Handbook of Uv Degradation and Stabilization [3 ed.] 1927885574, 9781927885574

Table of contents :
Cover
Handbook of UV Degradation and Stabilization
Copyright
Table of Contents
1 Introduction
2 Photophysics
3 Mechanisms of UV Stabilization
4 UV Stabilizers
5 Stability of UV Stabilizers
6 Principles of
Stabilizer Selection
7 UV Degradation & Stabilization
of Polymers & Rubbers
8 UV Degradation & Stabilization
of Industrial Products
9 Focus Technology − Sunscreens
10 UV Stabilizers and Other
Components of Formulations
11 Analytical Methods in
UV Degradation and
Stabilization Studies
12
Index
Back Cover

Citation preview

Handbook of UV Degradation and Stabilization 3rd Edition George Wypych, Editor

Toronto 2020

Published by ChemTec Publishing 38 Earswick Drive, Toronto, Ontario M1E 1C6, Canada © ChemTec Publishing, 2011, 2015, 2020 ISBN 978-1-927885-57-4 (hard cover); 978-1-927885-58-1 (E-PUB)

Cover design: Anita Wypych

All rights reserved. No part of this publication may be reproduced, stored or transmitted in any form or by any means without written permission of copyright owner. No responsibility is assumed by the Author and the Publisher for any injury or/and damage to persons or properties as a matter of products liability, negligence, use, or operation of any methods, product ideas, or instructions published or suggested in this book.

Library and Archives Canada Cataloguing in Publication Title: Handbook of UV degradation and stabilization / George Wypych, editor. Other titles: UV degradation and stabilization Names: Wypych, George, editor. Description: 3rd edition. | Includes bibliographical references and index. Identifiers: Canadiana (print) 2019014517X | Canadiana (ebook) 2019014520X ISBN 9781927885574 (hardcover) | ISBN 9781927885581 (PDF) Subjects: LCSH: Ultraviolet radiation-Handbooks, manuals, etc. | LCSH: Materials-Deterioration-Handbooks, manuals, etc. | LCSH: Materials-Effect of radiation on-Handbooks, manuals, etc. | LCSH: Stabilizing agents-Handbooks, manuals, etc. Classification: LCC TA418.6 .H36 2020 | DDC 620.1/1228-dc23

Printed in Australia, United States and United Kingdom

Table of Contents

i

Table of Contents 1

Introduction

1

2 2.1 2.1.1 2.1.2 2.1.3 2.2 2.2.1 2.3 2.3.1 2.3.2 2.3.3 2.3.4 2.4 2.5 2.6

Photophysics Nature of radiation Radiative energy Radiation intensity Radiation incidence Absorption of radiation by materials General principles Fate and utilization of absorbed energy Deactivation Intramolecular energy transfer Intermolecular energy transfer Luminescence Radiative processes involving dimers Modeling and photophysical data Illustrating examples

9 9 9 12 13 15 16 21 22 23 25 26 31 33 36

3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12

Mechanisms of UV Stabilization Absorption, reflection, and refraction Energy dissipation Radical deactivation and retardation of propagation of reactions chain Singlet oxygen quenching Degree of a hindrance Antioxidation Peroxide and hydroperoxide decomposition Acid neutralization Repairing defects caused by degradation Synergism Antagonism Effect of physical properties

37 37 44 48 51 53 55 58 61 63 64 68 71

4 4.1 4.1.1 4.1.2 4.1.3

UV Stabilizers Organic UV absorbers Benzimidazoles Benzoates Benzophenones

73 74 74 75 77

ii

Table of Contents

4.1.4 4.1.5 4.1.6 4.1.7 4.1.8 4.1.9 4.1.10 4.1.11 4.1.12 4.1.13 4.1.14 4.2 4.3 4.4 4.5 4.5.1 4.5.2 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 4.7 4.7.1 4.7.2 4.7.3 4.7.4 4.7.5 4.7.6 4.7.7 4.7.8

Benzotriazoles Benzotriazines Benzoxaxinones Camphor derivatives Cinnamates Cyanoacrylates Dibenzoylmethane Epoxidized oils Malonates Oxalanilides Salicylates Carbon black Inorganic screeners Fiber Hindered amine stabilizers Monomeric Oligomeric & polymeric Secondary stabilizers Phenolic antioxidants Phosphites and phosphonites Thiosynergists Amines Quenchers Optical brighteners Synergistic mixtures of stabilizers HAS+HAS Cinnamate+benzoate HAS+UV absorber Phosphite+phenolic antioxidant Phosphite+phenolic antioxidant+HAS HAS+HAS+phenolic antioxidant HAS+UV absorber+phenolic antioxidant Quencher+UV absorber

81 90 95 96 97 98 100 101 102 103 104 105 106 107 108 108 118 123 123 135 139 143 145 146 147 147 147 147 148 148 149 149 149

5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8

Stability of UV Stabilizers UV degradation Electronic structure Chemical reactivity Volatility Effect of temperature Oxygen partial pressure Pollutants Acid neutralization

151 151 153 154 155 157 158 160 161

Table of Contents

iii

5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19

Radical attack Diffusion and migration Grafting Polymerization and copolymerization Effect of pesticides Complexation and ligand formation Excited state interactions Sol-gel protective coatings Interaction with pigments Gas fading Effect of stress

161 162 164 165 166 167 169 169 170 171 171

6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10

Principles of Stabilizer Selection 1 Polarity Acid/base interaction Hydrogen bonding Process temperature Color Part thickness Volatility, diffusion, migration, and extraction Food contact Thermal stabilizing performance State

77 177 178 179 181 181 182 184 184 184 184

7 7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.1.6 7.1.7 7.1.8 7.1.9 7.1.10 7.1.11 7.1.12 7.1.13 7.1.14 7.1.15 7.1.16 7.1.17

UV Degradation & Stabilization of Polymers & Rubbers Polymers Acrylonitrile-styrene-acrylate Acrylonitrile-butadiene-styrene Acrylic resins Alkyd resins Cellulose-based polymers Chlorosulfonated polyethylene Copolymers Epoxy resin Ethylene-propylene copolymer Ethylene-propylene-diene monomer Ethylene-tetrafluoroethylene copolymer Ethylene-vinyl acetate copolymer Fluorinated ethylene-propylene Poly(3-hexylthiophene) Perfluoropolyether Polyacrylamide Polyacrylonitrile

187 187 187 189 194 199 202 204 206 207 211 215 217 218 220 222 223 224 225

iv

Table of Contents

7.1.18 7.1.19 7.1.20 7.1.21 7.1.22 7.1.23 7.1.24 7.1.25 7.1.26 7.1.27 7.1.28 7.1.29 7.1.30 7.1.31 7.1.32 7.1.33 7.1.34 7.1.35 7.1.36 7.1.37 7.1.38 7.1.39 7.1.40 7.1.41 7.1.42 7.1.43 7.1.44 7.1.45 7.1.46 7.1.47 7.1.48 7.1.49 7.1.50 7.2 7.2.1 7.2.2 7.2.3 7.2.4 7.2.5

Polyalkylfluorene Polyamide Polyaniline Polyarylate Polybutylthiophene Polycarbonate Poly(ε-caprolactone) Polyesters Polyetherimide Polyethylene Polyfluorenes Polyimide Poly(L-lactic acid) Polymethylmethacrylate Polymethylpentene Polyoxymethylene Polyphthalamide Poly(p-phenyl vinyl ketone) Poly(phenylene oxide) Poly(p-phenylene sulfide) Poly(p-phenylene vinylene) Polypropylene Polypyrrole Polystyrene Polytetrafluoroethylene Polyurethanes Poly(vinyl acetate) Poly(vinyl chloride) Poly(vinyl fluoride) Poly(vinylidene fluoride) Silicone Styrene-acrylonitrile Vinyl ester resin Rubbers Polybutadiene Polychloroprene Polyisoprene Polyisobutylene Styrene butadiene rubber

227 228 232 233 234 235 240 241 247 250 256 257 258 260 263 264 267 268 269 271 272 273 279 280 286 287 294 295 300 302 304 307 310 311 311 315 317 319 321

8 UV 8.1 8.1.1

Degradation & Stabilization of Industrial Products Adhesives Typical requirements

323 323 323

Table of Contents

8.1.2 8.1.3 8.1.4 8.2 8.2.1 8.2.2 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.4 8.4.1 8.4.2 8.4.3 8.4.4 8.5 8.6 8.6.1 8.6.2 8.6.3 8.6.4 8.7 8.7.1 8.7.2 8.7.3 8.7.4 8.8 8.8.1 8.8.2 8.8.3 8.8.4 8.9 8.9.1 8.9.2 8.9.3 8.9.4 8.10 8.10.1 8.10.2 8.10.3 8.10.4 8.11

Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Aerospace Important changes and degradation mechanisms Methods of stabilization Agriculture Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Automotive Typical requirements Lifetime expectation Important changes and degradation mechanisms Methods of stabilization Biology Coated fabrics Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Coatings and paints Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Composites Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Coil-coated materials Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Cosmetics Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Dental

v

324 324 325 327 327 327 328 328 328 329 329 331 331 331 332 332 335 336 336 336 336 337 339 339 339 340 341 343 343 343 343 344 345 345 345 346 346 348 348 348 348 348 351

vi

8.11.1 8.11.2 8.12 8.12.1 8.12.2 8.12.3 8.12.4 8.13 8.13.1 8.13.2 8.13.3 8.13.4 8.14 8.14.1 8.14.2 8.14.3 8.15 8.15.1 8.15.2 8.15.3 8.15.4 8.16 8.16.1 8.16.2 8.17 8.17.1 8.17.2 8.17.3 8.17.4 8.18 8.18.1 8.18.2 8.18.3 8.19 8.19.1 8.19.2 8.20 8.20.1 8.20.2 8.20.3 8.20.4 8.21 8.21.1

Table of Contents

Typical requirements Important changes and degradation mechanisms Door and window profiles Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Electrical and electronic applications Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Fibers and yarns Typical requirements Lifetime expectations Methods of stabilization Films Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Fishing nets Lifetime expectation Methods of stabilization Foams Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Food Typical requirements Important changes and degradation mechanisms Methods of stabilization Furniture Important changes and degradation mechanisms Methods of stabilization Geosynthetics Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Glazing Typical requirements

351 351 352 352 352 352 352 354 354 354 354 355 356 356 356 356 358 358 358 358 359 362 362 362 363 363 363 363 365 366 366 366 366 367 367 367 368 368 368 368 369 370 370

Table of Contents

8.21.2 8.21.3 8.22 8.22.1 8.22.2 8.23 8.23.1 8.24 8.24.1 8.24.2 8.24.3 8.25 8.25.1 8.25.2 8.25.3 8.26 8.26.1 8.26.2 8.27 8.27.1 8.27.2 8.27.3 8.27.4 8.28 8.28.1 8.29 8.29.1 8.29.2 8.30 8.30.1 8.30.2 8.30.3 8.30.4 8.31 8.31.1 8.31.2 8.31.3 8.31.4 8.32 8.32.1 8.32.2 8.32.3 8.33

Lifetime expectations Methods of stabilization Hair Important changes and degradation mechanisms Methods of stabilization Light emitting diodes Important changes and degradation mechanisms Medical supplies Typical requirements Important changes and degradation mechanisms Methods of stabilization Optical materials Typical requirements Important changes and degradation mechanisms Methods of stabilization Pharmaceuticals Important changes and degradation mechanisms Methods of stabilization Pipes and tubing Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Pollutants Important changes and degradation mechanisms Pultruded profiles Important changes and degradation mechanisms Methods of stabilization Pulp and paper Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Railway materials Typical materials Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Rotationally molded products Typical requirements Lifetime expectations Methods of stabilization Roofing materials

vii

370 370 372 372 373 374 374 375 375 375 375 376 376 376 376 377 377 377 378 378 378 378 378 380 380 381 381 381 382 382 382 382 382 384 384 384 384 384 385 385 385 385 387

viii

8.33.1 8.33.2 8.33.3 8.33.4 8.34 8.34.1 8.34.2 8.34.3 8.35 8.35.1 8.35.2 8.35.3 8.35.4 8.36 8.36.1 8.36.2 8.36.3 8.36.4 8.37 8.37.1 8.37.2 8.37.3 8.37.4 8.38 8.38.1 8.38.2 8.38.3 8.39 8.39.1 8.39.2 8.39.3 8.40 8.40.1 8.40.2 8.41 8.41.1 8.41.2 8.41.3 8.42 8.42.1 8.42.2 8.42.3 8.43

Table of Contents

Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Sealants Typical requirements Lifetime expectations Methods of stabilization Sheets Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Siding Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Solar cells and solar energy applications Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Space industry Typical requirements Important changes and degradation mechanisms Methods of stabilization Sporting equipment Typical requirements Lifetime expectations Methods of stabilization Tapes Typical requirements Methods of stabilization Textiles Typical requirements Important changes and degradation mechanisms Methods of stabilization Windshield Typical requirements Lifetime expectations Methods of stabilization Wire and cable

387 387 387 388 390 390 390 391 392 392 392 392 393 394 394 394 394 394 396 396 396 396 397 399 399 399 399 400 400 400 400 401 401 401 403 403 403 404 406 406 406 406 408

Table of Contents

ix

8.43.1 8.43.2 8.43.3 8.43.4 8.44 8.44.1 8.44.2 8.44.3

Typical requirements Lifetime expectations Important changes and degradation mechanisms Methods of stabilization Wood Typical requirements Important changes and degradation mechanisms Methods of stabilization

408 408 408 409 410 410 410 411

9

413

9.1 9.2 9.2.1 9.2.2 9.3 9.3.1 9.3.2 9.3.3 9.3.4 9.3.5 9.3.6 9.3.7 9.4 9.5

Focus Technology - Sunscreens Christine Mendrok-Edinger Introduction and history of sunscreens Photoreactions of UV absorbers in cosmetic sunscreens Reversible transformations Non-reversible transformations Ways of photostabilization in sunscreen products High optical density Triplet-triplet quenching Spatial separation of UV absorbers Coating of pigments Singlet quenching Solvent polarity Antioxidants Formulating for photostability Summary

413 414 415 417 418 419 419 423 424 425 426 426 428 430

10

UV Stabilizers and Other Components of Formulations

433

11 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.12.1 11.12.2

Analytical Methods in UV Degradation and Stabilization Studies Quality control of UV stabilizers Lifetime prediction Molecular weight Weight loss Color & gloss change Mechanical properties Morphology (microscopy) Impedance measurement Surface roughness Imaging techniques Chromatography Spectroscopy ESR DART-MS

439 439 440 441 441 442 442 443 445 445 445 446 446 447 448

x

Table of Contents

11.12.3 11.12.4 11.12.5 11.12.6 11.12.7 11.13

FTIR NMR UV MALDI-MS Acoustic emission Hydroperoxide determination

448 448 449 449 449 449

12 12.1 12.2 12.3 12.4 12.5

UV Stabilizers - Health & Safety Toxic substance control Carcinogenic effect Workplace exposure limits Food regulatory acts Safety concerns with nanoparticles

453 453 456 457 459 460

Index

465

1

Introduction

It will be customary to begin with the historical background of the first studies on UV degradation and stabilization, but chronologically it is more important to review the effect of ultraviolet light on living organisms such as plants, animals, and humans, and discuss their methods of prevention of degradation on UV exposure. Plants survived various levels of UV radiation from the beginning of their existence; moreover, they adapted to use it to harvest energy for biological transformations leading to the production of vital tissues and food required for their life. This suggests that plants must have developed various mechanisms of resistance against the destructive energy of UV radiation since they are composed of organic matter, which must be vulnerable to the high energy of UV radiation. It is known that UVB (280-320) can cause direct and indirect damage of deoxyribonucleic acid, DNA, but UVA (320-400 nm) can only cause indirect damage of DNA (formation of singlet oxygen and superoxide anion radicals that are the precursors of hydrogen peroxide involved in the generation of hydroxyl radical via Fenton-type reactions).1 DNA has absorption maxima in the UVC range (550

no

330-370

yes

PRINCIPLE OF DEGRADATION

The amount of energy absorbed by a molecule must exceed the bond energy in order to cause degradation This principle of degradation carries two important messages: • energy must be absorbed by a molecule for it to make any changes in molecular structure. This is the subject of further discussion in Section 2.2. • a change in the energy of radiation source will affect weathering. The radiation frequency is determined by the conditions under which it was formed. A hypothetical black body was postulated by Planck and used in the development of the law which bears his name. The black body, as it is postulated, can absorb and emit radiation of any wavelength. Its characteristics are shown in Figure 2.1. When the temperature of the radiation source increases, its emission spectrum is shifted to the left, meaning that it emits more UV and visible light. Figure 2.1 illustrates that the wavelength (or frequency of radiaFigure 2.1. Radiation intensity vs. radiation wavetion) depends on the conditions of emission. length and black body temperature.

12

2.1 Nature of radiation

2.1.2 RADIATION INTENSITY Table 2.3. Units used in measurement of radiation. Table 2.1 indicates that the energy of laser radiation is the same as the energy Quantity Unit of visible light or UV (depending on wavelength). But the fact that laser Radiant energy J radiation is substantially more intense Radiant energy density J m-3 (focused or concentrated) is central to -1 Radiant flux W (J s ) the following discussion. -2 Irradiance Wm Table 2.3 shows some units of radiation. A laser emits radiation from 1 mW (lasers frequently used in optical experiments) to 10 W (moderately powerful argon laser) and beyond. This power is emitted onto a very small surface area (laser light has high coherence, monochromaticity, and small beam width) usually in the range of 10 μ m2 to 1 mm2. Irradiance is calculated to be in the range of 107-108 W m-2 (in fact the illuminated surface area is limited by, and equal to, the wavelength of radiation, and power can be as large as 100 W, giving an irradiance of 1013 W m-2). If we compare these values with the mean intensity of sunlight on the Earth's surface (in the range of 103 W m-2), it is easy to understand the difference between these two sources of radiation and to explain the effects produced (surface etching by laser beam versus minor changes or no changes at all by sunlight). This illustrates the importance of the conditions under which the experiment is run and reported. It is useful to pursue the laser example. Laser light delivers 1012 to 1017 photons/cm3. At this intensity, several photons will react with a single atom, causing high levels of excitation. Laser light also induces very strong electric fields which can be as much as 100 gigavolts per meter. This inevitably causes changes in orientation, dipole formation, ionization, etc. of molecules, changing conditions of photochemical reactions and their mechanisms. The use of pulsed lasers, with their highly ordered (polarized) beams, can selectively excite the single isomer (in the mixture) which has the right configuration for energy absorption. This is why irradiation by chaotic radiation (e.g., sunrays) will produce totally different results than radiations of high intensities (e.g., lasers). The xenon lamp does not have enough energy to propagate through the entire thickness of material nor can it excite so many atoms at once. The consequence of increasing irradiance in a Weather-O-Meter from 0.35 W m-2, which is the typical UV radiation of daylight, to higher values is difficult to predict because the change in irradiance does not merely increase the number of excited molecules but also causes the random formation of higher excited states. The proportions of both are difficult to predict. In this context, it is significant to evaluate results of simulation of the effect of the uneven surface on results of exposure to UV.1 The modeling was verified by experiment. Flat and sinusoidal polymer surfaces were numerically simulated for their UV damage as a function of UV intensity, surface topography, and exposure time.1 Experimentally determined UV degradation rates for a unidirectional glass/epoxy composite were used to predict numerically the local rates of material degradation on sinusoidal epoxy surfaces

Photophysics

13

Figure 2.2. Surface topographies of unexposed (left) and UV exposed (right) for 1000 h surfaces of the epoxy specimens. Inserts show magnified SEM images of their surface topographies. [Adapted, by permission, from Lu, T; Solis-Ramos, E; Yi, Y; Kumosa, M, Polym. Deg. Stab., 154, 203-10, 2018.]

subjected to UV. The UV damage on uneven polymer surfaces reduced their surface roughness making them more planar.1 The degradation rates were the largest at the tips of the local heights of the surfaces.1 Modeling was subsequently verified by exposing neat epoxy specimens to UV in the air at 80°C for 1000 h (Figure 2.2).1 The surface roughness of the epoxy was reduced by about 12.5%.1 2.1.3 RADIATION INCIDENCE Two processes, reflection and scattering, determine the amount of energy crossing a surface of a specimen exposed to radiation. The geometrical relationship between incident, transmitted, and reflected beams is given by Snell's law (Figure 2.3): n 1 sin θ 1 = n 2 sin θ 2

[2.4]

where: n1 and n2 θ 1 and θ 2 Figure 2.2.

refractive indices the angles illustrated in

It should be noted that θ i = – θ 1 because the angle of incidence and the angle of reflection are equal. Table 2.4 gives refractive indices for some common materials. Using

14

2.1 Nature of radiation

Snell's equation, we can calculate that if an incoming beam has an incidence angle of 30o, Table 2.4. Refractive indices. an angle of transmission, θ 2 , is 19.5o (if n1 Material n equals 1 (for air), and n2 is assumed equal to 2.5). For any other angle of incidence, the Air 1.00 transmission angle is smaller than the inciGlass 1.50-1.95 dence angle if n2 > n1. If n2 < n1, there always Polymers 1.34-1.65 exists some critical angle of incidence above Water 1.34 which the beam is internally reflected. In practice, specimens are frequently exposed through glass or plastic and/or have a backup of metal, plastic, glass, or other materials. In such cases, refractive indices must be carefully considered when evaluating the effect of the internal reflection of the transparent cover or the energy retention in the material backing the specimen. Note that the refractive Figure 2.3. Reflection/refraction of radiation. index depends on radiation wavelength (refractive index generally decreases with an increase in wavelength from UV to IR). It would be helpful to know what proportion of light is reflected from the specimen’s surface and what proportion is transmitted into the specimen. Unfortunately, there is no such universal relationship. An understanding of Rayleigh (particle size T1-S0) is also smaller. Finally, because of triplet formation and as a result of intersystem crossing, the factors affecting the rate constants of intersystem crossing, k ISC and k ISC' , also affect the concentration of triplet states. In the proximity of a molecule containing an atom of high molecular charge (e.g., a heavy metal), spin-selection rules break down. This results in the enhancement of intersystem crossing rates and diminishes the quantum yields of singlet state processes such as fluorescence at the expense of the formation of more triplet states. Molecular oxygen can induce a similar effect by increasing the values of the rate constants: k ISC , k ISC' , and k p' . When the rates of non-radiative conversions are increased, the reactivity of molecules in the triplet state increases. The differences between the origins of fluorescence and phosphorescence are illustrated in Tables 2.6 and 2.7. Table 2.6 shows the energies of singlet and triplet states. Triplet energies are always considerably smaller than singlet energies. Table 2.7 shows the lifetimes of singlet and triplet states. Triplet state lifetimes are considerably longer than singlet lifetimes. Phosphorescence should have a longer lifetime and should appear at a higher wavelength (as it does). Studies show that singlet and triplet energies can be transferred over a distance as large as 20 Å. The maximum distance depends on orientation, the polarity of the solvent, and bonding type. Table 2.6. Energies of singlet and triplet states. Excited states

Singlet, kJ/mol

Triplet, kJ/mol

Acetone

n, π*

365

325

Benzophenone

n, π*

320

290

Benzene

π, π*

460

350

Photophysics

31

Table 2.7. Lifetimes of singlet and triplet states. Singlet, s Acetone

Triplet, s

-9

6x10-4

-12

6x10-3

2x10

Benzophenone

5x10

Benzene

3x10-8

6.3

2.4 RADIATIVE PROCESSES INVOLVING DIMERS Both the act of radiative energy absorption, followed by the promotion of a molecule to a higher energy singlet state level, S1, and a triplet state, T1, formed from a singlet in intersystem crossing, may result in emission: A∗ → A + hν Apart from these simple processes, a polymer may emit radiation from an excimer or an exciplex. Excimer emission occurs when an excited species forms an excited complex with a ground state species of the same kind: A∗ + A → ( AA )∗ → A + A + hν An excimer is a molecular dimer formed from a molecule in its lowest excited state (S1) and a molecule in its ground state (S0) or from two lowest excited triplets (T1). During singlet excimer decomposition, excimer fluorescence occurs, which differs from “normal” fluorescence because it lies in a region of longer wavelength, has no vibrational structure and is affected by temperature. Excimers formed from triplet states produce delayed fluorescence on decomposition. The delayed fluorescence is different from “normal” fluorescence because it has a longer decay time, depends on light intensity, and is sensitive to oxygen. The exciplex emission has a similar origin but results from the complex formed by an excited species and a species in the ground state: A∗ + B → ( AB )∗ → A + B + hν An exciplex, formed from an excited donor with an acceptor in the ground state, produces a fluorescent emission which depends on solvent polarity (fluorescence yield decreases with increasing solvent polarity). There are other possibilities: The dimeric excited species can be formed: A – A → ( A – A )∗ → A + A + hν Energy can be transferred along the chain by a migrating exciton: X∗ – X – X – X – X → X – X – X – X ∗ – X

32

2.4 Radiative processes involving dimers

Delayed fluorescence might be formed from triplet states: T 1 + T 1 → S 1 + S 0 → 2S 0 + hν To simplify further, we may consider all of the above reactions as reactions between an excited molecule (frequently called a sensitizer) and a molecule in the ground state (often called a quencher). The excited state may transfer energy to the ground state (if the quencher has lower energy than the sensitizer), producing the quencher molecule in the excited state at the expense of the sensitizer molecule returning to its ground state. Also, a singlet is produced from a singlet and a triplet from a triplet. These reactions may involve the formation of ionized molecules because an electron-rich donor may transfer electrons to an electron-deficient acceptor. In this reaction, a charge-separated pair of ions is formed. Both hole or electron transfers are possible. Primary processes might be followed by secondary (or chain) processes leading to energy or charge migration or to chemical reaction or to recombination. The properties of the polymer backbone also play a role. If a polymer backbone is conductive, then the charge can be transferred along with it, providing the distance is not long. If the backbone is non-conductive, space energy transfer will prevail. If the polymer can fold in a time shorter than its excited state lifetime, then intramolecular energy transfer is a probable mechanism. If polymers have numerous sensitizer groups (energy absorbing side groups), the energy can be transferred by energy exchange between neighboring sensitizers until the final transmission to the quencher occurs. This energy transmission can proceed along the chain but the distance of energy transfer can also be shortened in flexible polymers because of chain bending and the involvement of sensitizers from different segments of the chain. The simplest case occurs when sensitizers and quenchers are too distant from each other for energy transfer or excimer formation. The energy can only be transferred by radiative exchange or dipole-dipole interaction. It is theoretically possible to form a polymer from neighboring chromophores, which are unable to form excimers but are able to transfer energy on collision. Iinuma11 proposed this theory after studying a polymer containing 1,3,5-triphenyl-2-pyrazoline as a pendant unit. This group cannot form excimers and thus the theory was confirmed. Other models have been developed to show that energy is also transferred by collisions and that excimers are formed by energy migration. Polyvinylnaphthalene, polyvinylcarbazole, and some copolymers are thought to react due to collisions and excimer formation. The process is highly dependent on the concentration of excimer-forming sites and on the concentration of guest molecules. Energy transfer by collision following excimer migration has also been proposed. This has a very low probability because the excimer sites are not sufficiently numerous to allow the migration of excimer energy. Studies on polystyrene reveal that long-range dipole-dipole transfer from the excimer is a credible mechanism (Figure 2.16). When polystyrene contains free monomer, the emission occurs through the free monomer. If there is no free monomer, excimer emission is observed.

Photophysics

33

Figure 2.16. PS fluorescence spectra (control and film containing 1% styrene). [Data from Heisel, F; Laustriat, G, J. Chim. Phys., 66, 1895 (1969].

It is important that polymer blends are compatible. The compatibility enhances many properties including weatherability, but it is difficult to study. The measurement of excimer and exciplex fluorescence helps to measure the compatibility of polymers in blends at levels much below those at which phase separation occurs. The phosphorescence behaves according to the principles given in the fluorescence discussion, but the difference between the lifetimes of singlet and triplet states changes the proportions between the modes of energy utilization. Triplet state formation is a two-stage process and, thus, phosphorescence quantum yield, ψp, is a composite of triplet formation and decay:

Ψ p = Ψ ISC Ψ d where:

Ψ ISC Ψd

quantum yield of intersystem crossing quantum yield of triplet decay

Ψ p = [ k ISC ⁄ ( k F + k ISC + k d ) ] × [ k p' ⁄ ( k p' + k ISC' + k d' ) ] where

ψp kISC kF kIC kd kp’ kISC’ kd’

[2.15]

[2.16]

phosphorescence quantum yield rate constant of intersystem crossing ( S 1 → T 1 ) rate constant of fluorescence (S1) rate constant of internal conversion ( S i → S 0 ) rate constant of reaction (S1) rate constant of phosphorescence ( T 1 → S 0 ) rate constant of intersystem crossing ( T 1 → S 1 ) rate constant of reaction (T1).

The long-lived triplet state can be upgraded to the singlet state by the acquisition of thermal energy, but the energy gap between the singlet and the triplet states is of the order of kT.

2.5 MODELING AND PHOTOPHYSICAL DATA The sensitivity of polymeric materials to radiation of a defined wavelength has been measured over several decades.13-15 These studies have produced “wavelength sensitivity spectra”. These spectra are influenced by the type of reaction being monitored, the material structure, and composition. Two different terms are typically used to express results: • an activation spectrum which is a source dependent spectrum determined by exposure of samples to polychromatic radiation

34

2.5 Modeling and photophysical data

•

an action spectrum which is determined by exposure of samples to monochromatic radiation. These studies have provided information on the most dangerous wavelengths of radiation for a particular material. If we generate quantitative data which will give rate constants of degradation for different wavelengths, then it should be possible to model the degradation rate of particular material for which such data are available. This is a more theoretical than a practical assumption since materials generally have complex formulations, therefore there are many influences, not all of which are minor, which have to be considered to make such a model sufficiently precise to be useful. In spite of the complexity, such studies are needed for, over time, a body of data will be generated that will lead to better understanding. This will also put restrictions on equipment used for studies because such modeling studies will soon show that it is not right to compare data from, for example, exposure to fluorescent lamp (that misses many of these wavelengths of important radiation) with exposure to sunlight or exposure to xenon-arc lamp, which have a completely different spectrum. A study on modeling UV stabilization16 is an example of one such effort. The model used to evaluate the protective effect of stabilizers is based on Beer’s law and wavelength sensitivity: ΔP ( λ, x ) = E ( λ, x )S ( λ ) = tI 0 ( λ )10 where:

ΔP ( λ, x ) λ x E ( λ, x ) S(λ) t I0 ( λ ) Σε i ( λ )c i

– ( Σε i ( λ )c i )x

S(λ )

[2.17]

property degradation at depth of x due to irradiation by λ wavelength of radiation thickness of irradiated layer or depth of radiation penetration total energy received at depth x for wavelength λ the sensitivity of polymer to the wavelength λ exposure time incident light intensity the sum of absorbances of absorbing species in a sample (e.g., polymer, stabilizers, other additives, etc.)

