CA2133553C

CA2133553C - Apparatus and method for producing a web of thermoplastic filaments

Info

Publication number: CA2133553C
Application number: CA002133553A
Authority: CA
Inventors: Lloyd E. Trimble; Leon M. Zeldin; William J. Grubbs; John V. Francis
Original assignee: Fiberweb North America Inc
Current assignee: Fiberweb North America Inc
Priority date: 1992-04-10
Filing date: 1993-03-29
Publication date: 1999-02-16
Anticipated expiration: 2013-03-29
Also published as: MX9302051A; DE69303711D1; NO943813D0; EP0635077A1; JP3007157B2; WO1993021370A1; BR9306222A; AU4044593A; EP0635077B1; ATE140494T1; DE69303711T2; CA2133553A1; JPH07505687A; KR100189396B1; US5397413A; KR950701021A; ES2092304T3; DK0635077T3; NO943813L

Abstract

2133553 9321370 PCTABS00027 A slot draw attenuator apparatus and method are provided for producing webs of spunbonded thermoplastic filaments having improved cover even at low basis weights. The filaments are introduced to a slot draw attenuator having corona electrodes mounted in an elongate insulator bar and staggered and spaced along one wall of the attenuator slot near the exit end thereof. The corona electrodes are electrically connected to a high voltage source. The opposing wall of the slot is grounded. A corona is created in the attenuator slot so that the filaments are charged as they exit the attenuator. The electrostatic charge induces repelling forces in the filaments so that the filaments spread before they are randomly deposited upon a forming belt.

Description

V~!O 93/71370 ~ 1 3 3 5 5 3 Pcr/~s93to29l7 .

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`:; : : :APPA~I!US ANI:) MEI!XOD FOR PRODUCI~IG A
WEB ~OF THERMOPL~STIC FILA~ENTS

Field_o~ ~b~nvention " :: ~ The invention relates to an apparatus and ; method for producing a w b:of spunbonded thermoplastic .~"~ filaments,~and more~ particularly relates to an 5~ apparatus~and~method~for;producing a spunbonded web of enhanced~unlformity and~quality.
Back~round,of the :Invention The spunbondlng process~is widely~used 'for producing~nonwo:ven~fabrics~from thermoplastic ilaments.~ Spu~nbonded fabrics can be produced by many routes,~but~the~majorlty~ o~f spunbonding processes include~the~ba~sic~steps of:~extruding continuous f~ilament~s~of:~a~fiber-forming~thermoplastic polymer, uenching~the;~filamen~s, drawing or attenuating.thel ;
':: 15 ~filaments, usually by:~a high:velocity fluid, and deposit;ing th~e~fi~laments on a collection surface~to form~a web~
Manufacturers of spunbonded nonwoven fabrics have~long~sought to improve the.manufacturing process -ZO to achieve-higher productivity and better quality and r~ ' 1 WO93/21370 ~ ~ 3 3 5 S 3 PCT/US93/02gl7 ^ I

