Figure 1 – DV3T cone plate (AMETEK Brookfield, Middleboro, MA).Many types of instruments can be used to measure viscosity. There are two fundamental scientific devices that divide the rheology world (study of material deformation or flow behavior) into either kinematic or dynamic flow. The former utilizes a capillary viscometer—a glass tube vertically oriented with gravity driving fluid flow downward—to measure transit time between two reference points on the tube. The latter rotates a spindle immersed in the sample material and measures torque resistance (see Figure 1).
General industry preference is the rotational viscometer due to its ease of use and rapid test capability. Key questions to answer when considering what types of rotational viscometer are appropriate include the following:
- How much sample material is available for testing?
- Is a broad range of shear rates used in the test method important?
- Is temperature control of the sample required?
How many samples must be tested? Answers to these questions may dictate the selection of spindle type to make the viscosity measurement. Figure 2 shows an assortment of spindles found in labs. Each has a recommended sample volume to use for the viscosity test. Cone spindle is by far the one that requires the least amount of material, typically <2 mL. Others, like cylinder and disc, may need as much as 500 mL. Special chambers used with the cylinder can reduce sample size to 20 mL. The bottom line is that a cone spindle is limited to a very small sample size. That can be the ideal choice, provided there is sample homogeneity when testing such a small quantity of material.
Figure 2 – Examples of spindle types: cylinder, disc, cone, T-bar, and spiral.Test method must also be considered when making viscosity measurements. QC labs typically make a single-point test, which involves rotating the spindle at a specified speed for a defined time interval, then recording the viscosity value. R&D labs generally want to characterize the material, so spindle rotation at multiple speeds is necessary. This shearing action on the sample may correlate with forces applied to the material during processing (mixing, pumping, flow in pipe) or use by the operator (rubbing, spreading, or swallowing). Cone spindle provides the broadest shear rate range per revolution of spindle. Its use is therefore highly desirable in R&D labs. QC labs that perform two-points tests, where the specified shear rates are far apart (e.g., 10 sec-1 and 1000 sec-1), make cone spindle the preferred choice.
Viscosity of most materials is temperature-dependent. As temperature increases, measured viscosity will generally decrease. QC labs usually have a control temperature specification when qualifying production samples for acceptability. Common control values in industry are 20 °C and 25 °C; the medical and pharmaceutical industries may require body temperature or 37.6 °C. If the sample size is small, the time required to bring the material to a specified temperature is minimized. This advantage is maximized by using cone spindle. One word of caution worth mentioning is that cone spindle should also be tempered at the same temperature as the sample material before making the viscosity measurement.
The volume of samples requiring the viscosity test is an important consideration for QC labs. Time is precious, given the heavy workload that continues to grow every year. A comparison of cycle time for making viscosity measurement with each spindle type must include:
- Sample prep to place the sample in the instrument
- Time to run the viscosity test
- Instrument cleanup after the test.
Figure 3 – Cone and plate components.Again, cone spindle is the winner because of small sample size. The sample cup also requires cleaning, but this is much easier than handling all the surface area in a 500-mL beaker. A cone and plate viscometer has a compact assembly, as shown in Figure 3. The cone spindle attaches to a shaft in a long tube that extends downward from the instrument head. The sample cup attaches to the bottom of the tube. Ports on the tube are connected to a circulating water bath for temperature control of the sample. The cost of this system (instrument plus bath) is between $5000 and $10,000, depending on the model. Although more expensive than a standard viscometer with a cylinder or disc spindle, the time savings when running viscosity tests more than justifies the selection of cone and plate.
Samples sent to the author’s lab for evaluation by customers are usually evaluated by cone and plate. The reasons for choosing this viscometer type are the same as above. Total time needed to generate a flow curve showing viscosity versus shear rate is a matter of minutes (see Figure 4). Moreover, the shear rate range is sufficiently broad to show the shear thinning behavior of pseudoplastic materials (viscosity decreases as shear rate increases). If the customer requests multiple temperatures, it will obviously take longer because the circulating bath must equilibrate at each temperature. In addition, the gap between the cone spindle and sample plate must be adjusted to account for thermal expansion or contraction of the spindle/plate (generally required when the temperature difference between setpoints is >5 °C).
Figure 4 – Flow curve: viscosity versus shear rate.Conclusion
Cone and plate viscometers are becoming more popular throughout industry because of the many advantages described above. They have always been a preferred instrument in R&D labs. Now, productivity gains in QC testing are being noticed and word is spreading that high-volume workloads are best handled using cone and plate.
Robert G. McGregor is director, Global Marketing & High End Lab Instrument Sales, AMETEK Brookfield, Instrumentation & Specialty Controls Division, 11 Commerce Blvd., Middleboro, MA 02346, U.S.A.; tel.: 508-946-6200; e-mail: [email protected]; www.brookfieldengineering.com