Polymer rheology rotational rheometers

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 rheological behaviour of molten polymers is of prime importance as it relates to their microstructure and governs their processing characteristics [1]. Rotational rheometers, specifically cone-plate, parallel plate, and sliding plate rheometers are routinely used to characterize the linear viscoelastic properties of polymer melts. Small amplitude oscillatory shear experiments are employed to measure the storage (G ) and loss moduli (G"), which are related to the elastic and viscous character of the material, respectively, and the complex viscosity (77 ) as functions of angular frequency (a). 

The rheological behaviour of the two polymers was determined using classical techniques of rheometry, already described in Chapter II. 1 (rotational and capillary rheometers for shear viscosity and first normal stress difference measurements CogsweU method for the elongational viscosity). 

The cone-and-plate and parallel-plate rheometers are rotational devices used to characterize the viscosity of molten polymers. The type of information obtained from these two types of rheometers is very similar. Both types of rheometers can be used to evaluate the shear rate-viscosity behavior at relatively low vales of shear rate therefore, allowing the experimental determination of the first region of the curve shown in Figure 22.6 and thus the determination of the zero-shear-rate viscosity. The rheological behavior observed in this region of the shear rate-viscosity curve cannot be described by the power-law model. On the other hand, besides describing the polymer viscosity at low shear rates, the cone-and-plate and parallel-plate rheometers are also useful as dynamic rheometers and they can yield more information about the stmcture/flow behavior of liquid polymeric materials, especially molten polymers. 

Molten polymers are viscoelastic materials, and so study of their behaviour can be complex. Polymers are also non-ideal in behaviour, i.e. they do not follow the Newtonian liquid relationship of simple liquids like water, where shear-stress is proportional to shear strain rate. Unlike Newtonian liquids, polymers show viscosity changes with shear rate, mainly in a pseudoplastic manner. As shear rate increases there is a reduction in melt viscosity. This is true of both heat-softened plastics and rubbers. Other time-dependent effects will also arise with polymer compounds to complicate the rheological process behaviour. These may be viscosity reductions due to molecular-mass breakdown or physical effects due to thixotropic behaviour, or viscosity increases due to crosslinking/branching reactions or degradation. Generally these effects will be studied in rotational-type rheometers and the extrusion-type capillary rheometer. 

Figure 3.9 Plots of log rj versus log co (O) and log G versus log co (A) in oscillatory shear flow, and plots of log jj versus log y ( ) and log IVj versus log y (A) in steady-state shear flow for a commercial polystyrene at 200 °C. The data for jj and at low shear rates were obtained using the cone-and-plate fixture of a rotational-type rheometer, the data for jj and at high shear rates were obtained using a continuous-flow capillary rheometer, and the data for r and G were obtained using the paraUel-plate fixture of a rotational-type rheometer. Refer to Chapter 5 for details of the experimental methods employed to obtain the data. (Reprinted from Han, Rheology in Polymer Processing, Chapter 3. Copyright 1976, with permission from Elsevier.)...

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