Uses of Lyocell Fiber

Abstract

The present invention relates to the use of lyocell fibers (1) for producing nonwoven fiber fleece (10, 100). In order to produce thin nonwoven fiber fleece with sufficient mechanical properties, it is proposed to use lyocell fibers, wherein the fibers (1) have a cross-sectional aspect ratio of at least 1.8. The present invention further relates to the nonwoven fiber fleece (10, 100). In order 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 fibrillated lyocell fiber layers (11, 12) of fibrillated lyocell fibers (13), said fibrillated lyocell fibers (13) having a solid core (14, 110) and fibrils (15) extending from said core (14), said fibers and fibrils (15) being intermingled to form a fibrous fleece (10) in which the solid core (14) is embedded, 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

[0001] The present invention relates to the use of lyocell fibres in non-woven fibre fleece. In particular, the invention relates to the use of certain specific lyocell fibres in the manufacture of paper, for example filter paper or in particular paper to be used as battery separators. Existing technology

[0002] Batteries, including alkaline (primary and secondary) batteries and lithium-ion batteries, include a separator comprising a porous layer, which may include polymer fibers. Polymer film separators are most commonly used, but separators made from non-polymer inorganic fibers have also been used. Such separators are used to prevent electrical connection or short circuiting between the anode and cathode of the battery.

[0003] Cellulose fibers are widely used in battery separator papers due to their ability to absorb and retain electrolytes. However, some of these cellulose fibers, such as rayon or mercerized pulp, have poor fibrillation ability and therefore cannot produce battery separators with the desired properties in terms of density, porosity, and dimensional stability.

[0004] Cellulose fibers of the lyocell family are known for their ability to fibrillate and are used in battery separators. Lyocell fibers are spun from a solution of cellulose in a tertiary amine oxide.

[0005] Due to the fine, long fibrils, separators made from such fibrillated Lyocell fibers have an appropriate porosity, excellent ion mobility within the battery, and high battery efficiency. The fibrils are well interwoven during the papermaking process, forming a dense structure with low shrinkage and high dimensional stability. Furthermore, the average pore size is small, acting as a barrier to dendrites.

[0006] The use of lyocell fibers in battery separators has been disclosed in EP 0572921A1, US 2007 / 0014080A1, US 2010 / 0310921, and US 2009 / 0017385A1. WO 97 / 37392 discloses a battery separator made of a cellulose film formed from a solution of cellulose in an amine oxide. US 5,700,700 and DE 19855644 provide further prior art.

[0007] WO 2013 / 159948 and WO 2014 / 127828 A1 disclose the use of lyocell fibers having specific properties in battery separators.

[0008] US 3,318,990 discloses the use of viscose hollow flat fibers in glossy and transparent papers. The viscose fibers need to be modified with a water-swellable polymer to make them suitable for this purpose.

[0009] To achieve optimal performance, cellulose fibers used in paper, particularly for filtration applications and battery separators, need to be fibrillated prior to sheet production. This so-called refining process is extremely time- and energy-intensive. Besides fibrillation, refining cellulose fibers to a high degree of freeness has the detrimental effect of reducing the average length of the refined fibers. Consequently, the resulting separator sheet has lower mechanical properties.

[0010] The fibrillation of lyocell fibers occurs at the surface region of the fiber. This means that even if lyocell fibers are fibrillated to a high level of freeness, the central region of the fiber remains unfibrillated, forming a residual core from each individual fiber.

[0011] A sheet is formed when at least two (2) layers of fibrillated fibers are superimposed. This means that the minimum thickness of the sheet is proportional to the thickness of the fibrillated fibers used to construct it.

[0012] Standard lyocell fiber has a substantially circular cross section, and thus the circular cross section lyocell fiber of 1.7 dtex (dtex) has a diameter of about 12 μm in a dry state. After rough fibrillation, the residual core diameter is an average of 10 μm. The lyocell fiber with reduced diameter can be produced by making appropriate adjustments to the fiber spinning conditions. Lyocell fiber can also be fibrillated by refining to a high freeness of, for example, 80 ° SR (degree Schopper Riegler). The residual core diameter of the fiber after refining is about 2 μm less than the original diameter of the initial lyocell fiber. The smallest fiber diameter achieved so far in small-scale spinning trials is about 8 μm, and the gained residual core diameter achieved after refining these 8 μm diameter fibers is about 6 μm. Producing smaller fineness (and therefore smaller residual core diameter) is technically and economically challenging.

[0013] Invention Disclosure

[0014] There remains a need for non-woven fleeces, especially paper, that have sufficient strength even at very low thicknesses.

[0015] The object of the present invention is to provide such an improved nonwoven fleece.

[0016] This object is achieved by the use of lyocell fibers for producing nonwoven fiber fleece.

[0017] Preferred embodiments of the invention are set forth in the dependent claims.

[0018] Furthermore, the above-mentioned object is achieved by means of a nonwoven fiber fleece.

