The winner of this year’s SPC/SCS essay prize for the writer of the best essay on the SCS diploma course was Tayo O Olaleye, awarded for her essay on the subject of gums, thickeners and resins
Thickeners are materials used in the cosmetics industry to increase the viscosity of a product. The viscosity of a material can be described as its ability to resist flow, or its thickness. As consumers tend to equate viscosity with quality, viscosity is very important in cosmetics. Apart from gaining customer acceptance, an increase in the viscosity of a product is known to stabilise the product and prevent phase separation. The viscosity of a product is proportional to its ease of usage, which in turn, could aid efficient handling during manufacture.
Thickeners play a vital role in maintaining lather quality, delivery of actives and suspending insoluble ingredients, among others, and are usually used in small quantities (<1%). This paper highlights the different thickeners available on the market, their rheological properties, recommended applications and their limitations.
Rheological behaviours
Thickeners possess different rheological behaviours which it is vital to understand. These are:
Newtonian flow
This is the simplest of flow behaviours (eg water). In this case, the viscosity of the fluid is independent of the applied shear stress.
Pseudoplastic flow
A fluid that exhibits this behaviour (eg toothpaste) loses its viscosity with increasing shear rate. However, when the shear force is removed, the fluid immediately reverts back to it original viscosity.
Thixotropic flow
This is very similar to the pseudoplastic behaviour but is time dependent. Unlike the pseudoplastic behaviour, when a thixotropic fluid is sheared, it takes a while for it to attain its original viscosity – hours or days.
Plastic flow
In this case, the fluid has a yield point below which it will not flow. Yield point is the ability of a material to resist flow under stress. A force higher than this yield point has to be applied for the material to flow. This behaviour can be seen in mayonnaise.
Dilatant flow
This behaviour is characterised by an increase in viscosity with increased shear stress.
Types of thickeners
Thickeners are mostly categorised by their origin. The different types of available are:
I. Natural organic polymers
2. Natural modified polymers
3. Synthetic polymers
4. Electrolytes and polyethylene and alkanoamides
5. Resins
6. Inorganic polymers
I. Natural organic polymers
Natural gums have plant, animal or microbial origins, hence their chemical structures are based on proteins or polysaccharides.
These polymers consist of long chains of monomers and therefore have a high molecular weight. These monosaccharides are mainly six membered ringed pyranoses (eg glucose) and hexuronic (eg glucuronic acid) compounds. The thickeners used in the cosmetics industry are mostly plant derived so their chemical structures are similar to cellulose, the structural component of plants. Cellulose is linear in structure and some of the thickeners are branched (eg guar gum). Another difference is the substitution of the -OH on the end of cellulose with other functional groups. These chemical differences affect the properties of these thickeners and introduce variations to their reactivities and thickening abilities.
Carrageenan and xanthan gum are common examples of natural cosmetic thickeners.
Carrageenan
Carrageenan is a large polymer made up of galactose monomers. It is derived from red seaweed (Rhodophyceae) found in the North Atlantic, mostly of genus Chondrus crispus, Eucheuma cottonii and Eucheulma spinosum. Manufacture is by hot water extraction under mild alkaline conditions followed by drying or precipitation.
Carrageenan contains a sulphate ester functional group and is capable of forming helical structures with varying gelling properties. The three main types of carrageenan are kappa, iota and lambda, each differing in the number of sulphate ester groups. The forms also differ in their gelling properties at room temperature.
The presence of ester groups increases the hydrophilicity of carrageenan. Hydrophilicity of carrageenan is inversely related to its gelling power. Therefore, with increased hydrophilicity due to the presence of more sulphate ester groups, the less the gelling ability of the polymer. With water, the kappa form produces rigid but brittle gels, the iota forms soft gels and the lambda forms no gels with water but with proteins. The strength of the gels formed is also dependent on the types of counter-ions present.
At ambient temperatures, carrageenan is stable but the stability of its different forms increases with increasing sulphate ester groups; ie lambda is more stable than iota which is in turn more stable than kappa. However, carrageenan is unstable at pH values below 3.5 because the sulphate ester groups are prone to acid hydrolysis which accelerates with high temperatures.
Carrageenan exhibits a pseudoplastic flow behaviour where increased shear rate decreases its viscosity but removal of the force applied causes it to regain its initial viscosity. However, the kappa gel irreversibly loses its viscosity when sheared.
The use of carrageenan is limited by pH, temperature, solubility and the presence of cations in the system (Table 1). In order to achieve good gels with reduced brittleness, increased solubility in water and intermediate freeze-thaw stabilities, a blend of the three types is often used.
