Development of High Strength, High Water Resistant, Low‐Formaldehyde Emission Urea‐Formaldehyde Adhesive and Its Effect on the Properties of Wood Composites

ABSTRACT Urea–formaldehyde (UF) resin adhesive is one of the most widely applied in wood composite materials. However, it suffers from issues related to low water resistance and elevated levels of formaldehyde emission. In this study, polyamide–epichlorohydrin resin (PAE) was used as a modifier and crosslinking agent to prepare modified UF resin adhesives with high strength and water resistance using low molar ratio UF resin. The interaction between PAE, UF, and wood surface and the effect of PAE on the chemical structure, thermal stability, bonding strength, water resistance, adhesive layer characteristics, and formaldehyde emission of UF adhesive were studied. Compared with the pure UF adhesive, after adding 5% PAE, the dry bonding strength reached 1.405 MPa, which was 20.70% higher than that of the pure UF adhesive. At the same time, the wet bonding strength reached 1.188 MPa, which was increased by 33.33%. In addition, the formaldehyde emission of the prepared plywood was less than 1.5 mg/L, which met the E 1 standard. This work presents a novel approach to address the challenge of balancing water resistance and formaldehyde emissions in UF resin adhesives, expanding the industrial utilization of UF resin wood adhesives.