Polyethylene coextrusion

An approach that can be applied to laminates and coextrusions is to selectively dissolve and remove polymer layers by careful selection of solvents. Thus, the nylon layer in a polyethylene/nylon/polyethylene coextrusion can be isolated by boiling in xylene. Alternatively, the nylon could be removed by boiling in formic acid. Solvents for the selective removal of polymers are listed in Table 2. Acids or alkalis should be avoided on some polymers where there is a risk of reaction with the polymer. An example would be the use of concentrated sodium hydroxide solution on a metallized film comprising certain acrylic/ethylene... 

Tzoganakis, C. and Perdikoulias, J., Interfacial Instabilities in Coextrusion Flows of Low-Density Polyethylenes Experimental Studies, Polym. Eng. ScL, 40, 1056 (2000)... 

Nucrel [DuPont]. TM for an ethylene-meth-acrylic acid copolymer resin available for use in conventional extrusion coating, coextrusion coating, and extrusion laminating equipment designed to process polyethylene resins. Higher melt flow index results in fewer resin leaks. 

Griswold, P.D. Zachariades, A.E. Porter, R.S. Solid state coextrusion a new technique for ultradrawing thermoplastics illustrated with high density polyethylene. Polym. Eng. Sd. 1978,18 (11), 861. 

Introduction of hydroperoxide groups into polyethylene by coextrusion with air under pressure at 190°C in a 20-mm extruder with 3-min residence time. 

Linear low density polyethylene (LLDPE) is the predominant stretch wrap material, but films based on LDPE combinations, EVA and plasticised PVC are also found. Stretch materials can be produced by various processes, i.e. from lay flat tubing, casting, etc. The latter process gives orientation in the machine direction which assists stretch in the wrap around direction. Cling properties required for grip may be achieved by incorporating a cling additive or coextrusion to the outer surface(s) of a more tacky material, e.g. EVA. 

Coextrusion to form an effective barrier layer has been used by Plysu Containers Limited and Reed Plastic Containers, both producing tough solvent-resistant containers. The five-layer construction used by Plysu has HDPE both on the inside and outside, and a central barrier layer with adhesive layers on either side to allow lamination to HDPE. The Reedpac range of blow-molded containers developed by Reed Plastic Containers have polyethylene, polypropylene, polycarbonate, or PET combined with barrier resins such as polyamide, PVDC, and EVOH polymer. 

Du Font s thermoplastic polyester elastomer (TPE) Hytrel has been used in an underwater fiber optic cable produced by Shiplex Wire and Cable Corporation. The elastomer is used to position the fiber optics and is chosen because of its modulus properties that help prevent microbends in the fiber optics. The thermoplastic polyester elastomer is extruded over a central steel wire that imparts strength to the cable. Six fiber optic strands with a diameter of 5 mils each are then positioned and covered by a second Hytrel layer, which is added as part of a coextrusion with nylon to obtain greater abrasion resistance. Additional steel layers and a longitudinally formed copper tape are added, and the whole structure is then enclosed in a polyethylene jacket. 

Optical Properties ExxonMobil Exceed polyethylene can produce blown and cast films with low haze and high gloss. In coextrusion applications, two layers of ExxonMobil Exceed mEEDPE can encapsulate a material with poor optics, providing better overall optical properties. 

Film Processing Methods Extrusion, coextrusion, and Saran F- lacquer solution films. Multilayer Extrusion Saran resins are used in combination with a myriad of other polymers in flexible and rigid multilayer products. Multilayer cast and blown film coextrusion processes for Saran can be used with all polyethylenes, polypropylenes, and nylons. 

Coextrusion blow molding is often used to produce multilayer plastic containers. A multilayer parison is produced, and subsequent operations are identical to ordinary blow molding. Reuse of flash is more difficult since it will be multilayer. In many cases, the flash is used in an inner layer in the container, either alone or combined with other recycled plastic. High-density polyethylene (HDPE) laundry product bottles, for example, routinely are made with a three-layer structure in which the inner layer contains flash blended with postconsumer recycled HDPE. 

This coextrusion techniqne has been applied to the fabrication of breathable films, using polyethylene oxide and polyolefin fiUed with CaC03. As the layer thickness decreased, the water vapor transmission rate changed dramatically due to changes from a continuous to discontinuous layer structure. Microlayer extrusion of a number of polymers has been accomphshed, including combinations of high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE), ethylene-styrene copolymers and low-den-... 

Adhesion between the plies is partially mechanical since the materials are molten when combined, but the major bond has to be chemical (see Theories of adhesion). Where compatible materials are used, such as low-density polyethylene or high-density polyethylene (LDPE/HDPE), strong bonds can be achieved between the layers without additional means (see Compatibility). However, where the wider opportunities to use coextrusion are taken and multi-layers conceived with non-compatible materials such as polyamides, polyolefins, polycarbonate and polyethylene vinyl alcohol, adequate adhesion may be difficult to achieve. In these circnmstances, a specific polymeric layer may be used to act as a compatibilizing agent or Tie layer. A very thin layer of the extrudable adhesive has functional gronps to enhance the chemical bonding at the interface. 

Body materials that have been used include polypropylene, high density-polyethylene, or low-density polyethylene mixed with the bonding agent. The barrier material generally used is nylon, which may be either on the inside of the container or on the outside. Some of the advantages claimed for the coextrusion process are excellent aroma barrier, surface gloss, surface priming, scratch resistance (with the nylon on the outside), and chemical resistance and barrier to hydrocarbons (with the nylon on the inside). 

Interlayer with gradient color bands are made for automobile windshields (81-83). The band is printed on the interlayer surface or tinted by coextrusion with pigmented resin. Curvature for a windshield is accomplished by differentially stretching sheet heated at about 85-100°C over a tapered shaping drum. Rolls of sheet or precut interlayer blanks are usually stored or shipped refrigerated at 3-10°C, or shipped at ambient conditions with an interleaved thin sheet of plastic such as polyethylene to prevent blocking. 

An early description of the cast film process was published in Modem Plastics in 1952 [26] which was very similar to the cast-film scheme shown in Figure 6.19 which represents the process used today. The cast film process involves the extrusion of polyethylene through a die to form a thin molten layer of material that is drawn down to a thinner gauge based on the difference between the rate the molten polymer exits the extruder and the rate that the molten polymer is cooled onto a chill roll. The film cools very rapidly on the chill roll and film orientation is only in the machine direction. Optical properties are usually better with cast film as compared to blown film and line rates may be higher with the cast film process. A cast film line may also involve coextrusion of several layers of different types of thermoplastics to fabricate specialty films with improved properties. A... 

The motivation for this study was interface movement in the coextrusion of multilayer films. Figure 10.16 shows the computed and experimental deformation of the interface for flow of a low-density polyethylene (Dow 6411) through the same 61-cm channel. Here, only two layers were used. A three-dimensional finite element... 

New block and graft copolymers (Dipron) with good abrasion and heat resistance and good antifriction properties have been prepared in the USSR by coextrusion of polyethylene and polyamides [131]. Coextrusion has also been used to modify the relatively low impact strength at low temperature of polypropylene. A mixture of polypropylene and polyethylene has been processed at different conditions. The resulting materials have been compared to block copolymer produced by copolymerization [132]. The proper-... 

Metallocene polyethylene (mPE) plastomers 0.900-0.915 Coextrusion, heat seal layers Ethylene-vinyl acetate (EVA) resins with 9% vinyl acetate, ultra low density polyethylene (ULDPE), very low density polyethylene (VLDPE)... 

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