Co-hydrothermal carbonization of biopolymer and biomass wastes: Sustainable valorization into solid fuel and thermal filler.

The rapid growth of polylactic acid (PLA) consumption has created a disposal challenge because PLA degrades slowly under natural conditions and is rarely recovered through conventional recycling. This study proposes co-hydrothermal carbonization (co-HTC) with biomass as a practical valorization route to convert mixed PLA waste into functional hydrochar. Rice husk (RH) and spent coffee grounds (SCG), representing abundant agricultural residues, were used as co-feedstocks. Co-HTC enhanced carbonization and produced hydrochars with fuel characteristics comparable to lignite and bituminous coal. At a PLA:biomass ratio of 1:3, hydrochar yield, energy yield, higher heating value, and carbon retention showed positive synergistic effects relative to individual feedstock treatment. The hydrochars exhibited stable combustion behavior and improved fuel quality, indicating suitability as solid fuel substitutes. For practical application, hydrochars were incorporated into epoxy molding compounds (EMCs) used in electronic packaging. Incorporation of 30 wt% hydrochar significantly improved thermal conductivity, heat dissipation, and mechanical strength without compromising processability. These results demonstrate that co-HTC enables simultaneous management of bioplastic waste and biomass residues while generating value-added materials. The hydrochar can function both as a renewable solid fuel and as a thermal filler for polymer composites. This dual functionality provides a scalable circular-economy pathway for plastic-biomass waste streams.