Polypropylene foaming using supercritical carbon dioxide: a review on fundamentals, technology, and applications

Polymer foams are extensively employed in automotive, flame retardant, and food packaging owing to their lightweight, excellent insulation, high strength, thermal stability, and impact resistance. Polypropylene (PP) is regarded as an alternative to other thermoplastic polymers because of its low-cost, excellent heat resistance, and corrosion resistance. However, the inherent limitation stemming from the weak melt strength of linear PP, coupled with its low melt elasticity, presents significant challenges in the production of PP foams characterized by microsized and uniform cell distribution. To enhance the performance of PP foam, various modification methods such as long-chain branching, cross-linking, blending, and adding fillers are employed. This paper first provides an overview of the foaming process of supercritical carbon dioxide and analyzes the influence of chemical structure and crystal structure on PP foaming. Subsequently, methods to enhance PP foaming properties, such as branching, cross-linking, blending, and filler incorporation, are discussed. Various foaming technologies for PP, including batch foaming, extrusion foaming, and injection foaming, are delineated. Furthermore, the diverse applications of PP foams, encompassing automotive components, thermal insulation, flame retardancy, electromagnetic shielding, oil–water separation, and food packaging were outlined. Finally, the current research status of PP foams is summarized, and the prospects for high-performance PP are addressed.