Accurately measuring viscosity, or the resistance of a liquid to flow, is an important part of quality control in the cosmetics industry. Some liquids must have a certain type of viscosity to prove effective for their designed purpose. For instance, a lip balm must be highly viscous in order to proper adhere to and protect the skin. Conversely, a body mist must have very low viscosity so that it flows freely from the dispenser in an even manner. Some cosmetics must take on a specific viscosity to satisfy customer impressions and research suggestions; cosmetic products marketed "rich" or "luxurious" may have high viscosity while products touted as "light" or "natural" may have low viscosity.
Whatever the reason behind the product's design, it is critical to customer satisfaction to maintain a consistent and controlled quality of viscosity across production lines. Further, viscosity can impact a cosmetic product's chemical stability in its packaging over time. Therefore, given the many reasons why viscosity is important to the overall quality of cosmetic products, measuring viscosity has become key to successful cosmetic production companies.
Instruments and Viscosity
While there are numerous means by which to conduct viscosity testing at a cosmetic production facility, by and large the most prevalent test in the industry is the a mechanical viscometer. The precise type of viscometer needed for each cosmetic types varies upon the liquid's properties and intended viscosity. For instance, oil-based and water-based cosmetic compounds will do well to be tested with glass capillary or falling ball viscometers. A glass capillary viscometer measures the viscosity of the product by utilizing only gravity as the driving force. The falling-ball viscometer can be used for any Newtonian liquid or gas, and operates on the principle's of Newtonian physics as the ball travels through the cosmetic compound.
However, other cosmetic compounds, such as some elastic soaps should be tested with rotational viscometers. These non-Newtonian liquids measure viscosity of the liquid as a measure of the torque required to move an object through the cosmetic compound. There are numerous types of rotational viscometers available on the market with variable measurement range widths and precision. Ultimately, no matter what the type of viscometer, the machine will only measure viscosity of the cosmetic compound under one single condition. For more complex materials with viscosity that changes under certain conditions, it is necessary to utilize a rheometer.
For most cosmetic compounds, a viscometer represents a low-cost, precision instrument that is well-suited to the viscosity control testing required for consistent production. Typically, these devices are also small enough to move about the production area so that multiple products at multiple points in production lines may be tested with one unit on a routine basis. For smaller cosmetic companies, or even those that do not produce a large range of complex compounds, viscometers offer a uniquely valuable, versatile quality control device. However, many cosmetic companies will utilize a rheometer at the development stage, as they offer more data on how the compound's elasticity and viscosity will react under a broad range of conditions, including pressure and heat.
Of course, while it is highly advised to utilize a mechanical viscometer that has precise, reliable, and easily calibrated results, there are manual ways to test viscosity. With little more than a cup, or as much as a graduate cylinder and a small ball, it is possible to tell the viscosity of one liquid in relation to another liquid. These relative viscosities, however, have little meaning for internal documentation or quality control. Mechanical viscometers and rheometers will provide an absolute viscosity by which to measure future production lines.
For those developing cosmetics, it is necessary for serious developers to operate both a rheometer and a viscometer to isolate any compound instabilities or weaknesses, ensure product reliability and performance, and guarantee consistency and quality of lines for consumers and other stakeholders. The best machines will be able to give high-precision readings in only moments and offer easy-to-use functions, including absolute values, simplified clean-up, mobility, and broad measurement ranges. With the greater ranges of modern machines, the initial investment costs become drastically lower with the machine's versatility.
Manual Viscosity Tests
Finally, a person developing cosmetics may wish to initially employ manual viscosity tests while in the true envisioning phase of formulation. At this juncture, and prior to serious production concerns, it may be most economical and prudent to utilize the manual methods. Newtonian fluids do lend themselves very well to the simple formula that viscosity is equal to shear stress divided by shear rate. Or, for simple comparative viscosity, many developers with rank the compound's viscosity as a measure against the viscosity of water. Water, with an extremely low viscosity, is a good low comparative point for cosmetic compounds, which typically have formulas that "run" much slower than pure water.
Regardless of the means, it is clear that viscosity and the consistency of viscosity is important in the development and production of quality cosmetic products. By utilizing mechanic viscometers or rheometers, developers can assure precision readings and benchmarks for production from the onset of the production project.