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Siloxanes - Consumption, Toxicity and Alternatives

2 Application of silicones in Denmark

• 2.1 Raw materials production, import and export
• 2.2 Fields of application
  • 2.2.1 Data from the Danish Product Register
  • 2.2.2 Sealants used for construction
  • 2.2.3 Paints, inks and coatings
  • 2.2.4 Cosmetics and toiletries
  • 2.2.5 Cleaning agents and maintenance agents
  • 2.2.6 Mechanical fluids and heat transfer fluids
  • 2.2.7 Textile applications
  • 2.2.8 Process control and plastic additives
  • 2.2.9 Health-care applications
  • 2.2.10 Paper coating
  • 2.2.11 Other uses of silicone elastomers and resins
  • 2.2.12 Other uses of silicone fluids
• 2.3 Summary

2.1 Raw materials production, import and export

The statistics from Statistics Denmark (Danmarks Statistik) hold information on one commodity only for which it is explicitly stated that the commodity includes siloxanes or silicones.

Import, export and production of unprocessed silicones for the years 1998-2001 are shown in Table 2.1. The main part of unprocessed silicones is sealants packed in Denmark. As silixanes only account for a part of the sealants, the amount of silicones will be less than indicated.

Table 2.1 Import,export and production of unprocessed silicones (Statistics Denmark 1988-2002)

* supply = import + production - export

2.2 Fields of application

2.2.1 Data from the Danish Product Register

A list of siloxanes and siloxane-containing substances registered in PROBAS, the database of the Danish Product Register was retrieved. The retrieval comprises substances with chemical names including the text strings 'silox', 'silicone', 'methicon' or 'silsesquio'.

For each substance the total content of the substance in imported and exported products as well as in products produced in Denmark by application areas and branch was retrieved. The application areas registered in PROBAS are, however, not fully identical with the application areas defined in this study, and the data from PROBAS is only to some extent immediately applicable. In addition, only a part of the siloxanes in products is registered. Companies only have the obligation to submit information on the turnover of products to PROBAS, if the products contain substances classified dangerous (Bek. 439, 2002). Siloxanes are generally not classified dangerous. If the products contain dangerous substances, however, all constituents of the products are registered. The retrieval thus includes siloxanes in products in which other substances are classified dangerous.

In total 175 siloxanes or siloxanes-containing substances are registered as used in Denmark. The substances are listed in Annex 3. The total import, production and export of siloxanes registered in the Danish Product Register by application area is shown in Table 2.2. The total content of siloxanes in registered, imported products is 1,269-1,483 t in 78,000-90,500 t of products. The total content of registered products produced in Denmark is 162-1,143 t in 45,000-293,000 t of products. The data demonstrate the widespread use of siloxanes at low concentration in a very large product volume.

It should be noted that the supply calculated as import + production - export is only indicative. Siloxanes imported for production often will be registered in two different categories for import and production, respectively, e.g. imported emollients may be used for production of paints.

Table 2.2 The total import, production and export of siloxanes registered in the Danish Product Register by application area

* Supply = import + production - export
** Calculated from the non-rounded data

The substances which are registered with the largest volumes in imported products and products produced in Denmark are shown in Table 2.3. One substance is excluded for reasons of confidentiality. For most of the substances a large number of application areas and branches are registered, and the retrieval demonstrates that many of the siloxanes are used for a large range of different applications.

Table 2.3 Content of the 20 most used siloxanes in imported and produced products as recorded in the Danish Product Register.

Click here to see Table 2.3

2.2.2 Sealants used for construction

Silicone elastomers are widely used for sealants and rubbers. The different types of elastomers have been discussed in section 1.1.

Siloxane-based sealants can be organised into two main groups:

• One-component RTV silicones mainly used in the construction industry for sealants around windows and doors, in bathrooms, expansion joints between dissimilar materials, etc.
• Two-components RTV silicones mainly used for sealing of electronic components and sealed glazing units.

Besides the pure silicone sealants, a number of different hybrid sealants in which the siloxanes are blended with other polymers like polyurethanes, acrylics and isobutylene exist. In some of the sealants the siloxanes only account for a small part of the product.

Typical one-component RTV silicone sealants consist of the following components (Krogh 1999):

• 60-80 % siloxane;
• 5-7% cross-linking agent
• 20-30 % CaCO₂ filler;
• 1-6% silica filler;
• 5-20 % drying agents (siccatives);
• 0.05-0.1% organotin catalyst.

