Some properties of plywood panels manufactured from combinations of thermally densified and non-densified veneers of different thicknesses in one structure

Abstract

In this study, the effects of veneer thickness, the combination of different veneer thicknesses as well as non-densified and densified veneers with the same plywood structure, and adhesive amount on the physical and mechanical properties of plywood were studied. The plywood panels were made from rotary-cut beech veneer using urea-formaldehyde (UF) adhesive. Veneer sheets with thickness of 1.25 and 2.5 mm were subjected to the thermal pre-compression. Five-layers plywood panels were made from non-densified, densified, and combination of non-densified and densified veneers of different thicknesses with the same plywood structure at different adhesive amount (180 and 110 g/m2). The surface quality of the veneers is improved due to the thermal compression. The mechanical properties of plywood made of thin veneer were better than in samples made of thicker veneers. It concerned both non-densified and densified veneers. The bending strength values MOR (‖) = 116.56–193.86 MPa and MOR (ꓕ) = 57.53–96.60 MPa of all panels exceed several times the permissible values of strength 32 MPa for MOR (‖) and 5 MPa for MOR (ꓕ) according to EN 13353. The bonding strength values 1.51–3.22 MPa of all panels, with the exception of the panel made from non-densified veneer (0.77 MPa) with less adhesive amount, exceeds the permissible minimum value 1.0 MPa according to EN 314–2. The using of densified veneers increases the mechanical performance of plywood, but worsen thickness swelling and water absorption of panels. It is inappropriate to use high pressure level for achieving superior panel performance in bending, bonding and dimensional stability.