US3489895A - Regulated electrostatic charging apparatus - Google Patents

Description

Jan. 13, 1970 H. J. HOLLBERG 3,489,895

REGULATED ELECTROSTATIC CHARGING APPARATUS Filed Feb. 2, 1966 2 Sheets-Sheet 1 S 'n l Q H S k U1 ik we g? w i H n ha lw u): A

w N i 'AK T @u g I`\`Lg l m\\ Q 1 u @I MK A n m\%0 I) 22S* I n g 5y @www Jan 13, 1970 H. J. HOLLBERG REGULATED ELECTROSTATIC CHARGING APPARATUS 2 Sheets-Sheet B Filed Feb. 2 1966 mmf mlm United States Patent O 3,489,895 REGULATED ELECTROSTATIC CHARGING APPARATUS Herbert John Hollberg, Richmond, Va., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Filed Feb. 2, 1966, Ser. No. 524,624 Int. Cl. H011 37/26 U.S. Cl. Z50-49.5 9 Claims ABSTRACT OF THE DISCLOSURE This invention concerns a novel and useful corona discharge device for charging fibrous elements while maintaining a constant level of ion current flow between the electrodes of the device.

A corona discharge is the electrical discharge occurring in a gas surrounding a conductor when the potential gradient at a point in a non-uniform electric field exceeds the critical value for ionization causing a local `self-sustained discharge. It is a phenomenon particularly associated with sharp edged electrodes and essentially occurs in a limited region near the electrodes, the rest of the gap carrying a so-called dark current. The magnitude of the electric field gradient required to produce a corona discharge is the same as that required to produce arcing; the difference between the two being that in corona the critical value has been reached only in a limited region of the breakdown path between electrodes. Generally, the discharge device consists of two electrodes disposed opposite one another across a high resistance gap in a gaseous atmosphere, the distance being sufficiently small to promote corona discharge and sufficiently large to prevent disruptive spark discharge. The apparatus of the present invention is useful in supplying a corona field particularly in applying an electrostatic charge to fibrous elements.

Various methods have been described in the art for subjecting materials of high dielectric strength to a corona discharge. The treatment is especially useful for improving printability and adherability of polyethylene. In addition, corona discharge is very useful for applying an electrostatic charge to high dielectric -materials in the form of particulate fibers, continuous filaments or fibrous webs. The charge deposited on the fiber permits improved control of the fiber during processing. One satisfactory ap paratus for this purpose is described, for example, in Disabato and Owens, U.S. Patent 3,163,753, wherein one electrode comprises an ion gun, i.e., a row of parallel needles attached to a common conductor and the other electrode comprises a round bar.

In the development of a ash extrusion process based on the method of Blades and White U.S. Patent 3,081,519, methods have been developed for electrostatic control of bers. Various difficulties have arisen in controlling the fine brillar materials which are characteristic of this process. Accordingly, improvements have been made in the corona charging techniques. A flat target plate has been substituted for the round target bar of Disabato and Owens and the ion gun has been modified by inserting lCC high megohm resistors in series with the needle electrodes to level out ion flow from the needles to various portions of the target plate. The present invention concerns further improvements which have been devised to compensate for changes in the dielectric strength which occur in the gap between the electrodes during operation because of the deposit of polymeric material on one of the electrodes.

The flash extrusion process provides under certain conditions a highly fibrillated strand which may be spread into a web by means of a baffle and deposited in random overlapping layers on a moving belt as a nonwoven sheet. A further description of the sheet and its preparation may be found in the Steuber patent, U.S. 3,169,599. In a recent version of this sheet-making process the web is exposed to a corona discharge just after it leaves the baffie. A charge is applied to the fbril elements in the web as they pass over a flat plate which is one of the electrodes of a corona discharge system. The charged fibrils then repel one another so that a well dispersed network may be collected on the moving belt. The charge is removed by grounding the belt (if it is a conductor) or by applying an opposite charge to neutralize the iibrillar material.

It has now been found that a small amount of polymeric residue collects on the surface of the flat target plate, causing the dielectric strength in the gap between the two electrodes to gradually increase during use. The increase in dielectric strength causes a decrease in the current flow between the two electrodes, so that as time progresses a smaller-and-smaller charge is applied to the fibrillar material, and nonuniform webs are obtained. On the other hand, a sudden loss of the residue from the target plate may at times cause an increase in the current between electrodes, and this will promote increased charging of fibrils. These changes in fibril charge level can be compensated by adjustment of the applied voltage, but this must be done with care since excessive voltage may cause the fibrils to discharge at the trailing edge of the target plate by what is known as secondary ionization. Under such conditions the discharged fibers tend to clump together. It is obvious then that there is an optimum voltage and corona current level for obtaining uniform sheets. It has been found that the optimum control of fibril charge level can be maintained through control of the current reaching the target electrode from the pointed electrode.