~ uniformity of the spunbonded nonwoven fabric.
;; Maintaining the quality and uniformity of the fabric ~ ~ becomes a particular concern at higher production ¦~ speeds and when pro~ucing fabrics of low basis weigh~.
Several characteristics affect the quality and uniformity of spunbonded nonwoven ~abrics.
lament separation i5 the degr~e of separatlon of the individual filaments from one another. Good ~ilament separation occurs when th~
filaments are randomly arranged with limited parallel 1~; contact between the filaments. Ideally, no individual ¦~ filaments should be in parallel contact with another ilament, although, in practice, filaments tend to bé
in parallel contact over conslderable distances. Good -~ 15 filament separation is particularly important for light weight fabrics~,~where good coverage~is more difficult to achieve. Ropiness is the extreme state of poor , ~
filament separation. Large numbers of filaments in paral~lel twisted contact result in long strands in the fabric, whlch can causes holes or very thln areas in the~fabrlc. Splotchiness lS a relative large-scale non-uniformity in basis weight. A fabric having splotchiness is generally weak because of`the lower : ~ . i;
tensile strength of the thin araas of the fabric.
Also, a splotchy fabric ge`nerally has poor cover properties. ~
In the early spunbond processes which used round attenuator tubes to attenuate and draw the 1 ~ 2133S~
WQ93/21370 PCT/US93/~2gl7 -3_ filaments, achieving good uniformity and adequate cover presented significant challenges, particularly when manufacturers attempted to produce lighter weight w~bs or to produce webs at higher speeds or reduced cost.
¦ 5 The round attenuator tubes, often called Lurgi tubes, ~: typically use large quantities of high pressure air :that pro~ide the attenuation force for the filaments.
This results in high utility costs and high noise levels. Increasing the number of filaments in each - lO ~ube to increase productivi~y and to reduce the utility expense results in increased problems of poor filament separation, ropiness and webs having poor cover.
Many attempts have been made to overcome the above problems of filament separation, ropiness and 15 :splotchiness while~still preserving the tensile properties of nonwoven webs made from spunbonded hermoplasti;c filaments. For example, United States :: Patent Nos. 3:,2~6,678; 3,485,428 and 4,153,305 describe various apparatus and methods for mechanical and pneumatic oscillation of continuous filament bundles to sp~ead the~fi1aments as they are deposited on the collection surface. United States Patent No. 4,334,340 dèscribes~using~an a1r foil at the exit of a round attenuator tube to separate continuous filaments prior to thèir deposit on a forming wire. Forced air follows : ; the leading edge of the air foil and fil~ments striking ; the foil are carried by the forced air onto a forming ~: ~ , ., ~: l I

: W093/~137~ ~1335~ _4_ PCT/US93/02917 wire, resulting in a spreading of the filament bundle that promotes random deposit of the filaments.
Various electrostatic methods have been proposed to promote spreading of the filament bundle by applying an elec~ric charge to the filaments to cause the filaments to repel one another~ United States Patent No. 3,33~,992 describes triboelectric charging, in which the filaments are charged by rubbing contact with a suitable:dielectric material and repelling ~ 10 ~orces induced in the filament bundle cause the fllaments to separate as they exit a forwarding gun and prior to deposit o~ the forming wire. However, rubbing ~ .
~ contact:typically is not desirable fo~ more delicate ~, , webs, and:this method is al50 subject to lack cf raliability when ambient conditions change. The above-.
: :
: noted United:States Patent Nos. 3,338,992 and 3~296,678 : al50 de~Gribe electrostatically charging the filament :~: ~: :
;~: bundle with an ion gun or corona discharge device prior ;: to drawing and forwarding the filaments.
.~ 20 ~ United Sta~es Patent No. 4,~08,366 describes : a spunbonding process without the use of forced air attenuation,~ but which includes electrostatic treatment : of the filament bundle. The extruded filaments pass hrough an electrostatic charging zone and are drawn through a nip between elastomer covered draw rolls.
The charged filaments are propelled by the draw rolls , î~
into an el~!ctrostatic field generated between the rolls ~.
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and the collecting surface, which attracts the filaments to the collecting sur*ace.
United States Patent Nos. 3,163,753, 3,341,394, and 4,009,508, and PCT ~pplication No. WOs,107,530 relate to the use of corona electrodes for electrostat.ic treatment of filament bundles attenuated with round attenuator guns. In U.S. Patent No.

