Nonwoven sheet comprising machine direction oriented fibers

Abstract

The present invention relates to a method of manufacturing a nonwoven sheet comprising a first and preferably a second nonwoven base layer and a layer of machine direction oriented fibers, the method comprising: providing the first nonwoven base layer; preferably, providing the second nonwoven base layer; providing the machine direction oriented fibers and associating the resulting fibers to the first nonwoven base layer(s); and bonding together the layers to obtain the nonwoven fabric sheet. The invention further relates to a nonwoven sheet made by such a method, comprising a first and preferably a second nonwoven base layer and a layer of machine direction oriented fibers, wherein the layers are bonded together to form the integral nonwoven sheet.

Description

[0001] Nonwoven sheet comprising machine direction oriented fibers

[0002] The invention relates to a nonwoven sheet comprising a nonwoven base layer and a layer of machine direction oriented fibers, and a method for making such nonwoven sheet.

[0003] The hygiene industry uses nonwoven material on a large scale in hygiene products like baby diapers, adult incontinence products and feminine care products. Amongst different technologies for making nonwoven material in hygiene industry, the spunbonding technology has evolved into a dominant role. The basic principles of this technology comprise extruding a molten thermoplastic polymer through a spinneret to obtain a curtain of extrusion spun filaments, cooling and drawing (attenuating) the filaments in an aerodynamic device and depositing the solidified and drawn endless fibers onto an air permeable moving spinbelt. Before deposition, the fibers pass a diffusor to entangle the fibers and increase cross-machine directional orientation. The diffusor and the fact that the speed of the fibers exiting the diffusor is typically much higher than the travelling speed of the spinbelt contributes to a random fiber distribution with large proportions also oriented in cross-machine direction.

[0004] The terminology used in the preceding paragraph and the application altogether, in the context of spunbonding and the new technology of directionspinning introduced further below, distinguishes between “filaments” and “fibers”. The string of polymer that exits the spinneret is termed “filament”. When the filaments are drawn and cooled down, the polymer chains within the filaments are oriented in the length direction of the filaments and slowly crystalize, changing the internal structure. Once they have substantially reached their finally stretched state, and at a subsequent level, they are referred to as “fibers”. Other than when describing spunbonding or directionspinning, for example to describe meltblown fibers or stable fibers, reference is simply made to “fibers”.

[0005] The spunbonding technology has been known for more than 50 years, as evidenced, for example, by the disclosure of US 3,855,046 A. Since then, the process has continuously been optimized under various aspects.

[0006] Among such aspects have been a higher number of thinner fibers and a better uniformity at laydown. While in earlier generations of spunbonding processes 1000 to 2000 filaments per meter in cross direction would be spun, the number was raised to approx. 7000 to 8000 filaments in the newest generations, with thinner fiber diameters. This evolution required optimising filament cooling, drawing (attenuation) and laydown, but enabled creating thinner nonwoven sheets, in the sense of lower basis weight as expressed in grams per square meter, at equal performance. Performance can be in physical properties, like tensile strength and elongation, textile and soft feel, elastic properties, stiffness or fluid handling, with priorities depending on the particular application of the product. Another aspect is the adoption of bicomponent technology to make crimped fibers and produce loftier, softer and more stretchable nonwoven materials. Early works on bicomponent technology are disclosed, for example, in US 5,382,400 A and US 6,454,989 B1. Later improvements are disclosed, for example, in EP 3 246 443 B2, EP 3 246 44 B1 , EP 3 455 399 B2 and EP 3 521 495 B2. The latter of these references has a focus on making thin crimped fibers for more uniform laydown. Yet another aspect is the production of nonwoven materials including fibers formed from thermoplastic elastomers and having an inherent ability to be elastically stretched. Examples of corresponding technologies include EP 3 715 517 A1 , EP 3 867 437 B2, EP 4 180 018 A1 and WO 2023 / 083600 A1.

[0007] Owing to an ongoing increase in the line speed of machines for the production of hygiene products from nonwoven materials, a common requirement for any nonwoven material intended for such use is robustness and stability. This requirement is at odds with the competing requirements of reducing production cost for nonwoven materials and reducing their basis weight for more sustainability and, despite reported previous improvements in laydown uniformity, fiber number and fiber thinness, calls for further solutions.

[0008] Woven fabrics, also known as textiles, comprise oriented threads, which can be single fibers or yarns in warp or weft direction. This stands in contrast to nonwovens, where fibers are more or less randomly distributed over the machine direction (MD) and cross-machine direction (CD). Textiles have unique properties in terms of performance, but their production is relatively expensive and time-consuming, as it requires a stepwise process comprising manufacturing the thread and subsequently weaving the textile with exact control of each single warp and weft thread.

[0009] In view of the above considerations, it would be desirable to provide a nonwoven material that includes elements of woven fabrics for improved performance. It would further be desirable to provide a method for making such a material, which maintains the fast, robust and cost-efficient nature of spunbonding technology.

[0010] In this context, it has been taken into consideration that it has been previously known to include directionally oriented elements into nonwoven materials. For example, it has been described in, amongst many other references, US 2004 / 0005835 A1 and US 2021 / 0282979 A1 to combine nonwoven sheets and potentially pre-stretched elastic strands to impart elastically stretchable properties to nonwovens. In EP 3 974 572 A1 it has been described to use in-line extruded elastomeric polymer strips to form expressly MD oriented structural elements in extensible nonwoven materials. In EP 2 098 363 B1 a process for forming layered materials, which comprise nonwoven base layers and a layer of substantially parallel machine direction oriented fibers formed from extrusion spinning, has been described. The spinning machine is devoid of a drawing device in the form of Godet rolls or an aerodynamic device, and is devoid of a diffusor. The filaments are drawn by the tensile force when they are drawn into the nip between the consolidation rolls that combine the prefabricated nonwoven base layers and the machine direction oriented fibers to form the layered materials. The fibers are formed from thermoplastic elastomers and still sticky when the layers are combined, which renders the layered material stable without additional bonding. Similar solutions to the one of EP 2 098 363 B1 are disclosed in WO 2007 / 078344 A1 and EP 0 343 978 A2. As another variant, document WO 2005 / 065932 A1 discloses the idea of elastic machine direction oriented fibers associated with a meltblown base layer. With regard to the making of the machine direction oriented fibers in a curtain extruder, the reference refers to US 5,385,775 A, which discloses using a modified meltblowing machine that does not cool or draw the fibers. The tip of the meltblowing die is arranged close to the spinbelt and that the fibers are drawn by the spinbelt. A pair of consolidation rolls is used to associate the machine direction oriented fibers to a meltblown base layer.

