A novel pre-foaming strategy to manipulate carbon dioxide adsorption and desorption behavior for achieving low-density TPU foams

Microcellular foaming is one of the most promising method for preparing high-performance polymer foams. Gas diffusion and adsorption behavior can significantly affect the foaming behavior of polymers in microcellular foaming. However, it is still a challenging work to manipulate the gas diffusion and adsorption behavior for achieving desired performance of polymer foams. Herein, a novel pre-foaming strategy was proposed to manipulate carbon dioxide adsorption and desorption behavior for achieving low-density thermoplastic polyurethane (TPU) foams. It was demonstrated that pre-foaming is an efficient method for manipulating the diffusion and adsorption behavior of carbon dioxide in TPU matrix. Thanks to the newly created cellular structure by pre-foaming, both diffusion coefficient and adsorption amount increase gradually with increasing the pre-foamed expansion ratio of TPU. For the pre-foamed expansion ratio of 2.0, the gas solubility was increased by 92.8 %, the diffusion coefficients in adsorption and desorption were increased by 54.0 % and 111.0 %, respectively. Interestingly, pre-foaming can lead to a more perfect crystal structure, while destroying the hydrogen bond structure of TPU chains. Owing to the significantly increased gas adsorption capacity and greatly reduced cell growth resistance, the pre-foaming strategy can lead to remarkably increased expansion ratio of the TPU foams. All the TPU foams prepared without pre-foaming have a maximum expansion ratio less than 4, while those prepared with pre-foaming can have an expansion ratio larger than 16. This new microcellular foaming technique with pre-foaming provides a novel approach for preparing low-density thermoplastic elastomer foams.