Effects of Molar Ratio and pH on the Condensed Structures of Melamine-Formaldehyde Polymers.

The base-catalyzed melamine-formaldehyde (MF) reactions were studied in both diluted and concentrated solutions. The influences of F/M molar ratio and pH on the polymer structures were investigated based on the quantitative 13C-NMR analysis. The results show that both F/M molar ratio and pH influence the competitive formation of ether and methylene bridges. For the cases of F/M = 2.0, and 3.0, methylene bridge formation is minor in contrast to ether bridges either at pH = 9.3⁻9.8 or at 7.3⁻7.8. When the molar ratio was lowered to 1.0, methylene bridges became competitive with ether bridges at pH = 9.3⁻9.8, but the latter is still more favorable. When the lower molar ratio overlaps with the lower pH, significant changes were found. The content of methlylene bridges was over three times that of ether bridges with M/F = 1.0 and at pH = 7.3⁻7.8. The results in this study were compared with those previously obtained for base-catalyzed urea-formaldehyde reactions. It was found that molar ratio and pH influence the structures of the MF and UF polymers in similar ways. The different synthesis conditions of UF and MF resin were also addressed by comparing the structures of UF polymers with MF polymers.