Light-induced degradation mechanisms

In contrast to polyolefins, the majority of engineering plastics do have absorptions at wavelengths present in terrestrial sunlight, so that for these polymers photolysis can also play an important role. For these polymers in principle there are three mechanisms that can describe their light induced degradation ... 

Stutzmann, M (1985). Role of mechanical stress in the light-induced degradation of hydrogenated amorphous silicon, AppZ. Phys. Lett., Vol. 47,21- 23, ISSN 0003-8979... 

ON THE MECHANISMS FOR THE PHOTOINHIBITION OF THE ELECTRON TRANSFER AND THE LIGHT INDUCED DEGRADATION OF THE D1 PROTEIN IN PHOTOSYSTEM II... 

On the Mechanisms for the Photoinhibition of the Electron Transfer and the Light Induced Degradation of the D1 Protein in Photosystem II 349... 

The mechanism of light-induced degradation in PE and PP is autocatalytic oxidation of the polymer that is accompanied by main-chain scission (Singh and Sharma, 2008 Balaban et al., 1969). Solar UVR initiates this reaction by creating free radicals... 

Sterically hindered amines (HALS) play an important role as stabilizers against the light-induced degradation of polyolefins " the detailed mechanism of their action is still being discussed. Therefore, the method of triplet-sensitized... 

This Section wiii present the stabilization of polymers, because this topic is inseparable from the topic of degradation, above all in the area of thermal-oxidative and light-induced degradation. The chemical structure of stabilizers and their reaction mechanisms will not be presented - at least not in detail. For details on these topics please refer to the literature [38], [186], [518], [519], [520], [521], [522], [523] as well as [524]. 

Two commercial disazo disperse dyes of relatively simple structure were selected for a recent study of photolytic mechanisms [180]. Both dyes were found to undergo photoisomerism in dimethyl phthalate solution and in films cast from a mixture of dye and cellulose acetate. Light-induced isomerisation did not occur in polyester film dyed with the two products, however. The prolonged irradiation of Cl Disperse Yellow 23 (3.161 X = Y = H) either in solution or in the polymer matrix yielded azobenzene and various monosubstituted azobenzenes. Under similar conditions the important derivative Orange 29 (3.161 X = N02, Y = OCH3) was degraded to a mixture of p-nitroaniline and partially reduced disubstituted azobenzenes. 

The amount of TiO was varied, and the assembly was tested for its photocata-lytic activity using degradation of 2,4-xylidine as test reaction probe. The decrease in the concentration of xylidine and the corresponding increase in oxalate concentration were monitored through HPLC. A pseudo-first order kinetics was ob served in the photodegradation process based on the Langmuir-Hinshelwood mechanism. An inverse correlation was observed between the kinetic rate constant and the obtained Light-Induced Optoacoustic Spectroscopy (LIOAS) frequency maxima. 

Especially above room temperature many polymers degrade in an air atmosphere by oxidation that is not light-induced (heat ageing). A number of polymers already show a deterioration of the mechanical properties after heating for some days at about 100 °C and even at lower temperatures (e.g. polyethylene, polypropylene, poly(oxy methylene) and poly(ethylene sulphide)). 

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