Photochemical deterioration processes

From the viewpoint of prediction of service lives, the photochemical deterioration processes of polymers used as paints and finishes are theoretically analyzed based upon unsteady state dynamics. Theoretical results are compared with experimental data under natural and accelerated exposure. Infrared spectra and scanning micrographs show that the deterioration proceeds continuously inwards from the surface, but differently with the exposure conditions. Parabolic (/t ) law was derived approximately for the increase in the depth of the deteriorated layer of polymers with time. Paying attention to the influence of the deterioration of polymeric finishes, the parabolic law involving a constant term was also derived for the progress of carbonation of concrete. These parabolic laws well predict the progress of deterioration and explain the protective function of finishes on reinforced concrete. 

The photochemical deterioration processes of polymeric materials often used as paints and finishes are studied both theoretically and experimentally. Theoretical models for the deterioration processes are developed based upon unsteady state dynamics. The results were compared with experimental data under outdoor and accelerated aging... 

Both O2 and H2O2 can be analyzed in a FIA system consisting of a chemical reactor where the process takes place, a device for sample withdrawal that avoids contact with the atmosphere, injection into a CZE unit, where the analytes become separated in a short time, and an ELD unit for amperometric end analysis. The method was applied for determination of glucose and photochemical deterioration of ketoprofen (70), by measuring the H2O2 generated according to equations 16 and 21, respectively . 

Since most chemical reactions if carried to completion will result in deterioration of polymer properties, it is desirable to be able to ensure that all of the energy is dissipated in photophysical processes and to eliminate the photochemical reactions. However, in some cases, such as in the manufacture of photographic resists, it is desirable to maximize the photochemical effects. The polymer chemist is particularly concerned with the problem of how the relative efficiency of these various processes may be affected by the polymeric nature of the molecules he uses. 

Although the concepts outlined in this chapter are particularly appropriate for the interpretation of hydrolytic deterioration of cellulose, they show promise as an aid in the interpretation of thermal, photochemical, photolytic, and enzymatic degradation as well. Equations 3 and 4 are generally applicable to the scissioning process in linear polymers (33, 34). 

The oxidative deterioration of most commercial polymers when exposed to sunlight has restricted their use in outdoor applications. A novel approach to the problem of predicting 20-year performance for such materials in solar photovoltaic devices has been developed in our laboratories. The process of photooxidation has been described by a qualitative model, in terms of elementary reactions with corresponding rates. A numerical integration procedure on the computer provides the predicted values of all species concentration terms over time, without any further assumptions. In principle, once the model has been verified with experimental data from accelerated and/or outdoor exposures of appropriate materials, we can have some confidence in the necessary numerical extrapolation of the solutions to very extended time periods. Moreover, manipulation of this computer model affords a novel and relatively simple means of testing common theories related to photooxidation and stabilization. The computations are derived from a chosen input block based on the literature where data are available and on experience gained from other studies of polymer photochemical reactions. Despite the problems associated with a somewhat arbitrary choice of rate constants for certain reactions, it is hoped that the study can unravel some of the complexity of the process, resolve some of the contentious issues and point the way for further experimentation. 

Exposure of polymers to sunlight or to artificial light sources results in a more or less rapid deterioration of their physical and mechanical properties as a consequence of either primary and secondary photochemical processes or of subsequent photo-initiated thermal reactions. 

Polyamides are susceptible to degradation by heat, oxygen, light, and chemical agents. During melt processing, thermal, oxidative, and hydrolytic processes may be operative that may also contribute, in addition to photochemical degradation, to the deterioration... 

Lipid peroxidation is the primary mechanism by which food deteriorates upon storage in the presence of oxygen. This process of oxidation can be initiated enzymatically, by metal ion catalysis, or by photochemical processes, to name a few. Free radicals including peroxyl, alkoxyl, and hydroxyl have been implicated in the mechanism of lipid peroxidation. The changes in the quality of processed foods are manifested by... 

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