Carbomer

Carbomer is the umbrella name for a family of synthetic polymers built from acrylic acid, lightly cross-linked so that each molecule behaves like a microscopic sponge rather than a free-floating chain. As a raw material it arrives as a light, fluffy white powder. In a finished product you will never see it, because its entire job is to turn a thin, watery liquid into a clear, cushioned gel.

On an ingredient list it usually appears simply as Carbomer, sometimes followed by a number such as 934, 940 or 980. Those numbers refer to the grade and molecular structure the manufacturer selected, not to strength or quality. Closely related alkyl-modified versions — declared on packs as Acrylates/C10-30 Alkyl Acrylate Crosspolymer — carry oil-loving side chains as well, so they can hold oil and water together on their own. That is why they turn up in lightweight lotions and gel-creams.

Carbomer is a texture and stability ingredient rather than an active. It does not brighten, exfoliate or hydrate in its own right. What it does is give serums, gel cleansers, sunscreens, hair styling products and make-up their body, keep pigments, capsules and oil droplets evenly suspended, and stop a formula separating in the jar. Use levels are typically low — often well under 1% — so it makes up a small fraction of any formula it appears in.

Straight out of the drum, carbomer is acidic and its chains sit tightly coiled, so simply stirring the powder into water gives you a thin, cloudy dispersion and nothing more. The gel only appears once a formulator neutralises it with a base — sodium hydroxide, aminomethyl propanol and triethanolamine are common choices. That step strips protons from the acid groups running along the polymer backbone, leaving a chain studded with negative charges.

Like charges repel one another, so the coils spring open and stretch out, and the swollen chains trap water between them in a three-dimensional mesh. That mesh is what you feel as gel. Importantly, it has a yield point: at rest it holds its shape, but the moment you shear it between your fingers it thins and spreads easily, then re-sets on the skin.

Because the whole structure is charge-driven, it is sensitive to company. Salts and other electrolytes, or strongly positively charged ingredients, screen those negative charges and can thin the gel out. And because a carbomer molecule is enormous by skincare standards, it stays on the surface rather than passing into skin — it simply holds the rest of the formula in even contact with it.

Carbomer is a formulator's decision rather than a shopper's, but spotting it on a label tells you how the product will behave.

• Expect a gel, not a cream. Carbomer-thickened serums and gel moisturisers sit light on skin and absorb quickly, which tends to suit oily and combination skin, warm weather, and layering under SPF.
• Apply thin layers if you get pilling. Balling up under sunscreen or make-up can come down to how a gel is formulated as much as how it is applied, but using less, patting rather than rubbing, and letting each layer settle usually solves it.
• Mind what goes on top. Rubbing a rich balm or a silicone primer straight over a freshly applied gel is the most common cause of rolling.
• No frequency limit. Nothing about carbomer restricts how often you use a product — follow the directions on the pack.
• It plays well with actives. Carbomer is a thickener, not an active, so it does not react with vitamin C, retinoids or acids on skin — layer as the product directions tell you to.