[0011] Further, it has been taken into consideration that it has been previously known to draw extruded filaments in spunbond processes through a stretch (Godet) roll system instead of an aerodynamic device. The roll system would be located between a cooling device and a diffusor. As an example, reference can be made to DE 36 03 814 A1 or DE 41 02 650 A1 .

[0012] In view of the needs and considerations outlined above, the present invention relates to a method of manufacturing a nonwoven sheet comprising a first and preferably a second nonwoven base layer and a layer of machine direction oriented fibers, the method comprising: providing the first nonwoven base layer; preferably, providing the second nonwoven base layer; providing the machine direction oriented fibers and associating them to the first nonwoven base layer and preferably the second nonwoven base layer; and bonding together the layers to obtain the nonwoven fabric sheet. The invention further relates to a nonwoven sheet made by a method of the invention, comprising a first and preferably a second nonwoven base layer and a layer of machine direction oriented fibers, wherein the layers are bonded together to form the integral nonwoven sheet. A key aspect of the present invention, vis-a-vis prior art solutions like the ones of US 2004 / 0005835 A1 or US 2021 / 0282979 A1 , where prefabricated strands are combined with nonwoven sheets, is the in-line fabrication of the machine direction oriented fibers and their direct association with the base layer(s) upon production. This is in contrast to prefabricating the machine direction oriented fibers and later unrolling them from a spindle or the like.

[0013] According to the invention, the machine direction oriented fibers are provided by extruding a molten thermoplastic polymer through holes of a spinneret to obtain a curtain of extrusion spun filaments, followed by cooling and drawing (attenuating) the extruded filaments and associating the resulting machine direction oriented fibers to the nonwoven base layer(s) in the nip of a pair of consolidation rolls or a pair of calendering rolls. In an embodiment, the machine direction oriented fibers pass through a Godet roll system on their way from the spinneret to the consolidation or calendering rolls, meaning that the machine has one or a set of Godet rolls positioned before the consolidation or calendering rolls. This technology of making machine direction oriented fibers herein is designated as “directionspinning”. The machine direction oriented fibers can be synonymously designated “directionspun fibers” herein. In contrast to traditional spunbonding, the machine direction oriented fibers are not entangled in a diffusor or similar device, such that their substantially machine directional and parallel alignment is not deteriorated. Likewise, the filaments do not pass through an aerodynamic stretching device of significantly increased airflow speed, such that a drawing, i.e. stretching and attenuation is substantially effected by a speed increase of the filaments from the spinneret to the nip of the consolidation or calendering rolls. Primary cooling of the extruded filaments following their extrusion from the spinneret is preferably effected by a process air system as commonly applied also in traditional spunbonding, albeit at a largely reduced back pressure, which leads to reduced energy consumption. The air flow should still be sufficiently high to ensure the cooling / quenching of the molten resin, but no attenuation of the filaments is required. Guide plates for guiding an airflow in the area between the process air system for primary cooling and the consolidation or calendering rolls or, if any, Godet rolls can be present for air guidance, but should not lead to a significant increase in air flow speed. In order to reduce the amount of air flow around the Godet rolls, it is also possible to install a suctioning system to remove cooling air.

[0014] The maximum air speed during filament stretching, in a conventional spunbonding process, is typically at least 800 m / min, more typically above 1000 m / min. The directionspinning process herein, to the contrary, preferably uses maximum air speeds of below 600 m / min, more preferably below 300 m / min, or even below 100 m / min. Adjustments within that range are made depending on resin throughout, melt temperatures, a desired filament temperature at the consolidation or calendering rolls and, if any, the Godet rolls, and factors like the geometry of the quench chamber and the shape of the guide plates.

[0015] The Godet roll system, if present, can comprise a single roll, a pair or rolls, or a triplet of rolls, or can comprise two or more roll pairs or triplets or any combinations thereof. It can comprise two or more sub-systems that are spaced apart from each other. The rolls typically comprise a flat or mildly structured surface. They are preferably driven and controlled at a specific speed. Especially in the case of a single roll, to generate sufficient contact to enable their function, it is preferred to have a minimum wrap-around angle of >30°, preferably >45° or even >60°. In the case of two or more rolls, it is possible to use different surface speeds for each roll. The roll temperature may be controlled. In a preferred embodiment, the rolls can be cooled to aid filament cooling. To reduce a tendency to stick, the surfaces of the rolls can be coated with inert materials like fluoropolymers, for example polytetrafluoroethylene (PTFE), or resistant coatings like ceramic or hard-chrome.

[0016] The Godet roll system can lead to improvements in controlling the consolidated curtain of filaments coming down from the spinneret. It can impart more process stability and allows for stepwise filament drawing. Specifically, the filament can be drawn at a first ratio by increasing the filament speed from the speed of the filament just extruded at the orifice to the speed of the Godet roll system, and the cooled filament further pre-stretched (when elastic) or cold- drawn (when inelastic) at a second ratio by increasing, in a single step or in multiple steps, the speed from the speed of the pre-stretch roll system to the speed of the consolidation or calendering rolls, to create the machine direction oriented fibers.

[0017] In exemplary embodiments, the first ratio, corresponding to the ratio of the Godet roll speed to the extruded filament speed can be 3 to 400, preferably 5 to 300, more preferably 5 to 250. Due to the gravity of the molten polymer a minimum ratio of 3 and more preferably 5 between the filament speed at the orifice of the spinneret (the exit speed of the material through the capillaries, using the solid density of the polymer material for calculation) to the Godet system is expected as a minimum in order to sustain a higher pulling force against the gravity force of the polymer. The second ratio, corresponding to the ratio of the consolidation or calendering roll speed to the Godet roll speed, can be 1 ,5 to 10, preferably 1.8 to 9, more preferably 2 to 8. In the case of multiple Godet rolls or multiple spaced apart systems of Godet rolls, the second ratio is the product of two or more smaller ratios. The maximum ratio of stretching obtainable is depending on the polymer type and other processing parameters, like cooling efficiency and height position of the Godet system between the spinneret and the spinbelt. In terms of absolute numbers, the speed of the machine direction oriented fibers upon association with the nonwoven base layer(s) can, for example, be 100 to 1200 m / min, preferably 120 to 1000 m / min. The Godet roll speed can, for example, be 5 to 500 m / min, preferably 10 to 300 m / min.

