When we look at a metallized film, we see a bright shiny reflective surface, which seems very uniform. But what is the aluminum layer really like? How thick is it, and what is its real structure?

If we start with a typical metallized film with an Optical Density of 2, we can determine its thickness in two ways: with an electron microscope picture from the side or with surface-resistivity measurements. Electron microscope images will also show the structure or morphology of the aluminum layer. However, side view electron microscope pictures are hard (expensive) to take, and surface resistivity measurements are not as common as optical density measurements, so it is hard to make a good, direct thickness measurement or estimate.

Fortunately, in the U.S. Patent describing development of safe solar eclipse viewers, the data for both optical density and surface resistivity of the same sample can be found¹. Using this data set (values are opacity of 0.01% light transmission and a surface resistance of 0.80-1.80 ohms per square) and the relationship between film thickness and surface resistivity in the AIMCAL Metallizing Handbook², we can estimate the thickness of the aluminum layer deposited at a 2 optical density would range from 147 angstroms to 331 angstroms or 1.47x10^-8 meter to 3.31x10^-8 meter. These calculations demonstrate that the thickness of the metal layer is extremely small, and for aluminum with an atomic radius of 143.1x10^-12 meter³ (or a diameter of 2.86 angstroms) would represent approximately 52-118 atomic layers.

The paper written by Jamiesom and Windle⁴ evaluates the structure of the aluminum film formed on commercially-metallized, polyester film substrates. They find that the deposited layers on PET films are similar to aluminum layers deposited on glass and other inorganic substrates and that the diameter of the aluminum crystals are approximately the same as the thickness of the layer but that the crystal diameter is dependent on the vacuum level during deposition. So, generally we can say that the aluminum layer is composed of a polycrystalline layer one crystal deep.

The layer is formed by the condensation of aluminum atoms onto the substrate surface followed by the nucleation and growth of individual aluminum crystals, which impinge upon one another limiting their lateral growth.

How can I be sure that the aluminum layer is only one crystal thick? Figure 1 is a transmission electron microscope picture of an aluminum layer at high magnification. In it you can clearly see the many individual aluminum crystals, some are large and some are small. Some are dark due to scattering of the electrons away from the collector making it "dark."

If there were two layers of crystals, the electrons being scattered by the atoms in one crystal would be scattered again by the second crystal making a "moiré" pattern of lines (much like looking through two pieces of window screen). These patterns are seldom if ever seen in these pictures indicating the layer is only one crystal thick.

As is known from common metallizing experience, high-quality metallized films in all applications contain microscopic pinholes or voids in the metal layer. These voids are due to the physical nature of the metal layer and its formation, abrasion of thin metal layers over surface structure inherent to the substrate films and the presence of dust or other foreign particles on the film surface at the point of metallization.

During the metallization process it is possible that a surface site may not form a crystal nuclei and remain uncovered by an aluminum crystal or that the aluminum may be oxidized forming a "spurious" pinhole believed to be aluminum oxide⁵. Jamiesom and Windle⁴ also report the existence of visual pinholes when viewed with transmitted light, which do not correlate with defects in the substrate. The existence of the pinholes has been known for a long time, and the most extensive studies of pinholes have focused on the manufacture of extreme-ultraviolet, metal-film filters for use in spectrophotometers⁵ and for improving the barrier properties of metallized-PET and oriented-polypropylene films (BOPP)^5,6,7,8,9. These studies give a good indication of the causes and distribution of sizes of pinholes to be expected in metallized-aluminum layers.

With a better understanding of the aluminum layer of metallized films, you also gain a better understanding of the wonderful protection from light, moisture and oxygen they provide to packaged products.

1. Roger W. Tuthill, US Patent 3,897,140, "Multilayer Solar Filter Reducing Distortive Diffraction", July 29, 1975.

2. "Section 2: Electrical, Optical and Metal Thickness Relationships", Metallizing Technical Reference, 3rd edition, E. M. Mount III editor, Assn. of Industrial Metallizers, Coaters and Laminators, 2001.

3. J. Emsley, The Elements, Clarendon Press, Oxford, 1989, p. 12-13.

4. E. H. H. Jamieson and A. H. Windle, "Structure and Oxygen-barrier properties of metallized polymer film", J. Material. Sci., 18, (1983), p. 64-80.

5. W. R. Hunter, "The Preparation and Use of Unbacked Metal Films as Filters in the ExtremeUltraviolet", Physics of Thin Films, G. Hass, M. H. Francombe, J. L. Vossen, editors, Vol. 12, Fig. 3, p. 53, 1982.

6. R. S. A. Kelly, "High Barrier Metallized Laminates For Food Packaging", J. of Plastic Film & Sheeting, 3, (1987), p. 41-47.

7. U. Moosheimer and H.C. Langowski, "Permeation of Oxygen and Moisture Through Vacuum Web Coated Films", 42nd Annual Technical Conference Proceedings, Society of Vacuum Coaters, (1999), p. 408-414.

8. J. Specht, "Metallization: An End-User's Perspective", 41st Annual Technical Conference Proceedings, Society of Vacuum Coaters, (1998), p. 440-445.

9. Yializis, "High Oxygen Barrier Polypropylene Films Using Transparent Acrylate-A [l]_2O3 and Opaque Al-Acrylate Coatings", 38th Annual Technical Conference Proceedings, Society of Vacuum Coaters, (1995), p. 95-102.

Eldridge Mount

EMMOUNT Technologies

585/223-3996

emmount@msn.com

Based in Ft. Mill, S.C., the Assn. of Industrial Metallizers, Coaters & Laminators provides a broad range of services to the converting industry. If you have inquiries concerning products, equipment or material sourcing, converting capabilities or technical issues, the group's "Ask AIMCAL" service is available at its Web site: www.aimcal.org

For more information, contact AIMCAL at 803/802-7820, fax: 803/802-7821.