High-elastic and strong hexamethylene diisocyanate (HDI)-based thermoplastic polyurethane foams derived by microcellular foaming with co-blowing agents

Thermoplastic polyurethane (TPU) foams have exhibited promising prospect in many industries such as automobile, sportswear and packaging, due to their outstanding mechanical properties. However, the application of TPU foams prepared by microcellular foaming with CO2 as blowing agents is still limited, due to the serious shrinkage after foaming. Herein, in this study microcellular foaming with mixed CO2 and N2 as co-blowing agents was used to control the shrinking behavior of hexamethylene diisocyanate (HDI)-based TPU foams, and further, the effects of shrinkage, expansion ratio, and cell size on the mechanical properties of TPU foams were decoupled. The results show that the stretching degree of the molecular chain and the solubility of co-blowing agents play a vital role in stabilizing TPU foams. Foams with an expansion ratio of up to 16-fold can be prepared with both pure CO2 and co-blowing agents. The shrinkage ratio of TPU foams prepared with co-blowing agents is 6.3 %, while that of foams prepared with pure CO2 is 37.8 %. Moreover, it is also found that the mechanical properties of TPU foams with a smaller shrinkage ratio are much higher than those with a larger initial expansion ratio and a similar final expansion ratio.