3,163,753, the filament bundle is passed adjacent a ~ charged corona electrode while passing over a grounded : ~ 10 bar. In U.S. Patent No. 3,341,394, a corona is applied : while the filaments are under tension and before the : filaments enter the attenuation tube. In U.S. Patent -No. 4,00g,508, the filaments are subjected to ~ electrostatic treatment from a corona after they have : . 15 been discharged from the round attenuator tube and while the filaments impinge upon a target electrode for spreading:in the electric field. In PCT Application No. W0 9,107,530, the filaments exit a round attenuator tube and impinge upon the op~osed, converging deflector`
plates of a filament ~rans~er channel where the filament~ are mechanically spre~. Charging pins optionally are provided at the exit of the channel for electrostatic separation of th~ already mechanically : spread filaments. :
: 25 . ~ Various slot attenuato~s have been developed to : overcome the pr~blems and limitations of the~round .~ .
: : a~tenu~tor~. In a slot attenuator, or slot draw process, the multiple tube attenuators are replaced :: with a single slot~shaped attenuator that covers the .:
3 0 ! full width o~ the machine. A supply of air is admitted ; : into the slot attenuator below the spinneret face. The air procaeds down the attenuator channel, which narrows in width, creating a venturi effect to accelerate the -air flow and cause~ilament attenuation. The filaments exit the attenua~or channel and are collected on the forming wire. The attenuation air, depending on the : 8UBS~ITU~E S~EET

~2~335~ 3 -6- PCT/US93/~2917 type of slot draw process used, can bP directed into the attenuation slot by a pressurized air supply above the slot, or by a vacuum located below the forming wire. Slot drawing has various advantages over ~he Lurgi and other tube-shaped attenuator processes. The slot attenuator is self-threading in that the filaments fall out of the spin block directly into the slot attenuator. The high pressure air used by Lurgi devices is ~ot always required, thereby reducing noise ~ 10 and utility costs.
However, despite the advantages of the slot draw processj cover problems can still occur, particularly for lighter weight fabrics. The forced air stream can introduce turbulence at the point where the web is formed on the collection surface, which ~; adversely affects the quality of the web.
'7 Additlonally,~manufacturers are still attempting to produce webs at higher processing speeds, which compounds t.he problem. For example, U~S. Patent No.

4,753,698 descr~bes a technique for mechanically oscillating the rank of filaments exiting a slot draw attenuator and applying vacuum through the forming wire o fix the filaments in place. Coanda rolls set up à
pendular movement in the filament rank. However, the swinging velocity of the filaments at the reversal points is zero, and, unless special precautions are taken, pile-ups can occur at the reversal points. j-.
' ' r ~ c r o ~~ 0 2 1 3 3 ~) 5 3 r ~ p r ~ p ~
r t r t t r r ,~ In view of the advantages of the slot draw process , over prior filamen~ attenuation techniques, it would ~e ;i desirable to provide a slot-draw process capable of l 5 producing spunbonded fabrics having better cover 33 properties. Accordingly, it is an object of the present invention to provide a slvt draw process and apparatus for producing a spunbonded nonwoven web having improved cover properties~ More particularly, it is an object of the present invention to provide a slot draw process and apparatus capable of producing 1~ .
$: nonwoven webs having excellent cover characteristics, despite low basis weigh~ or ~igh processing speeds.
Su~mary_of the Invention In accordance ~ith the invention, a slot draw~
attenuator is provided with a corona device positioned ~; for electrostatically charging filaments leaving the attenuator so that el~ctrostatic repelling forces are 1~ induced in the ~ilaments to more uni~ormly spread the ¦ 20 filaments befor~ they are deposited on a collection I : ~urface to form a web. . -The slot draw a~tenuator, m~re particularly, has opposing side walls and op~osing end walls defining an elongate entrance slot for receiving the ~ilaments, an elongate exit slot ~rom which the filaments are ~ expelled, and a slot-shaped passageway extending `~. : between the entrance and the exit and through which the ~ilaments travel while being drawn and attenuated.
: Means is provided cooperating with the attenuator for 30 , lintroducing a flow ~f air through the slot-shaped , : passageway and for drawing and attenuating the filaments passing through the slot-shaped passageway.
A collec~ion surface is positioned adjacent the exit ; : slot of the attenuator ~or receiving S~BSTITUTE S~EET

