Title of Invention

APERTURED NONWOVEN AND PRODUCTION THEREOF

Abstract

Apertured nonwoven The present invention relates to a apertured nonwoven having a basis weight of 8 to 17 g/m comprising entangled continuous microfibre filaments having a linear density in the range from 0.05 to 0.40 dtex which are constructed, of at least two thermoplastic polymers having different hydrophobicities and have a pie or hollow pie cross section, from which the split filaments have been released, the apertures being clearly defined and free from split fibre filaments. FIG 1.

Full Text

Applicant: Carl Freudenberg, 69469 Weinheim Apertured nonwoven and production thereof Hygiene products, such as infant diapers, adult diapers or sanitary napkins, for absorbing body fluids are in principle constructed of an absorbent core, a liquid-impervious backsheet comprising a film or a web/film laminate and, facing the body, a pervious sheet material comprising a thin abrasion-resistant soft nonwoven or a vacuum-apertured film with funnellike, i.e. three-dimensional, openings. The vacuum-apertured film encloses the absorbent core with the largest aperture facing outwards, i.e. towards the body. The film material is constructed of hydrophobic thermo¬plastic polymer, such as polyethylene, polypropylene or a copolymer of polymeric vinyl acetate and ethylene (EVA). This ensures that the film surface is not wetted by the body fluid, that the body fluid is only conducted in the direction of the absorbent core and that any strikeback of the body fluid, for example in the event of 1 oading, movement or pr es sur e, i s prevented by the inwardly tapered apertures. As will be known, the absorbent core, as well as predominantly wood pulp, usually also contains superabsorbent polymer (SAP) particles. SAPs are capable of absorbing aqueous fluids in large amounts by substantially swelling to form a gel having a certain low gel strength. The presence of SAP has the advantage that weight is saved, making it possible to reduce the thickness of the absorbent core, and that the fluid cannot be rereleased under a confining pressure, substantially preventing leakages. But SAP also has the disadvantage of leading to the well-known gel-blocking, the more so the higher the SAP content. Gel-blocking #is the term used to describe the effect that fluid can be conducted only to a substantially reduced extent, if at all. But this problem has been solved by providing suitable constructions for the absorbent hygiene products. High-loft nonwovens or other very open structures which do not block on contact with fluid are positioned between the absorbent core and the topsheet. This interlayer absorbs fluid spontaneously from the diaper surface and conducts it uniformly. Such measures improve fluid management. As used herein, fluid management is the interplay of many variables, some of which have already been mentioned, directed to the goal of maximizing the wear comfort of the hygiene article. Sheet materials for the body-facing enclosure of the absorbent material include, as will be known, unapertured spunbond nonwovens and staple fibre nonwovens based on polyolefins. The fluid management of urine in the case of infant and adult diapers and of menstrual fluids in feminine hygiene is deemed very well understood. However, a diaper of the future should be capable of optimally managing not only urine but also fluid discharges from the intestines. Unapertured covering nonwovens have proved unsuitable for this purpose. The body fluid in question is a multiphase system with solid particles in various shapes and consistencies and the tendency to phase separate, especially at active surfaces or surfaces having a filtering and separating effect. These fluids will hereinafter be referred to as intestinal fluids. It has been determined that unapertured nonwovens are unsuitable for completely transmitting intestinal fluids and conducting them to the absorbent core. On the contrary, they have the tendency to cause solid and/or highly viscous fractions in the intestinal fluid to separate out on the diaper surface and possibly act as a barrier layer for subsequent body fluid having a thinner consistency. Not only the separation of coarser constituents as such but also the attendant blockade for continued fluid transmission are serious disadvantages of conventional diapers. Numerous proposals of better intestinal fluid management have therefore been made. They all require the use of apertured topsheets (covering nonwovens). The apertures should be clearly defined. Transverse struts of individual fibres or fibre assemblies or any fibre bridges have proved to be unfavourable. As well as apertured topsheets, diaper construction and the design of the open-structured nonwoven placed between the topsheet and the absorbent core should be adapted to the particular consistency and the associated properties of the intestinal fluid. Not only