Rheometer Design

The Weissenberg Rheogoniometer (49) is a complex dynamic viscometer that can measure elastic behavior as well as viscosity. It was the first rheometer designed to measure both shear and normal stresses and can be used for complete characteri2ation of viscoelastic materials. Its capabiUties include measurement of steady-state rotational shear within a viscosity range of 10 — mPa-s at shear rates of, of normal forces (elastic... 

The Weissenbeig Rheogoniometer (49) is a complex dynamic viscometer that can measure elastic behavior as well as viscosity. It was the first rheometer designed to measure both shear and normal stresses and can be used for complete characterization of viscoelastic materials. Its capabilities include measurement of steady-state rotational shear within a viscosity range of 10-1 —13 mPa-s at shear rates of 10-4 — 104 s-1, of normal forces (elastic effect) exhibited by the material being sheared, and of an oscillatory shear range of 5 x 10-6 to 50 Hz, from which the elastic modulus and dynamic viscosity can be determined. A unique feature is its ability to superimpose oscillation on steady shear to provide dynamic measurements under flow conditions all measurements can be made over a wide range of temperatures (—50 to 400°C). 

Details of solids rheometer design, operation and data analysis can be found in Whorlow (1992), Collyer and Clegg (1998) and Gunasekaran and Ak (2002). 

FIG. 15.23 Extensional rheometer, designed by Miinstedt (1979). A servo control system is used to maintain a specified extensional rate of strain or a specified tensile stress. Courtesy Society of Rheology. For a modern version, see Miinstedt et al., 1998. 

FIG. 15.24 Schematic representation of the four roller extensional rheometer, designed by Meissner (1972) to attain high Hencky strains. Two sets of rotary clamps are individually driven by two motors at constant rotation rates. The force in the sample is measured by a transducer F mounted on a leaf spring. From Barnes, Hutton and Walters (Gen Ref 1993). Courtesy Elsevier Science Publishers. 

The selection of the optimum chemorheological techniques can then be used to influence the choice of rheometer for chemorheological measurements. Final selection of the rheometer design must be determined by the considerations given in the previous section, as well as the following ... 

The Brabender plasti-corder flow test is used with screw injection molding of TS resins to graphically and accurately measure the flow as it relates to IM. Concern about whether a compound would have a sufficient flow life during its residence time in the barrel led to the use of this test. It is basically a torque rheometer designed to measure and record torque (in meter-grams) vs. the mixing time. It produces a flow curve that is a measure of the compound s processability. 

FIGURE 5.19 Schematic geometries of three different rheometer designs. The fluid under investigation in each case is shown in purple (a) spinning disk rheometer (b) cone-and-plate rheometer and (c) capillary rheometer. 

As a result of these difficulties, typical capillary rheometers measure pressure in or above the reservoir as indicated in Figure 6.2.1 or from the forces on a driving piston. (Different capillary rheometer designs are discussed further in Chapter 8.) To determine the true shear stress, a number of corrections must be considered. 

Many rheometer designs have been published. A number have been reviewed by Van Wazer et al. (1963), Whorlow (1980, 1992), Dealy (1982), and Collyer and Clegg (1988). In this chapter we concentrate on describing features that are conunon to cotmner-cially available instruments. Particular commercial instruments are used to illustrate specific designs however we do not attempt to list all the commercial rheometers. Such lists have been compiled by Whorlow (1980,1992) and Dealy (1982). 

Figure 8.2.6 also illustrates a sliding plate shear rheometer design. If a wall shear stress transducer is not used, then the total force on the fixed plate is usually measured with a strain gage load cell. The linear actuators used have time constants in the... 

Section 8.2 described how different rotary rheometers are designed to control and to measure rotation rate, angular position, torque, temperature, and other variables. Equally important is the analysis of these measurements, conversion of the raw millivolts to material functions. Twenty years ago this was all done by hand, but today commercial rheometers spit out materials functions like G and G" in real time. Data analysis software is becoming a more and more important part of rheometer design. We have already seen that the inertia correction algorithms illustrated in Figure 8.2.11 can significantly extend the performance of controlled stress rheometers. 

Such types of analysis and control will probably be the greatest area for innovation in future rheometer design. Fitting of con-... 

As indicated in Section 8.3, perhaps the most rapidly developing area of rheometer design is interactive software control. Significant advances have also been made recently in extensional... 

---