Ever since I entered the field of metallized-film product and process design in the mid-1980s, the key assumptions around metallized films have been changing dramatically. First it was assumed that metallized polyester (PET) and oriented polypropylene (OPP) would replace foil and that the barrier properties of metallized OPP would be equal to those of metallized PET. There were several other concepts and expectations around optical density (OD) levels, the impact of scratches and acceptable metal lift in laminations.

All of these are fairly settled questions now with realistic and functional choices for scratches, metal lift and optimum optical density defined. Also it's clear that metallized OPP and PET have not replaced foil in many areas, and metallized OPP does not have the barrier properties of metallized PET. Aside from the competitive tension between OPP and PET films based upon product thickness and yields (in²/lb or m²/kg), there remain strong, legitimate product protection reasons to choose between metallized OPP, PET and foil. Indeed, in some instances, it still makes perfect sense to select the lower-yield PET or foil rather than the high-coverage OPP in some metallized film and barrier-packaging market areas.

First, metallized OPP does not have the gas- or aroma-barrier profile of metallized PET, and neither are true replacements for foil based upon film properties. This is clearly shown in Figure 1, which maps out the oxygen and moisture barriers of foil¹ and metallized PET¹ and several metallized OPPs². This mapping teaches that the metallized films' barrier properties are controlled by the polymer surface on which the metallized layer is deposited³, ⁴, ⁵, ⁶. Neither metallized PET nor standard or high-barrier grades of OPP can replace foil across the board—based on film properties alone. This is not to say that metallized films haven't replaced foil, but in reality, replacement has been limited to those areas where foil represented overpackaging based on gas- and moisture-barrier requirements.

Indeed, in many applications the most significant barrier property of all metallized films is its light barrier⁷, ⁸, and here it can often compete effectively against foil. For many foods containing unsaturated oils will turn rancid by exposure to light much faster than by the mechanisms of dark oxidation via oxygen or moisture.

This, I believe, is one aspect of the slow growth and relatively low demand for clear barrier packaging in North America and other markets. Our foods taste better when packaged in metallized films, primarily due to the light barrier. That's why some of the first snack packaging was in cans. This may not be true in all regions of the world because of local tastes and the kinds of foods packaged, which is why there may be a demand for clear barriers in some regions and not others. This is also the greatest risk to clear, coated PET and OPP packaging-film markets where oxygen, aroma and moisture barrier will not be enough in the absence of a light barrier to maintain product freshness for today's ever-lengthening shelf life demands.

As this need for expanded protection is being realized, I believe the increasing demand for improved packaging will fuel the requirement for a four-component barrier profile of light first, moisture second followed by oxygen or chemical (aroma) barrier jockeying for third place—depending on the particular application. Why do I say this? Because how packaged products fail must be taken into consideration. Once this is understood, the market split between metallized PET, OPP and foil is easily understood, and the directions for development of new barrier technologies is clear.

In products most sensitive to light, such as photographic film or those requiring several years or decades of shelf life, laminated foil is clearly the material of choice. There are also many decorative-packaging applications for metallized films or foil where the least expensive material processible is the best choice. Otherwise for many products, an optical density (OD) of 2.0-2.4 is sufficient and yields optimum oxygen and moisture barrier properties⁶.

For foods which contain complex oils or delicate flavors but have high-moisture contents, such as coffee, and don't readily go stale, then metallized PET with its excellent oxygen and chemical barrier is the film of choice and trumps OPP's yield advantage. If oxygen or aroma barrier is of lesser importance, then metallized OPP is best due to its lower coverage cost (higher yield). This is especially true for products that go stale before becoming rancid.

Returning to Figure 1, there is a set of films with improved moisture barrier relative to metallized PET and standard metallized OPP. These are the high-barrier metallized OPPs. These films have improved moisture barriers but show a wide range in oxygen and chemical barrier². They are based upon the metallization of HDPE⁹ (HB film) and EVOH¹⁰ (UHB film) surfaces, and indeed MET-UHB has the potential to replace foil based on its gas- and moisture-barrier potential but not for ultimate light barrier. (We will always need foil). This UHB film also has a chemical-barrier profile², which could compete with the chemical barrier of metallized PET.

These films grew out of the demand for reduced moisture barrier to reduce staling in low moisture-content foods. Then a funny thing happened, the products indeed no longer became stale first, but became rancid, thus driving the demand for improved oxygen-barrier metallized films⁸. Metallized PET cannot compete in this market at this time due to its moisture barrier, which allows the food to change texture, by moisture loss or become stale through moisture gain before it becomes rancid.