numerous aperturing methods but also nonwovens and nonwoven composites are known. EP-A-0 215 684 describes the production of apertures in nonwovens by means of water jet technology. The support means used for the fibres and water jet treatment is not the well-known sieves, but drainage cylinders bearing projections. These are responsible for clearly defined apertures. US 5,628,097 describes another aperturing method and apertured products where the nonwoven is ultrasonically or thermally slit in the machine direction and stretched in the transverse direction by passages through a pair of intermeshing grooved rollers. The melt location slits are separated as a result and open up to form apertures. Described are nonwovens composed of staple fibres and continuous filaments, meltblown nonwovens and composites of staple fibres and continuous filaments with meltblown, referred , to for example as SM (for spunbond/meltblown composite) or SMS (for spunbond/meltblown/meltblown composite), An apertured nonwoven in the hygiene sector is required to provide not only intestinal fluid management but also very high whiteness or high hiding power and very high softness at least on the body-facing side. It is known that the two properties depend on the compliance and softness of the fibres used themselves. Compliance and softness increase with decreasing fibre linear density, so that it is obvious to use fine, very fine or even ultrafine fibres. Ultrafine fibres are also referred to as microfibres. These can be based on wovens or nonwovens. Similarly, meltblown nonwovens comprise microfibres in the size range of about 1-10 microns. Unicharm is known to manufacture an infant diaper which is covered with an apertured nonwoven which was produced by the specific water jet aperturing method briefly described above and comprises a composite of a PP-PE spunbond and a PP meltblown layer. This composite construction does make a contribution to improved intestinal fluid management, good softness on the meltblown side (= body side) and high hiding power. However, this composite construction and its production method also have serious disadvantages. The meltblown layer makes little if any contribution to the total strength or integrity of the composite. The weights are distinctly above those customary today. A weight reduction to below about 30 g/m2 appears to be impossible because of the high strength requirements in the machine direction for diaper fabrication. The large amount of material used is cost intensive. Considered by itself, the meltblown layer is not abrasion resistant and, in addition to the water jet treatment, has to be thermally attached to the spunbond support nonwoven to inhibit delamination tendencies. This in turn requires conjugated fibres having a centric or eccentric sheath component of lower-melting polymer than that of the meltblown layer. Nevertheless, this apertured SM composite on the soft M side falls far short of the abrasion resistance of a PP spunbond or embossing-bonded PP staple fibre nonwoven as used today in diapers and sanitary napkins. In other applications, such as sealing diaper leg cuffs or OR nonwovens, where abrasion resistance or nonlinting is required, only SMS can be used. With such a cover on the meltblown layer on the body-facing side, the advantages of the melt-blown layer will count for nothing. It is an object of the present invention to provide an apertured nonwoven which is superior to existing non¬wovens with regard to intestinal fluid management, which meets the demands for high opacity and higher compliance and soft feeling on the body-facing surface, which obviates the need for a construction in two or more layers and which requires a fibre material weight which is distinctly below that of apertured nonwovens used at present in diapers and sanitary napkins. It is a further object of the present invention to improve intestinal fluid management without impairing urine management. It is another object of the present invention to achieve fluid passage through the apertured nonwoven without the use of detergents and/or to reduce their use level to a fraction of the amounts customary in unapertured sheathing nonwovens. These objects are achieved according to the invention by an apertured nonwoven having a basis weight of 7 to 25 g/m2 comprising entangled continuous microfibre filaments having a linear density in the range from 0.05 to 0.40 dtex and constructed of at least two different filaments of thermoplastic polymers having different hydrophobicities and a segmented pie cross section, which have been released from fibres containing the filaments, wherein the apertures are clearly defined and free of isolated fibre filaments. The nonwovens of the invention are extremely low in weight yet very strong and, owing to the smallness of the fibre mass, exhibit very clearly defined hole structures. This makes it possible to ensure the rapid