DE19650608B4 - Method and device for separating and spreading drawn thermoplastic continuous filaments in nonwoven production by electrostatic charging - Google Patents

Abstract

method for separation and spreading of stretched, thermoplastic Endless filaments in nonwoven production by electrostatic Charging, characterized in that the rectified by a Air flow accompanied filaments immediately before the electrostatic Charging on the top of a wing-shaped baffle, in Filament direction between the blunt end and vertex below hit a sharp angle.

Description

The The present invention relates to a method and an apparatus for separation and spreading of stretched, thermoplastic Endless filaments by electrostatic charging in nonwoven production.

The description of the nonwoven production process with electrostatically charged filaments is given in the basic patents by di Sabato et al. ( US 3,163,753 ) and Kinney ( US 3,338,992 ). Here, the filaments are electrostatically charged by means of a corona discharge before the trigger. The filaments charged in the same direction repel each other, which leads to a better separation and spreading of the filament bundle in the depositing area and, as a consequence, to a better homogeneity and strength of the nonwoven thus produced.

A functional and efficient electrostatic charging and spreading of the filaments must meet partially contradictory requirements. First, the highest possible charge must be achieved so that the electrostatic repulsive forces against the other effective forces, such. B. flow resistance, inertial forces, bending forces, gravity, cohesion and adhesion forces can enforce. At the same time, however, the maximum charge is limited by the breakdown field strength of the air, as a result of which, for example, the surface charge of a flat surface can only reach a maximum of 2.7 × 10 ⁵ C / m ² .

Furthermore, it must be ensured that all filaments carry the same surface charge, otherwise unwanted electrostatic attraction forces arise. This is achieved, for example, by separating the filaments to be charged by a straight baffle plate designed as a counterelectrode, so that no filament covers the other during the charging process and all filaments to be charged are exposed to the same, high field strength during charging (Sternberg, ed. US 3,967,118 , Zeldin et al. US 5,225,018 A ).

There in such an arrangement but the moving filaments with the stationary Counter electrode inevitably come into intimate contact, are high Frictional forces, in connection with signs of wear the counter electrode the episode. In addition, the high thread / counter electrode friction leads to a troubled threadline with temporary congestion and compression effect, among them the fleece uniformity suffers. Above all, however, the charged filaments produce in the Counter electrode inevitably one equal "mirror" charge of opposite Polarity, a process commonly referred to as influenza. Through this influenza field become strong attraction forces of the counter electrode the filaments were exercised, the already small distance of the filaments to the counter electrode further reduce, for. B. in which they displace the air boundary layers and thus to a further increase in Thread / counter electrode friction lead. This can lead to the temporary sticking of the filaments. Thereby is not only the fleece uniformity permanently disturbed, but also drastically reduces the spread of the charged filament bundles, because the electrostatic repulsive forces at least partly by Influenzkräfte To get picked up. So here is the contradictory Task, on the one hand, the contact of the filaments with the counter electrode surface in time and spatially possible on the other hand, in order to minimize influence and contact friction a uniform and to achieve maximum charging of the filaments.

Piper et al. ( US 3,766,606 ) propose further, the drifting and towing air, which accompanies the filaments, to remove before filing the filaments from the sensitive nonwoven laying area by lateral discharge. This method is recommended in terms of a uniform fleece laying with maximum spreading of the filaments, especially in previously electrostatically charged filaments, because only by extensive elimination of the usually highly turbulent air flow, the electrostatic repulsion forces can be largely undisturbed effect. In this case, a congestion with increased static pressure is built up by a downstream of the electrostatic charge Konfusor with lateral openings and greatly reduced flow cross-section, so that the excess air escapes through the side openings. It goes without saying that such bottlenecks, in the example of only a few millimeters react even more sensitive to clogging, if the cited friction forces thread / counter electrode no uniform and continuous yarn path is guaranteed.

Reba ( US 4,334,340 ) describes a wing profile as a central element of a purely aerodynamic Filamentspreizapparatur without electrostatic charge. The chord of the wing is almost perpendicular to the direction of impact of the filaments. Over the blunt side of the wing, an additional airflow is directed over the wing to create a thick and spatially extended boundary layer on which the (uncharged) filaments "float" without friction and following the radially deflected air flow following sufficient time to spread, before being laid down to a fleece without any further spreading or air-squeezing measures.

The Target of the air supply it is here, a boundary layer separation along the wing profile to prevent. A uniform separation and spreading is not achievable.

Kamioka et al. ( US 4,380,104 describe an apparatus for separating filaments, wherein a corona discharge electrode is disposed within the extraction device in front of the baffle device and in which the baffle device may optionally also be provided with a corona discharge electrode. In this arrangement too, a considerable loss of the charge applied by the first electrode and thus only a slight spreading of the filaments takes place when passing through the discharge device. The attached directly to the baffle second electrode can cause at most a small additional spreading of the filaments by the simultaneously acting impact and loading forces. One of the baffle downstream gap with lateral air discharge, which could cause a further, additional spreading is not provided.

task It is the object of the present invention to provide a method and an apparatus for the separation and spreading of stretched, thermoplastic continuous filaments to show electrostatic charge, the above Disadvantages are completely or at least largely avoided.

