Upcycling Post-Consumer PET into Bio-Modified Unsaturated Polyester Resins for Low-Carbon, Weather-Durable Hemp-Fiber Composites

Abstract The transition toward circular and low-carbon materials demands strategies that recover post-consumer polymers while transforming them into higher-value, longer-life products. In this work, post-consumer recycled poly(ethylene terephthalate) (PCRPET) was used as a feedstock for unsaturated polyester resins via glycolysis–polyesterification using five polyol systems: propylene glycol (PG), diethylene glycol (DEG), bio-based 1,3-propanediol (bPDO), bPDO/PG, and bPDO/DEG. The resulting resins were reinforced with woven hemp fibers at a resin-to-fiber ratio of 80:20 (w/w, six plies) to produce composite laminates, which were evaluated for mechanical performance, accelerated weathering durability (QUV, up to 1000 h), outdoor thermal behavior, and carbon footprint. Glycolysis yielded structurally comparable hydroxyl-terminated oligomers across all polyol systems. During polyesterification, bPDO promoted chain growth, resulting in higher molecular weights, broader dispersity, and more heterogeneous crosslink density. Woven hemp fiber increased impact strength by more than fivefold across all formulations. UPR_bPDO_hemp fiber achieved the highest impact strength (6.39 ± 0.79 kJ/m2), exceeding the fossil-based reference composite, while UPR_bPDO/DEG_hemp fiber showed the most favorable flexural strength retention during prolonged QUV exposure. All bPDO-containing composites exhibited superior thermal buffering relative to the PG-only system, with peak temperature differences of up to 5.5 °C at noon. bPDO incorporation provided a nominal bio-based polyol content of up to 21.3 wt % and a radiocarbon-measured bio-based carbon fraction (Percent Modern Carbon) of 14.88 ± 0.25% for UPR_bPDO, while reducing the calculated CF by approximately 16% relative to the fossil-based neat resin reference. These results demonstrate the potential of PCRPET-derived, bio-modified hemp composites as durable, lower-carbon materials for outdoor applications.