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Sediment toxicity

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Description of key information

Toxicity to sediment organisms i not expected.

Key value for chemical safety assessment

Additional information

No experimental data is available on the sediment toxicity of UMA 121. However, based on the molecular structure and intrinsic properties of the substance, sediment is of minor relevance as target compartment for UMA 121 and no significant toxicity to sediment organisms is expected.

UMA 121 is the reaction product of the following substances:

- Methylendiphenyldiisocyanate, isomer mixture of 4,4’- and 4,2’-MDI (MDI) CAS No. 26447-40-5

- Dipropylene glycol (DPG) CAS No. 25265-71-8

- 2 -hydroxypropylmethacrylate (HPMA) CAS No. 27813-02-1

The reaction product is a solid resin containing the polymer HPMA-MDI-(DPG-MDI)n-HPMA as homologues of n = 0, 1, 2, 3, etc. (no upper limit defined) well as free HPMA.

The water solubility of UMA 121 was determined to be 0.4 mg/L in an OECD 105 study. However, taking into account that the water solubility is 130 g/L for HPMA (ECHA website, Registered substances) and 0.0002 - 0.002 mg/L for the shortest UMA homologue (n=0) (WSKOW v1.42 and WATERNT v1.01), it can be assumed that the 0.4 mg/L solubility is mainly due to HPMA and that the dissolution of UMA homologues is negligible. Additionally, it should be noted that the test substance was pestled to a powder for the purpose of the water solubility study. The resulting solubility value thus represents an absolute worst case compared to the water solubility of the substance in its normal resin form.

Release of UMA 121 to surface waters, and thereby exposure of sediment, is unlikely, as it will be removed in sewage treatment plants to a high extent. As stated in the “Guidance on information requirements and chemical safety assessment Chapter R.7b: Endpoint specific guidance”, insoluble chemicals will be removed in the primary settling tank or fat trap of sewage treatment plants (ECHA, 2012). Possible remaining amounts would be adsorbed to the activated sludge. However, as discussed above HPMA is far more soluble than the rest of the constituents and might thus dissolve into the water phase. But due to its ready biodegradability it would most likely be removed through biodegradation by activated sludge (ECHA website, Registered substances).

If UMA 121 were to be released into the aquatic environment, HPMA might dissolve into the aqueous phase. The UMA homologues (n=0 to n=∞) are poorly soluble in water and have high log Kow (> 5; KOWWIN v1.68) and log Koc values (> 5; KOCWIN v2.00). Apart from a possible release of HPMA, UMA 121 is thus expected to remain stable and inert in the aqueous environment. The toxic potential of HPMA and the UMA homologues is thus discussed separately below.

HPMA:

Due to its high solubility, HPMA may be released from UMA 121 into water. However, due to its high solubility and low log Kow, HPMA is likely to partition into the aqueous phase rather than sediment. Additionally, experimental data available on the toxicity of HPMA to aquatic organisms indicate low toxicity. No effects were observed at concentrations below 100 mg/L in acute studies with fish and Daphnia. For algae, both NOEC and ErC50 were > 97 mg/L (measured; nominal 100 mg/L). In a chronic study with Daphnia a NOEC of 42.2 mg/L was obtained.

UMA homologues (n = 0, 1, 2, 3, etc.):

Due to their low solubility in water (< 0.002 mg/L, WSKOW v1.42 and WATERNT v1.01), UMA homologues are not expected to be released from UMA 121 under environmental conditions. Even if minor amounts were to be released into natural waters, their uptake by organisms is considered unlikely. The smallest homologue HPMA-MDI-HPMA (n=0) has a molecular weight of 538.60 g/mol. For the larger homologues (n≥1) molecular weights are above 900 g/mol. All homologues have log Kow values above 5 (KOWWIN v1.68). These constituents are therefore unlikely to cross biological membranes. According to Lipinski’s “rule of 5”, developed to identify drug candidates with poor oral absorption, a molecular weight > 500 g/mol and a log Kow > 5 indicate a low potential of absorption after oral uptake. Toxicity to sediment organisms is thus unlikely due to minimal exposure via water and improbable uptake by organisms.

Furthermore, a Combined Repeated Dose Toxicity Study with the Reproduction / Developmental Toxicity Screening Test (OECD 422) conducted with UMA 121 did not reveal any test substance-related effect (Takawale, 2013). As stated in the “Guidance on information requirements and chemical safety assessment Chapter R.11: PBT Assessment”, the complete absence of effects in mammalian long-term studies is an indication that the compound is either chronically non-toxic and/or that it is not taken up to a significant extent.

In conclusion, UMA 121 is not expected to be of concern to sediment organisms, as it will remain inert in the aquatic environment, including sediment, except for possible release of HPMA, which will partition into the aqueous phase and is thus irrelevant for sediment organisms.