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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2003-10-13 to 2004-09-20
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
GLP compliance:
yes
Buffers:
The test system specified in the guideline requires distilled water, however, as the selected analytical method was 1H-NMR, it was necessary to use deuterated water to prepare the buffers. The relationship between pH and pD scales was established by Glasoe and Long. For solutions of comparable acidity of basicity the pH meter reading in D20 solution is 0.40 pH units lower than in H20 solution when calibrated against aqueous buffer standards. In this case: pD = pH meter reading + 0.40.
0.05 M buffer solutions were prepared to target pH 3.6, 6.6 and 8.6 by titration of formic acid (96%), sodium phosphate monobasic (99.0%) and boric acid (99.5%) solution, respectively, with sodium hydroxide (99.998%) solution. The formic acid, sodium phosphate monobasic, boric acid and sodium hydroxide were not deuterated.
Constant ionic strength was maintained for buffers by the addition of sodium chloride (99.999%). The ionic strength was not measured, but calculated based on buffer concentration/pH. Buffer solutions were made to known final volumes in polypropylene volumetric flasks with D2O. If necessary, final pH adjustments were made by dropwise addition of a concentrated sodium hydroxide or hydrochloric acid solution (prepared in D2O). The pH of each buffer solution was measured with a calibrated pH meter at the appropriate temperature and then converted to pD. Prior to use, the buffer solutions were sparged with argon gas for at least 5 min to exclude oxygen and sterilised through a 0.2µm filter.
Details on test conditions:
Kinetics experiments were conducted at the targeted pD 4, 7, and 9 at 25°C, pD 4 and 9 at 35°C and pD 4 and 9 at 10°C with buffer concentrations of 0.05M.
The experiment for pD 9 at 35°C was run in duplicate for repeatability purposes.
Buffers were thermostatted to ±0.1°C.
The test system consisted of buffered deuterated water (99.9 atom% D).
Due to the hydrolytically unstable nature of the test substance, the water miscible solvent ACN-d3 was used for application and distribution of the test substance in the test system. The acetonitrile was <1% (v/v) as allowed per the guideline.
Test systems: formic acid/NaOH for pD 4, sodium phosphate monobasic/NaOH for pD 7, and boric acid/NaOH for pD 9
Nominal initial concentration = 1x10-3 M (~150 mg/L). 
The hydrolysis reaction was initiated by adding an aliquot (95 µl) of the test substance solution (0.1 M MTMS in ACN-d3) to 10 ml of buffer (thermostatted to the appropriate temperature) using a gas tight volumetric syringe. The sample was immediately capped and inverted one time for mixing. Approximately 800 µl of the resulting sample was quickly transferred to an NMR tube and the 1H-NMR spectrum measured. The time between addition of the MTMS to the buffer and the first acquired spectrum was measured and recorded. To ensure the integrity of methyltrimethoxysilane, the test substance solution was analyzed by 1H-NMR before and after each set of hydrolysis kinetic experiments.
Number of replicates:
The experiment at pD 9 and 35°C was run in duplicate. The observed rate constants were 17.1420 and 18.3114 h-1 with a deviation of 3.3%. The deviation is slightly higher than the target 2.5% indicated in the OECD guideline because the NMR shims, resulting peak shape and resolution were better for replicate II than replicate I.
Statistical methods:
The hydrolysis of MTMS in dilute aqueous solution was observed to follow first-order kinetics for pD 7 and 9. The natural logarithm of the concentration (as peak intensity) was plotted as a function of reaction time. The observed rate constant, k, for the hydrolysis reaction is equal to the slope of a first-order regression line fitted to the data. The half-life of the hydrolysis reaction was calculated from the estimated rate constant according to the following equation: t1/2 = ln 2/k, where k is the reaction rate constant and t1/2 is the half-life of the test substance. Descriptive statistics such as average, average deviation, percent, and linear regression analysis were also performed.
Preliminary study:
The preliminary test at 50°C was not conducted since MTMS is considered hydrolytically unstable (t1/2 < 1 year).
Transformation products:
yes
No.:
#1
No.:
#2
Details on hydrolysis and appearance of transformation product(s):
Degradation products: Methylsilanetriol and methanol (CAS No. 67-56-1)
The hydrolysis product, methanol, was observed in  this study.  Based on the chemical structure of MTMS, this hydrolysis is expected to produce 3 moles of methanol and 1 mole of methylsilanetriol  (SEHSC (2008) Communication from T Hill, Scientific Programs Manager 
Silicones Environmental, Health and Safety Council.)
Key result
pH:
4
Temp.:
10 °C
DT50:
< 0.034 h
Remarks on result:
other: <2 min; hydrolysis too rapid to determine rate constant
Key result
pH:
4
Temp.:
25 °C
DT50:
0 - < 0.033 h
Remarks on result:
other: <2 min; hydrolysis too rapid to determine rate constant
Key result
pH:
4
Temp.:
35 °C
DT50:
< 0.021 h
Remarks on result:
other: <2 min; hydrolysis too rapid to determine rate constant
Key result
pH:
7
Temp.:
25 °C
Hydrolysis rate constant:
0.32 h-1
DT50:
2.2 h
Type:
(pseudo-)first order (= half-life)
Key result
pH:
9
Temp.:
10 °C
Hydrolysis rate constant:
1.86 h-1
DT50:
0.37 h
Type:
(pseudo-)first order (= half-life)
Key result
pH:
9
Temp.:
25 °C
Hydrolysis rate constant:
6.04 h-1
DT50:
0.11 h
Type:
(pseudo-)first order (= half-life)
Key result
pH:
9
Temp.:
35 °C
Hydrolysis rate constant:
17.7 h-1
DT50:
0.039 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: Average rate constant (rep I 17.1 h-1, rep II 18.3 h-1)
Conclusions:
Hydrolysis half-lifes at 25°C of <0.033 h, 2.2 h and 0.11 h were determined at pH 4, 7 and 9, respectively, in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.
Executive summary:

