What is Toluene di-isocyanate?
Toluene di-isocyanate (TDI) is an organic compound with a pungent, colorless, toxic odor that is sold as a mixture of 2,4 TDI and 2,6 TDI isomers. The 2,4 TDI isomer is found in pure form. This compound has 6 isomers, two of which are commercially important. All isomers are colorless, although commercial samples may be yellow due to impurities.
Toluene di-isocyanate is obtained from toluene in three steps. In the first step, toluene is nitrated with nitric acid and dinitro toluene is synthesized. This step determines the final TDI isomer ratio. In the next step, the nitro-toluene is hydrated and diamino-toluene (TDA) isomers are produced.
Finally, TDA reacts with phosgene and TDI is obtained. At this stage, hydrochloric acid is prepared as a by-product, which is one of the main sources of production of this acid in industry. Raw TDI is then converted to pure product by distillation.
Toluene di-isocyanate Mainly used for the production of flexible polyurethane foam, coatings, industrial coatings and adhesives. The isocyanate groups in the TDA react with the hydroxyl groups to form carbamate (urethane). In the 2,4 TDI isomer, position 4 is almost 4 times more active than in position 2 and can react better, but in the 2,6 TDI isomer, the two isocyanate groups react similarly due to the symmetry of the molecule.