JP2002327386A - Method for producing ultrafine fiber and sheet composed of ultrafine fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing ultrafine fibers dispersed in water from readily fibrillated fibers containing a vinyl alcohol polymer and an acrylonitrile polymer which have hitherto been difficult without producing fiber balls or bundles. SOLUTION: This method for producing the ultrafine fibers fibrillates fibers having 1-3 mm fiber length and comprising the vinyl alcohol polymer and acrylonitrile polymer in a ball mill.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ultrafine fibers and a sheet containing the same.

[0002]

2. Description of the Related Art As a method for obtaining ultrafine fibers comprising a vinyl alcohol-based polymer and an acrylonitrile-based polymer, a vinyl alcohol-based polymer and a polymer incompatible with the same are dissolved in a common organic solvent. Wet spinning to produce fibers, which are beater,
Japanese Patent Application Laid-Open No. 9-302 discloses a method of fibrillating with a refiner, a mixer, or the like, and obtaining a sheet by paper-making the fiber.
525. However, as described in this application, when the fibers are sufficiently fibrillated by the above-mentioned beater, refiner, mixer, or the like, fiber balls and binding are always generated, and uniform dispersion cannot be performed. However, when a sheet is produced by a wet papermaking method using the fiber ball, the fiber ball becomes a defect of the sheet, and there is a problem that thickness and density vary.

[0003] Due to the above-mentioned problems, this material is difficult to be formed into a sheet despite having excellent properties, and has been an obstacle to practical use.

[0004]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is to convert a fiber ball or a bundle from easily fibrillated fibers containing a vinyl alcohol polymer and an acrylonitrile polymer, which has been difficult in the prior art, as described above. It is an object of the present invention to provide a method for obtaining ultra-fine fibers dispersed in water without generating water. Another object of the present invention is to provide a sheet produced from the ultrafine fibers obtained by the above means. The present invention relates to a method for obtaining ultrafine fibers by fibrillating easily fibrillated fibers comprising a vinyl alcohol-based polymer and an acrylonitrile-based polymer, and a sheet comprising the same. These ultrafine fibers are excellent as battery separator materials, polishing materials, filtration materials, wiper materials, permselective membrane materials, water-repellent fabric materials, artificial leather materials, inkjet paper materials, binder materials, and the like.

[0005]

The present invention adopts the following constitution as means for solving the above-mentioned problems. The present invention includes the following aspects. [1] A method for producing ultrafine fibers, wherein fibers containing a vinyl alcohol-based polymer and an acrylonitrile-based polymer having a fiber length of 1 to 3 mm are fibrillated by a ball mill. [2] The method for producing ultrafine fibers according to [1], wherein the average diameter of the ultrafine fibers is 0.05 to 0.5 micron (μm). [3] B. E. T. The method for producing ultrafine fibers according to [1] or [2], wherein the specific surface area of the ultrafine fibers measured by a nitrogen adsorption method is 0.5 to 2.0 m ² / g. [4] including a step of treating fibers containing a vinyl alcohol-based polymer and an acrylonitrile-based polymer with hot water before fibrillating with a ball mill [1].
The method for producing ultrafine fibers according to [2] or [3].

[5] A sheet containing, as one component, ultrafine fibers obtained by fibrillating a fiber comprising a vinyl alcohol-based polymer and an acrylonitrile-based polymer having a fiber length of 1 to 3 mm with a ball mill. [6] The sheet according to [5], wherein the sheet is formed by a wet papermaking method. [7] The sheet according to [5] or [6], wherein the sheet has a thickness of 50 microns or less. [8] A sheet obtained by laminating layers containing ultrafine fibers produced by the method according to [1] to [4] by a wet papermaking method. [9] A sheet in which a layer containing ultrafine fibers is provided on the surface of the substrate by applying a slurry containing the ultrafine fibers produced by the method according to [1] to [4] to the substrate. [10] A separator for a secondary battery manufactured by the method according to [6].

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The easily fibrillated fiber comprising a vinyl alcohol polymer and an acrylonitrile polymer according to the present invention can be produced, for example, by a method disclosed in the following literature. That is, JP-A-9-302525
Can be used. That is, the vinyl alcohol polymer (A) and the polymer (B) incompatible with the vinyl alcohol polymer are dissolved in a common organic solvent, and the polymer (B) incompatible with the vinyl alcohol polymer is 1 to 20 μm. The spinning solution having a sea-island structure existing as a particle size of the above, and the obtained spinning solution is wet- or dry-wet spinning in a solidification bath comprising an organic solvent having a solidifying ability for the above-mentioned polymers and the stock solution solvent. , And further through a substitution bath consisting of alcohols, ketones and water,
An easily fibrillated fiber that is stretched 8 times or more after drying, and is obtained by using polyvinyl alcohol as the vinyl alcohol polymer (A) and polyacrylonitrile as the polymer (B) incompatible with the vinyl alcohol polymer. Can be used.

