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Rheology Testing of Polymers and theDetermination of Properties Using
Rotational Rheometers and Cap
Topics CoveredBackground
Factors That Affect the Rheology of Polymers
Melt Viscosity and Its Temperature Dependence
Die Swell or Extrudate SwellMelt Elasticity
Concentration of Additives
Characterising Flow Behaviour
Rotational Rheometers
Capabilities of Modern Rotational Rheometers
Flow Curves
Creep Tests
Stress Relaxation
Small Amplitude Sinusoidal Oscillatory Testing
How Viscoelastic Characterisation has Solved Real Processing Problems
Variability of Tube and Pipe Gauges in Extrusion ProcessesReducing Inconsistent Fibre Spinning Properties
Capillary Extrusion Rheometers
Determination of Die Swell
Applications of Capillary Rheometers
Extensional Properties
Capillary Rheometers and Processing Behaviour
Melt Fracture
Differences between Calculated Rheological Properties and Practice
Conclusion
BackgroundRheology is the science of studying the flow and deformation of materials rooted in the laws of elasticity and
viscosity proposed by Hooke and Newton in the late 17th Century. Thermoplastic polymer melts are widely used
in many modern industrial processes to manufacture a multitude of objects. Polymers are used because they are
relatively cheap to form into complex shapes in the molten state and therefore, we need to understand how they
flow when being processed.
Factors That Affect the Rheology of Polymers
Polymers are complicated materials to characterise rheologically because there are many factors that influence
their flow properties. Examples of factors that influence the flow behaviour may include: Processing temperature;
Rate of flow; Residence time etc.
Furthermore the rheological properties of polymers are in between those of a liquid and a solid. This leads to time
dependence of the flow properties and other important characteristics, some of which are discussed below.
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Melt Viscosity and Its Temperature Dependence
Melt viscosity is well known to be critically dependent on temperature. By lowering the temperature of a mould
until the part being produced has a matt finish, the processor can learn the minimum temperature (hence
maximum resin viscosity) at which the process can be run without surface defects becoming apparent. Reducing
the mould temperature saves energy and can reduce cycle times and so an understanding of the temperature
dependence of melt viscosity is very useful.
Die Swell or Extrudate Swell
Polymer melts are known to exhibit die swell when extruded. This phenomenon reveals itself as an increase of
diameter of an extrudate after exiting a die. The amount of die swell is related to the amount of elastic
deformation of the material at the inlet of the die. A further fact to be considered is that the degree of die swell
(more correctly extrudate swell) is dependent on the length of the die when material is extruded at constant
throughput. In other words polymer melts exhibit time dependency as the material forgets the elastic deformation
applied at the entrance of the die, the more time the material spends within the die the less die swell.
Melt Elasticity
Melt elasticity can also have profound implications for many other polymer processes such as:
Blow Moulding where the wall thickness of the blown component depends on the degree of swell that has taken
place during the extrusion process prior to the mould being closed.
Vacuum Forming or Thermoforming where the polymer must maintain a degree of elasticity to prevent the
material sagging before it is pulled by vacuum over the cold forming die. If the material does not have sufficient
elasticity it is likely to come into contact with the chilled die before the vacuum or pressure is applied.
Concentration of Additives
Polymer processing properties also depend on the concentration of lubricants, plasticisers, fillers and other
components in the compound being processed. From this brief introduction one can appreciate that proper
characterisation of polymer melt flow behaviour is likely to require sophisticated and versatile instrumentation.
Characterising Flow Behaviour
From the point of view of the rheologist polymer flow behaviour can be conveniently separated into three
components: Shear and extensional flows which are characterised by the corresponding viscosities and Elastic
behaviour which is characterised by measurement of modulus or swell ratios.
To fully characterise a material, instrumentation is required which has the capability of extracting these
parameters over a range of temperatures and shear/extension rates. Modern laboratory rheological testapparatus can be divided into two broad categories of rotational rheometers and capillary extrusion rheometers.
Rotational Rheometers
These instruments normally require a small specimen of the material to be tested in the form of disk typical
dimensions being 25mm diameter and 1mm thick. The sample is placed between a pair of parallel plates or upper
cone and lower plate whose temperature can be maintained by an external heating device such as a blown gas
oven or electrical heating of the plates.
