Many consumers will try a variety of personal care products in search of the perfect combination of performance, fragrance, feel, and price. When it comes to shampoo, consumers typically look for a shampoo that is thick and rich but still flows from the bottle and spreads easily, but at the same time one that is not too runny so product isn’t wasted. One main rheological parameter that correlates with the thickness and flow properties of a shampoo is the viscosity. The viscosity affects both the cleansing efficiency and the user perception of a shampoo product. It also influences the foaming properties, production filling, packaging, storage and long-term stability of the product.
The viscosity and thickness of shampoos and other personal care products such as shower gels are influenced by the polymers used in the formulation. Explains an article in International Journal of Research in Cosmetic Science, Advancements in polymers used in hair care: a review, the polymers used in hair care typically have low densities and are largely used as film formers, coloring agents, conditioning agents, thickeners and sun protection agents. Polymers made of long branched or unbranched molecules provide thickening effects by entanglement, cross-linking or cluster formation.
An article on the Royal Society of Chemistry web site explains the function of polymers in “2-in-1” shampoo/conditioner formulations:
“There are two basic approaches to gaining 2-in-1 functionality, according to Mort Westman, president of Illinois-based R&D consultancy Westman Associates. Both approaches employ anionic surfactants because they provide the best lather and cleansing, he says. ‘One approach is to mix cationic polymers in anionic shampoo. This was developed and commercialised in the late 1960s and early 1970s, and is still being done. The second approach is a micro-suspension of silicone in anionic shampoo. This is more effective than the cationic polymer approach. This has become the standard art of 2-in-1 shampoos.’
“…The conditioners are held in suspension by polymers, which bind to the surfactant molecules and hold on to the conditioning molecules. ‘The way the surfactant and polymer are bound is sensitive to water,’ Ryan explains. ‘So when you rinse your hair the surfactant separates from the polymer and conditioner. The polymer and conditioner are not soluble in water, and they precipitate and coat all the hairs.’ ”
Another interesting article on the use of polymers in shampoo can be found on the British Plastics and Rubber web site, which reports on an investigation into the use of a polymer derived from chitin, found naturally in the seafood shells, to replace synthetic polymers to make environmentally friendly shampoos and laundry detergents.
Other research has found that regardless of the formulation, consumers often have different opinions of the same product regarding viscosity because they usually put different amounts of energy into squeezing it out of the bottle or distributing it on themselves. Manufacturers modify the flow behavior of their products to appeal to different customer groups (female, male, and children) by using water-soluble polymers as modifiers.
In our next article, we will discuss how we tested three different commercial shampoos to show how products for those different customer groups differ rheologically and how to determine the parameter viscosity to ensure the shampoo meets customer expectations.
If you can’t wait, read The Rheological Behaviour of Shower Gel – What makes a product acceptable for a specific target customer? to learn how products for these customer groups differ rheologically and how the parameter viscosity can be determined with rheological instruments.