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In-line viscometry and rheometery: the options

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article image Piezo Axial Rheometer

THE need for in-line measurement of viscosity and other rheology related parameters has long been a requirement for many industries including those as diverse as coatings, food, polymer, mining etc.

Rheological properties can help a production engineer to evaluate the quality of the product in the process with respect to a predetermined criterion, according to Rheology Solutions .

On-line MFI

Melt Flow Index (MFI) is an often-used criterion for quality control, particular for the polymer industry, and this can now be achieved on-line, using the HAAKE PCR.

The PCR is specifically designed to provide the user with regular information on the MFI of a product during the production process, without the operator having to take a sample off-line to the laboratory for assessment. Production time and potential reprocessing of out-of-spec product can be minimised.

HAAKE PCR Process Control Rheometer - On-line QC viscosity measurement

On-line, single-point viscosity and temperature can be continuously monitored using the Marimex suite of torsional probes. These probes can be used in almost any industry, from low viscosity applications like some liquid foods or mining slurries, through to high viscosity applications, like pastes, concrete, and polymer melts.

These probes offer the opportunity for the process engineer to continuously monitor the process as it impacts on the material viscosity and they can be used in conjunction with most PLC control systems and can be easily mounted in either a tank or in a pipe.

They measure a single point viscosity (ie the viscosity at a single shear rate, of approximately 4s-1), which will deviate according to the flow properties of the material in the vessel or pipe.

User defined temperature and viscosity QC criteria can be input so that out-of-spec materials are quickly detected.

Marimex ViscoScope In-Line Viscometer - On-line viscosity curves for extrusion applications

A fuller definition of the material flowing through an extruder is often required. For most polymers and some foods, a pump combined with rheology dies can be placed at the end of an extruder, to provide a full flow curve for the material (ie viscosity data for a wide range of shear rates).

This allows a full picture of the flow behaviour to be known during production and so the suitability of the product for flow through a nozzle (high shear), into a mould (medium shear) and to fill the edges and corners of the mould (low shear) can all be determined using one online piece of equipment.

The rheology dies and melt-pump assembly is known as the HAAKE ProFlow. It is also possible to use a special extensional die in the HAAKE ProFlow assembly to provide relative extensional viscosity in cases where, for example, fibre spinning or other applications where the flow field is dominated by extensional forces occur.

HAAKE Proflow On-Line Melt Rheometer - On-line viscoelastic measurements

In some cases, steady shear viscosity, like that measured in the Marimex ViscoScope or HAAKE ProFlow systems is not as important as the viscoelastic properties of the materials. These properties, the viscous and elastic moduli and the complex viscosity provide us with information about the relative contributions of solid- and liquid-like components in the material to how it will flow and recoil.

There now exists a HAAKE Piezo Axial Viscometer (PAV), which can measure these properties continuously online. The user is informed at all times during the process about how the product is performing with respect to the viscoelastic flow properties, and QC variables, like the crossover point of the moduli (GC), or the polydispersity index (PI) can be used in conjunction with these measurements to evaluate the product during production.

Conclusion - The options

As a result of these many innovations, a suitable on-line solution can be offered for most applications, regardless of the type of industry.

Either a simple change in a set value can be monitored (HAAKE PCR, or Marimex ViscoScope), or a more complete picture of the material flow properties is possible (HAAKE ProFlow or HAAKE PAV), depending on the specific requirements of the user.

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