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What is a capillary rheometer?
A rheometer is an instrument used to measure rheological properties like shear stress, shear rate, and shear viscosity. Capillary rheometry is a technique whereby a sample undergoes extrusion through a die of defined dimensions and the shear pressure drop across the die recorded at set volumetric flow rates.
Capillary rheometers are comprised of a temperature-controlled barrel incorporating one or more precision bores fitted with capillary dies at the exit. Pressure transducers are mounted immediately above the dies to record the pressure drop as the material being tested undergoes extrusions through the dies.
What materials can I test using a capillary rheometer?
Polymer and rubber testing are historically the main areas that capillary rheometry is used in. However, applications for capillary rheometers cover a wide range of industries:
- Ceramics – to determine the flow and moulding properties
- Paints and inks – to characterize materials used in spraying and printing processes
- Foods – to determine mould filling, cooling and setting properties
- Personal care products – to assess container filling, spraying and pumping properties
How does the capillary rheometer differ from a melt indexer?
Traditionally, the melt flow indexer (MFI) has been the instrument of choice for quantifying melt flow properties in the plastics industry. MFI machines are typically inexpensive and easy to operate. They require a minimum of expertise to interpret the results, which more often than not is a single value. The basic test uses a single dead weight to record the amount of material extruded over a fixed period of time. As a result, the MFI quickly became the de facto standard in the plastics industry.
Capillary rheometers on the other hand can be used to determine a range of material functions. They can quickly and easily measure the flow properties of a material over the full range of forces, pressures, geometry, and temperatures that are encountered in real processes.
Modern capillary systems incorporate computer-controlled drive systems and software that allow measurements over a wide dynamic range and also provide all the elements necessary to simulate current processing machines. The standard capillary corrections can be automated in the software.
An MFI can be used for relative ranking of materials but is not capable of providing fundamental material properties and has a very limited test range.
As the applications for injection moulding , extrusion, and other processes become more demanding, the requirement for more extensive rheologcial testing has increased tremendously.
What sort of rheology information can I get from the capillary rheometer?
Shear viscosity is the main measure of resistance to flow and therefore affects real processes, especially when there are changes in temperature and flow rates. The most basic experiment performed on the capillary rheometer is a “viscosity flow curve” or “shear rate sweep”. The test sample is pushed by a piston driven at a defined speed through a capillary die and the pressure monitored above the die. The pressure builds up until an equilibrium condition is reached, at which point the pressure is recorded and the speed is changed to the subsequent measurement point. This process is repeated over a number of speeds, typically 5-10. The range of speeds is selected to correspond to the shear rate range of interest and normally covers more than 2 decades (>100 fold dynamic range) in a single test.
The fundamental rheological measurements at each speed are volume flow rate (defined by piston speed and piston diameter) and pressure drop. From these two measurements, a graph of viscosity vs shear rate is produced.
Less commonly considered or measured is the extensional viscosity, which can be easily obtained (simultaneously with the shear viscosity) when experiments are performed on the twin-bore capillary rheometer systems.
In addition to the above, there are several other specialist tests for specific processes, such as polymer melt strength for extrusion blow molding, film extrusion, wire-coating, and fiber forming; and for specific situations like time in melt state, degradation, cross-linking behavior, zero- shear viscosity for rapidly degrading samples, changes in volume as a function of pressure (PVT), detection of melt fracture, etc.
What is the shear rate range of the capillary rheometer?
The available shear rate range of any capillary rheometer is defined purely by the barrel and die diameters and the range of speeds over which the rheometer can be driven. For a given size barrel bore, a smaller bore die generates higher shear rates and stresses, whereas a larger bore die results in lower shear rates and stresses.
With rheometer barrels available in the range of 9.5 to 24mm in diameter, and commonly used dies from 0.25 to 3mm diameter, capillary rheometers are capable of shear rates of 0.001s -1 to over 5 ´ 10 6s -1.
What size dies should I be using?
To get fully developed flow in the die, a moderate length die of 15-20mm is recommended. Shorter dies tend not to facilitate fully developed flow, and longer dies results in higher pressures thus creating the need to change pressure transducers.
For performing the Bagley correction, the two die system is recommended; a pair of dies of the same diameter, one “long” and the other an “orifice” die of nominal 0.25mm length.
Capillary dies are defined in terms of the following:
- Capillary diameter D (bore)
- Capillary length L
- L/D ratio
- Entry angle to the die
- Any other special requirements (e.g. material, sealing arrangements, etc)
Die sizes are selected based on the following:
- Desired range of shear rates – defined by the die bore
- To cater for filled and non-uniform materials – conical entry dies aid flow of filled material into the die
- To carry out different tests such as wall slip or Bagley correction – multiple dies will be required
- When testing low viscosity fluids – narrow (fine) bore dies will be required
- For studying flow with different geometries (entry angle)
What is the Bagley correction and do I need to be using it?
