The effects of ionising radiation on polypropylene

Abstract

The interaction of ionising radiation with polymers is described and the literature relating; to the effects on polypropylene is reviewed. Oxidative and free radical reactions are discussed with particular reference to post-irradiationeffects.Isotactic and atactic polypropylene were δ and electron irradiated to doses of up to 20 megarad. Irradiations weremainly made in air. A series of other polymers were also irradiated in a preliminary survey. Molar mass measurements are used to measure the radiationyield for chain scission G (s). Irradiation at room temperature causes significantly more chain scission than at 195K. Additional chain scission occurs on storage following irradiation at 195 K. Free radical concentrations are determined by electron spin resonance, and the decay rates measured. The radical formed in air is a peroxy radical and in vacuo is a hydrocarbon radical. At77K in vacuo the radical is -CH2 - C* (CH3) - CH2 - but additional radicals are produced on warning to room temperature. The effects of increasing tenparature on radicals formed in air are described. Electron spin resonance studies on atactic polypropylene,and isotactic polypropylene in hydrogen, sulphur dioxide and nitric oxide are reported.. The melting temperatures, spherulite growth rates, and isothermal crystallisation rates of irradiated polypropylene are compared to those of the non-irradiated polymer. Crystallisation is found to proceed with an Avrami integer n = 2. At a given crystallisation temperature, the overall crystallisation rate of irradiated polymer is less than the non-irradiated, but spherulite growth rates are identical. Thermogravimetric analysis is used to assess the thermal stability of irradiated polypropylene in nitrogen, air and oxygen. Hydroperoxide analysis is used to show that several molecules of oxygen are absorbed for each initial radical, and that hydroperoxides continue to be formed for a long period following irradiation. Possible solutions for minimising irradiation and post-irradiation degradation are suggested, together with some problems for further study.