[0019] Preferred embodiments are again set forth in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Figure 1 Figure 6 shows:

[0022] Figure 1 A schematic diagram depicting a cross section of a lyocell fiber according to the present invention for making a nonwoven fiber fleece having a cross-sectional aspect ratio,

[0023] Figure 2 A schematic diagram of a cross section of a nonwoven fiber fleece fabric according to the present invention,

[0024] Figure 3a SEM micrograph of a nonwoven fiber fleece fabric in top view according to a first embodiment of the present invention,

[0025] Figure 3b Figure 3a A schematic diagram with outlines of a top view of

[0026] Figure 4a Figure 3 shows an SEM micrograph of a nonwoven fiber fleece fabric in a cross-sectional view.

[0027] Figure 4b Figure 4a A schematic diagram with outlines of a cross-sectional view of

[0028] Figure 5a SEM micrograph of a comparative nonwoven fleece from standard round lyocell fibers in top view,

[0029] Figure 5b Figure 5a A schematic diagram with outlines of a top view of

[0030] Figure 6a SEM micrograph of the nonwoven fiber fleece shown in FIG5 in a cross-sectional view, and

[0031] Figure 6b Figure 6a Schematic diagram with depicted outlines of a cross-sectional view in FIG. Detailed Description of the Invention

[0033] The present invention relates to lyocell fibers for use in making nonwoven fibrous fleece, whereby the lyocell fibers exhibit a cross-sectional aspect ratio of at least 1.8.

[0034] The cross-sectional aspect ratio of a fiber is defined as the ratio of the width to the height of the minimum circumscribed rectangle that encompasses the fiber's cross section. The minimum circumscribed rectangle is the smallest rectangle that circumscribes the perimeter of the fiber's cross section. The width of the circumscribed rectangle is thus measured along the longer dimension of the fiber's cross section.

[0035] In a further preferred embodiment, the lyocell fibers exhibit a cross-sectional aspect ratio of 2 to 10.

[0036] Surprisingly, it has been found that, compared to standard round lyocell fibers, lyocell fibers having a modified cross-section and therefore a cross-sectional aspect ratio as defined above, produced under comparable process parameters and at the same titer, require less energy and time to be refined to a comparable level. This saves both time and operating costs. Consequently, the use of such fibers allows for a more efficient manufacturing process.

[0037] The lyocell fiber with modified cross-sectional aspect ratio retains an unfibrillated solid core exhibiting a substantially elliptical cross-section after refining. The fibrillated lyocell fiber further comprises fine fibrils extending from the solid core.

[0038] In a preferred embodiment, the solid core of the fibrillated lyocell fibers exhibits a cross-sectional aspect ratio k as further defined below of at least 1.5.

[0039] The fibrils extending from the solid core may exhibit a distribution of widths ranging from about 100 nm to about 10 μm.

[0040] It has also been surprisingly discovered that when such fibrillated fibers are converted into a nonwoven fibrous fleece, the fibers are aligned within the resulting fibrous fleece so that the thinner axes of the fibers are perpendicular to the plane of the sheet. This enables the production of very thin nonwoven fibrous fleeces that still possess sufficient tensile properties, such as strength.

[0041] The term "lyocell" fibers, as is well known to the skilled person, refers to man-made cellulose fibers spun from a solution of underivatized cellulose in an organic solvent.

[0042] The most common type of lyocell fiber currently used, and a particularly preferred embodiment of the present invention, is lyocell fiber spun according to the amine oxide process. As is well known, the amine oxide process comprises at least the following steps: (1) dissolving cellulose in an amine oxide solvent, preferably at elevated temperature, to produce a solution; (2) spinning the solution (preferably at about 100°C) and drawing the shaped solution over an air gap; and (3) adding the shaped solution to a spinning bath and precipitating the cellulose.

[0043] Most preferably, the amine oxide solvent is aqueous N-methyl-morpholine-N-oxide (NMMO).

[0044] For the purposes of the present invention, the term "nonwoven fibrous fleece" refers to any fibrous fleece formed by entangling fibers or filaments and bonding them together by mechanical, thermal, chemical, or hydrogen bonding at the fiber level. In contrast, woven fabrics are formed by a weaving process involving yarns, but generally without bonding at the fiber level. The term "nonwoven fibrous fleece" includes nonwovens made by techniques such as hydroentanglement (hydraulic entanglement), needlepunching, and the like. The term "nonwoven fibrous fleece" specifically includes paper.

[0045] In a preferred embodiment of the present invention, the lyocell fiber is a flat fiber, ie has a substantially rectangular cross-section.

[0046] However, other lyocell fibers having modified cross-sections, whereby their cross-section deviates from a substantially circular cross-section, may also be used. Examples of such fibers include multilobal fibers, such as fibers having a "Y" or "X" shape, or other fibers having non-circular cross-sections, such as fibers having a figure-eight shape, as long as the cross-sectional aspect ratio of the fibers meets the above requirements.

[0047] In the case of flat fibers, lyocell flat fibers can be produced by spinning a cellulose solution through a die having rectangular openings. WO 2010 / 071906 discloses this way of producing flat fibers.

