CA1309853C

CA1309853C - Nonwoven fabric and method of manufacture

Info

Publication number: CA1309853C
Application number: CA000563156A
Authority: CA
Inventors: Leonard Robert Lefkowitz
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-03-31
Filing date: 1988-03-31
Publication date: 1992-11-10
Anticipated expiration: 2009-11-10
Also published as: EP0285376A2; GB2202873B; NO173621B; GB2202873A; NO173621C; ZA882210B; JPS6420371A; EP0285376B1; DE3877794T2; BR8801421A; NZ224050A; DE3877794D1; FI881493A0; FI881493A; FI93558C; EP0285376A3; AU1372388A; US4740409A; GB8807416D0; NO881343L

Abstract

ABSTRACT

This invention concerns a nonwoven fabric having knuckle-free planar surfaces comprising parallel linear machine direction yarns (10) residing in a single plane and interconnecting, cross-machine direction polymeric material (18) also residing in said plane and substantially entirely surrounding the machine direction yarns (10), the cross-machine direction material containing spaced apertures in the fabric. The invention is also directed to the method of producing such a nonwoven fabric.

Description

"` 1 309~53 NONWOVEN FABRIC ~ND MET~OD OF MAN~F~'TURE

This inven-tion relates to a nonwoven fabric and has parti.cular, -though not exclusive, reference to nonwoven paper machine forming fabrics, felts and dryer fabrics and S a method for the manufacture thereof.
For many years weaviny has been the principal method of constructing fabrics for use on paper machines. In the case of papermaker felts, some success has been achieved with needle punched nonwoven felts of the "fillingless"
type, (see for example, U.S. Patent No. 3,392,079). Such felts are made by winding spaced apart parallel machine direction yarns around tension rollers, covering the yarns with fibre batt, and needling the batt into the yarns from both sides to form a consolidated nonwoven felt.

Under low press loadings such fillingless felts operate satisfatorily; however, under high press loadings fillingless felts are not satisfactory because the machine ~irection yarns leave undesirable indentations in the paper sheet. Furthermore, such fillingless felts are prone to wi.dening during operation on the paper machine. Thus, such felts are seldom used on highly loaded presses.
Although nonwoven felts are known, it has not been , . . . .

, `` 1 ~09~53 hitherto possible to Make paper machine fo~mLng fabrics wi-thout weaving. A satisfac-tory forming fabric made by needling batt fibres into a parallel array of yarns in the same manner as fillingless felts i5 not practical, because such a product would lack sufficient surface uniformity and drainage, would tend to fill up with fines, and would inte~fere with release of t~e sheet after formation.
Nevertheless, the concept of producing a forming fa~rlc without weaving, has been the subject of con-tinuing interest due to the high cost oE manuEacturing woven forming Eabrics by present me~hods.
In -the case of paper machine forming fabrics, present trends are toward multi-layer woven Eabrics having coarse woven substrates made of thick yarns in the wear side of ~he Eabric, with small yarns woven in Eine mesh in the sheet f~rlning side of the Eabric. The coarse yarns are used to impart wear resistance to the fabric, while the fine mesh surEace is used -to achieve good sheet formation.
It has not been possible to make a woven forming fabric with a smooth sheet forming surface using the same coarse ~arns needed to achieve the desired wear properties. The multi-layer Eabrics, while pro~iding bo~h smooth surfaces and good wear properties, are difficult and time-consuming to manufacture due to their structural complexity.
Nonwoven fabrics and the manufacture thereof are well known. For example, U.S. Patents Nos. ~,259,399 and . ~ 4,285,748, disclose the preparation of nonwoven fabrics.

