by Cassandra Balentine
Advancements in digital printing technology enable almost limitless substrate capabilities. However, to ensure success, treatment is often necessary prior to printing, painting, or priming onto low-energy surfaces, like plastic.
Above: 3DT provides a complete line of corona and plasma surface treating systems, including standard and custom built systems and application development.
Corona, plasma, and flame treatments are all options that help change the surface energy of select materials to improve wettability and ensure proper adhesion of ink and certain finishing effects.
Surface treatments are important for digital printing in industrial settings as it can eliminate the need for expensive coated materials as well as improve image sharpness, color, and resolution. These treatments are used in a number of industries from automotive to labels and packaging to expand potential printable options.
Corona treatment is a high-voltage electrical discharge in atmosphere that reacts with surfaces like polymer or metal to make it more hydrophilic, shares Peter Lund Anderson, marketing manager, Tantec.
Corona treatments change the surface tension of non-absorbent substrates to allow ink or lacquer to adhere. The process is achieved by applying an electric discharge at close range in ambient air to a moving web through a series of electrodes.
Jeannette Woodman, marketing director, Vetaphone, explains that the charge modifies the molecular structure of the surface to improve adhesion.
Many materials, like polypropylene and polyethylene, have insufficient surface energy for printing and bonding applications. “These materials have many useful properties, which make them materials of choice, however, their poor wettability creates severe limitations,” offers Gary Kohlnhofer, sales engineer, 3DT LLC.
Corona treatment uses a generator and high-voltage transformer to create a high- voltage/high-frequency corona discharge applied through an electrode with small air gap onto the surface to be treated. Kohlnhofer says the result is a cloud of ionized air—or corona discharge—which is then used for the surface treatment of plastics and other materials.
“Consequently, a substance placed under the corona discharge is impacted by the electrons with energy two to three times that necessary to break the molecular bonds on the surface. The resulting free radicals rapidly react with the oxidation of products in the corona discharge or with adjoining free radicals on the same or different chain, resulting in a cross link. Oxidation of the surface increases the surface tension or surface energy allowing for better wetting by liquids and promoting adhesion,” explains Kohlnhofer.
Corona treatment is generally applied before printing, painting, or gluing. It also happens post press to enable further converting or finishing. It is performed both inline or offline.
Often used inline on web-based printing machines such as for treating label materials, Kohlnhofer says with inline corona treating systems, the web is threaded over a treater roller.
It is designed for use on any non-absorbent surface or material from film to foil. “Making it suited to 95 percent of all surface treatment requirements,” offers Woodman.
“The obvious benefit is greater adhesion of your paint, print, or glue. The ability to use cheaper raw materials when using corona treatment is a larger cost savings. With the enhanced adhesion, FDA approval of medical parts can be achieved as well as stringent automotive safety standards,” shares Lund Anderson.
There are challenges associated with corona treatment. Different substrates present individual issues, which can vary between suppliers and even between rolls. “Their chemical composition can vary so much that they perform in a totally different way when treated and two rolls of the same substrate from the same or different manufacturers will have different abilities to accept ink or lacquer—so ideally each roll needs to be tested before processing,” cautions Woodman.
Plasma treatments also aid in prepping a substrate for print.
Sometimes referred to as the fourth state of matter, Lund Anderson explains that plasma is an ionized air that can be performed in atmosphere or a vacuum plasma setting.
“Plasma is based on a simple physical principle. Matter changes its state when energy is supplied to it—solids become liquid, and liquids becomes gaseous. If even more energy is supplied to a gas, it is ionized and goes into the energy-rich plasma state, the fourth state of matter,” explains Bas Buser, application consultant, Plasmatreat.
Buser adds that plasma is not rare. “More than 99 percent of the visible matter in the universe is in the plasma state. It can be seen in its natural form on earth as lightning or as polar light in the Arctic and Antarctic, for example. During a solar eclipse, plasma can be observed as a bright circle of light around the sun.
The term plasma designates matter with a high, unstable energy level. When plasma comes into contact with solid materials like plastics and metals, its energy acts on the surfaces and changes important properties, such as the surface energy.”
In the manufacturing industry, Buser says this principle is used for selective modification of material characteristics. Treatment with atmospheric plasma energy causes a targeted and exactly adjustable increase in the adhesiveness and wettability of surfaces. This makes it possible to use new—even non-polar—materials and environmentally friendly, volatile organic compound-free paints and adhesives industrially.
Plasma surface treatment—also called in-air and atmospheric plasma—improves the wetting properties of polymeric materials including rubber, metals, glass, ceramics, and paperboard. The molecules of these difficult-to-bond materials are modified by the plasma process to obtain better adhesion without causing surface damage.