[0048] Alternatively, the fibers may be made by a method as disclosed in WO 2007 / 143761 by extruding the solution through a spinneret having several circular openings positioned adjacent to each other so that the filaments extruded through these openings fuse and form, for example, a substantially rectangular cross-section.

[0049] The fibers used in the present invention preferably exhibit a titer of 0.5 to 10 dtex, preferably greater than 1 dtex.

[0050] It has been found that, in particular in the case of flat fibers, fibers having a higher titer than standard lyocell fibers with a circular cross-section, for example a titer of 1.7 dtex, can be fibrillated and formed into a nonwoven fibrous fleece having a thickness less than that of a sheet formed from said standard lyocell fibers with a circular cross-section. This is due to the surprising discovery that these fibers align themselves horizontally, i.e. with their thinner axis perpendicular to the plane of the sheet.

[0051] As mentioned above, the lyocell fibers used according to the present invention surprisingly require less energy and time to be refined to a comparable fibrillation level. This saves both time and operating costs.

[0052] The purpose of refining is to fibrillate the fibers while minimizing the side effect of shortening the fiber length. Longer refining times / higher refining energies result in increased fiber length reduction, which in turn leads to poorer mechanical properties of the resulting nonwoven fiber fleece. It is therefore desirable to use lower refining energies and still achieve the desired level of fibrillation.

[0053] Lyocell fibers having a cross-sectional aspect ratio according to the present invention were found to have a better tendency to fibrillate than similar round standard lyocell fibers. Thus, it is apparent that during refining to the same freeness, the residual fiber length of flat fibers is not as impaired (reduced) as compared to round fibers. Consequently, nonwoven fleece fabrics made entirely of flat fibers exhibit much better mechanical properties than sheets made of similar fibrillated round fibers.

[0054] The tenacity, tear strength, and burst strength of nonwoven fiber fleeces made from fibers according to the present invention are improved compared to nonwoven fiber fleeces made from round standard fibers. In some embodiments, the tenacity can be increased by at least 50%, the tear strength can be increased by at least 100%, and the burst strength can be increased by at least 130%.

[0055] The present invention also relates to a nonwoven fibrous fleece comprising lyocell fibers, wherein the nonwoven fibrous fleece comprises at least two layers of fibrillated lyocell fibers, wherein the fibrillated lyocell fibers have a solid core and fibrils extending from the core, the fibers and fibrils being intermingled to form a fibrous fleece with the solid core embedded therein, whereby the solid core of the fibrillated lyocell fibers has an average cross-sectional aspect ratio k of at least 1.5.

[0056] In a further preferred embodiment, the solid core of the fibrillated lyocell fibers has an average cross-sectional aspect ratio k of at least 1.8, preferably at least 2.0, more preferably at least 2.2.

[0057] The average cross-sectional aspect ratio k of the solid core of fibrillated lyocell fibers is defined as

[0058] k=b / h, (1)

[0059] Where b is the average visible width of the solid core in a microscopic top view of the fibrous fleece, and h is the average visible height of the solid core in a microscopic cross-sectional view of the fibrous fleece.

[0060] By obtaining cross-sectional cuts of a fleece fabric, the solid core of the fiber is cut at various angles. Therefore, the width of the solid core (perpendicular to the longitudinal axis of the fiber) cannot be reliably determined from the cross-sectional view. Therefore, the determination of the average cross-sectional aspect ratio k needs to be split into separate determinations of the average visible width b and the average visible height h of the solid core.

[0061] The average visible width b of the solid core can be reliably determined by taking a top view of the fiber fleece (e.g., with the aid of an SEM micrograph) and obtaining several width measurements of the visible solid core perpendicular to the direction of fiber elongation and calculating their average. The measurements are taken at locations where the width of the solid core is substantially uniform, i.e., not at strongly fibrillated areas of the fibrillated fibers.

[0062] Alternatively, the average visible height h of the solid cores can be determined by taking a cross-sectional view of the fiber fleece (e.g., using an SEM micrograph). Since the solid cores in the fiber fleece extend in different directions, the solid cores are cut at different angles, and thus the cross-section of the solid cores may appear altered or even "smeared out." Therefore, to obtain a reliable and representative measurement of the solid core height, only a few individual, clearly identifiable solid cores should be measured and their average value calculated. Care must be taken not to take measurements in strongly fibrillated areas of the fiber.

[0063] The average cross-sectional aspect ratio k thus represents the ratio between the average width of the solid core and the average height of the solid core, measured at locations of the solid core having substantially uniform width and height. This measurement in turn is related to the cross-sectional aspect ratio of the (unfibrillated) lyocell fiber according to the invention, as measured by the smallest circumscribed rectangle circumscribing the perimeter of the lyocell fiber cross section.

[0064] However, due to the fibrillation of the lyocell fibers during the refining process, the cross-sectional aspect ratio of the solid core of the fibrillated lyocell fibers may change relative to the cross-sectional aspect ratio of the lyocell fibers before refining.