WO93/21370 3s5 3 -8- PCT/US93/02917 the filaments that are expelled from the attenuator to form a web. The corona device includes an electrode means that is carried on the walls of the attenuator -and is positioned for generat.in,g an electros~atically charged field across the slot-shaped passageway through which the filaments travel;
More specifically, the electrode means includes a seri s of point or wire corona electrodes that are carried by the exit slot on one of the : lQ opposing attenuator walls. These corona electrodes are located in a staggered relation to one another at spaced locations across the width of the wall of the at~enuator. A ground lS connected to the other ~:~ opposing wall of the attenuator. The high voltage ~ 15 power~source lS connected to each of the corona : ~ electrodes for producing a corona discharge, i.e. an electrical discharge in the air surrounding the corona electrode. The power is supplied through an electrical : conductor that is carried by an elongate insulator bar ~ .
. 20 a~tached to the attenuator wall. Each of the corona electrodes i5 mounted along the elongate insulator bar .
:: and is electrically connected to the electrical :
conductor~through a high voltage resistor. ! ~ .
The present invention also provides a method 25 of producing a web of thermop~astic filaments in which .
the ~ilaments are directed into and through an elonga~e , :~ slot-shaped passagaway while being attenuated and ~ dxawn. The filaments are electrostatically charged in : ' ' .
.
:

21335~3 W093/2137~ . . PCT/US93/02917 _ g ~
the passageway and are then expelled from the passageway while the repelling forces induced in the filaments by the electrostatic charge cause the filaments to repel one another, thus more uniformly ~:~ 5 spreading and distributing the filaments. The ; ~ filaments are then deposited on a collection surface ~o : form a web.
:
More specifically, the method includes - passing the f~ilaments through a corona zone wherein a lO ~high voltage is applied to a series of corona electrodes located aIong one of a pair of.opposing walls in the:slot~shaped passa~eway. The ~lectrodes gener~ate a corona in the s~lot-shaped passageway between the wall ca~rylng the electrodes and extending to the grounded other wall.
The apparatus and method of the invention are :capable of producing spun:bonded webs of enha~ced uniformity and quality~as compared to prior practiceO
Additlonally, by practice of this invention, it is ~ ,, possible to pr:oduce spunbonded nonwoven fabrics that have~acceptable;cover and tensile properties at basis weig~ts significantly lower than produced by previous apparatus;and methods.
Brief Descriptlon or the Drawin~s ~ 50me of the features and advantages of the nvention have been stated, other advantages will ~ be ome apparent as the description of the invention '" ~': ' : -~ ~ I

:~' W093/21370 2 ~3 ~S ~ ~ -10- PCT/US93/~2~17 proceeds, taken in conjunctlon wi~h the accompanying ~! drawings, in which: I
Figure l schematically lllustrates an apparatus for forming a spunbon~ed nonwoven web in accordance with the invention~
Figure 2 is perspëctive view of a portion of the apparatus of Figure l showing the slot draw attenuator;
i.3 Figure 3 is a transverse section through the t~
- lO slot draw attenuator, taken along line 2-2 of Figure 2 ; and showing the corona electrode assembly used for electrostatically charging the filaments;
Figure 4 is a longitudinal section through the corona eIectrode assembly taken along line 4-4 of Figure 3;
I Figure 5 is a perspective view of a portion showing a group of pin-shaped point electrodes mounted ~: in a mounting block for insertion into the corona electrode assembly;
0 Figure 6 is an enlarged fragmentary cross-sectional view of the corona electrode assembly taken from Figure 3 showing the attachment of the electrodes to high voltage re`sistors; and Figure 7 is a perspective view simïlar to !
Figure 5, showing an alternate form of corona electrode assembly. :1 ~: 'i, :r7 - .

