The selected modeling aspects of isocyanate prepolymer preparation reactions

An earlier kinetic diisocyanate-diol reaction model, accounting for the different chemical reactivities of the 2- and the 4-NCO groups in toluylene 2,4-diisocyanate (TDI), was used to model the nonstoichiometric polyaddition by which isocyanate prepolymers are prepared. The rate of the reaction and the chemical composition and molecular weight of the products forming in the reaction of 2,4-TDI with 1,4-butanediol carried out in the solution, were studied in relation to the initial-NCO to -OH group ratio. The kinetic parameters used in the model were the ones verified experimentally, viz., the constant k(ED) characteristic of the 2,4-TDI's 4-NCO group (reacting with the hydroxyl group) and the constant k(1) characteristic of the 2,4-TDI's 2-NCO group reactivity after the 4-NCO has reacted to yield a corresponding carbamate. The monomer ratio was found to affect considerably, but not monotonically, the molecular weight of the resulting prepolymers. In process simulations, the reaction rate attained maximum when the initial -NCO to -OH ratio was 1.8. When the isocyanate monomer was used in still greater excess, the species prevailing in the system were trimers of 2,4-TDI and 1,4-butanediol (Fig. 1) plus the unconverted diisocyanate that could not become urethanized because the hydroxyl components had run out.