Patent Publication Number: US-2013251916-A1

Title: Suction valve in a plasma coating apparatus

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to apparatus and methods for coating a container by means of a plasma treatment. 
     BACKGROUND 
     To reduce the permeability of container walls/walls of hollow bodies, e.g. in respect of undesired substances, it is advantageous to provide these walls with a barrier layer, for instance by the plasma-enhanced chemical vapor deposition (PECVD), as is described, for instance, in EP 0881197A2. 
     Such barrier layers are required, for instance, to reduce the transmission rates of gases through the plastic wall of a container. This allows, for instance, to minimize the loss of CO 2  from the filled product or the introduction of oxygen into the product. Also, it is possible to protect the product against substances escaping from the container material, which may change the color or taste of the product. 
     For the coating of containers by means of a plasma treatment, e.g. the interior plasma coating of plastic bottles, inter alia, a so-called high-frequency plasma may be used in so-called low-pressure systems. 
     Initially, the interior of the container is evacuated to a pressure in the range of 1-10 Pa. Then, a process gas is introduced through a gas lance into the area of the surface to be coated, e.g. the interior of the container. The layer—the so-called precursor—is formed from the process gas and allows the pressure inside the container to rise to 10-30 Pa or more. 
     This gas or gas mixture may be transferred in part or in whole into a plasma state by means of electromagnetic radiation, e.g. microwaves or high frequency, e.g. at 13.56 MHz, or other electric fields and, at the same time, be decomposed into its components. 
     In this case, for instance, a high frequency irradiated by a flat electrode outside the container is coupled to an electrically conducting gas lance and, with good pressure conditions in the container between 10 and 30 Pa, ignites a plasma in the interior of the container to be coated. 
     Portions of the process gas supplied into the interior of the container through suitable bores in the gas lance react plasma-enhanced in the gas phase or on the surface of the substrate to be coated, e.g. the inner wall of a plastic bottle, and condense on this surface forming a closed layer. 
     To prevent the ignition of plasma outside the bottles, this region is subjected to a higher pressure, e.g. 3000 to 4000 Pa, as compared to the pressure inside the container. To this end, the container may be pressed against a valve through which the interior of the container is evacuated to the process pressure of 1-30 Pa. 
     A problem arising in practice is that the plasma may burn not only in the container, but in an undesired manner also in the suction valve. This plasma uses up an undefined and undefinable part of energy, which is then no longer available in the container to decompose the process gases. It is difficult, in this case, to compensate this loss of energy, for instance, by a higher high-frequency power because this may lead to higher voltages at the electrodes. These higher voltages may, again, result in plasma discharges outside the container, so that the electric power available in the container is further reduced. 
     Another problem are the temperatures and the reactive gases generated by the plasma in the valve. They make the process even more difficult as the valves have to be cooled down actively to protect them against damage. Moreover, the sealing materials in the valve are affected by the reactive gases and need to be replaced more often. 
     If the valves are made of a plastic material they constitute in principle, as far as the conditions are concerned, a geometric prolongation of the bottle opening. If high frequency is applied, a plasma is ignited there, however, just like in the container itself. If the valve is made of metal and the individual components are grounded a plasma may develop in the interior all the same, as a hollow cathode may thus be formed. Besides, such hollow cathode plasmas have a particularly intensive plasma density, heating up the valve components especially strongly and, thus, leading to damages. 
     It is, therefore, an object of the present disclosure to improve an apparatus for the coating of containers by means of a plasma treatment, for instance the coating of plastic bottles, in particular with respect to minimizing undesired plasma ignitions and/or plasma burning in the region of the container opening. 
     SUMMARY 
     According to the disclosure this is achieved in some arrangements by an apparatus according to claim  1  and by a method according to claim  14 . Advantageous embodiments and further developments are defined in the dependent claims. 
