Rheometers

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 StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.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), StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.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.