Crystalline polymer microporous membrane, method for manufacturing the same, and filter for filtration

The present invention relates to a highly efficient crystalline polymer microporous membrane used for microfiltration of gases, liquids, 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 produced from cellulose ester as a raw material (see Patent Document 1 and the like), those produced from an aliphatic polyamide (see Patent Document 2 and the like), and polyfluorocarbon as a raw material. (Refer to Patent Document 3 and the like), those using polypropylene as a raw material (see Patent Document 4 and the like), and the like. These microporous membranes are used for filtration and sterilization of washing water for electronic industry, pharmaceutical water, water for pharmaceutical manufacturing process, food water, etc., and their applications and amounts have been expanding in recent years, and the point of particle capture. Therefore, highly reliable microporous membranes are attracting attention. Among these, the microporous membrane made of crystalline polymer has excellent chemical resistance, and in particular, the microporous membrane made of polytetrafluoroethylene (PTFE) has excellent heat resistance and chemical resistance. , The growth of the demand is remarkable.

Further, in Patent Document 5, the tape is elongated by uniaxially extending at a temperature lower than the crystal melting point of the polytetrafluoroethylene component and raising the temperature of the tape to a temperature higher than the crystal melting point of the polytetrafluoroethylene component. A method for producing a porous polytetrafluoroethylene article has been proposed, which comprises a step of amorphously fixing the tape and extending it in a direction perpendicular to the original elongation direction at a temperature exceeding the crystal melting point of the polytetrafluoroethylene component. According to this proposal, the filtration flow rate can be increased. However, the above proposal has a problem that the amount of the microporous membrane that can be filtered per unit area is small (that is, the filtration life is short).

Further, Patent Document 6 proposes a method for producing a crystalline polymer microporous film, which comprises a semi-baking step of applying thermal energy to the surface of an unfired film to form a temperature gradient in the thickness direction of the film. .. According to this proposal, multi-stage filtration is possible due to the asymmetrical micropores, and the filtration life of the microporous membrane can be extended. However, this proposal has a problem that it is not possible to produce a microporous membrane having an increased filtration flow rate. Further, in the method for producing a crystalline polymer microporous film by a semi-baking step, it is difficult to form a thin film because a temperature gradient cannot be formed on the film without a certain film thickness.

<patcit num="1"><text>U.S. Pat. No. 1,421,341</text></patcit><patcit num="2"><text>U.S. Pat. No. 2,783,894</text></patcit><patcit num="3"><text>U.S. Pat. No. 4,196,070</text></patcit><patcit num="4"><text>West German Patent No. 3,003,400</text></patcit><patcit num="5"><text>Special Table No. 11-515036 Gazette</text></patcit><patcit num="6"><text>JP-A-2007-332342</text></patcit><nplcit num="1"><text>"Synthetic Polymer Membrane" by R. Kesting, published by McGraw Hill</text></nplcit>

<p> An object of the present invention is to solve the above-mentioned problems in the past and to achieve the following object. That is, the present invention efficiently captures fine particles, does not clog, has a high flow rate, and has a long filtration life, and efficiently obtains the crystalline polymer microporous membrane and the crystalline polymer microporous membrane. It is an object of the present invention to provide a method for producing a crystalline polymer microporous membrane that can be produced, and a filter for filtration using the crystalline polymer microporous membrane.</p>

