Patent Publication Number: US-2022230854-A1

Title: Apparatus for indirect atmospheric pressure plasma processing

Description:
CROSS-REFERENCES 
     The following applications and materials are incorporated herein, in their entireties, for all purposes: European Patent Application No. 15191844.8, filed Oct. 28, 2015. However, such material is only incorporated to the extent that no conflict exists between the incorporated material and the statements and drawings set forth herein. In the event of any such conflict, including any conflict in terminology, the present disclosure is controlling. 
     TECHNICAL FIELD 
     The present disclosure is related to apparatuses and methods for indirect atmospheric pressure plasma processing, in particular where the substrate to be plasma processed is kept remote from the plasma discharge zone. 
     With indirect or remote plasma treatment of substrates, as opposed to in-situ plasma treatment, the substrate is not passed through the plasma discharge zone, in which an atmospheric pressure plasma is maintained between electrodes and activated species are formed. Instead, the substrate is positioned at a location remote from the plasma discharge zone and the plasma-activated species are transported to the remote location where they are made to react with the substrate. Remote plasma treatment is often preferred over in-situ treatment, in particular for cases in which in-situ plasma treatment would cause charging of the substrate surface and therefore undesirable interaction with the electric field of the plasma discharge. This is particularly the case for substrate materials having at least some degree of electrical conductivity. 
     INTRODUCTION 
     An apparatus for indirect or remote atmospheric pressure plasma processing is known from WO 2009/080662 2009 Jul. 2. The apparatus comprises a multitude of single micro-channels in which a plasma is formed and which are circumferentially arranged around a treatment zone. High gas velocities up to transonic flow conditions in the discharge zone are generated while maintaining moderate flow rates. The resulting superimposition of high drift velocity in the process gas flow and the inherent diffusion movement results in a prolonged displacement distance of activated species into the treatment zone. The treatment zone is cylindrical and wrapped or enveloped by the plasma micro-channels. A carrier gas with particulate material is made to flow through the treatment zone. The process gas with activated species admixes with the carrier gas in the treatment zone to perform a surface treatment of the particulate material. A drawback of the above apparatus is that the concentration of plasma activated species in the treatment zone is not uniform in a radial direction. 
     US 2003/0051993 2003 Mar. 20 describes an apparatus for atmospheric plasma processing of a PAN fiber. The PAN fiber is drawn through a cylindrical hull. A number of plasma discharge forming capillaries are arranged radially around the cylindrical hull. A drawback of the above apparatus is that the surface activation of the PAN fiber is low due to air entrained with the PAN fiber. For an effective plasma treatment, a long chamber is required with a large number of plasma capillaries, or the transport speed of the fiber must be kept low. 
     U.S. Pat. No. 8,227,051 2012 Jul. 24 describes in relation to  FIG. 2B  an indirect exposure plasma treatment of a carbon fiber. The fiber is pulled or placed into the exhaust flow from an atmospheric plasma device exposing the fiber to contact with the convected chemical active species generated by the plasma. The atmospheric pressure plasma device is configured to operate using background gas preferably comprising air, or any other oxygen containing gas mixtures including pure oxygen, that promotes the transport of short-lived reactive oxidative species to the fiber via a sufficiently high exhaust velocity. The plasma operating conditions including the size of the plasma volume, the composition of the processing gas, gas flow rates, and the energizing conditions of the electrical device generating the plasma, are adjusted to yield the desired surface modifications within the required residence time. Deleterious effects on fiber surface topography are minimized by the indirect exposure process because the fibers are located away from the bulk of the plasma and do not undergo direct ion bombardment. In the apparatus as depicted in  FIG. 2B  of the above document, an inhomogeneous treatment of the carbon fiber surface is obtained, since the side of the fiber facing the plasma discharge apparatus is more exposed to the plasma activated species than the side opposite the plasma discharge apparatus. As a result, the residence time of the carbon fiber must be prolonged, or the fiber must be turned and pulled a second time through the same apparatus. 
