The effect of thermomechanical activation on the properties of wood slabs without binders

Wood slabs obtained without binders from hydrodynamically activated crushed wood (sawdust) have high water resistance, but at the same time relatively low strength indicators. The structure formation of such plates is because fibrillation of wood particles occurs during activation. This increases the interfacial surfaces capable of forming intermolecular contacts between neighboring particles. Based on this, the strength of such plates can be increased because in the process of hydrodynamic processing, the degree of fibrillation of wood particles can be increased. It is proposed to carry out thermal modification of sawdust before their hydrodynamic treatment for this purpose. This method of preparation was chosen because when exposed to high temperatures, the components of the cell wall, namely hemicellulose, decompose. This breaks the continuity of the cell walls, increasing their porosity. As a result, during the hydrodynamic treatment, deeper structural changes will occur in the cell walls, increasing the interfacial surfaces. Experimental studies have confirmed the hypothesis.

Sawdust was processed at a temperature of 200 °C for 1–5 hours. After that, hydrodynamic treatment was carried out. Plates with a density of 950 kg/m³ were made from the resulting mass. 

The study of the porous structure of wood pulp and slabs, which was evaluated by the adsorption activity of iodine and methylene blue, showed the following. The number of pores, up to 2 nm in size, increases several times in proportion to the duration of heat treatment. In the finished slab, the number of such pores decreases, approaching thermally untreated wood, the number of pores, measuring 2–50 nm, also increases in proportion to the duration, and in finished slabs it decreases sharply.

The mechanical properties of plates made of heat-treated wood are significantly increased in proportion to the duration of processing. With a heat treatment duration of 5 hours, the static bending strength was 38.18 MPa, which is 72 % more than that of plates made of sawdust without heat treatment.

Thermal exposure lasting more than 3 hours reduces the amount of water absorption and swelling. When the swollen samples are dried, they return to their original dimensions and retain at least 80 % of the original strength. 

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