- 1 - Corona - Treatment

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1 - 1 - Corona - Treatment Corona treatment is a process used to modify the surfaces of plastic, metal and paper in order to improve the ability to bond with materials such as printing inks, adhesives and foams. The material to be treated is passed between an electrode assembly, designed for the specific application. An alternating voltage is applied between the electrodes, which is high enough to cause a glow-discharge. The air molecules become ionised in this process to form a corona plasma. The ions within the plasma move back and forth, in accordance with the direction of the current flow. When the ions bombard the material surface, the following changes to the surface occur: Craters, which are so small, that they are only visible under an electron microscope, are molten into the surface. These craters increase the surface area of the material and thereby increase the bonding ability to other materials. The bombardment of high-energy particles cause the long-stringed polymer chains on the material surface to rupture. The resulting unstable structures readily accept permanent chemical bonds to other materials. General information about corona treatment systems Generators AFS Generator G20S The development of high-voltage corona generators has made remarkable progress over the past few years. For some time now, equipment is available where extremely efficient, low-loss inverter concepts using new technologies have been realised, which in comparison to conventional generators with the same nominal power, produce up to 50% more treatment on the foil. The same modern generators, also have useful auxiliary facilities such as selfdiagnostics, trip and fuse-monitoring, and flashover-detection. Microprocessor control, intermittent operation and rotation detection are taken for granted. The use of a microprocessor interface makes it possible to remotely control and monitor the most important generator functions.

2 - 2 - Electrodes The choice of the treatment-side, i.e. the outer or inner side of the foil, or doublesided operation can be easily be selected by a switch, as every treatment-side is equipped with a separate high-voltage transformer. Typical Corona-treatment applications Corona-treatment is mainly used for the treatment of web materials. Extremely good results are also obtained with the treatment of pipes and cables. Ceramic electrodes with individually-sprung mountings Efficient foil-treatment stations, Regardless of the application, generally have some common features described below: The web runs over one or more rollers, which are usually coated with a dielectric (=insulating) layer. This roller is connected to ground potential. The highvoltage electrode is mounted parallel to the roller over the foil at a distance of 2mm. This distance and parallelism is extremely important, and a single adjustment point is desirable, as practical experience shows that systems with 2 or more adjustment-screws seldom result in the treatment electrode being set exactly parallel to the roller, and hence there are unequal treatment-results. The electrode assemblies are pivoted, so that thick sections of web material (>2mm), which sometimes pass through the station during machine start-up will not damage the electro0de bar. Corona-Treatment of foils As already mentioned, the treatment of foils the major area of corona-treatment applications. For nearly all treatment problems there are well-tested and reliable machines. The most important configurations are explained below. Corona-Treatment in the extrusion process Corona-treated plastics can only be badly or welded, or not at all. For this reason it is sometimes necessary to produce gaps in the treatment in the direction of web travel. This is attained through a segmented electrode bar, which permits electrode segments, generally 5 mm in width to be swung out of operation. The stations can be opened pneumatically, in order to facilitate threading the foil through the treatment station.

3 - 3 - Segmented electrode In extrusion installations more and more back-side effect free treatment is demanded; how this generally is reached is explained later. Corona-Treatment in converting application Here segmented electrodes are also indispensable because of the above mentioned reasons. In addition to this, the web material in bag machines often has to be cross-welded and in this case, perpendicular gaps in corona treatment are obtained by synchronous short-duration interruptions in the corona process (intermittent operation or skip-treatment). Corona-Treatment in the printing machine Gaps in the corona treatment in both the longitudinal and cross direction have to be possible. Because of the mainly high operating speed, the requirements for the intermittent operation system are very exacting. There are processor-controlled systems on the market, which interrupts the treatment exactly, precise and synchronous to the printing. For treatment near the anilox rolls and in hazardous atmospheres installations are available, which are constructed to ensure that no explosive gases can escape from the treatment station. Specially for this application, the systems largely conform to the legal regulations with respect to explosion-protection. Because of the high operating speeds, relatively high output powers are necessary. To avoid damage to the plastic surface due to local overheating from the corona electrode, several electrode assemblies may be necessary, so that the film gets treated several times in one operation. With rising power, the temperature of the roll also increases. To prevent the web material from shrinking, the treatment roller can be cooled. Corona-Treatment in the laminator Great importance is attached to reverse side effect free treatment. The treatment on the back of the foil arises through air pockets forming between the foil and the treatment-roll. Where there is an air gap, plasma occurs, and with this, corona treatment. There are several reasons why air gaps can occur: Creases in the foil, sometimes quite fine Unevenness in the treatment roller Slip between treatment roller and foil One solution for this problem is a banana roller or a bream spreader roller on the web inlet side, which helps to prevent creases from occurring, Air locks between the roll and the foil can be avoided by using a perfectly ground treatment roll with a pressure roll, which forces the web

