3,3'-dimethylbiphenyl-4,4'-diyl diisocyanate

Biodegradation in water and sediment: simulation tests

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

biodegradation in water and sediment: simulation testing, other

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH
1. HYPOTHESIS FOR THE ANALOGUE APPROACH
Substances whose physico-chemical, toxicological and ecotoxicological properties are likely to be similar as a result of structural similarity may be considered as analogue substances. In this case, experimental data of one substance may provide indications for the (eco-) toxicological profile of another, based on a read-across approach [1]. In the present situation an “analogue approach” has been chosen because target and source substances share same chemical structures and functional groups. This read-across is based on the hypothesis that source and target substances have the same type of (eco)toxicological effects based on common underlying mechanisms. This prediction is supported by physicochemical and toxicological data on the substances.
The target substance 3,3'-dimethylbiphenyl-4,4'-diyl diisocyanate (TODI, CAS 91-97-4) and the source substance 4,4'-methylenediphenyl diisocyanate (MDI, CAS 101-68-8) are both diisocyanates. The basic structures of the target substance TODI and the source substance MDI are the same. The common feature to all diisocyanates is that they consist of two N=C=O (isocyanate) functional groups attached to an aromatic parent compound. TODI consists of two additional methyl groups on the aromatic ring system.
Therefore, the read-across approach from the existing ecotoxicity studies and studies on environmental fate of the source substance is considered as an appropriate adaptation to the standard information requirements of the REACH Regulation for the target substance, in accordance with the provisions of Annex XI, 1.5 of the REACH Regulation.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
In regard to physico-chemical properties target and source substances are comparable. Both substances are organic solids. The target substance has a molecular weight of 264.27 g/mol. Its calculated vapor pressure is 0.0029 Pa at 25°C and the calculated log Pow is 6.05 at 25°C. The typical purity of the substance ≥99.8%. The impurities are ≤ 0.2%. The experimental determination of the water solubility was technically not feasible, because of the very fast hydrolysis of the substance. The estimated value is 0.59 mg/L at 25°C.
The source substance 4,4'-methylenediphenyl diisocyanate has a molecular weight of 250 g/mol. It has a vapor pressure of 0.001 Pa at 20°C and a measured log Pow of 4.51 at 22°C. The purity is considered to be ≥ 99%. Measuring the water solubility of is technically not feasible due to the low solubility and high reactivity with water. The estimated value for water solubility is 0.03 mg/L at 22 °C [2].
When biodegradation or ecotoxicological studies are conducted, TODI and MDI are expected to be rapidly hydrolyzed and polymerize to their transformation products (mainly insoluble polyurea) immediately after the initiation of the studies. Therefore, the study results for TODI/MDI substances reflect the biodegradability or ecotoxicological effects of their transformation products. The main common transformation compounds are polyureas, which are not expected to be biodegradable in the environment or toxic to fish, algae or microorganisms. To a minor extent the diamines TODA (3,3'-dimethylbiphenyl-4,4'-diamine, CAS 119-93-7) and MDA (4,4'-methylenedianiline, CAS 101-77-9) are formed which are not readily biodegradable. It was shown that the hydrolysis half-life of both diisocyanates at pH 7 and 37°C are in the same range (>4h).
3. ANALOGUE APPROACH JUSTIFICATION
This read-across is based on the hypothesis that source and target substances have the same type of (eco)toxicological effects based on common underlying mechanisms. This prediction is supported by physicochemical and toxicological data on the substances. Both target and source substances show a low oral and dermal acute toxicity. Both substances are classified for acute inhalation toxicity Cat. 4, for skin sensitization Cat.1 and for respiratory sensitization Cat.1. The target substance is not irritating to the eyes or skin. The source substance is irritative to skin, eyes and respiratory tract. TODI and MDI are not mutagenic, both are harmonized classified for Carc. 2. The target substance is classified for STOT RE Cat.2 after inhalation exposure. The target and source substance are not classified for reproductive or developmental toxicity.
No bioaccumulation is expected for both substances and its degradation products. TODI and MDI are not identified as PBT substances. No adverse effects have been reported for MDI in acute study in fish, algae, aquatic invertebrates and microorganism. Due to the rapid hydrolysis, actual exposure to these test organisms was predominantly the effect of formed polyureas [OECD SIDS, 2003]. The same is expected for exposure to TODI. TODI as such is hardly soluble in water, therefore Dimethylformamide was used as auxiliary solvent / dispersant in the acute toxicity test to fish, to aquatic invertebrate and to aquatic algae, respectively. Moreover, TODI showed to be highly unstable in aqueous media, with hydrolysis occurring virtually instantaneously on contact with water and the degradation product was found to be insoluble in organic and aqueous solvents. That means that (1.) the acute toxicity tests represent the worst case as the solubility in water was increased by an auxiliary solvent and (2.) a chronic exposure to the TODI, respectively to its degradation product is negligible due to the insolubility of both if no auxiliary solvent is used. On the other hand, the insolubility in water or aqueous solvents and the missing ready biodegradability of TODI (not so relevant because of rapid hydrolysis) and its degradation product and the calculated log Kow of 6.05 for TODI may indicate and trigger H413 classification for precautionary reasons. However, the estimated log Kow of the hydrolysis product TODA (log Kow= 2.34 - 3 .02), as well as the experimentally BCF of TODA (BCF < 83) does not indicate long-lasting effects in the aquatic compartment.
In conclusion, read-across from existing studies on the source substances and the degradation products are considered as an appropriate adaptation to the standard information requirements of the REACH Regulation for the target substance, in accordance with the provisions of Annex XI, section 1.5 of the REACH Regulation.
4. DATA MATRIX
ECHA (2017) Read-Across Assessment Framework (RAAF)
ECHA Homepage, disseminated dossier, 4,4'-methylenediphenyl diisocyanate (MDI), CAS 101-68-8
OECD SIDS, Initial Assessment Profile, Methylenediphenyl diisocyanate (‘MDI’), 11-14 November 2003