[0065] If the solid core exhibits an average cross-sectional aspect ratio k according to the present invention, a nonwoven fibrous fleece can be obtained in which the fibrillated fibers are aligned so that the thinner axis of the solid core is perpendicular to the plane of the nonwoven fibrous fleece. This allows very thin nonwoven fibrous fleeces to be obtained without compromising their mechanical strength and tensile properties.

[0066] Preferably, a nonwoven fibrous fleece is obtainable from the method for making a fibrous fleece using the lyocell fibers as defined above.

[0067] In yet another embodiment, the nonwoven fibrous fleece is paper.

[0068] The thickness of the nonwoven fiber fleece cloth may preferably be 20 μm or less, more preferably 10 μm or less.

[0069] In a further preferred embodiment, the nonwoven fibrous fleece consists essentially of lyocell fibers.

[0070] In another embodiment, the nonwoven fleece can be composed of a blend including lyocell fibers. A skilled artisan may therefore include other suitable fibers in the nonwoven fleece so long as the claimed effects are achieved. Such other fibers may include, for example, natural cellulose fibers such as wood pulp, hemp, sisal, flax, abaca, kenaf, esparto, etc., synthetic polymers such as polyesters, polyamides, polyvinyl alcohol, polyolefins, aramids, etc., or inorganic fibers such as glass fibers, etc.

[0071] In a further embodiment, the solid core of the fibrillated lyocell fibers exhibits an average height of 10 μm or less, more particularly 7 μm or less, preferably 4.5 μm or less. Due to this maximum height of the solid core of the flat fibers, very thin nonwoven fiber fleeces suitable as battery separators can be achieved.

[0072] In a still further embodiment, the solid core of the fibrillated lyocell fibers exhibits an average width of at least 15 μm, more particularly from 20 to 40 μm.

[0073] For example, if lyocell flat fibers having a denier of about 2.7 dtex and a cross-sectional aspect ratio of about 4 are used to make a fibrous fleece, the solid core of the fibrillated lyocell fibers exhibits an average height of 5-8 μm and an average width of 15-30 μm.

[0074] When using lyocell flat fibers with a titer other than 2.7 dtex, the average width and height of the solid core will vary accordingly, depending on the cross-sectional aspect ratio of the lyocell flat fibers according to the present invention.

[0075] In another aspect, the present invention provides a method for producing a nonwoven fiber fleece comprising lyocell fibers, comprising the steps of:

[0076] - providing a lyocell fiber having a cross-sectional aspect ratio of at least 1.8, preferably from 2 to 10,

[0077] - refining the lyocell fibers to form fibrillated lyocell fibers,

[0078] - forming a nonwoven fibrous fleece by entangling at least fibrillated lyocell fibers and bonding them together at the fiber level.

[0079] In a further aspect of the invention, the refined fibrillated lyocell fibres comprise a solid core and fibrils extending from the core.After refining, the solid core of the fibrillated lyocell fibres preferably has an average cross-sectional aspect ratio k of at least 1.5.

[0080] In a further aspect of the invention, the incorporation of fibrillated lyocell fibers at the fiber level involves the intermingling of fibers and fibrils to form a fibrous fleece having a solid core embedded therein.

[0081] For all aspects of the method of making a nonwoven fibrous fleece, the discussion of the preferred embodiments described above and below applies equally.

[0082] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

[0083] Figure 1 Schematic diagram depicting a lyocell fiber 1 for use in making a nonwoven fibrous fleece. The lyocell fiber 1 exhibits a substantially non-circular shape and can be circumscribed by a minimum circumscribed rectangle 2, which represents the smallest rectangle circumscribing the perimeter of the fiber's cross section.

[0084] The cross-sectional aspect ratio of the fiber 1 can be obtained by dividing the width 3 of the minimum circumscribed rectangle 2 by the height 4 of the minimum circumscribed rectangle 2. The width 3 of the fiber 1 is measured along the longer direction 5 of the fiber cross section, while the height 4 of the fiber 1 is measured along the shorter direction 6 of the fiber cross section. Figure 1 The fiber 1 schematically depicted in FIG. 1 has a cross-sectional aspect ratio equal to 2.26.

[0085] For simplicity, it is assumed that the fiber 1 extends along the y-axis, while the long direction 5 of the fiber cross section coincides with the x-axis and the short direction 6 of the fiber cross section coincides with the z-axis.

[0086] Due to the modified (flat) cross-section, the fibers 1 require much less energy and time to be refined to a comparable (freeness) level than standard round fibers. Thus, a more cost-effective production of nonwoven fiber fleeces can be achieved.

[0087] Such a nonwoven fiber fleece 10 is schematically depicted in Figure 2 The nonwoven fibrous fleece 10 comprises layers 11, 12 consisting of fibrillated lyocell fibers 13. The fibrillated lyocell fibers 13 thus consist of a solid core 14 and fibrils 15 extending from said solid core 14.

