## Viscosity

viscosity such as absolute viscosity, inherent viscosity, relative viscosity, apparent viscosity, intrinsic viscosity, specific viscosity, stoke viscosity, and coefficient viscosity. Other terms are reduced viscosity, specific viscosity, melt index, rheometer, Bingham body, capillary viscometer, capillary rheometer, dilatancy, extrusion rheometer, flow properties, kinematic viscosity, laminar flow, thixotropic, viscometer, viscosity coefficient, viscosity number, viscosity ratio, viscous flow, and yield value.

The absolute viscosity is the ratio of shear stress to shear rate. It is the property of internal resistance of a fluid that opposes the relative motion of adjacent layers. Basically it is the tangential force on a unit area of either of two parallel planes at a unit distance apart, when the space between the planes is filled and one of the planes moves with unit velocity in its own plane relative to the other. The Bingham body is a substance that behaves somewhat like a Newtonian fluid in that there is a linear relation between rate of shear and shearing forces, but also has a yield value.

Inherent viscosity refers to a dilute solution viscosity measurement where it is the ratio of the natural logarithm of the relative viscosity (sometimes called viscosity ratio) to the concentration of the plastic in grams per 100 ml of a solvent solution.

Relative viscosity (RV) is the ratio of the absolute solution viscosity (of known concentration) and of the absolute viscosity of the pure solvent at the same temperature. IUPAC uses the term viscosity ratio.

Apparent viscosity is defined as the ratio between shear stress and shear rate over a narrow range for a plastic melt. It is a constant for Newtonian materials but a variable for plastics that are non-Newtonian materials (Chapter 1).

Intrinsic viscosity (IV) data is used in processing plastics. It is the limiting value at an infinite dilution of the ratio of the specific viscosity of the plastic solution to the plastic's concentration in moles per liter; it is a measure of the capability of a plastic in solution to enhance the viscosity of the solution. IV increases with increasing plastic molecular weight that in turn influences processability. An example is the higher IV of injection-grade PET (polyethylene terephthalate) plastic can be extruded blow molded; similar to PETG (PET glycol) plastic that can be easily blow molded but is more expensive than injection molded grade PET and PVC for blow molding.

Specific viscosity is the relative viscosity of a solution of known concentration of the plastic minus one. It is usually determined for a low concentration of plastics such as 0.5g/100 ml of solution or less.

Stoke viscosity is the unit of kinetic viscosity. It is obtained by dividing the melt's absolute viscosity by its density. A centipoise is 0.01 of a stoke.

Coefficient viscosity is the shearing stress necessary to induce a unit velocity gradient in a material. In actual measurement, the viscosity coefficient of a material is obtained from the ratio of shearing stress to shearing rate. This assumes the ratio to be constant and independent of the shearing stress, a condition satisfied only by Newtonian fluids. With non-Newtonian plastics, values obtained are apparent and represent one point in the flow chart.

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