Isocyanates are organic compound with R-NCO functional groups. Two of the six possible isomers are commercially important: 2,4-TDI and 2,6-TDI. 2,4-TDI is produced in pure state, but TDI is often marketed as 80/20 and 65/35 mixtures of the 2,4 and 2,6 isomers respectively. All isomers of TDI are colorless, although commercial samples cand apper yellow. The iscocyanate functional groups in TDI react with hydroxyl groups in the polyol to form carbamate (urethane) links. 

TDI is mainly used in the production of flexible polyurethane foams. MDI is an aliphatic isocyanate, used for various applications such as rigid foams, CASE applications, used as binder and other applications.