Silicones in cosmetics – as bad as their reputation?

Silicones are widely used in the cosmetics industry and can be found in a multitude of products such as shampoos, conditioners, skin creams, and makeup. However, in recent years, they have come under criticism due to concerns regarding their environmental impact and their effects on the skin.

• They are said to be unnatural, prevent the skin from breathing, clog pores, and seal the hair.
• Furthermore, they are considered microplastics, are poorly biodegradable, and silicone in general is harmful to the environment.
• They are only used in cosmetics because they are cheap and merely make skin and hair appear more beautiful.

Everyone agrees on that ... ... ... right?

Why is that, actually?

In the end, is it perhaps not the case at all?

Let’s take a closer look at the topic that is surrounded by so many myths:

What are silicones?

Basic structure of polydimethylsiloxane - a silicone oil (INCI: Dimethicone)

 

Silicones are a group of chemical compounds that consist of silicon (Si), oxygen (O), carbon (C), and hydrogen (H). Unlike hydrocarbons, their backbone is not based on carbon, but on silicon and oxygen. This gives them completely different properties than hydrocarbon compounds (such as plant oils, paraffin, or beeswax).

Silicon is the most abundant element in the Earth's crust, so we are not going to run out of it anytime soon. It is found, for example, in quartz sand, which is also used to produce glass, among other things. To do this, one simply has to melt the sand.

In silicone production, it is not that simple: it requires several different chemical processes to manufacture a silicone product:

• Extraction of silicon
• Reaction of silicon with hydrochloric acid
• Hydrolysis
• Condensation
• Polymerization
• Cross-linking (e.g., in silicone elastomers, polysilicones, and silicone resins)

These steps are quite elaborate and also require a lot of energy—more than, for example, pressing almond oil from kernels. This makes silicones very specific and also expensive raw materials.

Silicones are therefore not a cheap substitute for vegetable oils

However, one should not forget that they are primarily used in construction, technology, or even in the kitchen (baking molds). Compared to that, only a small amount of silicone materials ends up in cosmetics.

They are also a huge group of substances and cannot always be compared with one another. There are very different groups of silicone substances, e.g.:

• Silicone oils
• Silicone elastomers (silicone rubber)
• Silicone powders
• Silicone resins
• Silicone emulsifiers (hydrophilic silicones)
• Chemically modified silicones (e.g., amino silicone oils in hair care)

Nevertheless, we want to look at what special features this group of substances generally has.

Advantages of silicones

Silicone is a material that is very stable. In chemistry, it is referred to as "inert." This means it does not react with other chemicals. Inert substances have the property of being non-toxic—they do not react with anything in our body and thus cannot cause any harm. Silicones used for cosmetics or medicine also have a high degree of purity, as they are extensively purified. For this reason:

Silicones are extremely well-tolerated by the skin

They have no allergy potential, unlike natural substances (in nature, there is always something that someone cannot tolerate). Therefore, they are often used in medical preparations, wound plasters, or lubricants that come into contact with mucous membranes.

Due to their special, silicon-based structure, silicones have further unique properties. One can actually imagine them a bit like liquid or elastic glass. Therefore, they are temperature-stable and water-repellent. They are also often used in coating agents or, as mentioned, in the kitchen (e.g., as a baking mold).

But another property makes them very special: as robust as they are, they are nevertheless air-permeable. This is due to their special, flexible, oxygen-containing molecular structure. This characteristic is also quite interesting in cosmetics. Silicone oils can be used, for example, for special protective creams against wetness or cold. They protect the skin better than oils or waxes, but still allow the skin to breathe. In this way, the skin is protected from moisture loss and environmental influences. Contrary to all claims that silicones have a strong occlusive effect, the opposite is the case:

Silicones are breathable and do not clog pores

Their superior protective effect is so pronounced that certain silicone oils (dimethicones) have been classified by the FDA (Food and Drug Administration) in the USA as an active ingredient for skin protection and regeneration.

Why are silicones used in cosmetics?

Their lightness makes them interesting for products for oily or blemished skin as well as in decorative cosmetics. They have no comedogenic potential and often feel much lighter than plant oils or ester oils. There are even volatile silicone oils, meaning those that evaporate on the skin—it doesn't get any lighter than that!

Besides these advantages, silicones are used primarily for one reason:

Silicones create a superior skin feel

This can also be justified by their chemical structure. Their molecular chains are much more flexible and mobile than hydrocarbon chains. Furthermore, they have a lower density, so they are actually lighter (in terms of weight) than most oils. In terms of skin feel, this manifests as a feather-light, silky sensation and very low greasiness. At the same time, they create a very smooth, lubricating film. However, the same applies here: there are various types of silicones—some richer and some lighter.

This makes them so interesting in hair care. They coat the hair fiber, protect the cuticle from friction, make the hair easy to comb, and produce brilliant shine.

