Corona Treatment in Practice

CORONA & PLASMA ReportNr. 125 E Corona Treatment in Practice

Plastic processing companies, especial- SOFTAL generators are equipped with non-conductive webs. However, for ly those producing and converting automatic control to eliminate irregu- the power rating of the corona equip- plastic film, are confronted every day larities in production. This controls the ment it is important to note that the with the problems of improving adhe- generator output and compensates effect of these electrodes on plastic, sion, quality, and of increasing the for changes in temperature, electrode for example, is poorer than the effect economic efficiency of processing gap, type, width and thickness of of metal electrodes. This must be com- machines. This report illustrates the material and dielectric thickness. It is pensated by increasing the output of technology behind the corona treat- then possible to have a turn down the generator or by using an addition- ment equipment, how it operates, and ratio of 10:1 with this system. al dielectric coating on the roller sur- how it can be applied to the produc- face, which will increase the efficiency tion processes in these companies. There are two variants of the elec- of this system. trode system, and their application High-frequency corona treatment is depends on the properties of the widely accepted as a process for material web. For non-conductive Electrode systems improving the adhesion of printing materials, for example plastic or paper, ink, lacquer, glue and other coatings metal electrodes are used. More than The effect of corona treatment basi- on plastic film, paper and metal foil. 14 years ago SOFTAL developed the cally depends on the electrode system. The reasons for its success are the highly effective MM multi-blade elec- The most common electrodes are sin- good results, controllability and easy trode which has now been patented gle blade electrodes, whose shape and handling. The techniques and the all over the world. configuration have an important effectiveness of the process have grad- influence on the efficiency. ually been improved to keep pace In this system the backing roller is with the developments of the produc- coated with a dielectric material such Over the past 12 years SOFTAL has tion machines. as silicone or ceramic, essential to paid a great deal of attention to the obtain regular, homogeneous corona development of electrodes, as experi- discharge. ence has shown that increasing generator output alone, to treat difficult The second variant of the electrode materials brings disadvantages in most system must be applied in cases where cases. Principle of corona equipment conductive materials are to be treated, such as aluminium, metallized paper, High generator outputs can only be The central features of the corona plastics, and laminates containing such obtained with a high ignition voltage equipment are a high-frequency gen- materials. In this case it does not mat- which leads to a separation of the erator, and an electrode configuration ter whether discharge on to the con- corona discharge into single sparks. mounted at a nominal distance of 1.5 ductive layer is direct or whether the This not only produces oxidation of mm from an earthed backing roller, conductive material is in a laminate or the plastic surface but at the same forming the corona station. on the reverse side of the material time decomposition products are no web. longer bound to polymer chains, and The generators are usually equipped adhesion problems occur, which is with IGBT amplifiers, and, in conjunc- The system for this application consists exactly the opposite effect that corona tion with a high voltage transformer, of dielectric electrodes, such as the treatment should produce. produce a sinusoidal output up to 40 SOFTAL KB (MMD) ceramic electrodes kVpk-pk at frequencies between 20 and filled with a metal conductor. The Our electrode development therefore 50kHz, depending an the type of gen- backing roller does not necessarily concentrated on transferring the erator. have a dielectric coating, since the entire available load as evenly and ceramic electrode provides the dielec- gently as possible to the surface of the The generated energy is discharged tric. material. The aim was to have uniform from the electrode system to the sur- discharge, avoiding long single sparks face of the material to be treated, as it While metal electrodes can only be and cooling the surface of the web at passes through the corona station used for non-conductive materials, it is the same time. This resulted in a com- between the electrode and the back- possible to use dielectric electrodes for pletely new concept in electrodes, the ing roller. the treatment of conductive as well as MM multi-blade electrode. Report

