A High-Performance, Fully Self-Contained, Rotational Viscometer

Routine viscosity measurements of materials are important in most industrial processes such as mixing and pumping, spraying and coating, extrusion, laydown, and leveling. Typically, a rotational rheometer is employed to measure the flow characteristics of most non-Newtonian systems; however, these instruments can be complicated and relatively expensive to purchase and maintain. They can require an operator with an advanced degree.

Figure 1 - MERLIN viscometer.

A lower-cost, simple-to-operate, rotational viscometer can provide a similar flow curve profile of these non-Newtonian materials if designed and produced to be application specific, incorporating a self-contained temperature control system, and interfaced with a user-friendly yet powerful Windows-based software package. This article describes the MERLIN self-contained, rotational viscometer (REOLOGICA Instruments, Bordentown, NJ) (Figure 1), which is capable of performing routine rheological tests—from quick, single-point checks to complete viscosity flow profiles, and yield stress determinations—without the need for complicated test method setup, laboratory facilities such as compressed air, and external fluid circulator and/or water connection for temperature control.

Figure 2 - Cone and plate, parallel plate, and bob and cup measuring systems for the MERLIN viscometer.

Table 1    -    Technical specifications for MERLIN viscometer

The MERLIN viscometer is well suited for investigating the mixing, stirring, and pumping behavior of coatings, emulsions, and dispersions, as well as for performing conventional flow and viscosity profile experiments. The innovative design incorporates a Peltier temperature control system that allows isothermal, step, and/or ramp temperature profiles. The DIN standard sample measuring systems of cone and plate, parallel plate, and bob and cup (Figure 2), coupled with a wide shear rate and torque range (Table 1), provide a measurable viscosity range from 1 to 1E08 cP. This is accomplished by using the DIN measuring system over the specified torque and/or angular velocity ranges of the MERLIN viscometer and then combining the results. 

The enhanced performance specifications of the viscometer enable the testing of a wide range of materials, including paints, coatings, inks, surfactants, polymer solutions, foods, pharmaceuticals, cosmetics, biochemicals, asphalt, molten polymers, adhesives, sealants, and petrochemicals.

Viscometer performance

Figure 3 - MERLIN viscometer Windows-based MICRA software.

To demonstrate the performance of the instrument with all three of the standard measuring systems—1) 30-mm-diam parallel plate, 2) 30-mm/5° cone and plate, and 3) 25-mm-diam bob and cup—a nominal 1000-cP silicone oil was evaluated at 25 °C. The parallel plate experiment was performed with a 0.5-mm gap, while the 5° cone and plate had a 0.150-mm truncation. Steady shear experiments were performed using the Windows™-based MICRA user interface software (Microsoft, Redmond, WA) (Figure 3) from high to low shear rate.

Figure 4 - Comparison of viscosity measurement for 1000 cP oil at 25 °C using three geometry setups: cone and plate, 30 mm; parallel plate, 30 mm; and bob and cup, 25 mm.

Figure 5 - Three independent runs for 1000 cP oil at 25 °C using bob and cup, 25 mm.

The resulting steady-state viscosity (Pa s) is plotted versus torque (mNm) in Figure 4 (torque is plotted on the abscissa rather than shear rate [s–1] to normalize the data for the three experiments). As expected, all three measuring systems show similar data. The ability to use different sample geometries permits the user to expand the capacity and range of the instrument and/or accommodate a wider range of materials and sample sizes/amounts. The bob and cup system is well suited for low-viscosity materials and/or low-shear-rate measurements since it provides more sample contact surface area and thus more torque signal. Three repeat runs are shown in Figure 5, where viscosity (Pa s) is plotted as a function of torque (mNm) for the bob and cup system, demonstrating the performance of the MERLIN viscometer over the working torque range of the instrument.

The bob and cup geometry was utilized to test several different viscosity standards covering the range 10–10,000 cP at 25 °C. All data are in acceptable ranges of viscosity, as shown in Figure 6. Again, the data are plotted versus torque to normalize the abscissa. 

Figure 6 - Viscosity as a function of torque for three Newtonian standard oils: 12,500; 1000; and 10 cP at 25 °C using bob and cup, 25 mm.

Figure 7 - Viscosity as a function of shear rate for 1000 cP oil at various temperatures: 10; 25; 50; and 80 °C using bob and cup, 25 mm.

Table 2    -    Temperature response of the MERLIN viscometer with bob and cup, 25 mm

Viscosity (Pa s) at several set temperatures for the nominal 1000 cP oil is shown in Figure 7 as a function of shear rate (s–1). The ability to control and/or systematically change the sample test temperature is paramount in making viable rheology and viscosity measurements. The viscometer’s built-in Peltier temperature control system supports all measuring systems, including cone and plate, parallel plate, and bob and cup. The temperature accuracy of the instrument using the bob and cup measuring system is summarized in Table 2. It is clear that the temperature was well calibrated throughout the range investigated. A thermal enclosure is available to enhance the performance at high temperatures.

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