Apparatus for Impregnating a Porous Medium Comprising Optimized Coated Electrodes

Abstract

Apparatus for impregnating a porous medium with powder, including a device able to generate an alternating electric field through the porous medium, the device including a first electrode and a second electrode, which are placed on either side of the porous medium, the apparatus being characterized in that: the first electrode is covered with a screen making contact with the electrode, said screen having a dielectric strength higher than 6 kV/mm; and the second electrode is covered with a protective layer, said protective layer having a superficial resistivity higher than 1×10.sup.12Ω/□.

Claims

1. An apparatus for impregnating a porous medium with powder, including a device able to generate an alternating electric field through the porous medium, the device including a first electrode and a second electrode which are placed on either side of the porous medium, wherein: the first electrode is covered with a screen coming into contact with the electrode, said screen having a dielectric strength higher than 6 kV/mm, and preferably higher than 9 kV/mm; the second electrode is covered with a protective layer, said protective layer being secured to the second electrode and having a superficial resistivity higher than 1×10.sup.12Ω/□, irrespective of the relative humidity level.

2. The apparatus according to claim 1, wherein the protective layer has a structural stability above 250° C.

3. The apparatus according to claim 1, wherein the screen in contact with the first electrode is covered with a protective layer coming into contact with the screen, said protective layer having a superficial resistivity above 1×10.sup.12Ω/□.

4. The apparatus according to claim 1, that wherein a screen is inserted between the second electrode and the protective layer, said screen having a dielectric strength greater than 6 kV/mm.

5. The apparatus according to claim 4, wherein the screen is covered with a protective layer, said protective layer having a superficial resistivity above 1×10.sup.12Ω/□.

6. The apparatus according to claim 1, wherein the electric field generated between said electrodes is comprised between 0.1 and 50 kV/mm.

7. The apparatus according to claim 1, wherein a specific device keeps the relative humidity level at less than 60% between said electrodes, when the electrodes are powered on.

8. The apparatus according to claim 1, further comprising a driving device for driving the porous medium between said electrodes.

9. A method for impregnating a porous medium, the method comprising: introducing the porous medium into an impregnating apparatus; heating the porous medium prior to the application of an electric field, wherein the impregnating apparatus includes: (i) a device able to generate the electric field through the porous medium, and (ii) a first electrode and a second electrode which are placed on either side of the porous medium, and wherein the first electrode is covered with a screen coming into contact with the electrode, said screen having a dielectric strength higher than 6 kV/mm, and preferably higher than 9 kV/mm; and wherein the second electrode is covered with a protective layer, said protective layer being secured to the second electrode and having a superficial resistivity higher than 1×10.sup.12Ω/□, irrespective of the relative humidity level.

10. A method for impregnating a porous medium, the method comprising: introducing the porous medium into an impregnating apparatus; drying the porous medium with dry air prior to the application of an electric field, wherein the impregnating apparatus includes: (i) a device able to generate the electric field through the porous medium, and (ii) a first electrode and a second electrode which are placed on either side of the porous medium, and wherein the first electrode is covered with a screen coming into contact with the electrode, said screen having a dielectric strength higher than 6 kV/mm, and preferably higher than 9 kV/mm; and wherein the second electrode is covered with a protective layer, said protective layer being secured to the second electrode and having a superficial resistivity higher than 1×10.sup.12Ω/□, irrespective of the relative humidity level.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0035] The embodiments will be better understood upon reading the following description, given solely as an indicative and non-limiting example, and with reference to the accompanying figures, in which the same references designate identical or similar elements and in which:

[0036] FIG. 1A is a longitudinal view diagram of one example embodiment of an impregnating apparatus;

[0037] FIG. 1B is a longitudinal view diagram of another example embodiment of an impregnating apparatus;

[0038] FIG. 1C is a longitudinal view diagram of another example embodiment of an impregnating apparatus;

[0039] FIG. 1D is a longitudinal view diagram of another example embodiment of an impregnating apparatus;

