Plasma Power

The term “atmospheric plasma” is creating a buzz throughout the mainstream converting industry. The promise of higher dyne levels, longer lasting dyne levels, superior adhesion performance, grafted surface chemistry, and an ability to apply variable chemistry to adapt to ever-changing application requirements are benefits worthy of converters' attention. However, potential users of atmospheric plasma in the printing, coating, and laminating industries are becoming confused by improper usage of the terms atmospheric plasma, chemical corona, and air plasma.

Corona discharges are not absolute plasmas. Although a corona discharge can be regarded as an atmospheric pressure plasma discharge, the surface modification provided by conventional corona discharges are exceedingly one-dimensional compared to gas phase atmospheric pressure plasmas that can offer an expanded range of surface modifications, both physically and chemically.

Corona

Corona systems convert standard utility electrical power into single-phase high frequency power. A corona is created between a precise air gap between the system dielectrics: the electrodes and ground roll. When air is exposed to different voltages, an electrical discharge develops. Voltage is applied to the electrode and ionizes the air in the air gap, creating a corona that will increase the surface tension of the substrate passing over the electrically grounded roll. High voltage is required to ionize air, and typically corona treating systems operate within an electrical voltage range of 10-20 kV.

Neutral molecules and electrically charged molecules collide, causing neutral molecules to become electrically charged, resulting in filamentary discharges or “streamers.” Such filamentary discharges create a non-homogenous cloud of ionized air.

When a substrate is placed under a corona discharge, electrons bombard the treatment surface with energies two to three times more than what is necessary to break the molecular bonds on the surface of most substrates. The resulting free radicals react rapidly with other free radicals on the same or different molecular chain, resulting in cross-linking. Oxidative effects on treated surfaces increase surface energy as a result of polar groups being created on the surface, primarily in the form of hydroxyl groups, carbonyl groups, amide groups, and carboxylic acid.

A byproduct of high voltage is the aforementioned filamentary discharges. These discharges prevent uniform treatment on a molecular level and, in fact, damage surfaces on a nano-scale. It is important to note that while this results in inconsistent treatment across the substrate surface, it usually is adequate for basic converting applications.

Exposure of corona treated surfaces to high levels of ambient humidity and temperature accelerates polymer side chain mobility and treatment degradation. Migration of slip additives can be accelerated and therefore also need to be taken into consideration when optimizing a process solution for converting corona treated substrates. Converters looking for improved corona treatment should consider high-definition corona systems in which optimized dielectrics are used to minimize filaments, pin-holing, and backside treatment potential.

Gas (Chemical) Corona

Gas or chemical corona treatment is electrically similar to corona processes. A process gas other than air, such as nitrogen, is ionized in order to increase the surface tension of primarily non-porous substrates. Because of the potentially lower breakdown voltage of process gases, gas corona treating systems can operate at an electrical voltage much less than 10 kV.

While these systems provide an improvement over air corona, they fall considerably short of the results achievable with atmospheric plasma. As we will see, atmospheric plasma creates a fourth state of matter, which differentiates its surface modification potential beyond what either a corona or chemical corona system can achieve.

Bridging Vacuum Plasma to Atmospheric Plasma

Atmospheric plasma systems are not enhanced corona discharge treating systems. The treatment attributes of atmospheric plasma systems were developed with low pressure vacuum plasma systems as the guide. The capabilities of a vacuum plasma system far exceed those of corona systems.

While the “fourth state of matter” can be achieved at both high and low temperature and pressure, it is non-thermal plasmas, and particularly non-thermal atmospheric pressure plasmas, that are best suited for continuous surface pretreatment of substrates.

The low density non-thermal plasmas characterized by vacuum plasma systems provide effective yet gentle surface treatment evenly across entire surfaces. In vacuum plasmas, the inter-bombardment of electrons, ions, vacuum ultraviolet, and UV rays combine to create profound surface modification effect. It is highly effective at etching and cleaning surfaces by removing organic material and creating chemical bonding sites on the surface. The controlled environment of a vacuum plasma system enables advanced treatment techniques, such as chemical vapor deposition, but they also are limited to applications in which batch processing is an acceptable means of production.

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Eight years ago a development project was undertaken to determine if the results achievable with vacuum plasma systems could be replicated with atmospheric plasma systems for high-speed converting applications. Extensive research and development led to a system that optimizes dielectric, surface etching, and surface modification characteristics of an atmospheric plasma surface treating system in a manner similar to vacuum plasma treatment systems.

