NONWOVEN CELLULOSE FIBER FABRIC WITH INCREASED WATER HOLDING CAPABILITY AND LOW BASIS WEIGHT

Abstract

It is described a nonwoven cellulose fiber fabric directly manufactured from a lyocell spinning solution. The fabric comprises a network of substantially endless fibers. The fabric exhibits a water holding capability of at least 850 mass percent. Further, the fabric comprises a mass per unit area which is smaller than 25 gram per square meter. It is further described a method and a device for manufacturing such a fabric, a product or composite comprising such a fabric, and various use applications for such a fabric.

Claims

1. A nonwoven cellulose fiber fabric, in particular directly manufactured from a lyocell spinning solution, the fabric comprising a network of substantially endless fibers, wherein the fabric exhibits a water holding capability of at least 850 mass percent, and the fabric comprises a mass per unit area which is lower than 25 gram per square meter.

2. The fabric according to claim 1, wherein the fibers have a copper content of less than 5 ppm and/or have a nickel content of less than 2 ppm.

3. The fabric according to claim 1, wherein different ones of the fibers are located at least partially in different distinguishable layers, wherein in particular the fabric comprises at least one of the following features: fibers of different layers are integrally connected at at least one inter-layer merging position between the layers; different ones of the fibers being located at least partially in different layers differ concerning fiber diameter, in particular differ concerning an averaged fiber diameter; fibers of different layers have the same fiber diameter, in particular have substantially the same averaged fiber diameter; fibers networks of different layers provide different functionality, wherein the different functionality in particular comprises at least one of the group consisting of different wicking, different anisotropic behavior, different optical properties, different liquid holding capability, different cleanability, different roughness, different smoothness, and different mechanical properties.

4. The fabric according to claim 1, wherein fiber networks in different layers have different merging factors, or fiber networks in different layers have at least approximately the same merging factor.

5. The fabric according to claim 1, wherein the mass per unit area is between 5 and 25 gram per square meter, in particular between 8 and 20 gram per square meter.

6. The fabric according to claim 1, wherein the fabric exhibits a water holding capability of at least 900 mass percent, in particular of at least 950 mass percent and further in particular of at least 1000 mass percent.

7. The fabric according to claim 1, wherein the fabric exhibits an oil holding capability of at least 800 mass percent, in particular of at least 1500 mass percent, and further in particular of at least 2000 mass percent.

8. The fabric according to claim 1, wherein the fiber network comprises at least one of the following features: different sections of the same fiber differ concerning fiber diameter so that a ratio between a largest fiber diameter of this fiber and a smallest fiber diameter of this fiber is more than 1.5; different fibers differ concerning fiber diameter so that a ratio between a largest fiber diameter of one of the fibers and a smallest fiber diameter of another one of the fibers is more than 1.5.

9. The fabric according to claim 1, wherein the fiber network comprises at least one of the following features: at least 3%, in particular at least 5%, of the fibers have a non-circular cross sectional shape having a roundness of not more than 90%; at least 1%, in particular at least 3%, of the fibers have a non-circular cross sectional shape having a roundness of not more than 80%, in particular of not more than 70%.

10. The fabric according to claim 1, wherein at least part of the fibers are integrally merged at merging positions, wherein in particular a merging factor of the fibers is in a range between 0.1% and 100%, in particular in a range between 0.2% and 15%, and further in particular in a range between 0.5% and 10%.

11. The fabric according to claim 1, wherein at least some individual fibers are twisted with each other and/or at least one fiber structure is twisted with another fiber structure.

12. A method of manufacturing nonwoven cellulose fiber fabric, in particular a fabric according to claim 1, directly from a lyocell spinning solution, the method comprising extruding the lyocell spinning solution through a jet with orifices supported by a gas flow into a coagulation fluid atmosphere to thereby form substantially endless fibers; collecting the fibers on a fiber support unit to thereby form the fabric; and adjusting process parameters of the manufacturing process so that the fabric exhibits a water holding capability of at least 850 mass percent, and the fabric comprises a mass per unit area which is smaller than 25 gram per square meter.

13. A device for manufacturing nonwoven cellulose fiber fabric directly from a lyocell spinning solution, in particular for manufacturing a fabric according to claim 1, the device comprising a jet with orifices configured for extruding the lyocell spinning solution supported by a gas flow; a coagulation unit configured for providing a coagulation fluid atmosphere for the extruded lyocell spinning solution to thereby form substantially endless fibers; a fiber support unit configured for collecting the fibers to thereby form the fabric; and a control unit configured for adjusting process parameters so that the fabric exhibits a water holding capability of at least 850 mass percent, and the fabric comprises a mass per unit area which is smaller than 25 gram per square meter.

14. A method of using a nonwoven cellulose fiber fabric according to claim 1 for at least one of the group consisting of a wipe, a dryer sheet, a filter, a hygiene product, a medical application product, a geotextile, agrotextile, clothing, a product for building technology, an automotive product, a furnishing, an industrial product, a product related to leisure, beauty, sports or travel, and a product related to school or office.