[0040] FIG. 2 shows the variation as a function of time of the temperature of the components in contact with the electrodes of an apparatus according to the prior art and the electrodes of an apparatus illustrated in FIG. 1;

[0041] FIG. 3 shows an alternative embodiment of the apparatus illustrated in FIG. 1A;

[0042] FIG. 4 shows an alternative embodiment of the apparatus illustrated in FIG. 3;

[0043] FIGS. 5A and 5B are longitudinal views of an apparatus generating a plasma, showing the distribution of the plasma between the electrodes, according to the prior art and according to one embodiment, respectively;

[0044] FIG. 6 shows an alternative embodiment of the apparatus illustrated in FIG. 4;

[0045] FIGS. 7A and 8A are longitudinal views of an apparatus generating a plasma, showing the distribution of the plasma between the electrodes as a function of the humidity level of a porous medium present between said electrodes;

[0046] FIGS. 7B and 8B are top views of the porous mediums shown in FIGS. 7A and 8A, respectively, after they have passed between the electrodes and been impregnated with powder.

DETAILED DESCRIPTION

[0047] As a reminder, this application aims to propose an apparatus for impregnating a porous medium with powder, having better resistance to aging and allowing the formation of a more homogeneous electric field to promote more regular impregnation of the powder in the porous medium.

[0048] One example embodiment of an impregnating apparatus 2 is shown schematically in FIG. 1A. According to this example, the apparatus includes two electrodes 4A and 4B facing and substantially parallel to one another. A first electrode 4A is in contact with a screen 8A characterized by a dielectric strength greater than 6 kV/mm.

[0049] The screen 8A makes it possible to electrically isolate the electrodes from one another. The thickness D8 of the screen can be adapted to support the electrode 4A. To that end, its thickness may be comprised between 1 and 20 mm. In this case, the screen 8A is a quartz plate whose thickness is equal to 5 mm.

[0050] A second electrode 4B of the apparatus is covered with a protective layer 10B characterized by an electric superficial resistivity greater than 1×10.sup.12Ω/□. The electric superficial resistivity or surface resistivity characterizes the capacity of a material to slow the circulation of a current on its surface in the presence of a difference in voltage. The electric superficial resistivity is measured according to standard “ASTM D 257-99” by using concentric electrodes described in FIG. 4 of the standard. The value of the electric superficial resistivity is expressed in Ohm or Ohm/□ to indicate whether it involves a surface resistivity. In other words, a surface having a high surface resistivity is characterized by a low mobility of the electrons on its surface.

[0051] The protective layer 10B in contact with the second electrode 4B therefore advantageously makes it possible to limit the movement of the electric charges on its surface, so as locally to avoid concentrations of charges that could form spots with high voltage promoting the establishment of electric discharges between the electrodes 4A and 4B. As a result, the electric charges present on its surface have greater difficulties in aggregating to form local spots with high voltage, relative to the electric charges present on the surface of a quartz screen, for example.

[0052] The thickness D10 of the protective layer 10B can be comprised between several hundredths and several millimeters. According to this example, the protective layer is made from silicone, the thickness of which is equal to about 1 mm.

[0053] The electrodes 4A and 4B are arranged so as to define a passage 14 for a porous medium 16. The apparatus may comprise an adjustable device of the rack or other type (not shown in the figures), making it possible to control the distance D14 separating the protective layers. This distance may be comprised between 1 and 50 mm. In the example considered here, the distance D14 is equal to 15 mm.

[0054] The electrodes 4A and 4B are preferably uniform conducting plates, so as to promote the establishment of a homogeneous electric field between them. These conducting electrodes can for example be made up of copper or aluminum plates, vacuum metal depositions, silver lacquer or any other appropriate conductors.

[0055] The contemplated embodiments are not limited to a particular form and arrangement of electrodes. The electrodes can be solid or openworked, and have multiple forms, such as concave or convex or tubular, and optionally comprise several conducting elements connected to one another. For example, embodiments may comprise discontinuous electrodes made up of a range of conductive strips, as described in document FR 2,933,327 on pages 5 and 6.

