Single-layer blown films

Figure 17.11 Examples of single layer blown film dies include side fed type (top left), bottom fed with spiders type (top center) and others are spiral fed types...

Several selected GLS Corporation elastomer products have proven well suited, especially as elastomer film and sheets in medical applications. Single-layer blown films can typically be produced down to 0.1-mm-layer thicknesses. TPEs are used as surgical elastomer wipes, as tapes for gauze bandages, for disposable surgical gowns, or for tourniquet bands. In addition to FDA compliance, easy sterilization properties as well as being free of latex and PVC, important TPE features are their excellent elasticity and soft, flexible surface. 

The film tube is collapsed within a V-shaped frame of rollers and is nipped at the end of the frame to trap the air within the bubble. The nip roUs also draw the film away from the die. The draw rate is controlled to balance the physical properties with the transverse properties achieved by the blow draw ratio. The tube may be wound as such or may be sHt and wound as a single-film layer onto one or more roUs. The tube may also be direcdy processed into bags. The blown film method is used principally to produce polyethylene film. It has occasionally been used for polypropylene, poly(ethylene terephthalate), vinyls, nylon, and other polymers. 

Blown film is made similarly to blow-molded articles the molten resia is extmded coatiauously as a parisoa, which is blowa to form a coatiauous film cyliader. This cyliader can be roUed up as a two-layer stmcture or sHt to form one large or two smaller single layers. 

The blown film process involves extruding a relatively thick tube that is then expanded or blown by the usual internal air pressure or the water quench process to produce a relatively thin film (Figure 5.6). The tube can be collapsed to form double-layer layflat film or can be slit to make one or two single-layer film webs. The water quench process is the generally preferred method of producing blown PP type film. 

Coextrusion is the process of forming an extru-date composed of more than one thermoplastic melt stream. The process came about because some service demands, particularly from the packaging industry, could not be satisfied by a single polymer although they could be met by a combination of polymers. Coextrusion was first practiced in the production of cast film and is now also used in blown film and sheet extrusion. The intention is normally to produce a laminar structure in which each layer contributes a key property to the overall product performance. Coextruded films may be very complex structures composed of many different functional layers, including tie layers whose purpose is to bond neighboring layers of limited compatibility. Five layers are not uncommon. However, side-by-side coextrusion is also possible. Fluoroplastics can be coextmded with other polymers such as ETFE and nylon. 

Multilayer structures are often used in packaging in order to take advantage of the properties of several polymers at once. Multilayer structures are made by lamination, coating, and/or coextrusion. Because of the multiple handling of materials and the thickness of the layers, laminations (see Chapter 8) may be relatively expensive materials. Coextrusion can be used to reduce the cost of multilayer plastic film and sheet. Coextruded packaging materials are multilayer plastic sheet or film constructions produced from more than one plastic resin in a single step, using either the cast or blown film process. In coextrusion, the materials never exist as separate webs. 

A brief coverage of the topic of blown film coextrusion is provided in this chapter. Coextmsion is the process of feeding a single die with two or more different polymer melt streams. Within the die, the various flow streams are combined to form a single-ply film comprised of the individual layers (Fig. 5.1). Because of the high viscosity of polymer melts, the individual layers tend not to mix but to retain their positions within the combined flow stream. In some cases, over seven layers of polymer are extruded into a film. For each type of polymer layer in the final structure, a different extruder is connected to the die (Fig. 5.2). 

Coextrusion is the simultaneous extrusion of two or more polymers through a single die where the polymers are joined together such that they form distinct, well-bonded layers forming a single extruded product. Coextrusion has been applied in film, sheet, tubing, blown film, wire coating, and profile extrusion. 

Coextruded films are produced by a tubular-blown film process and a flat-die, chill-roll casting process. Capital and operating costs for blown-film vs cast-film coextrusion lines are strongly dependent on product mix and utilization. Equipment suppliers provide comparative economic evaluations for specific products. Practical cast-film equipment has been discussed previously (3). Coextrusion dies are unique. Extruders used before the die and take-away equipment used afterwards are standard equipment for single-layer film manufacture of blown or cast film (see Extrusion). 

Another style of tubular-blown film die is the stackable plate die (Fig. lb). In this style of die, each layer is spread uniformly and formed into a tube in a single plate. Plates are then stacked on top of each other and the layers are added sequentially. This style of die is becoming popular for specific applications since the number of layers can be adjusted by simply changing the number of plates in the die. 

The coextrusion process is used to combine multiple materials into a single film (see Coextrusion). Both blown films and cast films may be coextruded in three, five, seven, or more layers. The combination of multiple materials in a single film allows a cost-eflective means of combining the performance properties of several polymers in a single film. One example would be the coextrusion of a... 

Many polymers are extmded through various styles of dies to produce monolayer and multilayer products. Coextmsion is a common method used for producing multilayer stmctures. Coextmsion is a process in which two or more polymers are extmded and joined together in a feedblock or die to form a single stmcture with multiple layers. This technique allows the processor to combine the desirable properties of multiple polymers into one stmcture with enhanced performance characteristics. The coextmsion process has been widely used to produce multilayer sheet, blown film, cast film, mbing, wire coating, and profiles (1-6). 

Artificial lipid bilayer membranes can be made [22,23] either by coating an orifice separating two compartments with a thin layer of dissolved lipid (which afterwards drains to form a bilayered structure—the so-called black film ) or by merely shaking a suspension of phospholipid in water until an emulsion of submicroscopic particles is obtained—the so-called liposome . Treatment of such an emulsion by sonication can convert it from a collection of concentric multilayers to single-walled bilayers. Bilayers may also be blown at the end of a capillary tube. Such bilayer preparations have been very heavily studied as models for cell membranes. They have the advantage that their composition can be controlled and the effect of various phospholipid components and of cholesterol on membrane properties can be examined. Such preparations focus attention on the lipid components of the membrane for investigation, without the complication of protein carriers or pore-forming molecules. Finally, the solutions at the two membrane interfaces can readily be manipulated. Many, but not all, of the studies on artificial membranes support the view developed in the previous sections of this chapter that membranes behave in terms of their permeability properties as fairly structured and by no means extremely non-polar sheets of barrier molecules. 

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