In his SPC/SCS prize winning essay Ulrich Sahre examined the ability of rheology modifiers to impart extensive secondary benefits to personal care formulations and ultimately enhance the product
The demand for multi-purpose ingredients in the cosmetics industry has inspired chemists to design new and innovative products. For example, many products offered by manufacturers nowadays act not only as thickeners or rheology modifiers but also impart many benefits to personal care formulations and their manufacturing. Besides the expected properties like effective thickening and stabilization, additional desired benefits include ease of use (eg liquid form, cold processing, self-dispersing, rapid wetting); flow control (thixotropy, pseudoplasticity); clear formulations; formulation flexibility (eg broad pH-range); compatibility with other ingredients; synergistic thickening
(eg with salts); enhanced aesthetic properties (eg appearance, texture); ease of application (spreadability, rub-in); enhanced sensory effects (eg conditioning, skin feel); cost effectiveness and yield value.
Rheology & cosmetics
Rheology is the science of the distortion and flow behaviour of matter. Knowledge of rheological behaviour helps the manufacturer to optimise his formulations in terms of application and usage properties.
There are five categories of flow behaviour: Newtonian, pseudo-plastic, thixotropic, plastic and dilatant (see Figure 1). Pseudoplasic and thixotropic flow behaviours are especially important for cosmetics.
Pseudoplastic flow behaviour shows a decreasing viscosity with increasing shear rate. As soon as the shear stress stops, viscosity recovers to its starting value. Thixotropic flow behaviour also shows a decreasing viscosity if shear stress is applied. Unlike in the case of pseudoplastic flow, the viscosity needs longer to recover to the initial value. Above flow behaviours can result in an apparently stiff cream being spread easily or roll-on antiperspirants being applied without dripping.
Yield Value is the initial resistance to flow under stress and is critical in order to achieve stability of certain physical forms, such as suspensions (avoids separation of particles due to differences in density), emulsions (prevents coalescence of droplets), foam (distributes gas bubbles uniformly) and surface cling (prevents dripping or flow off of fluids down vertical surfaces).
It is a common misconception that high viscosity of a product means suspension stability. A higher viscosity only slows down the rate of particle movement. Yield value is actually required to create a stable suspension.
Rheology modifiers used in cosmetic formulations can be classified into five groups: natural organic polymers, chemically modified natural organic polymers, synthetic organic polymers, mineral (inorganic) thickeners and miscellaneous thickeners.
Natural organic polymers
Carrageenan Carrageenan is an extract derived from seaweed or Irish Moss, chemically belonging to the family of polysaccharides connected with a branched polymer chain and functionalised with sulphate groups for better water solubility. It has a strong gelling power and shows a thixotropic behaviour in solution and gel states. Carrageenan is used in a broad range of applications but it is limited to a neutral or slightly acid environment and is incompatible
with cationic substances. Its texture characteristics in aqueous solutions include cohesiveness, consistency, creaminess, resilience, reduction in syneresis and improvement of the appearance of the final product. A broad variety of gel textures can be obtained ranging from strong and brittle to very elastic.
Additional functional properties include suspending, thickening and stabilization (because of the high capability to form matrices combined with strong electrostatic interactions).
Guar gum - Guar gum is derived from the seeds of the guar plant. It is a high molecular polysaccharide and acts as a thickener in cosmetic formulations while it is compatible with cationic ingredients. Modified guar gums are of interest to the cosmetics industry and will be described in more detail later.
Xanthan gum - Xanthan gum is a water-soluble excretion product of bacteria (Xanthomas campestris) and is a polysaccharide of glucose, mannose and glucuronic acid. Due to its anionic character it is not compatible with most cationics. Compared to solutions of other polysaccharides, solutions of xanthan gum are highly pseudoplastic but not thixotropic and show high viscosities even at low concentrations. Xanthan gum is a non-gelling, water binding thickener with a high stability towards temperature and pH-variation. It also acts as a volume and foam enhancer, emulsion stabilizer,
lubricant and suspending agent. Typical applications include toothpaste, shampoos and condititioners, lotions, creams, and shower gels.
