Weathering of Polymers and Plastic Materials 9783031332845

Table of contents :
Cover
Half Title
Weathering of Polymers and Plastic Materials
Copyright
Preface
Introduction
Contents
List of Figures
List of Tables
About the Author
List of Abbreviations and Acronyms
1. Degradation and Stabilization of Polymers
1.1 Degradation
1.1.1 Photodegradation
1.1.2 Degradation Mechanism
1.1.3 General Mechanism of Oxidative Degradation of Polymers
1.1.3.1 Initiation
1.1.3.2 Propagation
1.1.3.3 Ramification
1.1.3.4 Termination
1.1.4 Mechanism of Photochemical Degradation of Poly(Vinyl Chloride)
1.1.4.1 Initiation Mechanism with Formation of Free Radicals
1.1.4.2 Mechanism of Propagation by Peroxide and Hydroperoxide Groups
1.1.4.3 Mechanism of Initiation and Propagation by Free Radicals
1.1.4.4 Termination Mechanism by Formation of Reticulations
1.1.5 Degradation of Poly(Vinyl Chloride) During Processing
1.2 Stabilization
1.2.1 PVC Thermal Stabilizers
1.2.1.1 Tin-Based Heat Stabilizers
Tin Carboxylates
Stabilizers Based on Zinc Complexes
Co-stabilizers
Calcium Carbonate
1.2.2 Ultraviolet Protectors: Titanium Dioxide
1.2.3 Ultraviolet Absorbers
1.2.4 Processing Aids and Lubricants
1.2.4.1 External Lubricants
1.2.4.2 Internal Lubricants
1.2.4.3 Other PVC Processing Aids
1.2.5 Important Developments in Polymer Additives
1.2.5.1 Flame Retardants
1.2.5.2 Antioxidants
1.2.5.3 Heat Stabilizers
1.2.5.4 Light Stabilizers
1.2.5.5 Other Specific Stabilizers
1.2.6 Synergism and Antagonism Between Stabilizers
1.2.6.1 Synergism
1.2.6.2 Antagonism
1.3 Developments in Polymeric Materials and Composites Designed for Outdoor Applications
2. Natural Weathering
2.1 Fundamental Aspects That Influence Tests of Natural Weathering
2.2 Difficulties Inherent in Natural Exposure Studies
2.3 Variability Inherent to Natural Exposure Tests
2.4 Climate Characterization of an Exposure Site
2.4.1 Solar Radiation
2.4.1.1 Infrared Radiation
2.4.1.2 Visible Radiation
2.4.1.3 Ultraviolet Radiation
Variability of UV Radiation
2.4.2 Temperature
2.4.3 Water from Rain, Moisture, and Wetness
2.4.4 Other Environmental Factors
2.5 Synergism Between Atmospheric Agents
3. Outdoor Accelerated Weathering
3.1 Types of Natural Solar Concentrators
3.1.1 Q-TRAC
3.1.2 EMMA/EMMAQUA
3.1.2.1 Ultra-Accelerated Exposure Testing Devices
4. Artificial Accelerated Weathering
4.1 Generalities
4.2 Most Important Aspects and Experimental Parameters in Accelerated Artificial Weathering Tests
4.2.1 Programming and Control of Radiation in Accelerated Artificial Weathering Tests
4.2.2 Programming and Control of Temperature in Accelerated Artificial Weathering Tests
4.2.3 Programming and Control of Relative Air Humidity in Accelerated Artificial Weathering Tests
4.2.4 Presence of Solvents and Chemical Reagents
4.2.5 Material Characteristics
4.3 Types of Artificial Accelerated Weathering Devices
4.4 Types of Radiation Sources in Accelerated Artificial Weathering Devices
4.4.1 Carbon Arc Sources
4.4.2 Xenon Arc Sources
4.4.3 Medium/High Pressure Mercury Vapor Sources
4.5 Acceleration Rate
4.6 Inherent Constraints in Accelerated Artificial Weathering Studies
4.7 Variability of Artificial Accelerated Weathering Methods
4.7.1 Experimental Methods Used for Evaluating Variability
4.7.2 Reliability of Artificial Accelerated Weathering Methods
4.7.3 Simulation Ability
4.8 Correlation between Natural Weathering and Artificial Accelerated Exposure
4.8.1 Case Studies
4.8.2 Phosphor-Coated Low Pressure Mercury Vapor Sources
5. Instrumental Methods of Analysis and Characterization
5.1 Spectroscopic Techniques
5.1.1 FTIR
5.1.2 UV-Vis
5.1.3 X-Ray Methods
5.1.4 Other Spectroscopy Techniques
5.2 Microscopy Methods
5.2.1 SEM
5.3 Thermal Analysis Methods
5.3.1 DSC
5.3.2 TGA
5.3.3 DMA
5.4 Methods for Measurement of Appearance
5.5 Methods for Measuring Mechanical Properties
5.6 Chromatographic Methods
5.7 VOC Analysis
5.8 Hyphenated Techniques
5.9 Derivatization Techniques
6. Lifetime Prediction
6.1 Generalities
6.2 Reference Studies
6.3 Lifetime Prediction Methodology
6.4 Lifetime Prediction Models
6.4.1 Stochastic/Probabilistic Durability Models
6.4.2 Deterministic Methods for Service Life Prediction Based on Accelerated Testing Data
6.4.2.1 Generalities
Models for Property Decay Based on Kinetics
Models Based on Arrhenius Law
Models Based on Principle of Reciprocity
Mathematical Models
7. Statistical Data Treatment
7.1 Design of Experiments
7.2 Principal Components Analysis
7.2.1 PCA Application Example
7.3 Distribution Analysis
7.4 Regression Analysis
ANNEX: Standards (Tables AI.1, AI.2, AI.3, AI.4, AI.5, AI.6 and AI.7)
References
Recommended Bibliography
Index

Citation preview

Luís Eduardo Pimentel Real

Weathering of Polymers and Plastic Materials

Weathering of Polymers and Plastic Materials

Luís Eduardo Pimentel Real

Weathering of Polymers and Plastic Materials

Luís Eduardo Pimentel Real Laboratório Nacional de Engenharia Civi Lisboa, Portugal

ISBN 978-3-031-33284-5 ISBN 978-3-031-33285-2 (eBook) https://doi.org/10.1007/978-3-031-33285-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

In this book, a general review about weathering of polymers is provided. The degradation and stabilization of polymers are succinctly described, including references to synergism and antagonism between stabilizers, as well as the reactional mechanism resulting from accelerated weathering. It also addresses natural weathering, referring influence factors, difficulties, constraints, and variability issues inherent to natural exposure, climatic characterization, and synergism between atmospheric agents. Accelerated weathering is described, whether climatic or artificially conducted in the laboratory. Artificial accelerated weathering methods are widely mentioned, like programming and control of main factors, type of apparatus and radiation sources, constraints, as well as variability, reliability, and simulation ability of such methods. Correlation between natural exposure and accelerated artificial weathering is referred, presenting some case studies referred in literature. The main instrumental methods of analysis, commonly used for quantify degradation, are referred. Life prediction methodology and models are discussed. Finally, main method for statistical weathering data treatment are presented. Keywords  Polymers, Plastics, PV, PE, PP, Degradation, Stabilization, Durability, Analysis methods, Weathering, Lifetime prediction, Statistics Lisboa, Portugal

Luís Eduardo Pimentel Real

v

Introduction

Weathering is the adverse response of a material or product to climate. Thermoplastic polymers and composites are affected by weathering and, therefore, their ability to resist to the deterioration of their mechanical and aesthetic properties over long periods of exposure is a primary design factor and a key feature. A wide range of plastic-based materials are routinely used in outdoor applications, therefore requiring adequate weatherability and service lifetime, namely plastic building materials, furniture and surfaces, polymers used in transportation and in agricultural applications, coatings and paints for protection of outdoor surfaces, artwork, dyes, highway pavement markings and road signs; textile products, biopolymers, packaging, restoration and miscellaneous products used outdoors. Therefore, the study of the photo-degradation caused by weathering is a matter of great interest for plastic producers, scientific community, and final consumers. However, the subject is broad and complex, involving a range of aspects that are key settings for durability analysis and lifetime prediction of polymers, namely degradation phenomena (of which the mechanism and chemistry are influenced by the polymer nature and respective formulation), climatic agents and weathering variables affecting performance of polymers, as well as the correlation between the effects of natural and artificial weathering. To evaluate the weathering effects, it is also needed to know the most suitable instrumental methods of analysis for tracking the evolution of induced degradation in the polymeric materials. All these subjects are focused in this book, which purpose is not to explain everything in detail but rather to present the state of the art on “weathering of polymers and plastic materials.” In Chap. 1, the mechanism of degradation and stabilization of polymers is addressed, covering a reference to main stabilizers and the potential phenomena of synergism and antagonism between them. A reference to innovative polymer additives and some developments in polymeric materials designed for outdoor applications are also included, emphasizing a few selected cases. Then, in Chap. 2, natural weathering is widely described, covering topics related with atmospheric agents and environmental factors, variability, and difficulties

vii

viii

Introduction

associated with climatic characterization of the exposure sites. The phenomena of synergism between atmospheric agents is also addressed. Chapter 3 describes the outdoors accelerated weathering testing and use of solar concentrators. Next, in Chap. 4, laboratory artificial-accelerated weathering is widely detailed, covering topics related to programming and control of climatic factors, types of devices and radiation sources, inherent constraints in artificial weathering, variability, reliability, and correlation with natural exposure tests. Chapter 5 summarizes the most suitable methods of instrumental analysis to access and quantify (when possible) the degradation caused by weathering. In Chap. 6, lifetime prediction is the main topic, covering the reference to the most used methods for this purpose. Finally, Chap. 7 is dedicated to statistical data treatment of weathering data.

Contents

1

2

Degradation and Stabilization of Polymers . . . . . . . . . . . . . . . . . . . . . 1.1 Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Photodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Degradation Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.3 General Mechanism of Oxidative Degradation of Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.4 Mechanism of Photochemical Degradation of Poly(Vinyl Chloride) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.5 Degradation of Poly(Vinyl Chloride) During Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 PVC Thermal Stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Ultraviolet Protectors: Titanium Dioxide. . . . . . . . . . . . . . . 1.2.3 Ultraviolet Absorbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Processing Aids and Lubricants . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Important Developments in Polymer Additives. . . . . . . . . . 1.2.6 Synergism and Antagonism Between Stabilizers . . . . . . . . 1.3 Developments in Polymeric Materials and Composites Designed for Outdoor Applications . . . . . . . . . . . . . . . . . . . . . . . . . Natural Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Fundamental Aspects That Influence Tests of Natural Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Difficulties Inherent in Natural Exposure Studies . . . . . . . . . . . . . . 2.3 Variability Inherent to Natural Exposure Tests . . . . . . . . . . . . . . . . 2.4 Climate Characterization of an Exposure Site . . . . . . . . . . . . . . . . . 2.4.1 Solar Radiation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3 Water from Rain, Moisture, and Wetness . . . . . . . . . . . . . . 2.4.4 Other Environmental Factors . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Synergism Between Atmospheric Agents . . . . . . . . . . . . . . . . . . . .