[0056] According to another embodiment not shown in the figures, the first electrode and/or the second electrode can be replaced by a series of tubular electrodes with different sections, making it possible to apply an electric field through a porous medium passing between said electrodes. In this case, a first series of electrodes can be covered with a screen having a dielectric strength greater than 6 kV/mm, and the second series of electrodes can be covered with a protective layer having a superficial resistivity greater than 1×10.sup.12Ω/□. Like for FIGS. 1A to 1D, the first series of electrodes may optionally have, in addition to its screen, a protective layer having a superficial resistivity greater than 1×10.sup.12Ω/□, and the second series of electrodes may have a screen inserted between the electrodes and the protective layer, the screen having a dielectric strength greater than 6 kV/mm. The first and second series of electrodes may be inverted.

[0057] According to another embodiment not shown in the figures, the first electrode 4A may be cylindrical and pivot around its axis of revolution, the second electrode 4B may be made up of several tubular electrodes comprising a glass screen with a rectangular section, inwardly metalized, and positioned across from the first electrode 4A. The cylinder making up the first electrode 4A is covered with a layer of silicone serving as a protective layer 10A. The protective layer and/or the glass screen may have asperities or a relief on their surface so as to be able to store powder temporarily before releasing said powder into a space formed between the first and second electrodes 4A and 4B.

[0058] To allow the generation of an electric field E between the electrodes, they are connected to a same alternating voltage generator 6, able to deliver a voltage comprised between 1 and 100 kV, at a frequency comprised between 1 and 1,000 Hz.

[0059] FIG. 1B illustrates another embodiment of an impregnating apparatus 2′, including two electrodes 4A′ and 4B′ facing and substantially parallel to one another. A first electrode 4A′ is in contact with a screen 8A′ with characteristics similar to the screen 8A described above. A second electrode 4B′ and the screen 8A′ are respectively covered with a protective layer 10B′ and a protective layer 10A′. The protective layers are similar to the protective layer 10B. Advantageously, a protective layer is deposited on each electrode to strengthen the homogeneity of the electric field generated between said electrodes.

[0060] FIG. 1C illustrates another embodiment of an impregnating apparatus 2″, including two electrodes 4A″ and 4B″ facing and substantially parallel to one another. A first electrode 4A″ and a second electrode 4B″ are respectively in contact with a screen 8A″ and 8B″ with characteristics similar to the screen 8A described above. The screen 8B″ in contact with the second electrode 4B″ is covered with a protective layer 10B″ similar to the protective layer 10B. Advantageously, the screens 8A″ and 8B″ in contact with each electrode make it possible to generate, between said electrodes, an electric field with a higher amplitude relative to the impregnating apparatus 2′ shown in FIG. 1B.

[0061] FIG. 1D illustrates another embodiment of an impregnating apparatus 2′″, including two electrodes 4A′″ and 4B′″ facing and substantially parallel to one another. Each electrode is in contact with a screen 8A′″ and 8B′″, the screens respectively being covered with a protective layer 10A′″ and 10B′″. The screens and the protective layers have characteristics similar to those described above. According to this example embodiment, each electrode is in contact with a screen so as to improve the electric isolation between said electrodes to make it possible to increase the amplitude of the electric field between the electrodes. These two screens are each covered with a protective layer 10A′″ and 10B′″ to make it possible to homogenize the electric field between the electrodes 4A′″ and 4B′″.

[0062] The porous medium 16 passing between the electrodes making up the apparatuses described above may for example be an array of synthetic and/or natural fibers, a nonwoven or woven, paper, or even open-cell foam. The porous medium may for example be a needled mat made up of polyester or natural fibers such as cotton, hemp, wool or the like.

[0063] The powder 17 impregnating the porous medium 16 may be a powder of the thermoplastic or thermosetting type, such as a polyamide powder or an epoxide powder. The term “powder” may designate a mixture of powders of different types and with different particle sizes.

