Twin Screw Extrusion

Transcription

1 James L. White Eung K. Kim Twin Screw Extrusion Technology and Principles 2nd Edition Sample Chapter 9: Experimental Studies of Intermeshing Counter-Rotating Twin Screw Extruders ISBNs HANSER Hanser Publishers, Munich Hanser Publications, Cincinnati

2 9 Experimental Studies of Intermeshing Counter-Rotating Twin Screw Extruders 9.1 Introduction We now turn to experimental studies of flow in intermeshing counter-rotating twin screw extruders. There have been relatively few studies of this type in the literature. Furthermore, such experimental studies have not kept up with the development of machine technology. The experimental studies have, however, made certain aspects of the flow characteristics clear. These efforts will be reviewed in this chapter. In Section 9.2, we will summarize flow visualization investigations which seek to determine fluid motions. We turn in Section 9.3 to studies of pumping characteristics. Investigations of residence time distributions are summarized in Section 9.4. Research on melting of solid beds is contained in Section 9.5. Dispersive mixing is considered in Section Flow Visualization Flow visualization studies of intermeshing counter-rotating twin screw extruders have been reported by Jewmenow and Kim [1], Janssen and Smith [2 4], and by Sakai [5]. These investigations all confirm the positive displacement pump character of the motions. Jewmenow and Kim [1] have reported investigations in which a window was placed on a twin screw extruder. Aluminum flake markers were placed in a polyisobutylene process fluid. Photographs were taken through the windows and were used to determine streamlines. Janssen and Smith [2 4] reported studies on an extruder with a transparent polymethyl methacrylate barrel using an aqueous polyvinylpyrolidone solution. Sakai [5] has also made similar studies. 9.3 Pumping Characteristics There is a long history of experimental investigations of pumping characteristics of intermeshing counter-rotating twin screw pumps and extruders [4 18]. Kiesskalt [6], in a 1927 paper, studied the pumping of viscous Newtonian oils concluding that: Q V N Q eak ( p ) (9.1) C l i.e., there is a pressure dependent leakage between the clearances.

3 174 9 Experimental Studies of Intermeshing Counter-Rotating Twin Screw Extruders The earliest study of this type for a polymer melt is by Doboczky [8, 9] of Leistritz, who collected together data from a number of different twin screw machines. Doboczky compared the throughput data with Eq. (8.2b) which we repeat here: Q i NV C (9.2) for the positive displacement pump output of the machine. Doboczky found that the ratio of observed to theoretical output varied from roughly 50 to 90%, with 70% being a mean value. Doboczky also argued that the output of a twin screw extruder is about three times as high as that of a single screw extruder of a similar size and screw speed. A second study (1971) of the pumping characteristics of polymer melts was reported by Menges and Klenk [11 13] using polyvinyl chloride in a Schloemann AG Pasquetti Bitruder. It was reported that the throughput was proportional to the screw speed. The ratio of the experimental output to the theoretical predictions (Eq. (9.2) was found to be in the range of 37 to 41%. This result was only dependent on screw speed. A third study in 1976 was published by Janssen and co-workers [17], also using a Pasquetti type machine. The screw had a diameter of 47.7 mm and an effective screw length of 360 mm. The material investigated was polypropylene. Die pressures from 3 to 60 atmospheres were achieved. Throughput increased linearly with screw speed N. The specific output Q/N seemed to be independent of pressure developed over a wide range of melt temperatures and screw speeds, as can be seen in Figure 9.1. Figure 9.1: Relation of the ratio of specific extrusion rate (Q/N) to die pressure and mass throughput for the extrusion of polypropylene from Janssen et al. [17]

4 9.3 Pumping Characteristics 175 Janssen et al. [17, 18] hypothesized in their studies that the pressure development was associated more with the level of starvation than with the operating parameters of the machine. They were able to characterize this level of starvation by stopping the twin screw extruder while operating under steady conditions, cooling and then removing the barrel. Janssen et al. s results are summarized in Figures 9.2 and 9.3. Data obtained at higher screw speeds and higher temperatures show lower pressure rises. Hong et al. [19] have experimentally determined the screw pumping characteristics of three of the modules used in the Leistritz AG modular intermeshing counter-rotating twin screw extruder (see Section 7.12). These are shown in Figure 9.4 a, b, c together with the simulations for these elements of Hong and White [20] (Section 8.5). The agreement between simulation and experiment is good. Figure 9.2: Relationship between the die pressure and the number of fully filled chambers for extrusion of polypropylene at different screw speeds from Janssen et al. [17, 18]

5 176 9 Experimental Studies of Intermeshing Counter-Rotating Twin Screw Extruders Figure 9.3: Relationship between the die pressure and the number of fully filled chambers for extrusion of polypropylene at higher temperatures from Janssen et al. [17, 18] Dimensionless throughput Simulation Experiment a) Dimensionless pressure gradient Figure 9.4: Measured and simulated dimensionless screw characteristic curves for Leistritz intermeshing counter-rotating modules by Hong et al. [19, 20]; (a) thin-flighted screw; (b) thick-flighted screw; (c) shearing disc