The basic single-bore capillary rheometer “viscosity” test gives an apparent rather than a true measure of viscosity at the test temperature. A series of corrections is appropriate to derive the true viscosity. The twin bore instrument facilitates easy use of the Bagley correction.
The basic premise of the correction is that pressure changes are occurring at the entrance to the die, along its length, and at its exit. The entrance and exit pressures can lead to significant errors in the calculated shear stress if not assessed (up to 20-30%). The real magnitude of the entrance and exit pressures can only be determined if a second measurement is made. Rosand developed the technique of using a very short or “orifice” die to directly measure the entrance pressure. This is most efficiently done on twin-bore capillary rheometers.
The correction is strongly recommended when the data is to be used for design, flow simulation or fundamental studies. If the data is for comparison or ranking of materials then apparent viscosity values are usually quite sufficient and the Bagley correction need not be applied.
Do I really need a twin-bore capillary rheometer?
The original idea in developing the twin-bore capillary rheometer was to provide an efficient means of making the Bagley correction. Performing a twin-bore test requires significantly less time and effort when compared to sequential testing. With further software development the twin-bore’s capabilities have greatly expanded to include parallel comparative testing using the same size die to measure two materials simultaneously, and extended shear rate testing on a single material using two different diameter dies.
Single bore rheometers are well suited for situations where the testing is less demanding or if they are only needed for comparing samples.
Rosand single bore rheometers can be upgraded to twin-bore rheometers.
How much sample material do I need for the test?
The sample size is dependent on the barrel size and the shear rate of interest. Higher shear rates (higher volumetric rates) results in the use of more material. A sample size of 10-50g will be needed depending on the barrel size and the shear rates selected. The standard 15mm diameter barrel holds about 50cm 3 of sample.
What barrel size is best for the capillary rheometer?
The capillary rheometers are available with barrels in the range of 9.5 to 24mm in diameter.
Barrel diameter is usually a compromise. The larger barrel has the advantages of higher shear rates for a given die and more material in the barrel means more testing can be done.
The smaller size barrels requires less time for the test sample to reach thermal equilibrium (a critical point for thermally sensitive materials), and less material is needed for the test.
A barrel diameter of 15mm offers a good compromise considering the above mentioned points, especially as modern instruments have a wide dynamic speed range (>200,000:1) and dies are available with a wide range of bore sizes. Thus a wide range of shear rates can be covered with minimal risk of sample degradation during the thermal equilibration process.
What material should the rheometer barrel be made of?
The standard barrel is made of nitrided steel for hardness and toughness. Barrels from other materials may be required where the samples adhere to the steel making cleaning difficult e.g. PES and PEEK, where a sterile environment is required e.g. pharmaceutical and food, or where the sample may corrode the steel or degrade to produce corrosive products. Malvern can provide a chart of recommended barrel material for most commonly encountered polymers. Alternative choices of materials for the barrel are Hastelloy for PES, PEEK, and other corrosive materials, and stainless steel for pharmaceuticals, food products or other aqueous substances.
What is extensional viscosity?
In real processes, flow profiles are likely to contain both shear viscosity and extensional viscosity components. For complex fluids, the material properties in extension can be significantly different to the material properties in shear deformation. The importance of assessing extensional flow properties is increasingly being recognized. For instance, two materials may give processing differences despite showing similar shear viscosity data. Measurement of extensional viscosity can be the key to highlighting the differences between the samples.
In a capillary rheometer, extensional flow is realized in the converging flow region at the die entrance.
How does a capillary rheometer measure extensional viscosity?
When a twin-bore (Bagley corrected) test is performed, the data collected includes all the variables needed for calculating the extensional viscosity using the convergent flow (Cogswell) model. Extra care should be taken in the measurement of the orifice pressure drop, but the results from this method have been found to be in agreement with others and with other direct observations. A major advantage of generating extensional viscosity data by this method compared with alternative techniques is the ability to reach high extensional strain rates that are encountered in real production processes.
Should I get the bench top or the floor model capillary rheometer?
For a routine viscosity experiment, the same results will be obtained with the floor model and the bench top instruments, within experimental error. The choice of purchasing one over the other should be determined by the immediate testing needs and the likelihood of expanding into other testing/application areas.
Floor model capillary rheometers have higher force capacities (for extruding stiffer materials), larger barrels, larger working areas (for customizing speciality test attachments), and more available measurement options compared to bench top models. The greater frame strength of the floor model is also an important consideration in transient tests such as PVT.
What are the measurement options for the capillary rheometer?
There are several accessories available to suit particular applications or enhance the testing capability of the base unit. The main accessories are:
- Nitrogen purge – used to provide an inert, dry environment for testing samples prone to degradation.
