Recognizing product parameters and choosing appropriate ingredients to create a successful, robust formulation is half the battle. Personal care applications are formulated predominantly in aqueous systems. Though there are viable options to formulate and stabilize high performance, sophisticated systems enriched with active ingredients such as natural gums, starches and organic clays, acrylate-based polymers still dominate personal care applications due to their efficacy and the smooth texture and appealing sensory profile when formulating the aqueous phase. Moreover, these synthetic polymers can readily be structurally modified, broadening their application attributes for increasingly sophisticated modern formulations. When properly selected, these modified acrylate-based polymers offer formulator-desired aesthetic features which may contribute to viscosity building, particle suspension, emulsion stabilization and sensory perception.
Crosslinked acrylate-based thickeners such as anionic homopolymers or copolymers are commonly supplied in powder form. Crosslinked polyacrylic acid initially exists as collapsed coils when dispersed in water at low pH. The acidic groups of the polymer become ionized upon neutralization, imparting electrostatic repulsive forces that expand the collapsed coils into extended structures and modify the rheology of the aqueous phase. While intermolecular interactions with the surrounding medium such as hydrogen bonding can play a role, hydrodynamic space-filling, driven by charge repulsion and Donnan equilibrium, is the primary thickening mechanism.
Crosslinked acrylate-based polymers make versatile contributions in skin care formulations. Generally speaking, rheology performance and electrolyte tolerance of a polymer can be adjusted by the cross-linker type, degree of cross-linking and particle size. Hydrophobic modification of the polymer increases intermolecular interactions, which can increase yield value and improve suspension capability. In some cases, the polymers’ hydrophobic substituents can impart surface activity and interact with suspended oil droplets, offering additional emulsification support. Polymer characteristics not only deliver formulating solutions but also contribute to the sensory properties of the final product such as pick-up, spreadability and after-feel. Understanding a polymer’s rheological properties and its interaction with skin during application are the keys to tailor a product’s sensory profile.
Due to increasing skin health consciousness, consumers demand products with label-friendly ingredients and adequate amounts of functional actives. Many ionic skin care ingredients can have a detrimental impact on the viscosity and suspension performance of traditional acrylate-thickened systems. Electrolytes increase the ionic strength of solution, which shields electrostatic repulsion and results in a size reduction of the swollen microgels and collapse of the structure of the formulation. Furthermore, acidic functional additives such as alpha hydroxy acids (AHA), beta hydroxyl acids (BHA) and organic preservatives are only functional at low pH, which adds another level of complexity to formulations containing alkali-responsive polymers.
The simplest solution to recover electrolyte-related viscosity loss is to increase the use level of the acrylate polymer. However, this is not cost-efficient and excess polymer residue may create unwanted peeling and pilling. In some cases, adding naturally-sourced gums or nonionic thickeners can stabilize viscosity and suspension at low pH, but these ingredients can negatively impact sensory by adding stickiness and stringiness. Fortunately, advanced acrylate polymer technologies offer electrolyte tolerance or low pH thickening without sacrificing sensory properties. For instance, Carbopol Aqua SF-1 OS polymer and Carbopol Ultrez 30 polymer can alleviate these constraints while enabling the formulation of a broad range of textures from thin mists to rich emulsions or gels.
Carbopol Aqua SF-1 OS polymer (INCI: Acrylates copolymer) is an optimized acrylates copolymer targeting oil-in-water skin care applications. This polymer offers best-in-class suspension even at low use level, making stable low viscosity emulsions possible. It is electrolyte tolerant and highly compatible with titanium dioxide and iron oxide inorganic pigments, making this polymer an ideal thickening agent for high pigment loading sun care and color cosmetic applications.
Carbopol Ultrez 30 polymer is a cross-linked homopolymer of acrylic acid, effective over a broad pH range (pH 4.0-12.0), featuring enhanced electrolyte compatibility, and a rich sensory. It can withstand low levels of AHAs and BHA for low pH anti-aging and anti-acne applications. In Lubrizol’s recent concept development, The Sweet Escape, Carbopol Ultrez 30 polymer also demonstrated its ability to thicken polyol systems with high clarity, meeting anhydrous formulation inquires resulting from the current water scarcity trend.
Just like thickening agents, emulsifiers play a crucial role in stabilizing oil and water mixtures. These amphiphilic compounds lower the interfacial tension between two immiscible phases and establish an interfacial layer surrounding the dispersed internal droplet through adsorption. Emulsion stability may be attained by steric repulsion or electrostatic charge repulsion between the emulsified droplets. Emulsifiers used in leave-on skin care emulsions are mainly nonionic; and, therefore lack the ability to create charged micelles. To minimize coalescence of the emulsified droplets, using a blend of emulsifiers is recommended to achieve proper steric stabilization.
