Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

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. It is important to note that the MDI category approach for read-across of environmental and human hazards between the MDI substances belonging to the MDI category is work in progress under REACH. Therefore the read-across document should be considered a draft.

The test substances of the methylenediphenyl isocyanate category react quickly with water and hence will be rapidly hydrolysed upon contact with water, soil, sediment, etc.. The half-life for hydrolysis under heterogeneous reaction conditions is estimated to be 20 hours and in homogeneous solution is on the order of seconds. Hence, hydrolysis is considered to be the main removal mechanism in the different compartments. However, MDI is hydrophobic and poorly soluble in water and thus the heterogeneous reaction with water or soil is less rapid. The major product of such a reaction is a solid, insoluble polyurea. 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.

Due to the relatively low vapour pressures, and thus, low magnitude of vapour emission, for the MDI substances, atmospheric emission and degradation of vapour is considered to be of low importance for MDI substances. The half-life of 4,4'-MDI, based on predicted reaction with photochemically-generated hydroxyl redicals, has been estimated to be 0.92 days.

Phototransformation in water and soil for the test substance was assumed to be a negligible removal mechanism, compared to the predominant hydrolytic degradation pathways.

Biotic degradation is not considered to be the main removal mechanism. Due to the rapid reaction with water (hydrolysis T1/2 = 20 days), any attempt with biodegradation test to measure biological removal, will measure the biodegradation of the hydrolysis products. In an attempt with an inherent biodegradation test, it was found that PMDI (and mainly its hydrolysis products) is not readily biodegradable.

Due to the high reactivity of the substances of the MDI category with water, bioaccumulation tests can in principle not be performed with these substances. However, one bioaccumulation test (OECD 305E) with 4,4'-MDI and a mesocosm study with PMDI with an indication of bioaccumulation potential have been performed showing that MDI has no bioaccumulation potential in the aquatic environment.

No data are available on terrestrial bioaccumulation on any of the MDIs of the category approach. The reactivity of MDI with water, the low measured BCF in fish and the evidence of the mesocosm study, suggest that MDI has a low potential to bioaccumulate in terrestrial food-chains.

Due to the transient existence of MDI substances, only estimated values for water solubility and octanol-water partition coefficient (log Kow) can be determined and these numbers have little scientific value or relevance. This has been demonstrated for Toluenediisocyanate (TDI) which is similar to the MDI substances in structure and reactivity. A sensitivity analysis of the Level III fugacity model outputs for TDI showed that wide variation in model inputs such as water solubility , log Kow had negligible impact on the predicted environmental distribution and inter-media transport of such a reactive substance. This modeling analysis showed that, regardless of whether TDI (or in this case, MDI substances) are emitted directly to air, water, or soil, > 99.5% of the emission will remain and be degraded within the compartment to which emissions were made. Hence, the transport and distribution of the MDI substances are governed by their reactivity in environmental media, and properties such as water solubility, log Kow, and soil adsorption coefficient are of no real value and cannot be determined properly. No accumulation of the MDI substances is expected in any compartment.