CA1203972A - Method and apparatus for making a patterned non-woven fabric - Google Patents

Abstract

A B S T R A C T

"METHOD AND APPARATUS FOR MAKING A
PATTERNED NON-WOVEN FABRIC"
A non-woven fabric having a pattern defined by an array of discrete areas having a reduced fibre density but which are substantially free of perforations is produced by supporting a freshly wet laid web of the non-woven fabric on a porous surface and directing spaced jets of fluid against the unsupported side in order to displace fibres within discrete areas whilst maintaining in position a proportion of fibres that are within those areas and that are adjacent the porous surface. The fabric web may be supported on a Fourdrinier wire (1) and the jets of fluid (e.g. water) may be directed through the apertures in a perforated cylinder (6), the Fluid being supplied under pressure from a water-knife device (11). The apertures in the cylinder (6) preferably have a cross-section that increases in the direction of the water jets. Vacuum may be applied through the Fourdrinier wire (1) by means of a vacuum box (10) and vacuum may also be applied within the cylinder (6) from means (17) in order to remove excess water from within the cylinder (6).

Description

ET~lOD ANr APPARATUS FOR MAKI~'G A
PATTERNED ~'O~-llOVEN FABRIC"
Field of the Invention This invenLion relates to a method and apparatus for making patterned non-~lDven fabrics, for example paper for Lne ~,anu~acture or inrusion pDuches.
Background tn the invention nFusicn pouches, fDr example teabags and spice-b2gs are commonly formed as pouches of a nDn woven material (referrcd to hereinafter as "teebag paper that is permeable to water and to the beverage form2d by infusion, i.e. by the dlssolution of soluble solids in the content of the pouch, uporl the application of hot water thereto.
Teabag paper is generally a non-woYen web of a lisht weight permeable fibIous material made, for example, from sbaca pulp, sisal pulp, regenerated rayon, esparto grass pulpS long-fibred chemical wood p~Jlp or mixtuIes thereo,.
In order to permit the fabrication of a heat-sealed pouch, the fibrous matErial Jay comprise heat-sealable fibres such 2G as pol~olefins, e.g. polyethylene or polypropylene, or vinyl chlDride and vinyl acetate polymers or copolyrl~ro.
The heat-sealable bibles may constitute a discrete phase on, far example, a cellulosic base phase.
Teabag paper is current1y available in two types. one is a plain, non-woven web which is maze on on ordinary Fourdrinier wire. The other type is a patterned web, the pattern being formed by an array of discrete areas having a lower fibrs density than that of the rest of ttle web.
Teabao paper of the second type is formed on a wirs hrving pronounced knuckles, as described in British Patent Specification Jo. 1,102,246. Ho~Jever, in the course of - manufacturing the web, ihe knuckles cf the wire often wreak through the web and give rixe to clear holes cf the size of the knùckle.
It is alto known that perfGrated or reticulated non-

-2-woven materials can be produced by forming a ~et-laid Job supporting this on a perforated screen and forcing jets of fluid through the supported Deb. Such techniques are disclosed in British Patent 5peciFications No. 863,397 and No. 1,326,915 and United States Patent 3,4~5,706.
To be completely acceptable, teabag p2per must possess characteristics such as cleanliness, good absorbency, high wet strength and a sheet structure that permits rapid permeation of the beveravage; it is also found that many consumers have a preference for teabags formed from paper having a pattern thereon. Ho~lever, it is also irportant that the paper should not sift, that is it should prevent the pass2se therethrough of fine particles ("dust") of the tea or other solids contained in the bag or pouch. Clearly, however, ths presence of clear holes in the web Jill cause siFting Df the web. If one survey the filtering media produced by prior-art methods, it is found that they fall within the following categories: (i) products ~1ith a good pattern difinition but poor dust-retention properties, (ii) products with good dust-retention properties but a poorly defined pattern and (iii) products ~lith mediocre pattern definition and mediocre dust-retention properties.
Accordingly, there is a definite need for a patterned or decorative filter medium having a good pattern definition coupled with good filtration or sifting characteristics.
In the fDllo~ing text, the invention Jill be discussed primarily in terms of teabag paper; however, it should be understood that the invention can be applied to other non-woven filtration media, for example non-woven fabrics used in surgical face masks, coffee filters and the like.
Summary of the invention The present invention provides a method of producing a patterned non-woven fabric, which method comprises supporting a fob of a non-woven fabric against a porous

