PLASMA TREATMENT OF WEBS AND FILMS

By: E. Finson, Stephen L. Kaplan and L. Wood

ABSTRACT

Plasma surface treatment is rapidly gaining acceptance as a commercial tool for modifying the surface properties of molded polymers for a variety of subsequent conversion processes such as painting, printing, or bonding. Similar modifications are required in a variety of web and film applications. Only within the past five years has plasma equipment become available to treat film and web materials of commercial size and quantity. Cold gas plasmas utilize energy in the form of an electric field to dissociate a process gas under vacuum into a variety of excited species capable of modifying the surface of materials in the plasma environment. Cold gas plasma surface modification is substantially different than "corona" surface treatment by re-engineering the surface primarily through free radical initiated reactions as opposed to a "corona" generated shower of sparks. This paper discusses the myriad of different applications already commercial, as well as proposing a variety of possible future uses for plasma modification in the treatment of webs and films.

Keywords = Plasma Processing, Adhesion, Plasma Cleaning and Cleaning Polymers

INTRODUCTION

Plasmas are collections of highly excited atomic, molecular, ionic, and radical species, the bulk of which remain at room temperature. Although they can exist in liquids and solids they most commonly occur in the gaseous state, with stars, fluorescent lights, lightning and neon signs among the most familiar examples. This paper focuses on the types of gaseous plasmas used for surface modification of organic polymers. Plasmas used for these applications, although not fully ionized, are composed of ions, free electrons, photons, neutral atoms and molecules in ground and excited electronic states. Each of these components have the potential of interaction with surfaces upon which they come in contact. Plasmas can be employed to modify surface properties of a material without affecting the general characteristics of the base material.

Plasma is a proven, yet relatively under utilized technology which provides an efficient, economic, environmentally friendly, and versatile technique for improving desired surface properties of polymer materials. Extensive work in the past has centered around the plasma modification of plastic parts and the treatment of organic fibers1-7 typically to enhance adhesion. More recently, gas plasmas have gained interest for providing environmentally safe alternative to conventional surface treatment technologies8. Numerous applications have been developed for plasma enhanced chemical vapor deposition (PECVD) of high gas barrier coatings on film substrates to be use in flexible packaging9-11. Although there continues to be extensive interest in plasma deposition processes on polymer film substrates, plasmas can also be used for surface modification primarily oriented towards enhanced adhesion and improved wettability. Through plasma processing, it is possible to re-engineer the surface chemistry of polymer substrates to maximize the desired properties. This results in optimum design and performance of end products incorporating these materials. Equipment is now commercially able to process webs from 24 inches up to 60 inches in width.

THE NEED FOR TREATMENT AND LIMITATIONS
OF CONVENTIONAL SURFACE PREPARATIONS

Achieving adequate adhesion, whether bonding between polymers or the adhesion of coatings to polymers surfaces, is a recurring and difficult problem throughout the plastics industry. Designers of polymer substrates must often select specially formulated and/or more expensive polymer materials in order to insure satisfactory adhesion. In some cases, whole design concepts must be abandoned due to the prohibitive cost of the required materials/processes to achieve critical bonding requirements.

Historically, various surface treatments have been used to improve adhesion of coatings to plastics, including mechanical abrasion, chemical treatment, solvent swell followed by acid/caustic etching or corona discharge treatment. Often what works for one specific application will not be effective for another, thus specific treatments need to be developed for each. Mechanical abrasion is operator sensitive, operator intensive, dirty, and difficult to do on small parts or high-production volumes. Grit blasting also presents OSHA and environmental risks. Solvents pollute, present safety and expensive disposal problems, and often do not work. Acid etching, although more effective than solvent swelling alone, usually compounds handling problems, plus it is easy to over treat and damage parts. Corona treatment is often marginal in effectiveness and exhibits shelf life problems. Since a corona relies on ambient air, the process can change from day to day, and location to location. Complex shapes cannot be easily corona treated as the treatment quality is a function of the distance to the electrodes. In addition, the arcing prevalent in atmospheric (corona) discharge systems can damage the treated material either thermally or electrically. Each of these treatments has significant limitations, providing a strong driving force for the development of alternative technologies.

