One of the most common types of analytical devices is a capillary rheometer . It is utilized to analyze the flow properties of viscoelastic polymer materials subjected to processing conditions.
The rheometer is made up of:
- A heated barrel (at a specific temperature)
- A capillary die where the heated-softened plastic material is forced to flow by a piston
The piston is traveling down the barrel at different speeds, applying force and pressure to the melted polymer. The speeds of the piston (and the shear rates that result from it) are determined by the user and set at decreasing increments. The force/pressure required to maintain each of the speeds is recorded by means of either a pressure sensor or a load cell.
The LCR7001 can utilize both a pressure transducer and a load cell at the same time in order to generate information on the force applied to the polymer sample and also the pressure at the die entrance as a result. The signals produced, along with piston position is then relayed back to the rheometer`s software.
Pressure Sensor or Load Cell?
This is a common question among users. Simply speaking, a pressure transducer requires more frequent maintenance and cleaning than a load cell. The pressure transducer tip can be damaged by using improper cleaning tools as it is very sensitive. Furthermore, there could be some thermal losses from the pressure probe.
Yet, since the portion of the measured force/pressure is low for these materials, a pressure transducer produces more accurate data for testing materials of low viscosity, plus the frictional force from the piston tip can result in significant error.
The errors from the plastic leakage across the piston can also be avoided by reading the data from a pressure transducer. Additionally, by inserting the pressure transducer just before the capillary die, the barrel pressure drop from piston tip to the pressure sensor can also be avoided.
Usually the barrel pressure drop is assumed to be negligible in comparison with capillary pressure drop. This is due to the huge difference between the die and barrel geometries. Although, for short capillaries with large diameter, this error can be substantial. The rheometer gives more repeatable data, by receiving the data from a pressure transducer. This is confirmed in the below images.
Employing three different dies with different diameters, these flow curves are gathered from various shear sweep tests on low density polyethylene (LDPE) samples. The viscosity data for the first image was taken from the load cell reading and the viscosity data for the second image was obtained from the pressure sensor.
The repeatability of the data when utilizing a pressure sensor is better than a load cell. This is truer for high shear rates, as the barrel has higher pressure because of the longer effective length.
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
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