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However, fabrics proposed therein are not suitable for the applica-tions contemplated here.
With reyard to papermakers felts having porous incompressible woven substrates capped with porous fibre layers, known as batt on mesh felts, such felts require sufficient batting to reduce the extremes oE pressure that would otherwise occur over ~abric knuckles or weave crossovers. This is necessary since otherwise uneve~
pressure would reduce press dewatering efficiency and might have an adverse effect upon sheet quality.
When prior art woven felts are made with incompressible monofilaments, sucn base fabrics invariably contain intereconnected water flow passageways between yarns. Such passageways allow water to flow forward in the machine direction through the ~elt base fabric while the felt and paper sheet are subjected to hydraulic and mechanical pressure in the press nip. Such transverse water flow orward in the nip may contribute to increased sheet moisture and reduced pressing eficiency.
Transverse passageways in the woven base fabric also provide channels for the entry of air into the e~panding sheet and felt as they leave the press nip. It is suspected -that such air entry into the incompressible woven felt substrate may facilitate water transfer from within the sheet~felt interface back into the expanding paper shee-t as it emerges rom the press nip, thereby reducing .: . . ' ~ .' ' ' 1 309~53 press efficiency.
In the manufacture of filtration fabric belts for sludge concentration, as well as other purposes, it is often desired to produce a fabric having many small openings, made from yarns that are large enough to provide long service life. Where such yarns are thick monofilaments, they are difficult to weave close enough together to provide the fine mesh openings desired. A
compromise is often necessary, the compromise involving a trade-off to smaller yarns so that the desired mesh can be woven.
A nonwoven paper machine dryer fabric comprised of plastic monofilament spirals is also known. Each spiral is joined to its neighbour by means of a pin inserted through the intermeshed loops of the adjoining spiral to form an endless porous dryer fabric belt. As a result of this joining method, such belts are susceptible to failure should any one of the thousands of spiral connecting pins fail during operation on the paper machine.

Poxous plastic sheets are known wherein holes are created in the plastic sheet duriny formation, such as by extrusion of two sets of filaments at right angles to each other which are fused together at crossover points. Such sheets are comprised of unoriented polymer material, and if produced in the fineness needed for papermaking applications, they would lack sufficient dimensional stability to operate as endless belts on paper machines.

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, 1 309~53 This type of material may be subjected to stretching to orient the filaments to achieve increased stability, but at the price of causing the spaces between members to increase beyond the fineness needed for certain applications.
Dimensionally stable plastic sheet material may be rendered porous by means of perforating, drilling or the like; however, such fabrication methods preclude the use of combinations of materials that are particularly suited for specific tasks. For example, machine direction stretch resistance may best be satisfied with high modulus materials, whereas, cross-machine direction needs may call for materials that are resilient and of lower stiffness.
It is an object of the invention to provide dimensionally stable nonwoven fabrics for paper machines as well as other uses having machine direction reinforcing members and smooth surfaces.
It is also an object of the invention to provide fabric for paper machines and other uses with essentially no vertical or horizontal crimp, and with machine direction and cross-machine direction members lying substantially entirely in -the same plane~
It is a further object of the invention -to provide nonwoven felts for papermaking and other uses wherein the non-deformable base fabric layer contains respective fluid flow passageways perpendicular to the plane of the fabric.
It is a ~yet further object of the invention to provide felts or forming fabrics having fine fibre or foam sheet .... - ~

1 309~3 contact surfaces integrally bound to a nonwoven base fabric.
It is a still further object of the invention to provide nonwoven fabrics for papermaking as well as for other purposes having high modulus load bearing reinforcing elements disposed in the machine direction and substantially entirely encapsulated by porous polymeric matrix material characterised as having resistance to abrasive, chemical, or heat degradation.
It is additionally an object of the invention to provide nonwoven fabrics wherein the cross-machine direction polymeric matrix material may be selected from a wide range of materials, including at least some materials which are not formable into textile yarns.
This invention comprises a nonwoven abric having essentially no vertical or horizontal crimp, where machine direction and cross-machine direction members all reside in the same plane. More particularly, the nonwoven fabric of the invention comprises spaced apart tension bearing flexible monofilament, multifilament, or spun yarns extending substantially in a common direction and polymeric matrix material interconnecting and at leas~ partially encapsulating each of said yarns, the polymeric matrix having apertures therein extending through the fabric.
Forming fabrics for paper machines operate at high ~ .
speeds while under substantial machine direction (i.e., the direction in which the fabric runs) tension. In act, some :
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~, :, ", t 3~5~i machine positions are so sensitive to stretcl--that even a one percent extension may result in the need to remove the fabric from operation on the machine. In the case oE woven forming Labrics, such fabrics are subject to extension by two mechanisms; machine direction yarn stretch and machine direction yarn straightening through crimp interchange. In U.S. Patent No. 3,858,623, a woven ~abric construction taught which avoids stretch due to crimp interchange.

