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| Methylene diphenyl 4,4'-diisocyanate | |
|---|---|
| General | |
| Systematic name | 1-isocyanato-4- (4-isocyanatobenzyl) benzene |
| Other names | Pure MDI, methylenediphenyl 4,4'-diisocyanate, 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 | * |
| Properties | |
| Density and phase | 1.325 g/cm3, solid |
| Solubility in water | Reacts |
| in octane, benzene kerosene | soluble |
| Melting point | 37 °C (313 K) |
| Boiling point | 314 °C (587 K) |
| Hazards | |
| MSDS | External MSDS |
| EU classification | Harmful (Xn) |
| NFPA 704 | |
| R-phrases | , , |
| S-phrases | , , , |
| Flash point | 196 °C |
| RTECS number | NQ9350000 |
| Supplementary data page | |
| Structure & properties | n, εr, etc. |
| Thermodynamic data | Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Related Isocyanates | Toluene diisocyanate Naphthalene 1,2-diisocyanate Hexamethylene diisocyanate |
| Related compounds | Polyurethane |
| Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox | |
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 refrigerators 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
External links
- NIOSH Safety and Health Topic: Isocyanates, from the website of the National Institute for Occupational Safety and Health (NIOSH)
- Hazards of TDI, MDI, and HDI
- Isofact American Chemistry Council Diisocyanates Panel
- Azom Chemical database on Polyurethane chemistry
"Methylene diphenyl diisocyanate"