Mixing process extensional flow

LA. Utracki, Mixing in Extensional Flow, 14 Annual Meeting Polymer Processing Society, Yokohama, Japan, June 8-12 (1998)... 

The amount of LC added, processing condition, viscosity ratio of the blend components and the rheological characteristics of the matrix decide the size, shape, and distribution of the LC polymer domains in the matrix. Appropriate shear forces are required to obtain good mixing and extensional flow to deform the dispersed LC domains into fibrillar shape. The orientation of LC polymer microfibrils can be increased by drawing, which in turn, improves the mechanical properties of the blend. Discussion of blends processing will follow with several examples. 

The importance of extensional flow in the mixing process has been pointed out by Gotten (16) and thoroughly analyzed by Nakajima (8) in the case of carbon black-filled compounds. A steady elongational flow can be developed only if the extensional rate increases exponentially (versus time) (17). Nakajima demonstrated that this type of deformation induces an anisotropy of the material, enhanced in highly filled compounds or containing oriented fibers. Therefore, the steady state is nearly impossible and, with polymers, the elongational flow is not a pure deformation and necessarily involves a shear component. 

The behaviour of elastomers in internal mixers reflects consequently a combination of shear and extensional responses. Any model of the mixing process has to take account of both shear flow and transitory extensional flow, since for the latter the steady state cannot be achieved. Therefore the response of the elastomers in the earlier times of an elongational process gives information about the mixing behaviour. Using a high rate extensometer,... 

If shear thinning is the main phenomenon to be described, the simplest model is the general viscous fluid. Section 2.4. It has no time dependence, nor can it predict any normal stresses or extensional thickening (however, recaU eq. 2.4.24). Nevertheless, it should generally be the next step after a Newtonian solution to a complex process flow. The power law. Cross or Carreau-type models are available on all large-scale fluid mechanics computation codes. As discussed in Section 2.7, they accurately predict pressure drops in flow through channels, forces on rollers and blades, and torques on mixing blades. 

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