Shear sandwich rheometer

The rheological properties of gum and carbon black compounds of an ethylene-propylene terpolymer elastomer have been investigated at very low shear stresses and shear rates, using a sandwich rheometer [50]. Emphasis was given to measurements of creep and strain recovery at low stresses, at carbon black flller contents ranging between 20 and 50% by volume. The EPDM-carbon black compounds did not exhibit a zero shear rate viscosity, which tended towards in-Anity at zero shear stress or at a finite shear stress (Fig. 13). This was explained... 

Capillary rheometers are the most widely used rheological instruments for polymer melts. They are, however, generally limited to rather high shear rates. Rotational rheometers can provide data at lower shear rates. Cone-plate and parallel disc instruments have been popular with thermoplastic melts. Pressurized instruments, such as biconical or Mooney shearing disc instruments, are used with elastomers to prevent slippage [39]. Sandwich rheometers are used at the lowest shear rates and shear stresses. 

The details of the appearance of yield values differ depending upon how low the shear stress range where stresses are measured. Osanaiye et al. [21 ] measured the true yield values with a sandwich rheometer in the creep mode for rubber-carbon black compoimds. They foimd yield values of order between 560 Pa (EPDM/CB, 80/20 wt%) to 5,500 Pa (EPDM/CB, 50/50 wt%). Similar measurements were made by Li and White [22]. 

Studies of the shear viscosity of talc compounds [ 12,23,25,26] indicate that these compounds behave similarly to carbon black [24] and calcium carbonate [25]. Yield values are induced by volume loadings of 0.15 and higher (Fig. 2.13). Early studies determined yield values by extrapolation. The Osanaiye et al. sandwich rheometer [21], later upgraded by Kim and... 

So-called sandwich rheometers have sometimes been used in the study of rubber elasticity and melt viscoelasticity [141]. In a sandwich rheometer, twin sample plaques are placed in gaps formed by a central steel plate and two outer plates that are part of the same frame. These instruments are difficult to load and clean, and there is no direct control of the gap. Sliding plate melt rheometers were developed to make measurements of nonlinear viscoelastic behavior under conditions under which cone-plate flow is unstable, ie. in large, rapid deformations [ 142 ]. The sample is placed between two rectangular plates, one of which translates relative to the other, generating, in principle, an ideal rectilinear simple shear deformation. Creep tests can be carried out either by use of a feedback loop that generates a plate displacement that gives rise to a constant stress, or by use of a pneumatic drive, as in Laun s sandwich rheometer [143]. 

The oscillatory deep-channel rheometer described by Nagarajan and Wasan (227) can be used to examine the rheological behavior of liquid/liquid interfaces. The method is based on monitoring the motion of tracer particles at an interface contained in a channel formed by two concentric rings, which is subjected to a well-defined flow field. The middle liquid/liquid interface and upper gas/liquid interface are both plane horizon tal layers sandwiched between the adjacent bulk phase. The walls are stationary while the base moves. In the instrument described for dynamic studies of viscoelastic interfaces the base oscillates sinusoidally. This move ment induces shear stresses in the bottom liquid that are transmitted to the interface. The interfaces are viewed from above through a microscope attached to a rotary micrometer stage which is coaxial to the cylinders. 

Perhaps the conceptually simplest type of rheometer can be constructed by sandwiching a material to be tested between two or three parallel plates that are separated by a distance H, and moving one plate parallel to the others at a velocity V (Fig. 12). The shear rate 7 is VIH, For normal liquids this is not practical, but for elastomers and compounds it is very much so. Apparatus of this type have been designed and used by Zakharenko et al, [Zl],... 

In a parallel plate rheometer, shear flow is generated in a layer of fluid (thickness d) sandwiched between parallel discs (Figure 8.11). The fluid is retained in the gap by surface tension. Shear is created by rotating one disc at an angular speed Q, while the other is maintained stationary by applying a torque T. In this case, it does not matter which of the two discs is rotated, and in some designs both discs are rotated in opposite directions. Fluid elasticity creates a normal force F that attempts to separate the plates. T, Q, and F are the experimental measurables in this flow configuration. 

The cone-and-plate rheometer (Fignre 8.12) is a simple variant of the parallel plate device considered in Section 8.5. In a cone-and-plate rheometer, polymer samples are sandwiched between a blnnt cone and a flat plate. Shear is generated by rotating either the cone or the plate at an angnlar speed Q, while the other fixture is maintained stationary. Typically, the cone angle a is maintained small a < 6° to minimize measurement artifacts caused by secondary flow [18]. The main advantage of the cone-and-plate rheometer over the parallel plate device is the homogeneity of the shear field it creates... 

---