Document 6510169

Transcription

Document 6510169

Rheology Solutions for the
Polymer Industries
How To Measure
Yield Stress For
Polymer Industries
Rheo368
www.rheologysolutions.com
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Tim’s Top Tips - How To Measure Yield Stress For Polymere rmpIndustries
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Page 1
Tim’s Top Tips - How To Measure Yield Stress For Polymer Industries
Company Profile
Contact Details
Rheology Solutions Pty Ltd is a specialist sales and service
organisation dedicated to the science of materials
characterisation and are the exclusive Australian distributor for
the brand names HAAKE, NESLAB, PRISM and CAHN from
Thermo Fisher Scientific, Optical Control Systems, Schleibinger
Gerate and Marimex Industries Corporation range of equipment
and instruments, and they also distribute the Shimadzu range of
tensile testers and texture analysers.
Rheology Solutions Pty Ltd
Address: 15 -19 Hillside Street, Bacchus Marsh, Victoria, 3340
PO Box 754, Bacchus Marsh, Victoria, 3340
Phone: 03 5367 7477
Fax:
03 5367 6477
Email:
info@rheologysolutions.com
Website: www.rheologysolutions.com
Rheology Solutions recognises the importance of specialisation
and dedication to a specification science and as such provides
full technical support and service throughout Australia. The
company goal is to integrate industry experience and materials
characterisation techniques to provide practical solutions for
customers.
Rheology Solutions has an established applications laboratory
equipped with a comprehensive range of instruments to meet the
requirements of material characterisation. Specialist contract
testing services are also available and contracts can be tailored
to suit discrete tests or protracted testing requirements involving
a series of tests over a period of weeks or months.
Disclaimer
The information contained in this report is not intended for direct
use as a tool for process development. It is a guide only. This
document remains the property of Rheology Solutions Pty Ltd,
and may not be reproduced or altered in any way without the
written permission of the owners.
Written by Dr. T. Kealy
Technical Manager
Rheology Solutions Pty Ltd
© Rheology Solutions Pty Ltd 2007
A range of seminars and application specific workshops as well
as product launches and demonstrations are provided
throughout Australia. The seminars and workshops are designed
to meet the needs of specific customer and industry applications.
Rheology Solutions has its head office in Victoria and works with
a team of specialist sales and factory trained service personnel
throughout Australia. The combined experience of this team
ensures that Rheology Solutions are able to provide their
customers with access to the products to ensure that the right
technical support and service is provided.
Page 2
(4)
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Tim’s Top Tips - How To Measure Yield Stress For PolymerrmpIndustries
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Tim’s Top Tips – Rheology Solutions for the Polymer
Industries.
How To Measure Yield Stress
For Polymer Industries
Key Words: Rheology, rotational, liquid, viscosity, thixotropy, yield stress.
About The Author
Introduction
Tim has a background in
engineering and specifically in
rheology, with a B.Eng and Ph.D. in
Chemical Engineering and has held
postdoctoral research positions in
engineering rheology. Tim's
research has continued for the last
seven years and recent interests
and publications include the
application of rheology and rheometry to mineral, food, polymer
and surface coatings systems. His current position encompasses
the managment of customer contract testing and also includes
customer focussed education and training. Additionally he is
available to provide technical input for existing or proposed
materials characterisation systems for both laboratory and
production.
Often the polymer industries must overcome problems related to
(and often dominated by) the flow properties of their product,
though the relationships between these properties and
production related issues are not always immediately apparent.
It is the purpose of this series of articles, “Rheology Solutions for
Polymer Industries”, to help illuminate the issues faced by the
industry, how they relate to the flow properties of problem
materials and how they can be successfully measured and
controlled with a view to better processing.
Definitions
Yield stress is the minimum force required to initiate flow. It is
often seen at start-up of compounders, mixers and pumps, when
a higher than usual energy is required to begin rotation. This
peak in energy required is due to the yield stress, which causes
the material to display solid-like properties until the yield stress
has been exceeded. Once the yield stress has been overcome,
the material behaves like a liquid and will flow.
