Achieving maximum strength in oak wood (Quercus robur, L.) bonding: techniques for surface preparation and adhesive application

The growing use of wood in construction, driven by architectural trends favouring smaller cross sections, often compromises the material's physical and mechanical properties, especially in hardwoods like oak, which are prone to instability and moisture-related deformations. To counteract these issues, researchers have developed reinforced wood products that incorporate materials such as steel, aluminium, carbon fibre reinforced polymer (CFRP), glass fibre reinforced polymer (GFRP), and basalt fibres, which enhance strength and stability while reducing dimensional changes. Despite these advances, there is limited research on optimal bonding techniques, particularly surface preparation, which is crucial for effective gluing. To address this gap, this study aims to determine the most suitable mechanical surface preparation and adhesive to achieve satisfactory gluing of oak to oak. Understanding the optimal surface preparation and bonding techniques is a crucial first step before exploring the incorporation of nonwood implants in the next phase of research. Therefore, this study investigates the influence of three surface machining methods (planing, sanding, and face milling) on the performance of bonded Slavonian oak joints (Quercus robur, L.) in dry and artificially aged state (AA). The various machined surfaces were tested using five adhesives: polyvinyl acetate (PVAC), 1k polyurethane (PUR1), fibre-reinforced polyurethane (PUR2), 2k polyurethane (PUR3), and epoxy adhesive (ER). The surface properties of the wood and the bonding properties of the glued wooden joints were measured. The wetting angle was tested according to EN 828, the surface energy was calculated according to the Wu and OWRK methods, while the compressive shear strength test samples were prepared and tested according to the ISO 6238 standard in the dry and AA state. Visual designation of the main failure patterns and scanning electron microscopy (SEM) of adhesive line integrity and adhesive penetration were also used to evaluate the joint bonding properties. The sanded surface results in the best wettability and the highest surface energy, which may be attributed to changes in surface morphology and structure of chemical components on the wood surface. The strength of PVAc glued joints was affected only by different machining, ER and PUR1 were affected by different machining and/or by AA, while PUR2 and PUR3 were affected neither by different machining nor by AA. PUR types of adhesives have proven to be the most suitable for bonding moisture-resistant face-milled, planed, or sanded joints.