1 1 2 3 4 7 12 13 14 19 20 21 24 28 30 35 37 38 38 39 39 44 44 45 46 ix

x

Contents

3

Outdoor Accelerated Weathering ����������������������������������������������������������   47 3.1 Types of Natural Solar Concentrators����������������������������������������������   47 3.1.1 Q-TRAC��������������������������������������������������������������������������������   48 3.1.2 EMMA/EMMAQUA������������������������������������������������������������   49

4

Artificial Accelerated Weathering����������������������������������������������������������   53 4.1 Generalities ��������������������������������������������������������������������������������������   53 4.2 Most Important Aspects and Experimental Parameters in Accelerated Artificial Weathering Tests����������������������������������������   54 4.2.1 Programming and Control of Radiation in Accelerated Artificial Weathering Tests����������������������������   55 4.2.2 Programming and Control of Temperature in Accelerated Artificial Weathering Tests����������������������������������������������������   57 4.2.3 Programming and Control of Relative Air Humidity in Accelerated Artificial Weathering Tests����������������������������   57 4.2.4 Presence of Solvents and Chemical Reagents����������������������   58 4.2.5 Material Characteristics��������������������������������������������������������   58 4.3 Types of Artificial Accelerated Weathering Devices������������������������   59 4.4 Types of Radiation Sources in Accelerated Artificial Weathering Devices��������������������������������������������������������������������������   63 4.4.1 Carbon Arc Sources��������������������������������������������������������������   64 4.4.2 Xenon Arc Sources ��������������������������������������������������������������   64 4.4.3 Medium/High Pressure Mercury Vapor Sources������������������   66 4.5 Acceleration Rate������������������������������������������������������������������������������   67 4.6 Inherent Constraints in Accelerated Artificial Weathering Studies ��������������������������������������������������������������������������   68 4.7 Variability of Artificial Accelerated Weathering Methods����������������   69 4.7.1 Experimental Methods Used for Evaluating Variability ������   70 4.7.2 Reliability of Artificial Accelerated Weathering Methods ������������������������������������������������������������   74 4.7.3 Simulation Ability����������������������������������������������������������������   75 4.8 Correlation between Natural Weathering and Artificial Accelerated Exposure ����������������������������������������������������������������������   76 4.8.1 Case Studies��������������������������������������������������������������������������   79 4.8.2 Phosphor-Coated Low Pressure Mercury Vapor Sources ����   81

5

 Instrumental Methods of Analysis and Characterization��������������������   85 5.1 Spectroscopic Techniques ����������������������������������������������������������������   85 5.1.1 FTIR��������������������������������������������������������������������������������������   85 5.1.2 UV-Vis����������������������������������������������������������������������������������   86 5.1.3 X-Ray Methods��������������������������������������������������������������������   87 5.1.4 Other Spectroscopy Techniques��������������������������������������������   87 5.2 Microscopy Methods������������������������������������������������������������������������   88 5.2.1 SEM��������������������������������������������������������������������������������������   88

Contents

xi

5.3 Thermal Analysis Methods ��������������������������������������������������������������   88 5.3.1 DSC��������������������������������������������������������������������������������������   88 5.3.2 TGA��������������������������������������������������������������������������������������   88 5.3.3 DMA ������������������������������������������������������������������������������������   89 5.4 Methods for Measurement of Appearance����������������������������������������   89 5.5 Methods for Measuring Mechanical Properties��������������������������������   89 5.6 Chromatographic Methods����������������������������������������������������������������   89 5.7 VOC Analysis ����������������������������������������������������������������������������������   90 5.8 Hyphenated Techniques��������������������������������������������������������������������   90 5.9 Derivatization Techniques����������������������������������������������������������������   90 6

Lifetime Prediction����������������������������������������������������������������������������������   93 6.1 Generalities ��������������������������������������������������������������������������������������   93 6.2 Reference Studies������������������������������������������������������������������������������   94 6.3 Lifetime Prediction Methodology����������������������������������������������������   95 6.4 Lifetime Prediction Models��������������������������������������������������������������   96 6.4.1 Stochastic/Probabilistic Durability Models��������������������������   97 6.4.2 Deterministic Methods for Service Life Prediction Based on Accelerated Testing Data��������������������������������������   97

7

Statistical Data Treatment����������������������������������������������������������������������  105 7.1 Design of Experiments����������������������������������������������������������������������  105 7.2 Principal Components Analysis��������������������������������������������������������  106 7.2.1 PCA Application Example����������������������������������������������������  107 7.3 Distribution Analysis������������������������������������������������������������������������  108 7.4 Regression Analysis��������������������������������������������������������������������������  109

ANNEX: Standards (Tables AI.1, AI.2, AI.3, AI.4, AI.5, AI.6 and AI.7)������������������������������������������������������������������������������������������  111 References ��������������������������������������������������������������������������������������������������������  119 Index������������������������������������������������������������������������������������������������������������������  135

List of Figures

Fig. 2.1 Support for white specimens in natural exposure, facing south with a slope of 45°, installed on the terrace of a building located in LNEC, Lisbon���������������������������������������������������������������������� 36 Fig. 2.2 Metrological station for measurement of global and UV radiation at 45°, facing south, installed on the terrace of a building in LNEC, Lisbon���������������������������������������������������������������������������������� 36 Fig. 2.3 Illustration of a system used to measure wind speed and direction (LNEC, Lisbon)�������������������������������������������������������������� 37 Fig. 3.1 Q-TRAC devices at Q-Lab Arizona exposure site. (Image courtesy of Q-Lab Corporation, https://q-lab.com/)�������������������������������������������� 48 Fig. 3.2 EMMA devices in an ATLAS exposure laboratory site. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-­mts.com/) ���������������������������������������������������������������� 49 Fig. 3.3 Ultra-Accelerated Weathering System (UAWS) installed at ATLAS’ DSET exposure laboratory in Phoenix, Arizona. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/) ���������������������������������������������������������������� 51 Fig. 3.4 Ultra-Accelerated EMMA (UA-EMMA), at an exposure site. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/) ���������������������������������������������������������������� 52 Fig. 3.5 Ultra-Accelerated EMMA (UA-EMMA) – Hybrid, at an exposure site. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/) ���������������������������������������������������������������� 52

xiii

xiv

List of Figures

Fig. 4.1 Atlas weatherometer Ci65: (a) outside general view; (b) inside view�������������������������������������������������������������������������������������� 59 Fig. 4.2 Modern large sized Atlas indoor weatherometers with rotating rack: (a) Ci5000 Weather-­Ometer®, 12,000 W water cooled xenon arc lamp system with a total exposure area of 11,000 cm2; (b) Atlas Ci4400 Weather-Ometer®, 6500 W water cooled xenon arc lamp. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts. com/) ���������������������������������������������������������������������������������������������������� 60 Fig. 4.3 Modern small and medium sized indoor weatherometers with rotating rack: (a) Atlas Xenotest Series®. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/); (b) QLAB indoor weatherometer. (Image courtesy of Q-Lab Corporation, https://q-lab.com/)������������������������������������������������������������ 60 Fig. 4.4 Weaterometers with flat array for static horizontal exposures: ATLAS SUNTEST series®, with different dimensions, depending on the size of the test pieces (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/) ������������������ 61 Fig. 4.5 Weaterometers with flat array for static horizontal exposures: (a) QLAB Q-SUN XE-3 xenon test chamber; (b) QLAB Q-SUN XE-1 xenon test chamber (Images courtesy of Q-Lab Corporation, https://q-lab.com/) �������������������������������������������������������������������������������� 62 Fig. 4.6 Solar box: (a) exterior view; (b) open door view, illustrating exposed samples. (Image courtesy of CO.FO.ME.GRA. Srl, https://cofomegra.it)������������������������������������������������������������������������������ 62 Fig. 4.7 Modern fluorescent UV devices used for artificial accelerated UV weathering: (a) Atlas tester. (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/); (b) Q-Panel QUV. (Image courtesy of Q-Lab Corporation, https://q-lab.com/); (c) UV BOX (Image courtesy of CO.FO.ME.GRA. Srl, https://cofomegra.it)������������������������������������������������������������������������������ 62 Fig. 4.8 SEPAP chamber: (a) old model 12/24; (b) new model MHE [174]. (Images courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing, https://www.atlas-mts.com/)v �������������������������������������������������������������� 63 Fig. 4.9 Spectral distribution characteristic of mercury vapor lamps: (a) characteristic discrete peaks of the specially designed mercury vapor lamp of SEPAP chambers; (b) spectral distribution of a high pressure mercury vapor lamp (continuous spectrum with superimposition of maxima)���������������������� 67

List of Figures

xv

Fig. 4.10 Appearance of two different stabilized PVC formulations for outdoor applications in a dark background, in different photooxidation conditions: (a) reference samples, exposed at Lisbon; (b) 6000 h irradiated (xenon) using rain cycles (white specimens, showing only stains in samples stabilized with di-butyl tin maleate but not in samples stabilized with calcium-zinc); (c) 8000 h irradiated (xenon) without rain cycles showing total yellowing [101]������������������ 71 Fig. 4.11 FTIR difference spectra of a PVC film formulated for outdoor applications, during 7 years of natural exposure in Lisbon������������������ 72 Fig. 4.12 FTIR difference spectra of a PVC film formulated for outdoor applications, during continuous 6500 watt borosilicate filtered xenon arc (ATLAS weatherometer Ci 65), using two different experimental conditions: (a) 3350 h of continuous irradiation (no dark period) with water jets (18 min of rain each 2 h); (b) 4755 h of continuous irradiation (no dark period), without water jets (no rain)]������������������������������������������������������������������ 72 Fig. 4.13 FTIR difference spectra of a PVC film formulated for outdoor applications, during 4750 h of continuous irradiation, using a 2500 Watt borosilicate glass filtered xenon, without water jets and without dark period 101 ���������������������������������������������������������������� 73 Fig. 4.14 FTIR difference spectra of a PVC film formulated for outdoor applications, during 820 h of continuous irradiation, using medium-pressure mercury vapor 101 �������������������������������������������������� 73 Fig. 4.15 SFTIR difference spectra of a PVC film formulated for outdoor applications, during 910 h of irradiation, using a 313 nm fluorescent lamp, with a 4-h dark period each 12 h with condensation 101 ���������� 74 Fig. 4.16 Spectral distribution of fluorescent lamps with an emission maximum at 313 nm and 340 nm versus sunlight (UV range) (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing)������������������������ 82 Fig. 4.17 Spectral distribution of fluorescent lamps with an emission maximum at 351 nm versus sunlight behind glass (UV range) (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing)������������������������ 83 Fig. 6.1 Example of a regression function plot: Relationship between the time of artificial weathering with xenon radiation and the evolution of the decimal logarithm of the optical density at 1712 cm−1 of two different formulations of PVC films for outdoor applications: (a) PVC CZT, (b) PVC CZ������������������������������ 102 Fig. 6.2 Example of a longevity function plot: Decimal logarithmic relationship between the time of artificial weathering with xenon radiation and the evolution of the tensile strain at break of two different formulations of PVC sheets designed for outdoor applications: (a) PVC CZT, (b) PVC CZ�������������������������������������������� 102

xvi

List of Figures

Fig. 7.1 Probably plot for accumulated global energy during 7 years of natural weathering in Lisbon, determined using Minitab software [101] ������������������������������������������������������������������������������������ 109 Fig. 7.2 Residual regression analysis showing a logarithmic distribution between the natural exposure time and the respective evolution of optical density at 1712 cm−1 of a PVC film [101]�������������������������� 110 Fig. 7.3 Residual analysis of multivariable linear regression, showing a normal distribution of the evolution of strain at break of a stabilized PVC sheet, during natural exposure, as a function of climatic variables [101]������������������������������������������������������������������ 110

List of Tables

Table 2.1 Irradiance values contained in the standard EN ISO 4892���������������� 40 Table 2.2 Global irradiance values as a function of solar inclination���������������� 41 Table 4.1 Spectral distribution of a carbon source �������������������������������������������� 64 Table 4.2 Irradiances of a xenon arc lamp with various filter systems and their comparison with direct solar radiation measured behind a window�������������������������������������������������������������������������������� 66 Table 4.3 Spectral distribution of the MA 400 emitter�������������������������������������� 67 Table 4.4 Acceleration factors of a specific PVC formulation, based on measurements in the carbonyl region of the FTIR spectra, using different types of artificial accelerated devices������������������������ 81 Table 4.5 Relative spectral irradiance of type 1 UV lamps�������������������������������� 83 Table 4.6 Relative spectral irradiance of type 2 UV lamps�������������������������������� 83 Table 7.1 Principal component analysis of a PVC formulation under natural exposure, considering the evolution of the strain at break as a measured property ������������������������������������������������������ 108 Table AI.1 List of EN, ISO and EN ISO standards and technical recommendations regarding test methods for artificial weathering in polymers�������������������������������������������������������������������� 111 Table AI.2 List of ASTM standards regarding practices and test methods for artificial weathering of polymers������������������������������������������������ 113 Table AI.3 List of ASTM standards regarding practices and test methods for natural exposure and climatic weathering of polymers�������������� 115 Table AI.4 List of EN, ISO and EN ISO standards and technical recommendations regarding practices and test methods for natural exposure and climatic accelerated weathering of polymers�������������������������������������������������������������������������������������� 116

xvii

xviii

List of Tables

Table AI.5 List of standards to assess degradation caused by weathering in polymeric materials���������������������������������������������������������������������� 117 Table AI.7 List of standards establishing requirements for the material properties, including weatherability ������������������������������������������������ 118 Table AI.6 List of standards List of standards for calculation and computing colors for appearance measurements������������������������������������������������ 118

About the Author

Luís Eduardo Pimentel Real  Graduated in chemical engineering at the Instituto Superior Técnico (IST), Lisbon, Portugal. He holds a PhD in chemical engineering (IST, Portugal) and a PhD in chemistry (UBP, France). He is a researcher in National Laboratory of Civil Engineering (LNEC), where he has worked since 1990. His activity has been very dispersed and covers several fields, namely applied chemistry, degradation and stabilization of polymers, durability, environmental issues (radon and recycling), biocomposites, assessment of construction products, and technical advice. He is author and co-author of more than 200 publications, including scientific papers, communications, technical guides and reports. He has participated in several European research projects, and in specialized courses and technical training. He was responsible, for several years, for an accredited laboratory of plastics and composites. He was awarded the degree of specialist in metrology by the Portuguese Order of Engineers, for having carried out numerous activities in the scope of quality control of plastic materials and products, doing agreement activity and support for certification, technical auditing in plastic pipe factories located in Portugal and Europe, and participation in technical committees and working groups.

