A novel glyoxal-based resin with highly cross-linked networks: Reaction mechanism, characterizations and performance measurements

The formaldehyde-based adhesive is commonly used in wood industry; however, the emission of formaldehyde is a fatal defect, which endangers people`s health as well as pollutes the environment. Herein, the non-toxic and low-volatility glyoxal (G) is chosen to react with dimethylolurea (DMU) to prepare a novel and excellent water-resistant wood adhesive of glyoxal-dimethylolurea (G-DMU) resin. Quantum chemical calculations , structure characterizations, and performance measurements are carried out to study the reaction mechanism, and structure-performance relations of G-DMU resin with different DMU/G molar ratios. The results show that during the synthesis of G-DMU resin, the hydroxyl compounds N-p-G-DMU and N-p-G1-DMU, as well as the carbocation intermediates C-p-G-DMU and C-p-G1-DMU are formed by the addition reactions between DMU and protonated glyoxal (p-G and p-G1). The plywood bonded with G-DMU resin with the optimum DMU/G molar ratio of 1.2:1.0 has the best water resistance. Its wet shear strength is increased by 145% (cold water) and 193% (63 °C) over the DMU/G molar ratio of 0.8: 1.0. The analyses of structure characterizations reveal that the generation of C=N, C–O–C, N–C–N, and heterocyclic rings, simultaneously, the main cross-linked components concentratedly are characterized in 400–500 Da, 500–550 Da, 698 Da, and 712 Da regions. In addition, the high molecular weight oligomers at 658–712 Da, 874 Da, 1037 Da and 1213 Da promote the formation of a highly cross-linked network during the curing process. Meanwhile, these results are in good agreement with the results of quantum chemical calculations. Thus, this work establishes the theoretical foundation for the novel glyoxal-based resin of G-DMU resin.