Assessing the effect of a coating system on the rate of plywood moistening and changes in dimensions in relation to the change in relative air humidity

The paper summarizes results of an institutional research aimed at the evaluation of the effect of a coating system on the rate of plywood moistening and changes in dimensions in relation to changes in relative air humidity. Coefficients of dimensional changes were determined in relation to relative air humidity and effects were studied of the coating system on the rate of plywood moistening. All measurements were carried out in water-resistant combined plywood with surface coating and thickness of 8, 10, 12, 15 and 18 mm and in all-beech plywood 10, 12, 15 and 18 mm in thickness. Based on the results obtained it is possible to state that bending strength and modulus of elasticity are decisively affected by the construction of a sheet. Therefore, it is suitable to select the sheet structure according to requirements of a customer. It is known that surface layers transfer the greatest stress and that the stress of layers decreases towards the axis of symmetry. The fact can be used in designing the sheet structure, e.g. using veneers manufactured from cheaper species for central layers. Thus, the price and even density of plywood can be substantially reduced maintaining the required strength. By means of coats it is possible to reduce substantially penetration of moisture across the plywood edge. Comparisons between polyurethane coat U 2070 and acrylate coat AQUACOL V 2080 are demonstrated in a figure. Changes in the length and width of combined plywood and all-beech plywood amount to on average 0.020-0.025% per 1% change in moisture. Under upper values of the moisture range the changes are generally substantially lower. In practice, a suitable clearance for changes in the length and width of plywood amounts to 2 mm/m. Changes in thickness reach on average 0.4-0.5% per 1% changes in the moisture content.