US3790854A

US3790854A - Apparatus for removing static charge from webs of material

Info

Publication number: US3790854A
Application number: US00178313A
Authority: US
Inventors: K Dryczynski; H Kramer; D Messner; W Seifried
Original assignee: Kalle GmbH and Co KG
Current assignee: Kalle GmbH and Co KG
Priority date: 1970-09-10
Filing date: 1971-09-07
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05
Also published as: NL7111963A; DE2044828B2; CA942374A; DE2044828A1; LU63884A1; GB1361835A; FR2106420A1; BE772316A; FR2106420B1

Abstract

This invention relates to a method and apparatus for discharging a statically charged web of material. The method comprises bombarding the web with charge carriers produced by a highfrequency A.C. voltage of 1 to 1,000 kilocycles per second and having a current intensity of between a few Mu amp./cm of web width and several hundred Mu amp./cm of web width.

Description

United States Patent [191 Dryczynski et al.

[ Feb. 5, 1974 APPARATUS FOR REMOVING STATIC CHARGE FROM WEBS OF MATERIAL Inventors: Kurt Dryczynski; Heinz Kramer;

Dieter Messner; Walter Seifried, all of Wiesbaden-Biebrich, Germany Assignee: Kalle Aktiengesellschaft, Wiesbaden,

' Biebrick, Germany Filed: Sept/7, 1971 Appl. No.: 178,313

1 Foreign Application Priority Data Sept. 10, 1970 Germany 2044828 us; Cl 317/2 F,-317/4 Int. Cl Host 3/04, HOlt 19/00 Field of Search 317/2 R, 2 A, 2 c, 2 F, 4,

[56] References Cited UNITED STATES PATENTS 3,376,208 4/1968 Wood 317/4 3,634,726 l/l972 Jay 317/2 F 844,300 2/1907 Chapman 317/2 F 2,867,912 l/l959 Kritchever 317/2 F 2,879,396 3/1959 McDonald 317/2 F Primary Examiner-14. T. Hix Attorney, Agent, or Firm-James E. Bryan, Esquire ABSTRACT 29 Claims, 11 Drawing Figures PAIENIEB 5: 3.790.854

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T sum n ur 4 INVENTORS KURT DRY CZYNSKI HEINZ KRAMER DIETER MESSNER WALT ER SEIFRIED ATTORNEY v APPARATUS FOR REMOVING STATIC CHARGE FROM WEBS OF MATERIAL below, fof example, and as used inter alia as ele trically insulating films, charged webs cannot be processed at all, since they adhere at all points, or the adhering dirt is pressed through the material by the individual layers during winding, and this renders the material unusable.

Thus, there have been many attempts to remove the charge from webs before subjecting them to further processing. For example, use has been made of radioactive charge eliminators involving the ionization of the air by means of a radioactive isotope, and preferably, by means of an a-ray producer (e.g. polonium 210).

Apart from ra dio'active rays, X-rays of suitable wave length or other ionizing rays can be used for ionization of the air. If these charge eliminators are to be sufficiently effective, the intensities of the rays must be relatively high. In the practical use of apparatus of this kind it is therefore essential to provide and maintain expensive means affording protection against radiation. At the industrial level however, such means frequently prevent or interfere with the production cycle and the processing operation.

A further disadvantage arising in the use of the above-mentioned rays is the fact that, as a consequence of their high intensity, they cannot be used on all materials since they may cause changes in the material.

Also known for the removal of electrostatic charge are point ionizing apparatuses. When these are placed opposite a charged surface, they develop so great an electrical field strength at the gounded points that discharge occurs. A characteristic feature of these point ionizers is that before they become effective they require a certain minimum charge. Relatively small static charges are therefore not removed by such apparatus.

In addition to the passive point ionizers, active ionizers, operating on high voltage, are frequently used. Here, an A.C. voltage is applied between the points of the high-voltage ionizer and ground. This causes a corona discharge to occur at the points, whereby the air is in turn ionized and the charge is removed from the affected surfaces.

