US 3319309 A
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Description (OCR text may contain errors)
y 16, 1967 J. E. OWENS 3,319,309
CHARGED WEB COLLECTING APPARATUS Filed June 4, 1964 FIG.
4. POSITIVE "'37 0.0. SOURCE FIG-2 INVENTOR JOHN EDWARD OWENS ATTORNEY United States Patent Office 3,319,309 Patented May 16, 1967 3,319,309 CHARGED WEB CtDLLECTiNG APPARATUS John Edward Owens, Hockessin Hills, DeL, assignor to E. I. du Pont de Nemonrs and Company, Wilmington, Del, a corporation of Delaware Filed June 4, 1964, Scr. No. 372,629 4 Claims. (Cl. 28-1) This invention 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.
It is old in the art to make non-Woven sheet products from an uniform laydown of fibrous elements on a moving collecting surface. After collecting on the surface the fibers are coalesced by pressure, heat, solvents or the like into a coherent structure. Prior to the laying down of such elements it is sometimes convenient to place an electrostatic charge upon them and thereafter attract them to the collecting surface by an electrostatic charge of opposite polarity on the collecting surface, or by such a charge whose field encompasses the surface on which the elements are collected. Thus while the moving collecting surface may be metal and capable of acquiring and holding a suitable charge, it is often more convenient to employ a moving collecting surface which passes Within the field of a charged laydown plate. When the laydown plate is of opposite polarity to that imposed on the charged elements and is disposed on the side of the collecting surface opposite to that on which the charged elements are deposited, the attractive forces of the opposing charges cause the elements to adhere firmly to the collecting surface. A typical system with appropriate apparatus for charging and collecting fibrous webs on such a surface is described and claimed in US. application Ser. No. 372,- 623, filed of even date herewith to Hollberg and Owens. The Hollberg and Owens system is particularly suitable for the formation of sheet products by electrostatic laydown of a plexifilamentary web, a very light and fluffy material more particularly described and claimed in US. Patent 3,081,519 of Mar. 19, 1963, to Blades and White. When attempts are made to collect such light and fiuify materials on a collecting surface which passes over an oppositely charged laydown plate, difficulty has been experienced in disruptions and non-uniformities in the sheet product caused by electrostatic fibrous erections occurring after the formed sheet leaves the area of influence of the lay down plate. These have been observed to be caused by small residual similar electrostatic charges remaining on the fibers of the sheet, which result in repulsion on moving out of the field of the laydown plate.
It is an object of the present invention to provide novel and useful apparatus for removing residual electrostatic charges from electrostatically charged dielectric materials. Another object is to provide an improved collecting device for forming non-woven sheets from electrostatically charged fibrous webs.
These and other objects will become apparent in the course of the following specification and claims.
In accordance with the present invention apparatus is provided which provides a collecting surface capable of continuous motion in a given direction and disposed to collect a continuous deposit on one surface of electrostatically charged fibrous elements. This collecting surface passes through an electrostatic field, opposite in polarity to the charge on the depositing elements, generated by a laydown plate disposed on that side of the collecting surface opposite to the side on which the elements are deposited. The trailing edge of the laydown plate is a knife edge or a series of sharp points, i.e., a sharply tapered shape. A source of high electric potential is connected to the laydown plate. The high potential at the sharply tapered edge promotes formation of a corona discharge. As the collecting surface receives the charged elements they are held to the surface as a nonwoven 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 ad jacent vapors. As the sheet passes over the sharp trailing edge of the laydown plate, small residual charges are discharged to allow a smooth, nondisruptive transport to conventional bonding and windup.
This invention will be more readily understood by reference to the drawings.
FIGURE 1 is a crosssectional 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 of the present invention.
Referring particularly to FIGURE 1, a spinneret device 1 is connected to a source of polymer dissolved in an organic solvent. Polymer solution 2 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. 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 bafile 8 concomitantly with plexifilament 7, generally follows the path of advance of the plexifilarnent 7 from spinning orifice 5 to collecting surface 9, thereby creating a flow pattern within chamber 6 as indicated by the arrows. Bafile 8 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 baffle 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 fiow pattern in chamber 6 holds plexi filament 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 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 efficient operation will permit. Generally a clearance of from about 1 to about 2 inches is saticfactory although this will vary with the design and capacity of the particular equipment.
