Extrusion film orientation

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... 

It was discovered that a number of PVDF film properties, including electrical, could be altered by the extrusion and orientation conditions.The relationship between the extrusion and orientation process and the electrical properties of the film are important because of the application of PVDF films in miniaturized capacitors which are used in apparatus such as defibrillators. To compare the properties, extruded film, while in molten state, was brought in contact with chilled rollers at 80°C and cooled rapidly. The oriented film was stretched at 150°C in the longitudinal direction by a stretch ratio of 3.5, that is, its original length was increased by 3.5 times [Eq. (6.6)]. The dielectric constant and dissipation factor were measured as seen in Table 6.8. Orientation increased the dielectric constant and reduced the dielectric loss... 

R. W. Lusignea, Extrusion of oriented LCP film and tubing, published in Applications of High Temperature Polymers, edited by R. R. Luise, pp. 25 0, CRC Press, Inc., New York (1997). 

Polypropylene is the only high-volume thermoplastic that is processable by all major fabrication methods molding, extrusion, film, and fiber. Many new polypropylenes with improved engineering properties have been developed. The most promising are glass-reinforced polypropylene, foamed polypropylene, heat-resistant-polypropylene, and oriented sheeting. 

Virtually every polymer can be used in film form. Most thermoplastic polymer films are prepared by conventional extrusion techniques based on calendering, casting, and blown-film, or tenter-film systems. Other polymers, which cannot be easily melted, are formed into films by solvent casting. In the selection of a film for a particular application, the properties of the poljuneric materials must be considered in view of the application. Thermal properties, molecular characteristics, and crystallinity of the polymer affect processing and film properties. Additives influence extrusion and orientation processes and improve film properties. 

Molecular Weight and Molecular Weight Distribution. Molecular weight and molecular weight distribution affect extrusion and orientation processes and the physical properties of the film, eg, tensile strength, elongation, impact resistance, (qv) and optics (see Molecular Weight Determination). 

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 film can be further oriented uniaxiaUy via a secondary heated roll stacks, or oriented biaxially via a combination of machine direction and transverse direction stretching, which can be accomplished simultaneously or as a two-stage process. Greater detail of cast film extrusion and film orientation process, effects, and mechanics can be found in the referenced texts [18,19]. 

Other Vinyl Resins. One of the most important uses of poly(vinylidene chloride), PVDC, is for packaging film. The PVDC resin used for this purpose is usually a copolymer of vinylidene chloride and vinyl chloride. The film is produced by casting or by blown film extrusion. Biaxially oriented film is clear, with good strength and low permeability to moisture and other gases. This makes it especially useful for food packaging. 

Methods of Achieving Orientation. The most common ways of achieving orientation Tn semi crystal 1ine polymers are spinning from a gel, melt or solution, stretching in the solid state, or solid state extrusion. Biaxial orientation is found in blown films or in films prepared by passing a melt over a conical die. 

Blown film extrusion, online orientation, Raman spectroscopy, low-density polyethylene... 

Available as both film and sheet unless otherwise noted. All materials are available in PDA grades except for CTA, PVP, and PL EX = extrusion CL = calendering REG = regeneration CAST = casting BO = biaxial orientation. 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

A plastic film, 0.1 mm thick, is required to have its orientation in the transverse direction twice that in the machine direction. If the film blowing die has an outer diameter of 100 mm and an inner diameter of 98 mm estimate the blow-up ratio which will be required and the lay flat film width. Neglect extrusion induced effects and assume there is no draw-down. 

In an anisotropic material, the properties depend on the direction in which they are tested. For example, rolled metals, which are anisotropic, tend to develop a crystal orientation in the rolling direction. Thus rolled and sheet-metal products have different mechanical properties in the two major directions. Also, extruded plastic film can have different properties in the machine and transverse directions. These materials are oriented biaxi-ally and are anisotropic. (As reviewed above under EXTRUSION, Orientation). 

What process variables control the degree of orientation in a film manufactured via cast film extrusion and blown film extrusion ... 

Sometimes the hazy optical properties of polypropylene are a detriment to its use in thin films. To address this problem, as well as to improve physical properties and reduce material costs, we can orient the molecular chains by stretching the polymer film after it has left the extrusion die. There are two methods of orienting films, film blowing, shown in Fig. 11.6, and tentering, shown in Fig, 19.6. 

Processes such as film extrusion, fiber spinning, injection molding, and drawing tend to impart orientation to products made from semicrystalline polymers. Mechanical, dielectric, and optical properties, to mention only three, are often strongly influenced by orientation. X-ray diffraction offers a direct approach to studying crystallite orientation because the Intensity that is diffracted into a detector placed at an appropriate position is directly proportional to the number of crystal lattice planes that are in the correct orientation for diffraction. The principles of such measurements are well described in textbooks 0,2). 

ISO 1874-2 1995 Plastics - Polyamide (PA) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 15987 2003 Plastics - Film and sheeting - Biaxially oriented polyamide (nylon) films. 

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