Made with lyocell fibers

Abstract

The present invention relates to the use of lyocell fibers (1) for the production of nonwoven fibrous fleeces (10, 100). For the production of thin nonwoven fibrous fleeces with sufficient mechanical properties, the use of lyocell fibers (1) is proposed, the fibers (1) having a cross-sectional aspect ratio of at least 1.8. The present invention further relates to the nonwoven fibrous fleeces (10, 100). To produce a thin nonwoven fibrous fleece (10, 100) suitable for use as a battery separator, it is proposed that the fibrous fleece comprises at least two layers (11, 12) of fibrillated lyocell fibers (13), the fibrillated lyocell fibers (13) having a solid core (14, 110) and fibrils (15) protruding from said core (14), the fibers and fibrils (15) being entangled to form the fibrous fleece (10) and embedding the solid core (14) therein, whereby the solid core (14, 110) of the fibrillated lyocell fibers (13) has an average cross-sectional aspect ratio k of at least 1.5.

Description

Technical Field

[0001] The present invention relates to the use of lyocell fibers in non-woven fleece. In particular, the present invention relates to the use of certain lyocell fibers in the manufacture of paper for purposes such as filter paper or, in particular, paper for use as a battery separator.

Background Art

[0002] Batteries such as alkaline batteries (primary and secondary) and lithium-ion batteries include a separator that includes a porous layer that may include polymer fibers. Polymer film separators are most commonly used, but separators made from non-polymer inorganic fibers are also employed. Such separators serve to prevent electrical connection or short circuit between the anode and cathode of the battery.

[0003] Cellulose fibers are widely used in battery separator paper because of their ability to absorb and retain electrolytes. However, some of these cellulose fibers (such as rayon or mercerized pulp) have low fibrillation ability, and thus, a battery separator with desired properties in terms of density, porosity, and dimensional stability cannot be obtained.

[0004] Lyocell cellulose fibers are well-known for their fibrillation ability and are employed in battery separators. Lyocell fibers are spun from a tertiary amine oxide solution of cellulose.

[0005] Due to the fine and long fibrils, separators made of such fibrillated lyocell fibers have suitable porosity, very good ion mobility inside the battery, and high battery efficiency. The fibrils entangle very well during papermaking, form a dense structure with little shrinkage and high dimensional stability. Furthermore, the average size of the pores is small, which serves as a barrier to dendrites.

[0006] The use of lyocell fibers in battery separators is disclosed in EP0572921A1, US2007 / 0014080A1, US2010 / 0310921, and US2009 / 0017385A1. WO97 / 37392 discloses a battery separator made from a cellulose film formed from a cellulose amine oxide solution. Further modern technologies are presented in US5,700,700 and DE19855644.

[0007] WO2013 / 159948 and WO2014 / 127828A1 disclose the use of lyocell fibers having specific properties in battery separators.

[0008] US3,318,990 discloses the use of viscose hollow flat fibers in glossy transparent paper. The viscose fibers need to be modified with a water-swellable polymer to make them suitable for this purpose.

[0009] To achieve the most desirable performance, cellulose fibers used in paper, particularly filtration paper and battery separator paper, must be fibrillated before the sheet manufacturing process. This so-called refining process is extremely time-consuming and energy-intensive. In addition to fibrillation, refining cellulose fibers to a high degree of filtration has the detrimental effect of shortening the average length of the fibers being refined. The result is a decrease in the mechanical properties of the resulting separator sheet.

[0010] Fibrillation of lyocell fibers occurs in the surface region of the fiber. This means that even when lyocell fibers are fibrillated to a high level of filtration, the central region of the fiber remains unfibrillated, and a residual core is formed from each individual fiber.

[0011] A sheet is formed when at least two layers of fibrillated fibers overlap. This means that the minimum thickness of the sheet is proportional to the thickness of the fibrillated fibers that make it up.

[0012] Standard lyocell fibers inherently have a circular cross-section, and a circular cross-section lyocell fiber with a fineness of 1.7 dtex has a diameter of approximately 12 μm in a dry state. After extensive fibrillation, the remaining core diameter averages 10 μm. By suitably adjusting the spinning conditions of the fibers, lyocell fibers with reduced diameters can be produced. Lyocell fibers can also be fibrillated by refining to a high degree of filtration, e.g., 80°SR (Schopper Riegler degree). The remaining core diameter of the refined fiber is approximately 2 μm smaller than the original diameter of the initial lyocell fiber. The previous minimum fiber diameter achieved in small-scale spinning trials was approximately 8 μm, and the resulting remaining core diameter achieved after refining these 8 μm diameter fibers was approximately 6 μm. Producing smaller fineness (and therefore smaller remaining core diameters) is technically and economically difficult. [Overview of the project]

[0013] There is still a demand for nonwoven fleece, especially paper, that possesses sufficient strength even at very thin thicknesses.

