Carbomers, a family of crosslinked acrylic acid polymers, are essential ingredients in numerous products, including: pharmaceuticals; cosmetics and personal care items; household, industrial and institutional care products; printing inks; adhesives and coatings. For more than 50 years, formulators across multiple industries have relied on various carbomers to build viscosity, form gels, stabilize emulsions and suspend particles. When used correctly, carbomers help to build consumer-desired aesthetics into products while simultaneously enabling long-term shelf stability. Due to their utility, reliability and occasional ability to rescue doomed product launches by stabilizing poorly conceived formulas against separation, carbomers have been described by seasoned product developers as, “a formulator’s best friend.” This column will explore the chemistry and properties of carbomers that have earned them this well-deserved reputation.
Chemistry and Manufacture
Nomenclature: Numerous varieties of crosslinked acrylic acid homo- and copolymers exist, but the term carbomer is typically reserved to describe high molecular weight polymers of acrylic acid that are lightly crosslinked with allyl ethers of polyalcohols (see Figure 1).1, 2 Examples of such polyfunctional allyl ethers include tetraallyl pentaerythritol (TAPE) and hexaallyl sucrose, as shown in Figure 2. The National Formulary (NF) lists traditional carbomers individually according to their specific chemistry and properties, e.g., solution viscosity, using numbers associated with the trade names of early carbomers, e.g., carbomer 934 (see Table 1). For modern carbomers synthesized using benzene-free processes, the NF has adopted the terms carbomer homopolymer, carbomer copolymer, and carbomer interpolymer to describe the various species of carbomers employed as excipients in pharmaceuticals and OTC drug formulations. These terms are summarized in Table 1. In the European Pharmacopeia and Japanese Pharmaceutical Excipient monographs, the compendial names carbomers (EU) and carboxyvinyl polymer (JP) refer collectively to the various types of carbomers and carbomer copolymers. In contrast, the INCI Dictionary reserves the term carbomer for crosslinked homopolymers of acrylic acid, and it names crosslinked copolymers of acrylic acid with other comonomers as acrylate crosspolymers, e.g., acrylates/C10–C30 alkyl acrylate crosspolymer.
Monomers: The principal component of carbomer is acrylic acid, a commodity petrochemical derived from propylene gas feedstock. Commercial synthesis of acrylic acid typically involves a two-stage catalytic oxidation where propylene is reacted with air to produce acrolein as an intermediate, which is then further oxidized to yield acrylic acid.3–4 The polyallyl ethers employed as crosslinking monomers in carbomer synthesis are prepared via the base-catalyzed reaction of a polyhydroxy functional compound, e.g., sucrose or pentaerythritol, with an excess of allyl chloride to yield polyallyl ethers of varying degrees of substitution.5 In the allylation of sucrose, an average of five to six of the eight hydroxyl groups on the sucrose molecule are usually converted to allyl ethers, as shown in Figure 2b.
Precipitation polymerization: Carbomers are synthesized by free radical precipitation polymerization conducted in organic solvents.5–8 The solvents for this process are selected such that the monomers, initiators and other additives are soluble in the reaction medium, but the resulting polymer product is not. Historically, benzene was the preferred process solvent for the commercial synthesis of carbomers; however, due to the health and safety concerns associated with benzene, alternative solvent systems, such as n-hexane or mixtures of ethyl acetate and cyclohexane, are employed today in place of benzene.8, 9 The reactions are usually initiated thermally using organic peroxides as initiators, although oil-soluble azo initiators may also be employed.
A typical carbomer synthesis is shown in Figure 3.8 Acrylic acid and small amounts of TAPE and potassium carbonate (K2CO3) are initially dissolved in the ethyl acetate/cyclohexane co-solvent. The K2CO3 is added to neutralize a small percentage (typically ≤ 3%) of the acrylic acid groups, presumably to help promote precipitation of the resulting polymer in the cosolvent system. The mixture is heated to 50°C under a nitrogen atmosphere, and a peroxy initiator, such as di(2-ethylhexyl) peroxydicarbonate (predissolved in cosolvent) is slowly added to the reaction vessel over a period of six hours. As the polymerization reaction proceeds, the insoluble carbomer product precipitates from the solvent, and a slurry of carbomer particles in the solvent is formed. Upon completion of the reaction, the carbomer is isolated from the slurry, and the polymer solids are dried to yield the carbomer product in powder form.
