Thermal Decomposition Behavior of Blocked Diisocyanates Derived from Mixture of Blocking Agents

To improve the performance and reduce raw material costs, blocked isocyanates were prepared with mix ture of blocking agents in many industries. Three blocked isocyanates (adducts) namely -caprolactam/benzotriaz ole-blocked 4,4'-diphenylmethane diisocyanate (MDI), toluene-2,4-diisocyanate (TDI) and 4,4'-dicyclohexyl-methane diisocyanate (H12MDI) were synthesized. Six reference adducts were also prepared by blocking MDI, TDI, and H12MDI with -caprolactam (-CL) or benzotriazole. The reactions were carried out in acetone medium and dibutyltin dilaurate (DBTDL) was used as a catalyst. The progress of the blocking reaction was monitored by IR spectroscopy. De-blocking temperatures (dissociation temperatures) of these adducts were studied using DSC and TGA and the results were correlated. As expected, the thermal analysis data showed that de-blocking temperature of blocked aromatic isocyanates was lower than that of the blocked aliphatic isocyanates. The low de-blocking tem perature of blocked aromatic isocyanate could be due to electron withdrawing benzene ring present in the blocked isocyanates. It was also found that benzotriazole-blocked adducts de-blocked at higher temperature compared with -CL-blocked adducts. KCI Citation Count: 14