REO-Viscometer or Rheometer Making the Decision
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● ● PARTICLE ANALYSIS WWW.LABNEWS.CO.UK Viscometer or rheometer? Making the decision A viscometer can offer the solution for material, process or production tests that require simple flow measurements, whereas the performance of a rheometer allows far greater characterisation of flow and deformation behaviour. It is well worth taking into account the range of applications for which the instrument will be needed, and how differences in performance between a viscometer and a rheometer may affect their suitability for these applications. Instrument operation Most viscometers operate by rotating a spindle in the sample. Viscosity is determined by measuring resistance to this rotational force. Viscometers, in comparison to rheometers, are usually relatively simple instruments. Their simplicity of design and operation can offer advantages for operator ease of use, particularly within a busy QC environment. Spindle movement in a viscometer is in one direction, which allows the measurement of viscosity. Rheometers can apply oscillatory and rapid step changes in stress and strain, and can therefore determine viscoelastic properties (providing information on the structural properties of the sample) as well as flow properties. Viscometers employ a mechanical bearing that limits the speed and torque capabilities of the instrument, whereas rheometers generally use a low friction air bearing. The residual friction from the mechanical bearing can make the measurement of low viscosity materials difficult. Some of the latest viscometers use a variety of measurement geometries, enabling them to cover broader viscosity ranges. This is applicable to many fluid applications including paints, pharmaceuticals, petrochemicals and even asphalt. Rheometers, while generally more expensive than viscometers, are more versatile and have a much wider dynamic range for control and measurement parameters. In a typical stress and strain controlled rheometer a temperature control unit (TCU) is an integral part of the instrument rather than an option. Furthermore, there are several interchangeable TCUs available that cover different temperature ranges. Each TCU has been optimised for the best control and performance. An example of this is the Extended Temperature Cell, which is a forced convection oven. This provides rapid heating and cooling in an inert atmosphere to prevent sample degradation. In combination with a wider choice of measurement geometries, tests can be optimised for specific applications and sample types. The use of a low friction air bearing enables measurement of even low viscosity samples, whilst the inherent stiffness of the bearing also provides the capability to measure solids. With true stress and true strain control operating modes, a complete range of rheological tests (including creep, stress relaxation and multiwave oscillation) can be performed with full control of sample strain history. Measuring viscosity Rheometers function across a very wide range of shear rates enabling the simulation of real processes that occur over vastly different timescales, such as sedimentation and spraying. Shearing occurs whenever fluids flow through tubes and channels. The velocity is zero right at the wall surface and maximum at the centre. So the fluid is being sheared as it flows through a tube or channel. Table 1 shows the shear rates of processes that commonly require rheological measurement. Spraying, for example, is between 10 4 to 10 5 reciprocal seconds (s -1 ) and pumping is around 1 to 10 3 s -1 . Typically, a viscometer can measure in difference between samples that have the same viscosities. Dynamic measurements Virtually all materials show elastic as well as viscous properties. The measurement of sample viscoelasticity can provide vital information not given by viscometry alone. With a rheometer, tiny movements (small strain oscillations) can be used to measure viscoelastic properties without destroying the sample structure. Oscillatory testing generates a mechanical spectrum for the material, and this provides a unique behaviour fingerprint. the range of about 0.1 to 10 3 s -1 while a rheometer extends the measurement range from 10 -6 to 10 5 s -1 . Processes such as sedimentation are best suited to analysis with a rheometer because of its low torque capabilities. The high speed control of a rheometer also allows analysis of very high shear rate processes, such as spraying. The information in Table 1 illustrates that the measurement capabilities of a viscometer can also provide solutions for a wide range of applications, particularly if QC testing is the main requirement. Figure 2 provides an illustration of where the greater measurement range of a rheometer is useful. This example is for roller coating and shows a good roller coating sample (red) and a poor one (green). At shear rates below 10 4 s -1 both coatings have a similar viscosity. A viscometer would find no differences between these coatings. Two further examples of important viscometric measurements that can be made using a rheometer are yield stress and normal force. Yield stress Yield stress, the force required to make a sample start flowing, is a valuable measurement. It is the force that must be overcome when a pump is switched on; it holds paint in position after application to a wall; and it can also