Registration Dossier
Registration Dossier
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EC number: 242-177-9 | CAS number: 18297-63-7
Endpoint summary
Administrative data
Description of key information
Additional information
The registered (parent) substance 1,3-bis(trimethylsilyl)urea (CAS No. 18297-63-7) is highly unstable in water due to hydrolysis. Therefore, its water solubility is not measurable. However, the substance is described as poorly water soluble in the available aquatic toxicity studies. Besides water solubility, the substance is characterized by a low log Kow of 2.72 (OECD 117) and a low vapour pressure of 8.35E-03 Pa (OECD 104), indicating a low potential for volatility.
The substance is not readily biodegradable (OECD 301 F) but hydrolyses rapidly in contact with water under environmentally relevant conditions (DT50 < 10 min, pH 4, 7 and 9, OECD 111). The resulting transformation products are the silanol hydrolysis product trimethylsilanol (CAS No. 1066-40-6) and urea (CAS No. 57-13-6).
According to Guidance R.16 (ECHA, 2016), environmental effects are likely attributed to the hydrolysis products rather than to the parent itself for substances where the hydrolytic half-life is below 12 h and these effects should also be assessed. Since the parent substance hydrolyses so rapidly, the environmental hazard and risk assessment are based on the hydrolysis products trimethylsilanol (TMS) and urea.
Based on the available data summarized below, the environmental fate and hazard profile of the registered substance is considered predominantly attributable to the silanol hydrolysis product trimethylsilanol, since urea is readily biodegradable and is of low, overall concern for environmental and human health (OECD SIDS, 2002).
Hydrolysis product 1: silanol hydrolysis product trimethylsilanol (CAS No. 1066-40-6)
The silanol hydrolysis product trimethylsilanol (TMS) is characterized by a relatively high, estimated water solubility of 1000 g/L (QSAR) and an estimated volatility of 1850 Pa (QSAR). In the available aquatic ecotoxicity studies, it is considered to be a stable degradant of the registered (parent) substance, which could serve as a suitable representative of the test item in the available experimental aquatic toxicity tests. Furthermore, TMS also dimerises, depending on its concentration (and other factors), to form hexamethyldisiloxane and water. However, this process was not observed under the testing conditions relevant to the available experimental guideline studies.
From the available biodegradation study, it can be concluded that TMS is not readily biodegradable since the parent substance hydrolyses within less than 1 h to form TMS and urea, the latter of which, in turn, is known to be ultimately biodegradable (OECD SIDS, 2002). TMS is further characterized by a low, estimated log Kow of 1.14 (QSAR), and a low estimated log Koc in the range of 0.99 to 1.64 (QSAR), indicating a low potential for bioaccumulation and adsorption to soil and sediment.
Hydrolysis product 2: urea (CAS No. 57-13-6)
Urea is an important endogenous product of mammalian metabolism and has low, overall environmental and human health hazard profiles according to the OECD SIDS (UNEP Publications, 2002).
In the environment, it is expected to primarily distribute into the water (99.84%) and air (0.16%) compartments, based on modeling results (Mackay level 1). However, according to worldwide use patterns, the highest environmental exposure is to soil because of its wide use as fertilizer. Environmental exposure by consumer use is considered insignificant (OECD SIDS, 2002).
Urea is characterized by good water solubility (545 g/L) and low vapour pressure (1.6E-03 Pa). Thus, it has a low potential for volatilization to the atmosphere (HLC = 4.4E-08 atm m3/mol, OECD SIDS, 2002). Urea is inherently biodegradable within 24 h (93 – 98%) and biodegrades into ammonia and bicarbonate. In natural waters, most ammonia appears in the form of ammonium, which is transformed into nitrate and nitrite through bacterial action. Therefore, usually its degradation products nitrate, nitrite and ammonium can be measured (OECD SIDS, 2002). Urea has a low potential for bioaccumulation (log Kow = -2.11) and a low potential for adsorption to soil and sediment (OECD SIDS, 2002).
References:
OECD SIDS (2002); Urea, CAS 57-13-6, SIAM 2 Final Assessment Report, UNEP Publications
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