Despite their many external forms, all embodiments of this type of ionizer are based on similar construe-- tional features. They always include metallic points which operate with an A.C. voltage having a frequency of, preferably, 50 cycles per second. The geometric form and arrangement of the points, the type of metal and the thickness of the points can be varied within wide limits. Also available are certain special constructions in which the corona v,discharge takes place inside air jets, and the ions produced are blown by an air stream towards the surface from which the static charge is to be removed.

The inclusion of individual points in the construction of the abovedescribed active and passive ionizers leads, of necessity, to an uneven and irregular removal of charge from the materials to be treated. For example, it can be shown by applying suitably charged colored powder and also by means of fine resolution probe instruments, that on films of synthetic material, the discharge effect is greatest in the immediate vicinity of the points but is considerably less, and in some cases even .stems from the following fact: if the materials, for example films of synthetic material, from which the charge has been removed, are moved past the charge eliminators at high speeds such as frequently obtained in modern manufacturing processes, then in the direction in which the web of film is moved there occur periodically repeated patterns comprising zones from which the charge has been removed and zones from which the charge has either not been removed or removed to an insufficient extent.

With each A.C. voltage corona, the ions, positive and negative, according to the frequency, are continuously successively offered to the surface to be discharged. In the case of rapidly moving webs of film, with a high relative velocity between the eliminator and the material to be discharged, there therefore occurs a spatial separation between the zones of material bombarded by the negative and positive ions, this separation being dependent upon the speed. Furthermore, maximum ionization occurs only at the peakvalues of the A.C. voltage.

In the intermediate zero cross-overs of the sine wave on the other hand, all ionization effect disappears.

In the case of some of the more modern equipment, efforts have been made to eliminate these difficulties by, for example, replacing the sine-wave A.C. voltage by square-wave voltage. Although certain improvements can be effected in this way, it has not however generally been possible to solve satisfactorily the problem of uneven discharge of materials that are moved particularly rapidly.

The problem has thus arisen of providing a method whereby an even and satisfactory discharge of webs of material can be achieved in a technically simple manner without however affecting the chemical and/or physical properties of the treated object. 7 The present invention provides a method of discharging a statically charged web of material, wherein the web is uniformly discharged to the extent of at least 99 per cent of the original charge by bombardment, over the entire width of the web, under conditions having no deleterious effect on the material, with charge carriers as herein defined which are produced by a highfrequency A.C. voltage of between 1 and 1,000 kilocycles per second and having a current intensity of between a few 1. amp./cm of web width and several hundred p. amp/cm of web width.

The term charge carriers is to be understood to mean gas ions and electrons.

Obviously, to take into account the charge carriers that do not strike the web of material, the current intensity and/or the distance between the web of material and the location from which the charge carriers emanate must be so adjusted that for each unit of area at least that number of charge carriers are produced with a polarity opposite to that of the charge on the web as is necessary for discharging to the required extent.

With the method of the invention it is possible to discharge to the required extent webs of material that may carry a charge of up to the limiting field strength (approximately in the range of 1,000 to 2,000 kV/m). In many cases, residual field strengths of l per cent and less no longer have a critical effect and thus can be ignored. If, however, it is required to remove them completely, the method is preferably carried out several times in succession, in which case, as will be described later, various forms of apparatus can be used.

Frequencies of from 5 to 50 kilocycles per second are preferably used, and depending upon the way in which the method is applied, the frequency selected must in some cases be such that it does not occur as an interference frequency.

In a further embodiment of the method, the charge carriers are preferably produced with current intensities of between rt-amp. and 100 [L amp/cm of web width.

other Another of the method resides in producing the charge carriers not in air, as is customary, but in other readily ionizable gases. In some cases the gas also may be heated.

' The removal of charge in hot gases, preferably inert gases, results in further improvement in the removal of residual charges.

The invention also provides apparatus for performing the method. The apparatus includes at least one wire havinga thickness of less than 100 pand preferably 5 to 50 p. or system of such wires, which wire or wire system is dimensioned to extend over the entire widthof the web and arranged to be supplied with AC. under the conditions set forth above. A preferably adjustable A.C. generator is connected to the wire or wire system and the counter electrode is preferably grounded.

Depending upon the requirement as to the amount of any residual charge, the apparatus described may be repeated in series, a preferred arrangement being one in which the apparatus is fitted on both sides of the web.

