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Environmental fate & pathways

Hydrolysis

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Description of key information

The test substance is covered by the category approach of methylenediphenyl diisocyanates (MDI). The read-across category 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), and 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 majority of test data has been obtained from hydrolysis studies using heterogeneous conditions showing that the substances of the MDI category exhibit all a similar hydrolysis behavior when in contact with water. In total, 19 studies for 12 substances of the MDI category are available. The studies performed indicate a high level of confidence for predicting the behaviour of all substance in the MDI category. All category substances 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 the substances of the MDI category, complete within the first 72 hours within the organic phase. The conversion of the MDI Mixed isomers in the organic phase took the double time. Measurements of solution/extraction behaviour at two loading rates (100 and 10,000 mg/L) and under two stirring conditions (gentle and vigorous) were performed to explore whether factors such as test substance type/composition, loading rate, and stirring conditions are correlated with MDA concentrations. Only the available measurements of the solution/extraction behaviour at the loading rate of 100 mg/L and under gentle stirring conditions are summarized under this endpoint as these more closely reflect natural mixing conditions in surface waters.
Under heterogeneous conditions according to OECD guideline 120, parent MDI substances are rapidly hydrolysed to polyurea. These ureas were detected in the solid phase (precipitate) of the reaction mixtures under heterogeneous conditions but not in the aqueous phase. Only small amounts of MDA (<1 %) are formed. Its level is depending on a range of test conditions including pH, loading rate and level of stirring. 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 the substances of the MDI category, complete within the first 72 hours within the organic phase. For the 100 mg/L loadings, the average concentration is 0.40 mg/L or a yield of 0.40 %. The conversion of the MDI Mixed isomers in the organic phase took the double time. An important finding is that higher loadings show lower yield but the MDA released to water remains limited. As described in the conceptual model in the category Justification Document, MDA is formed rapidly after mixing both phases achieving a maximum concentration within 24 to 72 hours of their introduction to stirred water. Thereafter, the concentrations of MDA decreased with continued stirring. From the 4,4’-MDA control (Figure 50 of the Category Justification Document) it is apparent that MDA undergoes abiotic decay under these conditions. Overall, it can be concluded that only low amounts of MDA are formed with a maximum MDA yield of <1 % under the chosen conditions. The available data set comprising reliable studies performed under different conditions is sufficient and confirms a consistent and regular pattern of response (i.e. no trend) consistent with the hydrolysis MoA. Minor variations in levels of MDA produced are explained by the very low solubility of the MDI substances and the variations in the dispersed
phase surface area (such as observed in the solution/extraction behavior in water) and not due to structural differences between the substances of the MDI category. The principles of the heterogeneous reaction, are also illustrated in a supporting study of the fate of the pMDI substance in water by Yakabe et al. in 1994. At higher loadings of 400, 1000, and 10000 mg/L, where the source substance pMDI was vigorously stirred in water, the pMDI reacted with in-diffusing water and was converted into polyurea with an effective zero order reaction kinetics of all MDI constituents (being it monomeric MDI isomers or higher ring MDIs) with half-life times of ca 20 h. The rate was unaltered in sea water, and was rather slower with less agitation and significantly slower at 12°C. Hence, hydrolysis to polyurea is considered to be the main removal mechanism for MDI substances in environmental compartments. 


Hydrolysis testing (OECD 111):
In contrast, in studies performed under artificial homogeneous testing conditions (i.e. OECD 111) the 4,4’-MDI substance is fully solubilized in water, and MDI is quantitatively converted to MDA. Reliable hydrolysis data under homogeneous conditions are available for the three boundary substances 4,4’-MDI, pMDI and 4,4’-MDI/DPG/HMWP. Read-across is used to predict the outcome of missing studies on the other category substances on the basis that all substances contain a high content of mMDI. The hydrolysis half-life of 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. These trends are in agreement with observations by Castro et al. (1985). Comparable short half-lives and trends were observed for pMDI and 4,4’-MDI/DPG/HMWP. Moreover, the decay of 4,4’-MDI present in high levels in all three substances is not affected by the presence of other constituents containing NCO groups. Under these homogeneous conditions with a solvent present, 4,4’-MDA way 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, MDA-DPG-MDA) were formed. The hydrolysis of the oligomers (3-ring MDI and MDI-DPG-MDI) appears to be slower than that observed for free MDI but is nevertheless still rapid i.e. less than 5 minutes. A closer analysis of the test results revealed that very likely homogeneous conditions were not achieved for the oligomers because of their higher hydrophobicity. Overall, all three boundary 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.

Key value for chemical safety assessment

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

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