The cross-linking agents are tri- or tetrafunctional silanes containing hydrolysable Si-O or Si-N bonds. Most of the cross-linking agents react either spontaneously with the SiOH groups of the siloxane or with water. Curing of one-component RTV silicones starts when the compounds are exposed to atmospheric moisture during application. Depending on the used cross-linking agent different compounds are formed as cleavage product and will be released during the curing process (see section 1.1).

Tin catalysts are generally added to these systems to give complete curing and improve the properties.

In two-component RTV silicones, tetrafunctional alkoxysilanes are generally used as cross-linking agents in combination with tin catalysts (Ullmann 2003).

Consumption
In Western Europe 75-80% of silicones sealants are used in building and construction applications (Will et al. 2003). Based on Will et al. (2003), the total consumption of silicones for general purpose sealants in Denmark (building and construction) can be estimated at 920 t/year.

The knowledge center "Fugebranchens Samarbejds- og Oplysningsråd" does not hold any statistics on the consumption of silicone sealants in Denmark but estimates that the consumption pattern in Denmark is similar to the general Western European pattern. It is estimated by the organization that more than half of the traded elastomers are used for construction and glassing.

Imported products, recorded in the Product Register, contain in total 435-560 t silicones, whereas the content of produced products amounts to 170-1,120 t.

Siloxanes used
The Danish Product Register includes 53 different siloxanes (CAS no) registered as used in sealants. The non-confidential compounds are listed in Annex 5. The most used are polydimethyl siloxanes, vinyl group-terminated and hydroxy-terminated dimethyl siloxanes. More than half of the total volume is registered as "silicones and siloxanes" without more specific information of the siloxanes used.

For production in Denmark 0-130 t of dimethicone and 11-35 t siloxane polymers with silsesquioxanes are registered.

According to the knowledge center "Fugebranchens Samarbejds- og Oplysningsråd" silicones with acetic acid as cleavage products were formerly the most used, but they are to a large extent replaced by silicones with oxime and alcohol cleavage products.

Fate of the siloxanes
Sealants used for construction will after use most often be disposed of for incineration. Sealants adhered to non-combustive materials, e.g. applied between concrete elements or in bathrooms, may be disposed of to landfill or recycled together with the construction materials.

By the application of the sealants different volatile compounds will be released as mentioned above, whereas the release of siloxanes seems to be insignificant. Surplus sealant will mainly be disposed of to incineration.

2.2.3 Paints, inks and coatings

Siloxanes are widely used for paints and coatings.

The main application areas are:

• Coatings on masonry (silicone resins);
• High-performance paints;
• Antifouling paints;
• Defoamers, flow control agents and levelling agents in water-based paints.

Silicones are used as water repellents on mineral-based products like masonry, concrete and tiles for both interior and outdoor applications. The silicones can be applied as neat materials, in solvents or as water-based emulsions. Other silanes are used for this application area as well.

Siloxanes resins are used in high-performance paints to modify paints based on polyester, alkyd, epoxy and acrylic e.g. for anticorrosion. Silicones are used in high-temperature resistant coatings for exhaust pipes and stoves, household appliances and industrial applications.

A growing application area of silicones is antifouling paints used as alternative to TBT-containing paints. The silicone coating form a surface to which it is difficult for the fouling organisms to adhere.

Silicone liquids are widely used at low concentration in water-based paints in which the siloxanes improve the flow properties of the coating, eliminating wetting problems and thinning the coating edge.

Besides, silicones are used in small volumes in inks and dyes.

Consumption
Based on Will et al. 2003, the total consumption of silicone fluids for paint, coating and waxes can be estimated at 130 t/year. The consumption is growing by about 1.5% annually in W. Europe. Besides, water repellant coating and paints account for the major part of the consumption of silicone resins which can be estimated at 210 t/year. Roughly estimated some 300 t in total may be used for paints, inks and coatings. This figure represents the amount of siloxanes in products used in Denmark. The use of siloxanes for production of paints in Denmark has not been investigated.

According to the Product Register the total volume of siloxanes in imported products (46 product types) was 40-78 in 12,000-21,000 t of products. The total volume in produced and exported products was 7-426 t and 5-244 t respectively. Most of the products are registered as paint and lacquer without further specifications. As many water-based paints are not registered, the total volume in paints may be significantly higher.