passage of body fluids, especially intestinal fluids, with little if any addition of surfactants having a low surface tension (wetting agents) and to create a dry topsheet surface for diapers and sanitary napkins. The different filaments each have a linear density in the range specified above. The apertures preferably form a regular array and have an individual hole area of 0.01 to 0.60 cm2. The apertured nonwoven of the invention preferably has a strikethrough value after one minute of less than 3 sec. The ultimate tensile strength in the machine direction is preferably at least 30 N/5 cm. The rewet value is preferably less than 0.5 g. The nonwoven may be constructed using for example two different thermoplastic polymer filaments in a weight ratio in the range from 20:80 to 80:20. The construction of the fibre web will now be more particularly described with reference to two filaments Fl and F2. The invention also provides a process for producing such apertured nonwovens by laying down splittable pie or hollow pie continuous fibres whose cross section comprises at least two different thermoplastic polymers having different hydrophobicities in an alternating segmented pie arrangement to form a nonwoven, subsequently splitting and entangling the fibres to form entangled continuous filaments by means of high pressure water jets and subsequent aperturing of the resultant nonwoven using high pressure water jets. The aperturing is preferably effected on drainage and hole forming drums having protrusions on the surface. In what follows, first the polymers used for producing the nonwoven o f the invent i on and then the produc t i on process will be more particularly described. Of the two fibre polymers Fl and F2 at least one is hydrophobic and is preferably selected from polyolefins, such as polyethylene, polypropylene or copolymers thereof where one of the two is present in excess. The other may be not only hydrophobic but also hydrophilic, but is preferably not hydrophilic, but less hydrophobic than polypropylene. The more hydrophobic fibre polymer is herein designated Fl and the less hydrophobic fibre polymer F2. Fl preferably comprises polypropylene or polyethylene or blends thereof. F2 may be for example a fibre from the group of th^ polyesters, such as polyethylene terephthalate, polybutylene terephthalate, polypropylene terephthalate or a copolyester thereof and PE. Both Fl and F2 are as regards polymer selection not subject to any special restriction, except that they must be spinnable into conjugated fibres using existing spinbonding processes. Either or both of Fl and F2 may be made of thermo¬plastic elastomer. Examples of elastic polyolefins for spunbond nonwovens may be found in EP-A-0 625 221 and of metallocene-catalyzed LLDPE in EP-A-0 713 546, which also describes representatives of the less hydrophobic elastomers, such as polyurethanes, ethylene-poly-butylene copolymers, poly(ethylene-butylene) poly-styrene copolymers (Kraton), poly(adipate ester)s and polyetherester elastomer (Hytrel). It is known of these elastomers that spunbond nonwovens in meltblown or SMS combinations can be spun. The use of such elastomers in Fl and/or F2 enhances the softness and compliance of the apertured microfibre nonwoven. It has also been determined that only apertured nonwovens comprising entangled microfibre continuous filaments have the excellent properties with regard to fluid management. Apertured nonwovens comprising similarly entangled microfibre staple fibres do not attain these improved properties. Just on account of the processing on diaper machines (high tensile stress in the machine direction), the weight thereof would have to be on average tripled compared with the continuous fibre nonwoven, with distinct reductions in aperture quality, compliance, softness, abrasion resistance and fluid management. Similarly, additions of ingredients to the fibre polymer melt in the form of masterbatches for antistatic treatment, spin dyeing, delustring, softening, tackifying and flexibilizing the fibre, increasing or reducing the repellent properties with regard to liquids (such as water, alcohols, hydrocnrbons, oils), fats and multidisperse systems, such as intestinal fluids and other fluid body exudates, such as urine and menstrual fluid, are possible. Ingredients for altering the microfibre surface tension can also be applied subsequently to the generation or release of the microfibre filaments in the previously apertured nonwoven. Such materials are for example wetting agents dissolved or dispersed in water, which are used today to finish much diaper cover stock spunbond nonwoven for the purposes of better urine management. However, the nonwovens of the present invention preferably require no such wetting agents or only a fraction of the hitherto customary amount. The configuration of the apertures, i.e. their hole size, their shape, the arrangement of the individual