xix

List of Abbreviations and Acronyms

Å

Angstrom (unit of measurement of equivalent length 10-10 m) ABS poly (acrylonitrile butadiene styrene) APP ammonium polyphosphate ARXPS angular resolved X-ray photoelectron spectroscopy ATH aluminum trihydroxide Ba barium BHT butylated hydroxytoluene (C15H24O) Bo borosilicate glass filter BPT black panel thermometer C illuminant C C-C carbon-carbon bond (other bonds: C-H, C-Cl, Sn-C, Sn-S) Ca calcium CaCl2 calcium chloride CaCO3 calcium carbonate Ca/Zn, CZ calcium-zinc thermal stabilizer CB carbon black CBD cannabidiol antioxidant Cd cadmium CeO2 cerium oxide CIE Commission internationale de l'éclairage (International Commission on Illumination) CO2 carbon dioxide CTC charge transfer complex D65 illuminant D65 DBTM dibutyltin maleate DLO oxygen diffusion limited oxidation DMA dynamic mechanical analysis DOE design of experiments DSC differential scanning calorimetry xxi

xxii

EDS ELD EMMA

List of Abbreviations and Acronyms

energy dispersive spectroscopy equivalent light dose factor solar concentrator “Equatorial Assembly with Mirrors for Acceleration” EMMAQUA solar concentrator “Equatorial Assembly with Mirrors for Acceleration with Water” eq. equation ESCA electron spectroscopy for chemical analysis ESRI electron spin resonance imaging technique FR flame retardant FTIR Fourier-transform infrared FTIRS Fourier-transform infrared spectroscopy FWA fluorescent whitening agents GC gas chromatography GC/MS hyphenated gas chromatography – mass spectrometry GPC gel permeation chromatography GPC/FTIRS hyphenated gel permeation chromatography – Fourier-­ transform infrared spectroscopy HCl hydrochloric acid H2O water HALS hindered amine light stabilizers HDPE high-density polyethylene HFSF solar concentrator “high-flux solar furnace” Hg mercury lamps HMF hydroxymethylfurfural IR infrared radiation responsible by heating (in wavelength range from 780 to 3000 nm) IR-A infrared radiation in wavelength range from 780 to 1400 nm IR-B infrared radiation in wavelength range from 1400 to 3000 nm IRS infrared spectroscopy IST Instituto Superior Técnico (University of Lisbon) Lambda, λ wavelength LDPE low-density polyethylene LLDPE linear low-density polyethylene LNEC Laboratório Nacional de Engenharia Civil (Portuguese Nacional Laboratory of Civil Engineering) LPMM Laboratoire de Photochimie Moleculaire et Macromoleculaire (Clermont-Ferrand, France) LT-EMMA/EMMAQUA low-temperature EMMA/EMMAQUA m milli (1 × 10−3) + M metal ions (Fe2+/Fe3+, Mn2+/Mn3+, Co2+/Co3+, Cu+/ Cu2+,Ti3+/Ti4+, etc.) Mol molarity (concentration unit mol/kg, mol/l)

List of Abbreviations and Acronyms

MS MCB MDH MeCSTMIO

xxiii

mass spectrometry modified carbon black magnesium dihydroxide 9, 5-[2-(4-methoxycarbonyl-phenyl)-ethenyl]1,1,3,3-­ tetramethylisoindoline-­ 2-yloxy (a profluorescent nitroxide probe) Micro FTIR Fourier-transform infrared microscopy Micro FTIR/Raman hyphenated Fourier-transform infrared microscopy with Raman Spectroscopy MMT montmorillonite Na2Te sodium telluride NcTMs nanocomposite thermoelectric materials NH3 Ammonia nm nanometer (1×10-9 of a meter) NO nitric oxide NOx nitrogen oxides that are most relevant for air pollution O2 molecular oxygen O3 ozone P* polymer radicals PA polyamide, nylon PAS photoacoustic spectroscopy Pb lead PbTe lead telluride PC polycarbonate PCA principal component analysis PCL polycaprolactone pcr parts per hundred of resin PE polyethylene PET polyethylene terephthalate pH potential of hydrogen PHBV poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PLA polylactic acid PMMA poly(methyl methacrylate) PP polypropylene PPA polymer processing aids PS polystyrene PVC poly(vinyl chloride) PVC CZT PVC with thermal stabilizer of calcium/zinc and UV absorber Tinuvin P PVC CZ PVC with thermal stabilizer of calcium/zinc without UV absorber PVC-U unplasticized polyvinylchloride Q-TRAC solar concentrator of Q-Lab R alkyl group

xxiv

R*

List of Abbreviations and Acronyms

free radical (a radical may be referent to R*, ROO*, H*, HOO*, Cl*, etc.) Raman microscopy Raman spectroscopy combined with microscopy reac. reaction SAN styrene-acrylonitrile copolymer SEC size exclusion chromatography SEM scanning electron microscopy SF4 sulfur tetrafluoride SO2 sulfur dioxide SRG SAN-polybutadiene rubber graft SrTe strontium telluride TGA thermogravimetric analysis TiO2 titanium dioxide TMAO 1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-­ yloxyl (a profluorescent nitroxide) TMDBIO 1,1,3,3-tetramethyldibenzo[e,g]isoindolin-2-yloxyl (a profluorescent nitroxide) TMIO 1,1,3,3-tetramethylisoindol-2-yloxyl (hindered amine stabilizer analogue used as a profluorescent nitroxide) UAWS ultra-accelerated weathering system UBP Université Blaise Pascal UHMWPE ultra-high-molecular-weight polyethylene UV ultraviolet radiation (295-400 nm) UVA UV absorber UV-A UV in wavelength range from 315 to 400 nm UV-B UV in wavelength range from 280 to 315 nm UV-C UV in wavelength range from 100 to 280 nm UV-VIS ultraviolet-visible VOC volatile organic analysis Y may represent an undefined chemical structure WPC wood plastic composite WST white standard thermometer Xe Xénon arc lamp XPS X-ray photoelectron spectroscopy XRD X-ray diffraction Zn zinc ZnCl2 zinc chloride ZnO zinc oxide ZrO2 zirconium oxide

Chapter 1

Degradation and Stabilization of Polymers

1.1 Degradation In practice, any change of the polymer properties relative to the initial, desirable properties is called degradation. Once a freshly made polymeric material is exposed to further shear stress, heat, light, air, water, oxygen, radiation, or mechanical loading, chemical reactions start in the polymer, which have the net result of changing the chemical composition and the molecular weight of the polymer, leading to a change in the physical and optical properties of the polymer that can have adverse effects on the useful life of plastic products. Plastic materials, despite being properly stabilized, are subject to degradation due to weathering, especially those that are applied outdoors, as they are exposed to a wide variety of climatic variables, which affect their properties and characteristics at long term. Weathering due to exposure to the external environment consists of a complex set of processes, in which the combined action of ultraviolet radiation, heat, oxygen, and humidity is predominant and translates, in most cases, into loss of quality of the exposed surface. The most studied types of degradation are oxidation (when in presence of oxygen) and photodegradation (due to light action). Both processes are accelerated by complementary stress agents, namely, temperature. Most pure polymers are theoretically incapable of absorbing ultraviolet (UV) light directly. Trace amounts of other compounds within the polymer, such as degradation products or catalyst residues, can, however, absorb UV and initiate photodegradation. Once the process starts, it follows a chain reaction which accelerates degradation unless stabilizers are used to interrupt the degradation cycle. The polymer backbone can react via free radical reactions, which are very complex and can lead to numerous species depending on the nature of the radicals and the polymer structure. They originate irreversible chemical changes, like chain scission, unsaturation, branching, formation of oxidation products (like carbonyl groups) and chromophores, several types of © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. E. P. Real, Weathering of Polymers and Plastic Materials, https://doi.org/10.1007/978-3-031-33285-2_1

1

2

1  Degradation and Stabilization of Polymers

radical products, and cross-linking, which ultimately causes cracking, embrittlement, chalking, color changes, and deterioration of physical properties. The weather-induced degradation results from competing reaction of these different processes. Detailed kinetic considerations of both chain scission and cross-linking, photolysis reactions, and mechanisms of degradation and stabilization can be found in several publications Error! Reference source not found.-[8]. The general characteristics of weathering mechanisms and the photochemical evolution of polymeric materials were reported long time ago [9], as well as the chemical evolution originated by photoageing on traditional polymers (like polyethylene, polypropylene, polyvinyl chloride, and polycarbonate) [10]. Although the number and nature of light absorbing groups in a polymeric system are important aspects, there are also other factors involved that must be considered. Thus, photophysics, chemical chain reactions, polymer morphology, chain stiffness, crystallinity, etc., should also be considered [11]. The oxygen diffusion-limited oxidation (DLO) in polymeric matrices, which leads to preferential surface and nearsurface changes resulting in a heterogeneous distribution of property changes between the surface and bulk, during photoxidation, has been widely studied [12– 14]. Deficiency of oxygen in inner layers of a plaque of polymeric material, originates that, in some polymers, the concentration of activated chromophores and initiators and subsequently polymer radicals P*, created inside the plaque, decreases with increasing distance from the irradiated surface layer [13]. However, the oxidation profiles are affected by the particular polymer matrix (nature, transparency and opacity, crystallinity), the severity of the environmental stress, and material geometry [13, 14], originating a distribution of products strongly unsymmetrical, mainly in thick samples. Only very thin samples can have a uniform distribution of products throughout the depth of the sample, where the differences in concentration gradients between the near-surface and polymer bulk are quite small [14].

1.1.1 Photodegradation All plastics are damaged by the absorption of UV radiation by chromophores and in the activation of excited states in macromolecules. Chemical bonds in the polymer material are broken down through photodegradation initiated by solar radiation, which ultimately causes changes in appearance and deterioration of physical properties. Embrittlement (surface cracking), discoloration, and loss of transparency are the most obvious effects, but this is typically accompanied by loss of elongation at break. When a polymer is exposed to solar radiation, the energy absorbed by the polymer results in the formation of free radicals within the polymer by the dissociation of the C-H bonds in the polymer chains. The extent of this chemical reaction depends on the radiation exposure that is the quantity of ultraviolet light [15].

1.1 Degradation

3

Since oxygen is involved in the reaction process, there is an important balance between UV radiation and oxygen diffusion, and, of course, temperature since that will also determine the kinetics of reaction and the transport of reactive species. Under natural exposure conditions, there will be wetting and drying cycles and dark periods, during which the oxygen concentration in the material can occur and oxygen ingress can extend to greater depth [15]. A way of determining the photochemically effective energy is the measurement of photodegradation as a function of incident wavelength. The knowledge of the wavelength spectral sensitivity of the plastic formulations is important and has been studied by several authors [16, 17]. The peak sensitivity falls between 3000 and 3500 Å for most of the polymers [16]. Findings on light-induced damage to plastic materials, including wood-plastics composites and nanocomposites, were summarized and assessed by Andrady and co-workers [18].

1.1.2 Degradation Mechanism The description of the various stages of degradation depends directly on the knowledge that one has about the degradation mechanisms. In fact, when these mechanisms are known at a fundamental level, the relationship between the mode of breakdown of properties that makes the use of the material useless in service and the elementary defects of the material can be easily established through the kinetic laws of physical-chemistry [19]. The chemical evolution of the polymer must be studied in detail, along the thermally and photochemically initiated oxidations, and in particular recognize the successive oxidation steps that modify the polymer chains during weathering [20] and the variation of the concentrations of the intermediate products [21]. The evolution criterion adopted should be the formation of a photoproduct according to a known mechanism, as primary as possible. Its concentration can be determined by infrared or ultraviolet spectroscopy or by chemical determination (for example, hydroperoxide groups), measurement methods which are sufficiently sensitive to detect levels of oxidation which, although low, are nevertheless sufficient to significantly affect the polymer properties [22]. The agents of degradation, from artificial or environmental natural sources, has great influence in the degradation mechanisms, Thus, it is essential to know well the spectral distribution of the light used in photodegradation and evaluate if it contain radiation with wavelength λ  RSnY3 > R3SnY > > R4Sn. However, the stabilizing efficiency can be improved by using mixtures containing stabilizers of various suitable basic complexing agents, like carboxylates in the form of maleates (for example, those derived from dibutyltin maleate, DBTM). Tin Carboxylates Tin carboxylates, whose chemical structure is based on the Sn-O bond, are derivatives of carboxylic acids and can be classified into three main families [58]: dicarboxylates, R2Sn(O-CO-R)2, maleates and di-ester maleates, R2Sn(OCO-CH=CH-CO-OR)2, where R represents an alkyl group having 1–12 carbon atoms. The majority of tin carboxylates are liquid, an aspect that reduces the range of application. The maleates (II) are generally more effective than the simple carboxylates, since the maleic function also participates in the stabilization. However, certain maleates are difficult to process, and there is always the risk of generating maleic esters, which are watery products that are released during processing. Maleates are strong dienophiles, which is why they act through the Diels-Alder reaction, causing the interruption of the existing polyenic sequences in the degraded PVC chains [24]. DBTM undergoes a series of chemical transformations during PVC processing, leading to the parallel formation of maleic anhydride and alkyl maleic ester, during the cross-linking induction period, which shows the involvement of the latter product in Diels-Alder reactions, with development of conjugation in the polymer, which is evidenced by the formation of visible color [38]. Therefore, as with the carboxylates of group IIB metals, dialkyl tin maleates react with free HCl during PVC processing to form the corresponding carboxylic acid [38]. It was also found that during PVC processing, in the absence of stabilizers, the concentration of hydroperoxides increases rapidly [38]. In the presence of DBTM, the formation of hydroperoxides in the initial step is greatly reduced. The initial step of formation of peroxides is due, in part, to the reaction of oxygen with the radicals originated by mechanical-chemical degradation and in part to reaction 1.23,1 which occurs at molar ratios between ROOH and HCl of less than 1, followed by subsequent autoxidation of the allylic groups formed in the polymer.

