Unlocking the potential of enzymatic hydrolysis lignin with high cellulose content in poly (lactic acid) (PLA) composites: The role of cellulose and fractionation

The next generation of biorefinery aims to maximize the value of each fraction of biomass and valorization of lignin is a known challenge in carbohydrate-first approaches. For instance, high-value application of enzymatic hydrolysis lignin is crucial from the perspective of biorefinery profitability and sustainable circular bioeconomy, but is difficult to achieve due to the inherent structural heterogeneity of such lignin stream, especially with a high cellulose content. It is hypothesized that reducing the heterogeneity of lignin can improve its applicability. In this study, we propose a simple lignin fractionation strategy using 75% ethanol. The chemical composition, crystallinity, and morphological characteristics of each fraction were systematically characterized. These fractions were then compounded with polylactic acid (PLA) through thermal processing to investigate the mechanism by which cellulose residues influence the mechanical properties of the resulting composites. The 75% ethanol-soluble lignin fraction, featuring a low molar mass, exhibited better compatibility (93%) with the PLA matrix than the insoluble counterpart. Meanwhile, the ethanol-insoluble fraction, which contains 20% crystalline cellulose, exhibited the highest flexural modulus (4864.05 ± 112.34 MPa), higher Yang’s modulus (2203.98 ± 120.12 MPa), and rheological storage modulus. The synergistic effect of lignin and cellulose in enhancing the mechanical stiffness of PLA composites was further revealed using cellulase hydrolysis to remove cellulose prior to composite preparation. This study demonstrates that enzymatic hydrolysis lignin holds great potential for use in PLA composites with enhanced mechanical and melting rheological properties, opening up numerous opportunities for lignin-based thermoplastics and contributing to a sustainable bioeconomy.