JP2009061401A - Fiber structure for filtration filter and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fiber structure for filtration filter where an excellent impurity filtration function is expected by a microfiber containing VdF with ESD process and a method for manufacturing the same that excellently forms the fiber structure for filtration filter with the ESD process. <P>SOLUTION: A copolymer of VdF and HFP with MFR value of 0.11 g/10 min is prepared as the component of a solution material. It is preferable that the copolymer is a random copolymer with VdF of 75% or more and 92% or less and HFP of 8% or more and 25% or less. Next, the copolymer is dissolved so as to allow a resin concentration in the entire solution material to be 10 wt.% or more and less than 30 wt.% to a volatile solvent. This solution is supplied to an ESD device to obtain a nonwoven fabric 20 (fiber structure for filtration filter) by electrospinning. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fiber structure for a filtration filter using an electrospinning (ESD) method and a method for producing the same, and more particularly to a technique using ultrafine fibers containing VdF.

  In order to purify industrial ultrapure water typified by cleaning water for semiconductor devices and the like, in recent years, a filtration filter made of a nonwoven fabric formed by an electrospinning method (ESD method) has been used. Such a non-woven fabric filter has a relatively wide filter surface area in contact with water as compared with other filters such as a film structure, and therefore exhibits a highly efficient and highly efficient filtering action. In particular, if the ESD method is used, a non-woven fabric can be obtained with ultrafine fibers having a nano-order average fiber diameter called a so-called nanofiber, so that a non-woven fabric having a larger surface area than a general spinning method can be obtained. A high performance filtration filter can be expected.

  In the ESD (Electro Spinning Deposition) method, a solution material obtained by dissolving a fiber resin material is charged with a high voltage together with a metal injection needle, and the solution is discharged from the tip of the injection needle toward the grounded collection electrode surface. . The solution material is attracted to the surface of the collecting electrode by a strong electric field formed by the electric field concentration effect at the tip of the injection needle, and the solvent is vaporized and spun during flight. When this spinning process is continued for a certain period of time, a nonwoven fabric made of ultrafine fibers is formed on the collecting electrode plate (for example, Patent Document 3).

As a fiber material for a filtration filter, generally, an olefin material such as polyethylene or polypropylene, or a polyamide is used, but a fluorine resin material is used as one of high performance materials. Since the fluorine-based resin exhibits excellent anion characteristics due to fluorine atom ions, it has excellent impurity adsorption characteristics and is suitable as a filter material. PTFE, TFE, CTFE or the like is used as the fluororesin material, and fluororesin fibers containing any of these have been put into practical use.
JP 2006-92829 A JP 2004-308031 A JP 2006-144138 A J. et al. Polymer Science Part B, 39, 2598, 2001

  In addition to the above, polyvinylidene fluoride (PVdF) is known as a fluororesin that can be used as a filter material. PVdF is a fluororesin particularly excellent in impurity adsorption characteristics, and is currently used as a filter material in the form of a film. If this PVdF can be used as an ultrafine fiber, it is considered that a highly efficient pure water filtration filter can be realized.

  However, PVdF is a crystalline fluororesin and generally has a relatively low solubility in a solvent. For this reason, it is difficult to process into ultrafine fibers, and when a solution in which PVdF is dissolved is supplied to an ESD device under normal conditions, it is crystallized immediately after being discharged from the injection needle during electrostatic spinning and formed into a thread shape Therefore, it is difficult to spin.

As described above, a technique for efficiently spinning PVdF using an ESD method and manufacturing a filter using a nonwoven fabric made of the material has not yet been established.
The present invention has been made in view of the above problems, and its purpose is to provide a fiber structure for a filtration filter capable of exhibiting an excellent impurity filtering function with ultrafine PVdF-containing fibers using an ESD method. Another object of the present invention is to provide a production method for satisfactorily forming the filter filter fiber structure by an ESD method.

In order to solve the above-described problems, the present invention is for a filtration filter in which an average fiber diameter is 1 nm or more and less than 5 μm, and fibers made of a copolymer containing VdF and HFP are configured in a nonwoven fabric or a knitted fabric. A fiber structure was obtained.
Here, as the copolymer, a VdF-HFP copolymer can be used. The copolymer may have a random structure.

Further, the MFR value of the copolymer is preferably in the range of 0.1 g / 10 min to less than 3 g / 10 min.
The present invention is also a method for producing a fiber structure for a filtration filter using an electrospinning method, in which a solution material in which a copolymer containing VdF and HFP is dissolved is charged to one polarity, and the solution material is charged. Non-woven fabric or knitted fabric with fibers made of VdF-HFP copolymer having an average fiber diameter of 1 nm or more and less than 5 μm on the collecting electrode plate by discharging it from the injection needle toward the collecting electrode of the other polarity. The shape was to be formed.