6 9.3 Pumping Characteristics 177 Dimensionless throughput Simulation Experiment b) Dimensionless pressure gradient 0.5 Dimensionless throughput Simulation Experiment c) Dimensionless pressure gradient Figure 9.4: (continued)

7 178 9 Experimental Studies of Intermeshing Counter-Rotating Twin Screw Extruders 9.4 Residence Time Distributions Measurements of residence time distributions in intermeshing counter-rotating twin screw extruders were reported by Sakai [5,16], Janssen, Hollander, Spoor, and Smith [2,18], Rauwendaal [21], Wolf, Holen, and White [22], Potente and Schultheis [23], and Shon et al. [24]. The results of Sakai [5, 16] are summarized in Figure 9.5, which shows that the intermeshing counter-rotating twin screw extruder has a much narrower residence time distribution than the single screw extruder. Sakai associates this with both the C-chamber mechanism and the self-wiping screws. In the single screw extruder, the material is retained for much longer times associated with stagnant layers on the screw surface. Figure 9.5 also shows data for intermeshing co-rotating machines that was discussed in Chapter 6 and a non-intermeshed twin screw configuration which is not carefully defined. In the studies of Janssen et al. [18] a pulse containing radioactive MnO 2 was injected into polypropylene in a twin screw extruder. The decay 56 Mn to 56 Fe was detected using a scintillation crystal. Attention was given to the reproducibility of results. Janssen et al report their conclusions in terms of exit age distributions as shown in Figure 9.6. Wolf et al. [22] also used a radioactive MnO 2 tracer and applied it to a Krauss-Maffei Model KMD 90 machine (90 mm diameter screws). The machine described by Wolf et al. was being used commercially to extrude polyvinyl chloride at Kafrit Industries (Kibbutz Kfar Aza Israel). Figure 9.5: Residence time distributions of Sakai [5, 16]

8 9.5 Melting Phenomena 179 Figure 9.6: Exit age distribution of polypropylene being extruded from a Pasquetti intermeshing counter-rotating twin screw extruder after Janssen et al. [18] (reprinted with permission of the AIChE Journal) Rauwendaal [21] reported studies using an antimony oxide tracer, which was detected by X-ray fluorescence, on a Leistritz LSM GG 30/34 machine (34 mm diameter screws). He finds a much narrower distribution of residence times than in co-rotating twin screw machines. Shon et al. [24] in 1999 compared residence time distributions in four types of continuous mixers. The Leistritz LSM GG 30/34 intermeshing counter-rotating twin screw extruder had the narrowest residence time distribution as compared to modular co-rotating twin screw extruders and the Buss Kneader. All investigators have found the residence time distributions to be narrower than for single screw extruders or co-rotating twin screw machines. 9.5 Melting Phenomena Experimental studies of melting in intermeshing counter-rotating twin screw extruders have been few in number. Janssen [4] seems to have been the first to study the melting process in this machine. He notes that melting occurs more rapidly than in single screw extruders and seems to begin in clearances. Studies were subsequently made in the 1980s by Sakai [25, 26]. He also observed the more rapid melting in intermeshing counter-rotating machines as opposed to co-rotating machines. In a later paper comparing intermeshing co-rotating and counter-rotating twin screw extrusion, Lim and White [27] also note that melting occurs much more rapidly in intermeshing counter-rotating machines than in the co-rotating machine. This was also later found by Cho and White [28].

