6 - Rheology of Molten Polymers

Abstract

Rheology is a branch of engineering science, which deals with the relations between the stresses applied on materials and the resulting strains (deformations) and strain rates. Stress is force divided by area and strain rate is deformation per unit time. Polymer melts are fluids, which means they flow under the application of a tangential (shear stress). Viscosity (resistance to an applied shear stress) is the most important rheological property. Polymer melts are shear thinning materials, which means that their viscosity decreases as the shear rate increases. Polymer melts also exhibit elastic behavior and they are usually referred to as viscoelastic. This dual behavior is responsible for certain phenomena, such as stress relaxation after cessation of flow and extrudate swell. Extensional viscosity (resistance to stretching) is the second most important rheological property. Wall slip, storage modulus and normal stress differences are other important rheological properties. Accurate measurements of rheological properties are necessary for equipment/process design, troubleshooting and optimization purposes. Capillary viscometers and rotational rheometers are used for measurement of the rheological properties. Knowing the capabilities and limitations of various instruments is necessary for obtaining insights into the flow behavior of polymer melts for problem solving purposes. This chapter introduces these concepts and provides a valuable understanding necessary to deal with the science and technology of multilayer flexible packaging.