Shear rate rheological instrumentation

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). 

There are a number of techniques that are used to measure polymer viscosity. For extrusion processes, capillary rheometers and cone and plate rheometers are the most commonly used devices. Both devices allow the rheologist to simultaneously measure the shear rate and the shear stress so that the viscosity may he calculated. These instruments and the analysis of the data are presented in the next sections. Only the minimum necessary mathematical development will he presented. The mathematical derivations are provided in Appendix A3. A more complete development of all pertinent rheological measurement functions for these rheometers are found elsewhere [9]. 

Various methods are used to examine the viscosity characteristics of metallized gels. Two types that have received extensive application are the cone and plate viscometer and the capillary viscometer. Both instruments can measure rheological characteristics at high shear rates, and the former is useful for low shear rate measurements as well. 

A cone and plate rotational type viscometer is used to obtain rheological data in the low-to-medium shear rate range. It gives a constant rate of shear across a gap, and therefore, equations for this instrument are simple when the angle is small (less than 3°). For this reason the cone and plate viscometer has become a standard tool... 

In a wide-gap concentric cylinder instrument, the shear rate at a given location depends on the rheological behavior of the sample. This complicates, but does not prohibit, evaluation of shear rate. In a narrow-gap concentric cylinder instrument (bob radius/cup radius >0.97), the shear rate may be considered constant at the average value in the gap shear rate then depends only on radii and rotational speed, making its evaluation easy (Barnes et al., 1989). 

In both types of axial flow instrument, the shear stress at the duct wall is calculated from the duct dimensions and the pressure drop along the duct length, while shear rate at the same location is calculated from duct dimensions and volumetric flow rate. The wall shear rate depends on the rheological properties of the sample, complicating but not prohibiting its evaluation. 

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. 

Rheological measurements Two instruments were used to investigate the rheology of the suspensions. The first was a Haake Rotovisko model RV2(MSE Scientific Instruments, Crawley, Sussex, England) fitted with an MK50 measuring head. This instrument was used to obtain steady state shear stress-shear rate curves. From these curves information can be obtained on the viscosity as a function of shear rate. The yield value may be obtained by extrapolation of the linear portion of the shear stress-shear rate curve to zero shear rate. The procedure has been described before (3). 

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. 

A rheological instrument such as a viscometer can be used to evaluate t and 7 and hence obtain a value for the shear viscosity, 17. Examples of Newtonian fluids are pure gases, mixtures of gases, pure liquids of low molecular weight, dilute solutions, and dilute emulsions. In some instances, a fluid may be Newtonian at a certain shear-rate range but deviate from Newton s law of viscosity under either very low or very high shear rates (2). 

Parallel Plate Viscometer, This instrument resembles the cone and plate viscometer, except that it has a flat horizontal rotating plate in place of the cone. The shear rate within the narrow gap of the two plates is not as uniform as for the cone and plate viscometer. The limiting shear rates for the parallel plate viscometer are similar to those of the cone and plate instrument. This type of a viscometer is suitable for rheological measurements of suspensions and emulsions. 

Three different rheological measurements may be applied [36-39] (i) steady-state shear stress-shear rate measurements, using a controlled shear rate instrument ... 

The two main rheological properties of a suspension are the yield stress and the viscosity. Yield stress determines when the system becomes a fluid state and when is in a solid state, whereas viscosity determines the ability to flow. In this section, we start with the viscosity measurement. Although one can extract the yield stress from the complete viscosity-shear rate curve, it is helpful to measure the yield stress directly as well. The dynamic and transient measurements are also important for concentrated suspensions. However, because these two types of measurements can be blended into the measurements of the two main rheological properties with some modifications to the measuring instrument, we refer to their measurements only briefly when it is relevant to the discussion. 

Different techniques for the study of shear rheology of interfacial layers have been developed over the years however, they are mostly suited for liquid/gas inter faces. The early instruments were constructed to measure the interfacial shear viscosity under constant shear conditions. In more complex systems, nonlinear effects, shear-rate dependencies of the viscosity, and viscoelastic properties are... 

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