In flexible packaging, aluminum foil (hereafter foil) is the benchmark for barrier substrates when the requirements are protection against gases, moisture, and light. Throw in protection against grease, oil, and (many) chemicals and foil's preeminence is further established. No wonder foil has been a component in the packaging of foods, beverages, medicines, pharmaceuticals, cosmetics, and luxury items, among other categories.
At the thicker gauges, foil's permeation rates against gases and moisture are 0% and blockage against light is absolute; however, since the thicker the foil the more expensive, the challenge is source-reduction, choosing the thinness gauge consistent with protection requirements, because thinner gauges are more prone to pinholing. Additionally, beyond a certain thinness, foil can be troublesome to run for the converter and for the end-user. Another vulnerability of foil──one not limited to the thinner gauges──is flex-cracking, fracturing caused by repeated flexing, which can work its effects cumulatively and thereby can crop up unpredictably.
Despite the focus herein being on the protective prowess of foil, mention should be made of foil's aesthetics, for there is some overlap. The metallic sheen of foil imparts an eye-catching, premium aura to the packaging that transfers to the product and to the brand. That characteristic can be enhanced with clear, colored lacquers for a variety of lustrous results. Yet the smooth, pristine surface of foil is easily abraded and therefore needs coating and/or lamination to shield its aesthetic and protective properties.
As formidable as foil is, the competition is not deterred (to the good fortune of end-users). For the most part, a particular competing material might not replicate all of foil's properties but instead compete against one or more of those properties that are deemed most important to a targeted industry or application. Following is discussion of several such materials.Metalized films
It stands to reason that the deposition of aluminum on plastic film will yield a substrate with barrier properties bearing some resemblance to those of foil. However, since any plastic film can be metalized, the question is, when does the metalizing of films make sense? The answer derives from two factors. One is the cost of the film (separate from cost as a function of the amount of aluminum deposited). The other is the inherent barrier properties of the film, which whatever they happen to be, are increased by metalizing. Those two factors explain why packaging applications for metalized films are dominated by OPP (oriented polypropylene), PET (polyethylene-terephthalate), and polyamide (nylon).
Suppliers of metalized films promote their products as viable options to foil-containing structures by a number of comparisons. One is cost, sure; however, suppliers go to great lengths to emphasize that the cost advantage is not an exchange for unacceptable performance. Depending on the application and the film, a supplier might cite better puncture-resistance, better resistance to flex-cracking, reduced density, and fewer layers. The latter two carry sustainability credentials in terms of transportation costs (lighter weight) and energy consumption (fewer converting steps).Glass-coated films
Silicon oxide is deposited onto film to bestow some of the barrier properties of glass, principally against gas permeation. The theory is sound and compared to foil and metalized films, glass-coated films (like their namesake) provide see-through clarity. That same property, however, makes it difficult to (visually, at least) detect interruptions in deposition, in addition to making the films ineffective barriers to light. Moreover, the bugaboo of cost continues to stymie inroads into all but higher-margin products, such as in the medical industry. Nano-films
For the past several years, nanotechnology has been evaluated for its potential as a type of "smart packaging." It involves platelets so small that the unit of measure is the nanometer (one-billionth of a meter); hence, the platelets are as small as molecules, sometimes smaller. The composition of the platelets is mineral, clay, for example. But rather than being deposited on the film's surface the platelets are embedded throughout the film's thickness. The result is supposed to be an increased barrier to gases and moisture. Those molecules can't permeate through the layered platelets, necessitating that the molecules travel around and between. The winding path can amount to a distance that's multiple times greater than the thickness of the film. That being as it is, permeation is not prevented; rather, permeation is slowed. That pertains to permeation into as well as out of the packaging. Sufficient? Depends on how long the barrier is needed. Furthermore, nano-platelets typically don't render a film opaque, a decided disadvantage if barrier against light is required.
There are some high hurdles to be cleared before nanotechnology fulfills its potential regarding barrier films. One is technological: the need for greater control over the patterns in which the platelets are aligned and layered. Another is social: the need for greater knowledge about the potential effects of nano-particles on human health.EVOH
EVOH (ethylene vinyl alcohol) is a clear copolymer that can function as a gas barrier when incorporated into a multi-layer structure, either by coextrusion or by lamination. As such, films that are modestly priced but are poor gas barriers (for example, polyethylene and polypropylene) can be combined with EVOH for improved barrier. EVOH sees usage in the modified-atmosphere packaging of food, where visibility of the contents is required, but non-food applications are increasing. Arguably, the biggest drawback to EVOH is that it is hydroscopic, and its tendency to absorb moisture reduces its gas barrier properties under conditions of high humidity.Foil or an alternative?
Nothing has fueled the growth of flexible packaging more than have advancements in barriers, whether against gas, moisture, light, or other undesirables. For the immediate future, some applications will continue to favor foil, retortable packaging being a prime example. Nonetheless, many applications will be the objects of battles between foil and other barrier substrates.
Converters and end-users would do well to partner under the mutual objective of devising innovative ways to meet barrier requirements. Auxiliary to that would be the development of improved methodologies for measuring the associated permeation rates, coupled with greater knowledge of the conditions under which those rates play out. In all, stakeholders should be committed to knocking down the barriers to better barriers. Sterling Anthony
is a consultant, specializing in the strategic use of marketing, logistics, and packaging. His contact information is: 100 Renaissance Center-176, Detroit, MI 48243; 313-531-1875 office; 313-531-1972 fax; [email protected]