Hexamethyldisiloxane

SRP: The most favorable course of action is to use an alternative chemical product with less inherent propensity for occupational exposure or environmental contamination. Recycle any unused portion of the material for its approved use or return it to the manufacturer or supplier. Ultimate disposal of the chemical must consider: the material's impact on air quality; potential migration in soil or water; effects on animal, aquatic, and plant life; and conformance with environmental and public health regulations.

11 [ppm]

120 [ppm]

700 [ppm]

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 4,600(SRC), determined from a log Kow of 4.2(2) and a regression-derived equation(3), indicates that hexamethyldisiloxane is expected to have slight mobility in soil(SRC). Volatilization of hexamethyldisiloxane from moist soil surfaces is expected to be an important fate process(SRC) given a estimated Henry's Law constant of 0.0453 atm-cu m/mole(4). Hexamethyldisiloxane is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 42.1 mm Hg(5). However, adsorption to soil is expected to attenuate volatilization(SRC). No biodegradation data regarding hexamethyldisiloxane were found; however, dimethyl siloxanes in general are highly resistant to biodegradation(6). As a member of this class biodegradation is not expected to be an important fate process.

TERRESTRIAL FATE: Poly(dimethylsiloxane) (PDMS) fluids in intimate contact with many soils undergo siloxane redistribution and hydrolysis, resulting in the formation of low molecular weight cyclic and linear oligomers. Low molecular weight hydroxy-functional hydrolysis products are water soluble, and the cyclics and trimethylsiloxy-end-blocked oligomers are volatile, thus providing materials which can partition from the soil to the water and atmospheric environmental compartments(1). /Poly(dimethylsiloxane)/

AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 4,600(SRC), determined from a log Kow of 4.2(2) and a regression-derived equation(3), indicates that hexamethyldisiloxane is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon a Henry's Law constant of 0.0453 atm-cu m/mole(4). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 1.3 hours and 5 days, respectively(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered(5). According to a classification scheme(6), an estimated BCF of 340(SRC), from its log Kow(2) and a regression-derived equation(7), suggests the potential for bioconcentration in aquatic organisms is high(SRC). Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble(8), indicating that hydrolysis of hexamethyldisiloxane may be an important environmental fate process leading to products that will predominantly partition into the atmosphere. No biodegradation data regarding hexamethyldisiloxane were found; however dimethyl siloxanes in general are highly resistant to biodegradation(9). As a member of this class biodegradation is not expected to be an important fate process.

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), hexamethyldisiloxane, which has a vapor pressure of 42.1 mm Hg at 25 °C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase hexamethyldisiloxane is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 18 days(SRC), calculated from its rate constant of 1.4x10-12 cu cm/molecule-sec at 25 °C(3). Methyl siloxanes are transparent to UV radiation >290 nm(4) and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation(SRC).

Hexamethyldisiloxane was detected in two separate domestic waste sites in Munich Germany at concentrations of 0.01 and 0.31-0.45 mg/cu m(1). Hexamethyldisiloxane was also detected in the respective sewage treatment plants at concentrations of 0.14-0.17 and 0.18-0.20 mg/cu m, respectively(1).

The Koc of hexamethyldisiloxane is estimated as 4,600(SRC), using a log Kow of 4.2(1) and a regression-derived equation(2). According to a classification scheme(3), this estimated Koc value suggests that hexamethyldisiloxane is expected to have slight mobility in soil.

The mutagenicity of hexamethyldisiloxane (HMDS) was evaluated in bacterial Salmonella tester strains TA98, TA100, TA1535, TA1537 and TA1538 and yeast Saccharomyces tester strain D4, both in the presence and absence of added metabolic activation by Aroclor-induced rat liver S9 fraction. HMDS, diluted in ethanol, was tested for mutagenicity at concentrations up to 5.0 ul/plate using the plate incorporation method. The mutagenicity of HMDS was also tested in bacterial E. coli indicator organisms, although the concentrations used and numerical results were not reported. HMDS did not cause a positive response in any of the bacterial or yeast tester strains, either with or without metabolic activation.

The ability of hexamethyldisiloxane to induce specific locus mutations at the TK locus in cultured L5178Y mouse lymphoma cells (Mouse Lymphoma Mutagenesis Assay) was evaluated in the presence and absence of Aroclor-induced rat liver S9 metabolic activation. Based on preliminary toxicity determinations, both nonactivated and S9 activated cultures were treated with 0.0125, 0.0250, 0.0500, 0.1000, or 0.2000 ul/ml which produced a range of 104 - 32% total growth for nonactivated cultures and from 107 - 0.3% total growth for S9-activated cultures. None of the nonactivated or activated cultures produced mutant frequencies significantly greater than the solvent controls (ethanol).

Hexamethyldisiloxane was examined for DNA modifying activity in Escherichia coli strains W3110 polA+ and P3478 polA- (DNA repair deficiency assay) with and without metabolic activation provided by Aroclor-induced rat liver S9 fraction. The test article did not produce zones of inhibition in either strain when spotted on plates at volumes ranging from 0.001 to 5 ul in the presence or absence of metabolic activation.

