Flexible packaging

The process of laminating single-layer web materials to give flexible multilayer film structures has been an established method for many years. Alternate production techniques include extrusion coating and co-extrusion, where plastics are melted and extruded in thin layers through an extrusion die. 

Common laminating adhesives include solvent containing and solvent free, crosslinking polyurethanes, and two-componenf water-based polyurethanes - the latter have been increasing in importance in recent years because of environmental pressures. The choice of adhesive depends on the type of film to be bonded and on the end use application. In food packaging, for example, food regulations and the resistance of the adhesive (e.g. to boiling water) are also important. 

The prime advantage of aqueous polyurethane and polyacrylate dispersions over solvent-containing systems is that recovery or disposal of significant amounts of solvent is unnecessary. In the packaging industry, potential for residual solvent traces in the adhesive and subsequent migration into food are also major concerns. Therefore, solvent-containing adhesives have already been replaced by environmentally friendly water-borne adhesives in a number of applications. 

Film laminates are produced by coating adhesive on to one side of the primary film, drying, and then laminating a second film on to the dried adhesive layer under 

Addition of curative of this type results in a significant increase in adhesive strength however, covalent reaction occurs to a small extent owing to steric factors. The majority of the adhesion increase is attributable to the formation of hydrogen bonds predominantly formed between OH and NH groups and polar groups of the individual substrate. 

The primary disadvantages of flexible packaging are its lack of convenience for the user, and lack of strength. Flexible packaging has no appreciable ability to support a load, so secondary packaging must provide any strength that is required. Flexible packages tend to be difficult to open, and they are often impossible to reseal 

In 2004, Treofan GmbH developed a metallised version of its PLA biodegradable film that reduces permeability aromas, oxygen and water. The metallised Biophan PLA film is said to be suitable for packaging fatty foods such as butter and cheese, as well as for confectionery, where the mirror-like finish adds a decorative feature to the barrier properties. The metallised film meets both EU and US, Food Drug Administration food contact requirements. 

Plastic Suppliers Inc., a US extruder of blown film for labels and envelopes, has produced the world s first blown film from NatureWorks PLA. It was hitherto thought that PLA was unsuitable for blown film extrusion. Plastic Supplies claims that its EarthFirst film is 100% compostable, has high gloss, optimum clarity and transparency, high moisture vapour transmission rate, flavour retention, odour barrier, is breathable and is US Food and Drug Administration (FDA) compliant. Areas of application for EarthFirst include window carton film for food packaging, label film, floral wrap film, shrink film and envelope film. 

In 2004, Office Media (Tokyo) developed a new PLA film exhibiting vastly improved functionality as a packaging material. Through combination with other biodegradable plastics, the film s transparency, flexibility, heat resistance and impact resistance, have been balanced in multiple dimensions, and through adopting two-layer and three-layer structures, gas barrier properties have also been improved. Technology to eliminate the characteristic odour of PLA developed independently by Office Media, have also been applied. 

SBC film is growing into more and more film applications. It is proving to be well suited for fresh cut produce packaging, candy twist wrap, shrink films, flexible medical and decorative films. SBC can be extruded into either cast or blown film for use in flexible packaging applications. While some applications employ a monolayer SBC film, many films are co-extruded with SBC along with several layers of other materials in order to bring a combination of properties to the final application. 

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Flexible Packaging. Flexible packaging is composed of both single- and multilayer stmctures. The latter may be further subdivided into laminated, coated, and coextmded, or combinations of these. 

More than half of flexible packaging is used for food. Within foods, candy, bakery products, and snack-type foods, such as potato and com chips, use well over half of flexible packaging. Cheese, processed meat, shrink wraps, condiments, dry-drink mixes, fresh meats, and fresh produce represent smaller appHcations. 

Fabrication. Flexible packaging materials may be mono- or multilayer. Monolayer materials are usually films that have been produced by polymer resin melting and extmsion. 

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. 

A small quantity of flexible packaging material, usually oriented polypropylene, shrink polypropylene, or polyethylene, is used to overwrap paperboard cartons. The film is wrapped around the carton and sealed by heating. Products such as boxed chocolates, candies, and cookies are overwrapped, sometimes by a printed film. 

Numerous variations and other appHcations are common for flexible packaging materials, eg, oxygen-permeable wraps for fresh red meat and produce shrinkable, low oxygen permeabiHty bags for meat and rigid tray closures. 

A. L. Brody, Flexible Packaging of Foods, CRC Press, Inc., Boca Raton, Fla., 1972. 

M. L. Troedel, ed.. Current Technologies in Flexible Packaging ASTM Special Technical Publication 912, ASTM, Philadelphia, Pa., 1986. 

Within the scope of the original definition, a very wide variety of ionomers can be obtained by the introduction of acidic groups at molar concentrations below 10% into the important addition polymer families, followed by partial neutralization with metal cations or amines. Extensive studies have been reported, and useful reviews of the polymers have appeared (3—8). Despite the broad scope of the field and the unusual property combinations obtainable, commercial exploitation has been confined mainly to the original family based on ethylene copolymers. The reasons for this situation have been discussed (9). Within certain industries, such as flexible packaging, the word ionomer is understood to mean a copolymer of ethylene with methacrylic or acryhc acid, partly neutralized with sodium or zinc. 

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

This paper organizes some of the how and why of flexible package-food product interactions by discussing a few specific examples of food packaging development, as well as some overall factors of packaging material application. 

Among the commonly used flexible packaging materials, aluminum foil probably provides the most complete permeation barrier while paper is the most permeable. Although aluminum foil provides a barrier to moisture, gas, grease, and light, it usually needs protection from the contents of the package and from the environment since it is a soft metal and subject to chemical attack. 

The different food products discussed below illustrate compatibility/ incompatibility problems and solutions in flexible packaging. 

Trace Solvent Removal. Several papers were written by Nadeau (4) and by Gilbert (5) and co-workers on gas chromatographic methods for determining solvent traces remaining in flexible packaging films after printing or adhesive lamination. With proper equipment and techniques,... 

This paper describes an irradiation curing method which improves the mechanical, chemical, and thermal properties of multilayered flexible materials, increases the bond strength among the adhesively bonded layers, and provides flexible packages that can withstand the thermo-... 

Irradiation Conditions. The gamma (cobalt-60) radiation facility and the source calibration are described by Holm and Jarrett (4). Irradiation temperature was 21 (initial) - 40°C (final). The gamma source was calibrated with the ferrous sulfate/cupric sulfate dosimeter for a dose rate of 8 X 102 rads per second. Pouches were fabricated from multilayered materials and then irradiated while empty. The container used to hold the multilayered materials and the empty pouches during irradiation was a large size, flexible package that was sealed under vacuum prior to the irradiation. 

Therrtial Sterilization of Pouches of Beef. Methodology for food thermoprocessing in cylindrical metal containers assured food sterility in flexible packages. Beef slices (1.25 cm thick) were steam cooked to an internal temperature of 72°C and vacuum sealed in pouches to give a fill of 120 g. These pouches of beef were processed in a standard retort with complete water circulation and a superimposed air pressure of 1.7 X 105 Pa. The retort schedule—a 40-minute cook at 118°C plus come-up time followed by a 30-minute cooling time—achieved a F0 (lethality value) of 6. 

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