15. A product or composite comprising a nonwoven cellulose fiber fabric according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0083] FIG. 1 illustrates a device for manufacturing nonwoven cellulose fiber fabric which is directly formed from lyocell spinning solution being coagulated by a coagulation fluid according to an exemplary embodiment of the invention.

[0084] FIG. 2 to FIG. 4 show experimentally captured images of nonwoven cellulose fiber fabric according to an exemplary embodiment of the invention in which merging of individual fibers has been accomplished by a specific process control.

[0085] FIG. 5 and FIG. 6 show experimentally captured images of nonwoven cellulose fiber fabric according to an exemplary embodiment of the invention in which swelling of fibers has been accomplished, wherein FIG. 5 shows the fiber fabric in a dry non-swollen state and FIG. 6 shows the fiber fabric in a humid swollen state.

[0086] FIG. 7 shows an experimentally captured image of nonwoven cellulose fiber fabric according to an exemplary embodiment of the invention in which formation of two superposed layers of fibers has been accomplished by a specific process implementing two serial bars of nozzles.

[0087] FIG. 8 shows an image of a nonwoven cellulose fiber fabric with twists, intra fiber diameter variations, inter fiber diameter variations, in a substantially parallel manner coagulated fibers, and differently sized and shaped cavities.

[0088] FIG. 9 shows a single fiber with its internal structure comprising a plurality of fibrils.

[0089] FIG. 10 shows how a roundness of a fiber having a cross-section deviating from a circular can be calculated as the ratio between the inscribed and the circumscribed circles of the fiber cross-section fiber.

[0090] FIG. 11 shows a schematic image of nonwoven cellulose fiber fabric in which different types of merging positions and several crossing positions between fibers are illustrated.

[0091] FIG. 12 shows a schematic image of a nonwoven cellulose fiber fabric comprising two stacked and merged fiber network layers.

[0092] FIG. 13 illustrates a part of a device for manufacturing nonwoven cellulose fiber fabric composed of two stacked layers of endless cellulose fiber webs according to an exemplary embodiment of the invention.

[0093] FIG. 14 illustrates a nonwoven cellulose fiber fabric comprising three network layers.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0094] The illustration in the drawing is schematic. It is noted that in different figures, similar or identical elements or features are provided with the same reference signs. In order to avoid unnecessary repetitions elements or features which have already been elucidated with respect to a previously described embodiment are not elucidated again at a later position of the description.

[0095] Further, spatially relative terms, such as front and back, above and below, left and right, et cetera are used to describe an element's relationship to another element(s) as illustrated in the figures. Thus, the spatially relative terms may apply to orientations in use which differ from the orientation depicted in the figures. Obviously all such spatially relative terms refer to the orientation shown in the figures only for ease of description and are not necessarily limiting as an apparatus according to an embodiment of the invention can assume orientations different than those illustrated in the figures when in use.

[0096] FIG. 1 illustrates a device 100 according to an exemplary embodiment of the invention for manufacturing nonwoven cellulose fiber fabric 102 which is directly formed from lyocell spinning solution 104. The latter is at least partly coagulated by a coagulation fluid 106 to be converted into partly-formed cellulose fibers 108. By the device 100, a lyocell solution blowing process according to an exemplary embodiment of the invention may be carried out. In the context of the present application, the term lyocell solution-blowing process may particularly encompass processes which can result in essentially endless filaments or fibers 108 of a discrete length or mixtures of endless filaments and fibers of discrete length being obtained. As further described below, nozzles each having an orifice 126 are provided through which cellulose solution or lyocell spinning solution 104 is ejected together with a gas stream or gas flow 146 for manufacturing the nonwoven cellulose fiber fabric 102 according to an exemplary embodiment of the invention.

[0097] As can be taken from FIG. 1, wood pulp 110, other cellulose-based feedstock or the like may be supplied to a storage tank 114 via a metering unit 113. Water from a water container 112 is also supplied to the storage tank 114 via metering unit 113. Thus, the metering unit 113, under control of a control unit 140 described below in further detail, may define relative amounts of water and wood pulp 110 to be supplied to the storage tank 114. A solvent (such as N-methyl-morpholine, NMMO) accommodated in a solvent container 116 may be concentrated in a concentration unit 118 and may then be mixed with the mixture of water and wood pulp 110 or other cellulose-based feedstock with definable relative amounts in a mixing unit 119. Also the mixing unit 119 can be controlled by the control unit 140. Thereby, the water-wood pulp 110 medium is dissolved in the concentrated solvent in a dissolving unit 120 with adjustable relative amounts, thereby obtaining lyocell spinning solution 104. The aqueous lyocell spinning solution 104 can be a honey-viscous medium composed of (for instance 5 mass % to 15 mass %) cellulose comprising wood pulp 110 and (for instance 85 mass % to 95 mass %) solvent.

[0098] The lyocell spinning solution 104 is forwarded to a fiber formation unit 124 (which may be embodied as or which may comprise a number of spinning beams or jets 122). For instance, the number of orifices 126 of the jets 122 may be larger than 50, in particular larger than 100. In one embodiment, all orifices 126 of a fiber formation unit 124 (which may comprise a number of spinnerets of jets 122) of orifices 126 of the jets 122 may have the same size and/or shape. Alternatively, size and/or shape of different orifices 126 of one jet 122 and/or orifices 126 of different jets 122 (which may be arranged serially for forming a multilayer fabric) may be different. The orifices 126 may be arranged as one dimensional alignment of orifices 126.

