Swapping out silicone in our skin and hair products

More than most chemical markets, the cosmetic and personal care ingredient industry is subject to the feelings and fancies of everyday consumers. People tend to be conscious of the safety and environmental impact of the chemicals they apply directly to their skin and hair. It’s usually a good thing: the personal care sector often leads the way on sustainability, exerting influence on supply chains, novel chemistry, and industry practices.

That responsiveness, though, also makes the industry susceptible to fads. Fashionable ingredients take flight far beyond what their actual performance merits, and a bit of bad press can imperil whole categories before rigorous science starts to speak.

Silicones are in the latter camp as they face broadening bans and diminishing popularity despite what many describe as strong safety profiles, excellent performance, and modest environmental impact. Though they’d mostly rather not, chemical suppliers and consumer product makers are figuring out where and how they can swap out silicones in personal care goods.

Talking about silicones as a class is a bit misleading because they are a diverse group of chemicals rather than a single substance. Silicones are prized for their ability to make skin and hair feel smooth, light, and clean; the transfer resistance they impart to lipstick and other color cosmetics; and the way they spread easily across human surfaces.

Silicones are not, however, biobased or biodegradable, two adjectives of paramount importance in the modern beauty industry. “Silicones have fantastic functionality. And they’ve been around for 60, 70 years,” says Neil Burns, CEO of the personal care ingredient start-up P2 Science, which makes silicone replacements. “The key thing that really got folks looking at silicones is the lack of biodegradability.”

Chemically speaking, all silicones are polymer chains of alternating silicon and oxygen atoms with two hydrocarbon groups—usually methyls—attached to each silicon. The repeating unit is called a siloxane. Cyclic silicones are the simplest, rings with three to six dimethylsiloxane units. For example, decamethylcyclopentasiloxane, known as D₅, is a highly volatile ingredient used at high concentrations in solid deodorants.

Linear silicones are more complicated. Also known as dimethicones or polydimethylsiloxanes, the ones used in personal care range in length from just a few repeating units up to around 2,000, says Michael Fevola, vice president for R&D at Inolex, a chemical maker specializing in health and beauty ingredients, including silicone replacements. The shorter linear silicones flow like water, whereas the longer ones form a thick gum. In practice, Fevola says, the silicone ingredients used by the personal care industry are blends.

Functionalizing silicones by changing what’s attached to the silicon atoms unlocks nearly all the complexity and possibility that organic chemistry has to offer. Phenyl side chains add shine to hair and glow to skin, alkyl groups increase a silicone’s affinity for oils, and alcohol groups enhance water solubility and alter volatility.

Cyclic silicones are the most embattled. The European Union restricts D₄ (octamethylcyclotetrasiloxane) and D₅ to 0.1% or less in wash-off personal care products. It is expected to add D₆ (dodecamethylcyclohexasiloxane) to that restriction and extend it to leave-on products and other consumer goods as soon as this summer. The European Chemicals Agency lists D₄, D₅, and D₆ as persistent, bioaccumulative, and toxic.

Neslihan Utkan, who leads the Americas personal and home care division of the chemical maker Clariant, says the EU restrictions are driving consumer product makers to reformulate without silicones. She says her customers expect the bans to spread both in scope and geography, and they want to be ready. They also prefer to have one formulation they can sell around the world.

P2’s Burns says bans in the US are possible but not assured. “Things generally, in the field of sustainability, spread from Europe into the US,” he says.

So how good is the case for getting rid of silicones? It depends on where you look and exactly what question you’re asking.

Diane Dabkowski, a chemist and R&D manager at the home and personal care company Avon, says concerns from the public about silicones started in the 1990s, spurred by disclosures from the chemical industry.

The silicone maker Dow Corning, then a joint venture but now part of Dow, collaborated with the University of Rochester on a $30 million research program examining the safety of D₄, D₅, and D₆ as well as three light, linear silicones. Most products caused no ill effects on rodents, but high doses of D₄ resulted in some reproductive harm, inhibiting fetal implantation and causing smaller litters to be born. D₄ also caused some transient enlargement of rodent livers.

The study’s overall conclusion was that silicones are safe at hundreds or thousands of times the exposure that humans might encounter. Nonetheless, it led to an overnight exit from D₄ in most applications and drew scrutiny to silicones overall, Dabkowski says.

Die-hard critics of silicones are hard to find. Even among companies that sell silicone replacements, silicones are well regarded. People in the personal care industry say P2’s ingredients are probably the closest they have to a one-for-one drop-in replacement. Yet Burns says, “I’ve been in the chemical industry my whole life. Silicones are great-performing chemicals, and they’re certainly not—I don’t believe—harmful to human health.”

In addition to toxicity, regulators are concerned about the buildup of silicone waste in the environment. In the EU documentation supporting the bans, regulators argue that cyclic silicones persist in oxygen-poor aquatic environments and can accumulate in certain organisms at the bottom of the food web, such as plankton, where they can interfere with reproduction and longevity.

Though the academic literature is mixed when it comes to the harm to sea life, EU regulators and advocates for the bans argue that caution is the prudent choice because silicones take 50–500 years to break down completely into silica.