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surface; overlaying at least part of the supported web with an apertured member having a irst surface adjacent the web and a second surface remote from the web, the first surface having apertures therein each communicating with a respective apertuxe in the second surface by means of a passageway extending therebetween; and causing discrete streams of fluid to impinge upon the side of the web remote from the porous surface, characterised in that each stream passes through a respective passageway and has a cross-section smaller in area than the area of the respect-ive aperture in the first surface of the apertured member.
The invention also provides an apparatus for producing a patterned non-woven fabric, which apparatus comprises means defining a porous surface for supporting a non-woven web; an apertured member having a first surfaceadjacent the porous surface and a second surface remote from the porous surface, the first surface having apertures therein each communicating with a respective aperture in the second surface by means of a passage-way extending therebetween; and means for supplyingfluid to passageways in the apertured member to form a stream of fluid in each of those passageways in -the direction from the second surface to the first surface, characterised by an arrangement such that the streams of fluid each have a cross-section smaller in area than the area of the respective aperture in the first surface of the apertured member.
The streams of fluid that impinge on the web act to displace fibres from discrete areas of the web in directions substantially in the plane of the web whilst maintaining a proportion of fibres within those areas and adjacent said porous surface. The fibres that are not displaced from the discrete areas serve to bridge those areas and thus prevent the occurence of clear holes (as hereinafter defined).

Since the area of the aperture adjacent the web is greater than the area of the impinging fluid stream, there i5 a "void volume" ~Jithin the aperture not occupied by the fluid stream. It is believed that this allows displaced fibres - which are subject to the constraints imposed bv thc walls of the passageways - to accumulate therein until a condition of mechanical equilibrium is achieved, thereby avoiding clear holes. Of cnurse, it is not intended ihat the invention should be limited in any way by this hypo-1D thesis.
By "clear hole", there is meant an aperture or ~oidin the web that is significantly larger than the normal interstices between the fibres constituting the non-wo~en web. In practice, a "clear hole" is such bn aperture or void which would permit passage therethrough of fine particles ("dust") from the intended contents of an infus-ion pouch made from the fabric. In the case Or pa3er fo.
infusion pouches, the invention makes it possit)le to achieve a fabric which contains substantially no apertures or voids exceeding 450 microns in breadth. The upper linit for apertures or voids exceeding 450 microns in breath is realistically set, by means of the invention, at 7, (preferably 2nD) of the apertures or voids in the machine direction of the fabric, and 7O preferably 2r) in the ~5 cross direction.
The web of non-woven fabric produced by means of the present invention can be described as having a pattern defined by an array of discrete areas having a fiore density (i.e. fibres per unit area) less than that of the 3n web extending betweerl said discrete areas, said discrete areas being substantially free of clear holes (as herein-before deFined).
Brief description of _the drawings FIGURE 1 is a diagrammatic side cross-section of an exemplary apparatus for producing a patterned fabric in acco~danee ilk thy present invention;
FIGURE ? is longitudinal vie of the rneans for producing fluid streams within the machine of Figure 1;
FIGURE 3 is an enlargerl fragmentary elevation of the outer surface of an apertùred cylindPr ernpIoyed in the machine of Figure 1 to produce thestreams of fluid;
FIGURE 4 is a schematic representation of the proposed mechanism by which the pattern is produced in a non-woven fabric web in accordance with the present invention.
FIGURE 5 is a sectional view through an apertured cylinder similar to that shown in figure 3; and FIGlJRE5 to 9 are each a photomiorographic view of a sample of patterned teabag paper.
D scri tion of the referred embodiments e p p The non-woven fabrics employed in the practice of the present invention can be manufactured from any of thc fibres customarily use~l in the production of non-woven filtering rnedia7 for example fibres derived from ~Yood, abaca or rayon. Ilixtures of fibres can be used and it is also possible to have heat-sealable fibres either admixed with the base fibres or formed as a distinct phase on the base phase. The fibres will typically have lengths in the range from n. 1mm to 40mm.
Best results are obtained using a wet web, especially a freshly wet-laid web 9 although in principle it is possible to use webs formed by other methods, for example air-laid webs.
The means defining the porous surface can be, for example, a perforated or.otherwise foraminous sheet or plate; however, it is conveniently a mesh formed of strands of either metal (e.g. bronze) or a plastics material.
The mesh ran for example, be woven or knitted. The prey-erred means is a convential Fourdrinier papermaking wiry.
7h~ Fluid used in the streams (also referred to here-in as 'ijets"3 is generally a liquid and is preferably an .