Chemically modified natural organic polymers
Guar hydroxypropyltrimonium chloride -This cationic guar gum is a high molecular weight, water soluble polysaccharide modified with quaternary groups which imparts thickening and conditioning properties to formulations. It is suitable for shampoos and conditioners, lotions, creams, body washes and shower gels.
Hydroxyethylcellulose (HEC) - HEC is a reaction product of cellulose and ethylene oxide. Its properties depend on the degree of substitution and molecular weight. Because of this, a wide range of products exists. Additionally, modified types with special properties have been developed for specific applications.
Natrosol CS from Aqualon/Hercules, for example, is an HEC with the following characteristics: thickening, clarity, water binding, high salt tolerance, surfactant compatibility, alcohol compatibility, shear thinning, lubrication and suspension. Application areas include: shampoos and conditioners, hair colour, shave creams and gels, APDs, toothpaste, liquid soap, body wash, liquid make-up, eyeliner and mascara. The company’s Natrosol Plus CS and PolySurf products are hydrophobically modified cetyl-HECs with additional properties like broad pH stability, suspension of solids, co-emulsifier and non-tacky skin feel. They can be used in skin care, hair care and styling, hand and body washes and sun care.
Hydroxypropylcellulose (HPC) - HPC is a reaction product of cellulose and propylene oxide. Like with HEC a lot of different grades are available. Klucel from Hercules/Aqualon is an HPC and is thickening, clear, non-thixotropic, soluble in alcohols and glycols, thermoplastic, non-tacky and insoluble in hot water (>40°C). Its applications include hair styling products, shave creams and gels, aftershave products, APDs and facial masks.
Methylcellulose - This is the reaction product of cellulose and methyl chloride. Due to its more hydrophobic character it is used in high surfactant containing aqueous or organic solvent containing formulations. Properties of methylcellulose include water retention, cold water solubility, salt resistance, surface activity (colloid protective agent, emulsifier, dispersant), pH stability (3 - 11), shape retention (viscoelasticity), lubrication and film forming with good resistance to organic oils and esters.
Methylhydroxypropylcellulose - Methylhydroxypropyl cellulose is the reaction product of cellulose with methyl chloride and propylene oxide. Benecel and Culminal from Hercules/Aqualon possess qualities including thickening, clarity, salt tolerance, non-thixotropic, non-tacky feel, reversible thermal gelation at ~60°C, low surface tension, lather enhancement and stabilization and lubrication. These products can be used in lotions and creams, shampoos and styling, liquid soap and body wash.
Sodium carboxymethylcellulose (CMC) - Replacing OH-groups of cellulose with sodium monochloracetate produces CMC, also known as cellulose gum. Due to the anionic character it should not be used in combination with cationic ingredients. Blanose and Aqualon from Hercules/Aqualon are CMCs possessing such characteristics as: thickening, water binding, syneresis control, pseudoplastic flow, thixotropy, suspending and gelling agent, foam stabilizer, adhesion and lubrication. Applications are toothpaste, denture adhesives, hair dyes, liquid make-up, mascara and eyeliner.
Synthetic organic polymers
This family of rheology modifiers has many members. Most of them are polymers of acrylic acid - known under the CTFA name carbomer - and it derivates like co- or crosspolymers with different monomers. Carbomers need neutral-isation to show their thickening effect. One of the leaders in this sector is Noveon which offers a wide range of polyacrylate-based rheology modifiers under its Carbopol brand. This product range features liquid polymers, self-wetting polymers, easy-to-disperse polymers, co-solvent polymers and traditional polymers. Carbopol Aqua SF-1 [acrylates copolymer] is a liquid polymeric suspension in water designed to suspend, stabilize, thicken and enhance the appearance of surfactant-based personal cleansing products. Benefits are ease of use, suspension, stabilization, thickening, flow control, end-product clarity, pH flexibility, compatibility and pearlisation enhancement.
