Thermal aging of heterophasic propylene-ethylene copolymers: Spatial and temporal aspects of degradation based on ESR, ESR imaging, and FTIR

Heterophasic propylene-ethylene copolymers (HPEC) containing bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were thermally aged at 393 and 433 K. Two types of HPEC were examined, containing 25 and 10% ethylene (E), respectively, as ethylene/propylene rubber (EPR). Spatial and temporal effects of the aging process were studied by electron spin resonance (ESR) and ESR imaging (ESRI) of HAS-derived nitroxide radicals and FTIR of films prepared by compression molding. The spatial distribution of the HAS-derived nitroxide radicals was obtained by 1D ESRI. These experiments enabled the visualization of an outer region of thickness 100 mum that contained a lower amount of nitroxides and believed to arise from the loss of the stabilizer by diffusion ("blooming") and in chemical reactions during aging. Nondestructive ("virtual") slicing of the 2D spectral spatial ESR images resulted in a series of ESR spectra that indicated the presence of nitroxide radicals in two amorphous sites, fast and slow; the corresponding relative intensity varied with sample depth. 1D and 2D ESRI allowed the detection of faster degradation of EPR in the amorphous phase represented by the fast spectral component. Both ESRI and FTIR experiments suggested a faster degradation rate in HPEC containing 25% E compared to 10% E; moreover, a larger Tinuvin 770 content in the polymers led to less efficient stabilization. FTIR spectra indicated increased ordering of polypropylene segments in HPEC during aging at 433 K.