Photochemical and thermal degradation of films of dammar resin

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AbstractFilms of dammar resin were aged by exposure to light and heat. Changes in the films were followed by means of ultraviolet-visible light transmission, infrared and fluorescence spectrophotometry, and gel permeation chromatography. The solubility and acid value of the films were also monitored. Extensive autoxidation occurred only in the presence of light, although limited yellowing took place under such conditions. Autoxidation leads to the formation of products containing polar groups, particularly carboxylic acid groups; consequently, the oxidized resin is soluble only in relatively polar solvents. Cross-linking reactions lead to products with higher molecular weight; however, this does not cause the formation of a significant insoluble fraction. Yellowing is a secondary non-oxidative thermal process which takes place among autoxidation products. It is particularly strong when heat aging follows aging under light; it is not inhibited when an antioxidant is added during the heat aging step or when air is replaced by nitrogen during this step. Fluorescence under ultraviolet light as well is for the most part the result of thermal non-oxidative reactions. Aldol condensation among carbonyl species and dehydration reactions are likely to be responsible for yellowing and fluorescence. Yellow films can be bleached easily with light of wavelengths greater than 400nm (visible light).