Methylene diphenyl diisocyanate
4,4'-methylene diphenyl diisocyanate
Bayer Desmodur 44
|Molar mass||250.25 g/mol|
|Appearance||white or pale yellow solid|
|Density||1.230 g/cm3, solid|
|Melting point||40 °C (104 °F; 313 K)|
|Boiling point||314 °C (597 °F; 587 K)|
|EU classification||Harmful (Xn)|
|R-phrases||R20, R36/37/38, R42/43|
|S-phrases||(S1/2), S23, S36/37, S45|
|Flash point||212–214 °C (Cleveland open cup)|
|US health exposure limits (NIOSH):|
|C 0.2 mg/m3 (0.02 ppm)|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Methylene diphenyl diisocyanate, most often abbreviated as MDI, is an aromatic diisocyanate. It exists in three isomers, 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI, however, the 4,4' isomer is most widely used. This isomer is also known as Pure MDI. MDI reacts with polyols in the manufacture of polyurethane. It is the most produced diisocyanate, accounting for 61.3% of the global market in the year 2000.
Total world production of MDI and polymeric MDI is over 5 million tonnes per year (Mt/a in 2011). The largest producer is Bayer followed closely by Yantai Wanhua. Other major producers are BASF, BorsodChem, Dow, Huntsman, Nippon Polyurethane Industry, OCI. All major producers of MDI 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.
2 C6H5NH2 + CH2O → CH2(C6H4NH2)2 + H2O
Then, these diamines are treated with phosgene to form an MDI. The isomer ratio is determined by the isomeric composition of the diamine. Distillation of the MDI mixture give Polymeric MDI (a mixture of oligomeric polyisocyanates)[dubious ] and an MDI isomer mixture which has a low 2,4' isomer content.[clarification needed] Further purification entails fractionation of the MDI isomer mixture.
Reactivity of the isocyanate group
The positions of the isocyanate groups influences their reactivity. In 4,4'-MDI, the two isocyanate groups are equivalent but in 2,4'-MDI the two groups display highly differing reactivities. The group at the 4-position is approximately four times more reactive than the group at the 2-position due to steric hindrance.
The major application of 4,4'-MDI is the production of rigid polyurethane. Typically, one tonne of polyurethane foam needs 0.616 tonne of MDI and 0.386 tonne of polyol, with 0.054 tonne pentane as a blowing agent. These rigid polyurethane foams are good thermal insulators and used in nearly all freezers and refrigerators worldwide, as well as buildings. Typical polyols used are polyethylene adipate (a polyester) and poly(tetramethylene ether) glycol (a polyether).
4,4'-MDI is also used as an industrial strength adhesive, which is available to end consumers as various high-strength bottled glue preparations.
MDI is the least hazardous of the commonly available isocyanates but is not benign. Its very low vapour pressure reduces its hazards during handling compared to the other major isocyanates (TDI, HDI). However, it, like the other isocyanates, is an allergen and sensitizer. Persons developing sensitivity to isocyanates may have dangerous systemic reactions to extremely small exposures, including respiratory failure. Handling MDI requires strict engineering controls and personal protective equipment. Compared to other organic cyanates, MDI has a relatively low human toxicity. It is a potentially violently reactive material towards water and other nucleophiles.
- "NIOSH Pocket Guide to Chemical Hazards". National Institute for Occupational Safety and Health (NIOSH). id=0413.
- Randall, D.; Lee, S. (2003). The Polyurethanes Book. New York: Wiley. ISBN 978-0-470-85041-1.
- Gal, J. (2012-02-20). "To the Rescue". ICIS Chemical Business.
- Six, C.; Richter, F. (2005), "Isocyanates, Organic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a14_611
- Boustead, I. (2005). "Polyurethane rigid foam" (PDF). Eco-Profiles of the European Plastics Industry. Brussels: PlasticsEurope.
- US patent 6884904, Smith, A. K.; Goddard, R. J.; Paulsen, E. J. L., "MDI-based polyurethane prepolymer with low monomeric MDI content", issued 2005-04-26
- Allport, D. C.; Gilbert, D. S.; Outterside, S. M., ed. (2003). MDI and TDI: Safety, Health and the Environment: A Source Book and Practical Guide. Wiley. ISBN 978-0-471-95812-3.
- Almaguer, D. et al. (September 2006). "Preventing Asthma and Death from MDI Exposure During Spray-on Truck Bed Liner and Related Applications" (PDF). NIOSH Alert. The National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication No. 2006–149. Retrieved 2012-08-14.
- International Chemical Safety Card 0298
- IARC Monograph: "4,4'-Methylenediphenyl Diisocyanate"
- NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)
- Isofact American Chemistry Council Diisocyanates Panel
- Azom Chemical database on Polyurethane chemistry
- MDI and the Environment - 2005 presentation by Center for the Polyurethanes Industry
- International Isocyanate Institute
- Concise International Chemical Assessment Document 27