PLASMA SURFACE TREATMENT

A plasma is a low temperature glow discharge or a low pressure partially ionized gas consisting of large concentrations of excited atomic, molecular ionic and free radical species. Excitation of the gas molecules is accomplished by subjecting the gas, which is enclosed in a vacuum chamber, to an electric field, typically at radio frequency (RF). Free electrons gain energy from the imposed RF electric field, colliding with neutral gas molecules and transferring energy dissociating the molecules to form numerous reactive species. It is the interaction of these excited species with solid surfaces placed in the plasma which results in the chemical and physical modification of the material surface (see figure 1).

As an example of a plasma surface treatment, one of the more common processes to enhance adhesion is treatment in an oxygen plasma. An oxygen plasma can be very reactive and forms numerous active components. Even with this seemingly simple plasma you will find 0+, 0-, 02+,02-, 0, 03, ionized ozone, metastably-excited 02 and free electrons. As the components recombine, they release radiation, emitting a faint blue glow along with UV radiation. The photons in the UV region have enough energy to break the polymer's carbon-carbon and carbon-hydrogen bonds:

All of these active species react with the polymer surface, in addition to bombardment by photons, ions and neutral particles. This creates chemical functionality on the polymer surface by incorporating hydroxyl, carbonyl, and carboxylic acid groups. It is well documented that many conventional adhesives chemically bond to these groups allowing improved adhesion. The by-products of these reactions include C02,C0, H20 and hydrocarbons of low molecular weight that are readily removed by the vacuum system. These molecules can be excited by the RF field, but their effect on the reaction appears to be insignificant when used in an oxygen plasma.

The effect of a plasma on a given material is determined by the chemistry of the reactions between the surface and the reactive species present in the plasma. At the low exposure energies typically used for surface treatment, the plasma surface interactions only change the surface of the material; the effects are confined to a region only several molecular layers deep. The resulting surface changes depend on the composition of the surface and the gas used. Gases, or mixtures of gases, used for plasma treatment of polymers can include air, nitrogen, argon, oxygen, nitrous oxide, helium, tetrafluoromethane, water vapor, carbon dioxide, methane or ammonia. Each gas produces a unique plasma composition and results in different surface properties. For example, the surface energy can be increased very quickly and effectively by plasma-induced oxidation, nitration, hydrolyzation, or amination. Depending on the chemistry of the polymer and the source gases, substitution of molecular moieties into the surface can either make polymers wettable or totally nonwettable. The specific type of substituted atoms or groups determine the specific surface potential. For example, gases containing oxygen are generally more effective at increasing the polymer surface energy.

For example, plasma-induced oxidation of polypropylene increases the surface energy of 29 dynes/cm to well over 73 dynes/cm in just a few seconds. At 73 dynes/cm the polypropylene surface is completely water wettable.

For any gas composition, three competing surface processes simultaneously alter the plastic, with the extent of each depending on the chemistry and process variables. They are ablation, crosslinking and activation.

Ablation is similar to an evaporation process. In this process, the bombardment of the polymer surface by energetic particles (i.e.. free radicals, electrons and ions) and radiation breaks the covalent bonds of the polymer backbone, resulting in lower molecular weight polymer chains. As long molecules become shorter, the volatile oligomer and monomer by products boil off (ablate) and are swept away with the vacuum pump exhaust (Figure 2).

This process can be very useful in cleaning metals, removing surface finish from polymers or in removing weak boundary layers which may be present in certain processed polymers.

Crosslinking is done with an inert process gas (i.e. argon or helium). The bond breaking occurs on the polymer surface, but since there are no free radical scavengers, the molecule can do one of three things:

This process can be useful in preventing the exudation of certain additives to the surface of polymer substrates (Figure 3).

Activation is a process where surface polymer functional groups are replaced with different atoms or chemical groups from the plasma. As with ablation, surface exposure to energetic species abstracts hydrogen or breaks the backbone of the polymer creating free radicals. In addition, plasma contains very high-energy ultraviolet (UV) radiation. This UV energy creates additional similar free radicals on the polymer surface. Free radicals being thermodynamically unstable, quickly react with the polymer backbone itself or with other free radical species present at the surface to form stable covalently bonded atoms or more complex groups (Figure 4).

The new groups on the polymer surface alter its characteristics. Activation reactions are best done after a cleaning plasma to assure that the surface is free from contamination.

