Environmental Project No. 1031 – Siloxanes - Consumption, Toxicity and Alternatives

Siloxanes - Consumption, Toxicity and Alternatives

Summary and conclusions

Siloxanes are chemical compounds with a backbone of alternating silicium (Si) and oxygen (O) atoms, each silicon atom bearing one or several organic groups. Siloxanes are building blocks for silicone products or make part of other products, such as cosmetics or paint. In colloquial language the term silicones is often used synonymously with siloxanes.

The properties of the siloxanes and the silicone products depend on the length of the Si-O backbone, the chemical groups attached to the backbone and the presence of cross-links between the backbones. Silicone products are grouped into silicone fluids, elastomers and resins. Silicone fluids are used for a wide range of applications, silicone elastomers are mainly used for sealants and rubbers, and resins are mainly used for paints. The most common siloxanes are polydimethylsiloxane (PDMS) with different modifications.

Of particular interest to this study are the relatively small compounds: Siloxanes with a cyclic structure and linear siloxanes with a small Si-O backbone with a few Si-O moieties. The most common and the most investigated as to toxicity are octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). These compounds are widely used in cosmetic products and maintenance products (e.g. wax) under the name cyclomethicone - among other names. In the present report different names are used for the same compounds depending on the names typically used in the different contexts. Many of the compounds are volatile, and the users are directly exposed to the compounds when using the products, and the compounds are to a high extent released to the atmosphere or to wastewater.

Consumption of siloxanes in Denmark

The consumption of siloxanes by application area is shown in Table 1. The estimates are to a large extent based on information on the use of siloxanes in Western Europe under the assumption that the consumption pattern in Denmark of most products will resemble the general consumption pattern in Western Europe. Best estimates and an indication of the uncertainty of the best estimate are given.

The total consumption is estimated at approx. 3,100 t/year. Considering the uncertainty on applying the Western European consumption figures, the total consumption in Denmark is estimated to be within the range of 2,400-3,800 t/year.

The type of the siloxanes used is indicated in Table 1. The type can roughly indicate the potential for releases of the compounds to the atmosphere and wastewater. Volatile fluids are released to the atmosphere, whereas other fluids may end up in wastewater or released directly to surface water and soil. Elastomers and resins will mainly end up in solid waste.

The main application area is silicone sealants for construction, which account for about one third of the consumption. Besides, siloxanes are widespread, used in a vide range of products: In cosmetics and toiletries, paints, cleaning products, clothes, health-care products, etc. Often the siloxanes only account for a small part of the product, e.g. as defoaming agent.

A large number of different siloxanes are used within each application area. As an example the Danish Product Register includes 53 different siloxanes (CAS no.) registered as used in sealants and 98 different siloxanes used in paints and lacquers. About 200 siloxanes and siloxane derivatives are listed in the inventory of ingredients used in cosmetic products compiled by the European Commission (INCI 2000).

The specific siloxanes are often used in many different product types. The most widely used, polydimethylsiloxane, is in the Danish Product Register registered as being present in 159 product types. The most widely used of the cyclic siloxanes, octamethylcyclotetrasiloxane, is registered in 49 product types: Paints, cleaning agents, dyes, fillers, polishes, adhesives, etc. In most product groups the total registered amount is, however, quite small.

Table 1 Consumption of siloxanes in Denmark in 2002

Application area	Consumption Tonnes/year	Percentage	Uncertainty *	Type of siloxanes
Sealants used for construction	920	29	Low	Elastomers
Paints, inks and coatings	200	6	Medium	Resins, elastomers
Cosmetics and toiletries	240	8	Medium	Fluids, volatile fluids
Wax, polishes and cleaning agents	100	3	Medium	Fluids, volatile fluids
Mechanical fluids and heat transfer fluids	50	1.6	Medium	Fluids
Textile applications	380	12	High	Fluids, elastomers
Processing aids	470	15	Medium	Fluids
Paper coating	210	7	Medium	Fluids
Health care	110	4	High	Elastomers, fluids
Other uses of silicone elastomers	390	13	Medium	Elstomers
Other uses of silicone fluids	50	1.6	High	Fluids
Total	3,120	100

* Uncertainty indication:
Low : the right value is most probably within a range of x ±25%
Medium: the right value is most probably more than half and less than twice the best estimate
High: the right value may be less than half or more than twice the best estimate

Releases to the environment

The main source of releases of siloxanes to the air is volatile siloxanes used in cosmetics, wax, polishes, and to a minor extent in several other applications. No information of the quantity of volatile siloxanes for these applications has been available, but the volatile siloxanes may account for a significant part of the siloxanes used for cosmetics, and it is roughly estimated that between 50 and 200 t/year is released to the air. Siloxanes disposed of to municipal solid waste incineration are deemed nearly 100% to be mineralised by the incineration, and incineration plants are not considered significant sources of siloxane releases to the atmosphere.

