Carbopol
Properties keywords: water absorption, viscoelastic, shear thinning, yield stress
Analogue keywords: mudflows, crystal-rich magma
Common names: Carbomer, polyacrylic acid, PAA
General Information: Carbopol is a very high molecular weight, synthetic polymer of acrylic acid. It comes in a white powder form that readily absorbs and retains water, producing a transparent gel with highly efficient thickening properties. It is inexpensive and easy to prepare and clean.
Carbopol is commonly used as a thickening, dispersing or emulsifying agent and to modify rheology in pharmaceutical, cosmetic and paint formulations.
Properties
Carbopol is a hydrophilic solvent that swells to many times (up to 1000 times) its original volume with water retention. The properties of Carbopol gels (viscosity and yield stress) vary greatly with the concentration of Carbopol dissolved in the liquid (Figure 1).
It is a viscoelastic fluid with a yield stress of less than 10 Pa (Kim et al., 2003; Islam et al., 2004; Balmforth et al., 2006; Balmforth and Rust, 2009; Cochard and Ancey, 2009). Carbopol gels are shear thinning and follow a power law relationship (Figure 1) with an exponent between 0.48 and 0.68 (Kim et al., 2003; Islam et al., 2004). Cochard and Ancey (2009) describe Carbopol gels as following a Herschel-Bulkley rheology. Viscosity and yield stress do not vary greatly with temperature.
The density and thermal properties of Carbopol and water mixtures are approximately equivalent to those of water (Balmforth and Rust, 2009). Surface tension with air is approximately 0.066 N/m (Tran et al., 2015). Carbopol gels show excellent temperature stability and do not appear to exhibit thixotropy i.e. shear stress does not depend on strain-rate history (Cochard, 2007; Piau, 2007; Lubrizol technical data sheet-736).
Figure 1. a) Viscosity at 25C against Carbopol 674 concentration, from Lubrizol technical data sheet and b) strain-rate dependence of viscosity for different Carbopol solutions, from Islam et al. (2004).
There is a large range of Carbopol polymeric systems, resulting in highly variable viscosity, shear thinning behaviour and viscoelasticity.
Applications
Carbopol is not widely used as an analogue material in volcanology. It has been used to study the rheology and fluid dynamics of viscoplastic fluids, which include mudflows and crystal-rich magmas (Balmforth and Rust, 2009), in dam-break experiments by Balmforth et al. (2006) and Cochard and Ancey (2009). Almforth et al. (2009) found that Carbopol experiments were overly influenced by inertia and did not agree well with theory.
Tran et al. (2015) used Carbopol to study the dynamics of bubble motions in mud volcanoes, but did not find consistent bubble behaviours when comparing experiments by several different authors.
Balmforth and Rust (2009) used Carbopol’s elastic properties to study the possibility of oscillatory instabilities in thermal convection. They found that the yield strength of Carbopol inhibits convection and surmised that convection in magma chambers may therefore be limited to hotter, less crystallized areas.
Limitations and tips for use
In experiments Carbopol gels have shown wall slip effects (Balmforth et al., 2006; Cochard and Ancey, 2009). This can be overcome by coating surfaces with paint (see Cochard, 2007 for details). Carbopol is also very sensitive to pH and will dry out if not sealed (Cochard, 2007).
Carbopol polymers are not thought to pose a significant hazard to health, however, protective eye wear and clothing are advised and it should not be ingested.
Solutions can be affected by exposure to light, resulting in a decrease in gel consistency, and should therefore be kept in a closed container (Barry and Meyer, 1979).
To prepare Carbopol solutions the powder should be sifted slowly into rapidly stirred demineralized water to allow for each particle to completely wet out in the water. Mixing/homogenizing techniques that use extremely high shear rates should be avoided as this can break down the polymer and cause permanent viscosity loss. The solution should then be neutralized with a suitable base e.g. NaOH to result in gel expansion (e.g. Piau, 2007; Balmforth and Rust, 2009, Cochard and Ancey, 2009). Antifoaming agent may be added to reduce air entrapment in the solution (Lubrizol technical data sheet-736). Full details on Carbopol preparation can be found in Cochard (2007).
References
Balmforth NJ, Craster RV, Perona P, Rust AC, and Sassi R (2006) Viscoplastic dam breaks and the Bostwick consistometer. Journal of Non-Newtonian Fluid Mechanics 142, 1-3: 63-78
Balmforth N and Rust A (2008) Weakly non-linear viscoplastic convection. Journal of Non-Newtonian Fluid Mechanics 158: 42-43
Barry BW and Meyer MC (1979) The rheological properties of carbopol gels I. continuous shear and creep properties of carbopol gels. International Journal of Pharmaceutics 2: 1-25
Cochard S (2007) Measurements of time-dependent free surface viscoplastic flows down steep slopes. PhD thesis. Ecole Polytechnique Federale de Lausanne.
Cochard S and Ancey C (2009) Experimental investigation of the spreading of viscoplastic fluids on inclined planes. Journal of Non-Newtonian Fluid Mechanics 158: 73-84
Islam MT, Rodriguez-Hornedo N, Ciotti S, and Ackermann C (2004) Rheological characterization of topical carbomer gels neutralized to different pH. Pharmaceutical Research 21, 7: 1192-1199
Kim JY, Song JY, Lee EJ, and Park SK (2003) Rheological properties and microstructures of Carbopol gel network system. Colloid and Polymer Science 281, 7: 614-623
Lubrizol Technical Data Sheet TDS-736, edition August 2010, accessed January 2016, from: file:///Users/gljks/Downloads/TDS-736_Carbopol_674_Polymer.pdf
Piau J (2007) Carbopol gels: elastoviscoplastic and slippery glasses made of individual swollen sponges. Meso- and macroscopic properties, constitutive equations and scaling laws. Journal of Non-Newtonian Fluid Mechanics 144: 1–29
Tran A, Rudolph ML, and Manga M (2015) Bubble mobility in mud and magma volcanoes. Journal of Volcanology and Geothermal Research 294: 11-24