Treating stations with MM or KB (MMD) electrodes are manufactured in an open design, which means that there no enclosure is necessary for covering the treating station, essential for conventional treating stations. As a result, the threading of the web material is considerably easier and maintenance is simplified. This is achieved by covering the electrodes only. The ozone is extracted through an aluminium extraction tube onto which the insulated electrodes are mounted. Insulated covers on both sides of the electrode ensure protection against touching, efficient ozone extraction, and good cooling of the electrode system. The MM electrode is manufactured in different configurations, from Diagram 1 4 to 10 blades depending on the application. By design, the special shape of the electrode produces a smooth dis- A more important benefit of multi- water wetting (72 dyn/cm), depending charge which causes no damage to the blade electrodes lies in the fact that on the type of polymer. Evidence material. higher levels of surface tension can be shows that the design has a positive obtained than have been possible. As effect on adhesion. It has also been The advantages of these electrodes diagram 1 shows, the linear increases shown that in the case of pure poly- are obvious. As can be seen in diagram in surface tension approach limit val- mers, without significant slip addi- 1, less than half of the generator out- ues, meaning that increased generator tives, the surface effects obtained are put power is required, than with con- output will bring less and less more storage-proof than with conven- ventional metal electrodes, to obtain a improvement in treatment effect. tional electrode systems (see diagram surface tension of 42 dyn/cm on, for 2). There is a delay in the drop in sur- example, polypropylene films which This limitation can only be broken by face tension over the storage period are difficult to treat. On the other the design of the electrodes. With the and the residual values are higher, hand, the same surface tension can be multi-blade type, it is possible to have when the film is treated by the multi- obtained with the same generator surface tension up to the point of blade electrode system. output at twice the speed. This results in a considerable economic advantage, both in investment costs and in operating costs.

Furthermore, the 4-10 single blades, which heat up during operation, can expand independently and by this, bending can be avoided, and a uniform air gap held.

The comparison with dielectric electrodes is even more significant. These electrodes consist nowadays of ceramic, and operate against a bare backing roller. They are less effective and should therefore only be used in the corona treatment of metal foils or laminates containing metal, when used with a bare metal roll. Diagram 2 If the application requires a ceramic electrode for conductive materials and also nonconductive materials have to be treated, a silicone or ceramic coating on the roller is recommended in order to improve the efficiency on non-conductive materials (see diagram 1). Report

Essential features of corona Dielectric Outlook equipment Finally let us mention briefly the most In the future the aim of further devel- Easy operation, maintenance and reli- sensitive module in the corona system, opment of corona technology will be ability are the main criteria today for the backing roller dielectric. to optimise the generator technology selecting corona equipment. One and the electrode systems and in this important aspect of high-tech Although the dielectric is a necessary way to increase the efficiency of the machines with expensive downtimes is wearing part of the corona station equipment. Users, machine manufac- that the user can not accept failures and subject to frequent replacement turers, and the manufacturers of raw caused by additional equipment. The and repair, this can be influenced by materials can make an important con- first principle for the manufacturer of the selection of the dielectric material tribution by presenting their practical this equipment must be to face up to and its care. There is a choice mainly experiences, ideas and requests to the these conditions and use every techni- between two materials, ceramic and manufacturers of corona equipment cal means available to pursue without silicone. so that also this may be included in compromises the aim of maximum future developments. operation reliability, easy mainte- According to the state of the art, nance and simple handling. ceramic is the best material available as it is unbeatable with regard to tem- For this purpose the generators must perature resistance, high-voltage be short-circuit-proof and must strength and wear resistance. One dis- include practical functions and control advantage however is the investment features such as adjustable alarm sys- cost. Therefore the application must tem, fault indicator, power display, be considered as to whether the automatic generator tuning and infi- added costs of ceramic coating can be nitely variable output regulator. justified by saving expensive machine downtimes. It must be possible to install the equipment easily and within a short period The most commonly used dielectric is of time. This means that the equip- silicone rubber which has been devel- ment is delivered ready for installation oped so far that today it can have a with screened cables for power trans- service life of 6-12 months and more, if mission between the generator and it is well looked after. the station without complicated cable laying. Output transformers should be Silicone is available as a coating or also already mounted and connected to in the form of sleeves for various roller the corona station at the manufactur- sizes. In particular, sleeves for roller er so that high-tension cables are as sizes with a diameter of 100 - 200 mm short as possible and complex installa- are inexpensive and easy to handle. tion at the place of assembly is not Small defects can be repaired in min- necessary. utes with moisture interlacing silicone rubber. The corona stations should be made of rust-proof materials such as aluminium, stainless steel, ceramic or high quality plastic, like all SOFTAL standard stations. Safety features such as zero-speed switch, ozone extraction control and safety switches to control the electrode position are just as essential as the setting of the electrode gap during operation and the expansion possibility for electrodes on heating to prevent deformation. All installation components of the station must be easily accessible for cleaning in order to prevent surface leakage current and short-circuits. The major insulation components of the SOFTAL corona stations are made of arc-resis- tant ceramic. Report