In the fabric of this invention thexe is no crimp imparted to the machine direction yarns because they are not in-ter]aced with a second system of yarns as in woven fabric. Rather, the machine direction yarns lie perfectly straight within the surrounding and substantially encapsulating polymeric matrix material. Demand for cross-machine direction stability may readily be me~ by the matrix material, even though it is not oriented, as are khe high modulus machine direction yarns. Cross~machine direction loads are only a small fraction of machine direction loads. More important is the ability of the matrix material to provide the necessary long lasting wear surfaces needed for competitive performance in this application.
The matrix material may be selected from a wide variety of polymeric materials without regard for their ability to be formed into fibres or yarns. ~ne of the best materials in terms of wear resistance is polyurethane.

,.. ~,,. ,. : --" 1 309~)53 ~8--Even though this material is not available in usable yarn form, it could be used for the matrix material of fabrics of this invention intended for the forming fabric application. Other useful materials include polyesters, such as polyethylene terephthalate, polyamides, such as nylon, specifically nylon 6, nylon 6,6, or nylon 12, and polyethylene. The matrix material preferably has a melting temperature lower than that of the yarn employed.
In the case of felts for papermaking and like purposes, the porous matrix layer is comprised of non-deformable polymeric material. By non-deformable it is meant that any deformation that may take place during passage of the felt through the paper machine press nip would be minimal such that fluid passageways contained within the non-deformable matrix layer would remain open, thereby continuing to provide void space for the accommodation of fluid even under high pressure loading conditions.
The preferred papermaking feit of this invention would have a sheet contacting surface layer comprised of porous resilient fibres or foam attached to the base layer by fusion bonding or the like. Preferably, the fibrous surface layer would be mechanically interlaced as, for example, by needling prior to attachment to the base. This would give batt fibres some vertical alignment and would impart physical integrity to the batt prior to attachment.
It would also ensure that fibres from the top of the batt ' ~ ~0`~5:~
g as well as those from the bottom of the batt would become adhered to or embedded within -the non-deEormable matrlx materlal of the base layer.
Where fabrics of the i.nvention are intended for geotextile or filtration applications, porous fibres or foam may optionally be added to one or both sides of the yarn containing matrix material. ln the alternative, they may be positloned throughout -the matrix material itself, according to the application requirements.
In the case of nonwoven fabrics of the invention intended for the paper machine dryer section, machine direction tension bearing members may preferably be selected from among those materials noted for their hydrolytic stability and resistance to heat degradation..
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Materials such as Nomex or Kevlar (polymeric materials avai.lable from DuPont) may be considered, even though such materials are available only in spun or filament yarn form and would ordinarily, wear out rapidly in woven dryer fabric s-tructures. By encapsulating such yarns within the matrix material, they can be protected from abrasive wear and exposure to steam which otherwise would signiicantly reduce their service life.
Present dryer fabrics often use polyester monofilaments, despite their susceptibility to damage by hydrolysis. By encapsulation oi polyester yarns within the matrix material according to the invention, such yarns would be protected from steam exposure, thereby preserving * Reg. 'l' M.

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;~' 1 309~)3 , yarn strength better than in woven or spiral mesh nonwoven dryer fabrics.
In the case of dryer fabrics, the matrix material itself may be selected from among the entire spectrum of flexible polymeric compounds without regard to yarn forming ability of the material. Whereas conventional woven or spiral mesh dryer fabrics are comprised of textile yarns with the possible addition of resin treatments, the dryer fabric of this inven-tion may utilize a non-fibre forming matrix material if this would provide better service life or afford manufacturing or material economies. For example, silicone rubber may prove to be an ideal matrix material even though it is not presently available in yarn form for use in woven dryer fabrics.
Fabrics of this invention may be produced in either endless belt form or in flat form. Also, fabrics may be readily produced by use of a machine direction yarn of a material which possesses a melting temperature higher than that of the polymeric matrix material.
In other cases the matrix material may be a thermosetting plastic material, or it may be a resinous material which is water-reactive. In still other cases, the matrlx material may be a reaction molding compound which polymerizes almost immediately after being mixed together.

