Rheological, crystallization and foaming behaviors of high melt strength polypropylene in the presence of polyvinyl acetate

Polyvinyl acetate (PVAc) is a kind of CO2-philic materials with high solubility of CO2. For improving the supercritical carbon dioxide (Sc-CO2) foaming behavior of isotactic polypropylene (iPP), a high melt strength polypropylene (HMSPP) was prepared using styrene (St) as grafting monomer. The effect of PVAc on the preparation, rheological, crystallization and foaming behaviors of HMSPP was investigated. The high temperature gel permeation chromatography (HT-GPC) results showed that the PVAc had a promotive effect on melt grafting reaction. With the addition of PVAc, the weight-average molecular weight (Mw) of HMSPP increased from 217,158 to 240,733 g/mol. Thus, the HMSPP presented higher complex viscosity and storage modular, and lower loss angle, which indicated that the melt viscosity and melt strength of HMSPP was increased by adding PVAc. The crystallization behavior of HMSPP was investigated using differential scanning calorimetry (DSC). Double crystallization peaks were observed on the DSC cooling curves of HMSPP in the presence of PVAc, which was ascribed to incomplete molten of iPP with long chain branching (LCB) structure at low end melting temperature. Moreover, the prepared HMSPP exhibited better foaming behavior in the presence of PVAc. With the addition of PVAc, the average cell diameter of HMSPP decreased from 93 to 59 μm, and the cell density increased from 2.83 × 107 to 9.79 × 107 cell/cm3.