9 180 9 Experimental Studies of Intermeshing Counter-Rotating Twin Screw Extruders A more recent study by Wilczynski and White [29] identified the location of the initiation of melting as being in the calendering gap between the screws. Melting induced by the heated barrel surface was a secondary location of melting initiation. They concluded that the intense mechanical energy absorbed by the pellets in the calendering region was the primary cause of the initiation of melting. 9.6 Dispersive Mixing The relative ability of different types of intermeshing twin screw machines to disperse fillers seems first studied by Sakai, Hashimoto, and Kobayoshi [16] in the mid 1980s. They made their studies on a Japan Steel Works (JSW) TEX machine that had screws and kneading disc blocks. They studied the mixing of both talc and glass fibers. They report that the intermeshing counter-rotating does a better job of dispersing the small particles of talc but damages glass fibers to a greater extent as compared to a self-wiping co-rotating machine. In papers published in , Thiele and his coworkers [30 32] with American Leistritz describe the relative compounding characteristics of an intermeshing counter-rotating twin screw machine with Leistritz elements (Figure 7.14) and a self-wiping co-rotating machine, finding them similar. In 1993, Lim and White [27] published a study of the blending characteristics of intermeshing counter-rotating twin screw extruders of Leistritz design. Polyethylene-polyamide 6 blends were studied for different modular screw configurations. They found the intermeshing counter-rotating machine gave the finest phase morphology. In a 1996 paper, Cho and White [28] compared the blending of polyethylene and polystyrene melts in a modular intermeshing counter-rotating extruder of Japan Steel Works design and a self-wiping co-rotating twin screw extruder. They reached similar conclusions. In a 1999 paper, Shon and White [33] compared the breakage of glass fibers in a Leistritz LSM GG 30/34 intermeshing counter-rotating twin screw extruder with a modular co-rotating twin screw extruder and a Buss Kneader. The glass fibers were most seriously damaged in the intermeshing counter-rotating machine. In 2005, Shon et al. [34] published a study comparing the dispersive mixing of calcium carbonate in polypropylene for a Leistritz LSM GG 30/34 intermeshing counter-rotating, an intermeshing co-rotating twin screw extruder, a Buss Kneader, and a Kobelco NEX T. The most effective dispersive mixing was by the intermeshing counter-rotating twin screw machine. They represented their dispersion data through the kinetic expression of Eq. (6.7). It seems clear that the Leistritz GG machine is the most intense and effective mixer for additives, polymer blends, and particulate fillers. It also most severely damaged glass fibers.

10 References 181 References 1. S. D. Jewmenow and W. S. Kim, Plaste u Kautschuk, 20, 356 (1973). 2. L. P. B. M. Janssen and J. M. Smith, Proceedings of the Congress on Polymer Rheology and Plastics Processing, PRI/BSR, Loughborough, p. 160, September L. P. B. M. Janssen and J. M. Smith, Plastics and Rubber: Processing, p. 90 (1976). 4. L. P. B. M. Janssen, Twin Screw Extrusion, Elsevier, Amsterdam (1978). 5. T. Sakai, Góse Jushi, 29, 7 (1978). 6. S. Kiesskalt, Zeit VDI, 71, 453 (1927). 7. H. W. Selbach, Plastverarbeiter, 13, 595 (1962). 8. Z. Doboczky, Plastverarbeiter, 16, 57 (1965). 9. Z. Doboczky, Plastverarbeiter, 16, 395 (1965). 10. H. Markhenkel, Kunststoffe, 55, 363 (1965). 11. G. Menges and K. P. Klenk, Plastverarbeiter, 17, 791 (1966). 12. K. P. Klenk, Plastverarbeiter, 22, 33 (1971). 13. K. P. Klenk, Plastverarbeiter, 22, 105 (1971). 14. J. Stasiek, Plaste u Kautschuk, 21, 207 (1974). 15. L. P. B. M. Janssen, L. P. H. R. M. Mulders, and J. M. Smith, Plastics Polymers, 43, 93 (1975). 16. T. Sakai, N. Hashimoto, and N. Kobayoshi, SPE ANTEC Tech Papers, 33, 146 (1987). 17. L. P. B. M. Janssen, J. L. Pelgrom, and J. M. Smith, Kunststoffe, 66, 724 (1976). 18. L. P. B. M. Janssen, R. W. Hollander, M. W. Spoor, and J. M. Smith, AIChE J., 25, 345 (1979). 19. M. H. Hong, Q. Jiang and J. L. White, Int. Polym. Process., 23, 88 (2008). 20. M. H. Hong and J. L. White, Int. Polym. Process., 14, 136 (1999). 21. C. Rauwendaal, SPE ANTEC Tech. Papers, 38, 618 (1981). 22. D. Wolf, W. Holen, and D. H. White, Polym. Eng. Sci., 26, 640 (1986). 23. H. Potente and S. M. Schultheis, Int. Polym. Process., 4, 247 (1989). 24. K. Shon, D. Chang, and J. L. White, Int. Polym. Process., 14, 44 (1999). 25. T. Sakai, Japan Steel Works (JSW) Technical Report T. Sakai and N. Hashimoto, SPE ANTEC Tech. Papers, 32, 360 (1986). 27. S. H. Lim and J. L. White, Int. Polym. Process., 9, 33, (1994). 28. J. W. Cho and J. L. White, Int. Polym. Process., 11, 21 (1996). 29. K. Wilczynski and J. L. White, Int. Polym. Process., 16, 257 (2001). 30. W. Thiele, SPE ANTEC Tech Papers, 29, 127 (1983). 31. W. Thiele, W. Petrozelli, and D. Lorenc, SPE ANTEC Tech. Papers, 36, 120 (1990). 32. W. Thiele, W. Petrozelli, and C. Martin, SPE ANTEC Tech. Papers, 37, 1899 (1991). 33. K. Shon and J. L. White, Polym. Eng. Sci., 39, 1157 (1999). 34. K. Shon, D. Liu, and J. L. White, Int. Polym. Process., 20, 322 (2005).