Development of urea-glyoxal resin wood adhesives with multiple crosslinking network structures using epoxy resin

To mitigate the potential risks posed to human health and the environment by formaldehyde emissions originating from formaldehyde-based adhesives, adopting low-toxicity and biodegradable glyoxal as an alternative to formaldehyde in the synthesis of amino resin adhesives represents a direct, convenient, and promising strategy. Urea-glyoxal (UG) resins represent the most minimalist category within the family of glyoxal-based wood adhesives. However, their widespread application is limited due to inferior adhesive properties and inadequate water resistance. In this study, we enhanced the bonding properties of urea-formaldehyde (UG) by formulating EPn-UG wood adhesive, which integrates UG resin with epoxy resin (EP). Structural characterization revealed successful incorporation of the epoxy backbone into the low molecular chain of the UG resin via ring-opening reactions between the epoxy groups and amino groups on the UG resin. Additionally, the EPn-UG adhesive exhibits a lower curing temperature and higher storage modulus. At an EP addition level of 5%, the dry and wet strength of EP5-UG bonded plywood, measured after immersion in water at room temperature for 24 hours, were recorded as 1.65 and 1.02 MPa, respectively. This demonstrates a significant improvement of 34% and 149% compared to UG resin. Notably, the bonding strength exhibited a remarkable breakthrough from 0 to 0.61 MPa after immersion in water at 63°C for 3 hours. These findings suggest that the introduction of EP enhances the formation of a denser crosslinked network, effectively boosting both the bonding strength and water resistance of UG adhesives. This study presents a novel approach for investigating formaldehyde-free adhesives, further facilitating the industrial implementation of UG resin.