JPH0675221B2 - Contact brush charging device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for charging an insulating layer by means of a contact brush, and in particular, an individual fiber is electrically short-circuited or the electrical characteristics of the insulating layer are destroyed. To a contact brush charging device that does not cause physical interference.

BACKGROUND OF THE INVENTION In the currently commonly used electrostatographic reproduction apparatus, the photoconductive insulating member is charged to a negative potential and then exposed to the optical image of the original to be copied. This exposure discharges the photoconductive insulating surface in the exposed or background areas, producing an electrostatic latent image on the member corresponding to the image areas contained within the original document. The electrostatic latent image on the photoconductive insulating surface is then visualized by developing the latent image with a developing powder called toner in the art. During development, the toner particles are pulled by the carrier particles according to the charging pattern of the image areas on the photoconductive insulating areas to form a powder image on the photoconductive areas. This powder image is then transferred to a support surface such as copy paper and permanently affixed thereto by heat or pressure. After transfer of the toner image to the support surface, the photoconductive insulating surface is discharged and residual toner is removed, ready for the next imaging cycle.

In the above-mentioned electrophotographic apparatus currently used, various charging devices are used to charge the photoconductive insulating layer. For example, in commercial applications, various corona generators have been used in which a high voltage of 5,000 to 8,000 V is applied to a corotron device to generate a corona spray, imparting an electrostatic charge on the surface of the photoreceptor. Corona spray also produces ozone and other species that must be collected or neutralized.
Alternatively, the photoconductive insulating layer may be charged with a brush charging device that is in contact with it and a potential of about 1200V is applied during the charging process. This provides a constant voltage charging process, for example when 1000V is applied to the brush the insulating layer is charged to 800V. This is in contrast to corona generators, which are based on a constant current process and therefore have a large variation in the final potential formed on the photoconductive insulating layer.
Further, in the case of brush charging, even if the corona is small, the corona charging and the amount of specific ozone generated are drastically reduced. Moreover, brush charging is 100% efficient for current entering the photoreceptor, whereas corona charging is only about 10% efficient for current entering the photoreceptor. In general, contact brush charging has the advantage of being relatively unaffected by process speed and electrical history of the photoconductive insulating layer within the normal operating range. That is, in the contact brush charging device, when the photoreceptor is charged in one cycle and then discharged by being exposed to a light and dark pattern to generate different potential levels, the contact brush charging for the next copy is performed. Upon subsequent charging, the photoreceptor is only recharged to the initial uniform potential.

Contact brush charging offers these advantages, while
The various brush fibers, which are generally made of a conductive material, have the serious drawback that when they come into contact with imperfect areas of the insulating surface, these fibers can short-circuit at a portion of the insulating surface resulting in loss of output copy. In the commercial manufacture of photoreceptors (photoconductive insulating layers), it is very difficult to form an entire layer, whether drum-shaped or belt-shaped, so that the dielectric strength of the layer is exactly uniform over the entire surface. Have difficulty. Some defects in the photoreceptor often result from manufacturing defects, contamination, or other incompletely understood causes. In particular, a region having a relatively low dielectric strength that is visible or invisible to the naked eye is likely to occur. These defects are usually located within the layers of the photoreceptor and are randomly distributed throughout the layers. When such a photoreceptor is used in a commercial embodiment that includes a conductive fiber based contact charging device,
The fiber contacts the photoreceptor, and in the defect area of the photoreceptor, the high current density path creates an electrical short to the conductive backing on the photoreceptor layer. This short circuit causes localized heating, melting and oxidation of the polymeric material, and also releases the vaporized polymer gas, causing catastrophic and irreversible damage. This results in mechanical damage to the photoreceptor in the form of craters or holes in the photoreceptor layer. Initially, the scratch was an invisible defect, but gradually became visible as a 1-2 mm hole in the photoreceptor layer. This "pinhole" acts as a mechanical toner trap during the development cycle and develops as black spots, resulting in copy quality defects. Further, since the small specific portion does not behave as a photoconductive insulating layer, it may appear as an undeveloped area.