[0099] When the lyocell spinning solution 104 passes through the orifices 126 of the jets 122, it is divided into a plurality of parallel strands of lyocell spinning solution 104. A vertically oriented gas flow, i.e. being oriented substantially parallel to spinning direction, forces the lyocell spinning solution 104 to transform into increasingly long and thin strands which can be adjusted by changing the process conditions under control of control unit 140. The gas flow may accelerate the lyocell spinning solution 104 along at least a part of its way from the orifices 126 to a fiber support unit 132.

[0100] While the lyocell spinning solution 104 moves through the jets 122 and further downward, the long and thin strands of the lyocell spinning solution 104 interact with non-solvent coagulation fluid 106. The coagulation fluid 106 is advantageously embodied as a vapor mist, for instance an aqueous mist. Process relevant properties of the coagulation fluid 106 are controlled by one or more coagulation units 128, providing the coagulation fluid 106 with adjustable properties. The coagulation units 128 are controlled, in turn, by control unit 140. Preferably, respective coagulation units 128 are provided between the individual nozzles or orifices 126 for individually adjusting properties of respective layers of fabric 102 being produced. Preferably, each jet 122 may have two assigned coagulation units 128, one from each side. The individual jets 122 can thus be provided with individual portions of lyocell spinning solution 104 which may also be adjusted to have different controllable properties of different layers of manufactured fabric 102.

[0101] When interacting with the coagulation fluid 106 (such as water), the solvent concentration of the lyocell spinning solution 104 is reduced, so that the cellulose of the former e.g. wood pulp 110 (or other feedstock) is at least partly coagulated as long and thin cellulose fibers 108 (which may still contain residual solvent and water).

[0102] During or after initial formation of the individual cellulose fibers 108 from the extruded lyocell spinning solution 104, the cellulose fibers 108 are deposited on fiber support unit 132, which is here embodied as a conveyor belt with a planar fiber accommodation surface. The cellulose fibers 108 form a nonwoven cellulose fiber fabric 102 (illustrated only schematically in FIG. 1). The nonwoven cellulose fiber fabric 102 is composed of continuous and substantially endless filaments or fibers 108.

[0103] Although not shown in FIG. 1, the solvent of the lyocell spinning solution 104 removed in coagulation by the coagulation unit 128 and in washing in a washing unit 180 can be at least partially recycled.

[0104] While being transported along the fiber support unit 132, the nonwoven cellulose fiber fabric 102 can be washed by washing unit 180 supplying wash liquor to remove residual solvent and may then be dried. It can be further processed by an optional but advantageous further processing unit 134. For instance, such a further processing may involve hydro-entanglement, needle punching, impregnation, steam treatment with a pressurized steam, calendering, etc.

[0105] The fiber support unit 132 may also transport the nonwoven cellulose fiber fabric 102 to a winder 136 on which the nonwoven cellulose fiber fabric 102 may be collected as a substantially endless sheet. The nonwoven cellulose fiber fabric 102 may then be shipped as roll-good to an entity manufacturing products such as wipes or textiles based on the nonwoven cellulose fiber fabric 102.

[0106] As indicated in FIG. 1, the described process may be controlled by control unit 140 (such as a processor, part of a processor, or a plurality of processors). The control unit 140 is configured for controlling operation of the various units shown in FIG. 1, in particular one or more of the metering unit 113, the mixing unit 119, the fiber formation unit 124, the coagulation unit(s) 128, the further processing unit 134, the dissolution unit 120, the washing unit 118, etc. Thus, the control unit 140 (for instance by executing computer executable program code, and/or by executing control commands defined by a user) may precisely and flexibly define the process parameters according to which the nonwoven cellulose fiber fabric 102 is manufactured. Design parameters in this context are air flow along the orifices 126, properties of the coagulation fluid 106, drive speed of the fiber support unit 132, composition, temperature and/or pressure of the lyocell spinning solution 104, etc. Additional design parameters which may be adjusted for adjusting the properties of the nonwoven cellulose fiber fabric 102 are number and/or mutual distance and/or geometric arrangement of the orifices 126, chemical composition and degree of concentration of the lyocell spinning solution 104, etc. Thereby, the properties of the nonwoven cellulose fiber fabric 102 may be properly adjusted, as described below. Such adjustable properties (see below detailed description) may involve one or more of the following properties: diameter and/or diameter distribution of the fibers 108, amount and/or regions of merging between fibers 108, a purity level of the fibers 108, properties of a multilayer fabric 102, optical properties of the fabric 102, fluid retention and/or fluid release properties of the fabric 102, mechanical stability of the fabric 102, smoothness of a surface of the fabric 102, cross-sectional shape of the fibers 108, etc.

