Лако-красочные материалы — производство

Different mechanisms of alkaline catalysis have been suggested according to the alkali used. When caustic soda is used as the catalyst, the type of mechanism which seems the most likely is that which involves the formation of a chelate ring similar to that suggested by Caesar and Sachanen [7]. The chelating mechanism was thought to initially cause the formation of a sodium-formaldehyde complex or of a formaldehyde-sodium phenate
complex and is similar in concept to the mechanisms advanced for metal ion catalysis of phenolic resins in the pH range 3 to 7. However, while the cyclic metallic ion catalysis ring complexes have even been isolated [8], this is not the case for the sodium ring complex, evidence for its existence being rather controversial, the predominant indication being that it does not form [9].

When ammonia is used as a catalyst, the resins formed are very different in some of their characteristics from other alkali-catalyzed resins: the reaction mechanism appears to be quite different from the of sodium hydroxide-catalyzed resins. An obvious deduction is that intermediates containing nitrogen are formed. Several such intermedi­ates have been isolated from ammonia-catalyzed PF reactions [10-12] and hexamine prepared resins [13-16] by various researchers. Similar types of intermediates are formed when amines or hexamethylenetetramine (hexamine) are used instead of ammo­nia. In the case of ammonia the main intermediates are dihydroxybenzylamines and trihydroxybenzylamines, such benzylamine bridges having been shown to be much more temperature stable than previously thought and to impart particular characteristics to the resin [13-16].

These intermediates contain nitrogen and have polybenzylamine chains. They react further with more phenol causing splitting and elimination of the nitrogen as ammonia or producing eventually nitrogen-free resins. However, as benzylamine bridges have been shown to be much more temperature stable than previously thought, this requires a con­siderable excess of phenol and a high temperature, or heating for a rather long time. With phenol-hexamethylenetetramine resins of molar ratio 3:1, the nitrogen content of the resin cannot be reduced to less than 7% when heated at 210° C. When the ratio is increased to 7:1, the nitrogen content on heating at 210°C can be reduced to less than 1%. Contrary to what was widely believed it has been clearly demonstrated that in the preparation of PF resins starting from hexamethylenetetramine the di — and trihydroxybenzylamine bridges which are initially formed are very stable and are able to tolerate for a considerable length of time a temperature as high as 100°C [13] yielding in certain aspects (only) resins of upgraded characteristics. This behavior is closely tied to the reactions characteristic of hexamethylenetetramine to form iminomethylene bases [14-16], which are discussed in the melamine resins chapter in this volume (Chap. 32).

Ammonia-, ammine-, and amide-catalyzed phenolic resins are characterized by greater insolubility in water than that of sodium hydroxide-catalyzed phenolic resins. The more ammonia that is used, the higher the molecular weight and melting point that are obtained without cross-linking. This is probably due to the inhibiting effect of the nitrogen-carrying groups (i. e., — CH2-NH-CH3 or — CH2-NH2), which is caused by their slow rate of subsequent condensation and loss of ammonia. Ammonia, amines, and amides are sometimes used as accelerators during the curing of phenolic adhesives for wood products.