[0064] FIG. 2 illustrates an advantage related to the use of the protective layers described above in an impregnating apparatus. More specifically, FIG. 2 shows temperature variation curves measured on screens of an apparatus according to the prior art not including protective layers, and an apparatus illustrated in FIG. 1D. Of course, the temperature measurements are done under the same usage conditions and the same arrangements for both types of apparatus. More specifically, the panes of glass in contact with the electrodes have a thickness of 3 mm and are spaced apart by a distance of 20 mm. A voltage of 45 kV at 50 Hz is applied between the electrodes. FIG. 2 shows a sudden increase in the temperature of the glass screens after 15 minutes of use of the apparatus when they are not covered with protective layers as described above (curve 1), up to a temperature above 90° C., where the breakdown of said screens is observed. Conversely, when the glass screens 8A′″ and 8B′″ are respectively covered with a protective layer 10A′″ and 10B′″, the temperature of the glass screens does not exceed 60° C. (curve 2), and no electric discharge is observed between the electrodes 4A′″ and 4B′″. The protective layers therefore make it possible to limit the breakdown risk of the apparatus over time. More specifically, the greater the resistivity of the protective layers is, the lower this risk is, such that the apparatus has better resistance to aging.

[0065] Another advantage related to the electric superficial resistivity value of the protective layers lies in allowing the establishment of a more homogeneous electric field E in the passage 14 formed between the first and second electrodes of the apparatus. Indeed, the electric field is more faithful to the arrangement of the electrodes, since the electric charges created by the electrodes move only slightly on the surface of the protective layer(s). Thus, the distribution of the charges generated for example on the surface of the electrode 4B in FIG. 1A is substantially the same at the surface of the protective layer 10B defining the passage 14. The impregnation of a porous material can thus be better controlled.

[0066] As shown in FIG. 5B, an electric field of 4 kV/mm is generated that is more homogeneous between the first and second electrodes in the configuration of FIG. 1C, compared to a configuration of the prior art, in which the electrodes are only covered with a dielectric screen of the glass layer type, as illustrated in FIG. 5A. One can in fact see that between the electrodes, fewer electric discharges form, embodied by the light vertical strips in FIG. 5B corresponding to the configuration of the described embodiments, relative to FIG. 5A illustrating the prior art. FIGS. 5A and 5B have been done for configurations with the following parameters: [0067] metal electrodes 4A, 4B which are 5 mm thick; [0068] dielectric screen layer 8A, 8B made from glass 5 mm thick; [0069] protective layer 10B made from polytetrafluoroethylene (PTFE) 2 mm thick (present only in FIG. 5B); [0070] air knife 14 between the faces of the electrodes: 10 mm; [0071] electric field applied between the electrodes: 4 kV/mm AC sinusoidal at 50 Hz; [0072] relative ambient humidity: 75%; [0073] room temperature: 19° C.

[0074] According to one alternative of an impregnating apparatus 2 as shown in FIG. 1A, the apparatus may comprise a driving device 18 for driving the porous medium 16 as illustrated in FIG. 3. For example, this device may comprise a belt conveyor on which a porous medium can be deposited, so as to allow said medium to pass between the electrodes in the direction of advance F. The driving device may for example move the porous medium at speeds comprised between 20 and 500 m/min, which are higher than the impregnating speeds of the prior art.

[0075] According to an alternative of the apparatus shown in FIG. 3, the impregnating apparatus may include a specific device 20 known in the prior art as shown in FIG. 4, of the confinement chamber type, making it possible to monitor the characteristics of the gas present in the passage 14. The specific device may for example control the relative humidity level and keep it between 30% and 60%, preferably between 30% and 50%.

[0076] The composition of the gas in the passage 14 may also be controlled by the specific device 20 and for example comprise one of the following gases: argon, nitrogen, oxygen. The pressure of the gas may also be set by said device in a value range comprised between 10.sup.−7 and 1,000 hPa (10.sup.−7 and 1,000 mbar), preferably between 10.sup.−3 and 1,000 hPa (10.sup.−3 and 1,000 mbar).

