f t modules were f i r s t c l e a n e d i n K e s t e r 5345 R o s i n R e s i d u e Remover u s i n g a S o n i x IV Model S S - 1 0 4 U l t r a s o n i c C l e a n e r .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

418

POLYMERS

IN

SOLAR

ENERGY

UTILIZATION

S u b s e q u e n t l y , t h e s e same c e l l - s t r i n g a s s e m b l i e s were c l e a n e d i n K e s t e r 5120 v a p o r d e g r e a s i n g s o l v e n t u s i n g an E l e c t r o v e r t , Inc., D e g r e s t i l Model L C D - 1 8 v a p o r d e g r e a s e r . The a s s e m b l i e s were f i r s t i n t r o d u c e d t o t h e v a p o r z o n e , t h e n were d i p p e d i n t h e c o l d s o l v e n t t a n k and f i n a l l y were removed s l o w l y t h r o u g h t h e v a p o r z o n e . These c e l l s t r i n g s showed no d e l a m i n a t e d a r e a s o r b u b b l e s a f t e r b e i n g l a m i n a t e d w i t h EVA. C e l l s t r i n g s t h a t were o n l y swab c l e a n e d f o r f l u x removal showed b o t h b u b b l e s and d e l a m i n a ­ t i o n when l a m i n a t e d u s i n g i d e n t i c a l p r o c e s s p a r a m e t e r s . B e c a u s e f l u x i s s u c h a c o n c e r n , one c o n t r a c t o r i s e x p l o r i n g u l t r a s o n i c b o n d i n g ( 1 0 ) u s i n g p r e p u n c h e d aluminum i n t e r c o n n e c t s t h a t are attached t o e l e c t r o p l a t e d copper c e l l m e t a l l i z a t i o n w i t h a seam w e l d e r . Others are examining f l u x l e s s bonding con­ c e p t s , s u c h as v a p o r - p h a s e s o l d e r r e f l o w . Conclusions The (1)

(2)

(3)

(4) (5)

(6)

following conclusion Vacuum l a m i n a t i o n i s an a c c e p t a b l e p r o c e s s f o r m a n u f ­ a c t u r i n g v o i d - f r e e PV m o d u l e s , i f matched w i t h c o r r e c t m a t e r i a l s and u s e d w i t h a q u a l i f i e d c u r e c y c l e . C o n c e p t u a l d e s i g n o f a l a r g e (4 χ 4 - f t ) vacuum l a m i n ­ a t o r i n d i c a t e s t h e p o t e n t i a l f o r an i n e x p e n s i v e p i e c e of c a p i t a l equipment. M a t e r i a l r e s e a r c h by t h e E n c a p s u l a t i o n Task o f FSA has been a p p l i e d t o a c t u a l l a m i n a t e d s y s t e m s w i t h good results. One l a m i n a n t s y s t e m has been d e v e l o p e d t h a t shows e x c e l l e n t a d h e s i o n and r e s i s t a n c e t o d e l a m i n a t i o n a f t e r being soaked f o r 7 days i n c o l d w a t e r . Another system i s n e a r i n g f i n a l a c c e p t a n c e . Gel t e s t s a r e u s e f u l i n d e t e r m i n i n g p r o p e r c u r e c y c l e s . P e e l t e s t s as a measure o f l a m i n a t e a d h e s i o n a r e o n l y partially useful. Most l a m i n a n t s y s t e m s e x h i b i t e i t h e r v e r y low o r v e r y h i g h a d h e s i o n a f t e r a 7-day soak i n cold water. The soak t e s t may n o t be a s u f f i c i e n t p r e d i c t o r f o r 2 0 - y r s e r v i c e l i f e ; h o w e v e r , i t may be c o n s i d e r e d as a s c r e e n i n g t e s t f o r s y s t e m s t h a t s h o u l d receive additional e f f o r t . C o m p l e t e removal o f s o l d e r f l u x i s c o n s i d e r e d n e c e s s a r y t o ensure long-term laminant a d h e s i o n . A process change t o a v o i d s o l d e r f l u x i s t h e r e f o r e e n c o u r a g e d .

Acknowledgments The e f f o r t r e p o r t e d i n t h i s p a p e r i s b a s e d upon many c o n ­ c e p t s and m a t e r i a l s d e v e l o p e d by t h e E n c a p s u l a t i o n Task o f FSA. Paul W i l l i s of S p r i n g b o r n L a b o r a t o r i e s , I n c . , c o n t r i b u t e d b a c k g r o u n d i n f o r m a t i o n on t h e p r o c e s s i n g and t e s t i n g o f t h e m a t e r i a l s developed at Springborn.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

BURGER AND CUDDIHY

Vacuum Lamination

419

E. P. Plueddemann o f Dow C o r n i n g C o r p . c o n c e i v e d t h e c o u p l i n g a g e n t s and c h e m i c a l - b o n d i n g p h i l o s o p h y so v i t a l t o t h i s effort. Many o t h e r i n d u s t r y t e c h n o l o g i s t s a l s o c o n t r i b u t e d . The r e s e a r c h d e s c r i b e d i n t h i s p u b l i c a t i o n was c a r r i e d o u t by t h e J e t P r o p u l s i o n L a b o r a t o r y , C a l i f o r n i a I n s t i t u t e o f T e c h n o ­ l o g y , and was s p o n s o r e d by t h e U n i t e d S t a t e s Department o f Energy t h r o u g h an agreement w i t h t h e N a t i o n a l A e r o n a u t i c s and Space Administration.

Literature Cited 1. 2. *3. *4. *5.

*6. 7. 8. *9.

10.

D'Aiello, R. V., Quarterly Report No. 5, RCA Laboratories, DOE/JPL-954868-79/2, March 1979. Somberg, H., Quarterly Report No. 2, ARCO Solar, Inc., DOE/JPL-955278-79/2, July 8, 1978. Cuddihy, E., Encapsulatio Cost Goals, JPL Interna California, April 13, 1978. Maxwell, H . , Encapsulation Candidate Materials for 1982 Cost Goals, JPL Internal Document No. 5101-72, Pasadena, California, June 15, 1978. Cuddihy, E. (JPL), Baum, B., and Willis, P. (Springborn Laboratories, Low-Cost Encapsulation Materials for Terres­ trial Solar Cell Modules, JPL Internal Document No. 5101-78, Pasadena, California, September 1978. Cuddihy, E., Encapsulation Materials Status to December 1979, JPL Internal Document No. 5101-144, Pasadena, California, January 15, 1980. Bouquet, F . , Glass for Low-Cost Photovoltaic Solar Arrays, JPL Document No. 5101-147, Pasadena, California, February 1, 1980. (JPL Publication 80-12, DOE/JPL 1012-40). Photovoltaic Module Encapsulation Design and Material Selec­ tion, JPL Document NO. 5101-177 (in press). (JPL Publication 81-10, DOE/JPL 1012-60). Plueddemann, E. P., Dow Corning Corp., Chemical Bonding Technology for Terrestrial Solar Cell Modules, JPL Internal Document No. 5101-132, Pasadena, California, September 1, 1980. Rose, C. Μ., Quarterly Report No. 1, Westinghouse Electric Corp., AESD, DOE/JPL-955909-81/1.

