Blending Immiscible Viscous Fluids

Mixing two incompatible polymers together is regarded as a dispersive mixing process [1-3]. The dispersed phase of the blend exists as tiny droplets of a few microns 

The situation of a droplet within a continuous phase has been defined as follows [15]  

Taylor s work with Newtonian liquids showed that elongation of a droplet is favoured by low interfacial tension, larger particle diameter, matrix viscosity and high shear rates. Flumerfelt [17], using non-Newtonian fluids, showed that in a simple shear field, a spherical drop becomes ellipsoidal with the major axis and inclined at about 45° from perpendicular to the shear field. Depending on relative viscosities, a critical shear rate was reached in which the droplet broke up into smaller droplets. A minimum size was eventually reached below which break-up could not be achieved regardless of shear rate. 

As mentioned in Chapter 12, when mixing two polymers together by laminar shear, their viscosities should ideally be similar. Furthermore, mixing small quantities of a high viscosity material into a low viscosity polymer is more difficult than mixing a small quantity of a low viscosity polymer into a high viscosity polymer. 

This also applies to the break-up of droplets. According to Taylor, the breaking up of a single drop requires the viscous forces acting on the droplet to exceed the interfacial forces for a sufficient amount of time. This also indicates that a limit may exist in single screw extruders whereby under certain conditions, laminar shear flow fails to achieve a critical level of stress needed to break up the droplet. 

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Sundaraj, U., Dori, Y., and Macosko, C. W., Sheet formation in immiscible polymer blends model experiments on an initial blend morphology. Polymer 36,1957-1968 (1995). Swanson, P. D., and Ottino, J. M., A comparative computational and experimental study of chaotic mixing of viscous fluids, J. Fluid Mech. 213, 227-249 (1990). 

Static in-line mixers were developed for the intimate mixing of many mixtures, from solutions to heterogeneous mixtures (such as two immiscible liquids or gases in liquids) to viscous fluids, such as peanut butter [19], Commercial in-line mixers are tubes as small as 0.8 in. outside diameter by 6 in. long. Inside the tubes stationary elements are positioned to blend the components of the mixture in primarily a radial manner. By providing a reactor with a high surface-area-to-volume ratio, efficient heat transfer is allowed. Static mixers are used for reactions where good... 

At ambient and processing temperatures, elastomers are viscous fluids with persistent transport phenomenon. In immiscible blends, these lead to change in the size and shape of the elastomer phases and migration of the fillers, plasticizers, and curatives from one phase to another. These changes are accelerated by processing and plasticization but retarded by the ultimate vulcanization. Retention of the favorable properties of a metastable blend, which is often attained only at a select interphase morphology and filler/plasticizer distribution, thus requires careful control of both the processing and the vulcanization procedures. 

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