The data from the Product Register demonstrates that the siloxanes are used in low concentration in a very large volume of products.

Siloxanes used
Paints and coatings are the main application areas for silicone resins. According to the Product Register retrieval some of the main substances used for this field are polydimethyl siloxanes with silsesquioxanes (siloxanes with a Si/O ratio of 1.5) which form highly branched polymers.

Besides the compounds with silsesquioxanes, polydimethyl siloxanes are registered as the most used siloxanes for this application area.

Silicones with side groups containing among others polyethylene and polypropylene glycol butyl ethers are used at low concentrations (<1%) in many waterborne paintings, in which the siloxanes are used as defoamers, flow control agents and levelling agents.

Fate of the siloxanes
The major part of the siloxanes will be disposed of to incineration or landfills with the products to which the paint or surface coating is adhered.

A few per cent of the siloxanes in waterborne paints may end up in the sewage system by washing of brushes and paint pots. Besides, a minor part may be diffusively released to the surrounding by maintenance of painted surfaces.

2.2.4 Cosmetics and toiletries

Silicone fluids are widely used in cosmetics and toiletries for:

• Hair care products (mainly conditioners and hair styling products, but siloxanes may also be used in shampoos and colours);
• Antiperspirants and deodorants (mainly in stick deodorants, but siloxanes may also be used in aerosols, creams, gels and roll-ons);
• Skin care products (body care, facial care and sunscreen products);
• Shaving products (pre-shave and after-shave lotion and shaving foam);
• Decorative and colour cosmetic products (eye makeup, foundations, lipsticks and powder);
• Liquid soap and shower gels.

The major use is in hair-care products and antiperspirants.

About 200 siloxanes and siloxane derivatives are listed in the inventory of ingredients used in cosmetic products compiled by the European Commission (INCI 2000) (see Annex 1). The siloxanes and derivatives function in the cosmetics as emollients, antifoaming agents, viscosity-controlling agents, antistatic agents, binders, film formers, surfactants, emulsifying agents, humectants, antioxidants and additives.

According to a leading producer of silicones the primary benefits for incorpation of different siloxanes into hair-styling products are as follows (Berthiaume 1995):

Besides their technical properties, the siloxanes are used in the products, because they are generally nonsticky, nonoily, nonirritating, do not make marks on clothing and have a relatively low toxicity.

Consumption
Based on Will et al. 2003, the total consumption of silicones for cosmetics and toiletries in Denmark can be estimated at 240 t/year. The consumption is growing by about 2% annually in both Europe and the USA.

For the American market it is specified that 60% of the silicones used for cosmetics, toiletries and pharmaceutical preparations is used in hair, skin and other personal care products, whereas 32% is used in stick antiperspirants (Will et al. 2003).

The Danish branch organization for cosmetics and toiletries, SPT, has no specific information regarding the consumption of siloxanes in cosmetic products made in Denmark. It is however believed that the total consumption of cyclomethicone, which is one of the most used siloxanes in the cosmetic industry, is approximately 5 - 6 t/year in cosmetic products produced in Denmark. The typical content of siloxanes in the products is below 2 % of the final cosmetic product, but the content can according to SPT vary between 0.5-40%, depending on the products in which the siloxanes are used.

According to one of the suppliers of silicones, the siloxanes are more widely used in cosmetic products produced by the large international companies, whereas there has been a tendency to avoid the substances in the cosmetic products made in Scandinavia.

Siloxanes used
In a Danish survey of 328 hair-styling products in 2001, 25 different siloxanes were identified as ingredients (Annex 2). Hair-styling products are used for styling of the hair and do not include shampoos and conditioners. The most common siloxanes were dimethicone copolyol (82 products out of 328), cyclomethicone (24), dimethicone (21), phenyl trimethicone (18), amodimethicone (16), cyclopentasiloxane (15), and dimethiconol (12). About half of the found silicones were at the time of the reporting of the survey not included in the INCI list.

Shampoos, conditioners and stick deodorants are the main cosmetic products in which siloxanes are used. The presence of some of the same siloxanes mentioned above in stick deodorants, shampoos and conditioners on the Danish market has been confirmed by a short survey of products carried out as part of this project.

