JP2009061360A - Crystalline polymer microporous film, its manufacturing method, and filtration filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crystalline polymer microporous film which can efficiently capture fine particles, is hardly clogged and has improved scratch resistance while keeping performance of capturing fine particles and has long filtration life and to provide a method for manufacturing the crystalline polymer microporous film with which the crystalline polymer microporous film can be efficiently manufactured and a filtration filter using the crystalline polymer microporous film. <P>SOLUTION: The crystalline polymer microporous film has a crystalline polymer layer, which consists of a crystalline polymer and has a single structure, and many pores each penetrating the crystalline polymer layer in the thickness direction and is characterized in that each of many pores has a small diameter portion the average pore diameter of which is smaller than those of both open ends of each pore, is present in the inside of the crystalline polymer layer. This crystalline polymer microporous film is used in the filtration filter. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a crystalline polymer microporous membrane having high filtration efficiency used for microfiltration of gas, liquid, etc., a method for producing the crystalline polymer microporous membrane, and a filter for filtration.

Microporous membranes have been known for a long time, and are widely used for filtration filters and the like (see Non-Patent Document 1). Examples of such microporous membranes include those using cellulose ester as a raw material (see Patent Documents 1 to 7), those using aliphatic polyamide as a raw material (see Patent Documents 8 to 14), and polyfluorocarbon as a raw material. And the like (see Patent Documents 15 to 18) and those made of polypropylene (see Patent Document 19).
These microporous membranes are used for filtration and sterilization of electronic industrial cleaning water, pharmaceutical water, pharmaceutical manufacturing process water, food water, and the like. Therefore, highly reliable microporous membranes are attracting attention. Among these, the microporous film made of a crystalline polymer has excellent chemical resistance, and in particular, the crystalline polymer microporosity using polytetrafluoroethylene (hereinafter sometimes referred to as “PTFE”) as a raw material. Since the film is excellent in heat resistance and chemical resistance, the growth of the demand is remarkable.

In general, the filterable amount per unit area of the microporous membrane is small (that is, the filtration life is short). For this reason, when industrially used, in order to increase the membrane area, it is necessary to use many filtration units in parallel, and from the viewpoint of cost reduction of the filtration process, the filtration life is increased. Is needed. For example, as a microporous membrane effective for reducing the flow rate due to clogging or the like, an asymmetric membrane is proposed in which the pore diameter gradually decreases from the inlet side toward the outlet side (see Patent Documents 20 and 21).
In addition, a polytetrafluoroethylene multilayer porous membrane comprising a filtration layer having a small pore size and a support layer having a pore size larger than the filtration layer (see Patent Document 22), polytetrafluoroethylene emulsion dispersion on a polytetrafluoroethylene sheet The thing which apply | coated the liquid and extended | stretched (refer patent document 23) etc. is proposed.
However, Patent Documents 22 and 23 have a problem that cracks and defects are likely to occur in the microporous film when applied and dried. Moreover, since only the surface has a small pore diameter, there is a problem that a sufficient filtration life cannot be obtained.

  Moreover, according to Patent Documents 24 and 25, a double-hole sheet extrusion method can be used to produce a filter having a small pore diameter and a support layer having a large pore diameter that are completely integrated, and further having a filter layer inside. However, there is a problem that clogging is likely to occur at the boundary portion (portion where the pore diameter is discontinuous) between the filtration layer and the support layer. Further, since special pretreatment using a plurality of types of materials is necessary, the cost is increased.

  Patent Document 26 proposes an asymmetric microporous film having good scratch resistance due to an internal dense layer. However, the film-forming polymer is limited to specific soluble polymers such as polysulfone and polyethersulfone.

  Patent Document 27 proposes a method for producing an asymmetric pore diameter thin film by applying a temperature difference and a compressive force in the thickness direction of the tetrafluoroethylene resin thin film. However, in this proposal, since the heating temperature is as low as 250 ° C. to the melting point of the tetrafluoroethylene resin, it is impossible to form a hole having a desired shape.

  Therefore, it is composed of a crystalline polymer layer having a single layer structure, can capture fine particles efficiently, has no clogging, has a long filtration life, a method for producing the same, and a filter for filtration. It is the present situation that provision is desired.

US Pat. No. 1,421,341 US Pat. No. 3,133,132 US Pat. No. 2,944,017 Japanese Patent Publication No. 43-15698 Japanese Examined Patent Publication No. 45-3313 Japanese Examined Patent Publication No. 48-39586 Japanese Patent Publication No. 48-40050 US Pat. No. 2,783,894 US Pat. No. 3,408,315 U.S. Pat. No. 4,340,479 US Pat. No. 4,340,480 U.S. Pat. No. 4,450,126 German Patent Invention No. 3,138,525 JP 58-37842 A US Pat. No. 4,196,070 US Pat. No. 4,340,482 JP 55-99934 A JP 58-91732 A West German Patent No. 3,003,400 Japanese Patent Publication No.55-6406 Japanese Examined Patent Publication No. 4-68966 JP-A-4-351645 JP 7-292144 A JP-A-3-179038 JP-A-3-179039 JP-A-1-139116 Japanese Patent Publication No. 63-48562 Published by R. Kesting “Synthetic Polymer Membrane”, McGraw Hill (McGrawHill)

  An object of the present invention is to solve the above-described problems and achieve the following objects. That is, the present invention comprises a crystalline polymer layer having a single layer structure made of a crystalline polymer and a large number of pores penetrating in the thickness direction of the crystalline polymer layer, and the average pore diameter of the pores is In the thickness direction of the crystalline polymer layer, the presence of a fine small-diameter portion that is smaller than the average pore diameter at both ends allows fine particles to be trapped efficiently, and there is no clogging, and the scavenging resistance of the particle trapping ability. Crystalline polymer microporous membrane with improved filterability and long filtration life, method for producing crystalline polymer microporous membrane capable of efficiently producing crystalline polymer microporous membrane, and crystalline polymer An object is to provide a filter for filtration using a microporous membrane.

