Method Of And Apparatus For Applying An Electrical Charge To A Moving Sheet Of Flexible Material

Abstract

A sheet of flexible material such as photographic film or paper is passed between a pair of rollers of electrically conducting material. One roller is connected to ground, and there is applied to the other roller a direct current voltage of magnitude and sign such as to develop a desired electrical charge condition on the sheet. Advantageously, the other roller has a soft yielding coat thereon of an electrically partially conducting material such as surfactant-treated plush cloth which improves contact with the sheet and prevents sparking if the two rollers should come into contact with one another. The sheet can be in the form of a continuous web or an individual sheet of small dimensions which has been chopped from a web. The electrical charge on a sheet can be monitored by an electrostatic field meter or volt meter, and the sign and magnitude of the applied voltage can be regulated to maintain the desired charge condition. When the sheet of flexible material has a coating of electrically conductive material such as carbon, aluminum, silver or gold on one side it is not necessary to employ the grounded roller adjacent to the charged roller. Instead, the charged roller engages a first side of the sheet opposite the second side carrying the electrically conductive coating, and the second side subsequently passes over a grounded roller such as an idler roller.

Parent Case Text

This application is a continuation-in-part of our application Ser. No. 3,434 filed Jan. 16, 1970 and now abandoned.

Description

The present invention relates to a method of and apparatus for applying an electrical charge to a moving sheet of flexible material such as a continuous web or a small sheet of photographic film or photographic paper.

In the manufacture of such sheets, they are passed between and over rollers, or through other devices, which generate electrostatic charge on the moving sheets. When such a charged sheet is brought near a grounded object, sparking may occur which will be harmful to the sheet. For example, if the sheet has already been coated with sensitized emulsions, the spark may cause localized exposure of the emulsion. Also, sparks occuring in a dark room will expose much more material than the immediate piece being operated on. Furthermore, when such sparks occur on subbed photographic sheets which have not yet been coated with emulsion, they may burn the subbing layers so that the emulsion will not go on properly at a subsequent point in processing. Also, static electricity may be present in the form of islands of charge which will cause skips when the sheet is subsequently coated with emulsion. Static charges also may cause the attraction and accumulation of dust on the sheets, which will be detrimental to quality.

2. The Prior Art

Reference is made to U. S. Pat. No. 3,335,026 which was granted Aug. 8, 1967 to W. F. DeGeest et al. This patent describes the coating of an emulsion on a photographic sheet by passing the sheet continuously around a metallic coating roller adjacent to a pool of liquid emulsion, and charging the coating roller with a high voltage to cause the emulsion to be drawn up to the sheet where it forms a liquid layer. There is also provided a grounded metallic smoothing roller on the opposite side of the sheet, which is spaced a fixed distance from the periphery of the coating roller so as to smooth the liquid emulsion layer to the correct thickness and smoothness. The metallic coating roller is provided with an electrically insulating layer when a conductive sheet is being coated with emulsion. Such an insulating layer will hold a static charge and cause sparking when the two rollers come into contact with one another. This patent, however, does not propose a method and apparatus which will solve the problems set forth.

In British Pat. No. 976,027 there is described apparatus for applying a charge to a web by passing the web over a grounded roller while applying a voltage to the opposite side of the web through conductive bristles of nylon, brass, or the like which are in brushing engagement with the web. While this apparatus is operable, it has the disadvantage of possibly scratching the web, and of depositing particles from the bristles onto the web. Additionally, the bristles may attract and hold particles of dirt which may be deposited on the web. Another disadvantage is that if the web breaks or runs out, direct contact can occur between the bristles and the grounded roller, with the formation of sparks which are particularly detrimental when photographic film or paper is being processed in a darkroom.

Such problems are solved by our novel method comprising applying an electrical charge to a moving sheet of flexible electrically insulating material by transporting the sheet in an essentially dry condition over an electrically charged roller while connecting the opposite side of the sheet to ground. In one modification the sheet is passed between and in contact with a pair of first and second rollers acting as pinch rollers. The first roller has a conducting surface, such as a steel or other metal surface, in contact with one side of the sheet, and is electrically connected to ground. The second roller has an electrically conducting surface which engages the opposite side of the sheet. A direct current voltage is applied to the second roller, and thence through its conducting surface to the sheet, with the voltage being of such magnitude and sign as to develop a desired electrical charge condition on the sheet, for example neutral, or positively or negatively charged.

