Medical Device & Diagnostic Industry Magazine
MDDI Article Index

Originally Published February 2000

By increasing a material's surface tension, corona treating makes inks, films, adhesives, and other coatings more adherent.

Not long ago, the best and sometimes the only way to successfully print calibrations on such items as plastic syringe barrels, medicine bottles, or caps was to first expose the plastic to a flame, oxidizing it just enough to allow the printing ink or labeling to adhere to the treated surface. Although this flame-treating process was widely used, the problems inherent to the technique are numerous. The heat can easily distort the surface it's meant to treat if the flame is not properly adjusted initially and readjusted frequently and precisely. Complex three-dimensional objects may need to be manipulated several times and in several axes in order to achieve complete treatment, which can be tedious and time-consuming. The gas/air mixture must be closely monitored and adjusted to ensure any measure of consistency. In addition, the obvious hazard of working near an open flame is compounded by the risk of potentially noxious or toxic gases that could be emitted by the material being treated if it is overheated. And yet, for quite a long time, flame treating was virtually all that was available.

A current alternative is corona discharge surface treatment, which can eliminate many of the risks and other disadvantages associated with flame treating while adding significant benefits of its own. Corona discharge surface treatment allows substantial flexibility in the treatment of a wide variety of materials and products used in the medical industry. Application examples include syringe barrels, pill bottles, and catheter tubing that are direct printed; IV tubes that are treated and then coated for additional strength; and surgical gowns that are laminated with a protective film.

Bone-cement mixer components from Immedica Inc. (Chatham, NJ) are treated via an in-line corona discharge process.

The simplest definition of corona treating is that it is a way to increase the surface tension of a material. Increased surface tension means that other materials—such as a laminating film or printing ink—will adhere better to the treated material. This is accomplished by exposing the air near the material surface to a high-voltage electrical discharge—a corona—that causes the oxygen molecules in the discharge area to divide into their atomic form. These oxygen atoms are then available to bond with the molecules on the surface of the material being treated, thereby changing the surface molecular structure to one that is extremely receptive to inks, coatings, and various adhesives. Most film and sheet materials have a smooth, slippery surface (low surface tension). Corona treatment, in effect, chemically roughens the surface (raising the surface tension), allowing it to grab onto the ink, coating, or adhesive being applied. In reality, the resulting chemical bond that occurs is better than a simple mechanical bond with the surface.

In addition to eliminating the hazards of open flames and the high cost of labor-intensive sanding, corona treating can be more effective than flame treating or other methods of treating 3-D parts because it adapts more easily to complex configurations. In some cases, corona treating of an entire part can be done in a single split-second operation, with up to four discharge heads positioned to provide complete coverage of the surface of the part to prepare it for the application of labels, coatings, or direct printing of text or graphics.

DEFINING THE PROCESS

As explained above, corona treatment changes the surface molecules of a material in order to increase its surface tension and thus its ability to hold an ink or coating. Surface tension is measured in dynes per centimeter, and is commonly referred to as the dyne level of the material. In most cases, once the surface dyne level is approximately 10 dyn/cm higher than that of the liquid being applied, satisfactory wet out and adhesion of the ink or coating can be achieved.

Corona treating systems produce ozone as a byproduct, and various methods have been employed to trap or collect the ozone and treat or destroy it. Many ozone filtration and destruction systems have been plagued by a tendency to leak ozone into the atmosphere or surrounding area. However, newer ozone destruction systems—with greatly simplified designs that reduce the risk of leaks—have been developed that efficiently and reliably collect ozone and destroy it prior to exhausting it into the atmosphere. Exhaust air from these new systems contains less than 0.1 ppm of ozone, and the airflow and filter can be continuously monitored to ensure the system's effectiveness.

TESTING THE SURFACE TENSION

Different films and different inks and coatings present a wide range of surface adhesion incompatibilities that can be resolved by using a corona discharge surface treating system. Some materials and applications require higher levels of treatment—sometimes well above the typical 38- to 44-dyn range used in most processes. At the same time, a material requiring a high dyne level might be immediately followed by one that requires a surface tension well below 38 dyn, mandating not only a system flexible enough to accommodate such dramatic changes, but also a means of measuring dyne levels as accurately and quickly as possible.

