Registration Dossier
Registration Dossier
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EC number: 208-765-4 | CAS number: 541-05-9
Endpoint summary
Administrative data
Description of key information
The registered substance, hexamethylcyclotrisiloxane (D3), is not stable in water, which affects the approach to the determination of physicochemical properties.
In contact with water, hexamethylcyclotrisiloxane hydrolyses rapidly (half-lives of 2 minutes at pH 4 and 25°C, 23 minutes at pH 7 and 25°C, and 0.4 minutes at pH 9 and 25°C). The stated half-lives are for the ring-opening of the parent substance due to hydrolysis. The products of this reaction are also unstable in water, and so further hydrolysis reactions will follow; the intermediate hydrolysis products, 1,1,3,3,5,5-hexamethyltrisiloxane-1,5-diol (CAS 3663-50-1; EC No. 222-920-3; L3-diol) and 1,1,3,3-tetramethyldisiloxane-1,3-diol (CAS 1118-15-6; EC No. 214-258-9; L2-diol), with the final product being dimethylsilanediol (DMSD, CAS 1066-42-8; EC No. 213-915-7). The half-life for the hydrolysis of the L3-diol and L2-diol is approximately 200 hours at pH 7 and 25°C.
Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the intermediate and final silanol hydrolysis products are assessed instead.
The intermediate and final silanol hydrolysis products, L3-diol and DMSD, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (a colloidal suspension of small solid particles known as a sol) that may over time form an insoluble gel. A dynamic equilibrium is established. The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution.
D3 is a solid at standard temperature and pressure, with a melting point of 64°C, and a boiling point of 135°C. It has a density greater than that of water (1.19 g/cm3 at 25°C) and is moderately volatile (vapour pressure of 671 Pa at 25°C). It has low water solubility (1.6 mg/l at 23°C) and high estimated log Kow (4.4).
The substance is classified as highly flammable on the basis of a burn rate test (burning time 0.8 s; burn rate 7.5 inch/s). It has a self-ignition temperature of 386°C, is not explosive and is not oxidising.
The condensation reactions of silanediols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. At higher loadings, cross-linking reactions between the cyclic tetramers may occur. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. At loadings below 1000 mg/l DMSD, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above about 1000 mg/l causing the formation of insoluble polymeric particles and gels over time. Condensation of the intermediate silanol hydrolysis products is also possible, but unlikely to be significant below 1000 mg/l. Further information is given in a supporting report (PFA 2016am) attached in Section 13 of the IUCLID dataset.
The intermediate silanol hydrolysis product L3-diol is also hydrophilic (1700 mg/l at 20-25°C using a QSAR method); it is assumed for the purposes of assessment that the solubility of the L3-diol is similarly limited by condensation reactions to approximately 1000 mg/l. It has a log Kow of 3.2 (QSAR) and a predicted vapour pressure of 0.014 Pa at 25°C.
The saturation concentration in water of the final silanol hydrolysis product, DMSD, is therefore limited by condensation reactions to approximately 1000 mg/l. However, it is very hydrophilic (calculated solubility is 1E+06 mg/l at 20°C using a QSAR method) with a low measured log Kow of -0.38. It is not surface active. The silanol hydrolysis product is less volatile than the parent substance (predicted vapour pressure of 7 Pa at 25°C).
Reference : PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems. Report no: PFA.404.105.003. Owner company: Reconsile. Report date: Feb 16, 2016
Additional information
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