Adhesion theories in wood adhesive bonding: A critical review

Investigating the theories or mechanisms responsible for wood adhesive bonding has been an important aspect of wood science and technology research over the past century. Understanding the nature of adhesion in wood and wood-based composites is of importance because of the fact that wood is adhesively bonded in over 80 percent of its applications. For wood bonding, studying adhesion theories requires an understanding of wood material characteristics, surface science, polymer characteristics, and the interactions between polymers and surfaces. The state-of-the-art categorizes adhesion theories or mechanisms into seven models or areas. These are: mechanical interlocking; electronic or electrostatic theory; adsorption (thermodynamic) or wetting theory; diffusion theory; chemical (covalent) bonding theory; acid-base theory; and theory of weak boundary layers. The goal of this paper is to provide a concise, critical, state-of-the-art review on adhesion theories in wood adhesive bonding with an emphasis on factors influencing bond creation in wood-based material applications. Over 200 papers were reviewed and information is presented with recommendations for future studies on wood adhesion.