Volatile cyclosiloxanes (mainly octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane) are used in many stick deodorant and antiperspirant products. Mixtures of the compounds are in cosmetics designated 'cyclomethicones', but in some cases the term 'cyclomethicone' is also used for pure octamethylcyclotetrasiloxane (D4) or decamethylcyclopentasiloxane (D5). According to Will et al. the use has shifted from D4 towards D5 because of their supposed minor toxicity.

The Danish EPA has a database in which the content of chemicals in 766 cosmetic products on the Danish market is registered. Cyclomethicone is present in 64 of the 766 products within the product groups: suntan lotions, body lotions, hair-styling products, creams, lipsticks, children's makeup and deodorants (Pedersen 2004).

According to Will et al. 2003, polydimethylsiloxanes, either linear or cyclic are the most widely used silicones in skin-care products. According to producer web-sites (e.g. Basildon Chemicals, UK) the same siloxanes used in hair-care products may also be used for skin care.

Cosmetic products are in general not notified to the Product Register, and the database retrieval only revealed a few siloxanes used for this application area. The most common was dimethicone, polydimethyl siloxane, poly(oxy(dimethylsilylene)) and 3-hydroxypropyl methyl etoxylated dimethylsiloxanes/silicones.

According to Allan et al. (1997) 20,000 t volatile siloxanes was used in the USA for personal care applications in 1993. If it is assumed that the per capita consumption is the same in Denmark, the consumption of volatile siloxanes for this application area would be about 300 t. However, most probably it is lower.

Based on the present information it is not possible to split the total consumption on siloxanes of the different compounds, but the available information indicates a significant consumption of both volatile and non-volatile siloxanes.

Fate of the siloxanes
The non-volatile siloxanes used for this application area will mainly be discharged with wastewater and may constitute a major part of the sources of siloxanes in wastewater.

The volatile cyclosiloxanes used in cosmetic products are meant to evaporate during use and will mainly be emitted to the air. Based on American experience 92% of the volatile siloxanes should be emitted to the air (Allan et al. 1997).

2.2.5 Cleaning agents and maintenance agents

Silicone fluids are widely used in polishes and waxes for paints (e.g. car wax), rubber, plastics, silverware, leather products, etc. Silicones improve the ease of application of the wax and polish, provide water repellence and produce a quick shine.

Besides, siloxanes are used in smaller amounts in different types of cleaning agents and in softeners for laundry.

Consumption
According to the data from the Danish Product Register the total content in cleaning and maintenance products on the market amounts to approx. 47 t/year (Table 2.4). The main registered application areas are in polishes and waxes. However, only a part of the products may be registered in the Product Register, and the actual consumption may be significantly higher, e.g. as defoamers in different cleaning agents. The total amount of siloxanes as defoaming agents registered in the Product Register is 85 t, but it is not specified in which products the defoaming agents are used.

The market report by Will et al. (2003) does not specifically provide information on the W. European consumption of silicones for waxes and polishes, but paint, coatings and waxes in total are estimated at 130 t/year. If data for the U.S. market is applied on a per capita basis, the consumption with polishes and coatings would be approx. 170 t/year.

Based on the available data the consumption of siloxanes with cleaning agents and maintenance agents is roughly estimated at 100 t/year.

Table 2.4 Content of siloxanes in imported, produced and exported cleaning agents and maintenance agents according to the Danish Product Register

Siloxanes used
The Product Register holds information on 55 different compounds registered as used within this application area. The non-confidential compounds are listed in Annex 5.

According to the data from the Product Register the main compound is poly(dimethylsiloxane). This name may partly be used synonymously with many derivatives of dimethylsiloxane by the companies when notifying on the content of the products, if the exact siloxane compound is not known.

Polydimethylsiloxanes with aminofunctional groups are widely used for this application area (Will et al. 2003). The presence of aminoethyl and aminopropyl groups increases the water solubility and forms a bridge to organic substances in the product. The aminofunctional silicone fluids were developed to impart durability and detergent resistance through bonding to the paint film. Twelve different siloxanes with aminofunctional groups are registered in the Product Register for use within this application area. The total registered consumption of these compounds is approx. 10 t/year.