A wire system may include a plurality of wires arranged alongside and preferably parallel with one another, and/or located in different planes and offset from one another. An arrangement of this kind has .proved successful especially in the case of webs of material that. are moved particularly rapidly.

Another wire system may include a meandering disposition of the wires.

Since the very thin wires used in'the apparatus of the invention are very heavily loaded when current passes through them, it may be advantageous in the case of certain wire materials and large widths of web to mount the wire or wires in insulating support elements to prevent sagging and thus lack of uniformity in the wire arrangement in relation to the plane of the web.

In a further form of the apparatus, the wires are placed in protective elements which on the one hand are intended to prevent damage to the wires when, for example, a web tears, and on the other hand influence the flow of the charge carriers, i.e., the corona characteristic, to some extent, as a result of the way in which they are arranged in relation to the wires and depending upon whether the material used is conducting or non-conducting.

In a preferred arrangement, the protective elements consist of a base plate containing a number of channels corresponding to the number of wires. Depending upon design, the channels may be of particular crosssectional form, rectangular or semi-circular channels being preferred since these are easy to produce.

In the case of rectangular cross-sections, it has been found expedient so to arrange the wires so they are spaced by a distance of 5 to 10 mm, and preferably 6 to 8 mm from the side walls, and 10 to 25 mm, and preferably 15 to 20 mm, from the floor of the channels. In the case of semi-circular cross-sections, the distance between the wires and the floor of the channels is preferably up to 25 percent less than that separating them from the side walls of the channels.

In another form of the apparatus, a dielectric is fitted between the wires and the counter electrode so that depending upon material and thickness, the dielectric, for example in the form of thin films of synthetic material, enables the corona emission to be varied in a predetermined manner. Instead of, or in addition to, the protective elements, a conducting or non-conducting relatively wide-meshed protective grid may be fitted between the wires and the web, and the corona emission likewise may be influenced by this grid, where necessary by imparting an appropriate profile to it and by using particular materials of construction. For some applications it also has been found expedient for controlling the discharge characteristic to combine the grid and/or the protective elements with the counter electrode, or to use the protective elements themselves as the counter electrode. Particularly in the case of very long and very thin wires, preference is given to arrangements embodying means that apply tension to the ends of the wires. In this way, the wires, when under load, remain at a uniform distance from the plane of the web of material. In particular, springs are used as the tensioning elements because of the ease with which they can be fitted.

In a further form of the appatatus, there are provided means for generating the hot gases in the zone between the web and the wires. This may be achieved for example by means of heat radiators or hot-gas producers through which the hot gases can be blown into the zone.

Since in many cases in practice similar webs of material, the state of charge of which is'known, are consecutively treated when carrying out the method, it is frequently not necessary for the wires to be displaceable towards the plane of the web of material, but for the purpose of greater adaptability, an arrangement in which the level can be varied is preferred. In practice, it'has been found expedient with a generated current intensity of between 10 and p. ampJcm of webwidth to keep the distance between the wires and the web at 10 to 40 mm, and preferably 15 to 25 mm.

A particularly advantageous arrangement is constituted by a variant of the apparatus that uses a roll, since in many production lines rolls are in any case used as the elements for guiding the webs. In this case, the wire or wires are fitted inside at least two rotatable rolls containing openings, these rolls being arranged one behind the other or one above the other, as in the generally known roll systems. The number and area of the openings should be as great as possible so as to enable as many zones as possible to be discharged as the material passes over the roll. Those zones not discharged at the first roll are discharged at the second roll, the openings in the roll of course having to be so positioned during the travel of the material that in the case of subsequent rolls those zones that were not bombarded with charge carriers during the first pass because of the presence of areas between the openings, are then dealt with.

In a further form of the arrangement using rolls, the corona wires are distributed over the entire circumference of the roll, in which case the corona affects a lengthwise zone that depends upon the looping arc around the roll. The wires may be fitted in a fixed position or they may be mounted to rotate with the rolls, in the latter arrangement, current is supplied through slid- ;ing contacts.