There are many ways to measure surface tension. The simplest and most popular method—although somewhat subjective—is based on solutions that contain varying proportions of two chemicals, formamide and ethyl cellosolve. Each resulting solution will have a known surface tension level anywhere from 30 to 70 dyn/cm and testing stations generally have a selection of premixed and labeled solutions on hand.

The test involves applying the solution to the material to be tested. If the test solution wets the surface for approximately 2 seconds before breaking into droplets, the dyne level of the material is the same as that of the solution being applied. If the solution continues to wet the surface for longer than 2 seconds without breaking up, the dyne level of the material is higher than that of the test solution. In this case, higher-level solutions are used to continue to test the material until the exact level is determined. On the other hand, if the solution breaks into droplets in less than 2 seconds, the dyne level of the material is lower than that of the solution being used, and lower-level solutions are used to find the material's dyne level. Optimum results are obtained when the testing is done by starting with lower-surface-tension solutions and working up to higher-level mixtures until the surface-tension level is determined.

Corona discharge treatment can be used on complex part geometries to prepare a surface for direct printing of text or graphics.

This test can only provide an approximate result, since there is no reliable method of measuring at exactly what point the solution breaks into droplets. Human error in timing the breakup of the solution or in the initial application of the solution can affect the results. In addition, the same solution may act differently on different materials. Thus, a test that determines that two different materials are at the same dyne level does not ensure that the surface-tension level of the two materials is really the same. Nevertheless, the relative simplicity of this procedure combined with the fact that it can be used as a reliable benchmark once satisfactory printing or coating results are obtained has made it the most popular method of surface-tension-level testing.

Another method used to test dyne level is the test pen marker, or dyne test pen. Originally designed for use as a go/no-go test in solvent-based applications, the test pen method is very useful for checking overall consistency of treated materials, or for spot-checking pretreated films. The pen is used to mark the material, and the tester observes the reaction of the ink. At higher dyne levels, the ink lies flat in a continuous film, making it appear dark red in color, whereas on untreated film the ink breaks into tiny, nearly invisible droplets almost immediately.

A third method of dyne-level testing involves measuring the angle between a precisely controlled volume (droplet) of water and the surface of the subject material. A higher dyne level on the material causes the water droplet to wet the surface more readily, creating a larger angle between the side of the droplet and the material surface, which can be accurately measured with an optical comparator or similar device.

CORONA TREATER COMPONENTS

The components of a corona treating system include a power supply, a high-voltage transformer, and the treater station through which the material to be treated passes. The station itself typically comprises an electrode, an electrical insulator or dielectric, and a return path (ground), and it can be configured in a number of ways to accommodate different materials.

The power supply has a very simple function: to raise the frequency and voltage of the incoming power to levels sufficient to generate a corona in the station. Power supplies need to be monitored and controlled, since delivering the proper energy level is important to the characteristics of the resulting corona discharge and the surface-energy level attained on the surface of the material. Generally speaking, the higher the frequency (kHz) rating of the power supply, the lower the voltage required to deliver a given power to the corona discharge. A high-frequency/low-voltage combination is ideal, as a lower-voltage corona is less damaging to the insulators and dielectrics in the station and to the material being treated. However, not all high-frequency power supplies operate at lower voltages, so before purchasing a power supply it is important for device manufacturers to work closely with their supplier to ensure that the equipment can attain the output required for the materials being treated.

A relatively new development in power supply control is the power-density control system. This system provides a consistent treatment level by maintaining a constant power-density level in the corona discharge. The operator simply selects the treatment width and power-density requirement for a particular job or material being run, whereupon a sensor automatically detects the line speed and the system adjusts itself to the appropriate power level necessary to deliver the proper corona treatment to the material.

CORONA TREATER CONFIGURATIONS

The corona treating process can accommodate materials of various widths and types. In addition, it can be used for treating finished parts, such as plastic bottles and caps, medication containers, syringe barrels, plastic surgical instruments, and so on.

There are several basic corona treatment system configurations used for treating materials in web form. They are defined essentially by the location of the dielectric material in the station: conventional, bare roll, double dielectric, and convertible. The configuration that is best for a given application depends mainly on the material being processed.