Here, as a copolymer containing VdF and HFP, a random copolymer of VdF and HFP can be used.
Furthermore, as a copolymer, what was set to the range whose MFR value is 0.1 g / 10min or more and less than 3g / 10min can be used. As the solution material in this case, a material in which the copolymer is dissolved in an organic solvent in a concentration range of 10 wt% to 30 wt% is preferable.

  In addition, the MFR value mentioned in this application refers to the value measured according to ISO1133.

According to the present invention having the above configuration, ultrafine fibers containing VdF can be spun using the ESD method, and the production of a fiber structure for a filtration filter such as a nonwoven fabric having excellent filtration performance can be realized.
That is, as a result of intensive studies by the inventors of the present application, crystalline VdF is not used alone as a resin material, but rather is hexafluoropropylene (HFP) which is a fluororesin material with respect to VdF and has non-crystalline properties. By using a copolymer containing, the solubility in a solvent is remarkably improved, and the crystallinity of VdF, which becomes an obstacle in carrying out the ESD method, is suppressed.

  Specifically, in the present invention, by introducing an amorphous HFP component into a molecular structure as a copolymer in VdF, rapid crystallization even after the solution material is discharged from the injection needle when the ESD method is performed. Can be suppressed. The solution material is adjusted so as to disappear and solidify the solvent while exhibiting good fluidity when passing through the injection needle and maintaining the form when continuously discharged by the injection needle.

  As described above, the present invention is intended to obtain an ultrafine fiber containing PVdF in consideration of the spinning process peculiar to the ESD method, and for the filtration filter capable of exhibiting excellent filtration characteristics. A fiber structure can be realized.

Embodiments of the present invention will be described below, but the present invention is naturally not limited to these forms, and can be appropriately modified and implemented without departing from the technical scope of the present invention.
In addition, the constituent elements in the embodiments can be combined with each other within a consistent range.
(Configuration of filter unit)
FIG. 1 is a diagram illustrating a configuration of an ultrapure water filtration filter unit 1 according to an embodiment.

The unit 1 shown in the figure has a configuration in which a strip-shaped nonwoven fabric 20 is wound around a hollow porous axis 10 in the longitudinal direction.
Ring-shaped gaskets for fixing the nonwoven fabric 20 and preventing leakage of filtered water are provided on the side surfaces in the thickness direction of the nonwoven fabric at both axial ends of the unit 1.
The nonwoven fabric 20 shown in this figure is a fiber structure for a filtration filter made of ultrafine fibers containing VdF produced by an electrospinning (ESD) method, assuming an industrial ultrapure water filtration filter. is there. As an example of the size of the nonwoven fabric, the thickness can be set in the range of several tens to several hundreds μm and the basis weight can be set in the range of 5 to 750 g / m ² . As an example, the ultrafine fibers are composed of a copolymer having a random structure of vinylidene fluoride and hexafluoropropylene (VdF-HFP copolymer), and the average fiber diameter is 340 μm. Moreover, the molecular structure can be made into a random structure, for example in the range whose VdF is 75 mol% or more and 92 mol% or less, and HFP is 8 mol% or more and 25 mol% or less. Here, 20 is a non-woven fabric. However, after spinning, the ultrafine fibers may be wound once with a drum or the like and then reconfigured as a knitted fabric.

The porous shaft center 10 is a water collecting pipe through which filtered water is conducted, and is composed of a resin tube having a large number of holes on the peripheral surface. At the time of filtration, water filtered from the peripheral surface of the nonwoven fabric 20 is conducted to the hole, and the filtered water is circulated along the flow path 11.
Note that the shaft center 10 is not an essential component and can be omitted. In this case, for example, the non-woven fabric 20 is wound in a hollow state, and a filtered unit in which filtered water is circulated in the hollow portion can be obtained.

Further, in FIG. 1, the nonwoven fabric 20 is wound in a flat shape, but can be wound while being finely processed into a pleated shape. By devising in this way, a large filtration area can be secured.
Here, in the filter unit 1 of embodiment, the nonwoven fabric 20 is comprised as a fiber structure for filtration filters which consists of the ultrafine fiber (VdF-HFP copolymer fiber) containing VdF spun by ESD method. For this reason, compared with the nonwoven fabric which consists of the fiber spun by the conventional general spinning method (average fiber diameter is several hundred micrometer order or more), the surface area is ensured remarkably widely by adoption of ultrafine fiber, The impurities can be efficiently adsorbed and separated using the surface.

