Viscosity testing of cosmetics and personal care products is of vital importance for quality control in their production. Analyzing the viscosity at various shear rates to simulate the sample’s processability during production as well as application behavior to ensure customer satisfaction is one of several test methods in quality control.
This article explains how viscosity checks of cosmetics and personal care products can be performed with a rotational viscometer.
The property “viscosity” gives information on how thick a material is and how easily it flows. In scientific terms, viscosity is the measure of the internal flow resistance of a fluid. By comparing a high-viscosity material such as hair gel with a low-viscosity material such as body wash at the same temperature, the former flows slower than the latter (Figure 1).
Figure 1: Simple viscosity measurement
The viscosity is influenced by ambient conditions such as temperature and pressure. Every temperature change will influence the viscosity of a material but the extent of the influence is fluid-specific. Some fluids will change their viscosity by 10 % if the temperature decreases by 1 °C. Pressure does not have such a big influence on viscosity. However, significant pressure changes such as from 0.1 MPa to 30 MPa will cause a change of 10 % in the viscosity for most liquids.
Besides ambient conditions, external processes like pouring and pumping but also the inner structure of a material (e.g. shape and arrangement of molecules) influence the viscosity (Figure 2). Using a rotational viscometer the external forces are simulated through the set speed of rotation. Furthermore, viscosity is also influenced by the duration of the external process acting upon the material.
Figure 2: Influences on viscosity
Typically the viscosity of cosmetics and personal care products is determined with a “spring-type viscometer”, which allows quick and easy viscosity checks.
Generally, measurements performed with a spring-type viscometer have to be carried out between 10 % and 100 % torque. After waiting for at least five revolutions of the spindle the measurement can be stopped in order to obtain a valid result. There are many different methods available to determine the yield point of a sample; however all of them differ in their output results. Therefore, whenever results need to be compared (e.g. same sample from different batches) it is important that the same yield point determination method is always used.
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:
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[1] 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[2] users can use the corresponding shear rate values to each speed which helps in designing pumping processes.
Figure 3: Concentric cylinder geometry used with rotational viscometers. Precisely defined dimensions allow for calculating shear rate, shear stress, and absolute dynamic viscosity. Rc = radius of container [m]; Rb = radius of bob [m]; L = length of bob [m], ω = angular velocity [rad/s]
To analyze the viscosity at various speeds and the yield point (force required so that the sample starts to flow) the rotational viscometer has to be set at a low rotational speed first. After waiting for at least five revolutions of the spindle the measurement results can be taken. Afterwards the speed has to be increased stepwise with an appropriate waiting period for each measurement, generating a flow curve. To evaluate the yield point of the sample using a flow curve various mathematical model analyses exist (e.g. Bingham, Casson, Herschel-Bulkley).
Example: A shampoo sample measured on a spring-type viscometer with a concentric cylinder system showed a relatively low yield point (also called yield stress) of 1.158 N/m² using the mathematical model “Casson” (Figure 4).
Figure 4: Flow curve diagram and yield point determination of shampoo using the mathematical regression model “Casson”.
To evaluate the flow behavior of the sample it is useful to calculate the shear thinning index. The shear thinning index is an analysis method which calculates the degree of shear thinning by dividing the lowest speed by a speed which is ten times higher (e.g. viscosity at 10 rpm and 100 rpm). If the value is >1 the sample is shear thinning/pseudoplastic. If the value is <1 the sample is shear thickening/dilatant. The measured shampoo has a shear thinning index of 1.95 (Figure 5). The higher the ratio is the greater the shear-thinning behavior of the sample is pronounced.
Figure 5: Viscosity measurement of shampoo at various speeds to analyze the shear thinning index
Toothpastes are highly viscous substances with a complex structure. To guarantee a constant quality of the toothpastes, various rheological parameters have to be evaluated. Toothpaste has to show a shear-thinning flow behavior because the viscosity has to become lower when a force is applied, e.g. when squeezing it out of the tube. Additionally, toothpaste has to show time-dependent thixotropic behavior which allows its structure to recover as quickly as possible after being squeezed out of the tube, so that the paste does not run off the brush. In order to prevent the toothpaste from flowing out of the tube when no force is applied, the yield point has to be analyzed. These properties are controlled in certain amounts of ingredients. Throughout the production process additives which improve the shear-thinning behavior and enhance structural recovery have to be added to the formulation.
