Isocyanate

The isocyanate functional group

Isocyanate is the functional group of atoms –N=C=O (1 nitrogen, 1 carbon, 1 oxygen), not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more than one isocyanate group. An isocyanate that has two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for reactions with polyols in the production of polyurethanes. The largest use of isocyanates in industry involves the production of polymers. Polymers of common isocyanates are used in the manufacture of foams, paints, lacquers and in electrical insulation.

Chemistry

The isocyanate group reacts with the hydroxyl functional group to form a urethane linkage. If a di-isocyanate is reacted with a compound containing two or more hydroxyl groups (a polyol), long polymer chains are formed, known as polyurethanes.

The isocyanate group also reacts with the amine functional group. Reaction between a di-isocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas.

The isocyanate group can react with itself. Aliphatic di-isocyanates can form trimers, known as biurets.

The isocyanate group reacts with water to form carbon dioxide (CO₂). Carbon dioxide is used as a blowing agent (in situ) in order to produce polyurethane foams.^[1]

Isocyanates are intermediates in the Hofmann rearrangement, in which a primary amide is reacted with a hypohalite such as sodium hypobromite to form an isocyanate intermediate. The isocyanate is then hydrolyzed with water to yield the corresponding primary amine.^[2]

Common compounds

Methylene diphenyl 4,4'-diisocyanate (MDI)
Numbering of the ring atoms shown with blue numbers

A monofunctional isocyanate of industrial significance is methyl isocyanate (MIC), which is used in the manufacture of pesticides.

The global market for diisocyanates in the year 2000 was 4.4 million tonnes, of which 61.3% was methylene diphenyl diisocyanate (MDI), 34.1% was toluene diisocyanate (TDI), 3.4% was the total for hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), and 1.2% was the total for various others.^[3]

All major producers of MDI and TDI are members of the International Isocyanate Institute, whose aim is the promotion of the safe handling of MDI and TDI in the workplace, community and environment.

Hazards

Isocyanates are potentially dangerous irritants to the eyes and respiratory tract. No matter which chemical is used, all isocyanates must be treated as a highly hazardous chemical. The greatest risk when dealing with these chemicals involves inhalation of the fine vapours or droplets which irritate the nose lungs and thorax. Increased exposure to isocyanates can lead to sensitization causing workers to experience asthma attacks.

Common health effects caused by all isocyanates include eye irritation leading to watering and discomfort. Dryness of the throat and chest is common and difficulty breathing results from constant exposure, especially when in high concentrations. When the chemicals come in contact with the skin, reddening, swelling and blistering will occur and persist until completely washed off. Dermatitis is also linked to excessive exposure of isocyanates as well as liver and kidney dysfunction.

Isocyanates pose a particular threat when burned as carbon monoxide and hydrogen cyanide are formed. These gases are highly toxic and could be fatal if the area isn’t well ventilated. Contamination of isocyanates with water can create toxic gases, so they must be stored in airtight and dry containers. If contamination occurs, never seal the chemicals, as doing so will create unwanted pressure and may lead to an explosion.

Several isocyanates pose a greater risk such as Methylisocyanate as it can create tear gas when released. Methyl isocyanate is extremely toxic and can kill a worker even when exposed to only small concentrations. The toxic effect of this isocyanate is evident in the Bhopal Disaster which killed thousands due to high exposure. Another isocyanate which if not handled with care can cause severe harm is toluene diisocyanate.

Information on handling, personal protective equipment, exposure monitoring, transport, storage, sampling and analysis of MDI and TDI, dealing with accidents, and health and environmental information has been published.^[4]

See also

• Isothiocyanate
• Polyurethane
• Methylisocyanate
• Polyols
• Fire classes

References

1. ^ Paul Painter and Michael Coleman. Fundamentals to Polymer Science, An Introductory Text (Second ed.). pp. 39. 
2. ^ http://alpha.chem.umb.edu/chemistry/orgchem/CH20Handout.pdf, Ch20Handout, University of Massachusetts Boston
3. ^ Randall, D (2002). The Polyurethanes Book. Wiley. ISBN 0-470-85041-8. 
4. ^ Allport DC, Gilbert, DS and Outterside SM (eds) (2003).MDI and TDI: safety, health & the environment: a source book and practical guide. Chichester, Wiley.

External links

• NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)
• Health and Safety Executive, website of the UK Health and Safety Executive, useful search terms on this site — isocyanates, MVR, asthma
• [1] International Isocyanate Institute
• http://www.actsafe.ca/wp-content/uploads/resources/pdf/Isocyanates.pdf
• Isocyanates – Measurement Methodology, Exposure and Effects Isocyanates in Working Life Workshop Article (1999)
• Health and Safety Executive, Guidance Note (EH16) Isocyanates: Toxic Hazards and Precautions (1984)
• The Society of the Plastics Industry – Technical Bulletin AX119 MDI-Based Polyurethane
• Foam Systems: Guidelines for Safe Handling and Disposal (1993)
• An occupational hygiene assessment of the use and control of isocyanates in the UK by Hilary A Cowie et al. HSE Research Report RR311/2005. Prepared by the Institute of Occupational Medicine for the Health and Safety Executive