The solution This object is achieved according to the invention by a method and a Device according to the instructions of the claims.

The Method is characterized in that the rectified by a Air flow accompanied filaments immediately before the electrostatic Charging on the top of a wing-shaped baffle, in Filament direction between the blunt end and vertex below hit a sharp angle.

• – einen Prallkörper, welcher die Form der Oberseite eines Tragflügels hat und mit dem stumpfen Ende nach oben angeordnet ist,
• – eine Corona-Entladungselektrode mit Gegenelektrode und
• – einen seitlich offenen Konfusor, welcher aus einer einen sich verjüngenden Spalt bildenden Platte und Konterplatte besteht,

umfaßt.The device is accordingly characterized in that it is in the filament direction

• An impact body, which has the shape of the upper side of an airfoil and is arranged with the blunt end upwards,
• - A corona discharge electrode with counter electrode and
• A laterally open confuser consisting of a plate and counter plate forming a tapering gap,

includes.

application find the method and apparatus according to the invention fresh melt-spun, in known aerodynamically and / or mechanically stretched Filaments of polyolefins, polyester or polyamide, in particular of Polypropylene, polyethylene terephthalate, polyamide-6 or -6.6, with a single titer of preferably 0.5 dtex to 8.0 dtex.

The invention will be discussed in more detail below with reference to the drawings,
in which

1 an impact body according to the invention in the form of the top of an airfoil,
and

2 a device according to the invention, in which the impact body, counter electrode and confuser plate form a unit,
represent.

The Invention is based on the use of fluid mechanical phenomena, as they are observed at the top of airfoils. This one Candidate phenomena are in particular the flow of congestion, the Acceleration of the flow, the boundary layer structure and the separation of the boundary layer. These Invention uses these phenomena considering the special requirements of the electrostatic charge of Filaments, preferably with simultaneous discharge of the nonwoven storage disturbing Driving and accompanying air.

In the preferred arrangement, an impact body ( 1 ) with a surface contour which corresponds to the upper side of an airfoil, in the front region between the blunt end and the vertex, preferably in the front third ( 18 ), of the filament bundle ( 2 ) and the accompanying towing and blowing air at an acute angle. The cross section of the filament bundle ( 2 ) is relatively small before the impact and may be of any shape, but usually of circular or rectangular shape. As a result of the impact, the filaments are separated and forced into a one-sided curved plane which follows the contour of the wing-shaped impact body.

Preferably, the top of the wing, as in 1 shown the shape of a circle segment ( 1 ) with a radius R in the range of 150 mm to 500 mm, preferably 200 mm to 350 mm, and a segment angle α in the range of 20 ° to 40 °, preferably 20 ° to 30 °, with one side adjoining tangent ( 4 ). Here, the tangent is tilted by an angle β in the range of 5 ° to 25 °, preferably 5 ° to 15 °, relative to the solder.

The adjoining the vertex of the airfoil profile, expiring in a tangent region of the baffle body serves, as in 2 shown, simultaneously as a counter electrode. This part of the baffle or the entire baffle body is made accordingly of electrically conductive material, for example metal, and set by means of grounding or high voltage generator to a defined electrical potential. The counterelectrode opposite and perpendicular to the singulated filaments is a corona discharge electrode ( 3 ), for example one or more needle electrodes. The accelerated air flow at the convexly curved surface of the airfoil-shaped impact body helps overcome the frictional and influential forces of the filaments on the airfoil surface. In addition, the forming boundary layer reduces the contact of the filaments to the counter electrode.

After covering a distance of usually 15 mm to 100 mm, preferably 30 mm to 50 mm, the charged filament jet leaves the electrode, ie charging area. From the transfer point ( 19 ) further forced guidance of the charged filaments along the airfoil contour is no longer necessary since the electrostatic charge has already been completed. It is even extremely advantageous if the charged filaments detach as early as possible from the wing surface, as in this way friction and Influenzkräfte be minimized.

This is an essential aspect of the invention. It is known, that too on aerodynamically optimized wings an unavoidable boundary layer separation in the rear area occurs. This boundary layer separation is when used as Hydrofoil undesirable, because it reduces the lift and increases the flow resistance, which is why one strives by correspondingly streamlined design of the airfoil, the detachment point preferably far in the direction of flow to move backwards. In the case according to the invention, however, the opposite is true sought, i. it applies, electrostatic Influenzkräfte and frictional forces to minimize, which only by retention time and contact area reduction can be achieved. That means in the present case is an "aerodynamic bad "wing with early Flow separation desired. there but should necessarily be associated with the flow separation Vortex formation possible repressed be so no Filamentanhäufungen or bundles can arise. Surprisingly it is managed by the choice of the wing profile according to the invention, these usually to meet incompatible claims.