MTMS hydrolysis was followed by measuring its disappearance as a function of time by 1H-NMR spectroscopy. The hydrolysis was observed to follow pesudo-first order kinetics for pD 7 and 9, was pH dependent (faster at pH 4 and 9 than pH 7), and accelerated at higher temperature. The hydrolysis of MTMS for pD 4 was so rapid that insufficient data was obtained to determine the hydrolysis rate, however, the data were adequate for estimating the upper limit of t1/2.

According to the definition put forth in the test guidelines, the test substance was observed to be hydrolytically unstable (t1/2 <1 year) over a range of environmentally relevant pD conditions at 10.0, 25.0, and 35.0°C.

Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)

The model for hydrolysis at pH 7 has been developed for, and applies specifically to, di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and applies specifically to, organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
Key result
pH:
4
DT50:
0.2 h
Remarks on result:
other: 20-25°C
Key result
pH:
5
DT50:
0.3 h
Remarks on result:
other: 20-25°C
Key result
pH:
7
DT50:
2.6 h
Remarks on result:
other: 20-25°C
Key result
pH:
9
DT50:
0.1 h
Remarks on result:
other: 20-25°C
Conclusions:
Hydrolysis half-life values at 20-25°C of 0.4 h at pH 4, 0.3 h at pH 5, 2.6 h at pH 7 and 0.1 h at pH 9 were obtained using an accepted calculation method. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)

The model for hydrolysis at pH 7 has been developed for, and applies specifically to di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and apply specifically to organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Key result
pH:
4
DT50:
0.4 h
Remarks on result:
other: approximately 20-25°C for Constituent C1
Key result
pH:
7
DT50:
7.9 h
Remarks on result:
other: approximately 20-25°C for Constituent C1
Key result
pH:
9
DT50:
0.1 h
Remarks on result:
other: approximately 20-25°C for Constituent C1
Key result
pH:
4
DT50:
0.5 h
Remarks on result:
other: approximately 20-25°C for Constituent C2
Key result
pH:
7
DT50:
16 h
Remarks on result:
other: approximately 20-25°C for Constituent C2
Key result
pH:
9
DT50:
0.2 h
Remarks on result:
other: approximately 20-25°C for Constituent C2
Conclusions:
Hydrolysis half-lives of 0.4 h at pH 4, 7.9 h at pH 7 and 0.1 h at pH 9 for Constituent C1 and 0.5 h at pH 4, 16 h at pH 7 and 0.2 h at pH 9 for Constituent C2 and 20-25°C were obtained using an appropriate calculation methods. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)