Accordingly the objective of the present invention is to provide a means for maintaining a constant level of ion current flow between two electrodes of a corona charging system when such electrodes are subjected over a period of time to contamination by materials of varying dielectric strength.

The invention meets the desired goal with an apparatus adapted to receive fibrous elements continuously forwarded in a linear path of advance in a gaseous atmosphere and to apply an electrostatic charge to the elements while maintaining a constant level of ion current flow between two electrodes of a corona charging system compri'sing,

l) A direct current power supply producing an output voltage that is adjustable over a range of O to about l0() kilovolts, said output voltage developed between a high voltage terminal and a low voltage terminal;

(2) A corona discharge device consisting of a first electrode having a small radius of curvature and a second electrode having a large radius of curvature, the first electrode conductively connected to the high voltage terminal, and the second electrode conductively connected to the low voltage terminal, the electrodes being situated on opposite sides of the path of the fibrous elements, the distance between the electrodes being sufficiently small to promote corona discharge and suiiciently large to prevent disruptive spark discharge;

(3) A sensing means for determining the current ow in one of the conductive connections between one of the voltage terminals and the corresponding electrode, the sensing means having a current range suiciently large enough to indicate maximum current flow under normal operating conditions, the sensing means having adjustable high and low current sensors; and

(4) A control means operably connected to be responsive to the sensing means and operably connected to adjust the adjustable direct current power supply arranged to:

(a) increase the applied voltage across the electrodes when the current low is less than the low current sensor value,

(b) decrease the applied voltage across the electrodes when the current flow is greater than the high current sensor value, and

(c) leave the voltage across the electrodes unchanged when the current flow is in the range between the low and high current sensor values.

The invention will be more readily understood by reference to the drawings.

FIGURE 1 is a block diagram showing corona charging electrodes and the control installation for regulating current between the two electrodes.

FIGURE 2 is a similar diagram with additional cornponents and circuit details.

Considering lirst the block diagram of FIGURE 1, an adjustable direct current power supply 1 provides high voltage direct current. High voltage terminal 2 and low voltage terminal 3 provide direct current power for the circuit including multi-pointed electrode 4, target electrode 6, and meter-relay 9. The multi-pointed electrode 4 comprises a row of parallel needle points 5 attached to a common conductor preferably through high megohm resistors (e.g., each 600 megohms). The needle points are aimed at the trailing edge of target electrode 6. The target electrode is preferably a flat plate with a straight trailing edge. The voltage between terminals 2 and 3 is in the range 0 to about 100 kilovolts. During operation a corona discharge occurs between the two electrodes in the gaseous atmosphere between the two electrodes. The gas may be air, mixtures of air and solvent vapors, 100% vaporized solvent, or other gases. Fibrous material is passed between the two electrodes in the direction indicated by arrows 7. In the case of freshly spun fibers such as those prepared by flash extrusion, a small amount of polymeric material tends to collect on the surface of the target electrode. The major portion of the material, however, continues through the gap and is collected by means not shown.

The corona discharge between the two electrodes promotes ionization of the gaseous medium. These ions in turn promote charging of the fibers. The ibers are carried away from the electrodes bearing a charge in the range of 1 to 20 microcoulombs per gram. This in effect means that only a very small part of the current emanating from multi-pointed electrode 4 passes to the fibers. For example, this current might be in the range of 3 to 4 microamperes. The major part of the current, however, passes from the multi-pointed electrode 4 to the target electrode 6. In a typical case this current to the target electrode is 150 microamperes.

Depending on atmospheric conditions between the two electrodes and upon design-or other parts of the machinery, there may also be a leakage current of 20 to 30 microamperes. This current passes from the multi-pointed electrode to various other parts of the machinery and is eventually grounded. It will be seen then that the major portion of the current emanating from the multi-pointed electrode 4 passes to target electrode 6, then through conductor 8 to meter-relay 9. This loop of the circuit is com- 4 pleted by conductor 10, which is connected to the low voltage terminal 2 of the power supply. In an alternate arrangement conductor 10 and low voltage terminal 2 are independent grounded instead of being connected.

