MDI: Basics

MDI is the acronym of Methylene diphenyl Di-Isocyanate. This chemical product has in fact 3 isomers: 4, 4’ – MDI, 2, 4’ – MDI, and 2, 2’ – MDI.



MDI could be present in a polymeric form called Polymeric MDI, PMDI or MDI polymer. PMDI has more than just two aromatic cycles holding each an Isocyanates function; therefore PMDI functionality is larger than two whereas MDI monomers are strictly difunctional.

In general, pure MDI refers to a mixture where almost 98% is made up of the 4, 4’ – MDI isomer. The isolated 4,4’ – MDI isomer has a melting point about 40º C, which means that Pure MDI is a solid at ambient temperature.

Crude MDI, or technical grade MDI, refers to a mixture of the three types of isomers as well as the polymeric form. Crude MDI, however, is a brown liquid at ambient temperature.

In order to be handled easily, Pure MDI could be converted into a liquid by reacting 4, 4’ – MDI with a di-alcohol. This process will result into a derivative MDI product called Modified MDI or MDI Prepolymer.

In General, MDI is not used as often as TDI in the production of slab stock Flexible Polyurethane Foam. Even that figures show that MDI production in almost double than TDI, the main usage of MDI is in the molded Flexible Polyurethane Foam, Rigid Polyurethane, and CASE.