Microcellular foam of styrene-isobutylene-styrene copolymer with N₂ using polypropylene as a crystallization nucleating and shrinkage reducing agent

Abstract

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 10⁸ cells/cm³ and approximately 13 μm. PP can also impede the shrinkage of SIBS foams.