Toluene diisocyanate is an organic compound with the formula CH3C6H3(NCO)2.
Two of the six possible isomers are commercially important: 2,4-TDI and
2,6-TDI. 2,4-TDI is produced in the pure state, but TDI is often marketed as
80/20 and 65/35 mixtures of the 2,4 and 2,6 isomers respectively. It is produced
on a large scale, accounting for 34.1% of the global isocyanate market in 2000,
second only to MDI. Approximately 1.4 billion kilograms were produced in 2000.
Synthesis 2,4-TDI is prepared in three steps from
toluene via dinitrotoluene and 2,4-diaminotoluene. Finally, the TDA is
subjected to phosgenation, i.e., treatment with phosgene to form TDI.
This final step produces HCl as a byproduct and is a major source of
industrial hydrochloric acid. Distillation of the crude TDI mixture
produces an 80:20 mixture of 2,4-TDI and 2,6-TDI, known as TDI. Differentiation
or separation of the TDI can be used to produce pure 2,4-TDI and a 65:35 mixture
of 2,4-TDI and 2,6-TDI, known as TDI. Applications
The isocyanate functional groups in TDI react with hydroxyl groups to form
carbamate links. The two isocyanate groups in TDI react at different rates:
The 4-position is approximately four times more reactive than the 2-position.
2,6-TDI is a symmetrical molecule and thus has two isocyanate groups of
similar reactivity, similar to the 2-position on 2,4-TDI. However, since
both isocyanate groups are attached to the same aromatic ring, reaction of one
isocyanate group will cause a change in the reactivity of the second isocyanate
group. It is used in the production of flexible
polyurethane foams Hazards
The LD50 for TDI is 5800 mg/kg for oral contact and LC50 of 610 mg/m3 for the
vapour. Despite the indicated low toxicity, TDI is classified as “very
toxic” by the European Community. In the United States, the Occupational
Safety and Health Administration has set a permissible exposure limit with a
ceiling at 0.02 ppm, while the National Institute for Occupational Safety and
Health has not established a recommended exposure limit, due to the
classification of toluene diisocyanate as a possible occupational carcinogen.
Information is available on handling, personal protective equipment, exposure
monitoring, transport, storage, sampling and analysis of TDI, dealing with
accidents, and health and environmental themes. All major producers of TDI are
members of the International Isocyanate Institute, whose aim is the promotion of
the safe handling of TDI in the workplace, community, and environment.
See also Methylene diphenyl diisocyanate
Hexamethylene diisocyanate Isophorone diisocyanate
References External links
International Chemical Safety Card 0339 IARC Monograph: "Toluene Diisocyanates"
NIOSH Pocket Guide to Chemical Hazards NIOSH Safety and Health Topic:
Isocyanates, from the website of the National Institute for Occupational
Safety and Health International Isocyanate Institute
http:www.diisocyanates.org