Activation processes are very useful because they impart no change to the bulk properties or appearance. The surface chemistry can be specifically tailored to promote adhesion whether it be for printing, painting, or bonding of plastic surfaces. The surface energy can be increased (oxidative plasma) or decreased (fluorination plasma) depending on the desired result. Surfaces can also be altered to improve biocompatibility for tissue culture, immunoassay, or blood compatibility applications.

EFFECTIVENESS OF PLASMA PROCESSES

Treating a polymer with plasma can increase its surface energy by modifying the surface chemistry. Generally, greater surface energy translates to greater chemical reactivity and compatibility to adhesives, paints, inks, and deposited metallic film.

The enhanced surface reactivity is characterized in the laboratory by water wettability. Wettability describes the tendency of a liquid to spread over and penetrate a surface, and for impervious substrates is measured by the contact angle between the liquid and the surface. The relationship between contact angle and surface energy is direct; contact angle decreases with increasing surface energy.

Contact angle measurements are sometimes used as a general indication of the presence of contaminants. The cleaner the surface, the lower the contact angle a drop of water will make with the surface. For example, a substrate contaminated with silicones may form a contact angle of greater than 90 degrees. On the other hand, most plasma treated surfaces yield a contact angle of 20 degrees or less.

Bonding in manufacturing processes generally requires cleanliness and wettability to achieve good adhesion. Plasma cleaning and activation greatly increases the apparent bonding surface area, thus increasing the bond strength (Figure 6).

PLASMA EQUIPMENT CONSIDERATIONS

There are many different types of plasma treatment systems that have been used for plasma treatment or modification of polymers. While most plasma equipment consists of similar components, the design of the reactor chamber, the distribution of power, the excitation frequency and gas dynamics can all be important factors which influence the efficiency and end result of plasma treatment. Previous research has shown a strong correlation between excitation frequency and the efficiency of surface activation. Manufacturers of plasma equipment employing radio frequency (RF) excitation will either use low frequency (less than 400 kHz), or high frequency (13.56 or 27.12 MHz). For applications involving the treatment of plastic webs and films, 13.56 MHz is preferred providing the best combination of effectiveness and uniformity of treatment.

Another extremely important consideration is whether the material is treated in a primary or secondary plasma. Older equipment using large cylindrical barrels is typically a secondary plasma system. The plasma is created either between closely spaced paired electrodes or in the annulus between the vessel's outer wall and inner ring electrode. With this type of equipment, treatment of materials placed within the working volume depends on the diffusion of activated species created in the primary plasma into the inner chamber. Diffusion of these activated species is very dependent on pressure. The higher the pressure the shorter the distance an active species can travel before having an inelastic collision quenching its activity. This distance is referred to as the mean free path. When using a secondary plasma system, the concentration of active species typically varies throughout the working volume giving rise to non-uniform treatment (Figure 7). Rectangular chambers employing closely paired shelf sets also operate as a secondary plasma since the RF field is restricted between the paired shelf electrode and the material being treated is not with the RF field.

By contrast, all of Plasma Science equipment (including the commercial web treatment systems) utilize primary plasma which allows all items treated in the work volume to be exposed to the RF field. When working in the RF field, or primary plasma, the gas(es) are constantly being excited allowing polymeric materials to be exposed to a high and constant concentration of active species leading to very uniform treatment. In addition, since diffusion is not a mechanism, significantly higher pressures may be used allowing higher process gas flow rates. This assures that off-gases from the polymer materials are sufficiently diluted relative to the process gases resulting in adequate treatment with the desired process gas. The primary plasma is also very rich in ultra-violet (UV) radiation which is often an important initiation step in polymer reactions. Since UV radiation is line of sight, uniform treatment of webs or films can easily be obtained when working within a primary plasma. An example of a Plasma Science primary plasma reaction chamber is illustrated in Figure 8.

PLASMA TREATMENT RESULTS

Treatment in a plasma is an effective method of modifying polymer surfaces. Plasma is inexpensive, clean, safe and has no waste products that need special disposal. As early as 1969, researchers12 reported that plasma treatment of high density polyethylene, nylon 6 and polypropylene greatly improves their bondability, with bond failures often occurring in the polymer rather than at the adhesive/polymer interface. The following table presents a summary of their findings. This data generated on molded plastics bonded with epoxy adhesive shows improvement in the range of 400 to 1000% after plasma treatment. Interestingly, while all plasmas are effective, there is not a universal process for treatment as seen by differences in bond strength for oxygen and helium plasma exposure. This indicates that some polymer materials are sensitive to specific chemical functionalities.