[0014] The object of this invention is to provide such an improved nonwoven fleece.

[0015] This problem is solved by using lyocell fibers for the manufacture of the nonwoven fleece described in claim 1.

[0016] Preferred embodiments of the present invention are described in the dependent claims.

[0017] Furthermore, the above objective is achieved by the nonwoven fleece described in claim 6.

[0018] Again, the preferred embodiments are described in the dependent claims.

[0019] Preferred embodiments of the present invention are described below with reference to the drawings. FIGS. 1 to 6 show the following.

Brief Description of the Drawings

[0020] [Figure 1] It is a schematic view showing a cross-section of lyocell fibers for use in the production of a non-woven fiber fleece having a cross-sectional aspect ratio according to the present invention. [Figure 2] It is a schematic view of a cross-section of a non-woven fiber fleece according to the present invention. [Figure 3a] It is a top-view SEM micrograph of a non-woven fiber fleece according to the first embodiment of the present invention. [Figure 3b] It is a top-view schematic view of FIG. 3a with the contour line traced. [Figure 4a] It is a cross-sectional SEM micrograph of the non-woven fiber fleece shown in FIG. 3. [Figure 4b] It is a cross-sectional schematic view of FIG. 4a with the contour line traced. [Figure 5a] It is a top-view SEM micrograph of a comparative non-woven fiber fleece derived from standard circular lyocell fibers. [Figure 5b] It is a top-view schematic view of FIG. 5a with the contour line traced. [Figure 6a] It is a cross-sectional SEM micrograph of the non-woven fiber fleece shown in FIG. 5. [Figure 6b] It is a cross-sectional schematic view of FIG. 6a with the contour line traced.

Mode for Carrying Out the Invention

[0021] The present invention relates to lyocell fibers for use in the production of non-woven fiber fleeces, whereby the lyocell fibers exhibit a cross-sectional aspect ratio of at least 1.8.

[0022] The aspect ratio of a fiber's cross-section is defined as the ratio of the width to the height of the smallest circumscribing rectangle surrounding the fiber's cross-section. The smallest circumscribing rectangle is the smallest rectangle that circumscribes the fiber's cross-section. Thus, the width of the circumscribing rectangle is measured along the longitudinal direction of the fiber's cross-section.

[0023] In a more preferred embodiment, the lyocell fibers exhibit a cross-sectional aspect ratio ranging from 2 to 10.

[0024] Surprisingly, it was found that, compared to standard circular lyocell fibers, such lyocell fibers with a non-standard cross-section, and therefore the cross-sectional aspect ratio defined above, produced with the same fineness under equivalent process parameters, require less energy and time to refine to an equivalent level. This results in savings in both time and operating costs. Therefore, a more efficient manufacturing process can be obtained by using these fibers.

[0025] Lyocell fibers with an irregular cross-sectional aspect ratio retain an unfibrillated solid core after refining, which essentially exhibits an elliptical cross-section. Fibrillated lyocell fibers further contain thin fibrils protruding from the solid core.

[0026] In a preferred embodiment, the solid core of fibrillated lyocell fibers exhibits a cross-sectional aspect ratio k of at least 1.5, as further defined below.

[0027] The fibrils protruding from the solid core can exhibit a width distribution ranging from approximately 100 nm to approximately 10 μm.

[0028] Furthermore, surprisingly, it was found that when such fibrillated fibers are converted into nonwoven fleece, the fibers align within the formed fleece so that the thinner axis of the fibers is perpendicular to the plane of the sheet. This makes it possible to produce very thin nonwoven fleece that still possess sufficient tensile properties such as strength.

[0029] The term "lyocell" fiber, as is well known to those skilled in the art, refers to an artificial cellulose fiber spun from an organic solvent solution of underivativeized cellulose.

[0030] Today, the most common type of lyocell fiber, and a particularly preferred embodiment of the present invention, is a lyocell fiber spun according to an amine oxide process. It is well understood that the amine oxide process comprises at least (1) the step of dissolving cellulose in an amine oxide solvent, preferably at a high temperature, to prepare a solution; (2) the step of spinning the solution (preferably at about 100°C) to draw the formed solution onto an air gap; and (3) the step of adding the formed solution to a spinning bath to precipitate the cellulose.

[0031] The amine oxide solvent is most preferably aqueous N-methyl-morpholine-N-oxide (NMMO).

[0032] For the purposes of this invention, the term "nonwoven fleece" means any fibrous fleece formed by entangling fibers or filaments and bonding them together by mechanical, thermal, chemical or hydrogen bonds at the fiber level. In contrast, woven fabrics are formed by weaving processes that involve yarn, but are typically not bonded at the fiber level. The term "nonwoven fleece" includes nonwoven fabrics produced by techniques such as spun lacing (water flow entanglement) and needling. The term "nonwoven fleece" includes, in particular, paper.