Means for solving the problems are as follows. That is,
<1> A crystalline polymer microporous film having a crystalline polymer layer having a single layer structure made of a crystalline polymer and a large number of pores penetrating in the thickness direction of the crystalline polymer layer,
The crystalline polymer microporous membrane is characterized in that a small-diameter portion in which the average pore diameter of the pores is smaller than the average pore diameter at both ends is present in the crystalline polymer layer.
<2> The crystalline polymer according to <1>, wherein a difference (A−B) between a maximum average pore diameter A of both end opening average pore diameters and a minimum average pore diameter B of small-diameter portions is 0.5 μm or more. It is a microporous membrane.
<3> The crystalline polymer microporous membrane according to any one of <1> to <2>, wherein an average pore diameter of the pores is changed in a thickness direction of the crystalline polymer layer.
<4> The crystalline polymer microporous membrane according to <3>, wherein the change in average pore size is any one of continuously increasing and continuously decreasing.
<5> The crystalline polymer microporous membrane according to any one of <1> to <4>, wherein the crystalline polymer is polytetrafluoroethylene.
<6> An asymmetric heating step of heating one surface of a crystalline polymer film made of a crystalline polymer and removing heat from the heated surface to form a temperature gradient in the thickness direction of the crystalline polymer film;
And a stretching step of stretching a crystalline polymer film in a state where a temperature gradient is formed.
<7> The method for producing a crystalline polymer microporous film according to <6>, wherein the maximum temperature site in the temperature gradient in the thickness direction of the crystalline polymer layer is present inside the crystalline polymer film.
<8> The method for producing a crystalline polymer microporous film according to any one of <6> to <7>, wherein the heating is irradiation of electromagnetic waves on the crystalline polymer film.
<9> The method for producing a crystalline polymer microporous film according to <8>, wherein the electromagnetic wave is infrared.
<10> The method for producing a crystalline polymer microporous membrane according to any one of <6> to <9>, wherein the crystalline polymer is polytetrafluoroethylene.
<11> The method for producing a crystalline polymer microporous membrane according to any one of <6> to <10>, wherein the stretching is uniaxial stretching.
<12> The method for producing a crystalline polymer microporous membrane according to any one of <6> to <10>, wherein the stretching is biaxial stretching.
<13> A filtration filter using the crystalline polymer microporous membrane according to any one of <1> to <5>.
<14> The filter for filtration according to <13>, wherein a surface of the crystalline polymer microporous membrane having a larger average pore diameter side is used as a filter surface of the filter.

  The crystalline polymer microporous membrane of the present invention comprises a crystalline polymer layer having a single layer structure made of a crystalline polymer and a large number of pores penetrating in the thickness direction of the crystalline polymer layer. In the thickness direction of the polymer layer, the average pore diameter of the pores is smaller than the average pore diameter at both ends, and a dense small-diameter portion exists inside the crystalline polymer layer, so that fine particles can be efficiently captured, There is no clogging, the scratch resistance of the particle capturing ability is improved, and the filtration life can be extended.

The method for producing a crystalline polymer microporous membrane of the present invention includes at least an asymmetric heating step and a stretching step.
In the method for producing a crystalline polymer microporous membrane of the present invention, in the asymmetric heating step, one surface of a crystalline polymer film made of a crystalline polymer is heated and the heated surface is removed, thereby the crystalline polymer film. A temperature gradient is formed in the thickness direction. In the stretching step, the crystalline polymer film in a state where a temperature gradient is formed is stretched. As a result, the average pore diameter of the pores is smaller than the average pore diameter at both ends in the thickness direction of the crystalline polymer layer, and there is a dense small-diameter portion inside, so that fine particles can be captured efficiently. A crystalline polymer microporous membrane having no clogging and a long filtration life can be efficiently produced at low cost by a simple treatment process using a single material.

  Since the filter for filtration of the present invention uses the crystalline polymer microporous membrane of the present invention, it is possible to efficiently capture fine particles by performing filtration with the surface (surface) having a large average pore diameter as the inlet side. Can do. In addition, since the specific surface area is large, the effect that the fine particles are removed by adsorption or adhesion before reaching the minimum pore diameter portion is great, and the filtration life can be greatly improved.

  According to the present invention, the conventional problems can be solved, and it has a single-layered crystalline polymer layer made of a crystalline polymer, and a large number of holes that penetrate in the thickness direction of the crystalline polymer layer, Since the average pore diameter of the pores is smaller than the average pore diameter at both ends in the thickness direction of the crystalline polymer layer, and there are dense small-diameter portions inside, fine particles can be efficiently captured and clogged. A crystalline polymer microporous membrane that can be filtered at a high flow rate and has a long filtration life, and a crystalline polymer microporous membrane that can efficiently produce the crystalline polymer microporous membrane A method and a filter for filtration using the crystalline polymer microporous membrane can be provided.

(Crystalline polymer microporous membrane)
The crystalline polymer microporous membrane of the present invention comprises a crystalline polymer layer having a single layer structure made of a crystalline polymer, and a large number of pores penetrating in the thickness direction of the crystalline polymer layer. Other configurations are provided as necessary.