Table Of Contents
Introduction..........................................................3
Definitions ............................................................3
Background and discussion ..................................3
Measurement techniques and pitfalls ....................4
1. The vane technique ............................................
2. The stress ramp or CS technique..........................
3. The flow curve extrapolation technique ..............
Summary ..............................................................9
Other information for polymers industries ..........10
Polymers dictionary ............................................11
Information request form ....................................14
Background and Discussion
Yield stress has an impact on the polymers industry in a variety
of ways. In extrusion, compounding, pumping and mixing at
start-up, it must be overcome so that the screws or impellers can
rotate. In pipeline transport yield stress influences the velocity
profile of the material, and under some circumstances can cause
the material to flow as a solid plug carried by a lubricating liquid
layer at the wall where shear stresses are high. The yield stress
of a material dictates whether or not a solid fraction will settle
to the bottom of a tank, pipe or other container, and also
whether gaseous materials can rise through it. The yield stress
influences the ability of high solids pastes and melts to be
compressed and to flow and so on. Either a rotational
instrument (mostly described here) or a capillary flow instrument
can be used. Capillary flow tends to have a capability for high
pressure-driven flows (high shear rates and shear stresses) and
(5)
Page 3
is often used for polymer melts, rotational instruments can not
usually impose the extremely high shear a capillary instrument is
capable of, but are extremely useful for low shear measurements
and for dilute polymer systems.
• A container
This measurement should occur in an ‘infinite sea’ of
material (i.e. the walls and bottom of the container are far
enough from the sensor as to have no effect on the reading.
The minimum container size is illustrated in Figure 1.
Note: Working definitions are provided at the end of the paper.
1.2 Theory
Measurement Techniques
and Pitfalls
The theory behind this type of measurement is that the vane
turns extremely slowly, stretching the internal structure of the
material. As the structure is progressively stretched, the force
required by the viscometer to maintain its rotational speed
increases. As soon as the force applied in this manner exceeds
the yield stress of the material, its structure fails and the material
flows. As a result the forces required to turn the sensor diminish
again. In this way a peak shear stress is generated and the peak
occurs at the yield stress. Locating the peak provides a measure
of the yield stress.
1.0 The vane technique
This is the most straightforward technique for direct
measurement of the yield stress.
1.3 Experimental procedure
The experimental layout has been shown in Figure 1.
• The container is filled with sample and the sensor system is
lowered (or, more commonly, the sample is raised on a labjack or similar) until the vane is immersed as shown in Figure
1.
• A low rpm (below 0.5 rpm is usually fine) is chosen and the
sensor rotated.
• The stress build-up and decay is monitored and the peak
stress taken. This is the yield stress.
1.4 Results
The results of a vane test are shown in Figure 2.
Figure 1. Layout for yield stress measurement using the vane
technique
1.1 To make a measurement you will need:
• A viscometer or rheometer
Most mid-range viscometers will suffice, the key criteria are
good resolution of the torque and good range and control of
rotational speed, in particular a capability for constant, very
low rpm, rotation. Most CS rheometers are capable of this
kind of performance, as well as mid-range and high-end
viscometers.
.
• A suitable vane
Usually 4 or 6 blades and with a surface area large enough
to provide a torque reading within the specifications of the
viscometer.
Figure 2. Data from yield stress (τ0) analysis using the vane
technique
Page 4
(4)
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• Correct insertion of the vane into the material
If the vane is not correctly inserted, so that the shaft is at
right angels to the surface of the liquid, then inaccurate
measurements will be made.
1.5 Benefits of the vane technique
• Low level of disturbance
The presence of a yield stress indicates that a structure exists
in the material. The shape of the vane (low profile entering
the material) allows minimal disturbance to this structure.
This is a significant advantage over all other measuring
geometries.
• Settling or time dependent materials
Time dependent materials change their properties with time.
To repeatably measure the maximum yield stress, the
material should be allowed to equilibrate. Settling mixtures
have the opposite problem. If possible they should be
measured as quickly as possible, before appreciable
settlement has taken place.
• Can use a Controlled Rate (CR) viscometer
CR viscometers are comparatively low cost compared with
Controlled Stress (CS) rheometers, which invariably require
clean dry compressed air for near frictionless performance
and good precision at low stresses and strains. It is
important that the viscometer chosen has good resolution at
low rpm, since it is here that the structure can be gradually
‘stretched’ until it yields.
