The peroxides (as detected by an iodometric titration) formed by the o
xidation of polypropylene (PP) between 50 and 90-degrees-C consist of
two types: fast-decomposing and slow-decomposing. It is shown that whe
n both of these peroxides are present the oxidation rate is controlled
by the fast-decomposing peroxides. In the induction period the slow-d
ecomposing peroxides are formed initially; the decomposition of these
peroxides leads to primary oxidation products. Due to the restricted m
obility, these oxidation products will preferentially be oxidized, whi
ch leads to fast-decomposing peroxides. Chemical analyses show that th
ese peroxides are peracids. In the latter stage of the oxidation the p
eracids determine the oxidation rate. This causes a tremendous increas
e in the oxidation rate. Thus, the increase in the oxidation rate of P
P after the induction period between 50 and 90-degrees-C is not, as ge
nerally accepted, due to the accumulation of hydroperoxides, but can b
e ascribed to the faster decomposition of peracids which result from t
he oxidation of primary oxidation products.