EFFECT OF SCREW SPEED ON POLYETHYLENE-CALCIUM CARBONATE COMPOSITES PRODUCED USING TWIN AND QUAD SCREW EXTRUDERS Mansour M. Albareeki, Stephen Burke Driscoll, and Carol F. Barry Department of Plastics Engineering, University of Massachusetts Lowell, Lowell, MA Abstract The effect of ultra high screw speed on mixing was investigated using polyethylene microcomposites with 1 wt% calcium carbonate compounded on novel twin and quad screw extruders. The screws had similar designs and the screw speeds were 300 to 2000 rpm. Extruder type influenced the effects screw speed had on extruder residence time, melt temperature, drive torque, and head pressure. Parallel plate rheology indicated significant chain scission of the polymers and better filler dispersion at higher screw speeds of 900 and 1500 rpm, especially with the quad screw extruder. In the quad screw extruder, the lower melt temperatures and greater shear allowed better mixing at higher screw speeds than the twin screw extruder. The level of mixing in the quad screw extruder also depended on resin viscosity. Introduction Co-rotating twin screw extruders are the preferred method for commercial compounding of polymer systems, including mixing of fillers into polymers. When compounding with traditional co-rotating twin screw extruders, screw speeds have reached 1200 rpm and no issues have been reported during the compounding process [1-4]. With deeper screw channels, screw speeds up to 1800 rpm have been used for mixing highly-filled polymers [1]. In contrast, ultra high speed twin screw extruders, operated at screw speeds as high as 4000 rpm, have produced degradation of even relatively stable polymers, like polystyrene and polypropylene [5-6]. With these extruders, modification of the screw design and residence time has been a compromise between improving mixing and reducing degradation [7]. Moreover, fillers have tended to agglomerate, creating defects and leading to brittle failure in the resultant parts [7]. Compared to twin screw extruders, quad screw extruders - i.e., extruders with four parallel, fully intermeshing screws - have shown better dispersive mixing [8]. These improvements were attributed to the greater number of intermeshing regions ("bite points") which produce high shear stresses in quad screw extruders. The limited work reported for compounding with quad screw extruders, however, has focused on polymer blends mixed using low screw speeds [9-10] and color concentrates compounded using high speeds [8]. This study was an investigation of compounding a particulate-filled polymer composite with ultra high speed twin and quad screw extruders. The objective was to understand the effects of 1) screw speed (i.e., higher shear rates and short residence times), 2) the number of bite points, and 3) the number of kneading blocks in the screw design on the processing parameters and properties of the resultant composites. Materials Experimental Polyethylene-calcium carbonate composites were selected for this work. Calcium carbonate is a widelyused, relatively-low-cost inorganic filler with high specific heat capacity and thermal conductivity. Polyethylene-calcium carbonate composites have exhibited greater productivity, impact resistance, and barrier performance [11-12]. Due to the high surface area of the calcium carbonate particles, however, it is a challenge to disperse and stabilize calcium carbonate within the polymer matrix [13-16]. The five commercially-available polymers selected for this study were three low density polyethylenes (LDPE), one linear low density polyethylene (LLDPE), and one medium density polyethylene (MDPE). The reported densities and melt flow indices (MFI) are listed in Table 1. The calcium carbonate (CaCO 3 ) filler (Omya, grade: Omyafilm-SY) had a reported particle diameter of 0.5-50 µm. Table 1. Material Properties Material Density (kg/m 3 ) MFI (g/ 10 min) n E a (kda) LDPE 1 920 0.18 0.42 15.9 LDPE 2 920 0.65 0.50 32.2 LDPE 3 920 1.85 0.59 39.9 LLDPE 921 0.60 0.68 14.8 MDPE 936 0.30 0.51 11.6 The complex viscosity of the neat resins was measured at temperatures of 190, 205, and 220 C using parallel plate rheometry in accordance with ASTM D- 4440-15 [17]. As shown in Figure 1 and Table 1, the LDPEs exhibited overall complex viscosities that corresponded to their melt indices and had power law SPE ANTEC Anaheim 2017 / 1046
