What Is Pei-7?
Pei-7 is the cosmetic name for aziridine homopolymer, a synthetic chain of tiny three-membered nitrogen rings. Unlike ingredients that come from plants or minerals, Pei-7 is created entirely in a lab where chemists link individual aziridine units together in a controlled reaction. This results in a clear, water-soluble polymer that feels slippery at low levels yet thickens formulas when used in greater amounts.
The material first appeared in industrial coatings during the 1970s, prized for its ability to adjust flow without adding weight. Formulators in personal care soon noticed these same traits could help stabilize creams and gels, so Pei-7 found its way into cosmetics in the late 1980s as brands searched for alternatives to heavier waxes.
Manufacturing begins with purified aziridine produced from ethanolamine. The monomer is placed in a reactor, initiated with heat or a catalyst, then allowed to polymerize into chains of varying length. After neutralization and filtration, the polymer is diluted into a concentrated aqueous solution that ships to cosmetic labs.
You will most often see Pei-7 in rinse-off masks, sheet mask serums, lightweight moisturizers, anti-aging gels, hair conditioners and specialty treatments that need a silky yet non-greasy texture.
Pei-7’s Cosmetic Benefits/Uses
Pei-7 serves a single but important purpose in beauty formulas.
Viscosity controlling: By thickening water-based phases at very low concentrations, Pei-7 helps create smooth, even textures that stay put where you apply them. It keeps oil and water components from separating, suspends actives so they spread uniformly and gives products a pleasant glide without leaving a heavy film.
Who Can Use Pei-7
Because Pei-7 is a neutral, water-soluble polymer it is generally well tolerated by all skin types including oily, combination, dry and sensitive skin. It does not clog pores or leave an occlusive film so acne-prone users usually do fine with it.
The ingredient is synthesized entirely from petrochemical feedstocks with no animal-derived components, making it suitable for vegans and vegetarians. As always, finished products may differ so check the full ingredient list if cruelty-free sourcing is a priority.
No studies suggest Pei-7 poses a risk to women who are pregnant or breastfeeding when used topically in the tiny amounts found in cosmetics. That said this is not medical advice and anyone who is expecting or nursing should show the full product label to a healthcare professional before adding it to a routine.
Pei-7 is not known to increase photosensitivity and it plays well with most other skincare actives, from acids to retinoids, without destabilizing them.
Potential Side Effects/Adverse Reactions
Responses to topical Pei-7 vary from person to person. The effects below are considered possible yet unlikely for most users when the ingredient is used correctly by the formulator.
- Mild transient stinging or burning on very sensitive skin
- Temporary redness or flushing, usually resolving within minutes
- Rare allergic contact dermatitis presenting as itching, rash or swelling
- Eye irritation if the product accidentally gets into the eyes
- Buildup on hair shafts leading to dullness when high concentrations are used in leave-on conditioners
If any of these reactions occur discontinue use and consult a healthcare professional.
Comedogenic Rating
Rating: 0 / 5
Pei-7 is a large, water-soluble polymer that sits on the surface and rinses away easily so it does not block pores or trap oil. It lacks the fatty or waxy groups that usually contribute to breakouts, which is why formulators consider it non-comedogenic.
Suitable for acne-prone or breakout-prone skin.
As with any ingredient, the finished formula and how often it is used can influence pore clogging potential, but Pei-7 itself is about as low risk as it gets.
Summary
Pei-7 is a lab-made chain of aziridine rings that acts primarily as a viscosity controller. At tiny doses it thickens the water phase, stabilizes emulsions and keeps pigments or actives evenly suspended, all while giving a silky glide with no greasy afterfeel. These abilities make it a quiet workhorse in sheet mask serums, light gels, rinse-off masks and some conditioners.
It is not a headline ingredient so you will not see it splashed across marketing campaigns, yet chemists reach for it when they need reliable flow control without heaviness. Because it is water-soluble, free of fats and used at very low levels, safety profiles show minimal irritation or sensitization. Still, skin is personal so do a small patch test when trying any new product that lists Pei-7 to ensure it agrees with you.