By ELISE M. SANDERS
As a venue to improve the wettability of inks in the print industry, printers utilize surface-treatment technologies. These technologies allow adhesion of inks to plastics, polymers, films, and flexible packaging. Many treatment processes occur before substrates leave the factory, but further processing becomes a necessity due to a combination of uneven surface treatment and the limited longevity of the initial treatment. Popular processes today include corona, atmospheric plasma, cold gas plasma, and plasma-grafting treatments. How do these different treatments compare, what seems to be the best, and what is the future moving towards?
Corona treating preceded that of the other plasma processes, although theories of plasma treating have been around for the past four decades. Atmospheric plasma and cold gas plasma treatments both form a uniform layer of plasma bonded to the surface through oxidation; the main difference apparent is the use of a vacuum for cold gas plasma and none for atmospheric. Plasma grafting actually forms a whole new layer, or coating, onto the substrate to change the surface properties through polymerization. The future seems to be geared towards more forms of plasma treatment use. They currently hold a large portion of the market; however, corona treaters are still in contention. Other future venues incorporate multitreatment stations in one; combinations of plasma and corona treatments combined. Overall, plasma treatments present themselves as prominent methods to promote adhesion in impermeable substrates, both today and in the future.
Films, foils, & packaging
Products, perishables, and consumables have been enclosed in packages for centuries. Not only do packages contain their products, but they also incorporate advertising, durability, freshness, and consumption factors. Today, packaging producers turn to foils, polymers, cardboards, glass, and specially formulated materials in the consumer packaging industry, and printers increasingly rely on water-based inks for their products. These inks tend to resist adhering to select surfaces, and to counter this problem many substrates must be “treated.” In essence, treatments improve the surfaces to increase the “wettability” or willingness of inks to attach to substrates. Surface tension is a factor contributing to wettability, “a phenomenon that results directly from intermolecular forces between molecules of liquids.” (Hablewitz, 2005) A low surface energy creates poor wettability characteristics between inks and substrates. Substrate grades with a low surface energy include films, foils, and polymers. The surface energy of substrates can be measured in dyne/cm, and, in general, treatment processes increase the dyne level of substrates in order to increase the wettability. Suppliers often treat substrates prior to shipment to the printing facilities, but sophisticated treatment units are also available. Some printers feel that surface treating stations are an unnecessary waste of money if the substrate is pretreated. However, experts agree that the quality of the treatment should not be heavily relied on. “Treatment degrades over time, so that’s one issue (one has) to contend with.” (Gilbertson, 2004) Printing these substrates without sufficient treatment leaves printers with the possibility of ruined jobs and lost customers.
Three types of general treatment methods are currently available: mechanical treatments, physical treatments, and chemical treatments. Mechanical and physical processes are slowly becoming obsolete. They often required labor-intensive processes and output low production volumes. Physical treatments include water-based cleaning and evaporated acrylate coatings. Chemical treatments involve solvent cleaning, chemical etchings, chemical primers, flame treatment, UV/zone treatment, fluorination treatment, and gas plasma treatments. Gas plasma and corona treatments have proven to be prominent within the past few years. This ultimately leads to the question: why is the gas plasma procedure so respected, and will it affect the use of corona treatments in the future? To solve this question, three elite, specialized interviews were conducted; interviewees include Denis Dowling of Dow Corning, Rory Wolf of Enercon, and Rob Hablewitz of Pillar technology. Nine secondary sources were also consulted.
Corona treatment is a well-trusted “old favorite” for many facilities; however, plasma treatment is a current hot trend in treatment processes today. “Plasma can be defined as a partially or wholly ionized gas with a roughly equal number of positively and negatively charged particles. Some scientists have dubbed plasma the “fourth state of matter” because while plasma is neither gas nor liquid, its properties are similar to those of both gasses and liquids.”(“Plasma Applications”) Even though the idea has been around since the nineteen sixties, actual processing units were not built until recent years. Today, plasma treatments separate into four different categories: atmospheric plasma treating, cold gas plasma treating, flame plasma treating, and plasma grafting.