     An apparatus according to the disclosure for coating a container, e.g. a plastic bottle, by means of a plasma treatment may include at least one gas lance for supplying process gas into the container, and at least one suction valve for sucking off air or gas from the interior of the container. 
     The suction valve may have at least one recess, e.g. a centrally arranged one, for receiving or introducing the at least one gas lance into the container, and the suction valve may include at least one grounded, gas-permeable, electrical shielding element. The at least one grounded, gas-permeable, electrical shielding element is capable of preventing and/or suppressing nearly entirely the ignition and/or burning of plasma in the suction valve. 
     Avoiding or minimizing plasma ignitions and/or the burning of plasma in the suction valve has the advantage, inter alia, that the suction valve is thermally loaded to a smaller extent, for instance no active cooling is necessary, so that a longer service life of the suction valve can be obtained. Furthermore, minimizing undesired plasma ignitions and/or plasma burning processes in the suction valve or outside the container region to be coated has the advantage that an energy loss for the plasma used for the coating can be reduced. 
     The electrical shielding element around the at least one recess for receiving or introducing the at least one gas lance may have gas-permeable hollow body structures. Preferably, these gas-permeable hollow body structures may have average diameters which are equal to or smaller than the Debye length of the plasma generated during the coating so as to allow an effective electrical shielding effect in order to minimize plasma ignition processes and plasma burning processes in the suction valve. 
     The electrical shielding element may have a plurality of different structures. In particular, the electrical shielding element may be a gas-permeable, open-pored porous structure, with pores whose average pore diameter is, for instance, between 0.01 and 6 mm, preferably 3 and 4 mm. 
     In this connection, it is possible that this gas-permeable hollow body structure or, respectively, this gas-permeable, open-pored porous structure of the electrical shielding element may be made of a metallic foam, e.g. an aluminum foam, or of an electrically conducting ceramic foam, e.g. Al2O3/TiN, or of electrically conductive composite ceramics with carbon fibers, or of a plastic or polymer foam having electrically conductive properties, or of a combination of the aforementioned foams. 
     The electrical shielding element around the at least one recess for receiving or introducing the at least one gas lance may also have at least one meshed grid structure with average mesh diameters, for instance, between 0.01 and 6 mm, preferably 0.2 and 0.5 mm. It is, for instance, also conceivable that the suction valve includes a plurality of electrical shielding elements having a meshed grid structure which may be arranged in the suction valve in multiple layers with distances between 0.01 and 6 mm, preferably 0.5 and 1 mm, for instance, to prevent the plasma from being ignited and/or deposited between the layers. 
     In addition, it is conceivable that the electrical shielding element around the at least one recess for receiving or introducing the at least one gas lance is formed of concentrically arranged walls having radial intermediate partition walls, with average wall distances of the concentrically arranged walls ranging, for instance, between 0.01 and 6 mm, preferably 3 and 4 mm, and average distances of the radial intermediate partition walls, e.g. ranging between 0.01 and 6 mm, preferably 3 and 4 mm. 
     The electrical shielding element around the at least one recess for receiving or introducing the at least one gas lance may also be formed as a honeycomb structure, with average honeycomb diameters of the honeycomb tubes ranging, for instance, between 0.01 and 6 mm, preferably 3 and 4 mm. 
     The average length of the honeycomb tubes preferably should, in this case, be greater than the average honeycomb diameter of the honeycomb tubes. For instance, the average length of the honeycomb tubes may exceed the average honeycomb diameter of the honeycomb tubes by a factor of 4.0, 6.0, 10.0 or more, preferably 5 to 10. This has the advantage, inter alia, that the risk of a breakdown of the plasma along the longitudinal axis of the suction valve can be minimized. 
     The shapes of the cross-sections of the honeycomb tubes may be regular polygon shapes, e.g. a triangle, square, pentagon, hexagon, convex and/or non-convex inner wall shape, a star-shaped polygon shape, round shapes, e.g. a circular shape, elliptical shape, or a combination of the aforementioned shapes. However, hexagonal cross-sections are preferred so as to be able to obtain an optimized relationship between the honeycomb structure material, the honeycomb structure volume and the honeycomb structure stability. 