[0033] In a preferred embodiment of the present invention, the lyocell fibers are flattened fibers, i.e., have a substantially rectangular cross-section.

[0034] However, other lyocell fibers having irregular cross-sections, and therefore other lyocell fibers having cross-sections that deviate from an essentially circular cross-section, can also be used. Examples of such fibers include multilobal fibers, such as those having a "Y" or "X" shape, or other fibers having a non-circular cross-section, such as those having an "8" shape, provided that the cross-sectional aspect ratio of the said fiber satisfies the above conditions.

[0035] In the case of flat fibers, lyocell flat fibers can be produced by spinning a cellulose solution through a die having a rectangular opening. WO2010 / 071906 discloses this method for producing flat fibers.

[0036] Alternatively, fibers can be produced by a process in which a solution is extruded through a spinneret having several circular openings arranged adjacent to each other, as disclosed in WO2007 / 143761, and the filaments extruded through these openings fuse together to form, for example, an essentially rectangular cross-section.

[0037] The fibers used in this invention preferably have a fineness of 0.5 dtex to 10 dtex, and preferably more than 1 dtex.

[0038] In particular, in the case of flat fibers, it has been found that fibers with a finer density than standard lyocell fibers with a circular cross-section, for example, fibers with a fineness of 1.7 dtex, can be fibrillated to form nonwoven fleece, and the resulting nonwoven fleece has been found to be thinner than the thickness of a sheet formed from the standard lyocell fibers with a circular cross-section. This is due to the surprising discovery that the fibers are aligned horizontally, that is, the axis of the fiber in the direction of thinness is perpendicular to the plane of the sheet.

[0039] As described above, the lyocell fibers used in this invention surprisingly require less energy and time to be refined to an equivalent level of fibrillation. This results in savings in both time and operating costs.

[0040] The purpose of refining is to fibrillate the fibers while minimizing the side effect of fiber length reduction. Longer refining times / higher refining energy increase fiber length reduction, leading to a decrease in the mechanical properties of the resulting nonwoven fleece. Therefore, it is desirable to achieve the desired level of fibrillation using less refining energy.

[0041] It was found that lyocell fibers having the cross-sectional aspect ratio according to the present invention have a better fibrillation tendency than comparable standard circular lyocell fibers. Therefore, it is clear that the remaining fiber length of the flattened fibers is not impaired (reduced) compared to circular fibers during refining to a similar degree of filtration. Consequently, nonwoven fleece made solely from flattened fibers exhibits far better mechanical properties than sheets made from comparable fibrillated circular fibers.

[0042] The toughness, tear strength, and burst strength of nonwoven fleece made from the fibers according to the present invention are increased compared to nonwoven fleece made from circular standard fibers. In some embodiments, toughness can be increased by at least 50%, tear strength by at least 100%, and burst strength by at least 130%.

[0043] The present invention further relates to a nonwoven fleece comprising lyocell fibers, wherein the nonwoven fleece comprises at least two layers of fibrillated lyocell fibers, each having a solid core and fibrils protruding from the core, the fibers and fibrils intertwining to form a fleece, the solid core embedded therein, the solid core of the fibrillated lyocell fibers having an average cross-sectional aspect ratio k of at least 1.5.

[0044] In a more preferred embodiment, the solid core of fibrillated lyocell fibers has an average cross-sectional aspect ratio k of at least 1.8, preferably at least 2.0, and more preferably at least 2.2.

[0045] The average cross-sectional aspect ratio k of a solid core made of fibrillated lyocell fibers is defined as follows: k = b / h, (1) In the formula, b is the average visible width of the solid core when viewed from above under a microscope, and h is the average visible height of the solid core when viewed in cross-section under a microscope.

[0046] When a cross-section of fleece fibers is cut, the solid fiber core is cut at various angles, making it impossible to reliably determine the width of the solid core (perpendicular to the longitudinal axis of the fiber) from the cross-sectional view alone. Therefore, determining the average cross-sectional aspect ratio k must be separated into the independent determination of the average visible width b and average visible height h of the solid core.

[0047] The average visible width b of the solid core can be reliably determined by taking a top view of the fibrous fleece (e.g., by SEM microscopy), obtaining several width measurements of the visible solid core perpendicular to the direction of fiber elongation, and calculating the average value. The measurements should be taken in locations where the width of the solid core is essentially uniform, i.e., not in the strongly fibrillated regions of the fibrillated fibers.

[0048] On the other hand, the average visible height h of the solid core can be determined by taking a cross-sectional view of the fibrous fleece (for example, by using a SEM microscope). Because the solid core has a different elongation direction within the fibrous fleece, it is cut at various angles, and therefore the cross-section of the solid core may appear altered or even "smeared out." Therefore, to obtain a reliable and representative measurement of the solid core height, several characteristic and clearly identifiable solid core height measurements are taken and their average value is calculated. Care must be taken not to take measurements in strongly fibrillated areas of the fibers.

