Principle of Viscoelastic Behavior

The ease of deformation called plasticity characterizes the unvulcanized rubber compound, and causes the highly plastic rubber to flow easily. Viscosity is the resistance to plastic deformation or flow and, hence, the inverse of plasticity. Unvulcanized rubbers are not totally viscous but exhibit some elastic behavior. Rubber does not exhibit Newtonian flow as the shear rate is not proportional to the shear stress. Moreover, measurements of flow properties should be made at the shear rate of interest. 

As interestingly shown in Reference [1], to get an accurate idea of how a rubber behaves in the die of an extruder is a difficult problem that has not been solved by existing plasticity tests. Hence, plasticity tests have mostly been used as a check on the uniformity of repeated batches. 

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