DE7017574U - DEVICE FOR ELECTROSTATICALLY ATTACHING DIELECTRIC FILMS TO MOVING EARTHED SURFACES. - Google Patents

Description

EGG. DU PONT DE NEMOURS AND COMPANY 10th and Market Streets, Wilmington, Delaware 19 898, V.8t.A.

Electrostatic tacking device

dielectric foils to moving

earthed surfaces

When pouring molten, crystallizable thermoplastic film webs, these must be quickly cooled to a temperature below the freezing temperature in order to suppress the crystallization as far as possible. It is assumed that too strong a crystallization in the web disturbs the orientation, as a result of which cloudiness and thickness irregularities arise in the orientated Icze ^, * δ in general the extruded web is cooled, - ^ d-jtn the melted thermoplastic material Attempts to accelerate this process in the interest of a more effective and economical way of working have led to inadequate uniformity in thickness and width and to the formation of regularly recurring clouding patterns, which are known in technology as blind clouding.

Various methods have been used to overcome the difficulties associated with higher speeds

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to be overcome during the cooling process. One of the most successful methods is the adherence of the molten web to the cooling surface by placing the foil over either its whole An electrostatic charge is applied across the width or with the help of tip probes at the edges of the film. But now also the electrostatic attachment process at higher casting speeds increasingly ineffective, since a given the less the electrostatic force is able to adhere the film web to the cooling surface, the higher the speed, so that a layer of air is trapped between the web and the surface, whereby in turn, the speed of heat transfer is reduced will. Furthermore, higher speeds often lead to the formation of "sticky bubbles" where the web attaches the cooling surface hits, and these bubbles cause quality defects in the finished film.

Attempts have already been made to increase the electrostatic force generated by a wire electrode or by probes to increase the tension. Most of these attempts were unsuccessful because of the increase in tension in general leads to electrical breakdown between the electrode and the film web long before the film web has a sufficient one Charge can be issued in order to achieve a substantial increase in the attachment force of the electrode and the surface of the web destroys the electrostatic field of the electrode, which is responsible for the attachment force responsible for. Furthermore, the spark formation leads to the formation of sand holes in the freshly poured, still soft one Foil web, and these holes enlarge considerably when the foil is oriented.

Therefore, the one in electrostatic tacking has been available heretofore standing tack is not entirely satisfactory for these and other uses where high electrostatic Force to adhere a dielectric sheet or

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Foil on a moving surface is required . I.

The invention relates to a method for improving the effect uciT- elcktroötatiöciien Aiuieftung, öö dae »which produced Power for both high speed extrusion operations and other types the film treatment is sufficient, where a high tacking force is required.

The invention relates to a method for electrostatically adhering dielectric films to moving grounded surfaces, in which the film surface is electristatically charged by the film being passed close to at least one electrode, but out of contact with the same, which consists in that one the electrode in one? Gas holds _; in which a wire electrode current can be generated before the breakdown of at least about 4-0 microamps per cm of wire, as determined by the current generation test.

The difference between the voltage at the surge current strength and the voltage at the spark breakdown is preferably at least about 2 kV, also determined after the power generation test. ......

Fig. 1 is a partially cross-sectional side view a device with which the method according to the invention can be carried out.

Fig. 2 shows a tip probe which can be used in the method according to the invention, in section.

Fig. 3 is a schematic representation of an apparatus for Determining the effect of the method according to the invention.

FIG. 4 shows another device for carrying out the invention Procedural

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Pig. Figure 5 shows an apparatus for use in the power generation test.

In the method according to the invention, the electrostatic

or with tip probes aimed only at the edges of the film. In general terms, can one with the electrical device and the method of issuing the electrostatic charge according to U.S. Patents 3,223,757 and 3,068,528. Preferably will Direct current is supplied to the tacking device, and direct current of positive potential is particularly preferred.

When applying the method according to the invention to the cooling of a freshly extruded thermoplastic film web It is important that the electrostatic charge is close to the normal point of impact of the web on the cooling surface is brought to action. If the electrodes are placed too far from this normal point of impact, either a large amount of air is trapped between the film and the drum, or there is one less effective suppression of thickness fluctuations and constrictions.

