Which rheometer

It is very valuable to compare different rheometers in your own laboratory. Of course comparisons are helpful in checking instrument calibration or finding such errors as a mislabled cone angle. Juxtaposition of rheometers can often identify more insidious errors, such as secondary flows, slip, or evaporation. Comparisons are also very helpful in selecting which rheometer is most useful for a particular material, deformation, or temperature range. Comparisons give confidence to operators that they are really seeing material behavior and not some instrument artifact. 

Piston Cylinder (Extrusion). Pressure-driven piston cylinder capillary viscometers, ie, extmsion rheometers (Fig. 25), are used primarily to measure the melt viscosity of polymers and other viscous materials (21,47,49,50). A reservoir is connected to a capillary tube, and molten polymer or another material is extmded through the capillary by means of a piston to which a constant force is appHed. Viscosity can be determined from the volumetric flow rate and the pressure drop along the capillary. The basic method and test conditions for a number of thermoplastics are described in ASTM D1238. Melt viscoelasticity can influence the results (160). 

Bohlin also markets a computer-driven controlled stress rheometer, which determines viscosity as a function of stress and measures creep and... 

In another elongational rheometer (221), the specimen is suspended on the end of a flexible tape, which is wound onto a wheel turned by a servo-controUed torque motor. This design is the basis for the Gottfert Rheostrain instmment, with which strain rates of 5 x 10 to 2 ate possible. 

Chocolate does not behave as a tme Hquid owing to the presence of cocoa particles and the viscosity control of chocolate is quite compHcated. This non-Newtonian behavior has been described (28). When the square root of the rate of shear is plotted against the square root of shear stress for chocolate, a straight line is produced. With this Casson relationship method (29) two values are obtained, Casson viscosity and Casson yield value, which describe the flow of chocolate. The chocolate industry was slow in adopting the Casson relationship but this method now prevails over the simpler MacMichael viscometer. Instmments such as the Carri-Med Rheometer and the Brookfield and Haake Viscometers are now replacing the MacMichael. 

Bingham-plastic slurries require a shear stress diagram showing shear rate vs. shear stress for the slurry in order to determine the coefficient of rigidity, T], which is the slope of the plot at a particular concentration. This is laboratory data requiring a rheometer. These are usually fine solids at high concentrations. 

A capillary rheometer is another type of instmment, in which the uncured mbber is extmded through a small orifice and the change in dimensions of the extmdate is measured with a laser [2]. This instmment generates high shear rates, compared to Mooney rheometer. The capillary rheometer can thus represent flow of compounds on mbber processing machinery, such as injection molds. 

When compared to standard (open cavity) cone-plate or parallel disks rheometers, closed cavity torsional rheometers such as the RPA or the PPA have unique high-strain capabilities, which prompted us to modify the instmment in order to investigate the promises of FT rheometry, as outlined a few years ago by the pioneering works of Wilhelm. The technique consists of capturing strain and torque signals and in using FT calculation algorithms to resolve it into their harmonic components, as detailed below. 

Another typical example of inhomogeneity in rheometry is the oxidation of a polymer in a rotational rheometer in which a disk-shaped sample is held between metal fixtures. The oxygen enters the sample through the free surface (at the outer diameter) and diffuses radially inwards. The result is a radial gradient in properties which changes with time. If the reaction with oxygen results in... 

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