Providing enhanced rheological properties

Stable low pH viscous emulsion and gel formulations are often difficult to obtain. Traditionally, the most commonly used thickeners are synthetic, associative thickeners, which are frequently anionic in charge and hence, are typically incompatible with cationic components, especially quaternary ammonium salts. They can also be ineffective thickeners at low pH.

As a result, the formulator of low pH compositions, especially emulsions, has a limited choice of either nonionic thickeners, such as nonionic surfactants, or cationic thickeners. Nonionic thickeners are uncharged and thus are assumed to be less reactive, but nonionics tend to inactivate preservatives and in some cases, promote microbial growth. While some cationic polymeric rheology modifiers, such as hydrophobically modified aminoacrylate copolymers, are available commercially, their rheological properties are unpredictable or aesthetically unsatisfactory.

Thus, there is an ongoing need and desire for a cationic compatible polymeric rheology modifier for use in low pH formulations.

To meet this demand, Noveon introduces Carbopol Aqua CC polymer (INCI Designation: Polyacrylate-1 Crosspolymer), the newest member of the Carbopol polymer family.

This unique new polymer offers formulators cationic compatibility, smooth flow, shear thinning rheology, suspension capabilities and high clarity in applications below pH 6.0.

This novel polymer is surprisingly effective at thickening aqueous systems containing cationic ingredients (e.g. quaternary ammonium compounds and amines), cationic conditioning agents, softeners, surfactants, and other similar ingredients. Carbopol Aqua CC polymer is a multifunctional polymer comprised of

Amino substituents, which provide hydrophilicity and cationic properties at low pH.
Hydrophobic substituents which moderate the hydrophilicity.
Hydrophobically modified polyoxyalkylene substituents which provide associative properties.
Hydrophilic polyoxyalkylene substituents for controlled association and enhanced rheological properties.
Crosslinker.

The unique design of this polymer features an optimised balance of hydrophilic and hydrophobic character, with amino functionality which further activates ionic (cationic) characteristics at low pH (in use). Importantly, the polymer incorporates a proprietary modified hydrophobe package which enables controlled associative thickening and provides enhanced rheological properties. This novel feature clearly differentiates Carbopol Aqua CC polymer from typical associative polymers and makes it possible for formulators to create softer, smoother, more spreadable gels with the shear thinning rheology and suspension capabilities typical  of Carbopol polymers. Because of its cationic compatibility, functionality at low pH and desirable rheological properties, Carbopol Aqua CC polymer offers formulators a whole new dimension in formulating capabilities. Now, formulators can create a wide variety of product forms, such as non-pourable, stiff to soft gels, semisolid pastes to substantially solid sticks or bars, aerosolised foams to squeezable gels, as well as non-runny, yet flowable products that are suitable as pumpable sprays or in roll-on products and liquid lotions.

Typical properties
Appearance Milky white liquid
Total solids, as supplied (%) 20
Viscosity, as supplied (mPa·s) <50
pH, as supplied 7.8 – 8.5
2% Viscosity*, mPa·s 11,000 -18,000
2% Turbidity*, NTUs <20
Yield value (dynes/cm2) 1,000 – 2,000
2% active polymer in water, neutralised with glycolic acid to pH 4.0

Key benefits

Cationic compatibility.
Excellent clarity in low pH applications.
Smooth flow and shear thinning rheology.
Suspension and stabilisation capabilities.
Versatility through “back-alkaline thickening” in surfactant-based applications.
Synergistic thickening with salts and surfactants in surfactant-based applications.
Compatibility with anionic, cationic and non-ionic surfactants.
Broad applications fit.
Excellent after-feel.
Efficient, easy to use liquid form

Novel thickening mechanism
With Carbopol Aqua CC polymer, the carefully created balance of hydrophobic, semihydrophobic and hydrophilic character helps build viscosity without sacrificing the desirable, smooth flow rheology, clarity and suspension capability that is typical of other Carbopol polymers.