Photoreceptors that exhibit such a "pinhole" effect can have a variety of different shapes including plates, drums, flexible belts, and the like. Typical photoreceptors have one or more photoconductive layers on a support surface. The supporting substrate is itself conductive or is coated with a conductive layer, on which the photoconductive layer is coated. Alternatively, as a multi-layer photoconductive imaging photoreceptor,
It may consist of at least two electrically active layers, a photon generating or charge generating layer and a charge transport layer, each layer generally being applied on a conductive layer. See US Pat. No. 4,265,990 for further details of such layers. The entire patent is included here as reference material. In all of these various structures, some layers can be applied by vacuum deposition techniques for ultrathin layers. During some manufacturing processes, one or more layers may be exposed to contamination by foreign particles (dust) and other process deviations. And, small defects create the "pinhole" effect associated with the present invention.

(Prior Art) A typical device in which a conductive and thin carbon fiber is used in the form of a brush as a charging or transferring device is Japanese Patent Application No.
-102630 and 53-102631. Both of these applications disclose conductive materials having a resistivity in the range of about ^10-5 to ^10-3 ohms-cm, such as carbon fibers or stainless steel fibers.

Walkup U.S. Pat. No. 2,774,921 describes a brush charging device that electrostatically charges an insulating imaging surface for electrophotography. Walkup has recognized that when a highly conductive fiber contacts a hole or weak point in an insulating layer, it causes a short circuit, which is undesirable, especially in the case of printing plates for electronic printing. He also recognizes that the conductivity of the flexible element should not be too strong, nor should the resistance to electric current be too great. Ω ~ 100,000M
It has a surface resistance in the range of Ω. Walkup excluded materials such as copper, silver, and other common metals. The examples disclosed in column 3, lines 22 to 28 of the above-mentioned U.S. patent specification are papers, cloths, and papers which become slightly conductive by being metallized or coated with a hygroscopic salt such as glycerin or various salts. Includes seed plant fiber, glass cloth.

The inventors have found that the materials suggested by Walkup are all moisture sensitive materials and have the disadvantage of changing electrical resistance with relative humidity and temperature over time.
That is, any material described as appropriate by Walkup is appropriate at some point in time under certain conditions, but long-term use of materials that are sensitive to the presence of moisture can result in unpredictable behavior. In other words, as the water content increases, the specific resistance decreases and the fiber acts as a conductor and drops to a value that shorts the photoreceptor. Further, the inventors of the present invention have found that if the electrical resistivity of each fiber is maintained within a relatively narrow limit, the above-mentioned "pinhole" effect can be avoided, and a current suitable for necessary charging can be applied to the photoconductive insulating layer. I found what I was given.

Murray et al., U.S. Pat.No. 4,336,565, describes a brush charging device in which a brush is composed of a conductive carbon fiber filament to which an electric potential is applied to bring an electric charge onto an electrically insulating surface. There is.

An object of the present invention is to provide a device for contact-charging an insulating layer without causing pinhole defects in the insulating layer by forming an electric short circuit in the defective portion of the insulating layer.

Another object of the present invention is to provide a charging device in which the specific resistance characteristics of the charging device are substantially stable with respect to changes in relative humidity and temperature.

Another object of the present invention is to provide a contact charging device which is soft and does not destroy the insulating layer in a mechanical sense.

Another object of the present invention is to provide a charging device in which each fiber does not adversely affect the charging performance of the brush as a whole.

Another object of the present invention is to provide a contact brush charging device in which each fiber itself self limits the current flow.

According to the present invention, there is provided a contact brush charging device for charging an insulating layer, comprising a plurality of elastically flexible thin fibers arranged in a brush shape. The fiber is supported by supporting means such that each tip of the fiber is in contact with the insulating layer, and has an electrical resistivity of about 10 ² Ωcm to about 10 ⁶ Ωcm, and the specific resistance is A contact brush charging device is provided which is substantially stable to changes in humidity and temperature and is characterized in that the fiber is a partially carbonized polyacrylonitrile fiber.

(Example) Hereinafter, a preferable example of the present invention will be described.

Referring first to FIG. 1, an automatic xerographic copying machine 10 including the corona generating device of the present invention is shown as an example. The copier 10 shown in FIG. 1 shows various components used to make a copy from a document. While the apparatus of the present invention is particularly suitable for use in an automatic xerographic copier 10, the apparatus is equally suitable for use in a wide range of processing systems, including other electrostatographic systems, the application of which is shown in the particular embodiment illustrated. It will be clear from the description below that it is not necessarily limited to the examples.

