Two-Dimensional Nuclear Magnetic Resonance Analysis of Hydrogen-Bond Formation in Thermosetting Crystalline Urea–Formaldehyde Resins at a Low Molar Ratio

The unique phenomenon of crystalline domain formation in thermosetting crystalline urea–formaldehyde (UF) resins at low formaldehyde-to-urea (F/U) molar ratios is responsible for their poor adhesion as wood adhesives. The crystalline domains are apparently formed by hydrogen bonds between linear molecules in UF resins at low molar ratios. Clear evidence for the formation of linear molecules and subsequent hydrogen bonds in the UF resins using two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy is reported for the first time through our study. One-dimensional and 2D NMR spectroscopies were used to compare the short-range and long-range correlations between hydrogen, carbon, and nitrogen atoms in UF resin polymers prepared with high and low F/U molar ratios (1.6 and 1.0). Complete signal assignments for various structural units and interconnections among UF resin species are reported. The results revealed that the second addition of urea at the final step of UF resin synthesis split several linkages, such as branched methylene ether, oxymethylene (or acetal), branched hydroxymethyl urea, or uron, resulting in linear molecular structures, and showed the formation of hydrogen bonds in UF resins at the low molar ratio.