The high-barrier metallized films are successful because they add enough extra oxygen barrier to extend shelf life with gas flushing and improved hermetic sealing to minimize package-oxygen gain, which ultimately drives rancidity even under the diminished light levels inside the package. If a little more oxygen barrier is good, why not use the better UHB film of Figure 1? Simple, it's too expensive for the current shelf life expectations. This film is replacing foil only in markets where political action has resulted in tax disadvantages to foil-based packages due to packaging weight.

So, where should metallized films go next? In Figure 1, there's an apparent gap in the area of moderate-oxygen and high-moisture barrier films. Two possibilities exist for filling this niche: improved moisture-barrier metallized PET or improved oxygen-barrier metallized OPP. Several potential ways forward exist for both base materials. They are: (1) improved treatment or surface modification of HDPE⁸, ¹¹ or other polymer skins on OPP; and (2) improved polymer surfaces for PET by coextrusion, inline coating or perhaps improved treatment and surface functionalization.

It's possible that new surface-layer polymers for PET could be developed to enhance metallized-PET moisture barriers while maintaining current oxygen barriers. Alternatively, a moisture-barrier enhancement of the base sheet employing nanocomposite barrier technology¹² should be straightforward for PET polymers. Use of nanocomposite materials would not be as straightforward with polypropylene-based materials but are possible. Developments of this sort should drive the demand for new multilayer, coextruded PET films and spur polymer development for enhanced metallizing materials compatible with PET.

A metallized PET or OPP film with a high moisture- and oxygen-barrier profile would be a powerful competitor for Next Generation metallized films capable of improving packaging performance beyond today's measures. Due to PET's inherent chemical barrier and its well-developed manufacturing base and economics, a film as defined in Figure 2 could rapidly replace both coated and metallized OPP in many market segments.

Then, if the in-chamber overcoating technologies¹³ can be sufficiently developed by making advances in material development and if the process matures, the potential for a surface-printed, single-layer, metallized film could minimize the need for laminations in some markets—further reducing packaging costs. In the case where PET-film yield was still a problem, then the PET could be cavitated to increase yield and stiffness for more robust performance on packaging machinery.

A new surge in metallized-film development is just around the corner, especially through the potential gathering of several old, new and emerging technologies. Then, all we need is a truly easy way to open the bags.

1. Mapleston, P., "Barrier film coatings add performance options," Modern Plastics, Aug. 92, pp48-50

2. Mount III, Eldridge, M., Wagner, John R., "Aroma, Oxygen and Moisture Barrier Behavior of Coated and Vacuum Coated OPP Films for Packaging," TAPPI Press, Technical Proceedings of 1998 Polymers, Laminations & Coatings Conference, San Francisco, p875

3. Mount III, E. M., "Aroma and Moisture Barrier Behavior of Metallized Films," Flex-Pak 96, Chicago, Apr. 96

4. Mount III, E. M., "Aroma and Moisture Barrier Behavior of Metallized Films for Packaging," AMICAL Technical Proceedings, Oct. 97

5. Yializis, A., Mikheal, M.G., Ellwanger, R., Mount III, E. M., "Surface Functionalization of Polymer Films," Proceedings of the 42nd Annual Technical Conference of the Society of Vacuum Coaters (99), pp469-474

6. Yializis, A., Ellwanger, R., Harvey, J., "Barrier Degradation in Aluminum Metallized Polypropylene Films," Proceedings of the 40th Annual Technical Conference of the Society of Vacuum Coaters (97), pp371-375

7. J. Specht, "Metallization: An End-User's Perspective," Proceedings of the 41st Annual Technical Conference of the Society of Vacuum Coaters (98), pp440-445

8. Gavitt, I.,F., "Vacuum Coating Applications for Snack Food Packaging," Processings of the 36th Annual Technical Conference of the Society of Vacuum Coaters (93), pp254-258

9. Migliorini, R.A., Mount III, E.M., U.S. Patent 5194318, Issued March 16, 1993, "Multilayer Film With Metallized Surface"

10. Migliorini, R.A. U.S. Patent 5,153,074, Oct. 6, 1992

11. Mount III, E.M., Wagner, J.R., "Enhanced Barrier Vacuum Metallized Films," US Patent 5,981,079, Nov. 9, 1999

12. Trexler Jr., J. W., et al, "Process for making polyester/platelet particle compositions displaying improved dispersion," US Patent 6,162,857, Eastman Chemical Co., December 19, 2000

13. A. Yializis, "High Oxygen Barrier Polypropylene Films Using Transparent Acrylate-A [l]_2O3 and Opaque Al-Acrylate Coatings," Proceedings of the 38th Annual Technical Conference of the Society of Vacuum Coaters (95), pp95-102