High-performance and cost-effective poly(3-hydroxybutyrate)/microalgae biocomposites: synergistic chemical and mechanical modification transforming microalgae from a weakening filler to a reinforcement

Poly(3-hydroxybutyrate) (PHB) is a biodegradable, microbial produced polymer with promising potential as a sustainable alternative to petroleum-based plastics. However, its commercial viability remains limited due to the high production cost, reliance on hazardous extraction solvents, and low yields from bacterial fermentation. To address these challenges, this study investigates the incorporation of microalgae (MA) biomass as a cost-effective, biodegradable filler for PHB-based composites, with the aim of reducing extraction and purification costs while maintaining sustainability. However, direct use of biomass significantly deteriorates mechanical properties even at modest loadings and has long remained an unsolved barrier for algal valorization. Here, synergistic strategies to improve both chemical and morphological compatibility between MA and PHB are demonstrated. First, surface esterification was employed to enhance chemical affinity; then, ultrasonic disintegration was used to reduce the voids and defects in the final composites. When combined, these treatments enabled the composites to retain or improve mechanical performance, achieving increases of 17% in toughness and 22% in elongation compared with pristine PHB. In contrast, untreated biomass caused a substantial reduction of 77% in toughness and 62% in elongation. Biodegradation studies revealed that incorporating 25 wt % untreated MA accelerated degradation by up to 65% after 80 days, whereas ultrasonicated MA had a minimal effect, likely due to the dominant physical barrier imposed by PHB. This study demonstrates an economically and environmentally sustainable pathway to mitigate the challenges in algae-based composites while maintaining or even enhancing the performance, bioavailability, and biodegradability with practical and economic potential for commercial bioplastic applications.