<p> The means for solving the above-mentioned problems are as follows. That is, &lt;1&gt; The average pore diameter of one surface is larger than the average pore diameter of the other surface, and the average pore diameter continuously changes from one surface to the other surface. Crystalline polymer microporous property, characterized in that one of the surface sides has a microstructure consisting of a series of nodules interconnected by fibrils, and the aspect ratio (length / width) of the nodules is 25 or more. It is a membrane. &lt;2&gt; The crystalline polymer microporous film according to &lt;1&gt;, wherein the microstructure is a region within 90% of the total thickness of the film in the thickness direction from one surface. &lt;3&gt; The crystalline polymer microporous film according to any one of &lt;1&gt; to &lt;2&gt;, wherein the film thickness is 50 μm or less. &lt;4&gt; The crystalline polymer microporous membrane according to any one of &lt;1&gt; to &lt;3&gt; above, wherein the crystalline polymer is polytetrafluoroethylene. &lt;5&gt; An asymmetric heating step of heating one surface of a film made of a crystalline polymer to form a semi-baked film having a temperature gradient formed in the thickness direction of the film. The first stretching step of stretching the semi-baked film in the uniaxial direction and A sintering step in which the first stretched film is heated at a temperature equal to or higher than that in the asymmetric heating step and sintered. A method for producing a crystalline polymer microporous film, which comprises a second stretching step of stretching the sintered film in a direction orthogonal to the uniaxial direction. &lt;6&gt; The method for producing a crystalline polymer microporous film according to &lt;5&gt;, wherein the heating in the sintering step is at a temperature of 350 ° C. or higher. &lt;7&gt; The method for producing a crystalline polymer microporous film according to any one of &lt;5&gt; to &lt;6&gt;, wherein heating is irradiation of the crystalline polymer film with electromagnetic waves. &lt;8&gt; The method for producing a crystalline polymer microporous film according to &lt;7&gt;, wherein the electromagnetic wave is infrared rays. &lt;9&gt; The method for producing a crystalline polymer microporous membrane according to any one of &lt;5&gt; to &lt;8&gt;, wherein the crystalline polymer is polytetrafluoroethylene. &lt;10&gt; A filtration filter using the crystalline polymer microporous membrane according to any one of &lt;1&gt; to &lt;4&gt;.</p><p> In the crystalline polymer microporous membrane of the present invention, the average pore size of one surface is larger than the average pore size of the other surface, and the average pore size is continuously changed from one surface to the other surface. , The one surface side has a fine structure composed of a series of knots connected to each other by fibrils, and the aspect ratio (length / width) of the knots is 25 or more. The crystalline polymer microporous membrane of the present invention can efficiently capture fine particles over a long period of time, has a high flow rate, is not clogged, and has a long filtration life, so that a large amount of liquid or the like is industrially filtered. Suitable for.</p><p> The method for producing a crystalline polymer microporous film of the present invention is an asymmetric heating step of heating one surface of a film made of a crystalline polymer to form a semi-firing film having a temperature gradient formed in the thickness direction of the film. And the first stretching step of stretching the semi-baked film in the uniaxial direction, and sintering by heating the film after the first stretching at a temperature equal to or higher than the heating in the asymmetric heating step. It includes a step and a second stretching step of stretching the sintered film in a direction orthogonal to the first stretching. In the method for producing a crystalline polymer microporous film of the present invention, the microporous film of the present invention can be efficiently produced.</p><p> Since the filtering filter of the present invention uses the crystalline polymer microporous membrane of the present invention, fine particles can be efficiently captured by filtering with the surface having a large average pore size as the inlet side. Further, since the specific surface area is large, the effect of removing the fine particles by adsorption or adhesion before reaching the minimum pore size portion is large, and the filtration life can be greatly improved.</p>

<p> According to the present invention, a conventional problem can be solved, fine particles can be efficiently captured, no clogging, a high flow rate, and a long filtration life, a crystalline polymer microporous membrane, and the crystalline membrane. It is possible to provide a method for producing a crystalline polymer microporous membrane capable of efficiently producing a polymer microporous membrane, and a filter for filtration using the crystalline polymer microporous membrane.</p>

(Crystalline polymer microporous membrane) In the crystalline polymer microporous membrane of the present invention, the average pore size of one surface is larger than the average pore size of the other surface, and the average pore size is continuously changed from one surface to the other surface. The one surface side has a fine structure composed of a series of knots connected to each other by fibrils, and the aspect ratio (length / width) of the knots is 25 or more.

&lt;First feature of microporous membrane&gt; The first feature of the crystalline polymer microporous membrane of the present invention is that the average pore size of one surface is larger than the average pore size of the other surface, and the average pore size from one surface to the other surface is large. It changes continuously. The crystalline polymer microporous membrane has an average pore size on one surface that is larger than the average pore size on the other surface. That is, when the film thickness is "10", the average pore diameter in the thickness portion "1" from the surface is P1, and the average pore diameter in the thickness portion "9" is P2, P1 / P2 is 2 to 10,000. Is preferable, and 3 to 100 is more preferable. Further, in the crystalline polymer microporous film, the ratio of the average pore size of one surface to the other surface (one surface / the other surface) is preferably 5 to 30 times, more preferably 10 to 25 times. , 15 to 20 times is more preferable.

Here, for the average pore diameter, for example, a scanning electron microscope (Hitachi S-4000 type, vapor deposition is Hitachi E1030 type, both manufactured by Hitachi, Ltd.) is used to take a photograph of the film surface (SEM photograph, magnification 1,000 to 5,000 times). , The obtained photo was taken into an image processing device (main unit name: Nippon Avionics Co., Ltd., TV image processor TVIP-4100II, control software name: Ratoc System Engineering Co., Ltd., TV image processor Image Command 4198) and crystalline polymer. An average pore diameter can be obtained by obtaining an image consisting of only fibers and performing arithmetic processing on the image.