 ROOH + HCl ➔ RO* + H2O + Cl*

1

16

1  Degradation and Stabilization of Polymers

However, if HCl is partially removed from the system, as what happens in the presence of DBTM, then ionic decomposition of the hydroperoxides prevails, and it becomes an antioxidant [38]. This mechanism involves competition between homolytic and ionic reactions of HCl in PVC [38]. The rate of formation and decomposition of hydroperoxide is thus regulated by this competition, and assuming the existence of a constant oxygen diffusion rate in the system, a stationary concentration of hydroperoxide will be obtained, which will be equal to the concentration of HCl. However, below a critical level, the DBTM will be unable to keep the HCl concentration below the hydroperoxide concentration, making it no longer possible to maintain a steady state. In the circumstance, the presence of a small amount of CBD-type antioxidant would completely eliminate the hydroperoxides. It is important to point out that although the stabilization efficiency of tin carboxylates is lower than that of tin mercaptides (because the Sn-O bond is less reactive than the Sn-S bond), these stabilizers give PVC excellent weathering stability, which is why they are fundamentally used in products with outdoor applications. These compounds were developed for applications that require transparency and resistance to atmospheric agents, or for which stabilizers containing sulfur cannot be used [57, 58]. Finally, it should be noted that some solid stabilizers derived from DBTM have the advantage of eliminating the main drawbacks that traditional tin stabilizers have, allowing an optimal processing and obtaining products with a high Vicat softening temperature, suitable for piping products and for PVC window profiles. However, this additives may also confer plasticizing properties. Stabilizers Based on Zinc Complexes Additives of Ca/Zn and Ba/Zn constitute a new generation of thermal stabilizers for PVC, occupying a significant share of the European market. These stabilizers, which act according to a mechanism analogous to that of mixed Ba-Cd stabilizers, are used in combination with co-stabilizers without metals, organic or minerals [11, 27, 49, 57]. These compounds, in addition to contributing to obtaining an acceptable color and long-term stabilization, also improve the lubricating and plasticizing characteristics of the polymer. Liquid Ca/Zn stabilizers have been replacing lead in the profiles market, albeit with some additional cost. Ca/Zn stabilizers are also increasingly used in drinking water conduction systems, having been definitively adopted in Australia some 40  years ago, while in some European countries, namely, Belgium, Italy, and Bulgaria, their application in this area has been gradually increasing [43]. Ca/Zn stabilizers are also gradually replacing lead stabilizers in cable production [43, 45]. Ca/Zn stabilizers have characteristics practically equivalent to Ba/Zn stabilizers, acting according to a mechanism analogous and are used in combination with co-­ stabilizers without metals, organic, or minerals [27, 30, 49, 57].

1.2 Stabilization

17

In Europe, Ca/Zn has been considered as a preferred alternative stabilization system, both in terms of technical and processing requirements, light stability, and aging resistance. Some of its advantageous features are its compatibility with lead stabilizers (an important aspect during recycling) and the low cost required to adjust PVC processing machines. In addition, Ca/Zn-based thermal stabilizers also have the advantage of presenting less color variation after accelerated artificial weathering than Sn-based formulations, and, in turn, these present better behavior, in this aspect, than the old formulations based on Pb or Ba/Cd/Pb. One of the reasons that serve to justify these results is the fact that liquid thermal stabilizers, or those that are melted at the PVC processing temperature (Ca/Zn and Sn), present a more uniform final distribution than old stabilizers consisting of solid particles (based on Pb). The Ba/Zn and Ca/Zn complexes, in addition to allowing good thermal stabilization, also avoid the use of solvents and phenols [49]. Calcium carboxylates (usually in the form of stearates) are essentially HCl acceptors, while zinc carboxylates are also true thermal stabilizers. In the first stabilization step, Zn stearates react with chlorine from HCl, resulting in the formation of ZnCl2 and stearic acid [27].

Zn  OCOC17 H 35 2  2HCl  ZnCl 2  2 C17 H 35 COOH

(1.31)



Thus, in these systems, the appearance of stearic acid corresponds to the initial formation of HCl and consequent onset of unsaturation reactions, due to mechanical-­ chemical degradation. On the other hand, allyl chlorides react quickly with Zn(OCOC17H35)2/ Ca(OCOC17H35)2, but only in the presence of ZnCl2, so this reaction only takes place after capture of HCl in the initial steps of the degradation [27]. Cl | 2 CH 3CH 9 = CHCHCH 2CH 3  Zn  OCOC17 H 35 2  OCOC17 H 35  2 ___ 2 CH CH = CHCHCH CH  ZnCl 3

9

2

3

(1.32)

2

Part of the zinc chloride formed during the induction period is inhibited by reaction with unreacted calcium stearate [30].

Ca  OCOC17 H 35 2  Cl 2 Zn   OCOC17 H 35 2 Zn  Cl 2 Ca



(1.33)

The synergistic effect of the Ca/Zn mixtures is then interpreted by the exchange reaction indicated above, which reduces the rate of deterioration of PVC by decreasing the catalytic action of ZnCl2 on the mechanism of degradation and reconversion of zinc carboxylate, which becomes active again and which exerts a more effective stabilizing effect than calcium carboxylate [33]. However, it is important to note that the extent to which labile chlorine atoms are replaced depends on the stoichiometric ratio between compounds and stabilizers. In

18

1  Degradation and Stabilization of Polymers

fact, it has been found that if the stabilizer concentration is lower than that required for the complete reaction to take place (like, for example, happens after consuming all the stabilizer present in the PVC formulation), the stabilization reaction becomes reversible by the action of free HCl. In this circumstance, the stabilizing effect of the incorporated carboxylic groups decreases significantly, with regeneration of the original compound (unstable) and formation of the corresponding acid (RCOOH). Co-stabilizers Co-stabilizers are compounds which do not, by themselves, have a stabilizing effect but which significantly improve certain stabilization systems. The most used organic co-stabilizers are the following [11, 59]: epoxides, phosphoric acid esters (phosphites), polyols, phenolic antioxidants, 1,3-diketones, dihydro-pyridines, β-ketocarboxylic acid esters, and α-phenylindoles. Organic phosphites are important co-stabilizers in combination with Ca/Zn stabilizers, as these compounds are both antioxidants and anti-UV, as they eliminate hydroperoxides. They can also complex zinc chloride, be HCl acceptors and establish addition reactions with polyene sequences. The addition of phosphites in the liquid state makes it possible to reduce the viscosity of rigid PVC in the molten state, facilitating and smoothing its processing. However, they have the disadvantage of being, for the most part, susceptible to hydrolysis. Polyols, such as sorbitol, mannitol, trimethylpropane, and polyvinyl alcohols (penta and dipentaerythritol types), have a positive effect on the long-term thermal stability of PVC, as they complex, and therefore deactivate, zinc or calcium chloride [11, 59]. However, these compounds should not be added in concentrations greater than 0.2%, under penalty of causing coloration [11]. Epoxide compounds, used in concentrations of 0.2% to 6% (by mass), are economical and improve the thermal and photochemical stability of PVC formulations containing Ca/Zn stabilizers [59]. The most common compounds are epoxidized soybean oil, oleic acid epoxy esters, and tall oil2 fatty acids. Flaxseed, sunflower, and castor oils are also occasionally used, as well as epoxy resins. These compounds react with HCl and, under the catalytic influence of Zn ions, replace labile chlorine atoms. They also prevent sudden blackening of zinc-containing formulations. Epoxy oils and oleates also act as plasticizers, so when they are present in plasticized PVC, the amount of primary plasticizers must be reduced. Certain antioxidants may also be added to heat stabilizers to reduce or prevent the autoxidation that occurs during processing. Among these, organic tin compounds containing sulfur, phosphites, and phenolic compounds (such as bisphenol A and BHT) [59] stand out.

 Tall oil is a by-product of the pine wood pulping process.

2

1.2 Stabilization

19

Calcium Carbonate  As calcium carbonate (CaCO3) is a product that exists in large quantities in nature, therefore being cheap. when added in the right amount, it can be used as a non-­ functional filler, to reduce the cost of the final formulation, without, however, induce adverse changes in the properties of the final  plastic  formulation. However, as CaCO3 is normally marketed with a high degree of purity and good dispersion characteristics, it can also be used, in most cases, as a functional filler to obtain some improvements, such as a slight increase in the modulus of elasticity and resistance to impact, as well as a better surface finish in terms of gloss and excellent ­coloration [46].

1.2.2 Ultraviolet Protectors: Titanium Dioxide UV protectors are pigments that strongly absorb or reflect ultraviolet radiation and can function simultaneously as pigments or inert fillers. These additives have the drawback of altering the appearance of the product to which they are applied, so only opaque materials can be applied. This category includes carbon black, zinc oxide, and titanium dioxide (TiO2) [1]. Among them, it is only important to mention titanium dioxide, as it is the most frequently used. TiO2 has a protective effect. However, its application is only possible in white products, being widely used in PVC-U compounds, which confer whiteness and opacity, which is why they are also classified as UV protectors. In fact, TiO2 is an excellent UV protector for PVC, because it absorbs all radiation between 300 and 400 nm, that is, the range of UV radiation that reaches the Earth’s surface and that has enough energy to break part of the chemical bonds of plastics [26]. The smaller the size of its particles, typically in the range of 0.2–1 μm, the greater whiteness and opacity it presents. Pure TiO2 is very heat stable (even at PVC processing temperatures) and is nontoxic, extremely insoluble in water, and unaffected by atmospheric gases such as sulfur oxides and hydrogen sulfides [26]. TiO2 is marketed in two crystalline forms, anatase and rutile. The anatase form has a slightly blue tint and is less abrasive. TiO2 pigments based on the rutile crystalline structure, with a slightly yellowish hue, strongly absorb UV radiation and are more stable to this radiation than the anatase crystalline form. In fact, the rutile form has a higher refractive index of light and, therefore, a greater opacity, a factor that contributes to its preferential use. However, TiO2 in its raw state (extracted from ores, purified and ground) is a photocatalyst of degradation. TiO2 crystals use part of the absorbed UV radiation to generate free electrons, which can migrate to the surface of the pigment and then be transferred to the polymeric resin, directly or via hydroxide or hydroperoxide radicals in the presence of water, initiating redox reactions that end up degrading the

20

1  Degradation and Stabilization of Polymers

polymers [60]. The key factors for this photocatalytic activity of TiO2 pigments are UV radiation, oxygen, and water, and the reactive intermediates are the hydroxyl functional group and Ti (III). Thus, they may be responsible for the weakening of the surface layer of the products to which they are applied, this effect being accelerated by the humidity resulting from some atmospheric agents (dew, rain, hail). As a result of this undesirable characteristic, TiO2 manufacturers have developed techniques to reduce its oxidizing capacity, such as preferential production of the rutile form (with less photocatalytic activity), modification of the crystalline base (currently known as “lattice stabilization”) and surface application of suitable reinforcing amorphous inorganic fillers (salts of Al2O3, SiO2 and ZrO2, for example) [60, 61], which apparently protect the polymer from direct contact with TiO2. Despite these modifications, TiO2 does not completely lose its catalytic activity. There are reported in the literature [62] at least three possible mechanisms of photoactivated oxidation by TiO2. The first mechanism consists in the formation of an oxygen anion radical by electron transfer of photoexited TiO2 to molecular oxygen, reaction that involves an ionic annihilation process to form oxygen in the singlet state, which then destroys any existing unsaturation in the polymer chain. The second mechanism is the formation of reactive hydroxyl radicals by electron transfer from water, catalyzed by photoexcited TiO2. Ti3+ ions are reoxidized back to Ti4+ ions to start the cycle over again.  Finally, the third mechanism proposed by the literature consists of the reaction between an exciton (p), formed from the photochemical irradiation of TiO2, with surface hydroxyl groups, resulting in the formation of a hydroxyl radical. Simultaneously, oxygen anions are produced, which are adsorbed on the surface of the pigment particles. This combination of adsorbed radicals leads, in the presence of water, to the formation of reactive perhydroxyl radicals, which destroy the unsaturation of the polymer. Therefore, the ability of TiO2 particles to absorb UV radiation and the beneficial effects that this adjuvant causes in PVC formulations depend on several factors, namely, its concentration, its crystalline form, the degree of modification and reinforcement of the pigment, its degree of dispersion, the color of the compound, and the type of formulation used.