Furthermore, since the nonwoven fabric 20 is composed of ultrafine fibers containing VdF, which has a particularly high electronegativity, among the fluororesin fibers, the impurity adsorption characteristics are extremely excellent. Therefore, in the filter unit 1, good impurity filtration characteristics are also exhibited.
The ultrafine fibers constituting the nonwoven fabric 20 have an average fiber diameter in the range of 1 nm to 5 μm, more preferably 1 nm to 1 μm, and need not be highly uniform. However, from the viewpoint of preventing variations in product quality, it is desirable to have an average fiber diameter that is as converged as possible. In addition, an average fiber diameter can be suitably adjusted with conditions, such as the viscosity of solution material, and the internal diameter of the injection needle 217 of the apparatus 2 mentioned later.

In addition, although the filter unit 1 uses water as an object to be filtered, the present invention is not limited to this, and the filter unit 1 can also be used as a filtering means for liquids other than water, such as organic solvents.
(Configuration of ESD device 2)
Next, an apparatus for spinning an ultrafine fiber containing VdF and forming a nonwoven fabric using this will be described.

FIG. 2 is a partially cutaway view showing the internal configuration of the ESD device 2. The ESD device 2 includes a robot arm 200, a syringe 251, and a turntable 40 arranged in a box-shaped housing. The upper part of the housing is openable and closable so that the operator can access the above components as appropriate.
The robot arm 200 is a known three-axis robot arm, and a first arm 201, a second arm 202, and a third arm 203 are combined so as to be slidable along the x-, y-, and z-axis directions, respectively. The first arm 201 is coupled to the second arm 202 via a U-shaped slide base 211 fitted in a pair of guide slits 210 (one not shown). Similarly, the second arm 202 is connected to the third arm 203 via a slide base 214 fitted into the pair of guide slits 212 and 213.

The side surface of the first arm 201 is fixed to the inner wall surface of the apparatus via the base 230.
In the figure, reference numerals 220 to 222 denote articulated cable boxes which bend or extend following the movement of each arm. Inside the first arm 201 and the second arm 202, a motor unit for sliding the slide bases 211 and 214 along the longitudinal direction of the arms 201 and 202 is incorporated. Reference numeral 215 denotes a motor unit for moving the third arm 203 in the z direction with respect to the base 250. The operation of each motor unit is controlled based on a predetermined control program in a control unit (not shown) built in the lower part of the casing.

  The syringe 251 is a means for storing a resin solution used as a material for the nonwoven fabric inside, and is fixed to the third arm 203 at the peripheral surface side portion. An upper portion of the syringe 251 is provided with a cap 252 having an air hole, which is devised so that the discharge of the solution is not interrupted due to the negative pressure inside the syringe 251 when the apparatus is driven. On the other hand, an injection needle (injection nozzle) 217 made of a conductive material (for example, a metal material) is attached to the lower end of the syringe 251 and is disposed so as to face the collecting electrode plate immediately below the z direction. The injection needle 217 is held by the charging clamp 216 below the third arm 203, and a predetermined high voltage having a positive polarity is applied by the voltage application device 30 incorporated in the device 2. The discharge distance L (see FIG. 3) between the tip of the injection needle 217 and the collecting electrode 401 can be adjusted within a range of a maximum of 300 mm as the third arm 203 slides in the z direction. In the present invention, 50 mm or more and 200 mm or less is suitable and can be set to 120 mm, for example.

The inner diameter of the injection needle 217 can be appropriately adjusted in the range of 100 μm or more and 1.0 mm or less in accordance with the average fiber diameter of the fibers to be spun, but can be set to 330 μm, for example. In the present invention, a range of 150 μm or more and 510 μm or less is suitable.
Note that the syringe 251 and the injection needle 217 are not limited to one each, and at least one of them may be arranged in parallel over a plurality. Further, it is possible to directly discharge the solution material from the syringe 251 without using the injection needle 217. However, a high voltage can be applied between the solution material to be discharged and the collecting electrode 401 on the principle of the ESD method. Thus, for example, the syringe 251 is made of metal, and the solution material needs to be charged to the positive electrode side from the outside.