Like corona and gas corona, plasma is the electrical ionization of a gas. However, the plasma (glow) discharge creates a smooth, undifferentiated cloud of ionized gas with no visible micro-discharges or macro-filaments. Also unlike corona or gas corona, plasma is created at much lower voltage levels.

As mentioned, corona converts the substrate surface from a non-polar state to a polar state. Oxygen molecules from the corona discharge area are then free to bond to the ends of the molecules at the surface of the substrate being treated, resulting in an increase in surface tension.

The same description holds true for plasma with major exceptions. The rate at which electron bombardment occurs within a gas-phase plasma is up to 100 times greater. In addition, a significantly higher amount of ion bombardment initiates chain scission of molecules (on organic substrates) across the entire substrate surface. This result is increased surface etching and stronger bonding attributes across the web (see Table I).

As reactive gases are diffused toward the surface under the influence of electrical fields, low molecular weight materials such as water, absorbed gases, and polymer fragments are knocked off to expose a clean, fresh surface. At the same time, a percentage of the reactive components in plasma with sufficient energy bond to the freshly exposed surface, changing the chemistry of the surface and imparting the desired functionalities.

In addition to these surface reactions, plasma also facilitates the use of chemical gases that can produce controlled chemical reactions on the surface as well. Plasma technology also eliminates the possibility for backside treatment. The high-speed photos shown in Figure 1 capture the optical differences between corona and plasma treatment. The corona image shows the expected “filaments,” while the plasma treatment generates a homogenous discharge profile.

A major advantage of atmospheric plasma is its proven ability to produce long-lasting treatment results on low-polarity materials that would be unresponsive to corona treatment, such as silicone or fluoropolymer substrates. However, the ability to clean, etch, and functionalize surfaces has made atmospheric plasma a breakthrough solution for many industry-leading firms. In addition, the ability to address many difficult to treat applications while employing plasma systems that eliminate the significant generation of ozone created by corona discharge systems offers environmental returns on investment.

The use of variable chemistry in these systems allows tremendous versatility by optimizing reactive gases for the specific application. Next generation atmospheric plasma systems are being engineered for other performance features of vacuum plasma technologies, such as plasma-enhanced chemical vapor deposition.

New Evidence

A technical paper was released by Tampere Univ. of Technology at the 2008 TAPPI PLACE Conference in Portsmouth, VA. This paper was titled “The Influence of Atmospheric Plasma Treatment on Digital Print Quality of Extrusion Coated Paper.” It studied how corona, flame, and atmospheric plasma surface treatments compared relative to their effects on the surface properties of extrusion coating and, furthermore, on digital print quality. Figure 2 is an excerpt from this paper, which compares these processes relative to enhancing printability of toner to low-density polyethylene (LDPE)-coated paperboard.

Specific conclusions of the study are:

  • Surface treatments clearly increase surface energy of LDPE-coated paperboard by oxidizing its surface. Argon plasma is the most effective method based on the contact angle measurements.

  • Also surface analysis with electron spectroscopy for chemical analysis clearly shows the oxidizing effect of all surface treatments. Plasma treatments are more effective than traditional flame and corona.

  • Generally speaking, plasma treatments are more effective compared to corona. Same or even higher effect can be achieved with 50% lower efficiency compared to corona and flame.

  • All surface treatments evidently improve toner adhesion, which is very important in packaging applications. Plasma-treated surfaces have the lowest rub-off values.

  • The effects on print mottle are not yet advantageous. More development work clearly is needed with the treatment uniformity in order to improve print evenness. However, compared to corona and flame, plasma treatments are more uniform, giving more even print with lower print mottle and hence better visual quality.

As evidence mounts relative to the performance benefits of atmospheric plasma technology for specific applications, this author will make these results known so the converting industry can leverage atmospheric plasma's dynamic surface effects.

Rory A. Wolf is VP of business development for Enercon Industries Corp. With 27 years of experience in the printing and packaging industry, Wolf is chairman of the TAPPI PLACE (Polymers, Laminations, Adhesives, Coatings, and Extrusions) Div. and chairman for the Society of Plastics Engineers. He has published numerous technical articles for publications throughout the packaging and printing industries.

Table I. Comparative Contact Angle Measurements for Surface Treatment Technologies
Polymer TypeAdvancing Contact Angle Before TreatmentAfter N2 Vacuum PlasmaAfter N2 Atmospheric PlasmaAfter N2 Chemical CoronaAfter Air Corona
Polypropylene 110 deg 48 deg 50 deg 66 deg 76 deg

Note: All post-treatment contact angle readings conducted at the same watt density.

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The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s).


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