“Atmospheric plasma is created by combining reactive gas molecules and an electric field. This system uses one or more high-voltage electrodes, which charge the surrounding blown gas molecules, resulting in a highly ionized field that is forced onto the targeted surface. This ionized air stream creates a thermal property that reacts with the substrate and breaks the existing hydrogen bonds by introducing oxygen that recreates the chemical properties of the surface. The atmospheric plasma process causes an intensified reaction resulting in better wettability, stronger bonding, micro-cleaned surfaces, and eliminates unwanted backside treatment,” comments Kohlnhofer.
Woodman points out that plasma treatment is a modification of the surface tension of a substrate carried out in a closed and controlled environment. “It is both a chemical and physical process using dopant gas—typically nitrogen—that can be tuned to the requirements of the chemical make-up of more complex substrates.”
Similar to corona treatments, the plasma treatment process is most often executed pre-print, but sometimes used in post-print scenarios. “Plasma treating systems are often placed just prior to printing on inline printing production lines,” shares Kohlnhofer.
Woodman says plasma is an excellent choice for smaller print areas and high-speed printing. With the correct system and nozzle, it can treat 12 to 15 millimeters (mm) wide at speeds of up to 100 meters per minute (mpm). “You can also treat widths up to 50 mm at speeds of 30 to 40 mpm,” he offers.
Plasma treats both metals and plastics inline for smaller parts or using very large complex geometries in a vacuum chamber. “Many large automotive interior parts are done with vacuum plasma. Tubes, wires, metal housing, and EPDM profiles are treated inline with atmospheric plasma,” says Lund Anderson.
Plasma treatment is also highly suitable for three-dimensional (3D) plastic parts, thin film, rubber profiles, coated paperboard, and thicker materials such as foam and solid sheets of material. “This technology is useful in many industrial sectors including medical, automotive, air and space, packaging, converting, narrow web, and polymer film,” shares Kohlnhofer.
Wilson Lee, director of business development, Enercon Industries Corporation, feels as though plasma is generally the most versatile method of pretreatment for the largest range of surfaces. “Outside of a few fluoropolymers, plasma works on almost all films, plastics, glass, and metals. Not only is the plasma activating the surface, it can clean organics and some inorganics off of the surface. The result is a uniform pretreatment on most any surface prior to printing.”
There are advantages and disadvantages of plasma treatments in digital print environments. “Because atmospheric plasma is potential-free, one important advantage is that plasma treatment can be incorporated into conductive, semi-conductive, as well as non-conductive applications,” explains Kohlnhofer.
Lee says that the incremental cost of using plasma is pennies per hour. Integration is generally simple, as the nozzle can be mounted immediately prior to the printhead. “You can use plasma on very slow or extremely fast printi applications. The challenge tends to be on high-speed printing of larger print areas. The economics are generally better with corona or flame if the print width is more than a few inches wide.”
Woodman adds that plasma is capable of being fine tuned to match specific requirements that are not possible to achieve with any other surface treatment process. “The challenge is to justify the cost of this sophisticated technology.”
Flame treatment is a third option to prepare surfaces for printing. Flame surface treaters can be customized with burners designed to the required width of the application. “They are ideal for applications involving large surface areas, high line speeds, or complex surface geometries. Recent advancements in combustion control technologies make flame systems very reliable and safe to use with repeatable results,” explains Lee.
There are applications where flame is used to cure the print after it is applied; but for the most part, the flame treatment is applied to the part immediately prior to printing.
Flame is effective on many surfaces. “As long as the process speed is fast enough for the increase in temperature, it is a good solution,” shares Lee. He adds that flame is best suited for treating large areas fast. “The flame burner can be many feet wide if necessary. Or you can use a smaller burner in conjunction with a robot to treat 3D parts.”
Flame is also effective for treated contoured parts that may be harder to cure with corona or plasma. “Flame gives you more flexibility because the distance to the surface is less critical,” continues Lee.
The challenge tends to be the thermal impact on the material. “Flame may not be the best option on very thin materials or at slower processing speeds,” he admits.
Many surfaces require or benefit from pretreatments. Corona treatments are often applied inline and are most effective on non-absorbent materials. Plasma is a chemical and physical process used to selectively modify material characteristics of a surface. Flame treaters are ideal for large surface areas, high line speeds, or complex surface geometrics.
Surface treatments are useful in industrial applications where manufacturers are looking to successfully digitally print on materials with a low-surface energy. IPM
Oct2020, Industrial Print Magazine