Corona Treating for Coating Applications
Table of Contents
Maintaining high quality coating on paper, film, metalized film and foil substrates at production line speeds requires a method of enhancing substrate surface energy. Corona treatment will promote bond sites and increase surface energy without sacrificing the positive properties of the substrate. In coating applications, treater system design has undergone significant changes to accommodate lighter webs, higher line speed operation and advanced substrates. The range of developments has included the requirement for handling conductive substrates, nips at the treater roll, using the treater roll as a “pull roll,” and modifying the treater station design to reduce wrinkling and “backside” treatment. Several techniques have been tried in search of good way to accommodate conductive substrates and to reduce wrinkling and backside treatment. One approach, dual dielectric/coated roll technology, has become by far the primary method used to overcome these problems. A history of the various technologies used is discussed, and the current state-of-the-art in equipment, control parameters, and applications are explained.
Equipment Basics: The Corona Treating Station
In the early eighties, a treating station was developed that revolutionized the structure of both the electrode and the station. The principal element of this revolutionary Bare-Roll design was the first successful movement of the dielectric covering from the treater roll to the treater electrode (Figure 1). This change included the first use of ceramic as the dielectric medium. Covering a roll with ceramic was not technically feasible at that time. This allowed the station to have an “open” design while still providing operators with safety from electrical shock. It also allowed the station to treat both conductive and non-conductive substrates. These benefits were well recognized at the time.
Less recognized was the simultaneous development of an electrode assembly that moved the location of the ozone exhaust from the perimeter of an enclosed station to the end of the electrode assembly. This new approach enabled the electrode assembly to rotate to allow the passage of web splices and knots. An enclosure was no longer required to capture and contain the ozone for removal. Instead the ozone was captured and removed immediately at the point of production in the air gap. This change was given little recognition at the time, yet provided significant benefits. It greatly reduced the opportunity for ozone to seep into the work area and, ultimately, permitted the construction of Covered-Roll stations in an open design.
The Bare-Roll Dual Dielectric Station is a recent development that expands the capabilities of the Bare-Roll Station. The Dual Dielectric Station (Figure 2), which uses a special ceramic coating on the roll as well as a ceramic electrode, matches or betters the treating efficiency and effectiveness of any other system.
With one exception, the Bare-Roll Dual Dielectric Station provides all of the benefits of the original Bare-Roll Station. The exception is the fact that the special coating may, but seldom does, require repair or replacement. However, unlike the traditional Covered-Roll Station, the Dual Dielectric Station does not become unusable when the dielectric coating is pitted, cracked or pin-holed. Since the electrode is also ceramic covered, the station can continue to treat. The dual ceramic provides an additional benefit in that heat stress buildup during operation is shared, and therefore neither electrode nor roll coating is subject to the same level of heat stress as that found in Covered-Roll stations. This, of course, increases the long term operational reliability of the coated roll.
The Bare-Roll Dual Dielectric Station provides an additional, highly significant benefit over Bare-Roll and Covered-Roll stations. The Dual Dielectric Station greatly reduces wrinkling of ultra light substrates and thereby also reduces undesirable “back-side” treatment, i.e., treatment on the side opposite of that for which the treatment is intended.
Driving and Nipping the Treater Roll
The requirement to nip or to drive the treater roll is more often dictated by machine or substrate characteristics than by technical treatment demands. For example, treatment of substrates, especially ultra light weight substrates running under light tension, is seriously complicated by the tendency for these substrates to wrinkle under the corona discharge. The wrinkling not only causes a problem with the quality of the wound roll, but also produces backside treatment which can be detrimental to the final product.
Wrinkling and the resultant back-side treatment can be reduced by increasing the amount of wrap and tension of the substrate on the treater roll. For minor wrinkling problems this may be sufficient. For more serious wrinkling, the addition of a nip roll on the treater roll where the film enters the station can eliminate or mitigate the problem. The addition of a nip on the treater roll will require that the treater roll be driven, which will require a drive shaft on the roll. This may necessitate a complete rebuild or replacement of the station.
If there is any possibility that wrinkling may be a problem in your application, it is best to plan ahead and provide space for the inclusion of a nip roll, even if you are not having the nip roll installed immediately. In this case a drive shaft should be provided on the treater roll.
The majority of corona treatment stations do not require the treater roll to be driven. Therefore, most stations are not provided with a drive shaft. Even light substrates will provide sufficient friction and tension to rotate the roll at line speed. Even if the roll does not rotate when the substrate is moving and treater power is off, the roll will rotate when treater power is applied. This phenomenon is referred to as electrical pinning and is used by companies manufacturing what are called electrostatic pinning bars. These devices are sometimes touted as electrical nips, but they are not nearly as effective as a mechanical nip roll. Their real benefits are that they are small and can be added easily after the machine has been installed; but they are not replacements for nip rolls.
Since the corona discharge provides electrical pinning of the film to the treater roll, one might ask, “Why doesn’t it prevent wrinkling?” The answer is that even though the corona discharge will provide pinning power, in may cases, that pinning is accompanied by static electrical forces that cause the film to wrinkle. This effect is enhanced by the fact that the roll is not rotating at a rate exactly matching the line speed. Driving the roll will therefore reduce the possibility of wrinkling.
Driving the treater roll and, in some cases, adding a nip roll also can also be required when working with substrates that are highly extensible. These substrates require highly uniform and tightly controlled tensions throughout the process. Using the substrate to drive the treater roll can cause an unbalanced tension from one side of the station to the other and this imbalance may be sufficient to stretch the substrate. In some cases the machine design will require that the treater station be constructed as a ‘pull-roll.’ This requires that the treater roll be nipped and driven and provides the machine with a tension control point.