The vinyl alcohol polymer contains a vinyl alcohol unit, and may be a polymer obtained by copolymerizing monomers such as vinyl acetate, ethylene, and maleic anhydride. The acrylonitrile-based polymer includes an acrylonitrile unit, and may be a copolymer of units such as methyl acrylate, ethyl acrylate, and methyl methacrylate. The mixing ratio of the vinyl alcohol-based polymer and the acrylonitrile-based polymer is not particularly limited, but is preferably 20 to 80% by weight of the vinyl alcohol-based polymer, and is preferably 80 to 20% by weight of the acrylonitrile-based polymer.

Further, as the easily fibrillated fiber comprising the vinyl alcohol-based polymer and / or acrylonitrile-based polymer of the present invention, a fiber having a production method disclosed in JP-A-10-310931 can be used. That is, a polymer soluble in polyvinyl alcohol and dimethyl sulfoxide (DMSO) represented by polyacrylonitrile is dissolved in DMSO, and spun into a solidification bath mainly composed of methanol, and then the content of DMSO in the fiber is 2% by weight or less. After extracting DMSO from the fiber until it becomes, a predetermined amount of ketones having 4 or more carbon atoms typified by methyl isobutyl ketone is added to the fiber together with water, and a part of methanol is extracted and removed, and then the methanol concentration is 3 to 20. Fibers obtained by a method of drying fibers in a gas at a volume percentage of 150 ° C. or lower and then drawing the fibers can be used.

The easily fibrillated fiber comprising the vinyl alcohol polymer and the acrylonitrile polymer of the present invention preferably has a fiber length of 1 to 3 mm.
A 2 mm one is more preferred. A fiber having such a fiber length is obtained by cutting a continuous fiber bundle (yarn) into a fixed length. If the fiber length is less than 1 mm, the yield when fibrillated fibers are subjected to wet papermaking decreases, and the strength of the sheet tends to decrease, such being undesirable. On the other hand, if the fiber length is longer than 3 mm, fiber balls are liable to be generated during fibrillation, which is not preferable.

The easily fibrillated fiber comprising the vinyl alcohol-based polymer and the acrylonitrile-based polymer of the present invention has a fineness of 0.5 dtex (decitex) to 10 dtex.
Those having 0 dtex are preferable, and those having 1 dtex to 10 dtex are more preferable. Fibers within these ranges are excellent in handling or ease of fibrillation. The method for producing ultrafine fibers of the present invention is most characterized in that the easily fibrillated fibers having the above properties are fibrillated by a ball mill. As a method of fibrillating the easily fibrillated fiber, a general beater, a refiner, an attempt is made to fibrillate the above easily fibrillated fiber using a fibrillating means by the action of a rotating body in a slurry without using a medium such as a mixer. Then, in the case of mild fibrillation, remarkable fiber balls, twists, and bound fibers do not occur, but if fibrillation is promoted and divided into the smallest fibril unit, the fibers are extremely thin. , Fiber ball (spherical fiber bundle)
As described above, the physical properties of the sheet after sheeting may be impaired. Also, in a method of performing fibrillation by the action of a rotating body in a slurry using a medium such as beads or balls such as a sand grinder, similarly, it is inevitable that fiber balls, twists, and bound fibers are generated. .

The inventor of the present invention has observed in detail the problems caused by the fibrillating means as described above, and as a result of intensive studies on a method for improving the fibrillating means, the above-mentioned easily fibrillated fibers were accidentally obtained by wet ball milling. The present inventors have found that it is possible to divide the fiber into substantially the smallest fibril units without generating balls, twists, or binding fibers, and have completed the present invention.

[0013] The ball mill referred to here is a method in which a slurry of easily fibrillated fibers and a medium (balls, beads) are put in a container, and the container is externally rotated and / or vibrated to disperse and mix. , A device that performs division. Examples of such a ball mill device include a device in which a cylindrical container containing a slurry of easily fibrillated fibers and a medium is rotated on a turntable, or a device in which the container itself is fixed to a rotating shaft and rotates. However, particularly suitable for the purpose of the present invention is a vibrating mill of a type in which a gantry to which a container is fixed vibrates at high speed. In such a vibration mill, the medium repeats collision with the fiber at a high speed, so that fibrillation proceeds in a very short time, and since there is no movement of the rotating body inside the slurry, the fiber ball or the twist,
There is a feature that the generation of binding fibers is unlikely to occur.