RECEIVED November 22,1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26 Evaluation of Polyacrylonitrile as a Potential Organic Polymer Photovoltaic Material 1

2

PHILIP D. METZ, HENRY TEOH , DAVID L. V A N D E R H A R T , and WILLIAM G. WILHEL Brookhaven National Laboratory Upton, NY 11973

Thin film organic polymer semiconductors are suggested as an attractive option for cost-effective photovoltaic devices. This report first discusses the potential of organic polymer semiconductors to meet the electronic, physical and economic constraints imposed by the photovoltaic application. Then, recent results on one candidate material, polyacrylonitrile (PAN), are presented. PAN pyrolyzed above about 200°C displays the structural, electrical conductivity and optical properties expected of a one-dimensional semiconductor, including an optical absorption edge at ~1.0-2.0 eV. After pyrolysis at temperatures above about 350-400°C a sharp transition to more metallic behavior is observed, with high conductivity (~10° (ohm-cm) ), low activation energy (~0.1 eV), and broad optical absorption. Areas for further investigation are indicated. -1

The future of photovoltaic solar energy conversion as an a l ternative energy resource depends on the development of efficient low-cost large-area photoactive materials. While no suitable

1

Current address: State University of New York—College at Old Westbury, Department of Chemistry and Physics, Old Westbury, N Y 11568 Current address: National Bureau of Standards, Structure and Properties Group, Polymer Science and Standards Division, Washington, DC 20234 2

0097-6156/83/0220-0421$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

422

P O L Y M E R S IN

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UTILIZATION

c o s t - e f f e c t i v e m a t e r i a l e x i s t s today, recent research i n d i c a t e s that i t may be p o s s i b l e t o develop organic polymers with the semiconductive and p h y s i c a l p r o p e r t i e s necessary f o r p h o t o v o l t a i c a p p l i c a t i o n s . U n l i k e i n o r g a n i c semiconductors where the e l e c t r i c a l and o p t i c a l p r o p e r t i e s a r e f i x e d , the c h a r a c t e r i s t i c s of polymers may be a d j u s t a b l e by "property engineering" to enhance conversion e f f i c i e n c y . I n a d d i t i o n , organic polymer t h i n f i l m s are uniquely s u i t a b l e f o r mass production manufacturing, a c r u c i a l c o n s i d e r a t i o n i n the production of p h o t o v o l t a i c devices. Recently, there has been a great deal of i n t e r e s t i n semiconducting organic polymers, p a r t i c u l a r l y polyacetylene ( ( C H ) ) , as e l e c t r o n i c m a t e r i a l s f o r a p p l i c a t i o n s where low cost and l a r g e area are important. This report f i r s t discusses the p o t e n t i a l of organic polymer semiconductors to meet the e l e c t r o n i c , p h y s i c a l and economic c o n s t r a i n t s imposed by the p h o t o v o l t a i c a p p l i c a tion. Then, recent r e s u l t s on the s t r u c t u r a l , e l e c t r i c a l , and o p t i c a l p r o p e r t i e s of (PAN), a r e presented indicated. X

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Economics. The U.S. Department of Energy (DOE) has e s t i m a t ed that i n order t o be c o s t - e f f e c t i v e , the i n s t a l l e d system p r i c e for r e s i d e n t i a l p h o t o v o l t a i c systems i n 1986 must be $1.60 t o $2.20 p e r peak watt, i n 1980 d o l l a r s . Of t h i s , $0.80 p e r peak watt i s a p p l i e d to the p h o t o v o l t a i c c o l l e c t o r i t s e l f . Typical costs f o r current p h o t o v o l t a i c systems are $20.00 p e r peak watt, of which $10.50 per peak watt i s a l l o c a t e d to the c o l l e c t o r .(JO Although s t r i d e s a r e being made i n the development of s i n g l e c r y s t a l s i l i c o n p h o t o v o l t a i c d e v i c e s , the p o t e n t i a l f o r t h e i r low-cost manufacture remains an open question. The need to search f o r other m a t e r i a l s which may r e s u l t i n c o s t - e f f e c t i v e dev i c e s i s evident. The economic a t t r a c t i o n of an organic polymerbased p h o t o v o l t a i c device i s i t s use of small amounts of i n e x pensive m a t e r i a l and i t s s u i t a b i l i t y f o r mass production. Physical Properties. A p h o t o v o l t a i c device must withstand exposure to the environmental c o n d i t i o n s encountered i n the c o l l e c t i o n of s o l a r energy, i n c l u d i n g exposure t o u l t r a v i o l e t and v i s i b l e r a d i a t i o n and operating temperatures up t o 80°C. No o r ganic polymer m a t e r i a l which can meet these c o n d i t i o n s - and which has s u i t a b l e p h o t o v o l t a i c p r o p e r t i e s - has yet been i d e n tified. P o l y a c r y l o n i t r i l e has emerged as an i n t e r e s t i n g c a n d i date m a t e r i a l because i t can withstand these environmental c o n d i t i o n s , and may have the semiconducting p r o p e r t i e s required f o r a p h o t o v o l t a i c device.

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S u i t a b i l i t y f o r Mass P r o d u c t i o n . Thin f i l m organic polymers have unique p o t e n t i a l f o r the low-cost l a r g e - a r e a mass production of p h o t o v o l t a i c devices. Small amounts of inexpensive m a t e r i a l are required and mass production f a b r i c a t i o n processes s i m i l a r t o those already used i n the polymer converting i n d u s t r y , i n c l u d i n g f i l m manufacture, l a m i n a t i o n , metal c o a t i n g , and p r i n t i n g , may be a p p l i c a b l e . This i s one of the major a t t r a c t i o n s of a t h i n f i l m organic polymer p h o t o v o l t a i c d e v i c e . F e a s i b i l i t y of an Organic Photovoltaic Properties

Polymer-Based

Photovoltaic

Device -

For a s i n g l e gap p h o t o v o l t a i c device i t i s d e s i r a b l e that l i g h t above the i d e a l gap energy o f about 1.5 eV be s t r o n g l y ab­ sorbed. Polyacetylene ((CH) ) has a d i r e c t a b s o r p t i o n edge a t about 1.4 eV with a peak a b s o r p t i o n c o e f f i c i e n t of about 3 χ 1 0 cm" a t about 1.9 eV.^2 vantage i n p h o t o v o l t a i conductors such as c r y s t a l l i n e s i l i c o n , promising strong absorp­ t i o n of photons with energies above the band gap. The o p t i c a l a b s o r p t i o n of pyrolyzed PAN i s described below. There are two major problems concerning the f e a s i b i l i t y o f n o n c r y s t a l l i n e p h o t o v o l t a i c devices. The f i r s t i s that low c a r ­ r i e r m o b i l i t i e s may prevent most c a r r i e r s from reaching the junc­ t i o n region. The second i s that due t o the high d e n s i t y of s t a t e s i n the band gap of such m a t e r i a l s , most c a r r i e r s w i l l not even enter the conduction band. The s i t u a t i o n i s f u r t h e r compli­ cated by the p o s s i b i l i t y that the o p t i c a l and e l e c t r i c a l proper­ t i e s of polyacetylene a r e caused by s o l i t o n s (bond a l t e r n a t i o n domain w a l l s ) . (_3, 4) The f i r s t problem may be overcome by n o t i n g t h a t the high absorption c o e f f i c i e n t of amorphous m a t e r i a l s means that ( u n l i k e c r y s t a l l i n e s i l i c o n ) very t h i n "amorphous c e l l s can be f a b r i c a t e d so that most of the charge c a r r i e r s are photo-ex­ c i t e d w i t h i n the e l e c t r i c f i e l d r e g i o n and no i n t e r n a l d i f f u s i o n i s necessary."(5) Although i t i s not p o s s i b l e t o guarantee that a high d e n s i t y of s t a t e s w i l l not occur i n the gap o f organic polymer semiconductors, work on other disordered systems such as amorphous s i l i c o n i n d i c a t e s that the high d e n s i t y of s t a t e s w i t h ­ i n the gap i s not an i n t r i n s i c property. Thus, by property e n g i ­ neering research, i t may be p o s s i b l e t o overcome t h i s problem. An A1:(CH) Schottky device w i t h a quantum e f f i c i e n c y approaching u n i t y f o r photon energies above 2.8 eV and a conversion e f f i ­ c i e n c y of 0.3% at low l i g h t l e v e l s (0.21 mW/cm^) has already been reported.(6) The current saturated a t higher l i g h t l e v e l s , r e ­ ducing the e f f i c i e n c y , p o s s i b l y because of a space-charge buildup i n the d e p l e t i o n region.(6) This e f f e c t may a l s o have c o n t r i b u t ­ ed t o the low conversion e f f i c i e n c y . Research may y i e l d b e t t e r m a t e r i a l s and improved p h o t o v o l t a i c d e v i c e s . X

