Description
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.
Uses
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.
Definition
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.
Environmental Fate
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.
Toxicity evaluation
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.
Toxics Screening Level
The initial threshold screening level (ITSL) for 1,1'-methylene
bisi~ocyanatobenzene (also methylenediphenylene ester of isocyanic acid) is
0.2μg/m3 based on a 24-hr averaging time.