2. Natural modified polymers
These polymers are ether derivatives of cellulose. Cellulose has three hydroxyl groups per androglucose unit (AGU) available for modification. Cellulose is insoluble in nature so to attain a water soluble thickener cellulose is modified by reacting it with different chemical reagents such as ethylene oxide in hydroxyethylcellulose (HEC). Etherification of cellulose can occur at any of the three OH sites of the anhydroglucose unit to give compounds with varying solubility and hydrophilic properties.
The chemical characteristics of these thickeners are limited by the degree of substitution (DS), degree of polymerisation (DP), the molecular weight of the polymer, particle size of the polymer, the type of solvent used in the formulation and the starting cellulose backbone. An increase in polymerisation increases the length of the polymer backbone and leads to an increase in viscosity. The level of substitution of the hydroxyl group is defined by the degree of substitution and molar substitution (MS). DS is the average number of substituted hydroxyl group per AGU and MS is the number of moles of the substituent added per mole of AGU. Therefore increased level of substitution increases the molecular weight and viscosity of the polymer.
Examples of these cellulose gums include sodium carboxymethyl cellulose (Na-CMC), hydroxyethylcellulose (HEC) and methylcellulose.
Sodium carboxymethyl cellulose (Na-CMC)
Na-CMC is an anionic, water soluble polymer derived from the reaction of monochloroacetic acid and cellulose. Its physical properties are strongly affected by the degree of substitution. A DS value of 0.3 or less produces an Na-CMC only soluble in alkali whilst a DS of 0.6 gives a water soluble Na-CMC. The maximum solubility is however attained at a DS of 1.4. Na-CMC is insoluble in organic solvents. It is compatible with anionic and non-ionic systems but not cationic systems.
The order of manufacture is very important when dealing with Na-CMC particularly with formulations that contain electrolytes. Na-CMC is not compatible with trivalent cations as it crosslinks with the polymer or causes it to precipitate. Na-CMC is compatible with monovalent and divalent salts. Increase in DS decreases the sensitivity of Na-CMC to electrolytes. When formulating a product containing Na-CMC and electrolytes, the polymer should be dissolved first before any salts are added.
Na-CMC is stable within a pH range of 4-10. Below pH 4, free acid is produced which induces precipitation of the polymer. Above pH 10 the polymer is in danger of degrading. Thus outside this pH range, viscosity decreases. Na-CMC has a pseudoplastic flow behaviour which tends to switch to thixotropy with increasing DS. Its viscosity, however, decreases with increased temperature.
3. Synthetic Polymers
These polymers are acrylic acid-based polymers widely used in small quantities within the cosmetics industry. They are man-made and impart a pseudoplastic flow to aqueous-based cosmetics. These polymers exist as homopolymers, copolymers and cross-polymers.
Acrylic acid polymers are anionic hydrogels capable of swelling but do not dissolve in water. For swelling to occur, the polymers have to be neutralised using a base to form a water soluble salt. Polyacrylic acids are highly sensitive to electrolytes, pH and UV light. Degradation by UV radiation can be eliminated by the use of chelants or UV stabilisers. Examples include carbomers.
Carbomers
Carbomers are anionic acidic thickeners with high molecular weights. They exist unneutralised in a tightly coiled manner. Unneutralised carbomer has very little thickening potential and shows limited solubility in water. However, in the presence of water, the carbomer uncoils slightly. Sodium salts of carbomers are water soluble and are capable of crosslinking with other monomers. Carbomers are highly efficient in a pH range of 4-10.
To increase the thickening potential of carbomers, they have to be neutralised using a base. The choice of base used is dependent on the type of solvent used in the formulation. For aqueous-based solvents, inorganic bases such as KOH, NaOH or triethylamine (TEA) can be used. Base neutralisation introduces negatively charged carboxyl groups to the polymer backbone which causes the carbomer to uncoil as a result of the increased repulsive forces. This dramatically increases the thickening power of the carbomer but care has to be taken as over-neutralisation can lead to a decrease in viscosity. Carbomers exhibit highly pseudoplastic behaviours.
Creating carbomer dispersions requires mechanical force, but carbomers are very sensitive to shear. High shear can lead to an irreversible loss of viscosity. The limited solubility of unneutralised carbomers in water suggests a microbially robust dispersion.