Further, a wiring tube or the like may be provided on the cap 252 of the syringe 251 and the solution material may be continuously supplied from the outside through the tube.
On the other hand, immediately below the robot arm 200, a rectangular electrode plate 402 and a sheet-like collecting electrode 401 made of aluminum foil or the like are stacked in the same order in the upper direction. These 401 and 402 are grounded by wiring (not shown) inside the apparatus. Here, as the specification of the apparatus 2, the maximum applied voltage to the injection needle 217 can be set up to 80 kV, but in the present invention, the range of 15 kV to 50 kV is suitable.

In the device 2, the injection needle 217 side is a positive electrode and the collecting electrode 401 is grounded. However, these are settings for providing a relative voltage difference in order to discharge the solution material. Depending on the selection, the polarity may be reversed, or a voltage lower than that of the injection needle 217 may be applied without grounding the collecting electrode 401.
Note that an emergency stop button (both not shown) for urgently stopping the input display unit and the apparatus can be disposed on the front part of the housing 50, for example. As the input display unit, a liquid crystal display with a touch panel can be used. As a result, the operator inputs the setting of each driving condition of the robot arm 200 to the control unit.

Further, a joystick may be separately provided on the front part, so that the arms 201 to 203 of the robot arm 200 can be directly operated in the xyz axis directions.
In the apparatus 2, the robot 200 is not essential. For example, the syringe 251 is fixed in the xy direction (adjustable in the z direction), and the collection electrode 401 side is arranged on a known xy two-axis table. The collecting electrode 401 may be moved relative to the 251 side.
(About the manufacturing process of the nonwoven fabric 20)
The process for producing the fiber structure for filtration filter (nonwoven fabric 20) of the present invention can be exemplified as follows using the ESD device 2 described above.

  First, as a component of the solution material, a VdF and HFP copolymer having an MFR value of 0.11 g / 10 min is prepared. The copolymer is preferably a random copolymer having a VdF of 75 mol% to 92 mol% and an HFP of 8 mol% to 25 mol%. As a raw material of a commercial product having this configuration, for example, “Kynar 2801” manufactured by Tokyo Materials Co., Ltd. can be mentioned. If the MFR value of the copolymer is too large, the solution viscosity is lowered, and the solvent separation property is impaired during electrostatic spinning. If the MFR value is too small, spinning becomes difficult. Therefore, it should be noted that a resin material having an appropriate MFR value should be used.

The copolymer is dissolved in a volatile solvent so that the resin concentration in the entire solution material is 10 wt% or more and less than 30 wt%.
The volatile solvent may be any solvent that can partially or completely dissolve VdF at room temperature. For example, when 100% of DMF is used, or DMF and DMAC are mixed in a weight ratio of 75:25 in the same order. Things can be used.

  Other examples of usable solvents include acetone, tetrahydrofuran, MEK, dimethylformamide (DMF), dimethylacetamide (DMAC), tetramethyl urea, trimethyl phosphate, hexafluoroisopropanol, trifluoroacetic acid, etc., alone or in combination. Can be used. Other latent solvents (solvents that can be dissolved when the temperature is raised) include MIBK, butyl acetate, cyclohexanone, diacetone alcohol, DIBK, butyrolactone, tetraethyl urea, isophorone, triethyl phosphate, carbitol acetate, propylene carbonate, dimethyl phthalate, etc. Or more.

  Next, the adjusted solution material is stored in the syringe 251. On the other hand, as shown in FIG. 3, the operator adjusts the distal end position of the third arm 203 as shown in FIG. 3, and between the injection needle 217 attached to the distal end of the syringe 251 and the collection electrode 401. The discharge distance L is set. The set value of the discharge distance L at this time is set to such a value that the solvent can be substantially volatilized when the solution discharged from the injection needle 217 reaches the collecting electrode (for example, the solution concentration is about 23%, MFR When the value is 0.2 g / 10 min, the discharge distance L is set to 120 mm). If the discharge distance L is too short, fibers are not formed and the solvent remains. On the other hand, if the length is too long, the fiber density becomes sparse, and the fiber structure cannot be deposited well on the collecting electrode 401.

Specifically, the distance is adjusted by operating the third arm 203 of the robot 200 and operating the motor unit 215 to adjust the position of the third arm 203 along the z direction.
Next, the operator uses the input display unit or the like to set conditions such as applied voltage and voltage application time for the control unit (setting step). As an example, the DC application voltage can be 40 kV and the voltage application time can be 30 minutes.