     SUMMARY 
     An objective of aspects of the present disclosure is to overcome one or more of the above drawbacks. One objective of aspects of the present disclosure is to improve uniform and homogeneous plasma processing of the substrate surface. Another objective of aspects of the present disclosure is enabling a prolonged and more intimate contact between the reactive species exhausted from the plasma discharge and the substrate. Yet another objective is to improve plasma processing of the substrate surface, in particular for non-oxidative plasma treatments, i.e., treatments involving a substantially oxygen-free plasma forming gas. 
     According to a first aspect of the present disclosure, there is therefore provided an apparatus for plasma processing of a substrate transported continuously through the apparatus, as set out in the appended claims. Apparatuses according to aspects of the present disclosure comprise a first plasma torch. The first plasma torch comprises a first electrode and a second electrode arranged opposite the first electrode to define a first plasma discharge chamber between the first and second electrodes. The plasma discharge chamber comprises an inlet and an outlet for passing a plasma forming gas between the electrodes. The apparatus further comprises a control unit coupled to one or both the electrodes and operable to maintain an atmospheric pressure plasma discharge in the first plasma discharge chamber. The first plasma torch is therefore operable to exhaust plasma activated species through the outlet of the first plasma discharge chamber. 
     The apparatus further comprises an afterglow chamber downstream of the first plasma torch and in fluid communication with the outlet of the first plasma discharge chamber. A transport means is provided for continuous transport of the substrate through the afterglow chamber and such that the substrate is kept remote from the first plasma discharge chamber while being processed by plasma activated species exhausted from the outlet of the first plasma discharge chamber into the afterglow chamber. 
     According to a first aspect of the present disclosure, the afterglow chamber extends between a substrate inlet and a substrate outlet arranged at opposite sides of the outlet of the first plasma discharge chamber. The substrate inlet advantageously comprises an inlet aperture having a cross-sectional size substantially smaller than a cross-sectional size of the afterglow chamber. The cross-sectional size of the afterglow chamber can be assessed in correspondence of the outlet of the first plasma discharge chamber. The cross-sectional size can refer to an area, or clearance, such as a height, or diameter. Advantageously, the cross-sectional size is defined in a plane perpendicular to a transport direction of the substrate. Advantageously, the inlet aperture is aligned with a delimiting wall of the outlet of the first plasma discharge chamber, such that the afterglow chamber is made to extend at a downstream side only of the outlet of the first plasma discharge chamber. 
     The reduction in aperture of the afterglow chamber at the substrate inlet ensures that the afterglow zone is made to propagate further downstream along a transport direction of the substrate. An increased plasma treatment efficiency is thereby obtained. By appropriate selection of the aperture size, it becomes possible to reduce or minimize air entrainment by the substrate in the afterglow zone. 
     According to a second aspect of the present disclosure, which can be provided in addition to, or independently of the first aspect above, a second plasma torch is provided, which can be identical to the first plasma torch. The second plasma torch is aligned with and arranged opposite the first plasma torch, such that the outlets of the plasma discharge chambers of the respective plasma torches face each other and exhaust plasma activated species into the afterglow chamber interposed between the first and second plasma torches. A more intense afterglow stream is thereby provided, which furthermore allows for uniform treating continuous fibers along 360° of the circumference. 
     According to a third aspect of the present disclosure, there is provided a method for plasma treatment of continuous fibers, such as but not limited to carbon fibers and polymeric fibers. 