  The “single layer structure” means that there is no boundary in the crystalline polymer layer, and a multilayer structure formed by laminating or laminating two or more layers is excluded. Here, the presence or absence of a boundary between the crystalline polymer layer and the crystalline polymer layer is observed, for example, by observing a cut surface obtained by cutting the crystalline polymer microporous film in the thickness direction using an optical microscope or a scanning electron microscope (SEM). This can be confirmed.

  In the present invention, in the thickness direction of the crystalline polymer layer, the average pore diameter of the pores is smaller than the average pore diameter at both ends, and a dense small-diameter portion exists inside the crystalline polymer layer. Thereby, fine particles can be captured efficiently, there is no clogging, the scuff resistance of the particle capturing ability is improved, and the filtration life can be extended.

There are no particular restrictions on the shape, size, position, etc. of the small-diameter portion smaller than the average opening diameter at both ends, and it can be appropriately selected according to the purpose. The size is not particularly limited as long as it is smaller than the both-end opening average pore diameter, but the difference between the maximum average pore diameter A of the both-end opening average pore diameter and the minimum average pore diameter B of the small-diameter portions (AB). Is preferably 0.5 μm or more, more preferably 1 μm to 10 μm. If the difference is less than 0.5 μm, fine particles cannot be trapped efficiently, so that clogging is likely to occur and high flow rate filtration may not be possible. In addition, when both end opening average hole diameters are the same magnitude | size, either one average hole diameter becomes the largest average hole diameter A.
The position is not particularly limited as long as it is inside the crystalline polymer layer and can be appropriately selected according to the purpose, but it is preferably 5 μm or more in the thickness direction from the surface of the crystalline polymer layer.

Moreover, in this invention, it is preferable that the average hole diameter of a hole is changing to the thickness direction of a crystalline polymer layer.
The above-mentioned “the average pore diameter of the pores is changing in the thickness direction of the crystalline polymer layer” means that the horizontal axis indicates the distance d in the thickness direction from the front surface of the crystalline polymer microporous membrane (mainly (1) From the front surface (d = 0) to the back surface (d = film thickness) The graph is drawn with one continuous line (continuous), and the slope (dD / dt) of the graph is a negative area (decrease) or a positive slope (increase). If there is, the case where the inclination is 0 (zero) may be included. The case where the inclination is 0 (zero) in the entire microporous membrane (no change) is not included.
Among these, it is preferable that the average pore diameter of the pores in the crystalline polymer layer is either continuously increasing or continuously decreasing from the front surface to the back surface. It is particularly preferable that the graph from the front surface to the back surface continuously decreases and increases continuously.

  In the present invention, the surface of the crystalline polymer microporous membrane having the larger average pore diameter is referred to as “front surface”, and the surface having the smaller average pore diameter is referred to as “back surface”. However, this is merely a name given for convenience in order to make the description of the present invention easier to understand, and any surface may be a “back surface”.

Here, as shown in FIG. 1, the pore diameter of the pore 101a in the crystalline polymer microporous membrane of the present invention comprising the crystalline polymer layer 101 having a single-layer structure is smaller than the both-end opening average pore diameter, and has a fine small diameter. The part 101c exists inside the crystalline polymer layer, and the hole diameter of the hole 101a changes (continuously decreases and continuously increases) in the thickness direction of the crystalline polymer layer.
On the other hand, as shown in FIG. 2, it is a conventional crystalline polymer microporous film having a three-layer structure in which crystalline polymer layers 102, 103, and 104 are laminated, and has a small-diameter portion smaller than the average pore diameter at both ends. Although the layer 103 exists inside the crystalline polymer microporous membrane, it has a multilayer structure, and the pore diameters of the holes 102b, 103b, 104b are all in the thickness direction of each crystalline polymer layer. There is a portion in which the pore diameter changes stepwise in the thickness direction when the entire crystalline polymer microporous membrane is not changed.

  Here, the average pore diameter of the hole is, for example, a photograph of the film surface (SEM photograph, magnification 1,000 times to ~) with a scanning electron microscope (Hitachi S-4000 type, vapor deposition is Hitachi E1030 type, both manufactured by Hitachi, Ltd.) 5,000 times) and the obtained photograph is processed into an image processing apparatus (main body name: Nippon Avionics Co., Ltd., TV image processor TVIP-4100II, control software name: Ratok System Engineering Co., Ltd., TV image processor image command 4198 ) To obtain an image composed only of crystalline polymer fibers, and the average pore size is obtained by calculating the image.

-Crystalline polymer-
In the present invention, the “crystalline polymer” means a polymer in which a crystalline region in which long chain molecules are regularly arranged in a molecular structure and an amorphous region that is not regularly arranged are mixed. Such polymers exhibit crystallinity by physical treatment. For example, when a polyethylene film is stretched by an external force, a phenomenon that a transparent film becomes cloudy at first is recognized. This is because the crystallinity is expressed by aligning the molecular arrangement in the polymer in one direction by an external force.

The crystalline polymer is not particularly limited and may be appropriately selected depending on the intended purpose. Examples thereof include polyalkylenes, polyesters, polyamides, polyethers, and liquid crystalline polymers. Specific examples include polyethylene, Examples include polypropylene, nylon, polyacetal, polybutylene terephthalate, polyethylene terephthalate, syndiotactic polystyrene, polyphenylene sulfide, polyether ether ketone, wholly aromatic polyamide, wholly aromatic polyester, fluororesin, and polyether nitrile.
Among these, from the viewpoint of chemical resistance and handleability, polyalkylene (for example, polyethylene and polypropylene) is preferable, and fluorine-based polyalkylene in which the hydrogen atoms of the alkylene group in the polyalkylene are partially or entirely substituted with fluorine atoms. Alkylene is more preferred, and polytetrafluoroethylene (PTFE) is particularly preferred.
The density of the polyethylene varies depending on the degree of branching, the degree of branching is high, and the degree of crystallization is low density polyethylene (LDPE), the degree of branching is low and the degree of crystallization is high density polyethylene (HDPE). Any of these can be used. Among these, HDPE is particularly preferable from the viewpoint of crystallinity control.

