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Viscometers and rheological testing equipment

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RHEOLOGY Solutions supplies equipment for measurement of rheological, mechanical and flow properties in the laboratory.

The units, the Mooney-Line viscometer, the Oscillating Disc Rheometer (ODR) and the Moving Die Rheometer (MDR), are appropriate for quality control and product development.

They can reduce or remove subjectivity in estimating product quality, and help in estimating the potential of new products or changes in raw materials.

Rheological properties are important in the processing of most fluids, including high molecular weight plastics, elastomers and rubber.

Many of these substances, particularly rubbers and some high viscosity polymeric elastomers, are difficult to measure in conventional rheometers because of their high viscosity and the promotion of cross-linking, often through vulcanisation, during processing at higher temperatures.

After vulcanisation, the viscoelastic mixture is converted to an elastic material, whose properties include high resistance to deformation and strength. These substances must be properly characterised to ensure that they are not over- or under-processed.

The main equipment for assessing these properties in the rubber sector are, the Mooney-Line viscometer, the Oscillating Disc Rheometer (ODR) and the Moving Die Rheometer (MDR). These instruments vary in the complexity and the methods of measurement, and in many of the properties they measure.

The Mooney viscometer is used in the rubber and elastomer industry. Mooney viscometers are rotational (the rotor moves in a single direction at a constant rate) instruments, measuring Mooney viscosity (not viscosity in the true sense of the scientific term, but instead a measure of shearing torque averaged over a range of shear rates).

Mooney viscosity is a geometry specific, single low shear quantity. Mooney viscometers may be used to obtain data prior to, during and post cure.

In addition to Mooney viscosity, the stress relaxation index of samples may be measured (the time related decay in Mooney viscosity after movement in the test chamber has ceased), and the “Mould release test” for determination of the break force required to free the rotor from set material. This instrument’s measurement parameters have been standardised to allow better sample-to-sample comparisons, for example see ASTM D1646.

Industry standard tests, for proper quantification of sample properties, which Mooney viscometers should be capable of include ML(1+X) viscosity, scorch and delta Mooney (the difference between two predetermined scorch points) tests.

Good quality instruments have rotor and platen assemblies, which can also perform temperature measurements in addition to torque and time. Most measurements are made between 100°C and 200°C, but well engineered instruments can work in ranges of 35°C to 200°C.

As in the case of most rheological measurements, sample preparation is critical, and to obtain repeatable results, the same proportion of each ingredient must be used in each recipe, and consistent sample volume is crucial as these units are sealed using excess sample driven from the chamber by the closing platens.

The ODR has been largely superseded by the MDR, however the rubber and elastomer industry has many new and current users of this technology.

The ODR is an oscillatory rheometer, consisting of an oscillating disc, enclosed in an unsealed, stationary cavity. The disc oscillates at a fixed frequency and amplitude (generally determined by international testing standards) and operates in the same range of temperatures and pressure as the Mooney viscometer.

The ODR is a cure-meter, measuring the progress of cure, for example vulcanisation or crosslinking, through torque, temperature and pressure, and outputs viscosity, Tan Delta, viscous modulus, elastic modulus and cure rate.

Sample and disc size are quite large for these tests, however, resulting in a non-uniform temperature distribution through the chamber and disc.

As in the case of the Mooney instrument, sample volume and preparation are critical to obtain repeatable results. ODR instruments are sealed using excess sample driven from the test chamber by the closing platens.

The MDR may be automated if necessary and continued use of volumetrically consistent samples, and good handling procedures, ensure repeatability.

The MDR is a current generation cure-meter and is suitable for the rubber and elastomer industries. It helps overcome problems of thermal inertia, which has dogged the ODR.

MDRs have a cavity rather than an internally oscillating fixture, which is filled with sample and enclosed by a two piece, biconical die.

One part of the die oscillates at a specific frequency and amplitude (determined by international testing standards) and the torque, temperature and pressure (for samples producing gaseous by-products during cure) are measured.

Outputs include the viscous and elastic moduli, Tan Delta, cure rate and pressure.

Many processes run at constant temperatures, but these may vary depending on the formulation of the substance under manufacture. MDRs allow the user to preset a temperature, usually in the range of 100°C to 200°C, but some instruments can precisely control temperature in the band between 35°C and 250°C.

Industry standards usually specify an amplitude of 0.5°, but with an MDR, it is possible by changing the eccentric which drives the oscillating fixture to choose 1.0, 1.3 or 3.0°.

Volumetric sample cutters, like those produced by Prescott Instruments provide accurate sample handling and preparation. They can be used either with Prescott Instruments Mooney Line viscometers, Rheo-Line Oscillating Disc Rheometers, or Rheo-Line Moving Die Rheometers.

These sample cutters from Prescott Instruments, represented by Rheology Solutions, provide the facility for altering the volume of the sample, to tailor samples to test requirements.

Another potential problem for cure-meters, especially in the case of automated units, is the possibility of slip at the interface between the solid fixtures and the sample.

Slip is often overcome by the use of a semi-porus fabric layer at the interface. Prescott Instruments has developed an automated feeder system, which automatically feeds both sample and fabric through the instrument simultaneously. This allows laboratory personnel time to work on other tasks as testing proceeds.

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