After passing through the charging zone, plexifilament 7 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 direct current source 37. Due to the electrostatic forces, the plexifilainent 7 is attracted to surface 9 and clings to it in its arranged condition as a sheet 38 with sufiicient force to overcome the disruptive influences of whatever vapor blast may reach this area. Surface 9 carries sheet 38 out of chamber 6 through port 39. Flexible elements 40 across port 39 and also across port 41, which permits re-entry of the unloaded continuous belt, assist in retention of vapor within chamber 6. The sheet is then lightly compacted by compacting roll 44 and is collected on windup roll 42. 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 FIG. 2. As will be obvious from the illustration, the laydown plate 45 is in essence a flat conducting plate with one broad flat surface disposed parallel and adjacent to collecting surface 9. Preferably the leading edge 46 of plate 45 is rounded. A sharply tapered shape, i.e., knife-edge 47 is provided along the trailing edge of plate 45 to provide a corona discharge between the edge 47 and sheet 38 and a terminal for hook-up to power source 37 is provided at 48. In operation, as. can be seen more readily in FIG. 1, plexifilament 7 is deposited near the leading edge of plate 45. Sheet 38 is formed on laydown of plexifilament 7 upon collecting surface 9. The opposing electrostatic charges on plexifilament 7 and in the field of infiuence 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 largely neutralized. As sheet 38 passes over the knifelike trailing edge 47 of plate 45, small remaining residual charges are effectively discharged and on leaving the electrostatic field of plate 45, the sheet remains flat 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.
Although the collecting surface described herein is a continuous belt, it is obvious that a revolving drum may be employed. In such a system, the broad surface of laydown plate 45 is curved to conform generally to the configuration of the collecting surface. Any electrically conducting material may be used to form the laydown plate. Preferably, it is metallic. Stainless steel is suitable for this purpose. The material of construction of the collecting surface may be any non-conducting or semiconducting material. A semi-conductor such as carbon filled rubber is suitable. In addition it may be a nonconductor such as a felt or a fabric knitted or woven from a natural or syntheic fiber provided it is porous enough to provide a path for gaseous ions between the depositing web and the laydown plate.
The nature of the fibrous web and the charging element is not critical in the apparatus of the present invention. The apparatus of the present invention is particularly useful in combination with the system described in the Hollberg and Owens application previously identified and the ion gun of the corona discharge device described in greater detail and claimed in US. application to Owens filed of even date herewith and now abandoned. The nature of the filamentary structure to which the laydown system is applied may vary widely. Preferably, it is a plexifilarnent or a continuous man-made filament. The nature of the polymer from which the filament is formed may be any of an addition, condensation or natural'polymer. The intensity of charge to be applied to the laydown plate of the present invention will vary widely depending upon the physical dimensions of the particular system in which it is employed and the nature of and charge on the collected material.
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, a charged plate on one side of and conforming to the shape of the said continuous surface and adapted to generate an electrostatic field of opposite polarity in the path of the said continuous surface, the said plate extending transversely of and having an upper side and a lower side, the lower side converges upwardly and terminates at the upper side to form a sharp edge along the trailing end of the plate to provide a corona discharge between the said sheet and the said plate whereby flufiing up of said sheet is prevented, means for forwarding to and depositing electrostatically charged fibrous elements upon the side of the said surface opposite tothat on which the said plate is disposed, at a point upstream from the said trailing edge.
2. The apparatus of claim 1 wherein the said plate is metallic.
3. An apparatus for attracting, collecting and discharging a sheet of electrostatically charged fibrous elements which comprises: an endless driven belt having upper and lower reaches; an oppositely charged plate positioned beneath the upper reach and adapted to generate an elec trostatic field, the plate having an upper surface disposed toward the underside of the upper reach, and a lower surface having a first portion parallel to the upper surface followed by a second portion that angles obtusely toward and terminates at the upper surface to form a sharp edge along the trailing end of the plate to provide a corona discharge between the trailing end of the plate and the sheet whereby fluffing up of said sheet is prevented; and means for forwarding to and depositing electrostatically charged fibrous elements on top of the upper reach at a point upstream of the trailing edge of the plate.
4. The apparatus of claim 3 wherein the plate is metallic.
References Cited by the Examiner UNITED STATES PATENTS 2,686,141 8/1954 Sawyer 156-370 X 2,810,426 10/1957 Till et al 156-371 X 2,920,679 1/1960 Sittel 19-155 X ROBERT R. MACKEY, Acting Primary Examiner.
DORSEY NEWTON, Examiner.