What Is Cellulose Acetate Phthalate?
Cellulose Acetate Phthalate, often shortened to CAP, comes from cellulose, the natural fiber found in cotton and wood pulp. Chemists first turn cellulose into cellulose acetate by adding acetic acid, then react that with phthalic anhydride. The result is a white to off-white powder that dissolves in certain solvents and forms a clear flexible film when it dries.
CAP was introduced in the 1950s for use in pharmaceutical coatings, where its strong yet easily removed film made it ideal for protecting tablets. Formulators soon noticed the same properties could improve cosmetics, and by the late 1960s CAP began appearing in nail lacquers and hair sprays. Today manufacturers still follow a similar process: purify plant-derived cellulose, acetylate it, add phthalic anhydride under controlled heat, then filter and dry the finished polymer.
You will most often find Cellulose Acetate Phthalate in products that need a smooth durable layer on the skin or hair. Common examples include nail polishes, mascara, liquid eyeliners, long-wear foundations, facial masks and certain styling sprays. Its transparent film helps pigments stay put and keeps moisture or oils from breaking through too quickly.
Cellulose Acetate Phthalate’s Cosmetic Benefits/Uses
CAP acts as a film forming agent. When it dries it creates a thin invisible shield that anchors colorants and active ingredients to the surface, resists smudging and flaking and helps the product last longer through sweat, sebum and light water exposure.
Who Can Use Cellulose Acetate Phthalate
Because CAP sits on top of the skin and hair rather than sinking in, it is generally suitable for dry, normal, combination and oily skin types. Those with very oily or acne-prone skin who dislike any sensation of film might find heavy formulations containing CAP less comfortable, although the polymer itself is considered low risk for clogging pores.
CAP is produced from plant cellulose and synthetic phthalic anhydride, with no animal-derived components, so products using it are normally acceptable to vegans and vegetarians. As always, check the full ingredient list for other additives that may not be plant based.
No specific warnings link CAP to pregnancy or breastfeeding concerns. Its large polymer size limits absorption through intact skin, so it is viewed as low risk. Still, this is not medical advice and anyone who is pregnant or nursing should clear new cosmetic products with a healthcare professional first.
CAP does not make skin more sensitive to sunlight and it has no known interactions with UV exposure. It is also odorless and free from common allergens like gluten or nuts which makes it a versatile option for many users.
Potential Side Effects/Adverse Reactions
Responses to topical Cellulose Acetate Phthalate differ from person to person. The issues below are only potential effects and, when the ingredient is correctly formulated, most people will not notice any problems.
- Mild skin irritation such as redness stinging or itching in sensitive individuals
- Allergic contact dermatitis in the rare case of specific CAP sensitivity
- Eye irritation or watering if mascara or eyeliner containing CAP flakes into the eyes
- Respiratory discomfort if aerosol sprays with CAP are inhaled in an unventilated space
- Product buildup on hair or nails that can feel stiff or dull until washed off
If any negative reaction occurs stop using the product and seek advice from a qualified healthcare professional.
Comedogenic Rating
Rating: 1 / 5
Cellulose Acetate Phthalate earns a low score of 1 because its molecules are large and sit on top of the skin rather than entering pores. While the thin film it creates can trap existing oils if the formula is very heavy, the polymer itself does not block pores on its own. This makes it generally acceptable for people prone to acne or breakouts, especially when used in lightweight products.
Because CAP is almost always paired with volatile solvents that evaporate after application, it typically leaves little residue, further reducing the chance of pore blockage.
Summary
Cellulose Acetate Phthalate is mainly valued as a film former that locks pigments and actives in place giving makeup and styling products longer wear, better color payoff and mild water resistance. It does this by dissolving in the formula then drying to a clear flexible layer once applied.
The ingredient is moderately popular in nail polish, mascara, eyeliner and some long wear foundations but it is less common in everyday skin care where heavy film is not needed.
Safety reviews consider CAP low risk because it stays on the surface, has minimal irritation reports and is free of major allergens. Still, everyone’s skin is different so it is smart to patch test any new product that contains CAP to be sure it agrees with your skin.