indices (n) of 0.42-0.59. LLDPE and LDPE 1 had similar viscosities at low frequencies, but LLDPE showed less shear thinning - i.e., its power law index was 0.68. At low frequencies, MDPE had similar viscosity to LDPE 2, but had a power law index of 0.51. The activation energy for flow (E a ) was 10-16 kda for LDPE1, LLDPE, and MDPE, but greater (30-40 kda) for LDPE2 and LDPE3. resin temperature and pressure were determined by using a K type thermocouple and a Dynisco pressure transducer, respectively, located in the die adaptor. The residence time was measured as the time from the introduction of a colored pellet to the extruder until that color change was observed in the strands. Table 2. Compounding Parameters Parameter Units Value Barrel Temperature C 205 Die Temperature C 205 Feed Rate kg/h 1.68 Screw Speed rpm 300, 900, 1500, 2000 Characterization Figure 1. Complex viscosity of the neat polyethylenes. Compounding Composites of polyethylene containing 1 wt. % calcium carbonate were prepared by melt blending in (1) an ultra high speed co-rotating 15-mm-diameter twinscrew-extruder with a length-to-diameter (L/D) ratio of 60:1 (Technovel, model: KZW15TW-45/60MG-NH (- 4400)) and (2) an ultra high speed co-rotating 15-mmdiameter quad-screw extruder with an L/D ratio of 45:1 (Technovel, model: WDR15QD-45MG-NH (-2200)). Screws for the quad screw extruders contained four mixing sections with respective lengths of 15, 25, 15, and 55 mm (i.e., about 16% of the total screw length). For the twin screw extruder, the four mixing sections had the same composition, but were twice as long as those in the quad screw extruder; (the mixing sections were about 25% of the total screw length). Therefore, both the twin and quad screw extruders had the same number and type of mixing elements. Melt exiting the extruders passed through a single strand die for twin screw extruder and a double strand die for the quad screw extruder; when corrected for the number of screws, the dimensions of the die openings provided similar pressure drops in both extruders. The extruded strands were cooled in a water bath and then pelletized (Bay Plastics Pelletizer). The compounding parameters are listed in Table 2. For both extruders, the CaCO 3 and polymer powders were pre-mixed manually and then the premix was fed through the first feed port of the extruder using a volumetric feeder. The feed rate was held constant at 1.68 kg/h; the measured error was ± 5%. During each extrusion trial, drive torque was recorded from the control panel and the The rheology of the composites was determined using a parallel plate rheometer (TA Instruments, model: ARES). Samples were prepared by placing a ring and shim on the bottom plate of the rheometer, adding and preheating the sample pellets, and removing the extra melt to achieve a 2-mm gap between the two plates. Measurements were performed at a temperature of 205 C. Strain sweeps were used to identify the linear viscoelastic region and to study the breakdown of the composites' structure. These sweeps were performed at strains of 0.1 to 100% and at a frequency of 0.16 Hz (1 rad/s) for the linear viscoelastic region and 3 Hz (18.85 rad/s) for structure breakdown. Frequency sweep measurements were performed between 0.02 and 15.92 Hz (0.01-100 rad/s) at a strain extent of 10%. Results and Discussion Effect of Screw Speed on Extrusion Process As expected, extruder residence time decreased with increasing screw speed (Figure 2). Although the residence time was shorter for the quad screw extruder than the twin screw extruder, this change in residence time did not correspond directly to the screw length. The four parallel screws in the quad extruder provided a greater travel distance and the greater free volume decreased the effect of the kneading blocks on residence time. With partially filled screw channels, residence time depends on screw speed, but with completely filled channels, the residence time depends on V free /V, where V free is the free volume and V is the volumetric flow rate [18]. Preliminary calculations indicated that the relative residence time in the twin and quad screw extruders (t TSE /t QSE ) was proportional to L TSE V QSE /L QSE V TSE, where L and V are the extruder length and free volume in filled channels, respectively, in the twin and quad screw extruders; (free volume was calculated as shown in [19] and the quad screw extruder was treated as a twin screw extruder plus SPE ANTEC Anaheim 2017 / 1047