All corona treating installations require a source of controlled electrical power. Low-voltage 60 hertz electrical power is fed into an electrical device which raises the frequency. This high-frequency electrical power is applied to a step-up transformer that increases the voltage. The high voltage, high-frequency electricity is then discharged from an electrode through the web being treated to the electrically grounded metal roll.
Although the basic principles remain the same, many
improvements have been made over the past 30 years which greatly increase the capability and performance of the power supply. The earliest power supplies were motor generator types that proved unreliable for long-term continuous operation due to mechanical breakdowns. These were replaced by power supplies using a Telsa coil and spark gap to generate the high-frequency, high-voltage, electrical energy. These designs were an improvement over the motor generator systems but still left much to be desired in reliability due to the erosion of the spark gaps.
Next came solid-state power supplies using transistors as the power output device. However, early transistors had a limited output power capacity, and these units required as many as 16 transistors connected in parallel to achieve the required power levels. Although transistors are generally very reliable solid-state devices, the large number of devices required caused a proportional increase in the chance of a random failure.
The natural evolution of power supplies then led to the development of an inverter using silicon controlled rectifiers (SCR’s) as the power output device. SCR-type inverters have been in widespread use for the past 25 years and have proven to be very reliable.
Recent advances in transistor technology have led to a new generation of corona treating power supplies which provide increased power output in smaller sized enclosures. The ability of these new Insulated Gate Bipolar Transistors (IGBT’s) to handle higher current levels yet provide high speed on/off switching through logic control allows more precise control of treating power and frequency.
The advent of logic controllable power devices allowed the use of programmable microprocessor and minicomputer chips that provide reliable treatment levels on a variety of substrates. Improved control circuits for corona treater power supplies are now available which increase the consistency of corona treatment for enhanced adhesion of the extruded material to the substrate and, in post-treating applications, of inks, primers, coating and laminants to the extrusion coated surface. Incorporating microcomputer technology, these treaters with advanced control ensure this advanced control assures corona treating accuracy and consistency by automatically tuning the power supply power and frequency to all variables such as electrode, roll type, web thickness, and air gap (Figure 3).
The most significant design criterion involved in corona treatment equipment selection is sizing the system to meet the specific application requirements. Power supply size in kW is determined by the treatment power required by the most difficult-to-treat substrate to be coated. Treatment power is measured in Watt Density (Watts/Square Foot/Minute), which takes into consideration not only power level, but also the length of time the power is applied. If a given material is treated at a given watt density, its surface energy will be increased a given amount. This is the basic goal of surface treatment. However, both the ultimate surface energy achieved and the amount of increase are dependent on the material’s starting surface energy. For example, applying a watt density of 1.0 Watts/Square Foot/Minute on PET at 41 dynes may raise it to 46 dynes, but applying that same watt density of 1.0 to PET at 44 dynes may raise it to only 48 dynes. Although the final dyne level is higher in the second instance, the incremental increase is less because of the starting point dyne level.
As you might expect, different materials react differently to corona treating. Some materials, such as most polyesters, accept treatment readily and will exhibit a rapid increase in surface energy under relatively low watt density levels, say 0.5 to 1.0. Other materials, such as polyethylene with some additives, accept treatment less readily but will exhibit a significant increase in surface energy under moderate watt densities, say 1.4 to 1.8. Finally, materials such as polypropylene with slip additive, which are difficult to treat, may exhibit only moderate increases in surface energy under relatively high watt densities, say 2.0 to 2.5.
Treatment levels, i.e., watt density levels, vary considerably from application to application. The materials and watt densities selected for discussion here were chosen because they are somewhat typical of extrusion coating applications. However, treatment levels can also vary significantly based on additive load and other substrate characteristics.
Position of Treating Station
Most coating and laminating is done today using water-based adhesives and coatings. The use of water-based materials requires a substrate with a higher surface energy than when solvent-based coatings are used. Therefore, pretreated substrates should be used whenever possible and the pre-treated polymer should be treated again at the time of coating or lamination. Table 5 shows typical treatment levels for various materials.
Table 5 – Typical Watt Densities
|Solvent Coatings:||1.2 to 1.4 Watts/Square Foot/Minute|
|Water-Based Adhesives:||1.3 to 3.3 Watts/Square Foot/Minute|
|U.V. Coatings:||2.0 to 3.0 Watts/Square Foot/Minute|
|100% Solids Adhesives:||1.0 to 1.5 Watts/Square Foot/Minute|
Pre- and Post-Treatment
Most polymers are treated during the extrusion process. This treatment should be kept to a maximum of 44 dynes so the substrate does not “block” when wound. If the substrate contains additives such as slip or anti-block, the surface energy will decrease because the additives migrate to the surface covering up the treated material. When the substrate is later used in a coating or laminating process, the substrate should be re-treated on the process line to bring back the treatment and even raise it to higher levels. The treatment can be higher than 44 dynes at this time because the treated substrate will be covered with a coating or adhesive before being wound.
Post-treatment, i.e., surface treatment of coated substrates to facilitate follow-on converting process, is predominantly achieved using the corona process. Both the Pre- and Post treatment processes require several configurations of equipment to meet the widely varied technical and cost requirements found in extrusion coating applications.
Despite increased line speeds, the move to ever lighter webs and lower web tensions, and the introduction of co-extruded substrates, corona treating technology has advanced to offer both technical and cost effective solutions to surface treatment requirements for the substrate coating industry.
Written for Faustel by Enercon Industries Corporation