Polymer science

  

 

Bingham Plastic : A Bingham body

represents the case of an ideal plastic material which supports a finite stress before a strain is initiated.

 Thus, after the material reaches a definite Stress level known as the yield stress, sigma 0, a finite strain rate is generated, the Stress-strain rate relationship is assumed to be linear, as in Newtonian fluids, but only after attainment of the yield stress (sigma 0 ) . Shear stress equation for bingham plastic is given by:

Examples: clay suspensions, drilling mud. toothpaste, mayonnaise, chocolate and

mustard.

Pseudoplastic: 

The behavior of most real plastics is, however, characterized by non- linear relationships which are concave downward as shown by the pseudoplastic behavior. 

Most plastic melts, solutions or dispersions exhibit pseudoplastic flow patterns. At rest, long chain molecules of a plastic melt, solution or dispersion are believed to engage each other into stable associations as a consequence of chain entanglements and due to molar cohesion. At low shear rates, the stress required to overcome or undo the effect of chain entanglements is relatively high, but proportionately lower stress is required to allow the melt, solution or dispersion flow at a higher strain rate once the molecules are set in motion with respect to each other.

 The entanglement effect becomes virtually non- existent and almost Newtonian behavior assumes prominence at high shear rates depending on the exact nature of the fluid. materials. If allowed to stand at rest thereafter, the fluid materials set again. Such isothermal reversible sol-gel are as thixotropy. The equation for Pseudoplastic is given by:

Examples: Nail polish, whipped cream, ketchup, molasses, syrups, paper pulp in water, latex paint, ice, blood, some silicone oils, some silicone coatings, sand in water.

Dilatancy:

Concave upward curve in the shear stress vs. shear rate curve represents the behavior of dilatants substance. Such behavior is exemplified by some highly concentrated suspensions of certain fine powders in appropriate liquids and by some melts in rare cases where crystallization or structure formation occurs at high rate of shearing. Such systems usually resemble a Newtonian fluid at low shear rates, but beyond a specific shear rate, they sharply gain in viscosity which may finally reach a very high value. When set at rest again, they return to the flow behavior of a near-normal viscous liquid. This reversible flow pattern is called dilatancy. 

The equation for Pseudoplastic is given by:

1. • 

Examples: Suspensions of corn starch in water

Pseudoplastic are shear thinning while

dilatants are shear thickening.