5

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P r o p e r t i e s o f Pyrolyzed PAN S t r u c t u r e * The s t r u c t u r a l changes u s u a l l y a t t r i b u t e d t o the p y r o l y s i s (heat treatment i n i n e r t atmosphere) of PAN (7-11) a r e shown i n Figure 1. Above about 200°C, the unpyrolyzed polymer ( F i g u r e l a ) i s converted to a s i n g l y conjugated ladder ( F i g u r e l b ) , and then a t temperatures between 300-400°C t o a doubly conjugated ladder ( F i g u r e l c ) . While t h i s p i c t u r e i s an o v e r s i m p l i f i c a t i o n (7, 12), there i s evidence that i t i s f o r the most part c o n s i s t e n t w i t h experimental r e s u l t s up t o about 350-400°C. I n a d d i t i o n , a t temperatures above about 350-400°C NMR data des c r i b e d below suggest that c r o s s l i n k i n g occurs between the p r o t o nated carbons of the chains shown i n F i g u r e l c due t o hydrogen loss. The experimental r e s u l t s which lead t o these conclusions a r e summarized i n Table I Elemental analyses, conducted by a comm e r c i a l l a b o r a t o r y , ar r e l a t i v e numbers of atom bon atoms per monomer u n i t . IR s p e c t r a were obtained in-house using a P e r k i n Elmer 298 IR spectrophotometer. S o l i d probe magic angle spinning NMR experiments were performed a t the N a t i o n a l Bureau o f Standards. A l l of these analyses were performed on bulk ( i . e . pyrolyzed i n batches of ^ l g ) samples of 485,000 average molecular weight u l t r a p u r e PAN prepared i n a tube furnace evacuated by a mechanical vacuum pump, and are reported on i n det a i l elsewhere.(13) Table I a l s o summarizes e l e c t r i c a l c o n d u c t i v i t y and o p t i c a l a b s o r p t i o n experimental r e s u l t s obtained using t h i n f i l m s o f PAN s o l u t i o n c a s t i n dimethylformamide. As shown i n Table I , elemental, IR, and NMR analyses are a l l c o n s i s t e n t w i t h s t r u c t u r e l a f o r unpyrolyzed PAN. The NMR spect r a suggest a low c r y s t a l l i n i t y p o s s i b l y r e s u l t i n g from a l a c k of s t e r e o r e g u l a r i t y i n the s t a r t i n g m a t e r i a l . For the 220°C sample, the elemental a n a l y s i s confirms that no l o s s of n i t r o g e n has occurred. Increased IR a b s o r p t i o n between 1600-1200cm~l i n d i c a t e s conjugation (the a b s o r p t i o n i s too broad t o be more s p e c i f i c ) , w h i l e a decrease i n i n t e n s i t y of t h e C=tî a b s o r p t i o n a t 2240cm"* suggests that conjugation i s obtained by c y c l i z i n g the polymer r a t h e r than by d e s a t u r a t i n g the top c h a i n of Figure l a . The high hydrogen content (2.83 atoms per monomer u n i t ) confirms t h i s view. The e l e c t r i c a l c o n d u c t i v i t y o f t h i n f i l m samples pyrolyzed a t 220°C i s low, but the c o n d u c t i v i t y i s enhanced by doping (13, 14) as would be expected f o r a 1 - d i mensional conjugated c h a i n . Thus, a s t r u c t u r e intermediate between l a and l b i s i n d i c a t e d . Longer p y r o l y s i s time would be expected t o cause a more complete t r a n s f o r m a t i o n of l a t o l b . Bulk samples o f PAN pyrolyzed above about 250°C e x h i b i t a very sharp exotherm a s s o c i a t e d w i t h r i n g c l o s u r e ( i . e . thermal p o l y m e r i z a t i o n o f the n i t r i l e groups) (13, 15) and c o n s i d e r a b l e mass l o s s . This behavior i s not observed i n t h i n f i l m samples.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26.

METZ ET AL.

Polyacrylonitrile as a Photovoltaic Material

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In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Table I

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Experimentally Observed P r o p e r t i e s

P y r o l y s i s Temperature (°C) P y r o l y s i s Time (hr) Bulk Sample (^lg) Experimental

Unpyrolyzed

Results

- Most L i k e l y S t r u c t u r e ( s ) (see F i g u r e 1) - Elemental A n a l y s i s (Commercial Lab.) R a t i o of Atoms C:H:N:0 Normalized t o 3 Carbons - IR Spectra

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Thin F i l m Experimental

UTILIZATION

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Results

- Room Temperature E l e c t r i c a l C o n d u c t i v i t y (σ) (ohm-cm)" (see F i g u r e 2) - A c t i v a t i o n Energy (eV) (Slope of I n σ v s 1/kT) (see F i g u r e 2) - O p t i c a l A b s o r p t i o n Edge (eV) (see F i g u r e 3)

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In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Polyacrylonitrile as a Photovoltaic Material

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1()5 c m ) and an a b s o r p t i o n edge i n the range o f roughly 1-3 eV. H a l l m o b i l i t y has been reported f o r samples pyrolyzed above 600°C ( 1 8 ) , but d r i f t m o b i l i t y has not been reported (see 18 f o r an e s t i m a t e ) . Weak p h o t o c o n d u c t i v i t y has been observed. (17, 19) The charge t r a n s p o r t mechanism i n PAN i s poorly under­ stood. Some data suggest a c o r r e l a t i o n between twice the a c t i v ­ a t i o n energy of conduction ( s l o p e o f Ζησ vs 1/kT) and the o p t i c a l a b s o r p t i o n edge ( 1 3 , 16), w h i l e there i s a l s o evidence that σ ο,χ-1/4 (13 16), suggesting hopping v i a l o c a l i z e d s t a t e s near the -1

9

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Fermi energy. An aluminum-PAN Schottky j u n c t i o n has been r e p o r t ed. ^24) Enhanced c o n d u c t i v i t y v i a doping with halogen e l e c t r o n acceptors has been shown.(13, 14) Doping w i t h e l e c t r o n donors has not been reported. In s h o r t , there are some encouraging r e s u l t s , p a r t i c u l a r l y the a d j u s t a b l e o p t i c a l absorption edge and some discouraging r e s u l t s such as the low p h o t o c o n d u c t i v i t y . The charge t r a n s p o r t mechanism i s s t i l l poorly understood. An e v a l u a t i o n of t h e s u i t a b i l i t y of PAN as a p h o t o v o l t a i c m a t e r i a l r e q u i r e s b e t t e r data on charge t r a n s p o r t , doping and j u n c t i o n formation. General D i s c u s s i o n and

Conclusion

Thin f i l m organic polymer semiconductors are an a t t r a c t i v e o p t i o n f o r t h e development of c o s t - e f f e c t i v e p h o t o v o l t a i c dev i c e s . They are uniquely s u i t e d t o low-cost mass production and provide p o t e n t i a l f l e x i b i l i t m a t e r i a l s with b e t t e r p h y s i c a those c u r r e n t l y a v a i l a b l e are needed. This w i l l r e q u i r e a f a r b e t t e r understanding of the s t r u c t u r e and t r a n s p o r t p r o p e r t i e s o f these m a t e r i a l s . P o l y a c r y l o n i t r i l e i s an i n t e r e s t i n g m a t e r i a l f o r t h i s a p p l i c a t i o n w i t h evidence of semiconductive behavior and s t a b i l i t y a t elevated temperatures. The o p t i c a l absorption data i n d i c a t e t h a t the o p t i c a l absorption edge i n PAN may be a d j u s t a b l e , suggesting the p o s s i b i l i t y of f a b r i c a t i n g multigap c e l l s v i a t h i s o r a s i m i l a r m a t e r i a l . However, a f a r b e t t e r understanding of the s t r u c t u r e , microscopic t r a n s p o r t , and o p t i c a l p r o p e r t i e s o f PAN i s r e quired. This knowledge w i l l provide a b e t t e r understanding of the connection between polymer s t r u c t u r e and semiconductive beh a v i o r , p e r m i t t i n g the enlightened s y n t h e s i s of polymers o p t i mized f o r p h o t o v o l t a i c s . Acknowle dgment s The authors thank S. Aronson f o r valuable d i s c u s s i o n s on data a n a l y s i s and i n t e r p r e t a t i o n , and J . Andrews and F. Salzano for v a l u a b l e d i s c u s s i o n s and advice. Work performed under the auspices of the U.S. Department of Energy under Contract No. DE-AC02-76CH00016.

Literature Cited 1. 2.