[0107] Although not shown, each spinning jet 122 may comprise a polymer solution inlet via which the lyocell spinning solution 104 is supplied to the jet 122. Via an air inlet, a gas flow 146 can be applied to the lyocell spinning solution 104. Starting from an interaction chamber in an interior of the jet 122 and delimited by a jet casing, the lyocell spinning solution 104 moves or is accelerated (by the gas flow 146 pulling the lyocell spinning solution 104 downwardly) downwardly through a respective orifice 126 and is laterally narrowed under the influence of the gas flow 146 so that continuously tapering cellulose filaments or cellulose fibers 108 are formed when the lyocell spinning solution 104 moves downwardly together with the gas flow 146 in the environment of the coagulation fluid 106.

[0108] Thus, processes involved in the manufacturing method described by reference to FIG. 1 may include that the lyocell spinning solution 104, which may also be denoted as cellulose solution is shaped to form liquid strands or latent filaments, which are drawn by the gas flow 146 and significantly decreased in diameter and increased in length. Partial coagulation of latent filaments or fibers 108 (or preforms thereof) by coagulation fluid 106 prior to or during web formation on the fiber support unit 132 may also be involved. The filaments or fibers 108 are formed into web like fabric 102, washed, dried and may be further processed (see further processing unit 134), as required. The filaments or fibers 108 may for instance be collected, for example on a rotating drum or belt, whereby a web is formed.

[0109] As a result of the described manufacturing process and in particular the choice of solvent used, the fibers 108 have a copper content of less than 5 ppm and have a nickel content of less than 2 ppm. This advantageously improves purity of the fabric 102.

[0110] The lyocell solution blown web (i.e. the nonwoven cellulose fiber fabric 102) according to exemplary embodiments of the invention preferably exhibits one or more of the following properties:

(i) The dry weight of the web is from 5 to 300 g/m.sup.2, preferably 10-80 g/m.sup.2
(ii) The thickness of the web according to the standard WSP120.6 respectively DIN29073 (in particular in the latest version as in force at the priority date of the present patent application) is from 0.05 to 10.0 mm, preferably 0.1 to 2.5 mm (iii) The specific tenacity of the web in MD according to EN29073-3, respectively ISO9073-3 (in particular in the latest version as in force at the priority date of the present patent application) ranges from 0.1 to 3.0 Nm.sup.2/g, preferably from 0.4 to 2.3 Nm.sup.2/g
(iv) The average elongation of the web according to EN29073-3, respectively ISO9073-3 (in particular in the latest version as in force at the priority date of the present patent application) ranges from 0.5 to 100%, preferably from 4 to 50%.
(v) The MD/CD tenacity ratio of the web is from 1 to 12
(vi) The water retention of the web according to DIN 53814 (in particular in the latest version as in force at the priority date of the present patent application) is from 1 to 250%, preferably 30 to 150%
(vii) The water holding capacity of the web according to DIN 53923 (in particular in the latest version as in force at the priority date of the present patent application) ranges from 90 to 2000%, preferably 400 to 1100%
(viii) Metal residue levels of copper content of less than 5 ppm and nickel content of less than 2 ppm according to the standards EN 15587-2 for the substrate decomposition and EN 17294-2 for the ICP-MS analysis (in particular in the latest version as in force at the priority date of the present patent application).

[0111] Most preferably, the lyocell solution-blown web exhibits all of said properties (i) to (viii) mentioned above.

[0112] As described, the process to produce the nonwoven cellulose fiber fabric 102 preferably comprises:

(a) Extruding a solution comprising cellulose dissolved in NMMO (see reference numeral 104) through the orifices 126 of at least one jet 122, thereby forming filaments of lyocell spinning solution 104
(b) Stretching said filaments of lyocell spinning solution 104 by a gaseous stream (see reference numeral 146)
(c) Contacting said filaments with a vapor mist (see reference numeral 106), preferably containing water, thereby at least partly precipitating said fibers 108. Consequently, the filaments or fibers 108 are at least partly precipitated before forming web or nonwoven cellulose fiber fabric 102.
(d) Collecting and precipitating said filaments or fibers 108 in order to form a web or nonwoven cellulose fiber fabric 102
(e) Removing solvent in wash line (see washing unit 180)
(f) Optionally bonding via hydro-entanglement, needle punching, etc. (see further processing unit 134)
(g) Drying and roll collection

[0113] Constituents of the nonwoven cellulose fiber fabric 102 may be bonded by merging, intermingling, hydrogen bonding, physical bonding such as hydroentanglement or needle punching, and/or chemical bonding.

[0114] In order to be further processed, the nonwoven cellulose fiber fabric 102 may be combined with one or more layers of the same and/or other materials, such as (not shown) layers of synthetic polymers, cellulosic fluff pulp, nonwoven webs of cellulose or synthetic polymer fibers, bicomponent fibers, webs of cellulose pulp, such as airlaid or wetlaid pulp, webs or fabrics of high tenacity fibers, hydrophobic materials, high performance fibers (such as temperature resistant materials or flame retardant materials), layers imparting changed mechanical properties to the final products (such as Polypropylene or Polyester layers), biodegradable materials (e.g. films, fibers or webs from Polylactic acid), and/or high bulk materials.

[0115] It is also possible to combine several distinguishable layers of nonwoven cellulose fiber fabric 102, see for instance FIG. 7.