Using this and other model equations it has been possible to evaluate the protective action of different stabilizers based on measurements under varying conditions of exposure simulated by the use of different filters in xenon-arc exposures (Figure 2.17). Figure 2.17 shows the correlation between predicted and measured rates of yellowing. This study is valuable not only because it helped in the selection of suitable stabilizer but because it also demonstrated the application of sound photophysical principles. This type of approach encourages the experimenter to confirm that the exposure conditions are correct and that the appropriate properties were selected for monitoring. Results of some degradation studies cannot be explained by photochemical mechanisms. These data have been explained by proposing the formation of oxygen chargetransfer complexes which could possibly be peroxidation initiators.17-19

Photophysics

35

Although charge-transfer complexes were not isolated, the following formula and the mechanisms of the photodegradation of polyolefins have been proposed: hν CHCH2 CH2 CH H

O2

CHCH2 CH2 CH

+

H2O2

H

2HO

Charge transfer complex can be formed (as above) from a single chain or with the involvement of hydrogens from neighboring chains. It is proposed that numerous radicals are then produced, and this helps to explain why some materials lose their properties so rapidly. It will be important to verify these proposals by future studies. This mechanism has been postulated from time to time for over 60 years but has yet to be proven. More studies are required to determine the distances over which energy can be transported from excimers to internal and external acceptors.20 Studies of Figure 2.17. Comparison of experimental and predicted excited-state complexes21,22 have helped to values for stabilized PS samples. [Data from Allan, DS; explain the relationship between structure, Maecker, NL; Priddy, DB; Schrock, NJ, Macromoleand energy dissipation, and have led to the cules, 27, 7621 (1994).] development of new, advanced materials (e.g., electronically conductive polymers) but also to give information on the behavior of exciplexes. The final result of energy absorption depends on the balance of energy available in a particular molecule and the chemical structure of the excited molecule. The presence of other molecules in the neighborhood and the state of matter also favor a particular Figure 2.18. Excited-state proton transfer (ESIPT) in Tinuvin P type of conversion. Knowledge of molecule. [Adapted, by permission, from Paul, B K; Guchait, N, these complex relationships will Comput. Theor. Chem., 966, 250-58, 2011.] help our understanding of why materials fail and how to make them more durable.23-27

36

2.6 Illustrating examples

2.6 ILLUSTRATING EXAMPLES The exceptional photostability of Tinuvin P depends on its ability to undergo excited-state deactivation via an ultrafast excited-state proton transfer.28 Figure 2.18 shows the mechanism of excited state deactivation.28 The role of charge transfer interaction in the intramolecular hydrogen bonding has been evaluated and addressed under the provision of natural bond orbital analysis, which helped to find evidence confirming the mechanism.28 The development of LEDs operating upon soft visible light irradiation has opened new fields for polymer synthesis.29 Many novel photoinitiating systems based on organic and organometallic compounds with excellent visible light absorption have emerged for cationic photopolymerization.29 After absorbing visible light, the photoinitiator excited singlet state (or triplet state) should efficiently interact with additives.29 Long-lived singlet (or triplet) states of photoinitiators are decisive.29

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

Lu, T; Solis-Ramos, E; Yi, Y; Kumosa, M, Polym. Deg. Stab., 154, 203-10, 2018. Goswami, S; Haldar, C, J. Photochem. Photobiol. B: Biol., 153, 281-8, 2015. Lecamp, L; Lebaudy, P; Youssef, B; Bunel, C, Polymer, 42, 8541-47, 2001. Santos, B A M C; da Silva, A C P; Bello, M L; Gonçalves, A S; Rodrigues C R, J. Phochem. Photobiol. A: Chem., 356, 219-29, 2018. Stange, U C; Temps, F, Chem. Phys., 515, 441-51, 2018. Jablonski, A, Z. Phys., 94, 38-44, 1935. Frackowiak, D, J. Photochem. Photobiol., B: Biol., 2, 399-408, 1988. Al-shamiri, H A S; Khedr, M A; Sabry, M M, Optik, 182, 716-26, 2019. McKellar, JF; Allen, NS, Photochemistry of Man-made Polymers, Applied Science Publishers, London, 1979. Guillet, JE; Hoyle, CE; McCallum, JR, Chem. Phys. Lett., 54, 337 (1978).] Iinuma, F; Mikawa, H, Aviles, RG, Macromolecules, 12, 1078 (1979). Heisel, F; Laustriat, G, J. Chim. Phys., 66, 1895 (1969). Hirt, R; Searle, N, SPE RETEC, Washington, 1964. Mullen, PA, Searle, N, J. Appl. Polym. Sci., 14, 765 (1970). Pickett, JE, J. Appl. Polym. Sci., 33, 525 (1987). Allan, DS; Maecker, NL; Priddy, DB; Schrock, NJ, Macromolecules, 27, 7621 (1994). Gijsman, P, Angew. Makromol. Chem., 252, 45 (1997). Scott, G, Polym. Deg. Stab., 60, 215 (1998). Gijsman, P, Polym. Deg. Stab., 60, 217 (1998). Kopp, BS; Scranton, AB, Polym. News, 19, 280, 1994. Creed, D; Hoyle, CE; Jordan, JW; Pandey, CA; Nagarajan, R; Pankasem, S; Peeler, AM; Subramanian, P, Macromol. Symp., 116, 1, 1997. Jenekhe, SA, Adv. Mater., 7/3, 309, 1995. Croll, S; Skaja, A, Macromol. Symp., 187, 861-871, 2002. Gao, M; Xiao, B; Liao, K; Cong, YF; Dai, Y, Petroleum Sci. Techn., 24, 6, 689-698, 2006. Dever, JA; McCracken, CA, High Performance Polym., 16, 2, 289-301, 2004. Chin, JW; Martin, JW; Nguyen, T; Embree, E; Byrd, WE; Tate, JD, Antec, 2002. Conforti, PF; Garrison, BJ, Chem. Phys. Lett., 406, 6, 294-299, 2005. Paul, B K; Guchait, N, Comput. Theor. Chem., 966, 250-58, 2011. Xiao, P; Zhang, J; Dumur, F; Tehfe, M A; Morlet-Savary, F; Graff, B; Gigmes, D; Fouassier, J P; Lalevee, J, Prog. Polym. Sci., 4, 32-66, 2014.

3

Mechanisms of UV Stabilization 3.1 ABSORPTION, REFLECTION, AND REFRACTION Section 2.2 includes a discussion of two fundamental laws Grotthus-Draper’s principle and the second law of photochemistry and also gives the details of Beer-Lambert’s law. These are fundamental principles for this discussion. According to the Beer-Lambert’s law (subchapter 2.5), absorption of radiation depends on: • the intensity of the incident beam • path length • the concentration of absorbing species (chromophores) • extinction coefficient Figures 2.5 and 2.6 also show that the Beer-Lambert’s law is designed for monochromatic light and its absorption increases with a decrease in radiation wavelength. Finally, equation 2.6 gives the method of calculation of the combined intensity of radiation of polychromic radiation, which is the usual case of exposure of real samples. This is undoubtedly a good starting point, which can be further developed to answer pertinent questions. This part of the mechanism can be described simply, as follows: “Potential stabilizing materials are expected to reflect, absorb, or refract UV radiation without emission of radiation wavelengths, which may be harmful to the protected materials.” This raises many practical issues, as follows: • effect of material mixtures • cross-section of absorption • effect of dispersion • the action of organic absorbers • the action of inorganic particulates • attenuation of radiation throughout the cross-section of the sample • surface ablation • impact of particle size • conditions of reflection • conditions of refraction • influence of refracted and absorbed radiation (see the next section) The above topics describe practical aspects of mechanisms of absorption, reflection, and refraction, and they are discussed below in the above order.

38

3.1 Absorption, reflection, and refraction

UV absorber or screener does not exist in polymeric material alone, but it is dispersed within the matrix of the material to be protected. It is therefore pertinent that there is a competition for incoming radiation between UV absorber and other components of the mixture under study. The nature of this competition can be best explained by the following equation:1 A = ( a m c m + a a c a )b

[3.1]

where: A am aa cm ca b

absorbance at a particular wavelength absorptivity of matrix absorptivity of the absorber concentration of matrix concentration of the absorber thickness of the measured sample

This equation helps us to realize that both matrix and absorber compete for absorption of radiation. If we assume that A and b are unities and that the absorptivity of the UV absorber is 100 times higher than that of the matrix, then both matrix and absorber will absorb almost the same amounts of radiation if absorber concentration in the matrix is 1%. This shows that we need absorbers having much higher absorptivities than that of the matrix, but there will always be some residual radiation which will be absorbed by the matrix. This is the reason that the matrix cannot be completely protected by UV absorbers or UV Figure 3.1. Absorbance of polyimide film containing screeners added to the matrix. different concentrations of single-walled carbon nanoIn protection of wood, a combination tubes. [Data from Smith, J G; Connell, J W; Watson, K of organic and inorganic UV absorbers was A; Danehy, P M, Polymer, 46, 2276-84, 2005.] employed.2 The organic UV absorber filtered mainly UVB radiation (290 to 320 nm) whereas inorganic UV absorber mostly blocked UVA radiation (320 to 400 nm) and some UVB radiation.2 Figure 3.1 shows that absorbance increases with increased addition of carbon nanotubes because they play the role of radiation screener. It should be noted that the absorbance of nanotubes at a concentration of 0.08 wt% (the highest concentration on the graph) was only about 10 times larger than absorbance of the polymer. Comparing data from Figure 3.1 with the above example of absorptivities of matrix and absorber, SWNT in this example had absorptivity 12,500 times larger than polyimide, but polyimide containing 1 wt% of SWNT (it would be a very large concentration of such screener) still absorbs about 1% of incoming radiation.

Mechanisms of UV Stabilization

39

This analysis also shows that the use of UV absorbers and screeners gives limited protection to polymers, especially those having strong chromophoric groups (strong absorption in the UV range). The equation 3.1 can be generalized for more complex mixtures and different wavelengths of radiation, as follows:3 p

a ( λ i ) = ε 1 ( λ i )c 1j l + … + ε p ( λ i )c pj l =

 ε k ( λi )c kj l

[3.2]

k=1

where: a λ i j k p ε l c

absorbance wavelength index for the wavelength index for the number of components index for the molar extinction coefficient index for the number of components molar extinction coefficient sample thickness (or pathway length) concentration of components

This equation produces a sequence of equations for various wavelengths. These are fundamental equations used in so-called chemometrics, which is a subdiscipline of chemistry involved in the application of statistical and mathematical methods to problem-solving in chemistry (in this case, helping to collect maximum information from optical data in the application to photophysics).3 Absorption cross-section is a useful term because it helps to relate radiation intensity and absorption to the concentration of molecules:4 ln [ I 0 ( λ ) ⁄ I ( λ ) ] σ ( λ ) = -----------------------------------lC where:

σ(λ) I0 I l C

[3.3]

absorption cross-section in cm2 per molecule at a given wavelength, λ incoming radiation transmitted radiation optical path concentration in molecule cm-3

Absorption cross-section is very useful in comparison of data from different experiments because it helps to normalize conditions of experiments. Data give a perfect understanding of the effects of different wavelengths on a particular compound. One of the expectations of photochemical studies is that the radiation intensity corresponds to the photochemical change. Figure 3.2 shows the results of an experiment in which hydrogen peroxide concentration and UV radiation intensity varied and their effect on the kinetics of degradation of methyl tert-butyl ether was studied. In this simple experiment, a good linear relationship was obtained between supplied energy and rate of reaction, even though the concentration of hydroperoxide also varied. In more complex studies of materials containing a mixture of various products (especially polymers), there is always a danger that increased intensity (above solar radiation) may change reaction

40

Figure 3.2. Rate constant of reaction between radicals formed from peroxide and methyl tert-butyl ether at different average radiation intensities. [Data from Zang, Y; Farnood, R, Chem. Eng. Sci., 60, 1641-48, 2005.]

3.1 Absorption, reflection, and refraction

Figure 3.3. Absorbance vs. SWNT concentration in DMF. [Data from Kashiwagi, T; Fagan, J; Douglas, J F; Yamamoto, K; Heckert, A N; Leigh, S D; Obrzut, J; Du, F; Lin-Gibson, S; Mu, M; Winey, K I; Haggenmueller, R, Polymer, 48, 4855-66, 2007.]

kinetics and mechanisms. It is therefore still necessary to use a similar experiment to the one presented in Figure 3.2 to check the validity of the experiment. Carbon nanotubes are good UV radiation screeners; it was thus surprising to find that an increase in their concentration caused a reduction of absorbance (Figure 3.3). Further analysis of the phenomenon indicated that this reduction was caused by problems with their dispersion (Figure 3.4). Increased amounts of SWNT were more difficult to disperse, and therefore absorption suffered. This indicates that in stabilization processes, good distribution of stabilizer Figure 3.4. Absorbance vs. relative dispersion index of has a powerful influence on performance. SWNT in DMF. [Data from Kashiwagi, T; Fagan, J; Douglas, J F; Yamamoto, K; Heckert, A N; Leigh, S D; In the case of organic absorbers, dispersion Obrzut, J; Du, F; Lin-Gibson, S; Mu, M; Winey, K I; primarily depends on compatibility Haggenmueller, R, Polymer, 48, 4855-66, 2007.] between the stabilizer and other components of the formulation, but it also depends on technological processes of dispersion. Considering that stabilizers are used in small quantities, predispersion is always advisable.

Mechanisms of UV Stabilization

41

Good dispersion of inorganic stabilizers is even more difficult to achieve because it is complicated by properties of inorganic stabilizer (agglomerate formation, crystallinity, hardness, particle size, etc.), compatibility issues (e.g., acid/base interaction, polarity, etc.), and process conditions (intensity of mixing, mixing schedule, etc.).7 A sound process has to be developed to maximize the effect of stabilizer addition. Transmission of UV radiation through the sample is affected by absorption. Several quantities can be determined to evaluate the optical density of a material with and without stabilizer(s). These include: The mean free path represents the average distance between two successive interactions of photons in which the intensity of the incident photon beam is reduced by the factor of 1/e. This can be estimated using the value of the linear attenuation coefficient:8 1 MFP = --μ where:

μ

[3.4]

linear attenuation coefficient

The following relation represents the half-value thickness in which the intensity of the primary photon beam is reduced by half:8 ln 2 HVT = -------μ

[3.5]

During the processes of radiation passing through the material, the stabilizer may be partially rendered inactive (see more on this subject in Chapter 5) and matrix laden with degradation products, which frequently change matrix absorption and vulnerabilities to UV exposure. These two processes will make stabilization considerably less efficient. In some cases, stabilization is so ineffective that the exposure of the material to UV radiation causes processes of ablation. Ablation data can be analyzed using the Beer-Lambert law in the following form:9 1 F h = --- ln  ------ α  F T where:

α F FT

[3.6]

effective absorption/extinction coefficient flux of incoming radiation threshold fluence

Such processes are frequently observed when radiation fluence is too extensive for material to be able to prevent extensive damage (e.g., laser ablation). The effect of the organic absorber can be predicted from equation [3.1], but the effects of screener (inorganic particles) are more difficult to predict because they depend not only on particle size and other physical properties of screener but also on the ability to disperse agglomerates. Mie’s theory is usually used for understanding properties of screeners:10

42

3.1 Absorption, reflection, and refraction 2

 πr Qa ( m, X )n ( r ) dr σ = -------------------------------------------------3  ρ4 ⁄ 3πr n ( r ) dr where:

σ r m X n(r) ρ

[3.7]

specific attenuation cross-section particle radius complex refractive index, which is function of wavelength, λ size parameter, X = 2πr/λ size distribution particle density

If we take titanium dioxide as an example of the effect of particle size on wavelength absorption, we will observe the following:7 The particle size has a significant influence on the performance of titanium dioxide, both as a pigment and as a UV screener (absorber). For the pigment to have maximum opacity, the particle diameter must be equal to half of the wavelength (for a blue/green light to which the eye is most sensitive, the average wavelength is 460 nm; thus a particle diameter of 230 nm gives the maximum opacity). The color of the matrix (binder) has an influence here as well, and titanium dioxide must compenFigure 3.5. Absorbance of UV radiation at 290 nm vs. sate. For this reason, some grades of titaparticle size of ZnO. [Adapted, by permission, from nium dioxide are tailored to specific Goh, E G; Xu, X; McCormick, P G, Scripta Mater., conditions, and some are used to eliminate a 78-79, 49-52, 2014.] yellow undertone. This is done by the choice of particle size. Commercial grades have particle sizes in a range from 200 to 300 nm. The amount of titanium dioxide is also crucial. If too little titanium dioxide is added, the distance between particles is too large, and there is not enough opacity. If the amount is too high, it results in lower efficiency due to a particle crowding effect which causes particles to interfere with each other scattering efficiency. Because the optimum light scattering of titanium pigments occurs when the particle diameter is 0.23 μm, most pigments are manufactured to have the majority of particles closest to that in a range from 0.15 to 0.3 μm, depending on the application and the undertone required.7 Ultrafine grades are the exception. They typically have particle sizes in a range from 0.015 to 0.035 μm and, because of their small particle size, they are transparent to visible light but absorb in the UV range.7 The best grades for sunscreens (from the point of view of radiation absorption) have a particle diameter of 10 nm. At this particle size, they produce transparent looking sunscreens with excellent UV absorption qualities. Figure 3.5 shows that the UV absorbance of ZnO particles increases with increasing size in the range of 15-40 nm.11 The particles greater than 70 nm become opaque to UV

Mechanisms of UV Stabilization

43

radiation, whereas for particles greater than 70 nm, the absorbance decreases when size increases because of a lower particle density.10 The reduction of particle size to less than 40 nm has a detrimental effect on the UVA/UVB absorbance ratio.11 Both organic and inorganic absorbers are able to absorb energy. The fate of this energy is discussed in the next section. Inorganic particles may also reflect and refract incoming radiation. Reflection of radiation which occurs on the material surface is the most desired outcome because energy is reflected into the surrounding space and therefore it does not affect the material. If energy is reflected internally from the surface of an inorganic particle into, for example, a polymeric matrix, then this energy can be utilized for photochemical processes because radiation reflection does not affect its energy. Refraction occurs when a radiation wave travels from a medium having a given refractive index to a medium with another refractive index at an angle. At the boundary between the media, the wave's phase velocity is altered, usually causing a change in direction. Its wavelength increases or decreases, but its frequency remains constant. The change of direction depends on refractive indices according to Snell’s law: n sin θ ------------1- = ----2sin θ 2 n1 where:

θ1, θ2 n1, n2

[3.8]

angles of incidence and refraction indices of refraction

Refracted radiation retains some energy, but the energy and wavelength of refracted radiation are different than that of incident radiation and inversely proportional to the ratio of refraction indices: n λ1 ----- = ----2λ2 n1 where:

λ1 , λ 2

[3.9]

incoming and outgoing wavelength of radiation

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

Smith, J G; Connell, J W; Watson, K A; Danehy, P M, Polymer, 46, 2276-84, 2005. Nair, S; Nagarajappa, G B; Pandey, K K, J. Photochem. Photobiol. B: Biol., 183, 1-10, 2018. Hopke, P K, Anal. Chim. Acta, 500, 365-77, 2003. Horowitz, A; Meller, R; Moortgat, G K, J. Photochem. Photobiol A: Chem., 146, 19-27, 2001. Zang, Y; Farnood, R, Chem. Eng. Sci., 60, 1641-48, 2005. Kashiwagi, T; Fagan, J; Douglas, J F; Yamamoto, K; Heckert, A N; Leigh, S D; Obrzut, J; Du, F; Lin-Gibson, S; Mu, M; Winey, K I; Haggenmueller, R, Polymer, 48, 4855-66, 2007. Wypych, G, Handbook of Fillers, 4th Edition, ChemTec Publishing, Toronto, 2016. Kurudirek, M; Turkmen, I; Ozdemir, Y, Radiation Phys. Chem., 78, 751-59, 2009. Low, D K Y; Schmidt, M J J; Li, L, Appl. Surface Sci., 168, 170-74, 2000. Liousse, C; Cachier, H; Jennings, S G, Atmospheric Environ., 27A, 8, 1203-11, 1993. Goh, E G; Xu, X; McCormick, P G, Scripta Mater., 78-79, 49-52, 2014.

44

3.2 Energy dissipation

Figure 3.6. Energy dissipation by benzophenone UV absorber. [Adapted, by permission, from Schwalm, R, UV Coatings, Elsevier, 2007, pp 206-51.]

3.2 ENERGY DISSIPATION Excited-state intramolecular proton transfer, ESIPT, is commonly accepted as a mechanism responsible for energy dissipation in the organic UV absorbers. In general, it is a photo-induced prototropy between the enol and keto forms of an organic molecule possessing an intramolecular hydrogen bond.1 In the ground state, typical ESIPT molecules preferentially adopt the enol form, which is better stabilized by the intramolecular hydrogen-bonding.1 Upon photoexcitation, however, fast proton transfer reaction from the excited enol occurs to create the excited keto tautomer in a subpicosecond time scale.1 After decaying to the ground state, the keto-form reverts to the original enol-form via reverse proton transfer.1 Figure 3.6 shows some details of the mechanism of energy dissipation by a benzophenone stabilizer. Absorption of a photon of the UV energy by the absorber molecule at its ground state, S0, increases its level of energy to the excited state, S1. The energy dissipation from the excited state can follow different pathways of direct radiation-less deactivation, deactivation after intersystem crossing or tautomerization − all processes leading to the energy release in the form of heat without the formation of any radical species.2 Figure 3.7 shows that benzotriazole undergoes similar changes in energy dissipation (see also the mechanism of energy dissipation by Tinuvin P in Figure 2.18). Figure 3.8 shows a more elaborate mechanism of energy dissipation by triazine UV stabilizer. Simi-

Figure 3.7. Energy dissipation by benzotriazole UV absorber. [Adapted, by permission, from Schwalm, R, UV Coatings, Elsevier, 2007, pp 206-51.]

Mechanisms of UV Stabilization

45

Figure 3.8. Energy dissipation by triazine UV absorber. [Adapted, by permission, from Hayoz, P; Peter, W; Rogez, D, Prog. Org. Coatings, 48, 297-309, 2003.]

lar to the two other cases, radiation is absorbed, and the excited molecule of the stabilizer is elevated to a higher energy level. The energy is then dissipated by either fluorescence, radiationless deactivation, or proton transfer. Figure 3.9 shows the mechanism of energy conversion by particles of titanium dioxide.4 When titanium dioxide is exposed to UV radiation, it absorbs energy, and an electron is promoted from the valence band to the conduction band with concurrent formation of a hole, h+.4 -

TiO 2 → e + h

+

The separation of these two levels corresponds to the band gap energy, Ebg (the band gap energy of anatase TiO2 corresponds to a wavelength of 387 nm, in rutile to 405 nm, and ZnO to 384 nm).4 Migration of both the electron and the hole to the particle surface is then possible. From here, several processes may take place: hole-electron recombination, oxidation of an electron donor by the hole, or reduction of an electron acceptor by the electron.4 Also, water on the surface may react with the hole to form hydroxyl radicals, which are highly reactive species, and the electron may react with oxygen to produce the superoxide radical and then go on to form hydrogen peroxide via disproportionation.4 This scheme shows less safe energy conversions than described for the organic UV absorbers. Manufacturers of titanium dioxide and other photochemically active screeners

46

3.2 Energy dissipation

Figure 3.9. Photoreaction of titanium dioxide. [Adapted, by permission, from Cantrell, A; McGarvey, D J; Truscott, T G, Comprehensive Series in Photosciences, Vol. 3, Ch. 26, Photochemical and photophysical properties of sunscreens, Elsevier, 2001, pp 495-519.]

change surface properties of inorganic UV screeners by coating and doping to reduce the catalytic effect of their surfaces, which cause photodegradation of organic matrix in its vicinity, but all these modifications are not sufficient to completely prevent degradation (it can be only reduced). For this reason, these pigments are only used in the protection of materials which accept continuous renewal of surface (e.g., controlled surface chalking) or in the cases when the product protects another surface on which it has been coated (e.g., sunscreens). Details of action of titanium dioxide and other photochemically active pigments and nanoparticles can be found in a specialized monographic source.5 Commercial titanium dioxide nanoparticles were modified by pimelic acid to improve its interaction with isotactic polypropylene matrix.6 The modified particles dissipated two times more energy than the particles of non-modified titanium dioxide.6 Figure 3.10 shows the presence of a coating on the titanium dioxide surface.

Figure 3.10. TEM images of the titanium dioxide particles with pimelic acid coating. [Adapted, by permission, from Gonzalez-Calderon, J A; López-Esparza, R; Saldivar-Guerrero, R; Almendarez-Camarillo, A, Thermochim. Acta, 664, 48-56, 2018.]

Mechanisms of UV Stabilization

47

It is interesting and essential for the description of the mechanism of action of carbon black what happens with absorbed energy, but no such information is available from credible sources. It is known that carbon blacks having smaller particle sizes have better absorption and give better protection to polymers.7 Also, quinoid structures present on the surface of carbon black seem to participate in radical scavenging.8 Since carbon black gives long-term protection, it seems probable that if it retains its stabilizing properties for a long time that it is able to dispose of the absorbed energy without being affected. Also, mechanisms attributed to inorganic screeners do not play a role in the case of carbon black. REFERENCES 1 2 3 4 5 6 7 8

Seo, J; Kim, S; Lee, Y-S; Kwon, O-H; Park, K H; Choi, S Y; Chung, Y K; Jang, D-J; Park, S Y, J. Photochem. Photobiol. A: Chem., 191, 51-58, 2007. Schwalm, R, UV Coatings, Elsevier, 2007, pp 206-51. Hayoz, P; Peter, W; Rogez, D, Prog. Org. Coatings, 48, 297-309, 2003. Cantrell, A; McGarvey, D J; Truscott, T G, Comprehensive Series in Photosciences, Vol. 3, Ch. 26, Photochemical and photophysical properties of sunscreens, Elsevier, 2001, pp 495-519. Wypych, G, Handbook of Fillers, 4th Edition, ChemTec Publishing, Toronto, 2016. Gonzalez-Calderon, J A; López-Esparza, R; Saldivar-Guerrero, R; Almendarez-Camarillo, A, Thermochim. Acta, 664, 48-56, 2018. Suits, L D; Hsuan, Y G, Geotextiles Geomembranes, 21, 111-22, 2003. Pospisil, J, Polym. Deg. Stab., 34, 85-109, 1991.

48

3.3 Radical deactivation and retardation of propagation of reactions chain

Figure 3.11. Denisov cycle. [Adapted, by permission, from Haillant, O, Polym. Deg. Stab., 93, 1793-98, 2008.]

3.3 RADICAL DEACTIVATION AND RETARDATION OF PROPAGATION OF REACTIONS CHAIN The most well-known case of the radical deactivation (transformation or scavenging) is expressed by the so-called Denisov cycle (Figure 3.11).1 This easy to grasp scheme of conversions amplifies the most critical message for the stabilization with HAS. The nitroxyl radical essential in scavenging (deactivation) of radicalized polymer fragments (alkyl radicals) is produced from HAS by a simple reaction with peroxy radical, hydroperoxide, or singlet oxygen, and then it can be recovered to participate again in the next acts of scavenging. This scheme also stipulates that HASs begin their action, not before, but after some degradation already occurred, because they need some radicals to become activated. Some HAS are easily neutralized by acids or interact with acidic components of Table 3.1. pKa values of some HAS the formulation (acid/base interaction) which Commercial name renders them ineffective in some formulaStructure pKa tions (e.g., PVC). Table 3.1 shows the pKa >N-H 8-9.7 Tinuvin 770 constants of some HAS. The selection of par>N-CH3 7.5-8.2 Tinuvin 292 ticular HAS depends on properties of the sys>N-OR 4.2 Tinuvin 123 tem. The general rule stipulates selection of basic HAS for basic compositions and vice >N-C(O)CH3 2 Tinuvin 440 versa. Deactivation of HAS is due to acid impurities (atmospheric pollutants) of natural origin or arising from anthropogenic activities (nitrogen and sulfur oxides with high permeability into the polymer matrix), mineral acids in deposits such as acid rain or dew, acid compounds formed during the degradation of polymers (e.g., HCl from PVC), or additives (acid transformation products of thiosynergists in PO or dialkylthioglycolates in PVC, HBr from brominated flame retardants).2 HAS derivatives of low basicity can be used to reduce their deactivation by a sulfur-containing stabilizers.3 Basic HAS is also deactivated by acid fillers and acid cure catalysts in coatings.2 NOR-HAS has low pKa and therefore performs better in acidic environment,

Mechanisms of UV Stabilization

49

but it loses this advantage when it is transformed to nitroxides and forms salts (pKa = 7.49.6).2 It should be taken into consideration that HAS participates in peroxide and hydroperoxide decomposition which is discussed in Section 3.7. Radical deactivation is also considered to be a part of the mechanism of other UV stabilizers, such as • ferulic acid4-6 • phenoxy antioxidants7 • flavonoids8 • vitamins C and E and β-carotene9 • some natural extracts10 • carbon black11-13 • copper stearate14,15 Ferulic acid (4-hydroxy-3-methoxycinnamic acid) acts as scavenger of hydroxyl and peroxyl radicals and superoxide anions, inhibiting propagation of lipid peroxidation chain reaction.4 It also reduces the UVB-induced erythema, because of its high effectiveness in scavenging nitric oxide, and provides a high degree of skin protection acting as UVabsorber screen.4 Extract from rice bran has substantial concentrations of ferulic acid and tocopherols (vitamin E), which are both radical scavengers.5 Ferulic acid is used as a photoprotective ingredient in many skin lotions and sunscreens.6 Phenol-type antioxidants are known as radical scavengers. They also show autohomosynegism, resulting from cooperation between the parent phenolic antioxidant and its transformation products (benzoquinone and quinone methide) having a modified radical scavenging activity, which has an antioxidant supporting effect.7 Radical scavenging activity of flavonoids from Culcitium reflexum leaves extract was confirmed by bleaching of the stable 1,1- diphenyl-2-picrylhydrazyl radical (DPPH test) and peroxidation induced by the water-soluble radical initiator 2,2-azobis(2-amidinopropane) hydrochloride on mixed dipalmitoylphosphatidyl choline/linoleic acid unilamellar vesicles (LP-LUV test).8 Vitamins C and E and β-carotene are other natural products having radical scavenging capabilities.9 Various extracts containing tocopherols and other antioxidants were found to have radical scavenging activity, which was confirmed by the DPPH test.10 Carbon black is frequently mentioned11-14 as participating in radical scavenging, but no credible mechanism responsible for this action was ever proposed. Quinoid structures present on the surface of carbon black are thought to participate in radical scavenging as mentioned before. Radicals formed by Norrish type I cleavage of ketone groups, present in the polymers as anomalous structures, react with the copper ion to form inactive groups such as terminal carbon-carbon double bonds.14 If the chain propagating peroxy radical is scavenged before it abstracts a hydrogen atom from the polymer, stabilization occurs.16 This is accomplished by free radical scavengers.16 Free radical scavengers are designated as chain-breaking or primary antioxidants.16

50

3.3 Radical deactivation and retardation of propagation of reactions chain

REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Haillant, O, Polym. Deg. Stab., 93, 1793-98, 2008. Pospisil, J; Pilar, J; Nespurek, S, J. Vinyl Additive Technol., 13, 119-32, 2007. Gijsman, P, Polym. Deg. Stab., 145, 2-10, 2017. Rossi, C; Schoubben, A; Ricci, M; Perioli, L; Ambrogia, V; Latterini, L; Aloisi, G G; Rossi, A, Intl. J. Pharm., 295, 47-55, 2005. Santa-María, C; Revilla, E; Miramontes, E; Bautista, J; García-Martínez, A; Romero, E; Carballo, M; Parrado, J, Food Chem. Technol., 48, 83-88, 2010. Kullavanijaya, P; Lim, H W, J. Am. Acad. Dermatol., 52, 6, 937-58, 2005. Pospisil, J, Polym. Deg. Stab., 39, 103-15, 1993. Aquino, R; Morelli, S; Tomaino, A; Pellegrino, M; Saija, A; Grumetto, L; Puglia, C; Ventura, D; Bonina, F, J. Ethnopharmacol., 79, 2, 183-91, 2002. Andersson, C-M; Hallberg, A; Högberg, T, Adv. Drug Res., 28, 65-180, 1996. Contini, M; Baccelloni, S; Massantini, R, Anelli, G, Food Chem., 110, 3, 659-69, 2008. Allen, N S; Chirinos-Padron, A; Henman, T J, Polym. Deg. Stab., 13, 1, 31-76, 1985. Chirinos-Padrón, A J, J. Photochem. Photobiol. A: Chem., 49, 1-2, 1-39, 1989. Wiles, D M; Carlsson, D J, Polym. Deg. Stab., 3, 1, 61-72, 1980. Osawa, Z; Kobayashi, K; Kayano, E, Polym. Deg. Stab., 11, 1, 63-73, 1985. Osawa, Z, Polym. Deg. Stab., 20, 3-4, 203-36, 1988. Gugumus, F, Polym. Deg. Stab., 39, 1, 117-35, 1993.

Mechanisms of UV Stabilization

51

3.4 SINGLET OXYGEN QUENCHING Many materials do not absorb in the ultraviolet range; therefore they cannot undergo direct degradation process, but some admixtures may absorb UV radiation and cause indirect degradation of these materials:1

Sensitizer Sensitizer* 1

O2

+

light +

3

Sensitizer*

Quencher

Sensitizer*

+

1

O2 3

O2

O2

Quencher

+

+

Sensitizer Quencher*

Sensitizer

+

Quencher*

The schemes of reactions show that sensitizer may absorb radiation energy and get excited; excited sensitizer may react with triplet oxygen (a common form of oxygen in air) and produce singlet oxygen (high energy, reactive form of oxygen). Quencher may react with singlet oxygen and return it to the triplet state of oxygen. Also, the quencher may take energy from an excited sensitizer. Many different compounds can be sensitizers, including2 polynuclear aromatics, carbonyl groups, reaction products of antioxidants, dyes and pigments, etc. For material durability, quenching is an important process because it stops chains of reaction. Metal (especially nickel) chelates are commonly known quenchers of singlet oxygen.3 They perform with 1,4-polybutadiene3 and polyolefins and were useful in the protection of acid dyes.4 Nickel complexes of 4-benzoyloxybenzenesulfonic acid and its derivatives can be applied as effective stabilizers against the fading of indicator dyes for printing and imaging systems.4 Singlet oxygen is able to react with polymers having double bonds, according to the following reaction scheme:5 1

CH2CH CH

O2

CH CHCH OOH

Hydroperoxides are formed in this reaction, and they may begin a chain of photodegradation reactions. These reactions are common in polyolefins, which benefit from excited state quenching.6 Phenylformamidine light stabilizer is also considered to be capable of singlet oxygen quenching.7 Stabilizer was tested in polypropylene with positive results.7 Quenchers dissipate energy as heat, which is the most harmless outcome of energy dissipation.8 Water-soluble chitosan derivatives were obtained by Maillard reaction of glucosamine and low to medium molecular weight chitosan.9 A six-times reduction of the

52

3.4 Singlet oxygen quenching

molecular weight of water-soluble chitosan derivatives was determined, indicating the breakdown of the polysaccharide chain during the Maillard reaction.9 The polysaccharides quenched singlet molecular oxygen (1O2), with rate quenching constants correlating with the deacetylation degree of the samples, suggesting the important role of amino groups (−NH2) in the deactivation of 1O2.9 REFERENCES 1 2 3 4 5 6 7 8 9

Son, P N, Polym. Deg. Stab., 2, 295-308, 1980. Gugumus, F, Polym. Deg. Stab., 34, 205-41, 1991. Abdel-Bary, E M; Sarhan, A A; Abdel-Razik, E A, Polym. Deg. Stab., 18, 2, 145-55, 1987. Oda, H, Dyes Pigments, 48, 151-57, 2001. Scott, G, J. Photochem., 25, 83-90, 1984. Chirinos-Padrón, A J, J. Photochem. Photobiol. A: Chem., 49, 1-2, 1-39, 1989. Jiang-Qing, P; Cin, Q, Polym. Deg. Stab., 37, 195-99, 1992. Candlin, J P, Comprehensive Analytical Chemistry, Vol. 53, Ch. 3, 65-119, Elsevier, 2008. Vanden Braber, N L; Díaz Vergara, L I; Morán Vieyra, F E; Borsarelli, C D; Montenegro, M A, Int. J. Biol. Macromol., 102, 200-7, 2017.