An advantageous arrangement isone employing a roll made of thin resistive wires. This apparatus possesses the advantage that the openings are relatively large so that the use of two rolls generally suffices for the purpose of removing the' charge. In order also to enable the rear side of the film to be discharged, it is expedient to fit a further discharging apparatus opposite the roll. To enable operation in a particularly uniform manner, corresponding to the track of the web, this second charge-removing apparatus is preferably of curved form, the curvature in particular corresponding to the radius of the roll.

The following Examples 3 to 8 illustrate the invention, Examples 1 and 2 being given for the purpose of comparison:

LQRLE. .L

A commercially available point-type corona apparatus with a distance betweer. points of 20 mm and a wire thickness of 1.5 mm (the ends having points) was positioned 20 mm above, and over the entire width of, a p. thick web of polyester (as sold under the Registered Trademark HOSTAPHAN), moving at a speed of 100 meters per minute (m/min.). As a result of its passage through a roll system, the web of film material had a charge having a field strength of approximately 900 kV/m. The apparatus was operated at a high voltage of 6 kV and a frequency of 50 cycles per second. The current intensity was 2 p. amp./cm of web width. After passage through the corona zone, a residual charge having a field strength of 22 kV/m was determined by means of a surface probe instrument.

.ElAMBkEl Example 1 was rgeated u nder the same conditions ample 2, considerably larger intermediate zones not discharged were observed as a result of the low frequency. Since only certain zones had been relieved of charge, these films exhibited the same disadvantage as untreated webs of material, since the adhesion effect on particles of dirt for example is still present in the intermediate zones to the same extent or atleast to a much greater extent than would correspond to complete removal of the charge.

EXAMPLE .Emnl 3 w s rsaemi w th tbsdiff s w five parallel wires were used. The field strength of the residual charge was 2 kV/m.

E LAMPUQM 1 Example 4 was repeated but with the difference that 30 Example 5 was repeated but with the difference that apparatus as in Example 3 was arranged on both sides of the web of film material. The field strength of the residual charge was less than 1 kV/m.

ELEM QEJM A 6 u-thick film of polyester was passed through a system of rolls at a speed of m/min. The field strength of the charge on the film was 1,100 kV/m. The film was passed over four wire rolls having a diameter .of 500 mm, the first and-third rolls being in contact with the underface of the film, and the second and fourth rolls being in contact with its top face. The wires constituting the rolls were 1 mm thick and made of V4 A material, andonboths ides they were mounted in rfiigsat disaices apart of 2 mm. mcorfi wires hav ing a thickness of 30 p. were distributed over the upper half of each of the first and third rolls, and over the lower half of each of the second and fourth rolls. The wires were held fast and tensioned at both ends by springs. The other details were the same as those of Example 3. The driven rolls were so controlled that as the web passed through, there occured each time, in the case of the second roll of a pair, an aperture (gap between the wires) at that place where, in the case of the first roll of the pair, no discharge had taken place as a result of the web bearing on the wires. The field strength of the residual charge was I kV/m.

EXAMPLE 8 Example 7 was repeated but with the difference-that there was also positioned opposite each of the rolls a counter electrode which likewise comprised five 30 u-thick corona wires which were accommodated in a rectangular protective element. In this case, the field strength of the residual charge was only 0.2 kV/m.

As the Examples show, the method of the invention, in contrast to the known methods, enables an even and approaching complete discharge to be effected. Inter mediate zones in whichthe charge is not satisfactorily removed could not be found, despite the use of a fine resolution probe (for example, a probe as disclosed in German Pat. specification No. 1,281,573). Even at higher speeds of travel of up to approximatley 500 m/min. complete and even removal of the charge could be achieved.

Although the removal of static charge in accordance with the invention has been illustrated in the Examples using polyester films, the use of the method is not lim ited to this material. It can be applied in principle to all statically charged webs of material, for example, webs of paper as well as webs of synthetic material. However, it is advantageously used in the case of particularly sensitive webs such, for instance, as the thin polyester webs mentioned in the Examples. It will be readily appreciated that the current intensity, the gap and the frequency can be varied over a wide range within the scope of the invention in order to suit particular requirements.

The apparatus of the invention is illustrated diagrammatically by way of example in the accompanying drawings, in which:

FIG. 1 shows the principal components of one embodiment of discharge apparatus,

FIGS. 2 to 6 are plan views of different examples of the corona wire arrangement,

FIGS. 7 and 8 are elevations showing how the corona wires can be located in protective elements,

FIGS. 9 and 10 are cross-sections of discharge apparatus in roll form, and

FIG. 11 is a cross-section of an arrangement for discharge of both sides ofa web by means ofa roll system.