two additional screws without screw-barrel clearances at the sides of the screws). Further analysis is in process. Figure 2. Effect of screw speed on the residence time of For the quad screw extruder, increasing the screw speed from 300 to 2000 rpm produced a 25 C increase in melt temperature, with the temperature leveling off when the screw speed exceeded 1500 rpm (Figure 3). In contrast, the same increase in screw speed produced a linear 60 C increase in melt temperature for the twin screw extruder. These temperature increases were consistent with previous observations [8]. In the twin screw extruder, the melt flows in a tight figure-8 pattern, which creates significant friction at polymer-barrel wall and polymer-screw interfaces. For kneading blocks, this flow pattern caused significant increases in polymer temperature. The quad screw extruder, however, had a wider barrel design. With the same clearance between the screw and barrel walls, the open area in the extruder cross-sections was 1.69 cm 2 and 4.03 cm 2 for the twin and quad screw extruders, respectively. This change in design led to reduced friction as polymer flowed between the screws and barrel wall in the quad screw extruder. Figure 4 presents the three effects of screw speed on the head pressure. First, the head pressure for both extruders decreased with increasing screw speed. Second, the head pressure was significantly lower in the high speed quad screw extruder compared to the high speed twin screw extruder. These results were due to the levels of shearing in the extruders. The high speed twin extruder imparted high levels of shear, whereas the high speed quad provided less shear because of its greater free volume. The calculated free volumes for the twin and quad screw extruders were 165 cm 3 vs. 273 cm 3, respectively. Third, with a screw speed of 2000 rpm, the head pressure in the quad screw extruder was not constant; it varied from -0.1 to 0.1 MPa). This head pressure variation was attributed to the relatively low feed rate used in this work; this feed rate was selected to accommodate the twin screw extruder, but the quad screw extruder prefers higher throughputs. Figure 4. Effect of screw speed on the head pressure of Both extruders had the same 22-kW motors, but the greater number of screws in the quad screw extruder limited the maximum screw speed to 2100 rpm; (the maximum screw speed for the twin screw extruder is about 4500 rpm). The drive torque for quad screw extruder was lower than that for the twin screw extruder (Figure 5). This difference was attributed to the greater free volume and lower shear rates in the quad screw extruder. The former means that less power was needed to rotate the screws. The lower shear rates produced less shear thinning and reduced melt temperature increases in the quad screw extruder. Therefore, the drive torque for the quad screw extruder did not decrease as much with increasing screw speed as it did with the twin screw extruder. Effect of High Speed Extrusion on Mixing Figure 3. Effect of screw speed on the melt temperature of The relative viscosity of unfilled LPDE 2 compared to the viscosity of the unprocessed resin ( o ) was used to compare the effects of screw design of the twin screw SPE ANTEC Anaheim 2017 / 1048
and quad screw extruders (Figure 6). With the twin screw extruder, o increased at 900 rpm and then decreased below 1 for screw speeds of 1500 and 2000 rpm. For the quad screw extruder, the relative viscosity was about 0.5 for screw speeds of 300 and 900 rpm and also decreased for the two highest speeds. The lower melt temperatures and increased bite points in the quad screw extruder increased the shear stress applied to the polymer. This stress probably caused chain scission. Higher screw speeds also increased this stress, producing chain scission in both extruders. The increase in relative viscosity observed with the twin screw extruder may be due to the physical crosslinking of the polymer chains caused by the generation of free radicals [20]. rpm. This improvement in dispersion using the quad screw extruder was attributed to the greater number of bite points in the screw design as well as the lower melt temperatures. More bite points led to high shear stress which reduced filler particle size and increased the interface between the polymer and calcium carbonate filler. The higher temperatures at high screw speeds caused reductions in viscosity and poorer overall mixing. Figure 7. Effect of screw speed on the relative zero-shearrate viscosity of calcium carbonate-filled LDPE 2. Figure 5. Effect of screw speed on the drive torque of Figure 6. Effect of screw speed on the relative zero-shearrate viscosity of unfilled LDPE 2. Figure 7 presents the relative zero-shear-rate viscosity ( o ) of the LDPE 2 composites compounded on the twin and quad screw extruders. An o value greater than 1 indicated better dispersion of the CaCO 3 in the polymer melt. The quad screw extruder enhanced mixing at screw speeds of 300-1500 rpm, whereas the twin screw extruder only