What Is Polyquaternium-91?
Polyquaternium-91 is a man-made polymer created by linking together hydroxypropyl methacrylate and polyethylene glycol methacrylate, then treating the chain with ethyltrimonium chloride to give it a permanent positive charge. Because it is fully synthetic it is produced in controlled lab settings, which helps keep its purity consistent from batch to batch.
The polymer was introduced to the personal care market in the early 2000s as formulators searched for conditioning agents that would cling to hair without weighing it down. Earlier quaternary compounds could build up or feel greasy, so chemists tweaked the structure until they reached the balanced profile found in Polyquaternium-91.
Manufacturing starts with a water-based reaction that links the two methacrylate monomers into long chains. A quaternization step follows, where ethyltrimonium chloride attaches to the backbone, giving the finished polymer its cationic nature. After purification and drying the ingredient appears as a clear viscous fluid or granulated solid that dissolves easily in water.
Because of its ability to bond lightly to the hair shaft and form a smooth film, Polyquaternium-91 shows up most often in rinse-off and leave-in products such as shampoos, conditioners, hair masks, anti-frizz serums, color-protection lines and styling creams. It can also appear in sprays and gels where a soft flexible hold is desired.
Polyquaternium-91’s Cosmetic Benefits/Uses
In cosmetic formulas Polyquaternium-91 serves two main roles that work together to improve the look and feel of hair.
- Film forming: The polymer spreads over individual strands and dries into a thin breathable layer. This film smooths rough cuticles, reduces static, locks in moisture and offers mild resistance to humidity, helping styles last longer without stiffness.
- Hair conditioning: The positive charge of Polyquaternium-91 is attracted to the slightly negative surface of hair, so it deposits exactly where damage is greatest. This selective bonding boosts slip during wet combing, cuts down on breakage, leaves hair feeling softer and enhances natural shine.
Who Can Use Polyquaternium-91
Because it is a lightweight, water-soluble conditioning agent, Polyquaternium-91 suits all scalp and hair types including oily, dry and sensitive. It does not clog pores or leave an occlusive residue, so most people can tolerate it without discomfort. Those with very sensitive or allergy-prone skin should still pay attention to how their scalp feels after use, but the ingredient itself is regarded as mild.
The polymer is fully synthetic and contains no animal-derived substances, which makes it compatible with vegan and vegetarian lifestyles provided the finished product has not been tested on animals.
No specific warnings exist for pregnant or breastfeeding women; the molecule stays on the hair surface and is not expected to absorb through the skin in meaningful amounts. This is not medical advice and anyone who is pregnant or nursing should run any haircare product past a doctor to be on the safe side.
Polyquaternium-91 does not increase photosensitivity and can be used alongside sun exposure without extra precautions. It is also color-safe and can be layered with other common styling ingredients such as silicones, proteins or light oils.
Potential Side Effects/Adverse Reactions
Side effects from topical use of Polyquaternium-91 can vary between individuals. The following points cover potential reactions only; when the ingredient is properly formulated most users experience none of these issues.
- Scalp irritation or itching
- Redness or mild contact dermatitis in individuals allergic to quaternary compounds
- Eye irritation if the product accidentally runs into the eyes
- Product buildup leading to dull-looking hair when used in high concentrations or layered excessively
- Respiratory discomfort if inhaled in aerosol form during spraying
If any of these reactions occur stop using the product and consult a healthcare professional
Comedogenic Rating
Rating: 0 / 5
Polyquaternium-91 is highly water soluble, creates a breathable film and is designed to rinse away cleanly, all of which translate to an extremely low likelihood of blocking pores. It does not contain heavy oils or waxes that typically sit on skin and trigger comedones, and real-world usage reports rarely link it to acne flare-ups.
Suitable for acne-prone or breakout-prone skin.
Because this polymer stays mainly on the hair shaft and washes off with water, any residue that might contact facial skin is minimal, further supporting its non-comedogenic profile.
Summary
Polyquaternium-91 acts as a film former and hair conditioner. Its cationic charge lets it attach to damaged spots on the hair surface, smoothing cuticles, cutting static and adding light slip. Once deposited it forms a thin flexible layer that helps lock in moisture and protect against humidity so styles hold longer.
While not as famous as silicones or classic quats, this ingredient has carved out a steady niche in modern shampoos, conditioners, leave-ins and styling products aimed at a clean lightweight feel.
Industry and safety panel reviews rate Polyquaternium-91 as low risk for irritation or systemic absorption when used in rinse-off or leave-on formulas. Still, skin chemistry differs from person to person, so patch testing any new product is a smart habit.