Preferably, the filaments thus detached from the counterelectrode then become a laterally open confuser (or squeezing unit), which consists of a plate forming a filament-tapering gap (FIG. 4 ) and counter plate ( 6 ) is supplied. The confuser serves to divert the accompanying air, which disturbs the fleece deposit, to the side. The angle Φ between plate ( 4 ) and counter plate ( 6 ) is preferably adjusted (about 10 ° to 40 °), that in conjunction with the large separation angle of the filaments at the separation point ( 19 ) from the counterelectrode, the filaments in the confusor impact region (FIG. 20 ) against the counter plate ( 6 ) and thus experience an additional, mechanically caused spreading. The gap width at the confuser output is about 2 mm to 5 mm.

Advantageously, the confuser plate ( 4 ) a unit with the impact body ( 1 ) and the counter electrode, as an extension of the tangent region of the airfoil profile. In a further preferred embodiment, the lower half of the wing from the separation point ( 19 ) and the two Konfusorplatten in electrically non-conductive material, eg. As polyethylene, polycarbonate, perfluoropolyethylene or oxide ceramic, designed so as to prevent the braking effect of influenzieg mirrored charges.

Of the Advantage of this invention wing-shaped baffle plate across from known flat baffle arrangements manifests itself primarily in one enlarged spreading angle. With it opened the opportunity the number of exhaust nozzles and to reduce the separators and thus the energy consumption, especially the air consumption, as well as the fleece uniformity to increase, because both high air volumes and small spread angle because of Air turbulence and the low thread share coverage the fleece uniformity affect.

example 1

According to a known method (cf. US Pat. No. 3,655,862 by Dorschner) in round dies polypropylene filaments of isotactic polypropylene (MFI 25 g / 10 min) with a titer of about 2.4 dtex in an annular gap Laval propellant nozzle aerodynamically withdrawn and transferred to a circular filament guide tube with an inner diameter of 12 mm.

The filament and Verstreckluftstrahl hits at an acute angle to an impact body according to the invention with a grounded, metallic surface. The surface is curved on one side in the manner of a wing, the tangent of the wing inclined about 15 ° to the Lot. The radius of curvature of the impact body is 300 mm. In the lower part of the curvature passes tangentially into a flat surface, the at the same time forms one half of the two-part confuser. Opposite the impact and spreading region is the charging electrode, which is designed according to the prior art as a needle electrode at a distance of 20 mm.

The already through the impact mechanically vorgespreizten filaments go through the electrostatic charging zone exposed to 35 kV negative potential, dissolve at the detachment point from the counter electrode and then hit the opposite installed confuser counter plate. The dynamic pressure in the confuser leads to a lateral derivation of a considerable part of the filaments accompanying blowing air. After all The filaments leave the confuser through the 3 mm wide confuser gap in the form of a downwards opening Solid cone.

The spray cone cuts the filing screen belt 600 mm away from the confuser gap in the form of a non-woven ellipse ( 12 ) with the main axes 840 mm × 620 mm. If, instead, a known, straight baffle plate is used, then under the same conditions only a non-woven bumping ellipse with the main axes 430 mm × 410 mm is achieved.

Example 2

The Experimental arrangement was analogous to that of Example 1. The admission the exhaust nozzles however, was made with polyester filaments consisting of spun-form polyethylene terephthalate. The charging electrode was charged with a positive potential of 30 kV. It became similar achieved good results, as in Example 1 with polypropylene.

Claims (8)

Process for the separation and spreading of drawn, continuous thermoplastic filaments in nonwoven fabric production by electrostatic charging, characterized in that the accompanied by a rectified air flow filaments immediately before the electrostatic charge on the top of a wing-shaped baffle, in the filament direction between the blunt end and Vertex impinging at an acute angle. Method according to claim 1, characterized in that the Filaments after electrostatic charging a laterally open Confuser, which has a filament-tapering gap forms go through. Method according to claim 1 or 2, characterized in that the continuous filaments of polyolefins, Polyester or polyamide exist. Device for separation and spreading of stretched continuous thermoplastic filaments in nonwoven production electrostatic charge, characterized in that they are in filament direction - one Impact body, which has the shape of the top of an airfoil and with the blunt Arranged end up, A corona discharge electrode with counter electrode and - one laterally open Confuser, which from a tapered one Gap forming plate and counter plate consists, includes. Device according to claim 4, characterized in that the Top of the wing the shape of a circle segment with a radius in the range of 150 mm to 500 mm and a segment angle α in the range of 20 ° to 40 ° with it one-sided subsequent Tangent has. Device according to claim 5, characterized in that the Tangent at an angle β im Range from 5 ° to 25 ° opposite the Lot is tilted. Device according to claim 5 or 6, characterized in that impact body, counter electrode and optionally Konfusorplatte consist of a unit, with the under extended tangent the Confusorplatte forms. Device according to a the claims 4 to 7, characterized in that the baffle plate and the upper Area of the counter electrode made of electrically conductive material and optionally, the lower portion of the counter electrode and the Konfusorplatte off electrically non-conductive material.