Effects of the production parameters on formaldehyde emission and technical properties of plywood

At the laboratory, 43 different types of three-layer plywood were produced from beech, poplar and okoume. Formaldehyde emission was determined by WKI-bottle-method, specific gravity by DIN 52374 (1964), shear-tensile strength by TS 47 (1981) and DIN 53255, bending strength and modulus of elasticity by TS 3969 and DIN 52371 (1968). Amount of formaldehyde emission from beech plywood was found to be higher than those of poplar and okoume. Formaldehyde emission was decreased by the increase of veneer thickness» prportion of filler, extender, urea and melamine, press time and pressure, storing time of plywood and use of urea-formaldehyde resin with low formaldehyde proportion, but the type of fillers did not show any influence on the emission. On the other hand, specific gravity, shear-tensile and bending strengths and modulus of elasticity of plywood made of beech veneers were higher than those of the other wood species. Pressure and press time increased the mechanical properties, whilst urea addition was found to decrease the shear-tensile strength and the specific gravity and to increase the bending strength and the modulus of elasticity. By the increase of extender proportion, glue-bond strength of dry-tested poplar plywood increased, specific gravity decreased and bending strength and modulus of elasticity was seen not to be changed. Type and proportion of fillers did not have any influence on the specific gravity. The increase in the proportion of filler decreased the shear-tensile strength and the bending strength and the modulus of elasticity. Use of pine bark as filler did not show any influence on the shear- tensile strength. VIII