The other indicators are resistance to transverse matrix cracking and to thermal degradation and oxidation. While the initial degradation kinetics of the effects of edges and ply orientation can be evaluated using the methods above, the correlation of weight loss with mechanical performance is unsatisfactory. Some of these equations can be quite elaborate, even accommodating S-shaped curves similar to that seen in curve d Auerbach 1989 provides an excellent example. Similarly, a firstorder plot log concentration versus time that has two or more linear segments, has often been interpreted profitably in terms of a rapid initial first-order change superimposed on one or more slower ones. What, then, can I hope to accomplish in the present overview? Reproduced from Feller 1977 with permission of the publisher. Data at corrected for reflection, data at corrected for diffusion and reflection of irradiance Morand 1966.
Vink and van Veen 1978 cite a situation where the induction time in the stabilized case was almost directly proportional to the intensity. Likewise, the buildup of peroxides during thermal deterioration usually goes through a maximum; Feller and Enke 1982 illustrated this in the case of rubber cement. One of the earliest and most dramatic examples demonstrated more than forty years ago by Nolan et al. The problem is that once you have gotten your nifty new product, the Aging Photochemical And Thermal Aspects Research In Conservation Technical gets a brief glance, maybe a once over, but it often tends to get discarded or lost with the original packaging. These three conditions produce markedly different results in ranking eggs, predicting the texture of omelets, or describing the deterioration mechanisms of chickens.
It is currently possible to use a deterministic approach to predict the structural durability of metallic components for high-speed aircraft. Martin demonstrated this concept with a relatively simple systemthe photolytically initiated chainbreaking of polymethylmethacrylate. It is suggested that the conservator, researcher, and the student might benefit, though ultimately the work is written for the author himself. As observed with a microscope that has a micrometer eyepiece, the line of demarcation between the faded and unfaded portion of the paint becomes more sharply defined at higher concentrations of alizarin. Collision with neighboring molecules may dissipate the excitation energy so thoroughly that the quantum efficiency falls off. Thus, the behavior of the material at high temperature in the elastic-elastic, elastic-plastic, and plastic-plastic region must be studied. The effects of scale, geometry, surface quality, coatings, and diverse individual service conditions must be considered together with their possible synergistic interactions.
On exposure to a polychromatic source of light such as sunlight, both the yellowing and bleaching reactions occur simultaneously. As mentioned in Activation Spectra page 63 , the authors include a discussion of differences between activation spectra and action spectra. It has long been known, for example, that folding endurance decreases markedly in the early stages of thermal aging of paper, whereas tensile strength does not. Measurement of the Consumption of Oxygen. Thus, if one measures the change in concentration of a particular chemical end-product of the series, the kinetic analysis often will not reflect the result of one specific chemical step but instead will reflect the net result of the overall set of chemical reactions. Chapter 11 153 154 155 159 159 160 160 162 164 Toward the Future.
More recently, Catherine Bailie has been called upon in this effort and her cheerful, prompt response to hurried calls for help is much appreciated. The potential for such an oversight can be seen in the data of Wilson and Forshee 1959 regarding the degradation of cellulose acetate at 110 C. Kinetic Analysis of Change 31 In cases where there is an induction time, as illustrated in Figures 3. The wide range of stability in materials is one of the chief reasons why it is impractical to specify a standard accelerated-aging test that can effectively evaluate both highly fugitive and highly stable materials. Effects of Unfiltered and Filtered Xenon-Arc EmissionsLiterature on the Atlas Electric Devices Ci35 instrument presents data regarding the irradiance ranges under the various filter systems in the xenon-arc equipment, information summarized in Table 6. Matrix cracking can be characterized in terms of a saturation crack density, called the characteristic damage state that depends on ply properties, ply thickness, and stacking sequence Reifsnider and Highsmith, 1981. Admitting that there are difficulties, it is nonetheless useful to review the approaches that chemists and engineers have regularly employed in the effort to meet the objective of predicting long-term behavior and time to failure.
The applicability of this type of modeling must be verified. The synergistic effects of time, pressure, and atmosphere on composite degradation can be established following the approach employed by Pride et al. The characterization models and techniques are generally effective in describing the progression of particular degradation processes. What are the limitations that must be kept in mind concerning the application of very short wavelengths of ultraviolet radiation, correlation coefficients, and the Arrhenius and reciprocity relationships? Typically, these conditions have been identified by cyclic thermal or hygrothermal exposure testing Brunner, 1994; Sensmeier, 1994 , with test acceleration gained by rapid cycling, increased temperature range, or increased ply thickness multiple adjacent plies of one orientation. The number of quanta impinging upon a sample is usually determined by the use of chemical reactions whose quantum efficiency has been well established. Additional consideration and further refinements are certainly warranted Michalski 1987. The light-absorbing entities in trace contaminants may be carbonyl groups, double-bond unsaturation, oxygen-polymer Theoretical Aspects of Photochemical Deterioration 46 charge-transfer complexes, metallic impurities, and various catalyst residues McKellar and Allen 1979:13.
Much work has been done to evaluate fundamental materials aging responses for the isolated damage mechanisms identified in. The technical development of the concept, however, came from the Bureau. A pseudocreep curve was produced by placing line segments, having the slope represented by the steady-state creep rates, onto a creep strain versus time plot at the appropriate position with respect to the time axis i. These degradation mechanisms are discussed in. Materials of Moderate or Intermediate StabilityA challenging question facing the field of conservation science concerns whether to employ materials that fall in an intermediate class in the course of acceleratedaging tests. Presently their outdoor lifetimes depend on the use of photostabilizers in the case of plastics and on protective surface coatings in the case of wood. Brown 1991 states that a much greater appreciation of the mechanisms of degradation is needed.
Extensive references are provided to assist those who may wish to pursue various aspects in greater depth. The details of a technique using information from sharp cut-off filters to isolate the effects of narrow wavelength regions is described in Appendix D Martin and Tilley 1971. The primary absorption process is little affected by temperature, but varies greatly in effectiveness quantum efficiency. Paul Getty Trust, is committed to raising public awareness of the importance of preserving cultural heritage worldwide and to furthering scientific knowledge and professional practice in the field of conservation. Choosing an Acceptable and Unacceptable Degree of ChangeSubjective judgments of the point at which a material is too matte, too faded, or too yellow are bound to vary even among any group of experienced observers. This aspect of accelerated-aging tests is not always fully discussed. Berger 1972 states: Not every type of light absorption causes changes in the material.
Perhaps, at one-hundredth or one-thousandth of the rate induced by short wavelengths, one can consider, for all practical purposes, that the reaction has ceased. Reproduced with permission of the publisher. Influence of Temperature and Humidity on Corrosion of Protected Metals. One should not place too much emphasis on such slight differences in durability unless careful attention has been paid to the uncertainty and repeatability of the results Reedy and Reedy 1992. In evaluating the tendency of cellulose ethers to darken during thermal aging, Arney compared their behavior with that of rag paper, cellulose acetate, and newsprint Feller and Wilt 1990:51.
In contrast, the 365 nm radiation would have been absorbed primarily by peroxide groups and these had been shown not to be major initiators of photo-oxidation in this resin. It Changes with Age That Are Measured 16 is a major topic. The characterization of thermal degradation of a polymeric composite, under both oxidizing and inert conditions, is based on weight-loss measurements as a function of time and temperature. Khns study of the influence of humidity and temperature on fading involved an exposure of only about 800,000 lux hours Khn 1968. Martin 1984 , Martin and McKnight 1985 , McKnight et al. These data are in the range useful for design and also for setting realistic inspection intervals.