Supercritical CO2 foaming and shrinkage resistance of thermoplastic polyurethane/modified magnesium borate whisker composite

Abstract

Thermoplastic polyurethane (TPU) foam is eco-friendly prepared with supercritical carbon dioxide (CO2) as a blowing agent. Magnesium borate whisker (Mg2B2O5) modified by borate ester coupling agent (BE) (BE-Mg2B2O5) was introduced into TPU to modulate cell nucleation, bubble growth and foam stabilization depending on the intercalation structure of Mg2B2O5, entanglement effect of BE and increased physical crosslinking density. The modulation of Mg2B2O5 on the hard segment crystallization promoted hard segment’s (HS) microstructure, i.e., physical crosslinking density. The addition of BE-Mg2B2O5 enhanced the rheological properties, maximum tensile strength and elongation at break of TPU composites. Excellent mechanical property and strain-hardening behavior of TPU composites improved the foamability of TPU composites. Mg2B2O5 and entanglement points acted as heterogeneous nucleation sites to momentously increase cell density and reduce bubble size. Intercalation structure of Mg2B2O5, entanglement effect of BE and high physical crosslinking density effectively limited the TPU relaxation and memory recovery. BE-Mg2B2O5 around the cell wall acted as a barrier to slow down the diffusion rate of CO2 from inside the bubble to outside. The dual action synergistically reduced the shrinkage of TPU foam.