Torque rheometers with laboratory mixers and extruders are down-scaled production devices. They enable the replication of production processes in a laboratory setting. This type of measurement allows for the testing and comparison of materials in production conditions.
The Thermo Scientific™ HAAKE™ PolyLab™ OS System is a modular torque rheometer (see Figure 1). A CANopen Bus facilitates communication between measuring devices such as the torque sensor, mass temperature sensors, and mass pressure sensors, as well as the driving unit and computer.
Figure 1. HAAKE PolyLab System includes the HAAKE RheoDrive, mixer and extruder sensors. Image Credit: Thermo Fisher Scientific – Materials Characterization
Laboratory Mixers
Laboratory mixers are made up of a liquid or electrically heated measuring chamber, mixer rotors, and a feeding mechanism. For a mixer test, the chamber is heated to the desired test temperature. The rotors are set at a predetermined constant speed (Figure 2).
A feeding piston presses an exact quantity of test material into the empty mixer chamber. Throughout the mixing process, the required torque and sample temperature are measured and recorded.
Figure 2. Laboratory mixer—measuring principle. Image Credit: Thermo Fisher Scientific – Materials Characterization
A mixer rheogram plots the measured values versus mixing time. The mixer rheogram depicts the sample's melting, viscosity, cross-linking, and degradation behavior (Figure 3).
The graph depicts the melting behavior of a PVC dry blend. At the start of the test, the PVC powder is loaded into the mixer, causing an instantaneous torque rise.
The powder is then distributed in the mixer chamber, and some of the compound's components (such as waxes) melt due to the high mixer temperature. Both actions cause a decrease in torque.
Figure 3. Laboratory mixer rheogram. Image Credit: Thermo Fisher Scientific – Materials Characterization
Due to the increased mass temperature and shear energy, the PVC begins to mix into larger agglomerates. This increases viscosity, which in turn increases torque. This technique produces a second torque maximum. The PVC dry blend creates a homogenous melt.
Following the second torque maximum, the torque decreases until it reaches a constant value. The temperature increase induced by dissipation is balanced against the temperature drop caused by heat conduction through the chamber wall. The torque being adjusted here represents the sample's melt viscosity in relative terms.
By comparing different mixer rheograms, the effects of production and quality differences, recipe adjustments, and additive effects on a product can be investigated. The following example shows how changing stabilizer amounts affect the rheological behavior of a PVC dry mix (Figure 4).
Figure 4. Laboratory mixer rheogram showing the effect of different stabilizer proportions. Image Credit: Thermo Fisher Scientific – Materials Characterization
The comparison of the two curves reveals that the compound with the higher stabilizer concentration takes longer to melt than the compound with the lower stabilizer proportion. This is due to the stabilizer's lubricant-like properties. As a result, the sample with a larger stabilizer content can absorb less shear energy.
After both samples have completely melted, the two curves will run concurrently for an extended length of time. This means that the different stabilizers do not influence melt viscosity. Toward the end of the test, the sample with the lowest stabilizer proportion exhibits an earlier increase in torque.
This rise is caused by the cross-linkage reaction of the PVC sample, which is produced by PVC degradation. The sample with the higher stabilizer fraction exhibits this torque rise later, indicating that it is more stable.
Laboratory Extruder
PVC samples can also be tested in a laboratory setting using extruders.
A laboratory extruder can be outfitted with pressure sensors along its barrel. Pressure profiles along the extruder can be compared to identify whether a sample melts early (high-pressure profile) or late (low-pressure profile) (Figure 5).
A key advantage of a measuring extruder is its ability to be equipped with profile dies and additional components, such as tube and tape take-off units. Thus, production can be replicated on a small scale. The extrudate created can be used for further testing, such as mechanical, optical, or weathering tests.
Figure 5. Pressure built-up in a single screw extruder. Image Credit: Thermo Fisher Scientific – Materials Characterization
Extruder Capillary Rheology
When the extruder is equipped with rheological measurement dies such as a slit or rod-capillary dies, as well as a balance, the sample's absolute viscosity can be calculated. These measuring dies contain a precise geometrically defined flow channel (rod or slit geometry) where the pressure drop is measured (Figure 6).
The shear stress can be computed using this pressure drop and the flow channel geometry (Figure 7).
Figure 6. Capillary dies. Image Credit: Thermo Fisher Scientific – Materials Characterization
Figure 7. Rod capillary principle. Image Credit: Thermo Fisher Scientific – Materials Characterization
The output is measured using a computer-connected balance. The shear rate is computed from the output data, and the shear rate and shear stress are then used to compute the melt's viscosity (Figure 8).
Figure 8. Rod capillary die calculations. Image Credit: Thermo Fisher Scientific – Materials Characterization
Different shear rates can be achieved by gradually increasing the extruder speed. The generated viscosity curve depicts the flow behavior of the material under various flow conditions (Figure 9).
Figure 9. Capillary test measurement results. Image Credit: Thermo Fisher Scientific – Materials Characterization
The PolySoft evaluation software can be used to mathematically characterize the curve's path. Rheological model factors, such as Ostwald or Carreau, can be computed using regression analysis (Figure 10).
Knowledge of such data is useful, for example, for modeling flow channels and molds. Simulation software solutions such as Moldflow® or Cadmould employ Carreau model regression data to do computations.
Figure 10. PolySoft Software capillary rheometry regression analysis. Image Credit: Thermo Fisher Scientific – Materials Characterization
Summary
The HAAKE PolyLab OS Torque Rheometer System is sensitive and precise equipment for determining the processing behavior of PVC dry blends. PVC melt flow behavior can be investigated under production settings using laboratory extruders and rheological measurement dies.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials Characterization.
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