Carbomers are effective at increasing viscosity in aqueous systems at relatively low levels, with most carbomers capable of producing viscosities of 10,000–60,000 cP when used at only 0.5% w/w. Therefore, carbomers are commonly used as aqueous thickeners in various products. However, the real use of carbomer lies in its ability to impart high yield values to formulations.
Yield value is the resistance of a fluid to initial flow when stress is applied, and at rest, a tightly packed carbomer microgel network behaves as an elastic solid that resists deformation. The network does not begin to flow until a critical level of shear stress (i.e. yield stress) is applied, at which point the microgels can slide past each other, resulting in fluid flow. When heterogeneous phases, such as emulsion droplets, are dispersed in a carbomer-thickened formulation, the microgel network captures and stabilizes them against fluids at rest, ie, below the yield stress. However, when a stress in excess of the yield stress is applied, the fluid flows smoothly, enabling product dispensing and application. For this reason, carbomer is used to formulate countless products.
Carbomer is easily dispersed in water and mixtures of polar organic solvents and water, such as 70% w/w ethanol-water. When initially prepared, aqueous dispersions of hydrated carbomer particles are acidic and typically exhibit a pH of 2.5-3.5, depending on the polymer concentration. These dispersions do not have significant viscosity and may be cloudy prior to neutralization with alkaline pH adjusters such as sodium hydroxide or triethanolamine. After neutralization of the carboxylate groups, the carbomer becomes ionized and expands to hundreds of its original volume due to electrostatic repulsion between the negatively charged carboxylate groups and osmotic expansion due to trapped counterions times to obtain a microgel dispersion.
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