Exploring the potential applications of silicones in cosmetics and the recent concerns about their environmental friendliness

Maria Fernanda Gonzalez Prato | Biochemistry researcher

Pierre Trinh | Sustainability researcher

7th June 2024

Keywords: biodegradability; environment; stability; organosilicones; emulsions; silicone

Silicon is a widely used and versatile polymer in the cosmetic industry, known for enhancing the physical properties of a wide range of products such as sunscreen, shampoo, and color cosmetics. However, the use of silicones in cosmetic formulations remains a topic of debate, as there is a growing preference for "natural" or "biodegradable" ingredients. Concerns also arise from studies exploring their impact on the skin and their potential environmental toxicity. In an upcoming article, researchers Pierre Trinh and Maria Fernanda Gonzalez Prato will delve into the current understanding of silicone use in cosmetics, evaluating both the benefits and potential risks to the environment associated with these compounds.

Silicone's history of applications in cosmetic sector

Silicon is a widely used and versatile polymer in the cosmetic industry, known for enhancing the physical properties of a wide range of products such as sunscreen, shampoo, and color cosmetics. However, the use of silicones in cosmetic formulations remains a topic of debate, as there is a growing preference for "natural" or "biodegradable" ingredients. Concerns also arise from studies exploring their impact on the skin and their potential environmental toxicity. In an upcoming article, researchers Pierre Trinh and Maria Fernanda Gonzalez Prato will delve into the current understanding of silicone use in cosmetics, evaluating both the benefits and potential risks to the environment associated with these compounds.

Silicone, a class of hybrid inorganic and organic compounds, stands out for its versatility in personal care applications. Its unique chemistry and a wide range of valuable applications make it ideal for use in variable temperature ranges, maintenance of a low surface tension, large free volume, high compressibility, and low molecular weight, all of which are beneficial for skin care products (Mancuso et al., 2022)). Moreover, the broad range of synthesis and combination options for silicone allows for the creation of molecules tailored to specific applications, further enhancing its appeal.?

Silicone, a compound that rarely occurs naturally in its free state, was first discovered as an inclusion in volcanic material. However, it is primarily obtained through synthesis. The discovery and? commercialization of silicones have been marked by significant milestones. Several patents were filed in 1872, but it was the introduction of the Grignard approach in 1940 that revolutionized the production of silicones.? This direct method, using a copper catalyst, allowed for the efficient production of silicones from silicon and methyl chloride, leading to its rapid commercialization (Anthony. J O'Lenick Jr, Anthony J, 2008).?

Chemical Properties of certain silicones

Siloxanes, with their chains of alternating silicon and oxygen atoms, can be visualized as hydrides. They possess an amphiphilic nature, characterized by a polar inorganic backbone with strong Si-O bonds and non-polar organic side groups attached to the silicon atoms. This unique amphiphilicity is key to their properties. The polar Si-O segments can form hydrogen bonds with human skin, acting as hydrogen acceptors, while the non-polar organic groups (like CH3) can serve as hydrogen donors. This trait facilitates the adhesion between siloxane elastomers and human skin, making them valuable in applications requiring strong yet flexible adhesion (Mojsiewicz-Pieńkowska et al., 2016).

Siloxanes have a distinctive chemical structure that gives them a wide range of impressive properties, leading to their widespread use and advancement. One of the siloxanes' main characteristics is their chains' exceptional flexibility. This flexibility allows for several conformations and enables rapid conformational changes. This is due to the relatively long silicon-oxygen (Si-O, approximately 1.64 ?) and silicon-carbon (Si-C, approximately 1.88 ?) bonds, which, by creating a very low energy barrier for the Si-O-Si angle, combined with the absence of substituents on every other atom in the chain, allow for easy rotation around chemical bonds, contributing to the material's adaptability (Sayyed & Kulkarni, 2022).

Silicone polymers, which are commonly used in applications and processes involving water, remain insoluble in water and are often formulated as an emulsion for aqueous delivery. Such formulations are widely used in cosmetic and personal care products, where silicone enhances the functionality of hair conditioners and serums.

How have silicones been applied in cosmetic industry?

Utilization of silicone in makeup particularly started in 1950, Initially, dimethicone, the basic silicone fluid, was used in personal care application, being the easiest to process and handle, it was an excellent moisturizing ingredient due to its lower surface energy, while also producing an incredible gloss due to optical clarity and providing a “skin-feel” texture (Sayyed & Kulkarni, 2022)). Since the 1980’s, the demand for silicone has increased drastically, as they can be modified to serve specific purposes such as waterproofing, moisturizing, adhering color pigments, protecting hair, and providing smoothness.?

Additionally, silicones allow for excellent breathability on the skin due to their molecular structure, preventing any suffocation, and, in some cases, silicones are used as fillers to minimize the appearance of acne scars, demonstrating their non-pore-clogging properties. This means silicones are the backbones of cosmetic products (Sayyed & Kulkarni, 2022).

