US3320479A

US3320479A - Charged web collecting apparatus

Info

Publication number: US3320479A
Application number: US458358A
Authority: US
Inventors: Owens John Edward
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1965-05-24
Filing date: 1965-05-24
Publication date: 1967-05-16
Anticipated expiration: 1984-05-16
Also published as: NL6605750A

Description

y 1967 J. E. OWENS 3,320,479

7 CHARGED WEB COLLECTING APPARATUS Filed May 24, 1965 FIGJ T0 SOLVENT RECOVERY POLYMER -*-SOLUTION SUPPLY iEcAnvE 55 n c SOU RCE NEGATIV 5| Y 0.0.

1. POSITIVE 37 0.0. OUR

INVENTOR JOHN EDWARD OWENS ATTORNEY United States Patent 3,320,479 CHARGED WEB COLLECTING APPARATUS John Edward Owens, Hockessin, De]., assignor to E. L du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed May 24, 1965, Ser. No. 458,358 3 Claims. (Cl. 317-2) This application is a continuation-in-part of US. ap plication Ser. No. 372,629 filed June 4, 1964. The invention of the parent application relates to an apparatus for removing electrostatic charges from electrostatically charged dielectric materials, particularly those materials in the form of continuous filaments and especially those of a light and fluffy nature formed from a synthetic organic polymer. The invention of the present application is an improvement on the apparatus disclosed and claimed in the parent case.

In the apparatus of both the parent case and the present application, a collecting surface capable of continuous motion in a given direction and disposed to collect a continuous deposit of electrostatically charged fibrous elements on one face is provided wherein the collecting surface passes through an electrostatic field generated by a laydown plate electrode disposed on that face of the collecting surface opposite to the face on which the elements are deposited. The electrostatic field, produced by charges on the plate electrode opposite in polarity to the depositing elements, causes the elements to be strongly attracted to the collecting surface. It is of the essence of the invention in the parent application that the trailing edge of the laydown plate electrode be a knife edge or a series of sharp points, i.e. a sharply tapered shape. Thus, in operation, as the collecting surface receives the charged elements they are held to the surface as a non-woven sheet by electrostatic charge. As the collecting surface moves the non-woven sheet across the laydown plate the charge on the elements is largely neutralized by ionization of adjacent vapors. As the sheet passes over the sharp trailing edge of the laydown plate, small residual charges are largely discharged due to the corona discharge resulting from the high potential at the sharply tapered edge. Failure to use a sharp, trailing edge upon the laydown plate results in disruptions and nonuniforrnities in the sheet product caused by electrostatic fibrous erections occurring after the formed sheet leaves the area of influence of the laydown plate. While the apparatus of the parent application is satisfactory under most operating conditions, it has been observed that when operating in a closed chamber wherein solvent vapors are highly concentrated, particularly when the dielectric strength of the solvent gas is high, as in the case of trichlorofluoro-methane, it is particularly difiicult to neutralize the charge on the fibers. As a result, under these conditions, even when using the laydown plate of the parent application, the fibers tend to repel each other as they leave the vicinity of the laydown plate and then due to the motion of the collecting surface, the erected fibers tend to roll back toward the laydown plate. The rolling fibers eventually are caught by the nearby compacting roll and non-uniform worms or ridges appear in the finished sheet.

It is an object of the present invention to provide novel and useful apparatus for removing residual electrostatic charges from electrostatically charged dielectric materials in the presence of high concentrations of vapors of high dielectric strength. Another object is to provide an improved collecting device for forming non-woven sheets from electrostatically charged fibrous webs.

These are other objects will become apparent in the course of the following specification and claims.

Patented May 16, 1967 In accordance with the present invention an elongated anti-rollback electrode is added to the apparatus of the parent application aligned parallel to the trailing edge of the laydown plate electrode and situated on that side of the collecting surface upon which the fibrous elements are deposited, i.e. along the face of the collecting surface opposite to the face along which the laydown plate electrode is disposed. The anti-rollback electrode is charged opposite to the charge on the laydown plate electrode. Generally a structure having an arcuate surface with a low radius of curvature, such as a wire, is preferred for use as the anti-rollback electrode. The high potential between the sharply tapered trailing edge of the plate electrode and the anti-rollback electrode with a low radius of curvature, promotes formation of a corona discharge.