Capabilities of Modern Rotational Rheometers
Modern rotational rheometers are capable of a number of test types to allow full characterisation of a material
over a range of temperatures and flow rates. Examples of the types of the types of tests available are:
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Flow Rates
Creep Tests
Stress Relaxation
Small amplitude sinusoidal Oscillatory Testing
Flow Curves
Flow curves to measure the shear viscosity versus shear rate or shear stress. At sufficiently low shear rates a
constant value for the viscosity will be attained. This so called zero shear viscosity has been shown to depend on
the average molecular weight of the polymer and the length of the plateau (how high a rate before the viscosity
decreases) is known to reflect the width of the molecular weight distibution. Software packages are available to
determine the average molecular weight and molecular weight distribution from such data.
Figure 1. Flow curve for LDPE at 190C showing low shear rate plateau for viscosity. The magnitude of the zero
shear viscosity is determined by the average molecular weight of the polymer.
Creep Tests
Creep tests (application of constant stress for a defined period of time) allow an alternative means of determining
the zero shear viscosity. When combined with recovery testing (removal of the stress) these tests enable the
amount of elasticity in the sample to be measured because a material will with elasticity will recoil and attempt to
recover its original shape.
Figure 2. Creep (Blue) & Recovery (Red) Curve Polypropylene at 190C allow zero shear viscosity to be
determined and equilibrium recoverable compliance.
Stress RelaxationStress Relaxation tests apply an instantaneous deformation (strain) to the sample and record the time dependent
decay of stress with time. The rate of decay of the stress depends on the viscoelasticity of the polymer at the test
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scope of this short article, but the following examples are included to illustrate how viscoelastic characterisation of
polymers has solved real processing problems:
Variability of Tube and Pipe Gauges in ExtrusionProcesses
Oscillatory testing at low frequencies (below 0.1 Hz) revealed differences in the elastic modulus between different
batches of material. Clearly pipe gauge will depend on the degree of recovery of the polymer after being extruded
and so not surprisingly, the pipes and tubes with the higher gauge have greater elastic modulus.
Figure 5. Frequency Sweep data for two HDPE pipes. The sample with higher elastic modulus produced the larger
gauge pipe.
Reducing Inconsistent Fibre Spinning Properties
Low frequency oscillatory testing was able to show differences in the elastic properties of different batches of
material. No differences were observed in the viscosity, indicating the material was of consistent molecular
weight. The differences in elasticity at low frequency are related to differences in the molecular weight distribution
(MWD) with the result that the broader MWD results in increased molecular chain entanglement which hinders the
draw down process of the fibre spinning process. This in turn causes inconsistency in the final product.
Figure 6. Complex Viscosity as a function of frequency for good and bad PP Fibre samples. Note that no
discernable difference is evident.
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Figure 7. Storage Modulus as a function of frequency for good and bad PP Fibre samples. The bad sample had
more elasticity causing inconsistent fibre diameter.
Capillary Extrusion Rheometers
Advanced capillary extrusion rheometers comprise a temperature controlled barrel incorporating one or more
precision bores fitted with capillary dies at the exit. Melt pressure transducers are mounted immediately above
the dies to record the pressure drop as polymer melt is extruded through the dies at programmed flow rates. By
the use of a capillary die and an orifice or zero length die the shear and extensional viscosities of a polymer
melt may be determined simultaneously against shear and extension rates.
Determination of Die Swell
Additional accessories are available to record die swell by means if a laser scanning gauge and or extrudate melt
strength by passing the strand of polymer through a series of speed controlled nip rollers and recording the force
(melt tension) as a function of haul off speed.
Applications of Capillary RheometersAs a general rule, capillary rheometers are used to measure melt properties at higher shear rates than rotational
rheometers and allow determination of flow behaviour under typical processing conditions. A particularly
important consideration is the ability to measure extensional (elongational) properties at higher extension rates
than by other techniques (such as counter rotating pulley devices) and more importantly at extension rates
encountered on a processing line.
Extensional Properties
Figures 8 & 9 show both shear and extensional data, which illustrates an important and often neglected point:
Two polymers may have almost identical shear flow behaviour, but may exhibit considerably different extensionalproperties. As noted previously, many polymer processes (fibre spinning, blow moulding) are essentially
extensional processes and so determination of extensional viscosity is more important than measuring shear
viscosity.
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Figure 8. Shear viscosity versus shear rate. The data for the two rubbers is indistinguishable.
Figure 9. Extensional viscosity versus extension rate for the same materials shown in figure 8. There are clear
differences in the extensional.