~ 21335~3 ' wo 93J21370 PCr/USg3/02917 ~?, Description of the Illustrated Embodiment j! .In Figure l, reference lO generally indicates j an apparatus for producing a spunbonded nonwoven web of continuous filaments. The apparatus lO includes a melt 5 spinning section for producing continuous filaments of a thermoplastic polymer, including a feed hopper 12 for receiving the polymer raw material in granular or pellet form and;an extruder 13 for heating the polymer to a molten plastic state. The spunbonding process is lO applicable to a large variety of polymer resins, copolymers, and mixtures thereof, and the skilled artisan will recognize that the present invention is not restricted to the specific resins that may be used.
The~molten poIymer is directed from the 15~ extruder 13 at a~controlled, metered rate to a generally linear die head or spinneret 15 where the molten polymer is extruded as streams from fine die ; orifices to form continuous filaments F. The filaments ; are~quenched by a supply 16~of cooling air and are 20 directed to a~ sIot draw attenuation device 17 which co~ers~the ful1 width~of the spunbonding machine. A 3 supp1y~of air is adm1tted into the slot attenuator 17 below theispinneret face The air proceeds down the attenuator channel, wh1ch narrows in width in the 25 direction away from~the spinneret, creating a v nturi effect, causing acceleration of the air and attenuation of filaments. The filaments exit the lower end of the - : ,' :,: ~:~

..

' ~. W093~21370 2 13~ 5 5 3 PCT/US93/029~7 ;~ -12-:., attenuation device and are randomly deposited on an .endless forming belt 20 to form a web W.

The attenuation air, depending on the type of slot draw process used, can`.~e directed into the .!~ , _ attenuation slot by a pr~ssurized air supply above the slot, by a vacuum located below a forming belt, or by the use of eductors integrally formed in the slot. In ~: ~ the embodiment illustrated, the slot draw attenuator 17 includes an:eductor 22 which introduces air into the - 10 a~tenuator 17 between the inlet and exit ends thereof.
!~
:~ A corona devicel generally indicated by reference 1~, is located adjacent the exit end of the attenuator. The corona device generates a corona of :: ~
~ ionized air through which the filaments F pass as they ¦:~ 15 travel through:the attenuator, which introduces an : electrostatic charge on the filaments, causing the filaments to repel one another. The filaments thus separate and spread apart from one another as they exit : ~ the attenuator before being deposited randomly on th~
20 end~ess forming belt 20. The corona device is describe~ :
~: more;fuIly below with reference to Figures 2 through 7.
; : Endless forming belt 20 forms a driven loop ; 20' that has a generally horizontally extending~run 24 for supporting web W and for transporting the web from : 25 the initial lay-down point 26. ~uide rolls ~8 located , inside loop ~0' extend in substantially parallel relationship in the cross direction of the belt 20 for : supporting the belt. Belt 20 is preferably of a porous ,~
'.

~.1 .
~3 ~21335~ 1 WOg3f21370 ; PCT/US93/~2917 ~?, 13-or foraminous construction so that air from attenuator ~,~ 17 can pass through the ~elt and so that vacuum can be applied to the web W through the belt to provid,o enhanced control over the web during formation and transfer.
~:~
As shown in Figure l, as the web W reaches the downstream end of the belt 20, it is transferred from the belt and is advanced through a calender nip 32 formed between cooperating rolls 30 and 34. The filaments of the~we~b are thermally bonded together as ~they pass through~the calender nip~ Preferably, the ;
; ~~ one of the rolls has a smooth surface and cooperating roll is p~ovided~with a patterned surface so that thermal bonding~takes place at~discrete locations or points over the surface of ~he web.
After passlng through nip 32, the now therma~lly bonded~ web lS directed along ~he calender r~oll~surface to a wlndup roll 42. Windup roll 4Z may be~of~any conventional~type. In ~he embodiment shown, 20~ support~rolls~43~ and 44 support and rotate~the roll 42 of~spun~onded~nonwoven fabric.
Also shown in Figure l is a vacuum box ~8 nslde thè loop Z0' that applies a vacuum through belt 20~for holding and immobilizing the web W with respect to~the belt 20.~ Vacuum box~48 is a conventional sheet metal encIosure having a vacuum source connectea thereto through conduit 5Q. Also the vacuum box 4B may be used to facilitate the attenuation of the filaments, ;

wog3/~1370 ~33S S 3 PCT/US93/~2917 ~;

as was explained above, by drawing alr through the slot draw attenuator 17.
The slot draw attenuato,r~,l7 will now be described in more detail in con~,ec~ion with Figure 2.