     Methods for indirect or remote atmospheric pressure plasma treatment of a substrate are described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein: 
         FIG. 1  represents a cross section side view of an apparatus for atmospheric pressure plasma processing of a film substrate according to aspects of the present disclosure; 
         FIG. 2  represents a cross-section side view of another apparatus for atmospheric pressure plasma processing of a substrate according to aspects of the present disclosure; 
         FIG. 3  represents a cross-section view of yet another apparatus for atmospheric pressure plasma processing of a fiber according to aspects of present disclosure, comprising two oppositely arranged cylindrical plasma torches. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an apparatus  10  for plasma processing of a continuous substrate  16 , such as but not limited to films and foils, comprises a pair of oppositely arranged electrodes  11  and  12 . Electrodes  11  and  12  are planar and extend parallel to each other. They are spaced apart to define a plasma discharge chamber  13  between the electrodes  11 ,  12 . Advantageously, dielectric layers  14  cover one or both electrodes  11 ,  12  at the side facing the plasma discharge chamber  13 . In such case, the dielectric layers  14  form walls of the chamber  13 . Dielectric materials include borosilicate glass, quartz, and alumina. 
     Chamber  13  comprises an inlet  131  through which a plasma forming gas  133  is made to enter the chamber. The plasma forming gas is one which is able to create a plasma discharge in chamber  13  under an electric field generated by the electrodes  11 ,  12 . The plasma forming gas is advantageously a non-oxidizing gas, advantageously a gas which is substantially oxygen-free. Non-limiting examples of plasma forming gases are nitrogen (N 2 ), argon (Ar), helium (He) and neon (Ne), or combinations thereof. 
     The plasma forming gas is supplied to the chamber  13  at substantially atmospheric pressure. Suitable pressures may vary between about 0.5 bar below and about 0.5 bar above atmospheric pressure. The plasma forming gas may be supplied at ambient temperature (15° C.-30° C.) to the chamber  13 . Alternatively, it is possible to heat the gas stream  133  to an elevated temperature prior to supplying it to the chamber  13 . Elevated temperatures possibly range between 30° C. and 400° C., advantageously between 50° C. and 300° C. 
     The plasma forming gas enters the chamber at the inlet  131  and is made to flow along an axis  136  of chamber  13  until an outlet  132  arranged downstream of the electrodes  11 ,  12 . The inlet  131  and the outlet  132  of the plasma discharge chamber  13  are defined by the extent of the plasma discharges taking place in chamber  13 , i.e. it is assumed in the present description that the plasma discharge chamber  13  corresponds to and is delimited by the plasma discharge zone. Generally, the plasma discharge zone will be maintained in an area delimited by the electrodes  11 ,  12 . 
     It will be convenient to note that, since the electrodes are planar, the chamber  13  (as well as electrodes  11 ,  12  and dielectric layers  14 ) extends in a direction perpendicular to axis  136 , i.e. perpendicular to the plane of  FIG. 1 . The gap of chamber  13  between electrodes  11 ,  12  (between dielectric layers  14 ) typically is between 0.5 mm and 5 mm, advantageously 3 mm or less. 
     The plasma forming gas stream  133  can be loaded/enriched by at least one liquid or gaseous monomer added as a precursor to the plasma forming gas. The precursor can be activated by the plasma discharge to e.g. form radicals which initiate chemical reactions with the substrate  16 . A stream  134  of precursor can be injected in the plasma forming gas stream  133  by known methods, such as through an atomizer  135 , e.g. to form an aerosol which is carried with the plasma forming gas stream into the chamber  13 . Non-limiting examples of precursors are methane (CH 4 ) and acetylene (C 2 H 2 ). 
     The electrodes  11 ,  12  are coupled in an electric circuit including a control unit  15  which is operable to generate an electric/electromagnetic field between the electrodes  11 ,  12  that generates a plasma discharge in the chamber  13 . By way of non-limiting example, one electrode  12  can be connected to electric ground, whereas the other electrode  11  is supplied with an Alternating Current (AC) or pulsed Direct Current (DC) high electric voltage generated in control unit  15 . Suitable voltage differences between the electrodes  11 ,  12  range between 1 kV and 100 kV. Suitable frequencies (either AC or pulsed DC) range between 1 kHz and 200 kHz, advantageously between 5 kHz and 100 kHz. 
