Mechanism of yellowing

The temperature often determines the shape of action or activation spectra where both thermal and photochemical processes produce the same outcome. Both lignocellulose and poly(vinyl chloride), for instance, yellow on exposure to UV radiation as well as on heating. In the case of the latter substrate, even the mechanism of yellowing is the same. It is, therefore, necessary to include adequate controls in the experiments to estimate the relative importance of thermal processes in those exposures that cause combined photo-thermal degradation. Alternatively, additional samples might be exposed behind filters with... 

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