Studies on Unstable Structures in Urea-formaldehyde Resins Prepared in Strong Acid Medium

The variations of unstable structures in the synthesized urea-formaldehyde (UF) resin systems with the change of the molar ratios of formaldehyde to urea were discussed by ~ 13 C NMR spectroscopy. The results show that the relative contents of dimethylene ether groups, methylols and unreactive formaldehyde increased gradually with the increase of the molar ratios of formaldehyde to urea from 0.5 to 3.0, which implied that the structure of UF resins can be optimized by controlling the molar ratios of formaldehyde to urea in the process of synthesizing UF resin. It is optimal for the molar ratios of formaldehyde to urea in the range of 1.5 to 2.0, as evidenced by small amounts of unstable structures and significant amounts of stable structures in UF resins prepared.