Around 50% of sawn wood remains as a by-product during the processing into timber. A large part of these by-products consists of sawdust, which still contains a lot of intact fibers and would have great potential to be used for biocomposite materials. This study investigates the influence of different pretreatments on spruce sawdust in a two-step process, where chemical pretreatments like liquid hot water (LHW), organosolv (OS), and alkali (NaOH) are coupled with a mechanical pretreatment (refining). The idea is to customize a suitable fibrous material for creating a biocomposite with a natural binder as lignin instead of synthetic binders. The first part of the study focused on comparing the different chemical pretreatments and the influence of the refining time. Each chemical pretreatment resulted in a different partially solubilized lignocellulosic matrix profile, making the matrix’s fibers less or more accessible. In the second step, the material was treated in a refiner to fibrillate the embedded fibers, enhancing the flexibility and bonding properties. Paper sheets were produced to evaluate the mechanical properties of the obtained fibrous materials. Based on the results, the most promising material was selected for further investigation. The second part of the study focused on the initial testing of the produced fibrous materials as composites. First, the fibers produced in the first part were impregnated with lignin, forming composite bars through hot-compression molding. Finally, the tensile strength of the composites was determined. Especially with higher pretreatment temperatures and longer refining times, several promising combinations could be found for the two-step process.

Developing eco-friendly, high-performance fibers requires a deep understanding of the interplay between chemical and physical properties and processing conditions. Peracetic acid (PAA) pulping offers a sustainable alternative to conventional methods, decomposing into water and acetic acid, while providing higher selectivity for lignin removal and lower energy demand. This study aims to optimize PAA pulping conditions to maximize lignin removal while retaining hemicellulose and cellulose, thereby improving fiber quality for applications in biocomposites and paper products. PAA pulping was conducted under systematically varied conditions, with temperatures ranging from 70 to 90 °C and reaction times from 60 to 180 min at a 3 wt% solid load. The conditions were selected based on the reaction spectrum of PAA, which becomes feasible for pulping above 70 °C. To operate at atmospheric pressure and avoid excessive degradation, temperature was limited to 90 °C. The study (1) investigates the effect of these parameters on pulping efficiency, (2) evaluates chemical composition and structural changes through lignin content analysis, carbohydrate profiling, and fiber morphology characterization, and (3) determines mechanical performance through tensile testing of paper sheets before and after hot pressing. Optimal results at 80 °C for 120 min led to increased inter-fiber bonding (106.13 Nm/g), significant hemicellulose retention, and substantial lignin reduction. These findings underscore the potential of PAA pulping as an energy-efficient, sustainable method for producing tailored holocellulose fibers with applications in biocomposites and other renewable materials, highlighting a promising strategy for valorizing wood byproducts and reducing carbon emissions.

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.