The copier 10 shown in FIG. 1 uses a drum-shaped image recording member 12, the outer periphery of which is coated with a suitable photoconductive material 13. The drum 12 is rotatably supported in a body frame (not shown) by a shaft 14 and rotates in the direction of an arrow 15 to sequentially pass the drum-shaped image bearing surface 13 to a plurality of xerographic processing stations. . An appropriate drive means (not shown) is provided to energize and adjust the movement of the various mechanical parts that cooperate to record a faithful copy of the original input scene information on the final support 16 such as paper. It

First, the drum 12 rotatably moves the photoconductive layer 3 and passes it through a charging station 17 such as a contact charging brush 11 where electrostatic charges are uniformly deposited on the photoconductive layer 13 by a known method to prepare for image formation. Arrange. Then drum 12 is exposed station
When the charged photoconductive surface 13 is exposed to the light image of the original input scene information, the charge is selectively lost in the light exposed area, and the original input scene is recorded in the form of an electrostatic latent image. To do. After exposure, drum 12 rotates the electrostatic latent image recorded on photoconductive surface 13 to development station 19, where a conventional mixed developer is applied to photoconductive surface 13 of drum 12 to develop the latent image. Let it be a visual image. In general, suitable development stations include a magnetic brush development system with a magnetizable mixed developer having ferromagnetic coarse carrier particles and toner dye particles.

Sheet 16 of the final support material is a lifting stack support tray 20
It is supported in a stacked state. When the stack is in its raised position, the sheet separation feed belt 21 feeds individual sheets therefrom to the alignment pinch roll 22.
The sheet is then fed to a transfer station 23 which is in precise registration with the image on the drum. The developed image on the photoconductive layer 13 is brought into contact with the sheet 16 of final support material in the transfer station 23 and the toner image is transferred from the photoconductive surface 13 to the final support sheet.
Transferred to 16 contact surfaces. After transferring the image, if desired, paper,
The final support material, such as plastic, is conveyed to a separation station where the separation corotron 27 uniformly charges the support material and separates it from the drum 12.

After the toner image has been transferred onto the sheet 16 of final support material, the sheet bearing the image thereon is sent to a suitable fuser 24, which fuses the transferred powder image thereto. After the fixing process, the sheet 16 is sent to an appropriate take-out device such as the tray 25.

Most of the toner powder is transferred to the final support material 16, but some residual toner will inevitably remain on the photoconductive surface 13 after the transfer of the toner powder image onto the final support material. There, after transfer of the toner image onto the final support, the residual charge remaining on the drum is reduced by the corona generated from the precleaning corotron 28 according to the invention. The residual toner particles remaining on the photoconductive surface 13 after the transfer of the toner image are then removed by the purifier 26.

Generally, when copying is to be done in normal mode, the original to be copied is placed with the image face down on a horizontal, transparent see-through platen 30, and platen 30 is an optical system shown here as a Selfoc lens 18. The manuscript is transmitted through. The speed of the movable platen and the speed of the photoconductive belt are synchronized, and a faithful copy of the original is considered.

For the purpose of this application, the above description will be sufficient to show the general operation of an automatic xerographic copier in which the apparatus according to the invention can be implemented.

FIG. 2 shows a contact brush charging device in which a plurality of elastically flexible thin fibers 31 are wound around a support rod 32.
11 is shown. Each brush, evenly distributed along the length of the brush, is held in place on the rod by a U-shaped conductive outer shield 34, which has a pair of perforated tabs 36 at its ends. And provides a means for mounting and connecting the device to an electrical circuit. The brush may be used in a fixed position or, if desired, may be vibrated in a direction that intersects the direction of movement of the photoconductive insulating layer it contacts. Two or more brushes may be arranged in parallel to ensure the uniformity of electric charge. Although FIG. 2 shows a contact charging brush in the form of a flat bristle brush, it will be appreciated that the device of the present invention can also be used with a roller-shaped brush as shown in FIG. In the roller form, each fiber is mounted on, or woven or braided into, a conductive elastic base 38, which is wrapped around a conductive roller 40 connected to a power source. There is. Such a rotating brush maximizes the charge uniformity.

