m-tolylidene diisocyanate

Endpoint summary

Additional information

General discussion of environmental fate and pathways:

TDI reacts 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 under one minute and in homogeneous solution is on the order of seconds. Hence, hydrolysis is considered to be the main removal mechanism in the different compartments.

In general, the reaction product of an isocyanate group with water is a primary amino group and carbon dioxide. Since the primary amino group is a much stronger nucleophile than water, the amino group will immediately react with another isocyanate group to yield an urea group. The resulting urea group has two secondary amino groups, which also can react with further isocyanate groups to form cross linkages. This process results in the formation of hard, crusty, insoluble and inert polyurea.

For TDI, the rapid reaction with water leads to transient TDA. However, due to its strong nucleophilicity, intermediate TDA will immediately react with further isocyanate groups. Consequently, as long as there are free isocyanate groups available, temporarily generated TDA will instantly disappear from the medium. TDA can only be detected during the hydrolysis of TDI at very low TDI loadings (less than 10 mg/L or lower) and when efficient stirring of TDI into water is applied. Generally, the rate of reaction of 2,4-TDI was markedly dependent on stirring energy with half-lives at 27°C ranging from 30 sec with very efficient dispersion, to about an hour with moderate dispersion, and several days (dependent on surface area) when unstirred (Kitano et al., 1989, 1991; Yakabe et al., 1994, Yakabe et al., 1999). From the information provided above it can be concluded that no significant amounts of TDA are released from the hydrolysis of TDI under environmentally relevant conditions.

Hydrolysis of TDI leads to the formation of a solid crust of polyureas encasing unreacted material. This crust restricts ingress of water and egress of amine, and thereby slows hydrolysis and enhances the amine reaction with isocyanate, leading to an even higher yield of polyureas. Both reactions, the hydrolysis of TDI and the formation of polyurea are exothermic. The resulting reaction heat will stays within the encapsulating shell of polyurea. This further accelerates the hydrolysis of TDI and the fast build-up of polyurea. Since the latter is generated from the inside, the entire reaction can best be characterized as “self-annealing” process.

Due to the relatively low vapour pressures, and thus, low magnitude of vapour emission, for the TDI substances, atmospheric emission and degradation of vapour is considered to be of low importance for TDI substances. The half-life of 2,4-TDI, based on predicted reaction with photochemically-generated hydroxyl radicals, has been estimated to be 2days.

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 a relevant removal mechanism. Due to the rapid reaction with water (hydrolysis T1/2 = under one minute) resulting in inert polyurea, any attempt to measure biological removal with biodegradation tests will not deliver meaningful results.

Like for other isocyanates, sound bioaccumulation studies cannot be performed with TDI due to its high reactivity with water. However, one bioaccumulation test (OECD 305E) with 2,4-TDI and a mesocosm study with PMDI have been performed. These did not show any potential for bioaccumulation of the substances investigated. This supports the conclusion that isocyanates in general, including TDI, do not bioaccumulate in the aquatic environment.

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

Due to the transient existence of TDI 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. 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 is emitted directly to air, water, or soil, > 99.5% of the emission will remain within the compartment to which they were released. Hence, the (absent) transport and distribution of the TDI 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 TDI substances is expected in any compartment.