Colourless formaldehyde-free urea resin adhesives for wood panels

Amino resin precursors prepared by the addition of a new, colourless, non-volatile and non-toxic aldehyde, dimethoxyethanal (DME), to urea gave resins for boards that while able to harden were underperforming due to the lower reactivity of DME in relation to formaldehyde. Urea reacts with one and even two molecules of DME to form UDME and U(DME)₂ (called DU) but the subsequent cross-linking reaction to form bridges between two ureas, although existing as observed by CP MAS ¹³C NMR, was too slow at temperatures lower than 140 °C to be of significance for wood panel adhesives. However, addition of 20% isocyanate (pMDI) contributed to cross-linking of DU by its reaction with pMDI to also form urethane bridges, their existence being confirmed by CP-MAS ¹³C NMR. The adhesive resins so formed had excellent performance, were colourless, and produced boards that satisfied well the requirements of the relevant norms for interior panels (EN 120 and EN 312). The results were good enough to decrease the proportion of pMDI to 14% at pressing times starting to be of significance for industrial panel products. Formaldehyde emission, by perforator method was down exclusively to the formaldehyde produced by heating the wood chips. The panel emission was sufficiently low to even satisfy the relevant F^**** JIS A 5908 Japanese standard (JIS A 5908, 1994 ). These adhesives are colourless, as UF resins.