What Is Polyquaternium-18?
Polyquaternium-18 is a synthetic, water-soluble polymer classified as a quaternary ammonium compound. Its backbone is built from repeating units of ethylene oxide and propylene diamine that have been quaternized, giving the molecule a permanent positive charge. That positive charge lets it bind readily to the negatively charged surface of hair and skin. The ingredient was introduced in the late 1980s as chemists looked for gentler alternatives to traditional conditioning agents that could provide lasting smoothness without weighing hair down.
The manufacturing process starts with a controlled reaction between dimethylamine, epichlorohydrin and adipic acid derivatives. These building blocks are polymerized, then quaternized with methyl chloride to lock in the cationic sites. The final step is purification to remove residual salts and unreacted monomers, yielding a clear aqueous solution or dry powder.
Because of its strong affinity for keratin, Polyquaternium-18 shows up most often in rinse-off and leave-in hair products such as conditioners, masks, detangling sprays, styling gels and color-protection formulas. It can also be found in some facial masks, wash-off cleansers and moisturizing serums where a light film-forming effect is desired for a soft, hydrated feel.
Polyquaternium-18’s Cosmetic Benefits/Uses
Below are the main ways this ingredient improves personal care products
- Antistatic: The positive charge neutralizes static electricity on hair fibers, reducing flyaways and frizz. This results in smoother strands that are easier to comb and style, especially in dry or low-humidity environments.
- Film forming: It creates a thin, flexible coating that locks moisture into hair or skin. On hair the film adds light body, enhances gloss and helps protect against mechanical damage from brushing. On skin it imparts a silky finish and can boost the longevity of other active ingredients by keeping them closer to the surface.
Who Can Use Polyquaternium-18
Polyquaternium-18 is generally considered suitable for all skin and hair types, including dry, normal and combination. Sensitive skin also tends to tolerate it well because the polymer stays on the surface rather than penetrating deeply and has a low potential for irritation. Extremely oily scalps may notice product buildup more quickly, so lighter formulations or less frequent use can help prevent a weighed-down feel.
The ingredient is produced entirely through synthetic processes and contains no animal-derived raw materials, making it appropriate for vegetarians and vegans. As with most mainstream conditioning agents, it is also free of common allergens such as gluten or soy.
No data point to systemic absorption or endocrine disruption, so products containing Polyquaternium-18 are not expected to pose a risk to pregnant or breastfeeding individuals. That said, this is not medical advice; anyone who is expecting or nursing should clear any cosmetic routine with a qualified healthcare provider just to be safe.
Polyquaternium-18 does not increase photosensitivity, so it will not make skin or hair more prone to sunburn. It is odorless and colorless, works across the full pH range used in cosmetics and is compatible with most other ingredients, which further broadens its suitability.
Potential Side Effects/Adverse Reactions
Reactions to topical Polyquaternium-18 differ from person to person. The following points outline possible but uncommon side effects; most people will never experience them when the ingredient is used as intended by the product manufacturer.
- Mild skin or scalp irritation
- Allergic contact dermatitis in individuals already sensitized to quaternary ammonium compounds
- Transient eye stinging if rinse-off products accidentally enter the eyes
- Product buildup leading to dull or heavy hair when very high concentrations are used repeatedly without clarifying shampoos
If any of these effects occur stop using the product and seek guidance from a medical professional or pharmacist.
Comedogenic Rating
Rating: 0 / 5 – Polyquaternium-18 is a large, water-soluble molecule that sits on the surface of skin rather than sinking into pores, so it is not known to trap oil or dead cells that could spark breakouts. Its cationic charge also keeps it dispersed in water-based formulas, further lowering any clogging risk. Because of this, it is generally considered safe for acne-prone or easily congested skin. At very high percentages a sticky feel could hold on to grime if cleansing is skipped, but normal cosmetic use poses little concern.
Summary
Polyquaternium-18 is mainly valued for two things: it fights static by neutralizing the negative charge on hair and skin, and it forms a thin flexible film that locks in moisture adds slip and boosts shine. The positive charge of the polymer pulls it toward the keratin surface then its long chain structure lays down a smooth coating that stays put until the next wash.
You will see it most often in conditioners leave-in sprays and styling gels while it pops up only occasionally in skincare, so it sits in the middle ground of popularity—well known to formulators yet not a headline ingredient like hyaluronic acid.
Safety data show a very low rate of irritation or allergy and no evidence of systemic absorption. Still, every skin is unique so it is wise to patch test any new product that contains Polyquaternium-18 before making it a regular part of your routine.