The influence of pH on the melamine-dimethylurea-formaldehyde co-condensations: A quantitative 13C-NMR study

Abstract

1,3-dimethylurea (DMU) was used tomimic urea and tomodelmelamine-urea-formaldehyde (MUF) co-condensation reactions. The products of 1,3-dimethylurea-formaldehyde (DMUF), melamine-formaldehyde (MF), and melamine-1,3-dimethylurea-formaldehyde (MDMUF) reactions under alkaline and weak acidic conditions were compared by performing quantitative carbon-13 nuclear magnetic resonance (13C-NMR) analysis. The effect of pH on the co-condensation reactions was clarified. With the presence of the methyl groups in DMU, the appearance or absence of the featured signal at 54-55 ppm can be used to identify the co-condensed methylene linkage -N(-CH3) -CH2-NH-. Under alkaline condition, MDMUF reactions produced primarily MF polymers and the featured signal at 54-55 ppm was absent. Even though the co-condensations concurrently occurred, undistinguishable and very minor condensed structures with ether linkage were formed. Differently, under weak acidic condition, the relative content of co-condensed methylene carbons accounts for over 40%, indicating the MDMUF co-condensation reactions were much more competitive than the self-condensations. The formation of reactive carbocation intermediate was proposed to rationalize the results.