Effect of Different Aqueous Solvents with and Without Solubilized Lignin on the Swelling Behavior of Holocellulose Fibers.

The modification of lignocellulosic fibers through controlled swelling and impregnation plays a decisive role in tailoring their structure and reactivity for use in sustainable composite materials. In this study, holocellulose fibers were swollen in various solvents (sodium hydroxide at 2 and 4 wt% and ethanol-water mixtures at 0, 50, 70, and 100 wt%) to evaluate their impact on swelling and fiber characteristics. The pulp was produced with peracetic acid at 90 °C for 120 min from spruce wood chips and used for the swelling treatment. The fibers underwent swelling for 4 h in the different solvents, both without and with solubilized lignin at concentrations of 10 and 30 g/L, to investigate the impregnation ability of the fibers for lignin as a natural binder. Fiber morphology, lignocellulosic composition, and liquid retention values were analyzed to assess the effects of solvent-binder interactions on fiber swelling and lignin uptake. The results revealed significant differences in fiber characteristics influenced by both solvent choice and lignin presence, demonstrating the feasibility and optimization potential of a single-step swelling-impregnation process. These findings highlight key factors that can improve the uptake of natural binders in wood fibers, offering insights for effective fiber preconditioning in composite production.