ABSTRACT
Addition of diisocyanate to a difunctional hydroxyl compound (2) makes the basic reaction which yields linear polyurethanes. Other reactions, specifi c for isocyanates, also bring signifi cant contributions to the polyurethane manufacturing processes, especially to those processes which utilise linear polyurethanes at their initial processes and produce foamed materials, elastomers and coatings. Whether the consequences of those reactions become apparent or not, it is decided by both the structure of the monomers used, and by the conditions adopted for the polyaddition process: temperature, time, type and amount of catalyst (if any), presence of inhibitors which are employed to stabilise isocyanate prepolymers, auxiliary agents added (if any), initial molar ratio of monomers, and method of feeding (dosing) them. It is important to promote the reactions desired for the given process and to minimise at the same time the side reactions. In the foamed polyurethane manufacturing process, for example, the urethane chain growth reaction is important in which urea groups extend the chains to produce the matrix for the foam. Equally important is the reaction of free isocyanate groups which foams the polyurethane material with in situ evolved CO2 (equations 62):
In order to avoid that reaction, the polymerisation process should be carried out under inert gas blanketing, with the use of nitrogen or argon in most cases.
