Styrene-isobutylene-styrene copolymer (SIBS) is a thermoplastic elastomer with excellent chemical stability, biocompatibility, and low gas permeability. SIBS is a good candidate with a high melt viscosity and a high storage modulus to develop new lightweight elastomeric products. Foam injection molding with core-back operation is an efficient method to prepare SIBS foams. However, it is challenging to prepare a microcellular foam from neat SIBS by melt processing, such as foam extrusion or foam injection molding, because the hard segments cannot play a role in bubble nucleation sites in the molten state. Furthermore, a significant degree of shrinkage occurs after foaming. By introducing a semicrystalline polymer such as polypropylene (PP), the foamability can be improved in foam injection molding processes. By adjusting the foaming temperature to the crystallization temperature of PP, PP crystals provide bubble nucleation sites and increase the viscosity to suppress bubble growth.Microcellular foams with high cell density and small cell size were achieved at 10 8 cells/cm 3 and approximately 13 μm. PP can also impede the shrinkage of SIBS foams.

No abstract

Microcellular injection molding is an attractive method. However, their surface imperfections have been a major problem hindering wide industrial applications. Several methods have been proposed to improve the surface appearance of foams. In this study, we proposed a method to improve the surface appearance of polypropylene (PP) foams from the material property perspective, especially with regard to crystallization and viscosity. The basic idea of the surface improvement is to reduce the size of bubbles generated at the flow front, delay the solidification behavior of the polymer at the mold interface, squeeze the bubbles existing at the mold-polymer interface, and redissolve the bubbles into the polymer by holding pressure. Blending a low-modulus PP delays the crystallization of the polymers at the skin layer and solidification, taking enough time to squeeze the bubbles smaller. A sorbitol-based gelling agent, bis-O-([4 methylphenyl]methylene)-D-Glucitol, was used to increase the viscosity at a low strain rate to reduce the size of the bubbles generated at the flow front during the filling stage. The foam injection molding experiments demonstrated that the proposed method effectively improved the surface appearance of the foams. In particular, the surface appearance of the foams became almost equivalent to that of solid samples using low-modulus PP.