Лако-красочные материалы — производство

With regard to melamine-urea-formaldehyde, copolymers can be prepared which are generally used to cheapen the cost of MF resins, but which also show some worsening of properties. Copolymerization was proven by means of model compounds and poly­condensates [9]. MUF resins obtained by copolymerization during the resin preparation stage are superior in performance to MUF resins prepared by mixing preformed UF and MF resins, especially because processing of such mixtures is quite difficult [10]. The rela­tive mass proportions of melamine to urea used in these MUF resins is generally in the melamine:urea range 50:50 to 30:70 [11]. Melamine-phenol-formaldehyde resins, which in some respects show better properties than those of their corresponding MF and PF resins, have also been prepared [12-14]. Analysis of the molecular structure of those resins in both their uncured and cured states appeared to show that no co-condensates of phenol and melamine form and that two separate resins coexist. This is due to the difference in reactivity of the phenolic and melamine methylol groups as a function of pH. Also, in their cured state an interpenetrating network of the separate PF and MF resins, as a polymer blend, is formed, not a copolymer of the two [15-18]. Today MUF resins are produced in greater amounts than MF resins in the field of adhesives due to the relatively high cost of melamine: their formulation has progressed to such a level that often no difference in performance exists between a good MUF resin and a pure MF resin. MF resins are still more extensively used at this stage in the paper impregnation/laminates fields although both MUF copolymers as well as separate, double application of UF (paper core) and MF (paper surfaces) resins are making considerable inroads in this area. MUF resins instead totally dominate today in the wood adhesives field. Paper laminates and wood adhesives are the two main application areas of these resins.

A type of resin also used today is the so-called PMUF (or MUPF according to which author is writing) adhesives. These are fundamentally MUF resins in which a minor proportion of phenol (between 3 and 10%; phenol:melamine:urea by weight of 10:30:60 for example) has been assumed to have coreacted with to further upgrade weather resis­tance of the bonded joint. Unfortunately the alleged superior performance of such resins is

often only wishful thinking as the phenol has frequently not been properly reacted with the other materials, and consequently the PMUF resin will have a worse performance than a comparable top of the range MUF resin. This was confirmed by the demonstration that it depends exclusively on the resin manufacturing parameters and materials reaction order used whether or not the phenol coreacts within many PMUF adhesives, showing that often the phenol remains as a useless pendant group in the hardened aminoplastic (MUF) network without contributing at all to its performance [19,20] (Fig. 2).

The best reaction order necessary to obtain PMUF resins in which phenol makes a positive contribution to the performance of the hardened network has been reported [19]. PMUF resins are still used and some good resins of this type are indeed used in the unrealistic hope that they outperform equivalent MUF resins, when it has been shown clearly that they perform at best as a MUF adhesive presenting the same number of moles of melamine for the total moles of phenol plus melamine of the PMUF itself. The idea that the addition of small percentages of phenol to a MUF resin yields resins of better exterior durability is then an incorrect myth perpetuated in the wood panels industry. Newer formulations of MUF resins always outperform the corresponding PMUF. PMUFs are not bad resins, they are simply resins in which one of the materials, phenol, is often wasted for no purpose.