1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane

Endpoint summary

Description of key information

There are no in vitro or in vivo irritation data available for the registered substance. However, reliable data are available for a structural analogue. The key study for skin irritation was read-across from 1,1,1,3,5,5,5 -heptamethyl-3 -[(trimethylsilyl)oxy]trisiloxane. The study reported the test material to be not irritating to the skin of rabbits (DCC, 2009). The study was conducted according to OECD Guideline and in compliance with GLP.

The key study for eye irritation was read-across from 1,1,1,3,5,5,5 -heptamethyl-3 -trimethylsilyl)oxy]trisiloxane and reported the test material to be not irritating to the eyes of rabbits, in a study equivalent to current OECD Guideline but not compliant with GLP (Haruna 2001).

Key value for chemical safety assessment

Skin irritation / corrosion

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed (not irritating)

Eye irritation

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed (not irritating)

Respiratory irritation

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

In the key study for skin irritation, test animals were administered 50% or 90% solution of test material in olive oil onto intact or abraded skin (Haruna 2001). No signs of irritation were observed in any of the animals at either dose, at any observation time points of 1, 24, 48 and 72 hours after removal of the patches. No clinical signs were observed in any animals throughout the observation period. The body weights of all animals increased satisfactorily throughout the observation period. The study was conducted according to a Japanese protocol similar to OECD Test Guideline, but not in compliance with GLP.

In the key study for eye irritation, test animals received a single instillation of 50% or 90% solution of test material in olive oil into the eye (Haruna 2001). No eye irritation was observed in the cornea, iris or conjunctivae of the 3 animals at any observation time point in response to 50% or 90% solution, or in response to vehicle control of olive oil, applied to the other eye of the animals. No clinical signs were observed in any animals throughout the observation period and the body weights of all animals increased satisfactorily throughout the observation period.

Read-across justification

There are no available measured data for 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (CAS 3555-47-3) for skin and eye irritation. This document describes the analogue approach for fulfilling this endpoint by read-across from two source substances 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8) and dodecamethylpentasiloxane (CAS 141-63-9) for skin irritation and from one source substance 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8) for eye irritation, according to the Read-across Assessment Framework (RAAF) .

Read-across is proposed in accordance with RAAF Scenario 2: “This scenario covers the analogue approach for which the read-across hypothesis is based on different compounds which have the same type of effect(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst case.”

The read-across justification is presented (Table 5.6.4) according to RAAF scenario 2 assessment elements (AE) as outlined in Table B1 of the RAAF1:

Table 1: RAAF scenario 2 assessment elements (AE) as given in Appendix B (Table B1) of the RAAF1

AE A.1	Characterisation of source substance
AE A.2	Link of structural similarity and differences with the proposed Prediction
AE A.3	Reliability and adequacy of the source study
AE 2.1	Compounds the test organism is exposed to
AE 2.2	Common underlying mechanism, qualitative aspects
AE 2.3	Common underlying mechanism, quantitative aspects
AE 2.4	Exposure to other compounds than to those linked to the prediction
AE 2.5	Occurrence of other effects than covered by the hypothesis and Justification
AE A.4	Bias that influences the prediction

1.       AE A.1 Identity and characterisation of the source substance

The source substance, 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8), is a tertiary branched methylated siloxane structure of four Si atoms. The longest siloxane chain consists of three silicon atoms and two oxygen atoms, with a Si-O branch on the central silicon atom in the chain. The silicon atoms are fully substituted by methyl groups. Hydrolysis half-life values of 3.6 h at pH 4, 630 h at pH 7, 5.3 h at pH 9 and 20-25°C were obtained using an accepted calculation method.  

At pH 5.5 (the known pH of the skin), the hydrolysis half-life is expected to be between the values for pH 4 (3.6 h) and pH 7 (630 h). At pH 7 (the known pH of the eyes), the hydrolysis half-life is approximately 230 hours. The products of hydrolysis are trimethylsilanol (3 moles) and methylsilanetriol (1 moles).

The source substance has log Kow of 8.2 at 20°C (QSAR), water solubility of 0.00189 mg/l at 23°C (QSAR) and vapour pressure of 210 Pa at 25°C (OECD 104).

The source substance has log Kow of 9.268-9.508 at 20°C (OECD 123), water solubility of 7.0E-05 mg/l at 23°C and vapour pressure of 7.8 Pa at 25°C (QSAR).

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Therefore, 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 5 (5 h) and pH 7 (270 h) and at 37.5°C and pH 7 (relevant for eyes) is approximately 270 hours. The products of hydrolysis are dimethylsilanediol and trimethylsilanol.

2.       AE A.2 Link of structural similarities and differences with the proposed prediction

The registration, 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (CAS 3555-47-3), and the read-across substance, 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8), are structurally similar and are members of an analogue group of linear and branched siloxanes.

1,1,1,3,5,5,5-Heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8)  a is a tertiary branched methylated siloxane structure of four Si atoms, while 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (CAS 3555-47-3) is a quaternary branched methylated siloxane structure of five Si atoms. In both substances the longest siloxane chain consists of three silicon atoms and two oxygen atoms. The source substance has a Si-O branch on the central silicon atom in the chain, while the target substance has two Si-O branches on the central silicon atom in the chain. The silicon atoms are fully substituted by methyl groups.