Mechanisms of UV Stabilization

53

3.5 DEGREE OF A HINDRANCE Steric hindrance is one of the essential features of the design of effective stabilizer. It is important for the performance of several groups of stabilizers, such as: • HAS • phosphites • phenolic antioxidants • amine antioxidants Hindrance of the >NH group affects the physical quenching ability of HAS. Secondary HAS such as >NH do not quench 1O2 in practical applications.1 Nitroxides are also poor quenchers of 1O2 (the order of quenching is about 9.0x105 L mol-1 s-1).1 Model tertiary piperidine >NCH3 has a stronger quenching effect (~5.3x105 L mol-1 s-1) than the secondary amine >NH. The quenching can be attributed to oxidation (chemical quenching) at the N−CH3 group.1 1O2 quenching has only a small contribution to HAS performance in polyolefins, because a concentration of about 10% of the nitroxide in the solid matrix would be necessary to quench 50% of 1O2 at 1-nm collision quenching.1 Steric hindrance also explains why there are four methyl groups attached to two carbons next to nitrogen. It is because they better stabilize nitroxyl radical (than if there were for example 3 methyl groups).2 The stable radical can more efficiently trap alkyl radicals. Also, trans forms of HAS are better stabilizers than cis in polypropylene applications.2 Quenching occurs by long energy transfer (e.g., dipole-dipole interaction) and contact energy transfer.4 In long energy transfer, chromophore and quenchers are at distance of >5 nm, whereas in contact energy transfer they are 1-1.5 nm apart.4 Increased steric hindrance around phosphorous atoms in phosphites increases their hydrolytic stability.5,6 Steric hindrance by the bulky t-butyl groups stabilizes a phenoxy radical after phenolic antioxidant reacts with radicals:3

O

OH (H3C)3C

(H3C)3C

C(CH3)3

C(CH3)3

R CH3

CH3

and therefore cannot attack polymer chains.3,7 Steric hindrance of ortho-position to the hydroxyl group affects the radical reaction rate.8 Reaction rate with radicals increases with smaller steric hindrance, while the radical capturing capability against 1 mole of antioxidant decreases.8 This means that the higher steric hindrance to ortho-position restricts the coupling of the same homologs instead of the reaction with one more peroxy radical.8 One of the most common photocatalytic discolorations occurs when the antioxidant BHT (2,6-di-t-butyl-4-methylphenol) reacts with photogenerated hydroxide radicals to

54

3.5 Degree of a hindrance

produce long-lived radical species.9 These radical species couple to form stilbenequinone derivatives, which rapidly decompose during the continuation of UV exposure to yellow monocyclic products.9 Hindered amine stabilizers (HAS) exasperate this problem since amines create basic conditions that favor the production of the initial BHT radical complexes.9 Steric hindrance also prevents dimerization of the partially oxidized antioxidant monomer.9 Considering the photoluminescence behavior of the quinoid species, it is unclear whether the photoluminescence is affected by a direct quenching effect of the quinoid structures formed upon the oxidation-induced conversion of sterically hindered phenolic antioxidants.10 Some aromatic amines formed by oxidative coupling may also act as secondary antioxidants.11 Their reaction with peroxy radicals is controlled by steric hindrance.11 Nanosilica prepared by the co-condensation of tetraethoxysilane and N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane has irregularly shaped particles due to the steric hindrance of organic groups participating in the formation of Si-O-Si three-dimension network structure.12 It has both thermooxidative and UV stabilization effects on high-density polyethylene.12 The amino groups were distributed both on the surface and inside the nanoparticles.12 The amino groups on co-condensed nanosilica consumed carbonyl species and peroxides and generated hydroxylamines and nitroxide radicals which were able to eliminate free radicals thus restrained the degradation of high-density polyethylene.12 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12

Pospisil, J; Pilar, J; Nespurek, S, J. Vinyl Additive Technol., 13, 119-32, 2007. Son, P N, Polym. Deg. Stab., 2, 295-308, 1980. Candlin, J P, Comprehensive Analytical Chemistry, Vol. 53, Ch. 3, 65-119, Elsevier, 2008. Yousif, E; Hasan, A, J. Taibah Uni. Sci., 9, 4, 421-48, 2015. Bauer, I; Koerner, S; Pawelke, B; Al-Malaika, S; Habicher, W D, Polym. Deg. Stab., 42, 175-86, 1998. Minagawa, M, Polym. Deg. Stab., 25, 121-41, 1989. Pospisil, J, Polym. Deg. Stab., 20, 181-202, 1988. Lee, K-W; Hong, Z; Piao, F; Kim, Y-W; Chung, K-W, J. Ind. Eng. Chem., 16, 419-24, 2010. Jernakoff, P; Maynard, R B; Niedenzu, PM; Redkoles, L, Antec 2006, 119-26. Grabmayer, K; Wallner, G M; Beissmann, S; Braun, U; Steffen, D; Nitsche, D; Roeder, B; Buchberger, W; Lang, R W, Polym. Deg. Stab., 109, 40-49, 2014. Allen, N S; Chirinis-Padron, A, Polym. Deg. Stab., 13, 31-76, 1986. Lu, M; Liu, P; Zhang, S; Yuan, W; Yang, M, Polym. Deg. Stab., 154, 137-48, 2018.

Mechanisms of UV Stabilization

55

3.6 ANTIOXIDATION Scott1 divided antioxidant action into preventive and chain breaking and proposed the following general mechanism of their action (Figure 3.12).1 The chain-breaking mechanism (CB-A) involves removal of an alkyl radical by oxidation process with formation of a carbonium ion, and the CB-D mechanism involves reduction of alkylperoxyl radical with the formation of hydroperoxide, which is then decomposed by heat, light, and in the presence of metal ions.1,2

Figure 3.12. Mechanisms of antioxidation. [Adapted, by permission, from Scott, G, Polym. Deg. Stab., 10, 97125, 1985.]

Hindered phenols are a good example of the CB-D mechanism:2

R' X

R' OH

R''

+

OOR

X

O R''

+

HOOR

56

3.6 Antioxidation

Substituent groups affect the reduction of energy of the transition state and steric hindrance as discussed in the previous section. Alkylperoxyl radical scavengers are most efficient in the presence of excess oxygen.2 The reaction of quinones with alkyl radicals are an excellent example of the CB-A mechanism:2 X O

X O

X'

+

R

O

OR X'

Substituents (X, X’) have a favorable influence if they have electron-withdrawing or delocalizing character. CB-A antioxidants operate at optimum efficiency in oxygen deficiency or at high initiation rates.2 Hydroquinone can act through both CB-A and CB-D mechanisms.2 Figure 3.13 shows the operation of preservation and chain-breaking mechanisms with α-tocopherol used as an antioxidant. HAS can operate according to both mechanisms (preservation and chain breaking). Figure 3.14 shows the results of protection of PP film stabilized with different concentrations of HAS (Chimassorb 944) and exposed to long-term oxidation in a 135oC oven.3 Time to 0.2 absorbance by carbonyl increases linearly with an increased amount of HAS. To perform, HAS must first be converted into nitroxyl radical and one of the mechanisms of its formation includes direct reaction with oxygen.1 The mechanisms of chain-breaking with HAS participation abound. The most common is the case of reaction with peroxides, leading to the formation of nitroxyl radical and hydroperoxide.1

Figure 3.13. Antioxidative processes in the presence of α-tocopherol. [Adapted, by permission from Costa, L; Bracco, P, UHMWPE Biomaterials Handbook, 2nd Ed., Elsevier, 2009, pp 309-323.]

Mechanisms of UV Stabilization

Figure 3.14. Time to 0.2 absorbance by carbonyl groups formed in PP after exposure to 135oC oven vs. HAS concentration. [Adapted, by permission, from Gugumus, F, Polym. Deg. Stab., 44, 299-322, 1994.]

REFERENCES 1 2 3 4 5 6

Scott, G, Polym. Deg. Stab., 10, 97-125, 1985. Allen, N S; Chirinos-Padron, A; Henman, T J, Polym. Deg. Stab., 13, 1, 31-76, 1985. Allen, N S; Chirinis-Padron, A, Polym. Deg. Stab., 13, 31-76, 1986 Costa, L; Bracco, P, UHMWPE Biomaterials Handbook, 2nd Ed., Elsevier, 2009, pp 309-323.] Gugumus, F, Polym. Deg. Stab., 44, 299-322, 1994. Gugumus, F; Lelli, N, Polym. Deg. Stab., 72, 407-21, 2001.

57

58

3.7 Peroxide and hydroperoxide decomposition

3.7 PEROXIDE AND HYDROPEROXIDE DECOMPOSITION It has been shown that degradation mechanisms of thermal and photochemical oxidation are identical, except for the initiation step of the oxidation reaction.1 Thermal oxidation of most polymers (e.g., polyamide, polyolefins, polyvinyls, etc.) occurs predominantly through the homolytic scission of carbon-hydrogen bonds of the methylene groups.1 In the case of photooxidation, radiation with a wavelength lower than 290 nm can initiate the oxidation by the direct cleavage of the weakest bonds in the chain. On exposure to solar radiation, photochemical oxidation occurs if the polymer contains chromophores able to absorb radiation having a wavelength longer than 290 nm. Although solar radiation does not possess sufficient energy to cause direct homolytic scission of the C–N or C–C bonds, the radiation absorption can favor homolytic scission of the carbon-hydrogen bond (similar to thermal degradation), due to impurities, such as catalyst residues, metal ions, and carbonyl or peroxide species formed during high temperature processing. Hydroperoxide formation is the most distinctive reaction characterizing degradative processes. At the same time, it is not used to monitor the progress of degradation processes because the occurrence of simultaneous decomposition processes only permits us to determine the current concentration which is the difference between formation and decomposition. Several reactions describe hydroperoxide decomposition:2

POOH

PO + OH

POOH + PH POOH + POOH POOH + POH

PO + H2O + P PO2 + PO + H2O PO + PO + H2O

The reactions, written for polyethylene, include: monomolecular decomposition, bimolecular decomposition with another molecule of polymer, bimolecular decomposition of two molecules of hydroperoxide, and bimolecular decomposition with one molecule of already decomposed hydroperoxide-containing polymer.2 What is evident is that each act of hydroperoxide decomposition leads to two radicals (formed from one original radical). If this remains unchecked, rapid deterioration occurs. Polyolefins were difficult to stabilize until HAS was discovered. The major strength of HASs is their participation in peroxide decomposition, hydroperoxide decomposition, and carbon radical trapping. The reaction which forms a nitroxyl radical leads to the elimination of singlet oxygen. Both nitroxyl radicals and hydroxylamines formed from them may participate in reactions with peroxides:

Mechanisms of UV Stabilization

59

N O + POOH

N OH + PO2

N OH + PO2

N O + POOH

N OH + POOH N OP + P'O2

N O + PO + H2O N O + P'OOP

The last reaction is especially important because it regenerates the nitroxyl radical while peroxide is also eliminated. The use of HAS is efficient in most polymers. The effect of hydroperoxide formation is given in Figure 3.15. The rate of hydroperoxide formation at the early stages of UV exposure is a critical parameter governing the weatherability of a coating.3 However, once formed, the hydroperoxide photolytic stability appears to be similar, irrespective of the rate of formation.3 It is believed that the rate of hydroperoxide formation is governed by the chemistry of the coating.3 The hydroperoxide photodecomposition rate constant varied over only a small range for the different formulations, and thus, it was not considered to be of significant importance. It is believed to be governed by conditions of exposure (most likely temperature) and the effect of the staFigure 3.15. Relationship between the rate of hydroperbilizing system. oxide formation in polyester/melamine coatings and Figure 3.16 shows the influence of film ablation after 3 years of outdoor exposure in Queensland, Australia. [Adapted, by permission, from HAS on oxygen uptake during UV expoLukey, C A, Prog. Org. Coat., 41, 129-34, 2001.] sure of polyethylene.4 Substantial reduction of oxygen is caused by effective hydroperoxide decomposition by HAS. Phosphites effectively decompose hydroperoxides:5

ROOH + P(OR')3

ROH + O P(OR')3

60

3.7 Peroxide and hydroperoxide decomposition

Figure 3.17. Schematic diagram of autooxidation and stabilization cycles. [Adapted, by permission, from Voigt, W; Todesco, R, Polym. Deg. Stab., 77, 397402, 2002.]

Phosphites act as secondary antioxidants. They reduce the hydroperoxides formed from peroxy radicals to the corresponding Figure 3.16. Oxygen uptake by unstabilized and stabilized polyethylene with 940 ppm HAS versus exposure alcohol by a redox reaction, being simultatime in xenon weathering device. [Data from Gijsman, neously oxidized to the corresponding P; Dozeman, A, Polym. Deg. Stab., 53, 45-50, 1996.] phosphate.6 Combined HAS and phosphite in one molecule were synthesized, and they had even better performance in the decomposition of hydroperoxides.7 This combination also improved hydrolytic stability of phosphites. Figure 3.17 explains roles played by phosphites and phenolic antioxidants in the stabilization of polyolefins. Phenolic antioxidants scavenge alkoxy and peroxy radicals.6 By donating hydrogen radical they neutralize these kinds of radical species which are either formed through the harsh processing conditions in the presence of oxygen or by the cleavage of hydroperoxides.3 Phenolic antioxidants protect polymers during processing as well as in the end application as long-term thermal stabilizers.6 It is known that some PVC stabilizers, including sulfur-containing organotins and maleate type organotins, can also decompose hydroperoxides.8,9 Volatile peroxides are generated during the oxidation of polypropylene.10 They may consist of hydrogen peroxide and hydroperoxides, peroxides, and peroxyacids.10 After an induction period, the rate of release of peroxides was found to increase to a steady value.10 REFERENCES 1 2 3 4 5 6 7 8 9 10

Cerruti, P; Lavorgna, M; Carfagna, C; Nicolais, L, Polymer, 46, 4571-83, 2005. Gugumus, F, Polym. Deg. Stab., 69, 35-45, 2000. Lukey, C A, Prog. Org. Coat., 41, 129-34, 2001. Gijsman, P; Dozeman, A, Polym. Deg. Stab., 53, 45-50, 1996. Schwetlick, K; König, T, Polym. Deg. Stab., 24, 4, 279-87, 1989. Voigt, W; Todesco, R, Polym. Deg. Stab., 77, 397-402, 2002. Bauer, I; Körner, S; Pawelke, B; Al-Malaika, S; Habicher, W D, Polym. Deg. Stab., 62, 1, 175-86, 1998. Wypych, G, PVC Degradation and Stabilization, 3rd Edition, ChemTec Publishing, Toronto, 2020. Wypych, G, PVC Formulary, 3rd Edition, ChemTec Publishing, Toronto, 2020. Butler, C H; Whitmore, P M, Polym. Deg. Stab., 98, 471-73, 2013.

Mechanisms of UV Stabilization

61

3.8 ACID NEUTRALIZATION Hydrotalcite, magnesium–aluminum hydroxycarbonate, is a naturally occurring mineral having chemical composition Mg6Al2(OH)16CO3 4H2O exhibiting a layered crystal structure, which is comprised Figure 3.18. Acid neutralization by hydrotalcite. of positively charged hydroxide layers and [Adapted, by permission, from Kumar, B; Rana, S; interlayers composed of carbonate anions Singh, R P, eXPRESS Polym. Lett., 1, 11, 748-54, 2007.] and water molecules.1 The most common method applied to the preparation of hydrotalcite-like compounds is coprecipitation, which is based on the reaction of a solution containing both metal cations in adequate proportions with an alkaline solution.1 The products obtained by coprecipitation at low supersaturation are usually more crystalline in comparison with those prepared at high supersaturation conditions.1 However, the product crystallinity may be affected by various experimental parameters such as reaction pH and temperature, concentration of used solutions, flow rate during addition of reactants, hydrodynamic conditions in the reactor and/ or post-synthesis operations.1 Hydrotalcite acid-scavenging mechanism is presented in Figure 3.18.2 In addition to hydrotalcite, calcium and zinc stearates are known as popular acid scavengers. Acid scavenging is the primary mechanism of protecting PVC against autocatalytic degradation. Ca/Zn stearates and other compounds able to react with hydrogen chloride were long used as PVC thermal (and some as UV) stabilizers.3,4 But acid neutralization is not only peculiar to PVC but is required by many other polymers. EPDM/hydrotalcite composite was studied for photostability.2 Pristine polymer was more stable, but the presence of hydrotalcite was preferential if the material was to operate exposed to UV radiation in an acidic environment.2 Combination of phosphite and hydrotalcite gives better stability to poly(ethylene terephthalate).5 It should be noticed that phosphites are also acid scavengers, but they are not hydrolytically stable, and most likely hydrotalcite prevents their hydrolysis. Hydrotalcites play the same roles in some polyolefin formulations for outdoor use − they neutralize acids and prevent hydrolysis of phosphites.6 Calcium stearate and zinc stearate are also used in these formulations for acid neutralization.6 One of the reasons to use hydrotalcite in polyolefins is to prevent yellowing caused by magnesium chloride present as a part of a polymerization catalyst.7 Hydrotalcite added to polystyrene formulation did not modify photooxidation mechanisms of composition but had a some effect on the oxidation rate.8 Several combinations of hydrotalcite with other components of the formulation were also used.9-10 In sunscreen, hydrotalcite was intercalated with a UV absorber, which resulted in better retention of UV absorber and its isolation from skin contact (helps to avoid potential allergies).9 Similar reasons were behind the combination of hydrotalcite with ferulic acid in another sunscreen formulation.10 Anionic natural dyes were effectively protected by hydrotalcite from photofading.11

62

3.8 Acid neutralization

In PVC photodegradation the hydrotalcite replaces the hydrochloric acid with carbonic acid and reduces the CO2 evolution rate and the effect of humidity on degradation in the presence of titanium dioxide.12 The use of hydrotalcite benefits PVC formulations in this respect.12 Two UV absorbers (2-hydroxybenzoic acid and 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid) were intercalated into layered double hydroxides and used for modification of bitumen to improve its UV aging resistance.13 The oxidation of bitumen and generation and stacking of asphaltenes cluster during UV exposure were restrained by enhancement of UV absorptive ability of the intercalated absorber and improvement of its compatibility with bitumen.13 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

Kovanda, F; Kolousek, D; Cilova, Z; Hulinsky, V, Appl. Clay Sci., 28, 101-9, 2005. Kumar, B; Rana, S; Singh, R P, eXPRESS Polym. Lett., 1, 11, 748-54, 2007. Wypych, G, PVC Degradation and Stabilization, 3rd Edition, ChemTec Publishing, Toronto, 2020. Wypych, G, PVC Formulary, 3rd Edition, ChemTec Publishing, Toronto, 2020. Ashton, H C; Enlow, W; Nelen, T, Antec 2000, 2818-25. Tolinski, M, Additives for Polyolefins, Elsevier, 2009, pp 23-43. Pospisil, J, Polym. Deg. Stab., 39, 103-15, 1993 Leroux, F; Meddar, L; Mailhot, B; Morlat-Therias, S; Gardette, J-L, Polymer, 46, 3571-78, 2005. Perioli, L; Nocchetti, M; Ambrogi, V; Latterini, L; Rossi, C; Costantino, U, Microporous Mesoporous Mater., 107, 1-2, 180-89, 2008. Rossi, C; Schoubben, A; Ricci, M; Perioli, L; Ambrogi, V; Latterini, L; Aloisi, G G; Rossi, A, Intl. J. Pharm., 295, 1-2, 47-55, 2005. Kohno, Y; Totsuka, K; Ikoma, S; Yoda, K; Shibata, M; Matsushima, R; Tomita, Y; Maeda, Y; Kobayashi, K, J. Colloid Interface Sci., 337, 1, 117-21, 2009. James, S L; Robinson, A J; Arnold, J C; Worsley, D A, Polym. Deg. Stab., 98, 508-13, 2013. Xu, S; Yu, J; Wu, W; Xue, L; Sun, Y, Appl. Clay Sci., 114, 112-9, 2015.

Mechanisms of UV Stabilization

63

3.9 REPAIRING DEFECTS CAUSED BY DEGRADATION The principle of such a mechanism is to eliminate the defects induced during the photodegradation process. By induced defects, we mean a change in the molecular weight of the polymer, an introduction of groups which absorb radiation, change of the color of the polymer, or formation of groups which are vulnerable to photooxidative changes. Some mechanisms of this type are known but are used infrequently. One such effect can be achieved by balancing the chain scission and crosslinking reactions, which produces retention of constant molecular weight. Such cases have been reported for PVC formulations, where studies showed that there was no change in the molecular weight of polymer during the process of photodegradation.1 The molecular weight of other formulations decreased during irradiation.1 The difference in result is either an experimental error or the formulations were adjusted to balance the rates of chain scission and crosslinking. Double bonds, which can absorb radiation, undergo a photooxidative process, and change polymer’s color which was produced during PVC processing. Phosphites may substitute into the double bonds according to the equation: C CH CH

+ P(OR)3

O

C CH CH O

+HCl

C CH2 O

P(OR)3

+ RCl

CH

O P(OR)2

This reaction not only eliminates the double bond but does so at a very vulnerable location − the ketoallyl formation. Some PVC stabilizers also seem to be capable of eliminating ketoallylic chlorine. By doing so, they can break the sequence of conjugated double bonds that change polymer color. Organotin chloride is believed to cause double bond migration (or isomerization), which breaks the sequence of conjugated double bonds and preserves color: organotin chloride CH

CH

CH

CH

CH

CH

CH

Cl

CH

CH

CH

Cl

These are not numerous examples, but they may generate more interest in the search for other methods that would increase our ability to preserve the original properties of polymers. REFERENCES 1

Wypych, J, Poly(vinyl chloride) Stabilization, Elsevier, Amsterdam, 1986.

64

3.10 Synergism

3.10 SYNERGISM Numerous examples of synergism are available in practical applications and literature. They involve the following pairs of UV stabilizers: • HASs mixtures • HAS and UV absorbers • HAS, UV absorber, and phenolic antioxidants • HAS and amines • UV absorbers mixtures • UV absorbers and phenolic antioxidants • UV absorbers and dithiocarbamates • UV absorbers and Ni chelates • phosphite and phenolic antioxidant • phenolic antioxidants and dithiopropionate In many instances, synergism is claimed if experimental results cannot be readily explained, but the above long list suggests that there are real benefits offered by composition. Synergism is not a chemical property or mechanism until its nature is explained sufficiently to permit the use of stabilizer mixtures in a beneficial way. The discussion below shows details of synergisms of the above pairs as much as they can be explained by scientific data. Synergistic combinations of HAS have gained increasing importance for stabilization of polyolefins.1 Several critical general observations, worth noticing, were made, as follows:1 • 1:1 combination of two components is the most suitable for studies of synergistic properties (most likely because it is simply a starting-point) • results obtained for one grade of polypropylene (or by analogy any other polymer) were applicable to other grades of the same polymer • combinations of high and low molecular weight HAS most likely give synergistic compositions (unlike two HAS having similar molecular weights) • if synergism is observed it must result from a very specific interaction or specific mechanism of protecting components (e.g., protection of one stabilizer by another) From the above general points, it can be easily predicted that in the case of a mixture of two HASs, we can expect that synergistic effect is based on different diffusion rates of both components, which improves the balance between short- and long-term stabilities. Gugumus determined a property for individual stabilizers and compared it to the same property obtained from the study of the mixture. Polypropylene tapes were studied and the time T50 (or radiant energy E50) to 50% loss of tensile strength was compared. Figure 3.19 gives results for tapes studied in a Xenon-arc Weather-O-Meter with different proportions of HAS A (Tinuvin 770) and HAS B (Chimassorb 944) and Figure 3.20 gives results for exposures in Florida of tapes containing the same amounts of three different HASs in two polypropylene grades. Figure 3.19 illustrates the above-mentioned point that if synergism exists, it applies to different proportions of the component stabilizers (the proportion of stabilizers may increase the magnitude of the synergistic effect). Figure 3.20 shows that

Mechanisms of UV Stabilization

Figure 3.19. Length of exposure Xenon-arc WeatherO-Meter until 50% decrease in tensile strength for tapes having different proportions of HAS A (Tinuvin 770) and HAS B (Chimassorb 944). [Data from Gugumus, F, Polym. Deg. Stab., 75, 295-308, 2002.]

65

Figure 3.20. Effect of exposure of tapes made out of two different polypropylene grades and containing 3 different HAS (one sample is control). [Data from Gugumus, F, Polym. Deg. Stab., 75, 295-308, 2002.]

the grade of the polymer does not affect the performance of stabilizer, but it affects the magnitude of its effect. Figure 3.19 shows that a combination of low molecular weight HAS (Tinuvin 770) with polymeric HAS (Chimassorb 944) gives a synergistic mixture.7 In another study,2 Tinuvin 770 (low molecular weight) and Tinuvin 622 (high molecular weight) formed a synergistic mixture, which improved the stability of both polypropylene and polyethylene. It was also found that the stability studies conducted for traditional polymers agree with the results for metallocene polymers.2 In pigmented coatings, the optimization of UV radiation protection is best achieved by the right combination of UVA and HAS.3 The optimal ratio of UVA and HAS strongly depended on the concentration of pigments (acting as UV absorbers) used in the coating; that is, clearcoat requires higher amounts of UVA (and lower HAS), whereas opaque pigmented coatings require higher amounts of HAS (and lower UVA). Also, some HAS compounds are chemisorbed on the pigment surface, losing their free radical scavenging ability.3 The degree of chemisorption is a function of the HAS basicity as well as the nature of the pigment, that is, the surface charge.3 This information stipulates that in the case of HAS and organic and inorganic UV absorber, the nature of synergism is related to the opacity of a combination of both UV absorbers and interaction between HAS and inorganic screener or pigment. Similar observations were made in the stabilization of polypropylene.4 It is also considered, based on experimental data, that UV absorbers of benzophenone and benzotriazole types occasionally lose phenolic hydrogen during repeated UV absorption and they become quinoid compounds which do not participate in energy dissipation cycle.5 HAS is thought to be able to reduce the quinoid structure to the regular phe-

66

3.10 Synergism

nolic form which can exhibit its normal UV absorption activity.5 The combination Tinuvin 662 (polymeric HAS)/Chimassorb 81 (benzophenone) yields a quite pronounced synergism. In the presence of titanium dioxide, UV absorber no longer contributed to UV stability.4 However, HAS shows excellent performance.4 The nature of the synergism was not explained. In polypropylene studies, UV absorbers and HAS were found to form synergistic mixtures.6 It was concluded that HAS stabilized UV absorber.6 HAS was found to form synergistic compositions with amines.7,8 Bridged amines (e.g., a stearic acid salt of di-azo-bicyclo-octane, 7-nitro-1,3,5-triaza-adamante, and others) added at small concentrations (0.1%) can increase the stability of system stabilizerHAS by factor 2-3. No explanation of the nature of their synergism was offered, only a suggestion that both stabilizers act as quenchers of polymer oxygen charge transfer complexes.8 Similar effects were discussed elsewhere.7 UV-initiated interaction mechanisms between the phenolic antioxidant (Irganox 1330) and nine commercially used hindered amine light stabilizers were studied.9 Previously, the synergistic interaction was described as a result of the hydrogen transfer from a hydroxylamine derived from HALS to the oxidized form of a primary antioxidant, whereby the phenol was regenerated.9 This study showed that the synergism resulted from the formation of quinoid transformation products derived from Irganox 1330.9 The synergistic interaction was caused by the oxidized phenolic antioxidant, which acted as a UVabsorber.9 Benzophenone-type UV absorber was found to have synergism in compositions with phenolic antioxidants.10 It has been found that hindered phenols containing benzylic sulfur are more effective synergists at the same molar concentration than conventional hindered phenols.10 Antioxidants are shown to protect the UV absorber against hydroperoxides by catalytically destroying them and scavenging radicals formed from them.10 The UV absorber appears to deactivate excited species formed in the photodecomposition of oxidation products of antioxidants.10 In the combination HAS/phenolic antioxidants, HAS deactivates alkyl peroxycyclohexadienones (oxidation products of antioxidants which are photosensitizers) and regenerates phenolic antioxidants.6 Dithiocarbamates were found destroying hydroperoxides by which they were instrumental in protecting UV absorbers.11 The light-stable nickel thiolate antioxidants (particularly the dithiocarbamates and dithiophosphates) are slowly converted during photooxidation to lower molecular mass sulfur acids which are effective catalysts for peroxidolysis.10 Synergism occurs with UV absorber which protects the dithiocarbamate (nickel chelate) from photooxidation and the peroxide decomposer (nickel chelate) protects UV absorber from the destructive effects of hydroperoxides.10 Combinations of light stabilizers (HALS and UVA) with antioxidants (Irganox 1076 and Irgafos 168) have a synergistic effect in the protection of ABS against photooxidative degradation.12

Mechanisms of UV Stabilization

67

HALS, thiosynergist and phenolic antioxidant systems were investigated in polyolefins.13 The transformation pathways of HALS were influenced by the presence of thiosynergists (lack of aminoxyl radicals).13 HALS alone showed only a limited effect in protecting the polymer.13 The best performance was obtained with the ternary mixture HALS/thiosynergist/phenol with a degree of stabilization higher than the sum of the stabilization effects of the single components.13 Numerous examples of synergism can be found in PVC thermal and UV stabilization, which is too broad a topic to discuss here, but it is fully disclosed elsewhere.14,15 The guava byproduct extract standardized in ellagic acid presented synergy with the chemical UV filter (ethylhexyl methoxycinnamate) enhancing the solar protection factor of the phytocosmetic by 17.99%.16 Its antioxidant activity was observed in the presence of secondary metabolites such as phenols and flavonoids.16 It absorbed radiation of the wavelength in UVB range increasing the photoprotection efficacy in the cosmetic formulations.16 REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Gugumus, F, Polym. Deg. Stab., 75, 295-308, 2002. Gugumus, F, Polym. Deg. Stab., 44, 299-322, 1994 Schaller, C; Rogez, D; Braig, A, J. Coat. Technol. Res., 6, 1, 81–88, 2009. Gugumus, F, Polym. Deg. Stab., 50, 101-16, 1995. Mizokawa, S; Ohkatsu, Y, J. Japan Petroleum Institute, 50, 1, 1-7, 2007. Kikkawa, K, Polym. Deg. Stab., 49, 135-43, 1995. Pospisil, J; Pilar, J; Nespurek, S, J. Vinyl Additive Technol., 13, 119-32, 2007. Gijsman, P, Polymer, 43, 1573-79, 2002. Maringer, L; Roiser, L; Wallner, G; Nitsche, D; Buchberger, W, Polym. Deg. Stab., 131, 91-7, 2016. Scott, G; Yusoff, M F, Polym. Deg. Stab., 2, 309-19, 1980. Allen, N S; Chirinos-Padron, A; Henman, T J, Polym. Deg. Stab., 13, 1, 31-76, 1985 Santos, R M; Pimenta, A; Botelho, G; Machado, A V, Polym. Testing, 32, 78-85, 2013. Beissmann, S; Grabmayer, K; Wallner, G; Nitsche, D; Buchberger, W., Polym. Deg. Stab., 110, 509-17, 2014. Kohno, Y; Totsuka, K; Ikoma, S; Yoda, K; Shibata, M; Matsushima, R; Tomita, Y; Maeda, Y; Kobayashi, K, J. Colloid Interface Sci., 337, 1, 117-21, 2009. Wypych, J, Poly(vinyl chloride) Stabilization, Elsevier, Amsterdam, 1986. Milani, L P G; Garcia, N O S; Morais, M C; Dias, A L S; Conceição, E C, Rev. Bras. Farmacognosia, 28, 6, 692-6, 2018.