Unsurprisingly, the lifetimes of silicones in the environment depend heavily on what part of the environment you’re looking at. According to a recent review, cyclic siloxanes readily react in the air with ozone as well as nitrate and hydroxyl radicals to form small water-soluble silanols. In water, hydrolysis and photolysis reactions lead to half-lives between 17 and 315 days. The half-life in soil and sediment ranges from less than 1 day to more than 3 years (Environ. Sci. Pollut. Res. 2023, DOI: 10.1007/s11356-023-25568-7).

But this isn’t biodegradation. Isabel Almiro do Vale, global marketing and strategy director for personal care at Dow, says the industry focus on biodegradability puts silicones at an unfair disadvantage because their degradation mechanisms aren’t biological.

Common biodegradation tests measure how well microorganisms oxidize the carbon in a material into carbon dioxide, and that’s not a good fit for a silicon-based material, do Vale says. Dow is working with other silicone producers and regulators to develop and select the right analytical methods to determine the lifetimes of silicones in the environment.

“What we are doing at the moment is generating an extensive body of data about cyclics and linears,” do Vale says. “We are continuing to work with governments and regulatory agencies.”

While silicone advocates prepare their case, silicone replacements are eating away at the market share in personal care.

BASF, which doesn’t make silicones, has been working on alternatives for more than a decade, says Joel Basilan, the firm’s head of application technology for personal care in North America. “We know that this is a trending need, so we continue to innovate in this area.”

Basilan says the firm starts by understanding what effects a customer is looking to create and then customizes blends of starches, polysaccharides, gums, oils, and polymers to suit the need. It recently added artificial intelligence and a suite of digital tools called D’lite to help customers develop formulations. “For BASF, it’s important to bridge the performance between synthetics and naturals,” he says.

Even Dow, one of the largest producers of silicones, has replacement options. “Our strategy, independent of the chemistry, is to understand what the customers want,” do Vale says. She points to one recent launch, EcoSmooth universal fluid 1100, which is a biobased and biodegradable carrier fluid or solvent that can replace D₅ in some applications.

P2’s cosmetic ingredients that compete with silicones echo the dimethyl ether structural motif common to silicones, but they use carbon-based repeating units—terpenes such as citronellol, specifically—instead of silicon-based ones.

“This was just a hypothesis of ours when we first developed the products, but it turns out it’s that motif which gives these products the functionality with respect to emollience, slip, glide, gloss, shine—all these things which silicones do extraordinarily well,” Burns says. “Of course what we sell is not identical, but there’s enough functional overlap.”

The chemical distributor Univar Solutions has both silicones and silicone replacements. “If a customer is interested in a specific skin feel and they are not averse to using silicones, that’s probably the best option,” says Arnita Wofford, Univar’s global marketing and technical director for beauty and personal care. “The customer who comes in and says, ‘I want this long-wear, waterproof eyeliner, but I want it to be 100% natural,’ well, let’s take a step back. If you’re not willing to use any synthetic ingredients, you will not achieve that goal. So it’s also our job to keep them honest and manage their expectations.”

Dabkowski says Avon has evaluated the replacement offerings from numerous suppliers. In general-purpose lotions and products in which silicone is a minor ingredient, the replacements are OK, she says, but not when the silicone is part of the backbone of the formula. And the replacements come with functional trade-offs. Esters, including triglycerides, can impart shine to hair-styling products, for example, but they won’t dry as quickly and might give a greasier feel. Adding starches can reduce greasiness but increases weight and complexity.

And often, there’s an added cost. “Even though there have been price increases of the silicones due to the other demands for the material, it’s still very cost effective,” Dabkowski says. And major consumer product firms such as Avon don’t want to replace one excellent ingredient with multiple that, together, are pretty good but cost more.

Do Vale says it won’t work to completely ditch silicones, and there’s little need to. “I think what is missing in the conversation is to identify the subsets of chemistry where silicone is used but may no longer be required or where the industry would like to find alternatives,” she says.

Nikola Matic, vice president for chemicals and materials at the consulting firm Kline, has been watching and writing about silicone replacements since at least 2013. But he says it wasn’t until 2021 that cyclic silicone sales started to decrease, and then only in Europe. Now those numbers are going down in Europe and the US, but not worldwide.

Outside Europe, Univar’s customers are moving slowly away from silicones, Wofford says. “That shift requires formulation support, and not every brand is equipped to reformulate. It’s a time and capital investment,” she says.

However, Matic says, most new formulations are being made without silicones, especially cyclics, a trend that is likely to dampen demand over time. “The switch is going to take place. It’s going to happen in the US too. Formulations are global,” he says.

Clariant’s portfolio embodies that trend, Utkan says. The firm offers silicones, but its newer ingredients are natural and biobased. She also says a generational shift is happening. Baby boomers grew up with silicone and want that feel. Younger generations know it less, so they don’t miss it.

Demand for biobased personal care products is growing and looking like a long-term shift, not a fad, Matic adds. Silicones are derived from quartz through an energy-intensive process in which, with the exception of some new partially biobased offerings from Wacker Chemie, the carbon comes from petrochemical methanol. They don’t count as natural, Matic says, and that casts as much of a shadow on their future as any concern about their safety.

The tension around silicones and silicone replacements is an unsolved problem for the beauty and personal care industry, Dabkowski says. “You really can’t spin silicones into a natural,” but the replacements don’t match the performance or cost. “The jury is still out. I don’t know of any solution to tick all the boxes,” she says.