  When the operator completes the above steps, the apparatus 2 is driven. Before driving, the solution material of the syringe 251 is stored in a stationary state by a capillary in the injection needle 217, but a high voltage of 40 kV is applied between the injection needle 217 and the collecting electrode 401 by the start of driving of the apparatus 2. When applied, the solution material is also charged to the positive electrode, and a conical solution material portion called “Taylor Cone” is formed at the tip of the injection needle 217. Thereafter, when the surface tension of the solution material P at the tip of the injection needle 217 that has been stationary is broken, the solution material P is discharged toward the grounded collection electrode 401 (FIG. 3).

  The solution material P is continuously discharged toward the collecting electrode 401, contacts the air at high speed while flying in the electric field at the discharge distance L, and the solvent disappears instantaneously by vaporization. As a result, when the solution material P reaches the surface of the collecting electrode 401, only the components of the substantially copolymerized substance remain, thereby forming ultrafine fibers (nanofibers) containing VdF. The ultrafine fibers are deposited while being attracted to the surface of the collecting electrode 401 by electrostatic action. When this deposition amount becomes constant, the operator can carefully peel off the surface of the collecting electrode 401 as a fiber structure, which can be used as the nonwoven fabric 20.

  Here, in the apparatus 2, as shown in S of FIGS. 2 and 3, the ultrafine fibers formed by being ejected from the injection needle 217 side are electrostatically repelled from each other, and thus have a conical shape with respect to the surface of the collection electrode 401. Since it spreads, the nonwoven fabric 20 having a certain area can be obtained without moving the injection needle 217 or the like during discharge. However, in order to obtain the non-woven fabric 20 having a predetermined area and thickness, it is preferable to adjust so that the positions of the injection needle 217 and the collecting electrode 401 are relatively moved while the apparatus 2 is driven.

For example, in the case of the apparatus of FIG. 2, the first and second arms 201 and 202 of the robot 200 are adjusted by moving them in at least one of the xy directions, thereby adjusting the deposition position / area, etc. on the collection electrode 401. It is also possible to adjust.
In this embodiment, the nonwoven fabric 20 is directly obtained as the fiber structure obtained on the collecting electrode 401. However, the present invention is not limited to this, and the deposition of ultrafine fibers is divided into several degrees. You may comprise the nonwoven fabric 20 separately by processing, such as laminating | stacking these after obtaining a thing.

Further, the ultrafine fiber obtained on the collecting electrode 401 is once spun with a roller and knitted, whereby a knitted fabric for a filtration filter can be obtained.
As described above, when the ESD method is used, it is possible to relatively easily spin ultrafine fibers that are difficult to spin by other methods. In addition, the method can be implemented by a process under normal temperature and normal pressure conditions, and is relatively feasible. Furthermore, since a large-scale spinning device is unnecessary and spinning and non-woven fabric can be easily and continuously produced in a small ESD device, there is also an advantage that it can be manufactured efficiently at low cost.

Here, the present invention has a main feature in that an ultrafine fiber containing VdF can be spun using an ESD method, and a nonwoven fabric having excellent filtration performance can be produced.
That is, PVdF is a fluororesin having a good electronegativity and excellent impurity separability. Therefore, if ultrafine fibers are obtained as a non-woven fabric, production of a high-performance pure water filtration filter can be expected. Then, even if we try to spin a solution material containing a VdF homopolymer as a resin component by the ESD method, the solution material will clog in the syringe or injection needle, and the resin component will crystallize immediately after being discharged by the injection needle. As a result, a fine spherical resin lump (bead) is discharged. For this reason, it is very difficult to obtain ultrafine fibers having a constant average fiber diameter while being continuously discharged. As a cause of such problems, it is considered that PVdF is originally a crystalline resin and its solubility in a solvent is relatively low. Moreover, PVdF has a very high dielectric constant compared to other fluororesins (for example, the relative dielectric constant of PTFE, which is a typical fluororesin, is about 2.1, whereas the relative dielectric constant of PVdF is 8. The dielectric strength voltage is very high. For this reason, during electrostatic spinning, it cannot be effectively discharged unless a very high voltage is applied, and this property is considered to be an obstacle to performing the spinning process by the ESD method under normal conditions.

As a result of intensive studies by the inventors of the present invention on this problem, it is not possible to use crystalline VdF alone as a resin material, but to use a copolymer made of VdF and an amorphous fluororesin material (HFP). For example, the present inventors have found that the solubility in a solvent is remarkably improved and that spinning can be performed well even in an ESD method.
That is, PVdF by itself is too strong in crystallinity and exhibits properties unsuitable for the spinning process using the ESD method. However, non-crystalline HFP component is introduced into VdF as a copolymer, and VdF is introduced into the molecular structure. The crystallinity of the formation of the ultrafine fibers contained can be suppressed. This suppresses rapid crystallization even after the solution material is discharged from the injection needle, and adjusts the solution material that is continuously discharged to solidify while disappearing the solvent, thereby obtaining fiber processability. It is a thing.

