Physicochemical changes in stabilized, orientated polypropylene films during the initial stages of thermal oxidation

Orientated, stabilized, isotactic polypropylene films have been oven‐aged at 130°C for times shorter than required for their embrittlement. The structural changes occurring during this induction period have been studied using tensile tensiometric, density, thermal analytical (DTA and TGA), and infrared spectrophotometric techniques. After an initially rapid annealing, films show similar time‐dependent changes in tensile, density‐derived crystalline and thermogravimetric behaviors. Comparison with vacuum‐aged sample properties shows that these changes reflect the effects of oxidative attack of polypropylene chains. Infrared examination shows that, even after the longest aging times studied and close to the induction period for embrittlement, the presence of oxidized chain groups is only just discernible. Oven aging reduces post‐fusion oxidative onset and maximum mass loss temperatures determined by TGA. Kinetic analysis demonstrates that the apparent activation energies and Arrhenius factors are reduced during aging as the post‐fusion oxidation mechanism becomes more complex. The results suggest that, during the induction period, changes in both physical and chemical structural features of orientated polypropylene occur which have possible importance in determining the lifetime of the polymer in use