[0049] Therefore, the average cross-sectional aspect ratio k represents the ratio between the average width and average height of the solid core, where the width and height of the solid core are determined at substantially uniform locations. This dimension correlates with the cross-sectional aspect ratio of the (unfibrillated) lyocell fiber according to the present invention, which is determined from the minimum circumscribing rectangle that circumscribes the cross-section of the lyocell fiber.

[0050] However, fibrillation of lyocell fibers during the refining process can alter the cross-sectional aspect ratio of the solid core of the fibrillated lyocell fibers compared to the cross-sectional aspect ratio of the lyocell fibers before refining.

[0051] When the solid core exhibits the average cross-sectional aspect ratio k according to the present invention, a nonwoven fleece can be obtained, and the fibrillated fibers are aligned such that the axis of the solid core in the thin-thin direction is perpendicular to the plane of the nonwoven fleece. Therefore, a very thin nonwoven fleece can be obtained without impairing its mechanical strength and tensile properties.

[0052] Preferably, the nonwoven fleece can be obtained from the process of manufacturing a fibrous fleece using the lyocell fibers defined above.

[0053] In yet another embodiment, the nonwoven fleece is paper.

[0054] The thickness of the nonwoven fleece may preferably be 20 μm or less, and more preferably 10 μm or less.

[0055] In a more preferred embodiment, the nonwoven fleece is essentially made of lyocell fibers.

[0056] In another embodiment, the nonwoven fleece may consist of a blend containing lyocell fibers. Thereafter, those skilled in the art may include other suitable fibers in the nonwoven fleece, insofar as the claimed effect is achieved. Such other fibers include, for example, natural cellulose fibers such as wood pulp, hemp, sisal, flax, abaca, kenaf, and African honeysuckle, synthetic polymers such as polyester, polyamide, polyvinyl alcohol, polyolefin, and aramid, or inorganic fibers such as glass fibers.

[0057] In further embodiments, the solid core of the fibrillated lyocell fibers has an average height of 10 μm or less, more specifically 7 μm or less, preferably 4.5 μm or less. The maximum height of the flattened fiber solid core allows for the creation of a very thin nonwoven fleece, which is suitable as a battery separator.

[0058] In a further embodiment, the solid core of the fibrillated lyocell fiber has an average width of at least 15 μm, more specifically between 20 μm and 40 μm.

[0059] For example, when lyocell flat fibers with a fineness of approximately 2.7 dtex and a cross-sectional aspect ratio of approximately 4 are used in the manufacture of fiber fleece, the solid core of the fibrillated lyocell fibers exhibits an average height in the range of 5–8 μm and an average width in the range of 15–30 μm.

[0060] If lyocell flat fibers having a fineness other than 2.7 dtex are used, the average width and height of the solid core will therefore vary depending on the cross-sectional aspect ratio of the lyocell flat fibers according to the present invention.

[0061] In another embodiment, the present invention relates to a method for producing a nonwoven fleece containing lyocell fibers, - A step of providing lyocell fibers having a cross-sectional aspect ratio of at least 1.8, preferably 2 to 10, - A step of refining lyocell fibers to form fibrillated lyocell fibers, - The steps of forming a nonwoven fleece by intertwining at least fibrillated lyocell fibers and bonding them at the fiber level. This provides a method that includes [something].

[0062] In a further aspect of the present invention, the refined fibrillated lyocell fiber includes a solid core and fibrils protruding from the core. After refinement, the solid core of the fibrillated lyocell fiber preferably has an average cross-sectional aspect ratio k of at least 1.5.

[0063] In a further aspect of the present invention, the bonding of fibrillated lyocell fibers at the fiber level includes the entanglement of the fibers and fibrils to form a fibrous fleece in which a solid core is embedded.

[0064] The discussion of preferred embodiments described above and below applies equally to all aspects of the method for producing nonwoven fleece.

[0065] Preferred embodiments of the present invention will be described below with reference to the drawings.

[0066] Figure 1 shows a schematic diagram of a lyocell fiber 1 for use in the manufacture of nonwoven fleece. The lyocell fiber 1 has an essentially non-circular shape and can be circumscribed by a minimum circumscribed rectangle 2, which represents the smallest rectangle that circumscribes the periphery of the fiber cross-section.

[0067] The cross-sectional aspect ratio of fiber 1 can be obtained by dividing the width 3 of the minimum circumscribing rectangle 2 by the height 4 of the minimum circumscribing rectangle 2. The width 3 of fiber 1 is measured along the longitudinal direction 5 of the fiber cross-section, and the height 4 of fiber 1 is measured along the short direction 6 of the fiber cross-section. For fiber 1 schematically shown in Figure 1, the cross-sectional aspect ratio is 2.26.

