What Is Pei-45?
Pei-45 is the cosmetic name for a homopolymer of aziridine, better known in scientific circles as polyethylenimine. Picture a long, flexible chain made by linking many tiny aziridine rings together; that chain gives the ingredient its thickening power. Pei-45 is created through a controlled polymerization process where aziridine monomers are opened and joined, forming a water-soluble polymer with a medium molecular weight that delivers dependable texture control.
The material first attracted attention in the mid-20th century as chemists explored synthetic polymers that could enhance product feel without irritating skin. Over time manufacturers refined the production process to remove unreacted monomers, making the polymer gentle enough for leave-on and rinse-off cosmetics. Today Pei-45 is sourced from specialized chemical plants that follow cosmetic-grade purity standards.
You will most often spot Pei-45 in lotions, creams, facial masks, hair conditioners, styling gels, sunscreens and various anti-aging serums. Brands like it because a small amount can transform a runny formula into a silky lotion or a light gel into a cushiony cream.
Pei-45’s Cosmetic Benefits/Uses
Pei-45 is valued for one key role in personal care products.
Viscosity controlling: By tightening or loosening the internal structure of a formula Pei-45 lets chemists dial in the exact thickness they want. A stable viscosity keeps a product from separating in the bottle, helps active ingredients stay evenly distributed and gives users the smooth, luxurious feel they expect when they spread a cream or mask on skin or hair.
Who Can Use Pei-45
Because Pei-45 is a largely inert, water-soluble polymer it is considered suitable for all skin types, including dry, oily, combination and sensitive skin. Its main job is to manage texture rather than treat skin directly, so it is unlikely to trigger breakouts or exacerbate existing conditions. Very sensitive or compromised skin could react to trace impurities but this is rare with cosmetic-grade material.
The ingredient is fully synthetic and contains no animal-derived components, making it appropriate for both vegans and vegetarians. It is also produced without the use of animal testing when sourced from suppliers that follow modern cruelty-free standards.
Current safety data does not flag any specific risks for people who are pregnant or breastfeeding. Still, this is not medical advice and anyone who is expecting or nursing should show the full product label to a healthcare professional before use to be safe.
Pei-45 does not increase photosensitivity, so it will not make skin more prone to sunburn. It is stable across a wide pH range and pairs well with common skincare actives, so there are no special use-time restrictions or ingredient conflicts to note.
Potential Side Effects/Adverse Reactions
Side effects from topical Pei-45 can vary from person to person. The points below outline potential reactions, but they are uncommon when the ingredient is used at standard cosmetic levels and formulated correctly.
- Mild skin irritation – a temporary sensation of burning, stinging or redness can occur in very sensitive users, usually due to residual monomer traces or other formula components
- Allergic contact dermatitis – an allergy to polyethylenimine is rare but can appear as itchy, inflamed patches after repeated exposure
- Eye irritation – direct contact with the eyes may cause watering or discomfort, especially in high-concentration products such as hair styling gels
- Product buildup on hair – frequent use in leave-on hair products can lead to a dull feeling film that requires a clarifying shampoo to remove
If any discomfort, rash or other adverse effect appears while using a product that contains Pei-45 stop application promptly and seek advice from a medical professional.
Comedogenic Rating
Rating: 0/5. Pei-45 is a water-soluble polymer used at very low concentrations, so it rinses away easily and does not leave an oily or occlusive film that could trap debris in pores. Its molecular weight keeps it mostly on the surface and it lacks fatty chains that typically cause clogging. Because of this profile Pei-45 is considered safe for acne-prone or breakout-prone skin. Trace monomer residues could irritate exceptionally reactive skin but they do not increase comedogenicity.
Summary
Pei-45 is a texture manager that thickens and stabilizes cosmetic formulas by creating a flexible network within water phases, holding pigments and actives in even suspension and delivering a smooth glide during application. It is not a blockbuster household name but formulators appreciate the way a tiny dose can turn a thin blend into a luxurious cream, which keeps it quietly popular in lotions, masks, sunscreens and hair care products.
Current safety data shows Pei-45 to be low risk for irritation or sensitization when used at cosmetic levels, though highly sensitive users could react to minor impurities. As with any new skincare ingredient it is smart to perform a quick patch test when trying a fresh product that contains Pei-45 just to make sure your skin agrees with it.