Rheology, Part II: Rheology Modifiers

[Versione italiana]
[Back to the previous article: Rheology, Part I: Basic Concepts]

Polymers

What are polymers

Most rheology modifiers used in cosmetic products are polymers. Polymers are macromolecules composed of repeated units, called monomers. They exist in many different configurations. They can consist of repeating identical monomers (homopolymers) or two different monomers (copolymers). They can have different microstructures and architectures, for example they can be linear, branched, or cross-linked. Finally, the monomer units can be distributed in different ways: alternated (ABAB type), random, block (AAA-BBB type), or grafter (with monomer A being the backbone of the polymer and monomer B forming the branches or side chains). 

Polymers can be semi-crystalline or amorphous. The semi-crystalline polymers undergo crystallisation and melting transitions. All polymers (semi-crystalline and amorphous) undergo glass transition at a given temperature: below the glass-transition temperature, the molecular movements are not possible; above the glass-transition temperature, the polymers are rubbery and viscous and they can be manipulated (8). 

How do polymers generally work (9)

Polymers used as thickeners typically have flexible chains that can move and relax. When they are in solution at a certain concentration, they interact with each other and they undergo entanglement, which thickens the solution.

In very dilute conditions, there is no interaction between polymer molecules. The viscosity in this case increases with the concentration on the polymer and on its molecular weight and the fluid has Newtonian behaviour.

In concentrated conditions, polymer solutions show non-Newtonian behaviour (like discussed for emulsions). 

  • At low shear rate, the viscosity will be constant. There is no significant change in shape and orientation of the polymer chains or in the degree of entanglement.
  • With increasing shear rate, there is a shear-thinning region where the viscosity decreases. Here the rate of deformation causes the shape and orientation of the polymers to change, and the entanglement (interaction between polymer molecules) is hindered by the shear.
  • With high shear rates, the rate of deformation is so high that there’s no substantial observable change in the viscosity anymore.

When we use polymeric thickeners in cosmetics, we work in concentration ranges that allow this non-Newtonian behaviour. 

Thermoreversible polymers

For some polymers, the ability to thicken a solution is influenced by temperature. We can distinguish three types of thickeners in this regard:

  1. Temperature-independent thickeners: polymers like guar gum, xanthan and Carbomers maintain the gel viscosity upon heating.
  2. “Cold” thickeners: polymers like gelatin, pectin and carrageenan form gels that are melted upon heating. 
  3. “Heat” thickeners: polymers like methylcellulose, hydroxypropyl methylcellulose, hydroxypropyl cellulose and poloxamers form gels upon heating (3).

Polymers used in cosmetics

Polymers used as thickeners are typically classified according to their origin as natural, semi-synthetic and synthetic polymers. Here, I will further divide them into hydrophilic and hydrophobic polymers: hydrophilic polymers are the rheology modifiers we use to thicken the water phase of emulsions and hydrogels; hydrophobic polymers are the rheology modifiers that we use to make lipogels (to thicken oils). The information concerning their practical use in cosmetic products is from the relative technical data sheets from suppliers’ websites (10).

In some conditions, some of these polymers can even stabilise emulsions on their own, as they can partially absorb at the O/W interface and stabilise the dispersed droplets (11).

Hydrophilic polymers

Natural polymers

Starches. Starches are polymeric carbohydrates made of glucose monomers linked in α 1,4 linkages. The linear polymer is called amylose, whereas the branched is called amylopectin. The most used in cosmetics are corn, rice and tapioca starches. 

Use: add to the water phase, 1-3% should be sufficient to obtain a gel upon heating. I suggest using them only if you don’t have any other thickener available, or if you have a modified starch for cosmetic use. Starches supplied as food ingredients are not modified and their aqueous solutions are very susceptible of bacterial growth, they could shorten the shelf life of your product even in presence of preservatives. 


Guar gum. Guar gum is a galactomannan polysaccharide (composed of a mannose backbone and galactose side chains). It forms viscous solutions in water and it is stable over a pH range of 5-7. Viscosity is reduced in extreme pH conditions and above 80°C. Modified versions of guar gum (hydroxypropyl guar and guar hydroxypropyltrimonium chloride) are used as cationic conditioners (see later). 

Use: 0,5-1% in the water phase is sufficient to form a gel phase. It is not affected by ionic strength and is compatible with cationic conditioners like behentrimonium chloride and Esterquats. 


Caesalpinia Spinosa Gum. Galactomannan polysaccharide, can be used in cold and warm water as thickener.