68

3.11 Antagonism

3.11 ANTAGONISM Antagonism between stabilizers can be more detrimental to the final result than synergism, since we have seen that gains due to synergism are small, but antagonism may eventually undermine all efforts of stabilization. The following cases of antagonism were reported so far: • HAS and HAS • HAS and phenolic antioxidants • HAS and Ni dithiocarbamate • HAS and UV absorbers • inorganic fillers, screeners, and pigments and all stabilizers These effects are discussed below. Performance of many pairs of HAS stabilizers has been reported to find the best cases of synergism, such as some discussed in the previous section.1 In this study,1 there were also cases of no effect and antagonism (Figure 3.21). Figure 3.21 shows 3 combinations which all had a shorter time to 0.1 carbonyl absorbance, experimentally measured as compared with calculated values based on the additivity principle of results for individual components. Combinations A and B are for monomeric HAS, and combination C contains polymeric HAS.1 Information generated during research does not allow us to explain the reason for this antagonism.1 Some phenolic antioxidants and HAS form antagonistic mixtures. High-performance liquid chromatography with mass spectrometric detection was used for analysis of transformation products and monitoring of interactions between HALS and phenolic antioxidants.2 A strong antagonism between HALS and phenols was detected.2 Figure 3.21. Antagonistic mixtures of HAS (see text for explanation). [Data from Gugumus, F, Polym. Deg. Both stabilizer types were consumed faster Stab., 75, 295-308, 2002.] when used in combination, and once the concentration of the active form of the stabilizers dropped below a critical value, there was a rapid chemical change of the polymer.2 The reasons for this antagonistic behavior are not entirely understood, but there are several hypotheses brought forward:2-6 • HALS nitrosonium salt, a derivative of the parent amine, is the key intermediate for the observed antagonism as it leads to the fast consumption of the phenolic antioxidant and homolytic decomposition of hydroperoxides, thereby accelerating the degradation of polymeric materials • oxidation of phenolic antioxidants by nitroxyls from HAS

Mechanisms of UV Stabilization

• •

69

coupling between radicals derived from HAS and phenolic antioxidants inhibition of hydroperoxide formation by phenolic antioxidants, thus preventing the formation of nitroxyls by HAS • salt formation and/or reaction between oxidized acidic products from the sulfur compounds and HAS or their transformation products • sulfenyl radicals formed from thioesters may block nitroxyl radicals • BHT accelerates hydroperoxide decomposition by HAS which initiates autooxidation and accelerates degradation Each of the above arguments has found contradicting data. Antagonism between Ni diethyldithiocarbamate (hydroperoxide decomposer) and Tinuvin 770 is explained by the fact that hydroperoxide decomposer prevents the formation of nitroxyls.5 It was found7 that both benzophenones and benzotriazoles formed slightly antagonistic combinations with (2,2,6,6,-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) but that antagonism with benzophenone-type UV absorber has been stronger.7 It was suggested that this behavior could be associated with differences in the molecular structures of the UV absorbers.7 The bulky ortho substituent (heterocycle group) with an additional para substituent (CH3 group) on the phenyl ring in the 2'-hydroxybenzotriazole made it more stable for reaction with nitroxyl radicals.7 The strength of the hydrogen bonds formed between the hydroxyl group on the phenyl ring and the carbonyl group in 2-hydroxybenzophenone, and with the azo group in 2'-hydroxybenzotriazole differed, and this may have influenced the reaction with nitroxyl radical.7 A significant reduction in the induction period of oxidation of polypropylene composite was observed in the presence of montmorillonite. This is believed to arise from interactions between the additives and nanoclay.8 The interactions could involve the adsorption of additives, such as antioxidants, onto the clay. A dramatic shortening of the induction period of the oxidation was observed in the presence of the nanofiller, leading to a decrease of the durability of the nanocomposite.8 This unexpected result was attributed to the inhibition of the activity of the residual phenolic processing antioxidant.8 Antagonism is not only related to stabilizer mixtures but is also caused by combinations of photostabilizers with some other additives, such as brominated flame retardants, which make HALS less effective.9,10 Also HALS does not improve the stability of polymer films containing anatase because of strong antagonism.11 The observations discussed here are a part of broader influences related to interactions between organic additives and inorganic components of polymeric compositions, such as pigments, fillers, nanofillers, and UV screeners. These studies, although closely related to the topic of this book, are beyond as scope. Interested readers are directed to a monographic source,12 which discussed the matter in expected detail for this important subject. REFERENCES 1 2 3

Gugumus, F, Polym. Deg. Stab., 75, 295-308, 2002. Beissmann, S; Reisinger, M; Grabmayer, K; Wallner, G; Nitsche, D; Buchberger, W, Polym. Deg. Stab., 110, 498-508, 2014. Mizokawa, S; Ohkatsu, Y, J. Japan Petroleum Institute, 50, 1, 1-7, 2007.

70 4 5 6 7 8 9 10 11 12

3.11 Antagonism Kikkawa, K, Polym. Deg. Stab., 49, 135-43, 1995. Sedlar, J; Marchal, J; Petrus, J, Polym. Photochem., 2, 175-207, 1982. Takenaka, H; Mizokawa, S; Ohkatsu, Y, J. Japan Petroleum Institute, 50, 1, 8-15, 2007. Lucki, J, Polym. Photochem., 6, 273-91, 1985. Morlat-Therias, S; Mailhot, B; Gonzalez, D; Gardette, J-L, Chem. Mater., 17, 1072-78, 2005. Pfaender, R, Photooxidative stabilization of flame retarded polymers. Polymer Green Flame Retardants, Elsevier, 2014, 419-39. Pfaender, R, Polym. Deg. Stab., 98, 12, 2430-35, 2013. Scalarone, D; Lazzari, M; Chiantore, O, Polym. Deg. Stab., 97, 2136-42, 2012. Wypych, G, Handbook of Fillers, 4th Edition, ChemTec Publishing, Toronto, 2016.

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71

3.12 EFFECT OF PHYSICAL PROPERTIES There are many physical properties of stabilizers, which can be considered as strongly influencing their mechanisms of action. These include: • absorption spectrum • refractive index • color • diffusion coefficients • molecular weight • melting temperature • miscibility/solubility • viscosity • crystallization properties • acid/base characteristics These properties affect mechanisms of stabilizers action because they control absorption range, initial color, compatibility with polymer and additives, methods and uniformity of dispersion, and interaction with other components of formulations. These properties include some of the main characteristics, which are included in the description of different types of UV stabilizers in the next chapter.

4

UV Stabilizers Many different groups of chemical compounds are involved in UV stabilization of polymers. They can be divided as follows: • Organic UV absorber Benzimidazoles Benzoates Benzophenones Benzotriazines Benzotriazoles Benzoxaxinones Camphor derivatives Cinnamates Cyanoacrylates Dibenzoylmethanes Epoxidized oils Malonates Oxalanilides Salicylates • Carbon black • Inorganic screeners • Fibers • HAS Monomeric Oligomeric & polymeric • Secondary stabilizers Phenolic antioxidants Phosphites & phosphonites Thiosynergists Amines Quenchers Optical brighteners • Synergistic mixtures The chemical compounds belonging to each group are presented in the form of tables, which include typical representatives of the group and some relevant data. Comprehensive data on UV stabilizers are included in Databook of UV Stabilizers.

74

Introduction

In the section of mixtures, compositions of stabilizers manufactured by various companies are presented. These compositions include some of the most successful synergistic mixtures. Frequently their chemical compositions are given, which may be used as examples for other potential synergistic compositions, which can be designed to fulfill the special requirements of different applications. All information on the above products is based on the technical datasheets of manufacturers of these stabilizing components.

4.1 ORGANIC UV ABSORBERS 4.1.1 BENZIMIDAZOLES O HO

S

N

O N H

Chemical name: 2-phenylbenzimidazole-5-sulfonic acid Commercial name: Ensulizole Typical applications: day cream, sun screen, face cream, hand cream Food approval: 150

State: powder, granules

Density, g/cm3: 1.14

Water solubility, wt%: 97

Solubility, wt%: acetone − >50, benzene − 18, chloroform, methylene chloride − >40, ethyl acetate − 37, n-hexane − 300 nm. [Data from Piton, M; Rivaton, A, Polym. Deg. Stab., 55, 147-57, 1997.]

Figure 7.2. Relative rate of yellowing vs. irradiance used in laboratory testing. [Data from Pickett, J E; Gibson, D A; Gardner, M M, Polym. Deg. Stab., 93, 1597-1606, 2008.]

The formation of a ketone in the chain occurs in a cage reaction. The formation of an aldehyde group by the β-scission reaction is more likely than the formation of a ketone group. Polyethers formed in a crosslinking reaction are not very stable and are unlikely to accumulate.3 Electron withdrawing groups in α-position to a double bond cause several secondary reactions to occur with the products formed according to the previous four pathways.4 It is important to note that both chain scission and crosslinking result from these reactions, which irreversibly alter properties of rubber and thus the entire ABS copolymer. The degradation was found to be spatially homogeneous.2 Major effects occurred in the layer of the thickness of ~50 μm. This layer showed the disappearance of butadiene

UV Degradation & Stabilization of Polymers & Rubbers

191

Figure 7.3. 3D spacial ESRI images of HAS-derived nitroxides after 70 h (left and 643 h (right) of exposure of ABS to xenon arc radiation. [Adapted, by permission, from Lucarini, M; Pedulli, G F; Motyakin, M V; Schlick, S, Prog. Polym. Sci., 28, 331-40, 2003.]

units, and formation of hydroxyls and carbonyls.2 Figure 7.1 shows that radiation can penetrate thin samples (degradation on both sides of the sample), but it is, oxygen-controlled. Oxygen can penetrate to about 50 μm depth.6,7 In addition to radiation wavelength, irradiance is an important factor which may influence results and conclusion. Figure 7.2 shows the effect of irradiance on sample yellowing. Unlike several other polymers studied, ABS did not give an acceptable linear relationship, which may suggest that the high acceleration of degradative processes in the laboratory may produce results which do not simulate results of exposure in an outdoor environment. So far, the presented data and discussion shows that the mechanism of degradation of ABS is best studied under conditions which are typical of conditions of exposure to sunlight. ABS yellowing cannot be attributed to changes in polybutadiene domains. It was found to be caused in SAN domains (for details see Section 7.1.49).8 7.1.2.2 Mechanisms and results of stabilization Figure 7.3 shows 3D spacial ESRI images of HAS-derived nitroxides after 70 h (left) and 643 h (right) of exposure of ABS to xenon arc radiation.1 Tinuvin 770 (used in the study) apparently migrated to both sides of the plaque and was substantially exhausted on the exposed side after 643 h of exposure.1 A combination of light stabilizers (HALS and UV absorber) with phenolic antioxidants and trivalent Organophosphoric compounds gave the best results in ABS stabilization, although the synergistic effect becomes less efficient at longer degradation times.11 The Q-Sun Xe-3-Hs was useful to predict the outdoor weathering phenomena.12 Exposure of 1260 h in Q-Sun Xe-3-Hs corresponded to one year of outdoor exposure at Lisbon.12 The photooxidation of the ABS/PC blend is not a simple combination of the photooxidation of components but predominantly takes place at the ABS component.13 Because

192

7.1.2 Acrylonitrile-butadiene-styrene

of the interaction between the two components and the Fries rearrangement in the PC component, the ABS/PC blends have higher photostability than ABS alone.13 7.1.2.3 Data Table 7.2. Data on photodegradation and stabilization of acrylonitrile-butadiene-styrene Activation wavelength, nm: 320 & 3855 Products of photodegradation: radicals, peroxides, hydroperoxides, hydroxyls, carbonyls, chain scission, crosslinking, carboxyl and carbonyl end-groups Typical results of photodegradation: loss of mechanical properties (especially impact resistance) and yellowing Most important stabilizers: UVA: 2-hydroxy-4-octyloxybenzophenone; 2-hydroxy-4-methoxybenzophenone; 2-(2H-benzotriazol-2-yl)-p-cresol; 2-(2H-benzotriazole-2-yl)-4,6-di-tert-pentylphenol; 2-(2H-benzotriazole-2-yl)-4-(1,1,3,3-tetraethylbutyl)phenol; 2,4-di-tert-butyl-6-(5-chloro2H-benzotriazole-2-yl)-phenol; 2-[4,6-bis(2,4-dimethylphenyl)-1,3,5-triazin-2-yl]-5-(octyloxy) phenol; ethyl-2-cyano-3,3-diphenylacrylate HAS: 1,3,5-triazine-2,4,6-triamine, N,N’’’[1,2-ethane-diyl-bis[[[4,6-bis[butyl-(1,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl] bis[N’,N”-dibutyl-N’,N”bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-; bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate; 2,2,6,6tetramethyl-4-piperidinyl stearate; N,N’-bisformyl-N,N’-bis-(2,2,6,6-tetramethyl-4-piperidinyl)hexamethylendiamine; alkenes, C20-24-.alpha.-, polymers with maleic anhydride, reaction products with 2,2,6,6-tetramethyl-4-piperidinamine; 1, 6-hexanediamine, N, N’-bis(2,2,6,6tetramethyl-4-piperidinyl)-, polymers with 2,4-dichloro-6-(4-morpholinyl)-1,3,5-triazine; 1,6-hexanediamine, N,N'-bis(2,2,6,6-tetramethyl-4-piperidinyl)-, polymers with morpholine-2,4,6-trichloro-1,3,5-triazine reaction products, methylated Phenolic antioxidants: ethylene-bis(oxyethylene)-bis(3-(5-tert-butyl-4-hydroxy-m-tolyl)-propionate); 2,6,-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5,-triazine-2-ylamino) phenol; pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate); 2-(1,1-dimethylethyl)-6-[[3-(1,1dimethylethyl)-2-hydroxy-5-methylphenyl] methyl-4-methylphenyl acrylate; isotridecyl-3-(3,5-ditert-butyl-4-hydroxyphenyl) propionate; 2,2'-ethylidenebis (4,6-di-tert-butylphenol); 2,2’-methylenebis(4-ethyl-6-tertbutylphenol); 3,5-bis(1,1-dimethyethyl)-4-hydroxy-benzenepropanoic acid, C13-15 alkyl esters; phenol, 4-methyl-, reaction products with dicyclopentadiene and isobutene Phosphite: trinonylphenol phosphite; isodecyl diphenyl phosphite Other: 2,2’-thiodiethylene bis[3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate]; 4,4’-thiobis(2-tbutyl-5-methylphenol); 2,2’-thiobis(6-tert-butyl-4-methylphenol), 2,2’-(1,2-ethylenediyldi-4,1phenylene)bisbenzoxazole Mixture:10 benzylidene-bis-malonate+phenol octadecyl-3,5-di-tert.butyl-4-hydroxyhydrocinnamate+tris-(2,4-di-tert.-butyl-phosphite+(HAS) Concentration of stabilizers in formulations, wt%: UVA: 0.1-1; HAS: 0.05-1 Material and exposure conditions

pipe/Atlas’ WOM, 450 h

Change

Refs.

four times reduction in impact strength

References 1 2 3 4 5 6 7 8 9

Lucarini, M; Pedulli, G F; Motyakin, M V; Schlick, S, Prog. Polym. Sci., 28, 331-40, 2003. Bokria, J G; Schlick, S, Polymer, 43, 3239-46, 2002. Piton, M; Rivaton, A, Polym. Deg. Stab., 53, 3, 343-59, 1996. Wypych, G, Handbook of Materials Weathering, 6th Edition. ChemTec Publishing, Toronto, 2018. Pickett, J E; Gibson, D A; Gardner, M M, Polym. Deg. Stab., 93, 1597-1606, 2008. Piton, M; Rivaton, A, Polym. Deg. Stab., 55, 147-57, 1997. Carter, R O; McCallum, J B, Polym. Deg. Stab., 45, 1-10, 1994. Jouan, X; Gardette, J L, Polym. Deg. Stab., 36, 91-96, 1992. Davis, P; Tiganis, B E; Burn, L S, Polym. Deg. Stab., 84, 2, 233-42, 2004.

9

UV Degradation & Stabilization of Polymers & Rubbers 10 11 12 13

Kroehnke, C; Webster, J R; Gronmaier, E; Avar, L, US Patent 7,332,535, Feb. 19, 2008, Clariant. Santos, R M; Pimenta, A; Botelho, G; Machado, A V, Polym. Testing, 32, 78-85, 2013. Santos, R M; Botelho, G L; Cramez, C; Machado, A V, Polym. Deg. Stab., 98, 2111-15, 2013. Li, J; Chen, F; Yang, L; Jiang, L; Dan, Y, Spectrochim. Acta Part A: Molec. Biomolec. Spectroscopy, 184, 361-7, 2017.

193

194

7.1.3 Acrylic resins

7.1.3 ACRYLIC RESINS Additional information on acrylic resins can be found in the sections on copolymers, polymethylmethacrylate, and polyurethanes. 7.1.3.1 Mechanisms and results of degradation Poly(methyl acrylate/methyl methacrylate/butyl acrylate) copolymer dispersion was used for a topcoat on wood. 2-(2-Hydroxyphenyl)-benzotriazole, 2-hydroxyphenyl-s-triazine, and ZnO and TiO2 screeners were used to protect them.4 Scission of side chains containing carboxyl groups (with the production of carbon dioxide) and crosslinking were observed.4 The following changes were observed during exposure of acrylic resin coatings to Suntest conditions:5 • increase in absorption in the whole hydroxyl region • progressive decrease of the C–H stretching absorption • decrease of the carbonyl ester band • development of a small absorption possibly related to the formation of chain-end double bonds These studies also indicated that an apparent difficulty of accelerated weathering experiments arises from the fact that changes of the light intensity, combined with changes in the energy level of irradiance, may result in new reactions and may change the rate ratios of parallel or competitive responses.5 It must be stressed, therefore, that for the closest reproduction of natural outdoor aging conditions, it is necessary to employ radiation source matching the sunlight spectrum, such as the xenon arc lamp.5 Acrylic polymers may be made from a mixture of a relatively wide variety of monomers. This makes it essential to separate the effect of UV radiation on the polymer backbone from its impact on side groups. UV radiation below 300 nm may cause homolytic chain scission of the polymer backbone:8

resulting in a decrease in molecular weight. More frequently, chain scission or crosslinking occurs because of the formation of free radicals from radiolysis of bonds in side groups. These changes depend on the chemical structure of the functional groups available in the material. The carboxylic pendant group affects the formation of radicals:8

UV Degradation & Stabilization of Polymers & Rubbers

195

If acrylic acid is a component, a hydrogen radical is formed. When the carboxylic group is replaced by the ester group, three dissociation patterns are possible:8

Low-molecular-weight radicals may recombine or undergo further changes (this is usually the case), forming products frequently detected as volatiles:8

Macroradicals may also form gaseous products:8

and these are often found in the analysis of volatiles. Esters of higher hydrocarbons may produce unsaturated structures (e.g., poly(butyl acrylate) produces isobutylene). Macroradicals undergo further changes; some of them result in chain scission:8

196

7.1.3 Acrylic resins

which results in radical formation or unsaturation. In the absence of oxygen, crosslinking prevails:8

When oxygen is present, it causes both crosslinking and chain scission. When radicals react with oxygen, they form hydroperoxides which, after decomposition by UV or heat, produce carbonyl and hydroxyl groups. 7.1.3.2 Mechanisms and results of stabilization The matrix seems to play the dominant role in determining the lifetime of most commercially available UV absorbers.6 Strongly hydrogen-bonding media can disrupt the internal hydrogen bond that is essential for UV absorber stability, while matrices that are subject to rapid free radical oxidation can produce radicals that react with the absorbers and lead to their degradation.6 In studies of varnishes containing, in the acrylic formulation, different UVA, HAS, and pigments, it was found that the simultaneous presence of UVA, HAS, and pigment gives superior protection, compared to formulations which did not have one or more of these components.7 Combination of hydroxybenzotriazole and micronized titanium dioxide was used for protection of acrylic-based, clear wood coating.9 Organic UV absorber was very efficient in reducing the photooxidation of the acrylic binder.9 TiO2 only stabilized the high Tg component of the multiphase acrylic binder whereas the photooxidation of the low Tg component was accelerated.9 This may be explained by a photocatalytic activity of TiO2.9 TiO2 nanoparticles treated with 1H,1H,2H,2H-perfluorooctyltriethoxysilane may offer the promising prospect for applications in self-cleaning organic coatings.10 They have tendency for migration toward the coatings’ surface, during the film formation process.10 Residing on surface they cause a uniform degradation of the coating surface.10 The lignin photodegradation was lower for coated specimens, with slightly lower degradation for the specimens coated with acrylic paints modified with TiO2 and unmodified montmorillonite clay nanoparticles.11 A series of UV absorbing fluorine-silicone acrylic resin polymers containing different amounts of UV absorbent was prepared by solution polymerization, from 2-[3-(2Hbenzotriazol-2-yl)-4-hydroxyphenyl] ethyl methacrylate, vinyltrimethoxysilane, and hexafluorobutyl methacrylate as modifying monomers.12 The resin had high UV absorption, good thermal stability, and hydrophobicity.12 These super weather-resistant resins with heat resistance and low surface energy are expected to be used for long-term protective coatings.12

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High UV-shielding nanocomposite coatings were prepared from the acrylic emulsion and R-TiO2/or ZnO nanoparticles.13 The coatings containing 2 wt% R-TiO2/or ZnO nanoparticles and having a thickness of 45 μm had the best UV-shielding and photostability.13 The coatings shielded more than 98% (for R-TiO2) and 85% (for ZnO) of UV radiation in the range from 230 to 380 nm.13 After 60 cycles of UV/CON exposure, the color of coating samples protected by the R-TiO2 and ZnO nanocomposite coatings did not significantly change.13 7.1.3.3 Data Table 7.3. Data on photodegradation and stabilization of acrylic resins Products of photodegradation: hydroperoxides, hydroxyl groups, carbonyl groups, aldehydes, crosslinks, formaldehyde, methanol, hydrogen, carbon monoxide, carbon dioxide Most important stabilizers: UVA: 2-hydroxy-4-octyloxybenzophenone; 2-(2H-benzotriazol-2yl)-p-cresol; 2-(2H-benzotriazole-2-yl)-4,6-di-tert-pentylphenol; 2-(2H-benzotriazol-2-yl)-4,6bis(1-methyl-1-phenylethyl)phenol; 2,2’-methylenebis(6-(2H-benzotriazol-2-yl)-4-1,1,3,3tetramethylbutyl)phenol; 2-(2’-hydroxy-5’-methacryloxyethylphenyl)-2H-benzotriazole; 2-(2Hbenzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol; 2-[4-[(2-hydroxy-3-(2’ethyl)hexyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine; 2-[4-[(2-hydroxy3-dodecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine and 2-[4[(2-hydroxy-3-tridecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine; N-(2-ethoxyphenyl)-N'-(4-isododecylphenyl)oxamide Screeners: ZnO; cerium oxide, cerium–titanium pyrophosphate HAS: decanedioic acid, bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl) ester, reaction products with 1,1-dimethylethylhydroperoxide and octane; bis (1,2,2,6,6-pentamethyl-4-piperidinyl)-[[3,5bis(1,1-dimethylethyl)-4- hydroxyphenyl]methyl]butylmalonate; 2,4-bis[N-butyl-N-(1-cyclohexyloxy-2,2,6,6-tetramethylpiperidin-4-yl)amino]-6-(2-hydroxyethylamine)-1,3,5-triazine; bis(1,2,2, 6,6-pentamethyl-4-piperidyl) sebacate and methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate; bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate + methyl-1,2,2,6,6-pentamethyl-4-piperidyl sebacate Most important stabilizers: Phenolic antioxidant: isotridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate Optical brightener: 2,2’-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole) Mixtures: high intensity UV radiation (optical cable, LCD displays, acrylic storage media, and HID devices):1 HAS+either lactic acid, oxalic acid, or acetic acid, or their mixtures Concentration of stabilizers in formulations, wt%: 0.1-3 (UVA); 5-18 (screeners); 0.5-2 (HAS) Material and exposure conditions

Changes

Refs.

Ocular buttons from colorless acrylic resin(1- 4.4% reduction in microhardness 3.5 mm thick)/1008 h outdoors

2

Acrylic copolymer (Paraloid B72) applied on water protection efficiency was reduced stone/outdoor exposure in Florence, Italy from 96% to 60% on 60 month exposure

3

References 1 2 3 4 5 6 7 8 9

Yang, S-J; Abel, R, US Patent 7,407,998, August 5, 2008, Arkema France. Fernandes, A U R; Goiato, M C; des Santos, D M, Contact Lens Anterior Eye, 32, 283-87, 2009. Bracci, S; Melo, M J, Polym. Deg. Stab., 80, 533-41, 2003. Forsthuber, B; Grüll, G, Polym. Deg. Stab., 95, 746-55, 2010. Chiantore, O; Lazzari, M, Polymer, 42, 17-27, 2001. Pickett, J E; Moore, J E, Polym. Deg. Stab., 42, 231-44, 1993. Custódio, J E P; Eusébio, M I, Prog. Org. Coat., 56, 1, 59-67, 2006. Wypych, G, Handbook of Materials Weathering, ChemTec Publishing, 6th Edition, Toronto, 2018. Forsthuber, B; Mueller, U; Teischinger, A; Gruell, G, Polym. Deg. Stab., 98, 1329-38, 2013.

198 10 11 12 13

7.1.3 Acrylic resins Pazokifard, S; Esfandeh, M; Mirabedini, S M, Prog. Org. Coat., 77, 1325-35, 2014. Fufa, S M; Jelle, B P, Hovde, R J, Prog. Org. Coat., 76, 1543-48, 2013. Lei, H; He, D; Guo, Y; Tang, Y; Huang, H, Appl. Surf. Sci., 442, 71-7, 2018. Nguyen, T V; Dao, P H; Duong, K L; Duong, Q H; Le, T L, Prog. Org. Coat., 110, 114-21, 2017.

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199

7.1.4 ALKYD RESINS 7.1.4.1 Mechanisms and results of degradation Linseed oil is an essential component of alkyd drying resins. Figure 7.4 shows the changes in linseed oil on exposure to UV radiation in Suntest.4 The oxidative degradation of linseed oil is a part of the hardening process. During extended periods of artificial aging, corresponding to years of natural aging, the oxidation also affects the alkylic segments, leading to partial fragmentation of the structure.5 The gradual change of color was observed on exposure to UV radiation.7 Increase in the hydroxyl content and broadening of the absorption in the carbonyl region was observed by IR.7 7.1.4.2 Mechanisms and results of stabilization It is not efficient to use one light stabilizer for all shades from clear to semi-transparent to opaque pigmented systems.2 The following are the guidelines for different systems:2 • for clear non-pigmented systems, higher amounts of UVA (1-1.5 wt% on total paint) must be used with HAS (0.5 wt%) to prevent the formation of coating surface defects. In addition, the lignin stabilizer helps to keep wood color in its almost initial condition, to improve aesthetics and mechanical properties retention • for light and medium pigmented systems, a reduced amount of UVA (0.5%) is sufficient, due to the fact that the pigments themselves act as UV/VIS screeners and an increased amount of HAS (0.5-1.0 wt%). The lignin stabilizer adds further benefit to the overall performance • for dark and opaque pigmented systems, the use of UVA and lignin stabilizer adds no real improvement to the overall performance of the system as long as sufficient amounts of HAS (1-2 wt%) are used to avoid chalking and other undesired surface defects.

Figure 7.4. Change of unsaturation in linseed oil on exposure to UV radiation in Suntest. [Data from Lazzari, M; Chiantore, O, Polym. Deg. Stab., 65, 303-13, 1999.]

Figure 7.5. Rate of paint degradation based on chalking data for titanium dioxide having different densities of alumina coating. [Data from Gesenhues, U, J. Photochem. Photobiol. A: Chem., 139, 243-51, 2001.]

200

7.1.4 Alkyd resins

•

the synergistic effect of nano ZnO and the doped Ag provides a suitable pathway for the development of highly efficient photo-catalyst.8 The alkyd resin-based self-cleaning coating was formulated using 1 wt% Ag-doped ZnO nanoparticles as a pigment.8 In addition to the self-cleaning properties, the coating exhibits self-refreshing property which is essential for the long-lasting self-cleaning activity, as well as the disinfectant properties.8 Titanium dioxide is one of the most frequently used screeners in alkyd paints. Titanium dioxide has a strong photocatalytic activity which may be reduced by alumina (and other) coatings (Figure 7.5). The photostability of red synthetic organic pigments of three different chemical classes such as naphthol AS, diketopyrrolopyrrole, and quinacridone was investigated in acrylic and alkyd paints.9 The pigments were stable under the aging conditions.9 A relative enrichment of pigments was recorded on the surface, due to the photodegradation of the binders, which led to the formation of low-molecular-weight and volatile compounds.9 In the alkyd paints, bond cleavage was affecting the phthalate portion of the resin more than the oil portion.9 The pursuit of higher bio-based content in formulations does not necessary provide environmental benefits.10 Comparative life cycle assessment results for a 50%-bio-renewable content wood flooring coating showed improvements in six impact categories, but worse life cycle environmental performance for four categories, such as smog formation, eutrophication, acidification, and respiratory effects, compared to a conventional control coating with similar performance and durability.10 Replacing petrochemical components with renewable chemical substitutes should thus consider multiple environmental goals, not just the percentage of bio-based content.10 7.1.4.3 Data Table 7.4. Data on photodegradation and stabilization of alkyd resins Activation wavelength, nm: 3303 Products of photodegradation: carbonyl groups, loss of absorption by C=O and C−O groups of phthalate esters, chain scission, formation of chain-end unsaturations Typical results of photodegradation: loss of gloss, chalking Known influences of other factors: acid/base interactions can retard curing of air-drying systems1 Most important stabilizers: UVA: 2-hydroxy-4-methoxybenzophenone; 2,4-dihydroxybenzophenone; 2-benzotriazol-2-yl-4,6-di-tert-butylphenol; 2-(2H-benzotriazole-2-yl)-4,6-di-tert-pentylphenol; N-(2-ethoxyphenyl)-N'-(4-isododecylphenyl)oxamide; 2-[4-[(2-hydroxy-3-(2’ethyl)hexyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine6 HAS: decanedioic acid, bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl) ester, reaction products with 1,1-dimethylethylhydroperoxide and octane; 2,4-bis[N-butyl-N-(1-cyclohexyloxy-2,2,6,6tetramethylpiperidin-4-yl)amino]-6-(2-hydroxyethylamine)-1,3,5-triazine; bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate and methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate; 2-dodecyl-N(2,2,6,6-tetramethyl-4-piperidinyl)succinimide; polymer of 2,2,4,4-tetramethyl-7-oxa-3,20-diazadispiro [5.1.11.2]-heneicosan-21-on and epichlorohydrin Screener: TiO2 Phosphite: phosphoric acid, (2,4-di-butyl-6-methylphenyl)ethylester Concentration of stabilizers in formulations, wt%: 0.1-3 (UVA), 1-3 (HAS)

UV Degradation & Stabilization of Polymers & Rubbers

201

References 1 2 3 4 5 6 7 8 9 10

Schaller, C; Rogez, D; Braig, A, J. Coat. Technol. Res., 6, 1, 81–88, 2009. Schaller, C; Rogez, D, J. Coat. Technol. Res., 4, 4, 401-9, 2007. Kaempf, G; Sommer, K; Zirngiebl, E, Prog. Org. Coat., 19, 69-77, 1991. Lazzari, M; Chiantore, O, Polym. Deg. Stab., 65, 303-13, 1999. Gesenhues, U, J. Photochem. Photobiol. A: Chem., 139, 243-51, 2001. Ehrenstein, G W; Pongratz, S, Resistance and Stability of Polymers, Elsevier, 2013, 273-328. Cakic, S M; Ristic, I S; Vladislav, J M; Stamenkovic, J V; Stojilkovic, D T, Prog. Org. Coat., 73, 401-8, 2012. Hikku, G S; Jeyasubramanian, K; Jacobjose, J; Thiruramanathan, P; Ikeda, H, J. Colloid Interface Sci., 531, 628-41, 2018.V; Anghelone, M; Stoytschew, Jembrih-Simbürger, D; Schreiner, M, Microchem. J., 139, 155-63, 2018. Montazeri, M; Eckelman, M J, J. Cleaner Prod., 192, 932-9, 2018.