Carbopol Aqua CC polymer builds viscosity through the mechanisms of hydrodynamic thickening and controlled hydrophobic association, when pH is reduced to less than 6.0 through the addition of an acid. This activates the ionic functionality of the polymer (Fig. 1), causing the polymer to swell through charge repulsion (hydrodynamic volume expansion). Further, controlled hydrophobic association then occurs by the unique combination of semihydrophobic and hydrophobic moieties (side chains) at specific sites.

Acidic ingredients (such as lactic, glycolic, salicylic or fruit acids) used in personal care treatment products can serve both as active agents and acid swelling agents for the polymer to achieve the desired viscosity.

Versatility through “backalkaline thickening”
Carbopol Aqua CC polymer is further distinguished from many other rheology modifiers by its synergistic thickening with surfactants and salt, making it ideal for use in cleansing applications. Figure 2 demonstrates the thickening mechanism of Aqua CC in a surfactant-based system.1 The thickening, yield value and stabilising capabilities of Carbopol Aqua CC polymer can be maximised by adding sodium hydroxide (or other bases) to a neutralised surfactant-based system through the “back-alkaline thickening” procedure, as shown in Figure 3. By using “back-alkaline thickening”, the pH of the final product can be adjusted to as high as pH 7.0, as shown in Figure 4. This mechanism provides formulators with the versatility to tailor viscosity, clarity and yield value as desired and enables utility in a wider variety of formulations.

Synergistic thickening with salts and surfactants
Carbopol Aqua CC polymer offers excellent compatibility with anionic, cationic nonionic, and amphoteric surfactants. The polymer exhibits synergistic thickening with salt in surfactant systems. Low levels of sodium chloride serve to increase formulation viscosity when the polymer is formulated with suitable surfactant packages (such as SLES-2 and Cocamidopropyl betaine). Figure 5 compares the viscosity increase when Carbopol Aqua CC polymer is combined with sodium chloride, versus using sodium chloride alone.

Cationic compatibility
A conditioning styling gel formulation incorporating a cationic fixative agent, Polyquaternium 4, was developed and used to compare the rheological performance of Carbopol Aqua CC polymer versus Xanthan gum. Formulations are noted in the Table 2.

A noticeable difference in rheology and aesthetics is evident: superior viscosity (low and high viscosity as desired) and clarity can be observed in the formulations featuring Carbopol Aqua CC polymer. The photograph (Fig. 6) of film drawdown further demonstrates the aesthetic differences between the hair gels formulated with Carbopol Aqua CC polymer and those made with xanthan gum. The gel formulated with Carbopol Aqua CC polymer (pictured left) is markedly smoother, with sparkling clarity.

All formulations passed a 24-hour high humidity (90% Relative Humidity; 25°C) curl retention test demonstrating long lasting hold.

Broad applications fit
The cationic-compatibility and low pH functionality of Carbopol Aqua CC polymer enable formulators to thicken acidic, aqueous formulations and provide aesthetically smooth-textured products that flow and spread easily.

The polymer can be used to provide high clarity and rheological properties in both surfactant-based and water-based formulations to create:

Low pH facial and body treatments and scrubs.
Clear conditioning shampoos, bath gel, and cleansers.
Pearlescent shampoos and cleansers.
Anti-dandruff shampoos.
Conditioning styling gels.
Conditioning neutralisers for permanent waves.
Hair conditioning products.
Gel moisturisers, creams and lotions.

Formulating guidelines
Recommended use level: 1.0-3.0% solids (5.0-15.0 wt.% as supplied).
Recommended pH: < pH 6.0; optimal viscosity and clarity are achieved at pH 4 to pH 4.5.
Neutralisation: Carboxylic acids (glycolic acid, lactic acid, citric acid, etc).
Surfactant compatibility: Carbopol Aqua CC polymer is compatible with anionic, non-ionic, amphoteric and cationic surfactants.
Recommended surfactant active use levels: 8-22% active (surfactant mixtures) or as needed for desired levels of foaming, viscosity, and clarity.
Other compatibility: Carbopol Aqua CC polymer is compatible with cationic ingredients and fixatives, but is generally incompatible with anionic polymers.