[0018] The overall distance travelled by the filaments / fibers between the consolidation or calendering rolls is preferably at least 100 cm, more preferably at least 200 cm. This enables an appropriate distance for accommodating the necessary machinery, for the stepwise drawing I stretching, and for appropriate cooling of the machine direction oriented fibers before they are associated with the nonwoven base layer(s).

[0019] In one embodiment, the Godet rolls are mounted such that the distance to the spinneret on the one hand and the consolidation or calendering rolls on the other hand can be adjusted. This way, the absolute and relative distances over which the drawing and the further drawing I stretching are carried out can be adjusted. This allows for a more precise control and more variable adjustment of the stretch. In embodiments, the ratio of the distance from the spinneret to the Godet roll system and the distance from the Godet roll system to the consolidation or calendering rolls is 3:1 to 1:3. The absolute distance which the filaments travel from the spinneret to the Godet roll system can be 50 cm to 350 cm, preferably 100 cm to 300 cm. The absolute distance which the machine direction oriented fibers travel from the Godet roll system to the consolidation or calendering rolls can be 50 cm to 300 cm, preferably 100 cm to 200 cm.

[0020] The consolidation rolls used for associating the fibers to the nonwoven base layer(s) in one embodiment typically comprise a flat or mildly structured surface. The linear pressure applied to the structure of the machine direction oriented fibers and the nonwoven base layer(s) by the consolidation rolls, in one embodiment, can be 2 N / mm to 10 N / mm. The surface temperature of one or both consolidation rolls may be controlled to be 20°C to 140°C, preferably 50°C to 120°C. To reduce a tendency to stick, the surface of one or both consolidation rolls can be coated with inert materials like fluoropolymers, for example polytetrafluoroethylene (PTFE), or resistant coatings like ceramic or hard-chrome.

[0021] Irrespective of whether calendaring rolls are used instead of the consolidation rolls already for associating the fibers to the nonwoven base layer(s), as in one preferred embodiment, a bonding step is generally is necessary to obtain an integral sheet. Reliance on an inherent stickiness of even elastic fibers as in EP 2 098 363 B1 is not feasible with already cooled fibers that have been run through a stretch roll system, and would in any case be limited to a certain material group. Bonding also is a prerequisite for any setup where one or both of the nonwoven base layers are not pre-fabricated, but produced in-line. In preferred embodiments, the bonding comprises introducing (embossing) bonding points into the sheet by calender bonding. Typically, a calender comprises at least two calendaring rolls, where at least one roll comprises embossing projections on its surface. Both calender rolls can exhibit individual bonding projections for a dot-dot contact, or both can realize spirals as projections, again to create a contact without flat full contact rollers typically used when one roller only exhibits the bonding projections.

[0022] In a preferred embodiment, the machine direction oriented fibers resulting from the directionspinning process described above are associated to the nonwoven base layer(s) in the nip of the calendaring rolls. A pre-consolidation of the nonwoven sheet and corresponding consolidation rolls can be absent from the method and apparatus in this embodiment.

[0023] The linear pressure between the calendaring rolls, the "nip pressure", is usually significantly higher than the nip pressure of consolidation rolls. In embodiments, it can be 20 N / mm to 100 N / mm. The calender bonding can be ultrasonic bonding, where ultrasonic vibrations are introduced into the embossing projections on the surface of a calender roll, or thermal bonding, where the embossing projections are heated. The bonding points may be arranged in a systematic pattern or randomly distributed. The number of bonding points per cm2of the sheet may be lower than 100 and preferably lower than 80, and on the other hand higher than 10 and preferably higher than 20. The total area of the sheet surface taken up by the areal bonding points in one embodiment is less than 30 % and preferably less than 25 %, and on the other hand higher than 4% and preferably higher than 7%. The area of a single dot on the sheet may be 0,1 to 2 mm2.

[0024] Generally, dot shapes may be selected from dot shapes known in the art, such as round, oval, rectangular, diamond or rod. In an embodiment, a preferred variant are dots in the form of cross-machine directional rods such as those disclosed in WO 2017 / 148865 A1. Such a bond shape may lead to high tensile strength of the nonwoven sheet in the cross-machine direction, where tensile strength is not supported by the machine direction oriented fibers, without detriment to potential elastic properties of the nonwoven sheet in machine direction.

[0025] In embodiments, the method of the invention does not use an adhesive on the base layer(s) or machine direction oriented fibers before associating the machine direction oriented fibers to the base layer(s). Hence, the laminate is preferably free of adhesive at least on the surface of the base layer(s) that face the machine direction oriented fibers. In one embodiment, the capillary layout of the spinneret may be specifically configured to obtain a uniform distance of machine direction oriented fibers from one another. For example, as opposed to a standard layout for standard spunbonding, where capillaries are oriented along parallel machine directional lines, the lines may be slanted with respect to the machine direction. Angles can be up to 15°, for example. Angles, in one embodiment, can be such that capillaries of different capillary lines do not overlap in position, or such that there is an overlap.

[0026] In an embodiment, the capillaries of the spinneret may have a diameter of 0,2 to 0,7 millimeters and a length-to-diameter ratio of 1 :1 to 5:1.

[0027] The nonwoven base layer(s) can each individually comprise or consist of one or more spunbond nonwoven layers or meltblown nonwoven layers. The configuration can depend on an intended application. For example, for high flexibility, stretchability and loft, it can be preferred that at least one of the nonwoven base layer(s) comprises or consists of one or more spunbond layers formed from crimped multicomponent fibers. For tensile strength, it can be preferred that at least one of the nonwoven base layer(s) comprises or consists of one or more spunbond layers formed from linear monocomponent fibers or symmetric cross section bicomponent fibers. For liquid barrier properties, it can be preferred that at least one of the nonwoven base layer(s) comprises one or more meltblown layers. In general terms, layer sequences in the nonwoven sheet of the invention can, for example, be S-D-S, S-S-D-S, S-S- D-S-S, S-M-D-S, S-D, or S-S-D, or similar, where S stands for a spunbonded layer, M stands for a meltblown layer, and D stands for the directionspun layer of machine direction oriented fibers. In each case, the S may be a SBICO or SMOCO, where SBICO stands for a spunbonded layer of crimped or uncrimped multicomponent fibers and SMOCO stands for a spunbonded layer of linear monocomponent fibers.

