Abstract
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.
Highlights
Reactions of melamine with formaldehyde under certain conditions produce melamine-formaldehyde polymers, namely MF resins, which are widely used as molding compounds, surface coating materials, adhesives, etc. [1]
Does this mean that base-catalyzed MF reactions are faster than UF reactions and mainly produce methylene bridges? To balance the performances and cost, scientists have been trying to synthesize co-condensed melamine-urea-formaldehyde resin (MUF) during the past decades, especially some studies reported the synthesis of MUF resins under alkaline conditions [10,11,12,13]
Classic theory for UF resin synthesis pointed out that pH is the key factor that determines what reactions can occur, our experiments and theoretical calculations have clarified that the F/U molar ratio plays an important role that influences the competitive formations of different condensed structures [15,19]
Summary
Reactions of melamine with formaldehyde under certain conditions (pH, molar ratio, and temperature) produce melamine-formaldehyde polymers, namely MF resins, which are widely used as molding compounds, surface coating materials, adhesives, etc. [1]. MF resin used as wood adhesive is generally synthesized under alkaline conditions (pH = 8.0–10.0). Does this mean that base-catalyzed MF reactions are faster than UF reactions and mainly produce methylene bridges? To balance the performances and cost, scientists have been trying to synthesize co-condensed melamine-urea-formaldehyde resin (MUF) during the past decades, especially some studies reported the synthesis of MUF resins under alkaline conditions [10,11,12,13]. When the F/U ratio was lowered to 1.0, the methylene bridges appeared to be competitive with ether bridges This indicated that the molar ratio is an important factor that influences the competitive formation of the two condensed structures. In this study, MF reactions under different pHs and molar ratios were re-examined based on quantitative 13 C-NMR analysis of the reaction products
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