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

FIGURE 1 is a cross-sectional elevation indicating schematically the arrangement of the various elements in a system wherein the collecting system of the present invention is particularly useful.

FIGURE 2 is a perspective view of the laydown plate electrode.

FIGURE 3 is a much enlarged cross-section of the cylindrical anti-rollback electrode 50 of FIGURE 1.

FIGURES 4 to 7 show anti-rollback electrodes of various shapes.

Referring particularly to FIGURE 1, a spinneret device 1, is connected to a source of polymer dissolved in an organic solvent. Polymer solution 2 at a temperature above the solvent boiling point and under pressure is fed through extrusion orifice 3 into intermediate pressure or let-down pressure zone 4 and then through spinning orifice 5 into web forming chamber 6 which is at a much lower pressure and near atmospheric pressure. The extrudate from spinning orifice 5 is a plexifilament 7. Due to the pressure drop at spinning orifice 5 vaporization of solvent creates a vapor blast which, by virtue of impingement upon baffle 8 concomitantly with plexifilament 7, generally follows the path of advance of the plexifilament 7 from spinning orifice 5 to collecting surface 9, thereby creating a flow pattern within chamber 6 as indicated by the arrows. Baffle S is mounted on shaft 10 which in turn is oscillatably mounted in bearing 11 and is powered to oscillate by means not shown. While oscillation of the bafile is not essential, it is preferred in the preparation of wide sheets of plexifilamentary material. Target plate 13 and ion gun 14 are disposed on opposite sides of the path of advance of the plexifilament web, downstream from the web forming and mechanical spreading devices. Target plate 13 is so disposed that the vapor blast originating at 5 and the air flow pattern in chamber 6 holds plexifilament 7 in brushing contact with its charging surface. Target plate 13 is connected to ground through wire 15 and microammeter 16, the latter indicating target plate current. The position of needles 25 with reference to target plate 13 is important for efiicient operation. It will be apparent that the clearance between the needle points and plate 13 should be :as small as efi'icient operation will permit. Generally a clearance of from about 1 to about 2 inches is satisfactory although this will vary with the design and capacity of the particular equipment.

Plexifilament 7 passes through the charging zone, provided by high voltage negative D.C. source 35 operating through ion gun 14 and target plate 13, and is thereby provided with a negative charge. The charged pleXifilament is deposited upon a collecting surface 9. The surface illustrated is a continuous belt forwarded by drive rolls 36. The belt passes through an electrostatic field emanating from charged laydown plate 45 which receives its charge, opposite in polarity to that imposed on plexifilament 7, from high voltage direct current source 37. It will be understood, of course, that the direct current may have an oscillating component provided that the resultant polarity is not reversed during operation. Due to the electrostatic forces, the plexifilament 7 is attracted to laydown plate electrode 45 and clings therefore to collecting surface 9 in its arranged condition as a sheet 38 with sufficient force to overcome the disruptive influence of whatever vapor blast may reach this area.

As the electrostatically charged sheet 38 is carried along by collecting surface 9 its charge is partially neutralized by the ionized vapors developing under the influence of plate electrode 45. The substantial charge which remains on the fibers as the sheet passes the trailing edge 47 of the laydown plate electrode is then further reduced by means of a much higher ion concentration derived from the more intense corona discharge which occurs between the trailing edge 47 of the laydown plate electrode 45 and anti-rollback electrode 50. Electrode 50 is connected to a source of direct current 51 opposite in polarity to the power supply of laydown plate electrode 45. The in creased ion density between the sharp trailing edge 47 and the anti-rollback electrode 50 promotes much more rapid neutralization of the charges on the sheet 38 as the sheet passes over the trailing edge. The sheet 38 is lightly compacted by compaction roll 44 and is withdrawn from enclosure 6 through port 39 by means of wind-up roll 42. Flexible elements 40 may be provided to assist in retaining vapors within chamber 6. A conventional solvent recovery unit 43 may be beneficially employed to improve economic operation.