Referring to the drawings, FIG. 1 shows an impulse generator 1, which produces square wave impulses of a sequence frequency in the range envisaged by the invention, these impulses being amplified by an amplifier 2. The signal is amplified in a power amplifier 3 and passed to the primary winding of a high-voltage trans-' former 4. The high-voltage terminal of the secondary winding is connected to a corona wire 5. A counter electrode 6 is grounded. A web 7 of material is passed below the corona wire and at a certain distance therefrom. The effective value of the corona voltage is measured in kV on a high-voltage measuring instrument 8. The high-voltage amplitude can be varied at the impulse generator. The current intensity is determined in an independent measurement by means of suitable instruments, and adjustments in accordance with these measurements can be made.

In FIG. 2, the corona wires 5 are held in the tensioned condition in a frame 9. Springs 10, which apply tension to the wires are provided at both ends of the WII'CS.

In FIG. 3, a wire is mounted to zig-zag around points 11 provided on the frame. The tension is produced by means of a spring 10.

FIG. 4 shows four corona wires 5 which, in contrast to the arrangement seen in FIG. 2 however, are arranged closer together and are additionally held by means of three insulating support elements l2.

FIG. 5 shows five individual corona wires 5which' are each tensioned at each end by springs '10 and are staggered in relation to each other. This provides the advantage that, although the entire width of web is covered, shorter lengths of wire can be used.

FIG. 6 shows the n're arrangement seen in FIG. 2, but with a protective grid 13 also fitted.

FIG. 7 illustrates a rectangular form of protective element 14, and FIG. 8 shows a protective element of semi-circular form. The corona wires 5 are protected in channels 15 which are defined by walls 16 and a floor 17. When suitably arrang ed, the protective elements can be used directly as the counter electrode. Also, a dielectric, eg in the form of a lining of synthetic film, can be fitted between the corona wires and the interior of the protective elements. 7,

FIG. 9 shows a detail of a discharge apparatus in the form of a roll, and also shows a second discharge device disposed opposite the first. The web 7 of the material is passed over the roll which consists of an arrangement of rods 18. The corona wires 5 are fitted inside the roll and distributed around its entire circumference. A counter electrode, 'which consists of a 'protectiveelement 14 with a corresponding number of corona wires 5, is of curved shape. In the case of the roll here illustrated, the corona wires are preferably also rotatably mounted. V

FIG. 10 shows practically the same roll as that of FIG. 9, but with the corona wires 5 positioned only't'o correspond with the looping arc of the web 7 of material. In an arrangement of this kind, the corona wires are immobilized in relation to the rotatable roll.

FIG. 11 illustrates a roll arrangement I, II, III, and IV, for discharging both sides of the web of material, and by means of which particularly low residual charges remain. The rolls, which consist of thin rods 19, house co- 7 rona wires 5 and these are fixed in such a way that the corona is produced only over the looping arc of the web. The electrodes disposed opposite the rolls are the same as those shown in FIG. 9. Rollers 20 and 21 are guide rollers known per se.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. A method of discharging a statically charged web of material which comprises bombarding the web with charge carriers produced by a high-frequency alternating current of 5 to 50 kilocycles per second and having a current intensity of between a few [1, amp/cm of web width and several hundred p. amp/cm of web width.

2. An apparatus for discharging a statically charged web of material which comprises at least one un coated wire means having a thickness less than [L and being adapted to extend over the width of the web, means for supplying high-frequency alternating current of 5 to 50 kilocycles per second to said wire means, and counter-electrode means on the same side of the web as said wire means. I

3. A method as claimed in claim I, wherein the bombardment is carried out with charge carriers which are produced with a current intensity of 10 to 100 p. amp/cm of web width.

4. A method as claimed in claim 1, wherein the bombardm'ent is carried out with charge carriers which are produced in a' readily ionizable gas.

'5. An apparatus as claimed in claim 2, in which the wire thickness is to 50p.