The gases used for the process according to the invention are those Consider gases in which a wire electrode current before breakdown of at least about 40 microamps per cm Wire determined according to the power generation test can be generated.

A closed Test chamber with straight boundaries made of a transparent dielectric material, preferably acrylic resin ("Lucite") of 25.4 cm wide, 35.5 cm long and 25.4 cm Height made. At the bottom of the chamber, a 17.8 cm by 33.0 cm steel plate is placed on top of dielectric blocks. The insulated plate is through the wall of the acrylic resin

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kaatens electrically connected to a micro-ammeter and from the micro-ammeter to earth. A 0.2 mm thick and 22.9 cm long round stainless steel wire "302" is held at a distance of 6.35 ± 0.40 mm above the steel plate under sufficient tension to be deflected by stresses caused by the Test procedures are created to prevent. The wire is electrically connected to the outside of the test chamber. The surface of the steel plate is covered by a polyimide resin ("Kapton") so that a 2.54 cm wide and 17.8 cm long strip remains exposed under the wire, and the cover is arranged so that the longitudinal axis of the exposed strip intersects the vertical plane of the wire at an angle of 90 °. In order to facilitate the quick and complete exchange of the atmosphere in the test chamber, gas inlets and outlets are arranged at opposite corners of a surface of the test chamber.

To conduct the test, air with a moisture content of less than $ 5 is bubbled through the chamber for> 5 minutes. The gas to be examined is then fed into the chamber under a slight overpressure in order to ensure that there is a coherent atmosphere of the gas in question in the test chamber. There should be a measurable excess gas pressure at the gas outlet to ensure that an excess gas pressure is maintained in the chamber. The test gas is allowed to flow through the chamber for at least 5 minutes before the test is continued.

A voltage is then applied to the wire and gradually increased until the first reading of the microammeter is observed. At this point the voltage is registered and the current is then interrupted. This way of working is done twice repeated. From the voltages at which the first current readings ; observed on the micro-ammeter, the mean value is taken and this mean value is called the "voltage

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in the case of the S whe 11 ens t rom strength " ö.er- " designated gas.

Then the voltage is increased until between the wire and the exposed surface of the steel plate a! Spark übergc, -The The lowest voltage at which a spark is observed is recorded. This procedure is also repeated twice, and the average of the three readings is called the "voltage at spark breakdown".

The voltage applied to the wire is now set to 0.1 kV below the "voltage at spark breakdown" and the The associated current intensity displayed by the micro-ammeter is registered. Then the tension is twice more in the is set in the same way, and the mean value is taken from the three readings. This mean is referred to as "electrode current before breakdown".

Preferably, the difference between the voltage at the threshold amperage and the voltage at the spark breakdown should be be at least 2.0 kV for the gas in question. Then one needs the voltage applied to the tacking device not exactly under control to prevent sparking. * · ■

Typical gases in which a wire electrode current strength of at least 40 microamps per cm of wire can be generated are nitrogen, helium, air with a moisture content of less than about 5 % (hereinafter referred to as dry air), oxygen, dichlorotetrafluoroethane, dichlorodifluoromethane , Carbon tetrachloride vapor in dry air or nitrogen, hand tetrachloride vapor in nitrogen, and acetone vapor in nitrogen. The vapors of carbon tetrachloride, tetrachloroethane and acetone can be generated by permitting the carrier gas, e.g. dry air or nitrogen, through the liquid in question at room temperature.

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len leaves "until the carrier gas is essentially saturated with the steam is.

If the above gases are examined with the help of the power generation test, they all have wire electrode current strengths of at least 40 microamps before breakdown (C). The characteristic properties, namely the wire electrode amperage before breakdown (C), the voltage at the threshold amperage (V ₊₁ ) and the voltage at spark breakdown (V ^) are for the above gases as well as for other gases that are required for the purposes of the invention are unusable, summarized in the table below,