Applying ozone treatment, it is possi- Other applications tion, the surface tension of the mater- ble to keep the melt temperature of ial is lower than that of the ink and the plastic below 300 °C with the same Apart from the applications described the test must be repeated using ink laminating strength. The oxidation of before, there are a number of other with a lower surface tension. If the the plastic, otherwise produced by the applications, such as cup printing com- edges of the liquid remain intact for high melt temperature, of up to 320 bined with hole detector, coating longer than 2 seconds the test is to be °C and more, is caused solely by the machines for webs and sheets, cable repeated using ink with a higher sur- ozone and only on the coating side. printing, label printing, folding box face tension until the time of 2 sec- This means that a laminate produced machines and other special areas onds is obtained. Brushes for the dif- in this way has no sealing problems, which will not be covered in this docu- ferent test inks must be kept apart. since the exterior has less oxidation ment. because of the low melt temperature. The test ink bottles must be kept tight- The advantage, especially for the pro- ly closed and stored in a cool place as duction of foodstuff packing, is that far as possible. The inks should be dis- the effect on taste and smell, also Measuring surface tension carded and replaced after not more caused by oxidation at high melt tem- than 3 months, because, if they are in perature, is greatly reduced. There are two main methods of mea- constant use, water builds up in the suring surface tension in the plastics liquid and particles from the material The ozone treatment is mainly used processing industry, wetting angle surface adhere to the brush after for LDPE coating on paper, board and measurement and test ink measure- repeated testing, mix with the ink and aluminium, for example for milk pack- ment. change its composition. aging. In this case a dry bond strength of 400 g more per 15 mm on alumini- The wetting angle measurement Unfortunately, in many companies um as well as fibre tear adhesion on method is mainly used in laboratories, there is still too little attention paid to paper and board can be achieved. but the test ink method is one that is handling, storing and replacing the applied in practice and is common in test inks. This leads to unnecessary loss Additionally it should be mentioned the factory, at the machine. due to production waste or even cus- that the coating thickness also has an tomer complaints later. influence on the bonding strength. In wetting angle measurement, a With thicker LDPE coating, higher defined drop of water is applied to the Test inks are available in surface ten- bonding strength is achieved. surface of the material to be mea- sion ranges from 30 - 56 dyn/cm and as sured. There are different kinds of dyed distilled water of 72 dyn/cm. For some years now, particularly in measuring equipment on the market Japan, ozone plants have been in use and so the wetting angle of the water A simple method of ink test measure- for the production of flexible lami- drop to the surface of the material is ment is with a felt-tip pen. This nates with LLDPE coatings. These lam- defined in different ways. The smaller method should only be used for sur- inates can be produced much more the wetting angle, the higher the surface tension of 38 dyn/cm for rapid economically on extrusion coating face tension and vice versa. An results on the production line, for machines than on dry laminating untreated PE for example has a wet- example to determine which side of a machines requiring more energy. ting angle of 90°. film has been treated. Felt-tip pens are Several manufactures of raw materi- absolutely unsuitable for accurate als, have developed special types of Measuring surface tension with test measurements. LLDPE for this purpose and are pro- inks is done in mN/m, although the moting the use of ozone plants. traditional dyn/cm designation is still Measurements of corona treated more common. There are different paper is only possible within a limited LLDPE is mixed together with LDPE methods of measuring, but the results range as the liquid is immediately and coated on an anchor coated film, are the same for identical materials. absorbed by the paper or board. often polyamide and polyester, at a One method applied, and not only in However, it is possible to measure the temperature of 290 °C, treated before Europe, is the DIN 53 364 method effect by measuring the period of with ozone. The composite produced according to the German standard for absorption of a defined liquid drop. A in this way is in no way inferior to lam- measuring polyethylene and treated material will absorb the liquid inates produced in dry laminating polypropylene surfaces. This method is much quicker. machines with regard to laminating used in practice for other plastics also. strength, sealing strength and hot Another measuring method is the tack. American ASTM method which is simi- lar to the German DIN 53364.