6. An apparatus as claimed in claim 2, in which a plurality of wires are arranged alongside one another.

7. -An apparatus as claimed in claim 2, in which the wire means is arranged in zig-zag form.

8. An apparatus as claimed in claim 2, in which the wire means are mounted in insulating support elements.

9. An apparatus as claimed in claim 2, in which the wire means are fitted within protective elements.

10. An apparatus as claimed in claim 9, in which a protective element for the wire means is a plate containing channels corresponding in number to the number of wires.

11. An apparatus as claimed in claim 10, in which the channels are rectangular.

12. An apparatus as claimed in claim 10, in which the channels are semi-circular.

13. An apparatus as claimed in claim 10, in which the distance of the wire means from the side walls of the channels is 6 to 8 mm, and from the floor of the channels is 15 to 20 mm..

14. An apparatus as claimed in claim 12, in which the distance of the wire means from the portion of the channel constituting the floor thereof is up to 25 percent less than their distance from the portions of the channel constituting the side walls thereof.

15. An apparatus as claimed in claim 2, in which a dielectric is located between the wire means and the counter-electrode.

16. An apparatus as claimed in claim 2, in which a relatively wide-mesh protective grid is fitted between the web of material and the wire means.

17. An apparatus as claimed in claim 16, in which for controlling the discharge characteristic the protective grid is connected to the counter-electrode.

18. An apparatus as claimed in claim 2, in which the wire means has tensioning means at the ends thereof.

19. An apparatus as claimed in claim 18, in which the tensioning means are springs.

20. An apparatus as claimed in claim 2, in which means are additionally provided for heating the gas zone between the web of material and the wire means.

21. An apparatus as claimed in claim 2, so adapted that in the case of a generated current intensity of 10 p. amp. to p. amp/cm of web width, the distance of the wire means from the path of the web from which the static charge is to be removed is 15 to 25 mm.

22. An apparatus as claimed in claim 2, in which a plurality of wires are arranged in different planes and are offset from one another.

23. An apparatus as claimed in claim 2, in which the wire means are fitted inside at least one pair of rotatable rolls containing openings, the arrangement being such that the web from which the charge is to be removed can be passed over the rolls in succession.

24. An apparatus as claimed in claim 23, in which the wire means are distributed over the circumference of the rolls.

25. An apparatus as claimed in claim 23, in which the wire means are so held that they do not rotate with the roll.

26. An apparatus as claimed in claim 23, in which the rolls are made of thin wires.

27. An apparatus as claimed in claim 23, in which a further discharge apparatus is fitted opposite the rolls.

28. An apparatus as claimed in claim 27, in which the further discharge apparatus is of curved form.

29. An apparatus as claimed in claim 28, in which the' further discharge apparatus has a curvature corresponding to the radius of the rolls. =1:

Claims

1. A method of discharging a statically charged web of material which comprises bombarding the web with charge carriers produced by a high-frequency alternating current of 5 to 50 kilocycles per second and having a current intensity of between a few Mu amp./cm of web width and several hundred Mu amp./cm of web width.

2. An apparatus for discharging a statically charged web of material which comprises at least one uncoated wire means having a thickness less than 100 Mu and being adapted to extend over the width of the web, means for supplying high-frequency alternating current of 5 to 50 kilocycles per second to said wire means, and counter-electrode means on the same side of the web as said wirE means.

3. A method as claimed in claim 1, wherein the bombardment is carried out with charge carriers which are produced with a current intensity of 10 to 100 Mu amp./cm of web width.

4. A method as claimed in claim 1, wherein the bombardment is carried out with charge carriers which are produced in a readily ionizable gas.

5. An apparatus as claimed in claim 2, in which the wire thickness is 5 to 50 Mu .

7. An apparatus as claimed in claim 2, in which the wire means is arranged in zig-zag form.

11. An apparatus as claimed in claim 10, in which the channels are rectangular.

21. An apparatus as claimed in claim 2, so adapted that in the case of a generated current intensity of 10 Mu amp. to 100 Mu amp./cm of web width, the distance of the wire means from the path of the web from which the static charge is to be removed is 15 to 25 mm.

29. An apparatus as claimed in claim 28, in which the further discharge apparatus has a curvature corresponding to the radius of the rolls.