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. Production
Total world production of MDI and polymeric MDI is over 5 million tonnes
per year. 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.
The first step of the production of MDI is the reaction of aniline and
formaldehyde, using hydrochloric acid as a catalyst to produce a diamine
precursor under the chemical formulation:
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 gives polymeric MDI
and an MDI isomer mixture which has a low 2,4' isomer content. 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.
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 and
poly(tetramethylene ether) glycol. 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 but is
not benign. Its very low vapour pressure reduces its hazards during handling
compared to the other major isocyanates. 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. References
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
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