The rise in flexible-packaging applications and in the retail sale of pro-cessed, snack and convenience foods continues to drive the need for im-proved packaging barriers, which favors the selection of metallized films. Choosing the most appropriate metallized-film type can be determined from knowledge of product failure modes (stale, rancid or loss of flavor or nutritive value) and barrier-property profiles of the various metallized films. These factors are also be valuable for pharmaceutical packaging, which often times needs the same barrier profiles to maintain drug efficacy.

The growth in flexible packaging, in general, has been brought on by several factors. Among these are a need to reduce packaging costs, a new emphasis on packaging graphics for brand recognition to increase market share, consumer convenience, product protection and new or improved packaging materials. Some of the flexible packaging growth for food products is possible in large part due to the properties of metallized films, which have been improving steadily due to new product designs¹ and new metallizing process enhancements.²

These changes have improved barrier properties and allowed the effective replacement of existing packages such as cans, jars and boxes for food stuffs, as well as replacing existing packaging based on foil laminations and clear barrier technologies such as barrier polymer coatings. In some cases, the new packages maintain the barriers of the existing rigid package, and in other cases can lead to significant improvements in moisture retention and oxygen and/or aroma barrier.

The emergence of new package formats such as the standup pouch and the incorporation of zippers are helping to drive the switch from rigid to flexible. The combination of these new formats and im-proving metallized-film properties make metallized flex packs a natural choice.

In some sales regions, laws enacted to curb packaging use are also driving changes as packaging-weight reduction or structure simplification is rewarded with artificial cost savings in the form of reduced punitive taxes on packaging. While this approach can lead to higher consumer and packaging costs than are necessary, here again metallized films are at an advantage with their light weight, ability to combine several functions in one film and improving barrier profile of many metallized films.

What a food package needs

What makes metallized films especially suitable for food packaging is the unique combination of properties and attributes which can be designed into the film and the breadth of barr-ier property combinations and cost structures. However, determining the most suitable combination of properties and attributes (which is most cost-effective) does require improved knowledge of the food being packaged and the distribution cycle and environmental conditions to which the package will be exposed.

In general, the ability to answer the question of which metallized film is best for a particular application can be answered only by the knowledge of how the product fails. This is sometimes hard to determine.

Large-scale preparation and distribution of fully or partially prepared foods has helped increase the application of metallized films. Because of this, the food needs to be protected from changes in flavor and texture to maintain the just-prepared condition. Packaging of these products has evolved greatly over the years, and today the use of modified-atmosphere packaging and active packaging is commonplace. Consequently, the demand has grown for ever-increasing improvements in moisture, oxygen and flavor barrier as well as for improvements in package integrity needed to maintain the internal environment of the package.^3,4

Most conventional unmetallized film products cannot cost-effectively meet the stringent barrier profiles required. The improved food-package characteristics mentioned above do not generally mean a desire for a "metallized look" but rather to maintain the use of multilayer laminations with 100-percent ink coverage.

What is often forgotten in the description of metallized-film barriers is the key role played by the light barrier of the film in maintaining product freshness.³ Most organic materials are very sensitive to ultraviolet-light exposure and if left out will be degraded by free radical reactions (generally oxidation) induced by the effect of the UV light on the product). Thus, there is a complex interrelationship between the light, moisture and oxygen barrier, the food product and the desired shelf life, which will determine the optimum metallized-packaging material. Typically, the light-barrier level of metallized films is approximately 1-percent light transmission (An Ωptical Density of 2 minimum), which also results in optimized metallization-process speeds and film-barrier properties.⁵

Assuming the product's need for a light barrier exists, Figure 1 shows a mapping of oxygen- and moisture-barrier space for some product types. Maps of this type help differentiate between appropriate barrier-packaging property levels for various foods. For instance, coffee packaging is relatively complex and requires good oxygen and chemical barrier but not exceptional moisture barrier. It cannot get "stale" in a classic sense as it is not eaten directly (Carbon dioxide must be vented, however).

In contrast, dry foods with little fat to oxidize would need better moisture barrier and could use a "low oxygen barrier" film. Dry products with unsaturated oils need both a moisture and oxygen barrier but the level of oxygen barrier needed depends on the moisture barrier. For example, based on the moisture barrier used, will the product get stale before it gets rancid? If so, then oxygen barrier is not important. If the product gets rancid before it gets stale, then a better oxygen barrier would extend shelf life. This relative behavior for potato chips is shown in the product-failure mapping of Figure 2.

Having mapped out various product-failure modes, it is then possible to compare this with a film-product barrier mapping to highlight the most effective metallized film for the packaging application. Figure 3 shows a barrier mapping of various metallized films to match up with the particular application.

Surprisingly, not all metallized films have a good chemical (flavor and aroma) barrier because of the chemical resistance of the base sheet.⁶ ΩPP metallized films generally show poor chemical barrier while, on the other hand, metallized PET and metallized EVΩH-skinned ΩPP films do, because of the chemical resistance of the PET and the EVΩH in the base sheet.

Consequently, the base sheet of the metallized film must be chosen for its chemical resistance to the food product's flavors and aromas, or for the protection of the food from chemical contamination from the environment.

Ωnce the required barrier-property level is determined, the film must maintain its barrier during the conversion of the film into finished packaging as well as into the final package. Generally during lamination, the metallized barrier may be improved slightly if handled properly but can also be destroyed if lamination tensions and/or temperatures stretch the film above 2-percent elongation by exceeding the film's tensile strength. During conversion into the final package, elongation of the metal layer must also be controlled by proper film tension, but also by control of the bending of the laminate over forming collars, etc.

Metallized films should grow in volume for food packaging due to their multiple-barrier profile and the continuing shift from rigid to flexibles. In addition, they should continue to replace existing clear barrier products, such as PVDC and acrylic-coated films, for food packaging because the basic light-barrier protection of metallized films allows food to remain fresher longer.