By Melissa Donovan
Part 1 of 2
Manufacturing facilities use surface treatments to increase the adhesion of ink, paint, or primer onto materials. Beyond promoting adhesion, surface treatments also “clean, etch, and functionalize surfaces to remove contamination and in-crease surface energy,” explains Mark Plantier, VP marketing, Enercon Industries Corporation. Three popular methods of surface treatment include corona, flame, and plasma, however there are other lesser known options or derivatives of these processes. Which type of pretreatment process to undergo depends primarily on the application.
History on the Surface
Surface treatments have been used with analog technology such as flexography for many years. The introduction of digital printers in manufacturing environments heightens the need for pretreatment practices.
“As digital printing technology and the applications continue to evolve; corona, flame, and plasma are becoming an important part of the equation for adhesion success. In many cases printing would not be possible without inline sur-face treating and in other circumstances surface treating provides additional insurance that eliminates surface energy as a process variable,” shares Plantier.
According to Bas Buser, market manager printing, Plasmatreat GmbH, plasma treatment in particular has been used for many years in almost all areas, including automobile engineering, transport, electronics manufacturing, packaging technology, consumer goods, life sciences, textiles, and new forms of energy.
He believes new materials like plastics and recycled materials with undefined surface characteristics are one reason that surface treatments are thriving. “They have come onto the market and are less expensive, less weight, environ-mentally friendly, and offer a longer life.”
For plastic in particular, Greg Wood, president, Lectro Engineering Company Inc., notes the surfaces have traditionally been known to have poor adhesion because they are chemically inert and non-polar. Polyolefins like polyethylene (PE) and polypropylene (PP) always require surface treatment due to low surface energy.
Surface energy describes the surface of a given substrate like a plastic. The molecular force of attraction between a plastic and the ink determines the level of adhesion. The strength of attraction depends on the surface energy of the substrate. High surface energy substrates feature a strong molecular attraction, while low surface energy has weaker attractive forces.
PP, low-density PE, high-density PE, and BOPP have a surface energy of between 29 and 32 dyne/centimeter (cm), ac-cording to Kevin McKell, VP of sales, Vetaphone. However, requirements for different processes are higher. “Varying from printing with solvent-based inks at 40 to 42 dyne/cm to water-based inks at 46 to 48 dyne/cm, coating at 44 to 45 dyne/cm, and lamination at 46 to 56 dyne/cm.”
“However, with the increased use of water-based adhesives, primers, paints, and inks, there is a need to surface treat other types of plastics today. Surface treatment is often an afterthought of the design, molding, and manufacturing process,” he adds. In many cases, it may not be known surface treatment is necessary until adhesion fails.
Surface treatment equipment continues to advance in response. “The biggest change we have seen is human machine interface technology in corona treatment systems. This is important because it allows for today’s operators to have a simple understanding of these complex machines. The high-pace environment of our industry requires quick analysis and decisions,” advises Alyxandria Klein, marketing and sales director, QC Electronics, Inc.
Corona, Flame, and Plasma
Common surface treatments include corona, flame, and plasma. Each process differs, with their own benefits and challenges. Determining which to use is based on application.
Corona treatment—as explained by McKell—involves breaking up a substrate’s long molecular chains. In doing so, a multiplicity of open ends is created and free valances are formed. “This is done by directing a high-frequency electrical discharge at the plastic surface from close range. The electrons that are accelerated into the surface of the plastic disrupt the molecules by oxidizing it. The discharge splits the carbon molecules and breaks the oxygen into iodes, some of which enter the surface layer of the substrate and improve the bonding, while others form into ozone that needs to be extracted. The new carbonyl groups created have a higher surface energy, and this improves the chemical connectivity between liquid and plastic.”
Wood says advantages of using this pretreatment option are no shrinkage, warpage, scrap, or fire hazards. Conversely, the disadvantage of corona treatment, “is that the electrode which emits the discharge or treatment has to be in close proximity of the part to be treated. If there are several different parts to be surface treated, multiple electrodes and additional set up time is required,” he shares.
