Micro-crosslinked thermoplastic polyurethane foam with ultra-low density, excellent resilience and recycling by supercritical CO2/N2

Abstract

Supercritical CO₂ foaming can produce cushioning materials with excellent performance, such as thermoplastic polyurethane (TPU) foam. However, TPU foam generally face the issues of low mechanical resilience and hard recycling. The foaming properties of TPU and the dimensional stability of the product are affected by the molecular structure of the matrix. In this study, the diphenylmethane diisocyanate (MDI) masterbatch, a special modifier for TPU, was selected to prepare modified TPU with micro-crosslinking structure and chain extension. The micro-crosslinking structure limited the large-scale relaxation of molecular chains and enhanced the rheological properties and foaming behavior of TPU. The maximum expansion ratio of TPU foam was increased from 14.5 to 28.3. The shrinkage problem was optimized through CO₂/N₂ foaming. TPU foams with different stabilized expansion ratios (7–15.9) were produced by changing the N₂ partial pressure. The ultra-high expansion ratio and uniform cell structure provided high relative stress softening (＞95 %), low relative hysteresis loss (∆U/U≈5 %) and excellent resilience (∼70.1 %) for TPU foam. This product is promised to realize light-weighting and efficient energy recovery of TPU foam. After a closed-loop sustainable recycling and foaming process, the recyclable RTPU foam still maintained high expansion ratio and low energy loss.