Influence of different ratio of CO2/N2 and foaming additives on supercritical foaming of expanded thermoplastic polyurethane

Expanded thermoplastic polyurethane (ETPU) foam materials with superelasticity for footwear application were developed and investigated. ETPU foams with different ratios of CO2/N2 as foaming agents and ETPU foams prepared by pure nitrogen foaming with different kinds of foaming additives were prepared by supercritical (Sc) foaming technology, which further made into sheet forms through the steam molding process. TPU foam exhibits better cell structure, crystallization performance, and mechanical strength with the increased CO2 proportion. Further, the foams using 25% CO2–75% N2 and 50% CO2–50% N2, showed a lower density and excellent resilience and cyclic-compression performance. However, an increase of N2 ratio improves the dimensional stability and compression set of the foam. The introduction of foaming aids enhances the interaction between Sc-N2 and TPU. The ETPU foam prepared under the same conditions has a smaller density and a larger expansion ratio. Especially the ETPU foam prepared by Sc-N2 using monofluorodichloroethane as the foaming assistant as a co-blowing agent, the decrease in cell size (from 117.9 to 19.6 μm) and density (from 0.175 to 0.139 g/cm3) has observed. The study revealed the high potential of these materials in footwear applications and offered sound evidence for mass production in the industry.