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Environmental fate & pathways

Endpoint summary

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The test substance is covered by the category approach of methylenediphenyl diisocyanates (MDI). Hence, data of the category substances can be used to cover this endpoint. The read-across category justification document is attached in IUCLID section 13.

As a consequence of the chemical reactivity of the common aromatic isocyanate (NCO) functional group present on all substances of the MDI category and the high reactivity of this group with water, hydrolysis is the determinant of the fate of MDI substances in the environment. One of at least two aromatic isocyanate functional groups rapidly hydrolyses by dissolved water with the formation of inert polyurea. Formation of polyurea therefore represents the primary detoxification process in the environment under realistic (heterogeneous) environmental exposure scenarios. Two types of experimental studies were performed 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. 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 p articles. Under these conditions only trace levels of MDA are formed. In contrast, in studies performed under artificial homogeneous testing conditions (i.e. OECD 111) the source substance 4,4’-MDI is fully solubilized in water, and MDI is quantitatively converted to MDA. The formation of MDA only occurs under conditions where mMDI is first diluted into an aprotic organic solvent or is subjected to vigorous mixing which disperses the substances very effectively with artificially high specific surface area. Neither of these scenarios are representative of plausible realistic environmental exposure scenarios for any of the substances of the category. 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. Reliable hydrolysis data under homogeneous conditions are also 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 MDI substances collectively exhibit very low vapour pressure (< 0.1 Pa) and occur as either a dense crystalline solid or a very viscous liquid in their pure forms. Based on the vapour pressure data, there is very low potential for exposures to these substances in air unless aerosols are generated during spray applications or high-temperature processing operations. There are no measured photo-transformation data in air available for any of the MDI substances, but half-life in the atmosphere has been estimated using QSAR. Based on these QSAR estimationsit is predicted that the airborne MDI substance will have a rather short half-life as a consequence of ready degradation to inorganic compounds by hydroxyl radicals present in the troposphere. When considering both the low potential for atmospheric emission and the short half-life of MDI substances in this compartment, it is unlikely that significant amounts of the emitted substance will undergo deposition and further contaminate soil or water and sediment. Releases of the MDI substances to atmosphere might occur during production although they are expected to be low as exhaust gases produced during the manufacturing of MDI substances are treated by incineration or scrubbing. Where the fate of the MDI category substances in the atmosphere is of interest, their reaction with photochemically-generated hydroxyl radical is expected to be the most important process affecting their fate and lifetime. Reliable predictions of this reactivity are provided for all representative constituents of the category substances. Additional testing is therefore not suggested.
All substances of the MDI category possess aromatic isocyanate functional groups which are known to rapidly hydrolyse and polymerise to polyurea when added to water. Biodegradation is therefore not a relevant transformation process for the category substances. Nevertheless, biodegradation screening tests for the three boundary substances of the MDI category, 4,4’-MDI, pMDI and 4,4’-MDI/DPG/HMWP and Mixed isomers MDI/PIR have been conducted. When biodegradation studies are conducted on MDI substances, these substances are expected to be rapidly hydrolysed/polymerised to their transformation products (mainly insoluble polyurea) immediately after the initiation of the biodegradation studies. Therefore, the biodegradation study results for MDI substances reflect the biodegradability of their transformation products and not necessarily of the MDI substance constituents themselves. The main common (transformation) compounds (i.e. polyurea) are chemically “inert” and are not expected to be biodegradable in the environment. The minor common (transient) transformation compound MDA is shown in various simulation tests to be not persistent in soil and aquatic/sediment systems. Biodegradability of 4,4’-MDI and 4,4’-MDA mono-ureas was also investigated and not considered biodegradable.

The registrant will perform additional OECD 307 studies. The data will be generated on the two boundary substances of the category 4,4'-MDI/DPG/HMWP as well as pMDI. Four soils will be used, to demonstrate NER formation and identity of any "soluble" degradation products. Testing proposals for the OECD 307 studies with these two substances are added to the respective dossiers. A category approach will be used to fulfill the data requirement for the registered substance.

The substances of the MDI category hydrolyze immediately in contact with water to form polyurea. According to column 2 of Annex IX of the REACH regulation, studies on bioaccumulation do not need to be conducted if the substance has a low potential for bioaccumulation (i.e. an log Pow value equal or smaller than 3) and/or a low potential to cross biological membranes. Supporting evidence is provided by a fish bioaccumulation test, performed on 4,4'-MDI (CERI, 2002), in which no significant bioaccumulation has been observed. Though of limited meaning due to the instability of the test item in water, the study points to the absence of a potential for bioaccumulation. Furthermore, also in a mesocosm study with pMDI (Heimbach, 1993), no signs for accumulation of the test item in aquatic biota have been observed. For the minor common transformation compound MDA, a log Pow value of 1.55 and a BCF value below 15 L/kg has been determined (CITI, 1992; Macnab, 1999) indicating no or a low bioaccumulation potential. One (transient) mono-urea, represented by the diamino-monourea of MDA for which an experimental bioconcentration study (OECD 319A) (Ebersbach and Windisch, 2019) has been performed, did not indicate any bioaccumulation potential. No experimental data are available on the common transformation compound, polyurea of MDI, but these are not considered bioavailable based on their high hydrophobicity and large molecular weight and size and hence, have no potential to cross biological membranes. Based on these evidences, all MDI category substances are predicted to have low bioaccumulation potential in analogy to 4,4’-MDI and pMDI. Sufficient evidence is available to conclude that bioaccumulation studies are not warranted for the other substances of the MDI category. Nor the MDI substances, nor their hydrolysis products are considered B, therefore it can be concluded that the PBT and vPvB criteria are not fulfilled. The read across to the other substances of the category and the assessment are therefore conducted with high level of confidence.

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