Nanosilica Addition Dramatically Improves the Cell Morphology and Expansion Ratio of Polypropylene Heterophasic Copolymer Foams Blown in Continuous Extrusion

Currently, the preparation of polypropylene (PP) foam with a well-defined cell structure and a high expansion ratio is receiving increased attention. However, the present technical problems such as poor cell nucleation ability and weak melt strength of polymer resin, hinder the broader use of linear PP in foam production. In this study, a PP heterophasic copolymer with a linear structure was selected together with nanosilica to challenge the fabrication of PP foam with uniform cell structure, high cell density, and a high expansion ratio using CO2 as a physical blowing agent. Scanning electron microscopy (SEM) observation indicated that silica particles tended to aggregate in the PP matrix, but the multisilica aggregates with sizes from 80 to 350 nm were well dispersed in PP because of the addition of a coupling agent (CA). PP foam exhibited poor cell morphology and low cell densities of ca. 104–5 cells/cm3 at different die temperatures. An introduction of a small amount of nanosilica, 0.5 wt % and 1 wt %, dramatically improved the foaming behavior of PP, where the cell structure distribution of the resultant foams was uniform, and the cell density and foam expansion were high (i.e., 108–9 cells/cm3 and 16.9–19.5, respectively). Furthermore, the presence of nanosilica clearly broadened the foaming window of PP. By further increasing silica content, however, the foaming behavior of PP/silica nanocomposites became poor, especially at slightly higher die temperatures (i.e., above 140 °C), even though a high silica loading increased the number of heterogeneous nucleation sites. The effect of foaming on the dispersion of nanosilica in the PP matrix was also investigated.