MDI chemistry and production

The two main types of diisocyanates are aromatic and aliphatic isocyanates.
 
Methylene diphenyl diisocyanate, most often abbreviated as MDI, is an aromatic type.
 
MDI exists in three isomers: 2,2-MDI, 2,4-MDI, and 4,4-MDI. The 4,4 isomer is known as pure or monomeric MDI (mMDI).
 


4,4 isomer
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MDI types

MDI type Description  Form at room temperature
Polymeric MDI Contains monomeric MDI and products with higher molecular weight. Also known as crude or pMDI. Solvent free, yellowish-brown, low viscosity liquid material
Monomeric MDI Contains 98% 4,4-MDI. The remainder is 2,4 and 2,2-MDI. Known as pure or mMDI. White, crystalline solid material
Modified MDI Generated by catalysing from monomeric MDI and kept at high temperature, giving versatility in handling and properties. Light yellow liquid material
MDI prepolymers A mixed material that contains Polyol and monomeric MDI Slightly yellow liquid, free from any solid deposition
MDI variants Mixed material from monomeric, polymeric or modified MDI Yellowish-brown, liquid material

 

 
The growth of MDI has been linked to both GDP growth and new application growth of polyurethanes.
 
The key growth drivers in future will be:
  • Increasing use of polyurethanes for insulation, driven by government regulations, high energy costs and construction growth in developing regions such as Central and Eastern Europe
  • Continued use in cars but with new applications for MDI based foam and PU elastomers
  • Increasing production of shoe soles in Asia
  • Increasing production of synthetic leather
  • Increasing share of MDI in MDF, HDF and Particle Board

 

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Production process

 
The main feedstock for MDI is nitrobenzene which is produced by the nitration of benzene in either a continuous or batch process. The nitrobenzene is hydrogenated to aniline, which is then condensed with formaldehyde to form diphenylmethane diamine (MDA).
The polyamine is reacted with phosgene in a solvent such as monochlorobenzene to yield an isocyanate mix that largely corresponds to the composition of the polyamine.

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