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~ ~ 309~53 According to a still further broad aspect of the present invention there is provided a nonwoven fabric which comprises spaced apart linear yarns extending substantially in a common direction and polymeric matrix material interconnecting and at least partially encapsulating each of the yarns throughout the full longitudinal extent thereof. The polymeric matrix material has apertures therein in laterally offset disposi-tion relative to the yarns and extend through the fabric.
The invention will now be described further, by way of example, with reference to the al~c~panying drawings in which:
Fig. 1 is a plan view of an apparatus used to make the fabric of the invention;
Fig. 2 is a lateral cross-sectional view taken along line A--A' of Fig. l;
Fig. 3 is a partial, front, cross-sectional view taken on line B--B' of Fig. l;
Fig. 4 shows a part of Fig. 2 drawn to a larger scale;
Fig. 5 is a cross-sectional view taken on line C--C' of Fig. 4;
Fig. 6 is a plan view of a portion of a fabric constructed in accordance with the invention;
Fig. 7 is a cross-section through the fabric of the invention taken in the machine direction and on line D--D' of Fig. 6;
Fig. 8 is a cross-section through the fabric of the ~), ,~
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~-`1 3~9~53 invention taken in the cross-machine direction and on line E--E' of Fig. 6i Fig. 9 is a cross-section through the fabric of the invention in the cross-machine direction and on line F--F' 5of Fig. 6;
Fig. 10 is a view corresponding to Fig. 7 and shows a variant of the fabric having a layer of fibre batt adhered to its top surface; and Fig. 11 is a perspective view of the fabric of Fig. 10 10with a portion of the fibre batt removed for added clarity.
Referring now to the drawings, and particularly to Figs. 1, 2 and 4, an array of monofilament, multifilament, or spun yarns 10 is fed into extruder die 12 attached to extruder 14, which is filled with matrix material 18.
15Yarns 10 are pre-tensioned to provide better control. The extruder die outlet 16 is shaped to produce a ribbon-like tape having a side-by-side array of parallel yarns 10 encapsulated within molten plastic matrix material 18.
This can be ~een more clearly in Fig. 5.
20As illustrated in Figs. 1 and 4, newly formed tape 20 is immediately fed lrlto pinned section 22 of drive roll 24 while the plastic~matrix material 18 is still in a melted state. Drive roll 24 is driven by variable speed motor 26 at a speed to accommodate the output flow of extruder 14.
25Immediately subsequent to contact with the pinned section 22, the ~newly formed ~tape is subjected to conditions of heat and pressure by heat press roll 28, :

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1 3~$53 thereby forcing the still soft extrudate into the grooves 30 between pins 32 of pinned section 22. Yarns within the tape are guided into engagement with machine direction grooves 34, where they are surrounded and encapsulated by solidifying plastic matrix material. Plastic matrix material is likewise forced into cross-direction grooves 38, to form cross-direction interconnecting strutural members 40.
Top-most pin extremities 41 contact the outer surface 43 of heated roller 28. This forces plastic material from between these mating surfaces and causes holes 45 to be formed at each such contact point.
For flat fabric, pinned section 22 extends fully across the drive roll 24, so that the desired fabric width can be produced in a single pass. In such a case the material continues around the pinned roll until it cools sufficiently for complete solidification to occur whereupon it is pulled off the pinned roll by a fabric windup device.
Flat formed fabric thus produced may subsequently be formed into endless belts by conventional joining methods.
To produce wide endless fabrics suitable fox purposes such as paper machlne clothing without need for a subsequent ]ointing step, it is necessary to make repeated passes onto the pinned section 22 as shown in the drawings.
In such endless fabric production, the newly formed tape continues halfway around the pinned section 22 while it solidifies into hardened tape material 42. The hardened : ~ .

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., ' ' ~ ' 1 309g53 tape travels around tail .roll 44 and is then returned to a position on pinned section 22 immediately adjacent to the next successive wind of newly forming tape 20.
It should be understood that newly forming tape 20 and hardened return run tape 42 are actually successive portions of the same continuous tape. However, for purposes of clarification, they are referred to as if they were separate entities.

