Blow molding layers

Fuel tank applications utilizing coextruded blow-molded layers of ethylene vinyl alcohol (EVOH) in conjunction with high-density polyethylene (HOPE) have been in place for decades. In these applications, the EVOH layer is adhesively bonded to the HOPE with an acrylic copolymer adhesive. The EVOH acts as an impermeable membrane to gasoline and additives such that the anission permeation requirements for fuel tanks is met. 

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. 

A similar problem is presented by vehicle tires and certain blow molded bottles, which must be virtually impermeable to air and other gases. An example of the use of a very impermeable elastomers is butyl rubber. Because of its impermeability to gases, butyl rubber is used as a roof coating. With plastic bottles, different layers of both coinjected and coextruded plastics (Chapter 8) can be used to fabricate the bottle to make it impermeable to different vapors and gases depending on the barrier plastic included. 

Nylon barrier layers are used in packaging films and blow molded bottles. In these applications, their excellent resistance to oxygen permeation are valued when packaging greasy foods, such as potato crisps (chips), that rapidly turn stale when exposed to oxygen. 

Laminar materials are produced by a variety of techniques. Coextrusion blow-molding produces a number of common food containers that consist of multilayers such as layers consisting of PP-adhesive-PVA-adhesive-adhesive-PP. 

Multilayered articles can be made by coinjection blow-molding or coextrusion methods. A three-layer system generally contains a barrier layer sandwiched between two exterior layers. These are actually laminar products. In the coextrusion sequence, several extruders can be used to place the material into the mold. The multilayer container is then produced from blowing air into the preform. 

Extrusion. Sheet, film, and profiled articles are made by extrusion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextrusions, different combinations of plastics are separately but concurrently extruded to form layered sheet or film. In the packaging industry, specialty resins such as high barrier ethylene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer are additive, as opposed to the "averaging of property" in blends. Multilayers are also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

It is possible to blow mold several layers of material by coextrusion blow molding processes. By appropriate material selection, the various parts of the structure can be optimized for the best balance between properties and cost. 

THV can be readily processed by blow molding alone or with polyolefins. The olefin layer provides a structural integrity while THV provides chemical resistance and considerably reduced permeation.32... 

EVOH have superior gas barrier properties.80 89 They are often used as the internal layer in multilayer food packaging films, blow molded rigid containers, gasoline tanks for automobiles for a variety of purposes, etc. EVOH can be fabricated by the usual melt processing methods. The barrier properties of films decrease in the presence of moisture, so multilayer with protective polypropylene (especially biaxially oriented material), low-density polyethylene, nylons, or other moisture barrier films provides films or products that are useful even with liquids. The... 

Figure 6.2 Example of a 3-layer coextrusion parison blow mold head with die profiling [left) and example of a 5-layer eoextrusion parison blow mold head with die profiling (courtesy of Graham Machinery Group)...

Castillo, R., Coextrusion Die Design Doubles Number of Layers for Packaging Films and Blow Molding Parisons, SPE ANTEC, 2001. 

The third and most successful approach was accomplished on a wheel-type blow molding machine. Unlike the shuttle machine where only gravity acts on a downward extruded parison, a wheel machine draws or pulls the parison from the die. This allows extension in the extrusion direction and the opportunity to thin down the barrier layer and reorient the mica platelets. 

The best way for the reader to learn is to study actual examples of parts made by the blow molding of fluoropolymers. A majority of such parts have a multilayer construction to combine the properties of other plastics in a composite structure. The most common technique for producing multilayer sheet/film is coextrusion. The chemical resistance of fluoropolymers makes them attractive materials for inner layers of containers that come in contact with aggressive chemicals that can swell or degrade thermoplastics. 

Figure 6.59 Blow-molding head for four- or six-layer parison coextrusion. (Courtesy Bekum America Corp.)...

In another example, ] a coolant conduit is described that consists of multiple polymer layers that render the composite stmcture resistant to hydrolysis and pressure with high burst strength. Motor vehicle engines are an application for a tubular form of this conduit (Fig. 6.61). In a two-layer construction, the inner layer was comprised of a fluoropol5mier such as ETFE, FEP, or PVDF that has been functionalized for compatibility with the polyamide outer layer. Such a tube was formed by blow molding. 

Figure 6.63 A section of a two-layer blow-molded tube.> >...

Technology to successfully produce multilayer injection blow molded bottles is much newer than that to produce multilayer extrusion blow molded bottles. Heinz Inc. s ketchup bottle was the first U.S. example (Fig. 12.22). The use of PET bottles made by coinjection blow molding has grown rapidly. As is the case for coextrusion blow molding, the key is production of the parison. Once the multilayer parison is produced, the remainder of the process is essentially the same as for single layer materials. Even stretch blow molding can be used. 

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