Structure, properties and reaction mechanism of formaldehyde-free melamine-urea-glyoxal co-polycondensation resin

To reduce the harmful effects of free formaldehyde released from wood-based panel on human health and the environment, in this study, melamine-urea-glyoxal (MUG) resin was synthesized by using low-toxicity and biodegradable glyoxal instead of formaldehyde. It provides a comprehensive discussion on the structures, reaction mechanisms and properties of MUG resins with different molar ratios were compared and analyzed. FT-IR, ¹³C NMR, and XPS confirmed the co-polycondensation reaction between M, U, and G. However, with the change of raw material ratios, the structures of the MUG resins were less affected, the properties of the MUG resins were obviously different. When the molar ratio is R = n(M): n(U): n(G) = 0.16:0.30:1.0, thermal analysis showed that MUG plywood had better thermal stability. Evaluation of the gluing properties revealed that the dry and wet strengths of MUG adhesive bonded plywood reached 1.21 MPa and 0.91 MPa, respectively. In contrast to reported glyoxal-related wood adhesives, the MUG adhesive displayed much higher bonding strength, vastly superior water resistance. This study provides a new green pathway for the development of new urea formaldehyde adhesives, which helps to enhance their potential application value in the wood industry.