The plasma predecessor: corona
The major predecessor to plasma-treating processes, corona treating, still holds its spot in the market today. Corona treatments operate through the theory of highspeed oxidation. In this process, “energy of the corona breaks the molecular bonds on the surface of the nonpolar substrate. The broken bonds then recombine with the free radicals in the corona environment to form additional polar groups on the film surface. These polar groups have a strong chemical affinity to the polar inks and adhesives.” (Hablewitz, 2005) Simply stated, the treatment creates wettability by altering the dyne levels of the substrate to reach a compatible dyne level to the inks through high-voltage electrodes.
Although corona treating was originally designed in the 1950s for full treatments to substrates, many facilities utilize corona treating to “bump” treat inline to safeguard against loss of dyne level. “Bump” treatments serve as an economical avenue for printers when the corona process is available. For this process, a printer will purchase pretreated film. Because pretreated substrates have a propensity to decay and lose their specific dyne level during storage, they “bump treat.” The pretreated substrate allows the corona treater to use less power since the dyne level is only being recovered to its standard and not completely built up from the beginning (O’Neill, 2005).
The drawbacks for corona treatments include backside treatments, which in turn cause picking and blocking. “A typical corona treatment system is designed to produce a corona in the air gap between the high-voltage electrode and the ground roll. Corona will be produced anywhere there is air within this gap. The occurrence known as “backside treat” is the result of a corona being produced on both sides of the web, even though the treating system is only intended to treat one side.” (Habelwitz, 2005) Additionally, “the cause of backside treatment can be attributed to either material imperfections or operational problems. Wrinkles in film, scalloped edges, or “bagging” are all material conditions that promote backside treatment.” (Gilbertson, 2006) In the printing stage, a backside treat will result in “blocking” the ink on the substrate and also can create cases of “picking.”
Plasma treatment processes
Atmospheric plasma treatments followed in similar theories to corona treatments. Applications of atmospheric plasma include films, foams, nonwovens, wovens, fibers, metals, and powders. (Wolf, 2005) The idea of atmospheric treating involves similar processes of exposing nonpolar surfaces, forming them into polar surfaces, leaving the substrate with a glowing plasma discharge through similar oxidation practices (Finson & Kaplan , 1997). “Like corona, plasma is the electrical ionization of a gas. The plasma (glow) discharge creates a smooth, undifferentiated cloud of ionized gas with no visible electrical filaments. However, unlike corona, plasma is created at much lower voltage levels and temperatures.” (Wolf, 2004, p. 1) As seen in the examples, the lower voltage levels and temperatures give corona processes a more uneven surface as compared to atmospheric plasma.
Surface treated via corona (above) and surface treated via atmosphericplasma (below) (Wolf, 2004)
The rigid discharge (glow) across the corona-treated substrate often penetrates the substrate creating an undesired backside treatment. The even discharge from plasmas eliminates a possibility of backside treatments. Plasma introduction to the substrate also helps eliminate unstable molecules on the surface, virtually cleaning in the process of treating. This happens because of plasma’s concentration of high-energy UV radiation. “This UV energy creates additional, similar free radicals on the polymer surface. Free radicals, which are 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.” (Finson & Kaplan, 1997) Atmospheric plasma also treats a wide variety of substrates, minimizes ozone emissions, has a high and long-lasting dyne level, and treats in a uniform manner (“Atmospheric Plasma Treaters,” 2000).
Low-temperature gas plasma, also known as cold gas plasma, treats substrates through use of a vacuum. This differs from atmospheric plasma treatment primarily because of its use of a vacuum chamber. As substrates are fed through the unit, gas is dispersed within the vacuum chamber and is subjected to an electronic field. The field is set at a low radio frequency and an ambient temperature (hence the name “cold” gas plasma) (“Gas Plasma – a Superior Surface Modification,” 2004). This process excites surface molecules causing physical and chemical modification in the surface tension of substrates (Finson & Kaplan, 1997). On the positive side, gas plasma can create totally wettable or non-wettable substrates. This creates hydrophilic or hydrophobic materials—common processes that plasmas in general can perform.
