Trimethoxypropylsilane

Endpoint summary

Additional information

Trimethoxy(propyl)silane (CAS No. 1067-25-0) hydrolyses rapidly in contact with water (DT50 = 2.6 h, QSAR) to form the silanol hydrolysis product propylsilanetriol and the alcohol hydrolysis product methanol (CAS No. 67-56-1).

The ECHA guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). The TGD (EC2003) and ECHA guidance R.16 (ECHA 2010) also suggest that in case the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity.

Thus, the environmental hazard assessment in this dossier is based on the hydrolysis products propylsilanetriol and methanol rather than the parent substance.

 

Propylsilanetriol

For the short-term aquatic toxicity, experimental data for the substance itself is available for all three trophic levels (refer to Table 1 below). All tests were performed according to standard guidelines and in compliance with GLP. The test design was either static (aquatic invertebrates and algae) or semi-static (fish). In all studies, the test solutions were prepared from a stock solution, which was previously mixed for 18 h followed by filtration. Since the substance hydrolyses rapidly (DT50 = 2.6 h), it is therefore likely that the test organisms were predominantly exposed to the hydrolysis products propylsilanetriol and methanol in the test media. Methanol is not hazardous/toxic at concentrations relevant to the aquatic toxicity studies in this data set and therefore not considered contributory to any effects observed in the studies (OECD SIDS, 2004).

In all studies, no toxicity was observed up to the highest concentration tested (Table 1 below). The experimentally determined effect values are also expressed in terms of the silanol hydrolysis product, which were obtained by application of a correction factor of 0.74 for molecular weight (Mw (silanol) / Mw (parent) = 122.1/164.27).

 

For the long-term aquatic toxicity, experimental data is available for aquatic algae (Table 1 below). The test was static and the test solutions were prepared the same way as described above. Therefore, the test organisms were likely predominantly exposed to the hydrolysis products in the test media and the experimentally determined effect value is also expressed in terms of the silanol hydrolysis product. No effects were observed up to the highest concentration tested (Table 1).

The studies for the long-term aquatic toxicity to fish and aquatic invertebrates are not considered necessary based on the hazard profile derived from the short-term data set and the available long-term effect value. Furthermore, exposure is not significant according to Annex XI Section 3 and the risk characterisation ratio for water is very low (<< 1). What is more, the log Kow of the substance and its silanol hydrolysis product is low (< 3). A significant deviation from this overall hazard profile is not expected.

 

For the toxicity to microorganisms, one experimental study is available from the structural analogue substance trimethoxy(methyl)silane (CAS No. 1185-55-3), which was conducted according to standard testing guidelines and in compliance with GLP. The study was performed as a static test with an exposure duration of 3 h. The test material was directly added to the test solution. Therefore, no molecular weight correction factor was applied to the effect value obtained for the parent form because the substance has a DT50 of 2.2 h. No effects were observed up to the highest concentration tested (Table 1).

 

Methanol (CAS No. 67-56-1)
The second hydrolysis product methanol is not considered to be hazardous to aquatic organisms (Table 1). The effect concentrations derived for methanol in standard guideline studies are all well above 100 mg/L (OECD SIDS, 2004). Thus, methanol is not considered contributory to the overall aquatic toxicity of the target substance. Therefore, data for methanol are not presented specifically for each environmental toxicity endpoint within this dossier. 

 

Reference

OECD SIDS, 2004. Methanol - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.

 

Table 1. Overview of aquatic toxicity effect values for trimethoxy(propyl)silane (CAS No. 1067-25-0) and its hydrolysis products.

	Registered substance		Analogue source substance (read-across)		Common non-silanol hydrolysis product
	Parent	Silanol hydrolysis product*	Parent	Silanol hydrolysis product
CAS No.	1067-25-0	NA	1185-55-3	2445-53-6	67-56-1
	Trimethox(propyl)silane	Propylsilanetriol	Trimethoxy(methyl)silane	Methylsilanetriol	Methanol**
Mw	164.27	122.1	136.22	94.14	32.04
Acute Fish (LC50)	> 746 mg/L	> 554 mg/L	-	-	> 15000 mg/L
Acute Invertebrates (EC50)	> 816 mg/L	> 607 mg/L	-	-	> 10000 mg/L
Acute Algae (ErC50)	> 913 mg/L	> 679 mg/L	-	-	> 28 000 mg/L
Chronic fish (EC10)	Not available	Not available	-	-	No data
Chronic Invertebrates (NOErC)	Not available	Not available	-	-	No data
Algae (NOErC)	≥ 913 mg/L	≥ 679 mg/L	-	-	No data
Toxicity to microorganisms (EC10)	Read-across: CAS No. 1185-55-3	-	> 100 mg/L	See parent***	No data

* effect values of parent form expressed in terms of the silanol hydrolysis product. The value was obtained by application of a molecular weight correction factor of 0.74 (122.1/164.27).

** Source: OECD (2004). SIDS Methanol.

*** No molecular weight correction factor was applied to the effect value of the parent form because the substance has a DT50 of 2.2 h and the duration of the OECD 209 test was 3 h and the test item was directly applied at test start.