Лако-красочные материалы — производство

Low condensation PF resins have been coreacted under alkaline conditions with up to 42% molar urea on phenol during resin preparation to yield PUF resins capable of faster hardening times and presenting better performance than equivalent pure PF resins prepared under identical conditions [31-34]. The reason that urea reacts with relative ease with PF resols under alkaline reaction conditions can be ascribed to the relative reactivities toward methylol groups of urea and phenolic nuclei. A study has shown that there are definite pH ranges in which the reaction of urea unreacted — NH2 and — NH — groups with formaldehyde in competition with phenol or with the methylol groups carried by a PF resin, is more favorable than is autocondensation of the PF resin itself [31,33,45] (Fig. 5).

Figure 5 Log of reaction rate constant versus 1 /T (T in kelvins) for the reaction of ortho and para hydroxybenzyl alcohol with urea, detailing the relative rates of condensation with urea of ortho and para phenolic methylols in a PF resin.

It is the urea which contributes to the acceleration of resin cure by allowing the following.

(i) The preparation of greater molecular mass linear oligomers needing fewer steps to cure, hence curing faster. Copolymerization up to a certain level of urea addition brings an easier and much faster reaction to the mainly linear higher molecular weight oligomers; in a sense this is just a way to molecularly double very quickly the molecular weight of a PF resin while still maintaining the linearity of the higher molecular weight oligomers formed. The extent of copoly­merization, however, needs to be limited and cannot be brought to an excess otherwise the viscosity of the resin becomes unmanageable, although the pres­ence of free urea is able to control this trend.

(ii) The faster reaction of urea than phenol with the phenolic methylol group under alkaline conditions as well as the possibility of reaction with phenol of the methylol groups formed by the additional attack of HCHO on urea; the latter reaction drives the reaction of HCHO on urea, an equilibrium, in favor of methylol ureas and subsequent products [31,32].

The water resistance of these PUF resins is comparable to that of pure PF resins when used as adhesives for wood particleboard. Part of the urea (between 18 and 24% molar on phenol at a phenol:HCHO molar ratio of 1:1.7, but higher at higher HCHO proportions) was found by 13C NMR to be copolymerized to yield the alkaline PUF resin while, especially at the higher levels of urea addition, unreacted urea is still present in the resin. Increasing the initial formaldehyde to phenol molar ratio decreases considerably the proportion of unreacted urea and increases the proportion of the PUF copolymer. A coreaction scheme of phenolic and aminoplastic methylol groups with reactive phenol and urea sites based on previous model compounds work has been proposed, copoly­merised urea functioning as a prebranching molecule in the formation of a hardened resin network [31].

The PUF resins prepared are capable of further noticeable curing acceleration by addition of ester accelerators, namely glycerol triacetate (triacetin), to reach gel times as fast as those of catalyzed aminoplastic resins, but at wet strength values characteristic of exterior PF resins [31,32]. Guanidine carbonate has also been shown to be an accelerator of PF resins which, while yielding slightly slower gel times than triacetin when just added to a PF resin glue-mix, is also capable of giving glue-mix pot-lives on the order of several days, hence long enough to be premixed with the resin well before use [32]. Both triacetin and guanidine carbonate used as simple glue-mix additives increase the ultimate strength of the resin bond whatever the length of the curing time used for the purpose, this being confirmed both by thermomechanical analysis as well as by application to wood particle­board. Synergy between the relative amounts of copolymerized urea and ester accelerator is very noticeable at the lower levels of the two parameters, but this effect decreases in intensity towards the higher percentages of urea and triacetin [31,32]. The relative perfor­mance of the different PUF resins prepared under different conditions, allowed the pre­paration of wood particleboard bonded with accelerated PUF resins with the ability to achieve press times as fast as those of aminoplastic (UF and others) resins [31,32].