US3394435A - Apparatus for making a nonwoven web - Google Patents

Description

July 30, 1968 T. E. C. KNEE APPARATUS FOR'MAKING A NoNwovN WEB Filed may 51, 196e -nal-n Univ/UHU `mvENToR $24 .29 Tem-:Nee 5.o. KNEE United States Patent O 3,394,435 APPARATUS FOR MAKING A NONWOVEN WEB Terence E. C. Knee, Wilmington, Del., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Filed May 31, 1966, Ser. No. 554,099 2 Claims. (Cl. 19-156.3)

This invention relates to an apparatus for depositing fibrous materials being transported in a moving fluid medium, such as air, onto a moving receiver as a non- Woven web, and in particular to an apparatus wherein the fibrous materials, after deposition, are firmly held to the receiver so that they do not shift their position under the influence of air currents.

In the adaptation of the recently developed process for the preparation of continuous-filament webs, described in British Patent 932,482, to economical commercial production of wide nonwoven webs, it is necessary, in order to obtain the desired productivity, to use a plurality of spinnerets or other filament-producing devices and collect the outputs from these spinnerets as a nonwoven web of the desired width. The spinnerets may be located across the width of the web to be produced, and positioned so that the filaments, after having been molecularly oriented by drawing and electrostatically charged, can be forwarded toward the laydown zone and deposited in overlapping ribbons to form the nonwoven web.

The electrostatic charge is applied to the filaments to cause them to separate when the tension on them is released and thus permit the collection of uniform, nonblotchy nonwoven webs. Jet devices, which use a highvelocity stream of air flowing cocurrently with the laments through the device, are preferred for forwarding the electrostatioally charged filaments because the tension which the jet devices apply to the filaments is rapidly released as the filaments exit from the jet device and therefore the filaments are free to separate due to the applied electrostatic charge. When multiple jet devices are used, however, the air streams exiting from the jet devices and the electrostatic charge on the filaments cause interference between the filaments exiting from adjacent jet devices and prevents them from being combined uniformly into a non-directional web on the weblaydown receiver. It has been found that this interference between adjacent jet devices can be overcome by withdrawal of at least 5 times the amount of lair supplied to the jet devices through the foraminous receiver into a suction or plenum chamber located below'the weblaydown zone.

The withdrawal of this large quantity of air, however, increases the air movement across the web yand this causes the filaments to blow up, and the nonwoven web to curl, overlap and become non-uniform in the region where the freshly-deposited web leaves the suction area at the downstream side of the suction chamber.

In the instant method the aforementioned difficulties are overcome. The air stream exiting from the jet device and containing the bundle is directed toward a stationary depositing zone over the moving foraminous weblaydown receiver while suction is applied to the underside of the receiver in the :area beneath the depositing zone. Substantially all the air of the stream exiting from the jet passes through the foraminous receiver while the filaments contained inthe stream are retained on the receiver. As the filaments, now in the form of a deposited web, are carried downstream from the area of deposition by the moving receiver, suction is applied to the underside of the receiver in a second area that is contiguous to the first area beneath the depositing Zone. The suction 3,394,435 Patented July 30, 1968 ICC applied is greatest in the region immediately adjacent the suction area under the depositing zone and diminishes or decreases in intensity in the downstream direction. Thus a decreasing air flow gradient exits across the receiver from the region contiguous to the suction area under the depositing zone to the downstream region of said second suction area. The air flow is reduced to zero where the web leaves the second suction area.

The apparatus of this invention eliminates this shifting of the filaments after laydown and makes possible the high-speed deposition of filaments onto a moving foraminous rece-iver as a uniform nonwoven web. The apparatus comprises, in combination with a .primary suction chamber located below a web-laydown receiver for initially attracting land hold-ing filaments to the receiver, a secondary suction chamber located below the weblaydown receiver and contiguous with the downstream side of the primary suction chamber, the secondary suction chamber comprising means for providing a decreasing air flow gradient from the side of the secondary suction chamber contiguous to the primary suction chamber to the downstream side where the web leaves the suction zone of the secondary suction chamber.

The invention will be further understood by reference to the drawings in which FIGURE l is an isometric view showing in part an apparatus for producing la wide nonwoven web and outlining the placement of the primary and secondary suction chambers; and

FIGURE 2 is an enlarged cross-sectional view taken along line 2-2 of FIGURE 1 showing preferred embodiments of primary and secondary suction chambers.