The model for hydrolysis at pH 7 has been developed for, and applies specifically to di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and apply specifically to organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Key result
pH:
4
DT50:
0.3 h
Remarks on result:
other: approximately 20-25°C for Constituent D1
Key result
pH:
7
DT50:
5.9 h
Remarks on result:
other: approximately 20-25°C for Constituent D1
Key result
pH:
9
DT50:
0.1 h
Remarks on result:
other: approximately 20-25°C for Constituent D1
Key result
pH:
4
DT50:
0.4 h
Remarks on result:
other: approximately 20-25°C for Constituent D2
Key result
pH:
7
DT50:
12 h
Remarks on result:
other: approximately 20-25°C for Constituent D2
Key result
pH:
9
DT50:
0.2 h
Remarks on result:
other: approximately 20-25°C for Constituent D2
Conclusions:
Hydrolysis half-lives of 0.3 h at pH 4, 5.9 h at pH 7 and 0.1 h at pH 9 for Constituent D1 and 0.4 h at pH 4, 12 h at pH 7 and 0.2 h at pH 9 for Constituent D2 and 20-25°C were obtained using an appropriate calculation methods. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)

The model for hydrolysis at pH 7 has been developed for, and applies specifically to, di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and applies specifically to, organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Key result
pH:
4
DT50:
0.3 h
Remarks on result:
other: 20-25°C
Key result
pH:
5
DT50:
0.4 h
Remarks on result:
other: 20-25°C
Key result
pH:
7
DT50:
5.9 h
Remarks on result:
other: 20-25°C
Key result
pH:
9
DT50:
0.1 h
Remarks on result:
other: 20-25°C
Conclusions:
Hydrolysis half-life values at 20-25°C of 0.4 h at pH 4, 0.4 h at pH 5, 5.9 h at pH 7 and 0.1 h at pH 9 were obtained using an accepted calculation method. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)

The model for hydrolysis at pH 7 has been developed for, and applies specifically to, di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and applies specifically to, organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Key result
pH:
4
DT50:
0.3 h
Remarks on result:
other: 20-25°C
Key result
pH:
5
DT50:
0.4 h
Remarks on result:
other: 20-25°C
Key result
pH:
7
DT50:
5.9 h
Remarks on result:
other: 20-25°C
Key result
pH:
9
DT50:
0.1 h
Remarks on result:
other: 20-25°C
Conclusions:
Hydrolysis half-life values at 20-25°C of 0.4 h at pH 4, 0.4 h at pH 5, 5.9 h at pH 7 and 0.1 h at pH 9 were obtained using an accepted calculation method. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)

The model for hydrolysis at pH 7 has been developed for, and applies specifically to di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, 5 and 9 have been developed for, and apply specifically to organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Key result
pH:
4
DT50:
0.4 h
Remarks on result:
other: approximately 20-25°C for Constituent G1
Key result
pH:
7
DT50:
12 h
Remarks on result:
other: approximately 20-25°C for Constituent G1
Key result
pH:
9
DT50:
0.2 h
Remarks on result:
other: approximately 20-25°C for Constituent G1
Key result
pH:
4
DT50:
0.5 h
Remarks on result:
other: approximately 20-25°C for Constituent G2
Key result
pH:
7
DT50:
16 h
Remarks on result:
other: approximately 20-25°C for Constituent G2
Key result
pH:
9
DT50:
0.2 h
Remarks on result:
other: approximately 20-25°C for Constituent G2
Conclusions:
Hydrolysis half-lives of 0.4 h at pH 4, 12 h at pH 7 and 0.2 h at pH 9 for Constituent G1 and 0.5 h at pH 4, 16 h at pH 7 and 0.2 h at pH 9 for Constituent G2 and 20-25°C were obtained using an appropriate calculation methods. The result is considered to be reliable.

Description of key information

Hydrolysis half-lives

[Trimethoxy(methyl)silane, Constituent A]: <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 and 25°C (OECD 111)

 

[3-(Trimethoxysilyl)propylamine, Constituent B]: 0.2 h at pH 4, 2.6 h at pH 7, and 0.1 h at pH 9 and 20-25°C (QSAR)

 

[Disilyl(cycloalkylamine), penta(methoxy)-, Constituent C]: 0.4 h at pH 4, 8.1 h at pH 7, and 0.1 h at pH 9 and 20-25°C (QSAR)

 

[Trisilyl(alkylamine), octamethoxy-methyl-, Constituent D]: 0.3 h at pH 4, 6.2 h at pH 7, and 0.1 h at pH 9 and 20-25°C (QSAR)

 

[Tetrasilyl(alkylamine), decamethoxy-methyl-, Constituent E]: 0.3 h at pH 4, 5.9 h at pH 7, and 0.1 h at pH 9 and 20-25°C (QSAR)