- Tragethon Haul-Off – melt strength measurement and production simulation testing for fibre forming and blow moulding.
- Slot die – Has the advantage of not requiring the Bagley correction to obtain the true shear stress and the potential to generate information about melt elasticity.
- Laser die swell measurement – measures the extrudate diameter which is related to elastic memory and elasticity of the melt.
- Melt and die cutters – for enhanced die swell measurements.
Is the capillary rheometer capable of testing low viscosity materials?
The capillary rheometer can be adapted to test low viscosity materials such as paper coatings, inks, polymer solutions and even water (0.001Pas). The best results are generated at high shear rates, upwards of 10,000s -1, due to the low pressure values generated. The rheometer needs to be fitted with low range pressure transducers (down to 250psi) and small diameter dies (less than 0.3mm). The Bagley correction can be made by using different length capillary dies and utilising the automatic Historic Bagley Correction in the software.
Can I get viscoelastic information from the capillary rheometer?
The rotational rheometer is generally better suited for the determination of linear viscoelastic properties. In capillary rheometry, the most obvious elastic effect is post-extrusion swelling, commonly referred to as die swell or extrudate swell. It is interpreted as evidence of recoverable strain, but for most practical purposes, it is the comparative swell ratio that is of importance. The measurement is made using a laser micrometer that is mounted just below the die exit.
Information concerning elasticity of polymer melts may also be obtained by measurements of “hole“ pressure on a slot die and the special stress-relaxation test can provide information about the time-dependent decay of stress on cessation of material flow.
Is the rheometer capable of testing re-grinding material?
Yes, the capillary rheometer is indeed capable of testing regrind materials. In fact, it is usually of the utmost importance that the regrind material be tested to ensure that the material is still thermally stable and has some minimum flow characteristics for the process for which it is selected. The rheometer is also used to optimize the ratio of regrind to virgin material for the best cost/performance model.
Am I able to test PVC or other thermally sensitive samples?
PVC and other thermally sensitive samples can be tested on the capillary rheometer. Actually, in addition to generating viscosity curves, degradation studies are commonly performed on the rheometer. Considering the sensitive nature of the material, it is important to minimize thermal equilibrium time by selecting a small barrel diameter, such as a 12mm bore size.
The Rosand software has a number of test modes for determining thermal stability of materials.
Should I get a “cone and plate” rheometer or a capillary rheometer?
The instruments are complimentary and most well equipped material testing labs will have both. Rotational rheometers easily provide data on viscosity and elasticity in a variety of experiments, give precisely defined flow under low stress or strain conditions and are sensitive to the structural characteristics of the material.
Capillary rheometers on the other hand, provide critical processability data by more closely mimicking standard production processes. Several process related test options also allow for pre-production modeling.
The choice then comes down to the type of information needed and the use to which it is being applied. Rotational rheometers are used more for material development and fundamental studies. Capillary rheometers are ideal for processability testing and ranking materials in terms of their suitability for a process.
Is the rheometer capable of testing rubber?
Yes, the capillary rheometer is quite capable of testing rubber materials. Normally this is done without the curative in the rubber compound but this does not need to be the case. The temperature range of the rheometer is between 40-500 ° C with very precise temperature control, and so meets the requirements of rubber compounders. Due to the very viscous nature of some rubber, a floor standing rheometer (with its high force drive system) is a requirement for such testing.
A stress relaxation test and die (extrudate) swell measurements have been shown to be very important in rubber applications.
I am currently using an extruder with a capillary die for testing polymer samples, what additional information will I get from a capillary rheometer?
The capillary rheometer is designed for accurate and precise temperature and speed control, and is fitted with high accuracy pressure transducers. An extruder, by its very nature, is not a well controlled processor and can only be used to generate crude viscosity data, due to temperature control limitations, inconsistent flow to the die and excessive shear heating. Furthermore, the results can vary with the choice of screw and are subject to the problems of consistently feeding the extruder, which is not a trivial matter.
The dynamic range of an extruder is also not even remotely close to that of a modern capillary rheometer.
The more sensitive routines such as melt fracture detection, stress relaxation or a degradation test, will simply not be possible on an extrusion setup. Additionally, the full range of measurement test options for capillary rheometers are not available for operation with an extruder.
Is there a “viscosity standard” that I can use to confirm that the rheometer is in calibration?
Such a standard is not currently available. NIST has been evaluating a standard for the melt flow indexer (MFI), but to date one is not available for a capillary rheometer. The current recommendation on checking if the system is “in calibration” is that the user selects a stable reference material and periodically checks to see if the values are within acceptable limits. Malvern Instruments can provide a comprehensive instrument calibration and performance verification service – more details are available from your local service center . |
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