202

7.1.5 Cellulose-based polymers

7.1.5 CELLULOSE-BASED POLYMERS 7.1.5.1 Mechanisms and results of degradation In natural products, such as wood, cellulose is protected from degradation by lignin, which is a known absorber of UV and quencher,1 but when exposed alone, it undergoes many photochemical reactions discussed in detail elsewhere.2 Lignin, in a pure form, can be obtained as a co-product of bioethanol production.3 Radicals can be formed from glycosidic bonds (chain scission), hydroxymethyl group (hydrogen radical, hydroxymethyl radical), and hydroxyl group (hydroxyl radical).2 Pure native cellulose absorbs UV radiation strongly between 200 and 300 nm, but only very weakly up to 400 nm.4 Two pathways are important in cellulose degradation: oxidation of the hydroxyl side groups (changes in the color, polarity, solubility, and water absorption-desorption properties) and rupture of the glycosidic ether bonds between cellulose units (a decrease in the degree of polymerization, change in solubility, mechanical and other properties).4 For better weather performance, cellulose butyrate is preferred over cellulose acetate.5 7.1.5.2 Mechanisms and results of stabilization Ultraviolet absorbers and pigments provide protection against weathering of wood-plastic composites.6 The amount of protection can be influenced by photostabilizer concentration.6 The oxidation of lignin leads to the formation of p-quinone chromophoric structures, which is followed by the reduction of the p-quinone structures to hydroquinones, which causes photobleaching.7 High molecular weight diester HAS were found to be the most effective in controlling long-term fading and yellowing changes in wood-plastic composites.7 The addition of a benzotriazole ultraviolet absorber shows great synergism in controlling fading.7 Protection against harmful effects of UV component of solar radiation has been obtained by reaction of aminophenylsulfobenzotriazoles with the condensation product of 4-aminophenyl-sulfatoethylsulfone and cyanuric chloride.8 The UV absorbers with two different reactive groups (monochlorotriazine and aromatic vinylsulfone), capable of formation of covalent bonds with hydroxyl groups of cellulose, help to achieve long-lasting protection.8 The cellulose acetate butyrate in blend with polymethylmethacrylate reduced the effects of UV radiation by making polymethylmethacrylate more transparent to UV radiation while maintaining the amorphous structure of the blend.9

UV Degradation & Stabilization of Polymers & Rubbers

203

7.1.5.3 Data Table 7.5. Data on photodegradation and stabilization of cellulose-based polymers Spectral sensitivity, nm: 285 (cellulose nitrate),10 328 (rayon)11 Products of photodegradation: chain scission, radical formation Known influences of other factors: humidity, water, ozone Most important stabilizers: UVA: 2-(2H-benzotriazol-2-yl)-p-cresol; phenol, 2-(5-chloro-2Hbenzotriazole-2-yl)-6-(1,1-dimethylethyl)-4-methyl-; 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl1-phenylethyl)phenol; isopropenyl ethinyl trimethyl piperidol (cellulose diacetate),13 biphenyl cellulose (UV absorber fro paper),12 phenylbenzimidazole (reactive stabilizer for application in cellulosic textiles)14 Optical brighteners: 2,2’-(2,5-thiophenediyl)bis(5-tert-butylbenzoxazole) Mixtures:15 an ortho-hydroxy tris-aryl-s-triazine compound+hindered hydroxybenzoate compound+hindered amine compound containing a 2,2,6,6-tetraalkylpiperidine or 2,2,6,6-tetraalkylpiperazinone radical Concentration of stabilizers in formulations, wt%: 0.1-0.5 (UVA) Material and exposure conditions

Tenite Butyrate/Phoenix Arizona

Change

Refs.

after 36 months, tensile strength − 75% retention, elongation − 88% (black) and 11% (clear and colors) retention, impact strength − 84% retention (black)

5

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Donath, S; Militz, H; Mai, C, Holz Roh. Werkst., 65, 35-42, 2007. Wypych, G, Handbook of Materials Weathering, 6th Edition, ChemTec Publishing, Toronto, 2018. Cotana, F; Cavalaglio, G; Nicolini, A; Gelosia, M; Coccia, V; Petrozzi, A; Brinchi, L, Energy Procedia, 45, 52-60, 2014. Rubenziene, V; Varnaite, S; Baltusnikaite, J; Padlekiene, I, Understanding and Improving the Durability of Textiles. Chapter 6. Effects of light exposure on textile durability. Elsevier, 2012, 104-125. Tenite. Weathering of Tenite Butyrate. Eastman, May 1999. Stark, N M; Matuana, L M, Polym. Deg. Stab., 91, 3048-56, 2006. Mausher, M; Sain, M, Polym. Deg. Stab., 91, 1156-65, 2006. Akrman, J; Prikryl, J, J. Appl. Polym. Sci., 108, 334-41, 2008. Raouf, R M; Whab, Z A; Ibrahim, N A; Talib, Z A; Chieng, B W, Polymers, 8, 128, 2016. Hon, D N S; Gui, T L, Polym. Photochem., 7, 299-310, 1986. Saikia, C N; Ali, F; Goswami, T; Ghosh, A C, Ind. Crops Prod., 4, 233-39, 1995. Granstrom, M; Havimo, M; Heikkila, M; Kilpelainen, I, J. Mater. Chem., 19, 639-44, 2009. Khalikov, D K; Shanyavskii, I G; Kalontarov, I Y; Sanyukovich, G S, Vyssokomol. soyed., A19, 5, 1132-37, 1977. Kubac, L; Akrman, L; Burgert, L; Dvorsky, D; Gruner, P, J. Appl. Polym. Sci., 112, 3605-12, 2009. Sretranski, J A; Sanders, B M, US Patent 6,843,939, Jan. 18, 2005, Cytec.

204

7.1.6 Chlorosulfonated polyethylene

7.1.6 CHLOROSULFONATED POLYETHYLENE 7.1.6.1 Mechanisms and results of degradation Shear strength of seams increased significantly during the early aging exposures to xenonarc, and it remained at the higher level for the duration of the tests.1 This was true for both potable and industrial CSPE grades.1 Excellent seam durability is one of the main reasons for the selection of CSPE. The primary reaction of photodegradation of chlorosulfonated polyethylene is given below: SO2Cl {(CH2CH2)xCH2CH}yCH2CH Cl

n

hν -HCl

SO2Cl {(CH2CH2)xCH CH}yCH2CH n

It is quite similar to the photodegradation of PVC. It generates carbon-carbon double bonds in the main polymer chain and becomes the preferred site for further degradation or crosslinking in the polymer, leading to eventual brittleness of rubber.2 7.1.6.2 Mechanisms and results of stabilization Vulcanizates having excellent color stability and high hardness can be produced by increasing the amount of magnesia in a general purpose system to 20 phr and eliminating pentaerythritol.3 At this magnesia level, processing safety is marginal.3 Also, vulcanizates tend to stiffen and harden during outdoor exposure, whereas those containing less magnesia remain much more flexible.1 White products may have an initial pinkish cast, but the color disappears after short exposure to light.3 The silica filler contributes to water resistance.3 Water-resistant systems utilize organic lead salts as the acid acceptor in combination with MBT.3 Water resistant white vulcanizates may turn gray when exposed to sunlight.3 Acid acceptors play a dual role in the curing of chlorosulfonated polyethylene.3 They provide a readily available reactant for neutralization of acidic by-products of vulcanization (e.g., hydrogen chloride), which might otherwise catalyze polymer degradation; and they act as crosslinking agents, forming relatively weak ionic bonds.3 A variety of acidreactive polyfunctional materials will perform this role, but the acid acceptors primarily used are metal oxides, dibasic or tribasic organic lead salts, and epoxy resins.3 Magnesia (MgO) and litharge (PbO) are the most commonly used acid acceptors.3 Zinc oxide or zinc salts are undesirable because zinc chloride, formed during vulcanization, catalyzes degradation reactions that lead to poor weathering and heat resistance.3

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205

7.1.6.3 Data Table 7.6. Data on photodegradation and stabilization of chlorosulfonated polyethylene Products of photodegradation: HCl, crosslinking, double bonds Typical results of photodegradation: stiffening, yellowing Known influences of other factors: autocatalytic effect of HCl Most important stabilizers: carbon black, tetrakis(methylene (3,5-di-tert-butyl-4-hydroxyhydrocinnamate))methane,4 antiacids (see above) Concentration of stabilizers in formulations, wt%: several percents (unless carbon black is used for other purposes) (e.g., improvement of physical properties); 1 wt% (antioxidant) Material and exposure conditions

Longevity, years

Refs.

Reinforced geomembrane from Watersaver Co.

10 years warranty

Flat roof membranes

20 years warranty given by installers

2

Hypalon 40/Florida

10 years (100% modulus − 59.5 to 141.9% retention, tensile strength − 64.6 to 86.5% retention, elongation − 45 to 84% retention)

5

Hypalon geomembrane/outdoor Florida

water transmission rate after 20 years of exposure to direct sunlight was increased by 9%

6

References 1 2 3 4 5 6

Schoenbeck, M A, Geotextiles Geomembranes, 9, 337-41, 1990. Koerner, R M; Hsuan, Y G; Koerner, G R, Geomembrane Lifetime Prediction: Unexposed and Exposed Conditions, Geosynthetic Institute, June 7, 2005. Hypalon, Technical information, DuPont Performance Elastomers, Nov. 2, 2002. Peterson, A G, US Patent 5,523,357, Jan. 4, 1996, JPS Elastomerics Corp. Massey, L K, The Effects of UV Light and Weather on Plastics and Elastomers, 2nd Ed., William Andrew, 2006, pp 329-41. Ortego, J D; Aminabhavi, T M; Harlapur, S F; Balundgi, R H, J. Hazardous Mater., 42, 115-56, 1995.

206

7.1.7 Copolymers

7.1.7 COPOLYMERS Many copolymers are discussed in separate sections of this chapter. Also, many more are not included here. It is possible to extrapolate some properties from homopolymers to a combination of monomers in copolymers, but it is, at the same time, a very risky method because radicals and products of degradation coming from different segments of copolymer may also affect stability of other building units and therefore contribute to mechanisms which are not present in homopolymer degradation. It is, therefore, necessary to verify hypotheses by experimental studies before they can be applied in the assessment of the performance of an unknown material.

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7.1.8 EPOXY RESIN 7.1.8.1 Mechanisms and results of degradation The rate of chemical degradation for an amine-cured epoxy coating is always greater than that of the thickness loss.3 Photodegradation of an amine-cured coating is not a uniform thickness reduction (ablation), but it is an inhomogeneous erosion process, with the formation of localized nanometer-micrometer depressions and pits.3 Absorption of the energy of radiation in the range of 300-330 nm leads to the formation of radicals:5 CH3 C

O

O

CH3

hv

hv CH3

C

O

O

C

O

CH3

O

+

CH3

+

CH3

Pairs of radicals are formed from any of the two reactions, but the right-hand side reaction leads to chain scission and molecular weight reduction. The formation of a macroradical leads to further changes, most likely involving oxygen:5 O

O

H3C C

O

O2

H3C C OO

O

O

PH

O

H3C C OOH

O

+

P

O

hv H3C C O

O

+

β split

C O

OH

O + CH3

Chain scission reactions and the subsequent conversions of these low molecular weight products result in a variety of volatiles such as benzene, styrene, benzoic acid, benzaldehyde, and benzophenone.5 The unreacted epoxy rings are the terminal groups on chains. Hydrogen atoms around the oxirane ring are quite labile. Abstraction of a hydrogen atom leads to either alcohol, ketone, or aldehyde.6 Addition of nanoclay to epoxy resin composite improved its viscoelastic properties and increased activation energy of decomposition.7 The degree of UV degradation varied depending on the montmorillonite loading and the degree of cure.7 Formation of micropores was observed during degradation of epoxy resins.8 With further exposure, radicals recombine and form an impermeable surface layer with highly crosslinked structure.8

208

7.1.8 Epoxy resin

Figure 7.6. Surface morphology of epoxy after (a) 0 h (b) 300 h (c) 600 h (d) 900 h (e) 1200 h exposure to combined UV radiation and moisture by optical microscopy (100x magnification). [Adapted, by permission, from Afshar, A; Mihut, D; Hill, S; Baqersad, J, J. Compos. Mater., 52, 27, 3773-84, 2018.]

Two novel silphenylene-containing cycloaliphatic epoxy resins, 1,4-di [2-(3, 4-epoxycyclohexylethyl) dimethylsilyl] benzene and 1,3,5-tri [2-(3, 4-epoxycyclohexylethyl)

UV Degradation & Stabilization of Polymers & Rubbers

209

dimethylsilyl] benzene were synthesized.9 The resins exhibited much higher resistance to discoloration under UV irradiation than the commonly used epoxy resins containing diglycidyl ether of bisphenol-A.9 Corrosion resistance was improved by incorporating diketopyrrolopyrrole into epoxy resins.10 The single-coat epoxy system had UV stability and corrosion resistance.10 The organic dyes did not have a sensitizing effect on the epoxy matrix.11 The stability of the optical properties of the epoxy matrices exposed to the effects of different factors was found to depend on the nature of the epoxy polymer.11 The exposure to ultraviolet radiation, high temperature, and moisture had detrimental effects on the microstructure, strength, ductility, and toughness of epoxy and vinyl resins with more pronounced effects on the epoxy.12 The environmental damage to the microstructure of epoxy appeared primarily in the form of surface cavities and blisters while for the vinyl ester emerged mainly as microcracking events.12 The copper coating was an effective barrier in preventing environmental degradation of epoxy-based composites.12 Figure 7.6 shows changes in the epoxy morphology on exposure to UV radiation and moisture.12 After 300 h exposure, small pits, cavities, and cracks appeared on the surface of epoxy due to either mass loss caused by chain scission reactions or the ablation of the epoxy surface.12 The damage increased after 600 h of exposure and after 900 h of exposure pits and cavities gradually disappeared and the surface cracks linked together with small blisters showing after 1200 h.12 7.1.8.2 Data Table 7.7. Data on photodegradation and stabilization of epoxy resins Spectral sensitivity, nm: 300-330 Products of photodegradation: benzene, styrene, benzoic acid, benzaldehyde, and benzophenone Typical results of photodegradation: adhesion loss, brittleness, blister formation (especially in presence of water), yellowing, chalking Known influences of other factors: acid/base interactions can alter properties of products containing HAS1,2 (interference with acid-catalyzed crosslinking reactions); water Most important stabilizers: UVA: 2,4-dihydroxybenzophenone; 2-(2H-benzotriazol-2-yl)-pcresol; 2-benzotriazol-2-yl-4,6-di-tert-butylphenol Screener: nano-ZnO;4 nano-silica-titania4 Concentration of stabilizers in formulations, wt%: 0.1-0.5 (UVA); 0.07 (nano-ZnO) Material and exposure conditions

UV-LED stabilized with nano-ZnO

Longevity

171 h (97 h unstabilized)

Refs.

4

References 1 2 3 4 5 6 7 8

Schaller, C; Rogez, D; Braig, A, J. Coat. Technol. Res., 6, 1, 81–88, 2009. Schaller, C; Rogez, D; Braig, A, J. Coat. Technol. Res., 5, 1, 25-31, 2008. Rezig, A; Nguyen, T; Martin, D; Sung, L; Gu, X; Jasmin, J; Martin, J W, JCT Research, 3, 3, 173-84, 2006. Li, Y-Q; Yang, Y; Fu, S-Y, Composites Sci. Technol., 67, 3465-71, 2007. Monney, L; Bole, J; Dubois, C; Chambaudet, A, Polym. Deg. Stab., 66, 1, 17-22, 1999. Zhang, G; Pitt, W G; Goates, S R; Owen, N L, J. Appl. Polym. Sci., 54, 419, 1994. Tcherbi-Narteh, A; Hosur, M; Triggs, E; Owuor, P; Jelaani, S, Polym. Deg. Stab., 101, 81-91, 2014. Liu, F; Yin, M; Xiong, B; Zheng, F; Mao, W; Chen, Z; He, C; Zhao, X; Fang, P, Electrochim. Acta, 133, 283-93, 2014.

210 9 10 11 12

7.1.8 Epoxy resin Yang, X; Zhao, X; Zhang, Y; Huang, W; Yu, Y, J. Macromol. Sci., Part A: Pure Appl. Chem., 48, 692-700, 2011. Zeng, W; Qixin Zhou, Q; Haichang Zhang, H; Xiaoning Qi, X, Dyes Pigments, 151, 157-64, 2018. Laurinas, V C, IOP Conf. Ser.: Mater. Sci. Eng., 168, 012023, 2017. Afshar, A; Mihut, D; Hill, S; Baqersad, J, J. Compos. Mater., 52, 27, 3773-84, 2018.

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7.1.9 ETHYLENE-PROPYLENE COPOLYMER, EPR 7.1.9.1 Mechanisms and results of degradation Based on the results of many experimental studies, the following photochemical reactions take place:1 H CH2

C CH2

hν

CH2

CH2

C CH2

hν or Δ

CH2

CH2

C CH2

O2

OOH CH2

PH

O

hν or Δ

C CH2

+ P

C CH2 CH3

O CH2

C CH2

+

CH3

CH3

CH3 O

+ H

CH3

CH3 OOH

C CH2

hν

O CH2

C +

CH2

The oxidation reactions occur mostly in the propylene segments. Only weak oxidation occurs in the ethylene segments. The initiation of photooxidation in the propylene segment proceeds through hydrogen abstraction at the tertiary carbon atom where hydroperoxides are formed, leading to a variety of degradation products. The first set of equations shows the formation of the hydroperoxide. After exposure to UV or heat energy, hydroperoxides undergo homolytic dissociation, followed by the formation of carbonyl groups. During the degradation process, the carbonyl concentration increases rapidly, whereas the hydroperoxide concentration remains rather low (in fact, it is suggested that only hydrogen bonded hydroperoxides can be detected), implying that hydroperoxides decompose. The decomposition of a single hydroperoxide group gives rise to more than one carbonyl. Carbonyl groups can further absorb terrestrial UV radiation and form two radicals by the chain scission of the polymer backbone. It should be noted that if degradation proceeds with no oxygen, peroxyl radicals are produced by the photolysis of hydroperoxides which are present in the material. Figure 7.7 shows data on chain scission of polyethylene, polypropylene, and ethylene-propylene copolymer. It is noticeable that copolymer behaves as could be expected from a combination of two monomeric units. Similar data are for crosslinking after 3 weeks of exposure to QUV. When polymers are exposed for a longer period of time (6 weeks), surface degradation of polypropylene becomes more extensive than that of polyethylene and EPR degradation was almost identical to PP.2 7.1.9.2 Mechanisms and results of stabilization Figure 7.8 shows that UV is transmitted through the entire thickness of EPR plaque. ESR measurements permitted determination of HAS (Tinuvin 770) concentration, leading to the following conclusions:3 • the small intensity of UV radiation transmitted through the plaque thickness was sufficient for the formation of HAS-derived nitroxides on the non-irradiated side,

212

Figure 7.7. Depth profiles of LDPE, PP, and EPR scission rate after 3 weeks of exposure in QUV. [Adapted, by permission, from Shyichuk, A V; White, J R; Craig, I H; Syrotynska, I D, Polym. Deg. Stab., 88, 415-19, 2005.]

7.1.9 Ethylene-propylene copolymer, EPR

Figure 7.8. Transmittance of UV radiation at 310 nm through EPR plaque. [Data from Kruczala, K; Aris, W; Schlick, S, Macromolecules, 38, 6979-87, 2005.]

where the radical concentration was high because nitroxides were not consumed in the stabilization processes, and the rate of degradation was negligible • UV irradiation led to deterioration only on the irradiated side, and the nitroxides were consumed in the stabilization process It should be noted that conclusions, although maybe reflecting experimental data, should not be generalized, because if radiation proceeds through the entire thickness and oxygen is available on the other side of the sample, degradation should also occur on the other side. If it was not observed, it was either too small to notice or samples were exposed in such a way that oxygen was restricted on the back of the sample. 7.1.9.3 Data Table 7.8. Data on photodegradation and stabilization of ethylene-propylene copolymer Spectral sensitivity, nm: 300-360

Activation wavelength, nm: 300, 310

Products of photodegradation: free radicals, hydroperoxides, carbonyl groups, chain scission, crosslinking Typical results of photodegradation: loss of tensile and elongation Known influences of other factors: metals (e.g. iron salts) Most important stabilizers: UVA: 2-hydroxy-4-octyloxybenzophenone; 2,2’-methylenebis(6(2H-benzotriazol-2-yl)-4-1,1,3,3-tetramethylbutyl)phenol HAS: 1,3,5-triazine-2,4,6-triamine, N,N’’’[1,2-ethane-diyl-bis[[[4,6-bis[butyl(1,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]bis[N’,N”-dibutyl-N’,N”bis(1,2,2,6,6-pentamethyl-4-piperidinyl)Phenolic antioxidant: 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H, 5H)-trione

UV Degradation & Stabilization of Polymers & Rubbers Table 7.8. Data on photodegradation and stabilization of ethylene-propylene copolymer Concentration of stabilizers in formulations, wt%: 0.1-0.5

References 1 2 3

Wypych, G, Handbook of Materials Weathering, 6th Ed., ChemTec Publishing, Toronto, 2018. Shyichuk, A V; White, J R; Craig, I H; Syrotynska, I D, Polym. Deg. Stab., 88, 415-19, 2005. Kruczala, K; Aris, W; Schlick, S, Macromolecules, 38, 6979-87, 2005.

213

214

7.1.9 Ethylene-propylene copolymer, EPR

Figure 7.9. Mechanisms of EPDM degradation. [Adapted, by permission, from Kumar, B; Rana, S; Singh, R P, eXPRESS Polym. Lett., 1, 11, 748-54, 2007.]

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7.1.10 ETHYLENE-PROPYLENE-DIENE MONOMER, EPDM 7.1.10.1 Mechanisms and results of degradation Figure 7.9 shows the mechanism of degradation of EPDM. Compared with EPR, this mechanism is more complex, not so much because of differences in chemical structure, but because of EPDM applications including some products which require very high durability (e.g., roofing membranes), therefore EPDM is substantially more studied. Stability of EPDM depends on the third monomer (diene) type and amount. With an increasing amount of the third monomer, the UV stability of pure unvulcanized EPDM decreases.3 5-Ethylidene-2-norbornene, ENB,-containing EPDM is less sensitive to photooxidation than dicyclopentadiene, DCPD,-containing EPDM.3 However, DCPD-containing EPDM is more prone to UV crosslinking reactions, when compared with ENB-containing EPDM.3 Crosslinking of ENB-containing EPDM by dicumyl peroxide, DCP, substantially decreases the UV stability.4 The decomposition-product of DCP, acetophenone is not the reason for this UV stability decrease.4 It is the result of the presence of oxidation products inside the EPDM matrix after crosslinking, particularly hydroperoxides.4 7.1.10.2 Mechanisms and results of stabilization Degradation products in the presence of hydrotalcite (acid scavenger) were the same as when EPDM was degraded without hydrotalcite.1 The rate of EPDM degradation without hydrotalcite was faster than without it, but when degradation was conducted in an acidic environment, hydrotalcite presence hindered EPDM degradation.1 It is possible that hydrotalcite absorbs some additives which cause an increased degradation rate.1 The oxidation rate of EPDM can be efficiently reduced by antioxidants in thermooxidation conditions.6 In photooxidation conditions, antioxidants can induce EPDM degradation if they are not protected by UV absorbers.6 For this reason, the addition of UV absorber with antioxidants is a good practice.6 A multilayer film includes a fluoropolymer in the first layer; a UV resistant fluoropolymer adhesive layer, and EPDM in the third layer.7 The films find application in photovoltaic devices which are frequently degraded by exposure to UV if not sufficiently protected.7

216

7.1.10 Ethylene-propylene-diene monomer,

7.1.10.3 Data Table 7.9. Data on photodegradation and stabilization of ethylene-propylene diene monomer rubber Products of photodegradation: unsaturations and products of their degradation, crosslinks, chain scission, caboxylic acids, alcohols, aldehydes, and radicals Most important stabilizers: UVA: 2-hydroxy-4-octyloxybenzophenone; 2-(2H-benzotriazol-2yl)-p-cresol Screener: carbon black, titanium dioxide HAS: 1,3,5-triazine-2,4,6-triamine, N,N’’’[1,2-ethane-diyl-bis[[[4,6-bis[butyl(1,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]bis[N’,N”-dibutyl-N’,N”bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-; bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate Phenolic antioxidant: 2,6,-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5,-triazine-2-ylamino) phenol; pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate); 2-(1,1-dimethylethyl)-6[[3-(1,1-dimethylethyl)-2-hydroxy-5-methylphenyl] methyl-4-methylphenyl acrylate; 1,3,5tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione; 2,2'-ethylidenebis (4,6-di-tert-butylphenol) Other: hydrotalcite; 2,2’-thiodiethylene bis[3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate]; 4,4’thiobis(2-t-butyl-5-methylphenol); 2,2’-thiobis(6-tert-butyl-4-methylphenol); octylated diphenylamine, nickel dibutyldithiocarbamate5 Concentration of stabilizers in formulations, wt%: 0.15-0.7 (UVA), 0.02-0.05 (hydrotalcite), 0.05-1 (HAS); 0.0005-0.4 (antioxidant), 0.05-1 (thiosynergist) Material and exposure conditions

Nordel 1070 (white)/Florida 2 years

Change

Refs.

tensile strength − 76.2% retention, elongation − 98% retention

2

References 1 2 3 4 5 6 7

Kumar, B; Rana, S; Singh, R P, eXPRESS Polym. Lett., 1, 11, 748-54, 2007. Massey, L K; The Effects of Light and Weather on Plastics and Elastomers, 3rd Ed., William Andrew, Norwich, 2013. Snijders, E A; Boersma, A; van Baarle, B; Noordermeer, J, Polym. Deg. Stab., 89, 200-207, 2005. Snijders, E A; Boersma, A; van Baarle, B; Gijsman, P, Polym. Deg. Stab., 89, 484-91, 2005. Hewitt, N, Compounding Precipitated Silica in Elastomers, Elsevier, 2007, pp 311-44. Rivaton, A; Cambon, S; Gardette, J-L, Polym. Deg. Stab., 91, 136-43, 2006. Csillag, F J; Hong, K C, World Patent, WO2012033626, Saint-Gobain Performance Plastics Corporation, Mar. 12, 2012.

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7.1.11 ETHYLENE-TETRAFLUOROETHYLENE COPOLYMER Soiling effects of spectral light and solar transmittance decay of ethylene tetrafluoroethylene copolymer membranes after three and six months of exposure in Milano city outdoor urban conditions was studied with different tilt and orientation.2 Light transmission and solar heat gain coefficient were reduced by 4-8%.2 Electronic equipment was provided with an optical member (such as an optical film, an optical lens or a sealing material of a light emitting diode) having excellent transparency, heat and light resistance.3 A blend of two ethylene copolymers was protected against UV radiation by a surface layer of ethylene tetrafluoroethylene copolymer.4 This multilayer film was used in photovoltaic devices.4 7.1.11.1 Data Table 7.10. Data on photodegradation and stabilization of ethylene-tetrafluoroethylene copolymer Transmittance 300-400 nm, %: 92-94 Known influences of other factors: glass fiber reinforced grades are less stable1 Most important stabilizers: not used

References 1 2 3 4

Biron, M, Thermoplastics and Thermoplastic Composites, 3rd Edition, William Andrew, 2018. Mainini, A G; Poli, T; Paolini, R; Zinzi, M; Vercesi, L, Energy Procedia, 48, 1302-10, 2014. Taguchi, D; Ataku, M; Hamazaki, K, US Patent, US20140296367, Asahi Glass Company, Oct. 2, 2014. Prejean, G W; Samuels, S L, European Patent, EP2598331, DuPont, Jun 5, 2013.

218

7.1.12 Ethylene-vinyl acetate copolymer, EVA

7.1.12 ETHYLENE-VINYL ACETATE COPOLYMER, EVA 7.1.12.1 Mechanisms and results of degradation For vinyl acetate, the major reactions are:3 Norrish II (CH2CH2)n(CH2CH)m O

deacetylation

OH (CH2CH2)n(CH CH)m

O C CH3

+

mO C CH3 acetic acid

Norrish I (CH2CH2)n(CH2CH)m O

+

O C CH3

(CH2CH2)n(CH2C)m

+

mO C CH3 H aldehyde

In the Norrish II reaction, polyenes are formed in a manner similar to that which takes place during the PVC degradation mechanism. The formation of polyenes may lead to discoloration if their sequences are long enough (6 or more conjugated double bonds), but usually their sequences are too short to form deeply colored products. Polyenes may also undergo oxidation, which results in the formation of unsaturated carbonyls. The Norrish I reaction results in homolytic dissociation of the acetate bond. Each reaction produces different low molecular weight products, as indicated in the above scheme. Figure 7.10 shows that the molecular Figure 7.10. Molecular weight of EVA vs. exposure weight of unstabilized EVA decreases raptime in xenon arc Q-SUN1000. [Data from Jin, J; Chen, S; Zhang, J, Polym. Deg. Stab., 95, 725-32, idly on exposure to UV radiation from a 2010.] xenon lamp. It is also evident that increased concentration of vinyl acetate contributes to increased stability of copolymer. EVA exposed to gamma radiation had increasingly higher thermal stability with increased dose of gamma radiation received by the cable insulation.7 7.1.12.2 Mechanisms and results of stabilization The photolytic degradation rate can be reduced by both HAS (e.g., Tinuvin 770) and UV absorber (e.g., benzophenone) but results are not very encouraging because stabilizers are lost from material at a substantial rate. For this reason, oligomeric HAS are popular with EVA. EVA is the most widely used photovoltaic encapsulant. It contains additives to promote crosslinking and adhesion.8 They cause yellowing of the material over time, resulting in a loss of photovoltaic efficiency.8 Also, generation of acetic acid may cause serious

UV Degradation & Stabilization of Polymers & Rubbers

219

problems for thin-film high-efficiency cell materials (copper indium gallium diselenide).8 It has also been implicated in corrosion of aluminum interconnects in silicon panels.8 7.1.12.3 Data Table 7.11. Data on photodegradation and stabilization of ethylene-vinyl acetate copolymer Spectral sensitivity, nm: 2805 Products of photodegradation: hydroperoxides, hydroxyl groups, polyene sequences, aldehyde, acetic acid Typical results of photodegradation: yellow-brown discoloration,2 loss of mechanical properties Known influences of other factors: side acetate group of ethylene–vinyl acetate was hydrolyzed to the hydroxyl group under basic solution and the generated hydroxyl group self-catalyzed the hydrolysis of the neighboring acetate group6 Most important stabilizers: UVA: 2-hydroxy-4-octyloxybenzophenone; 2-(2H-benzotriazol-2yl)-6-dodecyl-4-methylphenol, branched & linear; propanedioic acid, [(4-methoxyphenyl)-methylene]-dimethyl ester HAS: 1,3,5-triazine-2,4,6-triamine, N,N’’’[1,2-ethane-diyl-bis[[[4,6-bis[butyl(1,2,6,6-pentamethyl-4-piperidinyl)amino]-1,3,5-triazine-2-yl]imino]-3,1-propanediyl]bis[N’,N’’-dibutyl-N’,N’’bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-; poly[[(6-[1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyl][2,2,6,6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl[2,2,6,6-tetramethyl-4piperidinyl)imino]]; 1,6-hexanediamine- N,N’-bis(2,2,6,6-tetramethyl-4-piperidinyl)-polymer with 2,4,6-trichloro-1,3,5-triazine, reaction products with N-butyl-1-butanamine an N-butyl2,2,6,6-tetramethyl-4-piperidinamine; butanedioic acid, dimethylester, polymer with 4-hydroxy2,2,6,6-tetramethyl-1-piperidine ethanol Phenolic antioxidant: pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) Amine: benzenamine, N-phenyl-, reaction products with 2,4,4-trimethylpentene Optical brightener: 2,2’-(1,2-ethylenediyldi-4,1-phenylene)bisbenzoxazole, C.I.F.B. 3671 Concentration of stabilizers in formulations, wt%: 0.15-0.7 (UVA), 0.15-1 (HAS), 0.2-1 (phenolic antioxidant), 0.1-0.5 (amine), 0.000025-0.00025 (optical brightener) Material and exposure conditions

Change

Refs.

Unstabilized EVA containing 14% VAc/Q-SUN1000, 400 h

tensile strength retention − 20%, elongation retention − 2%

4

Unstabilized EVA containing 18% VAc/Q-SUN1000, 400 h

tensile strength retention − 27%, elongation retention − 11%

4

References 1 2 3 4 5 6 7 8

Martini, T, Antec, 2002, pp 2525-29. Wu, C; Wicks, D A, Rev. Sci. Instruments, 76, 062212-1 to 062212-7, 2005. Wypych, G, Handbook of Materials Weathering, 6th Edition, ChemTec Publishing, Toronto, 2018. Jin, J; Chen, S; Zhang, J, Polym. Deg. Stab., 95, 725-32, 2010. Skowronski, T A; Rabek, J F; Ranby, B, Polym. Photochem., 3, 341-55, 1983. Ito, M; Nagai, K, Polym. Deg. Stab., 93, 1723-35, 2008. Boguski, J; Przybytniak, G; Lyczko, K, Radiat. Phys. Chem., 100, 49-53, 2014. Einsla, M L; Teich, C I; Bender, M T; Ottinger, J A; Greer, E C, Solar Energy Mater. Solar Cells, 165, 103-10, 2017.