The material of the medium (balls, beads, etc.) and the container can be appropriately selected from those made of ceramics, metal or resin. When the fibrillated fiber sheet is required to have high electrical insulation (for example, a separator of a battery or a capacitor), it is not preferable to use a metal medium or container. Examples of suitable materials include, in the case of ceramics, alumina-based, zirconia-based, and silicon nitride, and in the case of metal, steel and stainless steel.
In the case of resin, there are nylon and synthetic rubber. Such a resin is used in a ceramic or metal container or by coating a medium.

[0015] The medium (ball) preferably has a diameter of 1 to 30 mm. The filling amount of the medium is 10 to 8 of the internal volume of the container.
It is preferable to fill so as to satisfy 0%. The filling amount of the easily fibrillated fiber is 1.
6-17 g are preferred. For example, if the inner volume of the container is 3000
In the case of ml, 5 to 50 g is preferable. In the case of water, the ratio of the easily fibrillated fiber to the liquid to be added is preferably 50 g to 300 g per 10 g of the fiber.

The processing time when processing with a vibration mill is as follows:
Since it varies depending on the frequency, it cannot be unconditionally determined. However, when processing is performed at a frequency of 1,000 times / minute and an amplitude of 8 mm, 1 to
Preferably 30 minutes, particularly preferably 5 to 20 minutes. If the processing time is too short, sufficient fibrillation does not occur.
If the treatment time is too long, the fiber length and the fiber strength are reduced, which is not preferable. When performing the fibrillation treatment of the fiber by the above method, the addition of an appropriate dispersant may be effective in promoting fibrillation and preventing reaggregation. The fiber diameter of the ultrafine fibers fibrillated by the above method is preferably 0.05 to 0.5 micron at the main part, particularly preferably 0.1 to 0.3 micron. If the fiber diameter exceeds 0.5 μm, the ability to form a thin sheet and the performance as a battery separator, which are features of ultrafine fibers, are not sufficiently exhibited. Also,
If the fiber diameter is less than 0.05 microns, it may be difficult to form a sheet by a wet method. The fiber diameter of the ultrafine fibers can be measured using, for example, a scanning electron microscope.

The specific surface area of the fibrillated microfine fibers by the above method is preferably from 0.5~2.0m ² / g, particularly preferably 1.5 m ² / g from 0.8. If the specific surface area is less than 0.5 m ² / g, there is a possibility that the ability to form a thin leaf sheet, which is a feature of ultrafine fibers, and the performance of wiping a sheet formed therefrom may not be sufficiently exhibited. When the specific surface area exceeds 2.0 m ² / g, it becomes difficult to form a sheet by a wet method. The specific surface area of the ultrafine fibers fibrillated by the above method is measured using a specific surface area measuring device (for example, SA-6200 type continuous flow surface area meter manufactured by HORIBA, Ltd.) for the fibers obtained by drying the slurry. be able to.

The easily fibrillated fiber containing a vinyl alcohol-based polymer and an acrylonitrile-based polymer as main components contains a vinyl alcohol component, and therefore, depending on the use (for example, a separator of a lithium ion battery).
This can have undesirable effects. Also,
The vinyl alcohol component may cause foaming of the slurry during the fibrillation process or the sheeting process. If it is desirable to reduce the vinyl alcohol component for such a reason, the purpose can be achieved by treating the fibers with hot water before the fibrillation treatment and then washing with water. The temperature at the time of treatment with hot water is from 80 ° C to 100 ° C
, And more preferably 95 ° C to 100 ° C. The time for treating with hot water is preferably from 10 minutes to 60 minutes.

Known methods can be used for forming the ultrafine fibers obtained by the above method into a sheet. That is, a wet paper or nonwoven fabric manufacturing method or a dry nonwoven fabric manufacturing method can be used. Among these methods, a method suitable for forming the ultrafine fibers of the present invention into a sheet is a wet papermaking method. That is, this is a method in which ultrafine fibers dispersed in water are made on a wire by a paper machine, pressed, and then dried to produce a continuous web. According to this method, it is possible to produce a uniform sheet with a desired thickness and high productivity. According to this method, a very thin and homogeneous sheet having a thickness of 50 μm or less can be produced from the ultrafine fibers used in the production method of the present invention.

