Formula Troubleshooting: Acrylate-based Thickening

Aug 1, 2012 | Contact Author | By: Peter Tsolis, The Estée Lauder; and Marie Ollagnier, Lubrizol
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Title: Formula Troubleshooting: Acrylate-based Thickening
thickenersx acrylatesx polymersx
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Some of the first things a cosmetic scientist is introduced to are the different options available to build and thicken a formula. Although certain waxes, fatty alcohols and gums are available, most formulations rely on acrylate-based thickeners to provide the backbone of a formula’s design. These thickeners are commonly used to help with stabilization, suspension capabilities and aesthetics. Acrylate-based ingredients have been modified over the past decades, and their benefits are increasingly evident in many industries outside of cosmetics, such as paints, coatings, inks, textiles, etc.

These polymers have been hydrophobically modified, pre-neutralized and also made easier to disperse. In personal care, they are found in the majority of skin care and many hair care formulations, as well as pharmaceutical preparations. In fact, a 2007 study showed that the skin and hair care segments used more than 1.8 million pounds of carbomers and acrylate derivatives.1 In 2014, the global skin care market is projected to reach more than US $98 billion,2 and with strong growth forecasted in this market, the use of carbomers will undoubtedly grow. Also, skin care formulations have become increasingly sophisticated with the addition of actives, making the use of novel acrylate-based polymers more essential. Very often, the actives used in personal care contain low levels of electrolytes, which negatively impact the performance and texture provided by traditional acrylate-based polymers. Thus, there is a need for a new generation of acrylate-based polymers that are less sensitive to electrolytes and that provide viscosity, suspension and stability in such challenging systems.

ncoil as they are dispersed in water, and display different thickening and suspending capabilities depending on their size and cross-linking characteristics. Thus, changing the level or type of acrylic-based polymer allows formulators to optimize properties such as break, pickup and spreadability. Additionally, the ability of these polymers to impart high clarity and yield characteristics has made gel products a popular skin care form.3

The value of these materials is clear, but there also are some drawbacks that can affect their activity. High electrolyte systems and the addition of cationic ingredients interfere with the cross-linking and reduce their ability to provide structure and yield. This is frequently seen as a loss of viscosity, a lower yield value, the reduction of clarity in clear gels and separation. Certain processing parameters also must be adhered to in order to ensure these materials are not compromised, since most are shear-sensitive. In order to understand the pitfalls of acrylate-based thickeners, this article briefly reviews their mechanisms and considers formulation approaches to avoid these issues.

Structural Considerations

Most acrylate-based thickeners used in skin care are very high molecular weight cross-linked polymers of acrylic acid. Due to this particular structure, these polymers can thicken and modify the rheology of skin care products as well as provide suspension and emulsion stabilization. They may be anionic homopolymers or copolymers in powder form, and they hydrate and swell in water upon neutralization with a base.

Some hydrophobically modified copolymers provide further emulsion stabilization, as their hydrophobic alkyl chains can anchor into the oil droplets while their hydrophilic portion prevents coalescence and creaming.4 This is due to the formation of a swollen microgel network in the aqueous phase. For easier processing, some acrylate-based polymers are pre-neutralized in forms of sodium or ammonium salts, or prepared as liquid inverse-emulsion polymers in an oil carrier, enabling post-addition for final viscosity adjustments. Although the optimum pH range is between pH 4–9, some polymers of sulfonic acid derivatives can show a broader pH range, from pH 2–11.

Thickening Mechanism

The thickening mechanism and rheology properties of acrylate-based polymers are due to the hydrodynamic space filling by swollen polyelectrolyte microgels, i.e., swollen anionically charged polymer particles. The swelling behavior of these polyelectrolyte particles is dictated by the Donnan equilibrium,5 which is the equilibrium established between the osmotic pressure and the chemical potential, where the anionically charged polymer microgel acts as a semi-permeable membrane. In order to equalize the charge distribution as evenly as possible in the medium, an influx of water causes the particle to swell and the medium to reach its equilibrium.6

Electrolyte Systems

The addition of external electrolytes to a system, such as from the addition of an active, increases the ionic strength outside of the microgel and decreases the osmotic pressure. The swelling behavior of the polymer microgel is thus reduced and the gel collapses. This issue is one of the biggest challenges that skin care formulators face, since many active ingredients are electrolytic.

Until recently, the only solutions to improve the viscosity of systems containing high levels of electrolytes were to either incorporate higher levels of polyacrylic acid polymers, which is not always convenient or cost-effective, or to combine these acrylate-based thickeners with low levels of natural gums, such as xanthan gum, or with a nonionic thickener. This combination is sufficient to enable some electrolyte resistance but adding co-thickeners can have a negative impact on the rheology and texture of the emulsion, this limiting their use.

Fortunately, new technologies of highly cross-linked acrylic acid-based thickeners show improved electrolyte resistant properties, enabling formulas to reach higher viscosity in the presence of challenging electrolytic actives, and without impacting the aesthetics of the skin care product. This electrolyte sensitivity also explains why acrylate-based polymers perform more efficiently in deionized water as opposed to hard water that contains magnesium and calcium ions.

Processing

Cross-linked polyacrylic acid polymers can be dispersed in deionized water at room temperature as well as in the oil phase in a hot or cold process. Traditional powdered polymers display a rather long dispersion time, compared with easier-to-disperse or self-wetting polymers that wet much more rapidly. In the latter case, warm water (< 50°C) and gentle stirring can further reduce dispersion time.