[0028] There can also be more than one step of making machine direction oriented fibers. In other words, the step of providing the machine direction oriented fibers can be repeated. In terms of layer sequences, this leads to sequences like S-D-D-S, S-D-D, S-S-D-D, S-D-S-D-S, or the like.

[0029] The nonwoven base layer(s) can each individually be provided as prefabricated nonwoven sheets or formed in-line.

[0030] Forming in-line a base layer can, in the case of a spunbonded layer, include spinning fibers in a regular spunbonding process, including cooling and drawing the filaments in an aerodynamic device, passing the drawn and cooled fibers through a diffusor and depositing the fibers onto a target surface to form a fibrous web. Forming in-line a meltblown layer can comprise meltblowing fibers and depositing them on the target surface. Forming in-line a carded layer can comprise parallel laying or cross-laying a carded web of staple fibers onto the target surface. Whatever nature the nonwoven base layer is, when forming the first base layer, the target surface can be a moving spinbelt or an existing substrate layer. When forming the second nonwoven base layer, the target surface is the existing structure comprising the first nonwoven base layer and the machine direction oriented fibers. In-line formed web(s) of the nonwoven base layer(s) are preferably consolidated directly by passing the structure between two consolidation rolls. For description of roll configuration and settings, reference can be made to the description of the consolidation rolls above.

[0031] Prefabricated nonwoven sheets are already bonded structures that are typically manufactured off-line and, for provision in the inventive method, unrolled from a material roll and combined with the machine direction oriented fibers. For the first nonwoven base layer, a prefabricated layer can be co-fed into the nip of the consolidation or calendering rolls with the machine direction oriented fibers. For the second nonwoven base layer, a prefabricated layer can likewise be co-fed into the nip of the consolidation or calendering rolls with the machine direction oriented fibers, or combined with the structure comprising the first nonwoven base layer and the machine direction oriented fibers downline, for example downline of consolidation rolls at another pair of consolidation rolls or at a bonding station comprising a pair of calendering rolls. The prefabricated sheets are bonded in itself, but the overall bonding of the inventive sheet comprising the nonwoven base layer(s) and the machine direction oriented fibers is still necessary for the reasons explained above.

[0032] Combinations of fabricating one of the nonwoven base layers in-line and providing one nonwoven base layer as a prefabricated sheet are feasible. In embodiments, the first nonwoven base layer can be provided as a prefabricated sheet and the second nonwoven base layer formed in-line, or the first nonwoven base layer can be formed in-line and the second nonwoven base layer provided as a prefabricated sheet.

[0033] The machine direction oriented fibers can be monocomponent or bicomponent. The fibers or fiber components can be formed from any polymer suitable for the related technology of spunbonding. Polyolefins like polyethylene (PE), polypropylene (PP) or copolymers including polypropylene (coPP) or polyethylene (coPE), in particular polypropylene (PP) can be preferred to achieve desirable product performance. If high strength is desired, a low melt-flow rate polypropylene having a melt flow rate of, for example 2-25 g / 10 min, preferably 3-25 g / 10 min, more preferably 5-25 g / 10 min (ISO 1133, 230°C, 2, 16 kg), can be used. If elasticity is desired, for example, thermoplastic elastomers (TPE) like styrol-ethylen-butylen-styrol (SEBS) can be used. The thermoplastic elastomers may also be blended with other thermoplastic resins, for example with polyesters like polyethylenterephthalate (PET) or polylactic acid (PLA).

[0034] If the machine direction oriented fibers are bicomponent, the distribution of the components over the cross-section of the fibers is preferably symmetric, i.e. centric sheath-core. The symmetric bicomponent fibers do not crimp, but it allows, for example, to reduce stickiness of a fiber despite an elastic character when a core of thermoplastic elastomer is sheathed by a regular polypropylene, or to improve bonding while maintaining high strength when a core of a low melt flow rate polypropylene is sheathed with a polyethylene, a polypropylenepolyethylene copolymer or a higher melt flow rate polypropylene.

[0035] An asymmetric distribution like eccentric sheath-core, side-by-side or similar can also be used in bicomponent fibers, usually to induce crimp.

[0036] In embodiments, the linear mass density of the machine direction oriented fibers can be 300 denier (grams per 9000 m) or less, preferably 200 denier or less, more preferably 100 denier or less, yet more preferably 75 denier or less.. Exemplary ranges comprise 1 to 100 denier, preferably 10 to 50 denier, preferably 20 to 40 denier. Depending on the type of machine direction oriented fiber, denier ranges may differ. For example, an elastic fiber may have 30- 60 denier (relaxed, i.e. contracted state), while an inflexible fiber may have 4-20 denier. The number of machine direction oriented fibers per meter of sheet in cross-machine direction can be 500 or more, preferably 1.000 or more, more preferably 2.000 or more, or even 4.000 or more. Preferred upper limits depend on application and machine setup and can lie at 10.000, 8.000 or 6.000. The numbers of machine direction oriented fibers per meter of sheet in crossmachine direction (sheet width) given above lead to average center-to-center MDOF (fiber) spacing of 2 mm or less at 500 or more fibers / m(CD), 1 mm or less at 1000 or more fibers / m(CD), 0,5 mm or less at 2000 or more fibers / m(CD), 0,33 mm or less at 3000 or more fibers / m(CD) and 0,25 mm or less at 4000 or more fibers / m(CD). The average gap between fibers is even smaller owing to the lateral fiber dimension. For example, a 100 denier fiber typically has a diameter in the magnitude of approx. 0, 1 mm, so for such fibers, the average gap between such fibers would be approx. 0,1 mm less than the average center-to-center fiber distance.

[0037] The invention comprises embodiments where the filaments pass a Godet roll system before being associated with the nonwoven base layer(s) in the nip of a pair of consolidation rolls, and embodiments where the Godet rolls are omitted. Godet rolls may, in particular, be omitted where the machine direction oriented fibers are non-elastic. However, also in this case, using a Godet roll system may be preferred as it allows for a cold drawing of machine direction oriented fibers, which may improve their tenacity and modulus.

