Even veterans in the field seem to struggle when it comes to explaining why certain coatings and procedures work or fail. Here's a review of the basic principles, presented at the recent AIMCAL Fall Technical Conference.
Editor's note: This paper and the one which follows on page 48, presented at the 2002 AIMCAL Fall Technical Conference, have been awarded the Mateucci prize as Best Papers. A more extensive version of this article can be found in the proceedings of the 45th Annual Technical Conference of the Society of Vacuum Coaters, Orlando, Florida, 2002, ISSN 0737-5921.
There is hardly any application in thin films technology which is utilized as widely as barrier coating on packaging films. But despite this fact there still is not a comprehensive understanding of how and why barrier coatings actually work. In fact, even the biggest producers of barrier coated materials seem to struggle when it comes to explaining why certain coatings and procedures work or fail. Furthermore, it is apparent that sometimes even the most fundamental aspects of the function of barrier coatings are not well understood. In this report a review of some of the basic principals of Thin Film Barrier Coatings is presented. In addition it highlights some aspects of the function of barrier coatings that require more attention from the scientific community.
Diffusion
Packaging materials provide a barrier to gases and liquids, inhibiting either from reaching the other side of the said barrier. Gases, however, can penetrate through solids by the movement of single molecules or atoms, a process known as diffusion. Diffusion is the movement of matter through a medium driven by the kinetic motions of atoms and molecules. Directional diffusion--that is, the preferred flux of matter in a specific direction--is caused by nature's striving for equilibrium, and occurs in the opposite direction to the gradient of concentration of the particular diffusing species.
While the diffusion of gases within gases is very well understood, and can easily be expressed by mathematical equations, the diffusion of gases in solids is still an active field for research. Diffusion of gases in solids is influenced by many factors such as crystallinity of the solid, defects in the solid and the size thereof, polarity of the gas and solid as well as possible chemical interactions between the solid and the gas. Most data to date has been acquired empirically.
Diffusion through polymer packaging materials
Typical packaging materials are thin-film polymers. The three most commonly used polymers are polyethylene (PE), polypropylene (PP) and polyester (PET). Manufactured as packaging films and foils, they offer varying resistance to the permeation of gases. Table 1 lists the permeation and permeability of some polymers. Permeation is the actual amount of gas passing through a specific thickness of a material. The unit of permeability is a specific property of the material and is a measure for the ability of specific gases to permeate or diffuse through this material.
As can be seen, for most packaging applications these values are too high to achieve the goal of extending the shelf life of perishable foods. In order to improve on this situation, thin layers of organic or inorganic materials are deposited on top of the packaging film. Typically these coatings are applied in vacuum-coating chambers, and the most common these days is a thin layer of pure aluminum. An overview of different substrates and coating materials can be seen in Table 2.
Permeation models
When comparing the intrinsic values of the substrate materials and the thin film coating materials, one can see in Table 2 that the achieved barrier properties are lower than theoretically possible. In fact, microscopic examination of the coatings would reveal defects in these layers, which lower the barrier property of the coating. Research on the impact of such defects has been going on for many decades. Three different models for the calculation of the contribution of each barrier layer have been proposed: electric analogy, coverage and pinhole.
Electric Analogy Model:
The electric analogy model assumes that each layer in a barrier structure reflects a resistor for the gases to pass through. The model assumes that each layer has an average resistivity and does not consider defects. This model, however, does enable the determining of the individual contribution of barrier layers to the overall barrier structure.
Coverage Model:
The coverage model takes defects in the barrier layer into account. It calculates the barrier property of a deposited layer, based on the assumption that a barrier coating itself has perfect barrier property and that diffusion through the layer only happens in defects. It also assumes that diffusion through the barrier structure is one-dimensional, thus neglecting “cross”-diffusion in thicker substrates. It is, however, a reliable model for the determination of defect ratios on very thin substrates.