Recent concerns about silicones' environmental impacts

What is the problem with silicone?

Silicones are one of the most researched compounds that are used in consumer and commercial goods, and over 1,000 studies have been performed to examine their safety in several industrial processes, customers, and environmental contexts. A wide variety of conclusions have been obtained for 30 years, with the most outdated ones exposing their supposed risk for the environment and health, while more recent ones dismantle these studies.

Despite their wide applicability and exceptional physical and chemical properties, studies suggest that PDMSs may have been harmful to living beings and the environment. The most commonly used PDMSs, octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5), are typically found in cosmetic and personal hygiene products under the name of cyclomethicone, a type of endocrine disruptor that interferes with human hormone function (Montiel et al., 2019).

During the early 2000s, global production of siloxanes amounted to approximately 2,000,000 tons, and this figure has since grown to over 8,000,000 tons. The highest consumption rates are observed in China, North America, and Western Europe. Currently, nearly 50% of new skincare products contain at least one type of silicone, enhancing an increase in its research.

While historical studies raised significant concerns about the environmental and health risks associated with silicones, particularly D4 and D5, more recent research suggests that these risks may have been overstated. This evolving understanding indicates that the problem of silicone safety might not be as severe as previously thought. Meanwhile, certain scientists believe that there has been a widespread misconception that silicones are considered poorly biodegradable and that their accumulation poses a long-term risk not only for the environment but also for human health (Malka, M. 2023).

The more consistent analysis needed to uncover this issue

To see whether this is a misconception or not, the most common aspects in terms of product sustainability, including biodegradability within certain ecological conditions, potential impacts in such environments, and the ability to ensure the products’ stability, which is crucial in preventing high-risk substances from being disintegrated before actual applications, should be better comprehended and analyzed.

• Biodegradability of organosilicones in the atmosphere: In general, hydrolysis of PDMS produces dimethylsilanediol, which is biodegradable to carbon dioxide, silicon dioxide, and water by microbes. In addition to complementary, enzymatic cleavage, it may also depolymerize the chain ends, resulting in a significant drop in the polymer’s molecular weight. The rate of this depolymerization via hydrolytic degradation is typically rapid, and all siloxane linkages were broken within 21 to 30 days, leaving no trace of the silicone polymer. Similarly, because of its poor solubility and hydrophobic properties, silicone polymer is unlikely to collect in the aqueous compartment of the atmosphere, but instead, to be transported into the aqueous environment on transit to the wastewater treatment plant. Moreover, low-molecular-weight organosilicones (i.e., silanes, polysiloxanes, polycarbosilanes, and polysilsesquioxanes), which may eventually migrate to the atmosphere and react with the different atmospheric oxidant compartments, have a relatively short lifetime. These compounds can be completely removed from the atmosphere between 10 and 30 days (Graiver et al., 2003).
• Environmental impacts: In 2016, the SEHSC (Silicones Environmental, Health, and Safety Center), in collaboration with the EPA (Environmental Protection Agency), initiated an environmental monitoring program to analyze levels of D4 in the environment in order to obtain consistent data on the fundamental components used to produce octamethylcyclotetrasiloxane (D4), which helps the EPA assess this siloxanes’ environmental danger by utilizing real ambient concentrations rather than computer models to anticipate concentrations (ACC, 2016). Real-world data is being used by governments worldwide to guide chemical evaluations. In Canada, after reviewing the scientific evidence and environmental monitoring findings for D4, Environment Canada found that no limitations or product concentration limits were required for D4 usage in any application. The toxicologists determined that decamethylcyclopentasiloxane (D5) poses no negative environmental impact now or in the future, and the Canadian Environment Minister declared that no regulatory limitations on it are necessary (Canada Government, 2012).
• Ability to ensure the products’ stability: In the cosmetic field, the stability of the emulsions is a pivotal point not only in the development stage or to ensure the quality of the final product but also to make them highly marketable. In 2022, when comparing the stability of silicone-free emulsions to that of silicone-based ones, (Mancuso et al., 2022) discovered several key findings. From size distribution analysis, a better coherence of data appeared for silicone-based emulsion, as the size of the droplets was kept unchanged for an interval of 1?month.? Morphological investigation confirmed a homogenous droplet distribution for both samples. Silicones enhanced the viscosity, compactness, and strength of the cream, providing a suitable stability profile both at room temperature and when heated at 40°C.

Conclusions

Through various precisely-designed experiments,? recent studies have been able to conclude that silicone-based skincare products are highly biodegradable in different environments and play a key role in ensuring the products’ high stability compared to that of silicone-free ones. While numerous organosiloxanes degrade within a month in both atmospheric and aqueous environments, they pose no negative ecological impacts on those parts of Earth’s biosphere. In order to prove that skincare products containing silicones are superior to non-silicone ones, an in-depth understanding of the silicone-based biocompatibility is needed.

