Girish Mahajan (Editor)

Methylene diphenyl diisocyanate

Updated on
Edit
Like
Comment
Share on FacebookTweet on TwitterShare on LinkedInShare on Reddit
Related compounds
  
Polyurethane

Formula
  
C15H10N2O2

Melting point
  
40 °C

Molar mass
  
250.25 g/mol

Density
  
1.23 g/cm³

Boiling point
  
314 °C

Methylene diphenyl diisocyanate httpsuploadwikimediaorgwikipediacommonsthu

Appearance
  
white or pale yellow solid

Methylene diphenyl diisocyanate


Methylene diphenyl diisocyanate, most often abbreviated as MDI, is an aromatic diisocyanate. Three isomers are common, varying by the positions of the isocyanate groups around the rings: 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI. The 4,4' isomer is most widely used, and is also known as 4,4'-Diphenylmethane diisocyanate. 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.

Contents

Methylene diphenyl diisocyanate File4439methylene diphenyl diisocyanatesvg Wikimedia Commons

Global methylene diphenyl diisocyanate mdi industry 2016 market research report


Production

Methylene diphenyl diisocyanate File2239Methylenediphenyl diisocyanatesvg Wikimedia Commons

Total world production of MDI and polymeric MDI is over 5 million tonnes per year (Mt/a in 2011). The largest producer is Covestro (ex Bayer MaterialScience) 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.

Methylene diphenyl diisocyanate 4439Methylenediphenylene diisocyanate C15H10N2O2 ChemSpider

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

Methylene diphenyl diisocyanate DIPHENYLMETHANE DIISOCYANATE MDI

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.

References

Methylene diphenyl diisocyanate Wikipedia


Similar Topics