Abstract:
A corona treatment system ( 10   d ) has a corona treatment generator ( 11   d ), a first stationary electrode ( 12   d ) having a dielectric layer ( 18   d ) disposed thereon and a second roller electrode ( 13   d ) spaced from the first stationary electrode ( 12   d ) by a gap ( 15   d ). The second roller electrode ( 13   d ) has a layer of dielectric material ( 19   d ) disposed around a core ( 23   d ) and has a conductive layer ( 24   d ) disposed over a substantial portion of a length of the second roller ( 13   d ). A switch ( 20 ) is electrically connected between the conductive layer ( 24   d ) and an electrical ground, and the switch ( 20 ) is operable to switch the system between a grounded web mode and a dual dielectric mode.

Description:
TECHNICAL FIELD 
     The invention relates to rollers and electrodes for use in corona treating systems. 
     BACKGROUND ART 
     The prior art includes treatment of plastic films, papers, metal foils, and other materials using a high frequency, high voltage, electrical discharge, or corona. Early equipment employed a spark gap to generate the high frequency required. In the late 1960&#39;s, solid state generators were introduced which used SCR&#39;s (silicon controlled rectifier). The latest equipment uses IGBT&#39;s (insulated gate bipolar transistors) to provide high frequency switching within the power supply, also referred to herein as the “corona treatment generator.” 
     Referring to FIG. 1 a , a “conventional” corona treatment station  10   a  of the prior art is composed of a corona treatment generator  11   a , providing high frequency and high voltage, a stationary metal electrode  12   a , and an electrode roller  13   a  covered with a thin layer of dielectric material  14   a . The electrode  12   a  and roller  13   a  are separated by a small gap  15   a , usually about sixty mils, where the corona  16   a  is formed. Although the elements  12   a ,  13   a  are formed as cylinders, only their walls next to the corona gap  15   a  have been shown in section, it being understood that a full section would show a second thickness of layer  14   a , for example. The surface energy of a plastic web of material  17   a  is raised by direct contact with the corona  16   a . This improves the wetting and adhesion of inks or coatings applied in subsequent operations. 
     The electrode  12   a  in a conventional system is normally bare aluminum, steel, or stainless, in bar, tube, or flat-sided form. The electrode may be shaped (shoe form) to match the curvature of the covered roller. The electrode may also be one piece, for full-width treatment of the web, or broken into sections (segments) to allow adjustment of the area being treated. 
     Many types of materials have been used for the dielectric covering  14   a  on the roller in a conventional system. The oldest types are rubber compounds made from silicone and Hypalon™ polymers. Other coverings are made of epoxy and other thermoset resins (solid resin or composites with fibers), glass, and ceramics. 
     Inorganic coatings, such as glass and ceramic, provide durability and can also provide tolerance to the highest power densities in the corona since they are not combustible. These are more expensive than rubber coverings, for example. Epoxy and ceramic coverings are the most popular dielectrics for conventional systems. The thickness of the polymer coatings is typically 100 to 125 mils, while the thickness of glass and ceramic coatings is much lower, 40 to 80 mils (mainly due to cost). The practical range for all possible dielectric materials is from 20 mils to 500 mils in thickness. 
     Referring to FIG. 1 b , a bare roll form of corona treatment system  10   b  is shown. In this system  10   b , the roller  13   b  does not have a covering, so the system  10   b  is referred to as “bare roll”. The original dielectric on the upper electrode  12   b  was quartz but has been replaced with ceramic  18   b  (aluminum oxide) for improved temperature resistance. The bare roll electrode  12   b  is usually a tube (or several tubes connected in parallel) of ceramic  18   b , filled with metal powder  21   b , to avoid the thermal expansion problems of a solid metal electrode. The packed metal powder is connected directly to a high voltage lead from the generator. 
     The corona  16   b  of a bare roll electrode system  12   b  is always full width and cannot be adjusted. The bare roll system  10   b  has the advantage that the web  17   b  is in contact with a grounded roller  13   b . If a metallic web is being treated, the web  17   b  will not become energized, due to the grounding. The bare roll system  10   b  has the disadvantage that it is less efficient than other prior systems. Less input power is delivered from the corona treatment generator  11   b  to the web  17   b . Consequently, the power of a bare roll system  10   b  must be increased to equal the treatment of other systems, or the line speed must be reduced to reach the desired treatment level. 
     To overcome or reduce the limitations of the bare roll system, a dielectric cover  19   c  has been applied to the “bare” electrode roller  13   c . The covers  18   c ,  19   c  on both the electrode  12   c  and roller  13   c  are typically ceramic, although other materials could be used for the roller covering. The added dielectric  19   c  on the roller  13   c  shifts the power distribution toward the web  17   c , which increases the percentage of the input power actually used to treat the film, however, it does not provide a grounded web. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to improvements in corona treatment systems having all of the advantages of prior systems, including favorable power efficiency and grounding of the web, without the limitations of the prior systems. The invention provides a covered electrode roller, and a switching arrangement in which the corona treater system can be switched between a first operating mode as a bare roll system and a second operating mode as a dual dielectric system. 