Crosslinking and microgels: During the reaction, the polyfunctional crosslinking monomers copolymerize with multiple linear polyacrylic acid (PAA) chains as they propogate, leading to formation of a three-dimensional network of crosslinked PAA. In conventional bulk or solution polymerization processes, the crosslinking monomers would cause the reaction medium to gelate into a continuous mass of crosslinked PAA upon reaching high monomer conversion. However, in precipitation polymerization, the crosslinked PAA precipitates as fine particles and prevents macroscopic gelation from occurring. Thus, the crosslinking is confined to individual submicron-sized polymer particles. Each carbomer particle is actually one large macromolecule comprising many linear PAA chains that are crosslinked together. The tremendous size of these polymers impedes molecular weight (MW) determination of carbomers using conventional techniques, although the MWs of carbomers have been estimated to be on the order of 108–109 g/mol.10
Another important consequence of crosslinking in carbomers is that these macromolecules are not truly water-soluble. Instead, the mass of crosslinked hydrophilic PAA chains is only water-dispersible and water-swellable. Unlike noncrosslinked PAA, which dissolves to form solutions of polymer coils that overlap and entangle with increasing concentration, carbomers disperse in water and swell upon neutralization to form solutions of microgels that do not entangle with increasing concentration but instead form a network of closely packed microscopic “sponges.”11
Properties
Carbomers are typically supplied as fluffy, white, hydroscopic powders that may have a slight acetic acid odor. A variety of carbomers are available commercially, differing principally by the type of process solvent used (i.e., benzene vs. non-benzene), the type and level of crosslinker employed, and the addition of optional additives to improve wetting and dispersibility.12 Carbomers may also be supplied in preneutralized forms, e.g., as a sodium salt (INCI: Sodium Carbomer). Carbomers are considered to be nontoxic and exhibit little or no irritation potential to skin and eyes at the concentrations employed in cosmetics and personal care products.13–14 Impurities in these polymers may include residual polymerization solvents, unreacted monomers (e.g., acrylic acid), acetic acid, proprionic acid, polymerization initiator by-products and trace heavy metals.
Carbomers are readily dispersible in water and in mixtures of polar organic solvents with water, such as 70% w/w ethanol-water solution. When initially prepared, aqueous dispersions of hydrated carbomer particles are acidic and typically exhibit pH values of 2.5–3.5 depending on polymer concentration. Prior to neutralization with a basic pH adjuster, e.g., sodium hydroxide or triethanolamine, these dispersions do not possess significant viscosity and can be hazy. Upon neutralization of the carboxylic acid groups, the carbomer becomes ionized and swells to several hundred times its original volume due to electrostatic repulsions between the negatively charged carboxylate groups and osmotic swelling due to the captive counterions. The resulting microgel dispersions, sometimes referred to as mucilages, are clear fluids that display high viscosities and also exhibit high yield value.
Technology and Applications
Carbomers are efficient at building viscosity in aqueous systems at relatively low usage levels. For example, most of the carbomers listed in Table 1 are capable of building viscosities of 10,000–60,000 cP when used at only 0.5% w/w. Thus, carbomers are routinely employed as aqueous phase thickeners in a variety of products. However, the true utility of carbomers results from their ability to impart high yield value to formulations.
Yield value is the resistance of a fluid to initial flow when a stress is applied.11, 15–16 At rest, the closely packed network of carbomer microgels behaves as an elastic solid that resists deformation. The network does not begin to flow until a critical level of shear stress, i.e., the yield stress, is applied, at which point the microgels can slide past each other, resulting in fluid flow. When heterogeneous phases (e.g., emulsion droplets, pigments, pearlizers, opacifiers, air bubbles, silica abrasives, etc.) are dispersed in carbomer-thickened formulations, the microgel network entraps and stabilizes them against creaming and/or sedimentation while the fluid is at rest, i.e., below the yield stress. However, upon application of a stress that exceeds the yield stress, the fluid flows smoothly to enable product dispensing and application. For this reason, carbomers are used to formulate countless products, ranging from creams and lotions to hair styling gels to toothpastes, where suspension and stabilization of dispersed phases is critical.
Formulation Guidelines
Successful application of carbomers for thickening and yield value stabilization requires that they be used correctly. Because carbomers are pH-responsive polyelectrolytes with a pKa of ca. 6.0 ± 0.5, microgel swelling decreases dramatically below pH 5, resulting in loss of viscosity and yield value. Optimum performance is typically achieved in the pH range of 6–9. If excess base or other electrolytes (e.g., sodium chloride) are added to carbomer-thickened systems, the microgels will collapse due to polyelectrolyte effects (i.e., screening of the electrostatic repulsions by the excess ionic strength and balancing of the osmotic pressure between the inside and outside of the microgel), again leading to loss of viscosity and yield value. Additionally, multivalent ions (e.g., Ca2+, Mg2+, etc.) and cationic surfactants should be avoided when using carbomers to prevent formation of insoluble complexes.