The essential parts of an apparatus suitable for producing wide, continuous-filament nonwoven webs are shown in FIGURE 1. For clarity only one filamentspinning position including the means for electrostatically charging and drawing the filaments and only three filament-forwarding jet devices are shown. The addition-al positions :and jet devices are identical to those shown and are provided in the number required to produce the desired width of nonwoven web by overlapping the adjacent ribbons of nonwoven web laid down on the moving receiver. In FIGURE 1, filaments 10 are extruded from spinneret 11 and combined into a bundle on snub bar 12. Snub bar 12 may be cylindrical as shown or may have a convex or concave surface to cause divergence or convergence of the filament bundle, respectively. Alternatively, a comb device may be used in place of the snub bar 12 to regulate the width of the filament bundle and maintain it in the desired alignment with the draw rolls and jet device downstream there-l from. The filaments are then electrostatically charged by means of corona-discharge devices comprising target bars 13 and charge heads 14. The filament bundle is passed into light contact with slowly rotating target bars 13 which are posittioned adjacent char-ge heads 14 which have needle electrodes. A corona discharge is generated by applying a high electric potential to the electrodes and grounding the target bars.

The charged filaments pass over draw rolls 15 and then into stationary slot jets 16 which strip the filaments from the last draw roll and forward them toward the web-laydown receiver 17. The slot jets are provided with air through inlets 18. The jet devices are equipped with diffuser sections 19 to spread the filament bundles and provide elongated areas of deposition which are readily blended by overlapping about 67% into a wide, uniform nonwoven web.

The filaments are collected on moving foraminous receiver 17 above the primary suction chamber 20 as web 21. Air is withdrawn through the receiver into the suction chamber and then through exhaust duct 22 which is attached to a suction-blower system not shown. The air withdrawn through the receiver into the primary suction chamber serves to overcome the aerodynamic and electrostatic interference between laterally-adjacent jet devices. ln order to utilize electrostatic attraction as well as aerodynamic forces to attract and hold the charged filaments to the receiver, the receiver may be a grounded metal screen or alternatively, if a fabric belt is used, the surfaces of the primary and secondary suction chambers which contact the belt are grounded and a grounded metal plate is positioned below the belt in the region downstream from the secondary suction chamber.

The secondary suction chamber 23 is positioned adjacent to the downstream side 24 of primary suction chamber and is provided with means, not shown in FIGURE l, to provide a decreasing air-flow gradient in the direction of arrow 25. The air withdrawn t-hrough the receiver into the secondary suction chamber is controlled and graduated in such a way that the effect holding the filaments to the receiver decreases gradually from a high level at the primary suction chamber to lower levels over the secondary suction chamber, and to virtually zero as the nonwoven web leaves the suction zone of the secondary suction chamber. This gradient is essential and satisfactory results cannot be obtained merely by applying a high vacuum to both suction chambers. Ordinarily the air velocity differential in going from suction to no suction, i.e. the point where the web leaves the secondary suction zone should not exceed 50 Ift./ min. In going from first suction zone to the secondary suction zone the air velocity differential should preferably not exceed about 150 ft./min. The stepwise reduction in air velocity should preferably not exceed about 30 ft./min. per three inches of receiver width.

In FIGURE 2, the primary suction chamber 20 holds the filaments to the receiver from the time they touch until they are clear of the stream of air exiting from the jet devices. Virtually all of the air from the jet devices enters the primary suction chamber. In a preferred embodiment of the primary suction chamber, the chamber comprises an orifice plate 26, a perforated cover plate 27, and a multiplicity of independent fluid-distribution cells 28 forming a honeycomb structure between the orifice plate and cover plate. The cells are open at both ends, the lower end being in contact with the orifice plate and positioned over a single hole in the orifice plate, and the upper end being in contact with the cover plate. The cover plate may have an open area below each jet device conforming with and slightly larger than the pattern of the jet air striking the laydown receiver above the primary suction chamber. These open areas may consist of perforated portions in an otherwise solid cover plate or may be present in a mask plate positioned over the perforated cover plate.

The embodiment of secondary suction chamber shown in FIGURE 2 consists of an outer shell 29 which is solid except for exhaust duct 30, which is attached to an auxiliary suction-blower system not shown, and the top surface. The top consists of three elements plus a support frame. A uniformly perforated or foraminous plate 31, which is relatively open to air flow, is located directly below and in contact wit-h web-laydown receiver 17. Below plate 31 is mask 32 made of layers of a woven or nonwoven fabric, with increasing numbers of layers toward the exit edge of the secondary suction chamber, thereby providing a gradient of increasing resistance to airflow. Of course, a single layer of increasing thickness would be equally suitable. Below mask 32 is a grating 33 which supports mask 32 and offers little restriction to air ow. A series of bea-ms 34, which have no significant effect on the air ow, support the grating above the bottom of the secondary suction chamber. Since plate 31 is normally a light-weight material which is supported only by mask 32 in the region between the side walls of the suction chamber, it is desirable to have beams 34 adjustable so that the plate can be maintained in a level position across its entire width.

