Processes film extrusion

Polypropylene film is found in a variety of applications. Films are thin plastic sheets with thicknesses less than 250 pm. Applications for these films include candy mappers, plastic label material, electrical capacitors (which requires very pure polymer), and fiber glass insulation backing. There are two general categories of polypropylene films unoriented and oriented. Films are manufactured through either a cast or a blown film extrusion process. Oriented films exhibit improved strength relative to unoriented films. This means that they can be... 

Thermoplasticity. High molecular weight poly(ethylene oxide) can be molded, extruded, or calendered by means of conventional thermoplastic processing equipment (13). Films of poly (ethylene oxide) can be produced by the blown-film extrusion process and, in addition to complete water solubility, have the typical physical properties shown in Table 3. Films of poly(ethylene oxide) tend to orient under stress, resulting in high strength in the draw direction. The physical properties, melting behavior, and crystallinity of drawn films have been studied by several researchers (14—17). 

Cast film extrusion. In a cast film extrusion process, a thin film is extruded through a slit onto a chilled, highly polished, turning roll where it is quenched from one side. The speed of the roller controls the draw ratio and final film thickness. The film is then sent to a second roller for cooling of the other side. Finally, the film passes through a system of rollers and is wound onto a roll. A typical film casting process is depicted in Figs. 3.53... 

Figure 8-6. Schematic of cast film extrusion process (reprinted with permission from Tess and Poehlein, copyright 1985, American Chemical Society).

With the flexibility of the blown film extrusion process, one-step production of wide multilayer films is possible. Incorporating a built-in adhesive layer, PVDF films can be laminated directly onto various thermo and thermoset plastics or primed metal substrates. PVDF films can protect the esthetics of UV sensitive substrates while also providing a thermal, chemical, and abrasion-resistant barrier. Recently, white PVDF films have been commercialized that are of full opacity in the UV range of 290 00 Most recently,... 

The cast film extrusion process is schematically shown in Figure... 

Sheet material, i.e. material thicker than about 0.25 mm, is usually produced by using a slit-shaped die, whereas thinner material is often produced by a blown film extrusion process in which an annular die is used and air is blown into the centre of the tubular extrudate to blow it into a sort of bubble. At a certain distance from the die the polymer is sufficiently cool to solidify into a film, which is then flattened and collected on rollers. Figure 1.9 illustrates a blown-film system. 

Fig, 5. Schematic representation of the tubular film extrusion process. 

The gels shown in Figs. 11.38 and 11.39 were created in a blown film extrusion process. The polymer is an HDPE (ExxonMobil Paxon AA45-004) with a melt index of 0.35 gr/10 min. The gel in Fig. 11.38 is about 0.5 mm in diameter it has no significant discoloration. 

Monolayer Blown-Film Extrusion. One of the major applications for VDC resins is monolayer film produced via a blown-film extrusion process. Previously, VDC copolymer film extrusion was limited to just a few experts in the field. However, over the last several years, production of VDC-containing films has greatly expanded (195,196). Monolayer VDC copolymer blown-film lines are now commercially available from a number of manufactin-ers and a considerable number of lines have been recently installed aroimd the world, particularly in the People s Republic of China. 

Another very important idea is that drawing involves the stretching of a polymer network in which the junction points of the network are formed by physical entanglements. Even if crystallisation occurs, the deformation of the polymer network determines the overall constraints on the macroscopic deformation, which can occur during the fibre or film extrusion process and in subsequent tensile drawing or die drawing processes. The development of molecular orientation is a major factor in the development of physical properties such as tensile modulus and tensile strength and hence relates to the deformation of the molecular network. 

Cast film extrusion is used in manufacturing polypropylene films and requires greater surface pretreatment power density (possibly 2-3 times) compared to other polyolefin films. With blown film extrusion processes, polyethylene films are typically used and require pretreatment on both sides. Considerable amounts of slip additives, used to lubricate the surface of these films for processing ease, can be prevalent within the resin and migrate to the surface of the film within a few days after extrusion. Although there is potential for the additive to mask-over treatment, it is far more important to surface treat immediately after extrusion, since it will be practically impossible to do so after additive migration to improve surface properties sufficiently for ink, coating, or lamination adhesion. 

Kang, H. J. and J. L. White. 1990. A Double Bubble Tubular Film Extrusion Process of Poly /7-phenylene Sulfide (PPS). 48th SPE Annual Technical Conference, Dallas, TX, 36, 104-109. 

FIGURE 14.7 Schematic drawing of the blown-film extrusion process. (Data from U. S. I. Chemicals/Quantum, Petrothene Polyolefins A Processing Guide, 5th edn., National Distillers and Chemical Corporation.)... 

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