Gas V ₊ , kV V · ,, kV C, μ A / cm wire

Propane 7.0

Argon 4.2

Monochlorodifluoromethane 18.4

CO ₂ 7.6

moist air ^ ^a '6.0

Propyl alcohol vapor

in nitrogen 6.9

Toluene vapor

in nitrogen 5.6

Indoor air ^) 6.0

Nitrogen 7.4

Helium 0.8

dry air; ( ^c ) 5.8 acetone vapor in nitrogen 7.0

Oxygen 6.4

Tetrachloride hand steam

in nitrogen 7.0

Carbon tetrachloride

vapor in nitrogen 9.6

Carbon tetrachloride vapor in dry

Air 9.9

Dichlorodifluoromethane 18.2

Dihlortetrafluoroethane 18.4. 35, Ο ^ ^α; 62.8

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7.0 0 4.4 5.2 19.8 10.0 9.6 20.8 8.7 25.6 10.3 29.2 9.2 32.0 9L..6 .. 37.6 10.2 42.8 1.4 45.2 10.0 48.0 12.6 92.0 10.2 94.4 11.3 98.8 15.8 192.0 14.5 231.2 32.3 360.8

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Note "■ ·. ':" Η to the table:

^ ^a 'rel. ~ .ve humidity above 95 #:

^ 'relative humidity about $ 40 to $ 50;

^ 'relative humidity less than about $ 5; ^ 'upper stroke limit.

When carrying out the procedure according to the invention, the gas essentially surrounds the electrode. This can /. e.g. can be achieved by passing a stream of the gas in question over the electrode so that it is enveloped by the gas will. In order to facilitate the maintenance of the gas atmosphere around the wire electrode with minimal pressure, can the electrode is partially surrounded by a coating made of dielectric material in which an atmosphere of the gas is maintained. The sheath should of course have an open section that fits onto the freshly extruded Foil web is directed to the fact that the foil web can be given an electrostatic charge by the electrode, and it should not surround the wire or tip electrode so completely that there is a space charge of Ions forms. For further protection against the formation of a space charge in the casing or the dielectric The gas supply line should be the jacket or the gas supply line preferably be earthed. Because in an open casing the gas atmosphere within it is constantly dispersed the enveloping gas must continue to be continuously supplied to the area surrounding the electrode. Appropriate the rate of supply of the gas should only be sufficient to create an atmosphere around the electrode from the gas in question. A gas pressure that is the minimum pressure required to maintain the gas atmosphere only leads to a more rapid dissipation of the atmosphere surrounding the electrode and to formation a constant gas flow in the direction of the film web. It has been found that gas pressures that are sufficient to envelop the

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Electrode pressure required, hardly any improvement of electrostatic attachment beyond that caused by the initial rinsing of the electrode the gas atmosphere is achieved.

Pig. 1 is a partially cross-sectional view of a device which, within the meaning of the invention, for electrical _ Jatical attachment of a sheet of film over its entire width can be used across. An elongated transverse wire electrode 25 and a dielectric Gas supply line 31 with a slot opening 32 are like this arranged that the gases flowing out of the opening surround the wire. The melted film web 33 is from the The mouth 30 of the extruder is pressed onto the cooling drum 29 and comes into contact with the drum at the point of impact 27. The electrode placed above the point of impact is Verbvräen with the high voltage source 15. The gas flowing out of the pipe is supplied with sufficient overpressure, to constantly surround the wire with an atmosphere of the gas. In general, a gas flow rate is sufficient from the supply pipe of less than 2832 l / h. to keep the electrode in a gas atmosphere.

Fig. 2 shows a tip probe used in the method. jra js of the invention can be used. Here is the τ ¹ "*. E>" rode by a jacket 21 surrounding that, dielectric, insulating material from wärmebes / ■ .aigem there. By connecting 22 the electrode is connected to a (not shown) high-voltage source, and gas is fed to the jacket through the inlet 23 so that it surrounds the electrode tip 24.

Fig. 3 shows schematically a device for testing the adhesive force of the attachment device, the to be examined Foil 10 unwound from roll 11 and from spring balance 13 over the polished surface of a grounded block 12

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stainless steel is drawn. The spring balance shows the adhesive force which results from the electrostatic charges given to the foil by the electrode 14. The electrode H is connected to the high voltage source Ii). The electrode is of the type shown in Fig. 2. A gas stream flowing over the electrode towards the foil is generated from the gas cylinder 16, which is equipped with a valve and a pressure regulator 17 and via the flow meter 18 connected by the hose 19 to the electrode jacket is. The voltage applied to the electrode is set to the highest value that is without breakdown between the Electrode and the foil is possible.