[0068] For simplicity, we assume that fiber 1 extends along the y-axis, the longitudinal direction 5 of the fiber cross-section coincides with the x-axis, and the transverse direction 6 of the fiber cross-section coincides with the z-axis.

[0069] Due to its irregular (flattened) cross-section, fiber 1 requires far less energy and time to refine to an equivalent (filtration) level than standard round fibers. Therefore, nonwoven fleece can be manufactured more cost-effectively.

[0070] Such a nonwoven fleece 10 is schematically shown in Figure 2. The nonwoven fleece 10 includes layers 11 and 12 made of fibrillated lyocell fibers 13. Thereafter, the fibrillated lyocell fibers 13 consist of a solid core 14 and fibrils 15 protruding from the solid core 14.

[0071] According to the simplified schematic diagram in Figure 2, the fibers 13 are mainly oriented, i.e., extended along the y-axis, while the nonwoven fleece 10 extends in the x / y plane. However, in other embodiments, the fibrillated lyocell fibers 13 can be oriented along any direction in the x / y plane and therefore overlap randomly.

[0072] Layers 11, 12, and thus the entire nonwoven fleece 10 are formed via entangled fibrils 15, thereby embedding the solid core 14 in a dense network of entangled fibrils 15 that form the fleece 10.

[0073] The average cross-sectional aspect ratio k of the nonwoven fibers in the fleece 10, defined by equation (1) as the ratio between the average visible width b and the average height h of the solid core 14, is thereby at least 1.5. The solid core 14 can be equally circumscribed by a minimum circumscribing rectangle 16 representing the cross-sectional aspect ratio k. The visible width 7 can preferably be determined from a top view of the fibrous fleece 10, and the measurement should be made at various positions on various solid cores 14 to calculate its average visible width b. The determination of the visible width 120 of the solid core 110 is illustrated in Figures 3a and 3b for the fibrous fleece 100 according to Example 1, which is described in the paragraph of Examples below. Determining the width 7 from a cross-sectional view such as schematically shown in Figure 2 is less preferable because such a cross-sectional view may overestimate the width 7 due to the non-vertical cutting of the fibers 13. On the other hand, the height 8 of the solid core 14 can be determined from the cross-sectional view of the fiber fleece 10, and thereby the average height h should be calculated from measurements taken from several solid cores 14. Such determination of the height 130 of the solid core 110 from the cross-sectional view is illustrated in Figures 4a and 4b for the fiber fleece 100 according to Example 1.

[0074] It is preferable to determine the average visible width b and average height h by taking width 7 and height 8 measurements of at least five solid cores 14, more preferably at least eight solid cores 14. The top view for determining the width 7 of the solid core 14 preferably shows at least 0.2 mm of the fiber fleece 10. 2 , more preferably at least 0.4 mm 2 The cross-sectional view for determining the height 8 of the solid core 14 preferably covers a distance along the cross-section of the fibrous fleece 10 of at least 150 μm, more preferably at least 200 μm.

[0075] In a preferred embodiment, the average cross-sectional aspect ratio k of the nonwoven fleece is at least 2.0.

[0076] The fibers 13 are mainly aligned in the short direction of their cross-section along the z-axis 6, and therefore the majority of the fibers 13 lie essentially flat in the x / y plane. Thus, the nonwoven fleece 10 has a thickness d that is thinner than that of a nonwoven fleece formed from standard lyocell fibers with a circular cross-section. This makes it possible to obtain a much thinner fleece 10.

[0077] Other embodiments of the present invention not shown may be implemented within the scope of the claims. The figures and embodiments presented above are not intended to limit the scope of protection. [Examples]

[0078] The advantages of the present invention will be illustrated below using examples and comparative examples. However, it should be noted that the examples presented below are for illustrative purposes only and do not limit the scope of the present invention.

[0079] Therefore, lyocell fibers having the cross-sectional aspect ratio according to the present invention were produced by the production method described below.

[0080] Fiber production Cellulose pulp was mixed with an amine oxide / water solvent to produce a lyocell spinning solution. The water was evaporated under vacuum until the pulp dissolved. After dissolution, the pulp concentration in the solution was 13%. The resulting spinning solution was filtered and transported to a spinning pump.

[0081] The raw material was extruded by a spinning pump at a rate of 0.05 g / min per orifice at 115°C through a spinneret equipped with a deformed extrusion orifice to generate deformed cross-sectional fibers. The fibers were extruded from the spinneret into an air gap approximately 30 mm high. The fibers were then prepared in the air gap using a cross-draft at approximately 20°C with 8.6 g H2O / kg of dry air and a linear velocity of 4.2 m / sec.

[0082] Finally, the fibers were coagulated in a spinning bath containing a 25% amine oxide solution and then transported to a further finishing step via rollers. This produced fibers at a speed of approximately 30 m / min, achieving the desired fineness of 2.7 dtex. At suitable intervals, the produced fiber cords were cut and washed with desalinated water to remove all trace amounts of amine oxide from the fibers, yielding the tow.

