Plasma pretreatment for metallizing packaging film

It's a hot topic. What's driving the change and what impact does the new process have on the products converted?

By Consulting Technical Editor Eldridge M. Mount III -- Converting Magazine, 3/1/2001

During the last several years the metallizing industry has seen a surge of interest in the plasma pretreatment of metallizable films, particularly polypropylene-based films for packaging applications. This article will explore the source of the technology, why it is becoming important to the evolution of metallized packaging films, the impact of the process on existing films and how it may drive the development of new metallized products.

Existing pretreatment techs

While we talk about in-chamber pretreatment as if it's a new process for packaging films, it's worth noting that almost all films for metallization receive some form of pretreatment to the surface designed to receive the metal layer. Typically, this pretreatment is supplied by the film manufacturer and ranges from polymer coating, coextruding specific polymer skin layers and oxidative surface treatments such as corona, flame and a new atmospheric plasma process.¹,² These treatments or combinations of treatments have been practiced for many years and are necessary for several reasons. In some instances, they improve metal adhesion to the film surface, or improve the optical and physical uniformity of the metal layer, or the barrier properties of the metallized film. In reality what is occurring is the chemical modification or crosslinking of the native polymer film surface into an optimum or improved surface for metallization.

Dyne = surface energy

The increase in surface energy is measured by dyne level-surface oxygen concentration or contact angle of a reference fluid. The activation of the polymer surface is required to produce a dense, uniform metal layer. The impact of the surface treatment on the metal deposition is easily demonstrated by metallization of rolls containing spliced file samples of increasing surface treatment level.

During metallization, as the surface treatment increases at the splice transition and without changing the evaporation conditions, the optical density of the sample is found to increase. In metallizers with plasma-treatment systems turning the plasma on and off will show the same effect.³ From this, we infer that the refractive index (density) of the metal layer is increasing with increasing treatment level. This improvement in aluminum layer "quality" is most likely responsible for the improved barrier properties found for films with higher surface energy.

In some instances the films do not necessarily need additional oxidative surface treatment to improve the surface energy or to develop the optimum properties sought for the product.^4,5,6 In these cases, the film surface polymer-PETG, amorphous nylon, EVOH-controls the outcome of the metallization process and could be viewed as a priming or coating pretreatment.

However, for these polymers, it's often found that a light oxidative treatment improves the optical uniformity of the metal layer by removing or covering treatment patterns from static discharge tracks. The importance of the presence of surface treatment or a high surface energy polymer on OPP films for the development of barrier properties cannot be overemphasized.

Optimum surface composition

In many instances these products are expensive and difficult to make and may not be available commercially. However, they can define a target surface composition for the surface modification technologies to attempt to produce. If this is an accepted purpose of surface treatment, what's left is determining the best treatment process for optimizing the surface composition or surface free-energy level of a given film product.

For instance, it has been shown that the metallization of OPP films with an EVOH surface yields superior metallized oxygen and moisture barrier properties,⁷ which can challenge aluminum-foil properties.⁴ Extrapolating from this product, it might be possible, by treatment alone, to control the oxidation of a HDPE surface with corona, flame⁸ or plasma treatment to create a chemically equivalent metallization surface as the coextruded EVOH layer, but at a much lower manufacturing cost.⁹ One of the possible advantages of the new plasma pretreatment systems is that by controlling the reactive-gas blends and process conditions, they offer the chance to generate new surfaces under the metal layer than those possible with current standard treatment methods.

In-chamber plasma treatment

First, through discussions with suppliers of metallization equipment and plasma treaters, it's apparent that plasma pretreatment is not new, especially for those suppliers that market sputtering equipment or machines for capacitor applications. Indeed, according to leading metallizing equipment makers, technologies for film pretreatment in the metallizing chamber have been in use for years. However, the main uses were to pre-clean the web and for chemical modification of the film surface to improve bonding.

Generally this was accomplished with a planer magnatron-based technology with or without magnetic enhancement, and modified to use Argon gas to create a plasma to clean or etch expensive films to accept sputtered layers. In these cases, the process speed was not a limit, the cost of extra equipment and increased process complexity was warranted by the high price and quality demands of the products.

Today, however, use of in-chamber pretreatment is expanding from the low-speed sputtering applications into the high-speed process typical of aluminum metallization of packaging films. This change has been sparked by the realization, with existing film designs, of improved barrier properties, metal layer improvements,^2,10,11 and metal adhesion in the aluminized films produced. These improvements are brought about by the chemical modification of the film surfaces with the evolving pretreatment technologies.

