As you know from fluid dynamics, viscosity is the resistance of a fluid to flow. Elasticity is can be explained as the material’s ability to return its previous shape after a load application on it. As you understand that the Viscoelasticity term derives from these two terms. In this article we will explain the viscoelasticity on thermoplastic polymers.
Viscoelasticity is generally exist in thermoplastic polymers. Consider about a situation that there is a stress is applied on a material that is in the elastic region of this material. We expect that there will not a plastic deformation on this material after the load is removed. The material will return its original shape because of elasticity.
Also this material will show same elasticity when we apply the force in a specific time period. After the load is removed again, material will return to its original shape. But in viscoelastic polymers, there is an exception in this regard. Consider a situation that you apply a load that is lying inside the elastic region of viscoelastic material during a specific time period, when the load is removed, viscoelastic polymer will not return its original shape even load was in the elastic region. Polymer is plastically deformed with a load that is applied during a time but lying in the elastic region. So this behavior of thermoplastic materials called as viscoelasticity.
Viscoelastic behavior increases with the increasing temperature. After the glass transition temperature of the viscoelastic polymer, viscoelasticity will be more prominent. With the increasing temperature, polymer softens more.
Die swell in plastic extrusion is the major example of viscoelastic behavior of thermoplastic polymers. This phomenon occur at the nozzle of the plastic extrusion that extruded plastic’s diameter increases after leaving from the nozzle. Because there was a pressure on the molten plastic inside the die to extrude it.
This is the general and the basic expression of viscoelasticity in materials.
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