[0116] The nonwoven cellulose fiber fabric 102 may essentially consist of cellulose alone. Alternatively, the nonwoven cellulose fiber fabric 102 may comprise a mixture of cellulose and one or more other fiber materials. The nonwoven cellulose fiber fabric 102, furthermore, may comprise a bicomponent fiber material. The fiber material in the nonwoven cellulose fiber fabric 102 may at least partly comprise a modifying substance. The modifying substance may be selected from, for example, the group consisting of a polymeric resin, an inorganic resin, inorganic pigments, antibacterial products, nanoparticles, lotions, fire-retardant products, absorbency-improving additives, such as superabsorbent resins, ion-exchange resins, carbon compounds such as active carbon, graphite, carbon for electrical conductivity, X-ray contrast substances, luminescent pigments, and dye stuffs.

[0117] Concluding, the cellulose nonwoven web or nonwoven cellulose fiber fabric 102 manufactured directly from the lyocell spinning solution 104 allows access to value added web performance which is not possible via staple fiber route. This includes the possibility to form uniform lightweight webs, to manufacture microfiber products, and to manufacture continuous filaments or fibers 108 forming a web. Moreover, compared to webs from staple fibers, several manufacturing procedures are no longer required. Moreover, nonwoven cellulose fiber fabric 102 according to exemplary embodiments of the invention is biodegradable and manufactured from sustainably sourced raw material (i.e. wood pulp 110 or the like). Furthermore, it has advantages in terms of purity and absorbency. Beyond this, it has an adjustable mechanical strength, stiffness and softness. Furthermore, nonwoven cellulose fiber fabric 102 according to exemplary embodiments of the invention may be manufactured with low weight per area (for instance 10 to 30 g/m.sup.2). Very fine filaments down to a diameter of not more than 5 m, in particular not more than 3 m, can be manufactured with this technology. Furthermore, nonwoven cellulose fiber fabric 102 according to an exemplary embodiment of the invention may be formed with a wide range of web aesthetics, for instance in a flat crispy film-like way, in a paper-like way, or in a soft flexible textile-like way. By adapting the process parameters of the described process, it is furthermore possible to precisely adjust stiffness and mechanical rigidity or flexibility and softness of the nonwoven cellulose fiber fabric 102. This can be adjusted for instance by adjusting a number of merging positions, the number of layers, or by after-treatment (such as needle punch, hydro-entanglement and/or calendering). It is in particular possible to manufacture the nonwoven cellulose fiber fabric 102 with a relatively low basis weight of down to 10 g/m.sup.2 or lower, to obtain filaments or fibers 108 with a very small diameter (for instance of down to 3 to 5 m, or less), etc.

[0118] FIG. 2, FIG. 3 and FIG. 4 show experimentally captured images of nonwoven cellulose fiber fabric 102 according to an exemplary embodiment of the invention in which merging of individual fibers 108 has been accomplished by a corresponding process control. The oval markers in FIG. 2 to FIG. 4 show such merging regions where multiple fibers 108 are integrally connected to one another. At such merging points, two or more fibers 108 may be interconnected to form an integral structure.

[0119] FIG. 5 and FIG. 6 show experimentally captured images of nonwoven cellulose fiber fabric 102 according to an exemplary embodiment of the invention in which swelling of fibers 108 has been accomplished, wherein FIG. 5 shows the fiber fabric 102 in a dry non-swollen state and FIG. 6 shows the fiber fabric 102 in a humid swollen state. The pore diameters can be measured in both states of FIG. 5 and FIG. 6 and can be compared to one another. When calculating an average value of 30 measurements, a decrease of the pore size by swelling of the fibers 108 in an aqueous medium up to 47% of their initial diameter could be determined.

[0120] FIG. 7 shows an experimentally captured image of nonwoven cellulose fiber fabric 102 according to an exemplary embodiment of the invention in which formation of two superposed layers 200, 202 of fibers 108 has been accomplished by a corresponding process design, i.e. a serial arrangement of multiple spinnerets. The two separate, but connected layers 200, 202 are indicated by a horizontal line in FIG. 7. For instance, an n-layer fabric 102 (n2) can be manufactured by serially arranging n spinnerets or jets 122 along the machine direction.

[0121] Specific exemplary embodiments of the invention will be described in the following in more detail:

[0122] FIG. 8 shows an experimentally captured image of a nonwoven cellulose fiber fabric 102 according to an embodiment of the invention. FIG. 8 shows that the fiber network includes fibers 108 being characterized by different variations of diameter/titer and shape. This involves twists, intra fiber thickness variations, inter fiber thickness variations, in a substantially parallel manner coagulated fibers 108, and differently sized and shaped cavities.

[0123] It is mentioned that the mixture of thinner and thicker fibers may provide several welcome properties of the fabric 102. For instance thin fibers may yield a high capillarity between neighboring fibers which improves liquid holding capability. Thicker fibers may contribute to a high mechanical stiffness of the three dimensional fiber fabric structure. Such an increased fiber stiffness may be necessary if so called micro pores, which have been developed within the fiber network, do not collapse in particular because of adhesion forces between different fibers. In this context it should be clear that the collapse of such micro pores would decrease the liquid holding capability. Of course, any cavity, chamber, void, gap, or pore provides, due it's capillary forces, a more or less strong contribution to the liquid holding capability of the fabric 102.