2.0 The stress ramp or CS technique
This is a highly accurate technique for direct measurement of
the yield stress.
• Quick test
The vane test can be relatively quick, usually complete in a
matter of a couple of minutes, depending on the yielding
behaviour of the material.
• A CS rheometer
CS rheometers are usually high-end instruments, with air
bearings to allow near frictionless rotation and the
measurement and application of small forces.
• Intuitive interpretation of the data
The meaning of the peak is easily understood, and the
magnitude of the yield stress is immediately apparent.
• A container
For the vane, an infinite sea measurement is desirable, as
described before. For the other sensor systems the container
is predefined by the measuring system geometry.
• Simple measurement and simple analysis
The measurement is quite simple to execute and the data is
straightforward to process.
1.6 Potential problems with the vane technique
• A suitable sensor
A vane can be used, with the advantages as discussed in the
previous section.
Other rheological measurements (other than yield stress) are not
reliable using the vane, it should not be used for viscosity
measurements. One of the most common problems with the
vane sensor is that many users utilise it for measurements other
than yield stress. The sensor was not designed to measure any
other flow property.
Alternatively other (cone & plate, plate & plate or cup & bob)
type sensors can be used, shown in Figure 3. Plate and plate
and cup and bob sensors can be serrated to prevent slip for
multi-phase materials.
• Not enough data points
If insufficient data are collected there is a danger that the
peak stress will not be captured and so a lower than actual
yield stress will be measured.
• Temperature control
Yield stress often changes with temperature and
temperature control of such large volumes is often difficult.
Additionally, many vanes are manufactured for
measurements up to 10ºC. The limitations should be
checked with the manufacturer.
A
• Large agglomerates
Large agglomerates can occur in where a suspended phase
is present, these can interfere with the rotation of the vane.
(5)
Page 5
2.4 Results
The results of a pair of CS yield tests on different materials are
shown in Figure 4.
B
Figure 4. Data from yield stress (τ0) analysis using the CS
technique
C
Figure 3: Standard viscometric geometries (L to R) Cup & Bob,
Cone & Plate, Plate & Plate
2.5 Benefits of the stress ramp or CS technique
• Low level of disturbance
A vane can be used for this type of measurement, and the
advantages or restrictions associated with the vane
geometry are repeated here.
2.2 Theory
The theory behind this type of measurement is that the applied
stress is slowly increased and the movement of the sensor
monitored. Initially there is very slight movement, as the
structure stretches, and once the yield stress is overcome, the
rate of deformation increases dramatically. Plotting the
deformation vs. stress on logarithmic scales shows two distinct
linear regimes, one before and one after yielding. The
intersection of the two linear segments is the yield stress.
• Other information can be generated
If measuring geometries other than the vane are used, a CS
flowcurve can also be generated with the same equipment,
and a change in geometry is not necessary.
• Small sample volumes are required
The measuring gap is usually small, in particular for cone and
plate or plate and plate geometries.
The experimental layout has been shown in Figure 1 for a vane,
and is standard equipment set-up for the other sensor systems
(Figure 3).
• Stress controlled technique
Control of the shear stress (as opposed to the rotation)
means that the sample is deformed only according to the
stress imposed. A Controlled Rate (CR) or Controlled
Deformation (CD) technique instantaneously imposes a
deformation, which may mean that the yield stress is passed
before the first measurement point can be taken.
• The sample is loaded into the measuring geometry closed.
• A low shear stress is chosen and imposed on the material
through the sensor. The stress is gradually increased.
The meaning of the linear displacement regions is easily
understood.
• The deformation of the sample is monitored and the point at
which the sample begins to flow taken from the intersection
of the linear segments described previously. This is the yield
stress.
Page 6
• Simple measurement and simple analysis
The measurement is quite simple to execute and the data is
straightforward to process.
(4)
• Slip
Slip can be overcome by using profiled cup and bob or plate
and plate sensor systems..
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3.0 Flow curve extrapolation technique
This is a common technique for indirect measurement of the
yield stress.
For cone and plate, plate and plate and cup and bob
geometries temperature control is not difficult.