Screening a successful emulsifier combination for the system can be time consuming. The hydrophilic-lipophilic balance (HLB) system was created to aide in the selection of an effective nonionic emulsifier combination to match the calculated required HLB (RHLB) value of the oil to be emulsified. However, this tool can only be served as a simple formulating guideline since it was invented for ethoxylated or traditional nonionic emulsifiers. It has gradually become insufficient for the modern EO-free emulsifiers and formulas of increased complexity. Many formulators now seek to achieve long term stability with the addition of polymeric emulsifiers.
Pemulen polymeric emulsifiers are high molecular weight crosslinked acrylic copolymers. Hydrophobically modified with C10-30 alkyl acrylate comonomer, they are best described as hairy microgels when neutralized by a base.
These microgels can anchor their lipophilic portion into the oil droplets while their hydrophilic groups create a compacted gel network, thereby providing emulsion stabilization through associative stabilization and particle suspension. This comprehensive emulsion-stabilizing mechanism allows these polymeric emulsifiers to form cost-effective, oil-in-water emulsions with any oil phase, regardless of their required HLB. Due to the electrolyte sensitivity of these polymers, their microgels deswell quickly on contact with the electrolytes of the skin, providing immediate oil deposition and a refreshing sensory experience. Pemulen polymeric emulsifiers can be used to support the growing prevalence of conventional emulsifier-free claims. Because of their emulsion stabilizing efficiency, Pemulen polymeric emulsifiers can be used as a sole emulsifier at low use levels (0.1-0.4%) to replace traditional emulsifiers. The benefits of a conventional emulsifier-free formulation may include low irritation, light sensory and water-resistant properties as these polymers do not lead to re-emulsification upon rewetting.
Lubrizol currently offer three grades of acrylate-based polymeric emulsifiers. Pemulen TR-1 polymeric emulsifier generates a high viscosity emulsion, and can stabilize up to 30% of the oil phase. Pemulen TR-2 polymeric emulsifier provides low viscosity emulsions, and can stabilize up to 60% of the oil phase as it contains higher levels of hydrophobic groups than Pemulen TR-1 polymeric emulsifier. However, the downsides of the traditional Pemulen polymeric emulsifiers are the potential for long dispersion time, lack of efficiency at lower pH, and undesirable “cheese cake” texture which can occur through associative structuring with other co-ingredients such as fatty alcohol and fatty acids. To improve processing time and formulation flexibility, Pemulen EZ-4U polymeric emulsifier was created to serve market needs.
Pemulen EZ-4U polymeric emulsifier (INCI: Acrylates/C10-30 alkyl acrylate crosspolymer) is a fast-dispersing polyacrylate offering universal HLB independent emulsion stabilization up to 60% of oil phase over a broad pH range (pH 4.0-9.0). Its effective rheology modification provides low viscosity particle suspension at use levels as low as 0.05wt% with a quick-breaking, fresh sensory.
Cascading Sensory Impact
Sensorial perception of a product influences the consumer’s buying decisions, fosters consumer loyalty, and encourages repeat use. In addition to rheology modification and emulsion stabilization, acrylate-based polymers can also affect the sensory profile of a product. As a result of their space-filling mechanism, swollen microgels occupy the entire aqueous phase of the formulation. The characteristics of these microgels contribute to sensory attributes during the entire application cycle, including initial feel, rubout and after-feel.
Lubrizol’s latest sensorial formulating concept, Sensimap Climate, demonstrates that the electrolyte tolerance of the thickening polymer greatly impacts the initial application experience. Gel or emulsion formulations containing an electrolyte-sensitive acrylic-based polymer, such as Carbopol Ultrez 10 polymer (INCI: Carbomer), break rapidly on a salt-containing substrate such as skin. This quick break delivers a high degree of wetness, a light, spreadable touch and a fresh sensation. Alternately, formulations containing an electrolyte-resistant polymer, such as Carbopol Ultrez 30 polymer (INCI: Carbomer), retain greater structural integrity in the presence of electrolytes and therefore deliver richer, creamier aesthetics with longer play time. Formulators may mix polymers in the opposite sensory spectrum to deliver a distinctive sensory experience in the early application stage without altering the ingredient label.
Although product after-feel is mainly affected by the emollients used in the formulation, different types and use levels of the acrylate-based polymer may also contribute to after-feel effects, including film formation, peeling effect and instant tightening perception. By selecting individual ingredient features and the right combination of the components, a customized aesthetic experience can be obtained, which delivers tailored sensory properties at each stage of application and elicits targeted emotional responses from consumers during product use. To keep up with ongoing skin care technical inquiries of our customers, Lubrizol is committed to delivering solutions for effective thickening under challenging conditions, polymeric emulsification and cascading sensory impact.
About the Author
Joseph Shieh is regional technical marketing manager, skin essentials, Lubrizol Advanced Materials. More info: firstname.lastname@example.org, www.lubrizol.com