Use: 0,2-1% in serums and lotions; 0,5-2% in creams. Add to water by sprinkling slowly into water under stirring. Compatible with anionic and cationic ingredients, resistant to electrolytes.


Xanthan gum. Polysaccharide comprising glucose, mannose, and glucuronic acid. It is anionic and soluble in water. Xanthan gum solutions have pseudo-plastic flow profiles (shear-thinning). It can be used in synergy with most of the other thickeners, including guar gum, Caesalpinia Spinosa gum, and polyacrylates. 

Use: 0,5-2%. Add to the water phase:

  • Sprinkle xanthan gum powder into water under stirring, or
  • Disperse in glycerin first and then add water while stirring

It doesn’t work well with cationic conditioning agents. Addition of 0,5% sodium chloride can increase viscosity. 


Carrageenans or carrageenins. Family of polysaccharides that form helical strucutres, made of galactose units and anhydrogalactose (sulfated and nonsulfated). They are classified as mu, nu, lambda, kappa, iota, theta and xi forms according to the extent of sulfate substitution. 

They are soluble in warm water.

Use: Iota carrageenan 0,2-1,5%, to be dispersed in water at 45-50°C; Lambda carrageenan 0,2-2%, to be dispersed in glycerin and then added to warm water. 


Semi-synthetic polymers
Cellulose derivatives. 

Carboxymethyl (CMC) cellulose. Cellulose is a polysaccharide made of glucose units joined with beta 1-4 linkages. In CMC cellulose the hydroxyl units of the glucose monomers are all substituted with carboximethyl moieties. CMC cellulose is anionic and gives pseudo-plastic solutions. 

Use: 0,2-1% in emulsions, 1-3% in hydrogels, to be added to warm water. Incompatible with cationic conditioning agents; stable at pH 5-10; stable in presence of electrolytes and up to 60% alcohol. 


Hydroxyethyl (HE) cellulose is non ionic and compatible with most ionic ingredients, and stable at pH range 3-10. Solutions of HE cellulose are shear thinning and thixotropic. It hydrates at pH above 7: it can be useful to disperse it at a pH lower than 7 to ensure homogeneous dispersion, and then neutralising the pH to allow thickening. 

Use: 0,5-3%, add to warm water phase at pH below 7, then adjust to pH > 6 to ensure thickening. 


Hydroxypropyl methyl cellulose is a non ionic thickener that is often combined with other thickeners. 

Use: 0,2-1%, disperse in water and allow wetting, then mix. 0,2-0,5% is sufficient for creams and lotions, 1% for gels. 


Guar gum derivatives. 

Hydroxypropyl guar is a guar gum derivative often used in shampoos and lotions. It has good alcohol and salt tolerance and it is stable at pH 4-8. 

Use: 0,1-1,5%, to be added to the water phase; pH must be neutralised to increase viscosity. 


Guar hydroxypropyltrimonium chloride: cationic version of hydroxypropyl guar, used as thickening and conditioning agent in haircare products. 

Use: 0,1-0,5% in rinse-off products, 0,01-0,15% in leave-in products (Making Cosmetics says 0,2-2%). Dissolve in water while stirring; to thicken the solution, pH must be adjusted with citric acid below 7. 


Synthetic polymers

Polyacrilates (Carbomers & co.) Family of cross-linked acrylic acid polymers used as thickeners and gel makers. Carbomers are rather swellable than water-soluble: they disperse in water and upon neutralisation they swell and form a sort of network in the solvent. 

Carbomers are available with different commercial names depending on the type of process solvent used, the level of cross-linker employed, and the addition of additives to improve wetting and dispersibility. Most carbomers can be used also in hydroalcoholic gels (that contain up to 70% alcohol). 

Carbomer dispersions in water have a low pH. Upon neutralisation with sodium hydroxide the carboxylic acid groups are ionized and start to swell. This means that the thickening ability of carbomers is restricted to pH above 5. 

The presence of salts (electrolytes) induces a loss of viscosity because of de-swelling of the gel. This happens also if too much base is added in the neutralisation phase and then an acid is added to lower pH. 

Some polyacrylate-based thickeners that we can find: in addition to the Carbomers, we have TinovisⓇ GTC UP, CarbopolⓇ Ultrez 21, CarbopolⓇ Ultrez 30, PemulenⓇ tr1. Each one of these thickeners have their own requirements in terms of usage, pH range and interaction with other components of the formula. If you use them, be sure you read carefully the instructions given by the supplier on how to use them and on the incompatibility with surfactants, electrolytes, and pH-shifting ingredients. 


