High shear rheometer

A similar concept, shown in Figure 5.2.3, has been used for some time as a high shear rheometer for printing inks (Van Wazer et al., 1963, p. 296). As indicated in Figure 5.2.3, a thin, wide film is pulled through a narrow gap filled with fluid. It is assumed that the flow will keep the film centered. The velocity is determined by timing marks on the film. 

High Shear Viscometer Extmsion Rheometer Rheograph 2002 Rheo Tester RTR... 

The shear viscosity, especially as measured with capillary rheometers characterized by high shear rates, is hardly sensitive to material structure since the investigator usually has to deal with the substantially destroyed structure in the molten sample. Melt stretching experiments would normally provide much more information [33]. 

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. 

Capillary rheometers are used extensively to measure viscosity in the intermediate to high shear rate range. The rheometer has for all practical purposes a lower limit in viscosity measurement because of the plunger seals. These seals are shown on the bottom of the plunger in Fig. 3.16, and they induce a frictional resistance when they are pushed through the rheometer barrel. The piston force can be evaluated without polymer in the barrel, but it is always a source of error at low viscosities because of experimental variability. Moreover, barrel friction is one of the critical corrections that must be made when evaluating viscosity measurements... 

M 14. Merz, E. H., and R. E. Colwell A high shear rate capillary rheometer for polymer melts. ASTM Bulletin 232, 63—66 (1958). 

Zahler, G. G., and G. R. Murfitt High shear capillary rheometer. Brit. Plastics 36, 698-701 (1963). 

For smaller particles, smaller stresses are exerted. Thus, in order to predict sedimentation it is necessary to measure the viscosity at very low stresses (or shear rates). These measurements can be carried out using a constant stress rheometer (Carrimed, Bohlin, Rheometrics, Haake or Physica). Usually, a good correlation is obtained between the rate of creaming or sedimentation, v, and the residual viscosity rj 0), as will be described in Chapter 21. Above a certain value of ri(0), v becomes equal to 0. Clearly, in order to minimize sedimentation it is necessary to increase rj 0) an acceptable level for the high shear viscosity must be achieved, depending on the application. In some cases, a high rj[0) may be accompanied by a high rj (which may not be acceptable for apphcation, for example if spontaneous dispersion on dilution is required). If this is the case, the formulation chemist should seek an alternative thickener. 

Table IV shows the viscosity characteristics of these systems under high shear rates. The Severs extrusion rheometer, an efflux type instru-...

Figure 10. High shear rate viscosity characteristics of formulations I, II, and III from Table II (Serves extrusion rheometer)...

Ogorzalek (II) has studied in detail the effect of shear rate on the viscosity of organosols. His data for low shear (Brookfield) and high shear (Extrusion Rheometer) viscosity characteristics using alcohols as diluents are shown in Figures 36 and 37. 

Shear Rate Dependence. Experiments were performed on the In-stron rheometer and covered the shear-rate range 10 to 10 s Data for a 0.75-g dL" solution of polymer D-47 in 5 X 10 mol dm NaCl are shown in Figure 5. A low-shear limiting Newtonian viscosity, 1 (0), was observed. At high shear rates, the usual power-law behavior of polymers was observed with a power-law index of -0.86. This value is close to that predicted theoretically and observed for concentrated polymer solutions and undiluted polymers (13) and indicates an extensive network. 

The fibre-windup technique [Padmanabhan et al., 1996] can provide transient extensional-viscosity data using modified rotational shear rheometers. One end of the sample is clamped while the other end is wound around a drum at a constant rotational speed to achieve a given extension rate, with the rheometer s torque transducers being used to obtain the extensional viscosity. The technique is claimed to provide valuable extensional viscosity data for high viscosity liqmds. 

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