Recycling Polypropylene from Waste Polymers: A Review

Abstract Recycling is a critical component of sustainable waste management, turning waste materials into valuable resources and conserving energy, reducing pollution and decreasing the amount of material that ends up in landfills. Polypropylene (PP), as one of the most common recyclable polymers, is a kind of lightweight, high-strength and multi-purpose thermoplastic. This review presents the structure and major properties (stiffness, heat resistance, low water absorption, and finally chemical stability) of PP as well as an overview on the main commercial grades: homopolymer (HPP), random copolymer (RCP), and impact copolymer (ICP). Overall efficiencies, environmental aspects and polymer quality are compared for the basic mechanical processes, chemical methods and strategies into energy recovery. To stress out practical performance, tested data presented evidence that recycling can improve stiffness and flow properties: Young’s modulus goes from 1200 MPa (virgin PP) to 1400 MPa (recycled PP), +16.7%, whereas melt flow rate increases from 6.3 up to 11 g/10 min (+74.6%), as expected by the occurrence of chain scission phenomenon leading to a molecular-weight reduction during reprocessing (melt-compounding). With further reprocessing cycles the tensile strength and impact properties usually diminish, as does elongation at break even though it flexural modulus may be increasing (from increased brittleness and internal structural changes). Despite the fact that chemical recycling is more involved and more expensive compared to mechanical recycling, it’s highlighted. Finally, major PP applications are outlined, including fibers, films, sheets, molded products, packaging, and automotive components (e.g., bumpers, ducts, and battery casings), underscoring PP’s role in circular-economy strategies and global sustainability goals.