     Furthermore, also metal plates may be used as electrical shielding element, which may have a plurality of bores with average bore diameters of 0.01 to 6 mm, preferably 3 to 4 mm. The length of the bores may be a multiple of the average bore diameters, preferably exceed the average bore diameter by a factor of 5 to 10. 
     The electrical shielding element may be made of metal, e.g. aluminum, of electrically conductive ceramics, e.g. Al2O3/TiN, of electrically conductive composite ceramics with carbon fibers, of a plastic material having electrically conductive properties, or of a combination of the aforementioned materials. 
     The flow resistance of the suction valve when sucking off air or gas from the container primarily depends on the open cross-section of the suction valve, the frictional resistance on the inner wall of the suction valve and the air conduction in the valve. The rate at which the interior of the container can be evacuated to the desired process pressure is limited above all by the opening cross-section of the container itself. 
     Preferably, the opening cross-section of the suction valve may, therefore, have at least the same size as the opening cross-section of the container. Preferably, also the flow resistance of the suction valve during the evacuation may be equal to or smaller than the flow resistance through the container opening. 
     This has the advantage that the interior of the container can be evacuated to a desired process pressure, e.g. between 1 and 30 Pa, sufficiently fast, e.g. in less than 500 ms, so as to allow the treatment or coating of the containers at the speed at which they travel through the production process, e.g. on conveyor belts or holding clamps of a carousel. 
     The electrical shielding element may be formed of one piece or multiple pieces and cover in respect of its height at least 10%, 20%, 30% or 60% or more of the height of the suction valve. 
     However, the suction valve may also include a plurality of electrical shielding elements and cover in the sum of the heights of the individual electrical shielding elements at least 10%, 20%, 30% or 60% or more of the height of the suction valve. The electrical shielding element may, for example, be formed of a combination of a honeycomb structure, an open-pored porous foam or a grid, whereby the vertical distance between the different parts may be smaller than 1, 2 mm, preferably smaller than 0.5 mm. 
     In a method according to the disclosure for coating a container, e.g. a plastic bottle, by means of a plasma treatment a gas lance may be introduced through a recess of a suction valve sitting on the container opening into the interior of the container. Through the suction valve the interior of the container can be evacuated to a process pressure, for instance, of 1 to 30 Pa. Inside the container a process gas, which is supplied by the gas lance, can be transformed in part or in whole into a plasma, and the interior of the container can be coated by means of a plasma-enhanced chemical vapor deposition, e.g. with a gas barrier layer. One or more electrical shielding element(s) located in the suction valve prevent(s) and/or suppress(es) nearly entirely an ignition and/or burning of plasma in the suction valve or in a region not to be coated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures show by way of example: 
         FIG. 1  shows a container with a gas lance and a suction valve; 
         FIG. 2  shows a suction valve; 
         FIG. 3  shows a suction valve; 
         FIG. 4  shows a honeycomb structure; 
         FIG. 5A  shows a first exemplary cross-section for an electrical shielding element; 
         FIG. 5B  shows a second exemplary cross-section for an electrical shielding element; 
         FIG. 5C  shows a third exemplary cross-section for an electrical shielding element; and 
         FIG. 5D  shows a fourth exemplary cross-section for an electrical shielding element. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows by way of example a container  104  including a suction valve  100 . A gas lance  103  may be introduced into the container  104  through a recess  102  in the suction valve  100 . The suction valve  100  may have an electrical shielding element  101 , which has a height  105  that may extend, for instance, across nearly the entire height  106  of the suction valve  100 . 