Generator types

Several principal plasma generation technologies are in use today-the original DC- and AC-magnetron sources with reactive-gas blends and a new hollow-cathode AC plasma generator¹² with reactive-gas blends. All of the plasma technologies are available for retrofitting to existing metallizers, where space permits and in new equipment. The choice of technology, may be limited somewhat by manufacturer, but generally all are available.

The essential features of all the plasma generators are that they supply high-energy electrons in sufficient quantities to generate heavy ions and additional electrons from the gas blend, for bombardment of a film surface. In traditional magnetrons, the ions and electrons are constrained in a ring shaped magnetic field above the generator and the film is passed through or near the upper regions of the plasma (See Figure 1). These systems may also be magnetically enhanced to open the field lines towards the film.

In the hollow-cathode design, the magnetic field above the hollow cathodes are modified by permanent magnets placed behind the film as it passes through the treater. This has the effect of inserting the ions and electrons in the open magnetic field and directing them toward the film surface (See Figure 2) as the polarity of the generator cycles from positive to negative. Various electrical systems in conjunction with the gases fed into the plasma generators clean and activate the surface for chemical modification just prior to the deposition of the aluminum.

Results published by the various plasma-treater manufacturers all show improvements in metallized-film properties of barrier, metal adhesion and metal quality with different gas blends and flow rates. The future of the processes lies in determining which one allows the most flexibility and operating reliability in generating polymer-surface modifications for property enhancement beyond simply improved adhesion.

What took so long?

Why has it taken so long for this existing technology to find its way into packaging-film applications? My view, supported by the literature, is that only during the last seven to nine years has the true requirement for metallized oxygen barriers of consistently 45 cc/m² /day or lower been demanded by end users.¹³ Prior to 1991, the moisture barrier of standard aluminized-OPP packaging films was not sufficient to warrant the improved oxygen barrier, i.e. the product became stale before it became rancid or off flavored. While, the barrier properties could be met by metallized-PET films the cost was prohibitive.

As high-barrier, metallized-OPP base sheets improved in moisture barrier, the product failure mode of (for instance, oil-fried snacks) changed. As metallized-OPP films with oxygen barriers of 90-300 cc/m² /day were typical at that time,¹³ the product was found to be rancid or off-flavor before it was stale at the projected shelf life. Consequently, the demands of snack companies for oxygen barriers of 30 cc/m² /day was added to the metallized-product specifications.^13,14 These studies also showed the optimization of surface treatment of base sheets improved the barrier properties but at the expense of metal bonding.

All the treatment processes practiced by film suppliers suffer from treatment degradation due to the passage of time from film manufacture to the metallization chamber. As rolls are stored in wound form, the surface to be metallized may be contaminated from chemical additives in the film and contact with the opposite film surface. In addition, most oxidation techniques result in decreased surface energy with time due to the diffusion of chemical groups into the film surface.

A notable exception appears to be the new atmospheric-plasma treatment of polymer surfaces with helium/acetylene blends, which has been reported in the literature and found to be remarkably stable with time.¹ Consequently, plasma treatment at the point of metallization would seem to hold a great deal of promise. It: 1) can supply higher-treatment surfaces at deposition due to no aging, 2) provide better metal layers due to the surface cleaning, and 3) may supply better surface chemistry due to new gas blends for the surface frictionalization sought.

There is however a dark side to the technology, especially when used with existing films which are treated by the film manufacturer. As is known in the film industry and is shown in Reference 14 and in Figure 3 supplied by BPS LEYBOLD Systems, it's possible to overtreat film and cause a reduction in metal bonding. Thus when installing and starting up a new pretreatment process, it's worth working closely with suppliers to obtain film with reduced surface treatment to permit the plasma-treatment process to be optimized.

Conclusions

Plasma pretreatment of existing packaging films has demonstrated the potential for improving package performance and is emerging as a new process in the continuing evolution of metallized films. Innovative new plasma sources and process configurations coupled with new base film designs can be expected to drive further innovation and improved film properties for years to come, offering new end markets for metallized film in the packaging field.

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The author would like to thank the following companies for assistance in preparing this article: Galileo Vacuum Systems, Inc., East Granby, Conn.; General Vacuum Equipment, Charlotte, N.C.; and BPS Leybold, Hanau, Germany.

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Eldridge Mount, president of EMMOUNT Technologies, Fairport, N.Y., received an ME and PhD in chemical engineering from Rensselear Polytechnic Institute. He holds five U.S. and two European patents in the field of metallized and high-barrier metallized films. He has more than 20 years of industrial experience in the extrusion and orientation of PP and polyester films at Mobil Chemical and ICI Americas Films Divs. Eldridge can be reached at 716/223-3996, fax: 716/223-3480, e-mail: emmount@msn.com