Wednesday, July 29, 2009

Further Reaction of Polyisocyanate

A diisocyanate will react wit a dihydric alcohol to give a linear polyurethane. If either of the reactants is trifunctional a crosslinked polymer such as the polyurethane from a dihydric compound and triphenylmethane-p-triisocyanate is formed. In an adhesive, formation of the crosslinked polymer must of course be completed in the gluline. The adhesive therefore normally consist of two parts which are mixed together immediately before use, one part being the polyisocyanate (or prepolymer), the other a polyester or polyalcohol with terminal hydroxyl groups. In calculating the amount of polyhydric compound to be added stoichiometric proportions of isocyanate and hydroxyl groups would theoretically appear to be important, but in practice the ratio is not at all critical, although uncombined isocyanate is undesirable.

A considerable excess of polyester may be used to produce a more flexible adhesive, with some sacrifice of heat and chemical resistance. Adhesives based on mixtures of tolylene diisocyanate prepolymer and saturated polyester resins can be formulated to give a range of pot-lives, from one or two hours up to several hours. Since the polyurethane reaction is base catalyzed, a small amount of a tentiary amine such as N-methyl morpholine may be used to accelerate the curing reaction, especially at room temperature. Although the curing of a polyurethane adhesive is accelerated by heat, the effect of heat is not as marked as in the curing of the condensation polymers.

One interesting adhesive system (reffered to as a polyurethane) is that in which a glycidyl ether is used as solvent for the solid amine component. The alleged purpose of such an unusual solvent is as a possible "reactive modifier" presumably rather on the lines of the reactive diluent used with epoxy resins.