Surface treated via corona (above) and surface treated via atmosphericplasma (below) (Wolf, 2004)
Flame plasma introduces a flame into the process of treating. It can print on virtually any type of substrate, three-dimensionality is no hassle. This entails highly flammable gas to reach contact with air to create “an intense blue flame.” This process also utilizes oxidation as a method of polarizing molecules of substrates’ surfaces. Flame plasma creates very high dyne levels or surface tension, keeps up with high-speed presses, and has very long shelf-life properties (“Three-Dimensional Flame Plasma,” 2005). Unfortunately, experts have noted, “Flame treatment also has limitations for oxidative surface modification, difficulty in control, and a possibility of excessive thermal loads.” (Yializis & Markgraf, 2001) Again, flame plasma, too, is ozone free, and does not allow blocking, back treatment, or pinholes to occur.
Plasma grafting has capabilities to form on extremely difficult materials such as glass, metal, powders, and three-dimensional objects. (“Atmospheric Plasma Treaters,” 2000) The plasma grafting process includes more of a polymerization of the plasma onto the substrate as opposed to the oxidation utilized in the prior plasma processes (Finson & Kaplan, 1997). And as opposed to cold gas plasma and atmospheric plasma treatments, plasma grafting actually “grafts” a thin layer of plasma onto the substrate. Typically, a hydrocarbon-based plasma gas comes in contact with the substrate and forms onto the surface (“Atmospheric Plasma Treaters,” 2000). It relates to atmospheric plasma and cold gas plasma because of the evenly distributed plasma field. Unfortunately, it changes the bulk properties of the treated substrates (“Atmospheric Plasma Treaters,” 2000). Because the plasma must form to the substrate, the alteration of bulk properties is unavoidable. Plasma Grafting provides substrates with even plasma fields and adhesion properties as well as antibacterial and anti-fog components. It also guards against pinholes that occur when treatments adhere unevenly to the substrate; however, most plasma treatments guard against them.
What is most cost effective?
The largest conflict print shops deal with, in regards to treatment processes, is finding the most cost-efficient process for their plant. In many ways, corona treatments are the most cost-effective avenue for good surface treatment. Some companies boast that a corona treater is capable of paying for itself in a matter of months. This, I believe, is not true for the newer, top-of-the-line plasma treaters. They, in fact, are more expensive and sometimes are not even considered by small printers with insufficient funding for such a machine.
There is an upside to buying a plasma treater over a corona treater though. What must be considered is the money loss of a corona treater from bad runs with backside treatments and other such insufficiencies. The quality of a plasma treater pays for itself in customer satisfaction. Where a corona treater takes on average one minute to treat an allotted area of a substrate, an atmospheric plasma treater can take around one second. The faster a machine works while maintaining quality, the more cost efficient it is, and the more reliable it is for a customer.
What is the best?
Depending on the product demand, a printer can have good reason to choose a corona treater or a plasma treater. The deciding factor is the end use of the treated product and the customers’ demands for quality. As far as the present is concerned, corona, atmospheric plasma, cold gas plasma, and plasma grafting each have their own niche. The driving forces keeping corona alive are its economic value and easy upkeep. The cost of gasses naturally makes each of the plasma processes more expensive. However, the added value is a considerable tradeoff. The plasma processes treat a larger gamut of substrates, for example, fluoropolymers, threedimensional objects, and polypropylenes. Additionally, plasma treatments provide a higher surface-tension quality on substrates. Plasmas leave a uniform discharge and guard against pinholes, blocking, and back treatment. Corona treatment also has a limited shelf life, as the treatment breaks down with age. Specific differences between corona and atmospheric plasma (excluding the use of plasma) can be seen while the molecular excitement of atmospheric plasma is quicker, creates higher surface tension, and is more durable (Wolf, 2005). Both atmospheric plasma and flame treatment operate well at commercial press speeds. However, the ultimate choice of treatment should be guided by the size, type, and expected value of print jobs. Every process has its own positive and negative aspects.