In another modification, which is applicable to sheets having an electrically conductive coating on one surface eg. carbon or metal, the non conductive surface is in contact with the charged roller and the conductive surface passes over a grounded roller at a position remote from the charged rollers.

We have found it particularly desirable for an operator to determine the magnitude and sign of the static electrical charge on a sheet, and then to apply a voltage of sign and sufficient magnitude so as to neutralize the static charge, thus avoiding sparking when the sheet is subsequently brought near a grounded object. In another mode of operation, a sheet can be precharged to a selected voltage by our method at one station in a production line, and subsequently, after intermediate processing steps, can again be treated by our method to neutralize the charge on the sheet which may or may not have picked up additional charge.

The principles described above can be applied to a long continuous web, or to successive individual sheets of relatively small dimensions which have been chopped out of a long continuous web for packing together into a container.

As will appear hereinafter, important benefits are obtained when the conducting surface of the second roller is a partially-conducting non-insulating material of a soft and yielding nature which yieldingly engages the sheet. By an electrically partially-conducting surface we mean a surface having a surface resistivity between 1 .times. 10.sup.7 and 1 .times. 10.sup.12 ohm per square.

The apparatus for performing the method described above will be described more in detail hereinafter.

THE DRAWINGS

In the Drawings:

FIG. 1 is a schematic side elevational view showing one form of apparatus for developing a desired electrical charge condition on a long continuous web of flexible material, such as photographic film or paper;

FIG. 2 is a schematic side elevational view showing apparatus for chopping a continuous web of photographic material into individual sheets, which are then electrically neutralized and packed in juxtaposition to one another in a container; and

FIG. 3 is a schematic side elevational view of apparatus for charging a web having an electrically conductive coating on one side.

THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a long continuous web 11 of dry photographic film or paper is transported continuously between and in contact with a first metallic roller 13, which is connected to ground, and a second steel or other metallic roller 15 having a soft yielding coat 17 thereon of an electrically partially-conducting material which yieldingly engages the opposite side of web 11. Such a coat 17 provides better contact then would a metallic surface which would tend to have slight irregularities and would have other disadvantages as discussed below. The metallic surfaces are spaced from one another by the web thickness, but the soft yielding coat 17 will be in contact with roller 13 in the absence of the web.

The roller 15 operates in electrically insulating bearings (not shown) which insulate the roller from ground. A suitable variable power supply or source 19 of a direct current voltage is connected to the roller 15 and is provided with voltage sign and magnitude adjusters 21 and 23 for applying the correct voltage to the roller. Any source can be employed, such as batteries, or a D.C. generator, or a rectifier connected to an A.C. power line. Current flow is small, usually less than 1 milliampere.

While the correct voltage to apply can be empirically determined, we prefer to position an electrostatic field meter 25 adjacent to the web 11 after it has passed between the rollers, so as to continuously measure or monitor the electrostatic field resulting from any electrical charge on the web, and to display it on a meter 27. The operator observes the voltage and its sign on the meter 27, and adjusts the voltage supply 19 as needed so as to assure the correct charge and sign. An electrostatic field meter suitable for this purpose is described in the July 1950 issue of the "Journal of the Society of Motion Picture and Television Engineers" in an article by H. W. Cleveland on pages 37-44 entitled, "A Method of Measuring Electrification of Motion Picture Film Applied to Cleaning Operations."

The charge which has built up on a sheet 11 of photographic material may be quite large, resulting in electrostatic potentials of anywhere from 100 to 50,000 volts. The charge may have a positive or negative sign, depending upon the nature of the web material, and of the rollers or other apparatus through or over which it has been passing. Thus, to neutralize, it is necessary to apply a voltage to the roller of opposite sign by adjusting the variable power supply 19. The applied voltage is considerably less than the potential on the sheet because of the roller configuration employed.