The crystalline polymer microporous membrane of the present invention has a mode in which the average pore size continuously changes from one surface to the other surface (first aspect) and a mode in which it has a single-layer structure (first aspect). 2) are included. By further adding these aspects, the filtration life can be effectively improved.

In the first aspect, "the average pore diameter continuously changes from one surface to the other" means a distance d (from one surface) in the thickness direction from one surface on the horizontal axis. It means that the graph is drawn with one continuous line when the average pore diameter D is taken on the vertical axis. The graph from one surface (d = 0) to the other surface (d = film thickness) may consist only of a region with a negative slope (dD / dt &lt;0), or the slope may be negative. Area with zero slope (dD / dt = 0) may be mixed, or area with negative slope and positive area (dD / dt&gt; 0) may be mixed. Good. It is preferable that the region consists of only a region with a negative slope (dD / dt &lt;0), or a region with a negative slope and a region with a zero slope (dD / dt = 0) are mixed. More preferably, it consists only of a region with a negative slope (dD / dt &lt;0).

It is preferable that at least one surface of the film is included in the region having a negative inclination. In the region where the slope is negative (dD / dt &lt;0), the slope may always be constant or different. For example, when the crystalline polymer microporous membrane of the present invention consists only of a region with a negative slope (dD / dt &lt;0), it is on one side of the membrane rather than on the other side of the membrane. A mode in which dD / dt is large can be taken. Further, it is possible to take a mode in which dD / dt gradually increases (a mode in which the absolute value decreases) from one surface of the film toward the other surface.

The "single-layer structure" referred to in the second aspect excludes a multi-layer structure formed by laminating or laminating two or more layers. That is, the "single-layer structure" in the second aspect means a structure having no boundary between layers existing in the multi-layer structure. In the second aspect, it is preferable that the film has a surface having an average pore diameter smaller than the average pore diameter of the non-heated surface and larger than the average pore diameter of the heated surface.

The crystalline polymer microporous membrane of the present invention preferably has both the characteristics of the first aspect and the characteristics of the second aspect. That is, the average pore diameter of one surface of the film is larger than the average pore diameter of the other surface, the average pore diameter continuously changes from one surface to the other surface, and the film has a single-layer structure. Is preferable. With such a microporous membrane, fine particles can be captured more efficiently when filtration is performed from the surface side having a large average diameter, the filtration life can be greatly improved, and it is easy and inexpensive. It can also be manufactured.

&lt;Second feature of microporous membrane&gt; The crystalline polymer microporous membrane of the present invention has, as a second feature, a microstructure consisting of a series of nodules in which one surface side is interconnected by fibrils, and the aspect ratio (length / width) of the nodules. ) Is 25 or more.

The nodule means a mass of primary particles in which a plurality of fibrils are connected, and the diameter of the mass is larger than the diameter of the fibril and the diameter is 0.1 μm or more. The fibril means a fiber spun between two fused particles when a mechanical force is applied to each other.

The aspect ratio of the nodule means the average value of the length / width of the nodule. The aspect ratio (length / width) is 25 or more, preferably 50 or more. If the aspect ratio (length / width) is less than 25, the stretching effect of the nodule portion, which greatly affects the film thickness, is insufficient, so that the film becomes thick and the flow rate may deteriorate. Here, in FIG. 2B, C indicates the length of the nodule and D indicates the width of the nodule. The "length" and "width" of the nodule can be measured, for example, by surface photography (scanning electron microscope (SEM), light microscope, laser microscope, etc.).

The area ratio of fibril / nodule is preferably 99: 1 to 75:25, more preferably 99: 1 to 85:15. If the area ratio of fibril / nodule is less than (75:25), the flow rate may be insufficient due to the large number of nodules, and if it exceeds (99: 1), the number of holes is too large, resulting in a hole diameter. It can be too small. The area ratio of fibril / nodule can be measured by the following method. A photograph of the surface of the porous membrane is taken with a scanning electron microscope (Hitachi S-4000 type or Hitachi E1030 type) (SEM photograph, magnification 1000 to 5000 times). This photo was taken into an image processor (main unit name: Nippon Avionics Co., Ltd., TV image processor TVIP-4100II, control software name: Ratoc System Engineering Co., Ltd., TV image processor image command 4198) and separated into knots and fibrils. , Obtain an image consisting only of nodules and an image consisting only of fibers. The maximum nodule area was calculated by arithmetically processing the image consisting of only nodules, and the average diameter of the fibrils was calculated by arithmetically processing the image consisting only of fibrils (total area divided by 1/2 of the total circumference). The area ratio between the fibril and the nodule can be obtained from the ratio of the total area of the fibril image to the total area of the nodule image.