Non-volatile silicone fluids used in cosmetics, wax, polishes, cleaning products and for textile applications (softeners) will to a large extent end up in wastewater and be directed to wastewater treatment plants. The total release to wastewater is estimated at 200-700 t/year. By the treatment process the siloxanes mainly follow the sludge and are either spread on agricultural fields, incinerated or disposed of for landfills.

The major part of siloxanes used in silicone elastomers and resins in sealants, paints, rubbers, etc. is disposed of to incineration or to landfills with building materials. By the incineration the siloxanes are destructed.

Effects on human health and the environment

Only few siloxanes are described in the literature with regard to health effects, and it is therefore not possible to make broad conclusions and comparisons of the toxicity related to short-chained linear and cyclic siloxanes based on the present evaluation. Data are primarily found on the cyclic siloxanes D4 (octamethylcyclotetrasiloxane) and D5 (decamethylcyclopentasiloxane) and the short-linear HMDS (hexamethyldisiloxane).

These three siloxanes have a relatively low order of acute toxicity by oral, dermal and inhalatory routes and do not require classification for this effect.

They are not found to be irritating to skin or eyes and are also not found sensitizing by skin contact. Data on respiratory sensitization have not been identified.

Subacute and subchronic toxicity studies show that the liver is the main target organ for D4 which also induces liver cell enzymes. This enzyme induction contributes to the elimination of the substance from the tissues. Primary target organ for D5 exposure by inhalation is the lung. D5 has an enzyme induction profile similar to that of D4. Subacute and subchronic inhalation of HMDS affect in particular the lungs and kidneys in rats.

None of the investigated siloxanes show any signs of genotoxic effects in vitro or in vivo. Preliminary results indicate that D5 has a potential carcinogenic effect.

D4 is considered to impair fertility in rats by inhalation and is classified as a substance toxic to reproduction in category 3 with the risk phrase R62 ('Possible risk of impaired fertility').

The results of a study to screen for estrogen activity indicate that D4 has very weak estrogenic and antiestrogenic activity and is a partial agonist (enhances the effect of the estrogen). It is not uncommon for compounds that are weakly estrogenic to also have antiestrogenic properties. Comparison of the estrogenic potency of D4 relative to ethinylestradiol (steroid hormone) indicates that D4 is 585,000 times less potent than ethinylestradiol in the rat stain Sprague-Dawley and 3.7 million times less potent than ethinylestradiol in the Fisher-344 rat strain. Because of the lack of effects on other endpoints designated to assess estrogenicity, the estrogenicity as mode of action for the D4 reproductive effects has been questioned. An indirect mode of action causing a delay of the LH (luteinising hormone) surge necessary for optimal timing of ovulation has been suggested as the mechanism.

Based on the reviewed information, the critical effects of the siloxanes are impaired fertility (D4) and potential carcinogenic effects (uterine tumours in females). Furthermore there seem to be some effects on various organs following repeated exposures, the liver (D4), kidney (HMDS) and lung (D5 and HMDS) being the target organs.

A possible estrogenic effect contributing to the reproductive toxicity of D4 is debated. There seems however to be some indication that this toxicity may be caused by another mechanism than estrogen activity.

Effects which based on the reviewed literature do not seem to be problematic are acute toxicity, irritant effects, sensitization and genotoxicity.

Siloxanes are in general stable compounds that are very persistent in the environment. The cyclic siloxanes and small-chain linear siloxanes are bioconcentrated (bioconcentration factors for long-chained siloxanes have not been assessed). The estimated bioconcentration factors (BCF) of the small siloxanes range from 340 for HMDS to 40,000 for a phenylated trisiloxane (phenyl trimethicone). The small phenylated siloxanes seem to have very high BCF, and model estimates indicate that these substances are the most toxic for aquatic organisms.

