Better scCO2 Foaming of Polypropylene via Earlier Crystallization with the Addition of Composite Nucleating Agent

A method of earlier crystallization was applied to prepare high-performance polypropylene (PP) foam using supercritical CO2 (scCO2) with the addition of a composite nucleating agent (CNA). Increased crystallization point and decreased crystal size of PP were obtained in the presence of CNA. Earlier crystallization induced cell formation in an early stage of foaming; this, combined with enhanced heterogeneous nucleation caused by microcrystal growth, resulted in well-defined cellular structure. The cell size of PP/CNA foam decreased from 87 to 30 μm, and the cell density was more than 20 times that of the neat PP foam. Moreover, the first compression stress of PP/CNA foam increased from 125 to 180 kPa compared to neat PP foam. We proposed that earlier crystallization promoted the emergence of a large number of microcrystals at the beginning of the foaming process, which effectively suppressed cell growth and prevented cell rupture and collapse.