Photovoltaic System Definition and Development, Project Integration Meeting, Albuquerque, NM, October 21-23, 1980, SAND 80-2374, pp 12-15. Fincher J r . , C. R.; Ozaki, M.; Tanaka, M.; Peebles, D; Lauchlan, L . ; Heeger, A. J.; MacDiarmid, A. G. Phys. Rev. 1979, B20, 1589-1601.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

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Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Phys. Rev. Lett. 1979, 42, 1698. Rice, M. J. Phys Rev. Lett. 1979, 71A, 152. Adler, D. Sunworld 1980, 4, 18. Weinberger, B. R.; Gau, S. C.; Kiss, Z. Appl. Phys. Lett. 1981, 38, 555. Brennan, W. D.; Brophy, J. J.; Schonhorn, H. "Organic Semi­ conductors," Proc. Inter-Industry Conf., J.J. Brophy and J.W. Buttrey, Eds., 1962, p 159. Burlant, W. J.; Parsons, J. L. J. Polym. Sci. 1956, 22, 249. Topchiev, Α. V.; Geyderikh, Μ. Α.; Davydov, Β. E . ; Kargin, V. Α.; Krentsel, Β. Α.; Kustanovich, I. M.; Polak, L. S. Dok. Akad. Nauk. SSSR 1959, 128, 312. Becher, M.; Mark, H. F. Angew. Chem. 1961, 73, 641. Topchiev, Α. V. J. Polym. Sci. A: 1963, 1, 591. Monahan, A. R. J. Metz, P. D.; Teoh cal and Optical Properties of Pyrolyzed Polyacrylonitrile, (in preparation). Brokman, Α.; Weger, M.; Marom, G. Polym. 1980, 21, 1114 (note: These authors used PAN fibers "stabilized" in air at 220°C). Grassie, N.; McGuchan, R. Eur. Poly. J. 1970, 6, 1277. Teoh, H.; Metz, P. D.; Wilhelm, W. G. Mol. Cryst. Liq. Cryst. 1982, 83, 297. Hirai, T.; Nakada, O. Jpn. J. Appl. Phys. 1968, 7, 112. Suzuki, M.; Takahashi, K.; Mitani, S. Jpn. J. Appl. Phys. 1975, 14, 741. Ohigashi, H. Rep. Prog. Polym. Phys. Jpn. 1963, 6, 245. Helberg, H. W.; Wartenberg, B. Phys. Status Solidi A: 1970, 3, 401. Jacquemin, J. L . ; Ardalan, Α.; Bordure, G. J. Non-Cryst. Solids 1978, 28, 249. Airapetyants, Α. V.; Voitenko, R. M.; Davydov Β. E . ; Krentsel, B. A. Dok. Akad. Nauk. SSSR 1963, 148, 605. Etemad, S.; Heeger, A. J.; Lauchlan, L . ; Chung, T. C.; MacDiarmid, A. G. Mol. Cryst. Liq. Cryst. 1981, 77, 43. Chutia, J.; Barua, K. Phys. Stat. Sol., 1979, (a) 55, K13. Davydov, B. E . ; Krenstel, B. A. Adv. Polymer Sci. 1977, 25, 1.

RECEIVED November 22, 1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

27 Photovoltaic Properties of Organic Photoactive Particle Dispersions Polymeric Phthalocyanines R. BRANSTON, J. DUFF, C. K. HSIAO, and R. O. L O U T F Y Xerox Research Centre of Canada, 2480 Dunwin Drive, Mississauga, Ontario, L5L 1J9 Canada

Photovoltaic measurement silicon and germanium phthalocyanine particle dispersion in a Schottky barrier device. The optical, electrical and photoelectrical properties of these materials were found to depend strongly on the molecular stacking and separation distance of the phthalocyanine rings within the crystals. The electrical conductivity increased with a decrease in the separation distance between the phthalocyanine rings. Strong photoactivity is associated with a molecular arrangement in the solid state corresponding to a staggered parallel plane dimers. Both dihydroxy germanium and dihydroxy silicon phthalocyanine solids exhibited strong absorption peaks in the near infrared (870 nm) which indicates considerable intermolecular interaction is present in the solid state. Considerable effort has been made to improve the power conversion efficiency of solid-state organic photovoltaic cells in an attempt to find an inexpensive, efficient and stable solar cell for large scale terrestial use. (1) It is believed that this goal can be achieved only via a technological breakthrough in material design and crystal lattice architecture aiming at lowering the bulk resistance of organic semiconductors. A number of conducting metallophthalocyanine compounds have recently been reported. (2-6) It was shown that the electrical conductivity of these materials can be controlled by molecularly stacking the metallomacrocycle rings in a "face-to-face" orientation via covalent linkage. (3) Among the important precursors used in the polymerization of phthalocyanines are dihydroxy silicon, germanium and tin, which upon polymerization produce phthalocyaninato polysiloxanes, polygermyloxanes and polystannyloxanes. Polymeric phthalocyanines were noted for their intense colors, high thermal stability and a dramatic change in electric conductivity upon halogen oxidation or doping with o-chloranil. In spite of the extensive studies of the physical, structural and electrical properties of polymeric phthalocyanines, (2-8) however, there is a lack of photoconductivity data. We therefore, became interested in the synthesis and evaluation of these materials for solar cell applications. 0097-6156/83/0220-0437$06.00/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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In this paper, we report the results of a preliminary investigation of the optical, electrical and photoelectrical properties of a series of silicon and germanium phthalocyanines and a number of their polymers. EXPERIMENTAL Materials. Using the methodology developed by Kenney (2) dichlorosilicon and dichlorogermanium phthalocyanines, PcMCI , were prepared and then hydrolyzed to the dihydroxides, PcM(OH) as shown in Scheme I. Polymerization in the presence of diols produced the corresponding phthalocyaninato polysiloxanes and polygermyloxanes in high yield and purity (Scheme II). All materials were characterized by elemental analysis, x-ray, infrared and electronic absorption. Polyvinylacetate was obtained from Polyscience Inc. The conductive tin-oxide coated glass was from Pittsburg Glass Company 2

2

quinoline I G e C I

4

CI

CI aq N H pyridine 3

H

Ο

SCHEME I

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

27.

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AL.

Polymeric

439

Phîhalocyanines

SCHEME Π

Device Fabrication. Our approach required the fabrication of a photovoltaic device consisting of a thin film (0.8 /xm) particle dispersion of the phthalocyanine pigment in a polymer binder (7), sandwiched between Sn0 and a barrier electrode. The organic pigment films were prepared by coating a suspension of 0.18 gm pigment in 8 ml of a solution of 1.5% polyvinylacetate in methylene chloride, onto a precleaned heavily doped Sn0 substrate with a 2 mil draw bar. The resutling organic films contained 60% pigment by weight. The thickness of these films, measured by a Talysurf apparatus averaged 0.8 /im. A thin, semitransparent (1.2% transmission) indium electrode (90 nm) was vacuum deposited on the top of the organic film to complete the solid state junction as shown in Figure 1. 2

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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INCIDEN SEMI-TRANSPARENT IN ELECTRODE PIGMENT .PARTICLES DISPERSED IN PVA

j _ 1.0 μ NESA SUBSTRATE ΙΛΛΛΛΛΛΛΛΛΛΛΛΛΛΛΛΛΛΛΛ^ Figure 1

Side view of the photovoltaic device.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Polymeric Phthalocyanines

MEASUREMENTS Solubility. The solubilities of the polymers were tested using the solvents: dichlorobenzene, trichlorobenzene, methanol, dichloromethane, and 1chloronapthalene. (PcGeO(CH ) 0) and (PcGeO(CH ) 0) were sparingly soluble only in 1-chloronapthalene. These and other polymers were insoluble in the other solvents listed above. 2

6

n

2

12

n

Melting Points. All polymers had melting points of greater than 300°C. Polymers were examined under a microscope for changes in their crystal stucture during heating, but no changes were evident. IR Spectra. The IR spectra of the pigments dispersed in Nujol were taken on Beckman 5220 spectrometer germanium phthalocyanine C(CH ) C H 0). There was no hydroxyl peak detectable in the spectrum of the polymer, while an intense OH peak at 3500 cm' was clearly observable for PcGe(OH) . The absence of OH absorption in the polymers is indicative of at least moderate chain length. 3

