Sustainable processing of low-density polymers has recently been of increasing interest for engineering purposes where decreased density and increased material efficiency are desired. Polypropylene composites containing micro-silica and nano-silica were fabricated and tested to investigate the effect of filler size on the mechanical performance and cellular structure of the composites. The (PP) -silica composites were produced using a SG-20 twin parallel screw extruder and then compression-moulded into solid cylindrical samples. Subsequently, the compression testing was performed per ASTM D695 standard to evaluate the mechanical properties of the resulting polymer composites. PP composites containing various loadings of micro-silica (0–25 wt%) and nano-silica (0.25–6 wt%) were fabricated to investigate the influence of filler size and concentration on the mechanical properties and cellular morphology of the resulting foams. The resulting samples were then subjected to constrained batch foaming with nitrogen (N₂) at 168 °C and 5000 psi for 10 mins. The resulting compressive properties of (PP) show that silica reinforcement improved the compressive mechanical properties of (PP) composites when at optimal filler loading levels. In terms of the highest compression modulus and strength for micro-silica composites, this occurred at 10 wt%, however when considering the effectiveness of reinforcement from nano-silica composites was only at a concentration of 1.5 wt% because of increased specific surface area and better interaction between the filler and matrix. The solid samples were analysed using scanning electron microscopy (SEM) to Investigate the differences in the morphology of the broken edges of composite material and filler particle location, however the foamed samples showed continued improvement in the structure of the cellular material based on their morphology in relation to the addition of silica filler material. Furthermore, the introduction of nano-silica to the foamed sample material provided the smallest, most uniformly shaped cellular structures and demonstrated superior nucleating ability during the foaming process when compared to other foaming agents. The constrained foaming process decreased the sample density from 0.765 g/cm³ to 0.096 g/cm³, representing an 87.5% reduction, and expanded the sample volume by a factor of eight.

Keywords

Polypropylene composite foams, Micro-silica and nano-silica, reinforcement, Constrained batch foaming, Cellular morphology, Mechanical properties, Lightweight material