Rheometer

The rheological analysis of samples is a fundamental part of developing many types of products. Unlike a viscometer, a rheometer can actually measure sample properties at extremely low shear rates, as in sedimentation, or the high shear rates seen in pumping, mixing and application and making measurements in the correct shear range, we can adequately simulate a flow process and so differentiate good products from poor ones. The rheometer can also determine the effect of adding different quantities of an additive, or process changes and so be used to optimize the formulation and production of a product.

The rheometer not only measures the viscosity of the product at room temperature, but can also be used to evaluate the viscosity during a programmed temperature profile. This can also be used with polymers to evaluate processability and glass transition temperatures. Results are accurate with minimal time spent testing, as a pre-programmed analysis may be started and left to run unattended, or even overnight.

Methodology Overview

Rotational rheometers can accommodate many different measuring systems, although the most common are the cone and plate, the parallel plates, the coaxial cylinders and the torsional fixtures. In the case of the cone and plate or parallel plates, the sample is loaded onto a temperature controlled flat lower plate and an upper cone or flat plate is lowered onto the sample squeezing it into a defined space. After trimming away excess sample, the upper measuring system is then either sheared in one direction (viscometry) or oscillated rotationally (oscillation, as shown in Figure 1 below).

Viscometry can be used to investigate the Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress, ie the stress required to initiate sample flow, simulate a shearing process, measure shear stability or analyze how viscosity changes with temperature. Oscillation tests usually investigate the viscoelastic structure of a sample without breaking it down. Initially an amplitude sweep is run to determine how large an oscillation the sample can withstand before the structure breaks down, this is known as the Linear Viscoelastic Region (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.linear viscoelastic region. Once the Linear Viscoelastic Region (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.linear viscoelastic region has been determined, a frequency sweep, time sweep or temperature sweep may be performed to investigate how the viscoelastic structure and viscosity changes under dynamic conditions.

Rotational rheometers can accommodate many different measuring systems, although the most common are the cone and plate, the parallel plates, the coaxial cylinders and the torsional fixtures. In the case of the cone and plate or parallel plates, the sample is loaded onto a temperature controlled flat lower plate and an upper cone or flat plate is lowered onto the sample squeezing it into a defined space. After trimming away excess sample, the upper measuring system is then either sheared in one direction (viscometry) or oscillated rotationally (oscillation, as shown in Figure 1 below).

Viscometry can be used to investigate the Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress, i.e. the stress required to initiate sample flow, simulate a shearing process, measure shear stability or analyze how viscosity changes with temperature. Oscillation tests usually investigate the viscoelastic structure of a sample without breaking it down. Initially an amplitude sweep is run to determine how large an oscillation the sample can withstand before the structure breaks down, this is known as the Linear Viscoelastic Region (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.linear viscoelastic region. Once the Linear Viscoelastic Region (LVER)In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured.linear viscoelastic region has been determined, a frequency sweep, time sweep or temperature sweep may be performed to investigate how the viscoelastic structure and viscosity changes under dynamic conditions.

With the Rosand capillary rheometer, a sample is loaded into a cylindrical barrel that is pre-set at the required test temperature. A servo-drive controlled piston is then used to extrude the sample material through a cylindrical or rectangular slot die in place at the end of the barrel at a very controlled series of speeds (volumetric flow rate). The pressure drop across the die is continuously monitored and measured with a pressure transducer placed just above the die. A cut away is shown in Figure 1.  The Rosand capillary rheometers can accommodate a wide range of pressure transducers and dies, making them versatile for measuring a broad spectrum of sample types.  Typical sample viscosities can range from ink jet ink to highly-filled, high modulus rubber samples. The standard instrument’s temperature range is generally from ambient to 400°C (with cryogenic cooling and 500°C max temperature as option s). 

Capillary rheometers can be used to generate shear viscosity, extensional viscosity and elasticity measurements.  Also, there are modules for thermal degradation tests, flow – no flow testing, pressure volume temperature (PVT) tests, haul-off (fibre spinning), stress RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation, wall slip analysis and others. 

Generating a Viscosity Flow Curve

The constant shear test is designed to investigate the relationship between shear stress and shear rate for a material, with shear viscosity being the ratio of the two parameters. The test routine involves pre-setting the test temperature, then loading the sample with periodic tamping to ensure a uniform fill to reduce voids and air entrapment. Loading the sample is followed by further pre-test compressions to ensure the sample is de-aerated as much as possible and fully compacted. A series of discrete piston speeds (shear rates) are selected across the shear rate range of interest and the sampled extruded until pressure equilibrium is detected at each speed. Pressure is monitored during the test and shear stress calculated at each data pointed collected. To ensure accurate results of the true flow properties of the sample, the user has the option to apply up to two corrections associated with errors of entrance pressure and Non-NewtonianA non-Newtonian fluid is one that exhibits a viscosity that varies as a function of the applied shear rate or shear stress.non-Newtonian flow.