 

[Pentasilyl(alkylamine), tridecamethoxy-methyl-, Constituent F]: 0.3 h at pH 4, 5.9 h at pH 7, and 0.1 h at pH 9 and 20-25°C (QSAR)

 

[Methyl(methoxy) functional disiloxanes, Constituent G]: 0.5 h at pH 4, 12.8 h at pH 7, and 0.2 h at pH 9 and 20 -25°C (QSAR)

Key value for chemical safety assessment

Additional information

The registration substance is UVCB substance containing many constituents with a variety of structures and physicochemical properties (for example, water solubility ranging from 140 to 160000 mg/L); therefore, testing for hydrolysis rate would be technically difficult due to the requirement for a suitably sensitive analytical method. The calculated hydrolysis half-lives refer to the first alkoxysilane group to hydrolyse; once the first group has undergone hydrolysis, the subsequent hydrolysis steps are assumed to be faster. The constituents of the substance are methoxy-silanes. The alkoxysilane groups are susceptible to hydrolysis, producing silanols and methanol.

Under dilute conditions relevant for the environment, the hydrolysis of each constituent can be considered separately. The hydrolysis half-lives of each constituent are discussed below and the hydrolysis reactions are illustrated in the attached figure in Section 13. Where there are more than one constituent within a Block, the weighted average half-lives are reported in the 'key information' section above.

For some constituents, a measured hydrolysis rate is available and this is reported below instead of the predicted results.

Some constituents are outside the descriptor and/or structure domain of the QSAR prediction, therefore, there is greater uncertainty in the predicted values. Uncertainty in the predicted values for constituents C-G is considered in the chemical safety assessment by performing environmental exposure assessment and risk characterisation on the parent constituents as well as the hydrolysis product assessment entities.

Constituent A: Trimethoxy(methyl)silane, CAS 1185-55-3

Hydrolysis half-lives at 25°C of <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 were determined for trimethoxy(methyl)silane (Constituent A) in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable.

In a supporting reliable study, the stability of trimethoxy(methyl)silane in aqueous media under physiological conditions was investigated.

The rates of hydrolysis of 1000 ppm trimethoxy(methyl)silane were determined in water at pH 5.7, 0.15 molar (M) sodium-phosphate buffer (PBS), and 10% rat serum in 0.15M PBS at pH 7.4 and 37.4°C in soft glass reactors. In this study, the substance was hydrolysed in water, PBS, and PBS plus 10% rat serum at pH 7.4 and 37°C with half-lives of 24, 6.7 and 8.6 minutes respectively. This is also supported by a result in secondary literature of non-assignable reliability, which reports a half-life of 23 minutes at pH 5.7 and 37.4°C.

In another supporting study (CRL 2017), the hydrolysis of trimethoxy(methyl)silane was investigated under conditions designed to mimic the rat stomach after dosing the substance in corn oil. The half-life for disappearance of trimethoxy(methyl)silane applied in corn oil to gastric simulation buffer was 33 mins at pH 3 and 37°C and appears to be determined by phase transfer. The data suggest that, in the investigated system, hydrolysis occurs rapidly once trimethoxy(methyl)silane comes into contact with the aqueous layer and the rate determining step is the transfer of the trimethoxy(methyl)silane from the corn oil to the water. Combined recoveries of trimethoxy(methyl)silane and methanol (in mole equivalents of trimethoxy(methyl)silane; 3 moles methanol to 1 mole trimethoxy(methyl)silane assumed) were 87.5 to 104% and methanol content increased proportionally to the decrease in trimethoxy(methyl)silane. The study was conducted according to an appropriate test protocol and is considered reliable.

 

The hydrolysis products of trimethoxy(methyl)silane are methylsilanetriol (Silanol HP-X, 1 mole) and methanol (3 moles).

 

Constituent B: 3-(Trimethoxysilyl)propylamine, CAS 13822-56-5

The hydrolysis half-lives of 3-(trimethoxysilyl)propylamine have been predicted using validated QSAR estimation methods to be 0.2 h at pH 4, 0.3 h at pH 5, 2.6 h at pH 7, and 0.1 h at pH 9 and 20-25°C. The result is considered to be reliable. In a secondary source to which reliability could not be assigned, a hydrolysis half-life of 17.8 minutes at 25°C was reported; no information on pH was reported.

