In our last article, we discussed how consumers have expectations when it comes to shampoo. It has to feel luxurious but be able to flow out of the bottle, but not be so runny that product is wasted. There are certain expectations whether the shampoo is for men, women, or babies.
Rheology is the study of the flow of materials, and the deformation of materials under force and friction. Shampoo is definitely a product in which flow is part of its quality perception. One main rheological parameter that correlates with the thickness and flow properties of a shampoo is the viscosity (it’s resistance to flow). The viscosity of the shampoo not only affects the cleaning efficiency and user perception of the quality, but it also influences the foaming properties, production filling, packaging, storage and long-term stability of the product.
Shampoo formulations are complex, consisting of about 80 wt.% water and the remainder being surfactants, viscosity modifiers, preservatives, fragrances and colorants and performance additives. Each product type is made differently, depending on the market.
Today we will discuss how we tested three different commercial shampoos to show how products for different customer groups differ rheologically and how to determine the parameter viscosity to ensure the shampoo meets customer expectations. Those specific rheological properties will determine the richness or creaminess of the shampoo, thereby appealing to either male or female customers.
Three different commercial shampoos were tested on a viscotester rheometer with 35 mm plate/plate geometries to measure the thickness or the fluid’s resistance to flow. One of the shampoos was for men, one for women and one for children (infants). After carefully filling the shampoo on the lower plate and manually adjusting the measuring head to a pre-determined gap of 1 mm, the testing procedure was conducted in a shear rate range of 1 to 100 1/s. As can be seen in the figure below, the rheological fingerprint for the three products is quite different.
At low shear rates around 1 1/s the shampoo for men and the shampoo for children (infants) have about the same viscosity of about 12 Pas. The shampoo for women has a considerably lower viscosity of about 7.5 Pas. Male customers usually want to have an even richer (higher viscous) product than female customers that prefer slightly “creamier” (lower viscous) products. For child products, more functional aspects have to be taken into account as a child usually does not choose a shampoo according to its viscosity. However, the parents who buy the product intuitively “know” that a high viscous product will stay longer in the child’s hand when using it in the bathtub.
When applying higher shear rates the behaviour differs between the products. The “male“ product goes into non-Newtonian flow directly because of the higher molecular weight polymer additives that have been used in formulation to achieve a higher base viscosity, whereas the “female“ and “child“ product show Newtonian plateaus.
However the “female“ product leaves Newtonian flow at around 4 1/s to achieve lower viscosities when being used, while the “child“ product maintains a constant viscosity for much longer (up until about 20 1/s) so that less product is lost before starting the washing process.
As can be seen in the following figure, the measured viscosity is the same, independent of the measuring geometry.
This is due to the fact that the different processes will happen at different stress levels (as the customer groups apply different forces) and thus result in different shear rates. As no customer wants to experience the viscosity the product has at rest (rich and creamy) when they actually use the product, a shampoo has to be a non-Newtonian or better shear thinning fluid. To induce non-Newtonian flow and thus modify the flow behaviour towards the specific customer groups, water-soluble polymers are used as modifiers.
Additives have to be chosen wisely to formulate a product for a specific customer group. Determining the viscosity of a shampoo (or shower gel etc.) formulation is of utmost importance to understand if a product meets customer expectation.
To see more specifics, including instruments used and measuring routine, read The Rheological Behaviour of Shower Gel – What makes a product acceptable for a specific target customer?