Bond Strength, psi

SpectraTM, a highly structured polyethylene fiber produced by the Fibers Division of Allied-Signal Corporation, provides higher specific strength benefits than KevlarTM, albeit within a narrower temperature use range. However, due to its polyethylene structure it is very difficult to achieve adhesion. Obtaining maximum performance in advanced composites from fibers and fabrics made from Spectra TM can create difficult adhesion problems. Unless the fiber bonds well to the resin matrix, stress distribution will not be uniform and maximum benefit will not result. Plasma treatment makes possible the use of Spectra composites for structural applications as evidenced by the properties in the following graph.

Oxidative plasmas have also been used to drastically increase the adhesive bond strength of KaptonTM

polyimide films for electronic applications and this is shown in the graph below.

Fluoropolymers are a group of polymers in which part or all of the hydrogen has been replaced by fluorine. In general, fluoropolymers have an impressive array of engineering properties including outstanding temperature and chemical resistance. These properties make them a logical choice for use in a variety of polymer applications including medical, industrial, electronic and specialty engineering areas. In addition, many fluoropolymers have a very low coefficient of friction and this can be useful in many applications as a "non-stick" surface. However, this "non-stick" attribute creates other difficulties when it is necessary to coat, print, or bond to these materials due to their extremely low surface energy. In many cases it is nearly impossible to achieve adequate adhesion without some type of surface preparation. Plasma surface treatments have been used to improve adhesive bond strength to fluoropolymer substrates and this data is summarized on the graph below.

CONCLUSION

As early as 1969, plasma investigators reported that plasma surface treatment of polymer materials greatly improved their bondability. Cold gas plasma allows the user to re-engineer the polymer surface introducing functional chemical groups in a controlled manner. Because the process is conducted in a chamber where the atmosphere and process conditions are precisely controlled, the resulting modification is very reproducible. The plasma process provides a manufacturing tool that is both work place and environmentally clean and safe. In addition, the process appears to be very effective for all polymer classes and in a variety of forms. Recently, commercial equipment has been developed to plasma modify these materials in the form of webs and films which creates numerous new application areas.

Because plasma surface treatment of polymers offers performance, safety, and cleanliness as well as being cost effective, it is certain to continue to grow in use throughout the future.

REFERENCES

5. S. Masuda, "Surface Treatment of Plastic Material by Pulse Corona Induced Plasma Chemical Process - PPCP," Proc. 1991 IEEE Industry Applications Society Annual Meeting Vol. 1, 703, 1991.
6. J. Ryu, T. Wakida, and T. Takagishi, "Effect of Corona Discharge on the Surface of Wool and Its Application to Printing," Textile Res. J., 61(10). 595 (1991).

       7. J.H. Schut, "Plasma Treatment: The Better Bond," Plastics Technology Vol. 64, October (1992).

8. Kaplan S.L and Hansen, W.P., Plasma- The Environmentally Safe Method to Prepare Plastics and Composites

for Adhesive Bonding and Painting," SAMPE Environmental Symposium San Diego, CA, May, 1991.

9. Felts, J.T. and Grubb, A.D, J. Vac. Sci. Technol. A 10(4): 167S (1992).
10. Nelson, R.J., 35th Annual Technical Conference Proceedings, Society of Vacuum Coaters, Baltimore, MD., 1992, p.75

 

11. Felts, J.T., 36th Annual Technical Conference Proceedings, Society of Vacuum Coaters, Dallas, Texas, 1993, p.25.
12. Hal, J.R., Westerdahl, C.A.L., and Bodnar, M.J., "Activated Gas Plasma Surface Treatment of Polymers For Adhesive Bonding," Picatinny Arsenal Technical Report 4001, AMCMS Code 502E. 11.295, Picatinny Arsenal, Dover, NJ 1969.

SpectraTM – Ultra high molecular weigh polyethyle high modulus fiber produced by Allied Signal.

KaptonTM - Polymide film produced by Dupont.

PyraluxTM - Proprietary adhesive produced by Dupont.

Acknowledgement:

1.Originally published in the Society of Vacuum Coaters, 38th Annual Technical Conference Proceedings (1995) ISSN 0737-5921.