When performing wet papermaking, a sheet can be prepared by mixing other materials as necessary. That is,
Mix various wood pulp, synthetic pulp, synthetic fiber, regenerated cellulose fiber, inorganic fiber, binder fiber, pigment, filler, paper strength enhancer, yield improver, pH adjuster, dispersant, adhesive, defoamer, etc. Alternatively, it can be added.
Further, when the ultrafine fibers obtained by the above method are formed into a sheet by a wet papermaking method, a sheet in which a layer containing the ultrafine fibers and a layer of another combination are laminated by using a multilayer papermaking method is produced. It is also possible. It is also possible to form a layer containing ultrafine fibers on the surface of the substrate by applying the ultrafine fiber slurry to a suitable sheet substrate such as paper or film. The sheet containing the ultrafine fiber slurry produced by the method of the present invention as a component can be produced by a wet papermaking method and used as a separator for various batteries. In this case, materials other than the ultrafine fibers of the present invention can be mixed if necessary. Further, if necessary, the sheet may be calendered to adjust the thickness and densify the sheet.

[0021]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples.
The present invention is not limited by the following examples.
The evaluation of the fibrillation state of the fiber was determined by dispersing a small amount of the fiber after the fibrillation treatment into a 1000 ml transparent glass beaker with water and observing the dispersion. The thickness of the sheet was determined by averaging 10 places per sheet using a micrometer on a sheet made according to TAPPI Standard Test Method T-205, and calculating the standard deviation. ratio
The surface area was measured using a specific surface area measuring device (SA-6200 type continuous flow surface area meter manufactured by Horiba, Ltd.) for the fiber obtained by drying the ultrafine fiber. The fiber diameter was observed with a scanning electron microscope, and the average value was determined.

Example 1 10 g of chopped fiber (made by Kuraray Co., Ltd., fineness: 2.2 dtex, fiber length: 2 mm) composed of a vinyl alcohol polymer and an acrylonitrile polymer was put into a 3000 ml stainless steel pot together with 100 g of water. Alumina balls (medium) having a diameter of 5 mm were filled up to 60% of the container and capped. This pot is placed on a vibration mill (Chuo Kakoki
(MB-1 type), amplitude 8mm, frequency 1000
r. p. m. For 10 minutes to treat the fibers. The ball (medium) was separated by passing the slurry after the treatment through a mesh.

Comparative Example 1 A slurry was obtained in the same manner as in Example 1 except that the chopped fiber had a fiber length of 5 mm. Comparative Example 2 Water was added to 360 g of chopped fiber (made by Kuraray Co., Ltd., fineness: 2.2 dtex, fiber length: 2 mm) consisting of a vinyl alcohol-based polymer and an acrylonitrile-based polymer, and 23
It was beaten with a beater for 30 minutes according to TAPPI Standard Test Method T-200 to obtain a slurry.

Example 2 10 g of chopped fiber (made by Kuraray Co., Ltd., having a fineness of 2.2 dtex, fiber length of 2 mm) composed of a vinyl alcohol polymer and an acrylonitrile polymer was placed in 1500 ml of boiling water and boiled for 30 minutes. did. After cooling, 2
The fibers were filtered off using a 00 mesh filter cloth and washed thoroughly with water. 3000 ml of this fiber with 100 g of water
The container was placed in a stainless steel pot, and alumina balls having a diameter of 5 mm were filled up to 60% by volume of the container and capped. The pot was mounted on a vibration mill (MB-1 type, manufactured by Chuo Kakoki Co., Ltd.), and the amplitude was 8 mm and the frequency was 1000 r. p. m. For 10 minutes to treat the fibers. Balls were separated from the slurry after the treatment in the same manner as in Example 1.

Example 3 Using the slurries prepared in Examples 1 and 2,
Basis weight 12g / m according to TAPPI standard test method T-205
A handsheet No. ² was prepared. Comparative Example 3 Using the slurries prepared in Comparative Examples 1 and 2,
Basis weight 12g / m according to TAPPI standard test method T-205
A handsheet No. ² was prepared.

Example 4 A slurry of bleached kraft pulp adjusted to 300 ml of CSF was dehydrated on a wire of a hand-making apparatus shown in TAPPI Standard Test Method T-205 to form a wet paper layer, and Example 1 was further formed thereon. The slurry prepared in the above was poured and dewatered again, so that the basis weight of the bleached kraft pulp layer was 60 g / m ^2.
Thus, a laminated sheet having a basis weight of the ultrafine fiber layer of 10 g / m ² was produced.

[0027] Coating with a coating amount of 5 g / m ² on one surface of woodfree paper of Example 5 the slurry prepared in Example 1 was adjusted to a solid concentration of 5%, a basis weight of 100 g / m ² using a Meyer bar It was dried in a hot air drier kept at 100 ° C. for 5 minutes.