1.2.3 Ultraviolet Absorbers UV absorbers are additives that absorb strongly in near UV but are transparent to visible light. Thus, these substances generally do not change, or change little, the appearance of the product in which they are applied and allow to reduce the amount of UV radiation absorbed by the chromophores [59]. Ideally, they have a transmittance of 100% for visible light and 0% for wavelengths below 400 nm [9]. In reality, they present absorption bands in the near ultraviolet region, around 300 nm, but do

1.2 Stabilization

21

not allow obtaining transmittances below 20% [32]. These adjuvants work according to the Lambert-Beer law, so their ability to act depends on their concentration and, therefore, are not suitable for the stabilization of low-thickness films, for which higher concentrations are needed to obtain the same protective effect [1]. In these cases, for obvious reasons (limited solubility of the additive, influence on the physical behavior of the material, high cost of the additive, etc.), other solutions must be adopted, with another type of photochemical stabilizers. These additives are incorporated into plastics to compete with chromophores in the process of absorbing ultraviolet radiation, in relation to which they preferentially absorb it. The excited state that the UV absorber molecules reach is not reactive, and they return to the ground state dissipating their energy through reversible intramolecular rearrangement processes (called tautomerism) and/or in the form of heat by vibrational processes [11]. This process involves tautomeric structures (isomeric or mesomeric) in chemical equilibrium, that is, molecules that have the same molecular structure and molecular weight but that differ in the way the electrons are arranged. The most important classes of these substances are the hydroxybenzophenones and the benzotriazoles, these two being responsible for a big share of the total world market of photochemical additives [44]. The benzotriazole class includes strong absorbers over a wide range of UV, particularly between 300 and 350 nm, of which 2-(2-hydroxy-5-t-octylphenyl)benzotriazole [59] is a good example of tautomeric reaction mechanism of benzotriazoles. A tautomer absorbs a photon of UV energy, it transforms into the other tautomer, which in turn returns to its initial form after dissipating the previously absorbed energy in the form of heat. At the basis of this isomerization is the establishment of an intramolecular hydrogen bond. Under the circumstances, any disruption of this hydrogen bond implies a decrease in the protective effect, a situation that can occur when the polymeric matrix itself is capable of establishing a competitive hydrogen bond [1]. Therefore, it is not recommended to incorporate this type of additives in polymers containing strongly electronegative groups, but only in polymers with a nonpolar character, which can guarantee the maximum effectiveness of the protection process. In addition, all UV absorbers, with the exception of cyanoacrylates, act by intramolecular transfer of acidic protons, through a mechanism known as ESIPT (excited state intramolecular proton transfer), which can lead to undesirable chemical reactions with components of the polymeric matrix or with metallic residues.

1.2.4 Processing Aids and Lubricants PVC processing is only possible at temperatures well above its glass transition temperature. Under these conditions, it is normally necessary to use processing aids, including lubricants [41].

22

1  Degradation and Stabilization of Polymers

Lubricants are substances that are added to polymeric resins, in amounts rarely above 2 pcr, in order to facilitate their processing and handling, and have a technical and economic importance equivalent to that of thermal stabilizers [41, 42, 47]. Lubricants are classified as external or internal lubricants, according to the type of purpose they are intended for, and can be found among the following chemical species: polymers, saturated hydrocarbons (paraffin), alcohols and fatty acids (and their respective esters), esters of polyols, metallic carboxylates, amides, and silicones [42]. The behavior of a lubricant is mainly a function of its compatibility with the polymer, an aspect closely related to its polarity and the size of its molecules. Thus, it behaves as an internal lubricant in a polymer with which it is compatible and as an external lubricant in a polymer with which it is incompatible. Slightly compatible products are called internal/external lubricants, having a more or less pronounced external character depending on the concentration used. A polar substance, such as palmitic or stearic acid, can behave as an external lubricant in a polar polymer, such as PVC, due to the incompatibility of the long aliphatic chains of its molecules. The behavior of a lubricant is mainly a function of its compatibility with the polymer, an aspect closely related to its polarity and the size of its molecules. Thus, it behaves as an internal lubricant in a polymer with which it is compatible and as an external lubricant in a polymer with which it is incompatible. Slightly compatible products are called internal/external lubricants, having a more or less pronounced external character depending on the concentration used. A polar substance, such as palmitic or stearic acid, can behave as an external lubricant in a polar polymer, such as PVC, due to the incompatibility of the long aliphatic chains of its molecules. With the exception of PVC, the lubrication of thermoplastics is usually limited to the use of a single lubricant. With regard to PVC, before processing, the resin is added, together with the other adjuvants, to a lubricant system incorporating an external lubricant and one or more internal or internal/external lubricants. The definition of lubrication formulas for PVC should take into account possible interactions of thermal stabilizers, such as mercaptostannic compounds or epoxidized oils, which have internal lubrication capabilities. Possible changes in the behavior of the lubricant during the PVC extrusion process should also be considered, as with long-chain metal carboxylates, with which an external effect progressively occurs due to the release of their carboxylic acid. When defining the formulation, the occurrence of possibly negative secondary effects should also be foreseen, such as incrustations in the finished products or in the metallic parts of the processing machines, after cooling, due to the use of excessive external lubricants, extraction of dyes, opacification of a transparent PVC, or incorrect gelling of the PVC (with loss of mechanical properties). The type of lubricants used in PVC depends a lot on the type of thermal stabilizer incorporated in the formulation and the application for which the final product is intended, the most used being the following [19]: dicarboxylic acids, alcohols, and low-molecular-weight fatty acid esters; calcium stearate; stearic and hydroxy-stearic acid; oligomeric and high-molecular-weight fatty acid esters; and amide, paraffin, and polyethylene waxes.

1.2 Stabilization

23

1.2.4.1 External Lubricants External lubricants are substances whose aim is to avoid or mitigate the consequences of direct contact between the polymer and the hot metallic parts of the processing machine and molds, leading to superficial sticking, stagnation, and decomposition, through the formation of a protective surface layer [42]. In addition to these anti-adherent agents, there are also other external lubricants that are intended to facilitate the lubrication of finished products, called release or slip agents. Release agents can be mixed with the polymer before processing and must migrate to the surface during the injection phase or be sprayed or deposited on the surface of molds prior to injection. Slip agents are lubricants intended to reduce the friction caused by the sliding of the plates over each other, forming by exudation, at the temperatures of use, an external layer thin enough not to change the surface appearance. 1.2.4.2 Internal Lubricants Internal lubricants are substances intended to facilitate the flow of polymers by reducing their viscosity and also helping to disperse other additives present in the formulation [42, 47]. In the case of PVC, they play a very particular role. They make it possible to improve the rheological behavior of this polymer in the plastic state, helping to accelerate and facilitate its melting (gelation), without critically affecting the viscosity of the molten polymer [42, 47, 49]. 1.2.4.3 Other PVC Processing Aids There are also other processing aids, intended to lower the viscosity and to facilitate the flow of plastics in processing machines and moulds. Although these products also make it possible to accelerate the softening and melting of PVC and significantly facilitate its transformation during the extrusion phase [27], they are not classified as lubricants, as their chemical structure and their mechanisms of action are different [42]. These are high-molecular-weight polymers (between 1.2 × 105 and 2.5 × 106 g/ mole), normally copolymers of methylmethacrylate and other acrylates and methacrylates, and are added in amounts of up to about 3 pcr. The long chains of the acrylic polymer surround the shorter, less flexible chains of the PVC, interlocking them together during high-temperature processing. Acrylates increase elasticity and facilitate the deformation of the polymeric mass without breaking the chains, producing only a slight increase in viscosity [63].

24

1  Degradation and Stabilization of Polymers

1.2.5 Important Developments in Polymer Additives Development in stabilizers is driven by both regulatory and cost-performance issues. 1.2.5.1 Flame Retardants The most great improvement in stabilizers was made in the field of flame retardants (FR), which are a type of additives that do not influence weatherability, but of which the action is influenced by weathering. Moreover, in all plastics used outdoors, namely, in the wiring and cables, automotive, building, and construction (roofing, flooring, and insulation foams), flame retardants are important, and weatherability needs to be considered for assuring good outstanding performances. The subject of flame retardancy is wide, but there are not enough research about the durability and weatherability of flame-retarded products. The influence of aging on fire retardancy in different polymeric systems, with different types of halogen-free fire retardants, was investigated by Braun and his collaborators [64], who concluded that the degradation caused by aging is limited to the surface of the opaque materials used as samples in the research study, and that any bulk property like fire retardancy changes only by a limited amount. However, if fire retardancy mechanism is dominated by a surface mechanism, as for the intumescent formulations, an influence of weathering occurred. Some flame retardants even combine flame retardancy and light stabilizer functionality in the same molecule [65]. The subject of flame retardancy is wide, but there are not enough research about the durability and weatherability of flame-retarded products. Although literature about FR is extensive, it is not entirely inside of the main scope of this book. Readers interested in this subject can consult several reviews [66–69], as well as an interesting fire retardancy guide that was published by the author to support initial sample formulation of the external wood plastic composite (WPC) skin of facade kits developed in the scope of a funded European project [70]. Specialized references to different flame retardants are available in literature, namely, for ammonium polyphosphate (APP), used with polyethylene (PE) and polypropylene (PP) [71, 72]; metal hydroxides, like aluminium trihydroxide (ATH) or magnesium dihydroxide (MDH), coated or noncoated, used in polyolefins, PE thermoplastic or cross-linked copolymers, thermoplastic elastomers, poly(vinyl chloride) (PVC), rubbers, and thermosets [69, 71, 73]; melamine-based nonhalogenated flame retardants [71]; organic phosphorus compounds for thermoplastics and polyurethane foams [74]; and nanoparticles, in particular layered silicate nanocomposites [75, 76], as well as enzymatically and nonenzymatically synthesized polyborosiloxane copolymers [77].

1.2 Stabilization

25

1.2.5.2 Antioxidants Antioxidants are used to provide polymer protection both against oxidation during melt processing and through the product’s life cycle as a “long-term thermal stabilizer.” Such long-term thermal stabilizers differ from melt processing stabilizers in that they must function at temperatures considerably below the polymer melting point. Oxygen and sunlight are the principal degrading agents for hydrocarbon polymers during outdoor weathering. Thus, antioxidants can help polymers and composites to maintain properties during weathering. Special reference must be made, in the field of antioxidants, to the fairly advance in multifunctional antioxidants, which beneficially combines both primary and secondary antioxidant functions in one compound. Combining multiple stabilizing functions in one molecule eliminates the need to use co-stabilizers (e.g., phosphites, thioesters), greatly simplifying antioxidant storage, handling, and formulation [78]. Besides both primary and secondary antioxidant actions, these new multifunctional antioxidants have as main interesting properties a good storage stability, good high-­ temperature performance, excellent processing stability, and intramolecular catalized reduction of hydroperoxides [78]. Interesting developments have been made in the field of natural antioxidants, which cause less risk to human health, and therefore, they are of great interest in polymer industry. Since the majority of them have low thermal stability, making it difficult to mix them with the polymer by extrusion, some authors have been studying techniques to introduce additives into the polymer material without the need for extrusion, namely, by the addition of antioxidants within the polymerization reactor, which will be of great economic advantage [79]. Thus, in-reactor polymerization should be a promising technique for future industrial production. 1.2.5.3 Heat Stabilizers Heat stabilizers (mainly used in PVC formulation), which are used primarily to protect the polymer during processing, also prevent longer-term heat degradation in the end use. Consequently, this type of stabilizers is important during service use of products exposed outdoors. Suitable processing stabilizers positively influence the stability of the plastic against the harmful action of UV radiation, preventing the formation of oxidation groups sensitive to UV degradation in the polymer structure. There is synergy in the combination of processing stabilizers and properly selected UV stabilizers, implying that the combination of both results in greater stability than the one obtained with the individual systems. Tin-based heat stabilizers are mostly used in the USA. Tin mercaptide stabilizers have very good heat stability but poor light stability, and tin maleate or tin carboxylate-­based stabilizers offer better light stability but poorer heat stability. This can be overcome by the addition of titanium dioxide, which acts as a UV light

