DEMONSTRATION OF THE POSSIBLE COMPETING EFFECTS OF OXIDATION AND CHAIN SCISSION IN ORIENTATED AND STRESSED POLYPROPYLENES

A series of orientated polypropylene tapes and filaments with differing draw ratio were produced from two parent polymers containing antioxidant systems of different efficiencies. Lengths of tapes and filaments were exposed in air ovens at temperatures up to 130-degrees-C under ''free'' conditions, held at constant length or loaded to differing stress levels for prescribed times or to failure, whichever was earlier. Those morphological effects of oven ageing treatments as are identified from their subsequent tensile behaviors and densities have been reported elsewhere; this study reports the heat flux differential scanning calorimetric (HF-DSC) observations of unexposed and exposed samples. The HF-DSC curves of treated samples show, for the postfusion oxidative exotherm for oven-exposed samples, positive shifts to lower values compared with an unexposed sample, of the extrapolated onset temperature (T(on)). Analyses of the tensile, density, and HF-DSC data suggest that during the initial stages of thermal degradation in air, chain scission competes with oxidation. The rates of the former are enhanced by the application of stress, and scission is considered the origin of the observed deterioration in tensile behavior. Concurrent oxidation during oven exposures affects subsequent autoxidative behavior but has little immediate influence upon the mechanical properties. (C) 1994 John Wiley & Sons, Inc.