Microcellular nanocomposites based on millable polyurethane and nano-silica by two-step curing and solid-state supercritical CO2 foaming: Preparation, high-pressure viscoelasticity and mechanical properties

- Millable polyurethane foam was prepared by supercritical CO2.
- Viscoelasticity of polyurethane under CO2 was analyzed by high-pressure rheometer.
- The increase of crosslinking density depressed the CO2 diffusion.
- The effect of viscoelasticity on the cell structure depended on cell diameter.
- The tensile strength decreased linearly with cell diameter on a log-log scale.

The cell structure of microcellular millable polyurethane/nano-silica composites prepared by combining the solid-state supercritical CO2 foaming with the two-step curing were investigated in detail according to the high-pressure viscoelasticity of pre-cured polyurethane. The complex viscosity and storage modulus of pre-cured polyurethane saturated under supercritical CO2 increased with improving the crosslinking network induced by either pre-curing or nano-silica. Moreover, the raised crosslinking density could depress the CO2 diffusion and thus the reduction of the viscosity and modulus. It was shown that the matrix viscoelasticity impacted the cell morphology in different ways as the cell diameter was varied significantly. The compressive hysteresis results showed that the energy absorption of millable polyurethane elastomer was enhanced after the introduction of microcells. The tensile strength of millable polyurethane nanocomposite foam decreased with increasing the cell diameter, and displayed a linear relationship with cell diameter on a double logarithmic scale.

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