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High-Intensity Melt Mixing Technologies
There are a variety of melt mixing options available for combining fillers into thermoplastics, the more important of which are summarized in Table 1. Since their flexible design and operation also makes them suitable for a wide range of alternative compounding situations, which may exclude the use of fillers, only an overview of their construction is presented below. A range of texts providing more detailed information is given at the end of this entry.
Due to their effective mixing action, through the generation of the high shear stresses necessary to break down agglomerates, two-roll mills are widely used in laboratories and, to a limited extent, production environments, for combining additives into plastics and more especially rubbers. They comprise two counterrotating temperature-controlled rolls which cause the polymer to melt (in the case of thermoplastics), adhere to, and rotate with one of the rolls. As it passes through the narrow clearance between the rolls, material is subjected to intense shear, which facilitates dispersive mixing. The rolls may run at the same or differential speeds, thereby further influencing the local shear intensity. With this machine design, lateral cross-mixing must also be applied, generally by manual intervention, to impose a degree of convective mixing, ensuring compositional uniformity. A rotating bank of material located at the entrance of the nip gap between the rolls further augments the overall mixing effectiveness. Process variables include the adjustable nip gap between the rolls, the relative rotational speed (and hence friction ratio) of the rolls, and the mixing time.
Since two-roll mills have limitations in terms of scale and generally the need for significant operator involvement, internal mixers provide an alternative approach to mixing and are available in both batch and continuous designs. Analysis has shown that both shear and elongational melt flows occur, creating a unique and highly effective mixing action.
The polymer and filler are introduced into a temperature-controlled mixing chamber in the shape of a figure of eight containing two counterrotating specially configured rotors. A vertically mounted hydraulically driven ram is lowered onto the material, creating pressure and intensifying mixing. The rotor designs and their degree of intermesh critically affect the mixing action. This involves generation of high shear stress between the rotors and through the clearances between the rotors and walls of the mixing chamber, combined with convective longitudinal and lateral transfer of material throughout the mixing chamber. The other principal operational variables include the rotor speed, batch temperature, mixing time, ram pressure, and amount of material present in the mixing chamber (fill factor). With some filled compositions, the order and amount of component addition (simultaneous or sequential) can also influence the shear intensity and quality of mixing. Once mixing is completed after a predetermined time, material is discharged through an opening underneath the mixing chamber onto a two-roll mill or extruder for subsequent pelletization. Continuous variants are also available which contain mixing rotors similar in design to batch elements; however, materials to be mixed are continuously fed into the machine using counterrotating twin screws and then, after the mixing stage, are discharged through an exit opening, often into a single-screw extruder to allow for further homogenization, volatile venting, and die forming into strands or strip for pelletization. Some continuous internal mixer designs allow for direct addition of fillers or other additives into molten polymer.
Extrusion compounding is a well-established and often preferred method for combining fillers with thermoplastics, since the process is continuous and, with most machinery designs, offers considerable flexibility in terms of screw and barrel configuration to meet the requirements demanded from differing material types. There is a wide range of variants available based on single- and, in particular, twin-screw designs. Standard single-screw extruders have limited inherent mixing capability, developing only low levels of shear strain and stress. Their drag flow conveying action is also unsuitable for processing thermoplastics containing high filler levels. Nevertheless, these drawbacks can be partially overcome using modified screw designs, which impart a greater degree of distributive and, to some extent, dispersive mixing capability suitable for simple blending situations. Special grooved barrel designs can also aid conveyance of feedstock in the feed zone of the extruder. Static mixers, positioned between the end of the extruder and die, can be used to aid mixture homogeneity, by repeated passage of molten polymer through a tube containing motionless profiled elements. However, these designs generally only redistribute material and, being without a dispersive capability, are generally unsuitable for highly filled compositions.
For most purposes, specially designed compounding machinery is required of which ko-kneaders and twin-screw extruders have achieved greatest commercial importance. The ko-kneader is effectively a modified form of single-screw extruder, although its design and mode of operation are unconventional and somewhat distinctive. The screw used is not continuous but contains flights which are interrupted by three gaps per turn, and the barrel has three corresponding rows of stationary teeth projecting from its surface. During operation, the screw simultaneously rotates and reciprocates, causing the screw flights to pass forward and then backward between the teeth. This provides an interchange of material in both axial and radial direction and controllable shear stress between the teeth and screw surfaces, in addition to a forward conveying motion. Despite this apparent complexity, many design and operational variants exist, providing control and flexibility, including changing the screw and kneading teeth geometry, the length to diameter ratio of the screw, provision for downstream additive addition into the polymer, and a melt devolatilization capability. In order to produce pellets, the output from the ko-kneader is discharged into, and passed through, a single-screw extrusion pump.
The workhorse machine for filled thermoplastic compounding, however, is the twin-screw extruder. The two screws are located in a figure of eight-shaped barrel, and may be intermeshing or not, and rotate in the same, or opposite directions. As mentioned earlier, a feature of such machines is that they have effective material conveying characteristics, depending on degree of screw intermesh, which makes them particularly suitable for transporting filled compositions. In addition, most designs incorporate specially designed (kneading) elements within the screw profile to augment and intensify mixing and melting behavior. Their design (e.g., bi- or tri-lobal or segmented), position along the screws, number, and relative stagger can all influence shear intensity developed in the polymer during compounding. Machines are also designed to be multifunctional, incorporating downstream filler addition and melt devolatilization stages. Flexibility in machine configuration is achieved by using a modular screw and barrel assembly, which, in addition to allowing the length to diameter ratio of the machine to be changed, permits different functional tasks to be undertaken within the same machine. This feature gives an additional benefit, since screw and barrel sections in zones which are particularly prone to wear, such as in the feed end, can be easily and economically replaced. Although screws and barrels are normally specially hardened or protected with liners to make them wear resistant, this will inevitably occur when processing polymers with abrasive inorganic fillers, particularly at high loading levels.
Whereas closely intermeshing counterrotating twin-screw designs are in widespread use for processing thermally sensitive UPVC compounds, including filled variants, most general-purpose filled thermoplastic compounding is undertaken with corotating, so-called self-wiping screw machines configured with banks of the kneading elements described above. Some machines are also designed to have split barrels, which can be readily opened to expose the screws to aid cleaning and changes to screw assembly, or with two-stage operation, whereby the output from the twin-screw compounding section is fed into a crosshead single-screw extruder to enable melt pressurization before exiting through a die for pelletization.
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