Volatile cyclic dimethylsiloxanes are also widely used for polishes and waxes. In a recent Danish study of car polishes, hexamethylcyclotrisiloxane was released to the air from three out of 10 investigated products, whereas decamethylcyclopentasiloxane was released from 2 products. In the Danish Product Register 33 different products including cyclic dimethylsiloxanes (CAS no. 69430-24-6) are registered. The exact chemical compounds are not indicated, but the products most probably contain a mixture of different cyclic dimethylsiloxanes. The total registered consumption of these compounds is approx. 2 t/year.

Fate of the siloxanes
The major part of the siloxanes will, either by the application or later by washing of the products on which the polish is applied, be released to wastewater or to the ground.

A minor part of the siloxanes (e.g. the cyclic dimethylsiloxanes) may be released to the air by the application.

2.2.6 Mechanical fluids and heat transfer fluids

Silicones fluids are widely applied for the following applications:

• Lubricants and grease;
• Lubricating fluids for metal processing;
• Hydraulic fluids;
• Heat transfer fluids;
• Dielectric fluids (mainly dielectric fluids for fire-resistant transformers).

The siloxanes may be formulated into grease (with filler material) or emulsions (mixed with water). For many applications the siloxanes only account for a few per cent of the products.

Silicone fluids and greases are widely used in the automotive and the aircraft industries - from hydraulic damping and brake fluids to lubricate bearings, locks, linkages, instruments, etc. Silicone fluids and greases are generally applied when high temperatures, solvents or corrosive materials would destroy petroleum-based lubricants. (Will et al. 2003)

Consumption
Based on Will et al. 2003, the total consumption of silicones for mechanical fluids can be estimated at 110 t/year. In the market report, however, this includes also antifoaming agents and water repellants for textiles.

Products within this application area are to a large extent registered in the Danish Product Register. According to the database retrieval, the total consumption of siloxanes with these products was about 35 tonnes, the main part used in lubricants. The data only indicate a minor consumption of siloxanes with metal-working fluids, but a large part of the siloxane-containing fluids for this application may not be included in the register.

Siloxanes used
For all applications mentioned in Table 2.5, poly(dimethylsiloxane) is the main substance used according to the Product Register. It is, however, doubtful to what extent this name is used synonymously with all derivatives of dimethylsiloxane in the absence of exact knowledge on the siloxanes compound, when the companies are notifying on the content of the products.

According to Will et al. (2003) phenyl methyl siloxanes and fluorosiloxanes are more widely used for these application areas than the simple dimethylsiloxane fluids.

Fate of the siloxanes
Siloxanes used for this application area are mainly disposed of as chemical waste. For some applications direct releases to soil and discharges to wastewater may occur.

Table 2.5 Siloxanes in imported, exported and produced mechanical fluids and heat transfer fluids according to the Product Register 2003

Note: .. confidential

2.2.7 Textile applications

The main uses of silicones for textile applications are as follows:

• Softeners;
• Defoamers/antifoaming agents;
• Water repellants (liquids);
• Coatings (elastomers);
• Dry cleaning and laundry care.

The main application of the silicone fluids are as softening agent particularly in cotton and polyester/cotton substrates. They are used as emulsions or additives to other softeners.

Consumption
Based on Will et al. 2003 the total consumption of silicone fluids for the application area can be estimated at 340 t/year, broken down into 260 t in softeners, 50 t in defoamers/antifoaming agents 10 t in water repellants and 10 t other. The consumption of silicone elastomers for textile applications can - based on the same source - be estimated at 40 t/year.

In the Product Register a total of 50 t is registered in imported "textile impregnation products", whereas the content of produced and exported products were 0.3 and 0.2 t respectively. Siloxanes present in imported textiles will not be registered in the Product Register.

Siloxanes used
According to the Product Register retrieval the main compound used for textile impregnation is polymethylhydrosiloxane (methyl and hydrogen side groups). According to Will et al. 2003 primarily organo-modified silicone fluids are used as softeners in textiles.

Fate of the siloxanes
The siloxanes fluids used as softeners in the textiles will form a part of the final textile product and will to some extent be washed out by laundry ending up in the wastewater. In the technical data sheet for a new hydrophilic siloxane textile softener, DOW Corning states that traditional hydrophobic aminofunctional silicones lose their softening performance after 1-2 wash cycles (http://www.dowcorning.com/DataFiles/090007b5801286cb.pdf). It is not mentioned whether the silicones lose their softening performance because they are washed out.

