Abstract
Rheological properties related to the extrusion of polyolefins are the shear viscosity, the elongational viscosity, the slip velocity and their temperature- and pressure-dependencies. These properties are measured in the rheology lab mainly via a parallel-plate rheometer and a capillary rheometer. Then appropriate rheological models have to be used to account for all these properties. Such models are either viscous (e.g., the Cross model) or viscoelastic (e.g., the K-BKZ model). The latter gives the best fitting of the experimental data and offers excellent results in numerical simulations, especially in extrusion flows. Wall slip effects are also found and measured by rheometric flows. Modeling of extrusion flows should make use of appropriate slip models that take into effect the various slip parameters, including the effects of shear stress, molecular characteristics, temperature and pressure on the slip velocity. In this paper the importance of these properties in extrusion are discussed.
Highlights
Extrusion lies in the heart of polymer processing
Typical values reported for the activation energy of various polymers can range from as low as ≈ 22, 800 J/mol for high-density polyethylene (HDPE) to ≈ 83, 000 J/mol for HDPE to for low-density polyethylene (LDPE) to ≈ 116, 500 J/mol of polystyrene [19]
Capillary rheometry is extensively used in both industry and academia to assess the rheological behaviour of polymer melts at high shear rates as well before testing their processability in full industrial scale [1,3]
Summary
Extrusion lies in the heart of polymer processing. Essentially most polymer processing operations need an extruder to melt, mix and process polymers and their compounds [1,2]. In this article some important rheological properties related to extrusion are discussed These include the following: (i) the entry pressure in capillary extrusion important to capture the extensional behavior of polymer melts [5,6,7,8,9,10]; (ii) the effects of temperature and pressure on the rheological properties [11,12]; and (iii) the slip behaviour of polymers at solid boundaries [13,14,15,16,17,18]. (ii) to determine the entry pressure drop from the reservoir to the capillary die and use of this to evaluate the suitability of a rheological constitutive equation to predict it (Section 3.2) as well as to capture the important effects of extensional viscosity (Section 3.3). The possibilities of polymer melt wall slip are discussed and a comprehensive slip velocity equation is presented (Section 4) as an appropriate boundary condition for high-shear flows
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