Prior to re-entering pinned section 22, return-run tape-edge 48 adjacent to newly forming tape 20, is brought into intimate contact with the slotted outer surface 46 of extruder die 12 so that it is softened and fuses, in butting relationship, into adjacent melted edge 47 of newly forming tape 20. Both the return-run tape 42 and the newly formed tape 20 are then fed side-by-side adjacent to each other onto pinned section 22, where they are subjected to : heat and pressure by press roll 28, thereby improving the : bonding between the two tapes. Through this means, newly formed tape 20 is continuously joined to previously formed and hardened tape 42 into an endless porous belt 50 comprised of side-by-side tapes whose boundaries are barely discernible.
:In an alternative arrangement, not illustrated, the edge regions of~the newly formed tape, or indeed the tape as a whole, are of multi-stepped configuration, such that the newly formed tape 20 and return-run tape 42 form an overlapping, rather than a butt, joint, the extruder die ;

1 30q~53 12, pinned section 22 and press roll 28 being profiled, and the guiding of tapes 20 and 42 in the region of the extrusion die being modified, accordingly.
It may be found convenient, in some instances, to include further yarns at intervals transversely of the tape 20, such yarns extending from the longitudinal edges of the tape and being embedded in the adjacent edge of the return-run tape on bonding together of the two tapes.
The thickness of the machine and cross-machine direction members created by this method need not be the same. It is desirable in most cases that the yarn be fully encapsulated. It should be understood, however, that many variations in pin shape and spacing may be utilized to achieve particular construction objectives, dependent upon the end use application of the flat fabric or endless belt being produced.
For geotextile or filtration applications, fabrics of this invention may be further enhanced by the addition of a layer of porous membrane, open cell foam, fibrous matt, fibres, or the like to one or both sides of the yarn-containlng matrix material. Alternatively, foam or fibres may be positioned throughout the matrix mater1al itself. In the papermaking field, for example, a fine pore-size open-cell ~ foam may be added to the sheet-contacting surface of the fabric, to render the fabric suitable for use~as a wet press felt.
In some cases, the surface 43 of heated top roll 28 ~:
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1 309~53 may be embossed to impart a particular surface pattern to the fabric as it is formed.
It should be noted that the fabric of this invention differs from conventional woven fabrics in several respects. In particular, the requirement of woven fabrics that machine direction yarns pass over or under cross-machine direction yarns does not apply to the fabrics of this invention, As can be seen from Figs. 7 to 11, the matrix material encapsulating machine direction yarns lies in the same plane and interconnects with cross-machine direction matrix material. With this type of construction the homogeneous combination of matrix materials in machine and cross-machine directions ean provide independent non-interconnected void spaces between yarn elements. This factor may have special significance when the fabric is ; employed as a wet press felt for papermaking.
Fig. 6 shows a plan view of an embodiment o~ the invention. Here, it may be clearly seen that the fabric is in actuality a porous, reinforeed plastic composite wherein maehine direction yarns are the reinforcement elements and the surrounding matrix material contalns fluid passageways, fully encapsulates~ the yarns, and joins yarn to yarn to :: :
make the nonwoven fabric of the invention.
In Figs.~ lO and~ll, a fibrous surfaee layer Sl is ;~
shown integrally bound to matrix~material 52. In the preferred case, such bonding is~accomplished ~by presslng ~;
the heated~ fibrcus layer S~ ~down lnto the lower~ melting ,. ; :

, 1 3nq~3 temperature matrix material while partially melting this material, thereby allowing the batt fibres to become firmly embedaed within the matrix material.
In the discussion above, the problem of water flow in the press nip was addressed. It is thought that fabrics of the present invention will provide substantially more flow control than present felts. Water, having flowed through a fine porous sheet contact layer, may only pass vertically into the void spaces between machine direction and cross-direction fabric members since such void spaces are not interconnected but, rather, are surrounded by matrix material which forms effective barriers to the transmission of water in the transverse plane, and in particular, in the machine direction.
In another variation of the process, fabric of the invention is made entirely from sheath-core yarns having a core comprised of non-melting or high temperature melting monofilament or multifilament yarn and a sheath comprised of lower melting temperature fusable material. ~or example, the yarn core may be a high modulus nylon 6,6 multifilament and the yarn sheath may be nylon 12 material.
An array of side by side sheath-core yarns is fed into the machine direction grooves 34 of pinned roll section 22, thereat to be forced down into the grooves by heat and pressure by press roll 28. The sheath core monofilament cross section area is greater than the area of machine directlon groove 34, so that excess melted sheath material - ~

