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PBT assessment

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PBT assessment: overall result

PBT status:
the substance is not PBT / vPvB
Conclusion for the P criterion

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 (Chapter 2.4 of the CJD). 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 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. Based on experimental hydrolysis and indirect photolysis half-lives, the substances in the category are not considered to be persistent in the environment and are identified as not P. Based on the justification in the category approach, it is assumed that all MDI substances included in the category are not P. 


Conclusion for the B criterion

The substances of the MDI category have in common their high reactivity and unique combination of physico-chemical properties, which dictate a high degree of uniformity in their environmental fate and ecotoxicological effects. The same uniformity in reactivity and in physical-chemical properties will also govern their potential to bioaccumulate in biota and be advected between environmental compartments. The monomeric MDI constituents in the category substances represent the highest potential for exposure and mobility in the environment because of their lowest molecular weight and calculated octanol-water partition coefficient values of all possible constituents. Yet, studies of their bioconcentration in fish and mobility in soil show very low or no potential for accumulation and mobility in the environment. Considering the above, together with the fast hydrolysis of the MDI substances, there is no potential for significant bioaccumulation possible. Hence, the MDI substances do not fulfil the requirements for the B criterion and are identified as not B. Based on the justification in the category approach, it is assumed that all MDI category substances included in the category are not B. 


Conclusion for the T criterion

The acute aquatic toxicity potential of the substances of the MDI category is attributed to their potential to form transient MDA from transformation of their common mMDI constituents. The mMDI isomer and the 4,4'-MDI homopolymer substances have exhibited the lowest EL50 values of the category substances tested, as they have the highest (essentially 100 %) mMDI contents.  However, the water solubility limit of MDI is far above the criteria for T and on the basis of aquatic toxicity tests the MDI substances are identified as not T. However, according to Annex I of 67/548/EEC MDI is classified as STOT RE cat. 2, which automatically triggers a T. Based on this classification the MDI substances are identified as T.


Overall conclusion:

Generally, diisocyanates are known to hydrolyze readily in water to yield a carbamic acid intermediate, which decarboxylates to produce CO2 and an amine; the latter immediately reacts with more diisocyanate to yield a di-substituted urea as primary reaction product [Ullmann’s Encyclopedia of Industrial Chemistry]. Because the MDI substances are di-isocyanates, a nearly improbable and simultaneous hydrolysis of both isocyanate groups is necessary to form the diamine (e.g., Methylenedianiline; MDA), a minor hydrolysis product. The kinetically- and probability-favored urea formation reactions of this nature result in formation of polyurea solids which have high molecular weight, and are insoluble and non-reactive in the environment. Furthermore, and to a much lesser degree, also Mono-ureas of MDI (both NCO- and Amino-ended) are formed. This reaction dominates the environmental fate of diisocyanates, and therefore also impacts the hazardous properties of these substances as tested in the laboratory, and as observed in the environment. Approximated amounts in which the hydrolysis products of the MDI category members are occurring are: 



     > 90%


     approx. 8%

Mono-urea of MDI (both NCO- and Amino-ended); Methylenedianiline (MDA)

     approx. 1%

Details on the behavior of the substances of the methylenediphenyl diisocyanate (MDI) category in the aquatic environment are described in the Category Justification Document included in Section 13. Consequently, separate PBT assessments are provided for the following, relevant hydrolysis products: 

1. Polyurea 

2. Mono-ureas of MDI (both NCO- and Amino-ended) 

3. Methylenedianiline (MDA) 

No such assessment is performed for CO2, which is a naturally occurring gas. A PBT assessment is not required for those type of substances under Regulation (EC) No 1907/2006 (REACH). 

As demonstrated by the individual assessments of the hydrolysis products of the MDI category provided in the subsections under the PBT section, none of these reaction products is considered a PBT / vPvB substance.  Therefore, also the substances of the MDI category are not PBT / vPvB.