  As the polytetrafluoroethylene, polytetrafluoroethylene produced by an emulsion polymerization method can be usually used, and a finely divided polytetrafluoroethylene obtained by coagulating an aqueous dispersion obtained by emulsion polymerization. It is preferred to use ethylene.

  There is no restriction | limiting in particular as said polytetrafluoroethylene, According to the objective, it can select suitably, A commercial item can be used. Examples of the commercially available products include polyflon PTFE F-104, polyflon PTFE F-201, polyflon PTFE F-205, polyflon PTFE F-207, polyflon PTFE F-301 (all manufactured by Daikin Industries, Ltd.); Fluon PTFE CD1, Fluon PTFE CD141, Fluon PTFE CD145, Fluon PTFE CD123, Fluon PTFE CD076, Fluon PTFE CD090 (all manufactured by Asahi Glass Co., Ltd.); Teflon (registered trademark) PTFE 6-J, Teflon (registered trademark) TFElon (Registered Trademark) PTFE 6C-J, Teflon (Registered Trademark) PTFE 640-J (all manufactured by Mitsui DuPont Fluorochemical Co., Ltd.), and the like. Among these, F-104, CD1, CD141, CD145, CD123, 6-J are preferable, F-104, CD1, CD123, 6-J are more preferable, and CD123 is particularly preferable.

The crystalline polymer preferably has a glass transition temperature of 40 ° C to 400 ° C, more preferably 50 ° C to 350 ° C.
The crystalline polymer preferably has a mass average molecular weight of 1,000 to 100,000,000.
The number average molecular weight of the crystalline polymer is preferably 500 to 50,000,000, more preferably 1,000 to 10,000,000.

  The film thickness of the crystalline polymer microporous film is preferably 1 μm to 300 μm, more preferably 5 μm to 100 μm, and still more preferably 10 μm to 80 μm.

(Method for producing crystalline polymer microporous membrane)
The method for producing a crystalline polymer microporous membrane of the present invention comprises at least an asymmetric heating step and a stretching step, and further comprises a crystalline polymer film production step and, if necessary, other steps.

-Crystalline polymer film production process-
The crystalline polymer film production step is a step of producing a crystalline polymer film by producing a mixture in which a crystalline polymer is mixed with an extrusion aid, and extruding and rolling the mixture.
The crystalline polymer can be appropriately selected from those described above according to the purpose.
As the extrusion aid, a liquid lubricant is preferably used, and specific examples thereof include solvent naphtha and white oil. As the extrusion aid, commercially available products can be used, and for example, hydrocarbon oil such as “Isopar” manufactured by Esso Petroleum Corporation may be used. The addition amount of the extrusion aid is preferably 20 to 30 parts by mass with respect to 100 parts by mass of the crystalline polymer.

Paste extrusion is usually preferably performed at 50 ° C to 80 ° C. There is no restriction | limiting in particular about extrusion shape, Although it can select suitably according to the objective, Usually, it is preferable to make it rod-shaped. The extrudate is then rolled into a film. The rolling can be performed, for example, by calendaring with a calendar roll at a speed of 50 m / min. The rolling temperature can usually be set to 50 ° C to 70 ° C.
Then, it is preferable to remove the extrusion aid by heating and drying the film to obtain a crystalline polymer unheated film. There is no restriction | limiting in particular as heating temperature at this time, Although it can select suitably according to the kind of crystalline polymer to be used, 40 to 400 degreeC is preferable and 60 to 350 degreeC is more preferable. For example, when polytetrafluoroethylene is used as the crystalline polymer, 150 ° C. to 280 ° C. is preferable, and 200 ° C. to 255 ° C. is more preferable.
The heating can be performed by a method such as passing the film through a hot air drying furnace. The thickness of the crystalline polymer unheated film thus produced can be adjusted as appropriate according to the thickness of the crystalline polymer microporous film to be finally produced, and the thickness by stretching in the subsequent process. It is necessary to adjust in consideration of the decrease of
In the production of the crystalline polymer unheated film, the items described in “Polyfluorocarbon Handbook” (issued by Daikin Industries, Ltd., revised in 1983) can be appropriately employed.

-Asymmetric heating process-
The asymmetric heating step is a step of heating one surface of a crystalline polymer film made of a crystalline polymer and removing heat from the heated surface to form a temperature gradient in the thickness direction of the crystalline polymer film.
The above-mentioned “heating and heat removal of the heated surface” means that heating and heat removal are performed almost simultaneously on the same surface. A mode in which heat is removed after heating, a mode in which heat is removed from the beginning of heat removal, a mode in which heating and heat removal are performed. Although all the aspects which heat simultaneously are included, the aspect which removes heat after starting a heating is especially preferable.
The degree of heating and heat removal is preferably adjusted so that the surface temperature of the crystalline polymer film on the side to be heated and removed is maintained at 30 ° C to 327 ° C.
The maximum temperature site in the temperature gradient in the thickness direction of the crystalline polymer layer is not in the surface of the crystalline polymer film but in the inside, so that a dense small diameter site can be formed in the crystalline polymer layer. preferable.