Control of the current flowing from multi-pointed electrode 4 to target electrode 6 is effected in the following manner. The low and high limit setpoints 11 and 12 are adjusted by knobs 13 and 14 to the desired current level. The meter-relay is designed to measure currents in the range from 0 to 1000 microamperes, but not necessarily over the entire range. In a ash extrusion operation using linear polyethylene and trichlorofluoromethane as solvent in a closed system the meter-relay conveniently has a range of 0 to 250 microamperes.l

The power supply for the meter-relay is 115 volt, 60 cycle alternating current in a typical c-ase. This is shown coming through cable 15. Control signals produced by the meter-relay pass through control ca-ble 16 to reversible motor 17. An amplifier in a control module may be optionally used if the signals coming through cable 16 are relatively weak. This is shown in more detail in FIGURE 2 which will be described in subsequent paragraphs. The shaft 18 of the reversible motor 17 is connected mechanically to the regulator shaft 24 of the adjustable power supply 1. Control circuits in cable 16 are connected by proper circuitry to reversible motor 17 to cause rotation of shaft 18 in such a manner as to increase the power from power supply 1 when the sensors in the meter-relay indicate that the current level is below the low setpoint. Similarly when the current is above the high setpoint, the control circuits cause the reversible motor to rotate in the opposite direction thereby reducing the power from power supply 1. When the indicator needle falls between the high and low limit setpoints the shaft remains stationary. In order to guard against rotation of the power supply regulator beyond its limits, limit switches 19 are provided to de-energize reversible motor 17. These are actuated by collar 20 and pin 21, but, of course, are activated only during a breakdown of the operation. Pin 21 activates either a high or low limit switch by mechanical means 25. The apparatus may optionally be put on manual control by turning switch 23 to alternate cable 22. Manual pushbuttons 58 and 59 are arranged to raise or lower the direct current power supply by means to be described in respect to FIGURE 2.

The regulator 24 of the power supply may simply be the control shaft of a rheostat. On the other hand the adjustable direct current power supply may comprise two components as shown in FIGURE 2: (l) a vari-able input transformer, and (2) a direct current power pack or voltage-multiplier rectier.

FIGURE 2 illustrates a particular embodiment of the invention. Parts are numbered as in FIGURE l whenever they are the same. In this embodiment the regulation of the current input is achieved by means of a variable input transformer 40. Power for the entire system is provided from a 60-cycle 115 volt source through conductors 60 and 61. The AC power output from the transformer 40 passes through connectors 41 to direct current power pack 42. The power pack 42 is used to convert line voltage, usually 1 to 440 volts, 60 cycles per second, to the high potential D-C volta-ge necessary for the corona discharge that occurs between electrodes 4 and 6 during operation. In power pack 42 line voltage from cable 41 is connected to the primary winding of a step-up transformer (not shown). The voltage at the transformer secondary winding is increased severalfolld by a large turns ratio in the transformer, and it is electrically isolated from the primary winding. The secondary winding voltage is increased still further and converted to D-C by a voltage-multiplier rectiiier consisting of rectiers and capacitors (not Shown). The D-C output of the voltage-multiplier rectifier is connected between terminals 2 and 3 of the power pack. The potential .at terminal 2 of this power pack is in the rang of 0 to about kilovolts relative to ground. Terminal 3 is essentially at ground potential or a few millivolts away from ground if a meter is used to measure the leakage currents to ground. The electrical components of this power pack are installed in a container of transformer oil. The dimensions of the container should be sufficiently large enough to prevent corona and electrical spark discharge around the components while operating at high D-C potentials. Direct current is used for powering the electrodes because it creates a unipolar charging field at the target electrode.

The embodiment shown in FIGURE 2 includes a relay control module 47. This unit contains the circuitry for amplifying signals from control cable 16 to activate reversible motor 17. Various types of meter-relay may be used to monitor the target plate current. A particularly satisfactory meter-relay is the optical meter-relay. In this type of relay, control action is initiated by light-sensitive cells. Consequently, the signal pointer moves freely past setpoints and continually indicates signal strength even lafter control action has been initiated. There are no contacts to touch, and there is no interaction, mechanical or electrical, between the signal pointer and the setpointers. The two setpoints may be set -apart as little as 2 to 3 percent of full scale. Since the action at setpoint is analogue in nature rather than being the basic on/of action or locking contacts, true proportioning control is possible as the signal nears the setpoint, although such has not been illustrated in FIGURE 2. The meter-relay can thus be equipped to minimize isolation between the low and high setpoints.

