by Len Thibodeau, Brookfield Engineering Laboratories, Inc.
Quality control testing of viscosity usually involves a test methodology yielding one viscosity value. This application note recommends that additional viscosity testing should be considered in order to fully characterize rheological properties likely to be important to consumers of pharmaceutical creams and ointments.
Pharmaceutical ointments are typically engineered to have flow properties important to the physical performance of the product when used by the consumer. For example, most ointments are intended to be thick when standing to prevent them from flowing away from the intended area of use, and they must also become thin when rubbed so they are easy to apply.
Shear rate is a term used to describe the rate of flow of a fluid and is expressed in units of s-1. Shear rates from 0 s-1 to 10 s-1 describe nearly motionless fluid. The action of rubbing an ointment or cream into the skin can easily result in shear rates of over 1,000 s-1 and can be as high as 10,000 s-1. At high flow rates such as these, ointments usually exhibit much lower viscosity than they do when motionless. This property is known as shear thinning behavior and many products are engineered to behave this way.
This graph shows viscosity profiles of two ointments whose physical appearance is similar. However, a multi-step, programmed viscosity test clearly demonstrates significant differences between the two. The graph shows viscosity measured over a shear rate range of 37s-1 to 375s-1 then back to 37s-1. The entire measurement time for each test was 6.5 minutes. Both samples exhibit shear thinning behavior, indicated by the downward sloping curves. Also, sample #1 shows a significant loss of viscosity at the end of the test, while sample #2 shows only minor viscosity loss. This loss of viscosity is due to shearing and is termed thixotropy. To re-state this in terms of physical appearance sample #1 would appear to be a much more firm ointment prior to use than would sample #2. Both would feel smooth and creamy when rubbed, and both would appear to be similar in consistency after application. Sample #1 has permanently lost the firm appearance it had prior to application. Complete testing such as this allows the pharmaceutical manufacturer to see the affect on all aspects of product performance resulting from changes in formulation.
The addition of software to the modern viscometer enables the data acquired above to be to fit to one of several rheological models to objectively estimate parameters such as plastic viscosity (slope, or shear stress required to maintain constant flow) and yield stress (interecept, or the shear stress required to initiate flow). In the example shown, a Bingham model has been applied on the data from sample #1 above. Multi point viscosity profiles can better characterize flow behavior and provide more complete quality control than the single point viscosity tests that are commonly performed. The modern laboratory viscometer can automate this testing and provide data analysis parameters as well.