AL
Stopping the rubbing, equivalent to a very low or essentially “zero” shear rate, allows the material to recover to a higher viscosity, and helps ensure the product will stay on the skin. Viscosity measurements at various shear rates help quantify similarities or differences between various pharmaceutical ointments and creams, for example. The cone-plate rhe- ometer provides a convenient tool for testing small amounts of pharmaceutical products at different shear rates. Proper equipment selec- tion may allow significantly different materials to be tested with one instrument and geom- etry, merely by changing the testing speed or shear rate.
Figure 5 – Thixotropy: apparent viscosity, cP, decreasing with increasing time, minutes, at a constant speed of 1 rpm for a pharmaceutical ointment at 25 °C.
Some pharmaceutical customers use the mul- tispeed or “speed ramp” tests as an initial R&D tool. A simpler procedure may then be pro- vided to the QA/QC lab. One example is the use of cone-plate geometry to test a sample at one speed, waiting a few minutes before taking the data point. Shearing the material for some time helps to break down the structure; the viscos- ity readings decrease and then plateau after some time. Therefore, more consistent readings may be taken from sample to sample, after a suitable shearing time. Representative data are shown for the ointment in Figure 5. The data show that the viscosity plateaus within about 2 minutes for this material. Therefore, an ap- propriate, single-point QC test may be to shear each ointment sample at 1 rpm for 2 minutes before recording a reading.
Conclusion
Shear rate is important because it may sig- nificantly affect the viscosity and therefore the processability and applicability of various materials. In the example given here, higher shear rates during rubbing result in lower viscosities and thereby allow ointments and creams to be more easily spread by the user.
Huber USA Inc.
100 Centrewest Ct. • Cary NC 27513 Phone 1-800 726 4877 •
info@huber-usa.com www.huber-usa.com
Offer Ends July 31, 2017
David Moonay is sales engineer, rheology labora- tory supervisor, AMETEK Brookfield, Instrumenta- tion & Specialty Controls Division, 11 Commerce Blvd., Middleboro, MA 02346, U.S.A.; tel.: 800- 628-8139/508-946-6200; e-mail:
david.moonay@
ametek.com;
www.brookfieldengineering.com
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