| 4,4'-methylene diphenyl diisocyanate | |
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1-isocyanato-4-[(4-isocyanatophenyl)methyl] benzene |
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Other names
Pure MDI |
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| Identifiers | |
| CAS number | 101-68-8 |
| PubChem | 7570 |
| ChemSpider | 7289 |
| ChEBI | CHEBI:53218 |
| RTECS number | NQ9350000 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C15H10N2O2 |
| Molar mass | 250.25 g/mol |
| Appearance | white or pale yellow solid |
| Density | 1.230 g/cm3, solid |
| Melting point |
40 °C (313 K) |
| Boiling point |
314 °C (587 K) |
| Solubility in water | Reacts |
| Hazards | |
| EU classification | Harmful (Xn) |
| R-phrases | R20, R36/37/38, R42/43 |
| S-phrases | (S1/2), S23, S36/37, S45 |
| Flash point | 212–214 °C (Cleveland open cup) |
| Related compounds | |
| Related Isocyanates | Toluene diisocyanate Naphthalene diisocyanate Hexamethylene diisocyanate Isophorone diisocyanate |
| Related compounds | Polyurethane |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| 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.[1]
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Total world production of MDI and polymeric MDI is over 2 million tonnes per year (Mt/a). Major producers include BASF, Bayer, BorsodChem, Dow, Huntsman, Mitsui, Nippon Polyurethane Industry and Yantai Wanhua. 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.
MDI is prepared by "phosgenation" of a diamine precursor. 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 give 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.[2]
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.[1]
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.[3]
MDI is the least hazardous of the commonly available isocyanates but is not benign.[4] 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.[5] Compared to other organic cyanates, MDI has a relatively low human toxicity. It is potentially violently reactive material toward water and other nucleophiles.