Rheology plastic flow

Fig. 8-5 Rheology and flow properties of plastics (solid curve) and water (dotted curve).

PLASTIC FLOW7. A type of rheological behavior in which a given material shows no deformation until the applied stress reaches a critical value called the yield value. Most of the so-called plastics do not exhibit plastic flow. Common putty is an example of a material having plastic flow. 

Vehicles that exhibit the unusual property of Bingham-type plastic rheological flow are characterized by the need to overcome a finite yield stress before flow is initiated. Permanent suspension of most pharmaceutical systems requires yield-stress values of at least 2-5 Pa (20-50 dyn/cm ). Bingham plastic flow is rarely produced by pharmaceutical gums and hydrophilic colloids. National Formulary (NF) carbomers exhibit a sufficiently high yield value at low solution concentration and low viscosity to produce permanent suspensions. The carbomers, however, require a pH value between 6 and 8 for maximum suspension performance. The polymer is essentially incompatible... 

There are three main rheological properties of materials viscous flow, plastic flow, and elastic deformation. The stress deformation behavior of elastic materials is represented by a straight line through the origin. However, in this case, the... 

Flow flo [ME, fr. OEflowan, akin to OHGr flouwen to rise, wash, L pluere to rain, Gk plein to sail, float] (before 12c) v. (1) Resistance to movement by a liquid material and divided rheologically into four categories Newtonian (simple) flow, plastic flow, pseudoplastic flow, and dilatant flow. 

The rheological behavior displayed in Figure 17.7c is compatible with that in Figure 17.6c. It shows plastic flow The stress increases with increasing deformation until a yield stress is reached beyond which the material continues to deform without further increase of the stress (at constant dy/df). 

In compression moulding, the polymer is put between stationary and movable members of a mould. Under heat and pressure the material becomes plastic, flows to fill the mould and becomes homogeneous. Thermal and rheological properties of the polymer determines the necessary pressure and temperature, which typically is 150 °C and 6.9-20.7 MPa. A slight excess of material is placed in the mould to ensure its being completely filled. 

Viscosity, Plastic n Resistance to flow in excess of the yield value in a plastic material. Plastic viscosity is proportional to (Shearing Stress-Yield Value)/Rate of Shear. The coefficient of plastic viscosity is the force in excess of the yield value, tangentially applied, that will induce a unit velocity gradient. See Plastic Flow and Plasticity. (Shenoy, A., V., Thermoplastics Melt Rheology and Processing, Marcel Dekker, 1996)... 

One simple rheological model that is often used to describe the behavior of foams is that of a Bingham plastic. This appHes for flows over length scales sufficiently large that the foam can be reasonably considered as a continuous medium. The Bingham plastic model combines the properties of a yield stress like that of a soHd with the viscous flow of a Hquid. In simple Newtonian fluids, the shear stress T is proportional to the strain rate y, with the constant of proportionaHty being the fluid viscosity. In Bingham plastics, by contrast, the relation between stress and strain rate is r = where is... 

Rheology is the science of the deformation and flow of matter. It is concerned with the response of materials to appHed stress. That response may be irreversible viscous flow, reversible elastic deformation, or a combination of the two. Control of rheology is essential for the manufacture and handling of numerous materials and products, eg, foods, cosmetics, mbber, plastics, paints, inks, and drilling muds. Before control can be achieved, there must be an understanding of rheology and an ability to measure rheological properties. 

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