Plate 45 is shown in detail in a perspective view in FIGURE 2. As will be obvious from the illustration, the laydown plate electrode 45 is in essence a flat conducting plate with one broad flat surface disposed parallel and adjacent to collecting surface 9, i.e., conforming to the shape of the collecting surface. Preferably the leading edge 46 and side edges 49 of plate 45 are rounded. A sharply tapered shape, i.e. knife-edge 47 is provided along the trailing end of plate 45 and a terminal for hook-up to power source 37 is provided at 48. In operation, as can be seen more readily in FIGURE 1, plexifilament 7 is deposited near the leading edge of plate 45. Sheet 38 is formed on laydown o-f plexifilament 7 upon collecting surface 9. The opposing electrostatic charges on plexifilament 7 and in the field of influence of laydown plate 45 hold sheet 38 tightly to collecting surface 9. As sheet 38 moves along in the field of electrostatic influence of plate 45 its charge is gradually neutralized. As sheet 38 passes between the knifelike trailing edge 47 of plate 45, and anti-rollback electrode 50 the rate of neutralization is greatly increased, thereby, reducing effectively the charge level on fibers leaving the electrostatic field of plate 45. Consequently, the sheet remains fiat and undisturbed. When a laydown plate having a rounded trailing edge having a high radius of curvature is substituted for the laydown plate of the present invention, disruption of the sheet occurs by fluffing-up of light elements due to the presence of residual electrostatic mutually repelling charges that become manifest when the moving sheet leaves the field of influence of the laydown plate.

As discussed above, the finding-up which occurs at the trailing edge of the laydown plate is even more persistent in a gaseous atmosphere having a high dielectric strength, e.g. in a gas such as trichlorofluoromethane (Freon-11). Therefore, the improved apparatus of this invention has particular value when film-fibril material is flash-spun into a chamber containing principally a solvent gas such as Freon-11 and very little air. Accordingly, it has been found that the fibers can be controlled in a closed spinning chamber by providing an antirollback electrode disposed parallel to and near the trailing edge of the laydown plate electrode.

The length of the electrode 50 is approximately equal to the width of laydown plate electrode 45, both items having the approximate width of sheet 38 carried on belt 9. .Preferably electrode 50 is cylindrical as shown in FIG- URE 3 and consists of a single wire of 10 to mils diameter (0.010 to 0.100 in.), but preferably is 15 to 30 mils in diameter. The wire may also be multistrand. Another alternative is a sharp-edged bar having a knife edge as shown in FIGURES 4 and 5 with radius of ourvature less than about 50 mils (0.051 in.). However, this type of electrode requires special care of operation and construction in that the large mass of metal tends to attract webs. This attraction which occurs despite the apparent like charges on the web and on electrode 50 is believed to be due to formation of image charges in the massive electrode in response to the fiber charge. It is obvious then that the mass of the knife-edged bar must be kept to a minimum, i.e., electrode 50 should be generally cylindrical and have a short radius of curvature. Also, when a knife-edged bar is used as the anti-rollback electrode, a higher potential must be applied to obtain effective discharge of the fibers than is used for the wire type electrode. It is believed that the large mass of metal in the bar electrode distorts the electric field making the knifeedge electrode less efiicient than the charged wire electrode.

Another variation of anti-rollback electrode is a conducting bar having a series of conducting points extending in parallel from the bar along its entire length as shown in cross-section in FIGURE 6. A suitable electrode of this type has points with three mil radius at the tip.

FIGURE 7 is a front view of another variation of the anti-rollback electrode consisting of a wire screen having a solid metal shield 61 welded around three of its edges. The shield is made of bar stock or tubing having a large diameter to avoid unwanted corona discharge. The points 60 promote the corona discharge along the entire length of the electrode.

The anti-rollback electrode is located about 1 to 6 inches above the collecting surface. It may be placed directly over the trailing edge of the plate electrode or may be located as much as about 6 inches downstream. It should not, however, be so close to the wall of web forming chamber 6 (FIGURE 1) or to compaction roll 44 as to cause appreciable corona discharge to the chamber wall or to the roll.