apertures relative to each other (offset or in series, for example) and the open area on the one hand and also the extremely high compliance of the land between the apertures, which consists of entangled continuous microfibre filaments, and their very low weight permit this wetting agent reduction to the point where no wetting agent is needed at all. The drawing with Figures 1 to 6 illustrates the invention. Figures 1 to 6 indicate the shape of the individual openings K and their location in a sheet material. Fig. 1 shows K as an idealized opening in the form of an equilateral hexagon, a and b being identical. The distance o is the shortest distance between the centre c of the opening K and the edge a. The edges a and b are each at a constant distance g from each neighbouring K. Each opening K can be surrounded by a larger equilateral hexagon with edges e and f being arranged parallel to a and b. e = f in Fig. 1. This creates a honeycomb arrangement of the openings K. The edges a and b of an opening K are each in a parallel arrangement with the neighbouring edges a and b of the neighbouring openings K. The distance h is 0.5 g. The vertices formed by the touching edges a and a or a and b are rounded in the nonwoven. These roundings i and j of the vertices are illustrated in Fig. 1 for the case i = j. Owing to these roundings, the original distances d and e of the hexagon shorten to q and r. In Fig. 1 q is again = r. All roundings i and j can be so wide in the extreme case that a circularly round shape results for K, as depicted in Fig. 2. The openings K of Fig. 3 differ from those of Fig. 1 only in that b is distinctly longer than a and the rounding i is more pronounced than j. The roundings i and j can be so extended in the extreme case that the hexagonal K is converted into an elliptical shape, as depicted in Fig. 4. Hexagonal shapes for the openings K or shapes which result therefrom by rounding and the arrangement thereof as depicted in Fig. 1 to Fig. 4 are particularly preferable for fluid management. More particularly, equilaterally hexagonal openings K and their rounded derivatives always provide the shortest path for the body fluid from the diaper surface into the diaper interior. However, the invention is not restricted to such regular shapes and arrays. Other polygons and their rounded derivatives are conceivable for K as are irregular patterns of such or other openings. However, less utility is possessed by those openings and arrays thereof which give that part of the discharged body fluid furthest away from the opening edge a reason not to flow off rapidly through the openings K. Such arrays are reproduced in Figs. 5 and 6 by way of example. The distance from the most remote point w to the (rounded) corner of the rectangle is clearly larger than the distance h. The ratio u/h of the maximum distance to the opening K to the minimum distance should ideally be 1/1 and at worst not exceed 2/1. The individual hole area varies in the range from 0.01 to 0.60 cm2, preferably between 0.04 to 0.40 cm2. The individual apertures may all have the same shape and consistently the same hole area. But either or both may be different, subject to the abovementioned teaching of u/h not more than 2/1. The open area is in the range from 8 to 40%, preferably between 12 to 35%. The microfine entangled continuous filaments S form the frame L for the openings. The apertured nonwoven, as mentioned above, may contain surfactants imparting leachable, retardedly leachable or permanent hydrophilicity to it. These are advantageously applied wet-on-wet after the water jet aperturing. The application rate is between 0 to 0.60% by weight, based on the nonwoven weight, preferably between 0 and 0.20%. The dosage depends on the area of the individual holes and the total open area. The larger the two, the more the level of such surfactants can be lowered. For reasons of optimum biocompatibility, a surfactant content of 0% is desirable. It is particularly advantageous not to distribute the surfactant uniformly over the entire frame, but to limit it to the immediate neighbourhood of the hole periphery. This location will then inevitably be the starting point of a suction effect on the fluid that is directed towards the apertures. The multidisperse fluid system then does not suffer any dewatering or phase separation. Plugging of the apertures and deposits on the frame are prevented. The fluid acquisition and distributor layer which is placed between absorbent core and topsheet and which has likewise been rendered wetting additionally furthers the immediate removal of the body fluid from the diaper surface. Production of apertured nonwoven (topsheet) The process comprises a splittable pie or hollow pie fibre being laid down by spunbond web technology to form a nonwoven composed of continuous filaments. The cross sections of the unsplit fibres emerging from the spinneret consist of the two