, As shown, the att~nuator has,opposing walls 52 and 54 that define an entrance slot 56 for receiving the filaments F from spinneret 15 and an exit slot ~ from which the attenuated and drawn filaments are discharged. The opposlng walls 52 and 54 also define an - lO elongate slot-shaped passageway 60 (Figure 3) that extends between the entrance 56 and the exit 58 and through which the filaments F travel while being dxawn ~:~ and attenuated. Eductors 2~, associated with walls 52 and 54, inject air into the slot shaped passageway 60 and along a downward flow pa~h at a location just below , ~ the entrance slot 56. Air is dis~ributed to the ~ o eductors through manifolds 62 and 6~.
The corona device 18 is located adjacent the ' exit end 58 of the slot attenuator 17. As shown in ~ ~ 20 Figure~2, it includes a corona electrode assembly 66 : - that is carried by attenuator wall 52 and extends the full width of wall 5~ in the cross direction. The ëlectrode assembly 66 is connected to a high voltage ~`: power:source l9 and ~he opposite attenuator wall 54 is ~ .
25 grounded. ~ c The electrode assemkly 66 includes an ~' elongate bar 6~ formed of an electrical insulator with high dielectric strength, such as plastic. Insulator ~ 2133553 W093/.~.1370 . PC~/US93/02917 bar 68 is attached to the outer surface of attenuator : wall 52. As can be seen more clearly in Figure 3, the bottom edge of attenuator wall 52 terminates a short distance above the bottom Pdge of the opposing 5 ,attenuator wall 54 and the insulator bar 68 has a projecting shoulder portion 69 extending from the body :~ :of- the insulator bar 68 a distance corresponding to the thickness of~the wa~ll 5~ so that the inner exposed face of the shoulder portion 69 lies coplanar with the inner surface of attenuator wall~52. The projecting shoulder portion~69 of the insulator bar 68 thus forms the bottom portion:of the attenuator wall and is located directly;oppos~te: the oppcsing~grounded attenuator wall : 54.~Shoulder~pGrtion ~9 i:s shown:enlarged in Figure 6.
15~Located~ln the projectlng~shoulder portion:are cavities '70~i:n which~are:~mounted a series of spaced apart point el~ectrodes~ln~the:f.orm~of conductlve metal pins 72 with ends~which taper to~sharpened~polnts projecting into the~pa~saseway~ ~'o~a~short distance. The pins 72 are 2`0:~or:iented:toward~;~th~e:opposing grounded attenuator wall 5~ for~cre~atlng~a corona of ionized air across the entire`:passageway:~60~adjacent~the discharge end 58 of the a~tènuator;~'slot.~
eferring now to Figure 4, it will be seen that the:pins~72~are~arranged in gro'ups extending from a~mounting block 7~4~formed of~an electrically insulating material~with high dielectric strength. A
:sin~le mounting:block and~associated corona electrode 2~33~3 ~ 1-W093/21370 P~T/US93tO2917 ; , pins are shown in enlarged perspective in Figure 5.
The mounting blocks are seated on the floor of the cavity 70 and are arrang~d in ~wo vertically spaced apart rows e~tending the full width of the insulator bar. The mounting blocks in e~ch row are spaced apart . ~ :
from one another and the mQ~nting blocks in one row are arranged in offset or staggered relation to the : mounting blocks in the other row so as to insure that the electrically charged corona field produced by the corona electrodes is uniform and covers the full width : : :
. of the passageway 60 from left to right as seen in ;-Figure 4.
~ ~, The respective ~ins of each mounting block 7~