2

6

4

1

2

X-Ray Powder Diffraction. X-Ray Powder Diffraction was used primarily as a fingerprinting technique. The presence of sharp peaks in all of the spectra indicated that every polymer was crystalline, and that the crystalline structure of each polymer was different. The unit-cell dimensions do not appear to change continously with the increasing length of the alkane chain. All polymers differ from the starting materials PcGeCI and PcGe(OH) . The high degree of crystallinity indicates that the polymer chains are rigid or highly organized, presumably as a result of the large size and high symmetry of the phthalocyanine rings. It is not possible to determine the chain length of the polymers exactly using X-Ray Powder Diffraction; some examples are shown in Figure 3. It should be noted that the Xray powder diffraction lines observed in polymeric sample do not necessarily measure the degree of Pc-Pc interactions. 2

2

Bulk Optical Absorption of Phthalocyanine Particles. The visible and near infrared optical absorption of the phthalocyanine pigments was measured on a Cary 17 spectrophotometer using the technique described in Reference 8. A thin film of phthalocyanine dispersed in the polymer was coated onto 3 mil Mylar substrate using wirewound rods. The dry films were index matched by overcoating with a thick film of polyvinyl alcohol. The differential absorption between two films with different thicknesses were recorded to minimize reflection and light scattering losses. The solution absorption spectra of some pigments were also measured in a-chloronapthalene.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

442

P O L Y M E R S IN

(PcGeOC H C ( C H ) C H 0 J 6

1

J 1

4

1 4000

1

2

L

11

PcGe(OH)

3

1

6

4

1 —"1

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n

1

1

ι

2

1 3400

1

L 3000

J

1

1 2400

I 2000

WAVELENGTH μπι Figure 2

Infrared spectra of PcGe(OH) and (PcGeOC H C(CH ) C H 0) polymer taken in Nujol. 2

6

4

3

2

6

4

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

n

27.

BRANSTON E T A L .

443

Polymeric Phthalocyanines

PcGe(OH)

2

(PcGeO(CH ) 2

(PcGeO(CH ) 0) 2

4

(PcGeO(CH ) 0) 2

1 2

ÇH

n

n

3

[PcGeOC H -C- C H o] 6

4

6

4

n

_l

JL 40

30

20

L

10

26 Figure 3 X-ray powder diffraction of PcGe(OH) . 2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Photovoltaic Measurements. The dark and light current-voltage (J-V) characteristics of the Sn0 /phthalocyanine/ln cells were determined using an EG/G Polarographic Analyzer, Model 174A and recorded using Hewlett Packard 9845B desktop computer. 2

The light source was a 1 KW Oriel Solar Simulator equipped with AMO filters. Light intensities were measured by a Karl Lambrecht calibrated light probe. The unattenuated power flux delivered by the System was 208 mW/cm. Incident light intensity was varied using neutral density filters. The light source for action spectra measurements consisted of a 150 W Xe lamp and a A meter Jarrel Ash monochromator. The action spectra of the cell was measured as described previously (7). 2

l

RESULTS AND

DISCUSSION

Many organic pigment electrophotographic or photovoltaic devices. In these applications, the most important material parameters are (i) the photogeneration efficiency, φ, which is the average number of mobile electrons and holes generated per photon absorbed, (ii) the spectral response and (iii) the electrical conductivity. Photovoltaic techniques have recently (4) been shown to be useful for measuring photoconductivity, electrical conductivity and spectral response of organic materials. In our measurements the electrical and photoelectrical properties of a series of phthalocyanine polymers were determined by using these materials in a Schottky junction devices. Electrical Conductivity. Figure 4 shows a typical current-voltage curve of an Sn0 /PcGe(OH) , PVA/ln cell in the dark. These devices exhibit strong rectifying behaviour, with a forward bias corresponding to a negative voltage at the indium electrode with respect to the Sn0 . The forward biased current appears to be due to hole injection from Sn0 . At sufficiently high forward biased current the diode junction resistance approaches a constant value. This value is the series resistance R of the device which is dominated by the bulk resistance, R of the pigment. (10) The specific conductivity, σ, of the pigmented film can be calculated from: 2

2

2

2

s

b

L σ

=

(1) R . A where L is the thickness of the pigmented film and A is the area of the electrode. The specific conductivity of PcGe(OH) was found to be 3.5 χ 10" Q'cm' which is an order of magnitude lower than that of PcSi(OH) (2.3 χ 10" £2" cm" ). These results are in general agreement with those of Meyer and Wohrle (6). The electrical conductivity of PcGe(OH) was found to increase by two order of magnitude upon polymerization to (PcGeO) , see Table 1. This effect can be attributed to a decrease in the intermolecular spacing between the macrocyclic b

9

1

1

2

8

1

1

2

2

n

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

27.

BRANSTON E T A L .

Polymeric Phthalocyanines

445

N E S A / P c G e (OH) - Ρ VA / In 2

80

/

70 60 £T E υ

50

1

40

/ II h h 1 1 II

30 20

II

11 Ι ι

I ι

10 l

+2

Figure 4

I

/

I

S \

It

ι

-2

The introduction of hydrocarbon chains separating the phthalocyanine molecules with increasing length, results in a decrease of the electrical conductivity (see Table II). However, the decrease in σ was not catastrophic particularly for the long chain hydrocarbon, implying that the hydrocarbon chains are not all in trans­ configuration, permitting some intermolecular interaction between Pc rings to occur supporting Meyer and Wohrle recent results on polysilicon phthalocyanines (6).

TABLE I ELECTRICAL CONDUCTIVITY OF TETRAVALENT PHTHALOCYANINES

1.5 χ 1er

(PcGeO) PcGe(OH) n

7

3.5 χ 10"

9

2

7.4 χ 10" 6.0 χ 10*

(t-Bu) PcGe(OH) 4

11

2

PcSiCI PcSi(OH) 2

12

2.3 χ 10"

8

2

7

12

1

1

As shown in Tables I and II, σ varied from 10" to 10' Q'cm* for the series of material studied depending on the chemical nature and molecular packing of the molecules. The most electrically conductive silicon and germanium phthalocyanines were (PcGeO) , PcSi(OH) and PcGe(OH) . However, the resistivities of these materials were still several order of magnitude higher than that necessary for solar cell application. It has been shown previously that the conductivity of this class of materials can be lowered by many order of magnitude upon halogen oxidation (3) or doping with o-chloranil (6). Although doping might be useful in lowering the resistivity of these materials to the desired levels, the impact of doping on the photoconductivity of the samples is not known. n

2

2

TABLE II ELECTRICAL CONDUCTIVITY OF POLYMERIC PHTHALOCYANINES 1

(PcGeO) (PcGeO(CH ) 0) (PcGeO(CH ) 0) (PcGeOC H C(CH ) C H 0) n

2

2

6

4

4

12

n

n

3

2

6

4

n

σ (Ω* 1.5 χ 2.5 x 1.2 χ 1.2 χ

1

cm') 10 10' 10 10" 7

8

8

9

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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BRANSTON E T A L .