The frequency of sister chromatid exchange (SCE) was determined in L5178Y mouse lymphoma cells exposed in vitro to hexamethyldisiloxane with and without metabolic activation provided by mouse liver S9 fraction (induction of liver enzyme activity was not reported). The test article was administered at concentrations of 0.025, 0.05, 0.1 and 0.2 ul/ml in the absence of activation, and at concentrations of 0.025, 0.05 and 0.1 ul/ml in the presence of activation. A statistically significant (students t-test) increase in the mean number of SCE's/cell was observed at 0.05 ul/ml (p < 0.01) in the activated assays, and at 0.025 ul/ml (p < 0.01) and 0.1 ul/ml (p < 0.05) in the nonactivated assays. Statistically significant increases in the mean number of chromosomes/cell with SCE's, and in the mean number of SCE's/chromosome in chromosomes with SCE's were not observed under any condition. The test article was reported to be toxic to cells at concentrations greater than 0.2 ul/ml.

For more TSCA Test Submissions (Complete) data for HEXAMETHYLDISILOXANE (6 total), please visit the HSDB record page.

/HUMAN EXPOSURE STUDIES/ /Hexamethyldisiloxane/ was submitted to determine its ability to sensitize the skin of normal volunteer subjects using a repeated insult patch test. One-hundred subjects completed the study. Patches were applied to the infrascapular area of the back. The entire study extended over a six week period. It involved three phases: induction, rest, and challenge. The induction phases consisted of nine consecutive applications and subsequent evaluations of the test sites. The rest period lasted fourteen days. The challenge phase used identical patches applied to sites previously unexposed to the test material. Under the conditions employed in this study, there was no evidence of sensitization to the test substance.

/SIGNS AND SYMPTOMS/ Hexamethyldisiloxane has been noted to give off vapors which are transiently irritating to the conjunctiva, but no injury has been observed.

/OTHER TOXICITY INFORMATION/ Vapors have a very low toxicity for ... humans. ... /it/ irritates conjunctiva but does not attack the cornea.

Hexamethyldisiloxane's production and use as a chemical intermediate in the synthesis of silicone fluids, elastomers and fluorosilicone oils as well as in photolithography(1), personal care products and cosmetics(2) may result in its release to the environment through various waste streams(SRC).

AEROBIC: No biodegradation data regarding hexamethyldisiloxane were found(SRC, 2006); however dimethyl siloxanes in general are highly resistant to biodegradation(1). As a member of this class biodegradation is not expected to be an important fate process.

An estimated BCF of 340 was calculated for hexamethyldisiloxane(SRC), using a log Kow of 4.2(1) and a regression-derived equation(2). According to a classification scheme(3), this BCF suggests the potential for bioconcentration in aquatic organisms is high(SRC), provided the compound is not metabolized by the organism(SRC).

Section 8(a) of TSCA requires manufacturers of this chemical substance to report preliminary assessment information concerned with production, exposure, and use to EPA as cited in the preamble in 51 FR 41329.

Pursuant to section 8(d) of TSCA, EPA promulgated a model Health and Safety Data Reporting Rule. The section 8(d) model rule requires manufacturers, importers, and processors of listed chemical substances and mixtures to submit to EPA copies and lists of unpublished health and safety studies. Hexamethyldisiloxane is included on this list.

Used to make silicone fluids, elastomers, octamethylcyclotetrasiloxane, and fluorosilicone oils; Also used in cosmetics and in photolithography applications (adhesion promoter/priming agent); [HSDB]

Hydraulic fluid /Organo Silicate MLO 5277/

Chemical intermediate for silicone fluids and elastomers

Hexamethyldisiloxane is used in cosmetic and personal care products, in the production of octamethylcyclotetrasiloxane, as an adhesion promotor-priming agent for photolithography applications, and as an end blocking agent in the production of fluorosilicone oils.

/Used/ ... for headlights ... as a protective layer 20-30 nm thick ... effective diffusion barrier (condensed water resistance is 1000 times greater than without coating) ... . Other applications include the hydrophilic coating of contact lenses and the coating of medical materials to improve biocompatibility.

Used as antifoams in aqueous systems, in petroleum processing, and in laundry detergents ... In human and veterinary medicine ... used as antiflatulent agents ... Useful as lubricants (eg, for films, yarn, medical articles, wine corks, and fillers). /Polydimethylsiloxanes/

2023: 10,000,000 - <25,000,000 lb

This chemical is listed as a High Production Volume (HPV) (65FR81686). Chemicals listed as HPV were produced in or imported into the U.S. in >1 million pounds in 1990 and/or 1994. The HPV list is based on the 1990 Inventory Update Rule. (IUR) (40 CFR part 710 subpart B; 51FR21438).

(1977) At least 4.58x10+8 g (est.)

(1986) >1 million-10 million pounds

(1990) >1 million-10 million pounds

For more U.S. Production (Complete) data for HEXAMETHYLDISILOXANE (7 total), please visit the HSDB record page.

All Other Basic Organic Chemical Manufacturing

Disiloxane, 1,1,1,3,3,3-hexamethyl-: ACTIVE

... Used as a hydraulic fluid and comes in contact with heated metal surfaces of engines operating at high temperatures. /Organo Silicate MLO 5277/