For Carbopol Ultrez 21 Polymer [acrylates/C10-30 alkylacrylate crosspolymer] Noveon claims rapid wetting, high thickening effect, improved electrolyte tolerance, clarity, smooth aesthetic appearance, superior skin feel and a broad application usage featuring hair styling gels, lotions, sunscreens and shampoos.
A major disadvantage of all thickeners based on organic materials is their susceptibility to be attacked by microorganisms resulting in potential contamination, so the addition of preservatives is necessary.
Mineral (inorganic) thickeners
Clays - These are water-swellable layered silicates of crystalline type known as smectite which can be divided into three types: montmorillonite (bentonite), magnesium aluminum silicate (MAS) and hectorite. Hectorite and MAS are particularly important for the cosmetics industry. The advantages of hectorite over bentonite are its lower iron content, lighter colour, higher swelling capacity, greater gel strength and excellent skin feel.
All hydrophilic clays need time and energy (eg shear) during hydration to build viscosity. The capability to build up viscosity and to increase the yield value with hectorite correlates with increasing hydration resulting in a stronger colloidal structure.
Hectorites are used as a thixotropic thickener in the aqueous phase for applications like colour cosmetics, deodorants/antiperspirants and in sun, skin and hair care. Here, unlike organic thickeners, they provide a non-sticky and silky skin feel to cosmetic products, while other benefits include viscosity stability between pH 6 - 11, thermal and suspension stability, tolerance towards electrolytes and reduced phase separation.
Magnesium aluminum silicate (MAS) is a purified smectite clay with similar properties and benefits to hectorite. MAS shows synergistic effects in viscosity flow properties and yield value with carboxymethylcellulose (CMC) and xanthan gum. It is therefore often used in combination with these organic thickeners resulting in an improved economy, improved stability, yield value and thickening efficiency, stability at high temperature and improved aesthetics.
R.T. Vanderbilt’s Veegum line offers a wide range of clay thickeners and modified clays and Elementis Specialties supplies clays under its Bentone brand.
Modified (lipophilic) clays - These clays are modified through substitution of sodium ions of the clay with organic ammonium salts. Variations in the organic part strongly influence the area of application for the respective organoclay. They act as thixotropic thickeners for the oil phase of a formulation and need a polar activator as well as energy (eg very high shear) for optimum viscosity performance.
Elementis Bentone gels include a wide range of pre-dispersed and already activated products in various kinds of cosmetic solvents and volatile silicones. These give formulators the advantage that neither high shear nor an activator is needed. They are added at any stage of production and their claimed properties and benefits include being thixotropic and offering efficient pigment dispersion, viscosity control, reproducible rheology, better suspension of actives and pigments, prevention of phase separation or oil migration, enhanced temperature, viscosity and storage stability of emulsions and an excellent skin feel. Areas of application include colour cosmetics, lotions, sunscreens and APDs.
Fumed silica - Fumed silica is produced in a combustion process where silicon tetrachloride vapors are burned in a hydrogen oxygen flame of 1800°C. The resulting SiO2 molecules collide with each other and condense to form spherical particles by agglomeration. Final products are offered as surface treated or untreated grades with varying surface areas as well as different chemical and physical surface treatments to meet specific customer needs (eg hydropho-
bicity). Typical functions of fumed silica include: thixotropy, suspension stability, anti-caking in dry powder formulations, temperature stability of viscosity, moisture resistance, carrier function (eg for perfumes) and emulsification (act as secondary binder). Applications include toothpaste, dental powders, sunscreens, antiperspirants (aerosol and solid), fragrance (absorbing agent), nail polish (prevention of pigment settling), powders (maintaining free flow characteristics), lipstick (resistance of softening with increased temperature), make-up and creams, hair preparations (clear liquid shampoos).
