Synthesis and Deblocking Studies of Low Temperature Heat-Curable Blocked Polymeric Methylene Diphenyl Diisocyanates

One package heat curable polyurethane system typically requires blocked isocyanates with a relatively low deblocking temperature. In this study, a series of new aromatic secondary amines blocked polymeric methylene diphenyl diisocyanate (pMDI) was synthesized and characterized. Hot-stage FTIR, DSC and TGA spectroscopic methods were used to determine the deblocking temperatures of the blocked isocyanates. Cure reaction between the blocked isocyanates and a commercial polyl (PPG-2000) were carried out to establish structure and property relationship. Deblocking temperature and gel time of the blocked isocyanates based on N-methylaniline and N-methyl-o-toludine were lower than the blocked isocyanates with imidazole and methyl 4-methylaminobenzoate. Unexpected high deblocking temperature of imidazole and methyl 4-methylaminobenzoate was due to intra molecular hydrogen bonding. Solubility behavior of the blocked isocyanates was also studied using a series of polyols (PEG-400, Terathane-2000, PPG-2000, and castor oil).