[0038] Further details and advantages of the invention are described in the following with reference to exemplary embodiments and figures. The figures show:

[0039] Fig. 1 : a schematic illustration of a traditional spunbonding machine;

[0040] Fig. 2: a schematic illustration of one embodiment of a machine for directionspinning and making machine direction oriented fibers in a method of the invention;

[0041] Fig. 3: a variant of Fig. 2 with a different position of the Godet rolls;

[0042] Fig. 4: a variant of Fig. 2 with two sets of Godet rolls;

[0043] Fig. 5: a schematic illustration of another embodiment of a machine for directionspinning and making machine direction oriented fibers in a method of the invention;

[0044] Fig. 6: a schematic illustration of yet another embodiment of a machine for directionspinning and making machine direction oriented fibers in a method of the invention;

[0045] Fig. 7: a graph illustrating the correlation between spinbelt speed and fiber denier and basis weight of machine direction oriented fibers at a given polymer throughput per hole of the spinneret;

[0046] Fig. 8: an illustration of the layout of the layer of the machine direction oriented fibers;

[0047] Fig. 9: capillary layouts of spinnerets adapted for directionspinning;

[0048] Fig. 10: a process setup for making a layered nonwoven sheet of the invention in one embodiment;

[0049] Fig. 11 : a process setup for making a layered nonwoven sheet of the invention in another embodiment; Fig. 12: a process setup for making a layered nonwoven sheet of the invention in yet another embodiment;

[0050] Fig. 13: a process setup for making a layered nonwoven sheet of the invention in yet another embodiment;

[0051] Fig. 14: a detail view of the pair of calender rolls in the setup of Fig. 13, where the machine direction oriented fibers and the base layers are combined; and

[0052] Fig. 15: a stress-strain diagram for an exemplary polypropylene fiber.

[0053] A machine 10a for traditional spunbonding is illustrated in Fig. 1. Thermoplastic polymers from a feed system 11 are provided to an extruder 12, heated and extruded through a spinneret 13 to form a curtain of filaments 51a. A ventilation system 14a draws monomer exhaust air from the spinneret 13 and a process air system 14b, including a pressurized cabin, directs process air to the curtain of filaments 51 a. The process air typically enters through a honeycomb system in order to have a good air distribution, to stabilize the curtain of filaments 51a and to facilitate equal cooling and subsequent drawing of all filaments 51a. Subsequent to the process air system 14b, the filaments 51a and process air enter a stretching channel, comprising a so- called intermediate channel 15 and so-called SAS plates 16 for drawing and attenuating them. In intermediate channel 15, the air speed is increased, and the SAS plates 16 have an accurate position and precise distance, ensuring an equal airflow and thereby draw ratio across the width of the machine. During the drawing and further cooling, the polymer chains within the filaments 51 a are oriented in the length direction of the filaments and slowly crystalize, resulting in an increased tenacity in each single resulting fiber. Subsequently, the thus finally stretched endless fibers 50a enter a diffusor 17, also referred to as the distribution unit. Here, the airspeed and thereby the fiber speed is reduced, generating a controlled turbulence that will distribute the fibers 50a evenly on the air-permeable spinbelt 20, to which the combined fiber and air stream exiting the diffusor 17 is directed for fiber laydown and the formation of a fibrous web. The web is consolidated by a pair of consolidation rolls 18. The process air is suctioned off by a suctioning unit 19 below the spinbelt 20. The speed of the fibers 50a exiting the diffusor 17 is up to approx. 3-fold higher than the travelling speed of the spinbelt 20, contributing to their random distribution with high large proportions also oriented in cross-machine direction. In a traditional spunbonded nonwoven, a single endless fiber in one meter fabric length is thus up to approx. 3 meters long. A modified machine 10 for directionspinning and making machine direction oriented (= directionspun) fibers in a method of the invention is illustrated in Fig. 2. As in traditional spunbonding, thermoplastic polymers from a feed system 11 are provided to an extruder 12, heated and extruded through a spinneret 13 to form a curtain of filaments 51. Further as in traditional spunbonding, a ventilation system 14a draws monomer exhaust air from the spinneret 13 and a process air system 14b, including a pressurized cabin, directs process air to the curtain of filaments 51. In embodiments, the process air system 14b can also be used to remove air from the system (in the Figure, this would mean an inversion of the direction of arrows at 14b).

[0054] As compared to traditional spunbonding, however, the speed and back pressure of the cooling air is significantly reduced, mostly by omitting parts 15 and 16 in the narrow setup used in the traditional setup. An exemplary value can be a 50% reduction in cooling air flow rate as compared to traditional spunbonding, with a corresponding drop in back pressure by approximately 90%. As the cooling air generation is responsible for grossly half of the energy consumption of a traditional spunbonding machine (the other grossly half being the melting of the polymers and all line drives), this is very significant. In other words, the directionspinning leads to a reduced energy consumption.

[0055] In further contrast to traditional spunbonding, the modified machine 10 for directionspinning is devoid of an aerodynamic device, comprising stretching channel and diffusor, as shown in the traditional setup of Fig. 1. The process air is rather released to the surroundings and the filaments 51 are drawn to the nip of Godet rolls 21 , before further being cold-drawn (inelastic fibers) or pre-stretched (elastic fibers) to the nip of the consolidation rolls 18, together with typically, the spinbelt 20 and a nonwoven base layer on the spinbelt 20. Guide plates 22 for guiding an airflow in the area between the process air system 14b for primary cooling and the Godet rolls 21 can optionally be present for air guidance, but are arranged to not lead to a significant increase in the speed of the flow of process air, preferably lower than the Godet system speed and the line speed. This can be illustrated by giving numbers for the gap I distance between the plates 22. A distance between the plates 22 can be greater 80 mm to 500 mm in machine direction, preferably 150 mm to 350 mm, more preferably 200 mm to 300 mm. In contrast, in a traditional spunbonding setup as of Fig. 1 , a distance between plates defining the intermediate channel 15 and / or between the SAS plates 16 in machine direction can be between 15 mm to 80 mm. The filaments 51 are hence mechanically stretched without the use of an aerodynamic device or process air. The resultant fibers 50 are essentially oriented in the machine direction of the web and are essentially aligned parallel.

[0056] The set of Godet rolls 21 positioned before the consolidation roll 18 enables improved controlling the consolidated curtain of filaments 51 coming down from the spinneret 13. The Godet rolls 21 are preferably driven and temperature controlled, preferably cooled. For example, the surface temperature of the Godet rolls can be controlled to be in a range of 20°C to 80°C. A Godet roll temperature in the upper part of this range can reduce stretching forces for a given speed ratio between Godet rolls 21 and consolidation rolls 18. In general, the Godet roll temperature can be used to control the morphology of the material, like crystallinity, and to control stretching forces.

[0057] As illustrated in Fig. 3, the position of the Godet rolls 21 and hence the distance that the filaments 51 travel from the spinneret 13 until they get into contact with the Godet rolls 21 can be adjusted.