[0124] Twists may be given by at least two individual fibers which are twisted around each other in a spiral manner. Further, also a single fiber 108 which has a non-circular cross section and which is twisted (with itself) may be considered as to represent such a twist. Without being bound to a specific physical theory it is presently believed that any type of twisting increases overall capillary forces within the fabric such that the liquid holding capability is increased.

[0125] In the lyocell spinning procedure the fiber diameter respectively the fiber titer can be adjusted for instance by varying the size of the orifice through which the respective lyocell fiber is extruded. Also a variation of a gas flow (see reference numeral 146 in FIG. 1) accelerating the not yet coagulated fibers towards the (conveyor belt-type) fiber support unit 132 (see reference numeral 12 in FIG. 1) may be used for forming intra thickness and/or inter thickness variations of fibers 108.

[0126] A variation along the length of the fiber may be generated for instance by means of pressure and/or velocity variations of the gas flow which accelerates the extruded lyocell spinning solution towards the conveyor type fiber support unit (see reference numeral 132 in FIG. 1).

[0127] FIG. 9 shows an experimentally captured image of a single cellulose fiber 108 and its internal structure. The curved line denominated with reference numeral 108b is the so called shell of the fiber 108. As can be seen from this Figure, the internal structure of the fiber comprises a plurality of fibrils 108a. Due to the presence of the fibrils 108a also the internal structure of the fiber 108 can be considered as to represent a (submicron) channels which may provide a significant contribution to the liquid holding capability.

[0128] Experimental studies have revealed that surprisingly the described fibrils are a feature which is in particular pronounced in fibers having been produced by means of a lyocell spinning procedure. Specifically, a high homogeneity of the (submicron) fibril structure within the fiber 108 can be achieved. It has further been found out that an adjusted gas flow for the fiber coagulation process at a position, before the fiber coagulation is taking place, promotes the formation of fibril structures with channels in the submicron range, which are in particular suitable for accommodating liquids. The fibrils, which in this picture can be understood as to represent a system of submicron tubes provide a strong contribution to the liquid holding capability of the (internal structure of the) respective fiber.

[0129] At this point it is mentioned that a liquid absorption within the inner structure of the fibers goes along with a swelling process of the fibers. Compared with thermoplastic materials a carefully produced lyocell fiber exhibits within its interior a comparatively homogeneous capillary system of fibrils, which system extends along the fiber. This capillary system swells upon liquid absorption.

[0130] FIG. 10 shows how a roundness of fibers 108 having a cross-section deviating from a circular cross-section can be calculated as the ratio between an inscribed circle 280 and a circumscribed circle 282 of the cross-section of the fiber 108 according to an exemplary embodiment of the invention.

[0131] The minimum circumscribed circle 282 is defined as the smallest circle which encloses the entirety of the roundness profile of the cross-section of the fiber 108 illustrated in FIG. 10. The maximum inscribed circle 280 is defined as the largest circle that can be inscribed inside the roundness profile of the cross-section of the fiber 108 as illustrated in FIG. 10. In the context of this document, roundness can be defined as a ratio between a radius r of the inscribed circle 280 divided by a radius R of the circumscribed surface 282 (or vice versa). Roundness may be indicated by a resulting percentage value. In the present example, R2r and the roundness of the fiber 108 is therefore approximately 50%. For comparison, a circular cylindrical fiber 108 fulfills the condition R=r and has a roundness of one.

[0132] The curvature of the fiber shell within the drawing plane, i.e. at least substantially perpendicular to the longitudinal extension of the fiber, also contributes to a formation of (small cavities) which may have an impact on the overall capillary forces and accommodate liquid particles of water based and/or oil based liquids. As a consequence, also a deviation from a circular cross section may help the fabric to exhibit a large liquid holding capability.

[0133] Given a certain cross section the outer surface of the fiber increases with decreasing roundness respectively with increasing deviation from a circular cross section. Therefore, it should be clear that the roundness is also an important parameter which has an effect on capillary force and, as a consequence, also on the liquid holding capability of the fabric 102.

[0134] The cross sectional shape of the fibers may be adjusted for instance by the corresponding cross sectional shape of the orifice (denoted with reference numeral 126 in FIG. 1) through which the respective portion of the lyocell spinning solution is extruded. However, also other structural dimensions of the respective orifice may have an influence on the cross sectional shape of the resulting fiber 108.

[0135] FIG. 11 shows a schematic image of nonwoven cellulose fiber fabric 102 according to an exemplary embodiment of the invention in which on the one hand different types of merging positions 204 between fibers 108 and on the other hand non-merged crossing positions 220 between fibers 108 are shown.

[0136] The merging positions 204 comprise a merging point, wherein at least two different fibers intersect with each other, and a merging line along which fibers 108 are mutually aligned side-by-side over a portion of their length to form a superordinate fiber structure 206 with a diameter increase along the entire merging line. While the fibers 108 are integrally connected at the merging positions 204 and can be separated here only by destroying the fiber network, fibers 108 are only in frictional contact with one another at crossing positions 220 and can be freely moved here relatively to one another. Although merging and crossing between respectively two fibers 108 is illustrated in FIG. 11, merging and/or crossing can also occur between respectively at least three fibers 108.

