Title:
REMOVAL OF ELECTRIC CHARGES
United States Patent 3717791
Abstract:
Electric charges on moving webs are removed by condensing such a very small quantity of liquid within a confined region of the surface that the temporary increase in moisture remains very small compared with the total amount of moisture of the material.
Inventors:
Heyl, Gerhard (Leverkusen, DT)
Luttgens, Gunter (Leverkusen, DT)
Behr, Rolf (Leverkusen, DT)
Maus, Fritz (Odenthal-Hannenberg, DT)
Frenken, Hans (Leverkusen, DT)
Busch, Josef (Bensberg-Refrath, DT)
Bruck, Herbert (Leverkusen, DT)
Schaffer, Helmut (Opladen, DT)
Application Number:
05/125477
Publication Date:
02/20/1973
Assignee:
Agfa-Sevaert Aktiengesellschaft (Leverkusen, DT)
International Classes:
H05F3/00; H05F3/00; (IPC1-7): H05F3/00
Field of Search:
317/2R,2A,2C
Primary Examiner:
Hix L. T.
Claims:
We claim
1. A process for removing electric charges from a moving non-conductive foil comprising the steps of briefly condensing a small quantity of water on a restricted area of the foil by causing contact between the restricted area of the foil and a quantity of air having a dew point above the temperature of the restricted area whereby the electrical resistance of the foil is temporarily decreased by several powers of ten and electric charges are rapidly conducted away from the surface of the foil, the condensed quantity of water being capable of rapid evaporating the condensed water from the foil within a few seconds to dry the foil.
2. A process according to claim 1 characterized in that the charges in the region of increased conductivity are conducted away by means of a conductor which makes contact with the surface.
3. A process according to claim 1 characterized in the quantity of air which has a dew point above the temperature of the surface of the foil is briefly blown on to the surface of the material.
4. A process according to claim 1 characterized in that the surface of the foil is briefly cooled to such an extent that its temperature is below the dew point of the surrounding quantity of air.
5. A process according to claim 3 applied to moving foils of material, characterized in that an element of the surface of the foil is exposed to a stream of warm moist air for a length of time so that by the time the surface leaves the zone of warm moist air its temperature has risen above the dew point of the moist air.
6. A control process for achieving a specified and constant state of charge according to claim 3, characterized in that the state of charge is measured with a field intensity measuring device known per se after the surface of the foil has been discharged, and, if the state of charge exceeds a certain value, the absolute moisture of the quantity of air blown on to the surface of the material is increased accordingly.
7. An apparatus for removing electric charges from a moving web strip or foil of nonconductive material comprising a below ambient temperature cooling means disposed in heat exchange relationship with said material whereby a concentrated area of the surface of said material is cooled below the dew point of the air, and a small quantity of moisture is condensed upon the surface of said material whereby its electrical conductance is increased for discharging an electrical charge from said material.
8. An apparatus as set forth in claim 7 wherein said cooling means comprises a source of relatively cold air and a nozzle for directing said cold air from said source upon a concentrated area of said material.
9. An apparatus as set forth in claim 7 wherein said cooling means comprises a guide roller disposed in contact with said material and a cooling means for removing heat from said guide roller and said concentrated area of said material.
10. An apparatus as set forth in claim 7 wherein an electrically conductive roller is disposed in contact with said material after contact with said cooling means for discharging said electrical charge.
11. An apparatus as set forth in claim 8 wherein said nozzle is slotted.
12. An apparatus as set forth in claim 8 wherein a source of relatively warmer air is provided, a second nozzle being connected to said source of relatively warmer air, said second nozzle being directed upon a portion of said material after it has contacted said relatively cooler air whereby said material is heated above the dew point to dry it.
13. An apparatus as set forth in claim 12 wherein an electrically conductive roller is disposed in contact with said material between said sources of relatively colder and relatively warmer air for conducting said charge away from said material.
Description:
The invention relates to a process for the removal of electric charges from non-conductive materials by moistening the surfaces of the materials. The process according to the invention is especially suitable for use on continuously moving webs of material. When sheet webs, bands or foils consisting of a non-conductive material are being worked up, electric charges are liable to cause considerable difficulties and they constitute an uncontrollable source of danger.
A simple method of preventing electrostatic charges consists in working up the web of material in a very moist or wet state. For this purpose, the web is passed through a moist atmosphere at least at its critical points so that the electric conductivity of the whole material is greatly increased owing to the uptake of moisture. In many cases, however, this simple solution cannot be employed because subsequent working up of the web of insulating material must be carried out in a dry state. These requirements apply particularly to the multiple coating of photographic film supports with photographic emulsions if the individual layers are applied one after the other. The process of printing on insulating materials also generally requires an almost dry surface. It should also be noted that electrostatic charges are particularly critical in the case of photographically coated strips or webs because the charges are liable to cause uncontrolled discharges, so-called flash discharges, which result in exposure of the material and hence damage the product. When coating moving webs of insulating material with combustible solutions or dispersion (intaglio printing, magnetic sound tapes), electrostatic discharge phenomena constitute a serious hazard due to the risk of fire.
