Methylene diphenyl diisocyanate
Production
Major producers include BASF, Bayer, Dow, Huntsman, Mitsui and Yantai Wanhua. Total world production of MDI and polymeric MDI is over 2 million tonnes per year (Mt/a).[citation needed] 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.
There are six steps to the manufacture of pure 4,4′-MDI:
Nitration: Reaction of benzene with nitric acid and a catalyst, to form nitrobenzene
Hydrogenation: Reaction of the nitrobenzene with hydrogen and a catalyst, to form aniline
Aniline/Formaldehyde condensation: Reaction of the aniline with formaldehyde and a catalyst, to form methylene dianiline (MDA), also known as diaminodiphenylmethane (DADPM)
Phosgenation: Reaction of the MDA/DADPM with phosgene, to form an MDI mixture
Separation: Distillation of the MDI mixture to form Polymeric MDI (a mixture of oligomeric polyisocyanates) and an MDI isomer mixture which has a low 2,4′ isomer content
Purification: Fractionation of the MDI isomer mixture to form pure 4,4′-MDI and an MDI isomer mixture which has a high 2,4′ isomer content
Chemistry
The positions of the isocyanate groups influences their reactivities. 4,4′-MDI is a symmetrical molecule and thus has two groups of equal reactivity. 2,4′-MDI is an asymmetrical molecule and thus has two groups of different reactivities. The group at the 4-position is approximately four times more reactive than the group at the 2-position.
Applications
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.[citation needed] 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.
Safety
MDI is the least hazardous of the commonly available isocyanates, as it has a very low vapour pressure. This 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. MDI should not be heated or sprayed except with strict engineering controls and personal protective equipment. Compared to other Isocyanic acid derived Cyanate compounds it has relatively low human toxicity. It is a reactive material and reacts with hydrogen donors, in some cases violently. Its reaction with water produces carbon dioxide which can burst containers and produce aerosols. Since it polymerizes in the presence of water, its ecological risks are low.
Information on handling, personal protective equipment, exposure monitoring, transport, storage, sampling and analysis of MDI, dealing with accidents, and health and environmental information has been published
References
^ a b c Randall, David; Lee, Steve (2002). The Polyurethanes Book. New York: Wiley. ISBN 0-470-85041-8.
^ US6,884,904 (PDF version) (2005-04-26) Smith, Andrea Karen; Goddard, Richard Joseph; Paulsen, Evelyn Jennifer Lin, MDI-based polyurethane prepolymer with low monomeric MDI content.
^ Almaguer, Daniel; et al. (September 2006). “Preventing Asthma and Death from MDI Exposure During Spray-on Truck Bed Liner and Related Applications”. NIOSH Alert. The National Institute for Occupational Safety and Health. http://www.cdc.gov/niosh/docs/2006-149/. Retrieved November 10, 2008.
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. http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471958123.html
External links
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)
Hazards of TDI, MDI, and HDI
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
Categories: Isocyanates | Monomers | IARC Group 3 carcinogensHidden categories: All articles with unsourced statements | Articles with unsourced statements from May 2008
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