The reaction of isocyanates and water to form urea is essential to many polyurethane industrial applications. The reaction with water creates the essential stability and heat of reaction to make the kinetics of foam formation practical . In addition, it creates a gas reaction product (CO2) which provides an expanding volume helping create the porous structure of foams (Fig. 3.12). The initial reaction of water and isocyanate forms an unstable carbamic acid intermediate that immediately decomposes to amine and CO2 .
The reaction of amine with isocyanate is diffusion controlled forming urea and a significant enthalpy of reaction (Fig. 3.13) (ca. 40 kcal/mol) . It is reported that the
Figure 3.12 The reaction of isocyanate and water forming an amine.
Figure 3.13 The reaction of amine with an isocyanate to form urea.
use of catalyst has no kinetic effect on the rate of amine and isocyanate reaction and that the reaction of each NCO with water liberates a similar amount of heat as in the urethane reaction. Disparities in the literature may depend on experimental factors affecting the self associations that promote or hinder the formation of the new urea bond.
The reactivity of water with isocyanate relative to that of hydroxyls is generally regarded as following a path similar to that of alcohol hydroxyl requiring the concerted interaction of two water molecules across the isocyanate N = C bond. Quantum mechanical calculations have suggested that a third water molecule would actually provide an enhanced geometry for the reaction across the isocyanate . Certainly, the insolubility of most isocyanates with water will have significant effect on the observed rate of reaction and can conceivably account for much disparity of the measured rates found in the literature .
The kinetics of urea formation from the water reaction is of course a convolution of the initial kinetics of isocyanate and water which is slow unless catalyzed, followed by the reaction of the subsequent amine with isocyanate to urea which is diffusion controlled . An interesting phenomenon is the apparent first-order kinetic dependence on isocyanate concentration and second-order dependence on the amine. This second-order dependence on amine is realized for both aniline and aliphatic amines. This high order dependence coupled with the relatively low activation energy (~17.5 kJ/mol) account for the very high initial reactivity of these systems .