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EC number: 701-173-1
CAS number: -
The test substance reacts with water to form predominantly solid,
inert polyurea. Yakabe et al (1994) found that for loadings of 400 -
10,000 mg/L, and under heterogeneous conditions, a structural analogue
had a half-life of ca. 20h at 25°C.
Yakabe et al (1994) found that for loadings of 400 - 10,000 mg/L, and
under heterogeneous conditions, oligomeric MDI had a half-life of ca.
20h at 25°C when the disappearance rate was fitted to a zero-order
kinetic model. However the rate constant increased approximately
linearly with the nominal concentration of oligomeric MDI for all
constituents. For the two-ring constituents the rate constant was found
to be 0.5h-1 for a loading of 1000 mg/L oligomeric MDI.
The findings in the Yakabe et al (1994) study concur with the generally
accepted chemistry of hydrolysis of an isocyanate whereby the initially
produced carbamate decarboxylates to amine which then reacts very
readily with more isocyanate to produce a urea. For a diisocyanate of
low solubility such as MDI, the reaction system is usually heterogenous
and these reactions, leading to insoluble polyureas, occur in both
organic and aqueous phases and at the interface, where in each case
differing concentrations of reagents and intermediates lead to different
product distributions. Under conditions typical of many types of
environmental contact, i.e. with relatively poor dispersion of the
denser isocyanate, the interfacial reaction leads to the formation of a
solid crust encasing partially or unreacted material. This crust
restricts ingress of water and egress of amine, and hence slows and
modifies hydrolysis. An important consequence of general significance to
all aquatic tests of MDI, is that, in reality, it is the breakdown
products that are being studied and the technique used for introduction
or dispersal of the substance can affect the physical form and
composition of the product (although under all tested conditions the
predominant products are solid, insoluble polyureas).
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