Лако-красочные материалы — производство

In the pH range 3 to 7 the higher rate of curing of phenolic resins prepared by metallic ion catalysis is due to preferential ortho methylolation [17] and therefore also to the high proportion of ortho-ortho links of the uncured phenolic resins prepared by metallic ion catalysis. The faster curing rates of phenolic resins prepared by metallic ion catalysis is then due to the higher proportion of the free higher-reactive para positions available for further reaction during curing of the resin. The mechanism of the reaction [8] involves the formation of chelate rings between metal, formaldehyde, and phenols or phenol nuclei in a resin.

The rate of metal exchange is solution [8,18] and the instability of the complex formed determine the accelerating or inhibiting effect of the metal in the reaction of phenol with formaldehyde. The more stable complex II is, the slower the reaction pro­ceeds, to the formation of resin III. A completely stable complex II should stop the reaction from proceeding to resin III. If complex II is not stable, the reaction will proceed to form PF resins of type III. The rate of reaction is directly proportional to the instability or the rate of metal exchange in solution of complex II. The acid catalysis due to the metal ion differs only in degree from that of the hydrogen ion [19].

The effect of the metal is stronger than that of hydrogen ions, because of higher charge and greater covalence, since its interaction with donor groups is often much greater [19]. This allows phenolic resin adhesives to set in milder acid conditions. Most covalent metals ions accelerate the PF reaction. The extent of acceleration depends on the type of metal ion and the amount of it that is present. The capability of acceleration in order of decreasing acceleration effectiveness has been reported to be [11] Pbn, Znn, Cdn, Nin > Mnn, Mgn, Cun, Con, Co™ > Mn™, FenI» Ben, AlnI > CrnI, Con. The most

important conclusion to be drawn is that the accelerating effect is indeed present in both the manufacture of PF resin and its curing. Therefore, the fast rate of curing of high-ortho phenolic resins can be ascribed only partially to the high proportion of para positions available. The other reason for the fast rate of curing is that the metallic ion catalyst is still present, and free to act, in the resin at the time of curing. In such a resin, a considerable number of ortho positions (especially of methylol groups in ortho positions to the phenolic hydroxyls) are still available for reaction and capable of complexing.