A software upgrade closes the performance gap between research grade rheometers and standard benchtop viscometers.
Many development projects focused on formulation of new or improved materials need to qualify flow behavior for proper performance of the product. Examples include greases used in bearings, lubricants in engines, and thickening agents in foods. Companies that produce these fluids and semi-solid materials run extensive tests on flow behavior to include initial yield stress determination, flow curve profiling, and recovery after use. The requirement for detailed methods that can quantify viscosity of materials and related parameters has led to a number of expensive research rheometers with broad-based capabilities. The drawback, however, has not only been in price, but also in the time required to adequately train operators.
A recent breakthrough in the use of software to control standard benchtop viscometers has created an opportunity for more companies to afford some of the high-end capability characteristic of research grade rheometers. The new software algorithm, created by Brookfield Engineering, Middleboro, Mass., allows the viscometer to measure true yield stress, in addition to the typical flow curve profile. Given the low cost of this type instrumentation, R&D departments will be able to affordably address the issues mentioned above using their own equipment.
A rotational viscometer measures the torque required to rotate a spindle in a material. The spindle is driven by a motor through a calibrated spring; deflection of the spring is measured to quantify the amount of resistance to flow. By using multiple speeds of rotation and interchangeable spindles, a wide variety of viscosity ranges can be measured.
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Schematic of Rotational Viscometer
For a given viscosity, the viscous drag, or resistance to flow (indicated by the spring wind up), is proportional to the speed of rotation and the spindle’s size. The drag will increase as the spindle size and/or rotational speed increase. Measurements made with the same spindle at different speeds are used to detect and evaluate flow properties of the test material.
Conventional digital viscometers use continuous sensing transducers to monitor the torque signal as the spindle rotates. Programmable chips permit dedicated test programs to be stored directly in the viscometer head. These capabilities have led to a proliferation of software programs that transform the standard rotational viscometer, typically used for single point viscosity measurements, into a powerful analytical tool capable of broader rheological analyses.
One particular software program measures yield stress, or onset of flow, as the spindle first attempts to rotate in the material. The algorithm measures the wind-up of the spiral spring inside the viscometer head, compares it to the angular distance traveled by the rotating motor shaft, and then computes the stress and strain values associated with the sample material that is being tested. The vane spindle is key to the viability of this method.
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Typical graph is shown for yield stress test with viscometer and vane spindle running at low rotational speed.
The advance is revolutionary since this has not been done before with a standard benchtop viscometer. The price differential between the rotational viscometer with this new capability and a high end rheometer is substantial: $2,500 vs. $20,000+. The latter figure is typically higher once required software and accessories are included.
A more practical explanation for why this type of measurement is important may help.
Yield stress behavior
The yield stress is an important physical property used to characterize the initial flow behavior of liquids and semi-solid materials, such as pastes. The measured or calculated parameters are the yield stress and yield strain. The yield stress is the critical shear stress, applied to the material, which causes the onset of flow. The yield strain is the deformation, resulting from the applied stress, at which the flow starts.
Most materials are designed to have a target yield stress value, so that the behavior satisfies a specific customer need. Ketchup, for example, must flow out of a bottle when shaken or squeezed, but then solidify on the french fries. Shaking or squeezing the bottle stresses the ketchup so that it flows; after the ketchup settles on the fries, its structure rebuilds so that the ketchup “sits” in place rather than flowing off the fries like water.
Many water-based paints have low yield stresses. Brushing or spraying provides enough stress so that the paint flows easily and smoothly over a painted wall. However, a thin layer of applied paint, allowed to rest undisturbed on the surface, regains its structure quickly so that there is neither unsightly “dripping” afterwards nor rippled brush marks.
Adhesives can vary from low viscosity to high viscosity, depending on formulation and application. Thicker adhesives used for floor tiling are stiff when first put onto a trowel, but spread easily over the floor. This ease of application is important to the user, although the quality of the adhesive is possibly judged more by its “stiffness,” which relates to yield stress value.
Yield test data
As explained earlier, the operating principle of the rotational viscometer is to drive a spindle through a calibrated spiral spring connected to a motor drive shaft. When a vane spindle is immersed in the test material, the resistance of the material to movement is measured by observing increasing torque values as the motor rotates at low rotational speed. Choices of speed vary from 0.01 to 5 rpm, depending on the nature of the test material.
If the vane spindle did not move at all, a plot of the respective torque vs. time would yield a straight line. The data, however, often looks like the graph above because there is usually some deformation of the test material due to the increasing force imparted by the vane spindle. The maximum torque value is the yield stress. An algorithm in the firmware within the instrument converts the maximum torque value into a yield stress value, which is determined from the vane spindle geometry and the viscometer rotational speed.
The shear stress measurement range of the instrument (in Pascals) is determined by the size and shape of the vane spindle and the full-scale torque range of the calibrated spring. The slope of the line (stress data) prior to the start of flow is an indication of the “stiffness” of the material. A steep line suggests rigid material structure, whereas a low-angle line indicates softness.
Since Brookfield terminology is universally used to characterize standard rotational viscometers, consider the following spring torque viscometers:
The calculated shear stress measurement range for three vane spindles at the HB spring torque is further outlined:
Based on these performance parameters, a variety of materials can easily be measured varying from food products and hair gels to adhesive caulks and pastes.
This new software algorithm for yield stress measurement, which controls a standard benchtop rotational viscometer with continuous sensing capability, was initially developed in 2001. Field-testing by industry and academia commenced in 2001 and continues through 2004. New users have now confirmed applications on adhesives, gums, starches, beverages, and various food products. Standard rotational viscometers have started a new chapter in their role to help better characterize the comprehensive flow properties of materials.
—Robert G. McGregor
National Sales and Marketing Manager for Laboratory Products at Brookfield Engineering Labs
Brookfield Engineering Labs, 508-946-6200, www.brookfieldenginnering.com