 

The hydrolysis products of 3-(trimethoxysilyl)propylamine are (3-aminopropyl)silanetriol (Silanol HP-Z, 1 mole) and methanol (3 moles).

 

Constituents C: Disilyl(cycloalkylamine), pentamethoxy-

No measured data are available for the hydrolysis half-lives of Constituents C. Therefore, the half-lives have been predicted using validated QSAR estimation methods as follows:

Constituent C1: 0.4 h at pH 4, 7.9 h at pH 7 and 0.1 h at pH 9 and 20-25°C

Constituent C2: 0.5 h at pH 4, 16 h at pH 7 and 0.2 h at pH 9 and 20-25°C.

The final hydrolysis products under dilute conditions relevant for the environment are disilyl(alkylamine)-heptol (Silanol HP-Y, 1 mole) and methanol (5 moles) for Constituent C1 and disilyl(alkylamine)-heptol (Silanol HP-Y, 1 mole), methylsilanetriol (Silanol HP-X, 1 mole) and methanol (6 moles) for Constituent C2.

 

Constituents D: Trisilyl(alkylamine), octamethoxy-methyl-

No measured data are available for the hydrolysis half-lives of Constituents D. Therefore, the half-lives have been predicted using validated QSAR estimation methods as follows:

Constituent D1: 0.3 h at pH 4, 5.9 h at pH 7 and 0.1 h at pH 9 and 20-25°C

Constituent D2: 0.4 h at pH 4, 12 h at pH 7 and 0.2 h at pH 9 and 20-25°C

The final hydrolysis products under dilute conditions relevant for the environment are disilyl(alkylamine)-heptol (Silanol HP-Y, 1 mole), methylsilanetriol (Silanol HP-X, 1 mole) and methanol (8 moles) for Constituent D1 and disilyl(alkylamine)-heptol (Silanol HP-Y, 1 mole), methylsilanetriol (Silanol HP-X, 2 moles) and methanol (9 moles) for Constituent D2.

Constituent E: Tetrasilyl(alkylamine), decamethoxy-methyl-

No measured data are available for the hydrolysis half-lives of Constituent E. The hydrolysis half-lives have therefore been predicted using validated QSAR estimation methods to be 0.3 h at pH 4, 5.9 h at pH 7, and 0.1 h at pH 9 and 20-25°C. The result is considered to be reliable.

The final hydrolysis products under dilute conditions relevant for the environment are trisilyl(alkylamine) undecol (Silanol HP-W, 1 mole), methylsilanetriol (Silanol HP-X, 1 mole) and methanol (10 moles)

 

 

Constituent F: Pentasilyl(alkylamine), tridecamethoxy-methyl-

No measured data are available for the hydrolysis half-lives of Constituent F. The hydrolysis half-lives have therefore been predicted using validated QSAR estimation methods to be 0.3 h at pH 4, 5.9 h at pH 7, and 0.1 h at pH 9 and 20-25°C. The result is considered to be reliable.

The final hydrolysis products under dilute conditions relevant for the environment are trisilyl(alkylamine) undecol (Silanol HP-W, 1 mole), methylsilanetriol (Silanol HP-X, 2 moles) and methanol (13 moles)

Constituents G: Methyl(methoxy) functional disiloxanes

No measured data are available for the hydrolysis half-lives of Constituents G. Therefore, the half-lives have been predicted using validated QSAR estimation methods as follows:

Constituent G1: 0.4 h at pH 4, 12 h at pH 7 and 0.2 h at pH 9 and 20-25°C

Constituent G2: 0.5 h at pH 4, 16 h at pH 7 and 0.2 h at pH 9 and 20-25°C.

The final hydrolysis products under dilute conditions relevant for the environment are methylsilanetriol (Silanol HP-X, 2 moles) and methanol (4 moles) for Constituent G1 and 3-aminopropylsilanetriol (Silanol HP-Z, 1 mole), methylsilanetriol (Silanol HP-X, 1 mole) and methanol (4 moles) for Constituent G2.

 

The hydrolysis data for other substances used for read-across purposes in other endpoints are discussed below.

Hydrolysis of the read-across substance trimethoxy(propyl)silane (CAS 1067-25-0)

Data for trimethoxy(propyl)silane (CAS 1067-25-0) are read-across to Constituent A of the submission substance for the following endpoint: biodegradation in water. The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate section for the endpoint.