Conventional corona treating systems pass the web over a roll covered with an insulating material, such as a silicone rubber sleeve or ceramic coating. Suspended approximately 0.06 to 0.10 in. above the material is the electrode, which is typically made from bare aluminum and spans the width of the material where treatment is desired. Working in concert with the high-voltage transformer, the power supply generates a voltage in the range from 6000 to 10,000 V, which is applied to the electrode. The high voltage is sufficient to break down the air in the gap between the electrode and the surface of the material, creating a corona discharge across the entire width of the electrode. This conventional setup is the most efficient configuration, but it can only be used to treat materials that are nonconductive and contain no metal, foil, or other electrically conductive components.

A second corona system configuration positions the dielectric material on the electrode. Because the treater roll in this configuration is typically made from anodized or bare aluminum, it has become known as the bare-roll configuration. In such systems, the corona forms in the air gap between the dielectric covering on the electrode and the material being processed. Bare-roll treatment is useful for both conductive and nonconductive materials, but this configuration is always less efficient than a conventional configuration when nonconductive substrates are being treated.

A potentially less hazardous alternative to flame treating, the corona discharge process eliminates substrate heat distortion since it transpires at 260°F—below the kindling point of paper.

For more effective treatment of specialized, difficult-to-treat materials, the double-dielectric configuration is sometimes used. "Double dielectric" means that the dielectric covering is located on both the electrode and the treater roll. This configuration has become much more popular in recent years, as it can be very effective on new types of specialized substrate materials that are increasingly in demand.

For the contract converter, it can sometimes be difficult to predict what kind of job a customer might request. In order to stay competitive and maintain maximum flexibility, converting equipment must be able to accommodate a wide variety of materials. In addition, changeover from one material to the next must be quick and easy to accomplish, as many of today's jobs can be very short run and excessive down time between jobs is extremely costly.

Thus, a fourth type of corona treating system has been developed—the convertible configuration. A convertible station can be quickly and easily changed back and forth from a conventional to a bare-roll or double-dielectric system simply by flipping a selector switch. A variation of the convertible station, the double-side convertible, allows for the treatment of both sides of the substrate material in a single pass.

TREATMENT PROCESS COSTS

As the packaging marketplace becomes ever more competitive, the pressures on film producers and converters to control costs at all levels continue to escalate. In addition, customers are demanding higher and more consistent quality in package products, which tends to add to production costs. The challenge for packagers and converters is to adapt to new materials, provide a higher-quality product, deliver it faster, and bring costs down.

Determining the true cost of various corona treatment systems is more than simple dollars-and-cents accounting of purchase costs. While it's easy to compare the initial acquisition prices of several treatment systems when considering which to add to a production line, one area of cost control that is frequently overlooked or underappreciated is the ongoing cost of operation. That is, there is no advantage to purchasing a system with a 20%-lower price tag if it costs 40% more to operate every day.

Purchase prices for corona treaters can range from as low as $4000 for a very basic unit to nearly $200,000 for a high-speed, wide-web, state-of-the-art system with computer interface and remote monitoring and control. The actual operating costs of a treatment system depend on a number of factors, among them the overall efficiency of the system, local electricity rates, power supply and station size, line speed, web width, materials being treated, and desired treatment levels. Another part of the cost consideration is an awareness of how long it will take for the system to pay for itself or provide a return on the initial investment. In addition, operating costs of a system built 10 or more years ago will be higher than those for newer, more-efficient systems. It's entirely possible that replacing an old, inefficient treatment system with a newer, more-efficient one could pay for itself in energy savings within a few years. Also to be preferred are systems that can be easily and quickly customized or upgraded to meet new treatment, material, and production challenges.

CONCLUSION

Corona discharge surface treatment is an effective and efficient process that is commonly used to increase the surface tension of a wide variety of materials, parts, and packaging in order to provide a surface that is more receptive to inks, coatings, or adhesives. Highly consistent and controllable, the process is continually being adapted for new applications using both standard and innovative materials. When considering the purchase of a corona treating system, one should be certain to investigate the flexibility of the proposed unit as well as its overall efficiency. Selecting the proper system will ensure that the equipment does not become the limiting factor in the event of future manufacturing changes such as an alteration in materials or increased production rates.

Bruce Stobbe is president of Corotec Corp. (Farmington, CT), a supplier of corona discharge surface treating systems to the medical device and packaging industries. He has more than 20 years of experience in the design, manufacture, and application of corona treating equipment and accessories.

Photos courtesy of Corotec Corp.

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