26

1  Degradation and Stabilization of Polymers

blocker, but its application is restricted to white products. Darker colored siding and profiles require the incorporation of UVA light stabilizers. Thus, manufacturers have been developing new stabilizers to improve weatherability [80]. Europe currently uses mostly lead heat stabilizers in PVC pipes, window, and door profiles. Significant development efforts have been made to replace lead stabilizers, with more environmentally friendly mixed metal (mainly calcium/zinc) or organic stabilizers with comparable cost-performance to lead-based stabilizers. Although the organic stabilizers do not give as white a polymer product as lead or tin stabilizers, this has not been an issue in Europe where pipes are typically colored, compared to white pipes in the USA [80]. 1.2.5.4 Light Stabilizers Important stabilizers for improve weathering performance are the light stabilizers, which are used to protect the polymer from ultraviolet light degradation. A nonbasic light stabilizer, which perform extremely well with highly acidic PVC, contrarily to conventional hindered amine light stabilizers (HALS) due to their basicity, was developed by Ciba in 2008 [80, 81]. According to the manufacturer, this light stabilizer is being used in flexible roofing membranes, and rigid non-tin mercaptide formulations give better durability than twice the concentration of a UV absorber in PVC. Some manufacturers have developed light stabilizers designed to prevent yellowing of light colors and to minimize fading or chalking of dark colors [80]. A stabilizer system developed for improve durability and weathering resistance in PVC window profiles was developed by DuPont [82]. BASF claims to have developed a UV absorber (UVA), for long-term outdoor applications, with improved outstanding absorption capacity exceeding all other UV absorbers currently in use [83]. According to the manufacturer, this light stabilizer, based on highly stable chromophore of the triazine family, has a very high degree of absorption and a very broad absorption curve between 290 and 350 nm, nearly three times more than conventional benzotriazole and benzophenone UV absorbers. This UV absorber has great light stability, improved color, lower color deviation, high transparency, suitable for different thermoplastics, very good substrate compatibility, very low volatility, very good heat stability, low outgassing, low plate-out, and a normal influence on flowability, and it is well suited for long-­ term outdoor use of up to 20 years under very strong UV light exposure. BASF have developed also a new light stabilizer tailored for protecting film made from thermoplastic resins against overly fast degradation when exposed to UV light [84]. This cost-effective additive will help producers of agricultural films and masterbatchers to offer films with superior performance over a long period of time, even in the presence of severe concentrations of agricultural chemicals like elemental sulfur.

1.2 Stabilization

27

Croda Polymer Additives patented a newly developed family of inorganic UV absorbers that make use of ultrafine metal oxides (zinc oxide and titanium dioxide), providing UV protection and appreciably improved transparency relative to other commercially available metal oxide powders and nano-powders [85]. Through careful particle size control, the ultrafine metal oxide powder particles, which are smaller than the wavelength of visible light, have minimal effect on the transparency of the polymer matrix. Highly stabilized, these metal oxide particle dispersions resist reagglomeration unlike other surface-treated metal oxide powders, avoiding diffraction of visible light that causes haze and poor transparency. This new UVA does not migrate, leach out, or degrade, thereby providing longer-term UV protection than traditional organic UV absorbers. It has a higher degree of absorption than conventional benzotriazoles and benzophenones between 240 and 320 nm. Gabriel-Chemie GmbH, in collaboration with Ciba Specialty Chemicals (now part of BASF), has designed a unique UV stabilizer/flame retardant/color masterbatch for polypropylene stadium seating [86]. This combination of UV stabilizer and a flame retardant was previously not possible as light activated an unwanted chemical reaction between the halogenated FR and HALS which caused deactivation of the light stabilizing properties. This “all in one pellet” can be used in injection molding, blow molding, and extrusion processes. Americhem Inc. introduced a family of advanced high-performance UV stabilizers that includes a new stabilizer system for products of polyethylene terephthalate (PET) fibers, used in interiors and outdoor applications [87]. According to the manufacturer, this new stabilizer permits significant performance advances in UV protection for PET fibers with color, physical/mechanical properties’ retention and service life; all impressively improved. Artificial turf, which is increasingly important in international sporting events for its durability, ease of maintenance, and weather resistance, is also becoming an important application for BASF light stabilizers, based on hindered amine light stabilizer (HALS), used in high- and low-density polyethylene (HDPE and LDPE), linear low-density polyethylene (LLDPE), polypropylene (PP), and nylon (PA, polyamide) [88]. Researchers of Oulu’s University research unit of sustainable chemistry have developed a new synthetic and transparent bioplastic that protects from the sun’s ultraviolet radiation and made hydroxymethylfurfural (HMF) and furfural [89]. 1.2.5.5 Other Specific Stabilizers Some interesting developments were made in the fluorescent whitening agents (FWA, also called optical brighteners) [88]. These FWA work via a fluorescent mechanism, which absorbs light in the UV spectrum and re-emit most of the absorbed energy as blue fluorescent light of visible spectrum, between 400 and 500 nm, to yield a brighter and fresher appearance. FWAs are effective at very low concentrations in a variety of polymer substrates such as engineering plastics (e.g.,

28

1  Degradation and Stabilization of Polymers

polyesters, polycarbonate (PC), polyamides (PA), and acrylics), thermoplastic polyurethane, polyvinylchloride, styrene homo- and copolymers, polyolefins, adhesives, and other organic substrates. Research company NanoBioMatters, which specializes in nanoclay-based additives, has launched a nanomaterial that creates a tortuous path that increases barrier properties by reducing gas permeation rates through the plastic matrix and also demonstrates a synergistic improvement in UV barrier properties, decreasing UV transmission with increasing concentration of nanoparticle additive [90]. According to the manufacturer, low concentration of nanoparticle additive is needed to modify the properties of packaging materials without significant changes in density, transparency, and processing characteristics without significant changes in density, transparency, and processing characteristics. A series of new developments in additives for tracking degradation has been made. Indeed, new interesting approaches to follow the early stages of the PP degradation reactions (“induction period”), for radical-mediated damage arising from both UV and thermally initiated sources, are the use of profluorescent nitroxide probes, like TMDBIO, TMAO, MeCSTMIO, and HSTMIO [91–94]. These nitroxides possess a very low fluorescence quantum yield due to quenching by the nitroxide group; however, when the free-radical moiety is removed by reaction with alkyl radicals (to give an alkoxyamine), strong fluorescence is observed. Using spectrofluorimetry, the reaction of the nitroxide with polymer alkyl radicals during oxidation has been monitored. Through analysis of viscosity changes during processing, cumulative chain scission degradation may be estimated. Either chemiluminescence, ultraviolet-visible (UV-Vis) spectroscopy, or infrared spectroscopy (IRS) may be used to detect changes in the polymer. These probes have been used for monitoring degradation during melting process, but it seems that a similar approach may be considered to follow degradation during weathering. Polymer processing aids (PPA) may influence final weathering performance of a polymer. Daikin has been developing PPA, referred as DAI-EL™ PPA DA-810X (first-generation PPAs) and more recently the DAI-EL™ PPA DA-910 (third-­ generation PPAs) that permits to improve the quality of PE agricultural films while lowering melt fracture. These new PPAs developed by Daikin delay die buildup and reduce costs thanks to lower dosage of PPA and to a lower interaction with HALS, leading also to a faster coating process [95].

1.2.6 Synergism and Antagonism Between Stabilizers When preparing formulations, it is necessary to take into account several factors, such as compatibility between the additive and the polymer, the degree of dispersion of the additives, and the possibility of antagonisms (or synergisms) of actions, on the properties of the polymer or between the additives themselves. There are confirmed cases of synergism and antagonism between stabilizers and due to interaction between one or more stabilizers and a climatic agent [6]. How

1.2 Stabilization

29

much complex is the formulation of a polymer more is the probability of occurrence of antagonism phenomena. 1.2.6.1 Synergism Sometimes the combination of several different types of photochemical stabilizers allows a synergistic effect to be obtained. For example, the combination of a UV absorber with a radical scavenging agent or with a hydroperoxide decomposition agent allows obtaining a more efficient and synergistic stabilization, as it allows combining two different mechanisms of protection against photochemical degradation [1]. Other examples of synergism are the well-known improvements done by mixtures of hindered amines with thiosynergists. The existence of synergism can also result from the combination of photochemical stabilizers with other types of stabilizers. Thus, the association of an anti-UV absorbent with an antioxidant (phenol or phosphite type) allows synergism to be obtained, since the latter blocks the oxidation chains that the former “let pass” [1]. HALS also exhibit synergistic properties when used in combination with phenolic antioxidants [3]. Synergism is also found between modified carbon black (MCB) and a UV absorber (UVA) in polystyrene (PS), polyacrylonitrile-butadiene-styrene (ABS), and polymethyl methacrylate (PMMA) matrixes [96]. MCB was obtained by oxidation and hydroxymethylation reactions with conductive carbon black (CB), which originates introduction of hydroxyl groups on the surface of CB particles. The synergism was attributed to the formation of hydrogen bonds between MCB and UVA. Other examples of synergism among flame retardants [97–99], as well as among FR in the presence of nanoparticles [100], have been found, but only cooperation or antagonism is also possible, depending on the concentration of nanoparticles. 1.2.6.2 Antagonism An additive can improve a given characteristic in a polymer and, simultaneously, modify another property of that same polymer in an undesirable way. The combination of several different types of photochemical stabilizers can also lead to obtaining an antagonistic effect, in particular when one of them also has other types of stabilizing functions than photochemical ones. This is the case, for example, between carbon black and titanium dioxide in gray PVC products [1]. In fact, carbon black, as an antioxidant, inhibits the whitening caused by TiO2 and introduces several competitive mechanisms, largely influenced by exposure conditions, which are evidenced by the formation of color. Obtaining antagonistic effects can also result from the combination of different types of stabilizers. Thus, pigmentphotochemical stabilizer associations, for example, are generally not recommended, either because the pigment’s screen effect totally masks the homologous effect of

30

1  Degradation and Stabilization of Polymers

the organic stabilizer [1] or because antagonistic interactions lead to the deactivation of certain photochemical stabilizers, resulting from their reaction with the existing carboxylic functional groups on the surface of the pigment [1, 101]. There is also antagonism between HALS and other additives, as the basicity and reactivity of the former largely limit the usefulness of their application. For example, HALS react with hydrochloric acid and are therefore not used in PVC stabilization [3]. Another example of antagonism between stabilizers was reported by Xiang and co-workers [102]. A research work [103] showed that the combination of a UV absorber (Chimassorb 81) with an organotin mercaptide thermal stabilizer significantly accelerated color development because of the UV sensitivity of the organotin. Moreover, they found that the expected antioxidant effect of the mixture was not observed, perhaps because the Chimassorb 81 had been depleted by the methyltin mercaptide during the UV irradiation. In contrast, the results of this work showed that both Chimassorb 81 and the mixture of Chimassorb 81 with an organic calcium complex thermal stabilizer showed good behavior in inhibiting the photodehydrochlorination and photooxidation of PVC. Results of antagonism between a UV absorber of benzotriazole family (Tinuvin P) and a di-butil-tin-maleate (thermolite 410) and of synergism between the same light stabilizer and a calcium-zinc thermal stabilizer (MARK CZ 2001) in formulations of stabilized PVC for outdoor applications were found in a research work [103]. Other example of antagonism is the “pinking” phenomenon which frequently occurs in white PVC profiles, containing formulations that include heat stabilizers based on lead and titanium dioxide pigments, upon sunlight exposure. The short-­ chain conjugated polyenes formed due to degradation of polymer are responsible for the development of the pink color and result from interactions between the titanium pigment and the lead stabilizers. The authors that have investigated the PVC pink discoloration phenomena [104] have found that there was an increase in the production of hydroperoxy radicals at the onset of color development and that a trace lead dioxide accelerates the pinking. Similar phenomena, involving the formation of hydroxyl radicals, were reported in previous works [60–62, 105] and are known to be related with photoactivity of titanium dioxide pigments in the presence of water.

1.3 Developments in Polymeric Materials and Composites Designed for Outdoor Applications Polymer developments with increased weatherability should be remarked. Morrison and co-workers had developed an oxidation-resistant cross-linked ultrahigh-molecular-weight polyethylene (UHMWPE), which manufacture includes at least two different additives that synergistically increase the oxidation resistance of a normal cross-linked UHMWPE [106].