influence shelf life and structure perception, particularly for consumer products. The yield stress is an important factor in providing a thick and creamy texture, but a balance must be achieved because the yield stress will have a significant effect on the pumping requirements during processing. Normal force When a sample is sheared, elasticity can create a force perpendicular to the direction of shear. This is known as normal force and it is common with polymeric materials. Normal force testing can identify differences that may occur when a material is processed. An example of this is the application of paper coating. This is applied to the paper surface using a blade- coating process with a narrow gap, which generates high shear rates. If a coating sample has a high normal force, which acts to push the application blade away from the paper, the coating will be too thick. Often only normal force testing will show any Viscometers, in comparison to rheometers, are usually relatively simple instruments. Their simplicity of design and operation can offer advantages in terms of ease of use Deciding whether to purchase either a viscometer or a rheometer is not always straightforward. Here, Laboratory News looks at the factors that should be considered in order to make the right choice for the individual laboratory Gemini Rheometer Table 1: Shear rates of different processes Process Minimum Maximum Viscometer Rheometer shear rate (s -1 ) shear rate (s -1 ) Reverse gravure 10 5 10 6 ✔ Spraying 10 4 10 5 ✔ Blade coat 10 3 10 5 ✔ ✔ Mixing/stirring 10 10 3 ✔ ✔ Brushing 10 10 3 ✔ ✔ Pumping 1 10 3 ✔ ✔ Extrusion 1 10 2 ✔ ✔ Curtain coating 1 10 2 ✔ ✔ Levelling 10 -2 0.1 ✔ Sagging 10 -2 0.1 ✔ Sedimentation 10 -6 10 -2 ✔
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Transcript of REO-Viscometer or Rheometer Making the Decision
● ●
WWW.LABNEWS.CO.UK PARTICLE ANALYSIS
● ●
PARTICLE ANALYSIS WWW.LABNEWS.CO.UK
Viscometer or rheometer?Making the decision
Aviscometer can offer the solution for material, processor production tests that require simple flowmeasurements, whereas the performance of a rheometerallows far greater characterisation of flow and
deformation behaviour. It is well worth taking into account therange of applications for which the instrument will be needed,and how differences in performance between a viscometer anda rheometer may affect their suitability for these applications.
Instrument operationMost viscometers operate by rotating a spindle in the sample.Viscosity is determined by measuring resistance to thisrotational force. Viscometers, in comparison to rheometers, areusually relatively simple instruments. Their simplicity of designand operation can offer advantages for operator ease of use,particularly within a busy QC environment.
Spindle movement in a viscometer is in one direction, whichallows the measurement of viscosity. Rheometers can applyoscillatory and rapid step changes in stress and strain, and cantherefore determine viscoelastic properties (providinginformation on the structural properties of the sample) as wellas flow properties.
Viscometers employ a mechanical bearing that limits thespeed and torque capabilities of the instrument, whereasrheometers generally use a low friction air bearing. The residualfriction from the mechanical bearing can make themeasurement of low viscosity materials difficult. Some of thelatest viscometers use a variety ofmeasurement geometries,enabling them to cover broaderviscosity ranges. This is applicableto many fluid applicationsincluding paints,pharmaceuticals, petrochemicalsand even asphalt.
Rheometers, while generallymore expensive than viscometers,are more versatile and have amuch wider dynamic range forcontrol and measurementparameters. In a typical stress andstrain controlled rheometer atemperature control unit (TCU)is an integral part of theinstrument rather than an option. Furthermore, there areseveral interchangeable TCUs available that cover differenttemperature ranges. Each TCU has been optimised for the bestcontrol and performance. An example of this is the ExtendedTemperature Cell, which is a forced convection oven. Thisprovides rapid heating and cooling in an inert atmosphere toprevent sample degradation. In combination with a wider choiceof measurement geometries, tests can be optimised for specificapplications and sample types.
The use of a low friction air bearing enables measurement ofeven low viscosity samples, whilst the inherent stiffness of thebearing also provides the capability to measure solids. With truestress and true strain control operating modes, a complete rangeof rheological tests (including creep, stress relaxation andmultiwave oscillation) can be performed with full control ofsample strain history.
Measuring viscosityRheometers function across a very wide range of shearrates enabling the simulation of real processes that occurover vastly different timescales, such as sedimentation andspraying. Shearing occurs whenever fluids flow throughtubes and channels. The velocity is zero right at the wallsurface and maximum at the centre. So the fluid is beingsheared as it flows through a tube or channel. Table 1shows the shear rates of processes that commonly requirerheological measurement. Spraying, for example, isbetween 104 to 105 reciprocal seconds (s-1) and pumping isaround 1 to 103s-1. Typically, a viscometer can measure in
difference between samples that have the same viscosities.
