Effect of pre-pressing of veneer on the glueability and properties of veneer-based products

Abstract

The study used a physical and environmentally friendly method to enhance the properties of both veneer and plywood. The effect of pre-pressing process factors (pressure, temperature and time) on the compression ratio, surface roughness, tensile strength perpendicular-to-grain of rotary cut veneer sheets of birch (Betula pubescens), and on the shear strength of plywood made from these compressed veneer sheets were studied in laboratory tests. The effect of pressing factors (such as pressure and glue spread) on the properties of plywood made from pre-pressed veneer was also studied. Three-layer plywood panels were manufactured from pre-pressed and non-pre-pressed veneers using phenol formaldehyde glue resins. Test results revealed that the greatest influence on properties of veneer is the pre-pressing temperature followed by the pre-pressing pressure and pre-pressing time. Meanwhile, the veneer’s properties such as surface roughness can be improved, and the properties of plywood were also improved when the birch veneers were pre-pressed. Linear relationship between the compression ratio and surface roughness was found. The tensile strength of veneer was not significantly influenced by the pre-pressing factors. The pre-pressing temperature has the largest effect on shear strength of plywood. The initial veneer densification allows lowering both glue spread from 150 to 90 g/m2 (a 40% reduction) and pressing pressure from 1.8 to 1.0 MPa (a 45% reduction). The shear strength obtained for the tested plywood meets EN 314-1 requirements. It is expected that the results of this preliminary study will lead to new gluing approaches, which will result in significant performance improvements of veneer-based products.