In the future
Processes in the future will combine capabilities. Combination corona and flame plasma systems, and combination atmospheric plasma and plasma grafting systems are currently used today. The possibilities for the future are boundless. Another future technology to look for as a next step is the soon-to-be-introduced atmospheric pressure plasma deposition system, “Labline,” by Dow Corning of Dublin, Ireland. Dr. Denis Dowling of the Dow Corning surface engineering group has stated, “This project uses atmospheric plasmas plasma (Labline system) for the deposition of hydrophobic, hydrophilic, and oleophobic coatings. Dow Corning Plasma Solutions developed the system, at their plant in Cork. A unique property of the system is that the chemical functionality of liquid precursors is retained in the plasma deposited coating.” (Dowling, 2005)
Labline atmospheric plasma coating system (Dowling, 2005)
What does it all mean?
Plasma surface treatment revolutionized and is still evolving treatment systems and processes today. As stated previously, corona treatments do still stand strong as a method of treatment in printing plants today. Economic value currently gives corona appeal. However, the prices of plasma treatments are sure to fall as time goes by. The general process of fusing the surfaces of substrates with gases is heavily relied on today. Most treatment systems are predominantly built to run webs for films and packaging. Primarily, these products print via lithography and flexography. “According to the Flexible Packaging Association’s 2004 State of the Industry Report, the flexible packaging market is the second largest segment of the total U.S. packaging industry. It accounts for 17 percent of total U.S. packaging, just behind corrugated.”(“Focus Your Message to the Market That Drives Your Business,” 2005, p. 1) As markets for products that utilize surface treatments grow, it only leaves an opportunity for surface treatment corporations to sell more of their treatment units and for treatment units to continue to evolve.
Dowling, Dennis. Online interview. 16 Feb. 2005.
Hablewitz, Rob. Online interview. 25 Feb. 2005.
Wolf, Rory A. Online interview. 11 Feb. 2005
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Three-Dimensional Flame Plasma Treater. Enercon Surface Treating Systems . 28 Feb. 2005, from http://www.enerconind.com/treating/products/dyneAFlame.html
Finson, Eric, and Kaplan, Stephen L. (1997) 4th State . The Wiley Encyclopedia of Packaging Technology, Second Edition. 11 Feb. 2005,from http://www.4thstate.com/publications/surftreatmentPrint.htm.
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Gilbertson, Tom. (March 2004) Corona Treating on a Solvent Line? FLEXO, pages 30-31.
Gilbertson, Tom (2006) Only You Can Prevent Backside Treatment. Enercon Surface Treating Systems. 20 Jan. 2006, from http://www.enerconind.com/treating/eLibrary/techart/onlyYouCanPreventBacksideTreatment.html
O’Neill, Brendan (2006) Industry Insights: Corona Treating with Enercon and Pillar Technologies. Flexible Packaging. 20 Jan. 2006, from http://22.214.171.124/search?q=cache:CNyQPqzi6OgJ:www.flexpackmag.com/content.php%3Fs%3DFL/2005/11%26p%3D12+corona+treatment+better+print&hl=en
Wolf, Rory A. (1 May 2004) Plasma Treatment Boosts Adhesion. Adhesion & Sealants Industry. 11 Feb. 2005 from http://www.adhesivesmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2101,123458,00.html.
Yializis, A. and Markgraf, David A. (1 Feb. 2001) Atmosperic Plasma: The New Functional Treatment for Films. Paper Film & Foil . 28 Feb. 2005, from http://pffc-online.com/mag/paper_atmospheric_plasma_new/