Beyond the Zahn Cup
October 1, 2008
The standard coatings method for reading batch viscosity usually involves a Zahn or Efflux cup (Figure 1). This tried and true method involves testing for kinematic viscosity by dipping the cup into the batch and filling it with material. The user then draws the cup out of the material. When the cup breaks the surface, a stop watch is used to time the flow out of the hole in the bottom of the cup until the cup is empty. From this method, kinematic viscosity is then computed.
Sounds simple, and it is; no special skills are required to use a Zahn or Efflux cup. However, there are variables involved that can contribute to an inaccurate viscosity measurement. This is hardly ideal when an accurate viscosity is required on materials being produced in large and sometimes expensive quantities. Variables that can contribute to inaccurate readings include temperature considerations, operator error and maintenance issues such as a cup with a hole that has not been cleaned.
Also, what does one do if rheological data (flow behavior) is required? How can this information be gathered in a cost-effective manner? Rotational viscometers (Figure 2) provide accurate viscosity readings while also gathering rheological data.
Dip Cup MethodologyWhen using a Zahn cup, the operator, depending on the model of the cup, needs to insert a thermometer in the fluid to be measured, place a finger in the ring, lift the cup out of the fluid, and start a stopwatch when the cup breaks the surface, then stop the timing when the steady flow of the liquid from the orifice breaks. The reading, in seconds, is then converted to Kinematic Viscosity expressed in Centistokes (cSt) via this formula:
V = K (T-C) where
V = Kinematic Viscosity (cSt)
T = seconds
K and C are constants related to the cup being used
This is one method. Many readings are also reported in cup seconds and not Centistokes. From the drain time, a conversion chart can be used to correlate viscosity.
A Zahn cup’s accuracy is typically within 1 second of a known standard. For accuracy, each cup should have a correction, which is determined by comparing the measured viscosity to a viscosity standard. This needs to be appropriate to the size of the cup being used.
Rotational Viscometer AdvantagesKnowing the viscosity of your material is certainly important. But, what about performance factors and rheological information? What should you do if there is a need or requirement, not only for viscosity measurement, but for shear rate information, sag, thixotrophic behavior? These are equally important. What type of cost-effective method could be used to gather this information?
Rotational viscometers, appropriate accessories and powerful software can garner these types of parameters for less than $5000. Cost-effective, accurate, robust, reliable and, most importantly, repeatable rheological data can be produced and stored for analysis. There is no guess work, no subjectivity, with these instruments. Data, be it viscosity, shear rate or shear stress, is displayed on the screen of the instrument immediately and precisely. Data can be gathered in single-point measurements or, with the use of software, charted, stored and exported to Excel.
For defined shear rate testing, coaxial cylinder geometries are an option and can provide data at a reasonable cost. These are typically add-on options for generic rotational viscometers. For example, a Small Sample Adapter (Figure 3) can provide low to medium shear rate/shear stress information. When used in conjunction with software (Figure 4), professional charts and graphs can be produced. Thus, prior batch data can be called up and overlaid to compare to new batch testing. Furthermore, material flow characteristics can be plotted: viscosity against time, shear stress vs. shear rate, viscosity vs. rpm, etc. Thixotrophic behavior, a time sensitivity to shearing, can be plotted and analyzed, and temperature can be monitored and, with a programmable water bath, controlled to see what effects this would have on the material.
High Shear Rate MeasurementTo evaluate how a coating behaves when sprayed or brushed, high shear rate instruments are needed to test for this condition. There are a number of instruments that can perform this vital function (Figure 5). ICI produced a high shear rate cone/plate instrument for the coatings industry that would give, at 900 and 750 rpm respectively, shear rates of 12,000 and 10,000 reciprocal seconds. This type of high shear rate capability is needed for a spraying application, for example.
Obviously, it is essential to know the viscosity and rheological behavior of the material at these high shear rates. For data analysis, software packages are available that can plot and store data, create professional reports for presentation, export to Excel, and offer math modeling capabilities such as the Power Law.
Single-Point versus Multiple-Point MeasurementsSingle-point measurements, like those taken with a Zahn cup, can quickly give you a viscosity value. Rotational viscometers can give single-point viscosity as well as shear rate and shear stress, but a single-point measurement does not indicate a material’s flow characteristics. One quick method for solving this is called a Thix Index, which involves taking two measurements a decade apart in rotational speed. For example, a viscosity measurement is taken at 1 rpm; the second measurement is taken at 10 rpm. By dividing the low-speed viscosity reading by the high-speed viscosity reading, a Thix Index is generated. The higher the Thix Index, the higher the difference in viscosity, and the more shear thinning the material is and the easier it is to pump.
A more accurate indicator of a fluid’s behavior is to create flow curves or rheograms. To do this, multiple data points must be gathered. A speed ramp test, shown in Figure 6, is one the best indicators of how a material is behaving when sheared. By running a simple speed ramp test and collecting data over time, a material can be classified as shear thinning (pseudoplastic), or shear thickening (dilatant). More information can be gathered by plotting shear stress versus shear rate and using math models, such as a Bingham model which is typically used for paints to interpret the data.
To summarize, “a speed ramp” test is typically used to determine pseudoplasticity. By ramping the speed (shear rate) up and down, and plotting the data as viscosity versus shear rate, a curve is generated. If the data points fall back upon themselves as the shear rate is lowered, the material is time independent.
Time SensitivityTo determine the time sensitivity (thixotropy) of a material, a viscosity curve is generated at a constant rotational speed. Data is taken at specified time intervals. If the viscosity decreases with time (Figure 7), the material is time dependent (thixotropic).
By generating these types of flow curves on new batches of material and comparing these against known good samples, the quality and performance of the material can be ensured.
Cost-Effective Data AnalysisIn the final analysis, it is certainly up to the end user to determine if collecting rheological data versus Zahn cup kinematic viscosity data is pertinent. Certainly, all that data may not be necessary for some applications. However, when performance and quality cannot be compromised, data on how a material is pumped, sprayed, brushed or contained should be quantified. Accurate, repeatable measurements are a requirement, indeed a necessity, for this type of analysis. When a cost-effective method of gathering this information is available, this is even better.
The minimal cost outlay for a rotational viscometer can be under $2,000 and perhaps up to $5,000 if temperature control is implemented at the same time. The cost for the viscometer, pertinent accessories and software for data analysis (i.e., to determine if a material is suitable to be produced in thousands of gallons) is quickly recouped. Formulation, performance and production costs all reap the benefit gained through quick, easy and accurate flow property testing.
For more information, visit http://www.brookfieldengineering.com or contact firstname.lastname@example.org.