For trimethoxy(propyl)silane, hydrolysis half-lives at 20-25°C of 0.2 h at pH 4, 2.6 h at pH 7 and 0.1 h at pH 9 were predicted for the substance using a validated QSAR estimation method.

The hydrolysis products are propylsilanetriol (1 mole) and methanol (3 moles).

 

Hydrolysis of the read-across substance 3-aminopropyltriethoxysilane (CAS 919-30-2)

Data for the substance, 3-aminopropyltriethoxysilane (CAS 919-30-2) are read-across to Constituent B of the submission substance for the following endpoints: biodegradation in water, short-term toxicity to fish, short-term toxicity to aquatic invertebrates, toxicity to algae and toxicity to microorganisms. The hydrolysis half-lives and the formation of the same silanol hydrolysis product for the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For 3-aminopropyltriethoxysilane, hydrolysis half-lives at 24.7°C of 0.8 h at pH 5, 8.5 h at pH 7 and 0.15 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2001). At pH 4, the substance has a predicted half-life of 0.4 h at 25°C using a validated QSAR estimation method.

The hydrolysis products are 3-aminopropylsilanetriol (1 mole) and ethanol (3 moles).

 

Hydrolysis of the read-across substance triethoxy(methyl)silane (CAS 2031-67-6)

Data for the substance, triethoxy(methyl)silane (CAS 2031-67-6) are read-across to Constituent A of the submission substance for the following endpoints: short-term toxicity to fish, short-term toxicity to aquatic invertebrates and toxicity to algae. The hydrolysis half-lives and the formation of the same silanol hydrolysis product for the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For triethoxy(methyl)silane, hydrolysis half-lives at 20-25°C of 0.3 h at pH 4, 5.5 h at pH 7 and 0.1 h at pH 9 were determined using validated QSAR estimation methods.

The hydrolysis products are methylsilanetriol (1 mole) and ethanol (3 moles).

 

Hydrolysis of the read-across substance bis(trimethoxysilylpropyl)amine (CAS 82985-35-1)

Data for the substance bis(trimethoxysilylpropyl)amine (CAS 82985-35-1) are read-across to Constituents C-F of the submission substance for the following endpoints: biodegradation in water, short-term toxicity to fish, toxicity to aquatic algae and toxicity and toxicity to microorganisms. The hydrolysis half-lives of the two substances are relevant to this read-across, as discussed in the appropriate section for each endpoint.

For bis(trimethoxysilylpropyl)amine, hydrolysis half-lives at 20-25°C of 0.3 h at pH 4, 6.9 h at pH 7 and 0.1 h at pH 9 were determined using validated QSAR estimation methods. Alkoxysilanes containing amine groups hydrolyse more quickly than predicted due to an intramolecular catalysis mechanism. For example, 3-aminopropyltriethoxysilane (CAS 919-30-2) has a predicted half-life at pH 7 and 20-25°C of 20 h and a reliable measured value of 8.5 h. Therefore, the half-lives are considered to represent an upper-bound for the true hydrolysis rates.

The hydrolysis products are N,N-bis(trihydroxysilylpropyl)amine (1 mole) and methanol (6 moles).

Hydrolysis of the read-across substance N,N-bis(3-triethoxysilylpropyl)amine (CAS 13497-18-2)

Data for the substance, N,N-bis(3-triethoxysilylpropyl)amine (CAS 13497-18-2) are read-across to Constituents C-F of the submission substance for the following endpoints: biodegradation in water, short-term toxicity to fish, short-term toxicity to aquatic invertebrates and toxicity to aquatic algae. The hydrolysis half-lives of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For N,N-Bis(3-triethoxysilylpropyl)amine, a preliminary test on the hydrolysis of bis(triethoxysilylpropyl)amine was performed as part of the biodegradation in water test according to OECD Test Guideline 301 and in compliance with GLP. The hydrolysis test was performed in purified water for a test duration of 2 hours. The test item was observed to undergo hydrolysis to ethanol and the respective silanol hydrolysis product. Ethanol was analytically monitored at time points 0 hour, 0.5 hours, 1 hour and 2 hours in order to gather information on the hydrolysis rate of the test item. More than half of the test material (51 – 62%) was hydrolysed right after the preparation of the solution at time point 0 hour. 97-99% of the test material was hydrolysed after 0.5 hours, showing that the substance hydrolyses rapidly in water at alkaline pH (9.5) and room temperature.

