Vane rheometer

One very important point that must be considered in any rheological measurement is the possibility of slip during the measurements. This is particularly the case with highly concentrated dispersions, whereby the flocculated system may form a plug in the gap of the platens, leaving a thin liquid film at the walls of the concentric cylinder or cone-and-plate geometry. This behaviour is caused by some syneresis of the formulation in the gap of the concentric cylinder or cone and plate. In order to reduce sHp, roughened walls should be used for the platens an alternative method would be to use a vane rheometer. 

An example of a direct method is the vane technique (Collyer and Clegg, 1988). Here a vane rheometer is used (see Figure 4.4), in such a way that the vane attachment is used to monitor the shear response of a rotating fluid bath. 

Figure 4.5. Typical response from a Vane rheometer. Reprinted from Figure 9.10 (Collyer and Clegg, 1988).

The measurement of yield stress at low shear rates may be necessary for highly filled resins. Doraiswamy et al. (1991) developed the modified Cox-Merz rule and a viscosity model for concentrated suspensions and other materials that exhibit yield stresses. Barnes and Camali (1990) measured yield stress in a Carboxymethylcellulose (CMC) solution and a clay suspension via the use of a vane rheometer, which is treated as a cylindrical bob to monitor steady-shear stress as a function of shear rate. The effects of yield stresses on the rheology of filled polymer systems have been discussed in detail by Metzner (1985) and Malkin and Kulichikin (1991). The appearance of yield stresses in filled thermosets has not been studied extensively. A summary of yield-stress measurements is included in Table 4.6. 

Fig. 29. Measurements of yield stress with a vane device and Rheometrics controlled stress rheometer. The torque required to cause yielding is between 1.88...

Barnes, H. A. and Camali, J. O. 1990. The vane-in-cup as a novel rheometer geometry for shear thinning and thixotropic materials. J. Rheol. 34 841-865. 

The advantage of the vane technique is that material is allowed in between the prongs of the vane and in effect shears the material upon itself (rather than inducing shearing between a mechanical fixture and the material, as would be seen if a standard cup-and-bob or Couette rheometer were used). This eliminates the potential for slippage between a mechanical fixture and the material. A typical response is given in Figure 4.5. 

There are several direct methods of measurement of yield stress. The constant stress rheometer is most frequently used to determine value in shear. Dzuy and Boger [1983, 1985] used a rotational vane viscometer. Yield stresses in compression can be calculated from the unrelaxed stress values in parallel plate geometry. Its value in elongation has been directly measured as the critical stress value below which no sample deformation was observed during 30 minutes of straining in an extensional rheometer. 

ABSTRACT A rotational benchtop Rheometer with vane spindles can be used to measure the static yield stress behavior of materials. By running at different rotational speeds, the Rheometer data can be equated with the viscoelastic information determined by an oscillating rheometer. The rotational Rheometer offers a less expensive method suitable for Quality Control needs. 

Results from the food testing show that the Brookfield YR-1 Rheometer may provide correlation with an oscillating rheometer [1]. Therefore, we suggest that a standard benchtop Rheometer with vane spindle may be used for reliable collection of fundamental viscoelastic data of adhesive materials as well. 

FIG. 1—Rheometer with vane spindle for measuring yield stress (Broolfield YR-J). 

The operating principle of the YR-1 Rheometer is to drive a spindle through a calibrated spiral spring connected to a motor drive shaft (see Fig. 3). The vane spindle is immersed in the test materiaJ. The resistance of the material to movement is measured by observing increasing torque values as the YR-1 motor rotates. The amount of shaft rotation is measured by the deflection of the calibrated spiral spring inside the instrument. Spring deflection is measured with a rotary transducer. 

The test procedure involved using a Brookfield HB YR-1 yield rheometer with a V-72 vane spindle at the secondary immersion mark. The materials were tested in the container they came... 

The vane has been used in conjunction with controUed-stress rheometers to determine apparent yield stresses in eohesive elay suspensions [James et al., 1987] a similar technique has been reported for time-independent materials [Yoshimura et al., 1987]. It is important in this type of test that the material attains an equilibrium microstruetural state prior to test. The time, tg, required... 

Fig. 10.5 High-pressure attachment for the rheometer, consisting of a magnetic coupiing, a pressure head, a rotating vane geometry, and a cup. (Reprinted with permission from industriai and Engineering Chemistry Research, Voi. 42(25), N.M.B. Fiichy,...

An alternative procedure, to ensure no external force is applied to the powder bed by the vaned paddle, is to place the compacted sample on a balance and when the paddle is immersed in the powder to raise the vaned head slowly until the balance reading is zero. This dynamic method of bulk powder characterisation is allied to the rheological method for measurement of the viscosity of non-Newtonian fluids and suspensions. Commercial instruments based on the WSL cohesion tester are now available in the form of the FT4 Powder Rheometer (Freeman Technology) and the Stable Micro Systems Powder Flow Analyser (Stable Micro Systems). 

Characterization of the flow properties of aggregated suspensions is difficult due to long relaxation times and the potential for wall slip in a standard rheometer. A variety of techniques have been developed to reduce wall slip (Walls, 2003 Yoshimura, 1987) including roughening the rheometer tool walls and using a vane such that failure is cohesive as opposed to adhesive (i.e., between particles as opposed to between the slurry and the rheometer wall). As a result of these difficulties, measurements of parameters characterizing flow properties may suffer from poor reproducihiUty (BuscaU, 1987). 

This application uses the vane spindle with a controlled stress rheometer. Vane spindles are useful for low shear measurement of very high-viscosity, near-solid materials. In this case, a heavy flooring adhesive served as the sample. 

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