Development of Urea-Glyoxal Resin Wood Adhesives with Multiple Crosslinking Network Structures Using Epoxy Resin

Abstract

To eliminate the potential hazards to human health and the environment caused by formaldehyde released from formaldehyde-based adhesives at the source, the use of low-toxicity and biodegradable glyoxal instead of formaldehyde is a simple and promising strategy. Urea-glyoxal (UG) resins are the most spartan family of glyoxal-based wood adhesives. However, their widespread application is limited due to inferior adhesive properties and inadequate water resistance. The bonding properties of UG were enhanced in this study by preparing EPn-UG wood adhesive, which combines UG resin with epoxy resin (EP). The structure of EPn-UG polymer was characterized and tested using FTIR, NMR, XPS, DMA, and SEM. The results demonstrated the successful incorporation of the epoxy backbone into the low molecular chain of UG resin through ring-opening reaction between the epoxy group and amino group on the UG resin. At the same time, the EPn-UG adhesive exhibited a lower curing temperature and higher energy storage modulus. When the addition of EP reached 5%, the dry and wet strength (immersed in water at room temperature for 24 h) of EP5-UG bonded plywood were measured as 1.65 and 1.02 MPa, respectively, exhibiting a significant improvement of 34% and 149% compared to UG resin. The bonding strength achieved a breakthrough from 0 to 0.61 MPa after being soaked in water at 63 °C for 3 h. The findings suggest that the introduction of EP enhances the formation of a denser crosslinked network, thereby effectively enhancing the bonding strength and water resistance of UG adhesives. The present study provides a novel approach for the investigation of formaldehyde-free adhesives, and further facilitating the industrial implementation of UG resin.

Keywords: Urea-glyoxal resin, epoxy resin, Structure and property, Water resistance