It is the object of the invention to remove electric charges on the surfaces of non-conductive materials. The removal of charge should take place uniformly over the whole surface and the disadvantages described above of the methods of moistening hitherto employed should be obviated.
According to the invention, this problem is solved by means of the fact that a small quantity of liquid is quickly condensed within a confined area of the surface of the insulating material, the quantity of liquid being so small that the increase in the moisture of the material remains very small compared with the total amount of moisture present. It has been found that such a brief surface moistening is sufficient to about equalization of opposite charge carriers.
If the material carries an unipolar charge, the process according to the invention can be improved by conducting away the charges in the region of increased conductivity by means of a conductor which makes contact with the material.
Two alternative methods are provided for carrying out this brief moistening according to the invention. In the first method, air which has a dew point above the temperature of the surface of the material is blown briefly on to the surface of the material. In the second method, the surface of the material is briefly cooled to a temperature below the dew point of the surrounding air.
In the case of moving webs of material, a further technically important development of the invention consists in that a surface element of the web is exposed to a stream of warm, moist air until the temperature of the surface of the web lies above the dew point of the moist air when the surface leaves the zone of moist air. In this way, condensation takes place in the first section of the zone of moist air whilst drying begins in the second section. This effect may be important in cases where a condensed film of liquid is liable to give trouble at subsequent stages of working up.
In many cases, it is not necessary for the electrostatic charge to disappear completely and it is sufficient to keep the density of the charge on the surface constantly at a low value. For this purpose, the process according to the invention for removing the charge is combined with a control circuit. The control circuit is based on the principle that the state of charge is measured with a known field strength measuring device after the surface of the material has been discharged, and, if the charge exceeds a certain predetermined value, the absolute moisture content of the air blown on to the surface of the material is increased accordingly.
The apparatus according to the invention for carrying out the process on moving webs of material is characterized by a nozzle which extends over the whole width of the web and which emits a stream of cold air.
Another apparatus for brief cooling consists of a cooled web guide roller which removes heat from the web at its point of contact.
According to a further development of these apparatus, the contact device for discharging is in the form of a contact roller which is arranged in the region of increased conductivity.
At least one slot nozzle from which moist air streams on to the web is advantageously arranged in front of the contact roller.
A modification of the apparatus according to the invention is used for webs which run off a roll. In this case, the contact element is in the form of a contact roller which is adapted to lie on the roll. Furthermore, a slot nozzle from which moist air streams on to the web is arranged in front of the contact roller.
A further development of the apparatus according to the invention consists of a plurality of slot nozzles arranged along the length of the web. These nozzles are subdivided into a first group from which cold air is directed on to the web and a second group from which warm air or warm moist air is directed on to the web, and a contact roller discharging arranged in the region of the s second group of nozzles. With this arrangement, the web can be redried or conditioned after the discharging process. This is frequently necessary in the production of photographic materials.
In contrast to the known method of removing the charge by moistening the charged material, in which the total moisture content of the material is considerably increased, the method according to the invention of briefly moistening the material leaves the moisture content of the material practically unchanged. Photographic materials which had a high surface resistance temporarily became so conductive after application of the process of the invention that charge dissipation took place.
The process according to the invention and the apparatus according to the invention will now be described more fully with reference to examples and drawings. In the drawings:
FIGS. 1 to 5 are diagrammatic sketches of various pieces of apparatus;
FIG. 6 shows the principle of a measuring device for determining the changes in conductivity of web-shaped samples during and immediately after the brief moistening; and
FIGS. 7 to 9 are graphs of the results obtained, showing the changes in conductivity as a function of time.
FIG. 1 shows the simplest arrangement for briefly moistening a moving web of material. The web 1 is carried over guide rollers 2 and exposed within a confined area to a stream of cold air emitted from the nozzle 3. The web in this area is temporarily cooled to such an extent that after leaving the area of cold air, condensation takes place on its surface. The condensed moisture in this case is derived from the surrounding air in the atmosphere. An additional nozzle from which moist air or air from the surrounding atmosphere is directed on to the web may be arranged behind the nozzle 3 (viewed in the direction of movement of the web). This accelerates the condensation process. The dew point of the surrounding air or of the air emitted from the nozzle must lie above the temperature of surface of the web. The nozzles are in the form of slots and extend over the whole width of the web.
According to FIG. 2, the web 1 is carried over a cooled web guide roller 4 which removes heat from the area of web which extends over the angle of contact of the roller. When cooling has been effected, the web 1 is exposed to a stream of atmospheric air or moist air from the nozzle 5. This again causes the condensation process to be accelerated. In principle, condensation could again be effected here from the air in the surrounding atmosphere.
The same conditions as regards the necessary cooling and the air blown on to the material apply as in the first embodiment.
A similar arrangement is shown in FIG. 3. In addition, the earthed or grounded contact roller 6 is provided in this case in the region of increased conductivity.
