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In this article, the effects of different foam production times and temperatures on the mechanical, physical, and structural properties of polypropylene (PP) foam has been investigated. The microcellular PP foams were carried out using supercritical carbon dioxide (sc-CO2) as a physical foaming agent in a batch process. The samples were placed in a pressure vessel and were saturated with sc-CO2 at room temperature (23 - 25 °C) and saturation pressure of 40 MPa for 35 hours. Then, samples were removed from the vessel after releasing the pressure and were immersed in a glycerin bath for different times of 90, 180, and 270 seconds and different temperatures of 155 °C, 165 °C, and 175 °C. The mechanical responses of samples, e.g., flexural, Izod impact, and dynamic mechanical thermal properties, and physical characterization containing water absorption and relative density, were studied. Scanning Electron Microscopy (SEM) was applied to investigate the surface morphology. The cell size, cell density, and cell structure of PP foams were investigated by morphological parameters. X-ray diffractometer was utilized to evaluate the interaction effect on the samples. Results showed that in the semi-crystalline polymers such as polypropylene, the foaming temperature must be higher than the melting temperature of PP. A simultaneous examination of mechanical, physical, and morphological test results as well as cell structure properties indicate that the best temperature and time conditions for producing PP foams are the temperature of 175 °C and time of 270 seconds, respectively. These optimum conditions could be used in the industrial production of PP foams.

Microcellular plastics (MCPs) are manufactured through a batch process comprised of saturation and foaming stages. In the saturation process, gas molecules are dissolved into plastic in a high-pressure vessel. Following the saturation process, micro-cells are formed inside the plastic as the gas-dissolved plastic sample undergoes the foaming process. In this paper, we investigate the effects of repeating the batch process on the formation of MCPs. Because the plastic sample after the first batch process has developed microcells, these pre-existing cells are expected to affect the second round of the batch process. Of particular interest is the effect of repeated saturation at different saturation pressures. Experiments show that repeating the batch process can lead to favorable outcomes in terms of foaming ratio and cell morphology, which are otherwise unattainable particularly with a single batch process. [ABSTRACT FROM AUTHOR]