Acryloyldimethyltaurates. Polymeric thickeners based on acryloyldimethyltaurine and its salts. They allow gel formation at lower pH values compared to Carbomers, therefore they can be used also in presence of alpha- and beta-hydroxyacids. 

Examples of commercial products available are AristoflexⓇ AVC and AristoflexⓇ Silk. They can be dispersed in the oil phase (0,5-1%) in emulsions, or in the water phase (>1%) in transparent gels. Again, check the technical data sheet and instructions provided by the supplier for more detailed information on how to use them and on the incompatibility with other ingredients.


Hydrophobic polymers

In most products, we would use waxes to thicken oils. However, to obtain clear oil gels specific oil gellants should be used. They are not very common to find in suppliers’ shop, but here are some names you could find: Polyamide-3, Polyamide-4, Polyamide-6, ethylenediamine/stearyl dimer dilinoleate copolymer, and similar names. They can form clear oil gels with shear thinning properties. 

Film-forming polymers

Film-forming polymers are not used as viscosity enhancers, but as fixatives or in peel-off products. 

Polyvinylpirrolidone (PVP): used as hair fixative between 1-6% in the water phase (not very humidity-resistant)

Vinylpyrrolidone and vinylacetate copolymers (VP/VA copolymers): they form gels with hair fixative properties, used at 2-10% (in the water phase).

Arabic gum: used as fixative in makeup (eyeliner and mascara) and in hair styling products, use level 1-3% (Making Cosmetics suggests 1-10%), in the water phase. 

Polyvinylalcohol (PVA): used in peel-off masks and nail polish base coats. It can be dissolved in water (if it is a high molecular weight PVA, it will dissolve in water only upon heating). Upon water evaporation, it forms a film that can be peeled off. To allow and modulate evaporation of the solvent, alcohol is added to the formula. 10-20% PVA is sufficient to allow film formation; alcohol percentage varies according to the use: 10% in face masks, up to 20% in peel-off base coats (12). 

Inorganic thickeners

Alongside polymers, there is another small group of thickeners that can be used in cosmetic products: the inorganic thickeners. 

Inorganic thickeners are typically based on clays and silicas. They are supplied as powders and they can act as suspending or gelling agent. They can work with several mechanisms: hydration, cation exchange, formation of networks that thicken fluids (through hydrogen bonding or other interactions) (13). 

Bentonite clay: consists mainly of silica and aluminum, it can be used as filler, thickener or pigment binder. 

Use: 1-50%, add to the water phase and let swell. 

Magnesium Aluminum Silicate: smectite clay that can hydrate in water. 

Use: 0,5-3% in water phase (eventually in warm water to reduce hydration time).

Silica Dimethyl Silylate: oil-phase thickener, forms oil-gels. 

Use: in oil-phase, 0.1-30%. 

Hydrated Silica: used as thickener in toothpaste. 

Use: 20-30%.

References

(1) Rheology video series by Sam Morell on Youtube

(2) Barnes (2004), The rheology of emulsions. In Emulsions: structure stability and interactions (ed. Petsev)

(3) Lochhead (2017), The use of polymers in cosmetic products. In Cosmetic science and technology: Theoretical principles and Applications (eds Sakamoto, Lochhead, Maibach, Yamashita)

(4) Dicaprylyl ether: ECHA, European chemical agency, Dioctyl ether Registration Dossier

(5) Sasol Chemicals, Safety data sheet for NACOL Ether 8

(6) Castor oil e olive oil: de Vries (2017), The effect of oil type on network formation by protein aggregates into oleogels, RSC Advances, 7, 11803

(7) Caprylic/Capric Triglycerides: Wilmar International Ltd, Technical sheet for Caprylic/capric Triglycerides

(8) Polymer, Wikipedia Page

(9) Pal (1996), Rheology of emulsions containing polymeric liquids. In Encyclopedia of emulsion technology (ed Becher)

(10) Products’ technical data sheets provided by Glamour Cosmetics and Making Cosmetics

(11) Hennock (1984), Effect of xanthan gum upon the rheology and stability of oil-water emulsions, Journal of Food Science, 49, 1271

(12) Formulating Facial Masks, Cosmetics and toiletries 

(13) Polon (1970) The Mechanism of Thickening by Inorganic Agents, Journal of the Society of Cosmetic Chemists, 21, 347-363

Advertisement

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s