- February 01, 2003, Rory A. Wolf, Enercon Industries Corp.
Converters of hard-to-treat and thicker substrates will benefit from using an alternative to corona treating—the atmospheric plasma method.
The benefits of plasma treatment— such as higher treatment (dyne) levels, extended treatment life over time, reduced degradation of surface morphology, and the elimination of pin-holing and backside treatment — are well known. However, until recently, plasma treatment meant low-pressure plasmas that required ultra-sophisticated systems including vacuum pumps and sealed and containerized web handling equipment.
The recent introduction of high-density atmospheric plasma for polymer, paper, foil, foam, nonwoven, woven, and powder applications is significant. It is not just plasma at atmospheric pressure, it is plasma created and sustained in the atmosphere. As a result, web handling systems and web threading procedures are no different than they would be with a corona or flame treater.
How Plasma Compares to Corona
Corona treatment uses a high voltage, high frequency electrical discharge to ionize air to increase the surface tension of non-porous substrates. Normally, corona treating systems operate at electrical voltage of 10 kV.
Like corona, plasma is the electrical ionization of a gas, but with plasma the gas is selected dependent on the material being treated and the application being performed. The plasma (glow) discharge creates a smooth, undifferentiated cloud of ionized gas with no visible electrical filaments. Unlike corona, plasma is created at much lower voltage levels and temperature.
Corona converts the substrate surface from a non-polar state to a polar state. Oxygen molecules are then free to bond to the polar sites on the substrate being treated, resulting in an increase in surface tension.
The same description holds true for atmospheric plasma with a few exceptions. The rate at which electron bombardment occurs within high-density atmospheric plasma is up to 100 times greater. This increased electron activity forces a greater ion bombardment to the substrate surface. This results in increased etching, chain scission, and cross-linking on the substrate's surface, creating stronger bonding attributes across the length of the web.
In addition to these surface reactions, high-density atmospheric plasma can facilitate the use of gases, which can produce controlled chemical reactions on the surface as well. Gas molecules introduced in the plasma are propelled to the material surface. Low molecular weight material is cleaned from the surface while specific gas molecules bond to the surface. This can create a variety of desirable functionalities.
Atmospheric plasma technology also eliminates the possibility for backside treatment. The high-speed photos at left capture optical differences between corona and atmospheric plasma treatment that are sometimes invisible to the naked eye. The corona image shows the expected “filaments,” while the plasma treatment generates a smooth treat pattern.
Benefits of Atmospheric Plasma
There are three key benefits converters can turn into competitive advantages by using plasma treatment.
- Longer life treatments
—Substrates that have been atmospheric plasma treated hold their treatment levels far longer than corona treated surfaces. Longer treatment life will allow converters to take advantage of economies of scale during production, increase inventory life, and provide enhanced manufacturing flexibility.
- Higher treat levels allow for treatment of difficult-to-treat surfaces
—Atmospheric plasma treatment is a viable alternative for a variety of substrates that corona treating is ineffective at treating. For example, fluoropolymer-based materials like Teflon don't respond well to the corona process but do respond significantly to atmospheric plasma treatment. We have been able to raise the surface energy of fluoropolymer-based materials to 72 dynes/cm.
- Treatment of thicker substrates
— While substrates thicker than 0.125 in. usually do not respond well to the corona process, they can be treated by atmospheric plasma. Films, foams, and nonwovens, as well as fibers, metals, and powders, are all candidates for atmospheric plasma surface treatment.
The surface energies of materials treated by atmospheric plasma have been shown to increase substantially, thereby significantly enhancing the wettability, printability, and adhesion properties of these materials. Plasma systems also have the flexibility to be operated with variable chemistries and without the generation of ozone, pin-holing, and backside treatment. Application markets where atmospheric plasma systems have been found to contribute to new product developments include automotive, electronics, textiles, identification, packaging, apparel, and hardware.
A Nonwovens Trial
Plasma treatment on nonwoven substrates provides four primary beneficial effects:
- Cleaning Effect
— Combined with changes in wettability and texture for improved print quality, dyeability, and adhesion.
- Etching Effect
— Increases the micro-roughness of fiber surfaces.
- Free Radicals
— Induces secondary reactions, such as intermolecular cross-linking.
— Enables the deposition of chemistries with desired properties onto substrates.
Several polypropylene nonwovens with a 0.40 mil thickness were treated recently by the Enercon plasma system at atmospheric conditions. These nonwovens were treated on webs sized from 27-60 in. wide. Surface tension of the treated nonwovens was determined by surface tension test fluids markers (ASTM D-2578). The surface energy of these nonwovens was enhanced substantially after atmospheric plasma treatment.
The nonwovens were then printed with an image of the American flag in four-color process using photopolymer printing plates on a Mark Andy press with Akzo Nobel Hydrokett 3000 water-based ink. The anilox roll was 700-line screen with a 2.1 cell volume. The nonwoven material was printed in roll form at 200 fpm. The ink was dried in-line with a forced air at a temperature of 140 deg F.
Untreated, corona treated, and atmospheric plasma treated protocols for these nonwovens were then evaluated to determine the adhesion of the ink. A tape test was performed for each protocol with a ½ in. × 2 in. tape peel test using fresh transparent (clear) pressure-sensitive tape. The tape was applied to the printed side of the film and allowed to remain for 60 sec.
During the peel test, the untreated and printed nonwoven exhibited total ink adhesion failure, with all image ink removed with the tape. The corona treated nonwoven retained approximately 90% of the image ink at its surface. The plasma treated nonwoven displayed 100% ink adhesion, with no residue being removed from the surface.
The trial supports the role and efficiency of atmospheric plasma in functionalizing the surface of polypropylene nonwovens for improved water-based ink adhesion.
Textile materials successfully treated with this method include PP fibers, PP and PE nonwovens, PET fiber, Tyvek, nylon, wool, and textile yarns.
Rory A. Wolf is director-business development for Enercon Industries Corp. and serves as chair of the marketing committee for TAPPI PLACE Div. He has 24 years of sales and marketing management experience in the packaging industry including corrugated, point-of-sale display, folding carton, flexible packaging, electronic prepress, and surface treatment industry segments. Wolf holds a BBA degree from the University of Wisconsin-Milwaukee and an MBA degree from Marquette University. Contact him at firstname.lastname@example.org.
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