4 - 4 - material on to the treatment roll, directly in front of the treatment bar. Corona-Treatment of Pipes and Cables Large diameter plastic pipes are often heat-insulated with polyurethane foam. The surface of the pipes have to be treated, otherwise the foam does not adhere to the surface. The same also applies to printing inks. Here the coronatreatment is used with great success. The spreader roll and the treatment roll are normally driven by a tendency drive, to keep the slip as low as possible. Gaps in the treatment are not necessary, so continuous electrode bare are used The treatment effect of the ozone should not be underestimated; it is only possible to counteract to it by spatial separation of the treatment electrodes (as shown). With this application, the inner-surface of the pipe has to make contact with a grounded inner-electrode. With cables, only the conductor has to be grounded. An interesting side effect of the corona pipe or cable treatment is the automatic checking of leaks, or breaks in the insulation. As soon as there is hole in the wall of the material, the corona discharge flashes through the hole, and the generator shuts down with a flashover warning and alarm. The wall-thickness of the pipes is restricted to approx. 30mm, above this, the dielectric characteristics of the material prevent a sufficiently strong corona discharge from occurring.

5 - 5 - Ozone Concerns During corona-treatment, high concentrations of ozone are produced. This can not be prevented, as the development of ozone is a inevitable side effect of the airionisation. It is well-known that ozone is poisonous. Because of this, all possible measures must be taken to protect man and his environment. The active charcoal system is very simple to use, but it has the disadvantage that every three months, exhausted material accumulates with a volume of approx. 0,2 0,5 m³ from a single station. The saturated charcoal reacts with water producing an extremely aggressive acid. As the charcoal normally is disposed by throwing it away, this does not seem to be the best way to solve this problem. Superior results are achieved with special catalyst materials, whose life is approx. 5 times higher than charcoal, but their purchase is about three times more expensive. What remains, is the problem of disposal. Trends and future prospects The increasingly stringent regulations regarding the protection of the environment and the foods industry, especially in Switzerland force to use low-solvent or solvent-free inks (see Printing with water soluble colours"). These inks are more difficult to bond to plastic materials, however, so that the treatment requirements are much higher. Ever wider and faster installations require increasing generator performance and special electrode configurations, to provide suitable treatment without melting the foil. AFS Ozone destruction unit There are two systems for the destruction of ozone on the market, both of which operate similarly. One variant is, to draw the ozone-rich extracted air out of the treatment-station through a layer of active charcoal, the other one is, to use a special catalyst instead of the active charcoal. For fully automated applications, treatment stations have been developed, where the segments are positioned automatically, so that the treatment system is fully integrated into the process control system. In the field of the ozone destruction, new systems will soon be available whereby the ozone can be destroyed, without posing new disposal-problems.

6 - 6 - Why AFS Treatment- Stations? AFS Corona Station AV-250D There is world-wide competition in the field of the corona-treatment, but, as stated above, the different applications require various machine-layouts and configurations. The treatment of sensitive materials also require special types of machines. Special designs are necessary for high working speeds. Also if corona-treatment is desired only on one side of the foil, and no reverse-side effect is acceptable, special machine-layouts are necessary. Corona-treatment stations often are installed in new or already existing production-lines. Here it is important for the treatment-station to be able to be integrated into the complete system. The new generation of AFS corona generators moreover also fulfil many other requirements, which our competitors cannot offer at the moment. (For the details and the special advantages take a look at our brochures). As soon as special applications and special customer requirements have to be fulfilled, and sensitive materials have to be treated, just few manufacturers of corona-treatment systems are in the position of being able to offer long-term and trouble-free solutions. The strength of AFS is based on the fact that we are able to offer flexible and technical high-grade solutions for these applications working together with our customers, whereas our competitors are often are not able to do so. 23 representatives world-wide grant ideal customer care and prompt spare-part supply and a good service. Our installations world-wide are appreciated by our customers for their reliability and longevity. Furthermore we are the only manufacturer of treatment stations to offer a 2 year guarantee. Also special desires of the customers, such as skip-treatment and partial treatment in longitude direction, place great demands on the machine and the electronics.