Effect of thermal ageing on the optical properties of polyester resins cross-linked with styrene and divinylbenzene

Unsaturated polyesters containing 0, 5, 10, 15 and 20% of styrene and divinylbenzene separately were thermally aged at 100, 125 and 150°C. Changes in the optical properties—absorbance, transmittance and reflectance—were followed for a period of 960 h. On thermal ageing the clear cured blends underwent very considerable yellowing which increased with increase of ageing temperature and time; the absorbance increased whereas the transmittance and reflectance decreased. At these temperatures the deterioration in properties is always linear with time. The rate of discoloration of polyester-styrene and polyester-divinylbenzene systems depends upon temperature and percentage of cross-linking monomer.