Preparation of open-cell polymer foams by CO2 assisted foaming of polymer blends

- Phase transition was induced by CO2 saturation.
- Open-cell foams were prepared by CO2 foaming of co-continuous polymer blends.
- Mechanical properties were investigated.

This paper reports on a novel process for preparing open-cell polymer foams which consists in (1) blending a desired polymer with one which is much more CO2-philic and which is the dispersed phase, (2) tuning the sea-island type of morphology of the polymer blend to a co-continuous type by selectively swelling the CO2-philic polymer with CO2 saturation, and (3) selectively foaming the CO2-philic polymer by CO2 depressurization. More specifically, a blend composed of 70% of isotactic polypropylene (PP) and 30% of a copolymer of ethylene-propylene-diene monomer (EPDM) is chosen to show the working principle and potential of the above process. PP is a desired polymer and EPDM a CO2-philic one. During the CO2 saturation, the initially sea-island type of morphology of the PP/EPDM (70/30) blend evolves into a more or less co-continuous one due to the much more CO2-philic nature of the EPDM. Upon CO2 depressurization, foaming takes place primarily in the EPDM phase, leading to open cells. To achieve open-cells for the PP/EPDM (70/30) blend, it is necessary to search for an optimum CO2 saturation pressure and temperature window. Within this window, the cell size and the expansion ratio are in the range of 1.5-17.5 μm and 1.5-6.6, respectively. Tensile results show that the elongation at break of the CO2 saturated and foamed PP/EPDM blends is higher than that of the un-treated one, indicating that the co-continuous morphology of the PP/EPDM blend improves its toughness. Moreover, compression results indicate that the open-cell foams exhibit good flexibility and elasticity.