Preparation of Polyisocyanates and Prepolymers

Isocyanates can be made in a number of ways, but the commercial method is by phosgenation of an amine. As an example, in the preparation of tolylene diisocyanate (TDI), phosgene is passed into a heated solution of tolylene-diamine. The solvent commonly used is ortho-dischlorobenzene, an excellent solvent for both the phosgene and amine. A mixture of 2,4- and 2,6-tolylene-diamine isomers is preferred because the tolylene diisocyanate prepared from this is less likely to crystalline than that from the pure isomer. After distillation tolylene diisocyanate is obtained as a mobile liquid having a viscosity of less than one poise.

Tolylene diisocyanate is the most important polyisocyanate and its low viscosity would be a great advantage in adhesive applications were it not for the fact that in this (“pure”) form the substance is toxic and has a relatively high vapor pressure, making it unpleasant to handle. Furthermore, it reacts inconveniently rapidly rapidly with many hydroxyl compounds. These defects can be overcome to some extent by using a prepolymer made by combining the “pure” isocyanate with a small amount of hydroxyl compound in order to reduce the vapor pressure and reactivity. Unfortunately, the prepolymer inevitably has a higher viscosity, making it necessary to add a solvent to facilitate application.
The prepolymer is made by reacting the diisocyanate with polyhydric alcohol, such that the ratio of isocyanate to hydroxyl group is about 2:1, thus leaving one half of the isocyanate group available for subsequent reaction. To ensure a smooth reaction, an alcohol in which both monomer and prepolymer are soluble such as trimethylolpropane, is used. The formation of such a prepolymer is shown in the following equation:

A triisocyanate of interest as an adhesive is triphenylmethane p,p,p-triisocyanate. This is produce by phosgenation of para-triminotriphenylmethane. In the pure state it is a solid, whereas the technical grade is a viscose liquid, frequently used as a solution in methylene chloride.