447

Polymeric Phthalocyanines

Solid State Absorption. The solid state electronic absorption of organic pigments reveal important information on the molecular arrangement and extent of intermolecular interaction between molecules in the crystal. The solid state absorption properties of polycrystalline films of some silicon and germanium phthalocyanines and some polymers are shown in Figures 5-8. Figure 4 shows the solid state absorption spectra of a bis-silicon phthalocyanine linked by butanediol and a series of polygermanium phthalocyanines where the macrocyclic rings were separated by phenyl, biphenyl and bisphenol A. In these cases the absorption resembled the molecular solution absorption with the exception that it was slightly broader. These results clearly indicate that a very weak intermolecular interaction between molecules in the solid exists. The same is true for PcSiCI and PcGeCI . Figure 5 shows the solid state absorption spectra of PcGe(OH) and PcSi(OH) which is very different from the solution molecular absorption. The spectra consists of a broad absorption covering the wavelength region from 400-1000 nm, with two maxima one to the red and the other to the blue of the molecular absorption. The intense nea for PcGe(OH) and PcSi(OH) most recently for AlCIPc. In the case of x-H Pc a parallel plane dimeric structure of two neighbouring Pc molecules in a staggered configuration had been proposed by Sharp and Lardon (VYj. This stacking arrangement is very different from that of both α and β polymorphs of phthalocyanines. In the later forms molecules are projected directly along the stacking axis with equal spacing and in an eclipsed configuration. It is tempting to speculate that the crystal structure of PcGe(OH) and PcSi(OH) responsible for the near infrared absorption consists of parallel plane dimers (aggregate) in which the molecules are staggered. The solid state absorption of all other polymeric germanium phthalocyanine examined showed very weak or undetectable near infrared peaks. These results clearly indicate a strong intermolecular interaction between molecules in the solid state of PcGe(OH) and PcSi(OH) , while all other pigments showed much weaker interactions. It is interesting to speculate that the exciton splitting is related to enhanced photogeneration and that a strong intermolecular interaction is essential for high quantum yields of photogeneration, since x-H Pc, VOPc and ClAIPc all exhibit high photoactivity. 2

2

2

2

2

2

2

2

2

2

2

Photoconductivity. Figure 9 shows a typical current-voltage curve of an ln/PcGe(OH) PVA/Sn0 cell under (85 mW/cm ) AMO illumination. From the photocurrent-voltage curves under solar and monochromatic illumination, several fundamental parameters, such as the open circuit voltage (V ) the short circuit current (J ), the fill factor, as well as the dependence of J and V on light intensity can be extracted. The power conversion efficiency of the device can be estimated from: 2

2

2

QC

sc

s c

V

=

V

Jsc · oc ·

f

,

/

l

QC

(2)

American Chemical Society Library 1155 16th st. N. w. Washington, D. C. 20031

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

448

Figure 5

POLYMERS

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Solid-state absorption spectra of silicon and germanium phthalocyanine polymers.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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BRANSTON E T A L .

500

449

Polymeric Phthalocyanines

600

700

800

900

1000

λ (nm) Figure 6 Solid-state absorption spectra of PcGe(OH) and PcSi(OH) particles. 2

2

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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BRANSTON E T A L .

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Polymeric Phthalocyanines

I

I

I

500

600

700

I L_ 800

900

X(nm) Figure 8 Solid-state absorption spectra of PcGeO(CH ) 0) milling time in CH CI and of (PcGeO(CH ) 0) . 2

2

2

2

6

4

n

as a function of

n

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

452

Figure 9

POLYMERS

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Current-voltage curves for Sn0 /PcGe(OH) , PVA/ln cell in the dark and 2

2

under solar illumination.

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Polymeric Phthalocyanines

BRANSTON E T A L .

where I is the incident light intensity. The output of In/Pc, P V A / S n 0 photovoltaic cells under AMO solar illumination is given in Table III. The power conversion efficiency is very low (η ~10" %) typical of a high resistance photoconductor. 1; if TT annihilation depleted the phosphorescent triplet state then n < 1 would be observed. For a beautiful example of this type of behavior in polyvinylcarbazole films see Rippen, G.; Kaufmann, G.; Klöpffer, W. Chem. Phys. 1980, 52, 165 and references therein. 12. If 3Py* is populated by triplet excitons with a lifetime on the order of that of isolated Py* the effect will be an apparent lengthening of τ3Py*. 13. The absorption spectrum of vapor deposited CT complexes of Py-TCNB (no polymer matrix) is strongly red-shifted rela­ tive to naphthalene-TCNB (unpublished results of N. Kim). 14. According to H. Möhwald and E. Sackmann (Z. Naturforsch. 1974, 29a, 1216) the triplet state of Py-TCNB pairs in a 1:1 naphthalene-TCNB CT crystal is lower by 1400-2200 cm-1 than the naphthalene-TCNB triplet exciton. This energy gap is similar to what we observe nm) and the structureles e x

3

RECEIVED

November 22,1982

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

29 Photoelectrochemical Catalysis with Polymer Electrodes HOWARD D. M E T T E E Youngstown State University, Chemistry Department, Youngstown, OH 44555

Polymer electrode ally convert this t i a l , possibly with electrochemical assistance, using endoergic reactions such as water splitting, are of increasing interest as practical means of solar energy storage are sought. Developments in this field may be interpreted in terms of photocatalytic and electrochemical reactions catalysed by polymers. Emphasis is placed upon the roles of polymer films in protecting n-type semiconductors from anodic dissolution and in helping to under­ stand electron transfer mechanisms. Requirements of free-standing polymeric photoelectrochemical (PEC) catalysts are outlined. In 1972 Fujishima and Honda (1) focussed attention on the possibility of using visible light to photolytically sensitize the splitting of water into hydrogen and oxygen. This light driven, energy storing reaction has obvious attractions in that it produces a clean burning, re-cyclable fuel and it ultimately depends upon an infinite energy source. However, to couple ab­ sorbed solar energy to this thermodynamically uphill reaction requires photoelectrochemical (PEC) catalysts which will simutaneously oxidize and reduce water. PEC + hv solar 2 H0 y

>

4 H 0 + 4e~

•

1

2

2

• PEC* • 0 PEC* •

2

+ 4H

+ 4e

• 2H

2

_ + 4 OH

These catalysts must not only cause the birth of reactive inter­ mediates, which in some cases have been identified as Η', e", and OH' etc., but also prevent their back reaction until the O2 and H2 have formed. Obviously they must maintain their own chemical 0097-6156/83/0220-0473$06.50/0 © 1983 American Chemical Society In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

474

POLYMERS IN SOLAR ENERGY UTILIZATION

i n t e g r i t y i n the process. Thus the f u n c t i o n a l requirements of these PEC c a t a l y s t s are more demanding than those o f t h e i r phot o c a t a l y t i c c o u n t e r p a r t s , which serve only t o a c c e l e r a t e energet i c a l l y downhill reactions. In the i n t e r v e n i n g decade s i n c e Fujishima and Honda's paper a great deal o f chemical e f f o r t has gone i n t o the design and cons t r u c t i o n of these s p e c i a l PEC c a t a l y s t s . The r o l e o f polymers has been an important one a t a l l l e v e l s o f i n v e s t i g a t i o n whether the system be c o l l o i d a l m i c e l l e s , v e s i c l e s , microemulsions o r " m i c r o e l e c t r o d e s " , o r b u l k semiconductors. Without polymeric support f o r the c a t a l y s t s i n c o l l o i d a l systems f o r i n s t a n c e , important d i a g n o s t i c r e a c t i o n s could not be detected. More s i g n i f i cant f o r the polymer chemist, however, i s the i n c r e a s i n g l y cent r a l r o l e polymers a r e p l a y i n g i n the a c t u a l l i g h t a b s o r p t i o n , charge s e p a r a t i o n and p a r t i c l e flow dynamics that c h a r a c t e r i z e the intermediate chemistry p r i o r to H2 and O2 formation. A completely polymeri water s p l i t t i n g has no tem which can s u c c e s s f u l l y compete w i t h the e f f i c i e n c y and l o n g e v i t y o f p h o t o v o l t a i c a l l y - d r i v e n water e l e c t r o l y s i s (0 ^ 10%). Thus current e f f o r t s are addressed a t s e l e c t i n g a p p r o p r i a t e l i g h t absorbing agents, charge c r e a t i n g and s e p a r a t i n g media, and c a t a l y t i c environments which meet the necessary thermodynamic and k i n e t i c requirements. Of course the more p r a c t i c a l c o n s i d e r a t i o n s o f reasonable quantum y i e l d , d u r a b i l i t y and cost cannot be ignored e i t h e r . Broadly speaking, polymers have c o n t r i b u t e d both to the s t a b i l i t y o f PEC c a t a l y s t s and t o the understanding and c o n t r o l o f charge m i g r a t i o n and redox chemistry i n these systems. In the f u t u r e , polymers o f f e r an e x t r a degree o f s y n t h e t i c f r e e dom which may be e x p l o i t e d t o enhance the quantum y i e l d s , durab i l i t y and economic p r a c t i c a l i t y of PEC systems s t i l l f u r t h e r . A number of c l o s e l y r e l a t e d reviews have been p u b l i s h e d r e c e n t l y which h i g h l i g h t a number o f approaches t o s o l a r induced water s p l i t t i n g . Bard (2) has summarized the semiconductor des i g n c r i t e r i a f o r example. More d e t a i l e d reviews by Nozik (3) and Wrighton (4) consider the i n t e r p l a y o f the thermodynamic and k i n e t i c behavior o f these semiconductor u n i t s , and Wrighton has helped develop the concept o f surface m o d i f i e d , semiconductor e l e c t r o d e s . C a l v i n (5) and P o r t e r (6) have emphasized the c l o s e s i m i l a r i t y of PEC water s p l i t t i n g t o n a t u r a l p h o t o s y n t h e s i s , thereby l e a d i n g t o the study o f biomimetic systems a t the c o l l o i d a l l e v e l . Whitten (7) has considered photoinduced e l e c t r o n t r a n s f e r i n homogeneous s o l u t i o n s , very f r e q u e n t l y i n v o l v i n g chlorophyll-like sensitizers. P u b l i c a t i o n of the proceedings o f three recent symposia d e a l ing w i t h PEC processes i n the past two years i n d i c a t e s the h i g h l e v e l o f chemical i n t e r e s t i n these systems. Two i n the A.C.S. Symposium s e r i e s (8,9) consider the importance o f i n t e r f a c e s i n PEC systems, w h i l e a Faraday D i s c u s s i o n Volume (10) contains p r i n c i p a l l y semiconductor c o n t r i b u t i o n s . I t appears that the