Dynamic measurementsVirtually all materials show elastic as well as viscous properties.The measurement of sample viscoelasticity can provide vitalinformation not given by viscometry alone. With a rheometer, tinymovements (small strain oscillations) can be used to measureviscoelastic properties without destroying the sample structure.Oscillatory testing generates a mechanical spectrum for thematerial, and this provides a unique behaviour fingerprint.
the range of about 0.1 to 103s-1 while a rheometer extends themeasurement range from 10-6 to 105s-1. Processes such assedimentation are best suited to analysis with a rheometer
because of its low torquecapabilities. The high speedcontrol of a rheometer also allowsanalysis of very high shear rateprocesses, such as spraying. Theinformation in Table 1 illustratesthat the measurement capabilitiesof a viscometer can also providesolutions for a wide range ofapplications, particularly if QCtesting is the main requirement.
Figure 2 provides anillustration of where the greatermeasurement range of arheometer is useful. This exampleis for roller coating and shows agood roller coating sample (red)
and a poor one (green). At shear rates below 104s-1 both coatingshave a similar viscosity. A viscometer would find no differencesbetween these coatings.
Two further examples of important viscometric measurementsthat can be made using a rheometer are yield stress and normalforce.
Yield stressYield stress, the force required to make a sample start flowing,is a valuable measurement. It is the force that must be overcomewhen a pump is switched on; it holds paint in position afterapplication to a wall; and it can also influence shelf life andstructure perception, particularly for consumer products. Theyield stress is an important factor in providing a thick andcreamy texture, but a balance must be achieved because theyield stress will have a significant effect on the pumpingrequirements during processing.
Normal forceWhen a sample is sheared, elasticity can create a forceperpendicular to the direction of shear. This is known as normalforce and it is common with polymeric materials. Normal forcetesting can identify differences that may occur when a materialis processed. An example of this is the application of papercoating. This is applied to the paper surface using a blade-coating process with a narrow gap, which generates high shearrates. If a coating sample has a high normal force, which actsto push the application blade away from the paper, the coatingwill be too thick. Often only normal force testing will show any
The measuring geometry is oscillated sinusoidally about afixed position and stress and strain responses at differentfrequencies are recorded. This allows the elastic and viscouscomponents of the sample to be measured. Over differenttimescales, the viscous and elastic nature of the sample changes.For applications, including pouring and spraying, the sampleelasticity is critical as this relates to snap-back properties anddroplet formation. For polymers, dynamic measurements cangive details of molecular architecture, molecular weight andmolecular weight distribution.
As well as frequency sweeps, time and temperature sweepsprovide important information such as curing profiles andthixotropic rebuild. A thixotropic sample is one that exhibitstime-dependent shear thinning followed by recovery ofstructure. Typical thixotropic systems are sauces, paints andinks. Figure 3 shows an ink that has been subjected to a highshear rate to simulate application by a print head. The time forstructure rebuild in the ink is critical for dot retention anddefinition. This sample returns to a stable gel-state (elastically-dominated) after 12 seconds.
Summary Rheological measurements are essential for formulation, processand material control across all industries and applications.
A viscometer is a low cost instrument that is simple to use andcan offer portability for remote or field testing. It is highly suitablefor quality control testing and for on-line process control.
The rheometer represents a greater investment, but is essentialfor the true simulation of real processes and complete materialcharacterisation. The increased versatility and performance makeit an excellent tool for research, product and process development,as well as quality control testing.
Both instruments are complementary, and it is not uncommonwithin a single organisation to find viscometers used for QCtesting on products that have been developed using a rheometer.
