Mechanical Recycling of Hybrid Polypropylene/Recycled Carbon Fiber/Hemp Composite Wall Panels: A Comparative Study of Compression and Injection Molding

ABSTRACT

Circular economy increasingly influences sectors such as automotive, packaging, and building‐construction. The rise of circular economy in these sectors is driven by the generation of large volumes of waste that end up in landfills, resulting in a higher energy footprint. To address this challenge, circular economy seeks to extend the productive life of materials and components through recycling, upcycling, reprocessing, and repair, thereby minimizing waste and maximizing resource efficiency. This study focused on composite wall panels comprising polypropylene (PP), recycled carbon fibers (rCF), hemp fibers, and a minor addition of CO ₂ ‐derived MWCNTs at 0.1 wt%. The CO ₂ ‐derived MWCNTs were included as a minor fixed constituent based on prior LCA‐supported development of the original wall panel. The study primarily focuses on the recycling of composite wall panels made from these constituent materials. The original composite wall panels were shredded into processable granulates and reprocessed separately through compression and injection molding. The resulting recycled composites were evaluated using mechanical testing, DSC, μCT‐based void and fiber length distribution, and SEM‐based shredding‐induced damage. Compared with the original panels, compression‐molded composites showed reduced tensile strength and modulus by 69% and 57%, respectively, while injection‐molded composites showed reductions of 38% and 16%, respectively. Flexural strength and modulus showed larger reductions of 87% and 91% for compression‐molded composites and 77% and 81% for the injection‐molded composites. In contrast, impact strength increased by 42% after compression molding but decreased by 47% after injection molding. DSC showed that the PP melting transition was retained after reprocessing, but crystallinity decreased slightly after shredding from 30.6% to 27.5%, varied substantially after compression molding, and increased to 38.4% after injection molding. μCT results showed 21.14% and 2.19% voids in compression‐molded and injection‐molded samples, respectively. Overall, the results show that compression molding retained longer fiber lengths but resulted in high void content and structural heterogeneity, whereas injection molding produced greater fiber attrition but lower void content and better tensile/flexural property retention.