Generalized plastic fluid, properties

The removal of residual volatile components from polymers is an operation of some importance in the plastics industry. A generalized, although somewhat idealized, model for continuous, wiped-film devolatilization of viscous polymer melts is presented which relates devolatilization capability to important geometry, < perating, and fluid property variables. The applicability and limitations of the model are analyzed experimentally. The data support many aspects of the theory, but also reveal certain deficiencies in the model which should be considered in designing for maximum efficiency. 

Rubber and plastic melts can be considered, to a first approximation, as extremely high-viscosity fluids. This is only an approximation and it must be remembered that polymers generally show viscoelastic properties—a combination of viscous flow and elastic recovery. Viscosity, in turn, is the quantitative measure of resistance to flow under a given set of circumstances. The Greek letter that usually designates viscosity is Tj. For an ideal, Newtonian fluid, viscosity is simply the ratio between Shear Stress (t), the pressure placed on the fluid to create flow, and the Shear Rate (y), the rate of flow over time as seen in Equation 16C.1 ... 

A wide variety of nonnewtonian fluids are encountered industrially. They may exhibit Bingham-plastic, pseudoplastic, or dilatant behavior and may or may not be thixotropic. For design of equipment to handle or process nonnewtonian fluids, the properties must usually be measured experimentally, since no generahzed relationships exist to pi e-dicl the properties or behavior of the fluids. Details of handling nonnewtonian fluids are described completely by Skelland (Non-Newtonian Flow and Heat Transfer, Wiley, New York, 1967). The generalized shear-stress rate-of-strain relationship for nonnewtonian fluids is given as... 

Based on the magnitude of n and to, the non-Newtonian behavior can be classified as shear thinning, shear thickening, Bingham plastic, pseudoplastic with yield stress, or dilatant with yield stress (see Fig. 2 and Table I). The Herschel-Bulkley model is able to describe the general flow properties of fluid foods within a certain shear range. The discussion on this classiflcation and examples of food materials has been reviewed by Sherman (1970), DeMan (1976), Barbosa-Canovas and Peleg (1983), and Barbosa-Canovas et al. (1993). 

The surface absorption of a fluid (and subsequent plasticization of the polymer) is generally held to be the primary influencing factor as far as the effect of the fluid is concerned. The thermodynamics of the mixing process have been studied extensively in attempts to construct a material property-based predictive model. These attempts have not as yet been successful but some important relationships have been established (Hildebrand, Flory and Huggins etc.). A more general and accurate... 

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