Лако-красочные материалы — производство

The potential introduction of an intermediate reaction step at very acid pH inducing the formation of some uron in the preparation of UF resins of lower formaldehyde emission has caused some industrial interest [40], and today industrial UF resins manufactured with the introduction of a rapid, very acid step (pH 1-2) during preparation are available. The only published research work which can be found in the worldwide literature on this subject deals with the introduction of just such a strongly acid condensation step in the preparation of UF resins [40-42]. This work came to the conclusion that introduction of such an acid step can lead to UF resins of improved bonding strength [40-42] and also of lower postcure formaldehyde emission [2,3]. One of the marked effects of the introduction at lower reaction temperatures of the additional strongly acid condensation step was the formation of considerable quantities of uron [40-42], thus of the well-known structure of a cyclic intramolecular urea methylene ether [8,43]. Urons have been found by 13C NMR to be present as methylolurons, methyleneurons, or methylene ether urons, hence as struc­tures of the type shown below [40], and even more interestingly some completely substi­tuted urons appear to exist in the reaction mixture, this being the only case in which the existence of some form of tetrasubstituted urea has been noted [40] (Formula 3)

The favored pH ranges for the formation of urons in UF resin preparation were determined as being at pHs higher than 6 and lower than 4 at which the equilibrium

urons $ N, N0-dimethylolureas is shifted in favor of the cyclic uron species [40] (Fig. 5). Shifting slowly the pH during the preparation from one favorable range to the other causes a shift in the equilibrium and formation of a majority of methylolurea species, while a rapid change in pH does not cause this to any great extent [40]. Urea-formalde­hyde resins in which uron constituted as much as 60% of the resin were prepared and the procedure to maximize the proportion of uron present at the end of the reaction described [40]. Uron has been found to be present in these resins linked by methylene bridges to urea and other urons and also as methylolurons, the reactivity of the methylol group of this latter having been shown to be much lower than that of the same group in methylolureas. Thermomechanical analyses and tests on wood particleboard prepared with uron resins to which relatively small proportions of urea were added at the end of the reaction showed that these were capable of gelling and yielding bonds of considerable strength [40]. Equally, mixing a uron-rich resin with a low formaldehyde/urea molar ratio UF resin yielded resins of greater strength than a simple UF resin of corresponding molar ratio indicating that UF resins of lower formaldehyde emission with still acceptable strength could be prepared in this way [40]. As the ‘‘acid-step’’ industrial resins are not prepared under conditions as extreme as the research on the potential for uron introduction has shown to be possible, it is clear that in this direction there is some room for further improvement for UF adhesives.

The reopened structures reform the intramolecular uron methylene ether cycle as the pH reaches the acid range in which the cyclic structure is again stable. Thus, the uron structure is in equilibrium with the open dimethylolurea form and the pH range determines the direction towards which such an equilibrium is more or less shifted (Formula 4)

The rate of opening or closure of the cycle is not very rapid as, at the end of the reaction, when the pH is rapidly adjusted from the very acid to the alkaline range the proportion of cyclic structures present does not alter much, while during slow descent of the pH during the reaction, passing through the middle pH range causes the disappearance of the greater proportion of the uron cyclic structures, which then form again once the acid pH range is reached.