Methylene diphenyl diisocyanate

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4,4'-methylene diphenyl diisocyanate

General
Systematic name	1-isocyanato-4- [(4-isocyanatophenyl)methyl] benzene
Other names	Pure MDI 4,4'-methylene diphenyl diisocyanate 4,4'-diphenylmethane diisocyanate
Molecular formula	C₁₅H₁₀N₂O₂
SMILES	O=C=NC(C=C2)=CC=C2C C1=CC=C(N=C=O)C=C1
Molar mass	250.25 g/mol
Appearance	white or pale yellow solid
CAS number	[97568-33-7]
Properties
Density and phase	1.230 g/cm³, solid
Solubility in water	Reacts
in octane, benzene kerosene	soluble
Melting point	40 °C (313 K)
Boiling point	314 °C (587 K)
Hazards
MSDS	External MSDS
EU classification	Harmful (Xn)
NFPA 704
R-phrases	R20, R36/37/38, R42/43
S-phrases	S1/2, S23, S36/37, S45
Flash point	212–214 °C (Cleveland open cup)
RTECS number	NQ9350000
Supplementary data page
Structure & properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related Isocyanates	Toluene diisocyanate Naphthalene diisocyanate
Related compounds	Polyurethane
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Infobox disclaimer and 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. 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.^[1]

1 Synthesis
2 Chemistry
3 Production
4 Uses
5 Safety
6 See also
7 References
8 External links

[edit] Synthesis

There are six steps to the manufacture of pure 4,4'-MDI:^[1]

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

[edit] 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.^[1]

[edit] 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).

[edit] 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 refrigerators worldwide, as well as buildings. Typical polyols used are polyethylene adipate (a polyester) and poly(tetramethylene ether)glycol (a polyether).

[edit] 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.