Important Organic Isocyanate-Toluene Diisocyanate (TDI)

TDI industrial products used for polyurethane foam are mainly a mixture of 2,4- and 2,6-isomers with a mass ratio of 80:20 (TDI-80 or TDI-80/20), primarily used for flexible polyurethane foam. TDI-80 is the most widely used isocyanate in the production of flexible foam. A small amount of products with a mass ratio of 2,4- and 2,6-isomers at 65:35 (TDI-65 or TDI-65/35) are also used for high-load flexible foam, mainly for polyester-type flexible foam. In the early days, TDI was used to produce rigid polyurethane foam, but due to its high NCO content of up to 48%, which generates excessive heat and a strong odor during foaming, it is no longer used for rigid polyurethane foam production. Only a very small amount of TDI is used in the form of prepolymers or crude TDI for rigid foam. The advantage of TDI-based foam products is their good elasticity, and the density of TDI-based foam with similar formulations is lower than that of MDI-based foam.

At room temperature, TDI is a colorless or light yellow transparent liquid with a distinctive pungent odor. When stored below 10°C for a long time, it can form white crystals. The ratio of 2,4- and 2,6-isomers in TDI affects its physical properties, such as freezing point and viscosity.

The viscosity and density of toluene diisocyanate decrease with increasing temperature. At room temperature, TDI is slightly more viscous than water, with a viscosity three times that of water. The isomers 2,4-TDI and 2,6-TDI can be determined by measuring the peak heights at wavelengths of 12.25 µm and 12.75 µm in their infrared absorption spectra.