THE MECHANISM OF THE LOW-TEMPERATURE OXIDATION OF POLYPROPYLENE

The peroxides (as detected by an iodometric titration) formed by the oxidation of polypropylene (PP) between 50 and 90-degrees-C consist of two types: fast-decomposing and slow-decomposing. It is shown that when both of these peroxides are present the oxidation rate is controlled by the fast-decomposing peroxides. In the induction period the slow-decomposing peroxides are formed initially; the decomposition of these peroxides leads to primary oxidation products. Due to the restricted mobility, these oxidation products will preferentially be oxidized, which leads to fast-decomposing peroxides. Chemical analyses show that these peroxides are peracids. In the latter stage of the oxidation the peracids determine the oxidation rate. This causes a tremendous increase in the oxidation rate. Thus, the increase in the oxidation rate of PP after the induction period between 50 and 90-degrees-C is not, as generally accepted, due to the accumulation of hydroperoxides, but can be ascribed to the faster decomposition of peracids which result from the oxidation of primary oxidation products.