aqueous liquid, especially ~/ater, In che case us liquid streams, additives ma be employcd in order to achieve a desired viscosity.
To employ the method of this invention in a contin-uous manner, any appropriate means may be utilized toprovide relative movement between the web and the fluid streams impinqing thereon, In preferrred embodiments,the web is continuously advanced through the 70ne in which the fluid streams act; this may be easier to arrange than the convcrse 5~vstem wherein the apertured mer,~ber is moved alony a stationary Deb.
In order to obtain a clea, pattern, it is ~reforred that the fluid streams should impinge upon the web in a sinyle line across its width (i.e. in thc cross direction.
It is also preferred that the fluid streams should impinge upon the web in a series of pulses.
In principle, it is possible to utilize a perFo.ated sheet or plate as the apertured member. ~!o;:e~er ? in preferred embodiments, a perforated or aperturPd, hollow %0 cylin-Jer is employed. Such a cyllnder is advantageously supported over a continuously advancing porous support member for the non-woven web, the longitudinal axis of the cylinder being arranged parallel to the porous support surface and transversely with respect to the direction of q5 advance of the ~Isb. In other wordsa the cylinder is pref-erably supported for rotation about its longitudinal axis such that the outer surface of the cylinder comes inta clûse proximity to said porous surface. The web passes between the apertured cylinder and the porous surface, -~0 As mentioned above, the method of the present invention involves the use Or jets of fluid to displace .
unly a proportion of the fibres within discrete areas, Dne means of ensuring that a proportion of fibres is retained in position within said discrete areas is to form the passageways in the apertured member so that they '7~

are "flared", i.e. they increase in cross-sectional area in the direction from the second surface to the first surface (this being also the direction of flo~i of the jets in the passageways). The increase in area may be 7 1 neaA or ncn-lAnear.
Another rneans for achieving the requisite partial displacement of the fibres l~ithin the di-screte areas is to generate the fluid streams or jets such that each has a cross-section that is smaller in area than the area of the corresponding aperture in the second surrace. ilk such a fluid stream, it would be possible to utilize, say, a passageway with a constant cross-sectional area and still have the "void volume" referred to above. However, it can be advantageous to utilize such fluid streams in combination with the flared passageways described in the previous paragraph.
The referenses to the cross-sectional area of a stream of fluid relate in general to the cross-sectior1 of the stream irnmediately after entry into tl-e respective passageway.
It is also preferred to apply a vacuum to the ~/eb through the porous support member, particularly to a region o-f the web in register ~lith the region against which the fluid jets impinge. The vacuurn helps to retain fibres adjacent the porous support member (which fibres may become ternporarily lodged within the interstices of the support member, whereby said fibres resist to a certain extent the disturbing action of the fluid jets.
The fluid i5 conveniently supplied to the apertures 3D by means of a device that direets a sheet or "curtain") of fluid, preferably under pressure, to the said second surface of the member, to the face of the apertured member rer~ote from the web and from the porous support member. Vacuum means and/oL wiping means may be provided in order to remove the excess or surplus fluid, i.e. that which does not pass through the apertures.
Turning now to the accompanying dra-lings, the apparatus shown in Figures l and 2 cGr,p~lse3 a s~ppor L wire l which is continually advanced over rollers 2 and 3 in the machine direction indicated by arrow 4. The rate of advance may be, for example 9 from 4 to 415 metres per minute. In operation a fibrous web prodLIced at a down-stream location (not sho~in) is fed onto the support wire, which wire is preferably a standard Fourdrinier paper-m~king wire.
A gantry assembly indicated generally by 5 (see Figure 2) supports an apertured member in the forr,1 of a hollow metal cylinder 6. The cylinder is mounted at each end in bearings 7 for rotation about the longitudinal axis of said said cylinder 6. During operation, the cylinder 6 will rotate in the clock~lise direction 25 viewed in Fi9UL e l and as indicated by arrow 8. If required, the cylinder can be positively driven by appropriate means (not sho-~n).
A vacuum system lO is provided to supply vzcuum to the underside of the support wire in the regiorl 9.
Arranged within the apertured cylinder 6 is a "rluid knife" device 11, which device is adapted to direct a curtain of fluid perpendicularly to the internal surface 13 of the cylinder 6 in the region 9. The fluid knife 11 extends substantially along the length af thP cylinder so thst fluid jets will be directed against the supported fabric web along substantially its entire width, in the manner described hereinafter.
The fluid knife 11 comprises a reservoir 14 for high pressure fluid which is supplied to the system through conduit 141. The fluid under pressure passes from the reservoir 14 through a conduit 15 to a slot 16 from which the curtain of fluid 12 emerges. When the fluid is water, a flow rate of 2 to 20 m3 per metre of machine widthper hour has been found to be satisfactory.

