Toluene diisocyanate (2,4-/2,6- isomer blend)
Toluene diisocyanate is the most volatile of the major aromatic diisocyanates. It dominates flexible foam, and reaches coatings mainly as low-monomer adducts and prepolymers rather than as free monomer.
TDI is an aromatic diisocyanate: a toluene ring carrying two NCO groups (molecular weight about 174.2 g/mol, 48.3% NCO). The commercial “80/20” grade is a blend of 80% 2,4-TDI and 20% 2,6-TDI; 65/35 and essentially pure 2,4-TDI grades also exist. It is a colorless to pale yellow liquid with a sharp odor, boiling at about 251 °C.
Most TDI goes into flexible polyurethane foam. In coatings it is used after conversion into low-monomer polyisocyanates, for example TDI–trimethylolpropane adducts and TDI trimers that serve as fast-drying hardeners in wood finishes, furniture lacquers and some industrial coatings and adhesives. TDI-based prepolymers are also used in cast elastomers.
Because it is aromatic, TDI-based films yellow and chalk outdoors, so it is used where UV exposure is limited or the coating is overcoated with an aliphatic topcoat.
The two NCO groups in 2,4-TDI are not equally reactive: the group at the 4-position reacts several times faster than the one at the 2-position, which is shielded by the neighboring methyl group. Producers use this difference to make adducts and prepolymers with very little residual monomer.
TDI is a potent respiratory sensitizer and is toxic if inhaled; it is also a skin and eye irritant and is classified by IARC as possibly carcinogenic to humans (Group 2B). Occupational exposure limits are among the lowest of any industrial chemical (ACGIH TLV-TWA 0.001 ppm). Handle in closed systems with engineering controls and appropriate respiratory protection. In the EU, users of products containing more than 0.1% monomeric diisocyanate must complete mandatory training.
Store under dry nitrogen in sealed containers, protected from moisture and kept warm enough to avoid freezing.
Mostly in modified form. Coatings use TDI adducts, trimers and prepolymers with very low free monomer, mainly in wood finishes, industrial coatings and adhesives. Free TDI monomer is rarely used in field-applied coatings because of its volatility.
It is the isomer ratio: 80% 2,4-TDI and 20% 2,6-TDI by weight. Other commercial grades include 65/35 and essentially pure 2,4-TDI.
TDI is smaller, more volatile and has a higher NCO content (48.3% versus about 33.6% for 4,4′-MDI). MDI and its prepolymers dominate spray polyurea and polyurethane coatings, partly because they are far less volatile.
Yes. Like other aromatic isocyanates, TDI-based films yellow and chalk under UV light, so aliphatic isocyanates such as HDI and IPDI are used for light-stable topcoats.