[0083] The tofu was generated during a two-day trial, labeled, stored in plastic bags, and then used for the application of the softening agent.

[0084] To apply the softening agent, the tow was removed from its plastic bag and the remaining water was squeezed out using a foamer at 5 bar. The softening agent was prepared to achieve a softening agent application level of 0.2% (by weight relative to the fiber), and the tow was immersed in the aqueous softening agent solution. The finished tow was then dried in a drying cabinet at 65°C for 48 hours before being cut.

[0085] To prepare for cutting, the dried and finished tow was cut into 5mm lengths using a leaf guillotine. The cut, irregularly shaped fibers were stored in clear plastic bags and moved on to the refining process.

[0086] Refining For refining, the cut, finished, and dried deformed cross-sectional fibers were refined in a single-disc refiner with a plate spacing of 0.35 mm and a concentration of 0.6% to fibrillate the fibers. This refined the fibers to a filtration degree of SR80°. The dry content after refining and dehydration was approximately 20%. Finally, 500 g of the refined fibers were stored in a plastic bag and refrigerated at 4°C until nonwoven fleece was produced.

[0087] Production of nonwoven fleece For the production of the nonwoven fiber fleece according to the present invention, 100% of the refined cross-sectionally shaped fibers obtained according to the method described above were used. The dry content of the fibers was measured to be 18.74% before production. After determining the dry content, the amount of fiber required to produce 30 gsm nonwoven fiber fleece was determined and weighed. The weighed fibers were dispersed in a mixer with 1 liter of water at 3000 rpm for 45 seconds. The resulting suspension was then transferred to a disperser, further diluted with 2 liters of water, and sparged with gas for 10 seconds. The water was drained immediately after sparging, and the nonwoven fiber fleece was formed on a filter. A foil approximately 1 mm thick was placed on the resulting fiber fleece to create a lamination of filter, fleece, and foil, which was then vacuum-dried at 92°C for 10 minutes. The foil was then removed, and the fiber fleece was taken off the filter. The fiber fleece was stored in a transparent sheath until further analysis.

[0088] Measurement method To obtain a scanning electron microscope (SEM) image, take a picture of the nonwoven fleece fabric manufactured according to the method described above, approximately 1 cm from the center. 2 A sample of the specified size was cut out. The cut sample was sprayed with Au for 120 seconds. For measurement, an FEI Quanta 450 scanning electron microscope was operated at 5kV with the following settings: Spot 3, HV, EDT, WD10.

[0089] The breaking strength and elongation of the samples were measured according to DIN EN ISO 1924-2 2009. Prior to measurement, 50 mm × 100 mm samples were punched from the center of nonwoven fleece and conditioned at 23°C and 50% humidity for 24 hours. Measurements were performed using a ZWICK ROELL Z2.5 material testing machine with the following settings: load cell: 200 N, clamp gap: 80 mm, and travel speed of 20 mm / min. The measured values ​​were obtained as the average of 10 samples.

[0090] Tear strength was measured according to NWSP 100.2.R1(15). A 75mm x 150mm sample was punched from the center of the nonwoven fleece and conditioned at 23°C and 50% humidity for 24 hours. The measurement was repeated using a ZWICK ROELL Z2.5 material testing machine with the following settings: load cell: 200N, clamp gap: 25mm, and travel speed of 100mm / min, stopping when the travel length reached 40mm. The measured value was obtained as the average of five samples.

[0091] The bursting strength was measured according to WSP 110.5(05). A 100mm x 100mm sample was punched from the center of a nonwoven fleece and conditioned at 23°C and 50% humidity for 24 hours. The sample was fixed to the bottom of a ZWICK ROELL Z2.5 material testing machine, and a opposing ball was moved across the paper surface until a minimum force of 0.25N was measured. The ball was then moved slightly backward until a force of 0N was measured. The ball stamp was then moved through the sample at 300mm / min. The measurement was obtained as the average of five samples.

[0092] Results and Discussion A nonwoven fleece (paper) sheet containing flattened cross-section lyocell fibers was manufactured on a small scale according to the above production method. Microscopic examination by SEM showed that the fibers were aligned within the sheet such that the direction of their cross-sectional axis in the thinness direction was perpendicular to the plane of the sheet, and therefore aligned along the z-axis, as schematically shown in Figure 2.

[0093] The flattened lyocell fibers retained an unfibrillated core after refining, which essentially exhibited a non-circular cross-section. The width of the remaining core of the fibrillated flattened lyocell fibers was measured, and their thickness along the axis in the thin-thin direction (z-axis) was determined. Fibers initially spun with an average linear density (fineness) of approximately 2.7 dtex and a cross-sectional aspect ratio of approximately 3.6 showed a z-axis thickness of approximately 7–7.7 μm. After refining to 80°SR, the z-axis thickness of the remaining core decreased to a range between approximately 3 μm and 7 μm. When used to form paper sheets, the minimum achievable sheet thickness is proportional to the z-axis thickness of the flattened fibers.