The soft yielding partially-conducting coat 17 on the metallic roller 15 can be in the form of a sleeve, or a sheet, which can be shrunk on or held on by an adhesive layer. Preferably coat 17 is a cotton velour plush cloth which has been treated with a surfactant material to make it partially conductive of electricity, and is secured on the roll by an adhesive layer. A suitable surfactant is a sodium salt of an alkylaryl polyether sulfonate sold by Atlas Chemical Company as Triton X 200 E. Plush cloth is particularly good because the fibers provide uniform overall contact with the web surface without leaving small areas untouched, and because there is frictional electrification and a larger transfer of electrical charge than with some other coats. Other suitable partially-conductive materials are rubber containing carbon black, carbon-impregnated cloth, and polyurethane material which has been impregnated with a conductive surfactant.

The rollers are so positioned that the soft yielding coat 17 will be in contact with the surface of roller 13 in the absence of sheet 11. Such contact may occur if the web 11 should break, or run out. Undesirable sparking is prevented by the partially-conducting coat 17, whereas two metallic surfaces on the rollers could cause sparking.

The types of web materials that can be treated by the invention are many and varied such as cellulose triacetate film base or polyethylene terephthalate film base, which may or may not have been coated with sensitized emulsions, subbing layers, or antihalation backing layers. Other webs can be formed of photographic paper which may or may not have coatings of photosensitive emulsions, polyethylene or the like. Our novel method operates effectively when the web material is electrically non-conductive, is in a dry condition, and is of constant thickness before and after passing between the rollers (without a liquid or semi-liquid layer thereon).

The rollers 13 and 15 can be idler rollers, with the web 11 being transported by motor driven rollers located elsewhere; or if desired, either or both of the rollers 13 and 15 can be motor driven.

Our invention is operable whether the web is bent around roller 13 or 15, or passes straight through. The arrangement of FIG. 1, wherein the web is bent around roller 13 is particularly advantageous because essentially line contact of the coat 17 with the web occurs, resulting in superior efficiency of charge transfer and the ability to neutralize a large static potential by applying a smaller voltage to roller 13.

Now referring to the embodiment shown in FIG. 2, the rollers 13' and 15', the variable power supply 19', the field meter 25', and the voltage indicator 27' are all the same as described in connection with FIG. 1.

In this embodiment a web 11' of photographic paper or film is fed through a pair of driven rollers 31, 33 into a chopper 35 for chopping the web into individual sheets 37 of relatively small dimensions which are successively fed out of the chopper and between the rollers 13' and 15' where their static charges are neutralized. They then drop into a container 39 where they are piled up in close juxtaposition one on top of the other until the container is filled. Without the charge neutralizing apparatus, static electricity charges are apt to generate sparks when some of the individual sheets are piled on top of previous sheets, and these sparks have tended to impair quality and to cause exposure of other photographic products in the same room. The surfaces of rollers 13' and 15' will contact one another momentarily between successive sheets, but sparking on contact is prevented by the coat 17' of partially-conducting material.

We have found that a voltage of about -10 volts applied to roller 15' successfully neutralized the charge on sheets 37 which had been shown to exhibit +1,000 volts by meter 25' before neutralization.

The chopper 35 can be of any desired type for repeatedly cutting across the web on appropriate lines to produce finished sheets of the desired small size. One suitable chopper is described in U. S. Pat. No. 3,000,250 which was granted Sept. 19, 1961 to H. C. Altmann et al. Chopper 35 can be either hand operated or motor driven.

FIG. 3 shows apparatus for treating a web 11" whose lower surface carries a coating of an electrically conductive material such as carbon, or a metal such as aluminum, silver or gold. For example, photographic film often carries an electrically conductive carbon-containing antihalation layer coated on one surface.

When treating such a material it is not necessary to pass the web through a pair of pinch rolls as in FIGS. 1 and 2, thus avoiding problems caused by pressure such as possible marking of the web, and undesirable drag; and also permitting the apparatus to operate in more confined spaces than are required for FIGS. 1 and 2.

In this modification, the web 11" is bent around a covered roller 15", of the type previously described, with the web's electrically conductive coating on its first or bottom side out of contact with the roller, and the appropriate charge is applied to the non-conductive second or top side. Subsequently the web 11" passes over an electrically grounded metal roller 43 with the web's conductive bottom surface in contact therewith.