220

7.1.13 Fluorinated ethylene-propylene

Figure 7.11. SEM image of FEP surfaces irradiated with a nitrogen ion beam with an energy of (a) 80 keV, (b) 180 keV, (c) 250 keV, (d) 300 keV, (e) 350 keV, and (f) 380 keV. [Adapted, by permission, from Kitamura (Ogawa), A; Kobayashi, T; Satoh, T; Koka, M; Kamiya, T; Suzuki, A; Terai, T, Nuclear Instruments Methods Phys. Res. B, 307, 614-17, 2013.]

7.1.13 FLUORINATED ETHYLENE-PROPYLENE 7.1.13.1 Mechanisms and results of degradation Prolonged space exposure shifts the failure mode of the FEP from ductile necking to more brittle-like fracture.2 Figure 7.11 shows SEM images of the FEP surfaces irradiated at a fluence of 5x1015 ions/cm2 and various energies ranging between 80 and 380 keV.3 When the energy was lower than 300 keV, micro-protrusions were formed (Figures 7.10(a)–(d)). At energies higher than 350 keV, the protrusions were sparse, and the surfaces appeared to be almost smooth (Figures 7.10(e) and (f)).3 These morphological changes were due to main-chain scission and subsequent evaporation of FEP molecules.3 At ion beam energies below 300 keV, evaporation occurred only at local areas with low molecular density, resulting in the formation of micropores.3 At energies exceeding 350 keV, erosion in the entire irradiated area increased because the surface temperature was notably elevated.3 The thickness of the sample decreased as a result of irradiation.3 Flat TiO2 thin films were dip coated on contact lens polymer substrates (fluorinated ethylene propylene copolymer, FEP) (grain sizes in the range 10-20 nm).4 The films had the antimicrobial capability on exposure to UV irradiation.4

UV Degradation & Stabilization of Polymers & Rubbers

221

7.1.13.2 Data Table 7.12. Data on photodegradation and stabilization of fluorinated ethylene-propylene Most important stabilizers: not known to be used Material and exposure conditions

50 mm film/Florida 15 years

Change

tensile strength retention − 91%, elongation retention − 66%

Refs.

1

References 1 2 3 4

Massey, L K; The Effects of Light and Weather on Plastics and Elastomers, 2nd Ed., William Andrew, Norwich, 2007, p. 75-77. Jones, J S; Sharon, J A; Mohammed, J S; Hemker, K J, Polym. Testing, 32, 602-7, 2013. Kitamura (Ogawa), A; Kobayashi, T; Satoh, T; Koka, M; Kamiya, T; Suzuki, A; Terai, T, Nuclear Instruments Methods Phys. Res. B, 307, 614-17, 2013. Doran, N; Chen, W-F; Koshy, P; Ho, K K K; Sorrell, C C, Mater. Lett., 212, 134-8, 2018.

222

7.1.14 Poly(3-hexylthiophene), P3HT

7.1.14 POLY(3-HEXYLTHIOPHENE), P3HT 7.1.14.1 Mechanisms and results of degradation The photodegradation rate of P3HT depends on its microstructure (regioregularity).1 Highly-regioregular P3HT is more photostable because of its higher crystallinity and purity.1 Its molecular weight has no impact on its photostability.1 The photooxidation of P3HT leads to the formation of low-molecular-weight carboxylic acids that can diffuse and migrate out of the polymer films.1 Under UV radiation (365 nm), the π-conjugated system and the hexyl side chain are degraded almost simultaneously, involving Norrish-type reactions.2 Under visible light (525 nm) only the π-conjugated system is destroyed.2 Photooxidation of P3HT results in a decrease in absorbance as a result of photodecomposition.4 Oxides generated by the partial molecular scission of P3HT increase resistance of polymer solar cells, resulting in a decrease of their performance.4 Table 7.13. Data on photodegradation and stabilization of poly(3-hexylthiophene) Spectral sensitivity, nm: 365, 540 Products of photodegradation: low molecular weight carboxylic acids Typical results of photodegradation: degradation of π-conjugated systems, decrease in absorbance Known influences of other factors: Fe impurities Most important stabilizers: multiwalled carbon nanotube; UV absorber+HALS Concentration of stabilizers in formulations, wt%: 3

References 1 2 3 4 5

Dupuis, A; Chung, P W W; Rivaton, A; Gardette, J-L, Polym. Deg. Stab., 97, 366-74, 2012. Hintz, H; Sessler, C; Peisert, H; Egelhaaf, H-J; Chasse, T, Chem. Mater., 24, 2739-43, 2012. Ratha, R; Goutam, P J; Iyer, P K, Org. Electronics, 15, 1650-56, 2014. Aoyama, Y; Yamanari, T; Ohashi, N; Shibata, Y; Suzuki, Y; Mizukado, J; Suda, H; Yoshida, Y, Solar Energy Mater. Solar Cells, 120, 584-90, 2014. Cominetti, A; Salvalaggio, M; Malatesta, V, US Patent, US20130150502, Eni SPA, Jun 13, 2013.

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7.1.15 PERFLUOROPOLYETHER, PFPE 7.1.15.1 Mechanism and results of degradation UV radiation creates low energy electrons, which initiate the formation of radicals causing chain scission of PFPE.1 Main chain scission occurs due to UV degradation in the methylene (fluorine) chain and the hydroxyl end chain.1 Chain scission is accelerated by oxidation.1 Perfluoropolyether is used as UV resistant fluoropolymer adhesive in UV resistant clear laminates.2 PFPE obtained by photopolymerization contains acyl fluoride and carboxylic acid as an end-group.3 The presence of acyl fluoride and carboxylic acid end-groups make PFPE highly reactive and unstable.3 To prepare inert PFPE, it was stabilized by UV radiation of mercury lamp which removed acyl fluoride end-groups. Table 7.14. Data on photodegradation and stabilization of perfluoropolyether Spectral sensitivity, nm: 365 Products of photodegradation: chain scission Typical results of photodegradation: lower molecular weight products Known influences of other factors: Lewis acids Most important stabilizers: cyclic phosphazines

References 1 2 3

Lee, J; Chun, S-W; Kang, H-J; Talke, F E, Macromol. Res., 19, 6, 582-8, 2011. Csillag, F J; Hong, K C, World Patent, WO2012033626, Saint-Gobain Performance Plastics Corporation, Mar 15, 2012. Saxena, S; Malik, P; Tyagi, A K; Seshadri, G; Mandal, U K, Radiat. Phys. Chem., 156, 44-9, 2019.

224

7.1.16 Polyacrylamide

7.1.16 POLYACRYLAMIDE 7.1.16.1 Mechanisms and results of degradation UV irradiation of crosslinked polyacrylamide gels at 254 nm caused scission of the backbone and pendant unsaturated units forming acrylamide.1 Linear polyacrylamides were more stable under the same conditions of exposure. Only a small concentration of acrylamide (50 ppm) was detected.2 Cosmetic or dermatological sunscreen preparations contain one or more UV filters, a combination of thickeners of polyacrylate and polyacrylamide.3 These preparations are resistant to water and exhibit, despite low UV filters content, improved sun protection and advantageous stability.3 Nanoparticles have a core that is wholly or partially formed by at least one UVabsorbing organic compound in crystallized form and coated with an outer layer formed by at least one hydrophilic polymer, such as polyacrylonitrile.4 Nanoparticles are used for protection of textiles and fibers.4 Sunscreen composition containing UV filters and polyacrylamide.5 References 1 2 3 4 5

Caulfield, M J; Hao, X; Qiao, G G; Solomon, D H, Polymer, 44, 3817-26, 2003. Caulfield, M J; Hao, X; Qiao, G G; Solomon, D H, Polymer, 44, 1331-37, 2003. Bleckmann, A; Eitrich, A; Hun, D; Koch, F; Mummert, C, World Patent, WO2012084603, Beiersdorf AG, Sep 27, 2012. Poncelet, O; Ranard, O, World Patent, WO2011083447, Commissariat A L’energie Atomique Et Aux Energies Alternatives, Jul 12, 2011. Halpern, S; Simonnet, J-T; Shah, A; Candau, D; Roudot, A, US Patent, US20140170093, L’oreal, Jun 19, 2014.

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7.1.17 POLYACRYLONITRILE 7.1.17.1 Mechanisms and results of degradation Exposure to ion beam or UV radiation (UV radiation was slower to cause changes) caused the intensity of the CN stretching band at 2240 cm-1 to decrease with longer exposure times, while new broadbands arose at 3240 and 3500 cm-1, assigned respectively to NH2 and NH stretching vibrations.2 These features were associated with polyene formation, followed by intramolecular cyclization that resulted in the tautomeric equilibrium:2 CN

CN

CN R

R

R

N

N

N

H

H

H

NH2

R

N

N

N

N

N

N

H

H

H

N

N

N

NH

NH2

The occurrence of these changes can be easily confirmed by measurement of electric conductivity which increases with cyclization and formation of polyene conjugations which are known to conduct electrical current (e.g., polyacetylene, or UV degraded PVC). Pre-oxidized polyacrylonitrile significantly improved visible light photocatalytic activity of TiO2 nanoparticles.5 The pre-oxidized polyacrylonitrile possessed cycled ladder structure (see the above scheme) with conjugated groups.5 It can absorb more visible light to enhance visible light photocatalytic activity of TiO2 nanocomposites.5 The e-beam irradiated polyacrylonitrile fibers show varying conjugation and transformation of C ≡ N to C = N − groups as a function of the dose.6 The e-beam irradiation lowers the onset temperature, extends the exothermic reaction, and improves the thermal stabilization as a result of cyclization of PAN molecules.6 The warp threads made of PAN in black color are resistant to the effect of UV radiation and do not yellow even in a test with a xenon lamp with repeated exposure to several cycles over a total of 350 hours.7 UV-sensitive fibers were made from polyacrylonitrile with 10,12-pentacosadiyonic acid used as a dopant.8 The fibers undergo a complex color change (at lower doses blue color formation occurred, but at higher absorbed doses a red color appeared that finally changed to brown).8 The fibers can be used for UV absorbed dose detection.8

226

7.1.17 Polyacrylonitrile

7.1.17.2 Data Table 7.15. Data on photodegradation and stabilization of polyacrylonitrile Spectral sensitivity, nm: 270 and 3103 Products of photodegradation: polyenes, imides, hydroperoxides, lactones,4 amides4 Typical results of photodegradation: discoloration, electric conductivity Most important stabilizers: benzophenone, benzotriazole, and benzoates1

References 1 2 3 4 5 6 7 8

Wood, M; Hyun, J; Suhadolnik, J; Trainor, K; McCusker, M; Smith, A, US Patent 6,740,132, May 25, 2004, Ciba. Aggour, Y A; Aziz, M S, Polym. Test., 19, 261-67, 2000. Andreyeva, O A; Burkova, L A; Platonova, N V, Polym. Sci. USSR, 20, 12, 2721-28, 1988. Mailhot, B; Gardette, J-L, Polym. Deg. Stab., 44, 223-35, 1994. Luo, Q; Li, X; Li, X; Wang, D; An, J; Li, X, Catalysis Commun., 26, 239-43, 2012. Park, M; Choi, Y; Lee, S-Y; Kim, H-Y; Park, S-J, J. Ind. Eng. Chem., 20, 1875-78, 2014. Bauer, W, US Patent, US20140030949, Sattle AG, Jan. 30, 2014. Kozicki, M; Sąsiadek, E; Karbownik, I; Maniukiewicz, W, Sensiors Actuators B: Chem., 213, 234-43, 2015.

UV Degradation & Stabilization of Polymers & Rubbers

7.1.18 POLYALKYLFLUORENE 7.1.18.1 Data Table 7.16. Data on photodegradation and stabilization of polyalkylfluorene Spectral sensitivity, nm: 368-3831

References 1

Lee, R-H; Chen, W-S; Wang, Y-Y, Thin Solid Films, 517, 5747-56, 2009.

227

228

7.1.19 Polyamide

7.1.19 POLYAMIDE 7.1.19.1 Mechanisms and results of degradation Hydroperoxides are produced when the material is processed and they are potential initiation centers of photooxidation reactions. Scission of the amide linkage dominates photolytic reactions: H

hv

CH2CNCH2

CH2C

O

+

NCH2

O

H

This reaction is typical of both aliphatic and aromatic polyamides, and it is a starting point for further conversions. The carbonyl radical, after regrouping, forms volatile products: O CH2C

CH2

CH2CH2CH2

+

CO

CH2 +

CH2 CH2

Both radicals are capable of abstracting hydrogen from other molecules: CH2CNCH2

+

R

CH2CNCH

+

RH

and producing crosslinks: OH 2

CH2CNCH

CH2CNCH

CH2CNCH OH

The above radical is very stable and persists causing crosslinking. The crystalline structure of the polymer affects the probability of crosslinking. In the amorphous areas, the hydrogen bonds are less numerous, radicals migrate freely, and become involved in crosslinking reactions. The presence of oxygen during photolysis changes the mechanism of degradation. Individual reactions are favored and they cause an overall increase in the rate of degradation. The initial steps of photooxidation are similar to photolysis. Both carbonyl and amino radicals are formed. The next stage of photooxidation involves the formation of hydroperoxides from existing radicals: H CH2CNCH2

hv

H CH2CNCH

O

O -P H

CH2CNC O O

hv -H2O

O2/PH H

CH2CNCH O OOH

UV Degradation & Stabilization of Polymers & Rubbers

229

Many other reactions occur, and they explain the formation of low molecular substances which are detected in studies on polyamide photodegradation. Many of these mechanisms and changes of material structure and properties are discussed elsewhere.2 Changes in the sample morphology due to an increase in the mold temperature during molding resulted in a decrease of the weatherstability of the injection molded polyamide-6 under a load sufficient to induce creep.9 There was no difference in the weathering of the unloaded samples.9 This variation in the weathering of the samples under load is related to differences in the thickness and perfection of crystalline regions in the weaker non-equilibrium outer layers of the samples.9 7.1.19.2 Mechanisms and results of stabilization Combination of phosphite and phenolic antioxidant is sufficient for stabilization of interior automotive parts, but for exterior automotive parts a combination of carbon black, HAS, and phenolic antioxidant is required.4 N,N’-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1,3-benzenedicarboxyamide is an example of beneficial tailoring of the HAS molecule by “molecular recognition.” This HAS improved compatibility with an aromatic polyamide (aramid) by mimicking its construction unit.5 The light fastness of dyed fabrics from polyamide-6 can be dramatically enhanced by aftertreatment with some nickel arylsulfonates.6 Particularly, benzenesulfonate and 1naphthalenesulfonate give a very good effect.6 Phosphites cannot directly prevent depolymerization type reactions (e.g., hydrolysis), but they can function as scavengers of hydroperoxides generated by C−H bond scission and subsequent reaction with molecular oxygen.7 Phosphites can also function as acid scavengers or as acid group blockers.7 The presence of the HAS in polyamide-6,6 slows down its crystallization and increases the amount of the oxidizable amorphous phase, leading to a reduction in the efficiency of the stabilizer.8 The presence of the HAS also causes an increase in the amounts of carboxylic acids formed through the oxidation of acylperoxy radicals produced by reaction of the atmospheric oxygen with acyl radicals.8 The poor UV stability creates problems in applications of aramid fibers.16 Fe3+ was doped onto aramid fiber by coordinating with benzimidazole unit of the fiber structure.16 The tensile strength retention of the Fe-coordinated fiber was as high as 96% after UV irradiation, compared with 73% for the untreated fiber.16 7.1.19.3 Data Table 7.17. Data on photodegradation and stabilization of polyamide Spectral sensitivity, nm: polyamide-6: 290-310, 340-460 nm; aromatic polyamides 45 nm cannot penetrate nor permeate the skin.20 In addition to the particle size, some other influences are illustrated in Figure 12.1.20

462

12.5 Safety concerns with nanoparticles

Figure 12.1. Skin absorption of nanomaterials. NP = nanoparticle (non metal), MNP = metal nanoparticles, I = ions released. [Adapted, by permission, from Filon, F L; Mauro, M; Adami, G; Bovenzi, M; Crosera, M, Regulatory Toxicol. Pharmacol., 72, 2, 310-22, 2015.]

UV Stabilizers - Health & Safety

463

There is a discrepancy between studies of absorption of mineral nanoparticles such as ZnO by animals (much higher uptake detected) and humans.21 It is generally accepted based on current results that ZnO nanoparticles do not penetrate the viable epidermis or cause cellular toxicity in human skin.21 Because of the broad-spectrum UV filtering characteristics, UV stability, nonirritating nature, hypoallergenicity, and visible transparency of ZnO nanosunscreen is the preferred option for people with sensitive skin and allergic reactions to organic chemical UV filters.21 Two sunscreens with SPF of 50+ contained >50% in number of TiO2 particles exhibiting sizes between 1 and 100 nm.22 Sunscreen for creamy application complied with the safe limit of 25% in weight, so it did not pose a dermal risk; on the contrary, sunscreen used for sprayable applications may be of concern because of possible risks due to inhalation.22 Studies are still in progress, especially studies on the fate of nanoparticles which managed to enter a living organism. Animal studies are slightly more advanced but not sufficient to give us a clear picture of the safety of formulating with nanoparticles.

REFERENCES 1 2 3

4 5 6 7

8 9 10 11 12 13 14 15 16 17 18 19

Title 15 - Commerce and Trade. Chapter 53. Toxic Substances Control. Subchapter I - Control of Toxic Substances. Wypych, G, Handbook of Plasticizers, Wypych G., Ed., 3rd Edition, ChemTec Publishing, Toronto 2017. Chemical Hazard Data Availability Study. What Do We Really Know About the Safety of High Production Volume Chemicals? EPA’s 1998 Baseline of Hazard Information that is Readily Available to the Public Prepared by EPA’s Office of Pollution Prevention and Toxics, April 1998. EPA information on 2863 U.S. HPV Chemicals from 1990 IUR Update. Master Summary for the Chemical Hazard Data Availability Table, EPA. Identifying and Regulating Carcinogens, NTIS, November 1987. 2001 Toxic Release Inventory. Public Data Release. Appendix C. Basis of OSHA Carcinogen Listing for Individual Chemicals. Agents Classified by the IARC Monographs, Volumes 1–100. 2010. State of California. Chemicals known to the site to cause cancer or reproductive toxicity. March 2019 Documentation of the Threshold Limit Values and Biological Exposure Indices, 7th Ed. ACGIH, 2015 Supplement. Office of Response and Restoration, National Ocean Service, National Oceanic and Atmospheric Administration. Occupational Exposure Limits, March 2002. US Department of Labor. Occupational Safety & Health Administration. Standards - 29 CFR. Table Z-1. Limits for Air Contaminants. - 1910.1000. NIOSH Pocket Guide to Chemical Hazards. NIOSH Publication No. 2005-149. November 2018. Occupational Exposure Limits, OELs, Official J., L188, p.14, (1995). Code of Federal Regulations. Title 21. Food and Drugs. Chapter 1. Food and Drug Administration, Department of Health and Human Services. Herrling, T; Fuchs, J; Rehberg, J; Groth, N, Free Radical Biol. Medicine, 35, 1, 59-67, 2003. Tang, Y; Cai, R; Cao, D; Kong, X; Lu, Y, Toxicology, 406-407, 1-8, 2018. Baumann, L, Facial Plastic Surgery Clinics of North America, 26, 407-18, 2018. Lim, H W; Arellano-Mendoza, M-I; Stengel, F, J. Amer. Acad. Dermatol., 76, 3, Suppl. 1, S91-s99, 2017. Millington, K R; Osmond, M J; McCall, M J, J. Photochem. Photobiol. B: Biol., 133, 27-38, 2014. Corinaldesi, C; Marcellini, F; Nepote, E; Damiani, E; Danovaro, R, Sci. Total Environ.,

464

References 637-638, 1279-85, 2018.

20 Filon, F L; Mauro, M; Adami, G; Bovenzi, M; Crosera, M, Regulatory Toxicol. Pharmacol., 72, 2, 310-22, 21 22

2015. Wright, P F A, J. Investigative Dermatology, 139, 2, 277-8, 2019. Bocca, B; Caimi, S; Senofonte, O; Alimonti, A; Petrucci, F, Sci. Total Environ., 630, 922-30, 2018.

Index

465

Index Numerics 1,4-butadiene unit 189 1,4-cis-polyisoprene 317 1,4-polybutadiene 51 2,2,6,6-tetramethylpiperidine 261 2,6-di-t-butyl-4-hydroxytoluene 290 2,6-di-t-butyl-4-methylphenol 53 2'-hydroxybenzotriazole 313 2-hydroxybenzophenone 69, 290, 313 2-hydroxyphenylbenzotriazole 261 4-benzoyloxybenzenesulfonic acid 51 4-hydroxy-3-methoxycinnamic acid 49 A ab initio methods 154 ablation 41, 207, 209, 399 abrasion 378, 406 resistance 362, 403, 410 ABS 66, 160, 312 β-scission reaction 190 cage reaction 190 chain scission 190 crosslinking reaction 190 degradation 190 rate 189 electron withdrawing group 190 failure mechanism 378 pipe 378 yellowing 190, 191 absorbance 40, 56 absorbed energy 21, 47, 356 emission 18 utilization 25 radiation 29 absorber 39 molecule ground state 44 absorbing species 16, 37 absorption 15, 19, 37, 39, 40, 433 capacity 366 cross-section 37, 39 intensity loss 294 maximum 1

shift 177 range 71 recovery lifetime 152 spectrum 20, 28, 71, 265 theory 19 wavelength 178 /extinction coefficient 41 absorptivity 38, 283 acacia 252 accelerated aging 363, 388 studies 329 testing 346 weathering 165, 236, 387 accelerating factor 274 acceleration criterion 336 factor 394 accelerators 25 acceptor 25 concentration 154 acclimation response 2 acetate bond 218 acetic acid 273, 281 acetone 273 acetophenone 215, 281 acid 274 acceptor 204 chloride 315 cure catalyst 48 dye 51 filler 48 group blocker 229 neutralization 61, 161 paper 382 precipitation 332 rain 48, 160-161, 179, 331 resistance 387, 390 residue 161 scavenger 161, 167 scavenging mechanism 61 transformation product 48

466 /base balance 169 character 170 characteristic 71 interaction 41, 48, 178-179, 342, 433-435 mechanism 179 properties 178-179 acidic environment 48, 61 impurity 161 proton 179 acidification 200 acidity 152 acidolysis 265 acoustic emission 449 acrylic 194, 345, 381 acid 195 crosslinking 194, 196 film 371 latex 339 matrix 388 resin coating 194 acrylonitrile 166 -butadiene-styrene 189 -styrene-acrylate 187 data 188 action spectrum 34 activation barrier 21 energy 374 spectrum 33, 440 active center 154 layer 374 region 374 species 448 acute toxicity 454 acyl azide functionality 164 acylated microcrystalline cellulose 259 acylperoxy radical 229 additive 25 distribution 182 loss 156 solubility 162 -polymer interaction 162 adhesion 257, 340, 390 promoter 261

Index adhesive 323, 406 bond strength 340 damage 340 edge 323 joint 324 lifetime 324 expectation 324 performance 323 stabilization 325 surface morphology 326 yellowing 326 admixture 51 adsorption capacity 373 isotherm 5, 433 adverse conditions 4 aerospace 327, 390, 399 vehicle 327 aesthetics 410 afterglow 28 agglomerate 41 agglomerate formation 41 aggregated region 347 aggregation 4, 347 aging 163, 461 analysis 388 process 413, 447 temperature 358 agricultural chemicals 179, 251 film 155-156, 166-167, 179, 328, 358 materials 328 mulch lifetime 328 agriculture 328 air contaminants 459 limits 459 duct 384 aircraft 257 alcohol 207 aldehyde 207 end-group 241 algae 160, 276, 369 aliphatic polyester 242 polyurethane 288 alkanes 20

Index alkoxy radical 166, 307, 319 reaction 190, 311 alkyd color change 199 drying resin 199 hydroxyl content 199 paint 200 resin 199 stabilization 200 radical 48, 55, 56 alkylbenzene sulfonic acid 380 alkylperoxyl radical 55-56 allergic reaction 463 allergy 61 allowed transition 19 alumina 170 coating 200 aluminum flake 331 American Conference of Governmental Industrial Hygienists 457 football players 6 amide linkage 228 amine 64, 66, 143, 409 antioxidant 53 bridged 66 salt 434 amino group 52, 54, 253 radical 287 aminoacid 372 aminophenylsulfobenzotriazole 202 ammonium polyphosphate 436 amorphous area 228 phase 229, 240 region 160, 251, 258 segment 265 structure 358 analytical balance 441 anatase 69, 461 anhydride 250, 257, 315 anhydrosugar 411 animals 1 antagonism 68, 313, 437 antagonistic behavior 68

467 interaction 449 mechanism 179 mixture 68 antenna reflector 327 anthropogenic activities 48 impact 461 anti-aging mechanism 276 properties 253 antiblocking agent 436 antibonding 15 orbital 20 anticancer 350 antidiabetic 350 antifatigue agent 164 anti-inflammatory properties 5 antioxidant 2, 51, 53-54, 66, 164, 179, 215, 252, 360, 388, 409, 426, 446-447 absorption 170 activity 5, 179 depletion 181, 378 time 163 efficiency 170 potential 350 properties 350 supporting effect 49 antioxidation 55, 276 mechanism 55 antioxidative process 56 antiozonant 164 antistatic agent 434 properties 390 antitumor 350 antiviral 350 apolar solvent 169, 446 appearance 394 aquatic environment 380 aramid fiber 229, 356 Archean Earth 6 architectural glass 407 Arctic 3 aromatic carboxylic acid 169 ring 24, 27 unit 242

468 Arrhenius approximation 440 behavior 408 fit 378 law 160 modeling 181 artificial leather 336-337 aryl-carbonyl bond 152 arylsulfonate 229 ASA near-infrared reflectance 187 ascorbic acid 2, 5 asphalt shingle 387 asphaltene 387 cluster 62 ASTM 439-440 atmospheric pollutants 48 atomic layer deposition 275, 360 orbital 20 oxygen 300, 323, 327, 399, 441 flow 399 attenuation cross-section 42 autohomosynegism 49 automotive bumper 290 coating 331 chain scission 332 crosslinking 332 depth profiling 332 requirements 331 industry 407 seat foam 363 autooxidation 60, 69, 171 autooxidative degradation 265 aviation 327, 399 avobenzone 169, 414, 416, 419, 446, 449 photoinstability 419 Avogadro's number 10 avoidance mechanism 2 azimuth 3 azo group 69 B back-biting 273 backbone scission 300 backing material 14 bacteria 329 ballast tank 340

Index ballast tank coating 340 balloon 375, 399 bamboo 411 charcoal 366 fiber 394 band gap 45, 283 barium titanate 187 particle 187 basecoat 182, 332 basic moiety 179 toxicity information 454 basicity 48, 275 bathochromic shift 26 battery 354 beam energy 16 width 12 beech wood 442 Beer-Lambert law 16, 37 benzaldehyde 207, 281 benzene 207, 282 ring 265, 280, 297 benzenesulfonate 229 benzimidazole unit 229 benzoic acid 207, 281 anhydride 281 benzophenone 44, 65, 69, 77, 152, 157-158, 162, 164, 167, 169, 177, 183, 207, 310 derivative 294 stabilizer 156 -3 373 benzoquinone 49 benzotriazole 7, 44, 65, 81, 151, 153-154, 162-163, 168, 171, 202, 310, 315, 433-434, 446 moiety 180 ring 154 benzoyl group 165 benzyl cinnamate 413 hydroperoxide 247 radical 269 salicylate 413 bicycle helmet 385 bilberry 349 bimolecular decomposition 58

Index binder durability 340 bioactive compound production 3 biocide 179 biocompatibility 350 biocompatible 375 biocomposite packaging film 259 biodegradation 7 biological growth 160 biologically effective irradiance 6 biology 335 biosphere 6 bis-uracil derivative 337 bitumen 62 oxidation 62 black body 11 temperature 11 blackberry 349 bleaching 461 treatment 411 bleeding 164, 275 blend 261 amorphous structure 202 blister 209, 343, 346 blistering 346 blooming 162, 252 boat 385 Bohr 17 statement 18 theory 18 bomb blast resistant 370 cleavage 200 energy 11, 21 scission 229, 237 strength 11 bonded substrate 323 bonding 19 orbital 20 type 20 Born-Oppenheimer approximation 19, 21, 24 bound UV absorber 165 Brazilian bamboo species 350 breaking strength 362 bridging defect 374 brightness 382 brittle behavior 378

469 -like fracture 220 brittleness 204 brominated flame retardant 283 brown hair 372 Brownian motion 28 building joint 390 material 394 bulk carrier 340 erosion 258 mechanism 240 bumper 333 butadiene core 189 butyl methoxydibenzoylmethane 373 sealant 370 butylated hydroxyanisole 386 hydroxytoluene 386 C Ca/Zn stearates 61 cable 257, 408 insulation 218, 408 jacket 408 jacketing 409 sheath 384 cage reaction 265, 311 calcium carbonate 170, 382, 434 stearate 61, 167 cancer 456, 461 -causing drugs 456 canonical nucleobase 20 canvas painting 326 cap layer 237, 381 capacitor 354 capped sheet 392 capping 265 car component design 331 manufacture 331 washing 331 carbodiimide 244 carbon black 6, 47, 49, 229, 244, 251-252, 275-276, 337, 360, 369, 406, 434-435

470 shielding effect 435 structure 251 dioxide 282, 318 emission 194 nanofiber 327, 437 nanotube 38, 40, 155, 166, 400 radical trapping 58 steel substrate 380 tube 448 -carbon double bond 204 carbonation 387 carbonic acid 62 carbonium ion 55 carbonyl 196, 250, 253, 273, 297 absorbance 68 center 251 content 387 ester band 194 formation 190, 242 depth profile 190 group 2, 27, 51, 57, 211, 250, 291, 317, 410, 448 growth 329 radical 228 carboxyl 241, 250 carboxylic acid 250-251, 315, 319 carcinogen 456 list 456-457 carcinogenic effect 453 substance 456-457 cargo area 340 carotene, beta 5, 49 carotenoid 2-3 carrier transporting material 374 catalase 2 catalyst 28, 161 residue 58, 402 catalytic activity 155, 279 effect 342, 434 catastrophic failure 327 catheters 375 cationic photopolymerization 36 cedar 387 roofing shakes 387 cedar shingle 387

Index ceiling 458 board 384 cell cycle 3 degradation 397 membrane 1 wall 410 cellular toxicity 463 cellulose 202, 274 acetate 202 acetate butyrate 202, 261 butyrate 202 crystallinity 410 fabric 356 -based polymers 202 CeO2 nanoparticle 283 ceramic 387 tile 387 chain bending 32 branching 250 reaction rate 273 breaking 56 antioxidation 55 donor 253 mechanism 56 cleavage 237, 258 flexibility 22, 25, 26 scission 63, 171, 194-195, 202, 207, 209, 211, 223, 241, 250, 260-261, 265, 286, 296, 312, 317, 318, 328, 360, 403, 441 segment 295 stiffening 302 structure 25 -end hydroperoxide 319 chalking 199, 367, 385, 412 chaotic radiation 12 charge migration 32 transfer complex 27, 35, 66 interaction 36, 180, 435 chelate 51 chemical analysis 443 attack 329 change 23

Index conversion mechanism 177 degradation 343 group 253 interference 433 reaction 15, 32, 435, 441 reactivity 435 resistance 340, 378 structure 215 Substances Inventory 453 transformation 446 chemi-crystallization mechanism 240 chemiluminesce 446 chemisorption 65, 332, 433 chemometrics 39 chestnut 411 chitosan 51 chlorine atom 297 radical 295 chloromethylene 295 chlorophyll 3 concentration 3 chlorosulfonated polyethylene 204-205 chromatic change 261 chromophore 19, 26-27, 37, 53, 58, 170, 304, 382 absorption wavelength 26 photosensitization 160 chromophoric group 39 properties 27 structure 202 chronic toxicity 454 cisoid arrangement 179 cladding 343 clay 413 cleanup cost 407 clear laminate 223 clearcoat 172-173, 182, 290, 331-332 climatic conditions 440 cloud cover 3 coal tar coating 362 epoxy 339 coastal engineering structure 369 coated fabric 336

471 material 331 coating 48, 180, 197, 331, 333, 339, 410 layer 346 matrix 289 performance 289 requirements 339 specification 339 structure 340 surface 196, 290 thickness 346 weatherability 59 cobalt naphthenate 251 stearate 252 co-condensation 54 coextruded film 156 coextrusion 360 coffee beans 2 coherence 12 coil-coating 345-346 requirements 345 collagen crosslinking 414 fiber 349 collision 22, 32 diameter 25 probability 23 transfer 32 collisional transfer 25 color 71, 181, 348, 356, 410 change 202, 225, 288, 352, 442 development 288 difference 382 shift 170 stability 204 columnar structure 358 commercial sunscreen 415 compatibility 33, 40, 62, 71, 177-178, 275, 402, 440 competitive reactions 194 complex mixture 39 complexation 167, 168 composite 69 deck 343 durability 343 laminate 327 compression 171-172, 363

