Gluability of Thermally Modified Birch (Betula pendula Roth.), Aspen (Populus tremula), and Poplar (Populus x canadensis Moench) Veneers Using Commercial Adhesives

With the increasing demand for renewable materials worldwide,the use of wood has become increasingly appealing. However, wood has somedrawbacks, and modification is the approach that tries to reduce them.Thermal modification of wood is a sustainable method that enhances itsbiological durability and dimensional stability. However, it results indiminished mechanical properties, posing challenges for its bondingapplications. This study focuses on determining the glueing quality of 3-plyplywood produced from thermally modified aspen (Populus tremula L.), birch(Betula pendula Roth.), and poplar (Populus x canadensis Moench) veneers.Plywood samples were prepared using commercially available adhesives -phenol formaldehyde, melamine urea formaldehyde, hybrid polymer, andpolyurethane. The thermal modification was carried out under vacuum(TERMOVUOTO process) and in a water steam environment (WTT –hydrothermal process). Adhesion quality was evaluated in accordance withthe EN 314 standard. Results indicated that plywood from the TERMOVUOTOprocess-modified veneers yielded higher tensile-shear strength than plywoodfrom WTT-modified veneers. Polyurethane and phenol-formaldehydeadhesives proved to be the most suitable for bonding thermally modifiedveneers, whereas the hybrid polymer is unsuitable for this purpose. The studycontributes to optimising the use of thermally modified veneers in producingplywood for outdoor applications or 3rd Service Class, as per EN 1995-1-1:2004/AC:2006 - Eurocode 5.