Mechanical recycling and performance characterisation of insert-injection moulded homo-polypropylene single-polymer composites and foams

Abstract

Due to their easy recyclability and high sustainability, single-polymer composite (SPC) materials have been applied in various commercial products. Mechanical recycling can be efficiently conducted on SPCs without separation procedures. However, there is little research on the recycling of homopolymer-based SPCs. In this work, four injection moulded homo-polypropylene (PP) materials, including pure PP, microcellular PP foam (PPF), PPSPC, and PPSPC foam (PPSPCF), were recycled through shredding and hot-pressing. The mechanical properties of these recycled materials (R-PP, R-PPF, R-PPSPC, R-PPSPCF) were examined through bending and tensile tests, and the material's internal structure was revealed through thermal analysis, rheological tests, and morphological observation. The performance decline of R-PP demonstrates thermal-mechanical degradation. The microporous structure induces molecular chain stretching during shredding, and low-temperature hot-pressing can preserve fibres and the stretched molecular chain structure in the relative recycled materials while hindering molecular chain scission. Thus, R-PPSPC and R-PPSPCF showed superior mechanical properties compared to R-PP. However, hot-pressing conditions are essential to conserving the thermal stability of recycled samples. These findings have encouraging implications for the sustainable usage of recycled SPC materials.