From Polymer Waste to Composites: Thermoplastic Processing, Structural Development, and Material Properties

This article presents a critical review of chemical and technological approaches to the production of composites based on recycled polyolefins using thermoplastic processing. The aim of the review is to identify the relationships between feedstock condition, changes during processing, interactions between components, and material propertie. The main focus is on recycled polyethylene and polypropylene, including mixed and multi-component waste. The influence of the origin, contaminants and thermomechanical history of the raw materials on processability is analysed. Chain scission, branching and cross-linking are examined, together with their effects on molecular-weight characteristics, crystallinity and rheological behaviour. Methods for stabilising polymers and modifying composites using fillers and compatibilisers are summarised. The roles of extrusion, co-extrusion, injection moulding, calendering and compression moulding are analysed. It is shown that the content of recycled polymer and the melt flow index, taken separately, are insufficient for predicting properties. Environmental, economic and regulatory aspects are considered. The need for an integrated ‘chemical changes – structure – technology – properties’ approach is justified; this approach enables the targeted formation of the structure and regulation of the performance characteristics of composites through the selection of composition, modifiers and processing conditions.