different polymer components Fl and F2, which alternate like pie segments (normally 4 to 16 such pie segments) . The preferable prerequisite for subsequent splitting is to use such usually 2 polymer-chemically very different components as exhibit minimal adhesion at their common interfaces. But it is also possible to use chemically similar polymer components, for example polyethylene tere-phthalate and a copolyester or polypropylene and polyethylene, provided measures have been taken to reduce the adhesion at the interfaces of the two, for example by including a release agent at least in one fibre polymer component. A splittable fibre having a (round) void on the inside is referred to as a hollow pie fibre; otherwise it is known as a pie fibre. The linear density of the continuous filaments in the spunbond nonwoven prior to splitting is generally 1.0 to 4.0 dtex, preferably 1.6 to 3.3 dtex. The continuous filaments of the spunbond nonwoven are subsequently entangled and simultaneously split into the pie constituents by known methods of high pressure water jet technology (see for example EP-A-0 215 684) in a first after treatment stage. In the case of a pie fibre having* a linear density of 1.6 dtex and a total of 16 segments made up of 8 segments each of the two fibre polymers, microfibres having a linear density of 0.10 dtex are accordingly present after splitting. Since the invention is concerned with a very lightweight nonwoven, it is advantageous for the support to be used for the nonwoven not to be a sieve or support with apertures, but a completely unapertured support. As a result, the impact energy of the water jets being reflected back from this support can be utilized to minimize the energy loss. Aperturing is either followed by drying or advantageously, before drying, by the application, wet-on-wet, of surfactant for the purposes of surficial hydrophilicization. This can be accomplished by known methods of full bath impregnation, of face padding, of brushing or of printing. In a particular embodiment, the surfactant (wetting agent) is printed on in a pattern in such a way that only the aperture border regions of the fibre frame are affected. This requires the fabrication of specific printing screens which have to be adapted to the aperture pattern and particular control measures to maintain contour crispness for the wetting agent print during production. Inventive Example 1: A spunbond nonwoven having a basis weight of 13 g/m2 which is 100% 1.6 dtex pie fibre is laid down on a sieve. The pie fibre cross section is made up of an alternating sequence of 8 polypropylene segments and 8 polyethylene terephthalate segments. The size of the individual polypropylene segments is such that the weight fraction is 30% for the polypropylene and 70% for the polyethylene terephthalate. The unsplit continuous filament nonwoven is placed on a 100 mech drainage sieve and consolidated using a water jet pressure of 180 mbar and the continuous filaments are each split into their 8 microfibre segments of polypropylene and 8 microfibre segments of polyethylene terephthalate. Splitting produces equal numbers of polypropylene microfibre segments and polyethylene terephthalate microfibre segments. The polypropylene microfibre segments each have a linear density of 0.06 dtex and the polyethylene terephthalate segments each have a linear density of 0.14 dtex. The conversion of dtex into fibre diameter (idealized for a round cross section) gives a value of 2.36 micron for polypropylene (density 0.91 g/cm3) and a value of 4.42 micron for polyethylene terephthalate (density 1.37 g/cm3). After the splitting of the fibre by means of water jets, the sheet material is subjected to aperturing, likewise by means of high pressure water jets at a pressure of 70 kg/cm2. This is done using the drainage and hole forming drums with protrusions on the drum surface which are described in EP-A-0 215 684 instead of the otherwise customary drainage sieves. Drying provides a very soft, compliant nonwoven having clearly defined apertures. The individual apertures are all of an idealized circular shape and of equal size. The apertures are arrayed in an orthogonal grid having a grid spacing a, in each case with a further grid of apertures superposed in a face-centred arrangement. The radius r is 1.4 mm on average and the distance a = 6.0 mm. The open area OF is 34%, based on the total area. The apertured nonwoven was measured for ultimate tensile strength in machine direction according to EDANA 20.289, for liquid strikethrough time according to EDANA 150.3-96 and for coverstock wet back (also called rewet) according to EDANA 151.1-96. The strikethrough was repeated a total of 2 times after a delay time of 1 minute in each case without changing the filter paper layers. The reported values are each the averages of a total of 3 individual measurements. Results: Rewet: 0.09 g Inventive Example 2: The