[0058] As illustrated in Fig. 4, the machine 10 can also comprise two sets of Godet rolls, the Godet rolls 21 and the Godet rolls 21a, which are sequentially arranged between the spinneret and the point of lamination, e.g. the spinbelt 201 consolidation rolls 18.

[0059] Another embodiment of a modified machine 10 for directionspinning and making directionspun fibers in a method of the invention is illustrated in Fig. 5. The difference vis-a-vis the modified machines of Fig. 2-4 is that the modified machine of Fig. 5 is devoid of a Godet roll system and the filaments 51 are drawn straight from the spinneret 13 to the consolidation roll 18.

[0060] Yet another embodiment of a modified machine 10 for directionspinning and making directionspun fibers in a method of the invention is illustrated in Fig. 6. Here, the curtain of filaments 51 , after exiting the spinneret 13, is cooled with cooling air from one side only, as illustrated by the process air system 14b only being present at one side. Some of the the process air will exit the process at the opposite side, as illustrated by the smaller errors on the right hand side, while part of the process air will follow the fibers in a downwards direction. The monomer suction 14a is maintained all around the spinneret 13.

[0061] In the inventive setting as shown in Figs. 2-5, the filament stretch ratio and attenuation is influenced and controlled by the rate of the polymer extrusion through the spinneret 13, the speed of the Godet rolls 21 , and / or the speed of the spinbelt 20 / consolidation rolls 18. At a given polymer throughput per hole of the spinneret 13, the resultant denier of the fibers 50 thus varies, ultimately, with the speed of the spinbelt 20. Higher spinbelt speeds will result in lower denier, and lower spinbelt speeds will result in higher denier. The same applies to the basis weight of the resultant layer of machine direction oriented (= directionspun) fibers 50.

[0062] Fig. 7 illustrates this correlation by way of an example. The denier of the directionspun fibers is reduced as the “line speed", i.e. the speed of the spinbelt 20 I consolidation rolls 18 is increased. At 0,6 grams of polymer throughput per hole (= orifice) per minute (at hole diameter of 0,6 mm and circular holes), the denier range is from 46,0 denier at a line speed of 117 m / min to 4,6 denier at a line speed at 1173 m / min. In terms of basis weight, assuming a spinneret comprising 5700 holes per meter in cross-machine direction, the resulting basis weight of the layer of directionspun fibers, as measured in grams per square meter (“gsm” or “g / m2”), ranges from 29,2 gsm at 117 m / min to 2,9 gsm at 1173 m / min. (The denier refers to the denier at the position of the consolidation rolls. If, for example, an elastic machine direction oriented fiber retards later, then the denier will later increase).

[0063] The method of the invention offers a very flexible process and a possibility to generate a great variety of different fiber deniers at different conditions. The distance over which the first and second stretch occurs, meaning the distance between the spinneret and the Godet system on the one hand and between the Godet system and the spinbelt on the other hand, can be varied and are another parameter that adds a lot of flexibility to the inventive method.

[0064] Fig. 8 illustrates the layout of the layer of the directionspun fibers in terms of fiber thicknesses and open spaces between parallel fibers for different fiber stretch ratios.

[0065] Fig. 9 illustrates how the capillary layout of a spinneret may be configured to obtain a uniform distance of directionspun fibers from one another. In the left illustration, a standard layout for standard spunbonding is shown. The capillaries are oriented along parallel machine directional lines. The middle and left illustrations show capillary layouts that are optimized for directionspinning. Specifically, the capillaries are oriented along parallel lines that are a bit slanted with respect to the machine direction. Angles can be up to 15°, for example. In the middle illustration, the angle is small and capillaries of different capillary lines do not overlap in position. In the right illustration, the angle is bigger and there is an overlap.

[0066] Fig. 10 illustrates a process setup for carrying out a method of the invention to make a layered nonwoven sheet having a S-D-S sequence of layers in one in-line setting. For that purpose, two spunbonding machines 10a and a directionspinning machine 10 are sequentially arranged on a common spinbelt. A first traditional spunbonding machine 10a as per Fig. 1 for making a fibrous web forming for the first spunbond base layer is first in line. A directionspinning machine 10 for making the layer of directionspun fibers as per Fig. 2 is next in line. (The setup could likewise be realized with a directionspinning machine as per Fig. 3-6). A second traditional spunbonding machine 10a as per Fig. 1 for making a fibrous web forming for the second spunbond base layer is last in line. In each case, the layers formed are directly consolidated by a pair of consolidation rolls 18. In the case of the directionspinning machine 10a, the consolidation rolls 18 serve to associate the directionspun fibers to the web forming for the first spunbond base layer, and, if applicable, further pull on the directionspun fibers for additional cold drawing (inelastic fibers) or pre-stretch (elastic fibers) after the Godet rolls 21. The nonwoven base layers formed by traditional spunbonding machines 10a can each individually be formed from crimped multicomponent fibers or linear mono- or multicomponent fibers or different configuration, depending on the requirements in the product.

[0067] Fig. 11 illustrates an alternative process setup for carrying out a method of the invention to make a layered nonwoven sheet having a S-D-D sequence of layers in one in-line setting. For that purpose, as compared to the setup of Fig. 10, the second traditional spunbonding machine is replaced by a second directionspinning machine 10 (as per Fig. 2, but could likewise be a directionspinning machine as per Fig. 3-6).

[0068] Fig. 12 illustrates a further alternative process setup for carrying out a method of the invention to make a layered nonwoven sheet having a S-D-S sequence of layers. The setup of Fig. 12 does not comprise traditional spunbonding machines, but only one directionspinning machine 10 as per Fig. 2 (could alternatively be as per Fig. 3-6), and uses prefabricated spunbonded sheets 40, 41 that are unrolled from corresponding rolls to provide for the first and second nonwoven base layers of the sheet. The prefabricated spunbonded sheets and the directionspun fibers formed in the directionspinning machine 10 are all combined in the nip of consolidation rolls 18 associated with the directionspinning machine 10. The spinbelt may be absent in this embodiment.

[0069] In either of the embodiments of Fig. 10-12, the associated spunbonded base layer(s) and layer(s) of directionspun fibers are finally bonded by calender bonding at bonding station 30. The bonding is necessary to obtain an integral sheet. The calender comprises two rolls 31 , one of which comprises embossing projections on its surface. The calender rolls 31 act on the sheet by a certain linear pressure in the magnitude of, e.g., 40 N / mm and the embossing projections are heated to facilitate local polymer melting upon contact and the embossing of bonding points. Also, in either of the embodiments of Fig. 10-12, the products may be wound up to form a product roll on a winding station (not shown in Fig. 10-12) that is arranged downline of the bonding station 30.

