2,4,6-trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane

Endpoint summary

Description of key information

2,4,6-Trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane (F-D3) is a mono-constituent substance where the main constituent (trans geometric isomer) is present at concentrations≥72% w/w, and a named impurity (the cis geometric isomer) may be present at concentrations ≥0% -≤28% w/w.

F-D3 may be a solid or liquid substance at room temperature depending on the proportion of the geometric isomers. The melting point is influenced by the isomer in highest proportion; the trans form (99.5% purity) has a measured melting point of 35°C, when the form is 96% cis, the melting point is -1.9°C and at 85% cis, the melting point is approximately -7°C.

The substance has a predicted boiling point of 190°C and a predicted density of 1.2 g/cm³ at 20°C. F-D3 has a measured vapour pressure of 75.6 Pa and 88 Pa at 20°C and 25°C respectively and a predicted kinematic viscosity of 3.3 mm²/s at 20°C.

The substance is a liquid or low melting point solid depending on the ratio of the geometric isomers present. The flammability is therefore assessed based on available flash point data. F-D3 is not classified as a flammable liquid according to Regulation (EC) No. 1272/2008, on the basis of a measured flash point of 124°C at 100.6 kPa and predicted boiling point of 190°C. It has a measured auto-ignition temperature of 380°C at 1004.1 to 1021.1 hPa and is not explosive and not oxidising on the basis of structural examination.

F-D3 has a predicted log K_ow of 9 and predicted water solubility of 1.3E-06 mg/l at 20°C.

In contact with water, F-D3 reacts slowly (half-lives of >7.5 d at pH 5, 6 d at pH 7, 11 min at pH 9 and 25°C). The half-lives described are for the removal of parent substance by ring-opening. Complete reaction to the ultimate end products would take longer. 1,5-Dihydroxy-1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)trisiloxane and 1,3-tris[(3,3,3-trifluoropropyl)methyl]disiloxane-1,3-diol are the postulated intermediate hydrolysis products, while 3,3,3-trifluoropropylmethysilanediol is the final hydrolysis product, according to the following series of reaction:

[-Si(CH₃)(CH₂CH₂CF₃)O-]₃+ H₂O →   HOSi(CH₂CH₂CF₃)(CH₃)OSi(CH₂CH₂CF₃)(CH₃)OSi(CH₂CH₂CF₃)(CH₃)OH

HOSi(CH₂CH₂CF₃)(CH₃)OSi(CH₂CH₂CF₃)(CH₃)OSi(CH₂CH₂CF₃)(CH₃)OH + H₂O → HOSi(CH₂CH₂CF₃)(CH₃)OSi(CH₂CH₂CF₃)(CH₃)(OH) + CH₃Si(CH₂CH₂CF₃)(OH)₂

HOSi(CH₂CH₂CF₃)(CH₃)OSi(CH₂CH₂CF₃)(CH₃)(OH) + H₂O → 2CH₃Si(CH₂CH₂CF₃)(OH)₂

Silanediols may undergo condensation reactions to give siloxane dimers, oligomers and polymers, according to the scheme:

RR’Si(OH)2

RR’Si(OH)OSi(OH)RR’

RR’Si(OH)O[SiRR’(OH)O]nSi(OH)RR’

(where R is an alkyl or aryl side-chain)

The silanol hydrolysis product may undergo condensation reactions in solution to give siloxane dimers, and linear and cyclic oligomers. 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.

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. 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 above about 30 mg/l the concentration of the cyclic tetramer of the silanol hydrolysis product is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the cyclic tetramer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2013am) attached in Section 13.