Comparison of coupled chemical pretreatment and mechanical refining of spruce sawdust: fiber network properties and initial production of lignin-bonded biocomposites

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.