[0077] It should also be noted that depending on the amplitude and application time conditions of the electric field and the gas present in the passage 14, the presence of a plasma may cause changes to the surface tension of the materials present between the first and second electrodes (physiochemical change of the materials). This change in surface tensions may for example make it possible to increase the hydrophilic or hydrophobic nature of a material.

[0078] According to one alternative of the apparatus shown in FIG. 6, the impregnating apparatus 2 may comprise a pre-treatment device 22 making it possible to prepare the porous medium 16 before it is impregnated. The pre-treatment device may prepare the porous medium so as to control the value of its volume resistivity at a value above 10.sup.9 Ω.Math.cm, to promote the establishment of a homogeneous electric field between the electrodes. For example, the pre-treatment device can preheat the porous medium to lower its humidity level and/or to diffuse dry air through the porous medium. It is also possible to consider heating the electrodes in order to raise the superficial resistivity value.

[0079] The impregnating apparatus may comprise a device 24 for depositing powder 17 making it possible to deposit the powder in contact with the porous medium 16 before it passes between the electrodes 4A and 4B. The depositing device is preferably arranged between the aforementioned pre-treatment device and the electrodes.

[0080] This application also relates to a method for impregnating a porous medium with powder, consisting of applying an electric field comprised between 0.1 and 50 kV/mm to a porous medium 16 covered with powder 17.

[0081] The porous medium 16 may be a fibrous array, for example a nonwoven or woven, paper, or open-cell foam.

[0082] The powder can incorporate different components in terms of chemical composition or particle size, and additives or other complementary compounds intended to impart specific properties to the powder.

[0083] The impregnating method can comprise a preliminary step for pre-treating the porous medium 16 by heating or drying by blowing dry air through the medium, to make it possible to limit the attenuation of the electric field traversing said medium. This step can consist of lowering the humidity level of the medium when one of these components is known to have a certain humidity absorption level such as natural fibers, polyamide or moisture-sensitive anti-static oversprays. This heating and/or drying step advantageously makes it possible to increase the volume resistivity of the porous medium so as to disrupt the electric field as little as possible, to promote homogeneous impregnating of the powder in the porous medium. For example, to increase the volume resistivity of natural fibers making up the medium to a value above 10.sup.9 Ω.Math.cm, the medium may be dried beforehand. In the case where the medium includes synthetic fibers covered beforehand with an overspray, having anti-static characteristics in the presence of moisture in the air, a pre-treatment with dry air makes it possible to increase its volume resistivity above 1×10.sup.9 Ω.Math.cm.

[0084] FIGS. 7A and 8A respectively illustrate the distribution of an electric field encompassing a porous medium 16 with a relative humidity level of about 70% at 20° C. and a similar medium conditioned so that its level is equal to 32% relative humidity at 21° C. FIG. 8A shows a more homogeneous distribution of the electric field around the pre-conditioned medium relative to FIG. 7A. As shown by FIGS. 7B and 8B illustrating the distribution of the powder above the medium of FIGS. 7A and 8A, respectively, the powder is impregnated more homogeneously in the medium 16 when its humidity level is reduced.

[0085] Of course, the impregnating methods described here may be implemented by one or several impregnating apparatuses 2, 2′, 2″, 2′″ described above.

[0086] In conclusion, this application proposes an apparatus for impregnating a porous medium using an electric field. The apparatus advantageously comprises one or two protective layers, as well as one or two dielectric screens protecting the electrodes of the apparatus. The protective layers limit the movement of the electric charges on their surface, thus enabling precise control of the distribution of the electric field between said layers over space and time. As a result, the electric phenomena deteriorating the apparatus, such as the formation of electric discharges between the electrodes, are limited. The apparatus thus holds up better over time. The contemplated embodiments also allow the formation of a more homogeneous electric field between the electrodes so as to promote a more regular impregnation of a porous medium covered with powder passing between said electrodes.