Structure-gradient thermoplastic polyurethane foams with enhanced resilience derived by microcellular foaming

Polymer foams with gradient cellular structure show superior mechanical and physical properties, while developing polymer foams with tailored gradient-structures is still challenging. Herein, an environmentally-friendly foaming process with CO2 as blowing agents was developed for fabricating structure-gradient thermoplastic polyurethane (TPU) foams. In the unique microcellular foaming process, a two-step sorption procedure with a low CO2 pressure at the first stage and a high CO2 pressure at the second stage was used to generate gradient CO2 concentration in the solid TPU samples, which results in gradient cellular morphology in TPU foams. By varying the second sorption time, TPU foams with various gradient cellular morphology were prepared. Interestingly, the gradient cellular morphology helped to increase compression strength by 71.4% and modulus by 113.8%, to enhance resilience with a 24.7% reduction in energy loss coefficient and a rebound coefficient of 61.4%, and also to benefit shape-recovery after long compression deformation.