Experimental XPS investigation of thermal decomposition of polypropylene

By testing the melt index (MI) and tensile strength, the ageing rate of polypropylene ( PP) was analysed and compared. The appearance of the PP surface and the type of surface have been studied and analysed by SEM and X-ray photoelectron spectroscopy (XPS). The results showed that at 120 degrees C, the MI value increased gradually as the ageing time increased. This indicates that slow thermal decomposition had occurred and a short ageing period of 20 days did not affect the tensile strength of PP. This shows that the aging of PP is quite a long process and after 105 day aging, grooves emerged along certain directions on the PP surface as well as tiny cracks in a perpendicular direction. In the XPS experiments, the C1s spectroscopy of the original specimens includes three absorption peaks, C-C, C-O and C=O, which show light thermal aging that had happened in the raw materials while being stored and processed. After aging the C1s spectroscopy of specimens contained four peaks, C-C, C-O, C=O and O-C-O. Furthermore, the content of C-C had decreased while the relative content of the carbon in a high oxidation state (C-O, C=O and O=C-O) had increased. Therefore, O=C-O is a symbol of PP's thermal oxidated aging.