Carbomers are white-colored, ‘fluffy’, acidic, hygroscopic powders with a characteristic slight odor. A granular carbomer is also available (Carbopol 71G).
- BP: Carbomers
- PhEur: Carbomers
- USP-NF: Carbomer
Note that the USP32–NF27 contains several individual carbomer monographs
Acrypol; Acritamer; acrylic acid polymer; carbomera; Carbopol; carboxy polymethylene; polyacrylic acid; carboxyvinyl polymer; Pemulen; Tego Carbomer.
Chemical Name and CAS Registry Number
Carbomer [9003-01-4] Note that alternative CAS registry numbers have been used for carbomer 934 ([9007-16-3]), 940 ([9007-17-4]) and 941 ([9062-04-08]). The CAS registry number [9007-20-9] has also been used for carbomer.
Empirical Formula and Molecular Weight
Carbomers are synthetic high-molecular-weight polymers of acrylic acid that are crosslinked with either allyl sucrose or allyl ethers of pentaerythritol. They contain between 52% and 68% of carboxylic acid (COOH) groups calculated on the dry basis.
The BP 2009 and PhEur 6.4 have a single monograph describing carbomer; the USP32–NF27 contains several monographs describing individual carbomer grades that vary in aqueous viscosity, polymer type, and polymerization solvent. The molecular weight of carbomer is theoretically estimated at 7 * 10^5 to 4 *10^9.
In an effort to measure the molecular weight between crosslinks, MC, researchers have extended the network theory of elasticity to swollen gels and have utilized the inverse relationship between the elastic modulus and MC. Estimated MC values of 237 600 g/mol for Carbopol 941 and of 104 400 g/mol for Carbopol 940 have been reported.
In general, carbomer polymers with lower viscosity and lower rigidity will have higher MC values. Conversely, higher-viscosity, more rigid carbomer polymers will have lower MC values.
Bioadhesive material; controlled-release agent; emulsifying agent; emulsion stabilizer; rheology modifier; stabilizing agent; suspending agent; tablet binder.
Applications in Pharmaceutical Formulation or Technology
Carbomers are used in liquid or semisolid pharmaceutical formulations as rheology modifiers. Formulations include creams, gels, lotions and ointments for use in ophthalmic, rectal, topical and vaginal preparations. Carbomer grades with residual benzene content greater than 2 ppm do not meet the specifications of the PhEur 6.4 monograph. However, carbomer having low residuals of other solvents than the ICH-defined ‘Class I OVI solvents’ may be used in Europe.
Carbomer having low residuals of ethyl acetate, such as Carbopol 971P NF or Carbopol 974P NF, may be used in oral preparations, in suspensions, capsules or tablets. In tablet formulations, carbomers are used as controlled release agents and/or as binders. In contrast to linear polymers, higher viscosity does not result in slower drug release with carbomers. Lightly cross-linked carbomers (lower viscosity) are generally more efficient in controlling drug release than highly cross-linked carbomers (higher viscosity).
In wet granulation processes, water, solvents or their mixtures can be used as the granulating fluid. The tackiness of the wet mass may be reduced by including talc in the formulation or by adding certain cationic species to the granulating fluid. However, the presence of cationic salts may accelerate drug release rates and reduce bioadhesive properties.
Carbomer polymers have also been investigated in the preparation of sustained-release matrix beads, as enzyme inhibitors of intestinal proteases in peptide-containing dosage forms, as a bioadhesive for a cervical patch and for intranasally administered microspheres, in magnetic granules for site-specific drug delivery to the esophagus, and in oral mucoadhesive controlled drug delivery systems.
Carbomers copolymers are also employed as emulsifying agents in the preparation of oil-in-water emulsions for external administration. Carbomer 951 has been investigated as a viscosity-increasing aid in the preparation of multiple emulsion microspheres. Carbomers are also used in cosmetics. Therapeutically, carbomer formulations have proved efficacious in improving symptoms of moderate-to-severe dry eye syndrome.
Method of Manufacture
Carbomers are synthetic, high-molecular-weight, cross-linked polymers of acrylic acid. These acrylic acid polymers are cross-linked with allyl sucrose or allyl pentaerythritol. The polymerization solvent used previously was benzene; however, some of the newer commercially available grades of carbomer are manufactured using either ethyl acetate or a cyclohexane ethyl acetate cosolvent mixture. The Carbopol ETD and Carbopol Ultrez polymers are produced in the cosolvent mixture with a proprietary polymerization aid.