Plastics Technology

SEP 2018

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associated with color. They are commonly referred to as undis- persed or unmixed gels. Troubleshooting PE film extrusion processes where gels are appearing can be difficult due to the number of different types of gels. Because gels can originate from numerous sources, the troubleshooter must be able to identify the characteristics of the gel and recognize the likely source. Process changes must then be performed to mitigate the gel defects. GELS THAT OCCUR DUE TO 'MOFFAT EDDIES' Many PE film processes exhibit a low and continuous level of gels. These are typically oxidized and crosslinked PE material. They occur as black specks and brown soft gels. The gels typi- cally originate from regions of the screw that were stagnant, allowing the resin to have long residence times such that it will degrade. Photomicrographs of these types of gels are shown in Fig. 2. Typical gel sizes range from 50 to 250 microns. The most common source of these background gels is stagnant regions at the flight radii. The stagnant regions occur because of the formation of "Moffat eddies" that are due to small flight radii. A photograph of a screw with degraded resin at the flight radii is shown in Fig. 3. This degraded material will slowly separate from the screw and contribute to a constant and low level of gels in the film product. The resin will be in the Moffat eddies for extended periods of time, such that antioxidants cannot prevent the degradation. Moffat eddies are recirculations or vortices that occur at sharp corners, as shown in Fig. 4. That is, when fluid is put in motion with top-driven cavity flow, flow circulation is generated in the channel as shown in Fig. 4. A secondary circulation also develops in the corners of the channel, creating a low- velocity helical eddy that is outside the high-velocity flows of the main part of the channel. The Moffat eddies that create the degraded resin occur because the flight radii are too small for the depth of the channel. If the flight radii were larger, the Moffat eddies and consequent resin degradation would not occur. Flight radii sizes are shown in Fig. 5. Flight radii that are 1.5 times larger than the local channel depth are recommended for PE resins. POORLY DESIGNED MADDOCK MIXERS Poorly designed Maddock mixers are sources for two types of gels that can appear at a low and continuous level. The gel types include thermally oxidized gels, as shown in Fig. 2, and unmixed gels. As stated previously, unmixed gels are highly entangled species that are molten when they are discharged from the die, but solidify first upon cooling to produce a gel that appears as a solid polymer fragment. Photomicrographs of an unmixed gel going through the heating and stressing process are shown in Fig. 6. Unmixed gels can be easily removed from an extrusion process by subjecting all molten resin to a one-time high level of stress near the discharge of the extrusion screw. This stress is easily applied using a Maddock-style mixer with a relatively tight clearance on the mixing flight. A schematic of a Maddock-style mixer is shown in Fig. 7. The mixer is designed with pairs of inflow flutes and outflow flutes and a mixing flight with a narrow restriction. The resin flows into the inflow flute. Next, the flow is passed through the narrow restriction created by the mixing flight and the barrel wall. This QUESTIONS ABOUT EXTRUSION? Visit the Extrusion Zone. Troubleshooting PE film extrusion processes where gels are appearing can be difficult due to the number of different types of gels. Resin Degradation Flow A B 50 μm 50 μm FIG 2 Photomicrographs of gels that originated from stagnant regions of the screw, such as from Moffat eddies: a) a bright-field photograph, and b) the same gel photographed using epi- fluorescence illumination, indicating that the gel is highly oxidized. FIG 3 Shown here is degradation at the pushing and trailing flights of a screw running LLDPE. @plastechmag 63 Plastics Technology G E L S

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