Post-modification of ultra-low molar ratio urea-formaldehyde adhesive with different types and levels of organic acid hardener

Ultra-low (UL) molar ratio urea–formaldehyde (UF) adhesive is a class of UL formaldehyde-emitting adhesive systems for wood-based panels. However, the UL molar ratio exhibits poor reactivity, low cohesion strength, and inferior adhesion. This study aimed to enhance the performance of UL molar ratio UF adhesive via post-modification with the addition of different types and levels of organic acid hardener, such as acetic acid (AA), maleic acid (MA) and citric acid (CA) at 3 and 5% level based on the theoretical solids content of UF adhesive. The solids content, gelation time, viscosity, pH, and free-formaldehyde content decreased with the addition of a higher level of organic acids. This showed that the incorporation of organic acid enhanced the reactivity of the UL molar ratio UF adhesive via cross-linking of carboxylic acid groups and methylol groups or the free-formaldehyde as confirmed by the results of gelation time, pH, and free-formaldehyde content. Post-modification with 5% CA resulted in the lowest solids content, gelation time, viscosity, pH, and free-formaldehyde content. The adhesion strength values of plywood bonded with modified UF adhesive were greater than that of control UF adhesive. In addition, the FE of plywood bonded with modified UF adhesive was lower than that of control UF adhesive. This study showed that 5% CA could be added as a hardener of UF adhesive.