Viscosity measurement in the chemical industry

Before we get started with how to measure viscous materials, let’s have a quick look at how a rotational viscometer works. A typical rotational viscometer used for quality control measures the viscosity according to the “Searle principle”.

A cup is matched with a so-called spindle that is placed in the sample. The speed on the viscometer is preset and the spindle starts to rotate. The sample in the cup follows this movement and in further consequence the torque (force) required for turning the spindle against the fluid’s viscous forces is measured. A spring connects the motor of the viscometer and the spindle. The rotation (torque) of the spindle deflects the spring. Optical sensors detect the deflection and, as a result, we get the viscosity of the sample. Different spring-types of rotational viscometers allow measuring substances from low to high viscosity. In case of low-viscous substances, the spring needs to be sufficiently sensitive, whereas for samples in the high viscosity range, a more robust spring is required. In the next chapters, you will learn how to measure the viscosity of different chemical samples with such a spring-type viscometer.

Test method A: Determination of the apparent viscosity at 25 °C at a certain speed. Simply select a speed on the rotational viscometer so that the torque reading is between 10 % and 95 %. Once the viscosity reading has stabilized, stop the viscometer. When measuring thixotropic paints, the reading might not stabilize. If this is the case, the measurement has to be stopped after a specified period of time, e.g. one minute.

Test method B: Determination of the degree of shear thinning and thixotropy under changing speed conditions. This test method allows you to calculate the degree of shear thinning and thixotropy, which is especially important for the application behavior of paints and coatings. The shear thinning index and the degree of thixotropy can vary greatly between different paints, and is temperature-dependent. The diagram in Figure 2 displays the viscosity on the y-axis and the speed on the x-axis, which is useful for analyzing the flow behavior of the material.

If occasionally you want to measure the absolute viscosity instead of the relative viscosity, you have to use absolute measuring systems. If you use such concentric cylinder systems, you will only need a low sample amount for the test (approx. 1 ml to 60 ml).

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To ensure reproducibility of viscosity checks, make sure to use the same container in your measurements. Be careful not to introduce air bubbles while mixing the components of the adhesive. It is possible to apply a vacuum to eliminate bubbles prior to testing. Once the temperature has reached equilibrium, you can start with the viscosity measurement. The temperature of the adhesive has to be controlled with an accuracy of ±0.2 °C. In order to achieve the best accuracy, it is recommended to perform viscosity measurements at a speed resulting in a torque reading between 45 % and 95 %. Never carry out measurements at less than 20 % torque or more than 95 % torque. Stop the measurement when the viscosity stabilizes. If the sample shows thixotropic behavior and the viscosity therefore changes over time, stop the measurement after an agreed period of time. Viscosity measurements have to be performed as long as two consecutive measurements do not differ by more than 3 % from each other.

If the viscosity of adhesives was measured at different speeds, the flow behavior of the adhesive can be displayed in a diagram in which viscosity is plotted against the speed (see Figure 3).

Do you want to get practical insights into how you can use ViscoQC™ 100 to measure the viscosity of adhesives? Take a look at our application report.

To measure the viscosity of latex, use a spring-type viscometer. The determination of the apparent viscosity of latex is carried out according to ISO 1652. First, pour a portion of the test sample into a beaker with a diameter of at least 85 mm (3.35 inch). Take care to always use the same beaker for reproducible results. Place the beaker in a water bath maintained at 23 °C ±2 °C and stir the latex gently until temperature equilibration is reached. Slowly dip the spindle into latex and incline disc spindles so you do not tap air bubbles on the bottom side of the spindle. Further, a sufficient sample filling height is important: The tip of the spindle should be at least 10 mm above the vessel’s bottom and immersed as specified by the producer. If a spindle protector, also called “guard”, is used, immerse it first and the spindle afterwards. Attach the spindle and the guard securely to the viscometer and check the immersion depth. Finally, center the beaker.

Most times, a spring-type instrument for low-viscosity or medium/regular-viscosity samples is needed for viscosity measurements. Low-viscosity latex has to be measured at 60 rpm and medium-viscosity latex at 20 rpm (Figure 4). When performed at the suggested speed, the measurement should take from 20 to 30 s. If you perform the viscosity check for quality control purposes, always use the same spindle and speed combination.

Latex is known to show non-Newtonian flow behavior (shear thinning). For that reason it may be interesting to measure the viscosity of latex at different speeds. This allows you to calculate the viscosity ratios and characterize the thixotropic behavior of latex. This method is valuable for comparing the effects of thickening agents in latex.

As solvents usually have a very low viscosity (<100 mPa.s), they cannot be measured with the standard measuring system of rotational viscometers. To measure liquids with a viscosity lower than 200 mPa·s, a special measuring system, the so-called “double-gap measuring system (DG-MS)”, is recommended. It is an absolute measuring system according to DIN 54453, which means it has a defined shear gap. In this gap, constant shear conditions dominate. The high surface area (due to the double-gap) allows measuring low-viscosity substances down to 1 mPa·s. Only a small sample amount of approximately 7 ml is required for the test.

To measure the viscosity of a solvent in the DG-MS, the liquid has to be filled into the MS that already contains the measuring bob. Afterwards, remove the excess with, e.g., a syringe. The double-gap of the measuring system must only be filled with liquid. Mount the DG-MS on the viscometer to perform measurements at specified speeds. The viscosity test of the solvent should be performed in a torque range between 20 % and 95 %. For best accuracy, perform the measurement between 45 % and 95 % torque.

According to ISO 2555, the temperature of the solvent has to be controlled with an accuracy of ±0.2 °C during the test. The measurement can be stopped when the viscosity stabilizes. Viscosity measurements have to be performed as long as two consecutive measurements do not differ by more than 3 % from each other.

If the viscosity has been measured at different speeds, the Newtonian flow behavior of solvents can be displayed in a diagram in which viscosity is plotted against the speed (Figure 5).

To sum up, to measure the viscosity of viscous materials in the chemical industry in an appropriate and reliable way, it is important to follow an internal standard procedure or even available standards, taking the proper handling of the rotational viscometer in use into consideration. Quick viscosity checks are a valuable method to control the quality of incoming and final products. Ultimately, determining the viscosity of samples during production gives a beneficial insight into the processability, for example behavior during pumping and stirring. The rotational viscometer ViscoQC™ 100 helps you reach perfect consistency of your chemicals.

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