By Steve Carrington, Product Manager, Rheology Systems & Joanne Langridge, Applications Specialist, Malvern Instruments
Viscometers, in comparisonto rheometers, are usually
relatively simple instruments.Their simplicity of design and
operation can offeradvantages in terms of ease
of use
Deciding whether to purchase either a viscometer or a rheometer is not always straightforward. Here, Laboratory News looks at the factors that should be considered in order to make the right choice
for the individual laboratory
Gemini Rheometer
Figure 2: Measurements at high shear rate on twopressure sensitive adhesives which are applied by roller coating
Figure 3: Rebuild time for an ink after high shear rate deformation
Figure 1: Malvern Bohlin Visco88
Table 1: Shear rates of different processes
Process Minimum Maximum Viscometer Rheometershear rate (s-1) shear rate (s-1)
Reverse gravure 105 106 ✔
Spraying 104 105 ✔
Blade coat 103 105 ✔ ✔
Mixing/stirring 10 103 ✔ ✔
Brushing 10 103 ✔ ✔
Pumping 1 103 ✔ ✔
Extrusion 1 102 ✔ ✔
Curtain coating 1 102 ✔ ✔
Levelling 10-2 0.1 ✔
Sagging 10-2 0.1 ✔
Sedimentation 10-6 10-2 ✔
This article was first published in the August 2005 issue of LaboratoryNews. Visit www.labnews.co.uk for up to the minute news, features and
new products.
● ●
WWW.LABNEWS.CO.UK PARTICLE ANALYSIS
● ●
PARTICLE ANALYSIS WWW.LABNEWS.CO.UK
Viscometer or rheometer?Making the decision
Aviscometer can offer the solution for material, processor production tests that require simple flowmeasurements, whereas the performance of a rheometerallows far greater characterisation of flow and
deformation behaviour. It is well worth taking into account therange of applications for which the instrument will be needed,and how differences in performance between a viscometer anda rheometer may affect their suitability for these applications.
Instrument operationMost viscometers operate by rotating a spindle in the sample.Viscosity is determined by measuring resistance to thisrotational force. Viscometers, in comparison to rheometers, areusually relatively simple instruments. Their simplicity of designand operation can offer advantages for operator ease of use,particularly within a busy QC environment.
Spindle movement in a viscometer is in one direction, whichallows the measurement of viscosity. Rheometers can applyoscillatory and rapid step changes in stress and strain, and cantherefore determine viscoelastic properties (providinginformation on the structural properties of the sample) as wellas flow properties.
Viscometers employ a mechanical bearing that limits thespeed and torque capabilities of the instrument, whereasrheometers generally use a low friction air bearing. The residualfriction from the mechanical bearing can make themeasurement of low viscosity materials difficult. Some of thelatest viscometers use a variety ofmeasurement geometries,enabling them to cover broaderviscosity ranges. This is applicableto many fluid applicationsincluding paints,pharmaceuticals, petrochemicalsand even asphalt.
Rheometers, while generallymore expensive than viscometers,are more versatile and have amuch wider dynamic range forcontrol and measurementparameters. In a typical stress andstrain controlled rheometer atemperature control unit (TCU)is an integral part of theinstrument rather than an option. Furthermore, there areseveral interchangeable TCUs available that cover differenttemperature ranges. Each TCU has been optimised for the bestcontrol and performance. An example of this is the ExtendedTemperature Cell, which is a forced convection oven. Thisprovides rapid heating and cooling in an inert atmosphere toprevent sample degradation. In combination with a wider choiceof measurement geometries, tests can be optimised for specificapplications and sample types.
The use of a low friction air bearing enables measurement ofeven low viscosity samples, whilst the inherent stiffness of thebearing also provides the capability to measure solids. With truestress and true strain control operating modes, a complete rangeof rheological tests (including creep, stress relaxation andmultiwave oscillation) can be performed with full control ofsample strain history.
Measuring viscosityRheometers function across a very wide range of shearrates enabling the simulation of real processes that occurover vastly different timescales, such as sedimentation andspraying. Shearing occurs whenever fluids flow throughtubes and channels. The velocity is zero right at the wallsurface and maximum at the centre. So the fluid is beingsheared as it flows through a tube or channel. Table 1shows the shear rates of processes that commonly requirerheological measurement. Spraying, for example, isbetween 104 to 105 reciprocal seconds (s-1) and pumping isaround 1 to 103s-1. Typically, a viscometer can measure in
difference between samples that have the same viscosities.
Dynamic measurementsVirtually all materials show elastic as well as viscous properties.The measurement of sample viscoelasticity can provide vitalinformation not given by viscometry alone. With a rheometer, tinymovements (small strain oscillations) can be used to measureviscoelastic properties without destroying the sample structure.Oscillatory testing generates a mechanical spectrum for thematerial, and this provides a unique behaviour fingerprint.
the range of about 0.1 to 103s-1 while a rheometer extends themeasurement range from 10-6 to 105s-1. Processes such assedimentation are best suited to analysis with a rheometer
because of its low torquecapabilities. The high speedcontrol of a rheometer also allowsanalysis of very high shear rateprocesses, such as spraying. Theinformation in Table 1 illustratesthat the measurement capabilitiesof a viscometer can also providesolutions for a wide range ofapplications, particularly if QCtesting is the main requirement.