  The heating method is not particularly limited and may be appropriately selected according to the purpose. (1) A method of blowing hot air on the crystalline polymer film, (2) A method of contacting the crystalline polymer film with a heating medium, (3) A method of bringing the crystalline polymer film into contact with the heating member, (4) a method of irradiating the crystalline polymer film with electromagnetic waves, and the like. Therefore, electromagnetic wave irradiation is preferable. Examples of the electromagnetic wave include X-rays, gamma rays, electron beams, microwaves, infrared rays, and the like. Among these, infrared rays are particularly preferable because they are suitable for heating the surface layer.

There is no restriction | limiting in particular as said infrared rays, According to the objective, it can select suitably.
The general definition of the infrared can be referred to “practical infrared” (Human and History, published in 1992). The infrared ray means an electromagnetic wave having a wavelength of 0.74 μm to 1,000 μm, of which the range of 0.74 μm to 3 μm is near infrared, and the range of 3 μm to 1,000 μm is far infrared.
The type of infrared device is not particularly limited as long as it can irradiate infrared rays having a target wavelength, and can be appropriately selected according to the purpose. In general, near infrared rays are bulbs (halogen lamps), far infrared rays are A heating element such as ceramic, quartz, or metal oxide surface can be used.
In addition, if infrared irradiation is used, it is possible to perform asymmetric heating continuously in a flow operation industrially, and temperature control and maintenance of the apparatus are easy. Moreover, since it is non-contact, it does not cause defects such as cleanness and fluff.
The film surface temperature by the infrared irradiation can be controlled by the output of the infrared irradiation apparatus, the distance between the infrared irradiation apparatus and the film surface, the irradiation time (conveying speed), and the atmospheric temperature, and is set to the temperature at which the half-heated body is formed. However, 324 ° C to 380 ° C is preferable, and 335 ° C to 360 ° C is more preferable. When the surface temperature is less than 324 ° C., the crystal state does not change and the pore diameter control may not be possible. When the surface temperature exceeds 380 ° C., the crystalline polymer film melts and the shape is excessively deformed. Thermal decomposition of the crystalline polymer may occur.
The irradiation time of the infrared rays is not particularly limited, and is a time necessary for the target asymmetric heating to sufficiently proceed, preferably 30 seconds to 120 seconds, more preferably 45 seconds to 90 seconds, and 60 seconds to 80 seconds is more preferable.
The infrared irradiation in the asymmetric heating may be performed continuously, or may be performed intermittently after being divided into several times.

The heat removal method is not particularly limited and may be appropriately selected depending on the intended purpose. For example, (1) a method of blowing cold air on the surface of the crystalline polymer film, (2) a refrigerant on the surface of the crystalline polymer film Various methods such as a method of contacting, (3) a method of bringing a cooling member into contact with the surface of the crystalline polymer film, and (4) cooling by allowing to cool can be used. Among these, combinations when combined with the heating method described above From the viewpoint of easiness, (3) a method of spraying cold air on the surface of the crystalline polymer film is particularly preferable.
The temperature of the cold air is preferably 0 ° C. to 50 ° C., and the flow rate of the cold air is preferably 10 L / min to 2,000 L / min.

-Stretching process-
The stretching step is a step of stretching the crystalline polymer film in a state where a temperature gradient is formed.
The stretching is preferably performed in both the longitudinal direction and the width direction. Each of the longitudinal direction and the width direction may be sequentially stretched, or biaxially stretched simultaneously.
When sequentially stretching in the longitudinal direction and the width direction, respectively, it is preferable to first stretch in the longitudinal direction and then stretch in the width direction.
The stretching ratio in the longitudinal direction is preferably 4 to 100 times, more preferably 8 to 90 times, and still more preferably 10 to 80 times. The stretching temperature in the longitudinal direction is preferably 100 ° C to 300 ° C, more preferably 200 ° C to 290 ° C, and particularly preferably 250 ° C to 280 ° C.
The stretching ratio in the width direction is preferably 10 to 100 times, more preferably 12 to 90 times, still more preferably 15 to 70 times, and particularly preferably 20 to 40 times. The stretching temperature in the width direction is preferably 100 ° C to 300 ° C, more preferably 200 ° C to 290 ° C, and particularly preferably 250 ° C to 280 ° C.
The area stretch ratio is preferably 50 times to 250 times, more preferably 75 times to 200 times, and still more preferably 100 times to 150 times. When stretching is performed, the crystalline polymer film may be preheated to a temperature equal to or lower than the stretching temperature in advance.
In addition, after extending | stretching, heat setting can be performed as needed. The heat setting temperature is usually preferably not lower than the stretching temperature and lower than the melting point of the crystalline polymer.

  The crystalline polymer microporous membrane of the present invention can be used for various applications, and in particular, can be suitably used as a filter for filtration described below.

(Filter for filtration)
The filter for filtration of the present invention uses the crystalline polymer microporous membrane of the present invention.
When the crystalline polymer microporous membrane of the present invention is used as a filter for filtration, it is preferable to perform filtration with its surface (surface having a large average pore diameter) as the inlet side. That is, the surface side having a large pore size is used as the filter surface of the filter. In this way, fine particles can be efficiently captured by performing filtration with the surface (surface) having a large average pore diameter as the inlet side.
Further, since the crystalline polymer microporous membrane of the present invention has a large specific surface area, the fine particles introduced from the surface are removed by adsorption or adhesion before reaching the minimum pore diameter portion. Therefore, clogging is unlikely to occur and high filtration efficiency can be maintained over a long period of time.

The filtration filter of the present invention can be filtered at least 5 ml / cm ² · min or more when filtration is performed at a differential pressure of 0.1 kg / cm ² .
The filter for filtration of the present invention is preferably processed and formed into a pleated shape. Processing into a pleated shape has the advantage that the effective surface area used for filtering the filter per cartridge can be increased.

