We introduced the concept of viscosity in this post on fluid viscosity. In this follow-up article, we explain some of the key considerations every formulation chemist should consider when setting a viscosity specification.
Viscosity measurements are commonly used on release specifications for raw materials and cosmetic formulations because these types measurements are quick and easy to take. It is important for cosmetic chemists to understand what viscosity and rheology measurements mean in terms of formulation performance and stability in order to develop robust specifications. It may seem like an intimidating task to cover all the variations that can arise, but vigilance and experience will make you feel far more confident. Here are a few guidelines to use when developing viscosity (and other) specs for your new formulations.
Individual Contributors: When starting to develop viscosity specifications for a new formula first consider the rheological properties of your ingredients and what they are contributing to viscosity measurement. Thickeners can impart large increases in viscosity, but may lack sufficient yield value to stabilize emulsion droplets or suspend mica particles. Fatty alcohols can also build body over time due to their crystalline structure. So keep in mind, higher viscosity is not an indication of product stability. You have both chemical and physical instabilities at play.
Selecting a Target: Packaging and dispenser choice may impose parameters on viscosity and rheology. For example a depilatory should be easily squeezed from a tube but needs to remain on the consumer’s legs without sliding off for ten minutes. Viscosity can also provide sensorial signals to the consumer; a more viscous product can seem luxurious and more moisturizing (although it might not actually be). Be sure to assess these factors when considering viscosity targets.
Spindle, Speed, Time: You should choose a spindle and speed appropriate for your product. You also need to specify the number of rotations or time at which the reading is taken. Consult with literature to make the proper determination. Viscometer manufacturer Brookfield has some great reference pamphlets which you can easily obtain. However, it is not uncommon for a company to prefer measurements for many products are made with the same spindle, speed and time combination. Also, be sure to use a consistent container size for measurements. A 600 ml beaker is recommended, but if using a smaller container you should be sure all subsequent measurements are taken in from the same size container in order to make correlations.
Account for Raw Material Variability: Test multiple lots of your thickening ingredients, especially at the high and low end of their specifications, and any other ingredients which might have significant impact on the final formula. Do your purchasing agent a favor and test multiple suppliers of the ingredients if possible. Testing multiple sources allows for flexibility in purchasing and allows you become acquainted with the level of sales support and technical expertise each supplier offers. Slight differences in cost can be overcome by superior quality and reliable technical support in times when you need it most.
Stability considerations: Viscosity can change over product lifetime, and those changes may not necessarily be bad. Monitoring viscosity over the full array of stability testing will allow you to determine what range of readings is acceptable.
Scale-Up: Consider processing conditions and take measurements on samples from various stages of scale-up. The difference between mixing conditions in a beaker at the lab bench and in massive production tanks can cause profound differences in formulation viscosity. Homogenization, pumping and filling product can also contribute to changes in product viscosity, so measure viscosity and compare differences at different points in the manufacturing process.
Viscosity measurements are a key part of creating almost any cosmetic product. By considering all of these factors, you can make certain your compounders will make consistent, high quality products.