     The covering on the convertible electrode roller is preferably made of ceramic such as alumina, zirconia, or a blend of ceramic materials. Other embodiments may employ coverings of other inorganic materials, elastomers including silicone, and thermoset resins and composites, including epoxy and polyester resins. 
     The convertible electrode roller of the present invention an outer layer over the dielectric layer. This outer layer is conductive. The outer layer can either be grounded (bare roll mode) or ungrounded (dual dielectric mode). This outer layer behaves like a grounded bare metal roller while in the bare roll mode. The bare roll mode can be used for treating conductive or non-conductive webs, papers, films, and foils. While in the dual dielectric mode, the outer layer behaves like a metal foil web wrapped around a dielectric covered roller. 
     With the outer conductive layer connected to ground, the roller appears to the system to be a grounded metal roller. The capacitance of the roller-web combination is the same as in a bare roll system. The corona also appears the same as it does in a bare roll system with the same characteristics of smoothness or spikiness. 
     With the outer conductive layer isolated from ground, the system behaves like a dual dielectric treater system, using a ceramic covered electrode, of course. Normally, the corona in a dual dielectric system appears to be smoother than a bare roll or conventional system. In the case of the convertible roller electrode, the corona looks the same in either mode. The corona appears like a corona in a bare roll system. 
     Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follow. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however, are not exhaustive of the various embodiments of the invention, and therefore, reference is made to the claims which follow the description for determining the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a - 1   c  are longitudinal section views of several corona treatment systems of the prior art; 
     FIG. 1 d  is a longitudinal section view of a corona treatment system of the present invention; 
     FIG. 2 is a perspective view of an embodiment of the electrode roller for use in the system of FIG. 1 d;    
     FIG. 3 is a schematic detail view of the system of the present invention; 
     FIG. 4 is a transverse sectional view of the electrode roller of FIG. 2 with a brush contact device; 
     FIG. 5 is a detail sectional view taken in the plane indicated by line  5 — 5  in FIG. 4; 
     FIG. 6 is a detail sectional view of a second embodiment of the invention taken in the same plane as FIG. 5; 
     FIG. 7 is a detail sectional view of a third embodiment of the invention taken in the same plane as FIG. 5; and 
     FIG. 8 is a detail sectional view of a fourth embodiment of the invention taken in the same plane as FIG.  5   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention relates to the system  10   d  illustrated in FIG. 1 d , and more particularly to a corona electrode roller  13   d  illustrated in FIG.  2 . As shown diagrammatically in FIG. 3, the roller  13   d  includes a core  22   d , which may have tapered ends  23   d , one of which is seen in FIG.  3 . The core  22   d  is preferably made of steel. A bond coat  25   d  is applied to the core  22   d . A layer of dielectric material  19   d  as described previously in relation to FIG. 1 d  is applied over the bond coat  25   d  and the core  22   d , and a conductive layer  24   d  is applied over the dielectric layer  19   d . The conductive layer  24   d  is terminated about two inches short of the ends of the core  22   d . A switch  20 , shown schematically, connects the conductive layer  24   d  to the core  22   d  and to an electrical ground. By closing switch  20 , the outer conductive layer  24   d  is grounded, and this will ground a web  17   d  traveling over the conductive layer  24 . If the switch  20  is open, the outer conductive layer  24   d  is floating or ungrounded. 
     The covered electrode roller  13   d  of the present invention has a dielectric layer which can be in the range of 20 mils or more. The typical ceramic layer will be in the 20 to 40 mil range, but treating efficiency (in the dual dielectric mode) is improved if the coating is thicker (up to at least 125 mils). The resulting “layer” can be made of thinner layers or coatings applies in multiple passes in thermal spraying. 
     The dielectric on the convertible electrode roller is preferably made of ceramic such as alumina, zirconia, or a blend of ceramic materials. Other embodiments may employ dielectrics of other inorganic materials, elastomers including silicone, and thermoset resins and composites, including epoxy and polyester resins. 
     The convertible electrode roller  13   d  of the present invention provides an outer layer  24   d  over the dielectric layer  19   d . The outer layer is a good conductor. The outer layer  24   d  can either be grounded (bare roll mode) or ungrounded (dual dielectric mode). This layer behaves like a grounded bare metal roller while in the bare roll mode. The bare roll mode can be used for treating conductive or non-conductive webs, papers, films, and foils. 