  In addition, as the resin material of the ultrafine fiber containing VdF of the present invention, VdF and HFP copolymer have the highest feasibility, but a copolymer containing an amorphous resin component other than HFP may be used. For example, a non-crystalline fluororesin component (for example, a copolymer of a fluorine-containing olefin such as TFE (tetrafluoroethylene) or CTFE (chlorotrifluoroethylene) and a hydrocarbon olefin) is added to VdF and HFP. A copolymer may be used.

Furthermore, it is desirable that the copolymer has a structure (eg, a random copolymer) with a small number of VdF block portions in the molecular structure so that the crystallinity of VdF does not become noticeable when the solution is discharged.
<Experimental experiment on solution composition>
In order to investigate the optimum conditions for manufacturing a nonwoven fabric by the ESD method based on the present invention, ESD solution samples of Examples and Comparative Examples were prepared and examined.

  As the resin component used for the solution material, the samples used in the examples and comparative examples were all the Kynar series manufactured by Arkema (“Kynar” is a registered trademark of Arkema). Among these, VdF / HFP copolymer (flexible grade) was selected as the resin material of the examples. As the resin material of the comparative example, the copolymer or VdF homopolymer (medium viscosity / high viscosity grade) was selected.

Specific experimental methods and results for each sample are as follows.
In addition, as for the measuring method of MFR value, all shall be measured according to ISO1133.
(Example 1)
3 g of a copolymer of VdF and HFP (Kynar 2801, MFR value 0.2 g / 10 min) was mixed with 10 g of DMF (dimethylformamide) (manufactured by Nacalai Tesque) and completely dissolved by shaking with a paint shaker.

This solution material was put into the syringe of the ESD apparatus 2 shown in FIG. As setting conditions, an aluminum foil was used for the collection electrode, and each of the setting was performed by setting each of the injection needle inner diameter 0.33 mm, the applied voltage 40 kV, the interelectrode distance 120 mm, and the driving time 30 minutes. Thereby, the ultrafine fiber was laminated and the nonwoven fabric was obtained.
The obtained ultrafine fibers of the nonwoven fabric were observed with a scanning electron microscope (DS-130 manufactured by Topcon Corporation) at a magnification of 1500 times. Photo images at this time are shown in FIGS. FIG. 5 shows the magnification further increased to 10000 times that of FIG. As shown in these photographs, in Example 1, it was confirmed that good ultrafine fibers having a relatively stable fiber diameter were formed.

Furthermore, 30 locations were selected at random from the short fibers in the region shown in the microscopic image, and the fiber diameter was measured using a scale. As a result of averaging these data to obtain an average fiber diameter, the average fiber diameter of the ultrafine fibers constituting the nonwoven fabric was 0.34 μm.
(Example 2)
A mixed solution of 3 g of a copolymer of VdF and a copolymer of HFP (Kynar 2850, MFR value 0.11 g / 10 min), 7.5 g of DMF and 2.5 g of dimedylacetamide (DMAC) (both manufactured by Nacalai Tesque) Dissolved in. The solution material was electrospun in the same manner as in Example 1 to obtain a nonwoven fabric. As a result of examining the obtained nonwoven fabric, the average fiber diameter of the ultrafine fibers was 0.25 μm.

(Comparative Example 1)
3 g of VdF (Kynar 301F, MFR value 0.03 g / 10 min) was mixed with 10 g of DMF, and shaken with a paint shaker to completely dissolve.
This solution material was supplied to the ESD apparatus in the same manner as in Example 1 (conditions where the injection needle inner diameter was 0.25 mm, the applied voltage was 50 kV, and the distance between the electrodes was 120 mm), and a discharge was obtained on the collection electrode. The obtained discharge was observed with the scanning electron microscope. The photographic images at this time are shown in FIGS. 6 shows the magnification up to 1500 times, and FIG. 7 shows the magnification up to 10,000 times. As shown in these photographs, suitable fibers were not formed in Comparative Example 1, and many granular resin lumps (beads) were confirmed. Therefore, it was found that a nonwoven fabric with good fibers could not be obtained with this solution and apparatus conditions.