Flexible web applications are best suited for corona surface treatment. This includes products like packaging and labels on thin materials such as paper, foil, and fabric run on digital, flexographic, and gravure printing technology. “Corona is economic, effective, and easily integrated with printing systems ranging from narrow to wide web widths,” ad-vises Plantier.
“Corona is the most economical and diverse surface treatment process that is adaptable to most applications. It is best suited for high-speed printing presses, blown film applications, and extrusion coating applications,” agrees Klein.
Flame surface treatments are commonly used in relation to digital printing. “Flame treatment uses a hot oxidizing flame—usually natural gas or propane as fuel—in the range of 2,000 to 5,000 degrees Fahrenheit. It is brought into direct contact with the surface to be treated. The major advantages of flame are the ability to vaporize some contaminates on the plastic surface and a very fast treating rate,” explains Wood.
Negative associations with flame pretreatment processes, according to Wood, involve the flame’s inability to surface treat irregularly shaped parts. Another drawback is the safety hazard of working with an open flame near personnel in a manufacturing environment.
“Flame surface treaters are ideal for applications involving large surface areas, high line speeds, or complex surface geometries. A common application for flame treating prior to printing would be on larger bottles. Recent advancements in combustion control technologies make flame systems reliable and safe to use,” adds Plantier.
Plasma-based surface treatments center on a physical principal, according to Buser. “The term plasma designates matter with a high, unstable energy level. When plasma comes into contact with solid materials like plastic and metal, its energy acts on the surfaces and changes important properties, such as the surface energy. Treatment with certain plasma energy causes a targeted and exactly adjustable increase in the adhesiveness and wettability of surfaces. This makes it possible to use completely new materials and environmentally friendly, solvent-free paints and adhesives industrially,” he continues.
There are different types of plasma systems; Plantier explains that one is atmospheric. “When treating objects, a blown arc or blown ion plasma is often utilized. In this type of plasma treatment a concentrated discharge of ions clean, etch, and functionalize a surface to increase its wettability and bonding properties. These plasma systems are generally capable of covering an area up to three inches in single pass and are integrated with robotics, indexing, and conveyor systems prior to printing.”
Common applications include treating surfaces prior to printing inkjet lot and date codes, and printing on a variety of plastics with digital printing.
Beyond the Familiar
While corona, flame, and plasma surface treatments are the most common, a few other options available including chemical surface treatment and Lectro-Treat.
Chemical surface treatment involves chlorinated solvents, strong acids, or strong bases attacking the polymer surface to break bonds, create free radicals, and rough the surface. An example of this type of surface treatment is chlorinated priming, which Wood says is used in automotive painting of plastics. However, as EPA and OSHA rules become more stringent its use is decreasing.
Another type of surface treatment that Wood says differs from corona, flame, and plasma is suppressed spark treatment, or what he refers to as Lectro-Treat. It electrically treats the surface of 3D plastic objects. All types of molding processes, injection molding, blow molding, extruded, thermoformed, and vacuum formed plastic parts are viable candidates for the process.
“Lectro-Treat reaches a higher level of energy than all typical corona discharge devices by supplying what is referred to as directional plasma in the air,” explains Wood.
Plantier points out that primers are often used and applied manually to help adhesion, but usually corona, plasma, or flame surface treaters replace the need for primers because they guarantee a higher level of consistency. However, corona, plasma, and flame surface treatments are also used in conjunction with a primer and support the bonding of the primer with the material surface and ink.
Keep Holding On
The three common methods of surface treatment are corona, flame, and plasma, however other options are available. Each pretreatment process—when paired with the correct material and ink—promotes adhesion, increases surface en-ergy, and removes any contaminates on the surface prior to printing.
Used prior to digital printing with flexographic and gravure presses, surface pretreatments are not an unfamiliar presence in manufacturing facilities. However, with the increase of printable materials supported by digital printing, these surface treatments are being tested and manufacturers are creating equipment that truly enhances the experience.
In the next part of this two-article series, we discuss available surface treatment equipment, specifically used in con-junction with digital printing processes found in manufacturing environments.
Apr2019, Industrial Print Magazine