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

Abstract

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.