Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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While considered acceptable in many cases, they need not be tolerated and are usually attributable to minor screw-design flaws. Here's some advice on what to do about them. Reducing Low-Level Background Gels in PE Film Many polyethylene (PE) film products have a low level of back- ground gels that are considered acceptable for most end users and applications. These gels, however, do not have to be accepted. They exist due to minor design errors in the screw. Screw- design techniques are available that can reduce the occurrence of this type of gel. The term "gel" refers to any small defect that distorts a film. The low-level background gels at issue here typi- cally do not alarm the quality-control managers in the plant, and as such the film is considered "prime." There are many types of gels; these are the most common: 1) Highly o xidized polymeric materials that appear as brittle black specks; 2) Polymers that are crosslinked via an oxidative process; 3) Highly entangled polymeric material that is undispersed but not crosslink ed (unmixed); 4) Unmelted resin or solid polymer fragments; 5) Filler agglomerates from masterbatches; 6) A different type of resin or contaminant such as metal, wood, cloth fibers, insects or dirt. The two types of gels that are the focus of this article are low levels of crosslinked gels and unmixed gels. A crosslinked resin gel is typi- cally formed during an oxidation process, resulting in crosslinking of the resin chains and generation of discolored gels. Highly entan- gled gels are typically high-molecular-weight polymer chains that are entangled and thus are difficult to disperse during the extrusion process. When analyzed using a hot-stage micro- scope, the highly entan- gled gel type will melt as the stage temperature is increased. When the stage temperature is then decreased, the gel will recrystallize, creating the appearance of a gel as a solid polymer fragment. If the gel is exposed to a shear stress right after melting, the stress will often disentangle the chains such that they will not reform when cooled. Since these gels are not oxidized, they are not By Mark A. Spalding, Xiaofei Sun, Eddy I. Garcia-Meitin, and Stephen L. K odjie The Dow Chemical Company Gregory A. Campbell, Clarkson University/ Castle Associates Timothy W. Womer TWWomer and Associates LLC FIG 1 Photomicrographs of contaminants in film products: a) hard carbon speck from maleic anhridride plate-out; b) polyester contaminant; c) iron oxide particle; and d) a fiber. A B C D 100 μm 50 μm 20 μm 50 μm Defects T m = 265 C Core T m = 126 C 62 SEPTEMBER 2018 Plastics Technology PTonline.com Troubleshooting

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