The DIN 53 364 test method consists of wetting the surface of plastic films with a liquid. The test ink is applied thinly in stripes. The edges of these stripes must remain intact for approximately 2 seconds. If this is the case, the surface tension of the film corre- sponds exactly to the surface tension of the ink. If the edges of the liquid contract within 2 seconds of applica- Report

Function of corona process By intensive corona treatment these Applications substances will continue to oxidize Although corona treatment has been and cross-link so that adhesion The applications of corona treatment used in the manufacture and treat- becomes more certain. The corona are many, and range from pre-treatment of plastics for more than 35 „equalizes“ the aluminium surface in ment of drinking cups and yoghurt years, there was uncertainty for some the machine and transverse directions, beakers in cup printing machines, to time about the effect produced on the and helps to reduce wastage in the pre-treatment in extrusion coating surface of the plastic, and about the case of unevenly annealed foil. and laminating lines, as well as surface corona process itself. After detailed treatment of BOPP films up to 8,5 m research in this field, we know today wide in biaxial orienting machines. We that the corona treatment effect is shall go into further detail about the based on bombarding the surface of Power rating most common applications. the polymer with electrons. These leave the electrode and are accelerat- The power rating of the corona equip- ed under high tension towards the ment depends largely on the type of passing material web. In doing this material, width of web and the pro- Blown film extruder they collide with air molecules which duction speed. All three factors are transmit light and react in part to equally important. As far as the mate- The most frequent and at the same ozone and nitrogen oxide. When the rial is concerned, it is very important if, time the simplest application of coro- electrons come into contact with poly- for example, a plastic film is treated in- na treatment is the use with blown ethylene for instance, they have so line with an extruder in the freshly film extruders. In most cases the mate- much energy that they can break the extruded state or treated later on a rials are polyethylene films which nor- bond between carbon-hydrogen or processing machine after a certain mally have to be treated on both sides. carbon-carbon. Reactions with the period of storage. Treatment on an Although these films sometimes con- corona gas take place at these free extruder requires a much lower treat- tain considerable amounts of slip addi- radicals, mainly towards oxidation. ing power. tives, it is normally no problem to The functional groups thus formed are obtain surface tension of 44-46 dyn/cm polar and so provide the basis for In the treatment of stored films on the or more. At the time of treatment adhesion of applied printing inks, lac- other hand, an important factor is shortly after extrusion, the slip addi- quers, etc. whether they have been treated at the tives are still spread over the thickness time of manufacture on the extruder of the film and therefore are not a or not. Even traces of slip additives major obstacle. have adverse effects on the required rating. A negative effect of slip additives can be seen during storage, as they often The width of web and the speed are a lead to a rapid and dramatic drop in linear function of the power rating. surface tension, especially with thicker films. The unit of measurement for specific energy is watt x min/m?. This formula The reason for this is to be found in contains the necessary parameters the fact that the slip additives which such as generator output (Watts), pro- are intended to lubricate the surface cessing width (m) and machine speed of the film, tend to migrate to the film (m/min). surface within a few days and strongly reduce surface tension at the same At the SOFTAL technology centres in time. It is therefore all the more Germany and Japan we have simula- important to pre-treat films contain- tors to define specific energy for any ing slip additives on the extruder, since Reactions in the corona discharge required material. When the specific it is practically impossible to sufficient- energy necessary to treat a material to ly treat an untreated film with high In practice, the surface of aluminium a defined sur- foils, even when annealed, is not free face tension is from organic residue. Petroleum frac- known, the re- tions and additives are used as lubri- quired genera- cants when rolling foils and bands. tor output can Depending on the surface roughness, be calculated up to 20 mg/m? residue can be mea- for any speed sured. During the annealing process and treating the majority of the oil is removed by width. distillation and oxidation, but the surface is not in fact free from residue. Particularly at the centre of the web there remain rolling oil residues of some mg/m?. They are oxidized and therefore wet out, but have a negative effect on adhesion. Report slip additive content after storage, SOFTAL offers corona stations with Printing machines under economically acceptable condi- MM or the advanced SMM electrodes tions. with 7 or 10 stainless steel blades for One of the converting processes is this application. printing such films or other webs of The corona stations installed on blown For the power rating of the corona flexible material. Further corona treat- film extruders have one electrode on stations for blown film extruders and ment is often necessary, even though each side in most cases. We designed for cast film extruders it is certainly materials are used which have already the DM electrode for this application, advisable to provide for a surface ten- been corona treated during manufac- a variant of the multi-blade electrode sion of 42-44 dyn/cm on polypropylene ture. However, long periods of storage with four blades. films and approximately 44-46 dyn/cm mean that the surface tension is no on polyethylene films. These surface longer enough to guarantee satisfac- If required, the corona station can also tensions ensure that further process- tory adhesion of the ink. be fitted with segment electrodes ing can be done without problems, with 3 to 10 mm segments to leave the even after long storage times with low This process requires a surface tension desired strip width untreated for heat slip additive contents. They also proof at least 38-40 dyn/cm for printing sealing at a later stage. This is neces- vide a good basis for refreshing the inks containing solvents, and 44-46 sary because corona treated areas surface tension in-line with further dyn/cm for water based printing inks have poor sealing strength after heat converting processes. which are being used more and more sealing. in the printing industry for environ- mental reasons. If the surface tension The open construction of the SOFTAL falls below these values it is necessary corona station for blown film extrud- to refresh the treatment before print- ers offers the possibility of treating ing to ensure good adhesion between either the two inner sides or the outer the film and the ink. sides, if the tube has been cut to provide two single films.