provided this performance for speeds of 300 and 900 For all materials processed on the quad screw extruder, adding calcium carbonate improved the zeroshear-rate viscosity relative to the zero-shear-rate viscosity of the neat unprocessed polymer at a screw speed of 300 rpm (Figure 8). With further increases in screw speed, the relative viscosity seemed to depend on material structure. LDPE1, LLDPE, and MDPE exhibited continuous and significant decreases in relative viscosity with increasing screw speed. In contrast, LDPE2 and LDPE3 showed further increases in relative viscosity at screw speeds of 900 and 1500 rpm. These effects seem to correlate with the material viscosity. LDPE1 and MDPE had the highest viscosities and LLDPE showed a significantly lower degree of shear thinning. LDPE2 and LDPE 3 had lower viscosities than LDPE1. The cause for these observations is still under investigation. Figure 9 presents strain sweeps for neat and filled LDPE3 compounded in the quad screw extruder at screw speeds of 300 and 1500 rpm. In the linear viscoelastic region (strain of about 0-60%), the complex viscosity ( * ) of the filled material changed only slightly compared to the complex viscosity of the neat LDPE3. At higher strain rates, however, screw speed produced a difference in the complex viscosity. The sharp decrease in viscosity at 300 rpm indicated poor dispersion of the calcium carbonate dispersion. When the screw speed was increased to 1500 rpm, the complex viscosities of the neat and filled LDPE3 were similar, indicating better dispersion of CaCO 3 in the polymer matrix. Polymer characteristics also affected the complex viscosity-strain sweeps for the other materials. SPE ANTEC Anaheim 2017 / 1049
twin screw extruder. This difference was attributed to the greater free volume in the quad screw extruder. Figure 8. Effect of screw speed on the relative zero-shearrate viscosity of polyethylene-calcium carbonate composites compounded in the quad screw extruder. Figure 9. Strain sweeps for neat and filled LDPE3 compounded on the quad screw extruder at screw speeds of 300 rpm (top) and 1500 rpm (bottom). As shown in Figure 10, the complex viscosity-strain curves of CaCO 3 -filled LDPE2 also were affected by extruder type. For both extruders, increasing the screw speed increased the linear viscosity region (to higher strains), indicating better dispersion of the CaCO 3. With the quad screw extruder, the complex viscosity-strain curve became nearly linear at high strains; this change also suggested better dispersion of the CaCO 3. Conclusions When calcium carbonate was compounded into LDPE, LLDPE, and MDPE using ultra high speed corotating twin and quad screw extruders, higher screw speeds produced decreases in residence time, drive torque, and head pressure as well as increases in melt temperature. With equivalent screw designs, the quad screw extruder showed lower residence time, drive torque, head pressure, and melt temperature increases than the Figure 10. Strain sweep of filled LDPE2 compounded on the twin and quad screw extruders at screw speeds of 300 rpm (top) and 1500 rpm (bottom). Parallel plate rheometer results indicated significant chain scission of the polymers at higher screw speeds as well as with the quad screw extruder. The higher screw speeds, however, also produced better dispersion of the CaCO 3. In the quad screw extruder, the lower melt temperatures and greater shear allowed better mixing at higher screw speeds than the twin screw extruder. The level of mixing in the quad screw extruder also depended on the properties of the polymer matrix. Better dispersion occurred with the lower viscosity resins. Additional research is required to establish the relationship between processing and properties of the final composites. Measurement of the morphological, thermal, and mechanical properties of these microcomposites is in process. Acknowledgements The authors would like to thank Technovel Corporation (Osaka, Japan) for the use of their extruders. References 1. K. Kohlgrüber, Co-Rotating Twin-Screw Extruders: Fundamentals, Technology, and Applications, 2 nd ed. Carl Hanser Verlag, Munich, Germany (2012), pp. 276-278 2. P. Andersen, and F. Lechner, SPE-ANTEC Tech. Papers, 70, 988-992 (2012). 3. P. Peltola, E. Välipakka, J. Vuorinen, S. Syrjälä, and K. Hanhi, Polym. Eng. Sci, 46(8), 995-1000 (2006). 4. T. Villmow, B. Kretzschmar, and P. Pötschke, Compos. Sci. Technol., 70(14), 2045-2055 (2010). 5. A. Farahanchi, R. Malloy, and M. J. Sobkowicz, Polym. Eng. Sci., 56(7), 743-751 (2016). SPE ANTEC Anaheim 2017 / 1050
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