In FIG. 4, the web 1 (1') is run off a roll 7. The grounded contact roller 6 in this case lies loosely on the roll 7 and the web is passed between them. In front of the contact roller is the slot nozzle 5 from which warm moist air is blown on to the web. Since the dew point of this moist air again lies above the temperature of the surface of the web, condensation takes place in the area of contact with the grounded contact roller, so that the surface conductivity is increased and the charges can be conducted away. When the web is moved in the direction indicated by the reference numeral 1, the point at which the web leaves the roll lies on the contact roller 6, whereas when the web is moved in the direction indicated by the reference numeral 1' this point lies beyond the contact roller. In this case, therefore, separation of the web from the roll takes place at a later point in time. The contact roller 6 may, of course, also be arranged outside the roll 7, within the area of increased conductivity, but the arrangement shown in FIG. 4 is the most advantageous from a mechanical point of view.
FIG. 5 shows a further development of the apparatus according to the invention which consists of a plurality of slotted nozzles 3 and 5 arranged along the length of the web, strip or foil 1. These nozzles are subdivided into a first group of nozzles 3 from which cold air is directed unto the web 1 and a second group of nozzles 5 from which moist warmer air is directed unto the web, strip or band 1. A contact roller 2 for discharging electrical charges is arranged in the region of the second group of nozzles 5. Nozzles 3 are connected to and supplied from plenum 8, and nozzles 5 are connected to and supplied from plenum 9. With this arrangement, web or strip 1 can be redried or conditioned after the discharging process. This is frequently necessary in the production of photographic materials.
FIG. 6 shows an apparatus which allows rapid change to be made in the air conditions. Moreover, the surface resistance is measured at the same time. The sample 10, e.g. a strip of photographic film or paper with electrodes of colloidal graphite attached thereto, is situated in the lower part of the frame 11 which is electrically shielded. Air can be blown on to the sample 10 either from the nozzle 13 or from the nozzle 14 through the aperture 12 arranged over the center of the sample. These nozzles are connected to two separate air conditioning apparatus through flexible hosepipes (not shown). Rapid change in the air condition over the sample is brought about by moving the nozzles 13 and 14 by means of the pneumatically operated cylinder 15. The movements of the nozzles are recorded by the light barrier consisting of the lamp 16 varying transmittance light transmission strip 21 and photoelectric cell 17, and together with the output signal of the changes in the surface resistance on the sample supplied by the resistance measuring instrument 18 they are delivered to the multiple recording instrument 19 and recorded on the recording strip 20.
The apparatus shown in FIG. 6 may also be used to increase the moisture content of the air blown unto the surface of the material when the state of charge measured by the field intensity measuring device represented by instrument 18 exceeds a predetermined value. In that event instrument 18 actuates pneumatically-operated cylinder 15 to shift from a lower moisture content air blower nozzle 13 to a higher moisture content air blowing nozzle 14.
This apparatus was used for carrying out the three experiments described below.
EXAMPLE 1 (FIG. 7)
Air at a temperature of 10°C and containing about 1 g of water per kg of air (air condition I) was blown on to a paper sample which was coated with a photographic emulsion. The rate of flow of air from the nozzle was about 15 m/second. When the electric resistance had become established at a value which did not vary with time, air which was at a temperature of 40°C and contained 15 g of water per kg was blown on to the sample by shifting the nozzles. These values of temperature and water content correspond to a relative humidity of 32 percent and a dew point of 20°C (air condition II). Immediately after the change in the air condition above the sample, the variation in the electric resistance with time first showed a steep drop owing to the condensation of water vapor on the cold web of the material and then a gradual rise owing to the heating and hence drying of the web. Finally, the web is adjusted to the second air condition.
EXAMPLE 2 (FIG. 8)
In the next experiment, the time was measured on a 10 times larger scale. The recordings of the light barrier (not shown) indicated that the time required for changing the air condition was about 0.1 to 0.2 second. The first air condition I was the same as in Example 1, i.e. 1 g of water per kg at 10°C; the second air condition II was about 15 g of water per kg of air at 20°C. This corresponds to a relative humidity of 100 percent.
The time taken for adjustment of the resistance measuring instrument, i.e. the time in which, after a change in the measured value, the difference between the new measured value and the indicated value has dropped to the 2.7th part of the difference between the old and the new indicated value, was 50 msec. This clearly shows how rapidly the process takes place.
EXAMPLE 3 (FIG. 9)
In a third experiment, the compressed air cylinder and light barrier were replaced by a longer cylinder and longer barrier so that the second nozzle could be moved over and beyond the sample. The first nozzle had no air connection and instead, the sample was air conditioned by means of the ambient air which contained 5 g of water per kg at a temperature of 20°C (air condition I). The air ejected through second nozzle nozzle 14 (FIG. 6) to moisten the sample was at a temperature of 33°C and contained 27 g of water vapor per kg. Its relative humidity is 82 percent (air condition II). The length of time during which this air was blown on to the web was assessed at 40 msec. As shown in FIG. 9, a steep drop in the electric resistance is followed by a very brief period of low resistance. The resistance of the sample thereafter rises during air condition II and asymptotically approaches the state of equilibrium at 82 percent relative humidity. This rise is due both to drying of the small amount of water of condensation and to the diffusion of water into lower layers.