Cast film extrusion

Another area of application in the manufacture of films is cast film extrusion. As far as corona treatment is concerned, the application is not very different from that of blown film extrusion. Cast film extruders are often used in manufacturing polypropylene films and these require a considerably higher specific energy than polyethylene films. Depending on the type of raw material, the difference can be as much as a factor of three. Apart from In addition to plastic films aluminium that, cast film extruders usually pro- foils, usually of soft, annealed alumini- duce at higher speeds of up to 200 um, are also treated before printing. m/min and more. This means that gen- The use of KB ceramic electrodes in erators with higher output and 2 or the corona stations, as described more electrodes per side must be used. before, is needed for this purpose. In this case corona treatment can replace anchor coating which would otherwise be necessary to print on pure aluminium. Report

Laminating machines

The same rules apply to wet or dry laminating of flexible materials as to printing. It is recommended to treat the carrier web and the laminate web with corona since it is very important for both parts of the laminate to have good adhesive properties.

The corona station should be situated as close as possible to the coating or laminating unit in order to keep the risk of contamination of the treated surfaces on guide rollers to a mini- mum. Special pressurized types of corona station are available for operation in hazardous areas.

Extrusion coating and laminating machines

Another most important application In addition, it is often necessary to As mentioned earlier, corona treat- of corona treatment is extrusion coat- treat the plastic coating, usually LDPE, ment has a negative influence on the ing and laminating. Base materials for further processing. In this case, the sealing strength after heat sealing. For such as paper, cardboard, aluminium type of cooling cylinder used on the materials which have to be printed and plastic are used here in various laminator is important for the power and heat sealed on the printed side by forms. These are laminated together rating. The glossier a chill roll is, the further processing ,a uniform surface using a plastic melt, or are coated with more specific energy must be calculat- tension which must neither be higher plastic melt. All materials of the lami- ed for the treatment. nor lower than 37-38 dyne/cm is essen- nate must be corona treated in-line tial. This surface tension is sufficient before coating or laminating to For the post treatment of LDPE coated for a good adhesion of printing inks as achieve an acceptable degree of lami- paper and board the decrease of sur- well as for good sealing strength. A nating strength. face tension must be taken into con- uniform surface tension can only be sideration. Due to contamination of achieved by a speed dependent power In this case, the corona treater is the LDPE coated side by the uncoated control of the generator in order to installed as close as possible to the paper side the decrease of the surface ensure the same surface tension at dif- laminator respectively before the tension is much higher than on paper ferent speeds. This power control is an anchor coating unit. Depending on or board coated on both sides. optional device in all SOFTAL genera- the material and demand of bond Depending on the period of the stor- tors and can be used where necessary. strength, in many cases the LDPE coat- age until further processing it might ing can be carried out without anchor be necessary to carry out a second coating by use of the corona treat- treatment in-line with the processing ment with considerably lower costs. machine. Ozone treatment

On extrusion coating and laminating lines it is also possible to improve adhesion, and increase the speed at the same time, by additionally applying ozone treatment for oxidation of the plastic melt. Ozone, an intensive oxidizing agent, is produced in an ozone plant at a rate of up to 1000g/hour and blown with dry air at the extruder die on to the plastic melt which then oxidizes and increases its adhesive property on the ozone treated side.

The SORBEX ozone plants produced by SOFTAL in Germany and Japan are used extensively for LDPE coating and laminating and have also been in use for several years for LLDPE coating. Report

SOFTAL Corona & Plasma GmbH König-Georg-Stieg 1 21107 Hamburg Germany

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