Reaction mass of 1,1,1,5,5,5-hexamethyl-3-phenyl-3-((trimethylsilyl)oxy)trisiloxane and 1,1,1,7,7,7-hexamethyl-3,5-diphenyl-3,5-bis[(trimethylsilyl)oxy]tetrasiloxane and 1,1,1,9,9,9-hexamethyl-3,5,7-triphenyl-3,5,7-tris((trimethylsilyl)oxy)pentasiloxane

Hydrolysis

Key value for chemical safety assessment

Half-life for hydrolysis:

630 h

at the temperature of:

25 °C

Additional information

The requirement to conduct a hydrolysis study for the registration substance is waived in accordance with Column 2 of REACH Annex VIII because the substance is insoluble in water. The constituents of the substance have predicted water solubility values of <1 mg/l.

The constituents of phenyl silsesquioxanes are branched siloxanes with three to eight silicon atoms connected by oxygen atoms. The Si-O bonds are susceptible to hydrolysis.

The hydrolysis half-lives for the constituents of the substance have been predicted using validated QSAR methods developed for siloxanes.

The major and minor linear constituents of the substance are hydrolytically unstable with half-lives for siloxane degradation of 630 – >63 000 h at pH 7 and 20-25°C. The QSAR descriptor values for linear constituents 3-7 are outside the descriptor range for Si-O (see QPRF attached to IUCLID Section 5.1 for further details). The side-chain types represented in the QMRF training set include hydrogen, methyl and vinyl groups. Phenyl groups are not represented. However, the predictions are considered sufficient to indicate that the hydrolysis half-lives of these constituents are long.

The minor cyclic constituents of the substance are also expected to be hydrolytically unstable. Hydrolysis half-lives for the ring-opening step are read-across from the structurally analogous methyl-substituted cyclic siloxane, and range from 0.4 - >10,000 h. Subsequent hydrolysis half-lives of the ring-opened structure are predicted to be of ≥10,000 h at pH 7 and 20-25°C

The reaction rate is faster under acidic or basic conditions and half-lives at 20-25°C of 0.4 h to >10000 h at pH 4 and 0.1 - >10000 h at pH 9 are predicted for hydrolysis of the siloxane bond. The results are reported in Table 4.1.2. These half-lives refer to the hydrolysis of the first siloxane bond; complete hydrolysis to the ultimate hydrolysis products will take longer.

The ultimate hydrolysis products of the reaction are phenylsilanetriol and trimethylsilanol.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

k_obs= k₀+ k_H3O+[H₃O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

k_obs≈k_H3O+[H₃O⁺]

At pH 4 [H₃O⁺] =10^-4mol dm^-3and at pH 2 [H₃O⁺] =10^-2mol dm^-3; therefore, k_obsat pH 2 should be approximately 100 times greater than k_obsat pH 4.

The half-life of a substance at pH 2 is calculated based on:

t_1/2(pH 2) = t_1/2(pH 4) / 100

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT₅₀(XºC) = DT₅₀(T) * e^{(0.08. (T-X))}

Where T = temperature for which data are available and X = target temperature.

The half-lives at 37.5ºC and pH 7 (relevant for lungs and blood) and pH 2 and 37.5°C (relevant for conditions in the stomach following oral exposure) are calculated as shown in Table 4.1.2. At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 4 and pH 7.

Table: Hydrolysis half-lives in hours for the constituents of the registration substance

	Structural formula of constituent	Half-life at pH 7 and 25°C (h)	Half-life at pH 4 and 25°C (h)	Half-life at pH 9 and 25°C (h)	Half-life at pH 2 and 37°C (h)
Major linear constituents
Constituent 1	Monomer (OSiMe3)3SiPh	630	4	5	0.01
Constituent 2	Dimer (Me3SiO)2PhSiOSiPh(OSiMe3)2	6300	12	37	0.04
Constituent 3	Trimer Ph(Me3SiO)2Si(OSi(OSiMe3)Ph)OSi(OSiMe3)2Ph	63000	41	250	0.15
Constituent 4	Tetramer Ph(Me3SiO)2Si(OSi(OSiMe3)Ph)2OSi(OSiMe3)2Ph	>63000	139	1750	0.5
Constituent 5	Pentamer Ph(Me3SiO)2Si(OSi(OSiMe3)Ph)3OSi(OSiMe3)2Ph	>63000	472	>10000	1.7
Minor linear constituents
Constituent 6	Ph(Me3SiO)2Si(OSi(OSiMe3)Ph)4OSi(OSiMe3)2Ph	>63000	1600	>10000	5.9
Constituent 7	Ph(Me3SiO)2Si(OSi(OSiMe3)Ph)5OSi(OSiMe3)2Ph	>63000	5400	>10000	20