In skin care fumed silica helps to stabilize emulsions over a wide pH and viscosity range and provides a soft and smooth skin feel and improves the spreadability of TiO2-based sunscreens with reduced whitening effect. In shower/bath cleansing formulations fumed silica imparts a consumer perception of richness and creaminess. Due to high absorption properties it reduces greasiness, improves rub-in and leaves a smooth, dry after-feel in oil-rich products.
Miscellaneous thickeners
Salts - Salts, especially monovalent salts like sodium chloride, are cheap thickeners for surfactant containing formulations. The thickening mechanism is that ions of diluted salts promote agglomeration of micelles and also influence their size and shape. This increases the internal friction resulting in increased viscosity. Best thickening results can be achieved with fatty alcohol ether sulfates like sodiumlaurylethersulphate and NaCl. Unfortunately, some surfactants do not show a thickening effect when salts are applied. Examples are glycinates and a-olefinsulfonates.
Typical use levels of salts are 1-2%. Too high electrolyte levels can have a negative impact on viscosity. Typical applications for salt thickeners are surfactant-containing formulations like cleansers (eg shampoos and body wash). Salts usually do not influence the clarity of products. Salt thickened products generally show good spreadability and can be dosed easily.
Fatty esters of long chain poly-glycols - Members of this product class (eg PEG-4 to PEG-150 (Di-) laurate, oleate or stearate) act as viscosity modifiers even at low concentrations. Additional uses include emollients, opacifiers, spreading agents, wetting and dispersing agents. As an emulsifier they cover a wide HLB range. They are used in lotions, creams, make-up, bath oils, ointments, shampoos and conditioners and sunscreens.
Fatty acid amides & betaines - Fatty acid amides are nonionic reaction products of fatty acids (or derivates) with either monoethanolamine (MEA) or diethanolamine (DEA). Often used members of this family are lauramide DEA or cocamide DEA. Both can be found in personal cleansing products in combination with anionic surfactants. Beside their thickening properties they are excellent foam boosters and stabilizers.
Alkyl betaines are amphoteric reaction products of a tertiary amine (with one long organic chain)
and sodium monochloracetate. Coco betaine and lauryl betaine are often used with other surfactants in shampoos, bubble baths and shower gels. They show good viscosity increasing properties and good skin compatibility.
Alkyl amido betaines are produced by a condensation reaction of fatty acid with dimethylpropylamine followed by a reaction with sodium-chloracetate. They are often used in shampoos and conditioners and facial cleansers. The most prominent member of this family, cocamidopropyl betaine acts as a foam booster/stabilizer creating a light, lacy foam and imparts rinse-off and after-feel properties to personal cleansing products.
Various organic and silico-organic (co-) thickeners - Many personal care ingredients that impact the rheology of end products also exist. However, their main use is to enhance other properties. Examples are amine oxides (eg myristamine oxide, stearamine oxide) which, used in shampoos and conditioners, act as surfactants and give a dense, creamy foam. Phosphate esters (eg PPG-5 ceteth-10 phosphate) or amine amides (behenamidopropyl dimethylamine) impart conditioning effects in shampoos and conditioners.
Sulfosuccinates (disodium laureth sulfosuccinate) act as foam enhancers in personal cleansers. Glycol esters (glycol stearate/distearate) act as pearlising agents in cleansers. Organo-modified silicones like Velvesil 125 [C30-45 alkylated silicone polyether gel in cyclopentasiloxane] or SF 1642 [C30-45 alkyl dimethicone] are good examples for secondary thickening effects beside their main properties, including sensory enhancement, de-tackification and lubrication. They are used in various cosmetic formulations from colour cosmetics to sunscreens.
These are just some of the wide range of ingredients with thickening properties for the personal care industry, which also improve the aesthetics, flow characteristics and stability of the final product.