The effect of heat aging on acrylonitrile-butadiene-styrene (ABS) blends

Degradation of thick ABS materials at elevated temperatures is observed to be predominantly a surface effect which deteriorates the bulk mechanical properties under impact conditions. It is postulated that a critical level of surface degradation initiates polymer crazing in the rubbery butadiene phase, which propagates through the polymer, causing failure. A physico-mechanical study of the effects of heat aging on the rubbery phase in ABS reveals an increase in the loss modulus and tan delta, demonstrating a decrease in the free volume which may be due to cross-linking of fr-ee radicals formed by hydrogen abstraction and chain scission. Microscope FTIR spectroscopy of aged ABS surfaces shows that the degree of butadiene unsaturation decreases with time, and that carbonyl and hydroxyl absorbances increase as expected with thermo-oxidative degradation. Degradation of the rubbery polybutadiene phase in ABS may be initiated by hydrogen abstraction from the carbon a to unsaturated bonds, producing hydroperoxide radicals leading to carbonyl and hydroxyl products. Cross-linking is facilitated by the free radicals produced. Heat aging of ABS at elevated temperatures below the glass transition (T-g) of the SAN phase causes physical aging. This aging of the SAN phase provides thermal energy which allows for a structural reconfiguration of the glassy morphology towards an amorphous polymer. In this case, heat aging relaxes the intrinsic molecular stresses and causes a decrease in the initial rate of transition from the glass to the liquid when temperatures approach T-g. It is also possible that the physical aging of the SAN phase in ABS may contribute to the deterioration of the mechanical properties of the polymer.