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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: <1 min at pH 4, 7 and 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Additional information

Hydrolysis half-lives of approximately 0.2 minutes at pH 4, 0.3 minute at pH 7 and 0.1 minutes at pH 9 and 1.5°C were determined for the registered substance in accordance with OECD 111 (Miller and Kozerski 2001). The result is considered to be reliable and selected as key study. Similarly, hydrolysis studies conducted according to OECD 111 are available for other related dichlorosilane substances. These substances are fully hydrolysed within a few minutes at pH 4, 7 and 9 and 1.5°C. These are used as supporting data in the context of evidence from other available data for chlorosilane structural analogues.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at pH 7 and increase as the pH is raised or lowered. Therefore, the half-life at pH 2 (relevant for oral exposure), 4, 5.5 (relevant for dermal exposure) and 9 is expected to be <0.2 minutes.

The available half-lives were all measured at 1.5ºC. Reaction rate increases with temperature and the half-lives at 25ºC (the standard temperature used in OECD 111) studies and 37.5ºC (relevant for in vivo studies) are expected to be faster.

The above hydrolysis studies were carried out with the substance dissolved in water. Consideration of the rates of reaction with moisture in air is relevant for inhalation exposure. Experience in handling and use, as well as the extremely rapid rates observed in the available water-based studies, would suggest that rates of reaction in moist air will also be rapid. If any unreacted chlorosilane were to reach the airways, it would rapidly hydrolyse in this very moist environment.

A simulated nose-only exposure study (Houghton 2013) has been conducted to determine hydrolytic stability of dichloro(dimethyl)silane under conditions typical of nose-only vapour inhalation exposures. The vapour generation was on 1 day for 3 hrs 14 minutes; concentrations of parent material were measured at 30 minute intervals using gas chromatography (GC). The nominal concentration was 51 ppm. The mean temperature was 21.6°C and the relative humidity (RH) was 57%. 24% parent concentration remaining in the test atmosphere relative to nominal concentration was measured by GC. This indicates 76% hydrolysis of the parent substance had taken place by the time the test atmosphere reached the GC. It was concluded that at least 20-29% of the parent test article would be present in the breathing zone relative to the nominal concentration under typical conditions used for nose-only inhalation exposure of rats. It is therefore possible to expose rats in a nose-only study to parent chlorosilane, because the transit time from the substance reservoir to the nose is very rapid (<1 second), however, this is not considered to be representative of human exposure conditions.

The authors of this summary have used the information from this study to estimate a half-life for dichloro(dimethyl)silane in air of approximately 7 seconds (95% confidence limit = 3 -11 seconds), which is comparable to the half-life in water.

In a study of the acute toxicity to rats via the inhalation route (Dow Corning Corporation 1997), dimethyldichlorosilane was quantified in the exposure chamber using TCD detection and identification was confirmed using GC/MS. There was only about 15% of the nominal concentration of dimethyldichlorosilane left in the exposure chamber.  The test atmosphere contained an amount of chloride consistent with the nominal concentration of test article as determined via electrochemical detection.  Thus the majority of parent had hydrolysed in the test atmosphere at only 35% relative humidity.

Similarly, in a study to assessed stability of dimethyldichlorosilane vapour in air using gas-sampling FTIR (Dow Corning 2009), dimethyldichlorosilane was observed to be exteremly unstable in high relative humidity atmospheres. At 75% relative humidity level, a stable test atmosphere of the substance could not be generated. Also, in dry air, the substance had achieved 28% loss after 1 hour and 71% loss after 3.2 hours.

The significant extent of chlorosilane hydrolysis demonstrated in the studies with dimethyldichlorosilane is in good agreement with the theoretical capacity for hydrolysis in air under conditions typical of a rat repeated exposure test. Theoretically, air at 20°C and 50% relative humidity would have more than 100 times the amount of water necessary for complete hydrolysis of dichloro(dimethyl)silane:

Water content of air at 20°C = 17.3 g/m3  (100% humidity)

Assuming a 50% humidity = 8.65g water/m3= 8.65 mg water/l

Molecular weight of water = 18 g/mole; So 8.65 mg water/l = 0.48 mmol water/l

50 ppm HCl is the estimated upper exposure limit based on HCl corrosivity for a repeated exposure test

As dichloro(dimethyl)silane has 2 Cl groups this would be equivalent to 25 ppm HCl

Molecular weight of dichloro(dimethyl)silane= 129.06 g/mol

25 ppm dichloro(dimethyl)silane is equivalent to 132 mg/m3 or 0.001 mmol/l

 

Therefore, it can be concluded that the registered substance will hydrolyse very rapidly under conditions relevant for environmental and human health risk assessment. Additional information is given in a supporting report (PFA 2013ab) attached in section 13 of the REACH technical dossier.

The hydrolysis products of the registered substance are dimethylsilanediol and hydrochloric acid.

Hydrolysis of the read-across substance trimethoxy(methyl)silane(CAS 1185-55-3)

Data for the substance trimethoxy(methyl)silane(CAS 1185-55-3) are read-across to the submission substance dichloro(dimethyl)silane for appropriate endpoints (see Section 1.4).The silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections of the CSR for each endpoint.

For trimethoxy(methyl)silane, hydrolysis half-lives at 25°C of <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 were determined in accordance with OECD 111 (Miller J A 2004. Hydrolysis of methyltrimethoxysilane (CAS No: 1185-55-3).  Testing laboratory: Dow Corning Corporations. Owner company: SEHSC. Report no: 9877 -102. Report date: 2004-09-20).

The hydrolysis products are methylsilanetriol and methanol.

Hydrolysis of the read-across substance trichloro(ethyl)silane (CAS 115-21-9)

Data for the substance trichloro(ethyl)silane (CAS 115-21-9) are read-across to the submission substance dichloro(dimethyl)silane for appropriate endpoints (see Section 1.4).The silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections of the CSR for each endpoint.

For trichloro(ethyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were determined in accordance with OECD 111.(Miller and Kozerski 2001. Hydrolysis Screening Studies of HPV Chlorosilanes. Testing laboratory: Dow Corning Corporation, Midland, USA. Owner company: Silicones Environmental, Health and Safety Council of North America (SEHSC). Report number: 2001-I0000-50823. Report date: 2001-03-26).

The hydrolysis products are ethylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance trimethylsilanol (CAS 1066-40-6)

Data for the substance trimethylsilanol (CAS 1066-40-6) are read-across to the submission substance dichloro(dimethyl)silane for appropriate endpoints (see Section 1.4). The submission substance undergoes very rapid hydrolysis in water to produce dimethylsilanediol and hydrochloric acid.The silanol hydrolysis product of the submission substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

Trimethylsilanol is not expected to undergo hydrolytic degradation