• Rheometer or Viscometer
Most mid-range viscometers will suffice, the key criteria are
good resolution of the torque and good range and control of
rotational speed, in particular a capability for good control
and measurement at very low rpm, and small forces. The
better the control and measurement capabilities are at low
shear rates and shear stresses, the more accurate the final
result can be expected to be. To different degrees, most
viscometers and rheometers are capable of this kind of
performance.
• Sensor system insertion
Using computer control, the measuring gap is always closed
to the same point, at the same speed, making the
measurement very repeatable.
2.6 Potential problems with the stress ramp or CS
technique
• Can be time consuming
Compared with the vane technique, the CS technique can be
time consuming. The time taken can be reduced by
restricting the range of stresses to where the yield stress is
expected to lie and by judicious selection of the rate of
change of stress for the stress ramp.
• A suitable sensor system
A vane cannot be used. Cone and plate, plate and plate or
cup and bob type sensors can be used (Figure 3). Plate and
plate and cup and bob sensors can be serrated to prevent
slip for multi-phase materials.
• Sample loading
Closing the measuring gap can alter (sometimes irreversibly)
the structure of the material. Because of the shape of the
sensors and the small size of the measuring gap it is usually
impossible to avoid this using any measuring geometry other
than the vane.
• Theory
The theory behind this type of measurement is that a flow
curve is generated, which displays the relationship between
the shear stress and the applied shear rate (or vice versa for
a CS measurement). The curve is extrapolated back to the
shear stress axis (i.e. to shear rate = 0), and the intercept is
designated the yield stress.
If insufficient data are collected there is a danger that the
point of inflection will not be captured or there will be
insufficient data to evaluate one or both of the linear
segments of the curve, leading to poor estimation of the
point of inflection and so yield stress will be estimated
incorrectly.
The experimental layout is standard equipment set-up for the
other sensor systems.
• The sample is loaded into the measuring geometry closed.
• Agglomerates
Agglomerates or particles (more than 1/3 of the measuring
gap size, or 1/10 of the gap size for highly loaded materials).
is present these can interfere with gap closure for most
geometries and may cause an apparently excessive yield
stress because of solid particle(s) bridging the measuring
gap.
• A low shear rate is chosen and imposed on the material
through the sensor. The shear rate is gradually increased to
a preset value.
• The shear stress is monitored and a curve drawn through the
data, and extrapolated back to the stress axis. The curve can
be constructed by the investigator by hand, or fitted
according to some common fitting equation which allows for
the presence of a yield stress, like that of Bingham or
Herschel Bulkley etc.
• The stress intercept is designated the yield stress.
(5)
Page 7
• Sensor system insertion
Using computer control, the measuring gap is always closed
to the same point, at the same speed, making the
measurement very repeatable.
3.4 Results
The results of a flow curve extrapolation to determine yield stress
is shown in Figure 5.
3.6 Potential problems with the flow curve
extrapolation technique
• Can be time consuming
Compared with the vane technique, this technique can be
time consuming. It can be necessary to refine different curve
fits to find the best one.
• Reliability
This technique involves extrapolation of known data into an
area where behaviour is unknown. This is a dangerous
practice as the shape of the flow curve is not known with any
certainty outside the area in which a measurement has been
performed
• Accuracy
Depending on the curve fit, the quality of the data, and the
quantity of data, in particular at low shear rates, the
prediction of the intercept is unreliable at best.
Figure 5. Data from yield stress (τ0) analysis using the flow
curve extrapolation technique
• Sample loading
Closing the measuring gap can alter (sometimes irreversibly)
the structure of the material. Because of the shape of the
sensors and the small size of the measuring gap it is usually
impossible to avoid this using any measuring geometry other
than the vane.
3.5 Benefits of the flow curve extrapolation
technique
• Uses information often already generated
Often information on the interdependence of shear rate and
shear stress is already in existence from previous analyses.
If insufficient data are collected there is a danger that any
curve fit to the known data will be of poor quality, leading to
inaccurate prediction of the yield stress.
• Small sample volumes are required
The measuring gap is usually small, in particular for cone and
plate or plate and plate geometries.
