Reactions of isocyanates with water

The reaction between isocyanates and water leads to production of gaseous carbon dioxide and an urea group. This reaction is a very convenient source of a gas necessary to generate the cellular structure of polyurethane foams [1-26]  

The amine reacts very rapidly with other isocyanate molecules and generates a symmetrical disubstituted urea [1-3, 6-24]  

The reaction of isocyanate with water is more exothermic than the reaction with alcohols and the total heat release per mole of water is about 47 kcal/mol [1-3]. It is evident that one mole of water reacts with two -NCO groups, which is very important in order to calculate the correct quantity of isocyanate needed for polyurethane formulations. 

Water is considered, in polyurethane foam manufacture, as a chemical blowing agent, because the gas generation is a consequence of a chemical reaction. 

The reaction between isocyanates and alcohols or water is catalysed by tertiary amines with low steric hindrance, and some tin, lead or mercury compounds such as [1-3, 6-23]  

In general, isocyanates have a strong affinity to water, which makes them difficult to store. This reaction is not desirable in applications such as structural adhesives and sealants [15]. Isocyanate reacts with water giving carbamic acid, an unstable compound which spontaneously decomposes into a primary amine and carbon dioxide, together with a subsequent urea formation (Fig. 4). 

---

The fillers used in urethane formulations are similar to those used in sihcones calcium carbonate, talc, clays, and siUca are among the most common. On account of the undesirable reaction of isocyanates with water, fillers used in urethane formulations must be dry. 

Reaction of Isocyanates with Water and Carboxylic Acids... 

Scheme 4.13 Reaction of isocyanates with water and carboxylic acids.

The reactions of amines and isocyanates are important in adhesives because of the possible reaction of isocyanates with water. Because isocyanates react readily with water, raw materials used in formulating adhesives must be dry, and the compositions must be protected from moisture, including atmospheric humidity, during storage. The first reaction... 

FIGURE 3 15 Reaction of isocyanate with water to yield an amine and COr... 

The reaction of isocyanates with water is a complex one and may involve several possible mechanisms. Naegeli et al. [185] proposed the following possibilities... 

Volatilization of gases produced by the exothermic heat of reaction during polymerization such as occurs in the reaction of isocyanate with water to form carbon dioxide. 

This occurrence significantly affects the stoichiometry, not only from consumption of the isocyanate functionality in the process, but also by production of an amine product with two active hydrogens, which adds reactive functional groups to the glycol component of the polyurethane reaction. For these reasons care has to be taken to exclude moisture for the polyurethane applications described so far. However, for the production of flexible and rigid polyurethane foams the reaction of isocyanate with water is exploited to produce bubbles. 

The polyurethanes, although not used in large volumes, have specific applications of value for aerospace. Accordingly, a brief discussion of the chemistry and the material applications is in order. There are three chemical reactions of importance the reaction of hydroxyl groups with isocyanates to form urethanes (Reaction 1), the reaction of isocyanates with water to form amines (Reaction 2), and the reaction of amines with isocyanates to form ureas (Reaction 3). 

Catalysts. To make the final polyurethane, common catalysts are tertiary amines and organotin compounds, often used in mixtures. Amine catalysts favor the reaction of isocyanate with water, producing urea linkages and CO2, which acts as a blowing agent. On the other hand, organotin catalysts favor the isocyanate/hydroxyl reaction. Additional catalysts and catalyst decomposition products may be present if graft polyols are part of the mixture. The most common of these catalysts are free radical initiators. Phosphorus compounds may be present in the case of carbodiimide-modi-fied MDI. 

The reaction of isocyanate with alcohols is strongly exothermic (170-190 kJ/mol). One of the basic reactions in the urethane foam technology is the reaction of isocyanate with water with evolution of carbon dioxide and amine formation ... 

As the maximum amount of water is limited to around 4.8 parts per hundred of polyol (php) due to problems related to the dissipation of the heat generated by the strongly exothermic reaction of isocyanate with water, industrially-produced FPF density is limited to about 20 kg/m though the use of HFC coblowing agents may slightly reduce this density. 

The rest of the octadecyl isocyanate reacted with water, producing mainly dioc-tadecylurea, which was deposited on the fibers and contributed to the repellency durable only to mild laundering conditions. Because cellulosic fibers contain water, the side reactions of isocyanates with water cannot be avoided by solvent application [29,31] of the repellent. 

---