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Environmental fate & pathways

Hydrolysis

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Description of key information

MDI reacts with water to form predominantly inert polyurea. Yakabe et al (1994) found that for loadings of 400 - 10,000 mg/l, and under heterogeneous conditions, MDI had a half-life of ca. 20h at 25°C when the disappearance rate was fitted to a zero-order kinetic model. Howver the rate constant increased approximately linearly with the nominal concentration of PMDI 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 PMDI.

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Additional information

These findings 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).