Stabilization and Early Degradation of UV-Irradiated Heterophasic Propylene−Ethylene Copolymers Based on ESR, ESR Imaging, UV−Vis, and DSC:  Effect of Ethylene Content and UV Wavelength

Abstract

Heterophasic propylene−ethylene copolymers (HPEC) containing 1 wt % bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate (Tinuvin 770) as a hindered amine stabilizer (HAS) were aged by exposure of ≈3 mm thick plaques to UVA and UVB sources in weathering chambers at 318 K. Two types of copolymers, HPEC1 and HPEC2, were examined, which contained 25 and 10 wt % 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 (HAS-NO), by comparison of thermal transitions in original and aged samples using DSC, and by measuring the UV−vis absorption of HPEC films as a function of their thickness. ESR spectra indicated the presence of nitroxide radicals in two amorphous sites, fast (F) and slow (S). The spatial distribution of HAS-NO was obtained by 1D ESRI. Nondestructive (“virtual”) slicing of the 2D spectral−spatial ESR images resulted in a series of ESR spectra that reflected the variation of the F/S intensity ratio with sample depth. HAS-NO was detected on the nonirradiated side due to the small, but not negligible, intensity of UV light transmitted through the plaque; on this side the nitroxide concentration remains high because it is not consumed in stabilization processes, as the local rate of degradation is low. Both ESRI and DSC experiments indicated a higher degradation rate in HPEC2, which contained more polypropylene (PP), 90 wt %; in these samples the degradation was accompanied by an increase in the extreme separation in the ESR spectra, indicating slower dynamics as a result of the increased degree of crystallinity. The degradation was also more advanced in samples exposed to UVB radiation compared to UVA due to the higher UV absorption coefficient in the shorter λ range. The different effect of EPR content in thermal and UV aging of HPEC samples was discussed.

Principles of Stabilizer Selection

“…The radicals are concentrated mostly on the irradiated side of the sample in a layer having a thickness of about 1 mm. 25 For over 20 days of exposure, the radical concentration decreases slightly on the irradiated side and nitroxide radicals are present in increasing concentration on the backside. 25 This is because there is enough radiation passing through the sample to initiate formation of nitroxyl radicals on the backside but insufficient degradation occurs in the bulk of the sample to use nitroxyl radicals for stabilizing purposes.…”

“…25 For over 20 days of exposure, the radical concentration decreases slightly on the irradiated side and nitroxide radicals are present in increasing concentration on the backside. 25 This is because there is enough radiation passing through the sample to initiate formation of nitroxyl radicals on the backside but insufficient degradation occurs in the bulk of the sample to use nitroxyl radicals for stabilizing purposes. On the surface, there is sufficient radiation to cause degradation, therefore radicals are gradually being depleted on the surface as the current concentration of stabilizer becomes insufficient to prevent damage.…”

Analytical Methods in Uv Degradation and Stabilization Studies

“…27 Spatial and temporal effects of the aging process were studied by electron spin resonance and ESR imaging (ESRI) of HAS-derived nitroxide radicals, by comparison of thermal transitions in original and aged samples. 28 Nondestructive ("virtual") slicing of the 2D spectral-spatial ESR images resulted in a series of ESR spectra that reflected the variation of the F/S intensity ratio with sample depth. 28 HAS-NO was detected on the nonirradiated side due to the small, but not negligible, intensity of UV light transmitted through the plaque; on this side the nitroxide concentration remains high because it is not consumed in stabilization processes, as the local rate of degradation is low.…”

“…28 Nondestructive ("virtual") slicing of the 2D spectral-spatial ESR images resulted in a series of ESR spectra that reflected the variation of the F/S intensity ratio with sample depth. 28 HAS-NO was detected on the nonirradiated side due to the small, but not negligible, intensity of UV light transmitted through the plaque; on this side the nitroxide concentration remains high because it is not consumed in stabilization processes, as the local rate of degradation is low. 28 11.11.2 DART-MS Direct analysis in real time, DART, mass spectroscopy, MS, is a rapid, sample preparation free method.…”

“…28 HAS-NO was detected on the nonirradiated side due to the small, but not negligible, intensity of UV light transmitted through the plaque; on this side the nitroxide concentration remains high because it is not consumed in stabilization processes, as the local rate of degradation is low. 28 11.11.2 DART-MS Direct analysis in real time, DART, mass spectroscopy, MS, is a rapid, sample preparation free method. 29 In DART, a high-voltage helium plasma is created inside a ceramic flow cell, and ions are subsequently quenched at a flow through grid-electrode, leaving a stream of neutral and excited, metastable helium atoms.…”