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Issue 05/2016

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Processing By: Nick

Processing By: Nick Knowlton, NatureWorks LLC, Minnetonka, MN, USA Augie Machado, Brian Haight, Leistritz, Somerville, NJ, USA Twins help melting Melting efficiency performance for various polylactide resins in a co-rotating intermeshing twin screw extruder Torque % 100,00 90,00 80,00 70,00 60,00 50,00 40,00 3253 50 3754 00 4254 50 4755 00 5255 50 5756 00 RPM Figure 1: Torque % for Screw 2 Melt Temperature [ C] Neat Lubricated 231,00 230,00 229,00 228,00 227,00 226,00 225,00 224,00 223,00 222,00 3253 50 3754 00 4254 50 4755 00 5255 50 5756 00 RPM Neat Lubricated Figure 2: Melt Temperature for Screw 3 There are many grades of polylactic resins marketed under the Ingeo TM trade name. For commercial and technical reasons, several of the grades are supplied completely neat, without the addition of an external lubricant on the pellets. Most of the grades have this lubricant, which facilitates pellet flow through conveying systems, silos and dryers. Several studies have been performed comparing the differences in melting behavior, power load and melt temperature in single screw extruders, but to date no study has characterized the same parameters in a twin screw extruder. Twin screw extruders (TSEs) are a preferred manufacturing method to compound bioplastics. TSEs utilize modular barrels and screws. Segmented screws are assembled on splined shafts. The TSE motor transmits power into the gearbox/shafts and rotating screws impart shear and energy into the materials being processed. This arrangement allows for a wide range and refinement of many process applications. Material including PLA is converted through extrusion into a variety of products. These can include film/sheet for packaging, fibers and foamed parts, and medical devices. Polylactic acid (PLA) is heat and shear sensitive, as well as torque intensive. Changing sections of the segmented screws may allow the extruder to work more efficiently in melting, processing, and extruding PLA. Generally, with higher lubricant levels, the PLA melts less efficiently in the melting section of the TSE and the power load (a.k.a. torque) and melt temperatures are decreased. To this point, the effect of external lubricant on melting behavior, power load and melt temperature has not been studied on a TSE. TSE experiments processing PLA (Ingeo Biopolymer 4032D) were performed using different screw designs. PLA pellets with varying lubricant levels (Ethylene bis stearamide at zero, medium, and high levels) were processed on a ZSE 27 MAXX extruder (28.3 mm dia. screws, 5.7 mm flight depth, 1.66 OD/ID, 1200 max rpm). The TSE was set at 325, 400, and 600 rpm with a constant feed rate of 45 kg/hr. for screws 2 and 3 at each lubricant level. For screw 1, a flat temperature Figure 3: Conveying & Melting Section of a Twin Screw Extruder 64 bioplastics MAGAZINE [05/16] Vol. 11

Processing profile of 210 °C was used. For screws 2&3, the zones were set between 210 °C and 240 °C. Data was collected for seven minute samples, then averaged and graphed to see results. In order to minimize the audible crunching sound, Screw 1 was designed to replicate the screw used at NatureWork’s facility. The crunching was heard and it was determined that the use of surface lubricant with this screw configuration resulted in inadequate melting. Screw 1 could only process 35 kg/hr at low rpm due to high torque. Screw 2 was created, adding a longer melting zone with GFA instead of GFF elements that were in Screw 1. GFF elements are forward conveying and not self-wiping with higher free volume, while GFA elements are forward conveying and self-wiping. This change, along with an extended melting zone and a modified pitch transition in the kneading blocks, created more efficient pumping and mitigated the crunching sound. Overall, screw 2 had a median torque 4 % less and decreased the melt temperature by 7 °C compared to Screw 1. After running each condition on screw 2, screw 3 was created to, hopefully, increase the efficiency further. The melting zone was extended further adding tighter pitched GFA’s, and one kneading block was removed to decrease the energy and shear created. The torque decreased by an additional 6 % and the melt temperatures were similar to screw 2. As expected, when the lubricant content increased the torque and melt temperature both decreased. From screw 1, to 2, to 3, the torque decreased with higher lubricant levels which would allow for more efficient processing and less energy input into the material. Figures 1-2 show examples of trends from the run data where the torque and melt temperature decreased with increased lubricant content. In addition to the torque and temperature trends, the solid insert was removed in the 12D-16D section of screw 1 to visually see how the lubricant level effected the melt progression. As expected, with higher lubricant levels, the melt occurred later and there were still partially melted and un-melted pellets in the sample removed. After analysis of the extruded pellets for lubricant content and effect of properties, it was determined that the lubricant stayed in the product at consistent levels, and there was no significant effect on the properties. Overall, this test was successful in showing the trends and behavior of PLA in a TSE. As the lubricant level increased, both the torque and melt temperature decreased. The crunching sound associated with solids conveying and melting was mitigated through a change from screw design 1 to 2 and the efficiencies were further increased after changing from screw 2 to screw 3. After analysis, it was determined that the lubricant does not have a noticeable effect on physical properties of the PLA. Further testing will be performed to obtain additional data, further increase the efficiency of the screw design and confirm trends. 16F22 COMPOSITES EUROPE 11. Europäische Fachmesse & Forum für Verbundwerkstoffe, Technologie und Anwendungen Visions become reality. 29. Nov. – 1. Dez. 2016 Messe Düsseldorf Organised by Partners bioplastics MAGAZINE [05/16] Vol. 11 65

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