(Comparative Example 2)
3 g of a copolymer of VdF and HFP (Kynar 7201, MFR value 3 g / 10 min) was mixed with 7.5 g of DMF (manufactured by Nacalai Tesque) and 2.5 g of DMAC, and completely dissolved by shaking with a paint shaker. The solution material was subjected to electrospinning in the same manner as in Example 1 to obtain a discharged material (conditions of an injection needle inner diameter of 0.25 mm, an applied voltage of 50 kV, and a distance between electrodes of 120 mm). As a result of observing the obtained discharged matter with the scanning electron microscope, fibers were not formed, and many granular resin lumps were confirmed as in Comparative Example 1. Thereby, it turned out that the nonwoven fabric which consists of a suitable fiber is not obtained on these conditions.

(Comparative Example 3)
5 g of a copolymer of VdF and HFP (Kynar 2801, MFR value 0.2 g / 10 min) was mixed with 7.5 g of DMF (manufactured by Nacalai Tesque) and 2.5 g of DMAC, and completely dissolved by shaking with a paint shaker. . This solution material was supplied to the ESD device and driven by the same method as in Example 1 (conditions where the injection needle inner diameter was 0.33 mm, the applied voltage was 40 kV, and the distance between the electrodes was 120 mm). As a result, since the viscosity of the solution was high, the solution was not discharged from the injection needle and could not be spun.

(Comparative Example 4)
1 g of a copolymer of VdF and HFP (Kynar 2801, MFR value 0.2 g / 10 min) was mixed with 7.5 g of DMF (manufactured by Nacalai Tesque) and 2.5 g of DMAC, and completely dissolved by shaking with a paint shaker. . This solution material was supplied to the ESD apparatus and driven by the same method as in Example 1 (conditions where the injection needle inner diameter was 0.15 mm, the applied voltage was 40 kV, and the distance between the electrodes was 120 mm). As a result, the viscosity of the solution was too low to perform spinning.

  The experimental conditions and evaluation for each sample are summarized in Table 1 below.

From the results shown in Table 1, it was found that in Examples 1 and 2, the resin material was formed as ultrafine fibers. Of these, in Example 1, ultrafine fibers having a particularly stable average fiber diameter were obtained, and a nonwoven fabric having excellent uniformity was obtained. In Example 2, although a slight variation was observed in the fiber diameter as compared with Example 1, a nonwoven fabric composed of ultrafine fibers that could still be expected to have sufficient performance as the nonwoven fabric of the present invention was obtained.

  On the other hand, when VdF is used alone as the resin component of the solution material as in Comparative Example 1, the crystallinity of PVdF is too high and a resin lump is generated and spinning cannot be performed, so that a nonwoven fabric cannot be obtained. It was confirmed. Furthermore, when the MFR value was relatively low as in Comparative Example 1 and conversely, when the MFR value was high as in Comparative Example 2, none could be spun. Considering the reason why spinning was impossible in this way, it is possible that the fluidity of the solution material has a direct cause, but it is considered that the crystallinity of the resin material indirectly influenced the spinning result. It is done.

  On the other hand, as shown in Comparative Examples 3 and 4, even when a copolymer of VdF and HFP is used as the resin material and the MFR value is in an appropriate range, if the viscosity of the entire solution material is too high, the injection needle is clogged. On the other hand, there is a problem that the viscosity is too low to form a thread. Since the copolymer is used as the resin material, it is considered that the problem that occurred in Comparative Examples 3 and 4 appeared as a problem different from the crystallinity of the resin material. In other words, it was found that good spinning results cannot be obtained unless both the MFR value and the solution material concentration of the resin material are matched.

Next, after making the resin component and the resin concentration in the solution material constant, the influence of the DMF / DMAC mixed solvent on the nonwoven fabric when the solvent composition ratio was changed was investigated.
The results are shown in Table 2.

As shown in Table 2, nonwoven fabrics could be obtained when the ratio of DMF to DMAC was in the range of 50 to 100 and when 100% of DMAC was used (Examples 3 to 6). On the other hand, as shown in Comparative Example 5, when the ratio of DMF / DMAC was 25/75, the solution was solidified and spinning was impossible.

Further, in Example 3 using 100% DMF, spinning could be performed satisfactorily with a uniform average fiber diameter, and a nonwoven fabric was obtained. In Examples 4 to 6 using a mixed solvent of DMAC and DMF, Example 3 was used. In comparison, there was some variation in the average fiber diameter.
From this experimental result, when either DMF or DMAC should be selected as a solvent, it can be said that DMF is a more suitable solvent for carrying out the present invention.

  In addition, with respect to the solvent composed of DMF / DMAC, it was revealed that the solution material can be solidified and cannot be spun as shown in Comparative Example 5 in the case of a specific composition ratio. Whether the same phenomenon can be confirmed in other solvents is unknown at present. Therefore, regardless of the solvent used, it is desirable to confirm the flow state of the solution just before the ESD method is performed.