  Since the filter for filtration using the crystalline polymer microporous membrane of the present invention has such a characteristic that the filtration function is high and the life is long, the filtration device can be compactly assembled. In the conventional filtration apparatus, a large number of filtration units are used in parallel to cope with the short filtration life. However, if the filtration filter of the present invention is used, the number of filtration units used in parallel is greatly increased. Can be reduced. Moreover, since the replacement period of the filter for filtration can be extended significantly, the cost and time required for maintenance can be reduced.

  The filter for filtration of the present invention can be used in various situations where filtration is required, and is preferably used for microfiltration of gases, liquids, etc., for example, corrosive gas, various types used in the semiconductor industry. It is used for filtration and sterilization of gas, etc., washing water for electronics industry, pharmaceutical water, pharmaceutical manufacturing process water, food water and the like. In particular, since the filter for filtration of the present invention is excellent in heat resistance and chemical resistance, it can be effectively used for high-temperature filtration and filtration of reactive chemicals that cannot be handled by conventional filter for filtration.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

(Example 1)
-Production of polytetrafluoroethylene film-
100 parts by mass of a polytetrafluoroethylene fine powder (Daikin Industries, Ltd., “Polyflon Fine Powder F104U”) having a number average molecular weight of 6.2 million as a crystalline polymer, and a hydrocarbon oil (Esso Oil Co., Ltd.) as an extrusion aid “Isopar M”) was added in an amount of 27 parts by mass, and paste extrusion was performed in a round bar shape. This was calendered with a calender roll heated to 60 ° C. at a speed of 50 m / min to produce a polytetrafluoroethylene film. The resulting film was passed through a hot air drying oven at 250 ° C. to remove the extrusion aid, and a polytetrafluoroethylene film having an average thickness of 100 μm, an average width of 150 mm, and a specific gravity of 1.55 was produced.

-Production of semi-heated film-
The polytetrafluoroethylene film thus obtained was set in advance on a 50 ° C. roll (surface material SUS316) on one side with a near-infrared ray using a halogen heater with a built-in tungsten filament, so that the film surface temperature would be 360 ° C. did. Next, while blowing 10 ° C. cooling air at a flow rate of 500 L / min, near-infrared irradiation under the same conditions was performed for 1 minute (asymmetric heating treatment) to produce a semi-heated film. During this treatment, the surface temperature of the film was kept at 50 ° C.

-Production of polytetrafluoroethylene microporous membrane-
The obtained semi-heated film was stretched between rolls by 12.5 times in the longitudinal direction at 270 ° C. and once wound on a take-up roll. Then, after preheating the film to 305 ° C., both ends were sandwiched between clips and stretched 30 times in the width direction at 270 ° C. Thereafter, heat setting was performed at 380 ° C. The area stretch ratio of the obtained stretched film was 260 times in terms of stretched area ratio. Thus, the polytetrafluoroethylene microporous membrane of Example 1 was produced.

(Example 2)
-Production of polytetrafluoroethylene microporous membrane-
In Example 1, the polytetrafluoroethylene microporous film of Example 2 was prepared in the same manner as in Example 1 except that one surface of the polytetrafluoroethylene film was heated by far infrared rays using a ceramic heater. Produced.

(Comparative Example 1)
-Fabrication of crystalline polymer microporous membrane-
In Example 1, a polytetrafluoroethylene microporous membrane of Comparative Example 1 was produced in the same manner as in Example 1 except that the asymmetric heating treatment was performed in an atmosphere at 25 ° C. without blowing cooling air. .

(Comparative Example 2)
-Fabrication of crystalline polymer microporous membrane-
A polytetrafluoroethylene microporous membrane of Comparative Example 2 was produced in the same manner as in Example 1 except that the asymmetric heating treatment was not performed in Example 1.

(Comparative Example 3)
<Preparation of polytetrafluoroethylene microporous membrane>
-Preparation of preform-
As a crystalline polymer, 100 parts by mass of polytetrafluoroethylene fine powder (Asahi Glass Co., Ltd., “Fluon PTFE CD1”) having a number average molecular weight of 1,000,000, and a hydrocarbon oil (Esso Oil Co., Ltd., “ Isopar M ”)) 27 parts by weight was added. This was designated as paste 1.
Similarly, 100 parts by mass of polytetrafluoroethylene fine powder having a number average molecular weight of 10 million (Asahi Glass Co., Ltd., “Fluon PTFE CD123”) and hydrocarbon oil (Esso Oil Co., Ltd. M ") 27 parts by weight were added. This was designated as paste 2.
Next, paste 1 and paste 2 are spread as shown in FIG. 3 so that the thickness ratio (paste 1 / paste 2) is 4/1 (in FIG. 3, paste 1 is 5 and paste 2 is 4). And pressurized to form a preform 10 (see FIG. 4).
In the subsequent processing, the paste 1 (5) side was the “front surface” and the paste 2 (4) side was the “back surface”.

-Production of unheated film-
The obtained preform 10 was inserted into a paste extrusion mold as shown in FIG. 5, and a multilayer paste was extruded into a sheet shape. This was calendered with a calender roll heated to 60 ° C. to produce a multilayer polytetrafluoroethylene film. This film was passed through a hot air drying oven at 250 ° C. to remove the extrusion aid, and a polytetrafluoroethylene multilayer unheated film having an average thickness of 100 μm, an average width of 150 mm, and a specific gravity of 1.55 was produced.

-Production of semi-heated film-
Both surfaces of the polytetrafluoroethylene multilayer unheated film were heated at 345 ° C. for 1 minute using an oven to prepare a multilayer semi-heated film.

