Upgrading of MUF particleboard adhesives and decrease of melamine content by buffer and additives

Iminoamino methylene bases intermediates obtained by the decomposition of hexamethylenetetramine (hexamine) stabilized by the presence of strong anions such as SO 4 - and HSO 4 -, or "hexamine sulphate", have been shown to markedly improve the water and weather resistance of hardened MUF resins used as wood adhesives and of the wet internal bond (IB) strength performance of wood boards bonded with them. The effect has been shown to be induced by very small amounts, between 1% and 5% by weight of this material on resin solids content. This strong effect also allows the use of MUF resin of much lower melamine content while still allowing good performance of the bonded joints. As the effect is present also at the smaller proportion of hexamine as hexamine sulphate, it was shown that it was closely linked to the strong buffering action that hexamine sulphate has on MUF resins in the correct pH range for their hardening and not due at all to any increase in molar ratio of the resin as a consequence of hexamine sulphate addition. The reasons for the hexamine sulphate behaviour linked to its buffering action appeared to be due mainly to the shifting of the polycondensation ↔ degradation equilibrium of the resin to the left as a consequence of the increased pH induced by the buffering action. The presence of the buffer induces an increase in pH of between 25% and 50%, yielding faster hardening as a consequence of the higher SO 4 - ions content, but a much lesser extent of the simultaneous degradation reaction as a consequence of the higher pH. The network formed is then tighter and less touched by degradation resulting in lower thickness swelling of the board and a much greater strength after water attack of the panel. Some alternate systems presenting the same buffer action within the wanted parameters ranges were also shown to have a comparable effect.