Лако-красочные материалы — производство

Another recent approach which has shown considerable promise in markedly decreasing the percentage of PF adhesive solids on a board has been found almost by chance. It is based on the addition to the resin of certain additives capable of decreasing the percentage of any PF resin needed for bonding while still conserving the same adhesive and joint performance. These additives work best for melamine-urea-formaldehyde (MUF) adhe­sives, but give acceptable results for PF resins too. These additives are the acetals [41,42], methylal and ethylal being the two most suitable due to their cost to performance ratio, which do not release formaldehyde at pHs higher than 1 [43]. Methylal has according to results reported by the Enviromental Protection Agency (EPA) an LD50 value of 10,000 against that of 100 for formaldehyde, and is thus classed as nontoxic. The addition of these materials to the glue mix of a formaldehyde-based resin improves considerably its mechan­ical resistance and the performance of the bonded joint, mainly through its solvent action on the higher molecular weight colloidal fraction of the resin. Molecular, colloidal inter­actions causing diffusional hindrance problems are thus overcome and minimized by bringing the resin system to a homogeneous phase and as a consequence higher resin strengths are obtained. It has also been shown that the supposed existence of transaceti — lization reactions forming bridges in the resin hardened network which are better able to optimize the glue line and bonded joint viscoelastic dissipation of energy did not occur [44]. This is in general valid for MUFs, some UFs, and PFs, but the effect is particularly evident and particularly marked for the MUF resins [44]. However, still respectable (but more modest than MUFs) increases in PF resin strength of up to 25%, or alternatively decreases in PF resin solids content of as much as 20% while conserving the same performance, have been obtained in the case of wood particleboard [44].