     While in the dual dielectric mode, the outer layer  24   d  behaves like a metal foil web wrapped around a dielectric covered roller  13   d . With this layer, the voltage is confined to the roller  13   d  and is not transported beyond the treater station by the web  17   d . The dual dielectric mode is used for non-conductive webs only; papers and films, not foils. A low resistance is preferred in the outer layer  24   d  by selection of the material and layer thickness to ensure that it is well grounded while in the bare roll mode. The layer can be a conductive coating, conductive sleeve, conductive paint, or electroless metal plating, wrapped metal sheeting, wrapped wire layer, metal tube, or a thermal sprayed layer of a conductive metal, ceramic, or cermet. The preferred materials are resistant to oxidation and the erosive effects of direct constant exposure to the corona. These include conductive ceramics, cermets, nickel chromium and other alloys which are highly oxidation resistant. 
     The outer conductive layer  24   d  is typically thin and can carry a minimum current of one amp while being grounded on at least one end. If the outer conductive layer  24   d  is grounded at one end only in the bare roll mode of operation, it is sufficiently conductive that the voltage on the opposite end is close to ground potential, while the corona system is in operation. If the outer conductive layer is a thermal-sprayed coating, the layer is at least one mil thick, after any grinding or finishing operation, but is typically five mils or more in thickness. The preferred thermal spray coatings are stainless steels, nickel and nickel alloys, nickel chromium, and cermets containing chromium oxide and titanium dioxide ceramics. Coatings containing ceramics would typically be more than five mils in thickness. 
     The apparent capacitance of the roller-web combination is higher than expected. This is because the system senses the capacitance of the entire roller, not just the area under the electrode. The corona is in electrical contact with the outer conductive layer  24   d  in the area of the ceramic electrode. The outer conductive layer  24   d  is in electrical (capacitive) contact with the entire dielectric layer which forms a capacitor with the core. It is possible for the outer conductive layer  24   d  of the convertible roller to retain a stored charge after the corona is turned off. The outer conductive layer  24   d  must therefore be grounded before personnel come in contact with the roller, or attempt to make the ground connection to convert to the bare roll mode. 
     The consequence of this higher capacitance on the system is that the dielectric layer can be made much thicker than normal, for increased dielectric strength, without causing power factor problems to the system. 
     The corona treatment generator  11   d  converts low frequency power to a high frequency and high voltage. The frequency is normally in the range of 3 to 30 kilohertz. The most prevalent frequency used is 9.6 kHz, but the newest equipment using IGBT&#39;s is typically greater than 20 kHz. The RMS voltage delivered to the electrode is in the 10 to 15 kilovolt range, but can be somewhat higher or lower. The latest equipment tends to be lower in voltage. 
     The corona  16  is a hot plasma of ionized and highly energized gases resulting from the breakdown of air in the high voltage field between the electrode  12   d  and roller  13   d . The web  17   d  is transported through the corona  16   d  in contact with the covered roller  13   d . Various theories are given as to the actual chemistry of corona treating. The result is that corona treating increases the surface energy of the material surface being treated. This in turn promotes the adhesion and wetting of inks, for printing; or coatings, for coating and laminating applications. The degree of increase is directly proportional to the watt-seconds per square inch (time-power function) exposure to the corona, but varies widely with the type of material being treated, initial surface energy, and the type and level of additives, such as slip and antiblocking agents, on the surface of the material. 
     Referring to FIGS. 4 and 5, in a first example of the invention a roller  11   d  was constructed as follows. The core  22   e  was provided by a 3.5×12 steel tube core with 0.375 wall thickness, which was grit blasted. A bond coat  25   e  was then provided by plasma-spraying a 3-5 mil layer of Sulzer Metco 480 plasma sprayed bond coating. A ceramic layer  19   e  was then provided by plasma-spraying a 22-mil layer of Norton 110 gray alumina plasma sprayed ceramic. An outer conductive layer was then applied by spraying 2-mil layer of Sulzer Metco 480 as the conductive layer. The layer  24   e  was terminated two inches short of each end to prevent arcing to the core during the corona testing. The ends  23   e  of the roller  13   e  were tapered. The roller  13   e  was sealed and cured with an organic sealer to fill the pores in the plasma sprayed ceramic coating. After curing, the roller surface was lapped to remove excess sealer. 
     Insulating disks  26   e  (FIG. 5) are attached to opposite ends of the roller  13   e . Each disk  26   e  forms a V-shaped groove  27   e  with the ends of the roller  13 . A bead of silicone caulk or other insulating sealant  28   e  is placed in the groove  27   e.    
     In the preferred embodiment, electrical contact is made with the outer conductive layer  24   e  by a brush  29   e . The brush  29   e  can be pivoted away from the contact position when it is desired to switch the connection of the conductive layer to ground. Other contact arrangements for the roller, including those using end rings and brushes on the ends of the roller can also be employed. 