[0094] Therefore, despite their high fineness, fibrillated flat fibers can still be used to produce suitably thin separator paper. Depending on the spinneret used and the process parameters employed, a variety of shapes can be produced, as well as even finer fineness. This further reduces the remaining thickness, allowing for the production of even thinner sheets. The limiting factor is expected to be the desired mechanical properties of the resulting sheet, which can be improved by using flat fibers instead of standard round fibers.

[0095] These improved mechanical properties allow for the use of flattened fibers to produce remarkably stable, yet thin, sheets with a thickness of less than 10 μm.

[0096] [Example 1] An exemplary nonwoven fleece 100 (Example 1) according to the present invention was produced using 2.7 dtex lyocell fibers with an average cross-sectional aspect ratio of 3.57, an average width of 26.21 μm, and an average height of 7.34 μm, according to the production method described above. The width and height of the fibers were investigated by cross-sectional SEM micrographs of several randomly selected samples. No individual (unfibrillated) fibers with a cross-sectional aspect ratio of less than 3 were observed in these samples.

[0097] Figures 3a and 4a show SEM micrographs of the nonwoven fleece 100 manufactured in Example 1. Figure 3a shows a top view of the fleece 100, and Figure 4a shows a cross-sectional view. Figures 3b and 4b show traced contour lines of the SEM micrographs shown in Figures 3a and 4a, respectively.

[0098] In the top view of Figure 3a, several solid cores 110 of the fiber fleece 100 can be identified. As shown in Figure 3b, several characteristic portions of the solid cores 110 that exhibit substantially uniform width and do not lie in strongly fibrillated regions of the fibers were selected, and their visible widths 120 were determined. The selected portions of the solid cores 110 are marked with crosshairs in Figure 3a. The width measurements of the six selected portions are listed in Table 1.

[0099] Furthermore, the cross-sectional SEM micrograph shown in Figure 4a allows for the identification of several solid cores 110 embedded in the fibril network. As schematically shown in Figure 4b, four characteristic solid cores 110 were selected to determine the visible height 130 of the solid cores. This selected only clearly identifiable solid cores 110 that were cut at an angle close to 90° and did not show strong fibrillation. The selected solid cores 110 are also marked with crosshairs in Figure 4a. The height measurements of the four selected solid cores 110 are listed in Table 2.

[0100] [Table 1]

[0101] [Table 2]

[0102] Calculating the average values ​​of the width (120) and height (130) obtained from Tables 1 and 2, the nonwoven fleece 100 of the present invention in Example 1 exhibits an average width b of 15.55 μm for the solid core and an average height h of 6.87 μm for the solid core. Therefore, the average cross-sectional aspect ratio k of the solid core is 2.26.

[0103] Although not shown in the figures, in other embodiments of the present invention described above, the average cross-sectional aspect ratio k was determined to be 2.24, 1.92, and 2.04, respectively.

[0104] [Comparative Example 2] The comparative example nonwoven fleece 200 was produced from standard circular lyocell fibers having a fineness of 2.7 dtex, and this fleece was produced according to the production method described above. The circular lyocell fibers had an average cross-sectional aspect ratio of 1 and an average diameter of 15.1 μm.

[0105] Figures 5 and 6 show SEM micrographs of Comparative Example 2. Again, Figure 5a shows a top view of the fiber fleece 200, and Figure 6a shows a cross-sectional view. Figures 5b and 6b again show the traced contour lines of the SEM micrographs shown in Figures 5a and 6a, respectively.

[0106] From the top view in Figure 5a, several solid cores 210 can be identified within the fibrous fleece 200. From these solid cores 210, only those exhibiting substantially uniform width without strong fibrillation can be selected to determine the average visible width 220. Care must be taken not to consider fibrils protruding from the solid cores. As shown in Figure 5b, four characteristic portions of the solid cores 210 were selected, and their visible widths 220 were determined. The width measurements obtained in this manner are summarized in Table 3.

[0107] From the cross-sectional SEM micrograph shown in Figure 6a, several solid cores 210 can again be identified. Figure 6b shows the traced contour lines of the SEM micrograph in Figure 6a, highlighting five characteristic solid cores 210 selected to determine their visible height 230. Again, when selecting solid cores 210 for examination, it should be noted that only clearly identifiable solid cores 210 that are cut at an angle close to 90° and do not show strong fibrillation should be selected. The height measurements of the five selected solid cores are listed in Table 4.

[0108] [Table 3]

[0109] [Table 4]

[0110] Calculating the average values ​​of the width (220) and height (230) obtained from Tables 3 and 4, the nonwoven fleece 200 of Comparative Example 2, which uses standard circular lyocell fibers, exhibits an average width b of 10.54 μm and an average height h of 12.77 μm for the solid core. Therefore, the average cross-sectional aspect ratio k of the solid core (210) is 0.83.

