Isocyanate
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 reaction with polyols in the production of polyurethanes.
Chemistry
The isocyanate group reacts with the hydroxyl functional group to form a urethane linkage. If a diisocyanate 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 diisocyanate and a compound containing two or more amine groups, produces long polymer chains known as polyureas.
The isocyanate group can react with itself. Aliphatic diisocyanates 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
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 reactive to water, and are classified as sensitizers. Atomization, and heating over 115F, as well as direct skin contact of liquid or un-reacted solid (hot melt) can cause sensitivity to humans. Repeated exposure or those already sensitive to chemicals can increase sensitivity over repeated contact, potentially resulting in hypersensitivity. Isocyanates are generally 100% solids (no volatiles) and have very low vapor pressure, resulting in minimal if any vapors at room temperature. Cured Icocyanate is inert, with no emissions or health issues. For the safe handling of isocyanates, information is available from ISOPA[4], the European Diisocyanate and Polyol Producers Association
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 [5]
See also
References
- ^ Fundamentals to Polymer Science, An Introductory Text (Second ed.). p. 39.
{{cite book}}: Unknown parameter|authors=ignored (help) - ^ http://alpha.chem.umb.edu/chemistry/orgchem/CH20Handout.pdf, Ch20Handout, University of Massachusetts Boston
- ^ Randall, D (2002). The Polyurethanes Book. Wiley. ISBN 0-470-85041-8.
- ^ http://www.isopa.org ISOPA
5. Allport DC, Gilbert, DS and Outterside SM (eds) (2003). MDI and TDI: safety, health & the environment: a source book and practical guide. Chichester, Wiley. http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471958123.html
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