Increasing dimensional stability, stability and water resistance of wood products modified with urea solution

During the thermochemical modification of dry wood with impregnating compositions at elevated temperatures, the processes of impregnation and movement of impregnating substances under the action of capillary pressure, after absorption of the composition deep into the wood, and also under the influence of excess pressure in the autoclave predominate. The purpose of the work is to increase dimensional stability, water resistance and improve the performance properties of wood products modified with a urea solution. With a large pressure gradient, which is observed during modification, it is on the membranes of intercellular pores that gas bubbles form, which increase in size, and then burst under pressure and are filled with solution. To stabilize the shape and size of such wood, a urea solution was used and impregnated in a manually operated autoclave at elevated temperature and pressure. Wood modification with urea is associated with polycondensation processes of urea, lignin and readily available hemicellulose fractions. Urea enters into association with the hydroxyl groups of wood components to form hydrogen bonds. To obtain maximum density, and therefore, in the future, maximum strength, the moisture content in the wood must be greater than that required for reactions in which water is involved. Impregnation of wood with a solution of urea of varying concentrations (based on the mass of dry wood) followed by drying occurs when urea combines with the components of the wood, thereby compacting it, giving the workpieces increased dimensional stability and water resistance. Modification with urea occurs mainly due to the penetration of the solution through the pores of cellulose capillaries in the cell walls due to the pressure difference. For theory and practice the mechanism of moisture transfer in wood during its drying is important. This mechanism is different regarding free and bound water; at the beginning of the drying process, surface evaporation will cause a decrease in the moisture content of the outer layers. When the free moisture from the surface has been partially removed, a capillary pressure difference appears between the inner layers and the surface, ensuring the suction of free moisture to the surface as it evaporates. The drying rate in this period is constant and is determined by the intensity of moisture evaporation from the surface. When physically modeling the process of impregnation of coniferous wood (pine, spruce), the liquid spreads along the tracheids. When modeling the process of impregnation of hardwood (aspen), the liquid spreads mainly through large vessels in the early zone, flowing from vessel to vessel through pores and ladder perforations.

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