Rheometric Capillary Rheometry
The field of melt rheology is an area of concern not only to the researcher, but also to the producer and user of extrudable materials. Viscosity data help characterize material flow properties during processing operations. Data generated by melt characterization is useful in compound formulation and development as well as quality control assurance.
The rheometric capillary dies, round or slot, are designed to attach to any C.W. Brabender® extrusion system. Round capillary dies are manufactured to specifications outlined in ASTM D 5422, Measurement of Properties of Thermoplastic Materials by Screw Extrusion Capillary Rheometer.
The extrusion setup acts as a screw extrusion capillary rheometer by continuously extruding material through the die to collect pressure and mass throughput data. The data can then be conveniently analyzed using WINEXT Windows® based extrusion software when using our Plasti-Corder® Torque Rheometer, or by manual calculation. The information is used to determine pressure drop, apparent shear stress and shear rate. Flow corrections can then be applied to generate corrected shear stress, shear rate and melt viscosity.
The rheometric capillary extrusion is a simple test procedure when using WINEXT software. Material is fed into a laboratory extruder operating at a constant speed and temperature. The material experiences the work energy and melting process of a typical extrusion operation. Then, the following occurs:
In the round capillary die, material enters the round capillary insert located in the die. A pressure transducer and melt thermocouple monitor and record the pressure drop and temperature of the material before exiting the die. Mass throughput is determined by weighing extrudate over a timed interval. Subsequent measurements are made by varying the extrusion speed to obtain outputs at each pressure value. Substituting capillary inserts of varying L/D ratios meets the requirement for completing the test when using this die.
1.0 mm L/D ratios: 15:1, 20:1, 30:1
2.0 mm L/D ratios: 10:1, 15:1, 20:1
In the fixed dimension slot capillary die, the pressure drop is monitored and recorded between two pressure transducers. Mass throughput is determined by weighing extrudate over a timed interval. Subsequent measurements are made by varying the extrusion speed to obtain outputs at each pressure value.
160 mm x 20 mm x 0.8 mm
160 mm x 20 mm x 2.0 mm
WINEXT software compiles the data and analyzes it using classical flow equations. Poiseuille's equation for pressure drop can be solved for viscosity, where
Viscosity corrections can then be applied to obtain actual viscosity values. In the round capillary die, the Bagley correction, or shear stress correction, is applied to compensate for energy losses at the entrance of the capillary insert. The correction acts as an additional length of capillary. The Rabinowitsch correction, or shear rate correction, can be applied to data from both dies to compensate for non-Newtonian behavior of most polymeric materials.
The shear stress and shear rate calculations differ between the two die types. Calculations are based according to the die shape and dimension. After applying the correction factors, the WINEXT will tabulate the values and graph the information in a variety of formats. If necessary, data can be transferred to spreadsheets for more in depth investigation.