Pharmaceutical ointments are engineered to have rheological properties important to the physical performance of the product when used by the consumer. Most ointments are intended to be thick when standing to prevent them from flowing away from the intended area of use. High viscosity at near zero shear rate characterizes this behavior; determining the yield stress value quantifies this desired property. Ointments are also engineered to be easy to apply when rubbed. This is known as shear thinning behavior. Both characteristics of yield and shear thinning can be easily characterized using only a small volume of sample with a Wells-Brookfield Cone/Plate viscometer or rheometer.
For example, a Brookfield DV-III Ultra CP Programmable Rheometer with cone spindle CPE-40 enables low to high shear rate measurements with only°.5ml of sample material. Temperature control of the plate is easily accomplished by connecting to a Brookfield TC-501 Circulating Water Bath.
* While a particular model/version may be used as an example in this method, any current or past model/version from the same series may also be used. Please consult a sales associate to discuss the most current instrumentation and software available.
Graph 1 shows viscosity profiles of two ointments whose physical appearance is similar.
However, a single, programmed viscosity test clearly demonstrates significant differences between the two. The graph shows viscosity measured during a shear rate ramp from 37 sec-1 up to 375 sec-1 then back to the starting value. The entire measurement time was 6.5 minutes. While both samples exhibit shear thinning behavior, Sample #1 shows a significant loss of viscosity as a result of shearing action, while Sample #2 shows minor viscosity loss. Loss of viscosity due to shearing action is termed thixotropy. Such tests allow the pharmaceutical manufacturer to adjust formulations to consistently achieve the desired product performance.
The addition of Brookfield Rheocalc® software enables data acquisition and storage as well as the ability to fit rheological math models to the data to estimate parameters such as yield (the shear stress required to initiate flow) and plastic viscosity (the function of shear stress required to maintain constant flow). In this case, a Bingham analysis has been performed. This math model is one of several included with Rheocalc.
Figure 1: Bingham Math Model