are connected to high voltage power source 19 through a .
15~ resistor 76.~The reslstors are located in vertical : bores formed ln~the insulator bar 6~. The lower end of each~:resi:stor is elec~rically connected to the : ~ .
respective~pins 72:of a mounting block 74 through a : entral lead and~the upper end of the resistor is connected to an electrical:conductor or buss 78 which extends the full width of the insulator bar 6B to distribute a high voltage from power source 19.
Any high voltage DC source 19 ~ay be used to : establish the:electrostatic field between the corona ~, ~
: 25 electrodes and grounded opposing slot wall 54. The source should preferably have variable voltage settings up to at least about 50 kV and, preferably, (-) and (+) ;~ :
~ 212~
W~g3/21370 ~ J ~ PCT/US93/q2917 polarity settings to permit adjustments in establishing the electrostatic field.
;~ When the filaments pass through the corona, they become electrostatically charged, which causes the ~ ;
filaments to repel one another and to separate and to spread apart as they enter the free fall zone located ; between~the attenuato~r 17 and the forming belt 20 and continue to do so until deposited on the forming belt.
The free~fall~zone should be of sufficlent length to }O~provlde for the~deslred filament separation in the web.
Figure 7 shows ~an alternative form of the corona electr~ode wherein the electrodes are in the form ;of a~wlre rather~than~in~lviùual pins. Thus, as shown in Figure~7,~the~mounting block~74' has a corona I5~ electrode ~in~the~form~of`a wire 72' extendinq the length~of~the-moun~tlng block.

This-exàmpl~e~compares the physical properties of~;spunbonded webs~;of~arlous basis weights produced in 20~ acsordance~w~lth~the~;present invention uslng a corona~
device with`~webs of~comparabIe basis weight produced by similar processing conditions but without the corona The~results~tabulated below were achleved 2~5~ under~the~following~process conditions. A
polypropylene~polymer was melt extruded and drawn by a slot~draw~at~enuator at a filament speed of approximately lOOO to 3000 meters per minute. The ~ W~93/21370 2~33553 PCT/US93/02917 distance between the corona device and the forming wire was 350 - 600 mm. The distance between the tip of the pins and the opposite grounded conductive plate was 11 mm and a voltage OI from 8 to 30 kV was applied from a high voltage source to the pins. Additionally, a vacuum was applied to the forming wire of from 8 to 180 ~ mm of water and the forming wire traveled at : ~ approximately 5G to 200 meters per minute. Samples 1, 3 and 5 were:produced with the corona device operating and are thus ln accordance with the invention. Samples 2, 4 and 6 are control samples produced on the same apparatus under similar processing conditions, but with the corona device inoperative. Results achieved under these conditions are tabulated below.
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wog3~21370 213355 PCT/US93/02917 As can be seen from the comparative examples, the fabrics produced by practice of the present invention have dras~ically improved physical prcperties as compared to the control sample of comparable basis 5 weight. The tensile strength, both in the machine direction and in the cross direction, is significantly increased. Additionally, the percentage breakthrough is greatly reduced. The percentage ~reakthrough is a measurement of the level of penetration o~ certain size lO particles during a given time. The lower the , percentage breakthroughj the better the quality and cover properties of the web. As also seen from the table, the degree of formation greatly improves with electrosta~ic application. Formation is the visual 15 appearance of the web, indicating how uniformly the filaments are distributed throughout the entire web.
~.
This e~aluation also takes into consideration such defects as streaks, splotches, light spots or even holes, and the presence of ropiness. Formation is ; 20 evaluated by trained individuals visually on a ssale o~
0 to 5, with 5 being the best.
, ~; It should be understood ~hat the specific ; em~odiments described in detail hereinabove and illustra~ed in the drawings are specific examples of how~the pres~ent inven~ion may be practiced and that the invention is not limited to these specific embodiments.
Those modi~ications that come within the meaning and range of equivalence of the claims are to be included within the scope of the invention.