[0137] Without being bound to a specific theory it is presently believe that no or only a poor merging at a non-merged or only at a poor merged crossing position exhibits a barrier with regard to liquid travelling from one fiber to the other fiber. In case of a strong merging between at least two fibers (see e.g. reference numerals 204 in FIG. 11) a liquid transfer between the involved fibers is much easier. Even in case of an unfavorable spatial course of the fibers within the fabric, all fibers can accommodate liquid particles such that they swell and provide a contribution to liquid absorbing and holding capability.

[0138] FIG. 12 shows a schematic cross sectional view of a nonwoven cellulose fiber fabric 102 which, in accordance with the invention, comprises two stacked and merged network layers of interconnected fibers 108. A first network layer comprising first fibers 108 is denominated with reference numeral 200. The second network layer comprising second fibers 108 is denominated with reference numeral 202. Due to different processing parameter (values) the first fibers 108 can differ from the second fibers 108 in at least one physical and/or chemical property.

[0139] According to the exemplary embodiment described here the (average) diameter of the first fibers 108 is larger than the (average) diameter of the second fibers 108. This is indicated by the lower two details of FIG. 12. An important property of the interface between the layers 200, 202 is shown in the upper detail of FIG. 12, where an intra-layer merging point 204 is visible which integrally couples fibers 108 of both layers 200, 202 at the interface for increasing stability of the fabric 102 at the interface. Additionally, different ones of the fibers 108 being located in the different layers 200, 202 are integrally connected at at least one respective inner-layer merging position 204 (see again the two lower details of FIG. 12).

[0140] The fibers 108 located in the different fiber network layers 200, 202 and being formed with different average diameter may be provided with different functionalities. Such different functionalities may be supported by the different average diameters, but may also be further promoted by a respective coating or the like. Such different functionalities may for instance be a different behavior in terms of wicking, anisotropic behavior, different oil absorbing capability, different water absorbing and holding capability, different cleanability, and/or different roughness.

[0141] FIG. 13 illustrates a part of a device 100 for manufacturing nonwoven cellulose fiber fabric 102 composed of two stacked layers 200, 202 of endless cellulose fibers 108 according to an exemplary embodiment of the invention. A difference between the device 100 shown in FIG. 13 and the device 100 shown in FIG. 1 is that the device 100 according to FIG. 13 comprises two serially aligned jets 122 and respectively assigned coagulation units 128, as described above. In the embodiment described here, two coagulation units 128 are assigned to each one of the jets 122. In FIG. 12, one coagulation unit 128 is located on the left side of the path of the lyocell spinning solution 104 extending between the jet 122 and the fiber support unit 132 and the other coagulation unit 128 is located on the respective right side of this path. In view of the movable fiber accommodation surface of the conveyor belt-type fiber support unit 132, the upstream jet 122 on the left-hand side of FIG. 13 produces layer 200. Layer 202 is produced by the downstream jet 122 (see right hand side of FIG. 13) and is attached to an upper main surface of the previously formed layer 202 so that a double layer 200, 202 of fabric 102 is obtained.

[0142] According to FIG. 13, the control unit 140 (controlling the jets 122 and all the coagulation units 128) is configured for adjusting process parameters so that the fibers 108 of the different layers 200, 202 differ concerning fiber diameter by more than 50% in relation to a smallest diameter. Adjusting the fiber diameters of the fibers 108 of the layers 200, 202 by the control unit 140 may comprise adjusting an amount of coagulation fluid 106 interacting with the lyocell spinning solution 104. Additionally, the embodiment of FIG. 13 adjusts the process parameters for adjusting fiber diameter by serially arranging multiple jets 122 with orifices 126 (optionally with different properties) along the movable fiber support unit 132. For instance, such different properties may be different orifice 126 diameters, different speed of gas flow 146, different amounts of gas flow 146, and/or different gas flow 146 pressure. Although not shown in FIG. 13, it is possible to further process the fibers 108 after collection on the fiber support unit 132 e.g. by hydro-entanglement, needle punching, impregnation, steam treatment with a pressurized steam, and/or calendering.

[0143] Still referring to the embodiment illustrated in FIG. 13, one or more further nozzle bars or jets 122 may be provided and may be arranged serially along a transport direction of the fiber support unit 132. The multiple jets 122 may be arranged so that a further layer 202 of fibers 108 may be deposited on top of the previously formed layer 200, preferably before the coagulation or curing process of the fibers 108 of the layer 200 and/or of the layer 202 is fully completed, which may trigger merging. When properly adjusting the process parameters, this may have advantageous effects in terms of the properties of a multilayer fabric 102:

[0144] Without wishing to be bound to a specific theory, it is presently believed that the second layer 202 can be considered as a reinforcement of the first layer 200, which increases the homogeneity of the resulting nonwoven fabric 102. This increase of the mechanical stability can be further improved by fiber diameter variation (in particular inter-fiber diameter variation and/or intra-fiber longitudinal diameter variation of the individual fibers 108). When exerting deeper (in particular punctual) pressure (for instance provided by air or water), the cross-sectional shape of a fiber 108 can be further intentionally distorted, which may advantageously result in a further increased mechanical stability.

