STRUCTURE OF TRIMER-TYPE ISOCYANATE HARDENING AGENTS AND REACTIVITY OF THEIR ISOCYANATE GROUP

The structure of a trimer-type isocyanate hardening agent was examined using laser desorption ionization time-of-flight mass spectra (LDITOF -MS) analysis and molecular orbital calculations. The conventional tri mer-type agent of tolylene-2,4-diisocyanates (TDI) is confirmed to hav e higher multiples than three TDIs, and almost all these multiples con sist of odd-numbered TDIs. Odd-numbered multiples are formed by the se lf-polymerization of TDIs, as an isocyanurate ring structure, which is more stable than the uretdione structure, is formed. The lower reacti vity of the isocyanate groups of higher multiples is confirmed. This d ecreased reactivity of isocyanate groups as the number of multiples of TDIs increases is caused by the decrease in the atomic charge of carb on atoms in the isocyanate groups, which is attributable to the changi ng of the isocyanurate ring structure.