Viscometers and rheometers are essential tools in the polymer industry used to measure material properties during various stages of the vulcanisation process. Two commonly used instruments in the rubber industry are the Mooney Viscometer and Rotational Rheometer. In this article, we’ll compare these two instruments and discuss their respective advantages and disadvantages.
Viscometers and rheometers are used throughout the polymer industry to determine material properties before, during and after the vulcanisation process. The most commonly used versions in the rubber industry are a Mooney Viscometer and Rotational (Moving Die) Rheometer. Both apply a shearing force to a polymer sample within a sealed cavity. The reactionary torque of the sample is measured by a transducer and used to determine material properties. Test time, frequency, amplitude and temperature can all be varied to investigate more advanced material characteristics.
Prescott Instruments Mooney Viscometer
Prescott Instruments Moving Die Rheometer
Application of Shearing Forces
The main difference lies in how the shearing force is applied and how the resultant torque is interpreted.
A Mooney Viscometer uses a rotor embedded within a sample to exert a shearing force by full rotation. Testing is typically at sub-cure temperatures. The resultant torque measurement is used to determine viscosity using an arbitrary scale, Mooney Units (MU). Conversely, a Rotational (Moving Die) Rheometer does not use a rotor, but instead utilises an oscillating die to exert a shearing force on a sample. Temperatures are typically higher, as to induce vulcanisation. The torque signal (dNm) is used to express both cure and rheological/viscoelastic properties.
Mooney Viscometer Rotor
Moving Die Rheometer Die Cavity
Uses Of The Mooney Viscometer
The Mooney Viscometer is the industry standard test for pre-vulcanisation properties and has long been synonymous with the rubber industry. Typically, Mooney Viscosity (MU) is used as a yardstick the gauge the quality of raw materials, especially in grading natural rubber. Although Mooney Units use an arbitrary scale, results can be correlated empirically to other instruments. In particular, Scorch Time can be used as an indirect measure for cure onset properties.
Uses Of The Moving Die Rheometer
The Moving Die Rheometer (MDR) is used extensively throughout the rubber industry to measure the curing properties of any compound at a given temperature. The cure properties can be used for compound formula optimisation and to perform inter- and intra-batch quality control. Providing a snapshot of how and when vulcanisation occurs, the MDR cure curves are an invaluable resource to rubber technologists who need to take account of both pre- and post-cure processing effects during any production line.
Viscometer vs. Rheometer Comparison Table
|Mooney Viscometer||Rotational Rheometer|
Mini Mooney Viscometer
*Mooney Viscometer Variable Speed
|Moving Die Rheometer|
Mini Moving Die Rheometer
|Primary Measurement||Mooney Viscosity||Cure Properties|
|Vulcanisation||Uncured Polymer||Cured Polymer|
|Die Assembly||Sealed Cavity; Large or Small Rotor||Sealed Bi-Conical Cavity; Rotor-Less|
|Frequency||2 RPM (Std.)|
*Up to 20 RPM
**Up to 50Hz
|Amplitude||n/a||0.5° (Std), 1.0°, 3.0°|
**Up to 360°
|Shearing Action||Rotor||Moving Die|
|Torque Measurement||Mooney Units (MU)||Torque (dNm)|
|Typical Test Temperature||100°C||180°C|
|Typical Sample Volume||14.3 cm³||4.5 cm³|
|Standard||ISO No. 289 / ASTM D1646||ISO No. 6502 / ASTM D5289|
* and ** indicate advanced models
Rheometers and viscometers both use the principles of rheological flow to measure the viscosity of a fluid under a shearing force. Although both instruments have a lot in common in principle, the application of shearing forces, test conditions and sample size differentiate the rheometer and viscometer within the rubber industry.
While the Mooney Viscometer can be used to measure pre-vulcanisation properties and gauge the quality of a material, the Moving Die Rheometer primarily provides cure curves that can be used to assess and modify both the rubber compounding formulation and process.
Often used together but equally strong independently, the Mooney Viscometer and Moving Die Rheometer comprehensively measure the primary properties of rubber, making them both key pieces of equipment for any rubber laboratory or production facility.