Reduction of the thermal expansion of unsaturated polyesters by chain-end modification

The addition of the di-isocyanate in a stoichiometric amount comparable to the predicted number of free OH groups of the resin does not affect the curing process and the resin workability; the reaction monitored by FT-IR shows a complete chemical conversion of the OH groups at the end of the curing process. The resin modified with MDI is characterized by a reduced thermal expansion coefficient and by an increase of the glass transition temperature. Very modest effect was observed with IPDI while no changes could be detected with HDI. Experiments with variable amounts of MDI indicate that the decrease of the expansion coefficient is paralleled by the increase of the glass transition temperature. The observed effect is discussed and related to the formation of rigid bridges among the dangling chains of the polymer deriving from the reaction of the MDI with the OH groups.