-Production of microporous membrane-
The obtained multilayer semi-heated film was stretched between rolls by 12.5 times in the longitudinal direction at 270 ° C. and once wound on a winding roll. Then, after preheating the film to 305 ° C., both ends were sandwiched between clips and stretched 30 times in the width direction at 270 ° C. Thereafter, heat setting was performed at 380 ° C. The area stretch ratio of the obtained stretched film was 260 times in terms of stretched area ratio. Thus, the polytetrafluoroethylene microporous membrane of Comparative Example 3 was produced.

  Next, for each of the prepared polytetrafluoroethylene microporous membranes of Examples 1-2 and Comparative Examples 1-3, the average film thickness and the average pore diameter in the thickness direction from the front surface were as follows. Measurement and scratch resistance test were conducted.

<Measurement of average film thickness>
The film thickness of each of the polytetrafluoroethylene microporous membranes of Examples 1 to 3 and Comparative Examples 1 to 2 was measured with a dial-type thickness gauge (K402B, manufactured by Anritsu Corporation). In addition, arbitrary three places were measured and the average value was calculated | required. The results are shown in Table 1.

<Measurement of average pore diameter in thickness direction from front surface>
About each polytetrafluoroethylene microporous film | membrane of Examples 1-2 and Comparative Examples 1-3, the film thickness of a microporous film | membrane is set to 20, and 0 (namely, surface layer) part in the thickness direction from the front surface The average pore diameter at P0, the average pore diameter at 1 in the thickness direction as P1,..., The average pore diameter at 20 in the thickness direction as P20, and P0 to P20 were determined from SEM photographs of the film cross section. Here, the SEM photograph was taken with a scanning electron microscope (Hitachi S-4000 type, vapor deposition was Hitachi E1030 type, both manufactured by Hitachi, Ltd.), and the magnification was 1,000 to 5,000 times. An image consisting only of polytetrafluoroethylene fibers is taken into a processing device (main body name: Nippon Avionics Co., Ltd., TV image processor TVIP-4100II, control software name: Ratok System Engineering Co., Ltd., TV image processor image command 4198). The average pore diameter was obtained by computing the image.
Based on the values of P0 to P20 determined as described above, the average pore diameter (μm) at the distance (μm) in the thickness direction from the front surface was plotted. The results are shown in FIGS.

From the results shown in FIG. 6, the polytetrafluoroethylene microporous membrane of Example 1 has a small-diameter portion smaller than the both-end opening average pore diameter (maximum average pore diameter A = 5 μm) inside the single-layered crystalline polymer layer. There is a minimum average pore size B = 1 μm), and the average pore size of the pores changes in the thickness direction of the crystalline polymer layer (continuously decreases and increases continuously), and the crystalline polymer microporosity As a whole film, it was found that the hole diameter of the hole changed (decreased stepwise) in the thickness direction.
Further, from the results of FIG. 7, a small diameter portion (minimum average pore diameter B = 1 μm) smaller than the average pore diameter at both ends (maximum average pore diameter A = 5 μm) inside the crystalline polymer layer having a single-layer structure of Example 2. The average pore diameter of the pores changes in the thickness direction of the crystalline polymer layer (continuously decreases and increases continuously), and the entire crystalline polymer microporous membrane has It was found that the pore diameter changed (decreased stepwise) in the thickness direction.
On the other hand, from the results of FIG. 8, in the polytetrafluoroethylene microporous film of Comparative Example 1, the average pore diameter of the pores in the crystalline polymer layer having a single layer structure changes in the thickness direction of the crystalline polymer layer. It was found that there was no small diameter portion smaller than the average pore diameter at both ends in the crystalline polymer layer.
From the results of FIG. 9, the polytetrafluoroethylene microporous membrane of Comparative Example 2 has a single-layered crystalline polymer layer with a small-diameter portion smaller than the average pore diameter at both ends in the crystalline polymer layer. It was found that the average pore diameter of the pores did not change in the thickness direction of the crystalline polymer layer.
From the results shown in FIG. 10, the polytetrafluoroethylene microporous membrane of Comparative Example 3 is a microporous membrane having a three-layer structure, and the average pore size at both ends (maximum average pore size) is formed inside the microporous membrane. A layer (smallest average pore diameter B = 1 μm) having a smaller diameter portion than A = 3 μm) exists, but the average pore diameter of the pores in each crystalline polymer layer does not change in the thickness direction, and as a whole As a result, it was found that the average pore diameter changed intermittently.

<Abrasion resistance test>
(1) From the microporous membranes of Examples 1 and 2 and Comparative Example 1, a rectangle of TD direction (transverse direction) 180 mm × MD direction (film formation direction) 50 mm was cut out, and this was formed on a smooth plate. A porous membrane is attached with the back side facing up (Sample A). It is attached to the moving stage of the tensile tester so as to be horizontal (see FIG. 11).
(2) From the microporous membranes of Examples 1 and 2 and Comparative Example 1, cut into a rectangular shape with a size of 80 mm in the TD direction and 18 mm in the MD direction. The film was attached so that the surface of the conductive film was on top (Sample B). Further, a thread was attached to the center of the 18 mm side.
(3) After the sample B was placed on the sample A, the yarn of the sample B was fixed to the load cell via a pulley on the moving stage of the tensile tester (see FIG. 11).
(4) The moving stage was lowered at a speed of 200 mm / min.
(5) About the test part of sample A, the aqueous solution containing 0.01 mass% of polystyrene microparticles | fine-particles (average particle size of 1.0 micrometer) was filtered by making 0.1 kg of differential pressure | voltages. The presence or absence of particle leakage was evaluated. The results are shown in Table 2.