1.3 Developments in Polymeric Materials and Composites Designed for Outdoor…

31

Polylactic acid (PLA), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and polycaprolactone (PCL) with enhanced barrier properties to UV light, oxygen, and water were developed by Sanchez-Garcia and Lagaron [107], by means of incorporating an organomodified mica-based clay grade. With increasing clay content, the light transmission of these biodegradable biocomposites decreased by up to 90% in the UV wavelength region due to the specific UV blocking nature of the clay used. Increasing interest in competitive, sustainable, and biodegradable alternatives to petroleum-based plastics has encouraged the development of protein-based plastics. The final properties of protein bioplastics are heavily influenced by the protein source and can be related to the structural characteristics of the protein chains in the final product [108]. Protein plastics, manufactured through extrusion, produced materials with reasonable mechanical properties, with a similar tensile strength than synthetic plastics but are generally more brittle. Additives can also be used to improve its properties. However, further research into extrusion of proteins is required to produce bioplastics with less water sensitivity, high tensile strength, good ductility, better thermal stability, and increasing performance. Plastic composites are more than ever exhibiting significant successes in diverse markets. Developments were made in natural fiber-reinforced plastic composites, as WPCs. Since natural fibers are low cost, recyclable, eco-friendly, and biodegradable, they are considered as strong candidates to replace the conventional glass and carbon fibers. The chemical, mechanical, and physical properties of natural fibers vary depending upon the cellulosic content of the fibers which varies from fiber to fiber. The mechanical properties of composites are influenced mainly by the adhesion between matrix and fibers, and chemical and physical modification methods have been developed to improve the fiber-matrix adhesion resulting in the enhancement of mechanical properties of the composites (tensile, flexural, and compression) [109]. Additives such as fire retardants and light stabilizers may be added to improve properties like fire retardancy and durability. Outdoor durability depended on both the light stabilizer and the fire retardant added to the formulation [110]. The past few decades have witnessed an increasing demand for renewable energy technology and improvements in energy efficiency materials. Thus, a field that is receiving increasing research interest is the development of new materials and surface technologies and their contribution to energy efficiency, which presents economic benefits in the transportation and building sectors. This field of research and industrial development has been supported by the seventh framework research program of the European Union, namely, through the 2010 and 2011 calls for green cars and new energetically efficient buildings. Examples of green materials having improved surface properties are presented in an article review [111]. Some interesting developments were made in the field of direct thermal to electrical energy conversion via thermoelectricity, through nanocomposite thermoelectric materials (NcTMs) that are typically multiphased materials requiring high electrical conductivity, high thermopower, and low thermal conductivity [112]. Aiming to use the energy of environment in a useful manner, a research team made

32

1  Degradation and Stabilization of Polymers

up of material scientists, chemists, and physicists from Northwestern University have discovered a new efficient method for turning heat waste into electricity [113]. New high thermal insulation polymers aim similar targets of energy efficiency. New transparent PVC sheet, from 0.02 up to 1.0 mm thickness, with increased insulating properties due to the addition of certain inorganic particles, was patented [114]. The composition has high transparency, low haze, and cut-off infrared, and ultraviolet transmission to provide insulation properties. Increased research interest in energetically efficient materials has stimulated innovations in the building and construction industry, like the development of pultruded panels for ventilated facade of buildings. The design freedom allowed by composites, for outer building wrap covers, played for a good part in combining large dimensions and curved shapes, not possible using metallic materials. For such outdoor and long-term the technical performances of the composites are fundamental, because wide temperature variations need to be addressed. A successful architectural solution that had won the innovation award from the Construction Category at Composites 2011 is the ventilated facade of buildings developed by the partnership established between the companies 3B-the Fibreglass Company with the Italian P.C.R benefiting from the Advantex™ corrosion-resistant fiberglass technology [115]. Other innovations in this field developed are pultruded profiles incorporated into window frames providing both better thermal insulation and vandalism resistance compared to aluminum frames, glass pultruded rebar for concrete structures exposed to harsh environment like sea front or for special properties like electromagnetic transparent buildings and cooling tower components on which composites present increased durability [115]. Interesting developments were made in the field of stimuli-responsive materials and smart polymers. In line with energy efficiency concerns, thermotropic and organic thermochromic materials have been developed for adaptive solar control. Such materials are suitable for application in smart windows and insulation panels, which offer great potential for reducing CO2 emissions by allowing more efficient utilization of solar energy to heat a building in winter and block solar intrusion in summer. Polymer blends, hydrogels, resins, and thermoplastic films with a reversible temperature-dependent switching behavior are described in detail in a technical paper [116]. Inside this scope, smart polymer composites containing aggregachromic dyes of different nature and structure has been studied [117]. The dye-polymer interphase interactions depend on the equilibrium between the aggregated and the isolated forms of chromophores dispersed within the polymer matrix. Such interactions are easily perturbed by external solicitations like mechanical or thermal stresses, which induce a clear and rapid change in the composite’s optical properties both in absorption and in emission. These new stimuli-responsive composite materials, therefore, appear promising for the development of smart and intelligent polymer devices for sensing, and they may seem to be useful for changing insulation properties according to the temperature of the environment (which also depends on the amount of incident radiation).

1.3 Developments in Polymeric Materials and Composites Designed for Outdoor…

33

Nanomaterials have been a field of intense research. Some studies focusing the use of nanoclays (as montmorillonite, MMT) in plastic composites show that on exposure to UV radiation, the useful properties of the composites deteriorate faster than that for the unfilled polymer [118–121] and that nanoclays based on talc may catalyze the oxidative degradation [122]. Composites based on oxide nanofillers in polymers, namely, TiO2 and ZnO, show better behavior and may be promising [123–126]. Indeed, in a study, ZnO and CeO2 nanoparticles were compared to UV absorbers (Tinuvin 477 DW, 292, and 5151) commonly used to stabilize the color of wood products [127]. Results showed that nanoparticles (ZnO, CeO2) absorb UV light frequencies in a manner similar to common organic molecules and that the simultaneous use of both types of absorbers (organic and inorganic metal oxides) seems to create a protective synergy when degradation is only due to UV action. Inorganic nanoparticles are heavy and do not migrate and do not degrade as easily as the organic absorbers, allowing the maintenance of UV resistance of coating.

Chapter 2

Natural Weathering

The predominant factors in natural exposure are humidity, temperature, and solar (ultraviolet) irradiation. The severity of these factors will depend on geographical location. The weather is so variable from time to time and from place to place that even comparisons among outdoor tests obtained at different seasons, years, or locations have been inadequate, even considering the same orientation of the material being tested. Thus, an accurate knowledge of the ultimate outdoor exposure conditions of any material is an impractical expectation. Indeed, weathering is a complicated phenomenon, comprising not only the direct effect of heat, light, air, water and mechanical stress but also effects of the interaction of these elements. Measurement and registration of all main weather parameters are of decisive importance during outdoor weathering. The principal exposure parameters are temperature, relative air humidity, rainfall, rain period, humidity period, radiant exposure for global radiation, radiant exposure in the UV range, radiant exposure at 340 nm, black panel temperature, pH value of rain and dew, and concentration of air pollutants (such as O3, SO2, and NOx). This wide range of measurements is not easy to perform, and it is usually restricted to the major factors that affect polymer durability (solar radiation, temperature, and moisture/rain). The measurement of climatic parameters (minimum, maximum, and mean values) can also be used to develop accelerated weathering tests by permitting a more realistic simulation of outdoor conditions. A typical assembly, facing south with a 45° slope, to support samples subjected to natural exposure is presented in Fig. 2.1. Figure 2.2 shows a system for measurement of global and UV radiation at 45°, facing south, installed on the terrace of a building. Figure  2.3 shows a device used to measure wind speed and direction. There are devices to measure radiation in various directions (horizontal and vertical, for example), systems to measure precipitation, air humidity, ozone, among others, relatively common.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 L. E. P. Real, Weathering of Polymers and Plastic Materials, https://doi.org/10.1007/978-3-031-33285-2_2

35

36

2  Natural Weathering

Fig. 2.1  Support for white specimens in natural exposure, facing south with a slope of 45°, installed on the terrace of a building located in LNEC, Lisbon Fig. 2.2 Metrological station for measurement of global and UV radiation at 45°, facing south, installed on the terrace of a building in LNEC, Lisbon

Nowadays, there is a main concern about the effects of a depletion of the stratospheric ozone layer and the resulting increase in UV-B content in sunlight reaching the Earth’s surface. This particular subject that will have a definite impact on the use of materials in outdoor applications has been studied by some authors [18, 126]. The influence of different climatic agents during natural exposure has been studied abroad. The consistency and reproducibility of outdoor weathering data has been also studied [127, 128].

2.1  Fundamental Aspects That Influence Tests of Natural Weathering

37

Fig. 2.3  Illustration of a system used to measure wind speed and direction (LNEC, Lisbon)

The application of statistical analysis methods to evaluate such influence is known, like design of experiments [129] and principal component analysis [101, 130, 131]. Both methods aim reducing the set of parameters in processes involving a large number of variables, because in general a reduced number of factors account for most of the variability of the set of variables, thus simplifying the interpretation and analyzes of the experimental results (for instance, see Sects. 7.1 and 7.2).

2.1 Fundamental Aspects That Influence Tests of Natural Weathering The fundamental aspects that influence tests of natural weathering are the following: 1. Geographical location of the exposition site, namely, latitude, longitude, and altitude. 2. Climatic characteristics of the region where the exposition site is located. 3. Duration of the exposure. 4. Position of the specimens and their orientation in relation to the sun: For example, the exposure of specimens with an inclination of 45° to the south is less aggressive, in terms of ultraviolet radiation, than in the horizontal (0°), but it is more aggressive than in the vertical (90°) [132, 133]. 5. Cleaning of specimens and periodicity: It must be defined in advance whether the specimens should be washed or not. In case we choose to carry out a cleaning, we must define the respective frequency and which products should be used

38

2  Natural Weathering

in this process. The accumulation of substances on the surface of exposed specimens can have harmful effects (in the case of corrosive substances) or beneficial effects (inert dust that may eventually partially protect the material from the effects of radiation). Therefore, cleaning with water only, to remove dust and grease, is normally chosen, at least after the exposure has been completed and before measuring certain properties (e.g., brightness and color). 6. Location and composition of the material constituting the exposure holders: Placing the support next to the floor or on the terrace of a building involves different environmental conditions. On the other hand, materials have different radiative properties and affect panel temperatures differently, as well as the concentration of water and humidity. 7. Other details: The season in which the exposure begins or the pH of the rainwater are also important and may influence the results.

2.2 Difficulties Inherent in Natural Exposure Studies Measurements of temperature, humidity, and radiation outdoors can currently be carried out using automatic weather stations, not very sophisticated, with sensors for global radiation, ultraviolet radiation, temperature, and relative air humidity. However, given the huge amount of measurements, it is common to use only average values that can hardly reflect the severity of the weathering factors. Abnormal atmospheric conditions, in terms of wind and rain, can cause changes in the position and/or orientation of the exposed specimens, or even irreversible damage to them, an aspect that requires close surveillance and an adequate maintenance service of the supports and samples, requiring frequent visits to the exposure sites.

2.3 Variability Inherent to Natural Exposure Tests Several studies have made it possible to verify that when a certain number of specimens, considered identical (extracted from the same sample), are subjected to natural exposure in the same environment, it is common to obtain a great disparity in responses to degradation [133]. This situation is easily explained, because the results of a natural exposure vary according to: 1. The geographic location (i.e., due to climatic variations from region to region), during the same periods of the year. 2. The year of the exposure, in the same place and in the same period of the year. 3. The period of the year, in the same place and in the same year. 4. The duration of the exposure, in the same place, in the same period of the year, and in the same year.

2.4  Climate Characterization of an Exposure Site

39

In addition, the property whose level of variation is relevant to the material’s use in service also varies with the exposure environment. Thus, the dominant mode of degradation in a material exposed to a semidesert climate (Arizona type) is often associated with a loss of appearance caused by the intensity of ultraviolet radiation, while the dominant mode of degradation of the same material exposed to a climate semitropical (Florida type) is associated with a loss of surface protection caused by prolonged humidity.

2.4 Climate Characterization of an Exposure Site The climatic characterization of a site requires the determination of the value of climatic parameters such as solar radiation, temperature, humidity, wind, and atmospheric contaminants. The most important environmental parameters that, whenever possible, should be considered in a study involving natural exposure are the following: –– –– –– –– –– –– –– ––

Monthly average of maximum daily temperatures and relative humidity. Monthly average of minimum daily temperatures and relative humidity. Monthly average temperature and relative humidity. Maximum monthly amplitude of daily temperature and relative humidity variation. Monthly average amount of daily precipitation. Average duration of rain and dew. Monthly total solar radiation. Monthly total ultraviolet radiation.

