Melamine-formaldehyde curing acceleration by TiO_2-based silver-white pigments catalysis

摘要

The preparation of melamine-formaldehyde (MF) resins for paper impregnation for wood panels laminating was found to be catalyzed by TiO2-mica silver white pigments. The presence of small proportions of those greatly enhances the resin polycondensation during its preparation under the same conditions but without pigments. These pigments are composed mainly of TiO2-mica, as ascertained by X-ray diffraction (XRD), with the TiO2 being mainly in its rutile phase but also presenting some traces of the anatase phase. FTIR and C-13 NMR inferred clearly the presence of a catalytic effect of the TiO2-mica and appeared to indicate that the catalytic effect involved mainly the -NH2 groups of melamine and on formaldehyde. According to the peak reaction temperature measured by DSC, the highest catalytic activity of TiO2 on MF curing appears to be achieved at a 2% addition level by weight. Different possible explanations of this effect have been proposed, the most likely one being the catalytic effect caused by TiO2-promoted coordination complexes, the effect of acceleration being due to the Ti charge being stronger than that of H+, and the complex formed being unstable, thus not inhibiting the progress of the reaction.