If the present inventors configure the contact brush charging device with a fiber having a direct current electrical resistivity of about 10 ² to 10 ⁶ Ωcm and a resistivity that is substantially stable against changes in relative humidity and temperature. Have found that the above-mentioned pinhole defects and the accompanying copy quality defects can be avoided. Resistivity is 10 ² Ωc
Using a fiber material of m or less increases the amount of power consumed at the defect location, increasing the pinhole effect and the resulting defect in copy quality. The specific resistance is larger than 10 ⁶ Ωcm,
When a voltage of about 1200 V is applied, there is insufficient current flowing into the photoreceptor to complete proper charging of the photoreceptor used in the imaging process. The present inventors have also found that a preferable balance between the electrical resistivity and the conductivity within the above range can be obtained in a narrow resistivity range of about 10 ³ to 10 ⁵ Ωcm. Fibers with a resistivity below about 10 ⁵ Ωcm are the most stable in terms of increased resistivity. It should be noted that all resistive fibers tend to have a slight increase in resistivity over time. Fibers with a resistivity greater than 10 ³ Ωcm have better current surge limiting performance and are therefore less prone to pinholes. Although the materials having the above-mentioned general and preferable electric resistivity exist in the prior art, those materials have the properties as described in the Walkup patent and have a specific resistance to a change in relative humidity. Is not stable. The expression that the resistivity is substantially stable means that the magnitude of the change in resistivity is smaller than an order of magnitude based on the change in temperature and relative humidity over the normal operating range and aging. . So we are
About 60 ° F (15.6 ° C) to 90 ° F (32.2 ° C) and 10 to
Stability in the 80% relative humidity range is stated. Also, many of the materials used to date, such as stainless steel, brass and aluminized fiberglass, are too hard or too brittle to damage the photoreceptor when it comes into contact with it, or otherwise have desirable physical properties. It has no characteristics. In addition, some materials with the required properties are traditionally made by complex manufacturing techniques such as doping and adding pigments,
All of these mechanically degrade the mechanical properties of the material.

Since the fiber of the present invention is elastically flexible, even if the fiber is deflected by the sheet passing through the brush position, the fiber will elastically return to its original position after the trailing edge of the sheet has passed therethrough. Further, even if it has been compressed for a long time, if the compression is removed, the fiber returns to its original orientation. It is preferred that the fiber be relatively stiff and not brittle, i.e. soft, to reduce any physical degradation that may occur to the photoreceptor. Generally, the fiber has a tensile stress elongation of about 1.2% to 3% of its initial length before breaking. The resistive fiber of the present invention has a substantially circular cross section,
It has a diameter of about 5-50μ, preferably about 8-10μ, which makes the fiber less susceptible to breakage or breakage.

The fiber used in the contact charging brush of the present invention is a carbon fiber obtained from treatment at low heat treatment temperatures because it results in partial carbonization of the polyacrylonitrile (PAN) precursor fiber. It has been found that in such fibers, precise control of the carbonization temperature can be obtained by carefully controlling the carbonization temperature within certain limits. In this regard, note FIG. 3, which is a graph showing the resistivity and its dependence on the processing temperature of the carbonization process. Polyacrylonitrile precursor fiber is 1,000-180,0 by Stackpole, Celanese, etc.
It is manufactured commercially as a yarn bundle of 00 filaments. These yarn bundles stabilize the pan fiber in an oxygen atmosphere at a temperature of about 300 ° C and stabilize the total oxygen (preox-) P
It is partially carbonized by a two-step process that includes the steps of forming AN followed by carbonization in a higher temperature inert (nitrogen) atmosphere. The DC electrical resistivity of the resulting fiber is controlled by selecting the carbonization temperature. For example, as shown in FIG.
Carbon fiber with electrical resistivity of 10 ² to 10 ⁶ Ωcm is
It can be obtained by controlling the carbonization temperature in the range of about 500 ° C to 750 ° C. The fiber obtained through such a process is stable against changes in temperature and relative humidity, and its electrical resistivity does not change with relative humidity. This is in sharp contrast to the materials described above in connection with the Walkup patent, where electrical resistivity varies significantly with relative humidity. Further, the fiber is manufactured without the use of fillers, plasticizers, waxes, and other additives that can leach out of the fiber body and subsequently contaminate the photoreceptor. As a result, the fibers produced are substantially homogeneous in composition and relatively pure in the sense that there is no additive containing unbound species.
Also, all the nitrogen and oxygen left in the fiber is somehow bound to carbon as part of the residual polymer chain,
Or, they are connected to each other.