It has not been possible to identify studies on the fate of siloxanes in textiles.

2.2.8 Process control and plastic additives

Siloxanes are widely used for process control and as plastic additives:

• Surfactant (surface-active agent) in the manufacturing of polyurethane (PUR) foams;
• Antifoaming agent in the manufacturing and processing of products in a range of industries (food industry, pharmaceutical industry, chemical industry, etc.);
• Plastic additive (e.g. flame retardant, smoke suppressant and for providing a smooth surface);
• Release agent for plastics extrusion and moulding operations.

Silicone used as antifoams in textile industry is included in section 2.2.7.

The main application within this area is as surfactant used for manufacturing of polyurethane foams. The silicones add stability to the liquid foaming mixture so that collapse of bubbles is retarded, and flowability is increased.

Consumption
Based on Will et al. 2003 the total consumption as surfactants for PUR production can be estimated at 290 t/year, whereas the consumption for other applications can be estimated at 180 t/year. For processing aids, however, the consumption in Denmark may deviate significantly from the general consumption pattern in Europe.

According to the trade organisation Plastindustrien i Danmark, siloxane surfactants are used for production of all PUR products. The siloxane content of the products is approximately 0.75%. In total 35,000-40,000 tonnes PUR was produced in Denmark in 1999, corresponding to 260-300 t siloxanes per year.

Based on the information above the total consumption as processing aid is estimated at 470 t of which the main part is used for PUR production.

Siloxanes used
Siloxanes used as surfactants for PUR production are usually dimethylsiloxanes copolymerised with polyesters. Fifteen compounds are registered in the Product Register as used as surfactants. The most used is "Siloxanes and silicones, di-methyl,3- hydroxypropyl methyl, ethers with polyethylene-polypolene glycol methyl ether" (CAS 67762-85-0).

Fate of the siloxanes
The siloxanes in the PUR products are estimated mainly to be disposed of with municipal solid waste for incineration. A minor part - PUR used in cars -will mainly be disposed of with the fluff from the shredding process.

2.2.9 Health-care applications

Silicones are widely used in the following healthcare applications:

• Dental impression materials (impression moulds primarily used for crown and bridge reconstruction);
• Extrusion and tubing (drainage catheters, urological applications, bloodhandling equipment, etc.);
• Baby care (nipples for baby bottles and comforters, breast pumps and infant cup straws);
• Prostetics (e.g. liner and sockets);
• Control of releases of gastrointestinal gases (antiflatulant);
• Moulded parts (drainage accessories, external feeding, laboratory tubing, respiratory masks, drug delivery devices, etc.);
• Breast implants.

Consumption
Based on Will et al. 2003, the total consumption of siloxanes fluids for healthcare applications can be estimated at 50 t/year. The consumption of siloxanes elastomers can - based on the same source - be estimated at 60 t/year. The total consumption for this area is consequently estimated at 110 t/year.

The Danish Product Register will in general not include information on substances in health-care products.

Siloxanes used
Breast implants silicone gel consist mainly of dimethyl polysiloxanes, but low molecular weight siloxanes - both linear and cyclic - are included at low concentrations (IRG 1998). In the discussion of the possible health problems related to the use of silicone breast implants, diffusion of low molecular weight siloxanes plays an important role.

Simethicone (synonymous with polydimethylsiloxane) is stated to be the most commonly used antiflatulent, and acts by dispersing excess gas in the intestine (http://wiz2.pharm.wayne.edu/module/gastromed.html).

For other applications a large number of different siloxanes are applied.

Fate of the siloxanes
The fate of the siloxanes will be very different for the different applications.

As regards human exposure the use of siloxanes in breast implants, baby-care products (e.g. in nipples) and siloxanes used as antiflatulent is of particular importance.

The major part of the siloxanes will be disposed of with municipal solid waste or medical waste for incineration. A minor part, used as antiflatulent, will be released to municipal wastewater.

2.2.10 Paper coating

Silicones are used for paper coating, primaryly to coat release papers, films and foils and as a backing of pressure-sensitive adhesive labels and tapes.

Consumption
Based on Will et al. 2003 the total consumption for paper coating can be estimated at 210 t, broken down into thermal curing paper backing (140 t), UV curing for foil backing (40 t) and electron beam curing mainly for foils backing (20 t).

