High-Expansion Injection-Molded Thermoplastic Polyester Elastomer Foam with Oriented Cellular Structure and Anisotropic Compressive Properties

Thermoplastic polyester elastomer (TPEE) foam, characterized by low density, good dimensional stability, and anisotropic mechanical properties, has been successfully fabricated using the mold-opening foam injection molding (MOFIM) process. This was achieved using a matrix of chain-extending modified TPEE and supercritical carbon dioxide (scCO2) as the physical foaming agent. The reaction with the chain extender led to a noticeable increase in the molecular weight of TPEE, accompanied by the formation of a cross-linked structure, particularly evident with the addition of 0.5 wt % chain extender. This modification significantly enhanced the melt's elasticity and strength as well as facilitated the process of crystallization, contributing positively to the foamability of TPEE. The resultant injection-molded TPEE foam displayed a fairly high expansion ratio of 16-fold and a uniform and dense cellular structure, characterized by a cell density of up to 2.97 × 108 cells/cm3 and an average diameter of less than 50 μm. Additionally, the obtained TPEE foam exhibited a distinct oriented morphology. In cyclic compression tests, the TPEE foam displayed anisotropic behavior, with 70% reduced residual strain and 100% heightened compression strength along the cell orientation direction. The high energy loss coefficient of TPEE foam suggests its potential utility in energy absorption applications, such as sports equipment, packaging, and transportation.