Die extrusion behavior

DIE EXTRUSION BEHAVIOR OF CARBON BLACK-FILLED BLOCK COPOLYMER THERMOPLASTIC ELASTOMERS... 

We now briefly describe some common viscometric methods to characterize the shear flow in capillary viscometers used frequently for observing the die extrusion behavior of most polymer systems. Detailed descriptions of these techniques and the other theoreticals may be found in some well-organized textbooks and monographs [32-38,41 5]. 

The paper written by the authors [44] was thought to be the only one reported and published for the die extrusion behavior of carbon black-filled black copolymer... 

The main objective of this chapter was to cover the die extrusion behavior of block copolymer thermoplastic elastomers filled with carbon black. Thus, shear... 

Schreiber and co-workers have noted very persistent history effects in linear polyethylenes (69). Fractions which have been crystallized from dilute solution required times of the order of hours in the melt state at 190° C in order to attain a constant die swell behavior upon subsequent extrusion. The viscosity on the other hand reached its ultimate value almost immediately. The authors concluded from this result that different types of molecular interactions were responsible for elastic and viscous response. However, other less specific explanations might also suffice, since apparent viscosity might be relatively intensitive to the presence of incompletely healed domain surfaces, while die swell, requiring a coordinated motion of the entire extrudate, might be affected by planes of weakness. It would... 

The power-law index plays an important role in melt flow. It is obvious that the high flow rate results in the increase of the die pressure. However, for a Newtonian fluid (such as water, n = 1), a 10 X increase in pressure is accompanied by a 10 X increase in flow rate. For a non-Newtonian fluid with n = 1/2, a 10 X increase in pressure is accompanied by a 100 X increase in flow rate. For n = 1/3, a 10 X increase in pressure results is accompanied by a 1000 X increase in flow rate. For n = 1/4, a 10 X increase in pressure is accompanied by a 10,000 increase in flow rate. For n = 1/5, as in the case of a bad regrind above, a lOX increase in pressure results in a 100,000 increase in flow rate This is essentially applicable for any extrusion die. Hence, the power-law index of a hot melt essentially determines its extrusion behavior. 

Liang, J. Z. (2008) Effects of extrusion conditions on die-swell behavior of polypropylene/diatomite composite melts. Polym. Test, 17, 936-940. 

Dekker [3] studied the effect of various flight geometries on solids conveying performance. He proposed that many extrusion instabilities might be due to internal deformation of the solid bed. Internal deformation is more likely to occur when the internal coefficient of friction of the polymer particles is low. Spherical particles tend to have a lower internal coefficient of friction than non-spherical (e.g. cylindrical) particles and are, therefore, more susceptible to internal solid bed deformation. This may explain the often observed difference in extrusion behavior between strand pelletized and die-face pelletized material. 

Super-extrusion is a technique whereby the shear stress in the die land is above the critical shear rate for melt fracture see Section 7.5.3.2. This is possible with polymers that exhibit a second stable region above the melt fracture region. Linear polymers such as HDPE, EEP, and PEA exhibit super-extrusion behavior. The melt fracture behavior can be determined on a capillary rheometer by running a polymer melt at different shear rates and observing the corresponding condition of the extru-date. A typical flow curve for a linear polymer is shown in Eig. 11.43. 

Melt Viscosity. The study of the viscosity of poljmier melts (47-62) is important for the manufacturer who must supply suitable materials and for the fabrication engineer who must select polymers and fabrication methods. Thus, melt viscosity as a fimction of temperature, pressure, rate of flow, and poljmier molecular weight and structure is of considerable practical importance. Polymer melts exhibit elastic as well as viscous properties. This is evident in the swell of the polymer melt upon emergence from an extrusion die, a behavior that results from the recovery of stored elastic energy plus normal stress effects. Theoretical developments include a constitutive equation that correctly captures nonlinear behavior in both elongation and shear (63,64). 

Li et al. [103] studied the die swell behaviors of PS, EPDM, and PS/EPDM (80/20) by using a special ultrasonic oscillation extrusion system developed in their laboratory. These authors investigated the effects of ultrasonic intensity and screw speed on die pressure, volume flow rate and apparent viscosity of polymers, as well as die swell. During extrusion, the apparent viscosities of the blends depended not... 

There is a continuing interest to improve and extend the fimctional properties range of dairy proteins to provide both health benefits and their characteristic physical behaviors under different temperature, moisture, and pH conditions so that they may be included in foods that ordinarily do not contain them. One such research area is the extrusion texturization of whey proteins, which have resulted in dairy proteins with new characteristics imparted by a controlled texturization process, depending on the application desired (Hale et al., 2002 Manoi and Rizvi, 2008 Onwulata, 2009 Onwulata et al., 1998). Protein texturization is a two-step process that involves, first, the unfolding of the globular structure (denaturation) and, second, the alignments of the partially unfolded structures in the direction of mass flow in the extruder. The surface characteristics are imparted at the extruder die as the molten mass exits (Onwulata et al., 2003a). 

Chemical considerations lead to two effects that influence the stability of reactive extrusion - the gel effect and the ceiling temperature. Gel effects increase the conversion by an autocatalytic behavior. If the gel effect occurs completely in the screws it stabilizes the process, but if it occurs near or in the die it may have a destabilizing effect. The occurrence of a ceiling temperature slows down the reaction and so has a direct, negative influence on hydrodynamic stability. 

Polymers can be confined one-dimensionally by an impenetrable surface besides the more familiar confinements of higher dimensions. Introduction of a planar surface to a bulk polymer breaks the translational symmetry and produces a pol-ymer/wall interface. Interfacial chain behavior of polymer solutions has been extensively studied both experimentally and theoretically [1-6]. In contrast, polymer melt/solid interfaces are one of the least understood subjects in polymer science. Many recent interfacial studies have begun to investigate effects of surface confinement on chain mobility and glass transition [7], Melt adsorption on and desorption off a solid surface pertain to dispersion and preparation of filled polymers containing a great deal of particle/matrix interfaces [8], The state of chain adsorption also determine the hydrodynamic boundary condition (HBC) at the interface between an extruded melt and wall of an extrusion die, where the HBC can directly influence the flow behavior in polymer processing. 

The non-Newtonian behavior of plastic melt makes its flow through a die complicated but controllable within certain limits (Chapter 3). Simplified flow equations are available to account for the non-Newtonian melt behavior. They provide an excellent foundation using an empirical approach that pertains to extrusion die channels of different shapes. 

In the extrusion die, the flow is driven simply by the pressure drop, as shown in Figure 13.23 and Figure 13.24. This type of flow can be analyzed relatively easUy, even for complicated rheological behavior. We will look at two geometries in detail flow between two flat plates and flow in the annular space between two cylinders. Both these geometries are used commonly to make floor tiles, cylindrical pipe pieces, and by cutting in half, semicylindrical roof tiles. 

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