In the Danish Product Register a total of 9 t siloxanes were registered as surface treatment agents for paper, cardboard, etc.

Siloxanes used
In the Danish Product Register 32 different siloxanes are registered as agents for paper coating. The main siloxanes are poly(dimethylsiloxane) and poly(dimethylsiloxane), reaction products with silica.

Fate of the siloxanes
Siloxanes used in paper coating will be disposed of to municipal solid waste for incineration.

2.2.11 Other uses of silicone elastomers and resins

Other uses of silicone elastomers and resins not covered elsewhere include the following:

• Automotive applications (hard-coat coatings for protecting of polycarbonate windows, mirrors and headlamp lenses, protecting of plastic trim, electronics, airbag coatings, seat belt adhesives);
• Electrical fittings (cable accessories and insulators);
• Domestic appliances (gasketing, electronics, sealing and potting applications in household appliances like owens, irons and refrigerators) ;
• Consumer goods (diving masks and other scuba equipment, protective masks, earplugs, baking tins, consumer products packaging, showerhead membranes, soft-touch products (e.g. pencils), spatulas, etc.);
• Food/packaging (fruit labels, bakery papers, wrappers for candy, chewing gum, meat and frozen food);
• Business machines (small computer keypads);
• Mouldmaking (reproduction of complicated shapes).

Consumption
Based on Will et al. 2003 the total consumption with special purpose sealants can be estimated at 390 t/year, broken down into automotive applications (130 t), electrical fittings (80 t), domestic appliances (50 t), consumer goods inclusive of packaging (50 t), business machines (20 t) and mould-making (20 t).

Siloxanes used
A large number of different siloxanes are used for this application area.

Fate of the siloxanes
The siloxanes in other elastomers will mainly be disposed of for incineration with municipal solid waste or waste from management of electrical and electronic waste. The main part of the silicones used for automotive applications will be landfilled with waste from shredders.

2.2.12 Other uses of silicone fluids

Silicone fluids may be used for different applications not included above.

These applications include use in reprography, impregnation of water-proof insulation materials and probably other uses.

The consumption of silicone fluids for other applications is, with a sidelong glance at Will et al. 2003, roughly estimated to be of the order of magnitude of 50 t/year.

2.3 Summary

The present information on the use of siloxanes in Denmark in 2001 is summarised in Table 2.6.

The main application areas are sealants for construction (29%), processing aids (15%) and textile applications (12%).

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.

Table 2.6 Consumption of siloxanes in Denmark 2002

* 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

In the present study it has not been possible to obtain a detailed split of the different types of siloxanes on application areas. A study of the fate of the siloxanes in the USA may though give an indication of the fate of the siloxanes in Denmark (Allen et al. 1997). The main source of siloxanes to the air is the volatile siloxanes of which 92% is released to the air by use. Polydimethylsiloxane with modifications (silicone oils) are the main source of siloxane releases to wastewater, as about 25% of the total is discharged to wastewater treatment plants. Another 21% is "dispersed" and may end up in water bodies, on the ground, etc.

Silicone resins and elastomers mainly end up in solid waste for incineration, landfilling or recycling.

Table 2.7 Environmental loadings of industrial siloxanes for the USA in 1993 (based on Allen et al. 1997) (x 1000 t)

* Modified polydimethylsiloxanes include: methyl(hydrido)siloxanes, methyl(vinyl)siloxanes, methyl(alkyl)siloxanes, methyl(phenyl)siloxanes, methyl(trifluoropropyl)siloxanes, and methyl(aminoalkyl)siloxanes. These are treated as a group, because their physico-chemical properties (which dictate their fate) are similar to PDMS and to each other.

The major release/disposal routes and risk of consumer exposure is summarised in Table 2.8 below.

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 data for the USA indicates that 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 for 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 is either spread on agricultural fields, incinerated or disposed of to landfills. According to Fendinger et al. (1997) approximately 97% of the polydimetylsiloxane will be bound to the sludge by the wastewater treatment, while the remaining 3% will be discharged to surface waters. It indicates that the main sources of discharge to surface waters in Denmark, as is the case for many heavy metals, are precipitation-dependent discharges which are discharged directly to surface water without treatment.

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

Table 2.8 The major release/disposal routes and risk of consumer exposure

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Version 1.0 September 2005, © Danish Environmental Protection Agency