[0070] Combinations of the embodiments of Fig. 10-12, allowing, for example, the manufacture of an S-D-D-S or an S-S-D-S sheet, or using in-line forming of one of the spunbonded base layers and using a prefabricated sheet for the other one of the spunbonded base layers, are feasible and also encompassed as variants of the invention.

[0071] In an example, the directionspinning technology can be used to reinforce nonwoven sheets in terms of their MD tensile strength.

[0072] The embodiments including a Godet roll system, as per Fig. 2-4, are particularly suited, as they allow to use the effect that cold-drawing of a substantially inelastic fiber, e.g. a PP fiber can increase its tensile strength. To illustrate this effect, Fig. 15 shows a stress-strain diagram for a polypropylene (PP) fiber that, in an unstretched state, has a linear mass density of 7,25 denier. The different curves in the diagram represent different individual tests. As can be seen, the tensile strength increases significantly as the fiber is cold-drawn, and the stretch can generally be up to 400% without breaking the fiber. The force required to stretch the fiber in an unstretched state is about 3 cN, and rises to about 3- fold (9 cN) at a 400% stretch, with the fiber denier at the same time decreasing to 1 ,80. Already at 200-250% stretch, the increase in tensile strength is significant, at low risk of fiber break. The increase of tensile strength and at the same time reduction in linear mass density is highly desired to obtain fabrics with low basis weight and high machine directional tensile strength.

[0073] As an example, an SDS product can be produced, with two standard polypropylene (PP) spunbonded base layers of 5 g / m2basis weight each and a D-layer of machine direction oriented polypropylene (PP) fibers of 2 g / m2can be produced in a line of a setup as shown in Fig. 12. The density of the polypropylene used is 0,9 g / cm3. The line speed can be 600 m / min. To cold-draw the directionspun fibers by a factor of 400% at the Godet system, the Godet roll speed is adjusted to 150 m / min (reference to the first roll in the case of multiple rolls). The 2 g / m2D-layer can be made up of different numbers of fibers having different linear mass densities. Options are given in the following Table 1.

[0074] Table 1

[0075] Generally, the options with high fiber numbers of 4500 and beyond give products where the directionspun fibers are optically less obvious, given their lower basis weight and closer distance.

[0076] In another example, the directionspinning technology can be used to impart MD elastic behaviour to nonwoven sheets.

[0077] In that respect, an embodiment as shown in Fig. 13-14 can be particularly suited. The respective process setup is for making a layered nonwoven sheet having a S-D-S sequence of layers and having high elasticity especially in machine direction. Like the setup of Fig. 12, also the setup of Fig. 13 does not comprise traditional spunbonding machines, but only one directionspinning machine 10 as per Fig. 2 and uses prefabricated spunbonded sheets 40, 41 that are unrolled from corresponding rolls to provide for the first and second nonwoven base layers of the sheet. In this embodiment, the prefabricated base layers 40, 41 are combined with the directionspun fibers 50 not in a nip of consolidation rolls, but rather directly in a nip of calendering rolls 31. Consolidation rolls or spinbelts are absent in this embodiment. The Godet roll system 21 in this embodiment is illustrated as a one-roll system with a wrap-around angle of around 90°, but could also encompass more rolls.

[0078] The calendering rolls, as visible in Fig. 14, comprise a structured roll 31a and a flat roll 31 b, which are rotated in opposite directions, as indicated by the curved arrow markings. The rolls 31a, 31 b act on the combined sheet by a certain linear pressure in the magnitude of, e.g., 40 N / mm and the embossing projections are heated to facilitate local polymer melting upon contact and the embossing of bonding points.

[0079] The speed of the Godet roll 21 , which in this embodiment is illustrated as a one-roll system with a wrap-around angle of around 90°, but could also encompass more rolls, is smaller than the speed of the calendering rolls 31 , such that the directionspun fibers 50 As mentioned above, this embodiment, which includes a Godet roll system 21 and combines the directionspun fibers 50 and the base layers 40, 41 in the nip of a pair of calendering rolls 31 , is particularly suited in the context of using directionspun fibers formed from an elastic material, e.g. elastic TPE fibers, to obtain a nonwoven sheet having elastic properties, especially in the machine direction. In this context, the Godet rolls 21 can be operated at a speed that is lower than the speed of the calender rolls 31 to pre-stretch the directionspun fibers 50, which, at the position of the Godet rolls 21 , are already formed and cooled down such as to have elasticity, such that they pre-stretch (as indicated by the arrows pointing away from each other on the left side of the rolls 31a, 31b in Fig. 14) and then contract after having been associated to the basis layer(s) at the bonding I calendering station 30. The base layers 40, 41 can be formed from stretchable material, like spunbonded materials from crimped (asymmetric bicomponent) fibers, or can be formed from material with lower stretchability, like spundbonded material from uncrimped (monocomponent or symmetric bicomponent) fibers. In an embodiment, the base layers 40, 41 can be fed to the nip of the calendering rolls without pretension or with smaller pre-tension than the directionspun fibers. In such configuration, the base layers 40, 41 will be pulled short (as indicated by the arrows pointing towards each other on the right side of the rolls 31a, 31b in Fig. 14) by the elastic retraction force of the elastic directionspun fibers and exhibit a certain wrinkled state, as shown in Fig. 14. Having base layers 40, 41 formed from material with inherent lower stretchability in this context may have the advantage of obtaining materials that are stretchable in machine direction but less stretchable in cross-machine direction.

[0080] As an example, an SDS product can be produced, with two spunbonded base layers 40, 41 and a D-layer of directionspun styrol-ethylen-butylen-styrol (SEBS) fibers 50 of 30 g / m2and a 200% pre-stretch can be produced in a line of a setup as shown in Fig. 13. A certain minimum basis weight for the directionspun fibers 50 in the elastic application is required to impart a certain elasticity to the fabric. The line speed (= speed of the calendering rolls 31) can be 100 m / min. To pre-stretch the directionspun fibers by a factor of 200% at the Godet system 21 , the Godet roll speed is adjusted to 50 m / min. The 30 g / m2D-layer can be made up of different numbers of directionspun fibers 50 having different linear mass densities. Options are given in the following Table 2.