[0088] according to Figure 2 In the simplified schematic diagram in FIG, the fibers 13 are primarily oriented, i.e., extend, along the y-axis, whereas the non-woven fibrous fleece 10 extends in the x / y plane. However, in other embodiments, the fibrillated lyocell fibers 13 may extend in any direction in the x / y plane and thus be randomly overlapped.

[0089] The layers 11 , 12 and thus the entire nonwoven fibrous fleece 10 are formed by mixed fibrils 15 , whereby a solid core 14 is embedded in a dense network of mixed fibrils 15 to form the fibrous fleece 10 .

[0090] The average cross-sectional aspect ratio k of the nonwoven fibers in the fleece 10, defined by formula (1) as the ratio between the average visible width b and the average height h of the solid cores 14, is therefore equal to at least 1.5. The solid cores 14 can also be circumscribed by a minimum circumscribed rectangle 16, which represents the cross-sectional aspect ratio k. The visible width 7 can preferably be determined from a top view of the fiber fleece 10, and measurements should be taken at different positions of different solid cores 14 to calculate their average visible width b. For the fiber fleece 100 according to Example 1 as described in the Examples section below, in Figure 3a and Figure 3b The determination of the visible width 120 of the solid core 110 is shown in FIG. Figure 2 The width 7 is measured from the schematically depicted cross-sectional view in FIG, because such a cross-sectional view may overestimate the width 7 due to non-perpendicular cuts of the fibers 13. On the other hand, the height 8 of the solid core 14 can be measured from a cross-sectional view of the fiber fleece 10, whereby the average height h of several measured values ​​of the solid core 14 should be calculated. For the fiber fleece 100 according to Example 1, Figure 4a and Figure 4b 1 shows how the height 130 of the solid core 110 is measured from a cross-sectional view.

[0091] The average visible width b and the average height h are preferably determined by taking measurements of the width 7 and height 8, respectively, of at least 5 (five) solid cores 14, more preferably at least 8 (eight) solid cores 14. The top view used to determine the width 7 of the solid cores 14 preferably covers at least 0.2 mm of the fiber fleece 10. 2 , more preferably at least 0.4 mm 2 The cross-sectional view used to determine 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 of at least 200 μm.

[0092] In a preferred embodiment, the nonwoven fibrous fleece has an average cross-sectional aspect ratio, k, equal to at least 2.0.

[0093] The fibers 13 are primarily aligned in their cross-section in the short direction along the z-axis 6 . Consequently, the vast majority of the fibers 13 lie substantially flat in the x / y plane. The thickness d of the nonwoven fibrous fleece 10 is therefore lower than that of a nonwoven fibrous fleece made of standard lyocell fibers having a circular cross-section. This results in a much thinner fibrous fleece 10.

[0094] Other embodiments of the invention not depicted in the drawings may be implemented within the scope of the claims. The drawings and embodiments presented above do not limit the scope of protection. Example

[0095] The advantages of the present invention are demonstrated below using examples and comparative examples of the present invention. However, it should be noted that the examples given below are merely illustrative and do not limit the scope of the present invention.

[0096] Thus, the lyocell fiber having a cross-sectional aspect ratio according to the present invention has been produced by the production method described below.

[0097] Fiber production

[0098] Cellulose pulp is mixed with an amine oxide / water solvent to produce a lyocell spinning solution. The water is evaporated under vacuum until the pulp dissolves. After dissolution, the pulp concentration in the solution is 13%. The resulting spinning solution is filtered and transferred to a spinning pump.

[0099] The dope was extruded from a spinning pump through a spinneret with modified extrusion holes at 115°C and a rate of 0.05 g / min per hole to produce fibers with modified cross-sections. The fibers were extruded from the spinneret into an air gap approximately 30 mm high. In the air gap, the fibers were then conditioned at approximately 20°C under a crossdraught of 8.6 g H2O / kg dry air and a linear velocity of 4.2 m / s.

[0100] Finally, the fibers are coagulated in a spinning bath containing a 25% amine oxide solution and then conveyed via rollers to a further finishing step. Fibers are thus produced at a speed of approximately 30 m / min to achieve the desired fineness of 2.7 dtex. The resulting fiber cables are cut at appropriate intervals and washed in demineralized water to remove any traces of amine oxide from the fibers and obtain their tows.

[0101] Tows were produced accordingly during the two-day trial, labeled and stored in plastic bags for transport to the application of the soft finish.

[0102] To apply the soft finish, the tow was removed from its plastic bag and the remaining water was pressed out using a Foulard at 5 bar. A soft finish was prepared and the tow was immersed in the aqueous finish solution to achieve a finish application level of 0.2% (weight on fiber). The finished tow was then dried in a drying oven at 65°C for 48 hours before being transferred to cutting.

[0103] For cutting, the dried and conditioned tow was cut into 5 mm lengths using a leaf guillotine. The cut cross-section modified fibers were stored in clean plastic bags and transferred to refining.

