Scared sili: is tougher regulation required for silicone?

Silicones, like plastics, were invented in the 20th century and have since become a colourful, omnipresent part of our lives. Silicones are made of repeating units of siloxanes: silicon-oxygen chains. In their many forms – oils, gels, foams and solids – they are valuable in electronics, medicine, transport and other industries; significantly, their unique sets of properties help improve energy efficiency and reduce waste (they tend to be very light and durable, and make excellent thermal insulators and lubricants).

“Silicone is a very key raw material for future development,” says Pierre Germain, secretary general of CES-Silicones Europe. “Just to take the example of CO2 reduction; if you use one tonne of CO2 to produce silicone you will save nine times this in the application.”

Silicone rubber in particular is a popular consumer material: its biocompatibility, inertness, low toxicity and smoothness render it perfect for intimate uses such as in baby feeders and sex toys, while its resistance to extreme temperatures, flexibility, and stain-resistance make it ideal in kitchenware. Recently, it has been proposed as an environmentally friendly replacement for plastic: single-use sanitary products containing non-biodegradable plastic can be substituted with a silicone menstrual cup, tea bags with a silicone tea strainer, and plastic drinkware with silicone equivalents.

Simon Holmes of Silicones UK has noted an “upward trend” of commercial silicone products: “Now more than ever people are trying to cut down on use of plastics; they are looking for new materials to use that are maybe more sustainable. Maybe that’s linked to [the upward trend],” he suggests.

While silicones may be enjoying a moment as a consumer darling, their building blocks – siloxanes – are at the centre of an ongoing dispute between regulators and industry.

Siloxanes are used to make silicones, but are also added directly to consumer products: notably, they lend smoothness to personal care products like shower gel. It is in these applications – wash-off cosmetics – that siloxanes attract the most concern. Wash-off siloxanes enter wastewater, through which they are released into the environment and consumed by fish, much like plastic microbeads. Unlike plastic microbeads, however, siloxanes are chemicals that can seep directly from the gut of a fish into the rest of its body.

In laboratory studies, scientists demonstrated that the common siloxane octamethylcyclotetrasiloxane (D4) was toxic to fish at concentrations as low as one part per billion as well as disrupting hormones and potentially impairing fertility in rodents; D4 was heavily reduced in personal care products in the 1990s as a result. Many manufacturers replaced it with its cousin decamethylcyclopentasiloxane (D5), which was associated with weaker toxic effects. The two siloxanes were later restricted in wash-off products.

Following warnings from the UK health and safety watchdog that siloxanes were accumulating and persisting in the environment, the European Chemicals Agency (ECHA) took the controversial decision last year to classify D4, D5 and dodecamethylcyclohexasiloxane (D6) as substances of very high concern (SVHCs). Consequently, D4, D5 and D6 have been placed under European chemicals regulation (REACH): all three will be limited to 0.1 per cent by weight in wash-off products in the EU from February 2020, with other products containing non-trivial traces of D4 and D5 also set for a bloc-wide market ban.

According to Holmes, the silicones industry has been “hit a bit cold” by their core ingredients being placed under REACH, with companies hurrying to reassure their customers that their products are safe.

Meanwhile, the Global Silicones Council (including CES-Silicones Europe) is taking legal action to challenge the ECHA decision. Germain argues that the classification of D4, D5 and D6 as SVHCs is inappropriate; he cites the fact that the evidence used to support the decision was laboratory-based, and the methods used were developed for carbon-based compounds and thus inappropriate for the “totally different chemistry” of silicon-based compounds.

“In the environment, the behaviour of the substances could be very different from in artificial test conditions,” Germain says. He strongly believes that the decision could needlessly hamper the development of new technologies with medical and environmental benefits. “My personal view is that we have studied [siloxanes] so much that if there was a big issue, we would have seen it [...] Only Europe has this classification. I think this puts us in a bad position, especially for the development of new technologies.”

The behaviour of siloxanes in the environment is a difficult and niche area of research, in no small part because contamination of environmental samples by researchers’ own equipment and personal care products is notoriously hard to minimise.

A small team of scientists at the Norwegian Institute for Air Research (NILU), however, has been investigating the accumulation of siloxanes in the Scandinavian country’s lakes and fjords. While siloxanes contaminate both air and water, the researchers are most concerned about the latter: airborne siloxanes degrade under sunlight, and air-breathing animals are better than aquatic animals at eliminating siloxanes from their bodies. The researchers consider the ECHA classification rational as a “precautionary principle” but stress that much remains unknown.

“The effect of [bioaccumulation] is often very hard to predict. While acute toxicity is possible to test in the laboratory, chronic toxicity resulting from long-term exposure to low concentrations of chemicals is very difficult to evaluate in advance,” says Dr Ingjerd Sunde Krogseth, an NILU scientist. “If we additionally know that chemicals are persistent, we know that if we stop the emissions one day, it will take a very long time before they disappear from the environment.”

A major and contentious issue is how – or rather whether – siloxanes are transferred up the food chain, with major studies producing contradictory results.

Krogseth suggests this could be affected by how the siloxanes are released into the water, the characteristics of the different aquatic environments, and what species make up the food web. “This makes the question very difficult, and we still need more research to really understand what’s going on,” she comments.

While the silicone industry hopes that the ECHA will reverse its classification of D4, D5 and D6 as SVHCs with the presentation of real-world evidence, in reality, there is no precedent for a removal of a chemical from under REACH. The looming restrictions placed on siloxanes will, in turn, raise further questions for researchers as industry adjusts to comply with regulations.

“We need to keep monitoring the siloxanes to see how the regulation will affect their concentrations in the environment and to pay attention to which substances will replace D4 and D6, and whether they will have any consequences for the environment,” Krogseth advises.