Reason / purpose for cross-reference:

read-across source

Remarks on result:

other: No %Degr. indicated. Based on the short half-life in water, the substance is essentially unavailable for biodegradation.

Reference 2

Endpoint:

biodegradation in water: sediment simulation testing

Data waiving:

exposure considerations

Justification for data waiving:

the study does not need to be conducted because direct and indirect exposure of sediment is unlikely

Transformation products:

not specified

Reference 3

Endpoint:

biodegradation in water and sediment: simulation testing, other

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

test procedure in accordance with national standard methods with acceptable restrictions

Qualifier:

according to guideline

Guideline:

other: OECD Guideline 302 C (Inherently Biodegradability)

Key result

Remarks on result:

other: No %Degr. indicated. Based on the short half-life in water, the substance is essentially unavailable for biodegradation.

Methylenediphenyl diisocyanate (MDI) is inherently reactive in water, with a predicted half-life in solution of a minute or less. Because of this high reactivity, MDI can have only a transient existence in water, and as such, is essentially unavailable for uptake and bioaccumulation or for biodegradation.

Conclusions:

By applying published data of a supporting substance the degradation in surface water is not relevant for TODI. TODI has - based on the analoge behavoiour of MDI - only a transient existence in water, and as such, is essentially unavailable for uptake and bioaccumulation or for biodegradation.

Executive summary:

TODI is highly insoluble in water. Once in contact with water, TODI hydrolyses quite fast. Resulting degradation products are considered to be inert and insoluble polyureas. Thus, neither TODI nor its degradation products are available for uptake or bioaccumulation or for biodegradation.

Reference 4

Endpoint:

biodegradation in water: simulation testing on ultimate degradation in surface water

Data waiving:

study technically not feasible

Justification for data waiving:

the study does not need to be conducted because the substance is highly insoluble in water

Transformation products:

not specified

Description of key information

The study does not need to be conducted because TODI is highly insoluble in water. Additionally, once in contact with water, TODI hydrolyses quite fast. Resulting degradation products are considered to be inert and insoluble in water. Thus, neither TODI nor its degradation products are available for uptake or bioaccumulation or for biodegradation.