Treatment of wood-based residues for the use in biocomposites

Abstract

Each year, the wood processing industry produces millions of tonnes of by-products such as sawdust,shavings, and wood chips. These materials are often turned into low-value products, including pellets for heat generation or particleboard bonded with synthetic adhesives. While such uses allow for some energy recovery or recycling, they also release CO2, rely on petrochemical binders, and do not take full advantage of the mechanical advantages of intact wood fibers. In the context of circular bioeconomy,these residues represent an abundant and renewable resource, with strong potential for sustainable,high-value applications.This thesis addresses the gap in research on converting sawmill by-products into structurally intact,holocellulose-rich reinforcement fibers for fully bio-based biocomposites. The work began with asystematic comparison of mild chemical pulping methods such as Organosolv (OS), alkali (Soda), and liquid hot water (LHW) pulping, to explore different solvents for fiber separation. Since these methods required additional mechanical refining, an alternative chemical process was tested: peracetic acid (PAA) pulping. This method aimed to produce holocellulose fibers at their natural length (~4 mm insoftwoods) while keeping the fiber (tracheid) structure intact. PAA pulping showed the best potential,combining effective lignin removal with high hemicellulose retention, fiber structure preservation, and less mechanical pretreatment requirement.A custom-designed 10 L glass reactor system was developed to scale up PAA pulping. This semiautomatic setup enabled larger, reproducible pulp batches, ensuring consistent feedstock for comprehensive material characterization, while avoiding metal-induced catalytic decomposition ofPAA. The scale-up was crucial to generate sufficient batch volumes to investigate modification,analytical and mechanical testing, and possible composite fabrication while maintaining processcontrol within one batch. Different process parameters were tested to find the optimum combination between hemicellulose retention, mechanical performance, complete lignin removal, and fiberintegrity.In a subsequent study, the focus shifted to fiber modification, specifically examining swelling behavior—a critical factor for binder penetration and fiber–matrix bonding. Various swelling agents,including water, ethanol–water mixtures, and sodium hydroxide solutions, were evaluated for their ability to uniformly expand holocellulose fibers without degrading cellulose or hemicellulose. In asecond step, these swelling agents were then tried as carrier solvents for lignin impregnation, enablinga one-step swelling and impregnation process to streamline production.The optimized PAA-treated fibers showed high hemicellulose content, preserved structure, and greater surface reactivity, making them well-suited as load-bearing elements in lignin-based biocomposites. Mechanical tests confirmed the fibers' competitive tensile strength and strong interfiberbonding.The novelty of this work lies in integrating sustainable pulping, targeted swelling, and natural binderimpregnation into one process, supported by the design of specialized equipment for larger-scale PAApulping. This combination enables the use of mixed sawmill residues to produce structurally intact,bio-based reinforcement fibers. By demonstrating a scalable, low-impact process that valorizes acurrently downcycled industrial by-product, this thesis supports the move toward petroleum-free composite materials for structural and semi-structural applications, contributing to the development of a more sustainable, circular materials economy.