A comprehensive review on adhesion interactions between formaldehyde-based adhesives and wood biopolymers on surface

Formaldehyde-based adhesives, such as urea - formaldehyde (UF) resins and phenol - formaldehyde (PF) resins, play a vital role in the wood-based composite industry because of their excellent bonding strength, fast curing, thermal stability, and cost-effectiveness. These adhesives bond with wood biopolymers (cellulose, hemicellulose, and lignin) through various types of adhesion mechanisms, including mechanical interlocking, secondary forces (notably hydrogen bonding), and, in rare cases, covalent bonding. The complex physicochemical interactions between formaldehyde-based adhesives and wood biopolymers are influenced by several factors, including the surface chemistry of wood, the heterogeneity of its cell walls, moisture content, and surface preparation. These variables significantly influence bond strength and long-term durability. The presence of reactive hydroxyl groups of cellulose and hemicellulose, as well as phenolic groups in lignin, are critical to the adhesion process, allowing for chemical and physical interaction with adhesives. This review provides an extensive study of the surface adhesion mechanisms of formaldehyde-based adhesives to model wood biopolymers. This review addresses current challenges and identifies potential strategies to enhance adhesive performance in wood-based composite applications by examining these interactions, the roles of chemical structures, and key factors influencing surface adhesion.