472 compression mode 171 set 390 concentration 39 profile 446 concrete pavement 391 condensation polymerization 165 conditions of exposure 440 conduction band 45 conductive 32 caulking 354 elastomer 354 pathway 340 polymer 35, 341 confocal microscopy 445 conformation 179 conformational isomer 179 conidia production 3 conjugated double bond 27, 63, 242, 282, 295, 315 polymer 272 conjugation 26 length 374 constant current stress 374 contact angle 258, 286, 359, 388 energy transfer 53 container 385 contaminant 25 contamination 340 controlled accelerated degradation 328 release 185 surface chalking 46 conversion 289 efficiency 397 cool material 187 roof 387 roofing material 388 cooling effect 387 copolyester sheeting 441 copolymer 206, 252 copolymerization 165, 265 copper coating 209 stearate 49

Index coral reef 461 corpuscle 17 corpuscular description 9 corrosion 341, 378, 390, 445 performance 290 protection 345 resistance 209 test 384 -protective coating 331 corrosive substance 331 cosmeceutical 348-349 cosmetic formulation 67 stability 155 cosmetics 335, 349, 461 cotton 337, 382, 403 fabric 232 Coulombic transfer 25 Council on Environmental Quality 453 covalent attachment 434 covalent bonding 433 cover plate 396 crack 274 formation 444 orientation 444 propagation 449 surface area 444 cracking 305, 343 crazing 291 creep 229 critical value 340 crop propagation film 328 cropping film 328 crosslink 160, 228, 241, 281 density 305 formation 250 crosslinked network 313 crosslinker 318 crosslinking 63, 190, 194, 196, 204, 228, 240-241, 250, 261, 296, 311, 315, 360, 374, 403, 441, 448 agent 204 probability 228 reaction 317 crust 159 crystal imperfection 417 crystallization properties 71

Index crystalline material 154 polymer 28 region 229, 258 residue 258 structure 159, 228, 302, 443 crystallinity 41, 160, 222, 240, 251, 259, 265, 289, 329, 340 crystallite size 283, 286 crystallization 259, 261 cultivation conditions 328 cultural heritage object 382 cure 390 conditions 172 cured rubber 337 cut-off filter 6 cyanoacrylate 98, 170 cyanobacteria 160 cyanobacterial mats 3 species 3 cyclic reduction 367 cyclization 225 cycloaddition 153, 417 cyclohexanedienone 162 cytotoxicity 461 D damage binding protein 5 damaged skin 461 dark skin 5 data 453 daylight spectrum 295 de Broglie 17 De mayo reaction 417 deacetylation degree 52 deactivation 21-22, 24, 48, 151, 170 after intersystem crossing 44 mechanism 20 process 151 dealkylation 417 debonding 343 decabromobiphenylether 283 decarbonylation 241 decarboxylation 241 decking 343 decomposition 31, 207, 319 temperature 259

473 decorative film 367 decorative foil 367 defect 444 center 302 intensity 154 repair 63 degradable network 268 degradation 11, 348 acceleration 2 byproduct 161 cause 442 depth 410 kinetics 39 mechanism 24, 58, 187, 410 product 41, 211 product accumulation 2 rate 13, 157, 251 time 374 timing 328 degradative damage 24 process 58, 191, 441 reaction 265 dehydrochlorination 296 delamination 237, 360, 370 Delaney clause 456 delayed fluorescence 29-31 delivery 2 vehicle 268 delocalizing character 56 delustrant 403 Denisov cycle 48 dental 351 implant 351 deoxyribonucleic acid 1, 21, 335 deployable solar panel 327 depolymerization 229 deposition time 286 depth profile 212 profiling 172, 242, 448 derivative UV spectrophotometry 449 deterioration 442 developmental and reproductive toxicity 454 device efficiency 407 lifespan 374

474 dew 48 dialkylthioglycolate 48 diazo pigment 447 dicumyl peroxide 215 dicyclopentadiene 215 dielectric constant 354, 426 fluid 354 loss 354 diethylstilbestrol 380 differential thermogravimetry 265 diffraction 9 diffusion 2, 162, 177 coefficient 71, 159, 162-163, 181, 275 control 181 distance 23 rate 23, 64, 162, 178, 290 -limited reaction 158 diketopyrrolopyrrole 209 dimensional change 387 stability 165, 385, 412 dimeric excited species 31 dimerization 4, 54 dimethylacrylate 16 dimethylene group 304 diphosphite 252 dipole formation 12 -dipole interaction 32 transfer 32 direct additives to food 459 analysis in real time 448 cleavage 58 oxidation 319 radiation-less deactivation 44 directional control 25 dirt 369 accumulation 276 particles 444 pickup 390 discoloration 164, 218, 287, 296, 315, 324, 367, 396, 442 discrete energy levels 17 disinfectant properties 200

Index disinfestation 382 dispersion 37, 40, 71, 178, 258, 440 index 40 disproportionation 45 dissociation 21 energy 305 disulfide cleavage 372 metathesis 313 dithiocarbamate 64, 66 dithiophosphates 66 dithiopropionate 64 diurnal changes 2 DNA damage 4 signaling downstream 3 lesion 1 mutations 461 repair pathways 4 process 3 replication 2 transcription 2 donor 25 door profiles 352 dopant 225, 279 doped polymers 23 dosimeter 270 dosimetry 302 double bond 26, 241, 258, 282, 313, 317 concentration 318 migration 63 substitution 63 vicinity 315 drain element 368 drug delivery components 375 release rate 258 dry climate 352 drying time 382 ductility 363 durability 288, 339, 400, 410 durable nitroxide radical 447 dust 396, 441-442 deposition 396 formation 184 dwell time 173

Index dye 51 bonding 403 durability 403 fastness 403 dynamic equilibrium 26 E Earth 6 ecotoxicity 454 eggplant 329 Egypt 6 elastic recovery 390 elastin 349 electric 354 conductivity 225, 341, 385 field 12 power cable 409 electrochemical activity 341 impedance spectroscopy 445 mechanism 341 electroluminescent material 272 electrolyte 340 electromagnetic radiation 9, 18, 20 intensity 18 theory 9 electron 17, 20, 45 acceptor reduction 45 donating group 153 donor oxidation 45 energy 19 exchange 30 low energy 223 migration 45 motion 19 paramagnetic resonance spectroscopy 372 spin 20 resonance 305, 447 imaging 446 spectroscopy 348 trajectory 18 transfer 32 quencher 270 reaction 426 -acceptor system 154 -deficient acceptor 32 -donating character 180 group 154

475 electronic component 354 configuration 20 energy dissipation 20 excitation 20 factors 24 level 18 matrix element 24 relaxation 21 spectrum 18 state 19 structure 15, 153 transition 15, 19, 25 moment 19, 20 wave 9 wavefunction 19 electronics 354 electron -rich donor 32 -withdrawing character 56, 180 group 154 electrophoretic deposition 380 electrospun poly(ε-caprolactone) 240 electrostatic interaction 24 elevated temperature 325, 396 elongation 270, 276, 406, 408, 443 embrittlement 157, 305 time 158 Emergency Temporary Standards 457 emission 9, 32 control 456 polarization 19 spectrum 28 wavelength 17, 374 emitted energy 17 encapsulant 396 encapsulation 185, 275 end group 223, 265 endonuclease 2 endothermic process 5 end-product 319 energy 14 absorption 12, 18, 22, 35 amount 25 conversion 45, 288, 447 mechanism 26

476 difference 15, 28 dissipation 21, 23, 29, 35, 44, 151, 153, 169, 177 cycle 65 mechanism 44 outcome 51 rate 152 emission 18 emitted 28 exchange 25, 32 fate 9 gap 24, 30, 33 internal conversion 414 level 15, 17-18, 21, 44-45 migration 23, 29, 32 mismatch 25 orbital 20 quenching 22 reduction 56 release 44 retention 14 saving 388 sources 10 state 15 level 21 supply 24 transfer 9, 14-15, 18, 26, 30, 53 distance 35, 53 mechanism 25 probability 24-25, 30 transmission 32 utilization 33 wave form 17 enol form 44, 169, 288 environmental benefits 200 conditions 328 damage 209, 343 exposure 343, 399 factor 328, 352 Protection Agency 453 enzyme 3 -mediated hydrolysis 329 EPA 453 EPDM 61, 215 antioxidants 215 application 215

Index diene stability 215 hydrotalcite 215 mechanism of degradation 215 UV crosslinking reaction 215 epidermal cell 1 skin layer 372 transmittance 2 epidermis 463 transmittance 2 epidural catheter 375 epoxidized soybean oil 165 unsaturated fatty acid 297 epoxy 13, 183, 343 activation energy of decomposition 207 chemical degradation 207 coating 339 crosslinked structure 207 degree of cure 207 discoloration 209 group 317 macroradical formation 207 matrix 399 micropores 207 morphology 209 optical properties 209 resin 207 ring 207 system 209 UV absorption range 207 EPR 212, 408 carbonyl concentration 211 degradation 211 irradiated side 212 photochemical reactions 211 photooxidation initiation 211 UV transmission 211 erosion dispersion mechanism 274 process 207 erythema 49 ESIPT 153, 169, 179 deactivation process 180 ESR 172, 211, 264, 280 imaging technique 447

Index ester 274, 319-320 group 360 linkage 241 esterification 265 esthetics 343, 400 ETFE electronic equipment 217 soiling effect 217 ether group 360 etherification 265 ethylene propylene copolymer 296 segment 211 tetrafluoroethylene copolymer transmittance decay 217 propylene copolymer 211 data 212 diene monomer 215 tetrafluoroethylene copolymer 217 vinyl acetate copolymer 218, 396 alcohol 252 ethylhexyl methoxycinnamate 67 eugenol 366 Europe 362 European legislation 348 Union 458 eutrophication 200 EVA 185, 218, 448 gamma radiation 218 photolytic degradation rate 218 polyenes 218 vinyl acetate concentration 218 evaporation 164, 181, 346 evergreen species 4 evolved gases 257 excess oxygen 56 excessive exposure 1 excimer 25, 31-33, 35 emission 31-32 energy 32 fluorescence 31 formation 32 migration 32 site 32

477 excipient 377 exciplex 25, 31, 35 emission 31 fluorescence 33 excitation 9, 20, 25, 28 energy 20, 414 disposal 21 lifetime 25 radiation 29 wavelength 29 excited chromophore 169 complex 31, 35 deactivation 36, 154 enol 44 intramolecular proton transfer 44, 153 keto tautomer 44 molecule 18, 24, 32, 35 sensitizer 51 singlet 26 state 21, 151 species 18, 25, 28 state 4, 12, 15, 20, 31-32, 44, 151, 154, 177 deactivation 36 density 25, 26 longevity 25 proton transfer 35, 154 quenching 51, 169 triplet 24, 30-31 state 420 exciting radiation 28 exciton 31 migration transfer 25 expansion joint 363, 391 exposed geotextile 369 exposure conditions 340 source 440 time 12, 158, 172, 251, 343, 445 exterior glazing 370 extinction coefficient 16, 37, 153 extraction 275 resistance 183 extreme ultraviolet solar measurement 399 extrusion pressure 435 exudation 165 exuding stabilizer 332

478 eye protection 5 F fading 51, 202 failure mode 220 fatigue 343 load 384 fencing rail 343 Fenton-type reaction 1 FEP erosion 220 evaporation of molecules 220 main-chain scission 220 morphological changes 220 surface 220 ferulic acid 49, 61 fiber 161, 183, 240, 356, 403, 436 reinforced polymer 381 surface 240 -dominated longitude properties 343 -reinforced polymer matrix 343 fibroblasts 414 filler 69, 433 interaction 170 surface 185 film 183, 358 ablation 59 composition 172 lifetime expectation 358 requirements 358 thickness 244 filtering 372 final energy state 17 fire treatment 387 firefighter clothing 336, 356 fishing line 362 net 362 rod 343 fixity ratio 327 flame retardancy 436 retardant 69, 155, 161, 170, 337, 435 flavonoid 1, 2, 5, 49, 67, 294, 350 flexibility 340, 378, 390, 406 flexible roofs 336 flexural strength 274 Florida climate 394

Index Flory-Huggins equation 162 fluence 41 fluorescence 24, 26, 28-30, 33, 45, 177, 288, 416 decay 152 time 31 intensity 23, 372 lifetime 23 quantum yield 28 yield 31 fluorescent emission 28-29, 31 light 29 radiation 29 fluorinated ethylene-propylene data 221 ethyl-propylene 220 fluorination 300 fluoropolymer 154, 215, 261, 345 flux intensity 16 fly ash 160 foam 363 requirements 363 sealant 363 fogging 332 food 366 and Drug Administration 459 contact approval 164 packaging 448 pigment 324 regulatory acts 453, 459 storage 366 forbidden transition 19-20, 26, 28 foreign object damage 343 formaldehyde 264-265 scavenger 265 formic acid 265, 281 formulation 433, 441 four-point bending 172 test 171 Franck-Condon factor 24 principle 19, 23 free monomer 32 particle 18

Index radical 165, 169, 172, 232, 253, 305, 348, 372, 433, 447, 461 attack 251 chain mechanism 250 oxidation 196 scavenging 1 scavenger 2, 49 volume 162, 251 fraction 162 -radical grafting 164 Freundlich adsorption 5 Fries rearrangement 233 frozen morphology 397 FTIR 280, 287, 295, 315, 347, 448 functional group 19, 250 theory 180 functionalization 290 functionalized titanium dioxide 276 fundamental form 9 law 15 fungal community 3 fungi 3, 160, 329, 336, 410 furniture 294, 367 fusion heat 162 G gallic acid 350 gamma irradiation 164, 448 radiation 218, 261, 286, 372 rays 9 gangway hood 384 garbage bag 358 garden furniture 367 gas chromatography 446 fading 160, 171 gaseous products 159, 304 gear weight 362 gel 441 formation 270 genetic integrity 3 genomic information 21 genotoxic effect 461 geocomposite 368 geogrids 368

479 geomembrane 155, 162, 181, 255, 368 thickness 181 geometry 179 georgid 368 geosynthetic 368-369 membrane 409 requirements 368 geotextile 162, 276, 368 containers 368 germinating seedling 2 germination 3 glass 403 glass reinforced polymer 343 transition 166 temperature 283, 327, 346, 399 /epoxy composite 12, 343 glazing 370 application 371 lifetime 370 requirements 370 unit 370 global oxidation kinetics 274 gloss 352, 367, 382, 385, 410 change 442 loss rate 352 glucosamine 51 glutathione 2 reductase 2 glycation 461 glycosidic bond 202 golf balls 400 gradual degradation 374 grafting 164 condition 189 graphene 344 nanoplatelet 276 oxide 180, 290 graying 352 green barley 2 greenhouse blind 329 film 167, 251, 255, 328-329 grid electrode 448 Grotthus-Draper principle 15, 37 ground state 20-21, 31-32, 44 molecular oxygen 295

480 growth 3 guaranteed performance 396 guava extract 67 guest molecule 32 H hair 372-373 damage 373 follicle 372 photodamage 5 half-value thickness 41 HALS 48 concentration 402 nitrosonium salt 68 hard segment 360 hardening process 199 hardness 41, 204, 259, 443 hardwoods 410 harmless energy 356 HAS 53, 56, 58, 64-68, 157, 160-162, 166, 169, 171, 178, 182-183, 202, 229, 237, 252, 290, 297, 329, 346, 362, 433, 443-444, 449 antagonistic mixtures 68 basicity 65 concentration 57 deactivation 48 depth profile 182 efficiency 434 low basicity 369 mobility 164 performance 178 pKa values 48 reactive 164 specification 439 stabilizer 68 tertiary 170 hazards 458 HCl scavengers 297 HDPE 163, 252, 385 geomembranes 368 membrane 368 oxidation induction time 155 Health and Human Services 457 and safety 454 hazard 184 heat 44, 354, 356 insulation coating 371

Index resistance 204 heavy atoms 24 Heisenberg's uncertainty principle 18 helium atom 448 beam 448 plasma 448 helix-distorting lesions 2 helmet 385 hemophilicity 258 herbal rosemary 386 heterocyclic imidazole 297 heterojunction 397 hexafluorobutyl methacrylate 196 High Production Volume chemicals 453 vacuum 300 -temperature processing 165 higher energy state 21 hindered amine light stabilizer 66, 360, 409, 447 nitroxide 447 stabilizer 54, 108, 446 phenol 55, 66, 252 hindering effect 300 historical background 1 hokey stick 343 hole 45 migration 45 transfer 32 -electron recombination 45 holocellulose 411 homolysis 152, 287, 295, 312 homolytic decomposition of hydroperoxides 68 dissociation 211, 218 scission 58 homopolymer degradation 206 host polymer 157 hot-melt 323 HPLC 281, 420 human 1 epidermal keratinocyte 372 hair 372 skin 5, 335, 413, 418 skin model 461

Index humidity 62, 257, 294, 354, 372, 394, 403 cycle 343 resistance 345 hybrid layer 170 UV absorber 180 hydrocarbon 160, 195 hydrochloric acid 62 hydrodynamic conditions 61 hydrogen 282 abstraction 207, 211, 228, 241, 280, 287, 295, 300, 315, 319 atom 310 bond 69, 196, 228 formation 154 strength 161 bonding 154, 179-180, 196 potential 156, 180 solvent 152 chloride 204 out-of-plane bending 151 peroxide 1, 45, 60 concentration 39 radical 60, 195, 202 stretching vibration 151 transfer 66 hydrogenated epoxy resin 401 hydrolysis 229, 242, 259, 265, 315 stabilizer 244 hydrolytic degradation 258 hydroperoxide 27, 48, 55-56, 66, 196, 211, 250, 253, 269, 272-273, 287, 317, 367, 448-449 cleavage 60 concentration 449 decomposer 69 decomposition 49, 58, 69, 168, 250, 283 degradation product 378 formation 58-59 inhibition 69 rate 59 group 180 homolysis 317 photodecomposition rate constant 59 photolytic stability 59 route 317 scavenger 229, 237

481 hydrophilic 305, 380 anatase 388 hydrophilicity 258 hydrophobic 380 matrix 178 modification 187 organic contaminant 283 product 185 rutile 388 surface 388 hydrophobicity 258, 412 hydroquinone 56, 202 hydrotalcite 61-62, 161, 215 crystallinity 61 hydrothermal reaction 383 stress 340 hydroxyapatite 168 hydroxybenzophenone 161, 166 photoproducts 233 hydroxybenzotriazole 161, 179. 196 hydroxycinnamic acid 1, 4 hydroxyl 241, 253, 319 group 196, 202, 247, 269, 317 radical 1, 45, 202, 300 region 194 side group 202 hydroxylamine 54, 58, 252-253 hydroxymethyl group 202 radical 202 hydroxyphenyltriazine 170, 237 hyperelastic behavior 291 hyperpigmentation supression 350 hypoallergenicity 463 hypsochromic shift 26 I ionization 21 image analysis 446 imaging techniques 445 Immediately Dangerous to Life and Health 458 immobilization 180, 433 immune function 413 impact accident 385 modifier 312

482 resistance 376, 387 strength 265, 352, 394, 443 impermeable layer 159 impurities 30, 58, 304, 340 impurity chromophore 448 incidence angle 14 critical angle 14 incident beam 16 angle 16 photon beam 41 radiation wavelength 14 incoming radiation 15, 38, 290 incompatibility 433 indicator dye 51 indirect additives to food 460 degradation 51 induced oxidation 319 induction period 69, 164, 296, 435 time 163, 368 industrial emissions 394 film 358 sites 394 inert atmosphere 257 inflammation 414 infrared 9 energy 24 radiation 9, 11, 15 inhibitor 253 initial color 71 concentration 332, 373 deposition 290 energy state 17 initiation 250, 273, 280 ate 56 step 250 initiators 25 injury 453 ink penetration 382 inner-shell excitation 355 radiation damage 355

Index inorganic particulates 37 screener 65 stabilizer 41 -organic matrices 170 installation 408 insulation 257, 354 foam 363 roof sheet 384 integrated circuit 355 interaction 71, 169, 399 mechanism 66 intercalated absorber 62 intercalation 62 interfacial bond hydrolysis 346 interference 9 interior automotive part 229 interlayer 406 intermolecular energy transfer 21, 25-26 hydrogen bond 177 overlap 30 process 26 internal conversion 20-21, 24, 28, 33, 416 rate 414 energy conversion process 237 reflection 14 stress 332 tensile stress 346 International Association for Research on Cancer 456 internuclear distance 19 separation 19 intersystem crossing 21, 24, 26, 28-33, 44, 151 rate 30 intramolecular conversion 23 energy transfer 21, 23-24, 29, 32 hydrogen bond 20, 44, 154, 169, 177 bonding 36, 44, 154, 179-180 process 22, 26 proton transfer 153, 179 transition 24

Index intra-ocular implant 375 lens 375 intrinsic attenuation 376 intumescent fire protective coating 339 iodine value 297 iodometric titration 449 ion beam 225 ionic bond 204 ionization 12 efficiency 449 -mass spectrometry 446 ionized molecule 32 ionizing radiation 300 ionosphere 399 iron impurity 434 irradiance 24, 194, 274, 352 level 319 irradiated polyurethane foam 363 side 182 irradiation dose 382 time 251 irregularly shaped particles 54 irritation 458 ISO 439-440 isomerization 21, 63 isopropenyl acetate 412 isosorbide-based polycarbonate 238 J Jablonski diagram 21-22, 24, 151 joint design 390 prosthesis 232 K kayak 385 keratin 372 radical 372 keratinocyte 372 keto form 44 /enol-tautomerism 416 -chromophore 397 -enol phototautomerism 4 ketone 49, 207, 250-251, 311, 315, 319 ketonic species 282

483 kinetic energy 19 kink resistance 378 knot stability 362 kojic acid 350 L labeling 385 labile hydrogen atom 241, 251 laboratory studies 440 lacquer 410 lactone 252, 319 laminated glass 370 unit 370 land use 385 landfill cover system 368 liner 368 lanthanum 161 large volume injection 446 L-ascorbate 386 laser ablation 41 beam 12 desorption/ionization 449 fluence 258 surface etching 12 laurel stain 442 layer thickness 183 layered crystal structure 61 double hydroxide 383 LCD blacklight film 354 leaching 162, 164, 368 leave inclination 3 orientation 5 lecithin 366 LED lamp 233 leveling 390 Lewis acid 154 life cycle assessment 200 expectancy 296 lifetime 29, 32, 152, 260, 328, 340, 343, 345-346, 348, 352, 356, 369-370, 378, 396, 406 estimate 408 expectation 385

484 prediction 157, 315, 369, 440, 445 warranty 305 ligand formation 167, 168 light absorption 2 cover 384 emitting diode 217, 233, 374, 376 intensity 31, 40 spectrum 9 stabilization 237 transmission 396 wave 9 -harvesting pigment 4-5 -mediated development 2 -scattering particle 329 lighting lamp 376 lignin 165, 202, 252, 258, 274, 275, 349, 411 darkening 349 fraction 382 oxidation 202 photodegradation 196 removal 410 stabilizer 199, 412 stabilizing concept 411 yellowing 382 linear attenuation coefficient 41 linen 403 linseed oil 199 lipid peroxidation chain 49 lipidic peroxidation 5 lipids 1 lipophilic compound 366 lipstick 348 liquid chromatograph 446 chromatography 68, 446 system 23 litharge 204 lithography 354 liverwort 3 living organism 1, 463 species 335 load condition 343 local cohesive stress 340 long energy transfer 53

Index -lived radical species 54 -term oxidation 56 lotion 348 low friction coefficient 336 molecular substance 229 weight product 274 luminescence 21, 23-24, 26 quantum efficiency 23 luminescing 24 lysosomal damage 461 M macromolecular architecture 251 morphology 448 macrophase separation 397 macroradical 264, 281, 295 macroscale chemical reaction 26 magic angle NMR 448 magnesia 204 magnesium chloride 61 -aluminum hydroxycarbonate 61 magnetic field 9 Maillard reaction 51, 52 main chain 273 maintenance frequency 390 MALDI 247 -MS 449 manganese ligand 168 mar resistance 345 marine environment 276, 369 paint 341 Mars 6 mascara 6 mass loss 209, 263, 399 spectrometer 446 spectrometric detection 68 spectroscopy 446 masterbatch 184 material durability 28 protection 9 rigidity 23

Index surface 160, 401 thickness 158 mathematical interpretation 9 modeling 340 relationship 445 matrix 43, 196 -dominated transverse properties 343 maximum absorption coefficient 342 opacity 42 Maxwell’s theory 9 mean free path 41 mechanical action 343 deformation 173 performance 442 strength 406 stress 251 mechanism 64, 443 medical devices 375 implant 258 surface 258 silicone elastomer 305 melanin 372 pigmentation 5 melt fracture 435 melting point 181 temperature 71, 181 membrane 302, 387 mercury lamp 10, 16, 189, 223 radiation 257 merocyanine 349 mesophilic bacterium 251 metabolic activity 372 function 335 metal 387 cluster 305 cushion 384 deactivator 252 ion 58 /polymer interface 346 metallic thin film 399 metallocene polymer 65

485 methanol 273 methoxycinnamate 153, 169 methoxydibenzoylmethane 153, 169 methyl group 269 methacrylate 165 orange 380 tert-butyl ether 40 methylene group 235, 288, 311, 317, 319 methylmethacrylate 355 methyltin mercaptide 180 microbalance 441 microbial enrichment 461 microcracking 327, 343 event 209 microemulsion polymerization 279 microorganisms 335, 410 microporous resin 185 micro-protrusions 220 microscopy 443 microspheres 165 microwave activation 165 irradiation 251 mid air fires 257 Mie scattering 14 theory 41 migration 2, 162, 164, 177, 274, 289-290, 385, 440 mildew 339 mineral acid 48 miscibility 71 mitochondrial damage 461 mixing intensity 41 modeling 12 studies 34 moisture 343 molar decadic extinction coefficient 16 extinction coefficient 16, 39, 154 ratio 178 volume 162 mold temperature 229 molded part 171 molecular bonding 15

486 charge 30 collision 30 dimer 31 dissociation 21 mass 402 mass distribution 265 mobility 22, 372 orbital 20-21, 30 orientation 160 oxygen 30, 237 recognition 229 structure 15, 24, 161 transition 15 vibrations 414 weight 25, 52, 63, 71, 156, 171, 180, 194, 207, 218, 222, 268, 317, 374, 441 decrease 258 molecule compressed condition 23 mobility 22 momentum 18 monochromacity 12 monochromatic light 37 radiation 34, 260 monofilament 402 monomer 32 ratio 189 monomeric HAS 275 monomolecular decomposition 58 monophosphite 252 Monte Carlo simulation 25 montmorillonite 69, 170, 434 clay nanoparticles 196 loading 207 morphological analysis 240 morphology 229, 443 mud 413 mulch film 328 multilayer film 358 sheet 392 multiphase acrylic binder 196 multiphotonic excitation 160, 274 multiple consecutive cycles 387 multi -season greenhouse 167

Index -walled carbon nanotube 327 museum room 310 mutagenic potential 6 mutagenicity 454 mutation 335 N nano-cesium zincate 238 nanoclay 69, 207, 344 nanocomposite 69, 279 nanofiller 69, 155 nanomaterials skin absorption 462 nanoparticle 253, 391, 460-461 absorption 463 safety concerns 460 nanoreinforced film 327 nanosilica 54, 253 nanotitania 328 naphthalate unit 242 National Institute for Occupational Safety and Health 456-457 Toxicology Program 456 natural bond orbital analysis 36 extract 49 near-satellite environment 399 network structure 54 neutralization 204 neutralization reaction 161 newsprint 382 nickel 51 chelate 64, 66, 168, 290 complex 261 diethyldithiocarbamate 69, 157 dithiocarbamate 68 quencher 145, 329, 457 thiolate antioxidant 66 nicotinamide adenine dinucleotide phosphate 4 NIOSH 456 nitric oxide scavenging 49 nitrogen dioxide 160, 171 nitrous oxide 403 nitroxide 49, 53, 211 concentration 447 distribution 182 mapping 446 photolysis 153

Index radical 54, 182, 253 nitroxyl polarity 178 radical 48, 56, 58, 161-162, 434 concentration profile 182 regeneration 59 stabilization 53 NMR 448 solid state 315 NMR 448 non -bonding 15 -fading formulation 171 -linearity 327 -radiative energy dissipation 23 process 21, 29 recombination rate 374 -radical cyclic mechanism 260 -reversible transformation 417 -sag 406 NOR HAS 48, 179, 369 Norrish I 218, 241, 282, 312, 417 cleavage 49 II 241 mechanism 258 reaction 218 scission 251 primary process 241 -type reaction 222 nuclear kinetic energy 19 operator 24 motion 19 power plant 408 nucleic acid 335 nucleotide excision repair 5 nutrient acquisition 3 O oak 252 Occupational Exposure Limits 458 eye protector 376 health standard 456 Safety and Health Act 456

487 Administration 456-457 ocean-like conditions 274 octyl salicylate 5, 373 octylmethoxycinnamate 373 oestrogenic activity 380 oil tanker 340 Okazaki Large Spectrograph 260 okra 2 oligomeric HAS 275 opacity 42, 65, 382 ophthalmic application 291 lens material 376 optical brightener 164, 181, 325, 355, 366, 382 data 39 density 419 device 407 fiber 376 requirements 376 pathway length 407 properties 286 optimal fluence 261 impact protection 376 ratio 65 optimum timing 328 optoelectronic device 233 orbital 19 orbital momentum 20 organic absorber 37, 40 coating 445 dye 209 matrix 46 pollutant 380 -inorganic hybrid coating 170 organometalic barbiturate complex 297 organomontmorillonite 434 organotin 60 stabilizer 394 orientation 12, 30 origin 48 original condition 410 orthodontic adhesive 324, 351 oscillation 19 OSHA 456-457

488 outdoor application 302 environment 385 exposed paint 290 exposure 187 testing 440 outward surge 23 oxalanidine 103, 332 oxidation 53, 211, 223, 242, 302, 315, 325, 343 induction period 434 process 55, 251, 446 product 312 rate 61, 159-160, 274, 300, 433 reaction 58 initiation step 58 reduction 244 oxidative induction time 181 process 1 oxirane ring 207 oxybenzone 461 oxygen 25, 196, 228, 242, 269, 447 addition 310 charge-transfer complex 34 concentration 263 diffusion 159-160, 171, 312, 315 diffusion coefficient 160 ingress 397 partial pressure 158 permeability 244, 311 pressure 159 quenching 25 starvation 158 triplet state 51 uptake 60, 158, 270, 273 -control 191 oxygenase 5 oxyradical 264 ozone 257, 327 hole 3 P P3HT absorbance decrease 222 photodecomposition 222 photodegradation rate 222 regioregularity 222

Index packaging 358 film 360 material 337, 363, 366 paint 179, 339, 391 degradation rate 199 drying 340 film 346 requirements 339 panchromatic UV absorption 297 pantograph cover 384 paper 382, 448 stability 382 stabilizers 383 paraffin 354 parallel spin 21 paramagnetic behavior 260, 305 part thickness 182, 333 particle crowding effect 42 density 42-43 diameter 275 radius 42 size 37, 41-42, 47, 275, 461 path length 183 pathogenicity 3 patterned surface 268 Pauli exclusion principle 21 pavement damage 391 p-benzoquinone 169 pendant group 317 pendant unit 32 pentaerythritol 161 pentaerythritoltetravalerate 297 perception mechanism 2 pathway 2 perester 319 perfluoroether data 223 perfluorooctane 380 perfluorooctanoic acid 380 perfluorooctyl-trichlorosilane 407 perfluoropolyether 346 perfluoropropionic acid 380 periimplantitis 351 permanent change 15 paper 382

Index permeability 340 permissible exposure limit 456-457 peroxidase 2 peroxidation 5, 49, 373 initiator 34 peroxide 40, 253 crosslink 281 decomposer 66, 261 decomposition 49, 58 species 58 peroxidic photoproduct 153 peroxidolysis 66 peroxy group 273 radical 48-49, 53, 54, 60, 211, 295 peroxyacid 60 Perrin kinetics 22 model 23 volume 23 pesticide 155, 161, 166-167, 179, 329 resistant additive 167 petroleum bitumen 387 pH 26, 61, 343, 396 pharmaceutical products 377 phase segregation 397 separation 33 phenolic antioxidant 49, 53, 60, 64, 66, 68, 123, 157, 160, 162, 170-171, 229, 433 compound 169, 350 consumption 68 hydrogen loss 65 hydroxyl 411 oxidation 155, 179 specification 439 phenoxy antioxidant 49 radical 269 phenyl radical 269 ring 153, 180 phenylene ring 283 phenylformamidine light stabilizer 51 phenylpropanoid derivatives 2 phosphate antioxidant 252

