Cast-film

As pointed out earlier, PTFE homopolymer cannot be processed by melt extrusion because of its extremely high melt viscosity. Thus, other methods, such as skiviug 

FIGURE 6.6 PTFE stadium roof. (Courtesy of St. Gobain Corporation.) 

The equipment used is again a vertical coater with heated zones, very similar to the coating tower for fabric [14]. The speed of the belt is slow, about 0.3 to 1 m/min (1 to 3 ft/min), and there are no applicators used to remove excess dispersion. The amount of coating picked up by the belt is controlled mainly by the solids content of the dispersion and by the belt speed. A production machine is built with multiple stages. Thus, after a film is sintered, it is recoated in the next stage. At the end of the machine, there is a device designed to strip the finished films from both sides of the belt and to wind them up into rolls. 

The advantage of this method is that each layer can be made from a different type of dispersion. For example, clear and pigmented layers can be made or the 

FIGURE 6.7 PTFE roofing elements at the Munich Airport Center. (Courtesy of Skyspan Europe GmbEl.) 

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Cast babbitts Cast film Cast film process Casting... 

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). 

Cast films provide a high clarity, heat sealable film and are primarily used as an overwrap for boxes and other packaging. These films have a lower density than cellophane and provide a longer product shelf life. Properties of polypropylene films are given in Table 12. 

Eig. 7. Extmsion of roU-cast film A, die inlet B, cast-film die C, air gap with molten web D, casting roU E, stripping roU E, idler roU G, edge-trim sHtter ... 

One of the requirements of this process is that the melt maintain good contact with the chill roU, ie, air must not pass between the film and the roU. Otherwise, air insulates the plastic and causes it to cool at a rate different from the rest of the plastic and this spoils the appearance of an otherwise satisfactory product. The melt should not emit volatiles, which condense on the chill roU, reduce heat transfer, and mar the film s appearance. The cast film process allows the use of a higher melt temperature than is characteristic of the blown film process. The higher temperature imparts better optical properties. 

Film. Nylon film can be pioduced as eithei tnbulai oi cast film. In tubular film, melt is extmded through a screen pack and a tubular die, and a... 

Deformation of a solution usually either by spreading or by extrusion as used in making cast film and certain synthetic fibres and filaments. 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

An important application for polypropylene is film tape. This is made by slitting unoriented film (cast or blown) into tapes 2 or 3.5 mm wide and stretching under heat about seven-fold. With cast film the orientation is more completely monoaxial and there is a tendency for the film to split along its length (fibrillate). Tubular film does not self-split so easily and also has a somewhat softer feel. Such tapes may be woven into sacks and these have... 

Copolymers of vinylidene chloride with 5-50% acrylonitrile were investigated by IG Farben during World War II and found to be promising for cast films. Early patents by ICC and Dow indicated that the copolymers were rigid, transparent and with a high impact strength. 

Because polycarbonates are good light absorbers, ultraviolet degradation does not occur beyond a depth of 0.030-0.050 in (0.075-0.125 cm). Whilst this is often not serious with moulded and extruded parts, film may become extremely brittle. Improvements in the resistance of cast film may be made by addition of an ultraviolet absorber but common absorbers cannot be used in moulding compositions because they do not withstand the high processing temperatures. 

Film generated by depositing a layer of liquid plastic onto a surface and stabilizing by evaporating the solvent, by fusing after deposition or by cooling. Cast films are generated from solutions or dispersions. 

P. C. Wu, Metallocene ethylene based resins in cast film extrusion and application, MetCon 94 Proceedings, USA, May 1994. 

The experimental procedure consisted of casting the varnish on glass plates by means of a spreader bar having an 0-102 mm (0 004 in) gap this produced a wet film 0-051 mm (0-002 in) thick that yielded a dried film of 0-025 mm (0-001 in). This standard thickness was used throughout and resistances are quoted in cm. The cast films were dried for 48 h in a glove box followed by a further 48 h in an oven at 65°C. 

When samples of about 1 cm were taken from a single cast film of 100 X 200 mm of a number of paint and varnish films, their resistances varied with the concentration of potassium chloride solution in one of two ways (Fig. 14.2). Either the resistance increased with increasing concentration of the electrolyte (inverse or / conduction) or the resistance of the film followed that of the solution in which it was immersed (direct or D conduction). The percentage of / and D samples taken from different castings varied, but average values for a number of castings were 50% D for the pentaerythritol alkyd and the tung oil phenol formaldehyde varnishes, 57% for urethane alkyd, 76% for epoxypolyamide and 78% for polyurethane varnishes... 

Union Carbide Corp produced the first of the synthetic papers in the late 1960s. Since that time other examples of synthetic papers include DuPont s Tyvek nonwoven paper and Van Leer s Valeron cross-laminated film. This market is now dominated by a few large, worldwide ventures with proprietary processing techniques that extend the use of single and multilayer extruded blown film or cast film. 

The pumping pressure required on the melts entering the different designed die heads differs to meet their melt flow patterns within the die cavities. The pressure usually varies as follows (1) blown and lay-flat films at 13.8-41.3 MPa (2000-6000 psi) (2) cast film, sheet, and pipe at 3.5-27.6 MPa (500-4000 psi) (3) wire coating at 10.3-55.1 MPa (1500-8000 psi), and (4) monofilament at 6.9-20.7 MPa (1000-3000 psi). 

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