To facilitate hydration, a chelating material such as EDTA can be added to scavenge metal ions in the water phase. Once mixed, the polymer dispersion in water is strongly acidic, i.e., pH 2–3, and hazy since the unneutralized polymer form exhibits low swelling behavior in water.7

However, subsequent neutralization causes the refractive index inside and outside the polymer microgel to become closer, improving the clarity of the dispersion. Another concern for improper swelling of the polymer may be pH variation due to a portion of the polymer being under-neutralized. For the pre-dispersion of the polymer in an oil phase, the temperature of the oil phase must be below 70°C to avoid plasticization of the polymer. Steam in a closed kettle also can cause plasticization, and caution must be taken to ensure that no polymer lumps are sitting on the surface, and that they do not get the chance to hydrate. Fully hydrating the polymer before neutralization is essential in order to prevent the formation of hydrogels that are visible to the naked eye, i.e., “fish-eyes,” after the neutralization stage.

Neutralization of the polymer with a base is a critical step to achieve optimum thickening. Common inorganic neutralizers such as sodium or potassium hydroxides are cost-effective solutions to reach high viscosity in skin care products.8 The neutralizing agent must be carefully selected in order to obtain the best performance, and the choice depends on the content of the formulation. For example, inorganic bases are not recommended in systems containing high levels of solvents such as ethanol, since the solubility of the base decreases with the polarity of the medium and causes the polymer to precipitate. Amines-based neutralizing agents such as triethanolamine (TEA) or tris amino are preferred in more complex systems where viscosity adjustment is challenging, as well as in the presence of hydrotropes.

The neutralization step, and to a larger extent the entire mixing process, is important in working with acrylate-based thickeners. These polymers can be sensitive to very strong and prolonged mechanical shear during processing. Neutralization after homogenization and gentle stirring after neutralization are recommended to minimize viscosity loss. Pre-neutralized acrylate-based thickeners, for which the neutralization step may be suppressed, can be dispersed in the water phase using a cold process. They can be added at different points during the process and used to adjust viscosity during the final steps.

Sensory Contribution

Finally, these different acrylate-based polymers can influence the sensory profile of a system as they contribute to the texture, consistency and initial feel of an emulsion. They can also have some impact on the after-feel and the drag of a product. Depending on the polymer used, the application experience of a product can vary as it interacts with the electrolytes on the skin. Emulsions thickened with electrolyte-sensitive, cross-linked acrylic acid polymers break rapidly in contact with the electrolytes of the skin, releasing water and providing a refreshing sensation. On the other hand, emulsions thickened with more electrolyte- resistant polymers appear creamier. The selection of the appropriate acrylate-based rheology modifier enables the formulator to tailor the appearance and initial feel of a formulation.

It is important to address any issues and to try different polymers and sequencing to determine which system works best. Direct addition to the water or indirect addition to the oil each have certain advantages and should be attempted depending on formula type. In most cases, certain actives should be added after neutralization to allow the acrylate-based polymer to fully develop and to avoid interfering with cross-linking. This is generally at a stage when the formulation is either cold or cooling. Adding actives that have the ability to drastically change the pH, or that have a high level of electrolyte content, can disturb the thickening and consistency of the product.

Conclusions

As with all materials, the benefits of acrylate-based polymers are realized once scientists understand the parameters in which they must work. Though the process seems straightforward—i.e., hydrate, neutralize and thicken, a formulator must look at the material’s capabilities and limitations. Choosing the proper polymer that works best in the formula is the first step, and attempting several materials may be necessary to ensure formulation success.

References

  1. A Eldib, www.icis.com/Articles/2008/ 03/31/9112296/us-polymer-use-in-personal-care-hits-15m-study.html, Eldib Engineering and Research Inc., Berkeley Heights, NJ, USA (2007)
  2. Global beauty and personal care: State of the industry: Beauty and personal care, available at www.euromonitor.com/beauty-and-personal-care, Euromonitor International (2012)
  3. Measurement and understanding yield value in personal care formulations, technical sheet TDS-244, available at www.lubrizol.com/PersonalCare/Literature/TechnicalDataSheets.html, Lubrizol Advanced Materials (Jan 2002)
  4. Carbopol polymers general overview, available at: www.lubrizol.com/PersonalCare/Literature/TechnicalDataSheets.html, Lubrizol Advanced Materials (2008)
  5. Donnan equilibrium, in World of Microbiology and Immunology, KL Lerner, BW Lerner and G Cengage, eds, vol 1, Detroit, MI, USA: Gale (2003)
  6. Principles of polymer science and technology in cosmetics and personal care, Cosmetic Science and Technology Series, ED Goddard and JV Gruber, eds, Marcel Dekker, NY vol 22 (1999)
  7. Cosmetics: Science and Technology, B Sagarin and GR Strianse, eds, Wiley Interscience: Hoboken, NJ, USA, Vol 2, 2nd edn (March 1972)
  8. Neutralizing Carbopol and Pemulen Polymers in Aqueous and Hydroalcoholic Systems, technical data sheet TDS-237, available at: www.lubrizol.com/PersonalCare/Literature/TechnicalDataSheets.html, Lubrizol (Sep 2009)

All websites accessed on June 25, 2012.

 

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Biography: Peter Tsolis, The Estée Lauder Companies

Peter Tsolis

Peter Tsolis has held various positions within The Estée Lauder Companies R&D for the past 14 years, ranging from innovation to business and brand development. He is an active member of the Society of Cosmetic Chemists and has presented on skin care formulation, delivery systems and new technology. His research interests include innovative technology, optimizing formulas and marketing.

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