The control cable 16 in FIGURE 2 carries power for meter-relay 9 and carries the various signals from the meter-relay to the control module 47 which amplies the signal and operates an internal relay for the proper response from reversible motor 17. The alternating current power is supplied to the relay control module through conductors 60 and 61. Tie line 48 provides a common power supply from line 60 for the various command circuits to the reversible motor 17. Terminals 49 are always powered during automatic operation. Terminals 50 and 51 are connected internally to relay contacts that are operated by amplified command signals from meter relay 9, these signals being interrupted only when the meter pointer is above the upper limit or below the lower limit.

The reversible motor, as shown schematically in FIG- URE 2, has separate field coils 52 and 53. A suitable capacitor is connected across the two field coils to cause the motor to operate by shifting the phase of this supply voltage across one coil. The circuitry of meter-relay 9, control module 47, and motor 17 is connected so that the high and low relays are both energized when the signal pointer is within the setpoints. This provides dynamic braking of the motor by shorting out the phase shifting capacitor 54 and energizing both windings 52 and 53 on the motor. When either the high or low relay is deenergized, the connectors from terminals 50 and 51, respectively, will be de-energized. This will in turn remove the short across capacitor 54 and permit the armature 55 of the motor to turn in the proper direction to adjust variable transformer 40 so as to adjust the target electrode current and bring the signal pointer back within limits.

When terminals 50 and 51 are both energized, the motor 17 remains inactive, since under these conditions the capacitor 54 is shorted out and the phase relationships between coils 52 and 53 are such as to cancel one another.

Another feature, which may optionally be included, is a provision for manual control. To operate manually switch 23 may be moved to connect conductor 22 to conductor 60 whereupon the power for the control module is cut off. The system may then be controlled manually by depressing spring loaded manual switch 58 or 59 to open one of the two circuits. Switch 59 when opened causes motor 17 to operate so as to increase the potential across electrodes 4 and 6. Switch 58 when opened causes the potential to decrease across the electrodes.

The apparatus may also include high end limit switch 56 and low end limit switch 57. These are to prevent the motor from turning the variable transformer beyond its mechanical stops.

The reversible motor 17 is an instrument-type motor which turns at less than l0 revolutions per minute (rpm.) when activated. A motor speed of 1.4 r.p.m. is satisfactory. However, the motor speed should be chosen to give a response to match the type of electrode and the meter-relay used so that excessive hunting or oscillation is avoided and so that reasonably quick response is obtained.

Other optional equipment shown in FIGURE 2 include a microammeter 43 calibrated in kilovolts provided between the two output terminals of the direct current power pack. The meter is in series with a resistor 44 of appropriate size to limit the meter current for full scale reading at maximum kilovolt output. This resistor must be protected from electrical breakdown and can be installed with the power pack components. The meter will generally indicate a gradual increase in voltage during several days operation if the power pack is adjusted to maintain a set level of target electrode current. This voltage increase is provided automatically by the invention to compensate for the layer of polymer which collects on target electrode 6 and which in the absence of the invention would c-ause decreasing current passage. An equivalent voltmeter can be used.

The electrode circuit may optionally be provided with an ammeter 45 for measuring leakage currents to ground. The ammeter is suitably a microammeter or milliammeter. Neon lamp 46 serves to protect the meter and as a safety device to hold connector 10 at a safe potential with respect to ground in the event of discontinuity in the meter loop. The current reading for meter 45 is equivalent to the current leakage from the high voltage loop to ground plus the current transmitted from multi-pointed electrode 4 to the ber. The total current for the system may be found by adding the current recorded in meter-relay 9 and the current recorded in ammeter 45.

The electrodes employed may be the particular types shown herein (i.e., an ion gun and a iiat target pl-ate) or any other types suitable for use in a corona discharge device adapted to apply an electrostatic charge to fibrous elements. The present invention may be used to improve these devices which encounter the collection of a residue on the electrodes. For example, the apparatus including a round target bar electrode of Disabato and Owens, U.S. Patent 3,163,752 can be adapted to be an embodiment of the present invention. However, it is important that the electrodes used be suitable to produce corona discharge. This is assured by having one electrode with a very small radius of curvature, i.e., about 0.001 to about 0.005 inch, and a second electrode having a large radius of curvature, i.e., greater than about 0.250 inch.

The power source should provide direct current with a potential in the range of 0 to about 100 kilovolts to maintain optimum web charge levels during spinning operation. The term about is used to indicate an approximate upper limit, and one skilled in the art will realize that a potential somewhat above kilovolts may be suitable with out departing from the heart of this invention.

