Plywood manufacturing using various combinations of hardwood species

This study evaluates the potential of various hardwood combinations in plywood production in response to increasing wood demand and a changing roundwood supply in Central Europe. Six different combinations of nine-layer plywood were produced using 2 mm rotary-cut veneers from lime (Tilia spp.), Norway maple (Acer platanoides), European hornbeam (Carpinus betulus), Sycamore maple (Acer pseudoplatanus), mountain ash (Sorbus aucuparia), and European beech (Fagus sylvatica) with phenol–formaldehyde adhesive, and they were compared to silver birch (Betula pendula) plywood as a reference. The raw densities of the test panels varied between 0.85 and 1.04 times the reference density (795 kg m-3). Flexural strengths (the modulus of rupture, MOR) ranged from 68 N mm-2 to 104 N mm-2 for a parallel fibre orientation and 44 N mm-2 to 61 N mm-2 for a perpendicular fibre orientation of the top layers. The modulus of elasticity (MOE) ranged from 7160 N mm-2 to 11,737 N mm-2 for the parallel fibre orientation and from 4366 N mm-2 to 5575 N mm-2 for the perpendicular orientation. The tensile shear strength varied between 0.91 and 1.69 times the reference (1.49 N mm-2). The thickness swelling after 24 h was higher in all variants than the reference (6.4%), with factors between 1.39 and 1.64. A significant effect was observed when layers with a lower density were arranged on the outside and those with a higher density in the core, resulting in a more uniform density distribution across the cross-section after hot pressing. This created a levelling effect on mechanical and physical properties, especially the modulus of rupture (MOR) and the modulus of elasticity (MOE). Overall, the evaluated hardwood combinations demonstrated comparable properties to the birch reference and industrially produced birch plywood.