The Rheological Significance of Entrance Pressure Drop in Extensional Viscosity

Extensional/elongational viscosity measurements, also known as melt strength, are of rheological importance in any plastic stretching process.

For example:

• Fiber spinning
• Blown film extrusion
• Blow molding
• Sheet extrusion

Properties of Polymers

This is because the plastic is constantly expanding or stretching in those processes. Two polymers may have identical shear flow behavior, but they could exhibit different extensional flow behavior depending on their branching, molecular structure, etc.

The polymer melt stretches along the streamlines at the entrance to the die in a capillary rheometer, because the diameter of the die against that of the barrel is extremely small. The flow of polymer melt from the barrel of a capillary rheometer to the orifice die is illustrated in the image below.

During transition at the die entrance the polymer melt faces extensional deformation. Capillary dies with larger entrance angle or smaller diameter increases the size of extensional deformation.

Dynisco^® Capillary Rheometer

Using Cogswell`s equations^[1,2] Dynisco® capillary rheometer (LCR) calculates the extensional viscosity of polymer materials at a wide range of extensional rates (specifically the rates corresponding to those in a plastic processing line).

When utilized for this task, the rheometer requires information on the amount of the entrance pressure drop which is a result of the elasticity nature of polymer materials.

The entrance pressure falls and extensional characteristics can be measured by undertaking two shear sweep tests on the rheometer utilizing two different dies possessing the same diameter but of different lengths.

By calculating the figure on top, the extensional viscosity level of the material can be analyzed, plus if it is increasing or decreasing with extensional rate.

References

1. F. N. Cogswell, Trans. Soc. Rheol 16:383 (1972).
2. F. N. Cogswell, Polym. Eng. Sci. 1:46 (1972).

This information has been sourced, reviewed and adapted from materials provided by Dynisco.

For more information on this source, please visit Dynisco.