q g The width of the slot is preferably from 25 JJIn to 80 em and is typically about 5Ll urn.
Associated with the fluid knife 11 is a vacuum system 17 in which a ~acuurn (for example, of 5U to 33û mm Hg) is drawn via a vacuum slot 18. The vacuum system 17 serves to dray us surplus or exeess ,luid (i.e. tne fluid from the fluid curtain 12 that does not pass through the apertures irl the cylinder 6); by this means, flooding of the system is avoided. The excess fluid drawn up by the vacuum system 1n 17 can be discharged via any appropriate means (not shown.
s indicated in Figures 3 and 4, the outer surface 21 of the cylinder or roll 6 is provided with a rPgular array of apertures 20 communicatincl with corresponding apertures in the inner surface 13 by means of passageways 2~. The apertures 20 in the outer surface l of the cylirlder can be of any desired shape, for example square, rectangular, diamond-shaped, oval, circular or star-shaped. the walls of the passageways 22 diverge in the direction From inner surface 13 to outer surface 21. Thus, the area of each aperture 2D in the outer surface l is greater than the area of the corresponding aperture at the inner surface 13.
he fluid curtain 12 may, in some embodiments, have a thickness (determined by the width - i.e. the dimension in the machine direction - of the slot 16) greater than the machine-direction dimension of the apertures in the inner surface 13 of the cylinder G In such cases, the fluid curtain 12 will strike thP inner surface 13 of the cylinder 6 and a proportion of the fluid will pass into the passage-ways 22 in the form of discrete streams or jets. The cross section of each jet will then be determined by the area of the respective aperture in the inner surface 13.
However, it is preferred that the width of the fluid curtain be less than the dimension, in t!-e machine direction, of the apertures in the inner surface 13. Thus, as clearly shown in Figure 4, there is a void space 23 between the fluid stream or jet 24 and the diverging side waLls of the pass-a~eway 2~.
Generally, the edge of each apcrture 20 in the 'zone of influence' 9 will be in contact with the web.
In other words, the p-ss2~;ay2 throJ3h W hi-h the fluid jets 24 directed are sealed off by the web During operationj and again as shown in Figure-4, it appears that the impinging jet 24 displaces a proportion Or the fibres in web 25, the displaced fibres tending to accumulate as at 26 in the void spaces 23. As mentioned, ii is thougi1t that the displacement of fibres proceeds until a mechanical equilibrium is achieved with respect to the displ3ced and accumuiated fibres. At the point of equilibrium, fibres within the areas covered by apertures 20 are retained in position to give discrete areas ~7 having a rcduced fibre density compared with the web ln the regions between the areas impinged upon by the fluid jcts. The areas of reduced fibre density retain the integrity associated with the untreated web and are therefore free of the clear holes produced in the prior-art methods owing to the passage of the fluid jets completely thr'ough the Deb British Patent 836,397), or owing to the breakthrough of wire knuckles (British Patent 1,102,246).
The vacuum applied to the web through the Fourdrinier wire 1 by means of the lower vacuum system 10 can aid in maintaining the integrity of the web in the areas 27 by lodging the fibres within the interstices of the Fourdrinier wire. The vacuum applied may be, for example, from sn to 33û mm Hg.
The vacuum system 10 also acts to remove the fluid supplied as jets after the latter have caused fibre dis-placement. This removal is important in ordsr to avoid further, unwanted disruption of the fibres.
Since the cylindrical roll 6 rotates in concert with 9'7~

