Performance and reaction mechanism of zero formaldehyde-emission urea-glyoxal (UG) resin

In order to eliminate the harm of formaldehyde (F) from panels bonded with UF resin to environment and human health at the source, the low volatile and nontoxic aldehyde of glyoxyal (G) was chosen to react with urea (U) to prepare the wood adhesive of urea-glyoxal (UG) resin to substitute for urea-formaldehyde (UF) resin. The urea-glyoxal (UG) resin was synthesized under weak acid conditions (pH 4-5). The bonding strength of the bonded plywoods was tested, and the curing process of UG resin was studied by dynamic mechanical analysis (DMA). Some initial acid-catalytic reactions involved in the synthesis of the UG resin were theoretically investigated at BLYP/DND/COSMO of quantum chemistry using density function theory (DFT). Furthermore, the UG resin was characterized by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS). The results show that the bonded plywood with dry shear strength of 0.98 MPa could be directly used as interior decoration and furniture material. The addition reaction of either G or C1 with U undergoes through a concerted mechanism represented by a four-member ring transition state with a notable barrier (above 130 kJ/mol). The reactions of U with different protonated forms of p-G and p-G1 have two main pathways to form C-p-UG and C-p-UG1 with lower energy barriers of 30-40 kJ/mol. The main assignments of MALDI-TOF-MS of UG resin are corresponding to the intermediate products of C-p-UG1, C-p-UG, UG1, UG or between each other. (C) 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.