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- 1 309~53 is forced into cross direction grooves 38 to form the cross directional interconnecting strutural members 40.
The following are intended to illustrate -the invention, by way of example only:-Example 1 An endless porous belt was formed utilizing an experimental apparatus of the type described above. A
matrix material comprised of nylon 12 (melting temperature 171 C) was used to encapsulate an array of seven 0.36mm diameter high modulus polyester monofilament yarns (melting temperature 246C). The yarns were fed into the extruder die to emerge completely encapsulated by the matrix material. The extruder and die were operated at 196 C, sufficiently hot to melt the matrix nylon material but not hot enough to melt or adversely effect the polyester monofilament yarns.
An 86mm diameter x 12.7mm wide pinned section of the drive roll having 15 pins per cm in the machine direction and 12.6 pins per cm in the cross-machine direction was utili~ed. The pins each had a rectangular, flat tip with a surface of 0.18mm in the cross-machine direction and 0.25mm in the machine direction. The pins were 0.81mm high ln both directions, i.e., the grooves between pins were of equal depth in both directions.
The pinned section had machine direcion grooves that were large enough fully to accommodate the monofilament ,.. , . . . . : , . . .

1 309~53 yarns to be encapsulated in the plastic matrix, i.e., these monofilaments could fit into machine direction grooves and be recessed midway down into the space between pins.
To produce endless fabrics, a newly formed tape containing seven monofilament yarns and measuring approximately 4.7mm wide was produced. This tape was squeezed down into the grooves between pins by the heated press roll and solidified by the time it had travelled half way around the drive roll. The hardened tape was guided around the tail roll, then against the slotted die edge, and finally back onto the drive roll immediately adjacent to the newly formed tape then being produced.
Each newly created tape wind was fed back onto the pinned roll section in the manner described above. With each subsequent wind of the belt, the tapes would advance progressively further down the drive roll surface away from the pinned sectlon. This process was repeated until an endless belt had been formed which measured 229mm wide x 1143mm in loop length. At that point the extruder and drive roll were stopped, the apparatus was dismantled, and the endless belt was removed.

Example 2 A belt similar to that described in Example 1 was formed usln~g .305mm diameter~polyester monofilaments and nylon 12 as the matrix material. The procedure of Example 1 was followed, except that excess plastic material was :

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:' '' ~ '' ~ 30q~3 utilized to fill one outside machine direction groove on each edge of the newly formed tape completely. Plastic matrix material also filled in all of the cross-machine direction grooves within nine machine direction groove widths.
With seven monofilaments occupying adjacent grooves, a tape of nine groove widths, measuring 6mm wide, was produced and repeatedly wound around the apparatus in the manner described above to form an endless porous belt 10having the same physical characteristics as that of Example 1 except for reduced machine direction strength. This reduced strength was due to the fact that smaller yarns were used and there were fewer yarns present in the fabric.
It is not essential that a tape be made first and then 15built up and joined. Flat material of the invention could be joined into an endless belt.
The invention is not limited to the exact features as hereinbefore disclosed, since alternatives will readily present themselves to one skilled in the art.
20Thus, for example, the matrix material may be provided in the form of low melt polymeric sheet material which is brought into alignment-with the array o~ yarns in advance of the roller nip and which meets and flows in the region of the nip,~thereat at least partially to encapsulate the 25yarns. In building up a wide band of nonwoven fabric, a formed matrlx/yarn tape is looped round a tall roll for re-presentation to the roller nip in adjacent overlapping 1 3;~53 disposition relative to the incoming portion of polymeric sheet material and overlying yarn arrays, and passes through said roller nip with said incoming portion and associated yarns. By arranging that the array of yarns is disposed centrally of a continuous length of polymeric sheet material of a width three times that of the said array and that the extent of overlap is equal to the width of the yarn array, successive passes of a 'formed' matrix/yarn tape through the roller nip will result in a nonwoven fabric wherein the body comprises a matrix formed by the fusion of three layers of polymeric material and a single layer of yarn. The edge regions of such fabric will, of course, be incomplete, but such edge regions may be trimmed as necessary. The substantial overlap between successive lengths of polymeric sheet material approaching the roller nip does ensure intimate fusion between such successive lengths, thus to enhance the integrity of the fabirc in the transverse direction thereof.