489 phosphatidylcholine liposomes 5 phosphite 53, 60-61, 63, 135, 161, 229, 237, 252, 325 composition 171 hydrolytic stability 53 hydroperoxide decomposition 59 phosphonate 171 phosphonite 135, 252 phosphorescence 24, 26, 28, 30, 33 emission 29 quantum yield 30, 33 photoaccelerator 260 photoaging properties 391 photobleaching 202, 397 photo-catalyst 200, 232 photocatalytic activity 200, 232, 244, 275, 461 discoloration 53 effect 352 efficiency 380 enamel 380 reaction 170 photochemical activity 252 breakdown 169 change 39 conversion 26 decomposition 272 mechanism 34 oxidation 58 process 11, 22, 43 reaction 15, 25, 202, 242 reaction irreversible 153 sensitivity 3 studies 39 transformation 283 yield 3 photochemically active additives 25 species 25 photochemistry 15, 29 second law 37 photochromic compound 380 photoclick polymerization 313 photo-crosslinkable 313 photodamage 21 photodecay rate 426

490 photodecomposition 66 photodegradant nanoparticle 461 photodegradation 258, 283, 388 half-time 183 initiation 235 mechanism 358 process 63 reaction 51 photodegradative change 274, 305 photodegraded PVC 296 photodynamic reaction 4 photoexcitation 44 photoexcited impurities 280 state 169 photofading 61 photo-Fries mechanism 287 products 237 reaction 236, 360 rearrangement 235, 237, 287 photogenerated hydroxide radical 53 radical pair 289 UV absorber 237 photografting 164 photo-induced electron 154 radical initiator 374 photoinitiating system 36 photoinitiator 36, 235, 260, 290, 318 photoinstability 414 photokinetics 415 photoluminescence 54 photolysis 228, 264, 274, 282, 304 photolytic degradation 153 process 171 reaction 228 photomorphogenic response 2 photon 15, 17-18, 24 absorption 19, 44 energy 15, 18 interaction 41 photooxidation 58, 189, 215, 222, 253, 272, 274, 282, 315, 317, 319-320, 352, 449 process 311, 446

Index rate 244 reaction 228 photooxidative change 319 degradation 329 reaction 260 stability 360 stress 5 photophysical conversion 22 data 33 principles 34 properties 9 reaction 274 photophysics 9, 15, 39 fundamental principles 9 photopolymerization 223 reaction 16 photoprotection 67 photoprotective ingredient 49 properties 349 photoreaction 2 photoreactivity 417 photoresist 374 photosensitive material 153 species 274 photosensitization 274 photosensitizer 66, 251 photosensitizing chromophore 4 photostability 20, 169, 200, 222, 261, 294, 356 photostabilizer 434 concentration 202 photostabilizing effect 165 efficiency 252 grade 352 photosynthesis 6 photo-tautomerism 20 phototendering 169, 403 photothermal degradation 397 phototoxic effect 461 potency 461 phototransformation rate constant 434

Index photovoltaic backsheet protection 302 cell 233, 397 devices 215, 217 photoyellowing 282 phthalate portion 200 phthalic acid 233 phthalimide 247 physical quenching 22 phytocosmetic 67 Pickering emulsion template method 258 piezoelectric sensor 449 pigment 51, 69, 179, 199, 202, 332, 340, 433, 435 content 347 surface 65 charge 65 pigmented coating 65 system 199, 411, 412 pimelic acid 46 pinking 296, 352 pipe 378, 409 piperidine nitrogen 434 pitting 394 pKa constants 48 Planck 9 constant 18 law 10, 17 plane wave 407 plant 1 biomass production 4 germination 328 growth 3 proactive reaction 4 plasma treatment 263 plasticizer 25, 164, 184 point defect 374 polar medium 177 solvent 426 polarity 30, 41, 152-153, 177, 202 polarization 9 pollutant 380, 410 pollution prevention 454 poly(3-hexylthiophene) 222 data 222

491 poly(acrylonitrile-styrene-acrylate) 388 poly(butyl acrylate) 195 poly(butylene adipate-co-terephthalate) 329 poly(ε-caprolactone) 240 poly(ethylene naphthalate) 241 poly(ethylene terephthalate) 61, 159, 185, 241, 244, 358, 367 poly(lactic acid) 366 poly(l-lactic acid) 258 bulk erosion 258 crystallinity 258 degradation mechanism 258 medical implant 258 photosensitizers 258 silver nanoparticles 258 poly(phenyl vinyl ketone) 268 poly(phenylene oxide) 269-270 chain scission 269 data 270 hydroperoxidation 269 stabilization 270 poly(p-phenylene sulfide) 271 data 271 poly(p-phenylene vinylene) 272 benzaldehyde end-groups 272 light-emitting devices 272 organic solar cells 272 singlet oxygen 272 poly(urethane-urea) 360 poly(vinyl acetate) 294, 326 poly(vinyl butyrate) 370 poly(vinyl chloride) 48, 61, 63, 180, 204, 225, 295, 336, 343, 345, 408, 434, 449 decomposition 297 degradation 295, 394 mechanism 218 dehydrochlorination 296 film 164 photodegradation 62 roofing membrane 296 soft transparent 337 stabilization 296-297 stabilizer 63 thermal stability 161 stabilizers 352 titanium dioxide 296

492 UV stabilization mechanism 161 poly(vinyl fluoride) 300, 336, 345 conjugated carbon double bonds 300 data 300 degradation 300 hydroperoxides 300 low orbit space environment 300 photovoltaic cells 300 stabilization 300 poly(vinylidene fluoride) 302, 336, 346 data 302 degradation 302 lifetime performance 302 polyacetylene 225 polyacrylamide 224 crosslinked 224 linear 224 nanoparticles 224 sunscreen 224 polyacrylonitrile 225 black color 225 cyclized ladder structure 225 data 226 photocatalytic activity of TiO2 225 polyalkylfluorene 227 polyamide 58, 169, 228, 273, 346, 362, 403 automotive parts 229 crystallization 229 data 229 fabric light fastness 229 hydroperoxides 228 photodegradation 229 radicals 228 stabilization 229 tubing 375 polyamide 6 185, 229, 358 polyamide-6,6 229 polyaniline 232, 337 polyaramid 445 polyarylate 233 data 233 hydroxybenzophenone 233 photo-Fries reaction 233 UV stability 233 polybenzimidazole 297, 445 conjugated structure 297

Index polybutadiene 189, 311-313, 315, 317, 321 crosslinked 311 data 314 degradation 311 electron withdrawing groups 311 photooxidation 311 stabilization 313 polybutylthiophene 234 data 234 polycarbonate 170, 185, 235, 237, 371, 407, 447 film 238 humidity 236 hydroperoxides 235 hydrophobicity 237 photo-Fries rearrangement 235, 236 photooxidation 236 radiation wavelength 236 sheet 237, 392 surface roughness 237 UV penetration 236 polychloroprene 315 data 316 degradation 315 marine aging 315 thermal degradation 315 polychromatic radiation 16, 33, 37 polydimethylsiloxane 304, 337 polydispersity 441 polyene 218, 297, 397 conjugation 225 polyester 59, 233, 241-242, 273, 310, 336, 345-346, 381 chain recombination 242 scission 242 degradation 241 fabric 337 film 329, 355, 366 hydroperoxides 242 inorganic filler 242 oxidation rate 242 photodegradation process 242 photolysis 241 quinonoid groups 242 scission reactions 241 silicone-modified 345

Index polyetherimide 247 chain scission 247 data 249 hydroxyl formation 247 methylene radical 247 oxidative process 247 polyethylene 54, 58-60, 65, 159, 181, 185, 211, 250-251, 270, 273, 379, 394, 399, 408 carbonyl index 251 chain scission 250-251 chlorinated 408 chlorosulfonated 204, 408 crosslinked pipe 379 degradation 251 fusion enthalpy 251 glycol 164, 409 high-density 54, 251, 253, 411, 446 hydroperoxide 250 linear low-density 251-252, 328 low-density 157, 212, 251, 255, 443-444 film 251 oxygen access 251 photolysis 251 photooxidation 250 photosensitizer 251 polar groups 251 thermooxidative processes 250 ultrahigh-density 232 molecular weight 362 wastes 251 polyfluorene 256 absorption maximum 256 polyfunctional material 204 polyhedral oligomeric siloxane 276 silsequioxane 283 polyimide 38, 257, 399 data 257 film 38, 257 gloss loss 257 surface 257 roughness 257 wire 257 polyisobutylene 319, 324 chain scission 319 data 320 degradation 319

493 hydroperoxides 319 oxidation mechanism 319 photooxidation 319 polyisoprene 315, 317-318 chain scission 317 reaction 318 data 318 degradation 317 hydroperoxide homolysis 317 tertiary hydroperoxide 317 polymer absorbance 38 backbone 32, 194, 211 blend 33 chain 53, 251 coating 445 density 160 film 399 additive loss 156 formulation 156 matrix 165, 169, 253 oxidation stability 434 processing additive 254 surface 242 synthesis 36 polymeric film 448 membrane 387 polymerization 165, 315, 337 degree 202 polymethylmethacrylate 166, 202, 260-262, 358, 371, 376, 445, 448 chain scission 260 quantum yield 260 degradation mechanism 260 ESR spectrum 260 side chain scission 260 stabilization 261 UV absorption 152 polymethylpentene 263 polynuclear aromatics 51 polyolefin 7, 35, 51, 53, 58, 60, 64, 67, 164, 170, 367, 409 discoloration 160 pipe 379 yellowing 61

494 polyoxymethylene 264 data 266 hydrogen radicals 264 hydroperoxides 264 photooxidative processes 265 stabilization 265 polypeptide cleavage 372 polyphenols 2 polyphthalamide 267 polypropylene 51, 53, 56-57, 60, 65-66, 69, 157-158, 160, 162-163, 171, 185, 211-212, 273-274, 276, 283, 369, 436, 443, 446, 449 biaxially oriented 360 chain-end methyl ketones 273 composite 274 degradation mechanism 273 depth profile 159 discoloration 274 exposure conditions 274 film 354 geotextile 369 hydroperoxide concentration 274 hydroperoxides 273 hydrophobicity 274 isotactic 46 molecular weight 159 multifilament 171 photooxidation 273 photothermal oxidation 160, 274 scission 159 stabilization 275 tape 64, 275, 402 thermal processing 273 ZnO 275 polypyrrole 279, 337 nanoparticle 279 polysaccharide chain 52 polysilane 374 polysilylene 374 polystyrene 7, 29, 32, 61, 164, 168, 171-172, 280, 283-284, 312, 363, 449 degradation 280 film 283 fluorescence 33 hydrogen abstraction 280 hydroperoxide decomposition 282 oxidation 283

Index segment 321 yellowing 35 -maleic anhydride copolymer 164 polystyryl radical 280, 282 polysulfide 313 polytetrafluoroethylene 286, 336 data 286 sheet 286 thin film 286 polythiophene 341 polyurea 291 polyurethane 287- 288, 291, 324, 345-346, 381, 406 clearcoat 289 gloss 289 coating 290, 342 coil coating 347 crystallinity 289 degradation 287, 289 electrocoating 290 exposure 289 foam 291, 387 hydroperoxide group 289 network 290 roofing foam 363 stabilization 289, 294 stabilizer migration 290 topcoat 180 polyvinylcarbazole 29, 32 polyvinylnaphthalene 32 polyvinyltoluene 29 pomegranate 2 pore formation 159 porphyrins 4, 24 post-irradiation effect 382 post-synthesis operations 61 potlife 390 potassium permanganate 251 persulfate 251 potential field 19 powder coating 233 generation 385 predispersion 40 preferential adsorption 185

Index preservation 310 mechanism 56 pressure 164 sensitive adhesive 324 preventive antioxidation 55 conservation 310 primary antioxidant 252 hydroxyl group 290 photon beam 41 protective mechanism 2 quantum yield 28 primer 410 adhesion 410 principle of degradation 11 printability 382 printable 400 probable transition 19 problem solving 39 process additive 154 conditions 41 rate 24 profile dimension 400 requirements 352 promotion 23 prooxidant 251, 367 propagation 250 retardation 48 propyl gallate 386 propylene segment 211 -ethylene copolymer 182 protection 47, 376 longevity 419 protective activity 446 protein 1, 372-373, 461 damage 372 degradation 5, 373 proton NMR 448 transfer 20, 45, 151, 180 process 151 reaction 44 protonated form 152

495 protonic acid 178 prototropy 44 pseudo-second-order kinetics 5 pulp 382 composition 448 pulsed laser 12 deposition 286 pultruded profile 381 structure 381 punicalagin 2 purity 222 pyrolysis 411 pyrimidine dimers 1-2 Q quantum mechanics 18 principles 19 of electromagnetic radiation 17 state 18 theory 9, 274 yield 28-30, 283, 434 quencher 22-23, 25-26, 30, 32, 51, 53, 66, 202 concentration 23 diffusion rate 23 quenching 22, 51, 53, 313 effect 54 efficiency 23 rate constant 23 sphere 23 quinoid compound 65 species 54 structure 47, 49, 54, 179 transformation 66 quinomethane structure 282 quinone 56 derivative 153 methide 49 R radiant energy 382 radiation 351 absorption 16 attenuation 37 curable coating 342 damage 335 dose 3

496 emission 37 energy 10 fluence 41 frequency 9, 11 incidence 13 intensity 11-12, 39 level 3 quantum 15 reflecting pigment 346 reflection 43 refraction 43 screener 38 stability 375 sterilization 375 units 12 velocity 9 wave 43 wavelength 9-11, 14, 17, 34, 37, 39, 191, 318, 382 -induced autoxidation 372 radiationless conversion 21 deactivation 45, 151 transition 24 radiative conversion 23 energy 9 absorption 31 exchange 32 process 29 transfer 25 radical 180, 241, 260, 264, 295, 315, 317, 321, 447 attack 161, 189, 311 capturing capability 53 complex 54 deactivation 48- 49 formation rate 305, 447 strain 173 generation 348 lifetime 305 mechanism 447 power 348 reaction 372 rate 53 reactivity 300

Index recombination 171, 332 scavenger 49, 232, 261, 315, 346 scavenging 47-49, 253, 276, 434 ability 65 activity 49 species 44, 310 transformation 48 yield 172-173 -radical recombination 289 rail pad 384 railway 384 materials 384 rain 396 water gutter 384 rainwater protection 411 Raman imaging spectroscopy 446 rancidity 348 random walk process 25 rapid deterioration 58 raspberry 349 rate quenching constant 52 Rayleigh scattering 14 theory 14 reaction 21, 29 chain 51 kinetics 39 mechanisms 40 pathway 29 rate 39, 273 oxygen species 5, 461 UV-absorber 165 reactive light stabilizer 289 reactivity 177, 304 received radiation 5 recombination 32, 281 efficiency 26 of photo-induced electron 154 rate 26 Recommended Exposure Limits 457 recovery stress 399 time 152 recrystallization 265 recycled content 387 recycling 259

Index red dyazo type condensation pigment 237 organic pigment 345 rose 349 synthetic organic pigment 200 refinish 333 refinishing coating 331 reflectance spectrophotometry 449 reflected radiation 6 reflection 13-14, 37 conditions 37 refracted radiation 43 refraction 14, 37 conditions 37 refractive index 13-14, 42-43, 71 regulations 453 reinforcement 400 relaxation modulus 327 remote wavelength conversion 233 removal 2, 6 renewable chemical substitute 200 replenishment 2 reprocessable rubber 313 resonance fluorescence 29 resorcinol 233 phthalate 237 polyarylate 233 respiratory effect 200 tract 458 retention degree 289 retina 5 reversal 2, 6 reverse proton transfer 44 reversible hydrogen bond 356 transformation 415 riboflavin 4 ribonucleic acid 1 rice bran 49 ring conjugation 154 opening 180 polymerization 258 risk 453 assessment 454 RNA polymerase 5

497 road cone 385 roofing 387 membrane 215 products requirements. 387 rope 362 rotational mode 15 state 19 rotomolded products requirements 385 stabilizers 386 rotomolding 385 rulemaking procedure 456 S safeguarding children’s health 454 safety glazing 370 sage extract 386 sagging properties 414 salt 257 bath 376 formation 69 sample preparation 440 sample surface 17 thickness 17 sandstone 261 satellite dish 407 measurement 399 saturated hydrocarbon 387 scattering 13-14 center 407 effect 14 efficiency 42 scavenging 48 Schrödinger equation 19 scission number 172 scratch resistance 378 scratching 332 screener 39, 178, 412 screening 313, 440 efficiency 178 requirements 440 sculpture 310 durability 310 sea urchin 4 seal 370

498 sealant 363, 390-391, 406 adhesion 406 application 390 conductive 390 durability 390 lifetime 390 warranty 390 requirements 390 runway 390 stabilizers 391 windshield 390 sealed glass unit 370 sealing material 217 seam 204 durability 204 seawater 276, 369, 461 SEC/MALDI 247 second law of photochemistry 15 secondary antioxidant 252 hydroxyl group 290 optics 376 reaction 190 sedimentation 347 segment conjugation length 374 selection criteria 19-20 self -cleaning coating 196, 200 -healing 313, 374 composite 363 -refreshing property 200 SEM 444 image 13, 444 sensitized fluorescence 29 sensitizer 22, 25, 32, 51 group 32 sequential steps 189 service exposure conditions 343 life 162-163, 251, 368, 384, 387 shape memory polymer 327, 399 polyurethane 391 recovery 363 rate 327, 399

Index sheet 392 requirements 392 stabilizers 393 shingle 387 short -range transfer 25 -Term Exposure Limit 458 -wave solar UV 6 -wavelength UV irradiation 380 shrinkage 305, 394 side chain degradation 372 scission 194 group 32, 194, 317 siding 394 requirements 394 silanol formation 315 silica 170, 204, 436 coating 169 fiber 376 gel 433 hydrolysis 315 layer 342 silicon bond 305 dioxide 329 silicone 304, 345 adhesive 324 degradation 304 gel 305 glue 305 macroradicals 304-305 oil 354 silk 382, 403 silos fabric 336 silsesquioxanes 276 silver nanoparticle 253 simulation 399 sinapate esters 1 single photon 15 singlet 19, 26 energy 30 excimer decomposition 31 excited state 425 lifetime 425 lifetime 30

Index molecular oxygen 52 oxygen 1, 48, 51, 58, 152, 168, 169, 251, 295 deactivation 52, 168 formation 25 quenching 51 quencher 420 quenching 425 state 21, 30, 33, 36 level 31 quencher 420 to singlet transition 19 -singlet energy transfer 420 sinking speed 362 sinusoidal polymer surface 12 site competition 435 sizing 382 skin 413, 447 cancer 5, 335, 413-414 care products 426 cell 5 damage 461 darkening 349 penetration 14 pigmentation 5 protection 49 skis 400 slate 387 sludge 446 smog formation 200 Snell's equation 14 law 13, 43 sodium azide 5 soft segment 360 visible light irradiation 36 softwood 382, 410 soil parameter 328 soiling 160 solar cell 222, 396, 407 efficiency 397 module 325 stability 397 cream 153 filter 350 heat gain coefficient 217

499 heating 160 modules 305 panel 323, 385 photovoltaic system 385 protection factor 67 radiation 39, 58, 260, 280, 362, 388, 396, 440 reflectance 187, 387 spectral irradiance 399 system 396, 400 requirements 396 stabilizers 397 transmittance decay 217 sol-gel coating 169-170, 178, 183 solid-state reaction 166 solubility 71, 202 solvent 25, 30, 184, 257 non-hydrogen bonding 152 polarity 31, 426 resistance 345 soot 160, 441 UV degradation 160 sorptive extraction 446 space application 305, 323, 327, 356, 399 conditions 441 energy transfer 32 environment 300 exposure 220 solar panel 324 station 399 spatial electron spin resonance imaging 446 transfer 25 specific interaction 64 mechanism 64 wavelength 15 spectral absorption coefficient 17 flux 17 intensity 17 light 217 lines 17 spin 21 coating 170 inversion 21 multiplicity 20

500 overlap integral 19 -orbit coupling 19 interaction 24 -selection rules 30 spontaneous emission 18 sporting equipment 400 requirements 400 stabilizers 400 spray foam insulation 363 stabilization 9, 41, 252, 406 cycle 60 mechanism 187 package 369 stabilizer addition 41 amine 143 benzophenone 77 benzotriazole 81 chemical composition 74 reactivity 154 compatibility 164 concentration 182, 252 cyanoacrylate 98 degradation 181 probability 153 distribution 40 excited state 169 hindered amine 108 loss 155, 177, 181 mixture antagonism 69 oxalanilide 103 phenolic antioxidant 123 phosphite 135 phosphonite 135 physical properties 71 quencher 145 selection principle 177 solubility 157, 162, 164 thiosynergist 139 triazine 90 type 332 volatility 155 quality control 439 synergistic mixtures 147 stabilizing composition 392

Index stadium seating 367 stain 391, 410, 442 resistance 339, 345 staining 390, 406 stainless steel wire 384 standard classification system 453 starch blend 251 retrogradation 251 static mechanism 22 nuclei 19 stearoyl benzoyl methane 161 stent 386 steric hindrance 53-54, 56 t-butyl group 53 sterically hindered phenol 265 sterilization 386 Stern-Volmer equation 23 kinetics 22 stiffness 362 stilbenequinone derivative 54 stimulated emission 18 Stokes' shift 374 storage 363 tank 385 strain 172 -induced hardening 173 stratosphere parameter 358 stratospheric ozone layer 6 stress 171, 313, 329, 340, 358 formation 396 frozen-in 173 level 374 stringing 390, 406 structural defect 310 deformation 20 stability 240 styrene 33, 207 styrene acrylonitrile copolymer 189, 307 degradation 307 photooxidation 307 -rich phase 189 styrene butadiene rubber 321 data 321

Index degradation 321 photodegradation 321 styrenic cap layer 381 sublimation 164 substrate 340 sulfenyl radical 69 sulfonamide 162 sulfur oxide 48, 171 sun care market 418 protection factor 350 value 349 sunblinding effect 369 sunburn 413 suncream formulation 153 sunlight 12, 15, 191, 260, 295, 323, 380 exposure 375 spectrum 194 sunproof 385 sunrays 236 sunscreen 42, 46, 61, 153, 348-349, 391, 413-415, 446-448, 461 allergy 350 formulation 417, 424, 430 optical density 419 lotion 349 photostability 350, 428 photostabilization 418 preparation 224 release 461 superhydrophobic 286 fabric 337 superoxide anion 49 radical 1 dismutase 2 radical 45 superposition 447 supersaturation 61 supplied energy 39 surface 242 ablation 37 area 185, 401 catalytic effect 46 cavity 209, 343 character 14 color 411

501 corrosion rate 396 crack 266 cracking 159, 385 defect 199, 411-412 degradation 211, 378 erosion 258 flame spread 394 layer 244 modification 258 morphology 263 phenomenon 410 quality 381 renewal 46 roughness 13, 286, 343, 380, 445 structure 14 topography 12-13, 343 surfactant 340 surrounding space 43 survival 3 rate 4 sustained stress 343 swimming water environment 376 switching temperature 391 symbiosis 461 symbiotic dinoflagellates 461 symmetry properties 20 synergetic effect 191, 399, 449 synergism 64, 66, 68, 202, 275, 296 synergist 66 synergistic effect 64, 66, 161, 173, 200 magnitude 64 interaction 261 mixture 65-66, 74 syntactic foam 363 synthetic fiber 362 T talc 170, 433-434, 446 tannin 253 extract 252 tape 65, 401 requirements 401 stabilizers 401 tar 362 tautomeric structure 177, 416 tautomerism 4, 151, 177, 180 tautomerization 44

502 tear strength 403 technical textiles 403 TEM 46, 444 temperature 59, 61, 251, 290, 319, 327, 340, 343, 352, 363, 403 cycle 300, 343 fluctuation 370 strain 346 temporal effect 447 temporary charge trapping 354 tennis rackets 400 tensile modulus 327 strain 173 strength 64-65, 258, 270, 378, 399, 403, 408, 443 retention 229 tension 171-172 teratogenicity 454 terminal location 26 vinyl bond 260 termination 250, 273 tertiary carbon 300 atom 211 hydrogen 273 hydroperoxide 317 piperidine 53 testing method 440 tetraethoxysilane 54 tetraethylorthosilicate 261 tetramethylpiperidinol sebacate 275 textile 403-404 requirements 403 stabilizers 404 thermal cycling 327, 397 performance 327 degradation 265, 352 studies 441 energy 33, 180 absorption 30 expansion mismatch 396 properties 346 gravimetric analysis 441

Index history 290 insulation coating 371 insulator 384 oxidation 58 performance 324 shock resistance 387 stability 297, 378, 411 stabilization 265 stabilizer 157 stress 370 transition 447 thermally activated process 171 thermochromic coating 388 dye 388 thermogravimetry 156 thermooxidation 274, 320 thermooxidative protection 275 stability 252, 402 thermoplastic elastomer catheter 375 polyurethane 291 starch 251 vulcanizate 337 thick sample 332 thickness 212, 448 change 183 loss 207 reduction 207 thin-film coating 407 thin sample 332 thioesters 162, 170 thiol 313 -ene 313 chemistry 318 click reaction 240 thiosynergist 48, 67, 139, 161, 325 Threshold Limit Value 457 tide 369 tie layer 381 tile adhesive 323 time scale 44 time-of-flight mass spectrometry 446 tin stabilizer 352 Tinuvin P photostability 36

Index ultrafast excited-state proton transfer 36 tissue integration capability 258 titanium dioxide 42, 62, 66, 155, 160, 164, 170, 179, 183, 200, 283, 296, 342, 345, 349, 352, 360, 388, 394, 433-434 aggregated region 347 alumina coating 199 density 199 coating 179, 342, 376 concentration 347 depth distribution 347 energy conversion 45 micronized 196 nanoparticles 342 opacity 42 optimum light scattering 42 particle 342 size 42 photoactivity 435 photocatalysis 160 photoreaction 46 photoreactivity 416 transparent 42 titration 449 tocopherol 49 ToF-SIMS 163 tolerance 2, 6 topcoat 290, 346 surface 347 topcoat-primer interface 347 torsional libration 151 toughness 378, 408 toxic substance control 453 Substances Control Act 453 traffic paint 339 transcription-coupled repair 5 transformation pathways 67 product 68 transistor 354 transition moment 19 probability 19, 24 temperature 391 translational mode 15

503 motion 25 translesion synthesis 2 transmission angle 14 transmittance 358 transmitted beam 16 transparent cover 14 transportation 363 tree orchard 328 triazine 44-45, 90, 171, 371, 434 cycle 356 nitrogen atom 20 trim coating 331 trimethoxybenzene 270 triplet 19, 26, 152 decay 33 energy 30 transfer 437 excited state lifetime 425 formation 30, 33 lifetime 30 longevity 152 oxygen 51, 168 reaction 29 state 21, 29, 31, 33, 36 formation 33 lifetime 29-30 quenching 419 to triplet transition 19 -triple annihilation 29-30 quencher 423 quenching 29, 419 mechanism 423 tryptophan 372 degradation 5, 373 tubing 378 tyrosinase activity 350 U ultimate strain 399 strength 327 ultrasonication 380 uncertainty 18 in momentum 18 in position 18 principle 18 undecenoic acid 251

504 under-the-hood 333 undertone 42 underwater application 270 uneven surface 12 uniaxial tension 172 United States Congress 453 unpaired electron spin 29 unsaturated double bond 189 polyester 241, 310 sequences 295 structure 195 unsaturation 196, 250, 311 urea bond 360 urethane linkage 360 urolithin A 2 useful lifetime 329 UV 9, 268 absorbed dose detection 225 absorber 4, 38, 61-62, 64-66, 68-69, 161, 166, 170-171, 183, 237, 244, 290, 294, 300, 302, 310, 315, 360, 366, 388, 401, 403, 410, 412-413, 440, 448 absorptivity 38 combination 419 concentration gradient 290 degradation 152, 294 depletion 158, 300 diffusion rate 290 distribution 332 dose 360 hydroperoxide protection 66 lifetime 196 molecule 169 photodegradation 290 polymerizable 165 reduction 153 regeneration 275 spatial separation 423 specification 439 stability 196 surface layer 183 synergistic mixture 153 absorbing/deactivation effect 153 absorption 27, 261, 276, 297 activity 66 spectrum 27, 440

Index absorptivity 391 damage 12-13 degradation 9, 343 rate 12 durability 337 energy 1, 44 absorption 276 enhancement 3 erythema 413 exposure 59, 327 filter 373 loss 348 filtering efficiency 342 instability 374 intensity 12, 343 irradiated material 264 irradiation 166, 253, 313 power density 279 level 4 lithography 355 measurement 236 penetration 158 photoelectron spectroscopy 154 protection 153, 187, 232, 259, 329, 378 protective compound 4 protector 350 radiation 9, 15, 199, 222, 225, 300, 343, 376 components 14 energy 15 intensity 39 penetration 291 protection 65 stabilizing effect 223 transmission 41, 376 range 51 resistance 275, 342, 390 resistant film 244 fluoropolymer adhesive 223 screener 38, 40, 69 screening agent 349 shielding 1, 337, 358 spectrophotometry 449 stability 209, 238, 252, 396, 401 stabilization 34, 54, 154, 157, 240, 457 mechanism 37 stabilizer 73, 160, 359, 367, 439

Index polymerizable 283 stability 151 thermal properties 181 vapor pressure 156 sunscreen protection 2 transmittance 302, 358 varnish 367 -induced lesion 5 -sensitive fiber 225 -shielding 197, 283 UVA light emitting diode 374 UVB 413 UVB and UVA protection performance 418 UVB detection dosimeter 270 UVB radiation 4 V vacuum ultraviolet radiation 300, 399, 441 valence band 45 van der Waals forces 433 vapor pressure 156 varnish 196 velocity 400 of light 10 Venus 6 vibration 19 vibrational energy dissipation 23 level 24 factors 24 mode 15 overlap 19 integral 19, 24 relaxation 21 state 19, 23 structure 31 vibronic coupling 24 vineyard 328 vinyl 253, 448 acetate 218 ester 209, 381 resin 310, 343 hydroperoxide formation 310 initial oxidation 310 unstable impurities 310 UV-resistance 310 group 250-251

505 ketone polymer 268 vinylene bond 272 vinylidene group 250, 448 vinyltrimethoxysilane 196 viral invasion 3 virulence 3 viscosity 30, 71, 390 visible 9 light 9, 11, 15 absorption 36 region 283, 374 vitality 3 vitamin C 49 vitamin D activation 5 vitamin E 49, 386 volatile degradation product 159 volatiles 195, 207, 264, 273, 282 volatility 180, 275 volatilization 165, 368 vulcanization 446 W wall board 384 warehouse 161, 171 warp resistance 394 warranty 343 washing fastness 337 wastewater 446 water 242, 282, 336 availability 4 condensation 343 intrusion 324 pipe 384 quality 380 resistance 204, 408 resistant 348 shortage 4 use efficiency 4 wave mechanic 20 wave's phase velocity 43 wavefunction 19 symmetry 20 wavelength 16-17, 39, 274 distribution 236 range 14, 391 sensitivity 34, 260 spectrum 33 shift 28

506 wax 409 weakest bond energy 29 wear 343 weather conditions 440 resistance 385, 388 stability 331, 394 weathered surface 274 wood 410 weathering 33, 204, 253 conditions 440 cycle 440 data interpretation 440 effect 261 performance 180 requirements 339 resistance 276 standards 339 Weather-O-Meter 152, 336 weight loss 296, 336, 441 West Africa 362 wettability 258 change 263 white hair 372 light emission 302 whiteness 244 whitening 367 wind 396 load resistance 394 resistance 387 window glass 384 profile 352 windscreen 324 windshield 323, 406-407 lifetime expectation 406 requirements 406 wiper 406 wire 408

Index and cable 337, 409 lifetime expectation 408 requirements 408 stabilizers 409 wood 164-165, 202, 294, 410, 412, 442 applications stabilizers 411 degradation 410 depth 410 flooring coating 200 flour 274, 297, 434 impregnating solution 411 products requirements 410 surface 387, 410 /PVC composites 297 -adhesive bond 387 -plastic composite 202 wool 403 fiber 461 workplace exposure limits 453 wrinkles 414 X xenon arc lamp 12, 34, 194 x-ray fluence 261 Y yarn 356 yellow undertone 42 yellowing 34, 187, 202, 270, 282, 288, 315, 321, 352, 382 yellowness index 382, 403 yield strain 173 Young’s modulus 251, 259 Z zinc 244, 275, 283, 443, 461 borate 394 carbamate 158 nanoparticle 200 oxide 166, 204, 275, 302, 360 stearate 61 zirconyl chloride 261