Geometry, rheological instrumentation

The application of the theoretical treatment depends on the ability to measure the extrusion force and rate. Most commercial extruders do not allow for these types of measurement. Normal rheological equipment, such as cup-and-bob or cone-and-plate, do not have a suitable geometry or instrumentation to handle materials of the consistency normally used. A ram extruder is a suitable experimental design. 

The vane viscometer is yet another form of the concentric cylinder instrument, in which the bob is replaced by a rotor with four blades or vanes each attached by one edge to a vertical shaft, at 90° intervals around the shaft (Figure 22.7). This geometry, which can be used either with a cup or in the infinite sample mode, is particularly useful for measuring yield stress, and can also be used to measure the rheological properties of non-Newtonian liquids. Its advantages are described by Gunasekaran and Ak (2002). 

Viscometers of relatively complex geometry, for example the Ostwald glass U-tube viscometer, can be used to measure the viscosity of Newtonian liquids, which is independent of shear rate and time, after calibration with a Newtonian liquid of known viscosity. Such instruments cannot be used for Theologically characterizing non-Newtonian liquids, and therefore cannot be classed as rheometers, as geometrical complexity prevents evaluation of shear stress and shear rate at a given location independently of sample rheological behavior. 

A sphere vibrating at a specific frequency can be used to obtain magnitudes of G and G" at a specific frequency. Such an instrument was used to follow sol-gel transition of 5,7, and 10% starch dispersions (Hansen et ah, 1990). However, such instruments seem to have a limited range of oscillation frequencies (e.g., 676-680 Hz). In addition, the reliability of the data obtained in comparison to data from dynamic rheological tests in which cone-plate, parallel plate, and concentric cylinder geometries have been used needs to be established. 

In the last decade of the nineteenth century, Maurice Couette invented the concentric cylinder viscometer. This instrument was probably the first rotating device used to measure viscosities. Besides the coaxial cylinders (Couette geometry), other rotating viscometers with cone-plate and plate-plate geometries are used. Most of the viscometers used nowadays to determine apparent viscosities and other important rheological functions as a function of the shear rate are rotating devices. 

The flow curves can be established for different concentrations and different molar masses of HA samples, and at different temperatures for a better insight into the molecular properties of polymers. Fig. (14) shows results of a series of rheological tests of HA polymers with different molar masses at different concentrations. Fig. (14, left panel) shows the flow curves for three different HA samples with the Mw values of 850 kDa, 600 kDa, and 400 kDa. Fig. (14, right panel) exhibits the flow curves for an HA sample at four different concentrations ranging from 0.11% to 2.16%. The flow curves are obtained by using an AR 2000 stress-controlled rheometer from TA Instruments (New Castle, DE, USA). A cone/plate geometry is used. The rotor was made of the acrylic material, 4 cm of diameter and 1° of cone angle. The measurements were performed at 20 °C. 

Dynamic mechanical properties are measured to evaluate melt rheology of thermoplastics with and without additives which may modify rheological characteristics of these compositions. " Dynamic oscillatory shear rheometers are used for these purposes. Two geometries of test fixtures are used including parallel plates and cone and plate. Instrument use for these measurements must be capable of measuring forces (stress or strain) and frequency. Temperature must be controlled in a broad range and various modes of temperature sweeps should be available. Sample geometry is not specified but it should be suitable for measurement in particular experimental setup. 

Rheological measurements were made with rectangular gel samples in a torsion geometry. The gel samples had dimensions of approximately 12 x 4.5 x 28 mm. The measurements were made on a Rheometric Scientific ARES instrument at a frequency of 1 Hz and a scan rate of 2 °C/min. An environmentally controlled chamber permitted determination of the modulus over temperatures from -100 to 70 C. Strain sweeps were conducted at various temperatures to ensure that the modulus was independent of strain. 

The melt rheological analysis probed the two polyacetal resin samples for differences in their melt viscosities as a function of shear rate. The instrument used was a TA Instruments AR-1000 controlled stress rheometer with a parallel plate geometry (Figure 15-1-1). 

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