It is to be appreciated that the surface configuration of the pinned roller will require modification to accommodate the different thicknesses of polymeric material/yarn at the roller nip on start up of the process~

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Claims

1. A nonwoven fabric comprising spaced apart linear yarns extending substantially in a common direction and polymeric matrix material interconnecting and at least partially encapsulating each of said yarns throughout the full longitudinal extent thereof, the polymeric matrix material having apertures therein in laterally offset disposition relative to the yarns and extending through the fabric.

2. A nonwoven fabric as claimed in claim 1, wherein the yarns exist in a generally parallel form and the polymeric matrix material interconnecting said yarns extends generally perpendicular thereto and to the matrix material serving to at least partially encapsulate said yarns.

3. A nonwoven fabric as claimed in claim 1 or 2, wherein the encapsulating matrix material forms a flat surface at one face at least of the fabric.

4. A nonwoven fabric as claimed in claim 1 or 2, wherein: the polymeric matrix material interconnecting the at least partially encapsulated yarns defines a surface outwardly of the plane of the encapsulated yarns at one face of the fabric.

5. A nonwoven fabric as claimed in claim 1, wherein the apertures in the matrix material comprise respective individual flow passages perpendicular to the general plane of the fabric.

6. A nonwoven fabric as claimed in claim 5, wherein the apertures are uniformly spaced.

7. A nonwoven fabric as claimed in claim 1 or 2, wherein the matrix material has a melting temperature lower than that of the yarn.

8. A nonwoven fabric as claimed in claim 1, wherein the matrix material is porous.

9. A nonwoven fabric as claimed in claim 8, wherein the matrix material comprises a polyurethane or a fluorinated polymer.

10. A nonwoven fabric as claimed in claim 1 or 2, wherein the yarn matrix material comprises a non-deformable polymeric material.

11. A nonwoven fabric as claimed in claim 1 or 2, wherein the yarn has a high modulus.

12. A nonwoven fabric as claimed in claim 1 or 2, wherein the yarn is wholly encapsulated by the matrix material.

13. A nonwoven fabric as claimed in claim 1, wherein the yarns comprise sheath-core monofilaments having a core comprised of a polymeric material of high modulus and a sheath comprised of polymeric material having a melting temperature lower than that of the core material.

14. A nonwoven fabric as claimed in claim 13, wherein the sheath defines the polymeric matrix material aforesaid.

15. A nonwoven fabric as claimed in claim 1, wherein at least one surface of the fabric is covered by a porous layer.

16. A nonwoven fabric as claimed in claim 15, wherein the porous layer comprises a textile batt.

17. A nonwoven fabric as claimed in claim 1 or 2, comprising a papermakers fabric.

18. The method of producing a nonwoven fabric comprising the steps of moving an array of spaced side-by-side yarns in the longitudinal direction thereof and into contact with a moving polymeric matrix material, passing said yarns and matrix material into a roller nip to cause said yarns to become at least partially embedded in said matrix material, and setting the fluid yarn/matrix structure, the rollers defining the said roller nip contacting one with the other at spaced intervals in the axial and peripheral directions thereof thus to form apertures in the matrix material on passage therethrough.

19. The method as claimed in claim 18, wherein the matrix material exists in a liquid state on engagement with the yarns.

20. The method as claimed in claim 18 or 19 including the further steps of presenting the set yarn/matrix structure, in the form of a moving loop and at a position in advance of the roller nip, in edge-to-edge relationship with a further array of yarns and moving polymeric matrix material, advancing said structure, and the adjacent yarns and matrix material, through the roller nip thereat to form a further perforated yarn/matrix structure intimately fused with the said set such structure.