Methylene diphenyl diisocyanate

4,4'-methylene diphenyl diisocyanate

4,4'-methylene diphenyl diisocyanate
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Names
IUPAC name 1-isocyanato-4-[(4-isocyanatophenyl)methyl] benzene
Other names Pure MDI 4,4'-methylene diphenyl diisocyanate 4,4'-diphenylmethane diisocyanate Bayer Desmodur 44 Methylene bis(4-phenyl isocyanate)
Identifiers
CAS Number	101-68-8 Y
ChEBI	CHEBI:53218 N
ChemSpider	7289 Y
Jmol 3D model	Interactive image Interactive image
PubChem	7570
RTECS number	NQ9350000
InChI InChI=1S/C15H10N2O2/c18-10-16-14-5-1-12(2-6-14)9-13-3-7-15(8-4-13)17-11-19/h1-8H,9H2 N Key: UPMLOUAZCHDJJD-UHFFFAOYSA-N N
SMILES O=C=NC(C=C2)=CC=C2CC1=CC=C(N=C=O)C=C1 O=C=N\c1ccc(cc1)Cc2ccc(\N=C=O)cc2
Properties
Chemical formula	C15H10N2O2
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)
Solubility in water	Reacts
Vapor pressure	0.000005 mmHg (20°C)
Vapor pressure	{{{value}}}
Related compounds
Related Isocyanates	Toluene diisocyanate Naphthalene diisocyanate Hexamethylene diisocyanate Isophorone diisocyanate
Related compounds	Polyurethane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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 (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.

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 (a mixture of oligomeric polyisocyanates) 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 (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 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.

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)
• 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

• 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