Physical measurements of yield stress, creep, and viscosity can guide R&D teams to produce desired product formulations.
Figure 1: Brookfield controlled stress rheometer with cone/plate geometry. All figures: Brookfield Engineering Laboratories
Driven by marketing and manufacturing input, R&D teams are responsible for characterizing new formulations for flow behavior. Marketing-based end-user requirements for product performance include how paint coats a surface, whether it spreads easily, and how it levels afterwards; the lubricating action of grease in a bearing; squeezing pastes out of tubes and applying/spreading the paste on a surface; and the clinginess of sauces and dressings to salads and other food items.
Manufacturing input anticipates processing issues, which include the challenge of moving product between tanks, mixing and blending ingredients together, and delivering product to fill containers where final packaging takes place.
Three physical properties R&D teams investigate when evaluating new formulations are yield stress, creep, and viscosity. During processing, yield stress values are used to calculate the startup torque required in the motor for the mix tanks, while viscosity affects the flow rate in the outlet nozzle at the fill line. When the customer uses the new formulation, yield stress may simply be the perceived "thickness" of the material in the container; viscosity is associated with how well the material flows during use; and creep characterizes whether flow continues after the intended fluid movement has been completed.
This article explains how these properties can be measured and technical challenges that may arise in standardizing on a method for use by quality control (QC).
Initial resistance to movement is the essence of yield stress. Measuring the amount of force it takes to start a fluid moving is the most direct way to quantify yield stress. A controlled stress rheometer is the best tool for making this measurement (Figure 1). After the spindle is brought into contact with the material, a torque ramp is applied to the spindle.
At the outset, when the torque values are relatively low, the spindle only moves within the elastic limits of the fluid since the fluid's internal structure is stronger than the external force being applied to the spindle by the motor torque. As soon as the instrument detects non-elastic spindle movement, the amount of torque applied to the spindle at that instant is captured and converted into a yield stress value.
Various spindle geometries are available for making yield stress measurements, and there is no single rule of thumb on preference. The best approach is to experiment with the different geometries—coaxial cylinder, cone/plate, vane spindles—and decide based on the data that shows consistent repeatability.
An important technical consideration is the rate at which the stress ramp is applied. This means how quickly the stress is incremented from zero to a predetermined maximum value. Results for the yield stress measurement will vary as a consequence. A rapid ramp generally produces a higher yield stress compared to a more gradual ramp, which results in a lower yield stress value. Therefore, it is important to do some experimentation to decide which ramp rate works best in terms of repeatable data.
Once a material starts to flow, will it continue if the initial force that caused movement continues to work on the fluid? This property, known as creep, is measured by applying a constant force to the material and monitoring spindle movement using a controlled stress rheometer. Figure 2 shows the type of graphical plot obtained when making a creep measurement.
The action of gravity on materials is the most ubiquitous example of this everyday phenomenon. When food items are coated with a sauce or dressing, movement will continue if gravity is stronger than the yield stress within the fluid. The undesired consequence is that the food item loses its coating. The formulator will therefore use thickening agents in the sauce or dressing to prevent this from happening. The amount of thickening agent needed to prevent creep is determined experimentally by preparing different concentrations and running experimental tests with a controlled stress torque value equivalent to gravity. When spindle movement is negligible, the formulation may be deemed successful because the dressing will hold its position.
This parameter may be best understood since manufacturing companies are more likely to already make viscosity checks in accordance with an established QC specification. The most common method is to use a rotational instrument, similar to the device shown in Figure 1. Choice of spindle and speed are defined. The measured viscosity value must fall between two limit values for the test to pass. This is called a "single point" viscosity check.
Most fluids are pseudoplastic, which means the measured viscosity decreases as the speed of spindle rotation increases (Figure 3). With the automation available in today's viscometers and rheometers, the use of two or more speeds to make viscosity measurements is worth consideration. The additional information gained is an understanding of the "degree of pseudoplasticity" that exists in the fluid. R&D teams are quite comfortable running this type of test to characterize flow behavior. Whether it has value for QC is something for R&D to determine. The advantage is that it may identify a suspect batch of material that a "single point" test would fail to detect.
Today's instruments can run these tests automatically. A possible shortcut is to consider a "two point" test, which selects a low rotational speed and a high rotational speed that is at least an order of magnitude greater. The respective viscosity values are turned into a ratio, which gives a value greater than 1.0 and provides a quick reference on the "degree of pseudoplasticity". This calculation, known as "Thix Index", has been used by some companies for many years. If a research group is not sure about moving toward a multiple speed test that produces a curve similar to Figure 3, this is a reasonable compromise to consider.
Testing for fluid flow behavior probably requires more than just a viscosity measurement. R&D teams must decide what tests work best. Viscosity alone may not give enough information. Yield stress is important to manufacturing because this determines the startup power requirement for pump motors. It may also be important to the customer, relating to the appearance of product or the initial force required to use the product, such as squeezing a tube. Creep will tell if the product holds position after it has flowed, such as a bead of adhesive remaining in place after application.