The charge applied by roller 15" on the second side induces a charge of equal magnitude and opposite polarity in the conductive coating on the first side. Hence the charge exists as a double, or dipole layer and cannot be measured by field meters. For this reason a voltmeter 41 is mounted about 1 mm from the web adjacent grounded idler roller 43 as a basis for determining the magnitude of the charge to be applied by roller 15" to develop a desired charge condition on the web.

Successful tests have been conducted with the FIG. 3 modification using rollers 15" covered with ordinary cotton cloth and with plush cloth, both cloths having been treated with well known surfactants to make them semi conductive. Among the surfactants are Tamol brand sodium salt of sulfonated napthalene (Rohm & Haas) and Catanac brand long chain quaternary ammonium salt (American Cyanamid Co.). Another suitable roller material is formed of synthane phenol formaldehyde resin filled with cloth and milled with Tamol surfactant.

A web 11" that exhibited a charge of plus 75 volts at voltmeter 41 was brought back to zero by applying a negative charge of -400 volts to a roller 15" covered with Tamol treated cotton cloth; brought to plus 200 volts by applying a positive charge of 600 volts.

A semi conductive plush covered roller 15" was somewhat more effective, and a semi conductive synthane roller somewhat less effective.

There are several modifications of the invention described above which will be readily apparent to persons skilled in the art. Among the modifications are the following cases:

A. Arrangement of rollers illustrated in FIGS. 1 and 2, with the voltage being applied to the soft covered roller 15 or 15', and the metal roller 13 or 13' connected to ground. To neutralize or reduce the charge on a sheet, a voltage of the opposite polarity to that on the incoming sheet is applied.

B. Arrangement of rollers illustrated in FIGS. 1 and 2 with the voltage being applied to the metal roller 13 or 13', and the soft covered roller 15 or 15' being connected to ground. To neutralize or reduce the charge on a sheet, a voltage of the same polarity as that on the incoming sheet is applied.

C. Case A wherein the positions of rollers 13 and 15 (or 13' and 15') are interchanged, but with electrical connections and operation being the same as Case A.

D. Case B wherein the positions of rollers 13 and 15 (or 13' and 15') are interchanged, but with electrical connections and operation being the same as Case B.

E. Arrangement of rollers and mode of operation as shown and described with reference to FIG. 3.

To summarize some of the improvements provided by the present invention:

Spark discharges are prevented when a sheet of photographic material is grounded, thus avoiding spark damage to the sheet itself, and avoiding the exposure of other photographic materials in the same dark room.

The formation of detrimental islands of high charge density on a sheet of photographic film or paper base is prevented, thus improving subsequent coating operations.

Accumulation of dust particles on a sheet of photographic material can be prevented by reducing the static potential which tends to attract and hold dust particles.

Spark discharges are avoided when two rollers are brought into contact, as when a web breaks or runs out, or between spaced sheets which are passing between the rollers.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

1. A method for neutralizing the static electrical charge on a photographic material to prevent sparking comprising the steps of: transporting said photographic material through the nip of first and second interengaged rollers in which one of said rollers has an electrically conducting surface, the other roller has a soft yielding semi-conducting surface, and one of said first and second rollers is connected to ground; measuring the static electrical charge on the photographic material when it exits from said nip, and in response to said measuring step; applying a direct current voltage of a desired magnitude and sign to the other of said first and second rollers to neutralize the static electrical

2. In an apparatus for neutralizing the static electrical charge on a photographic material to prevent sparking, the combination comprising: a first roller having an electrically conducting surface; a second roller having a soft yielding surface of semiconducting material in engagement with said first roller; means for applying to one of said rollers a direct current voltage of an adjustable magnitude and sign; means for electrically connecting the other of said rollers to ground; means for transporting a photographic material through the nip formed by said first and second rollers; means adjacent said photographic material when it exits from said nip for measuring the electrical charge on said material; and means responsive to said measuring means for adjusting the magnitude and sign of said direct current voltage applied to said one roller whereby the

3. The invention according to claim 2 wherein said soft yielding surface of semi-conducting material comprises a cotton velour plush cloth which has

4. The invention according to claim 2 wherein said direct current voltage is adjusted substantially between -400 and +600 volts.