 

Minor cyclic constituents
	Structural formula of constituent	Half-life for ring-opening at 25°C / h (result at pH 7 based on read-across from analogous cyclic siloxane)			Predicted half-life for hydrolysis of ring-opened structure at 25°C / h
		pH 7	pH 4	pH 9	pH 7	pH 4	pH 9
Constituent C3	Ph(Me3SiO)Si(OSi(OSiMe3)Ph)OSi(OSiMe3)Ph	0.4	0.1	0.1	>10,000	22	96
Constituent C4	Ph(Me3SiO)Si(OSi(OSiMe3)Ph)2OSi(OSiMe3)Ph	69-144	1.3	1.1	>10,000	76	664
Constituent C5	Ph(Me3SiO)Si(OSi(OSiMe3)Ph)3OSi(OSiMe3)Ph	1590	5.8	11.5	>10,000	256	4590
Constituent C6	Ph(Me3SiO)Si(OSi(OSiMe3)Ph)4OSi(OSiMe3)Ph	~9600	15.2	51.9	>10,000	870	>10000
Constituent C7	Ph(Me3SiO)Si(OSi(OSiMe3)Ph)5OSi(OSiMe3)Ph	>10,000	>15.5	>53.7	>10,000	2950	>10000

 

Hydrolysis of read-across substances for other endpoints are now discussed:

1.1.1.1.1     Hydrolysis of the read-across substance 3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9)

Data for the substance 3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9) are read-across to the registration substance, phenyl silsesquioxanes, for appropriate endpoints. The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of phenyl silsesquioxanes are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For 3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane, hydrolysis half-lives at 25°C of 1.9 h at pH 4, 1.4 h at pH 5, >200 h at pH 7 and 2.0 h at pH 9 were determined using a validated QSAR estimation method.

The ultimate products of hydrolysis are Dimethylvinylsilanol and Phenylsilanetriol.

1.1.1.1.2     Hydrolysis of the read-across substance dodecamethylcyclohexasiloxane (CAS 540-97-6)

Data for the substance dodecamethylcyclohexasiloxane (CAS 540-97-6) are read-across to the registration substance, phenyl silsesquioxanes, for appropriate endpoints. The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of phenyl silsesquioxanes are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For dodecamethylcyclohexasiloxane, hydrolysis half-life at 25°C of >1 year at pH 7 was determined using a generally accepted scientific method (Dow Corning Corporation,2009b).

The ultimate product of hydrolysis is dimethylsilanediol.

1.1.1.1.3     Hydrolysis of the read-across substance decamethyltetrasiloxane (CAS 141-62-8)

Data for the substance decamethyltetrasiloxane (CAS 141-62-8) are read-across to the registration substance, phenyl silsesquioxanes, for appropriate endpoints. The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of phenyl silsesquioxanes are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For decamethyltetrasiloxane, hydrolysis half-lives at 25°C of 14 h at pH 5, 728 h (30.3 days) at pH 7 and21.1 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation,2009a).

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.

1.1.1.1.4     Hydrolysis of the read-across substance decamethylcyclopentasiloxane (CAS 541-02-6)

Data for the substance decamethylcyclopentasiloxane (CAS 541-02-6) are read-across to the registration substance, phenyl silsesquioxanes, for appropriate endpoints. The rate of hydrolysis of the two substances is relevant to this read-across.

For decamethylcyclopentasiloxane, hydrolysis half-lives at 25°C of 9.3 h at pH 4, 1590 h at pH 7 and 24.8 – 31.6 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation,2006a). 

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol. 

1.1.1.1.5     Hydrolysis of the read-across substance

1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9)

Data for the substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9) are read-across to the registration substance, phenyl silsesquioxanes, for appropriate endpoints. The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of phenyl silsesquioxanes are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For 1,1,1,5,5,5-Hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane, hydrolysis half-lives at 25°C of 2-4 h at pH 4, 200-630 h at pH 7 and 2-5 h at pH 9 were predicted.

The ultimate products of hydrolysis are phenylsilanetriol and trimethylsilanol.