Melamine–formaldehyde curing acceleration by TiO₂-based silver-white pigments catalysis

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 ¹³C 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.