Synthesis, characterization and thermal dissociation of 2-butoxyethanol-blocked diisocyanates and their use in the synthesis of isocyanate-terminated prepolymers

A series of blocked diisocyanates has been synthesized from toluene diisocyante (TDI), isophorone diisocyanate (IPDI), hexamethylene diisocyanate (HDI), 4,4 '-diphenylmethane diisocyanate (MDI) and 2butoxyethanol. The synthesis of blocked diisocyanate adducts was confirmed by Fourier transform infrared, H-1 NMR, electron impact mass spectrometry and nitrogen analysis. Differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and carbon dioxide evolution were used to determine the minimum de-blocking temperatures. De-blocking temperatures determined by these three techniques were found to be in the order DSC > TGA > CO2 evolution. The effect of different metal catalysts on thermal de-blocking reaction of the blocked diisocyanates was studied, using the carbon dioxide evolution method. It was found that iron(III) oxide has the maximum catalytic activity on de-blocking. The solubility of the blocked diisocyanate adducts was determined in different solvents. The study revealed that at 30 degrees C blocked IPDI and HDI adducts show better solubility than adducts based on TDI and MDI. Isocyanate-terminated prepolymers of blocked diisocyanates and hydroxyl-terminated polybutadiene (HTPB) were prepared. The storage stability and gelation times of the prepolymers were studied. Results showed that all the diisocyanate-HTPB compositions are stable at 50 degrees C for more than three months. However, aliphatic diisocyanate-HTPB compositions require greater gelation time than aromatic diisocyanate-HTPB compositions at their respective de-blocking temperatures. (C) 2007 Society of Chemical Industry.