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Hydrolysis

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The requirement to conduct a hydrolysis study for the 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 consitutents of the substance are hydrolytically unstable with a 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 analoguous 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.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[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:

kobs≈kH3O+[H3O+]

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

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

t1/2(pH 2) = t1/2(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 Silsesquioxanes, Phenyl for appropriate endpoints (see Section 1.4). The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of Silsesquioxanes, Phenyl is 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 Silsesquioxanes, Phenyl for appropriate endpoints (see Section 1.4). The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of Silsesquioxanes, Phenyl is 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 Silsesquioxanes, Phenyl for appropriate endpoints (see Section 1.4). The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of Silsesquioxanes, Phenyl is 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, Silsesquioxanes, Phenyl for appropriate endpoints (see Section 1.4). 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. 

The following information is taken into account for any hazard / risk / persistency assessment:

Hydrolysis half-life: 2.5 – 4.2 h at 20-25°C and pH 7 for hydrolysis of methoxy groups of constituents 4 - 6; 14 d - >1 year at 20-25°C and pH 7 for degradation of constituents 1-3 and intermediate (methoxy-hydrolysed) products of constituents 4-6. (QSAR).

1.1.1.1.5     

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, Silsesquioxanes, Phenyl, for appropriate endpoints (see Section 1.4). The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the constituents of Silsesquioxanes, Phenyl is 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.