Recommendations

• Recommendations for a sustainable brand

The brands should stop greenwashing the products as ‘’silicone-free’’, which obviously leads consumers to misunderstand that the mentioned products are safer than those containing silicone. Therefore, if cosmetic brands want to sustainably advertise their products, the advisory role of independent research institutions or scientists (in this case, material scientists and micro-/nanotechnologists, or sustainability researchers with an extensive background in biochemistry, cosmetic industry, or material science) is vital. By providing consumers with reliable scientific findings, cosmetic brands will be able to make sustainable claims and make actual impacts. Furthermore, based on robust scientific databases, cosmetic brands should promote certain biodegradable synthetic ingredients, offering manufacturers a feasible chance to create finished goods with high functionality and negligible environmental footprints.

• Recommendations for a sustainable manufacturer

In order to synthesize ethical goods, manufacturers should put in a high effort to adapt suitable fabrication processes, especially in the preparation of raw input ingredients. Typically, it is estimated that extracting a kilogram of silicone from silica requires up to 235 NJ, or roughly 65,300 watt hours, making it extremely energy intensive and leading to the abundant release of carbon dioxide. However, in 2017, a University of Wisconsin–Madison chemistry professor came up with a new and more sustainable way to make silicon at much lower temperatures (at 650 degrees Celsius). This method, mimicking the Hall-Héroult process, derives the silicon from the calcium silicate, which can be dissolved in molten salts such as calcium chloride. Molten salt can then melt at a relatively low temperature and still dissolve calcium silicate. A “supporting electrolyte” of calcium oxide aids the speedy transfer of oxygen. All of these compounds are cheap and common. Therefore, by adapting more effective processes, the factories can not only reduce their cost of production, but also promote long-term sustainability.

References

1. Mancuso, A., Tarsitano, M., Udongo, B. P., Cristiano, M. C., Torella, D., Paolino, D., & Fresta, M. (2022). A comparison between silicone‐free and silicone‐based emulsions: Technological features and in vivo evaluation. International Journal of Cosmetic Science, 44(5), 514–529. https://doi.org/10.1111/ics.12800?
2. Anthony J. O’Lenick Jr., Anthony? J. Silicones for Personal Care, 2nd Edition, 2008, scientificspectator.com/documents/book service/Anthony O’Lenick Silicones for Personal Care.pdf.?
3. Mojsiewicz-Pieńkowska, K., Jamrógiewicz, M., Szymkowska, K., & Krenczkowska, D. (2016). Direct Human Contact with Siloxanes (Silicones) – Safety or Risk Part 1. Characteristics of Siloxanes (Silicones). Frontiers in Pharmacology, 7. https://doi.org/10.3389/fphar.2016.00132?
4. Sayyed, A., & Kulkarni, R. (2022). Silicone chemicals in cosmetics applications and their implications to the environment, health and sustainability. Research Gate. https://www.researchgate.net/publication/365761015_Silicone_chemicals_in_cosmetics_applications_and_their_implications_to_the_environment_health_and_sustainability?
5. Montiel, M. C., Máximo, F., Serrano‐Arnaldos, M., Ortega‐Requena, S., Murcia, M. D., & Bastida, J. (2019). Biocatalytic solutions to cyclomethicones problem in cosmetics. Engineering in Life Sciences, 19(5), 370–388. https://doi.org/10.1002/elsc.201800194
6. Bernaue, U, et al. (2016). Scientific Committee on Consumer Safety SCCS OPINION ON decamethylcyclopentasiloxane (cyclopentasiloxane, D5) in cosmetic products. https://doi.org/10.2875/841314
7. Malka, M. (2023, May 8). Silicones in cosmetics: risks & alternatives. Ingredients? Wellness. https://ingredientswellness.com/blogs/news/silicones-in-cosmetics-risks-alternative.?
8. Graiver, D., Farminer, K., & Narayan, R. (2003). A Review of the Fate and Effects of Silicones in the Environment. Journal of Polymers and the Environment, 11(4), 129–136. https://doi.org/10.1023/a:1026056129717.?
9. ACC. (2016). New Website Highlights Silicones’ Socioeconomic, Health and Environmental Benefits. Chemycal.com. https://chemycal.com/news/8dec1ea4-40c7-4f33-ad3c-3c829a21277e/New_Website_Highlights_Silicones_Socioeconomic_Health_and_Environmental_Benefits
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11. ??Tenenbaum, D. (2017). New technique could slash energy use in making silicon. Wisc.edu. https://news.wisc.edu/new-technique-could-slash-energy-use-in-making-silicon/?