Measuring the viscosity of body wash or shampoo is an important part of quality control. To meet customers’ expectations the liquid has to be applied easily to the skin while still adding a “rich” feeling. Liquids which are considered “rich” usually have a higher viscosity (i.e. are thicker) than “light / natural” products.
Thickness and flow properties are directly linked to viscosity and influence the cleansing efficiency, users’ perception, foaming properties, production filling, packaging, storage, and long-term stability. These products are generally composed of approximately:
- 80 % water
- 10 % surfactants
- 5 % viscosity modifier
- 2 % preservatives, fragrances, and colorants
- 3 % performance additives
Water-soluble polymers act as viscosity modifiers (i.e. thickening agent) to increase the viscosity. The required viscosity of shampoo and body wash also depends on the target group: Depending on whether they are aimed at women, men, or children the products come with different product thicknesses at rest in order to enable the consumers to apply different amounts of pressure on the tube for squeezing the product, e.g. shampoo, out. Body wash and shampoo mostly have a shear-thinning flow behavior that enables them to be squeezed out of the tube without forming stripes. Moreover, in order to improve the long-term stability during storage the viscosity of the liquid should be as high as possible, since the higher the viscosity is the lower the sedimentation of dispersed particles will be.
To be able to forecast the viscosity during pumping, application, and in the bottle at rest, the viscosity has to be determined at different speeds with a rotational viscometer. Tests can be either performed according to the ASTM D2196 standard with a relative spindle, which is usually used in a 600 mL beaker, or with a concentric cylinder system (Figure 3). The advantages of concentric cylinder systems are that a much lower amount of sample is needed and due to the defined shear gap according to the standard ISO 3219 users can use the corresponding shear rate values to each speed which helps in designing pumping processes.