When performing the examination, the slide is through the spring balance is pulled off the roll over the block without gas flowing through the electrode santel = under tensile stress the film first adheres to the block and then slides over the block. With the spring balance measured the force required to make the film slide. Then the electrode is exposed to a gas atmosphere and the force required to overcome the sticking is determined in turn.

Fig. 4 shows a preferred embodiment in which a grounded, insulated, second electrode is used to surround the wire electrode with gas. In this picture it is streaming the gas flow 40 supplied from the cylinder 16 through the second electrode 41 and a dielectric insulator 42, so that it essentially surrounds the wire electrode 25.

Fig. 5 shows a device for testing gases according to the Power generation test. The test chamber 50 made of transparent dielectric material has a gas inlet 51 and a gas outlet 52 at opposite corners of one of its side walls. Rests on the floor of the test chamber the blocks 54 the steel plate 53. The steel plate is through

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the wire 55 passed through the wall of the test chamber is grounded via the micro-ammeter 56. The steel plate is covered by dielectric foils 57 to such an extent that an uncovered gap remains free in the middle of the plate. The wire electrode 58 is stretched over the uncovered part of the steel plate and perpendicular to the longitudinal axis thereof. The distance from the wire electrode to the surface of the steel plate is kept constant by the measuring and setting blocks 59. The tension of the wire is kept constant by the tension spring 60 connected to the power source 61.

The method according to the invention is arbitrary to pinning dielectric films applicable. Preferred films are those made from organic thermoplastic polymers; ZcB = polyesters such as polyethylene terephthalate, polyolefins such as polyethylene and polypropylene, polymers and Copolymers of vinyl acetate, polymers and copolymers of vinylidene chloride, polyamides, cellulose esters and ethers, polymers and copolymers of styrenes, rubber hydrochlorides and polycarbonates. The procedure is particularly suitable for cooling crystalline polymers, especially polyethylene terephthalate, because the stretching and the resulting optical properties of these films when used at high speeds be improved.

Furthermore, the method according to the invention can be applied to the treatment use other dielectric foils, e.g. when coating or printing on paper, cellulose glass and thermosetting Resins such as polyimide resins ("Kapton").

The mechanism for the process according to the invention is responsible for the significant improvement achieved is not yet fully understood. So far it has been assumed that the increase in voltage at the supply electrode leads to a

• *

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Increase in attachment force leads. However, shows one of the invention preferred gases, namely helium, have a relatively low "voltage at spark breakdown". The invention beJTuiit on ueJi Föatötellung deJC- Bödeutüiig der Strölijstark, which can be generated at the tacking wire electrode. The exact relationship between a high generated current and the increased attachment force, however, given the uncertainty as to the mechanism involved in the electrostatic tacking plays hard to define. In any case, the process causes a significant and unexpected one Improvement in adherence force, which is evident from the following examples.

Examples 1 to 4

A 25.4 μ thick polyethylene terephthalate film is used for the investigation in the apparatus shown in FIG. 3 using the tip electrode shown in FIG. The electrode is at a distance of 1.8 cm from the foil surface.

The current supply to the electrode is set in all cases & vf the highest value which can be reached without sparking. The gases listed in the table below are used to surround the electrode, ie with a flow rate of 283 l / h. are supplied to maintain an atmosphere of the gas in question around the electrode. In all cases, the force required to overcome the adhesion of the film to the block is determined with the spring balance. The results are compared with those of an experiment in which room air, which is supplied at the same speed, surrounds the electrode.

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example gas Maximum tension
without spark image
dung, kV To overcome
of sticking
required
Maximum strength, g 1 air 14th 246 2 helium 7.5 425 3 oxygen 16 565 4th nitrogen 16 453 Example 5

Comparative example

One works naoh Examples 1 to 4, but with Monoohlordifluoräthan than gas. This gas has a wire electrode current before breakdown of about 10 μA / cmφ die The maximum voltage that can be applied to the device without sparking is 15 kV and the maximum force is j uip to overcome clinging, 246 g. From this it follows that there is no improvement compared to room air with this gas will.