From the results shown in Table 2, the microporous membrane of Comparative Example 1 had poor durability because the dense layer was present on the back surface, and particle leakage occurred in the scratch resistance test.
In contrast, Examples 1 and 2 were found to have excellent durability because there was no particle leakage even after the abrasion resistance test because the dense small-diameter portion was inside the microporous membrane.

<Filtration test>
Next, the filtration test was done about each crystalline polymer microporous film | membrane of Examples 1-2 and Comparative Examples 1-3. First, an aqueous solution containing 0.01% by mass of polystyrene latex (average particle size 0.17 μm) was filtered with a differential pressure of 0.1 kg. The results are shown in Table 3.

From the results of Table 3, the microporous membrane of Comparative Example 3 was substantially clogged at 800 ml / cm ² . The microporous membrane of Comparative Example 2 was not measurable because of the large pore size and insufficient particulate supplementation.
In contrast, the microporous films of Examples 1 and 2 and Comparative Example 1 is capable of filtering each ^{1500ml / cm 2, 1500ml / cm} 2, 1500ml / cm 2, until the crystalline polymers of the present invention It has been found that using a microporous membrane significantly improves the filtration life.

  The crystalline polymer microporous membrane of the present invention and the filter for filtration using the same can capture fine particles efficiently over a long period of time, improve the scuff resistance of the particle capturing ability, and have heat resistance and chemical resistance. Therefore, it can be used in various situations where filtration is required, and is suitably used for microfiltration of gases, liquids, etc., for example, corrosive gases, various gases used in the semiconductor industry, etc. It can be used for a wide variety of purposes, such as water filtration, electronics industrial cleaning water, pharmaceutical water, pharmaceutical manufacturing process water, food water, etc., sterilization, high temperature filtration, and reactive chemical filtration.

FIG. 1 is a schematic view showing an example of the crystalline polymer microporous membrane of the present invention. FIG. 2 is a schematic view showing an example of a conventional crystalline polymer microporous membrane. FIG. 3 is a diagram showing an example of the production process of the crystalline polymer microporous membrane of Comparative Example 3. FIG. 4 is a view showing an example of a preformed body of Comparative Example 3. FIG. 5 is a diagram showing another example of the manufacturing process of the crystalline polymer microporous membrane of Comparative Example 3. FIG. 6 is a graph showing the relationship between the average hole diameter of the holes and the distance from the surface in Example 1. FIG. 7 is a graph showing the relationship between the average hole diameter of the holes and the distance from the surface in Example 2. FIG. 8 is a graph showing the relationship between the average hole diameter of the holes and the distance from the surface in Comparative Example 1. FIG. 9 is a graph showing the relationship between the average hole diameter of the holes and the distance from the surface in Comparative Example 2. FIG. 10 is a graph showing the relationship between the average hole diameter of the holes and the distance from the surface in Comparative Example 3. FIG. 11 is a diagram illustrating an example of a tensile tester used for the scratch resistance test.

Explanation of symbols

4 First layer 5 Second layer 8 Lower mold 10 Preliminary molded body 15 Polytetrafluoroethylene unheated film 21 500 g plate with film attached 22 Film 23 Horizontally mounted sample table 24 Pulley 25 Moving stage of tensile tester 26 Load cell 27 Tensile tester 101 Crystalline polymer layer (single layer)
102, 103 Crystalline polymer layer (multilayer)
101a hole 102b, 103b hole

Claims (13)

A crystalline polymer microporous membrane having a crystalline polymer layer having a single layer structure made of a crystalline polymer and a large number of pores penetrating in the thickness direction of the crystalline polymer layer,
A crystalline polymer microporous film characterized in that a small-diameter portion having an average pore diameter of the pores smaller than the average pore diameter at both ends is present in the crystalline polymer layer.   2. The crystalline polymer microporous membrane according to claim 1, wherein a difference (A−B) between a maximum average pore diameter A of both end opening average pore diameters and a minimum average pore diameter B of small-diameter portions is 0.5 μm or more. .   The crystalline polymer microporous membrane according to any one of claims 1 to 2, wherein an average pore diameter of the pores varies in the thickness direction of the crystalline polymer layer.   4. The crystalline polymer microporous membrane according to claim 3, wherein the change in average pore diameter is either continuously increasing or continuously decreasing.   The crystalline polymer microporous membrane according to any one of claims 1 to 4, wherein the crystalline polymer is polytetrafluoroethylene. An asymmetric heating step of heating one surface of the crystalline polymer film made of the crystalline polymer and removing heat from the heated surface to form a temperature gradient in the thickness direction of the crystalline polymer film;
A method for producing a crystalline polymer microporous membrane comprising at least a stretching step of stretching a crystalline polymer film in a state where a temperature gradient is formed.   The method for producing a crystalline polymer microporous membrane according to claim 6, wherein the maximum temperature site in the temperature gradient in the thickness direction of the crystalline polymer layer is present inside the crystalline polymer film.   The method for producing a crystalline polymer microporous film according to any one of claims 6 to 7, wherein the heating is irradiation of electromagnetic waves on the crystalline polymer film.   The method for producing a crystalline polymer microporous film according to claim 8, wherein the electromagnetic wave is infrared.   The method for producing a crystalline polymer microporous membrane according to any one of claims 6 to 9, wherein the crystalline polymer is polytetrafluoroethylene.   The method for producing a crystalline polymer microporous membrane according to any one of claims 6 to 10, wherein the stretching is uniaxial stretching.   The method for producing a crystalline polymer microporous membrane according to any one of claims 6 to 10, wherein the stretching is biaxial stretching.   A filter for filtration, characterized in that the crystalline polymer microporous membrane according to any one of claims 1 to 5 is used.