     The plasma setup of  FIG. 1  is referred to as a parallel plate dielectric barrier discharge apparatus. The apparatus operates as a plasma torch which creates plasma activated species in the plasma discharge chamber. These species are carried by the plasma forming gas stream to the outlet  132  of chamber  13  where they are made to react with the substrate  16 . It will be convenient to note that, although dielectric barrier discharge plasma processing provides advantageous operation, the present disclosure is not limited thereto and other kinds of plasma discharge, such as e.g. glow discharge or corona discharge may be contemplated. 
     Generally, the plasma activated species exiting the plasma discharge chamber retain their reactivity for a short period. A zone directly downstream of the outlet  132  of the plasma discharge chamber  13 , where electromagnetic fields that sustained the plasma are absent or insufficient to maintain any plasma discharge, but where the plasma activated species are still reactive, is referred to as the afterglow zone. In the afterglow zone, the plasma activated species exiting the plasma discharge chamber react with other molecules, such as substrate molecules or recombine with molecules present in the plasma forming gas or other gas present in the afterglow zone. 
     The plasma treatment apparatus  10  is designed to treat substrate  16  in the afterglow zone, at a location remote from the plasma discharge chamber  13 . To this end, substrate  16  is transported in proximity of the outlet  132  of the plasma discharge chamber  13 , but without entering or contacting chamber  13  or the plasma discharge. Generally, the transport direction of substrate  16  is perpendicular to the axis  136  of flow of the plasma forming gas in chamber  13 . By way of example the substrate  16  may be unwound from spool  165 , guided along guide/tensioning drums  163  and  161  upstream of the plasma torch  10  and further along guide/tensioning drums  162  and  164  downstream to eventually be wound on a take-up spool  166 . 
     According to an aspect of the present disclosure, a chamber  17  is provided downstream of the plasma discharge chamber  13 , which allows for confining the afterglow. In the example of  FIG. 1 , the afterglow chamber  17  is defined/delimited by the substrate  16  on the one hand (in the assumption that substrate  16  is an impermeable film), and a confinement wall  174 , advantageously made of a dielectric material, arranged opposite substrate  16  and advantageously parallel thereto. Substrate  16  is transported at a side opposite the outlet  132  of plasma discharge chamber  13 . Wall  174  extends from outlet  132  along a direction advantageously parallel to the transport direction of substrate  16 . The substrate  16  and wall  174  hence form a channel-shaped chamber  17  which advantageously guides the afterglow stream along the substrate  16 . Afterglow chamber  17  comprises an inlet  171  for the plasma activated stream in fluid communication with and which advantageously corresponds to the outlet  132  of chamber  13  and an inlet  172  for the substrate  16 , separate from inlet  171 . Substrate inlet  172  is advantageously located at an upstream side of outlet  132  opposite wall  174 , such that chamber  17  extends at the downstream side of the outlet  132  only. Both the afterglow stream and the substrate are transported in an advantageously same direction until outlet  173  of the chamber  17 . 
     A shielding member  175  is advantageously provided at the substrate inlet  172 . Shielding member  175  defines a wall  176  which advantageously extends between the outlet  132  of the plasma discharge chamber  13  and the substrate inlet  172 , and reduces a clearance G 1  between substrate  16  and the shielding member  175  at the substrate inlet  172  compared to a height G 2  of the afterglow chamber  17 . Height G 2  can conveniently be assessed at the outlet  132 , or further downstream, particularly in cases where the afterglow chamber would have a constant cross section. Wall  176  is advantageously aligned with a wall of the outlet  132 . 
     One advantage of shielding member  175  is to ensure that the chamber  17  and hence the afterglow zone extends to the downstream side of the outlet  132  only. This results in a prolonged and more intimate contact between the reactive species present in the afterglow zone and the substrate  16 . Another advantage of shielding member  175  is to reduce and/or minimize air entrainment by substrate  16  into the afterglow chamber  17 . Air comprises oxidative species, such as oxygen, which neutralize the plasma activated species leading to reduced efficiency of the plasma treatment. Furthermore, the air entrained by the substrate  16  forms a boundary layer on the substrate surface hindering contact with the reactive species present in the afterglow zone. 