The viscosity of paste-like samples, such as toothpaste, should preferably be measured with vane spindles (Figure 6). Vane spindles have the advantage that the samples’ structure will not be destroyed during immersion of the spindle and also slippage of the sample is reduced in contrast to measuring systems according to ISO 3219[2]. Vane spindles should be used in a sample container with a diameter of at least twice the vane diameter. The depth of the container should be greater than or equal to the vane spindle diameter.
Figure 6: Vane spindles
To analyze the viscosity of toothpaste at various speeds a speed ramp from low to high speeds has to be performed with a rotational viscometer. Using vane spindles, 10 rpm is the maximum speed which must be used for the test to avoid potential occurrences of turbulences that would falsify the measurement results.
To measure the static yield point with a rotational viscometer a special vane technique exists. Here, the sample is sheared at a constant low speed and the resulting torque or shear stress is measured respectively. If no torque increase is detectable anymore, the sample starts to flow. The maximum shear stress measured is called the static yield point and can be detected via a peak in a graph (Figure 7). Shearing of the sample before starting the measurement should be prevented since if the sample is pre-sheared prior to the yield point test the dynamic yield point is measured instead of the static yield point. The speed during the yield point measurement should be as low as possible and must not exceed 5 rpm. To compare the yield point of different samples the same measurement method has to be used.
Figure 7: Static yield point determination of toothpaste with vane spindles (red line indicates the yield point)
End consumers of hair gels expect various functionalities of the product. It has to bond the hairs together and maintain a long-lasting styling. Some additionally want to have a conditioning effect of the gel. Film-foaming adhesives like polymeric resins and fixatives are responsible for the bonding effect. Thickeners like carbomers are added to the gel to provide stiffness and a long holding power. The hair-conditioning effect can be reached by adding e.g. oils or glycerin to the formulation. The quality of the gel can be controlled through the amount of each of its ingredients. After checking the viscosity of the hair gel using rotational viscometers, additional ingredients can be added to reach the desired quality/viscosity of the product.
Hair gels have to have a shear-thinning property to ensure even application in and on the hair. The correct shear-thinning property can be easily tested with a speed ramp from low to high speeds. Viscosity values have to decrease with increasing speed to show a shear-thinning behavior. To be able to compare the shear-thinning property from batch to batch different mathematical analysis models like the Power Law and shear thinning index exist.
Another important property of hair gel is its yield point. The yield point influences the processability of the gel during manufacturing (e.g. pumping, filling) and affects the application (pressing the gel out of the tube). Moreover, hair gels with a higher yield point are perceived to have a higher quality. To measure the yield point of hair gel with a rotational viscometer the vane technique is recommended (Figure 8). The technique has already been described above.
Figure 8: Yield point detection of hair gel with the vane technique (red line indicates the yield point)
Liquid make-up has diverse rheological properties to satisfy consumers. Producers have to control the yield point to determine the amount of force required to press the liquid out of the tube. Further, the liquid must show a thixotropic behavior because after pressing it out of the tube the viscosity has to recover to its initial state again, so that it does not run off the finger or application brush. In the next step the liquid make-up must show a shear-thinning flow behavior to enable easy and even spreading on the face.
An easy method to control the quality of the liquid make-up is performed by using a rotational viscometer. To study the shear-thinning flow behavior and yield point in one go a shear rate ramp with DIN spindles according to ISO 3219[2], DIN 53019[3], or DIN 54453 (withdrawn) has to be performed. Measurements start at a low rotational speed and end with a high rotational speed (e.g. 10 rpm to 100 rpm) with various data points in-between.
Afterwards the yield point can be calculated using various mathematical analysis models, such as Bingham, Casson, and Herschel-Bulkley (Figure 9). The regression model which matches the curve the most should be used to analyze the yield point.
Figure 9: Flow curve diagram and yield point determination of liquid make-up using the mathematical regression model “Herschel-Bulkley”.
Simple viscosity checks with a rotational viscometer ensure perfect consistency of the end product to meet customers’ expectations. Measurement of the viscosity at different speeds gives insight into the flow behavior of personal care products. Yield point testing helps to figure out the force required for e.g. pumping, filling, and application of cosmetics. The viscosity, flow behavior as well as the yield point can be controlled through the formula of the product and thickening agents. More complex test methods to analyze the structure of cosmetics and personal care products can be performed with a rheometer.