[0145] On the other hand, intended merging between fibers 108 of the fabric 102 according to FIG. 13 can be triggered so as to further increase the mechanical stability of the fabric.

[0146] FIG. 14 illustrates a nonwoven cellulose fiber fabric 102 comprising three network layers. A first (lower) fiber network layer is denominated with reference numeral 200. A second (middle) fiber network layer, which is formed on top of the first fiber network layer 200, is denominated with reference numeral 202. A further (upper) fiber network layer, which is formed on top of the second fiber network layer 202, is denominated with reference numeral 202. As has already been mentioned above, the fabric 102 may comprise more than three stacked fiber network layers.

[0147] As can be further seen from FIG. 14, the three fiber network layers 200, 202, 202 have different thicknesses. The first fiber network layer 200 has a first thickness t1. The second fiber network layer 202 has a second thickness t2. The further fiber network layer 202 has a third thickness t3.

[0148] The fabric described in this document is characterized by a high water holding capability in connection with a low basis weight. Specifically, despite of the low basis weight of less than 25 gram per square meter a large water holding capability of at least 850 mass percent can be achieved.

[0149] In particular for applications requiring certain haptic characteristics suitable specific basic properties of a fibrous web made from the described fabric can be tailored towards the specific application. Such a specific basic property may be for instance a specific liquid absorbing, holding and/or releasing management combined with a soft haptic characteristic of a cover layer of the fabric. In particular, the above described design parameter of fiber thickness variations allows to produce a desired combination of softness, mechanical stability, liquid absorbing and holding capability, wicking (speed), etc.

[0150] It should be noted that the term comprising does not exclude other elements or steps and the use of articles a or an does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

LIST OF REFERENCE SIGNS

[0151] 100 device for manufacturing nonwoven cellulose fiber fabric [0152] 102 nonwoven cellulose fiber fabric/web like fabric [0153] 104 lyocell spinning solution [0154] 106 coagulation fluid [0155] 108 fibers [0156] 108a fibril [0157] 108b fiber shell [0158] 110 wood pulp [0159] 112 water container [0160] 113 metering unit [0161] 114 storage tank [0162] 116 solvent container [0163] 118 washing unit [0164] 119 mixing unit [0165] 120 dissolving unit [0166] 122 jet [0167] 124 fiber formation unit [0168] 126 orifices [0169] 128 coagulation unit [0170] 132 (conveyor belt-type) fiber support unit [0171] 134 further processing unit [0172] 136 roll [0173] 140 control unit [0174] 146 gas flow [0175] 200 merged layer/first network layer [0176] 202 merged layer/second network layer [0177] 202 merged layer/further network layer [0178] 204 intra-layer merging position/intra-layer merging point/intra-layer merging line [0179] 206 superordinate fiber structure [0180] 220 non-merged crossing positions [0181] 280 inscribed circle [0182] 282 circumscribed circle [0183] r, R radius of inscribed circle respectively circumscribed circle [0184] t1, t2, t3 layer thicknesses

[0185] In the following, examples for producing variations in the merging factor are described and visualized in the table below. Different merging factors in the cellulose fiber fabric may be achieved by varying the coagulation spray flow while using a constant spinning solution (i.e. a spinning solution with a constant consistency), in particular a Lyocell spinning solution, and a constant gas flow (e.g. air throughput). Hereby, a relationship between the coagulation spray flow and the merging factor, i.e. a trend of merging behaviour (the higher the coagulation spray flow, the lower the merging factor), may be observed. MD denotes hereby the machine direction, and CD denotes the cross direction.

TABLE-US-00001 Specific Hand Fmax Fmax Coagulation Merging MD CD Total cond. cond. Sample spray flow Factor [mN [mN [mN MD CD ID l/h % m.sup.2/g] m.sup.2/g] m.sup.2/g] [N] [N] 1.0 10 9.20 n n n 45.6 10.0 1.1 60 5.65 48.8 38.1 43.4 43.6 33.4 1.2 100 3.29 31.1 23.6 27.3 37.8 29.4 1.3 140 2.93 36.5 17.3 26.9 31.8 24.9 1.4 180 2.48 17.5 16.4 16.9 26.9 20.9 1.5 220 2.34 19.1 13.6 16.3 22.7 21.0 1.6 260 1.98 15.2 11.9 13.6 22.8 20.4 1.7 350 0.75 2.2 2.0 2.1 22.4 19.2

[0186] The softness (described by the known Specific Hand measuring technique, measured with a so-called Handle-O-Meter on the basis of the nonwoven standard WSP90.3, in particular the latest version as in force at the priority date of the present patent application) may follow the above described trend of merging. The tenacity (described by Fmax), for example according to EN29073-3, respectively ISO9073-3, in particular the latest version as in force at the priority date of the present patent application, may also follow the described trend of merging. Thus, the softness and the tenacity of the resulting nonwoven cellulose fiber fabric may be adjusted in accordance with the degree of merging (as specified by the merging factor).