     As yet a further advantage, shielding member  175  avoids the necessity that the entire plasma processing zone be put under inert atmosphere. Therefore, aspects of the present disclosure allow for reducing gas consumption and therefore allow more economical plasma processing. 
     Advantageously, the clearance G 1  is equal to or smaller than 50% of the clearance G 2 , advantageously equal to or smaller than 30%, advantageously equal to or smaller than 20%, advantageously equal to or smaller than 10% of clearance G 2 . The clearance G 1  is advantageously equal to or smaller than 2.5 mm, advantageously equal to or smaller than 1 mm, advantageously equal to or smaller than 0.5 mm, advantageously equal to or smaller than 250 μm. The clearance G 1  can be as small as 10 μm. 
     Advantageously, the clearance G 2  is equal to or smaller than 10 mm, advantageously equal to or smaller than 7 mm, advantageously equal to or smaller than 5 mm. G 2  is suitably at least 1 mm. 
     Advantageously, the afterglow chamber  17  extends over a distance L 2  between the outlet  132  of the plasma discharge chamber and the outlet  173 . The length L 2  of the afterglow chamber is advantageously at least 100 mm, advantageously at least 200 mm, advantageously at least 500 mm. 
     It will be convenient to note that either one or both the afterglow chamber  17  and tunnel of the substrate inlet  172  can have a constant cross-section. 
     In an aspect of the present disclosure, it is advantageous to have substrate  16  pass through the substrate inlet  172  in a contactless manner. That is, substrate  16  enters the afterglow chamber  17  without contacting the shielding member  175  or the shielding wall  176 , such that a clearance G 1  is advantageously always present. 
     In order to further reduce air entrainment, shielding member  175  advantageously extends a distance L 1  upstream along the transport direction of substrate  16 . The clearance G 1  may be maintained along the entire length L 1  of shielding member  175 . As a result, the substrate inlet  172  may be shaped as a tunnel with clearance G 1 , instead of just being an aperture or diaphragm. The length L 1  of the tunnel is advantageously at least twice the clearance G 1 , advantageously at least three times G 1 , advantageously at least five times G 1 . A suitable length L is 10 to 20 times G 1 . 
     In one aspect, the air entrainment by the substrate  16  through the substrate inlet  172  can be substantially completely suppressed by using a gas knife as shown in  FIG. 2 . Gas knife  18  injects a stream  181  of an inert or non-oxidizing gas, such as nitrogen gas, at the inlet  172 . The stream  181  impinges on the substrate  16  to remove any entrained air. 
     Referring to  FIG. 2 , in case substrate  16  would be porous, a channel wall  177  is advantageously arranged opposite wall  174  and outlet  132  to confine the afterglow chamber  17 . The substrate  16  is transported along the afterglow chamber  17  between walls  174  and  177 . It will be convenient to note that in such case the clearances G 1  and G 2  are determined as from wall  177  instead of substrate  16 . 
       FIG. 2  shows an alternative type of plasma torch  20 , which differs from the plasma torch of apparatus  10  in that electrodes  11  are arranged at opposite sides of a central electrode  12 . The outer electrodes  11  are advantageously supplied with high voltage, whereas the central electrode  12  is connected to ground. The central electrode  12  can comprise an internal lumen  121  advantageously extending until the outlet  132  of the plasma discharge chamber. The stream  134  of precursors is supplied through the internal lumen  121  and injected directly in the afterglow zone (chamber  17 ), where the precursors can react with the plasma activated species exhausted from the plasma discharge chamber. Such a setup is particularly suited in cases wherein it is not desired that the precursors be broken down by the plasma discharge. 