Silicones are extremely hydrophobic (water-insoluble). Due to their water-repellent ability, they can protect hair from humidity. Even the smallest amounts in a shampoo or conditioner adhere very well to the hair. This is because they prefer to stick to the hair rather than remain dissolved in water—they don't like that at all. However, this also has a consequence:

Silicones accumulate in the hair and cause a build-up effect

This is one of the rumors that corresponds to the truth. Therefore, it is advisable to use silicone-containing products sparingly. If a build-up effect occurs, one should use a deep-cleansing shampoo or perhaps skip the conditioner once or twice. This way, the silicone film does not become too heavy and the advantages prevail.

In fact, the bad reputation of silicones probably even comes from this corner: there used to be a 2-in-1 shampoo on the market that probably overdid it with the conditioning effect and contained too much silicone. Over time, the hair became so heavily sealed that hairdressers complained that hair dye or perm products were no longer working as well. This stigma was then likely transferred to skincare, and the myth spread that silicones had a sealing effect. However, we now know that the opposite is the case.

Another aspect that silicone oils have in common is:

Silicones have low surface tension

This means they are very fluid and spread very well. This is also the reason why they are used in technical fields as lubricants. They glide very well and creep into the smallest spaces.

In cosmetics, we can take advantage of this because they spread very well on the skin and ensure that products can be applied effortlessly. Even at low concentrations, they unfold their effect. Their low surface tension also means that silicones have a defoaming effect. They can easily spread over a liquid and form a thin layer on its surface. When silicones get into a foamy liquid, they distribute themselves over the foam bubbles and break them up due to their low surface tension. This reduces the foam and the liquid unfolds its defoaming effect.

This effect is used specifically in emulsions to prevent "whitening" during application. The phenomenon of whitening is nothing more than the emulsifier in the product mixing with air and foaming up briefly when applied to the skin. Adding even a small amount of silicone oil to the formulation completely eliminates this effect!

Solubility of silicone oils

Incidentally, due to their different chemical structure, surface tension, and polarity, silicone oils are sometimes difficult to dissolve and mix in hydrocarbon-based oils. In a cosmetic formulation, silicones therefore essentially represent a third phase alongside the water and oil phases. However, they can sometimes be processed with certain cosmetic oils and emulsifiers. There are also special, modified silicone raw materials that are indeed miscible with water (silicone emulsifiers) or hydrocarbons (organosilicones), or that allow for miscibility. When compatible oils and silicone oils are used together, silicones can indeed be integrated into the oil phase and can meaningfully complement it with their special properties.

Silicones in the environment

So they don't harm us, but what about silicones in the environment?

Silicones are, as already mentioned, very inert. They react poorly with other things and cannot be metabolized by bacteria. The consequence is:

Silicones are not biodegradable...

 

... but what does "biodegradable" actually mean?

"Biodegradable" is not generally the same as "degradable." It refers much more to the fact that a substance can be broken down and metabolized by microorganisms. This does not mean, however, that there are no other ways a substance can be broken down—e.g., chemically or by UV radiation.

In fact, there are studies indicating that silicones can be broken down in the atmosphere by sunlight into their smallest components:

• Silicon dioxide (sand)
• CO₂
• and water

So, these are not substances that wouldn't also occur in nature. The same applies to the soil. Through pressure and naturally occurring acid clays, silicones are also broken down into their components. However, this does not happen overnight. While substances classified as "readily biodegradable" are largely broken down within 30 days, this can take up to 2 years for a silicone—just as a comparison.

However, what is important for a substance is not only rapid degradability, but also its environmental compatibility. And we have already mentioned this—silicones are completely harmless and cannot cause any harm—they are neither toxic to the environment nor in bodies of water. Here, the comparison with glass is again appropriate: it degrades slowly, but we would never think that a piece of glass in the forest poisons the soil.

Therefore, silicone cannot be compared with plastic (hydrocarbon plastic)—the basic chemical structure is completely different—which breaks down into the smallest parts and can accumulate in the environment as well as in organisms. Moreover, hydrocarbon-based plastics sometimes take thousands of years to decompose.

Silicone is not plastic and is therefore not part of the microplastic debate

Silicones in water

Something similar applies to wastewater. When we wash a silicone-containing product off our skin, it first ends up in the drain, in the wastewater, and thus sooner or later in the sewage treatment plant. Because it is very hydrophobic, it can be easily filtered out of the water because it prefers to get stuck in the filter rather than remain in the water.

After water purification, the sewage sludge is dried and burned in the waste incineration plant. During this process, silicon dioxide (sand) is formed from the silicone, which of course cannot burn. Here, it can indeed happen that the sand in the plant's turbine leads to a sandblast-like effect, which can cause damage.

Silicones can pose a problem in waste incineration plants

However, it does not leave behind toxic substances when burned (of course, there are exceptions for specially modified silicones). Silicones are therefore not a problem for the environment per se. With the exception of their energy-intensive production and possible technical problems in waste management, there are no major concerns in this regard.

All in all, silicones are performance raw materials with special properties that cannot simply be replaced by other substances in cosmetics. It is certainly not always necessary to integrate a silicone raw material into a cosmetic product, but there are cases where this is quite sensible. And then it is good to know that one does not need to be afraid of them.

Quite the contrary ...