     The convertible roller electrode  13   e  of the test embodiment was exposed to a corona using a 5 kW Pillar P1000 Corona Generator tuned to suitable settings. The roller was rotating in all cases. A ceramic covered electrode, eight inches long, was used for the testing. 
     The first test was with a corona gap of about 0.25 inch. For test purposes, the outer metal layer  24   e  on the test roller was connected to the grounded core  22   e  with a wire, in place of the brush  28   e , to simulate a bare roll treater. The electrode was offset so that part of the corona was outside the metal layer on the bare N110 ceramic. The corona over the metal was more spiky than over the ceramic. A maximum power of about 0.20 kW was achieved. 
     The gap was reduced to about one sixteenth inch. The corona spikes were smaller and more numerous. The power was the same. 
     The wire to the core was disconnected to simulate a dual treater system. At a one sixteenth inch gap, the corona looked the same as described above. A maximum power of about 0.22 kW was achieved. 
     A ceramic covered corona treater roller with a conventional 60-mil thick layer of Norton 110 gray alumina was tested to compare the appearance of the corona with the convertible roller. Even though the ceramic was almost three times as thick, the maximum power achieved was about 0.20 kW. The appearance of the corona was softer and less spiky than the convertible roller. 
     A bare metal core was substituted for the ceramic covered roller. The appearance of the corona was the same as the convertible roller and the power was again about 0.20 kW. 
     This testing confirmed that the operation of a dual mode convertible electrode roller. The corona formed the same on the outer metal layer of the convertible roller whether it was grounded to the core or floating, disconnected from ground. 
     As shown in FIG. 6, another embodiment of the convertible roller electrode  30  preferably utilizes a rubber or composite material for the dielectric layer  31 . The roller has inner and outer shells  32 ,  33  which are spaced about 125 mils apart. Epoxy or room-temperature-cure silicone rubber  31  is cast into the cavity (probably under vacuum to eliminate air bubbles in beyond the end  34  of the inner roller core shell  33 . The insulator disk  36  is attached with silicone caulk to prevent arcing from point A to point B. A switch  20 , as seen in FIG. 1 d , is utilized in a circuit between point A and point B to switch the roller between the grounded mode and the dual dielectric mode. An outer metal sleeve  37  has an oxidation resistant layer of material to prevent oxidation of the core material (probably light weight aluminum). 
     In the embodiment in FIG. 7, an outer metal sleeve  37  has been replaced with a 20-mil thick layer  38  of thermally sprayed metal or cermet. Using temporary forms, a composite material is wound past the end  40  of the core  41 ,  42  to provide a simplified end cap  52  with superior dielectric strength and arc resistance. Electrical contact is made to the conductive layer  38  by spring  43   a  and electrode  43 . The electrode ring  43  can be connected to an electrical ground by a wire, crimped connectors on each end and a couple of wing nuts, connecting point A to B. A switch  20 , as seen in FIG. 1 d , is utilized in a circuit between point A and point B to switch the roller between the grounded mode and the dual dielectric mode. This embodiment is very low cost, lightweight, and easy to fabricate. 
     In the embodiment in FIG. 8, a composite tube  44  is used as a dielectric roller. Electrical contact on each end is made by electrode  48  and spring  49  which is situated in a groove formed between the electrode  48  and a tapered end of the roller  44 . The tube  44  has a conductive layer  45  on the inside surface to provide the grounded electrical function of the core  45 ,  51 . The tube  44  also has a composite dielectric layer  46  and an outer conductive layer  47 . An end cap  53  similar to the end cap  52  in FIG. 7 is used for the embodiment of FIG.  8 . The dual mode of operation can be controlled by an electrical connection between point A and point B shown in FIG. 8. A switch  20 , as seen in FIG. 1 d , is utilized in a circuit between point A and point B to switch the roller between the grounded mode and the dual dielectric mode. This embodiment has the advantage of low cost, easy replacement, no need for spare cores, and reduced shipping charges (only the tubes, not the cores, need be shipped as replacements). 
     The dielectric constant and thickness of the covering affects the capacitive reactance of the roller-electrode combination and thus the efficiency of the corona treatment station. Usually the generator circuitry allows or provides adjustment of the power factor for various roller coverings. This is accomplished by adjustments to the output inductor (step-up transformer) or the frequency of the system. Silicone has a low dielectric constant in the range of 3.0-4.0. Ceramic is in the high range, about 9. Most other materials are in the midrange, 5.0 to 6.0. The biggest variation in roller capacitance is between a thick silicone cover and a thin ceramic cover. The capacitance of the roller is an important variable in the treatment efficiency of dual dielectric treater systems. 
     The above has been a description of the detailed, preferred embodiments of the apparatus of the present invention. Various modifications to the details which are described above, which will be apparent to those of ordinary skill in the art, are included within the scope of the invention, as will become apparent from the following claims.