Sustainable Reprocessing of Thermoset Composite Waste into Thermoplastics: A Polymer Blend Approach for Circular Material Design

Thermoset composites provide excellent strength but pose major recycling challenges due to their crosslinked structure. In this study, epoxy–polyurethane–glass fiber (EPG) wastes were mechanically recycled into low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polyamide-6 (PA6) matrices to produce second-generation thermoplastic composites (STCs). Fillers at 10–50 wt% were processed by single-screw extrusion and compression molding, and the resulting composites were comprehensively characterized. For LDPE, the tensile modulus increased by ~286–589% and tensile strength increased by 40–47% at 20–30 wt% loading, though ductility decreased at higher levels. HDPE composites showed a ~347% rise in modulus and ~24% increase in strength, but performance declined with more than 40 wt% filler. PA6 offered the most balanced outcome, retaining ~70% of its neat tensile strength while achieving an ~300% modulus improvement at 40 wt% loading. Thermal stability was strongly enhanced, with char residue at 700 °C rising from 0.4% to 38.7% in PA6 and from ~2.5% to 33–46% in polyolefins. In contrast, crystallinity decreased (e.g., LDPE 62.2% → 23.7%), and impact strength dropped at a loading above 30 wt%. Overall, the results demonstrate that EPG wastes can be reprocessed into functional composites without compatibilizers, with PA6 providing the most robust property retention at high filler contents.