research in The Getty Conservation Institute Accelerated Aging Photochemical and Thermal Aspects conservation Robert L. Feller Editor: Dinah Berland Production coordinator: Anita Keys Cover designer: Garland Kirkpatrick Designer: Irina Averkieff Illustrator: Ad • Infinitum Printer: Edwards Bros., Ann Arbor, Michigan ©1994 The J. Paul Getty Trust. All rights reserved. Printed in the United States of America Library of Congress Cataloging-in-Publication Data Feller, Robert L. Accelerated aging : photochemical and thermal aspects / Robert L. Feller. p. cm. -- (Research in conservation ; 4) Includes bibliographical references and index. ISBN 0-89236-125-5 1. Materials--Testing. 2. Accelerated life testing. 3. Materials--Effect of radiation on. 4. Materials--Effect of temperature on. I. Title. II. Series. TA410.F36 1994 94-29424 620.1´1228--dc20 CIP The Getty Conservation Institute The Getty Conservation Institute, an operating program of the J. Paul Getty Trust, is com- mitted to raising public awareness of the importance of preserving cultural heritage worldwide and to furthering scientific knowl- edge and professional practice in the field of conservation. The Institute conducts research, training, documentation, and conservation activities in several major areas including objects and collections, archaeological sites and monuments, and historic buildings and sites. Research in Conservation This reference series is intended to make avail- able the findings of research conducted by the Getty Conservation Institute and its individual and institutional research partners, as well as state-of-the-art reviews of conservation litera- ture. Each volume covers a separate topic of current interest and concern to conservators. Other volumes in the Research in Conserva- tion series include: Cellulose Nitrate in Conser- vation (1988), Statistical Analysis in Art Conservation Research (1988), Evaluation of Cel- lulose Ethers for Conservation (1990), Protection of Art from Atmospheric Ozone (1990), Epoxy Resins in Stone Conservation (1992), and Airborne Parti- cles in Museums (1993). Accelerated Aging Photochemical and Thermal Aspects Contents Robert L. Feller Foreword . xi Preface . xiii Acknowledgments. xv Objectives of Accelerated Aging . xvi Introduction. 1 Chapter 1 Classes of Stability . 5 Range of Stability . 5 Class of Stability . 6 Statistical Approach to the Probability of Failure . 9 Materials of Moderate or Intermediate Stability . 10 Summary . 11 Chapter 2 Changes with Age That Are Measured . 13 Chemical versus Physical. 13 Mechanism . 15 Sensitivity of Measurement . 16 Choosing an Acceptable and Unacceptable Degree of Change . 18 Summary . 19 Chapter 3 Kinetic Analysis of Change . 21 Equations Expressing Changes in the Concentration of Reactants . 22 “Order” of Reaction . 24 Sequential, Simultaneous, and Reversible Reactions. 28 How Properties Change in Time . 30 Summary . 35 Chapter 4 Prediction of Useful Lifetime . 37 Empirical Equations . 37 Change According to Laws of Chemical Kinetics . 37 Sizmann and Frank Method . 38 Statistical Analysis of Probability of Failure . 39 Correlation . 39 Controls . 42 Summary . 43 Chapter 5 Theoretical Aspects of Photochemical Deterioration. 45 To Activate Molecules, Radiant Energy Must Be Absorbed. 45 Photolysis . 47 The Primary Process . 49 Secondary Processes . 51 Reciprocity Principle. 53 Reactions Proportional to the Square Root of Intensity . 54 General Influence of Temperature in Photochemical Reactions . 54 Initiation . 55 Depth of Penetration of Light into Coatings. 56 Summary . 61 Chapter 6 Influence of Wavelength . 63 Activation Spectra. 63 Effects Observed over an Extensive Range of Wavelengths. 64 Relatively Broad Regions of Sensitivity in the Ultraviolet . 65 Sharp Dependence on Wavelength . 67 Depolymerization of Poly(methylmethacrylate) and Other Acrylic Polymers . 71 Influence on Induction Time . 73 Apparently Nonspecific Response to Decreasing Wavelength . 73 The Idea of a Threshold of Photochemical Activity. 76 Action of 280–315 nm Wavelengths . 78 Wavelength Sensitivity in the Buildup and Dissociation of Peroxides. 83 Fundamental Difference Between Thermally Induced Auto-oxidation and Photo-oxidation . 87 Summary . 88 Chapter 7 Practical Aspects of Testing . 91 Light Sources . 91 Selection of Limited Spectral Ranges . 96 Sharp Cut-off and Narrow-Band-Pass Filters. 98 Stress . 99 Experimental Strategies . 100 Special Considerations . 103 Simple Test for Ultraviolet Emissions . 113 Recently Prepared versus Aged Samples . 113 Summary . 114 Chapter 8 The Effect of Moisture in Oxidative Degradation . 115 Normal Response . 115 Irregular Response . ..