Лако-красочные материалы — производство

MUF resin can be used as a cold-setting wood laminating adhesive for glulam and fin­gerjointing by the use of adequate acid hardeners. In all semiexterior and protected exter­ior structural applications where a clear/invisible glue line is preferred for aesthetic reasons then a MUF adhesive is preferred to the classical PRF adhesives used for this purpose. It is then more a question of fashion cycles, but notwithstanding this MUF resins have taken a considerable hold today in Europe (contrary to North America where PRFs are by far preferred) and confidence in them for this application has been steadily growing.

PRF ‘‘honeymoon’’ fast-set, separate application adhesives for exterior-grade struc­tural glulam and fingerjointing have now been used industrially for about twenty years [1,8] in several relevant variations developed over the years. MUF resins are now taking the same ‘‘honeymoon’’ direction: the use of a melamine resin and a resorcinol separate component system [47] has been reported. However, for all the improvements made to the commercial MUF resins of this type in all their different variations, they were still based on some resorcinol or resorcinol-aided component. Thus, using as one component a MUF resin of high melamine content and resorcinol as a second component is just unusual in its use of a MUF rather than a PRF resin; a very acceptable resin concept but for the fact that it is coupled with a phenol such as resorcinol. The coupling of an acid-setting MUF adhesive and of resorcinol might well present no advantages or even some potentially serious disadvantages. It has been shown for example that thermosetting PMUF resins do not present a better performance than equivalent MUF resins and that often, depend­ing on their sequence of manufacture, present instead a much worse performance. There are very well-defined technical and chemical reasons for this [19,20] that boil down to the relevant differences in reactivity of the two materials, namely the phenol (here resorcinol) and melamine. The reactivity of melamine and even urea at the acid-setting pHs they need is much greater than that of any phenol, even resorcinol, as this pH range is that of the lowest reactivity of any phenol. Thus, even resorcinol runs the risk of being linked very little to the MUF matrix, especially in a fast-setting system such as a honeymoon, and at best it will remain as a bypassed pendant side group not able to fully achieve the function for which it has been added.

More recently an exclusively MUF-based honeymoon adhesive for glulam and fin- gerjoints has been developed and reported in which one component is a high performance MUF resin, while the second separate application component is based on just slightly acidified water thickened to the same viscosity of the first component by the addition of 1.5% carboxymethyl cellulose (CMC) [48,49] (Fig. 9). The system has also been tried successfully in industry for both fast production of fingerjointing (Fig. 10) and glulam and also for the fast production of ambient temperature pressed plywood [48,49]. MUF — based, honeymoon-type, fast-setting, separate application adhesive systems which do not need any resorcinol are then capable of performing as adhesives for structural exterior — grade joints and glulam and of satisfying all the requirements of the relevant adhesive specifications for such an application. The parameters that were shown to be determining are mainly the performance of the MUF resin, if and once an excellent resin formulation is

available both the ratio of melamine to urea and the molar ratio having a lesser effect, performance only starting to drop lower than the requirements of relevant standards when M:U weight ratios fall well below 20:80 and of the order of 10:90. Addition of resorcinol at these failing levels while improving slightly the performance did not solve the problem; resorcinol addition then does not allow specification requirements to be satisfied [48,49].

At the higher M:U ratios such as M:U = 47:53, but even at lower melamine contents, addition of resorcinol does not improve the results at all, its addition again revealing itself superfluous. The reasons for such a behavior are those already presented and explained above. The MUF honeymoons present all the other usual advantages associated with honeymoon adhesives, namely high curing rate, long pot life, tolerance to higher moisture content of the substrate, and tolerance to even quite severe imbalances in viscosity and proportions between the two components.