Figure 2 provides anillustration of where the greatermeasurement range of arheometer is useful. This exampleis for roller coating and shows agood roller coating sample (red)
and a poor one (green). At shear rates below 104s-1 both coatingshave a similar viscosity. A viscometer would find no differencesbetween these coatings.
Two further examples of important viscometric measurementsthat can be made using a rheometer are yield stress and normalforce.
Yield stressYield stress, the force required to make a sample start flowing,is a valuable measurement. It is the force that must be overcomewhen a pump is switched on; it holds paint in position afterapplication to a wall; and it can also influence shelf life andstructure perception, particularly for consumer products. Theyield stress is an important factor in providing a thick andcreamy texture, but a balance must be achieved because theyield stress will have a significant effect on the pumpingrequirements during processing.
Normal forceWhen a sample is sheared, elasticity can create a forceperpendicular to the direction of shear. This is known as normalforce and it is common with polymeric materials. Normal forcetesting can identify differences that may occur when a materialis processed. An example of this is the application of papercoating. This is applied to the paper surface using a blade-coating process with a narrow gap, which generates high shearrates. If a coating sample has a high normal force, which actsto push the application blade away from the paper, the coatingwill be too thick. Often only normal force testing will show any
The measuring geometry is oscillated sinusoidally about afixed position and stress and strain responses at differentfrequencies are recorded. This allows the elastic and viscouscomponents of the sample to be measured. Over differenttimescales, the viscous and elastic nature of the sample changes.For applications, including pouring and spraying, the sampleelasticity is critical as this relates to snap-back properties anddroplet formation. For polymers, dynamic measurements cangive details of molecular architecture, molecular weight andmolecular weight distribution.
As well as frequency sweeps, time and temperature sweepsprovide important information such as curing profiles andthixotropic rebuild. A thixotropic sample is one that exhibitstime-dependent shear thinning followed by recovery ofstructure. Typical thixotropic systems are sauces, paints andinks. Figure 3 shows an ink that has been subjected to a highshear rate to simulate application by a print head. The time forstructure rebuild in the ink is critical for dot retention anddefinition. This sample returns to a stable gel-state (elastically-dominated) after 12 seconds.
Summary Rheological measurements are essential for formulation, processand material control across all industries and applications.
A viscometer is a low cost instrument that is simple to use andcan offer portability for remote or field testing. It is highly suitablefor quality control testing and for on-line process control.
The rheometer represents a greater investment, but is essentialfor the true simulation of real processes and complete materialcharacterisation. The increased versatility and performance makeit an excellent tool for research, product and process development,as well as quality control testing.
Both instruments are complementary, and it is not uncommonwithin a single organisation to find viscometers used for QCtesting on products that have been developed using a rheometer.
By Steve Carrington, Product Manager, Rheology Systems & Joanne Langridge, Applications Specialist, Malvern Instruments
Viscometers, in comparisonto rheometers, are usually
relatively simple instruments.Their simplicity of design and
operation can offeradvantages in terms of ease
of use
Deciding whether to purchase either a viscometer or a rheometer is not always straightforward. Here, Laboratory News looks at the factors that should be considered in order to make the right choice
for the individual laboratory
Gemini Rheometer
Figure 2: Measurements at high shear rate on twopressure sensitive adhesives which are applied by roller coating
Figure 3: Rebuild time for an ink after high shear rate deformation
Figure 1: Malvern Bohlin Visco88
Table 1: Shear rates of different processes
Process Minimum Maximum Viscometer Rheometershear rate (s-1) shear rate (s-1)
Reverse gravure 105 106 ✔
Spraying 104 105 ✔
Blade coat 103 105 ✔ ✔
Mixing/stirring 10 103 ✔ ✔
Brushing 10 103 ✔ ✔
Pumping 1 103 ✔ ✔
Extrusion 1 102 ✔ ✔
Curtain coating 1 102 ✔ ✔
Levelling 10-2 0.1 ✔
Sagging 10-2 0.1 ✔
Sedimentation 10-6 10-2 ✔
This article was first published in the August 2005 issue of LaboratoryNews. Visit www.labnews.co.uk for up to the minute news, features and
new products.
