Promoting effect of crystallization on the foaming behavior in polypropylene homopolymer/polypropylene block copolymer blends

Polypropylene (PP) is a kind of semi‐crystalline polymer so it is hard to foam with supercritical carbon dioxide (SCCO₂). We used polypropylene block (PP‐B) copolymer as a modifier to improve the crystallization behaviors and foaming performance of polypropylene homo polymer (HPP). HPP, PP‐B, and a series of HPP/PP‐B blends were characterized by differential scanning calorimeter (DSC), X‐ray diffraction, and polarized optical microscope. Results show that both the crystallization behaviors and melt strength have influence on the cell structure. The crystallization temperature of PP‐B is about 7°C higher than that of HPP and that the crystallization behavior of HPP/PP‐B blends is similar to that of PP‐B. Much denser and smaller size spherulites are observed in PP‐B and HPP/PP‐B blends than in HPP, and the crystal structure is unchanged after blending. Scanning electron microscope results show that much more uniform, smaller cells can be obtained for the HPP/PP‐B blends. The crystal nuclei formed earlier can act as physical crosslink points, increasing the melt strength and improving dramatically the cell structure and morphology of the HPP/PP‐B blends. Furthermore, the best cell structure and morphology was achieved for HPP/PP‐B blends with the ratio of 70/30 under the same foaming conditions. POLYM. ENG. SCI., 56:1175–1181, 2016. © 2016 Society of Plastics Engineers