26447-40-5

Methylene diphenyl diisocyanate (MDI) is a white, odorless diisocyanate that may also occur in crystalline form. It exists in three isomers, (2,20-MDI,2,40-MDI, and 4,40-MDI), however, the primary technical/commercial form of MDI is actually polymeric MDI, which is a mixture that contains 25–80% monomeric 4,40-MDI as well as oligomers containing three to six rings and other minor isomers, such as the 2,20-isomer the 4,40 isomer. MDI is most widely used and accounted for 62% of the global market in 2000.

MDI is used to produce polyurethane foams for application in a wide range of retail, commercial, and industrial uses to protect cement, wood, fiberglass, steel, and aluminum surfaces such as truck beds, trailers, boats, foundations, and decks. MDI is also used as a high strength adhesive in industrial and consumer applications.

ChEBI: Diphenylmethane-4,4'-diisocyanate is a diisocyanate consisting of diphenylmethane with two isocyanate groups at the 4- and 4'-positions. It has a role as a hapten and an allergen. It derives from a hydride of a diphenylmethane.

MDI is the least hazardous of the commonly available isocyanates, however, it is not without toxicity. It has low vapor pressure, which reduces hazards during handling compared to the other major isocyanates, e.g., TDI, HDI. The complex nature of MDI composition and reactions in the environment often makes interpretation difficult. Release of MDI to the atmosphere is low due to its low volatility and since MDI readily degrades to inorganic compounds in the presence of hydroxyl radicals, its atmospheric half-life is short. High exposures involving MDI in ambient environments are expected to be rare. There is no information about levels of various forms of MDI in the ambient air. When MDI is released to water, it reacts readily with water to form mixtures of diisocyanates and amines, which then readily react with more MDI to produce inert, solid, insoluble polyurea. MDI has a transient existence due to its reaction with the water to produce predominantly insoluble polyureas. The evidence indicates that MDI accumulation through the aquatic food chain is extremely unlikely, as might be expected considering the very low solubility and high reactivity of MDI with hydroxyl radicals in aqueous solution.

A definitive mechanism of MDI toxicity is unknown; however, it is speculated that humoral and cellular immunity may be involved in the pathogenesis of hypersensitivity due to isocyanate sensitization responses of both animals and humans.