Лако-красочные материалы — производство

As noted previously, strong or weak bases that are sometimes present in the polyols will catalyze the urethane reaction. The effect of catalysts on the isocyanate reaction is well documented. Indeed, the first reported examples occur in the literature well before urethanes became a commercially significant class of compounds. The first use of a catalyst with an isocyanate was reported by Leuckart in 1885 [23]. Other early reports

(а) (Ь)

Figure 9 Structure of (a) triethylenediamine and (b) triethylamine.

were from French and Wirtel (1926), who used triethylamine to catalyze the reaction of phenols with 1-naphthylisocyanate [24]. Baker and Holdsworth (1947) detailed the mechanism of the urethane reaction [25].

Commercial catalysts consists of two main classes: organometallics and tertiary amines. Both classes have features in common in that the catalytic activity can be described as a combination of electronic and steric effects. Electronic effects arise as the result of the molecule’s ability to donate or accept electrons. For example, in the tertiary amines, the stronger the Lewis base, generally the stronger the polyurethane catalyst. Empty electronic orbitals in transition metals allow reactants to coordinate to the metal center, activating bonds and placing the reactants in close proximity to one another.

Steric effects arise from structural interactions between substituents on the catalyst and the reactants that will influence their interaction. The importance of steric effects can be seen by comparing the activity for triethylenediamine to that of triethylamine. The structure of triethylenediamine (see Fig. 9) forces the nitrogens to direct their lone electron pairs outward in a less shielded position than is true of triethylamine. This results in a rate constant for triethylenediamine that is four times that of triethylamine at 23°C [1].

Organometallic complexes of Sn, Bi, Hg, Zn, Fe, and Co are all potent urethane catalysts, with Sn carboxylates being the most common. Hg catalysts have long induction periods that allow long open times. Hg catalysts also promote the isocyanate-hydroxyl reaction much more strongly than the isocyanate-water reaction. This allows their use in casting applications where pot life and bubble-free parts are critical. Bismuth catalysts are replacing mercury salts in numerous applications as the mercury complexes have come under environmental pressure.

Catalysts will not only accelerate reaction rates but may also change the order of reactivity. Table 3 illustrates this behavior. These data indicate that amines do not affect the relative reactivities of different isocyanates and show that Zn, Fe, and Co complexes actually raise the reactivity of aliphatic isocyanates above aromatic isocyanates.