Facile Construction of Polypropylene Foam with Dense Oriented Cell Structure and Enhancing-ductility Mechanism

Common polypropylene (PP) foam exhibited intrinsic inferior ductility and mechanical strength, which restricted its application as lightweight structural parts. To improve the ductility and mechanical strength of PP foam, the highly oriented PP component with fibrillar structure (cold-drawn fiber, CDF) was blended with isotropic PP resin to prepare PP/CDF foam via a facile foam injection molding process. During foaming at 170°C, CDF was not melted completely. Then, the remaining fibrils acted as shish structure and induced the crystallization of PP chains on its surface to form oriented kebab crystals, and the viscoelasticity and melt strength of the system were significantly improved, which promoted the nucleation of the bubble and the elongation of the cells, constructing a dense oriented cell structure with a large number of slender cylindrical cell walls for the foam. Such cell wall structure was prone to large plastic deformation and continuous elongation under load, which reduced stress concentration and absorbed a significant amount of tensile energy. As a result, a ductile and strong PP/CDF foam was successfully constructed, which exhibited roughly 465% increase in strain at break and 15% increase in tensile strength, respectively compared with PP foam.