Effect of temperature on thermal, mechanical and morphological properties of polypropylene foams prepared by single step and two step batch foaming process

This study reports the influence of foaming temperature on morphological and thermo-mechanical characteristics of polypropylene (PP) foams prepared using two different methods of batch foaming at low saturation pressures. In the first method, involving single-step pressure-induced-foaming (PIF), the solid specimen was heated to different temperatures in a high-pressure vessel, and then saturated CO₂ under pressure. Depressurization then led to foaming. In the second method, involving a two-step temperature-induced-foaming (TIF), the solid specimen was saturated with pressurized CO₂ at room temperature for a specific period of time and then the CO₂ laden specimen was immersed in hot glycerol bath at different temperatures for foaming. SEM micrographs of the fractured foamed specimens were employed for measurement of cell-size distribution. The average cell sizes ranged between 3 and 310 μm in specimens obtained using PIF, while in case of foams prepared by TIF, the cell sizes ranged between 30 and 70 μm. The cell sizes achieved in TIF are significantly smaller and more uniform as compared to those in PIF foams. The density (0.393–0.186 g cm⁻³) of PIF foams was seen to decrease with increasing foaming temperature; in contrast, in case of TIF the density remained more or less unchanged around 0.43–0.47 g cm⁻³ with changes in foaming temperature. The foamed specimens were characterized in uniaxial compression; the stiffness (elastic and collapse moduli) and compressive strengths of both the PIF and TIF foams were seen to decrease with increase in cell-size. The magnitude of plateau-regime stresses within the compressive stress-strain response showed strong correlation with the foam cell-wall thickness. The crystallinity of the foamed specimens was observed to decrease with increase in foaming temperatures. The thermal stability of both PIF and TIF foams in general showed improvement compared with the PP matrix.