Recommended order of addition

In surfactant-based formulations:

1 Add Carbopol Aqua CC polymer to the water portion of the formulation.
2 Add primary surfactants (anionics) and secondary surfactants (amphoterics).
3 Add remaining ingredients (silicones, EDTA, color, fragrance and preservatives).
4 Decrease pH using an acid (citric, glycolic, lactic, and inorganic acids, etc). Formulation becomes clear at a pH less than 6.0.
5 If desired, “back alkaline thickening” can be employed, up to pH 7.0 In surfactant-based formulations containing a cationic conditioning polymer (Polyquaternium, cationic guar, etc).

1 Add cationic polymer and Carbopol Aqua CC polymer to the water phase.
2 Premix surfactants as a separate phase.
3 Add the Carbopol Aqua CC polymer and cationic polymer dispersion slowly to the surfactant phase, using mild agitation.
4 Add remaining ingredients (silicones, EDTA, colour, fragrance and preservatives).
5 Decrease pH using an acid (citric, glycolic, lactic, and inorganic acids, etc). Formulation becomes clear at a pH less than 6.0.
6 If desired, “Back Alkaline Thickening” can be employed, up to pH 7.0.

Formulation examples

Sulfate-free, acne cleanser (CL-F0014)
This sulfate-free, salicylic acid facial wash with nourishing beads demonstrates the cationic compatibility, thickening, suspension and clarity provided by Carbopol Aqua CC polymer in a low pH application. This formulation presents a gentle cleanser with a proven acne-fighting ingredient that treats blemishes and helps prevent breakouts, without irritating skin.

Clear conditioning shampoo
This high performance clear shampoo combines the benefits of botanical extracts, a cationic conditioning polymer, and SilSense Q-Plus and A-23 Specialty Silicones to condition the hair and provide a smooth feel and healthy appearance. CarbopolAqua CC polymer provides luxurious rheology and high clarity.

Summary
Carbopol Aqua CC polymer (INCI Designation: Polyacrylate-1 Crosspolymer), the newest member of the Carbopol polymer family, provides formulators with important new capabilities in a multifunctional rheology modifier: cationic compatibility, smooth flow, shear thinning rheology, suspension capabilities and high clarity in low pH applications. The novelty of this multi-purpose polymer suggests its use as a thickener/ rheology modifier; as a stabiliser of emulsions with silicones, oils, emollients and fatty alcohols; for suspending insoluble ingredients; for film-forming, conditioning and moisturisation; as a spreading aid or carrier for enhancing the efficacy, deposition or delivery of chemically and physiologically active ingredients and cosmetic materials; and as a vehicle for improving the psychosensory and aesthetic properties of a formulation in which these ingredients are included.

Additionally, the cationic character of this novel polymer at low pH offers the potential for use as an antistatic agent. Under certain conditions, it may also provide biocidal, anti-microbial, or other preservative activity.

Cationic-compatibility and low pH efficacy in a Carbopol polymer – what was once only imagined is now a reality: Carbopol Aqua CC polymer.

Reference
1 Bjorn Lindman and Kyrre Thalberg., Polymer- Surfactant Interactions – Recent Developments (chapter 5) in Interactions of Surfactants with polymers and proteins, E.D. Goddard and K. P. Ananthapadmanabhan., Ed., CRC Press, Florida, 1993.

Acknowledgements
Dave Weaver, Carol Kyer, Debby Filla, Duane Krzysik and Toby Dunlap.

Author biographies
Allen Park is a marketing manager for new products in the personal care and pharmaceuticals business unit of Noveon. Julie Shlepr serves as global director of marketing for Noveon Personal Care. Krishnan Tamareselvy, Ph.D. is a research associate in the Noveon Personal Care business unit.