Example 6 A battery cell was prepared by using lithium cobalt oxide as a positive electrode active material and non-graphitizable carbon as a negative electrode active material, and using the slurry prepared in Example 3 using the slurry of Example 2 so as to be in contact with each electrode. Was prepared. Then, 0.5 parts of a mixed solvent of propylene carbonate: ethylene carbonate = 1: 1 was added.
An electrolyte in which M / l of LiClO ₄ was dissolved was permeated into the cell to produce a lithium ion battery.

Comparative Example 4 In the same manner as in Example 6 except that the sheet prepared in Example 3 was replaced with the sheet prepared using the slurry of Comparative Example 2 (beater treatment) in place of the sheet prepared in Example 3. To produce a lithium ion battery.

[0030]

[Table 1]

As shown in Table 1, when the fibrillation treatment is performed by the method of the present invention (Examples 1 and 2), a fibrillated fiber which is uniformly dispersed without generation of twist or fiber ball can be obtained. . However, when the fiber length deviates from the preferred range (Comparative Example 1), a significant twist occurs due to the treatment. Further, when a beater which is an inappropriate method was used as a fibrillating means (Comparative Example 2), fiber balls were generated even if the fiber length was appropriate.

[0032]

[Table 2]

As shown in Table 2, the sheet prepared in Example 3 had a small thickness variation, was uniform, had no holes (pinholes), and was in a good condition. On the other hand, Comparative Example 3
In the sheet prepared in the above, fibrillation was not performed uniformly, so that the sheet prepared therefrom had a remarkable variation in thickness, and the state of the sheet had many defects and was not preferable.

As shown in Example 4, the ultrafine fiber slurry produced by the method of the present invention can be combined with bleached kraft pulp used in ordinary papermaking to form a composite sheet. This composite sheet had a uniform surface state without peeling or the like. As shown in Example 5, the slurry produced by the method of the present invention could be applied on paper in a conventional manner to produce a coated paper. The coated paper had a smooth and uniform surface.

The lithium ion batteries prepared in Example 6 and Comparative Example 4 were subjected to a charge / discharge test in which the batteries were charged to 4.1 V and discharged to 2.7 V. Although the battery capacity did not decrease even after repeated charge and discharge, the battery of Comparative Example 4 was internally short-circuited at the first charge, and could not be charged or discharged thereafter. From these results, it was found that the sheet produced by the method of the present invention had good characteristics as a lithium ion battery separator. On the other hand, it has been found that sheets made without using the method of the present invention are not preferred as lithium ion battery separators, probably due to their non-uniformity.

[0036]

According to the method of the present invention, a uniform ultrafine fiber slurry can be prepared from easily fibrillated fibers comprising a vinyl alcohol-based polymer and an acrylonitrile-based polymer, which has been conventionally difficult. Also,
Sheets prepared from this slurry are uniform and free of defects and have favorable properties as battery separators and the like.

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Claims (10)

[Claims] 1. A method for producing ultrafine fibers, comprising fibrillating a fiber containing a vinyl alcohol polymer and an acrylonitrile polymer having a fiber length of 1 to 3 mm with a ball mill. 2. The average diameter of the ultrafine fibers is from 0.05 to 0.5.
The method for producing ultrafine fibers according to claim 1, wherein the diameter is 5 microns. 3. B. E. T. Claims a specific surface area of the ultrafine fibers was measured by a nitrogen adsorption method is 0.5~2.0m ² / g 1
Or the method for producing ultrafine fibers according to 2. 4. The method for producing ultrafine fibers according to claim 1, further comprising a step of treating fibers containing a vinyl alcohol-based polymer and an acrylonitrile-based polymer with hot water before fibrillating with a ball mill. 5. A sheet containing, as one component, ultrafine fibers obtained by fibrillating fibers made of a vinyl alcohol polymer and an acrylonitrile polymer having a fiber length of 1 to 3 mm with a ball mill. 6. The sheet according to claim 5, wherein the sheet is formed by a wet papermaking method. 7. The sheet according to claim 5, wherein the sheet has a thickness of 50 microns or less. 8. A sheet obtained by laminating layers containing ultrafine fibers produced by the method according to claims 1 to 4 by a wet papermaking method. 9. A sheet in which a layer containing ultrafine fibers is provided on the surface of a substrate by applying a slurry containing ultrafine fibers produced by the method according to claim 1 to a substrate. 10. A separator for a secondary battery manufactured by the method according to claim 6.