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

29.

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475

semiconductor approach i s a dominant one at the moment, and i t should not be a t a l l s u r p r i s i n g to f i n d out that polymers have c o n t r i b u t e d mainly to r e s e a r c h i n t h i s area. This i s not to sug­ gest that work l i k e that of Regen and co-workers (11a), who have photopolymerized v e s i c l e w a l l s and thereby extended the t y p i c a l v e s i c l e l i f e t i m e from 48 hours to more than two weeks, i s i n c i ­ d e n t a l . Indeed no one can f o r e t e l l at t h i s moment what the f i n a l forms of s u c c e s s f u l PEC c e l l s w i l l be, and c o l l o i d a l s u r f a c t a n t systems may w e l l be among them. However, polymers have been most e x t e n s i v e l y a p p l i e d to b u l k photoelectrodes and i t i s t h i s sub­ j e c t that p r i n c i p a l l y occupies t h i s review. O p e r a t i o n a l Background Figure 1 d e p i c t s the p h y s i c a l arrangement of a t y p i c a l PEC c e l l w i t h a polymer coated photoanode (e~ pass from the e l e c t r o ­ l y t e through the polyme tocathode. The polyme or p h y s i c a l l y adsorped to the s u b s t r a t e , and i t f r e q u e n t l y con­ t a i n s a redox couple to be conductive. The c o n d u c t i v i t y of the polymer l a y e r may be due to a m e t a l - l i k e wide energy band (eg. p o l y p y r r o l e ( l i b ) ) , or may occur through a narrow energy window r e s u l t i n g from s p e c i f i c e l e c t r o a c t i v e s i t e s w i t h i n the f i l m (eg. f e r r o c e n e ) . The s u b s t r a t e may be a noble metal or g r a p h i t e , i n which case the s e n s i t i z e r i s embedded i n the f i l m , or a semicon­ ductor which may then assume the r o l e of s e n s i t i z e r . M e t a l e l e c ­ trodes are o f t e n used i n c o n t r o l experiments to d i s t i n g u i s h f i l m behavior from that of semiconductors. An i l l u s t r a t i o n of a PEC c e l l of t h i s type, which operates i n r e v e r s e , may be found i n the work of R u b i n s t e i n and Bard (12). The w e l l known duPont polymer N a f i o n was dip coated on a g r a p h i t e e l e c t r o d e f o r t h i s experiment, and then immersed i n a s o l u t i o n of Ru(bpy)3Cl2 (bpy = 2 , 2 ' - b i p y r i d i n e ) . C y c l i c voltammograms o f t h i s t r e a t e d e l e c t r o d e showed broad o x i d a t i v e and r e d u c t i v e waves c l o s e to 1.2V vs NHE of the p o t e n t i a l sweep, c h a r a c t e r i s t i c o f the presence of the Ru(bpy)3^+ complex sequestered i n the polymer f i l m . However, when o x a l a t e i o n was added to the s u p p o r t i n g e l e c t r o l y t e , which i o n i s only o x i d i z e d at h i g h e r p o t e n t i a l s by p l a i n N a f i o n , three e f f e c t s were noted; namely, an enhanced o x i ­ d a t i v e current ( m o r e ^ R u ( b p y ) R u ( b p y ) 3 ) , a damped r e t u r n Z

minescence due to Ru(bpy)3 "K*J. Thus i t appears that the power­ f u l Ru(bpy)3 oxidant c a t a l y t i c a l l y o x i d i z e s o x a l a t e w i t h s u f ­ f i c i e n t excess energy to produce an 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 . E l e c t r o c h e m i c a l Polymers. PEC experiments w i t h polymer i n ­ t e r f a c e s a r i s e from e l e c t r o c h e m i c a l work by Anson (13a, b, c) Kuwana (14), M i l l e r (15) and others. This f i e l d has been review­ ed by Murray (16) and S n e l l and Keenan (17). A study by Anson and Oyama (13b) i l l u s t r a t e s s e v e r a l general p r i n c i p l e s of f i l m s .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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CATALYST

t POLYMER

Î SUBSTRATE

Figure 1. A photoelectrochemical c e l l w i t h a p o l y m e r / e l e c t r o l y t e i n t e r f a c e c o n t a i n i n g a l i g h t absorbing s e n s i t i z e r (S) embedded i n the polymer. L i g h t absorption may enable a redox r e a c t i o n o f (R) d i s s o l v e d i n the e l e c t r o l y t e . When a semiconductor i s the s u b s t r a t e , i t i s a l s o o f t e n the s e n s i t i z e r . (WE and CE denote working and counter e l e c t r o d e s ) .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

29.

477

Photoelectrochemical Catalysis

METTEE

Their work i n c l u d e d c o a t i n g p y r o l y t i c graphite e l e c t r o d e s w i t h p o l y ( 4 - v i n y l p y r i d i n e ) and immersing them i n aqueous s o l u t i o n s o f R u ( e d t a ) 0 H 2 . A slow and steady growth of the redox waves of the R u / couple was observed as the complex entered what was otherwise an e l e c t r o c h e m i c a l l y quiet f i l m . C o n t r o l experiments showed that when i n the f i l m , the R u i o n c h e m i c a l l y coordinated w i t h p y r i d i n e by l i g a n d s u b s t i t u t i o n . This work showed that t h i c k durable polymer f i l m s could conveniently be made, and that t h e i r e l e c t r o c h e m i c a l a c t i v i t y could r e s u l t from the redox coup l e s sequestered from the ambient s o l u t i o n . The p r i n c i p l e of "mediated" e l e c t r o n t r a n s f e r , whereby e l e c trons are passed from the reduced form of a r e l a t i v e l y negative redox couple to the o x i d i z e d form of a r e l a t i v e l y p o s i t i v e couple, has been demonstrated to occur between two polymer l a y e r s o f s l i g h t l y d i f f e r e n t R u ( b p y ) 3 complex polymers by Murray and coworkers (18). This k i n d of stepwise u n i d i r e c t i o n a l e l e c t r o n t r a n s f e r may be very s i g n i f i c a n c e l l s which seek to separat Ru*I(bpy)3 complexes are f r e q u e n t l y used as c y c l i c PEC c a t a l y s t s i n water s p l i t t i n g experiments. Some d e t a i l s of t h i s experiment are thus i n f o r m a t i v e . To demonstrate u n i d i r e c t i o n a l charge flow v i a e l e c t r o n media t i o n , Murray's group e l e c t r o c h e m i c a l l y polymerized complexes [ R u ( b p y ) ( v p y ) 2 ] , A, and [ R u ( b p y ) ( v p y ) C l ] + , B, on Pt e l e c trodes i n CH3CN. (vpy i s 4 - v i n y l p y r i d i n e . ) The order of deposit i o n of t h e f i l m s i s c r u c i a l of course s i n c e [RU '2+(A)] = +1.23 and E [Ru ( B ) ] = +.76 V vs SSCE and the inner f i l m mediator (poly(A)) would not be expected to move e l e c t r o n s u p h i l l . The r e s u l t s are summarized i n Figure 2, where i t i s c l e a r that both redox waves a s s o c i a t e d w i t h the outer f i l m couple (poly (B)) are m i s s i n g i n the dual l a y e r system ( F i g 2 ( b ) ) . The (A + B) copolymerized s i n g l e f i l m e l e c t r o d e ( F i g 2 ( c ) ) shows the e l e c t r o n i c presence of both couples at the Pt/polymer i n t e r f a c e . Thus the inner polymer l a y e r has seemingly screened communication between the h o l e ( h ) a t the Pt s u r f a c e and the redox couple i n the outer f i l m . 3+/2+ E l e c t r o n i c mediation, the passage of e~ from one Ru couple to another, i s shown by c o n s i d e r i n g the d e t a i l s of F i g u r e 2(b). One important feature i s the o x i d a t i v e s p i k e , or prewave, i n the f i r s t o x i d a t i v e scan, and i t s absence from subsequent scans i f the r e t u r n p o t e n t i a l i s not swept negative past about - I V . A second i s the pronounced cathodic s p i k e on the l a r g e r e d u c t i o n wave i f t h i s negative scan i s c a r r i e d out subsequent to a p o s i t i v e one. In the f i r s t anodic scan, the Ru ions i n both polymer l a y e r s begin as R u . When the e l e c t r o d e p o t e n t i a l f i r s t passes +1.15 V, the outer l a y e r Ru +(B) -> R u ( B ) i s s i g n a l l e d as a sharp s p i k e , and the inner l a y e r R u ( A ) -* R u ( A ) a c t s as a mediator. When ~ +1.23 V i s passed, the remaining Ru (A) R u ( A ) occurs. However, on the r e t u r n scan, only 111