Alternatives to siloxanes for cosmetics and maintenance products

Traditionally when talking about substitution, the siloxanes have been on the positive side, e.g. as alternatives to PCBs. The development of alternatives to siloxanes has mainly focused on siloxanes used in cosmetics and breast implants. Until now the absence of siloxanes in cosmetics has not been a competition parameter in Denmark, but many - and in particular American producers - use the Internet for advertising "silicone-free" hair care and skin care products.

As cosmetics and maintenance products are among the most significant product groups as to consumer exposure and releases to the environment, the assessment of alternatives to siloxanes has focused on these groups.

The siloxanes have a number of properties which are not easily matched by alternatives. For soaps and leave-on products (lotions and creams for skin) the siloxanes e.g. can give the product the combination "smooth and soft feeling" on the skin combined with the sense that the product does not feel greasy on the skin after application. In particular the properties of the volatile cyclic siloxanes are difficult to substitute. The price of the alternatives ranges from the same as the price of the siloxanes to approximately the double price. The use of alternatives will in general not require changes in production equipment.

Alternatives to siloxanes in cosmetics identified by enquiries to Danish producers and suppliers are listed in Table 2.

The substitution of siloxanes has not had the particular attention of the producers of cleaning and maintenance agents. Siloxanes used in cleaning agents, waxes and polishes are in general different from the siloxanes used in cosmetics, although some of the wanted properties are the same, for example shine, spreadability and antifoaming. The identified alternatives are therefore also quite different from the alternatives developed for cosmetic products. As for alternatives to siloxanes in cosmetic products it is the general opinion in the cleaning agent trade that siloxanes have some special qualities that cannot easily be found in alternatives. These qualities are in particular as solvent, emulsifier, anti-soiling and defoaming agents.

Identified alternatives to siloxanes antifoaming agents are non-ionic mineral oils (tensides), paraffin oils, vegetable oils and block polymers consisting of polyethylenglycol and polypropylenglycol. Alternatives to amino-functional dimethylsiloxanes in polishes are lipophilic tensides. It is, however, difficult to assess to what extent the alternatives actually match the properties of the siloxanes. Advertisements for silicone-free polishes and waxes can be found, but the reason for mentioning that they are silicone-free is usually technical.

Table 2 Identified alternatives to siloxanes in cosmetics from Danish producers and suppliers.

Name of alternative	CAS-no for alternative	Alternative to	Used in	Market situation	Price
Neopentylglycol heptanoate	N/A	Dimethicone	Conditioners and leave-on products	Not sold to Danish manufac- turers of cosmetics yet	Approximately 100 DKK/kg, compared to app. 50 DKK/kg for dimethicone
Isodecyl- neopentanoate	60209-82-7	Cyclomethicone	Conditioners and leave-on products. Perhaps also shampoos and cream soaps	Not sold to Danish manufacturers of cosmetics yet	Approximately 100 DKK/kg, compared to app. 45 DKK/kg for cyclomethicone
Glycol distearate	627-83-8	Cyclomethicone and dimethicone in cream soaps. (do not have exactly the same properties)	Cream soaps	Have been used in Danish products for the last 2-3 years	Approximately half price of cyclomethicone and dimethicone (20-25 DKK per kg)
Different vegetable oil components - e.g. dicaprylyl carbonate	N/A	Dimethicone, cyclomethicone and other siloxanes. (do not have exactly the same properties)	Creams and lotions - do not have the foam reducing effect that some siloxanes have in creams and lotions	Can be found in products in Denmark	Approximately the same price level as the siloxanes
Diethylhexyl carbonate	N/A	Cyclopentasiloxan	Lotions and emulsions	The alternative is already sold to manufac- turers of cosmetics, but can so far not be found in products sold in Scandinavia	Slightly less than cyclopenta- siloxan
Hydrogenated polydecen*	68037-01-4	Cyclomethicone in composition with paraffin oils	Leave-on products	Have been sold in Denmark the last two years and can be found in products in Denmark	Not possible to estimate as it cannot directly substitute the siloxanes

* Is used as alternative to cyclomethicone and paraffin, but do not substitute all properties
N/A CAS No. has not been available - the substances are not included in the 1st update of the inventory of ingredients used in cosmetic products (INCI 2000)