Many equivalent modifications will be apparent to those skilled in the art from a reading of the above without a departure from the inventive concepts.

What is claimed is:

1. Apparatus for attracting, collecting and discharging a sheet of electrostatically charged fibrous elements comprising in combination a continuous surface adapted to move continuously in a given direction and providing on one face, a path of laydown for the said fibrous elements, a charged plate conforming to the shape of the said continuous surface disposed along and extending transversely of the face of the said surface opposite to that providing the said path of laydown and adapted to generate an electrostatic field of opposite polarity in the path of the said continuous surface, the said plate having a broad side disposed toward said surface, the said plate extending substantially completely across the width of the said path of laydown and terminating at its trailing edge in a sharply tapered shape adapted to provide a corona discharge between the said sheet and the said plate, a charged elongated electrode positioned above and extending substantially across the width of the said path of laydown, proximate to the said trailing edge of the said plate, said electrode having an arcuate surface of low radius of curvature disposed toward said trailing edge and being adapted to generate an electrostatic field opposite in polarity to 5 6 that generated on the said plate, and means for forwarding References Cited by the Examiner to and depositing electrostatically charged fibrous ele- UNITED STATES PATENTS ent th 'd thofl d ta oitu team g figg ifgg f ay Own a p n PSI 844,300 2/1907 Chapman 317-2 2. The apparatus of claim 1 wherein the said elongated 5 2,483,542 I 8/1949 Hooper electrode is a Wire 3,122,634 2/1964 Klng 317262 X 3. The apparatus of claim 1 wherein the said elongated p electrode is situated immediately downstream from the MILTON HIRSHFIELD Primary Exammer' said trailing edge of the said plate. I. A. SILVERMAN, Assistant Examiner.

Claims

1. APPARATUS FOR ATTRACTING, COLLECTING AND DISCHARGING A SHEET OF ELECTROSTATICALLY CHARGED FIBROUS ELEMENTS COMPRISING IN COMBINATION A CONTINUOUS SURFACE ADAPTED TO MOVE CONTINUOUSLY IN A GIVEN DIRECTION AND PROVIDING ON ONE FACE, A PATH OF LAYDOWN FOR THE SAID FIBROUS ELEMENTS, A CHARGED PLATE CONFORMING TO THE SHAPE OF THE SAID CONTINUOUS SURFACE DISPOSED ALONG AND EXTENDING TRANSVERSELY OF THE FACE OF THE SAID SURFACE OPPOSITE TO THAT PROVIDING THE SAID PATH OF LAYDOWN AND ADAPTED TO GENERATE AN ELECTROSTATIC FIELD OF OPPOSITE POLARITY IN THE PATH OF THE SAID CONTINUOUS SURFACE, THE SAID PLATE HAVING A BROAD SIDE DISPOSED TOWARD SAID SURFACE, THE SAID PLATE EXTENDING SUBSTANTIALLY COMPLETELY ACROSS THE WIDTH OF THE SAID PATH OF LAYDOWN AND TERMINATING AT ITS TRAILING EDGE IN A SHARPLY TAPERED SHAPE ADAPTED TO PROVIDE A CORONA DISCHARGE BETWEEN THE SAID SHEET AND THE SAID PLATE, A CHARGED ELONGATED ELECTRODE POSITIONED ABOVE AND EXTENDING SUBSTANTIALLY ACROSS THE WIDTH OF THE SAID PATH OF LAYDOWN, PROXIMATE TO THE SAID TRAILING EDGE OF THE SAID PLATE, SAID ELECTRODE HAVING AN ARCUATE SURFACE OF LOW RADIUS OF CURVATURE DISPOSED TOWARD SAID TRAILING EDGE AND BEING ADAPTED TO GENERATE AN ELECTROSTATIC FIELD OPPOSITE IN POLARITY TO THAT GENERATED ON THE SAID PLATE, AND MEANS FOR FORWARDING TO AND DEPOSITING ELECTROSTATICALLY CHARGED FIBROUS ELEMENTS UPON THE SAID PATH OF LAYDOWN, AT A POINT UPSTREAM FROM THE SAID TRAILING EDGE.