Heat seal

The abrasion resistance of ionomers is outstanding, and ionomer Aims exhibit optical clarity. In composite structures ionomers serve as a heat-seal layer. 

Some cast (unoriented) polypropylene film is produced. Its clarity and heat sealabiUty make it ideal for textile packaging and overwrap. The use of copolymers with ethylene improves low temperature impact, which is the primary problem with unoriented PP film. Orientation improves the clarity and stiffness of polypropylene film, and dramatically increases low temperature impact strength. BOPP film, however, is not readily heat-sealed and so is coextmded or coated with resins with lower melting points than the polypropylene shrinkage temperature. These layers may also provide improved barrier properties. 

Coextrusion. An increasingly popular technique to produce tailored film or sheet products is to coextmde one or more polymer types in two or more layers of melt (6). In this fashion the benefits of specific polymer types or formulations may be combined. Thus high cost barrier resins may be combined with a low cost thicker layer of standard resin to achieve an optimum barrier film at lower cost. Thin sUp-control layers may be used on the surface of a bulk layer of opticaUy clear resin to obtain an aesthetic film with good handleabUity. Lower melting outer layers may be used to provide heat sealing for polymers that seal with difficulty by themselves. 

Other Techniques. The FEP resin is bonded to metal surfaces by the appHcation of heat and pressure it can be heat sealed or hot-gas welded. Heating FEP at 260°C and allowing it to cool slowly results in stress relieving, or annealing. The FEP film is used to weld PTFE-coated surfaces. 

Textile uses are a relatively stable area and consist of the lamination of polyester foams to textile products, usually by flame lamination or electronic heat sealing techniques. Flexible or semirigid foams are used in engineered packaging in the form of special slab material. Flexible foams are also used to make filters (reticulated foam), sponges, scmbbers, fabric softener carriers, squeegees, paint appHcators, and directly appHed foam carpet backing. 

Attempts have been made to perform thermal retorting ia a gas barrier flexible pouch or tray. The retort pouch, under development for many years, has a higher surface-to-volume ratio than a can and employs a heat seal rather than a mechanical closure. Similarly, plastic retort trays have higher surface-to-volume ratios and are usually heat seal closed. Plastic cans iatended for microwave reheating are composed of bodies fabricated from multilayer plastic including a high oxygen barrier material, plus double-seam aluminum closures. 

Specific Teasile Elongation, Heat-seal Water-vapor Gas... 

Linear Low Density Polyethylene. Films from linear low density polyethylene (LLDPE) resias have 75% higher tensile strength, 50% higher elongation-to-break strength, and a slightly higher but broader heat-seal initiation temperature than do films from LDPE. Impact and puncture resistance are also improved over LDPE. Water-vapor and gas-permeation properties are similar to those of LDPE films. 

Oriented polypropylene film has exceUent water-vapor barrier but poor gas barrier properties exceUent clarity, or opacity in newer forms and good heat-seal properties in packagiag appHcations. 

Ethylene vinyl acetate copolymer (EVA) forms a soft, tacky film with good water-vapor barrier but very poor gas-barrier properties. It is widely used as a low temperature initiation and broad-range, heat-sealing medium. The film also serves for lamination to other substrates for heat-sealing purposes. 

Some flexible packaging is fabricated by converters into bags and pouches. Bag material is either small monolayer or large multiwall with paper as a principal substrate. Pouches are small and made from laminations. Bags usually contain a heat-sealed or adhesive-bonded seam mnning the length of the unit and a cross-seam bonded in the same fashion. 

Applications. Preformed bags are opened by the packager, filled with food product, and closed by adhesive, heat-sealing, clipping, stitching,... 

In a variant of the horizontal form/fill/seal operation, the material, moving in a horizontal direction, is folded on itself vertically. Vertical sections of the two faces are heat-sealed to each other to form a pouch, which may then be filled. The pouch, usually made from film or paper bonded to aluminum foil plus a plastic laminant and heat sealant, is closed by a heat seal. This type of pouch gives high moisture and oxygen protection and is used for moisture-and flavor-sensitive condiments and beverage mixes. 

Hot tack strength is the abiUty of a heat-seal layer to hold together while molten, before the seal cools and sets up. This is a technically important property which is difficult to measure reproducibly in the laboratory. Owing to the reinforcing effect of ionic bonding on melt strength, ionomer sealing layers provide superior performance in a wide spectmm of appHcations. 

For skin packaging, ionomer films provide outstanding protection, especially for sharp objects. Films can be drawn down tightly around regular shapes without penetration. Characteristics of heat seals can be adjusted so that convenient peelable seals are obtained (31—35). 

Commercially available containers for use with parenteral products include single-dose ampuls that are heat sealed and opened by snapping at the point of least diameter, vials for multidose use, and botdes and pHable bags that are used for large volumes such as needed in intravenous infusions. Container size can vary from 1 mL to 1 L. Generally volumes up to 100 mL are available as ampuls or vials. 

In contrast to most extmsion processes, extmsion coating involves a hot melt, ca 340°C. The thin web cools rapidly between the die and nip even at high linear rates. Both mechanical and chemical bonding to substrates are involved. Mechanical locking of resin around fibers contributes to the resin s adhesion to paper. Some oxidation of the melt takes place in the air gap, thereby providing sites for chemical bonding to aluminum foil. Excessive oxidation causes poor heat-sealing characteristics. 

Vinylidene Chloride Copolymer Latex. Vinyhdene chloride polymers are often made in emulsion, but usuaUy are isolated, dried, and used as conventional resins. Stable latices have been prepared and can be used direcdy for coatings (171—176). The principal apphcations for these materials are as barrier coatings on paper products and, more recently, on plastic films. The heat-seal characteristics of VDC copolymer coatings are equaUy valuable in many apphcations. They are also used as binders for paints and nonwoven fabrics (177). The use of special VDC copolymer latices for barrier laminating adhesives is growing, and the use of vinyhdene chloride copolymers in flame-resistant carpet backing is weU known (178—181). VDC latices can also be used to coat poly(ethylene terephthalate) (PET) bottles to retain carbon dioxide (182). 

Materials are also blended with VDC copolymers to improve toughness (211—214). VinyHdene chloride copolymer blended with ethylene—vinyl acetate copolymers improves toughness and lowers heat-seal temperatures (215,216). Adhesion of a VDC copolymer coating to polyester can be achieved by blending the copolymer with a linear polyester resin (217). 

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