Synergistic Effects of Ammonia Addition and Melamine Copolymerization on the Structure and Properties of Urea–Formaldehyde Resin for Wood Bonding

ABSTRACT Urea–formaldehyde (UF) resin is the most widely used adhesive in the wood‐processing industry due to its low cost, excellent bonding performance, and nearly colorless glue line. However, UF resins and UF‐bonded products release formaldehyde, a harmful pollutant that poses risks to indoor air quality and human health. Substantially reducing these emissions without compromising bonding performance remains a major challenge. This study develops a low‐formaldehyde‐emission UF resin (formaldehyde‐to‐urea molar ratio = 0.98) through a combined approach of 4 wt.% ammonia modification and 4 wt.% melamine copolymerization, achieving a formaldehyde emission of only 0.23 mg/L. Structural analysis reveals that the addition of ammonia consumes formaldehyde, which reduces the hydroxymethyl group content, branching density, and curing rate, while increasing the residual urea content. As a result, plywood bonded with the ammonia‐modified UF resin exhibits lower formaldehyde emissions but compromises water resistance. Subsequent melamine copolymerization effectively enhances water resistance and further reduces the formaldehyde emission from 0.26 to 0.23 mg/L. This synergistic strategy provides an effective solution for significantly lowering formaldehyde emissions while maintaining acceptable bonding performance that meets the requirements of Chinese Standard GB/T 9846‐2015, demonstrating strong potential for industrial application.