• Agglomerates
Agglomerates or particles (more than 1/3 of the measuring
gap size, or 1/10 of the gap size for highly loaded materials).
is present, these can interfere with gap closure for most
geometries and may cause an apparently excessive shear
stress during the flow curve measurement because of solid
particle(s) bridging the measuring gap.
The meaning of the extrapolation is easily understood.
• Simple measurement
The measurement is quite simple to execute but data
processing is not always straightforward.
• Slip
Slip can be overcome by using profiled cup and bob or plate
and plate sensor systems.
For cone and plate, plate and plate and cup and bob
geometries temperature control is not difficult.
Page 8
(4)
Summary
Table 1 summarises the possibilities for measuring yield stress
using the techniques discussed. Each of the techniques is ranked
between 0 and 5 for each of the potential issues and solutions,
where:
5 = Excellent
2 = Possible
4 = Good
1 = Difficult
Determining the most suitable type of measurement or
instrument is not simply a matter of adding up the ranking for
each. Rather, identify which measurement technique, variable etc
is most relevant and appropriate for your application/product.
Often, more than one technique is required to ensure
consistency, reproducibility and accuracy is achieved.
3 = Adequate
0 = Not Possible
Table 1: Assessment of strengths/weaknesses for each technique
Technique:
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Vane
Stress Ramp
Flow Curve
Extrapolation
Rapid
5
3
3
Easy
4
3
2
Accurate
4
5
2
Direct determination of yield stress from measurement
5
5
0
Small sample volume
2
4
4
Temperature control
2
5
5
Both
Rheometer
Both
Cost
5
3
4
Large variety of sensors
3
4
5
Measures materials with large particles & agglomerates
4
2
2
Structural disruption on loading avoidable
5
2
2
5
5
4
3
Measurement
Measuring system
Rheometer or viscometer
Number of Participants
Single operator
5
Results
Intuitively comprehended
4
* Depending on the test, these parameters may be viewed
alternatively as either a strength or as a weakness
(5)
Page 9
Contact Details
Other Notes Available in the
Tim’s Tips - Rheology Solutions
for Polymer Industries Series are:
• How To Measure Flow and viscosity curves (Rheo364)
• How To Measure Thixotropy (Rheo366)
Other Information Available
for Polymer Industries include:
• Rheology Solutions for Polymer Industries Information Kit
• Applications Laboratory and Contract Testing Capabilities
Statement for Polymer Industries
Head Office Rheology Solutions Pty Ltd
• Technical Literature for Polymer Industries
Address: 15 -19 Hillside Street, Bacchus Marsh, Victoria, 3340
PO Box 754, Bacchus Marsh, Victoria, 3340
Phone: 03 5367 7477
Fax:
03 5367 6477
Email:
Managing Director – Pat Griffin
Email:
patgriffin@rheologysolutions.com
Technical Manager – Dr Tim Kealy
Email:
timkealy@rheologysolutions.com
Service Engineer – Richard Donaldson
Email:
richardd@rheologysolutions.com
service@rheologysolutions.com
Focused on providing our customers with materials characterisation
solutions through knowledge, experience and support.
Page 10
(4)
Polymer Dictionary
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Industry Term:
Definition:
Bingham.
A flow curve exhibiting a linear relationship between shear rate and shear stress, with a
positive intercept on the stress axis. The curve is of the form:
Governing Properties:
Rheology Solutions Instrument:
Measured using a flow curve generated on CS rheometer or a CR viscometer.
HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Industry Term:
Definition:
Controlled Deformation.
Mode of operation for a rheometer or viscometer. Controls the deformation of the sample.
CD mode is usually available using a CS rheometer or a CR viscometer.
Industry Term:
Definition:
Controlled Rate.
Mode of operation for a rheometer or viscometer. Controls the shear rate imposed on the
sample.
CR mode is usually available using a CS rheometer or a CR viscometer.
Industry Term:
Definition:
Industry Term:
Definition:
Industry Term:
Definition:
Controlled Stress. (Stress Controlled Technique).
Mode of operation for a rheometer or viscometer. Controls the shear stress imposed on
the sample.
CS mode is usually available using a CS rheometer but not on a CR viscometer.
HAAKE RheoStress, HAAKE MARS.
Deformation.
The movement of an element of a material, usually as a result of some applied force.
