Methylene_diphenyl_diisocyanate

Methylene diphenyl diisocyanate

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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. The 4,4' isomer is most practically useful, and is also known as Pure MDI. MDI is reacted with a polyol in the manufacture of polyurethane. It is the most produced diisocyanate, accounting for 61.3% of the global market in the year 2000.

Production

Major producers include BASF, Bayer, Dow, Huntsman, Repsol, Shell Chemicals, and Tosoh. Total world production of MDI and polymeric MDI is over 2 million tonnes per year (Mt/a).

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. 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 be not be heated or sprayed except with strict engineering controls and personal protective equipment. Compared to other other Isocyanic acid derived Cyanate compunds 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.