There are a large number of inorganic (silica, bentonite) and cellulose-based thickeners. In addition, acrylic acid polymers are mainly preferred in cosmetics.
Carbomers are often white, loose, hygroscopic powders. Classically, they are polyacrylic acid carbomers, with which there is more than 50 years of experience in cosmetic applications. Carbomer polymers are high-molecular, non-linear polymers of acrylic acid that are cross-linked with a polyalkenyl polyether. Chemically speaking, they are homopolymers of acrylic acids that are cross-linked with polyalcohol allyl ethers.
When dispersed in water, they have a pH value of around 2.5. In the neutral and slightly alkaline range, the carboxyl groups form salts that can bind considerable amounts of water. Depending on the degree of polymerization and cross-linking density, different properties can be achieved.
Production: In principle, one can start with acrylic acid, which is radically polymerized, or with the corresponding polyacrylonitrile precursors, which are saponified. Polyacrylic acids can also be produced by oxidative polymerization from acrolein and hydrogen peroxide. In a special case, polyacrylic acid is obtained by radical polymerization of acrylic acid in the presence of peroxides (hydrogen peroxide, peroxodisulfate, dibenzoyl peroxide), azo compounds or other radical formers. Allyl ethers of pentaerythritol or sucrose (0.75 to 2%) are used as crosslinking agents for the copolymerization.
Use: Carbomers and their sodium salts are used in cosmetic products in particular for emulsion stabilization and to control the rheological properties in concentrations of up to 50%. (Thickening) In aqueous and isopropanol-containing transparent gels, the carbomer concentrations are normally 0.5%. After dispersion in water and neutralization with alkali hydroxides or amines, carbomer dissolves clearly and forms a gel. The optimal pH value that shows the best thickening effect is, depending on the type, between pH 5 and pH 10. Some carbomers have a relatively clear thickening effect even at pH 4. The polyanionic colloid, which is initially “coiled”, undergoes a certain “uncoiling” when dispersed in water and a “stretching” of the molecules when neutralized.
Typically, the various types of carbomer are manufactured using organic solvents, such as benzene, ethyl acetate and cyclohexane. The residual solvent content varies from approximately 0.01% to 1% depending on the type of carbomer.