     The plasma torch  20  can be provided both as a parallel plate device, with planar electrodes  11  and  12 , or as a cylindrical device, wherein electrodes  11  and  12  are circular and concentric, extending along axis  136 . 
     Referring to  FIG. 3 , for cases in which the substrate  16  is to be plasma treated at both sides, it is advantageous to provide two plasma torches  31  and  32  arranged oppositely one another. Plasma processing apparatus  30  therefore comprises a first plasma torch  31 , similar to anyone of the torches already described above. Plasma torch  31  shown in  FIG. 3  is cylindrical and may have a same structure as torch  20  shown in  FIG. 2 . A second plasma torch  32 , advantageously identical in structure as torch  31 , is aligned with torch  31 . Torch  32  comprises electrodes  21  and  22  spaced apart to define a plasma discharge chamber  23 . A dielectric layer  24  is advantageously provided between either one of the electrodes and the plasma discharge chamber  23  as described. Plasma torches  31  and  32  are aligned on a same axis  136  and such that the respective outlets  132 ,  232  of the plasma discharge chambers  13 ,  23  are facing each other. The plasma activated species from plasma discharge chambers  13  and  23  are therefore exhausted towards each other in the afterglow chamber  17 . 
     The afterglow chamber  17  is arranged between the outlets  132  and  232 , and extends from the outlets downstream along a transport direction  26  of the substrate  16 . The afterglow chamber  17  therefore receives plasma activated species from both plasma torches  31  and  32  so that a highly concentrated and uniform afterglow zone in chamber  17  can be obtained. The substrate  16  enters chamber  17  from a substrate inlet  172  having a reduced clearance as described above. 
     The plasma apparatus  30  is particularly suited for plasma processing of fibers, which require a 360° treatment of the fiber surface. In such case, torches  31  and  32  can be cylindrical, with concentric electrodes  11  and  12 , and  21  and  22 , all aligned on axis  136 . With cylindrical plasma torches, the afterglow chamber  17  can be cylindrical as well, with fiber  16  being transported along the axis of the cylindrical chamber  17 . In such case, wall  174  is advantageously tubular with circular cross-section. 
     A cylindrical afterglow chamber can comprise an upstream end at the outlets  132  and  232  of the plasma discharge chambers, which is defined by a shielding member  175  closing chamber  17  except for a small aperture through it which forms the substrate inlet  172 . Substrate inlet  172  is advantageously aligned with the axis of tube  174 . By so doing, the afterglow is conveyed through tube  174  in the same direction as the substrate  16  to obtain a longer afterglow zone along the substrate  16  and therefore a longer contact time. 
     It will be convenient to note that the values for the clearances G 1  and G 2  as indicated above advantageously apply to the diameters of the inlet  172  and the tube  174 . By appropriate selection of dimension of the plasma torches  31  and  32 , and the processing parameters such as plasma forming gas flow, a uniform afterglow zone in chamber  17  can be obtained allowing for a uniform 360° treatment of the fiber  16 . 
     Elements of the plasma processing apparatuses described in relation to  FIGS. 1 through 3  can be interchanged. In particular, two parallel plate plasma torches as in  FIGS. 1 and 2  can be arranged oppositely as with the plasma torches  31  and  32  of  FIG. 3  to obtain an afterglow channel  17  with rectangular cross-section and uniform afterglow zone, allowing the simultaneous treatment of a plurality of fibers. 
     Advantageously, the wall  174  and/or  177  of the afterglow chamber  17  is at least in part made of a transparent material, such as quartz glass. The transparent wall allows for checking the color and/or the length of the afterglow zone, which may be an indication of the purity of the gases used. 
     Apparatuses according to aspects of the present disclosure are particularly useful for plasma processing of carbon fibers. The fibers are drawn or pulled through the afterglow chamber and made to react with reactive species present in the afterglow zone. The fibers do not enter or come in contact with any of the plasma discharge zone(s) and do not suffer from charging effects due to the plasma discharge.