While the above-described embodiment of the secondary suction chamber is preferred because of excellent performance, simplicity and low cost, other methods for obtaining a gradient lof resistance to air flow can also be used. For example, the open area in the perforated plate 31 may be gradually decreased by decreasing the number and/ or size of the perforations in the direction of travel of the receiver. Alternatively, a honeycomb structure of the type described above in the primary suction chamber may be used, with the holes in orifice plate lbeing decreased in size in the direction of travel of the receiver.

eparate primary and secondary suction chambers with separate suction-blower systems are preferred because they provide great versatility to the laydown operation and precise control over the air-ows. These factors are of considerable importance because the preparation of various types of non-wo-ven products can require the use of different process conditions in the spinning and subsequent treatments of the filaments. These different process conditions may require different velocities of jet air. For each jet air velocity, there are optimum levels of air Withdrawal for both the pri-mary and secondary suction chambers and these optimum levels can be readily obtained With separate chambers. If, however, only a single type of nonwoven web is to be produced, it is possible to use a single suction chamber with two portions corresponding to and functioning in accord with the above-described primary and secondary suction chambers, and with both of these portions being connected to the same suctionblower system. It is intended that this type of suction chamber come within the scope of the present invention.

What is claimed is:

1. Apparatus comprising a movable foraminous weblaydown receiver on which to form a nonwoven web, a plurality of means -for discharging a stream of air containing a plurality of fila-ments onto one face of said receiver, a primary suction-chamber located at the opnosite face of said receiver and opposite the said discharging means, for withdrawing air that is directed at the web-laydown receiver by said discharge means and that passes through said web-laydown receiver, and a secondary suction-box located below the web-laydown receiver and contiguous with the downstream side of the primary suction-chamber, the secondary suction-chamber having means for successively increasing resistance to ow from the side of the secondary suction-chamber contiguous to the primary suction-chamber to the downstream side where the web leaves the suction zone of the secondary suction chamber.

2. The apparatus of claim 1 wherein the means providing an increasing resistance to air flow comprises layers of fabric increasing in number toward the exit edge of the secondary suction chamber.

References Cited UNITED STATES PATENTS 2,859,506 ll/l958 Slayter 28ll 2,940,135 6/1960 Heritage 19-l56.3 2,993,239 7/1961 Heritage lO-l56.3 3,114,939 12/1963 Labino 19-l56.3

MERVIN STEIN, Primary Examiner'.

I. C. WADDEY, Assistant Examiner.

Claims (1)

1. APPARATUS COMPRISING A MOVABLE FORAMINOUS WEBLAYDOWN RECEIVER ON WHICH TO FORM A NONWOVEN WEB, A PLURALITY OF MEANS FOR DISCHARGING A STREAM OF AIR CONTAINING A PLURALITY OF FILAMENTS ONTO ONE FACE OF SAID RECEIVER, A PRIMARY SUCTION-CHAMBER LOCATED AT THE OPPOSITE FACE OF SAID RECEIVER AND OPPOSITE THE SAID DISCHARGING MEANS, FOR WITHDRAWING AIR THAT IS DIRECTED AT THE WEB-LAYDOWN RECEIVER BY SAID DISCHARGE MEANS AND THAT PASSES THROUGH SAID WEB-LAYDOWN RECEIVER, AND A SECONDARY SUCTION-BOX LOCATED BELOW THE WEB-LAYDOWN RECEIVER AND CONTIGUOUS WITH THE DOWNSTREAM SIDE OF THE PRIMARY SUCTION-CHAMBER, THE SECONDARY SUCTION-CHAMBER HAVING MEANS FOR SUCCESSIVELY INCREASING RESISTANCE TO FLOW FROM THE SIDE OF THE SECONDARY SUCTION-CHAMBER CONTIGUOUS TO THE PRIMARY SUCTION-CHAMBER TO THE DOWNSTREAM SIDE WHERE THE WEB LEAVES THE SUCTION ZONE OF THE SECONDARY SUCTION CHAMBER.

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