wire 1 and the supported web 25, and since the outer surfa^e 21 parts cleanly from the ~veb as the latter moves out of the region 9, there is no disruption oF the fibres, as ~lould otherwise be caused iF there were relative movement oF the 5 cylinder and the web.
It will be appreciated that, as the drum rotates, any given part of the fluid curtain 12 will periodically strike solid areas of the inner surface 13 instead of entering a passage-lay 2~. Thus, the fluid jets 24 are 10 formed intermittently or as a series of pulses; this determines at least in part the distribution or psttern in the treated web of the areas of lower fibre density.
~lith a circumferentialspeed of 200 m/minute, a typical apertured drum 6 of 12 inch (30.4B cm) diameter has been 15 calculated to intcrrupt the fluid curtain 12, at an given position, at a rate of 1452 times per second or per metre idth oF cylinder.
The dimensions oF the apertures ~0 will generally be from 0.1 mm to 1 n mm, for instance from 1 mrn to 5 mm.
By way of example, a cylinder 6 has been used having a thickness of 0.36 mm and passagcways-of rectangular cross-section. The apertures 20 in the outer surface 21 were 1.7~ x ~.39 mm and those in the inner surface 13 were 1.10 x 1.71 mm, the longer dimension in each case being in the ~5 machine direction. The apertures 2n were G.34 mm apart in the machine direction and û.50 mm apart in the cross directlon.
In another exemplary cylinder 6, of 0.40 mln thicl;ness, the apertures were each in the shape of a rhomous (I~igure 3), arranged with the longer diagonal in the machine direction.
The diagonal of the apertures in the inner surface fore measured at n. 9û and 1.24 mm, from which the rhombus side were calculated to be 0.77 mm. The sides of the apertures ~0 were found to be 1.57 mm, the acute angles of the rhonbus being about 70 or 71. the centres of adjzcent apertures wore ~57 mm apart in the machine direction and 2.~0 m;~
apart in the cross direction.
In general the ratio of the area of each aperture 2n to the area of the correspontling aperture in surface 5 13 is from 1.2~ tn I, for example frorn 2 to 5.
In Figure 5, an alternative construc-ion of the apertured cylinder 6 is sho~n9 in wl1ich the ells defini,n3 the passageways 22 have a curved prr.~file. ~;o~:ever, the walls still define a flared passage for the fluid jets 24.
1C The invention is applicable to the production of pattrrned non-~Joven webs from a variety oF fibres. ~o.Jever, when the fluid is, or comprises, water it ie pre,erred that the web-forming fibres shall contain a significant proportion (preferably ~0c to 1nO by l~eight) of hydrophilic fibres, 15 which will become plasticized in aqueous solution and ~/ill thus be more readily enmeshed in the inters'ic2s of the porous surface. The basis ~eir~ht of' the pstLe~nec' product - can vary widely, a suitable range being from to ~5 gsm-(graMs per square metre).
The apertured roll assembly, in order that it shall be capable of continuous operation at hirih'speed, should be constructed of a rigid material, For exa.m?le nickel.
This rigidity is desirable to ensure that the resulting ' product has a uniform pattern despite the f2rces exerted 25 on the cylinder due to its rotation and due to the application of the high pressure flujd. The thickness of the cylinder Hall may be for example, from n.1mm to 2r~m, preferably 0.~5 - 0.7 mm and especially 0.35 - Q.4 mrn.
'The outer surface l of the'cylinder should also be 30 sufficiently smooth to prevent the undesirable accumulation of fibrous material wl1ich may lead to the blocka9e of the apertures ~û.
It is desirable for the perForated cylinder G to remain a constant distance from-the support lire in order 35 to achieve uniformity of the resulting product This .