[0081] Table 2

[0082] Generally, the options with high fiber numbers of 9000 or beyond can be less desirable for reasons of process stability. Moderate numbers like 4500 can be preferred, as these are standard spinnerets, the ratio exit speed to Godet speed will deliver a stable spinning process and the distance between the directionspun fibers is sufficiently small to create a homogenous product. Two consecutive spinnerets can be used for making a D-D-arrangement and can use different settings than a single spinneret for directionspinning.

[0083] The pre-stretched elastic directionspun fibers will contract after leaving the calender rolls 31 and before being wound up at station 70. Cooling rolls 60 can be used to enhance material consolidation and short-pulling. The speed of winding up at station 70 can hence be lower than the speed of calender rolls 31 . On the other hand, it can be higher than the speed of the Godet rolls 21 . The speed difference between the calender rolls 31 and the winding station 70 can be at least 20%, allowing for a fabric contraction and a basis weight increase of the D-layer of, correspondingly, at least 20%. A method to find appropriate winding up speeds can be measuring the sheet tension before rolling up. A sufficiently low value is representative of the elastic potential of the directionspun fibers having been exploited.

[0084] Of course, the embodiment as shown in Fig. 13-14 can also be used in the context of nonelastic directionspun fibers, e.g. for cold-drawing as described above.

[0085] To improve uniformity of the contraction over the entire width of the sheet, the machine may comprise a series of at least two haul-offs positioned between the bonding station 30 and winding station 70. The distance between subsequent haul-offs can, for example, be 2 meters or smaller. Also, in the interest of uniformity of contraction, a speed control of the winding up station 70, rather than a tension control, can be preferred.

[0086] A sheet with elastic directionspun fibers can, in an embodiment, also be subjected to activation post production. The activation is understood as a machine-directional stretching of the sheet. The activation can loosen or weaken the base layer(s) and make them more stretchable. The extent of the activation has an influence on the extent to which the final sheet can be elastically stretched in machine direction. An option to activate the sheet comprises passing the sheet through a mill comprising a pair of interacting rolls whose surfaces comprise interlocking cross- directional ribs and grooves. An alternative option are a series of consecutive haul-offs operated at different speeds. The activation can be such that, for example, the sheet is stretched by 10-300 %, preferably by 20-200%, more preferably by 50-150% of its original dimension. As to timing, for example, the activation can take place at least two days post production, where the crystallization process will essentially be complete.

Claims

Claims1. A method of manufacturing a nonwoven sheet comprising a first and preferably a second nonwoven base layer and a layer of machine direction oriented fibers, the method comprising: providing the first nonwoven base layer; preferably, providing the second nonwoven base layer; providing the machine direction oriented fibers by extruding a molten thermoplastic polymer through holes of a spinneret to obtain a curtain of extrusion spun filaments, cooling and drawing the filaments, and associating the resulting machine direction oriented fibers to the first nonwoven base layer and preferably the second nonwoven base layer in the nip of a pair of consolidation or calendaring rolls; bonding together the layers to obtain the nonwoven fabric sheet.

2. The method of claim 1 , wherein the machine direction oriented fibers pass through a Godet roll system on their way from the spinneret to the consolidation or calendering rolls.

3. The method of any preceding claim, wherein the machine direction oriented fibers do not pass through a diffusor or an aerodynamic stretching device between the spinneret and the consolidation or calendering rolls.

4. The method of claim 2 or 3, wherein at least one roll of the Godet roll system is driven and controlled at a specific speed and / or cooled.

5. The method of any one of claims 2 to 4, wherein the ratio of the distance from the spinneret to the Godet roll system and the distance from the Godet roll system to the consolidation or calendering rolls is 3:1 to 1 :3.

6. The method of any preceding claim, wherein the overall distance travelled by the filaments between the spinneret and the consolidation or calendering rolls is at least 100 cm, more preferably at least 200 cm.

7. The method of any one of claims 2 to 6, wherein the ratio of the Godet roll speed to the extruded filament speed is 3 to 400, preferably 5 to 300, more preferably 5 to 250; and / or the ratio of the consolidation or calendering roll speed to the Godet roll speed is 1 ,5 to 10, preferably 1 ,8 to 9 and more preferably 2 to 8.

8. The method of any preceding claim, wherein the bonding comprises introducing bonding points into the sheet by calender bonding, preferably in the pair of calendaring rolls in whose nip the machine direction oriented fibers are associated to the first nonwoven base layer and preferably the second nonwoven base layer, wherein preferably a shape of the bonding points is a shape of cross-directional rods.

9. The method of any preceding claim, wherein the first and / or the second nonwoven base layer is a spunbond nonwoven layer.

10. The method of any preceding claim, wherein the first and / or the second nonwoven base layer is formed in-line.

11. The method of any preceding claim, wherein the first and / or the second nonwoven base layer is provided as prefabricated nonwoven sheet, unrolled from a material roll and combined with the machine direction oriented fibers.

12. The method of any preceding claim, wherein the machine direction oriented fibers comprise thermoplastic polyolefins, in particular like polyethylene (PE), polypropylene (PP), or copolymers including polypropylene (coPP).

13. The method of claim 12, wherein the linear mass density of the machine direction oriented fibers is 1 to 50 denier, preferably 2 to 40 denier, more preferably 4 to 20 denier.

14. The method of any one of claims 1-11 , wherein the machine direction oriented fibers comprise thermoplastic elastomers (TPE), in particular thermoplastic olefins (TPE-o) such as, for example, styrol-ethylene-butylene-styrol (SEBS).

15. The method of claim 14, wherein the linear mass density of the machine direction oriented fibers is 10 to 100 denier, preferably 20 to 80 denier, more preferably 30 to 60 denier.

16. The method of any preceding claim, wherein the number of machine direction oriented fibers per meter of sheet in cross-machine direction is 1 .500 to 10.000, preferably 3.000 to 8.000, more preferably 4.000 to 6.000.

17. A nonwoven sheet made by a method of any preceding claim, comprising a first and preferably a second nonwoven base layer and a layer of machine direction oriented fibers, wherein the layers are bonded together to form the integral nonwoven sheet.

18. The nonwoven sheet of claim 17, wherein the layer sequence in the nonwoven sheet is selected from S-D-S, S-S-D-S, S-S-D-S-S, S-M-D-S, S-D, S-S-D, S-D-D-S, S-D-D, or S- S-D-D, where S stands for a spunbonded layer, M stands for a meltblown layer, and D stands for the layer of machine direction oriented fibers.

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