1 1 1

1 1

1 1 1

II

2+

2

2

3 +

F

o

3 + / 2 +

g u r f

+

2 +

2

3+

2+

2+

3+

3+

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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POLYMERS IN SOLAR ENERGY UTILIZATION

Figure 2. a) A double polymer l a y e r on a P t substrate w i t h energy l e v e l s used to i n t e r p r e t voltammograms (b) and ( c ) . Scan (c) i s that o f a s i n g l e l a y e r o f copolymerized complex A and B. Adapted from Murray et a l . ( 1 8 ) .

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

29.

479

Photoelectrochemical Catalysis

METTEE

3+

2

the inner l a y e r r e d u c t i o n R u ( A ) -> Ru +(A) occurs s i n c e the R u (B) i s e n e r g e t i c a l l y and s p a t i a l l y i n a c c e s s i b l e (cf F i g . 21(a)). Unless e l e c t r o n s whose p o t e n t i a l i s more negative than +.76 V can be s u p p l i e d to the outer f i l m , the R u ( B ) s i t e s w i l l remain trapped there. Subsequent anodic scans do not r e v e a l the presence of Ru +(B) i n the outer l a y e r , as few i f any are present. The outer l a y e r h o l e s at +.76 V may be discharged, and the o x i d a t i v e prewave returned, however, i f e i t h e r the monomer o f Ru +(B) i s added to the e l e c t r o l y t e (E '[Ru +/ +(B)] = +.76 V) o r i f the p o t e n t i a l i s swept negative enough to reduce bpy° -»· bpy i n the inner l a y e r l i g a n d s . (See redox values i n Figure 2 ( a ) ) . In t h i s l a t t e r case a pronounced cathodic spike s i g n a l s the r e duction of R u ( B ) -> R u ( B ) i n the outer l a y e r , at a p o t e n t i a l n e a r l y 2 V more negative than necessary to e f f e c t the r e d u c t i o n ! One obvious c o n c l u s i o n to be drawn from these experiments i s that these polymer f i l m s are e s s e n t i a l l y i n s u l a t i n g except i n the r a t h e r narrow energy rang couple. Another i s tha a laminate polymer system, thereby mimicking the r o l e played by the p o t e n t i a l gradient w i t h i n the d e p l e t i o n l a y e r of conventional p-n semiconductor j u n c t i o n s . F u r t h e r d i s c u s s i o n of the o x i d a t i v e and r e d u c t i v e prewaves may be found i n the work of Meyer (19). Examples of the propagation of a redox r e a c t i o n through polymers f i l m s may be found i n the work of M i l l e r (15a). J

3+

2

2

3

2

m

3+

2+

P h o t o c a t a l y t i c Polymers. The e l e c t r o c h e m i c a l experiments c i t e d above were chosen from a vast body of recent polymer coated e l e c t r o d e work. L i k e w i s e the f i e l d of polymer supported photoredox c a t a l y s t s i s a l s o broad and has a more extended h i s t o r y . P o s s i b l y a common l i n k a g e between e l e c t r o c h e m i c a l and photochemic a l c a t a l y s e s , a s s i s t e d by polymers, can be t r a c e d to o x i d a t i o n of a s c o r b i c a c i d (AH2). In 1966 Davidov (20a) found that l i g h t exposed, p o l y a c r y l o n i t r i l e (PAN) c o n t a i n i n g s o l u t i o n s of AH2 consumed oxygen i n a measurably d i f f e r e n t manner than s i m i l a r s o l u t i o n s without AH2. (Simple photoabsorption of 02 a l s o o c c u r s ) . In 1977 Kuwana and coworkers (14) found that RF discharged vapors of b e n z i d i n e (BZ) l e d to the formation of a f i l m on graphite e l e c trodes (PBZ) which e l e c t r o c h e m i c a l l y c a t a l y s e d the same o x i d a t i o n . (Naked graphite e l e c t r o d e s functioned very p o o r l y ) . hv

e'

In Polymers in Solar Energy Utilization; Gebelein, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

POLYMERS IN SOLAR ENERGY UTILIZATION

480

I t may be that t h i s coincidence i s nothing more than f o r t u i t o u s , but i t i s worth n o t i n g that whether a chemical o x i d i z i n g o r r e ducing p o t e n t i a l i s produced at the molecular l e v e l by the a c t i o n of l i g h t o r an e l e c t r i c a l f i e l d , the r e s u l t i s e s s e n t i a l l y the same. An important d i f f e r e n c e remains, however, between e l e c t r o and p h o t o c a t a l y t i c processes. I n the e l e c t r o c a t a l y t i c case, the c r e a t i o n o f the charge gradient i s the i n i t i a l a c t , the one l e a d ing to charge s e p a r a t i o n and thereby chemical p o t e n t i a l . On the other hand, p h o t o l y t i c a l l y e x c i t e d molecules may undergo numerous d i s s i p a t i n g processes such as fluorescence and i n t e r n a l convers i o n , not to mention i r r e v e r s i b l e photochemistry. Thus i t i s ess e n t i a l t o reduce these d i s s i p a t i v e processes t o a minimum, and to the extent that polymers can do t h i s and promote r a p i d charge s e p a r a t i o n , polymers w i l l continue t o f i n d a u s e f u l f u n c t i o n . I t i s u s e f u l to note here that nature has a l r e a d y accomp l i s h e d the goal o f l i g h t i o n centers of Rps..sphéroïdes, These l a r g e macromolecular assemblies c o n t a i n organized u n i t s o f cytochrome C ( c y t C), b a c t e r i o c h l o r o p h y l l dimers ( ( B C h l ^ ) , bacteriopheophytin (BPh), a metal-ion f r e e c h l o r o p h y l l , an i r o n complexed quinone (FeQ) and a second quinone molecule (Q2). The l i g h t absorbing and charge s e p a r a t i n g sequence, c r e a t i n g i n the end both o x i d i z i n g and reducing power on opposite s i d e s of a membrane, i s c u r r e n t l y v i z u a l i z e d as f o l l o w s (20b), where the p r i mary donor and acceptor appear i n [ ] , and the time s c a l e i s given on the r i g h t . Species

Time (picoseconds)

cyt

C,[(BChl) ,BPh],QFe,Q

cyt

Ψ hv C,[(BChl)*,BPh],QFe,Q Ψ

2

2

0

2

0

+ cyt

C,[(BChl) ,BPh*],QFe,Q Ψ 2