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distance is dependent upon the degree of bridging (i.e.
the extent of the Deb areas connecting the areas of reduced fibre density) that is required and also on the nature of the web itself. The optimum posi-tion of the cylinder 6 is such that the outer surface 21 of the cylinder 6 is close to (generally within one-eighth inch or 3 mm) or in contact with the top surface o; the fibrous web, which web is preferably in a llet condition. If the gap between the cylinder 6 and the support wire 1 is too narrow, the stock or web Jill be compressed and this ma hinder the effecti~-displacrment of the uppermost fibres. If, on the other hand, the gap is too large the resulting product may become diffuse (i.e. it may have an ill-defined pattern structure or possibly no pattern at all) as the zone of influense of the fluid jets becomes less effective.
The practice of the present invention is illustrate in the following Example.
Example r A typical freshly wet laid teabag web, at 17 gsrn (air dry), comprising abaca fibre 35~, wood pulp fibre 40 and synthetic, heatseal fibre 25Q2 by ~leight5 was supported on a synthetic, Fourdrinier-type wire with a count of 87 strands per inch for the warp and 72 strands for the weft.
This web was fed into the "zone of influence" (region 9) the apparatus, shown diagramatically in Figures 1 and 2.
The web had an approximate consistency of 20~ ~ibre and 8U3 water immediately before entering region 9~ vacuum of 288 mm of mercury was applied via vacuum box 10, and a similar vacuum applied via slot 18. The perforated cylin-dcr possessed apertures with a courlt of 32 per squarc cm ineach direction. The dimensions of these apertures were 0.7 x 1.0 mm when viewed from the inner surface of the cylinder and were tapered from the external surface to give an aperture approximately 502 larger at the outer surface Of the cylinder;

A range of products were made by varyins the flo;~ o, the fluid, in this case water at 10C, in the range of 2-12 cubic metes per mete width of the web per hour.
the resultant prDducts, after drying, are shown in photo-graphs B?, B3 ancl B4 ~Fig~res 7,Q acd 9).
In Tables 1 and 2 which follow, there can be seen the comparative results of the pore size distribution for the webs, as measured by an optical image analyser, and the pcrcentage sifting of tea dust by the webs when subjected 1n to a tea sifting test using commercial tea. The pore size distribution results listed in Table 1 give the frequencv o; holes measured at particular chorcJ iengths. The sifting list records the percentage of tea which passes through thy web compared with the amount passing a standard wire r,esh sieve.
It will be noted that the incidence of apertures havinn a breadth grPater tharl 45~ micIons in web 53 is 6.7.o in toe cr~ss-direction (CD) and 9.4Cu in the machine direction (MD), which is higher than is acceptable for use in infusion pouches. This is verified by the comparat-.~el~
high seepagc figure or this web (see Table I). The incid-ence of clear holes (breadth 450 microns) in web B2 is 6.9~ (Cn) or 6.5Do (MD); in web By the incidence of such clear holss is 0.5O (CD~ or 1.7Uo ~llD), which i5 reflected ~5 in the excellent tea-dust retention result The results clearly show that a web of "controlled open-ness" can be produced without the generation of gross holes corresponding to the aperture size in the cylinder.
To illustrate the invention further, the web, examples of which are shown in photographs B2, B3, and By, was subjected to a sheet splitting process which divides the web along its thickness approximately into halYes. the photograph A1 shows clearly that the top half of the web possesses distinct holes whereas the lower half of the web, which is supported on the porous wire9 is undisturbed (see Figure 6).

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TEA DUST RET~TION Cl-lARACTERISTICS
Seepage Sample R~ 130 Sample B2 SO
Sample B4 - 35 Modifications and variations of the illustrative embodiments are of course possible within the scope of the present invention. For instance, it may be desirable to have areas of the outer surface of the cylinder that are free of apertures. Thus, it is possible to block off an area of, say, 1 cm , in the shape of a letter or other symbolO This imparts an image of that symbol to the ~;/eb surface, for example for decorative or identification purposes.
Determination Or suitable values of the variate parameters - e.g. the machine speed, the degree to ~Jnich the passageways 22 are flared, or the fluid pressure can be readily carried out by the skilled person for an qivrn case.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A web of patterned, non-woven fabric, particularly for an infusion pouch, characterized by being constituted by a sheet material having a pattern defined by an array of discrete areas having a fibre density (that is to say, fibres per unit area) less than that of the web extending between said discrete areas, said discrete areas being substantially free of openings or holes having a breadth greater than 450 microns.

2. A web according to claim 1, characterized in that the upper limit for the number of openings or holes having a breadth greater than 450 microns is fixed at 7% of the total number of openings and holes in the machine direction of the fabric and at 7% of the total number of openings and holes in the crosswise direction.

3. A web according to claim 1, characterized in that the upper limit for the number of openings or holes having a breadth greater than 450 microns is fixed at 2% of the total number of openings and holes in the machine direction of the fabric and at 2% of the total number of openings and holes in the crosswise direction.