We also sell the new EMS Viscometer , a high-tech electromagnetic spinning viscometer for tricky applications like adhesives, blood, and expensive perfumes. Viscosity is the measure of a substance's resistance to motion under an applied force.
The result is typically expressed in centipoise cP , which is the equivalent of 1 mPa s millipascal second. Shear stress is the force per unit area required to move one layer of fluid in relation to another. Shear rate is the measure of the change in speed at which intermediate layers move with respect to one another.
Isaac Newton, the man to discover this formula, thought that, at a given temperature and shear stress, the viscosity of a fluid would remain constant regardless of changes to the shear rate. He was only partly right. A few fluids, such as water and honey, do behave this way.
We call these fluids Newtonian fluids. Most fluids, however, have viscosities that fluctuate depending on the shear rate. These are called Non-Newtonian fluids. There are five types of non-Newtonian fluids: thixotropic, rheopectic, pseudoplastic, dilatant, and plastic. Different considerations are required when measuring each of these fluid types.
By moving two fluid layers against each other the fluid movements create a resistance stiffness and internal friction , referred to as flow resistance. Viscous forces are created by moving several layers of fluid at different velocities. This causes faster flowing layers to slow down and slower flowing layers to speed up.
These processes follow the laws of physics and can be defined in equations whereby shear rate velocity gradient and shear modulus are factors. Viscosities are always dependent on temperature. For example, viscosity measurements are performed to optimize a paste-like substance with regard to consistency or to optimally adjust it for the production line. ZwickRoell's back extrusion fixture offers an alternative solution to rotation viscometers. The principle of the back extrusion fixture is based on the displacement of the test material through an annular gap between piston and back extrusion cell.
The cell is filled with the test substance and the piston is lowered into the container, pressing the material up through the annular gap. When the piston is raised, the material flows back through the annular gap into the cell.
The force is measured in the down and up cycle at a defined position. The dynamic viscosity is evaluated from the measured force difference and the flow rate of the liquid in the annular gap.
The ZwickRoell system can repeat this cycle as often as desired with freely adjustable shear rates. The test can be optimally adapted to the test material, and the viscosity can be measured across the entire shear range with one single test. And to top it off another advantage testXpert III test program especially designed for viscosity testing:.
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. Quick single-point viscosity checks of paints and coatings with a rotational viscometer are used for quality control.
The viscosity of paints and coatings further provides in-depth insights into the characteristics of different kinds of application. Two test methods are described in this standard:. 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. 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. Have a look at our application report. The viscosity of an adhesive is often adjusted to fit its purpose. Viscosity checks of adhesives are usually carried out according to ISO The standard ISO describes a test method for determining the apparent viscosity of resins in a liquid state using a single cylinder type rotational viscometer.
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. 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. 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.
Take a look at our application report. Viscosity is typically used to characterize latex and describe its processability. When it is important to know the response to mechanical force such as mixing and pumping, viscosity checks can provide important details.
A change in the viscosity of latex allows monitoring the vulcanization procedure and the reaction of additives. 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 First, pour a portion of the test sample into a beaker with a diameter of at least 85 mm 3.
Take care to always use the same beaker for reproducible results. Slowly dip the spindle into latex and incline disc spindles so you do not tap air bubbles on the bottom side of the spindle. Attach the spindle and the guard securely to the viscometer and check the immersion depth.
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