In addition, a preliminary hydrolysis study was conducted as part of an adsorption study according to OECD Test Guideline 106. The half-life of the substance was determined in soil eluates of 5 different soil types having neutral pH values (5.5 - 7.2). The parent substance was analytically monitored via LC-MS at time points 0 hour, 0.25 hours, 0.5 hours, 1 hour, 2 hours, 4 hours, 6 hours and 24 hours. The hydrolysis half-lives were between 1 - 5.6 hours, indicating rapid hydrolysis of the substance in soils of neutral pH.

In conclusion, using the evidence from both studies as weight of evidence; it can be concluded that the substance hydrolyses rapidly and within the trigger value for fast hydrolysis of 12 hours given by the guidelines.

The hydrolysis products are N,N-bis(trihydroxysilylpropyl)amine (1 mole) and ethanol (6 moles).

Hydrolysis of the read-across substance triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5)

Data for the substance, triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5) are read-across to Constituent B [3-(trimethoxysilyl)propylamine] of the submission substance for the toxicity to microorganisms endpoint. The silanol hydrolysis products and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate section for the endpoint.

Triethoxy(3-isocyanatopropyl)silane, has two hydrolysable groups, trimethoxy (-OCH3) and isocyanate (-N=C=O). The isocyanate group is expected to hydrolyses very rapidly, for example the hydrolysis half-lives of 2,2,4(or 2,4,4)-trimethylhexane-1,6-diisocyanate were measured in accordance with OECD 111 test method and in compliance with GLP. Very rapid hydrolysis following pseudo-first order kinetics with the following half-lives was determined:

pH 4 - 8.08 min at 10°C, 3.81 min at 20°C and 2.51 min at 30°C

pH 7 - 12.1 min at 10°C, 4.88 min at 20°C and 2.15 min at 30°C

pH 9 - 5.78 min at 10°C, 1.93 min at 20°C and 0.74 min at 30°C

For triethoxy(3-isocyanatopropyl)silane, this means very rapid hydrolysis to form 3-aminopropyltriethoxysilane (CAS 919-30-2) as an intermediate hydrolysis product and carbon dioxide. The hydrolysis half-lives of 3-aminopropyltriethoxysilane (CAS 919-30-2) are discussed above.

The ultimate product of the hydrolysis reaction under dilute conditions is 3-aminopropylsilanetriol (1 mole). The other hydrolysis products are ethanol (3 moles) and carbon dioxide (1 mole).

Hydrolysis of the read-across substance N-(3-(trimethoxysilyl)propyl)ethylenediamine (CAS 1760-24-3)

Data for the substance, N-(3-(trimethoxysilyl)propyl)ethylenediamine (CAS 1760-24-3) are read-across to constituent B of the submission substance for the long-term toxicity to aquatic invertebrates endpoint. The hydrolysis half-life and formation of similar hydrolysis products are relevant to this read-across as discussed in the appropriate section for the endpoint.

For N-(3-(trimethoxysilyl)propyl)ethylenediamine, measured hydrolysis half-life values of 0.1 h at pH 4 and 24.7°C, 0.025 h at pH 7 and 24.7°C, and 0.32 h at pH 5 and 24.7°C was determined for the substance in accordance with OECD 111. The result is considered to be reliable. At pH >7, the half-life became too rapid (<90 s) to measure using the methodology of the study.

In other secondary sources to which reliability could not be assigned, a hydrolysis half-life of 0.016 h at pH 7 and 24.7°C was reported. Also, a hydrolysis half-life of 24.1 h at 25°C was reported, information on the pH was not stated.

The hydrolysis products are N-(3-(trihydroxysilyl)propyl)ethylenediamine (1 mole) and methanol (3 moles).

Hydrolysis of the read-across substance trichloro(ethyl)silane (CAS 115-21-9)

Data for the substance, trichloro(ethyl)silane (CAS 115-21-9) are read-across to Constituent A of the submission substance for the long-term toxicity to aquatic invertebrates endpoint. The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across as discussed in the appropriate section for the endpoint.

The hydrolysis half-life of trichloro(ethyl)silane was in turn read-across from trichloro(methyl)silane (CAS 75-79-6), hydrolysis half-lives of <1 minute at pH 4, pH 7 and pH 9 at 1.5°C were determined for trichloro(methyl)silane in accordance with OECD 111 test method. (Dow Corning Corporation, 2001).

The hydrolysis products are ethylsilanetriol (1 mole) and hydrochloric acid (3 moles).