apertured nonwoven of Inventive Example 1 was padded with an aqueous emulsion of a nonionic wetting agent based on polysiloxane by the full bath method in a pad-mangle. The add-on was 0.042% by weight of solids after drying. This pattern gave the following test results: Ultimate tensile strength in machine direction: 30.2 N/5 cm Rewet: 0.31 g Comparative Example 1: A meltblown layer of 2 0 g/m2 was spun onto an embossing-bonded polypropylene spunbond having 2.2 dtex continuous filaments and a basis weight of 10 g/m2. The average diameter of the microfibres making up the meltblown layer was 3.82 micron. The welded area of the embossing-bonded spunbond was 5.2%. This two-ply laminate was hydroentangled by the method described in Inventive Example 1 and subsequently apertured on a conventional 20 mesh sieve belt. The open area worked out at 18.4%. This two-ply nonwoven was likewise very soft, but revealed distinct deficits with regard to ultimate tensile strength and strike-through compared with the test values measured in Inventive Examples 1 and 2. Strikethrough and rewet were each measured on the PP meltblown side. Ultimate tensile strength in machine direction: 25.4 N/5 cm Rewet: 0.10 g The strikethrough values are distinctly too high for a topsheet. Comparative Example 2: The sample of Comparative Example 1 had 0.40% of nonionic wetting agent based on polysiloxane applied to it. As the measured results show, this does distinctly reduce* strikethrough, but rewet increases disproportionately. Such a high rewet is not acceptable in a diaper. Results: Rewet: 2.35 g The meltblown layer lends high softness to the top-sheet. In the presence of wetting agent, however, this meltblown layer acts like a sponge. Such a construction thus proved unsuitable as coverstock for an underpad. Comparative Example 3: The 2-ply construction described in Comparative Example 1 is subjected to a water jet treatment corresponding to Inventive Example 1. The average radius r of the holes after water jet aperturing was r = 1.28 mm. The spacing a remained unchanged at a = 6.0 mm. This provided an open area OF = 28.6%. Results: Rewet: 0.10 g The strikethrough values are again too high. Claims 1. Apertured nonwoven having a basis weight of 8 to 17 g/m2 comprising entangled continuous microfibre filaments having a linear density in the range from 0.05 to 0.40 dtex and constructed of at least two different filaments of thermoplastic polymers having different hydrophobicities and a segmented pie cross section, which have been released from fibres containing the filaments, wherein the apertures are clearly defined and free of fibre filaments. 2. Apertured nonwoven according to Claim 1, characterized in that the apertures form a regular atray and have an individual hole area of 0.01 to 0.60 cm2. 3. Apertured nonwoven according to Claim 1 or 2, characterized in that in the nonwoven the ratio of the maximum distance of points on the nonwoven surface to the next aperture to the minimum distance is in the range from 1:1 to 2:1. 4. Apertured nonwoven according to any of Claims 1 to 3, characterized in that the open area is 8 to 40%. 5. Apertured nonwoven according to any of Claims 1 to 4, characterized in that the apertured nonwoven is constructed of polyolefin and polyester filaments in a weight ratio in the range from 20:80 to 80:20. 6. Apertured nonwoven according to any of Claims 1 to 5, characterized in that the nonwoven is impregnated with 0 to 0.60% by weight, based on the nonwoven weight, of at least one surfactant. 7. Apertured nonwoven according to any of Claims 1 to 6, characterized in that the strikethrough value after one minute is less than 3 seconds, the rewet value is less than 0.5 g and the ultimate tensile strength in the machine direction is at least 30 N/5 cm. 8. Process for producing apertured nonwovens according to any of Claims 1 to 7 by laying down splittable pie or hollow pie continuous fibres whose cross section comprises at least two different thermoplastic polymers having different hydrophobicities in an alternating segmented pie arrangement to form a nonwoven, subsequently splitting and entangling the fibres to form entangled continuous filaments by means of high pressure water jets and subsequent aperturing of the resultant nonwoven using high pressure water jets. 9. Process according to Claim 8, characterized in that the aperturing is effected on drainage and hole forming drums having protrusions on the surface. 10. Use of apertured nonwoven according to any of Claims 1 to 7 as topsheet in hygiene products such as diapers or sanitary napkins. 11. Apertured nonwoven having a basis weight of 8 to 17 g/m2 substantially as hereinbefore described with reference to the accompanying drawings. 12. Process for producing apertured non wo vens substantially as hereinbefore described with reference to the accompanying drawings.

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