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Description of key information

In a read-across approach based on a tests performed with 4,4'-MDI and polymeric MDI, the hydrolysis according to OECD 111 half time is < 5 minutes at 20°C. The reaction with water followed zero order kinetics. The hydrolysis product MDA is recovered by about 100%. In such dilute systems, isocyanate functions are generally converted to amino functions.

Key value for chemical safety assessment

Half-life for hydrolysis:
5 min
at the temperature of:
20 °C

Additional information

The test substance is covered by an analogue approach. The read-across justification document is attached in IUCLID section 13.

Two types of experimental studies are presented under this endpoint explaining the hydrolysis behavior.

Firstly, exposure in heterogeneous conditions, under which the water insoluble MDI substances form droplets suspended in aqueous phase (i.e OECD 120; determination of solution/extraction behaviour in water). Note that this type of study is not a standard required but used as supporting information to give an insight about the behavour of MDI and its derivatives in e.g. acute ecotox studies.

Secondly, exposure in homogeneous conditions (i.e. OECD 111), under which the insoluble MDI substances are artificially solubilized in solution. In this regard it should be noted that such homogeneous testing conditions do not exist in the natural environment and can only be achieved under highly artificial laboratory conditions.

Testing of solution/extraction behaviour in water (OECD 120):
In the studies performed under heterogeneous testing conditions (i.e. OECD 120), reaction of the NCO group with water occurs rapidly at the boundary layer of the MDI substances droplet and by water diffusing onto and into the organic phase forming inert polyurea particles. Under these conditions only trace levels of MDA are formed. The results have been obtained from hydrolysis studies using heterogeneous conditions (in concentrations above the water solubility) showing that source substance 4,4'-MDI and target substance MDI MT exhibit a similar hydrolysis behavior when in contact with water. Source and target substance contain high levels of mMDI and have aromatic NCO groups which under heterogeneous testing conditions are hydrolysed to polymeric ureas with only minimal formation of MDA. The key relevant findings of hydrolysis information obtained from the measurement of solution/extraction behaviour (heterogeneous test conditions) in water show that the transformation processes, which are relevant to aquatic exposure and toxicity of MDI type aromatic diisocyanates, complete within the first 72 hours within the organic phase. concentrations.

Hydrolysis testing (OECD 111):
In contrast, in studies performed under artificial homogeneous testing conditions (i.e. OECD 111) the 4,4’-MDI and pMDI source substances are fully solubilized in water, and MDI is quantitatively converted to MDA. For the analogue read-across approach, reliable hydrolysis data under homogeneous conditions are used from 4,4’-MDI and pMDI. Read-across is used to predict the outcome of the source substance to the target substances on the basis that all the substances contain a high content of monomeric MDI. The hydrolysis half-life of monomeric 4,4-MDI under homogeneous conditions was extremely short and ranged between 11.9 and 17.6 seconds over the pH range 4 to 9 at 20°C. Acidic conditions do not accelerate MDI hydrolysis, but there is a slight enhancement of the hydrolysis rate under alkaline conditions. Comparable short half-lives and trends were observed for pMDI. Moreover, the decay of monomeric MDI present in high levels is not affected by the presence of other constituents containing NCO groups as e.g. in pMDI. Under these homogeneous conditions with a solvent present, 4,4’-MDA was by far is the most common transformation product formed. The yield of the common compound MDA was essentially 100% under the highly dilute and stirred reaction conditions studied. In addition, trace amounts of higher aminic compounds (three-ring MDA in pMDI) were formed. The hydrolysis of higher constituents (e.g. 3-ring MDI) appears to be slower than that observed for free MDI but is nevertheless still rapid i.e. less than 5 minutes.

Overall, the two source substances show rapid hydrolysis with a half-life of less than five minutes at 20°C, over the pH range of 4 to 9. The dependence of hydrolysis rate constant on temperature (10, 20, and 30°C) was evaluated and showed the expected doubling of rate constant with each 10°C increase in temperature according to the Arrhenius rate law. 

Notwithstanding that only hydrolysis under heterogeneous conditions will be truly relevant for hazard and risk assessment for the aquatic environment, the assessment of this endpoint addresses hydrolysis under homogeneous conditions (OECD 111) as required by REACH. A half-life of 5 min. is considered for the chemical safety assessment.