Thermooxidative degradation of high melt strength polypropylene (HMS-PP)

Abstract

Modified properties of polyolefins, including iPP, must find an explanation in terms of the basic irradiation processes: crosslinking , branching and degradation. HMS-PP in pellets form was synthesized by the gamma irradiation (60Co) of PP under a crosslinking atmosphere of acetylene with doses of 5, 12.5 and 20 kGy. After irradiation the samples were submitted to a thermal treatment at 90 °C for 1h to eliminate residual radicals [1,2]. A common approach for characterizing ageing response in a more practical time frame is to conduct accelerated ageing tests at temperature above the expected service limits. The thermal stability of the HMS-PP was evaluated after thermal ageing of dumbbell samples using a stove at temperature of 120 °C, in presence of air at different periods of time. The surface of PP , pristine and modified, (i.e., irradiated), was studied using optical microscopy (OM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X ray scattering. The effects of elevated temperature ageing were evaluated in HMS-PPs exposed surfaces according to the order: HMS-PP 20 >12.5 >5 kGy, showing intense crack formation in all the exposed surfaces due to thermooxidative degradation. The results were compared with previous work [1] where no evidence of thermoxidation was reached, exposition applied test at temperature of 90°C. The new condition is more conclusive about the effect of the thermooxidative degradation in the irradiated materials.