The rate of deformation (shear rate) due to an applied force is dependent on the resistance
of the material - it's viscosity in the case of a liquid, or rigidity in the case of a solid.
HAAKE ViscoTester 550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS, Marimex
ViscoScope.
Flow Curve.
A flow curve is a plot showing the relationship between shear rate and shear stress.
Measured using a CS rheometer or a CR viscometer.
(5)
Page 11
Industry Term:
Definition:
Herschel Bulkley.
A flow curve exhibiting a shear thinning relationship between shear rate and shear stress,
with a positive intercept on the stress axis. The curve is of the form:
Measured using a flow curve generated on CS rheometer or a CR viscometer.
Industry Term:
Definition:
Rheology.
The flow and deformation of matter.
N/A
Industry Term:
Definition:
Rheometer.
An instrument designed for the measurement of viscous and viscoelastic flow properties
at specified temperature and atmospheric conditions, by measuring the force required to
move one layer over another without turbulence.
Rheometers often have air bearings, making them highly sensitive to small variations in
load or displacement and can operate in rotation or oscillation for Controlled Rate or
Controlled Stress modes. Some rheometers have mechanical bearings, but in general they
do not have the required sensitivity to make good use of CS mode in these cases and can
not run oscillatory measurements well (or at all).
HAAKE RheoStress, HAAKE MARS.
Industry Term:
Definition:
Industry Term:
Definition:
Industry Term:
Definition:
Page 12
Shear Stress.
This is the force per unit area imposed on an element of fluid.
The shear stress is dependent on the geometry of the fluid element and can be measured
by a CR viscometer and may be imposed by a CS rheometer.
Thixotropy.
Thixotropic fluids show shear thinning behaviour combined with a time dependency. The
viscosity of a thixotropic fluid drops when subjected to a constant shear rate for a period
of time. The viscosity of thixotropic fluids often recovers substantially over a period of time
after the shearing forces have been removed.
Thixotropy depends on the rate of structural recovery in the material. It can be measured
using a flow curve on a CR or CS instrument, or by measuring the recovery of the moduli
after shearing on a CS rheometer.
Vane Technique.
A test for measuring yield stresses using a special 'star' shaped rotor.
Usually depends on the solids loading in the slurry or suspension and the interaction
between them. The presence of viscosity modifiers in the slurry can also effect yield point.
Measured using the vane technique and a CR viscometer.
HAAKE RotoVisco, HAAKE ViscoTester 550.
(4)
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Viscometer.
An instrument for measuring the viscosity of a liquid at specified temperature and
atmospheric conditions, by measuring the force required to move one layer over another
without turbulence; also referred to as viscometer.
Viscometers usually have mechanical bearings in their motor and generally operate in
rotational mode only.
HAAKE ViscoTester 550, HAAKE RotoVisco.
Viscosity.
The resistance to flow of a fluid.
Viscosity is the shear stress divided by the shear rate. These are measured on a CR
viscometer or CS rheometer using a flow curve.
HAAKE ViscoTester VT550, HAAKE RotoVisco, HAAKE RheoStress, HAAKE MARS.
Yield Stress.
The minimum shear stress required to initiate flow in a fluid.
Governed by the structural properties of the material at rest, measured by extrapolation
using a flow curve, or using the vane technique, both on a CR or CS instrument. It can
also be measured using a CS rheometer by a stress ramp.
Notes:
• ViscoTester 550 and RotoVisco are controlled rate viscometers, RheoStress is a controlled stress rheometer, MARS is a modular
R&D Controlled Stress Rheometer, all of which are HAAKE brand names of Thermo Fisher Scientific (Karlsruhe, Germany) GmbH.
• ViscoScope torsional motion viscometer is a brand name of Marimex Industries Corporation.
Disclaimer
The information contained in this report is not intended for direct use as a tool for process development. It is a guide only. This
document remains the property of Rheology Solutions Pty Ltd, and may not be reproduced or altered in any way without the written
permission of the owners.
© Rheology Solutions Pty Ltd 2007
(5)
Page 13
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Tim’s Top Tips – Rheology Solutions for the Polymer Industries.
How To Measure Yield Stress
For Polymer Industries
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Technical Literature for Polymer Industries
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(4)

Document 6510169

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