Although films of dammar resin cannot be stabilized against photochemical degradation in the presence of ultraviolet light with currently available additives, a remarkable stabilization is obtained with a hindered amine light stabilizer (HALS) if the ultraviolet component of the light source is eliminated. Even in the absence of UV light the effect of HALS can be improved by further addition of a benzotriazole ultraviolet absorber. It is proposed that the UV absorber functions as a radical scavenger under such conditions. Films of dammar containing 2% Tinuvin 292 and 3% Tinuvin 328 remain intact when aged in a xenon arc fadeometer under exclusion of UV light for as long as 3700 hours, while they normally break down in a few hundred hours. After such a period of accelerated aging the original components of the resin are all still intact and the films remain removable with cyclohexane. No yellowing occurs during heat aging after a preceding period of prolonged fadeometer aging. Changes in the films were observed using ultraviolet-visible light and infrared spectrophotometry, gel permeation chromatography, gas chromatography /mass spectrometry and solubility tests. The ‘cut-on’ wavelength of the UV filter should not be lower than c. 400nm for best results.
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