Nanocellular TPU composite foams achieved by stretch-assisted microcellular foaming with low-pressure gaseous CO2 as blowing agent

Microcellular foaming is an advanced eco-friendly foaming technology with carbon dioxide as blowing agent. Generally, high-pressure supercritical blowing agents is needed to achieve microcellular structure with cell size less than 10 μm, which places high requirements on the foaming equipment and brings safety issues, thereby resulting in high production and maintenance costs. Herein, the stretch-assisted microcellular foaming technology was developed to prepare TPU, TPU/nanographite, and TPU/PMMA foams with significantly refined cellular structure. The results demonstrate that stretch can dramatically promote cell nucleation. For the neat TPU, the cell density can be increased by more than 8 orders of magnitude by applying a linear strain of 50 % in microcellular foaming. Increasing the amount of stretching can continue to refine the cell morphology. Compared with the neat TPU foam, the TPU/nanographite composite foam and the TPU/PMMA composite foam show further refined cellular morphology under the same foaming condition. Surprisingly, with low-pressure (300 psi) gaseous carbon dioxide as blowing agents, microcellular TPU and TPU/nanographite foams, and nanocellular TPU/PMMA foam were achieved by the stretch-assisted microcellular foaming. Furthermore, the modified classical cell nucleation theory by considering the role of elastic distortion energy in cell nucleation was proposed to clarify the cell nucleation behavior in the stretch-assisted microcellular foaming process. According to the theory, the synergistic effect of stretch-induced cell nucleation and heterogeneous phase-induced cell nucleation is the underlying mechanism of the significantly refined cellular morphology.