The stable nature of the electrical resistivity with respect to temperature and relative humidity contrasts with most polymer fibers where conductivity is obtained by adding carbon black or other physically mixed additives in one form or another. Is. The use of polymer fibers generally results in carbon black and other additives ultimately deposited on the photoreceptor.

The fibers according to the invention are very densely packed and give a high fiber packing density (free fraction of fibers per unit area). Generally, the contact brush charging device of the present invention has a fiber packing density of about 5 × 10 ⁴ to 4 × 10 ⁶ fibers / square inch. These fibers are suitable for weaving,
It can therefore be woven into a woven fabric if desired. In operation,
Each individual fiber acts as a charging element without mechanically eroding the photoreceptor or otherwise damaging it. Therefore, the contact charging brush according to the present invention has a long working life.

The preferred carbon fiber used in the practice of the invention is commercially available from celanese as CELECT 675. These fibers are made through a variety of processes generally taught in the literature. For details of the process used to manufacture this carbonized fiber, reference is not made to: "Production of carbon fiber from polyacrylonitrile at low temperature", D, E. Cagliostro, Textile Research Journal , 1980 10
Tsuki, pp. 632-638; "Explanation of carbonization process of polyacrylonitrile fiber from the viewpoint of electrical characteristics", L, Brehmer
Et al., Plaste und Kautschuk, 1980, Vol.27, No.6,309-313.
Section; and “Electrical Resistance of Carbon Fiber”, DBFischba
ck et al., Seattle, Washington, University of Washington, Department of Mines, Metallurgy and Ceramic Engineering, FB-10, 191-192.

FIG. 3 shows the change in the specific resistance depending on the processing temperature as a logarithmic display of the specific resistance versus the processing temperature. From this graph,
It is clear that precise control of resistivity can be obtained by controlling the temperature of carbonization. Also, as mentioned above, the preferred fiber used in the practice of the present invention is stable, its resistivity does not change with relative humidity or temperature, and the fiber itself is highly flexible, thin in diameter,
Virtually no compression hardening occurs and the elongation is only 1.2-3%.

The table below shows the contact brush charging performance for several fibers with different DC electrical resistivity. In each example, the brush had a fiber packing density of about 40,000 fibers / square inch and was constructed with the same geometric dimensions. The conductive graphite and stainless steel fibers were made according to the Mandless winding method of Swift et al., US Pat. No. 4,330,349. That is, a double-sided backing foam adhesive tape is placed on the mandrel, the fiber is wrapped around the mandrel, then double-sided backing foam adhesive tape and yet another fiber are alternately wound onto it, finally A brush having 4 layers of fiber winding was obtained. An aluminum strip was then placed on the outer tape, enclosing and laminating the brush to provide electrical contact to the fiber. The tip of the brush was trimmed with a cutting cutter to have a protruding length of about 0.5 inch (1.27 cm). The back of the brush was then coated with a conductive silver paint to ensure electrical contact to all fibers and to seal each fiber within the brush.

Stackpole fibers were supplied as 40,000 fibers / inch 4 inch (10.2 cm) wide tow. After these tows are manually laminated with double-sided foam tape, aluminum strips are used to enclose the laminate to form the roots or handles of the brushes and electrical contacts to the fibers. The tows protruded about 1 inch (2.54 cm) from the aluminum strip and were trimmed with a cutting cutter to typically have a projected length of about 0.5 inch (1.27 cm).

Each 4-inch long brush with a 0.5-inch (1.27 cm) free fiber length is then described in US Pat.
Each was tested by gluing it around a rotating drum having a photoconductive surface and contacting the photoconductive surface as described in 265,990. During one revolution of the drum, the applied voltage to the brush was ramped from 0 to -1500 V, causing a linearly increasing charge density to be deposited on the photoreceptor, which was measured by an electrometer as the drum rotated. V (photoreceptor)
The inclination of the line to the V (applied) becomes the magnitude of the charging ability.
In the case of the brushes of the invention, this slope is very close to 1, ie the surface potential of the photoreceptor follows the applied voltage irrespective of other process variables such as speed, thus obtaining a "constant voltage charge". If the resistivity of the carbon fiber is too high, the slope of the charging curve becomes much smaller than 1, and V
(Photoreceptor) increases less than V (applied). This means that V (application) must be increased to achieve the same result as with a more conductive brush, which results in the loss of one of the advantages of brush charging.
Conversely, if the conductivity of the fiber is too high, pinhole damage or photoreceptors will occur during charging, and the size and number of such pinholes will increase as the conductivity of the fiber increases.
Each performance is summarized in the table below.