       FIG. 2  shows by way of example the opening  213  of a container  212  with a suction valve  200  sitting on same. A gas lance  201  may be introduced through a recess  211  of the suction valve  200  into the container  212 . The suction valve  200  may comprise a plurality of electrical shielding elements  203 ,  204 ,  205 , which may have different heights  206 ,  207  and  208  and distances  209 ,  210  from each other, e.g. distances  209 ,  210  between 0.01 and 6 mm, preferably 0.5 and 1 mm. The sum of the individual heights  206 ,  207  and  208  of the electrical shielding elements  203 ,  204 ,  205  can cover at least 10%, 20%, 30% or 60% or more of the height  202  of the suction valve  200 . 
     Moreover, the electrical shielding elements  203 ,  204 ,  205  may be different from each other in respect of material and structure. For instance, the electrical shielding element  205  may be made of an open-pored porous metal foam, the electrical shielding element  204  may be made of a honeycomb structure, and the electrical shielding element  203  may be made of a grid structure. 
       FIG. 3  shows by way of example the opening  313  of a container  305  with a suction valve  300  sitting thereon. A gas lance  301  can be introduced through a recess  304  of the suction valve  300  into the container  305 . The gas lance  301  may be additionally fixed by a holder  303 . The suction valve  300  may comprise an electrical shielding element  302  formed of a honeycomb structure. The average length of the honeycomb tubes  306  may be greater than the honeycomb diameter of the honeycomb tubes  306  by a factor of 1.5, 2.0, 4.0, 6.0, 10.0 or more, preferably 5 to 10. The honeycomb diameter  307  of the honeycomb tubes  306  may be between 0.01 and 6 mm, preferably between 3 and 4 mm. See in this respect also  FIG. 4 , illustrating by way of example the honeycomb diameter  401  and the length  402  of a honeycomb tube in a honeycomb structure  403 . 
       FIGS. 5A ,  5 B,  5 C,  5 D show by way of example different cross-sectional shapes of different exemplary electrical shielding elements. 
     In  FIG. 5A  the electrical shielding element  500  has, for instance, an open-pored porous structure around the recess  501 , whereby the pores  502  may have average pore diameters between 0.01 and 6 mm, preferably between 3 and 4 mm. 
     In  FIG. 5B  the electrical shielding element  600  has, for instance, a structure of concentric walls  602  with radial intermediate partition walls  603  around the recess  601 , whereby the average distances between the concentric walls  602  and the radial intermediate partition walls  603  may each be between 0.01 and 6 mm, preferably between 3 and 4 mm. 
     In  FIG. 5C  the electrical shielding element  700  has, for instance, a grid structure with meshes  702  around the recess  701  whose average mesh diameter may range between 0.01 and 6 mm, preferably between 0.2 and 0.5 mm. The geometry of the meshes  702  may be a regular or an irregular one. 
       FIG. 5D  shows by way of example an electrical shielding element  800  having a honeycomb structure around the recess  801 , with average honeycomb diameters of the honeycomb tubes  802  which may range, for instance, between 0.01 and 6 mm, preferably between 3 and 4 mm. The honeycomb cross-section may be hexagonal, as is illustrated. However, other cross-sectional shapes are possible as well, such as regular polygon shapes, e.g. a triangle, square, pentagonal, convex and/or non-convex inner wall shape, a star-shaped polygon shape, round shapes, e.g. a circular shape, elliptical shape, or a combination of the aforementioned shapes. 
     Structures having hexagonal honeycomb cross-sections are preferred, inter alia, in order to advantageously obtain an optimal relationship between the honeycomb tube opening cross-section and the honeycomb tube wall cross-section, and, for instance, in order to allow the easy production of the honeycomb structure by means of folding and joining methods, e.g. of a film material. 
     It is noted that the exemplary circular outer contour of the electrical shielding element and the suction valve, respectively, is a consequence of the adaptation to the usually circular container opening shape. It is, therefore, easily possible to also adapt the geometry of the suction valve and the electrical shielding element to non-circular container openings, e.g. rectangular container openings.