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Methylene Diphenyl Diisocyanate
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Methylene diphenyl diisocyanate, most often abbreviated as '''MDI''', is an Aromatic Diisocyanate . It exists in three Isomer s, '''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.


SYNTHESIS


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 a 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.


PRODUCTION


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


USES


4,4'-MDI has only one major application, that 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 Refrigerator s worldwide, as well as buildings. Typical polyols used are Polyethylene Adipate (a Polyester ) and Poly(tetramethylene Ether)glycol (a Polyether ).


SAFETY


MDI is the least hazardous of the commonly available isocyanates, as it has a very low vapour pressure. This makes handling it relatively safe. However, appropriate precautions should be taken.


SEE ALSO



REFERENCES