[0104] fine grinding

[0105] During refining, the cut, shaped, and dried cross-section-modified fibers were refined in a single-disc refiner with a plate spacing of 0.35 mm and a consistency of 0.6% to fibrillate the fibers. This resulted in the fibers being refined to a freeness of SR 80°. The dry content after refining and dewatering was approximately 20%. Finally, 500 g of the refined fibers were stored in plastic bags and refrigerated at 4°C until the production of nonwoven fleece fabrics.

[0106] Nonwoven fiber fleece production

[0107] For the production of non-woven fiber fleece according to the present invention, 100% of the refined cross-section modified fibers obtained according to the above method are used. The dry content of the fiber measured before production is 18.74%. After measuring the dry content, the fiber amount required for producing 30gsm non-woven fiber fleece is measured and weighed. The weighed fibers are dispersed in a mixer with 1 liter of water at 3000rpm for 45 seconds. The resulting suspension is then transferred to a disperser, where it is further diluted with 2 liters of water and bubbled with gas for 10 seconds. Immediately after bubbling, the water is drained to form a non-woven fiber fleece on top of the filter. A foil of about 1mm thickness is placed on the resulting fiber fleece to create a filter, fleece and foil stack, which is then dried in a vacuum at 92°C for 10 minutes. After this, the foil is removed and the fiber fleece is removed from the filter. The fiber fleece is stored in a transparent sheath until further analysis.

[0108] Measurement method

[0109] In order to obtain SEM (scanning electron microscopy) micrographs, a 1 cm section was cut from the middle of the nonwoven fiber fleece fabric prepared according to the above method. 2 The cut samples were spray coated with Au for 120 seconds. For the measurements, a FEI Quanta450 scanning electron microscope was operated at 5 kV with the following settings: Spot 3, HV, EDT, WD10.

[0110] The breaking force and elongation of the samples were measured according to DIN EN ISO 1924-2 (2009). Before measurement, 50 mm x 100 mm samples were punched from the center of the nonwoven fleece and conditioned at 23°C and 50% humidity for 24 hours. Measurements were performed using a ZWICK ROELL Z2.5 materials testing machine using the following settings: dynamometer: 200 N, grip gap: 80 mm, traverse speed: 20 mm / min. The measured values ​​were obtained as the average of 10 samples.

[0111] Tear strength was measured according to NWSP 100.2.R1 (15). 75 mm x 150 mm specimens were punched out of the center of the nonwoven fleece and conditioned at 23°C and 50% humidity for 24 hours. The measurements were again performed using a ZWICK ROELL Z2.5 materials testing machine using the following settings: dynamometer: 200 N, grip gap: 25 mm, traverse speed of 100 mm / min, stopping at a traverse length of 40 mm. The measured values ​​were obtained as the average of 5 specimens.

[0112] The burst strength is measured according to WSP 110.5 (05). For this purpose, a 100 mm x 100 mm sample is punched out of the middle of a nonwoven fleece and conditioned at 23° C. and 50% humidity for 24 hours. The sample is fixed to the bottom of a ZWICK ROELL Z2.5 material testing machine and an opposite ball is moved to the paper surface until a minimum force of 0.25 N is measured. The ball is then moved slightly backwards until a force of 0 N is measured. A ball stamp is then moved through the sample at 300 mm / min. The measured value is obtained as the average value of 5 samples.

[0113] Results and Discussion

[0114] Nonwoven fiber fleece (paper) sheets containing flat cross-section lyocell fibers were produced on a small scale according to the above production method. Microscopic examination by SEM showed that the fibers were arranged in the sheet so that the direction of their thinner cross-section axis was perpendicular to the plane of the sheet, so that Figure 2 Schematically depicted as being arranged along the z-axis.

[0115] Flat cross-section lyocell fibers retain an unfibrillated core exhibiting a substantially non-circular cross-section after refining. The width of the residual core of the fibrillated flat cross-section lyocell fibers was measured and their thickness along the thinner axis (z-axis) was determined. The initially spun fibers, having an average linear density (fineness) of approximately 2.7 dtex and a cross-sectional aspect ratio of approximately 3.6, exhibited a z-axis thickness of approximately 7 to 7.7 μm. After refining to 80° SR, the z-axis thickness of the residual core decreased and was approximately 3 to 7 μm. When used to form a paper sheet, the minimum achievable sheet thickness is proportional to the z-axis thickness of the flat cross-section fibers.

[0116] Thus, despite its high fineness, the fibrillated flat fibers can still be used to produce suitably thin separator papers. Depending on the spinneret used and the process parameters employed, different geometries and even smaller finenesses can be produced. This, in turn, further reduces the residual thickness and enables the production of even thinner sheets. However, the limiting factor is expected to be the desired mechanical properties of the produced sheet, which in turn is improved when using flat fibers instead of standard round fibers.

[0117] Due to their improved mechanical properties, flat fibers can be used to produce very stable but thin sheets with a thickness of less than 10 μm.