[0111] Although not shown in the figures, in other comparative examples using standard circular lyocell fibers, the average cross-sectional aspect ratio k was determined to be 0.91, 1.04, and 1.31, respectively.

[0112] Table 5 (below) summarizes the parameters and mechanical properties of the nonwoven fleece of Example 1, as well as the improvement in these properties compared to the nonwoven fleece of Comparative Example 2, which was made from standard circular lyocell fibers.

[0113] Consideration Refining reduced the average width b and average height h of the solid cores compared to the (unrefined) fiber dimensions of both the flattened (inventive) and rounded (comparative) lyocell fibers in Examples 1 and 2. Furthermore, the average cross-sectional aspect ratio k of the solid cores of the flattened lyocell fibers decreased to 2.27 compared to the cross-sectional aspect ratio of 3.57 for the unrefined flattened lyocell fibers. However, this decrease varies depending on the refining process and its parameters, and may even differ within a single batch. In other examples (not shown), the cross-sectional aspect ratio remained uniform or even slightly increased. Such changes in the cross-sectional aspect ratio are expected and not significant, as long as the average cross-sectional aspect ratio k of the solid cores remains within the range defined in the claims.

[0114] As can be seen from Table 5, by using irregularly shaped flattened lyocell fibers to manufacture nonwoven fleece, the energy input required to refine the fibers can be reduced by 67.5%, while the tensile properties are significantly improved compared to standard round lyocell fibers.

[0115] [Table 5]

Claims

1. The use of lyocell fibers for the manufacture of nonwoven fleece (10, 100), characterized in that the fiber (1) has a cross-sectional aspect ratio of at least 1.

8. The cross-sectional aspect ratio is defined as the ratio of the width to the height of the smallest circumscribing rectangle surrounding the cross-section of the fiber, as used in this context.

2. The use according to claim 1, characterized in that the fiber (1) is a fiber with a flat cross-section.

3. The use according to claim 1, characterized in that the fiber (1) is a cross-sectionally irregular fiber having a non-circular cross-section.

4. The use according to any one of claims 1 to 3, characterized in that the fiber (1) exhibits a fineness ranging from 0.5 dtex to 10 dtex.

5. The use according to any one of claims 1 to 3, characterized in that the nonwoven fleece (10, 100) is paper.

6. A nonwoven fiber fleece comprising at least two layers (11, 12) of fibrillated lyocell fibers (13), wherein the fibrillated lyocell fibers (13) have solid cores (14, 110) and fibrils (15) protruding from the cores (14), and the fibers and fibrils (15) are intertwined to form a fiber fleece (10), in which the solid cores (14) are embedded, thereby the solid cores (14, 110) of the fibrillated lyocell fibers (13) have an average cross-sectional aspect ratio k of at least 1.

5. The average cross-sectional aspect ratio k of the solid core (14, 110) of the fibrillated lyocell fiber (13) is defined as follows in a nonwoven fleece. k = b / h, (1) (In the formula, b is the average visible width of the solid core when viewed from above under a microscope, and h is the average visible height of the solid core when viewed in cross-section under a microscope.)

7. The nonwoven fiber fleece according to claim 6, characterized in that the solid core (14, 110) of the fibrillated lyocell fiber (13) has an average cross-sectional aspect ratio k of at least 1.

8.

8. The nonwoven fleece according to claim 6 or 7, characterized in that the nonwoven fleece (10, 100) is paper.

9. The nonwoven fleece according to claim 6 or 7, characterized in that the nonwoven fleece (10, 100) has a thickness d of 20 μm or less.

10. The nonwoven fleece according to claim 6 or 7, characterized in that the nonwoven fleece (10, 100) contains at least 20% by weight of lyocell fibers (13) in its dry mass.

11. The nonwoven fleece according to claim 6 or 7, characterized in that the nonwoven fleece (10, 100) is essentially made of lyocell fibers (13).

12. The nonwoven fleece according to claim 6 or 7, characterized in that the nonwoven fleece (10, 100) consists of a fiber blend comprising lyocell fibers (13) and other fibers, wherein the other fibers are selected from the group consisting of natural cellulose fibers, synthetic polymer fibers, and inorganic fibers.

13. The nonwoven fiber fleece according to claim 6 or 7, characterized in that the solid core (14, 110) has an average height of 10 μm or less.

14. A nonwoven fiber fleece obtained by a process for manufacturing a fiber fleece using lyocell fibers according to any one of claims 1 to 3.

15. The nonwoven fleece according to claim 14, wherein the nonwoven fleece (10, 100) is characterized by claim 6.

16. Battery separator paper comprising the nonwoven fleece (10) described in claim 6 or 7.

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