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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: <<1 hour at pH 4, 7 and 9 and 25°C (estimation)

Key value for chemical safety assessment

Half-life for hydrolysis:
1 min
at the temperature of:
25 °C

Additional information

In a preliminary study conducted in accordance with OECD 111 test method (Weissenfield 2005), tri(isopropyl)silyl acrylate was observed to hydrolyse immediately in aqueous buffer solutions. In order to ascertain complete dissolution of the test substance in solution, the hydrolysis product, tris(1-methylethyl)silanol (TIPSOH) was analysed using GC-FID. 100% hydrolysis reaction was observed after 2.4 hours incubation and no traces of the parent substance tri(isopropyl)silyl acrylate was observed even in the initial test solutions (2 ml samples taken by syringe, extracted in acetone and analysed by GC-FID, the elapsed time between sampling and solvent extraction was not reported) at pH 4, pH 7 and pH 9 and 50°C.

Reliable studies according to OECD 111 are available for two related triacetoxysilane structural analogues (Dow Corning Corporation 2001). The acetate groups are fully hydrolysed within seconds at pH 4, pH 7 and pH 9 and 25°C. The measured half-lives at pH 4, pH 7 and pH 9 and 25ºC are all limit values.

The existing data for the hydrolysis of the two structural acetoxysilanes analogues have been evaluated. The evaluation looks into the primary effects (type of bond to Si) and into the secondary effects (electronic and steric effect of R-groups) and a QSAR model for alkoxysilanes. These factors are used to extrapolate the effect of secondary factors for the half-life prediction for tri(isopropyl)silyl acrylate.

The available evidence suggests that a substance which has the structural feature Si-O-C(O) CH2R such as tri(isopropyl)silyl acrylate will hydrolyse extremely rapidly (half-life significantly less than 1 hour) at pH 7 and 25˚C to Si-OH and the corresponding carboxylic acid. No quantitative predictions have been performed for other pH values, though it can be stated that under these conditions, the hydrolytic half-life is expected to even be shorter than that at pH 7. At higher temperatures (relevant for in vivo conditions), the hydrolytic half-life is expected to be even shorter than that at 25°C.

The work is described in the expert report (PFA, 2014) attached to the Key Endpoint Study Record in Section 5.1.2 of the REACH technical dossier.

It is not possible to attempt a quantitative prediction of rate or half-life, but there is no need to do so as the chemical safety assessment is not sensitive to this uncertainty within this range.

The hydrolysis of Si-O bonds is acid- or base catalysed. The above discussion investigates the likely half-life of the compound at pH 7 and 25°C. The rate of reaction is expected to be slowest at pH 7 and increase as the pH is raised or lowered. Therefore, the hydrolysis half-life at pH 4 and 25°C and at pH 9 and 25°C is expected to be significantly lower than 1 hour. The rate of reaction also increases with temperature. Therefore, at 37.5ºC and pH 7 (relevant for lungs and blood and in vitro and in vivo (intraperitoneal administration) assays), at 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), and at 37.5ºC and pH 5.5 (relevant for dermal exposure), the half-life for hydrolysis is expected to be far less than1 hour.

No quantitative predictions or extrapolations are made for the substance at these pH values and temperature, because the predicted half-life for pH 7 and 25°C is already significantly below 1 hour, and the accuracy of the prediction does not allow the half-life to be predicted with more accuracy than this.

Additional information is given in a supporting report (PFA 2013ac) attached in Section 13.

The other reported study (Dow Corning Corporation 2003) was not designed to yield a quantitative measure of hydrolysis half-life at pH 4, 7 and 9. An in-vitro study of the test substance was conducted at pH 1.29 and 37°C. The relative molecular weight distribution (MWD) of the hydrolysis and condensation products was determined using a Gel Permeation Chromatography (GPC). Characterisation of both the hydrolysis and condensation products was determined using both Nuclear Magnetic Resonance (NMR) and Electrospray Ionisation Mass Spectrometry (ESI-MS). At pH 1.29 and 37°C, the test substance were observed to hydrolyse very rapidly and the hydrolysis products from the test substance underwent continuous, condensation reactions to produce higher molecular weight cyclic and linear siloxanes


The products of hydrolysis for tri(isopropyl)silyl acrylate are tris(1-methylethyl)silanol and acrylic acid.

Reference:

PFA (2013ac) Peter Fisk Associates, Hydrolysis: Type 1b. Acetoxysilanes analogue group.