The conventional conductive graphite fiber described above is Hercule
cels, Celanes and Union Carbide, Celanes Celiones, Chyatam, NJ, respectively.
n 6000 carbon fiber; Un, Chicago, Illinois
ion Carbide Thornel 50 and 300 (PAN) carbon fiber; Hercules, Wilmington, Del.
Magnamite AS4 PAN base graphite fiber. Stainless Steel Fiber, Schl, Rochester, NY
Available from egel. Fibers 1-6 are PAN fibers made with Stackpole and carbonized at the indicated temperatures for Xerox.

As is clear from the table, the pinhole effect was recognized in the conductive fibers (conventional conductive graphite, stainless steel and Panex 30). Stackpole Nos. 1, 2 and 3 were too insulative to give a reliable and uniform charge.

Stackpol No. 4, 5 showing a specific resistance of 1.8 × 10 ^{3 to} 4.0 × 10 ^6.
And 6 charged photoreceptors, of which Stackp of No. 5 and 6
An acceptable charge was obtained for the ole fiber.

(Effect of the Invention) As described above, according to the present invention, it is possible to select a material which can be commercially produced and has a long life which is compatible with the surface of the photoreceptor, and which does not cause the above-mentioned "pinhole effect". A charging device is provided. Further, the specific resistance of the brush charging device can be controlled by the carbonization temperature. The brush thus obtained is soft and does not cause damage to the photoreceptor in a mechanical sense. Moreover, the current in one fiber during a short circuit is grounded on the photoreceptor without reducing the voltage across the brush due to the high resistivity of each fiber, so the fiber is self-limited in terms of current. , Any shorting of individual fibers does not adversely affect the charging performance of the brush. Moreover, the preferred fibers according to the present invention do not deposit any debris or debris on the photoreceptor and do not abrade the photoreceptor.

All patents and publications referenced herein are incorporated herein by reference in their entireties.

Although the present invention has been described in detail above with particular reference to contact brush charging devices used in electrostatographic reproduction machines, it will be appreciated that brush charging devices can be applied in many different fields.
For example, it can be used as a static eliminator brush or as a biased and non-biased photoreceptor device. It will be appreciated that various changes may be made from the above detailed description without departing from the spirit and scope of the invention. All such modifications made by those of ordinary skill in the art are intended to be within the scope of the appended claims.

[Brief description of drawings]

1 is a schematic sectional view of an automatic electrostatographic copying machine equipped with a brush charging device according to the present invention; FIG. 2 is a perspective view of an embodiment of a charging brush according to the present invention; and FIG. 3 is a preferred fiber according to the present invention. 4 is a isometric view of another embodiment of the charging brush according to the present invention. 11 ... Contact brush charging device, 13 ... Insulating layer, 31 ... Fiber, 34 ... Supporting means, 38 ... Conductive sleeve.

Claims (6)

[Claims] 1. A contact brush charging device for charging an insulating layer, comprising a plurality of elastically flexible thin fibers arranged in a brush shape, each of which has its tip in contact with said insulating layer. And having an electrical resistivity of about 10 ² Ωcm to about 10 ⁶ Ωcm, the resistivity being substantially stable to changes in relative humidity and temperature. Is a partially carbonized polyacrylonitrile fiber, a contact brush charging device. 2. The first fiber according to claim 1, wherein the plurality of fibers are arranged in a uniform distribution along the length of the brush.
The device according to the item. 3. The apparatus of claim 1 wherein said fiber is substantially homogeneous in composition. 4. The apparatus of claim 1 wherein said fiber has a generally circular cross section and a diameter of about 5 microns to about 50 microns. 5. The fiber is disposed in a brush and has about 5
× 10 ⁴ to 4 × 10 ⁶ lines / square inch (about 78 lines / mm ² to 620
A device according to claim 1 having a density of lines / mm ² ). 6. A device according to claim 1, wherein said fibers are arranged in the form of a rotating brush around a cylindrical conductive sleeve.