Spotlight: How to … test material viscosity to ensure quality

Materials World magazine
,
1 Jul 2019

Cole-Parmer Global Product Marketing Manager Mary Vessele and Content Specialist Dr Julia Lock discuss material fluidity.

Adhesives, chemicals, biofuels, paints, petroleum production and pharmaceutical industries use viscometers to test the viscosity of materials. The chocolate industry is dependent on viscosity testing as the texture and flow of chocolate can be complex to manufacture and maintain. For example, the thickness of chocolate when it is in a melted state will impact the finished solid state. To achieve the correct density, texture and taste of chocolate when it becomes solid, careful consideration to the viscosity of the product when it is liquid is vital.

Accurate results

Rotational viscosity technology can measure a range of viscosity from 200-106 million, with an accuracy range of ±1% to ±0.2%. The measurement of a product’s viscosity is based upon immersing a specifically selected spindle to a sample of the product. A vortex is created as the spindle rotates. The torque required to rotate the spindle at a set speed is measured, based upon the amount of drag created from the spindle moving in the liquid. The instrument records the resistance to rotation and the measured torque value is mathematically converted into a viscosity value in scientific units called centipoise (cP) or milliPascal-seconds (mPa.s).

Water is the reference material that establishes the benchmark value of 1cP for its viscosity with all other materials subsequently measured by their viscosity in comparison to water. The rotational viscometer device will record and store up to 10,000 data points, and with the software provided, users can instantaneously view flow curves, plot up to four data sets simultaneously, and graphically present data. These results can be downloaded to a spreadsheet for further analysis and/or records. In food or any material, it is important to track batches to ensure they contain the same viscosity level throughout production to achieve optimal quality.

Spindle design

Flow resistance increases with the speed and size of the spindle. Different samples and materials require different types of spindles, depending on the available sample size, the torque range used, temperature needs, and material consistency. A parallel plate or cone spindle is used for thick and more viscous samples, a concentric cylinder spindle for thinner, less viscous pourable liquids, while a T-bar is an option for gel-like materials.

Temperature control

The temperature of the material sampled will also affect its viscosity value. The sample’s temperature should be controlled within ±0.05°C,  while taking a reading to achieve accurate viscosity. Viscometers have built-in temperature probes, which will display the temperature on the unit, to help the user maintain control of samples.

Speed and time

The speed and the amount of time the spindle is rotating need to be considered, as samples can be impacted by both factors. The spindle should rotate at least five turns before a value is taken and the duration the spindle rotates should be dictated by a defined period of time. Some fluids are more sensitive than others to the amount of time they are sheared, which will result in a change in viscosity. Modern viscometers allow the user to set the speed up to 250RPM and can offer 2,600 speed selections. The display on the device will provide readouts of speed, spindle, viscosity, percentage of torque, shear rate, and shear stress. When the sample density is entered, the device will also calculate and display kinematic viscosity. The device will calculate the shear rate – rotation speed of the spindle – and measure shear stress – the torque needed to rotate the spindle – and plot the points on a graph. The slope of that line is the viscosity at a specified temperature. All of the information provided on the device monitor can be downloaded to a computer and for use in spreadsheets. Tracking the viscosity of a sample and being able to produce a detailed report is essential to ensuring the best quality of a product.

Viscosity across industries

Viscometers play an important role in the a range of industrial appliactions. Viscosity is measured either in process or afterwards to qualify the end product for acceptability and quality standards. Many times, samples are selected off the production floor and brought into the quality control lab for testing to ensure the quality is maintained throughout the entire process. Regular monitoring can help offer information that can be used to influence quicker and more informed decision-making on the factory floor.