Shear, effects rheometer

Capillary rheometer ASTM D3835-90 Measure shear effects on viscosity at high rates Given sample conditioning (ASTM D618), given pressures or flow rates, determine viscosity via various corrections Determine dwell time that does not influence results (due to curing)... 

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. 

Therefore, it is more suitable to measure the shear sensitivity of polypropylene. Capillary rheometers or oscillatory shear flow rheometers are widely used for that purpose. Moreover, an investigation of elongational flow properties of molten PP can be used to check, for example, the presence of long-chain branching in some speciality grades of PP (to study the strain-hardening effect). 

The model system used by Mabille et al. [149, 150] was a set of monodisperse dilute (2.5 wt% of dispersed oil) emulsions of identical composition, whose mean size ranged from 4 p.m to 11 p.m. A sudden shear of 500 s was applied by means of a strain-controlled rheometer for durations ranging from 1 to 1500 s. All the resulting emulsions were also monodisperse. At such low oil droplet fraction, the emulsion viscosity was mainly determined by that of the continuous phase (it was checked that the droplet size had no effect on the emulsion viscosity). The viscosity ratio p = t]a/t]c = 0.4 and the interfacial tension yi t = 6 mN/m remained constant. 

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). 

Melt fracture occurs when the rate of shear exceeds a critical value for the melt concerned at a particular temperature (that is, the critical shear rate ). There is a corresponding critical shear stress and the relevant point on the flow curve (or the shear rate-shear stress diagram) is known as the critical point. It is believed that it is reached in the die entry region (that is, where material is being funnelled from the die reservoir into the capillary of a capillary rheometer)—which, in an extruder, corresponds with the point at which melt moves into the die parallel portion of the die. Some further complicating effects may occur at the wall of the die. 

Capillary rheometers measure the effect of pressure on volumetric flow through a cylindrical capillary. They are popular in practical work because shear rate and flow geometry are similar to conditions in extrusion and injection molding. They cover a wide range of shear... 

The effect of the type of impact modifier on the melt flow of a PVC window profile formulation as a function of shear rates encountered during extrusion was investigated and the relationship between the melt flow and mechanical properties of the profiles evaluated. A Rheoplast Capillary Rheometer with a pre-shearing device was employed to investigate the melt viscoelastic properties of the formulations and the performance of the formulations in terms of post-extrusion shrinkage, surface gloss and enthalpy relaxation discussed. 2 refs. 

Influence of addition of free polymer on the rheology of the suspensions Fig. 3 shows the effect of addition of PEO (Mjj=20,000, 35,000 and 90,000 respectively) on the extrapolated yield value obtained from the shear stress-shear rate curves using the Haake Rotovisko. On the other hand. Figure 4 shows the results obtained using the Deer rheometer in which Tg was directly obtained. Although the trend obtained is the same, the yield values obtained using the Deer rheometer are significantly... 

After shearing of the heated mixed WPI/CWM dispersions during the up ramp in the rheometer, the apparent viscosity at 300 s increased with starch mass fraction. That increase reflected the dominant effect of the still intact CWM granules in the structure disrupted during the shearing of the blends (Figure 4-41). 

Other errors, which could influence the results obtained, are, for example, wall effects ( slipping ), the dissipation of heat, and the increase in temperature due to shear. In a tube, the viscosity of a flowing medium is less near the tube walls compared to the center. This is due to the occurrence of shear stress and wall friction and has to be minimized by the correct choice of the tube diameter. In most cases, an increase in tube diameter reduces the influence of wall slip on the flow rate measured, but for Newtonian materials of low viscosity, a large tube diameter could be the cause of turbulent flow. ° When investigating suspensions with tube viscometers, constrictions can lead to inhomogeneous particle distributions and blockage. Due to the influence of temperature on viscosity (see Section Influence Factors on the Viscosity ), heat dissipated must be removed instantaneously, and temperature increase due to shear must be prevented under all circumstances. This is mainly a constructional problem of rheometers. Technically, the problem is easier to control in tube rheometers than in rotating instruments, in particular, the concentric cylinder viscometers. ... 

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