When preparing carbomer-thickened formulations, care must be taken to ensure uniform dispersion of the carbomer in order to avoid grainy textures and the formation of “fish eyes,” i.e., partially hydrated agglomerates of carbomer particles that fail to completely disperse.17 Dispersion of traditional carbomers usually requires that the powders are slowly sprinkled into the dispersion medium with rapid stirring; for commercial-scale compounding, powder dispersers may also be employed. Alternatively, carbomers can be dispersed in nonsolvents, e.g., the oil phase of an emulsion, and then added to the aqueous phase containing the neutralizing agent. Extremely high shear mixing with homogenizers or colloid mills can lead to shear degradation of the carbomer microgels and should be minimized or avoided altogether. Modern “easy-to-disperse” carbomers have reduced the complexity associated with carbomer dispersion. These carbomers incorporate a steric stabilizing agent, usually an ethoxylated nonionic surfactant with a block or comb configuration, into the carbomer during precipitation polymerization.12 The resulting carbomers are easily wetted when added to aqueous media but hydrate slowly, enabling smooth, uniform dispersion of the carbomer.
References
1. Carbomer, Official Monographs, in the United States Pharmacopeia 34-National Formulary 29, United Book Press Inc., Baltimore, USA (2011) pp 1465–1473
2. Carbomer, Monograph ID 5092, in the International Cosmetic Ingredient Dictionary and Handbook, 13th ed, Personal Care Products Council, Washington, DC USA (2010)
3. G Swift, Acrylic (and methacrylic) acid polymers, in Encyclopedia of Polymer Science & Engineering, John Wiley & Sons Inc., Published online (Mar 15, 2002) pp 79–96 http://onlinelibrary.wiley.com/doi/10.1002/0471440264.pst009/abstract (Accessed Aug 23, 2011)
4. W Bauer Jr, Acrylic acid and derivatives, in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons Inc., published online (Jun 20, 2003) pp 342–369 http://onlinelibrary.wiley.com/doi/10.1002/0471238961.0103182502012105.a01.pub2/abstract (Accessed Aug 23, 2011)
5. US 2798053, Carboxylic polymers, HP Brown, assigned to The B. F. Goodrich Company (Jul 2, 1957)
6. US 2923692, Mucilaginous composition comprising salt of crosslinked carboxylic polymer and method of preparing the same, JF Ackerman and JF Jones, assigned to The B. F. Goodrich Co. (Feb 2, 1960)
7. US 2980655, Polymerization process, JA Glass and JF Jones, assigned to The B. F. Goodrich Co. (Apr 18, 1961)
8. US 4923940, Polycarboxylic acids with higher thickening capacity and better clarity, CC Hsu, assigned to The B. F. Goodrich Co. (May 8, 1990)
9. TEGO Carbomer 340 FD, Product Bulletin D 02/09, Evonik Goldschmidt GmbH: Essen, Germany, (Feb 2008)
10. Molecular weight of Carbopol and Pemulen polymers, Lubrizol Technical Data Sheet, TDS-222, Lubrizol Advanced Materials: Cleveland, OH USA (Jul 10, 2008)
11. JV Gruber, Synthetic polymers in cosmetics, Chapter 6 in Principles of Polymer Science & Technology in Cosmetics and Personal Care, ED Goddard and JV Gruber, eds, Marcel Dekker Inc., New York USA (1999) pp 217–275
12. US 5288814, Easy to disperse polycarboxylic acid thickeners, CJ Long II, Z Amjad, WF Masler III and WH Wingo, assigned to The B. F. Goodrich Co. (Feb 22, 1994)
13. RL Elder, ed, Final report on the safety assessment of Carbomers-934, -910, -934P, -940, 941, and -962, J Am Coll Toxicol 1(2) 109–141 (1982)
14. Annual review of cosmetic ingredient safety assessments–2001/2002, Cosmetic Ingredient Review Expert Panel, Int J Toxicol 22 (Suppl. 1) 1–35 (2003)
15. Measurement and understanding of yield value in personal care formulations, Lubrizol Technical Data Sheet, TDS-244, Lubrizol Advanced Materials: Cleveland, USA (Jan 2002)
16. M Cloitre, Yielding, flow, and slip in microgel suspensions: From microstructure to macroscopic rheology, Chap 11 in Microgel Suspensions, Fundamentals and Applications, A Fernandez-Nieves, H Wyss, J Mattsson, and DA Weitz, eds, Wiley-VCH Verlag GmbH & Co., Weinheim, Germany (2011) pp 285–310.
17. Dispersion techniques for Carbopol polymers, Lubrizol Technical Data Sheet, TDS-103, Lubrizol Advanced Materials: Cleveland, USA (Jan 2002)
