Cell structure and hardness evolutions of TPU foamed sheets with high hardness via a temperature rising foaming process

The foams based on TPU with high hardness were prepared by temperature rising foaming. The effects of hardness on thermal behavior, mechanical and rheological properties, gas solubility and diffusivity of TPU were discussed. It was found that TPU with higher hardness have lower CO2 solubility and smaller diffusivity. TPU foamed sheets with various expansion ratio and cellular morphology were fabricated using different foaming parameters. It was easier to prepare TPU foams with low density for TPU with higher hardness. At suitable foaming temperatures, foams with similar density but different cell sizes could be prepared by altering saturation pressures. Finally, the hardness of the foamed samples was systematically compared. It was demonstrated that the hardness of TPU foams was affected by foam density, resin hardness and cell structure. [Display omitted] •Hardness has a significant effect on the thermal behavior and mechanical properties of TPU.•TPU with higher hardness has lower CO2 solubility and diffusivity.•At high foaming temperatures, increasing the saturation pressures will cause severe shrinkage of TPU foams.•The hardness of TPU foams is affected by foam density, resin hardness and cell structure.