SHEAR RATE continued
Figure 4 presents data for the above samples. The cream data are in red, while the ointment data are in blue.
The cone-plate
rheometer provides a convenient tool for
testing small amounts of pharmaceutical
products at different shear rates.
The apparent viscosity of each sample de- creases greatly with increasing shear rate. Viscosity increases as the shear rate decreases again, but to values less than those mea- sured in the initial, increasing-rate ramp. This
response is thixotropy. Structure in the mate- rial breaks down over time and needs time to recover. The non-Newtonian behavior, in these cases, is useful—the viscosity decreases as the materials are rubbed on the skin, for example, easing application. Stopping the rubbing, and ceasing the shearing, allows the viscosity to increase, helping the ointment or cream to stay on the area.
materials for consumer use, whether the prod- ucts are pharmaceutical creams, ointments, or other. A high viscosity may help an ointment stay in place once it is applied, while a lower viscosity may help a cream spread more easily.
The pharmaceutical industry manufactures and tests a wide variety of commercially im- portant products. Material behavior is often non-Newtonian. To reduce costs, many us- ers try to test smaller quantities of materials. Cone-plate or cone-and-plate geometry may be used in this case, because it provides well- defined shear rates and typically requires small sample amounts. Figure 3 shows a cone- plate rheometer.
A pharmaceutical cream and ointment were tested using the HBDV3TCP rheometer, with the CPA-40Z cone spindle and CPA-44PYZ sam- ple cup (AMETEK Brookfield, Middleboro, MA). This particular geometry requires only 0.5 mL of sample. The sample cup’s water jacket was connected to a bath to maintain the proper experimental temperature. All experiments were run at conditions ensuring on-scale read- ings. Since the ointment was far more viscous than the cream, slower speeds—lower shear rates—were used, given the instrument’s spring torque range and the cone used.
Figure 3 – Left: cone-plate rheometer. Right: sample cup and cone spindle, with Vernier dial for gap-setting.
Figure 4 – Pharmaceutical cream (red) and ointment (blue); apparent viscosity, cP, versus shear rate, s-1
, at 25 °C. AMERICAN LABORATORY 36 JUNE/JULY 2017
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56