As carbomers are anionic in nature, they are incompatible with cationics, PEGs, PVP resins and polyethoxylated surfactants. This definitely generates a problem if carbomers must be used in a formulation containing any of the aforementioned excipients. To prevent the formation of insoluble complexes, the carbomer should be partially neutralised before the other ingredients are added to the formulation. Carbomers are also incompatible with divalent and trivalent electrolyte salts. Increase in the occurrence of these salts leads to a decrease in viscosity. This however is important in topical creams. The presence of salts on the surface of the skin causes the reduction in viscosity of the cream, aiding spreadability.
| Table 1: Chemical properties of different forms of carrageenan | |||
| Kappa | Iota | Lambda | |
| Effect of cations | Gels most strongly with potassium ions | Gels most strongly with calcium ions | Non-gelling |
| Type of gel | Strong & brittle with syneresis | Elastic & cohesive without syneresis | Non-gelling |
| Water solubility | Na salts are soluble in hot or cold water, Ca & K salts are soluble above 60˚C | Na salts are soluble in hot or cold water, Ca salts for a thixotropic dispersion in cold water, K salts are soluble above 60˚C | All salts are soluble in hot or cold water |
| Freeze-thaw stability | Not stable | Stable | Stable |
4. Electrolytes & polyethylene & alkanoamides
Electrolytes (eg sodium chloride) are often chosen as thickeners over others to minimise cost. They are mainly used in surfactant-based systems (particularly anionic surfactants) to increase viscosity. Electrolytes increase the ionic density of the formulation and modify the micelle structure of the surfactant. Increase in viscosity is dependent on the size, level, pH and ionic strength of the electrolyte and the size, shape and ionic charge on the micelle. A high level of electrolyte can lead to irreversible loss of viscosity. To attain a substantial increase in viscosity, an electrolyte with the same cation as the surfactant should be used (eg NaCl).
Alkanolamides (eg coconut diethanolamide) are non-ionic thickeners used in surfactant-based systems. Alkanolamides increase viscosity by changing the position of the viscosity/electrolyte salt curve. The use of alkanoamides is, however, limited by the formation of carcinogenic nitrosamine as a resuit of available free amines.
Polyethylene glycol (PEG) is used to thicken mild surfactant systems. PEGs generate more stable viscosity than alkanoamides and electrolytes and the gels are stable in extreme temperatures and pHs.
5. Resins
Resins are polymeric thickeners mostly used for hair care products to improve their holding capacities. There are four main classes – PVP & PVP/VA, methyl vinyl ether derivatives, crotonics and acrylates.
Acrylates are the most used resins in cosmetics usually made up of acrylic acid or its derivative as a monomer. Its function is to improve the holding power of products at high humidity. It can also introduce plastic flow rheology to a product.
6. Inorganic polymers (organoclays, silicas, Al/Mg hydroxide stearate)
Clays & organoclays
Clays are naturally occurring thickeners. They are finely divided, hydrated silicates of Al, Mg or calcium with minute amounts of lithium and iron. Smectite clays are the most important types of clay in the cosmetics industry because of their ease of dispersion and ability to swell in water. Smectite clays have two different crystal structures – dioctahedral (eg bentonite) and trioctahedral (eg hectorite). Smectite clays are made up of three layered platelet structures comprising aluminium or magnesium oxide layers between two layers of silicon dioxide. These platelet structures have positively charged faces and negatively charge edges. Upon dispersion in water, the faces of the platelet become electrically charged and form an electrostatic interaction with cations in water (eg Na+) forming a thixotropic gel.
Organoclays are synthetically modified smectite clays formed as a result of a reaction of smectites with quaternary compounds.
Silicas
There are two main classes of silicas used in cosmetics: fumed silicas and hydrated silicas. Fumed silicas (<30mu in size) are manufactured by steam hydrolysis of silicon tetrachloride at elevated temperatures. They contain fewer -OH groups than hydrated silicas and are capable of thickening both aqueous and non-aqueous systems. Thickening arises from the formation of hydrogen bonds between the silica particles to form a network and hydrogen bondings with water. Fumed silicas are highly sensitive to high shears and pH. Within a pH range of 1-7.5, fumed silicas are stable and have a good thickening power. Above pH 7.5, there is an increase in negative charges which results in electrostatic repulsion between the particles, hence a reduced thickening effect.
Al/Mg hydroxide stearate
This is a hydrophobic thickener with a crystal lattice made up of alternating layers of anionic stearic acid and cationic aluminium or magnesium hydroxide. These thickeners swell in non-aqueous systems, increasing viscosity. Al/Mg hydroxide stearate is used to produce colourless, odourless oil-based gels. It increases melting points of non-aqueous systems, provides products with water repelling properties and improves the stability of w/o emulsions. The gel formed is resistant to broad temperature ranges. This stearate thickener aids the suspension of actives in the system. It is therefore often used in colour cosmetics and antiperspirants.