Capillary Rheometers and Processing Behaviour
Capillary rheometers are often used to examine processing behaviour, rather than determine rheological
parameters: Two examples could be determination of regions of flow instability and measurement of wall slip or
critical stress.
Melt FractureFlow instability or melt fracture is generally the result of tensile stress when the melt flows from a large
cross-section to a smaller one. If the tensile stress becomes large enough, the melt fractures. The effect of melt
fracture becomes less noticeable as the length of die is increased and as the die temperature is increased.
Increasing die length damps out the effect of the cross-section change at the entrance of the die and increasing
temperature reduces the viscosity and also the stress at the same shear rate. In a capillary rheometer a region of
melt fracture is revealed as a regular oscillation of the melt pressure signal as shown below. The melt effectively
fractures and then reforms with the effect that adjacent elements have experienced different extensional histories
and so will swell differently upon exiting the die.
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Figure 10. Evidence of Melt Fracture is shown by the oscillating pressure signal. The material is Polypropylene
measured at 190C.
Differences between Calculated Rheological Propertiesand Practice
A fundamental assumption when calculating rheological properties with a capillary rheometer is that the material
at the wall of the capillary die is stationary this is the so-called stick condition. In practice polymer melts
deviate from this situation at a critical stress and the material flows as combination of shear flow superimposed
onto a plug flow. Wall slip and determination of the critical stress can be analysed in a capillary rheometer by
measurement of flow curves at the same temperature for at least three sets of capillary dies with the same length
to diameter ratio. For a material not experiencing wall slip identical shear stress versus shear rate profiles will be
generated.
In the case of wall slip occurring, shear stress will decrease as the die diameter increases at constant shear rate.
Analysis of the flow data allows the slip velocity and critical stress to be determined. These parameters are
often required by computational fluid dynamics software packages along with shear and extensional viscosity data
to predict the flow of melts in moulds and extrusion profiles. The two examples above show how a capillary
extrusion rheometer may be used to help predict the processing performance of a polymer melt. Other test
regimes are also possible: Determination of polymer degradation by multiple flow curve measurements or
viscosity versus time; Measurement of critical temperature for flow to commence at constant extrusion pressure;
Stress relaxation after flow cessation; Melt compressibility at constant temperature etc.
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Figure 11. Rheograms for HDPE at 200C. The line of constant stress reveals evidence of Wall Slip.
Figure 12. Slip velocity versus shear rate for HDPE at 200C. Slip velocity is calculated by Mooneys method.
Conclusion
Polymer melt rheology is a complex subject that requires careful experiment design in order to obtain the
information needed to meet an investigators requirements. Rotational rheometers are the preferred choice when
the requirement is to obtain information concerning the molecular structure and how this affects processing
characteristics. In particular, the ability to easily extract information about the average molecular weight and
molecular weight distribution via measurement of the viscoelastic properties makes the rotational rheometer a
powerful tool. The capillary rheometer extends the shear rate range attainable in the laboratory beyond that
available in a rotational instrument and allows the flow properties to be measured under typical processing
conditions. In addition, the ability to readily determine both the shear and extensional properties under real life
conditions provide the polymer producer and processor with information that is vital to the successful use of a
polymer melt. Finally, the capillary rheometer enables processing problems to be investigated in a controlled
environment without the need to stop.
Source: A Rheological Viewpoint of Thermoplastic Melts, Application Note by Malvern Instruments.
For more information on this source please visit Malvern Instruments Ltd (UK) or Malvern Instruments (USA).
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Date Added: Apr 13, 2005
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Company BackgroundMalvern is a leading supplier of analytical solutions for particle characterization and rheological applications.
Advanced measurement technologies are combined with robust mechanical designs and comprehensive data
handling and automation software, to provide systems that are relevant across a wide range of industrial and
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Material characterization data such as size distribution, particle shape, zeta potential, molecular weight, and bulk
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Technologies used include laser diffraction, image analysis, laser Doppler electrophoresis, static and dynamic light
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Operating from headquarters in the UK, Malvern has an established network of subsidiary organisations, local offices
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A highly developed and continually expanding programme of web-based information, education, training and
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Characterizing nanoparticles
Since the physical and optical properties of nano-sized particles are related strongly to their size, there is a growing
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The technique of dynamic light scattering (DLS) is ideally suited to the determination of the size of particles in the
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measurement of samples at much higher concentrations than is possible using conventional DLS instruments where
a 90 detection angle is the norm. The Zetasizer Nano series also offers combined size and zeta potential
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