The low voltage terminal may be connected to a microammeter or milliammeter to measure currents reaching ground. However, in an optional modification, the low voltage terminal may be grounded.

Although various types of sensing means can be used, it is important that the sensing means used be able to indicate maximum current ow under normal operating conditions. The range of the sensing means determines, to a large extent, the sensitivity and resolution of control for the particular ilow rate of material to be electrostatically charged.

The aforesaid factors are important in use of the present invention for charging fibrous Webs and for avoidance or correction of variations in ion current ow between two electrodes of the type previously described. By use of the present invention a constant level of ion currentl flow can be maintained.

The present invention is useful in the corona discharge-type charging of discrete dielectric materials including synthetic organic materials such as polyethylene, polyethylene terephthalate, polyhexamethylene adipamide, the caproamides, the acrylic iibers, and indeed all of the synthetics suitable to be employed in the textile art capable of holding a charge. Similarly non-synthetic organic materials may be used. The materials may be in the form of continuous filaments, brillated strands, nonwoven sheets, films, etc. The present invention is particularly useful in charging webs of a continuous brillated strand described in U.S. Patent 3,081,519 to Blades and White.

Many equivalent modifications of the above invention will be apparent to those skilled in the art from a reading of the above without a departure from the inventive concept. The scope of the invention is as dened in the claims.

What is claimed is:

1. An apparatus adapted to receive fibrous elements continuously forwarded in a linear path of advance in a gaseous atmosphere and to apply an electrostatic charge to the elements while maintaining a constant level of ion current flow between two electrodes of a corona charging system comprising,

(l) a direct current power supply producing an output voltage that is adjustable over a range of to about 100 kilovolts, said output voltage developed between a high voltage terminal and a low voltage terminal,

(2) a corona discharge device consisting of a rst electrode having a small radius of curvature and a second electrode having a large radius of curvature, said first electrode conductively connected to said high voltage terminal, and said second electrode conductively connected to said low voltage terminal, said electrodes being situated on opposite sides of said path of said fibrous elements, the distance between said electrodes being sufficiently small to promote corona discharge and suiciently large to prevent disruptive spark discharge,

(3) a sensing means for determining the current flow in one of said conductive connections between one of said Voltage terminals and the corresponding electrode, said sensing means having a current range sufciently large enough to indicate maximum current iiow under normal operating conditions, said sensing means having adjustable high and low current sensors, and

(4) a control means operably connected to be responsive to said sensing means and operably connected to adjust said adjustable direct current power supply arranged to:

(a) increase the applied voltage across said electrodes when said current ow is less than said low current sensor value,

(b) decrease the applied voltage across said electrodes when said current flow is greater than said high current sensor value, and

(c) leave the voltage across said electrodes unchanged when said current flow is in the range between said low and high current sensor values.

2. The apparatus of claim 1 wherein said first electrode is an ion gun consisting of a row of parallel conducting needles iixed rigidly along a line arranged transverse to and completely across the said path of advance with the points of the said needles aimed at the second electrode which is a flat target plate.

3. The apparatus of claim 1 wherein said sensing means is an optical meter-relay in the 0-1000 microampere range.

4. The apparatus of claim 1 wherein said sensing means is in said conductive connection between said low voltage terminal and said second electrode.

5. The apparatus of claim 4 wherein said low voltage terminal and said second electrode are each conductively connected to ground potential and said sensing means is in said conductive connection between said second electrode and ground potential.

6. The apparatus of claim 1 wherein said control means is a reversible motor electrically connected to the said sensing means.

7. The apparatus of claim 6 wherein said operable connection is a mechanical connection of the motor shaft of said reversible motor to the adjustment means of said adjustable direct current power supply.

8. The apparatus of claim 4 further comprising an ammeter conductively connected between ground potential and said conductive connection between said sensing means and said low voltage terminal to measure leakage current in addition to web charging current.

9. The apparatus of claim 1 further comprising a voltmeter between said two terminals of said power supply to indicate any change in voltage due to a build-up of residue on said electrodes.

References Cited UNITED STATES PATENTS 914,892 3/ 1909 Storer 323-1 3,163,753 12/1964 Disabato et al. 3,320,479 5/1967 Owens. 3,335,274 8/1967 Codichini et al 250-49.5

RALPH G. NILSON, Primary Examiner S. C. SHEAR, Assistant Examiner

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