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Claims

WHAT IS CLAIMED IS:

1. An apparatus for producing a web of thermoplastic filaments comprising:
a) a slot draw attenuator having opposing side walls and opposing end walls defining an elongate entrance slot for receiving filaments, an elongate exit slot from which the filaments are expelled, and a slot-shaped passageway extending between said entrance and said exit and through which the filaments travel while being drawn and attenuated;
b) means cooperating with said attenuator for introducing a flow of air through the slot-shaped passageway and for drawing and attenuating the filaments through the slot-shaped passageway;
c) a collection surface positioned adjacent said exit slot of said attenuator for receiving the filaments that are expelled from said attenuator to form a filamentary web; and d) corona means positioned for electrostatically charging the filaments that are expelled from said attenuator so that repelling forces are induced in the filaments to more uniformly spread the filaments before they are deposited on said collection surface to form a web.

2. The apparatus according to Claim 1 wherein said corona means includes electrode means carried on said walls of said attenuator and positioned for generating an electrostatic field through which the filaments pass as they travel through said slot-shaped passageway of said attenuator.

3. The apparatus according to Claim 2 wherein said electrode means includes a series of corona electrodes carried by one of said opposing attenuator walls, said corona electrodes being located at spaced locations along the length of the slot-shaped passageway, a ground connected to the other of said opposing attenuator walls, and a high voltage power source connected to each of said corona electrodes.

4. The apparatus according to Claim 3 wherein said electrode means includes an elongate insulator bar carried by said one attenuator wall adjacent said exit slot, and an electrically conductive buss carried by said insulator bar, and wherein said corona electrodes are mounted at spaced locations along said elongate insulator bar and are electrically connected to said conductive buss.

5. The apparatus according to Claim 4 wherein said corona electrodes each comprise a pin having a sharpened point facing into the slot-shaped passageway and a high voltage resistor electrically connecting the pin to said buss.

6. The apparatus according to Claim 4 wherein said corona electrodes each comprise a wire facing into the slot-shaped passageway and a high voltage resistor electrically connecting the wire to said buss.

7. The apparatus according to Claim 3 wherein said corona electrodes are located in staggered relation to one another at spaced locations across the width of said one wall.

8. The apparatus according to Claim 1 further comprising means for extruding filaments of a thermoplastic polymer.

9. The apparatus according to Claim 8 wherein said filament extruding means comprises a spinning beam.

10. The apparatus according to Claim 1 further comprising means for bonding the filaments together after they have been formed into a web on said belt.

11. The apparatus according to Claim 10 wherein said filament bonding means comprises a calendar nip for bonding the filaments together after they have been formed in to a web on said belt, said apparatus further comprising a windup roll for winding the spunbonded web after passage through said calendar nip,

12. The apparatus according to Claim 1 wherein said filament drawing and attenuating means comprises an adductor for inducing a flow of air through said slot-shaped passageway for drawing and attenuating the filaments.

13. A method for producing a web of thermoplastic filaments comprising directing a plurality of filaments into and through an elongate slot-shaped passageway while attenuating and drawing the filaments as they travel through the passageway, electrostatically charging the filaments as they travel through the slot-shaped passageway, expelling the electrostatically charged filaments from the elongate slot-shaped passageway while permitting the repelling forces induced in the filaments by the electrostatic charge to more uniformly spread the filaments, and depositing the thus spread filaments on a collection surface to form a web.

14. The method according to Claim 13 wherein the step of electrostatically charging the filaments comprises passing the filaments through an electrostatic field formed by a corona.

15. The method according to Claim 14 wherein the step of passing the filaments through an electrostatic field includes applying a high voltage to an electrode located along one of a pair of opposing walls in the slot-shaped passageway and generating a corona in the slot-shaped passageway between the electrode and the opposing wall of the slot-shaped passageway.

16. The method according to Claim 15 wherein the step of applying a high voltage to an electrode comprises distributing the high voltage among a series of corona electrodes extending into the slot-shaped passageway at spaced locations along one wall of the passageway.

17. The method according to Claim 13 further comprising the steps of advancing the collection surface as the filaments are deposited thereon to form a web and thermally bonding the filaments to form a unitary web.