The target and source substance share a common hydrolysis product, trimethylsilanol. (Poly)silicic acid is the other hydrolysis product for the target substance and methylsilanetriol for the source substance. These two substances both hydrolyse in similar rate to 4 moles or 3 moles of trimethylsilanol.

Table 2: Physico-chemical properties

Property		Target substance		Source substance
Substance name		1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane		1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane
CAS number		3555-47-3		17928-28-8
Hydrolysis half-life at pH 5.5 and 37.5°C		>1.9 < 74 h		> 3.6 h < 630 h
Hydrolysis half-life at pH 7 and 37.5°C		>74 h		230 h
Silanol hydrolysis product		Trimethylsilanol and (polysilicic acid)		Trimethylsilanol andmethylsilanetriol
LogKow value		9at 20°C (QSAR)		8.2 at 20°C (QSAR)
Vapour pressure		27.6 hPa at 20°C (OECD 104)		210 Pa at 25°C (OECD 104)
Water solubility		0.1507 µg/l at 20-25°C (measured)		0.00189 mg/l at 23°C (QSAR)

 

3.       AE A.3 Reliability and adequacy of the source study

In a skin irritation study with 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane, test animals were administered 50% or 90% solution of test material in olive oil onto intact or abraded skin (Haruna 2001). No signs of irritation were observed in any of the animals at either dose, at any observation time points of 1, 24, 48 and 72 hours after removal of the patches. No clinical signs were observed in any animals throughout the observation period. The body weights of all animals increased satisfactorily throughout the observation period. The study was conducted according to an appropriate OECD Test Guideline and in compliance with GLP.

In an eye irritation study with 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane, test animals received a single instillation of 50% or 90% solution of test material in olive oil into the eye (Haruna 2001). No eye irritation was observed in the cornea, iris or conjunctivae of the 3 animals at any observation time point in response to 50% or 90% solution, or in response to vehicle control of olive oil, applied to the other eye of the animals. No clinical signs were observed in any animals throughout the observation period and the body weights of all animals increased satisfactorily throughout the observation period. The study was conducted according to an equivalent to OECD Test Guideline and in compliance with GLP.

4.       AE A.4 Bias that influences the prediction

Data on the source substance 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8) were read-across to the registered (target) substance 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (CAS 3555-47-3). The source substance and the target substance have similar chemical structure and physico-chemical properties. Both substances hydrolyse at similar rate to the same Si-containing hydrolysis product, trimethylsilanol. However, the non-silanol hydrolysis products are different, (poly)silicic acid and methylsilanetriol for the target and source substance, respectively. Despite that, their toxicological properties are expected to be similar, with similar acute toxicity.

No other data for relevant substances were available. These substances are the closest structural analogues to the target substance; other siloxanes show consistent results (PFA, 2013u).

5.       AE A.2.1 Compounds the test organism is exposed to

All substances hydrolyse at similar rates to the same or similar Si-containing hydrolysis product. Therefore, the test organism is might be exposed to both the parent substance or hydrolysis products, trimethylsilanol, dimethylsilanediol, (poly)silicic acid, trimethylsilanol and methylsilanetriol.  The source and target substances have been profiled using the OECD QSAR Toolbox v. 4.1. The three substances and their silanol hydrolysis products show similar profiles for all toxicological endpoints. No alert for acute toxicity was detected by OECD QSAR Toolbox v.4.1.

The irritation potential of (poly)silicic acid is well established. No classification for skin or eye irritation has been assigned to the substance.

6.       AE A.2.2 and A.2.3 Common underlying mechanism, qualitative and quantitative aspects

No toxicity data are available for the target substance 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (CAS 3555-47-3), therefore data are read-across from the structurally analogous substance 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8) and dodecamethylpentasiloxane (CAS 141-63-9) for skin irritation and 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8) for eye irritation . These substances hydrolyse at similar rate to give the same or similar hydrolysis products. The non-silanol hydrolysis products, (poly)silicic acid, trimethylsilanol and methylsilanetriol, are not expected to be relevant for this endpoint. Moreover, they have similar physico-chemical properties. Thus, these substances are expected to have similar toxicity profiles.

7.       AE 2.4 Exposure to other compounds than to those linked to the prediction

The target substance, 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (CAS 3555-47-3), has a named impurity impurity at 5-20% is 1,1,1,5,5,5-hexamethyl-3-[(trimethylsilyl)oxy]-3-trisiloxanyl tris(trimethylsilyl) orthosilicate (CAS 18602-90-9 and synonym of Q2M6 or M6Q2). No toxicological data are available for M6Q2 however based on the weight of evidence for linear and branched siloxanes acute toxicity is not expected (PFA, 2013u).  The source substance, 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8), does not have any impurities of toxicological concern.

The test substance in the study with the source substance, 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 17928-28-8), has a purity of 99.9%.

The test substance in the study with the source substance, dodecamethylpentasiloxane (CAS 141-63-9), has a purity of 99.4%.

8.       AE 2.5 Occurrence of Other Effects than Covered by the Hypothesis and Justification

Not relevant.

 

[1]European Chemicals Agency (ECHA) (2015) Read-across Assessment Framework. Appendix B, Scenario 2.

PFA (2013u). Peter Fisk Associates, Mammalian toxicity of linear and branched siloxanes Analogue Report, PFA.300.002.008

Justification for classification or non-classification

Based on the available read-across information, 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane does not require classification for skin or eye irritation in accordance with current Regulation (EC) No 1272/2008.