Twin-screw systems

Reactive extrusion takes place when the twin screw system is designed to incorporate one or more chemical reactions during the process, such as grafting or even polymerization. Grafting is the process of attaching pendant molecules onto the backbone of a polymer chain. For example, a small concentration of maleic anhydride can be incorporated into polypropylene to make the... 

Expressions for the limiting shape factors when the width of the channel is small relative to the depth (W H ) are given hy Booy [29]. However, this type of channel geometry is generally not encountered in commercial twin screw systems. Numerical simulation of the flow and heat transfer in twin screw extruders is covered in Chapter 12. Section 12.3.2 discusses 2-D analysis of twin screws, and Section 12.4.3.3 deals with 3-D analysis of flow and heat transfer in twin screw extruders. Since 2000, major advances have been made in the numerical methods used to simulate twin screw extruders. The boundary element method now allows full 3-D analysis of flow in TSEs. A significant advance in the finite element method is the mesh superposition technique that allows analysis of complicated geometries with relative ease. This is discussed in more detail in Chapter 12. 

Avalosse et al. [91] used the simulation program to further study the co-rotating twin screw system with and without kneading blocks, Fig. 12.29. 

Another important aspect is the bulk density of the initial blend. If the bulk density is too low, the feeding unit, which is usually based on a twin-screw system, becomes the limiting part of the equipment with regard to throughput and screw All degree. Therefore, the formulator is requested to balance between sufficient flow properties, bulk density, and particle size distribution which should be similar for all ingredients in order to avoid demixing. 

Venting can be carried out in either single- or twin-screw extruders. In either case, the removal of the material to be vented involves a complex interaction of heat, mass, and momentum transfer. Analyses based on various assumptions exist for both single- and twin-screw systems. 

Figure 2.18 Schematics (a) of the apparatus for twin screw extrusion combined to electrospinning for the production of composite nanofibers (features are not to scale), (b-d) Photographs of the twin-screw system (b), of the extrusion unit coupled to the spinneret (c) and of the die (d). Adapted with permission from Ref. 221, Nanotechnology, 2008,19, 165302. Doi 10.1088/0957-4484/19/16/165302. Copyright 2008, lOP Publishing Ltd.

Fig. 19.28. Twin-screw systems. (Courtesy Werrter Pfieiderer Corp.)...

TABLE 8.1 Extrusion Data for the Twin-Screw System... 

We are now in position to calculate the remaining unknowns for the single-screw extruder, which are N and pQ. Using the data for the twin-screw system given in Table 8.1 we can calculate N 2 (= Ti/tfi). The unwound channel lengths are... 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Create a table listing two or more advantages and disadvantages of each of the following compounding systems batch mixers, continuous mixers, single screw extruders and twin screw extruders. 

One of the earliest published studies on extraction in twin-screw extruders was conducted by Todd (1974). In this work devolatilization was conducted under vacuum using two different polymeric systems, polystyrene in one and polyethylene in the other. In the case of polystyrene, styrene was not used as the volatUe component so as to avoid problems associated with further polymerization or depolymerization instead, use was made of mixtures of thiophene and toluene or ethylbenzene. Todd found good agreement between the measured exit concentrations of the volatile component and the predicted values using Pe = 40 in the solution to Eq. (38) (see Fig. 15). The value of 5 in Eq. (39) was not reported and it is not known whether a value was chosen to provide a fit with the data or whether it was known a priori. In any event, what is clear is that the exit concentration varies with IVwhich suggests that mass transfer is occur-... 

Werner (1980) has studied devolatilization in corotating twin-screw extruders when the volatile component was stripped from the polymeric solution by applying a vacuum to the system. Rough estimates of the equilibrium partial pressure of the volatile component in the feedstream for each of the systems studied by Werner indicate that this pressure was less than the applied pressure, which means that bubbles could have been formed. Figure 17 shows the influence of the externally applied pressure on the exit concentration for a methyl methacrylate-poly(methyl methacrylate) system of fixed concentration. Note that the exit concentration decreases as the pressure is decreased, but seems to approach an asymptotic value at the lowest pressures studied. Werner also reported that at a fixed flow rate and feed concentration the exit concentration did not vary with screw speed (over the range 150-300 min" ), which also suggests that ky alay, is independent of screw speed. Figure 18 is a plot of data obtained by Werner on an ethylene-low-density poly(ethylene) system and also shows that decreases in the applied pressure result in decreases in the exit concentration, but here a lower asymptote is not observed. 

Fig. 18. Experimentally measured values of the exit concentration as a function of pressure and water content for the system ethylene-low-density polyfethylene). Data were obtained in a twin-screw extruder by Werner. (Reproduced with permission from Werner, 1980.)...

Mack, M., Corotating Intermeshing Twin-Screw Extruder Berstorff s System, Chapter 5 in Plastics Compounding Equipment and Processing, Todd, D.B. (Ed.), Hanser Publishers, Munich (1998)... 

The twin-screw extrusion (TSE) technology is utilized extensively in the plastics and food industries to complement continuous manufacturing of products. Ghebre-Sellassie et al. [37] and Keleb et al. [36] have cited the application of this technology in the continuous production of pharmaceutical products. Commercially available TSE provide the great Lexibility required for effective continuous mode of operation. The schematic of such a system is shown in Figure 23.6. The twin-screw extruder... 

Figure 10.40 Particle tracking inside a co-rotating double flighted twin screw extruder. In order to plot the flow lines, the system is viewed from a coordinate system that moves in the axial direction at a speed of Rftshuj).

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