[Study on natural photo-oxidation degradation of polypropylene nanocomposites by infrared microscopy]

Polymer nanocomposites have attracted great attention because of the demand for polymer materials with high performance and high functionality at the same time. Nano CaCO3 and SiO2 can be used to improve the rigidity of polypropylene (PP), which is quite importance for biaxial orientation PP film. However, the effect of these nano fillers on natural photo-oxidation degradation of PP has not been studied. In the present paper, the natural photo-oxidation profiles along the depth of PP and PP nanocomposites bars were studied with infrared microscope (IM) as well as polarizing lighu microscope (PLM) and scanning electron microscope (SEM). The results indicated that nano CaCO3 and SiO2, especially the latter, accelerated the oxidation rate of PP significantly, with more and deeper surface cracks. Larger amount of nano filler gave rise to greater oxidation degradation. The existence of nano CaCO3 and SiO2 helped to diminish the spherulite and broaden the transition region from amorphous to crystal The oxidation development along the depth in PP nanocomposites depended on the diffusion rate of oxygen and underwent two stages. In the first stage, the oxidation degree increased with time because of the low density in the transition region. In the layer under this region, the oxidation is difficult to occur owing to the high density of crystals and oxygen starvation. Therefore, the oxidation depth profiles decreased to near zero at nearly the same depth--about 200 microm from the exposed surface, corresponding to the transition region in PLM observation, namely surface oxidation layer. The high degree of oxidation in the surface layer gave rise to crack formation, and these surface cracks helped further oxidation in PP nanocomposites. In the second stage, the interconnection of surface cracks resulted in the fracture of the seriously oxidized surface oxidation layer. Consequently, the oxidation of the subsurface layer began and the depth profile developed to deeper place. In PP, the much thinner surface amorphous layer and the relatively perfect crystallization under it were responsible for the higher stability during the natural exposure.