Examples 6-15

In Examples 6 to 15, polyethylene terephthalate is extruded from the melt at constant speed onto a cooling drum. The cooling drum has a diameter of 183 cm and the extruded film is 41.2 cm wide. The thickness of the film on the cooling drum varies; with the speed of rotation of the drum.

In Example 6, a single 0.2 mm thick stainless steel wire electrode is located 0.95 cm above the surface of the cooling drum in the position which gives the best adhesion. A positive direct current with the highest voltage that is possible without spark formation is applied to the wire electrode, and the speed of the drum is increased to the maximum value that can be achieved without

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"Anheftblasen" between the surface of the drum and that of the extruded T \ ^::: ^ take ground. The result

<wi · <η <- ^ AA ^. ~ * **. VA «4 -« * n «-—" *! -
llXOOU UJ.COCO XVWXX ΜΙΛ'ΛΧ V CXO LLW1X9 , m / min. .O _- t Λ - example 6th gas Indoor air Film thickness, μ 188 Highest drum speed 24.4 Voltage, kV 9.2 Amperage, μ A / cm wire 12th

In Examples 7 "to 9, the procedure of the Example 6 with the difference that instead of the bare wire electrode of Example 6, the electrode shown in FIG. 4 is used Electrode device used. The electrode device is in the same place as the bare electrode, and the same voltage is applied to the electrode. In Examples 8 and 9 the device is so operated with oxygen or nitrogen fed so that the wire electrode is almost completely surrounded by the gas. These gases do one massive increase in the amount of ions forming, resulting in a periodic increase in force across the width of the film leads. This results in fluctuations in the point of impact, which in turn reduces the maximum speed which can be achieved without the formation of attachment bubbles.

Example 7 8 9

gas Indoor air oxygen * 32.0 nitrogen Film thickness, μ 152 152 9.2 152 Highest drum speed 18th speed, m / min. 33.6 32.0 Voltage, kV 9.2 9.2 Amperage, μ A / cm wire 15th 36

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Examples 10 to 12 work according to Examples 7 to 9 with the difference that the voltage is at its maximum value is increased, which can be achieved without spark formation. The increased tension results when using oxygen and Nitrogen leads to such a strong increase in the formation of ions that there is an evenly increased adhesion force at the point of impact and the maximum speed that can be achieved without the formation of attachment bubbles is increased.

Example

gas
Pole thickness, μ

Highest drum speed, m / min.

Voltage, kV current, μ A / cm wire

. 10

11th

12th

Room air oxygen nitrogen 127 127 127

■ 36.1
10.5
36

37.4
10.8
38

37.4 11.3 32

Examples 13 to 15 work according to Examples 10 to 12 with the difference that the position of the tacking device is set to the most favorable position, in contrast to the arrangement of the device at the point of best bare wire performance.

example

13th

14th

15th

gas Indoor air Oxygen ί Embroidery ;O Pole thickness, μ 120 120 120 Highest drum speed speed, m / min. 46.0 49.0 49, 0 Voltage, kV 10.1 10.9 10, 7th

Amperage, μ A / om wire

28

40

32

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Claims (3)

EGG. du Pont de Nemours May 11, 1970 and Company F-1988-R S chux z claims 1. Apparatus for electrostatic adhesion of dielectric Foils on moving earthed surfaces, with the sioh approximately above the point of impact of the Extrusion press exiting film web on the grounded surface in motion a sxch over the entire width the film web extending to a high voltage source connected electrode is, characterized in that in the vicinity of the electrode (25, 20) a dielectric Gas supply line (31, 21) is arranged such that the electrode is enveloped by the gas flowing through the gas supply line. 2. Device according to claim 1, characterized in that the electrode is a wire electrode (25) and the dielectric Gas supply line is designed as a tube (31) arranged above the electrode, one parallel to one another having a longitudinal slot (32) extending towards the electrode and directed towards the electrode. 3. Device according to claim 1, characterized in that the electrode is a tip electrode (20, 24) and the dielectric Gas supply line al3 surrounding the electrode jacket (21) is formed. - 16 - ■ 701757m 9.10.7Π