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Silicones in the Environment

Silicones give qualities to an enormous range of products that enhance the enjoyment, quality and safety of modern life. We need to be sure, however, that these benefits are not gained at the expense of the natural world.

Silicone producers together form a major sector of the global economy. As such, protecting wildlife and the natural environment is a priority. The members of CES fully research and test all our products and share results with national and international regulators. On this basis we are convinced that our products are safe when used as intended.

Silicones in the Environment

There are several classes of silicones that may potentially be found in the environment on account of their applications, but the two principal groups are volatile methylsiloxanes (VMS) and polydimethylsiloxanes (PDMS).

The picture below summarises our knowledge on their behaviour in the main environmental compartments of water, sediment, soil and air.

Click on soil, air, water and sediments to get more information

Soil

Polydimethylsiloxanes (PDMS) degrade when they are added to agricultural or other soils as a component of sludge. This is a complex process that is initiated by contact with the clay component of the soil. It is fastest under dry conditions, but once the silicone molecules are 'unlocked', biological degradation can follow. Experiments show that the degradation products may either evaporate into the air or, depending on the soil type, degrade further in the soil. In either case the ultimate degradation products are silica and carbon dioxide, thus completing the PDMS life cycle. A similar mechanism has been shown to operate for VMS during degradation in the atmosphere.

In view of the significance of the soil compartment in the life cycle of PDMS, a number of studies have been conducted on various soil-living animals and plants. For example, there was no evidence of uptake or adverse effects on worms or crops such as wheat or soybeans grown in sewage sludge amended soils containing PDMS.

Air

Depending on the application, volatile methylsiloxanes (VMS) may evaporate into the air before entering other environmental compartments such as water or soil. In air, they are degraded in the presence of sunlight, ultimately to silica, water and carbon dioxide.

Experiments also show that siloxanes do not contribute to ground level ozone pollution, and because of this, have not been classified as Volatile Organic Compounds (VOCs). On account of their relatively short atmospheric life they do not reach the upper atmosphere to affect the ozone layer.

Water

For uses such as in shampoos, conditioners and detergents, which naturally enter wastewater, the aquatic compartment is of key importance. However volatile methylsiloxanes (VMS) are rapidly lost into the air and polydimethylsiloxanes (PDMS) do not dissolve in water. Therefore, it is very rare to detect either of them in rivers or lakes.

Nevertheless, studies on the effects of PDMS in the environment have been conducted using relatively high concentrations. No significant effects on fish or other aquatic life have been seen.

In fact, silicones can be used to protect wildlife. A major such use was the treatment with PDMS of otters caught in a large oil spill in Arctic waters. After using detergents to remove the oil, a PDMS coating gave their fur the necessary protection against the freezing waters until their natural waterproofing system could recover. They would not otherwise have survived.

Sediments

As sediments are an integral part of the aquatic environment, several studies have focused on the fate and effects of silicones in sediments. During wastewater treatment, in particular non-volatile silicones bind tightly to particulates. They are thus removed from wastewater during treatment and are processed as part of the sludge.

The sludge is normally either sent to landfill, incinerated or used to improve the quality of soils used for agriculture or other purposes (e.g. for golf courses, landscaping, etc.).

A minor percentage (less than 5%) of silicones adheres to suspended solids in the water outlet of treatment plants and may become part of river sediments. In laboratory experiments with PDMS on a number of sediment-living organisms, such as worms and insect larvae, no adverse effects were seen even at high concentrations.

Monitoring and Testing

Environmental monitoring and testing are central to the commitment of the silicone industry to environmental protection. Via such programmes, the silicone industry identifies how its products enter and leave the environment, quantifies exposure levels and evaluates possible risks. They also help to highlight further research requirements.

Analysis

An in-depth understanding of the fate and effects of substances in the environment depends on high quality analysis and accurate test results that deliver consistent and comparable data. To ensure such information, CES has developed test methods to extract and detect silicones in samples collected from the environment. In most cases silicones are absent from samples.

References

  • Chandra G(Editor), The Handbook of Environmental Chemistry, Volume 3 Anthropogenic Compounds Part H. Organosilicon Materials, Springer Verlag, 1997.
  • Stevens C., Powell D.E., Mäkelä P, Karmans C.Fate and Effects of Polydimethylsiloxane (PDMS) in Marine Environments, Marine Pollution Bulletin Vol 42, No 7, pp 536-54.
  • Stevens C., Environmental Degradation Pathways for the Breakdown of Polydimethylsiloxanes, Journal of Inorganic Biochemistry 69 (1988) 203-207.
  • Stevens C., Environmental Fate and Effects of Dimethicone and Cyclotetrasiloxane from Personal Care Applications, International Journal of Cosmetic Science 20, 297-305 (1988).