Bio-based, switchable adhesives for wood-wood and wood-metal composites

Adhesives that enable bonding and especially debonding on demand have attracted rapidly growing interest in the last decade because they can overcome the problems associated with conventional adhesives in separating, recycling, and repairing. Furthermore, in the wood industry the demand for adhesives based on natural biopolymers is high. The objective of this study was to develop a bio-based, switchable polyurethane adhesive for multilayer wood composites. The switchability of the adhesive is based on a reversible crosslinking mechanism. Through the integration of special thermo-reversible crosslinking units into the polymer structure, the cohesion can be switched OFF when temperature rises and switched ON by cooling down. The final properties of the adhesives, such as viscosity and flexibility, were adjusted through the resin composition and the raw materials. The research included the determination of thermal parameters as well as the identification of appropriate joining parameters. Furthermore, mechanical parameters such as modulus and strength were determined. The reversibility could be demonstrated for at least three cycles and the wood composites and the wood-metal composites could be separate by hand when the switching temperature is exceeded. The aim is to use this adhesive to enable an easier recycling of wood-metal composites and to produce reshapable wood-wood composites.