Upgrading of MUF polycondensation resins by buffering additives. II. Hexamine sulfate mechanisms and alternate buffers

Iminoamino methylene bases intermediates are obtained by the decomposition of hexamethylenetetra- mine (hexamine). These are stabilized by the presence of strong anions such as SO24 and HSO 4 , that is, “hexamine sulfate.” The effect of hexamine sulfate was closely linked to the strong buffering action it has on MUF resins. Its role is mainly to induce regularity of the reaction and the stability of conditions during resin networking, due to the buffer. Shifting of the polycondensation a degradation equilibrium to the left appeared to be the determinant factor. This was a consequence of maintaining a higher, constant pH during curing, due to the buffer action. The modulus of elasticity (MOE) increases the curves of hexamine sulfate-catalyzed MUF resins, confirming this trend. The resins are faster curing than when catalyzed by ammonium sulfate. The effect is valid within the narrow buffering range of pH’s used for resin hardening. Polycondensation is far too slow to occur at a much higher pH, and degradation is, instead, more predominant at much lower pH’s. The network formed is then more crosslinked and less tainted by degra- dation when curing occurs within the correct pH range. The result is a much better performance of the wood board after water attack. The effects induced by hexamine sulfate effects are of longer duration than those of other potential buffers. This is due to the hexamine sulfate heat stability under standard hot curing conditions of the resin. Alternate systems were found and shown to have a comparable effect.