Next, the effect on the spinning process when the resin concentration in the solution material was changed while the resin material and the solvent composition were made constant was examined. The resin concentration was changed between 9.09 wt% and 33.33 wt%.
The results are shown in Table 3.

From the results of Table 3, as in Comparative Example 4, it was confirmed that when the resin concentration was 9.09 wt%, the viscosity of the solution was too low and the yarn was not formed well. On the other hand, in the case of Comparative Example 3, it was confirmed that when the resin concentration was 33.33 wt% or higher, the viscosity of the solution was too high and the solution was not discharged from the injection needle.

Therefore, it can be said from this result that in order to carry out the present invention, there is a resin concentration range for appropriately adjusting the viscosity of the solution. Specifically, from the results of Examples 1 and 7 and other experimental results, it is considered that approximately 10 wt% or more and 30 wt% or less is suitable.
Further, considering together with the results of Examples 1 and 2 in Table 1, a range of about 0.1 g / 10 min to less than 3 g / 10 min is appropriate as the MFR value that can be taken by the resin material of the present invention. As the upper limit, it is considered appropriate to be 0.2 g / 10 min or less. The range of the MFR value of the resin material should be set together with the above-described resin concentration range in consideration of the results of the respective examples.

From the above experiments and considerations, the superiority of the present invention was confirmed.
In the above embodiment, the configuration using the VdF-HFP copolymer as the ultrafine fiber copolymer is exemplified, but a polymer (PVdF) may be used as the VdF component.

  The fiber structure for a filtration filter of the present invention can be used as a nonwoven fabric incorporated in a filter unit in, for example, an industrial ultrapure water filtration filter for purifying precision device cleaning water.

It is a figure which shows the structure of the ultrapure water filtration filter unit using the nonwoven fabric of embodiment. It is a perspective view which shows the structural example of the ESD apparatus utilized by embodiment. It is a figure which shows typically the mode at the time of the drive of an ESD apparatus. It is a microscope picture of the nonwoven fabric of Example 1 produced with the ESD apparatus. It is a microscope picture (enlarged photograph) of the nonwoven fabric of Example 1 produced with the ESD apparatus. It is a microscope picture of the discharge material of the comparative example 1 produced with the ESD apparatus. It is a microscope picture (enlarged photograph) of the discharge material of the comparative example 1 produced with the ESD apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ultrapure water filtration filter unit 2 ESD apparatus 10 Axis center 20 Non-woven fabric (fiber structure for filtration filter)
DESCRIPTION OF SYMBOLS 30 Voltage application apparatus 40 Collection electrode unit 50 Apparatus housing 201 1st arm 202 2nd arm 203 3rd arm 217 Injection needle (injection nozzle)
251 Syringe 401 Collection electrode (aluminum foil)

Claims (8)

A fiber structure for a filtration filter, wherein an average fiber diameter is 1 nm or more and less than 5 μm, and fibers made of a copolymer containing VdF and HFP are formed into a nonwoven fabric or a knitted fabric. The fiber structure for a filtration filter according to claim 1, wherein the copolymer is a VdF-HFP copolymer. The fiber structure for a filtration filter according to claim 1 or 2, wherein the copolymer has a random structure. The fiber structure for a filtration filter according to any one of claims 1 to 3, wherein the MFR value of the copolymer is in a range of 0.1 g / 10 min or more and less than 3 g / 10 min. A method for producing a fiber structure for a filtration filter using an electrospinning method,
By collecting the solution material in which the copolymer containing VdF and HFP is dissolved in one polarity, and discharging the solution material in a fibrous shape from the injection needle toward the other polarity collecting electrode, the collecting electrode A method for producing a fibrous structure for a filtration filter, characterized in that a nonwoven fabric or a knitted fabric is formed of fibers made of VdF-HFP copolymer having an average fiber diameter of 1 nm or more and less than 5 μm on a plate. The method for producing a fiber structure for a filtration filter according to claim 5, wherein a random copolymer of VdF and HFP is used as the copolymer containing VdF and HFP. 7. The method for producing a fiber structure for a filtration filter according to claim 5, wherein the copolymer has a MFR value set in a range of 0.1 g / 10 min or more and less than 3 g / 10 min. . 8. The fiber structure for a filtration filter according to claim 7, wherein the solution material is prepared by dissolving the copolymer in an organic solvent in a concentration range of 10 wt% or more and 30 wt% or less. Method.