rheology testing of polymers and the determination of properties using rotational rheometers and cap

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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.

Rheology Testing of Polymers and the Determination of Properties Using Rotational Rheometers and Cap

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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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Malvern Instruments Ltd

Enigma Business Park, Grovewood Rd

Malvern

Worcestershire, WR14 1XZ

United Kingdom

PH: 44 (0) 1684 892456

Fax: 44 (0) 1684 892789

Email: [email protected]

Visit Malvern Instruments Ltd Website

Company BackgroundMalvern is a leading supplier of analytical solutions for particle characterization and rheological applications.

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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

support now plays a fundamental role in delivering on Malverns commitment to share its expertise and experience

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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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Particle size Dispersion stability Zeta potential Rheological properties

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rheology testing of polymers and the determination of properties using rotational rheometers and cap

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