[0118] Example 1

[0119] An exemplary nonwoven fiber fleece 100 (Example 1) according to the present invention was produced using 2.7 dT Lyocell fibers having 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. Fiber width and height were investigated by cross-sectional SEM micrographs of several randomly selected samples. In these samples, no single (unfibrillated) fibers having a cross-sectional aspect ratio of less than 3 were found.

[0120] exist Figure 3a and Figure 4a , a SEM micrograph of the nonwoven fiber fleece fabric 100 prepared in Example 1 is shown. Figure 3a Depicts a top view of a fiber fleece 100, and Figure 4a Describe its cross-sectional view. Figure 3b and Figure 4b Display separately Figure 3a and Figure 4a Depicted outlines of the SEM micrographs depicted in .

[0121] exist Figure 3a In the top view of FIG, several solid cores 110 of the fiber fleece 100 can be distinguished. Figure 3b As depicted in FIG, a plurality of individual segments of the solid core 110 that exhibit a substantially uniform width and are not located in the heavily fibrillated regions of the fiber are selected and their visible widths 120 are measured. Figure 3a The width measurements of six selected segments are reported in Table 1.

[0122] Also in Figure 4a In the cross-sectional SEM micrograph shown in FIG, many solid cores 110 embedded in the fibril network can be distinguished. Figure 4b As schematically shown in FIG, four individual solid cores 110 are selected for determining the visible height 130 of the solid cores. Thus, only clearly discernible solid cores 110 that are cut at an angle close to 90° and that do not show strong signs of fibrillation are selected. Figure 4a The height measurements of the four selected solid cores 110 are listed in Table 2.

[0123] Table 1: Measurements of the width (120) of selected solid cores (110) in Example 1

[0124]

[0125] Table 2: Measurements of the height (130) of selected solid cores (110) in Example 1

[0126]

[0127] The average values ​​of the width (120) and height (130) calculated and measured according to Tables 1 and 2 show that the nonwoven fibrous fleece fabric 100 of the present invention of Example 1 has an average solid core width b of 15.55 μm and an average solid core height h of 6.87 μm. Therefore, the average cross-sectional aspect ratio k of the solid core is equal to 2.26.

[0128] In other embodiments of the invention according to the above description which are not depicted in the drawings, average cross-sectional aspect ratios k of 2.24, 1.92 and 2.04 were determined, respectively.

[0129] Comparative Example 2

[0130] Comparative Example of a nonwoven fiber fleece 200 produced from standard round lyocell fibers having a fineness of 2.7 dtex. The fleece was produced according to the above-described production method. The round lyocell fibers had an average cross-sectional aspect ratio of 1 and an average diameter of 15.1 μm.

[0131] In FIG5 and FIG6, SEM micrographs of Comparative Example 2 are shown. Figure 5a Again depicting a top view of the fiber fleece 200, and Figure 6a Display its cross-sectional view. Figure 5b and Figure 6b Display separately Figure 5a and Figure 6a Depicted outlines of the SEM micrographs depicted in .

[0132] from Figure 5a In the top view in FIG, several solid cores 210 can be identified in the fiber fleece 200. From these solid cores 210, only those showing segments of at least substantially uniform width without strong signs of fibrillation are selected for determining the average visible width 220. It must be noted that fibrils extending from the solid cores are not taken into account. Figure 5b , four individual segments of the solid core 210 were selected and their visible widths 220 were measured. The width measurements thus obtained are summarized in Table 3.

[0133] from Figure 6a In the cross-sectional SEM micrograph shown in , again numerous solid cores 210 can be discerned. Figure 6b depiction Figure 6aThe SEM micrograph in FIG. 4 outlines and highlights five individual solid cores 210 that were selected for determination of visible height 230. When selecting solid cores 210 for examination, care should again be taken to select only clearly discernible solid cores 210 that were cut at angles close to 90° and that did not exhibit strong signs of fibrillation. The height measurements of the five selected solid cores are listed in Table 4.

[0134] Table 3: Measurements of the Width (220) of Selected Solid Cores (210) in Comparative Example 2

[0135]

[0136] Table 4: Measurements of the height (230) of selected solid cores (210) in Comparative Example 2

[0137]

[0138] The average values ​​of the width (220) and height (230) measured by calculation according to Tables 3 and 4 show that the nonwoven fiber fleece fabric 200 of Comparative Example 2 using standard round lyocell fibers exhibits an average width b of the solid core of 10.54 μm and an average height h of the solid core of 12.77 μm. Therefore, the average cross-sectional aspect ratio k of the solid core (210) is equal to 0.83.

[0139] In other comparative examples using standard round lyocell fibers not depicted in the drawings, the average cross-sectional aspect ratios k were determined to be 0.91, 1.04, and 1.31, respectively.

[0140] Table 5 (below) summarizes the parameters and mechanical properties of the nonwoven fiber fleece of Example 1 and the improvements in said properties compared to the nonwoven fiber fleece of Comparative Example 2 made from standard round lyocell fibers.

