Production of Films and Sheets

Polymer foils and sheets possess unique properties including a high aspect ratio, a great flexibility and physical adhesion to ubiquitous surfaces [125]. When such films are produced from biodegradable or blocompatlble materials, they can serve as coating material for biomedical Implants. 

Melt pressing is a first technique which results in thin polymer films. Polymer granules are heated between electrically heated plates, which can be forced against each other. Since this is a discontinuous process, it has little industrial application. However, if the mould plates are micro-structured, structured polymer sheets can be developed. In this case, the term hot-embossing is generally applied. 

Bubble blowing can be seen as an alternative for melt extrusion. In this technique, a screw extruder forces the material through an annular die. At the same time, an inert gas stream passes through the inner tube to blow the extruded material into a cylindrical bubble. This bubble is then collected by a series of rollers to produce a continuous film. 

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The release effect of the LDPE film used for backing self adhesive coatings can be considerably enhanced by the addition of 2-3% amide wax. It is also available as an antiblocking system in granular form. This antiblocking system is also used successfully in the production of film and sheeting of injection-molded articles from EVA copolymers. 

High line speeds The attractiveness of film and sheet products depends on their low costs, which is determined pardy by line speeds. Processing aids can allow higher line speeds without their own costs negating the overall process savings that are gained. 

Stiffness of the films and sheeting can be measured as the tensile modulus of elasticity. Droop or drape tests may be used, particularly for multilayer products. The stiffness is strongly influenced by thickness (to the third power) and temperature, and is important to the processing of film in printing, coating, or end use appHcations where it affects the "hand" of the product. 

Tables 2—5 Hst some typical properties or ranges of properties for the more common film and sheet products. Although these values are good for comparative purposes, actual performance tests are best to determine suitabiHty for use. Properties of multiple-layer films or sheets in laminar stmctures cannot always be predicted from values for the individual polymer layers. Use conditions of stress, temperature, humidity, and light exposure all strongly influence performance. Film and sheet manufacturers can recommend product combinations or variations that may provide significant performance advantages to the user.

The basic methods for forming film or sheeting materials may be classified as follows melt extmsion, calendering, solution casting, and chemical regeneration. Of special note is the use of biaxial orientation as part of the critical manufacturing steps for many film and sheet products. 

Solution Casting. The production of unsupported film and sheet by solution casting has generally passed from favor and is used only for special polymers not amenable to melt processes. The use of solvents was generally very hazardous because of their flammabiUty or toxic nature. The cost of recovery and disposal of solvents became prohibitive for many lower price film appHcations. The nature of the drying operations leads to problems with solvent migration and retention that are not problems with melt-processed polymers. 

Table 6 shows the sales estimates for principal film and sheet products for the year 1990 (14). Low density polyethylene films dominate the market in volume, followed by polystyrene and the vinyls. High density polyethylene, poly(ethylene terephthalate), and polypropylene are close in market share and complete the primary products. A number of specialty resins are used to produce 25,000—100,000 t of film or sheet, and then there are a large number of high priced, high performance materials that serve niche markets. The original clear film product, ceUophane, has faUen to about 25,000 t in the United States, with only one domestic producer. Table 7 Hsts some of the principal film and sheet material manufacturers in the United States.

Extrusion. In general, extmsion is the process of forcing a polymer melt through a die (104,105). Typical extmsion appHcations include initial resin pelletization after manufacture and production of film, sheet, pipe, tubing, and insulated wire. The HDPE extmsion temperature is around 150°C, the pressure 40—50 MPa (5800—7250 psi). An extmsion production line usually consists of an extmder (mono- or twin-screw) with a die at the end, a cooling and shaping device, a pulling device (a roUer), and a cutter. 

Polyester film consumes 7% of production. When coated with a chemical emulsion, it is used as x-ray and microfilm when coated with a magnetic emulsion, it is used for audio and video tapes and when coated with an adhesive, it is used for wrapping and sealing tapes (see Films and sheeting). 

Poly(vinyl chloride-i o-vinyl acetate) [9003-22-9] has found appHcation in flooring, phonograph records, protective coatings, fibers, and some films and sheeting. Because of their low viscosity and good processabihty, such copolymers constitute the bulk of the vinyl tile market. The total production of PVC copolymers in 1989 was 113,500 t (73) (see Vinyl polymers). 

The specimen is mounted in a punch-type shear fixture, and the punch (1 in. diameter) is pushed down at a rate of 0.05 in./min until the moving portion of the sample clears the stationary portion. Shear strength is calculated as the force per area sheared. Shear strength is particularly important in film and sheet products where failures from this type of load may occur (Fig. 2-21). This property can be used for comparison with other materials and for determination of the forces needed for punching openings (holes, etc.). 

Basically the calendering process is used in the production of plastic films and sheets. It converts plastic into a melt and then passes the pastelike mass through roll nips of a series of heated and rotating speed-controlled rolls into webs of specific thickness and width. The web may be polished or embossed, either rigid or flexible (9). One of its sheets major worldwide markets is in credit cards. At the low cost side these lines can start a million. A line, probably the largest in the world... 

Film and sheet can, in principal, be made by calendering or by extrusion. Factors that govern the advantages and disadvantages (limitations) of each process can interact in a complex way. Factors to be considered include (1) type of material to be processed, (2) quantity of product to be produced, (3) thickness... 

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