Upcycling Secondary Cellulosic Fibres and Post-Industrial Recycled Polypropylene from Household Appliance Manufacturing into Circular Composites with Enhanced Dimensional Stability

This study addresses the development, thermal characterisation, and dimensional stability of circular composites based on a post-industrial recycled polypropylene (rPP) matrix from household appliances. The matrix was reinforced with natural (B600 and BC300) and recycled (SIS250, PWC500, and ZZC500) cellulosic fibres at 10%, 20%, and 30% dosages, using 3% MAPP as a compatibilizer. Processing was conducted via twin-screw extrusion and injection moulding. Systematic evaluations (TGA-DTG, DSC, and DIL) showed a minor reduction (2.6–5.1%) in thermal degradation onset, maintaining a safe processing window. Cellulosic fibres generally promoted matrix crystallization through heterogeneous nucleation, increasing matrix crystallinity up to 39.3% in composite B600-30 compared to 20.4% for the reference rPP matrix. Furthermore, the reinforcements acted as dimensional stabilizers, reducing the linear thermal expansion coefficient (α40−120) by up to 28.9% (ZZC500-30) while increasing thermal conductivity by up to 32.7% (B600 series). This approach demonstrates a viable upcycling strategy, yielding high-secondary-content circular composites by combining post-industrial recycled polypropylene matrices with reinforcements (virgin wood cellulose benchmarks (B600 and BC300), pre-consumer recycled cellulose (PWC500), post-consumer recycled cellulose (ZZC500), and recycled technical sisal (SIS250), complemented by 3 wt.% commercial MAPP compatibilizer.