[0141] Discussion

[0142] For the flat (the present invention) and round (comparative example) lyocell fibers from Example 1 and Example 2, due to refining, the average width b and height h of the solid core are reduced compared with (unrefined) fiber size. Compared with the cross-sectional aspect ratio of the unrefined flat lyocell fiber of 3.57, the average cross-sectional aspect ratio k of the solid core of flat lyocell fiber is also reduced to 2.27. But this reduction will change according to the refining process and its parameters, and even may be different in a single batch. In other embodiments (not shown), it has been observed that the cross-sectional aspect ratio remains unchanged or even slightly increases. This variation of the cross-sectional aspect ratio meets expectation and is not critical, as long as the average cross-sectional aspect ratio k of the solid core is within the scope of the claims.

[0143] As can be seen from Table 5, by using flat lyocell fibers with a modified cross section to make nonwoven fiber fleece, the energy input for refining the fibers can be reduced by 67.5% compared to standard round lyocell fibers, while at the same time significantly improving the tensile properties.

[0144] Table 5: Parameters and properties of Example 1 and Comparative Example 2

[0145]

[0146]

Claims

1. The use of lyocell fiber for manufacturing non-woven fiber fleece fabric (10, 100), characterized in that The fibers (1) have a cross-sectional aspect ratio of at least 1.8, and the solid core (14, 110) of the fibrillated lyocell fibers (13) formed by refining the fibers (1) has an average cross-sectional aspect ratio of at least 1.5 k .

2. The use according to claim 1, characterized in that The fiber (1) has a cross-sectional aspect ratio of 2 to 10.

3. The use according to claim 1 or 2, characterized in that The fibers (1) are flat cross-section fibers.

4. The use according to claim 1 or 2, characterized in that The fiber (1) is a cross-section-modified fiber having a non-circular cross-section.

5. The use according to claim 1 or 2, characterized in that The fibers (1) exhibit a titer of 0.5 to 10 dtex.

6. The use according to claim 5, characterized in that The fibers (1) exhibit a titer of at least 1 dtex.

7. The method according to any one of claims 1, 2 and 6, characterized in that The non-woven fiber fleece (10, 100) is paper.

8. A nonwoven fibrous fleece comprising at least two fibrillated lyocell fiber layers (11, 12) of fibrillated lyocell fibers (13), said fibrillated lyocell fibers (13) having a solid core (14, 110) and fibrils (15) extending from said core (14), said fibers and fibrils (15) being intermingled to form a fibrous fleece (10) in which the solid core (14) is embedded, whereby the solid core (14, 110) of the fibrillated lyocell fibers (13) has an average cross-sectional aspect ratio of at least 1.5 k wherein the fibrillated lyocell fibers (13) are formed by refining lyocell fibers.

9. The nonwoven fiber fleece fabric according to claim 8, characterized in that The solid core (14, 110) of the fibrillated lyocell fiber (13) has an average cross-sectional aspect ratio of at least 1.8 k .

10. The nonwoven fiber fleece fabric according to claim 9, characterized in that The solid core (14, 110) of the fibrillated lyocell fiber (13) has an average cross-sectional aspect ratio of at least 2.0 k .

11. The nonwoven fiber fleece fabric according to any one of claims 8 to 10, characterized in that The non-woven fiber fleece (10, 100) is paper.

12. The nonwoven fiber fleece fabric according to any one of claims 8 to 10, characterized in that The nonwoven fiber fleece (10, 100) exhibits a thickness d of 20 μm or less.

13. The nonwoven fiber fleece fabric according to claim 12, characterized in that The nonwoven fiber fleece (10, 100) exhibits a thickness d of 10 μm or less.

14. The nonwoven fiber fleece fabric according to any one of claims 8 to 10 and 13, characterized in that The nonwoven fibrous fleece (10, 100) contains at least 20% by weight of lyocell fibers (13) in dry mass.

15. The nonwoven fiber fleece fabric according to any one of claims 8 to 10 and 13, characterized in that The non-woven fiber fleece cloth (10, 100) is composed of lyocell fibers (13).

16. The nonwoven fiber fleece fabric according to any one of claims 8 to 10 and 13, characterized in that The nonwoven fibrous fleece (10, 100) is composed of a fiber blend comprising lyocell fibers (13) and other fibers, wherein the other fibers are selected from natural cellulose fibers, synthetic polymer fibers and inorganic fibers.

17. The nonwoven fiber fleece fabric according to any one of claims 8 to 10 and 13, characterized in that The solid core (14, 110) has an average height of 10 μm or less.

18. The nonwoven fiber fleece fabric according to claim 17, characterized in that The solid core (14, 110) has an average height of 7 μm or less.

19. The nonwoven fiber fleece fabric according to claim 18, characterized in that The solid core (14, 110) has an average height of 4.5 μm or less.

20. A nonwoven fibrous fleece obtainable by use of the lyocell fiber according to any one of claims 1 to 7 for producing a fibrous fleece.

21. Battery separator paper comprising the nonwoven fiber fleece (10) according to any one of claims 8 to 20.

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