Pinhole Model:
The pinhole model is a mathematical model for diffusion through defects in barrier coatings, taking into account the diffusion in all directions in the substrate. The solution of the three-dimensional differential equation for the diffusion through the pinhole then describes very clearly the relationship between pinhole size, pinhole density and thickness of the substrate.
Two interesting conclusions of the pinhole model are 1) that many small pinholes have a much bigger impact on the degradation of the barrier than a few large defects with the same defect area, and 2) that the impact of pinholes and defect area increases if the substrate thickness increases.
Interface phenomena
Surface characteristics before the coating process:
Another important factor that determines the quality of the vacuum deposited barrier layer is the condition of the substrate surface prior to coating. Many of the commonly used polymers exhibit a low degree of surface energy, inhibiting the creation of a good layer of barrier material during the coating process.
It has been shown that different forms of pre-treatment can overcome this problem. One of the most common technologies used today is the exposure of the substrate surface to ionized gases, i.e. plasmas. Treatment with ionized gases can lead to physical and chemical alteration of the substrate surface. If the right parameters for treatment are chosen, values like adhesion and/or barrier behaviour can be dramatically improved. By studying the effects behind these improvements, it becomes more and more obvious that chemical reactions at the boundary between the coating and substrate–the interface–can create a chemistry that may have its very own characteristics in terms of barrier property. Research into this aspect is ongoing.
Also of importance is the surface morphology of the chosen substrate. Typically it is said that a smooth, flat substrate gives better barrier.
Coating process:
The coating process itself has a big impact on the substrate properties. After all, during the deposition process there is a considerable amount of energy impinging on the substrate surface. Depending on the coating process used, there are different sources of energy which work on the substrate surface:
- Radiation:
- Heat radiation from the evapo rator boats or the crucible
- Electron radiation from reflected electrons during e-beam evaporation
- Electron-, ion- and UV-radation from plasma during sputtering or plasma enhanced processes
- Heat of condensation and fusion, as well as specific heat from the material deposited.
- Kinetic energy of material impacting the surface, specifically during sputtering.
Changes to the surface due to high energy impact could include certain degradation effects on the outer layers, thus reducing barrier or adhesion properties.
Converting:
Finally, after the barrier coating has been applied there is still opportunity to alter the properties of the coating during the converting process.
Most obvious is the degradation of the barrier layer due to scratches and/or pinholes imposed during winding, slitting and lamination processes.
More important, however, might be the possible changes during the actual lamination process. There is a large variation, depending on whether cold or hot seal adhesives are used, how high the actual lamination temperature is, and duration and level of the actual nip.
Also, there is evidence that certain adhesives have interaction with the barrier materials, forming a new interface and altering the structure of the barrier layer and hence the properties.
Conclusion
The function of the Barrier Coating is a very complex matter, as is evident by the enormous amounts of publications and the ongoing research in this area.
The basic principles however, as described in this paper, should enable the converter to develop the required understanding of the impact of his procedures on the properties of his product.
At the same time, it is obvious that there is still a big piece missing, specifically when it comes to the change in barrier behavior during subsequent converting procedures. For this, additional research, particularly of the interface phenomena, is required and this seems to have drawn the interest of several scientists working in this field.
Wolfgang Decker earned his Ph.D. in Mechanical Engineering at the Technical University Braunschweig, Germany, 1997, graduating on a project for production of transparent barrier coatings. He joined Toray Plastics (America) Inc. in 2001. He is now Technical Manager for Metallizing and Converting at the N. Kingstown, R.I. plant.
Dr. Decker holds five patents in the area of thin-film coatings and surface functionalization, and has over 30 publications in the field. He is a member of the Society of Vacuum Coaters Technical Advisory Chairs for Emerging Technologies and Vacuum Web Coatings, as well as the AIMCAL Vacuum Web Coating Committee.
Dr. Bernard Henry is presently an Associate Director of Research of Nanocomposites in the Dept. of Materials of the University of Oxford, England, U.K. Dr. Henry obtained his Ph.D. in Materials Science from Imperial College, University of London.
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