2.4.1 Solar Radiation Global radiation is the total solar radiation that is received on a flat horizontal surface located on the Earth’s surface, in the wavelength range [280  nm, 3000  nm, measured according to a perpendicular vertical incidence, with the sun at 90° [134]. This range comprises the following areas of the solar spectrum: UV-B ultraviolet radiation (280–320 nm), UV-A ultraviolet radiation (320–380 nm), visible radiation (380  m to 780  nm), IR-A infrared radiation (780 at 1400  nm), and IR-B (1400–3000 nm). Radiation outside the wavelength range [280, 3000 nm] is null, since it does not reach the Earth’s surface [135]. Clearly, the intensity and spectra power distribution of solar radiation vary also widely with the hour of the day, the day and season of the year, the altitude, the latitude and geographical location, as well as with the clarity of atmosphere (cloud cover, dust, etc.). A month’s exposure in July or August at any location is not the same as a month’s exposure in December or January. Even at one location, the

40

2  Natural Weathering

variation in solar radiation from year to year can be as great as the total radiation for a whole month. Therefore, even a year as a unit for timing exposure is variable and cannot be used for direct comparison of samples, unless they were simultaneously exposed during the same period. Due to seasonal and local variations in the intensity of the sun radiation, it is recognized that “time of exposure” is not a good basis for assessing outdoor exposure of samples. Even at constant level of total incident radiation, degradation proceeds faster in summer than in winter, probably due to the higher temperature as well as to increased ultraviolet intensity [136]. Indeed UV radiation varies much more than the total radiation, presenting also maximum intensity in the summer. Furthermore, the shorter the wavelength, the greater the change with season and local conditions and especially with the time of the day [137]. When there is much scattered radiation, the proportion of short- to long-wavelength ultraviolet increases. Indeed, all plastics are damaged from the effects of ultraviolet (UV) radiation. Chemical bonds in the polymer material are broken down through photodegradation which ultimately causes changes in appearance and deterioration of physical properties. Surface cracking and/or discoloration are the most obvious effects, but this is typically accompanied by loss of elongation at break. Table 2.1 shows the irradiance values indicated in the international standard EN ISO 4892 [138], along the solar spectrum, measured under the conditions mentioned above. The solar constant (annual average of total irradiance) varies according to the Earth-Sun distance, and variations of ±3% can be obtained (e.g., 1.4  kW/m2 in January and 1.3 kW/m2 in July). The variation of the solar spectrum with the seasons leads to a “cutoff” of the solar spectrum at 295 nm in summer, while in winter, this same lower wavelength limit occurs at 310 nm [135, 139]. This phenomenon leads to the conclusion that materials that are only sensitive to photochemical degradation between 295 and 310 nm do not degrade during the winter period. Other terms that are important to mention are the concepts of sky radiation, reflected radiation, net radiation, and daytime illumination [134]. Sky radiation is the global radiation, diffused by the atmosphere, that is received on a flat horizontal surface protected from direct solar radiation. Reflected radiation is the portion of the total radiation that is reflected by the Earth’s surface, measured on a flat horizontal surface facing downwards. Net radiation is the difference between incident and reflected radiation from the Earth’s surface. Daytime illumination is the radiation measured at the Earth’s surface between 510 nm and 610 nm. Table 2.1  Irradiance values contained in the standard EN ISO 4892

Wavelength range, λ, nm 300–320 320–400 400–800 800–2450 Total

Irradiance, W/m2 4 70 604 412 1090

2.4  Climate Characterization of an Exposure Site Table 2.2  Global irradiance values as a function of solar inclination

Solar inclination angle, ° 5 10 90

41 Total solar irradiance, W/m2 52 480 1120

The radiation from the sky also varies as a function of the wavelength and height of the sun. Thus, as the angle of solar inclination increases, the radiation from the sky decreases. Table 2.2 shows the values of solar radiation measured, as a function of the solar inclination, on a day with a clear sky [135]. In the case of measurements carried out with the sky completely covered, the measured radiation intensity is about 20% of the value measured in a cloudless sky situation. 2.4.1.1 Infrared Radiation Infrared radiation (IR) corresponds to the spectral range of electromagnetic radiation with wavelengths above visible light, and can be classified as IR-A (780 nm-1.4 µm), IR-B (1.4-3 µm ) and IR-C (3 µm-1 mm).  Infrared radiation contributes for the heating of the objects irradiated, triggers thermal effects, particularly in dark pigmented polymers, but does not initiate photochemical reactions, and, consequently, it does not induce the degradation of the polymers. 2.4.1.2 Visible Radiation Visible radiation, between 380 and 780 nm, makes up approximately 52% of total radiation, also contributes to heating, but is capable of initiating photochemical processes, increasing photodegradation by sensitisation of visible light-absorbing chromophores or additives. 2.4.1.3 Ultraviolet Radiation It is well known that ultraviolet radiation promotes the primary photochemical process and plays the major role in degradation. The area of the spectrum with the greatest photochemical effect is that of UV-C radiation (λ 290 nm to >310 nm causes a higher conversion of oxygen to carbonyl, because the absorption of the CTCs above 290 nm decreases with increasing wavelength, which means that when a glass filter is used (wavelength > 310 nm), oxygen uptake via the CTC mechanism becomes less important, and normal oxidation becomes more important. Their

80

4  Artificial Accelerated Weathering

results confirm that conversion of oxygen to carbonyl during outdoor weathering is even lower than that during accelerated weathering using a UV filter, because outdoor weathering takes place at a lower temperature than accelerated weathering, which results in a higher stability of the CTCs, so that during outdoor weathering, more oxygen is converted via the CTCs and less through normal oxidation. In a research study comparing the results found in outdoor weathering and indoor accelerated weathering conditions, Diepens and Gijsman did not obtain coincident acceleration factors for two different photooxidation products of bisphenol A polycarbonate, suggesting that the ratio between photo-Fries rearrangements and photooxidation reaction is different [196]. Rabinovitch and Butler have compared color weatherability of various PVC compounds, determined in the QUV accelerated weatherometer and with outdoor exposure in Arizona, Ohio, and Florida [197]. The authors analyzed the effect of the PVC compound base formulation and color on correlation of the QUV to the outdoor weathering, and they concluded that QUV does not accurately rank PVC compounds in weathering performance. Contrarily, Wernstahl and Carlsson have established correlations between Florida exposure and accelerated exposure programs for automotive coatings using an acceleration factor, the equivalent light dose factor (ELD), based on the light dose needed to cause a certain reduction in gloss value [193]. They conclude that difference in temperature dependence of the gloss reduction for different coatings is one of the main causes for poor correlation between accelerated and natural weathering and that to achieve excellent agreement between accelerated and natural weathering testing, one should either carry out the test at a moderately raised temperature level or perform tests at two different temperature levels to establish the temperature dependence of the gloss reduction. In a study that compares the results obtained in a UV chamber equipped with 300-W mercury vapor Osram Ultra-Vitalux lamps with respect to the natural outdoor conditions in Southern Finland, the authors have determined the acceleration factors for the artificial exposures in the chamber for the doses integrated over the wavelengths 290–315 nm (UV-B), 315–400 nm (UV-A), and 290–400 nm (UV), and they concluded that these factors were quite different for the different wavelength ranges [190]. They also concluded that UV chambers equipped with mercury vapor lamps cannot be used as a direct substitute for the commercial chambers employing xenon arc lamps or fluorescent lamps, because the strongly peaked maxima in the radiant output may induce abnormal changes that would not occur in the normal service environment of the materials, especially if the maxima in the action spectra of the degradation processes under study match with the maxima of the artificial radiation used. In a weathering study developed by the author [130], using four different types of TiO2 pigmented PVC formulations, designed for outdoor applications, containing calcium carbonate (CaCO3), two types of thermal stabilizers (based on calcium/zinc or dibutyltin-maleate), and various additives, longevity functions, correlations, and acceleration factors are calculated for the evolution of optical density at 1712 cm 1 and elongation at break (which are well correlated), during the weathering at ten

4.8  Correlation between Natural Weathering and Artificial Accelerated Exposure

81

Table 4.4  Acceleration factors of a specific PVC formulation, based on measurements in the carbonyl region of the FTIR spectra, using different types of artificial accelerated devices System ATLAS Ci-65 (Xe/Bo/Water/dark periods) SOLAR BOX (Xe/Bo) ARALAB FITOCLIMA 700 EDTU (Xe/Bo/water/dark periods) SEPAP 12/24 (Hg) QUV 313 (UV-A)

Acceleration factor 14 ± 3 21 ± 7 12 ± 3 98 ± 22 132 ± 62

different set of photooxidative conditions, obtained from five different accelerated weathering apparatus and a natural exposure for 7 years. The acceleration factors are dependent of the photooxidation conditions (source, filters, climatic parameters, nature of polymer, and stabilization degree). However, it is possible to obtain different acceleration factors in different weatherometers, even for the same polymer and formulation, the same irradiation source, and the same calibration conditions, which demonstrates that there is an inherent variability in each chamber and in each material that cannot be completely controlled or eliminated. Table 4.4 shows the acceleration factors obtained during artificial weathering in different photooxidative conditions, using various weather apparatus, based on comparison of optical density at 1712 cm 1 on natural exposure and artificial accelerated weathering. The results presented in Table  4.4 demonstrate that mercury vapor radiation sources generally allow to accelerate photooxidative degradation much faster than xenon arc sources. This characteristic makes the use of the SEPAP chamber more advantageous to perform the artificial weathering of polymers, provided that the type of degradation phenomena induced by the respective radiation sources is representative of the natural exposure conditions, which, however, was found not to always happen in relation to stabilized PVC formulations.

4.8.2 Phosphor-Coated Low Pressure Mercury Vapor Sources The spectral distribution of low pressure mercury arc UV lamps (fluorescent UV lamps) is achieved through careful selection of the type of phosphor coating on the inner surface of the lamp and the nature of the glass used in the construction of the external tubes. Accelerated weathering with fluorescent UV lamps are the scope of several standards [214, 215, 216]. Devices with fluorescent UV lamps may be used in tests varying light/dark cycles, temperature, condensing humidity, water sprays, and irradiance control.

82

4  Artificial Accelerated Weathering

Although the spectral range of fluorescent lamps is limited to the UV region and does not fully reproduce the solar spectrum, tests with these lamps are widely used in the laboratory, mainly for relative rating comparisons between materials (screening and formulation comparison tests in severe exposures), regardless of the validity of this comparison with their performance in service conditions and with the accuracy of the simulation relative to natural exposure. There are several different types of fluorescent UV lamps that have unique spectral characteristics. Fluorescent UV-B lamps, with a peak around 313 nm, have nearly all of their energy concentrated between 280  nm and 360  nm, and a large percentage is at wavelengths shorter than what is present in natural sunlight. These UV-B lamps are more recommended in development work, when aiming to obtain a quick response, not always according to what happens in the real world, since the results obtained by the use of UV lamps are not necessarily related directly with the results obtained under natural exposure conditions [215]. The large amount of short-wave UV and the lack of long-wave UV and visible radiation may cause mechanisms of degradation significantly different from those of the “natural” tests. As the type 1 lamp emits a significant amount of radiation at wavelengths below 300 nm, which are not present in solar radiation reaching the Earth’s surface, it is well suitable for use in materials used in aerospace technology [215]. UV-A lamps, with a peak around 340 nm or around 351 nm, have nearly all of their energy concentrated between 300 nm and 370 nm. The UVA-340 lamp simulates the direct solar radiation below 325  nm. The spectral distribution at lower wavelengths of a UVA lamp with maximum at 351 nm is similar to that of sunlight behind glass. Figure 4.16 shows the spectral distribution characteristic of low pressure mercury vapor fluorescent lamps with a maximum emission at 313  nm (type 2) and 340 nm (type 1A) versus the component UV of sunlight.

0.8 0.7 Irradiance (W/m2/nm)

Fig. 4.16 Spectral distribution of fluorescent lamps with an emission maximum at 313 nm and 340 nm versus sunlight (UV range) (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing)

Sunlight

UVA-340

UVB-313

0.6 0.5 0.4 0.3 0.2 0.1 0.0 250

275

300

325

350

375

Wavelength (nm)

400

4.8  Correlation between Natural Weathering and Artificial Accelerated Exposure

83

Figure 4.17 shows the spectral distribution characteristic of low pressure mercury vapor fluorescent lamps with a maximum emission at 351 nm (type 1B) versus the component UV of sunlight behind a glass window. Tables 4.5 and 4.6 show the relative spectral irradiance of these two types of lamps [215].

1.0

Irradiance (W/m2/nm)

Fig. 4.17 Spectral distribution of fluorescent lamps with an emission maximum at 351 nm versus sunlight behind glass (UV range) (Image courtesy of Atlas Material Testing Technology and AMETEK Measurement, Communications & Testing)

0.8 0.6 0.4 0.2 0.0 250

275

300

325

350

375

400

425 450

Table 4.5  Relative spectral irradiance of type 1 UV lamps

Wavelength range, (nm) Relative spectral irradiance, % 270