The microstructure is preferably a region within 90% of the total thickness in the thickness direction from one surface, and more preferably within 80%. If the microstructure exceeds 90% of the total thickness in the thickness direction from one surface, the dense layer may become too thin and the catchability may deteriorate.

The film thickness of the crystalline polymer microporous film is preferably 50 μm or less, more preferably 45 μm or less. If the film thickness exceeds 50 μm, the flow rate may be insufficient. The film thickness can be measured using, for example, a 1/1000 mm dial thickness gauge (manufactured by Teclock Co., Ltd., product number SM1201), and any five points in the film are measured, and the average value is referred to as "film thickness". did.

-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 a non-crystalline region in which long chain molecules are not regularly arranged are mixed. , Such a polymer develops crystallinity by physical treatment. For example, when a polyethylene film is stretched by an external force, a phenomenon is observed in which the initially transparent film becomes cloudy. This is because crystallinity is exhibited 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 polyalkylene, polyester, polyamide, polyether, liquid crystal polymer and the like, and specific examples thereof include polyethylene and the like. Examples thereof include polypropylene, nylon, polyacetal, polybutylene terephthalate, polyethylene terephthalate, syndiotactic polystyrene, polyphenylene sulfide, polyether ether ketone, total aromatic polyamide, total aromatic polyester, fluororesin, polyether nitrile, and the like. Among these, polyalkylene (for example, polyethylene and polypropylene) is preferable from the viewpoint of chemical resistance and handleability, and fluorine-based poly in which the hydrogen atom of the alkylene group in the polyalkylene is partially or wholly replaced by a fluorine atom. Alkylene is more preferred, and polytetrafluoroethylene (PTFE) is particularly preferred. The density of the polyethylene changes depending on the degree of branching, and the one with a high degree of branching and a low degree of crystallinity is low density polyethylene (LDPE), and the one with a low degree of branching and a high degree of crystallinity is high density polyethylene (HDPE). It is classified as, and any of them 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 fine powder of polytetrafluoroethylene obtained by coagulating an aqueous dispersion obtained by emulsion polymerization. It is preferable to use ethylene.

The polytetrafluoroethylene is not particularly limited and may be appropriately selected depending on the intended purpose, and a commercially available product 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, and 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) PTFE 62XT, Teflon (Registered trademark) PTFE 6C-J, Teflon (Registered trademark) PTFE 640-J (both manufactured by Mitsui DuPont Fluorochemical Co., Ltd.), etc. can be mentioned. 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 or melting point of 40 ° C to 400 ° C, more preferably 50 ° C to 350 ° C. The mass average molecular weight of the crystalline polymer is preferably 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.

(Method for manufacturing crystalline polymer microporous membrane) The method for producing a crystalline polymer microporous film includes an asymmetric heating step, a first stretching step, a sintering step, and a second stretching step, and includes a crystalline polymer film manufacturing step, and further, if necessary. It includes other steps.

-Crystalline polymer film manufacturing process- The crystalline polymer film is produced by preparing a mixture of the crystalline polymer with an extrusion aid, past-extruding the mixture, and rolling the film. The crystalline polymer can be appropriately selected from the above-mentioned ones according to the purpose. As the extrusion aid, it is preferable to use a liquid lubricant, and specific examples thereof include solvent naphtha and white oil. As the extrusion aid, a commercially available product can be used, and for example, a hydrocarbon oil such as "Isoper" manufactured by Esso Petroleum Co., Ltd. may be used. The amount of the extrusion aid added is preferably 20 parts by mass to 30 parts by mass with respect to 100 parts by mass of the crystalline polymer.

The paste extrusion is usually preferably performed at 50 ° C to 80 ° C. The extruded shape is not particularly limited and may be appropriately selected depending on the intended purpose, but it is usually preferable to have a rod shape. The extruded product is then rolled into a film. The rolling can be carried out, for example, by calendering at a speed of 50 m / min with a calendar roll. The rolling temperature can usually be set to 50 ° C to 70 ° C. After that, it is preferable to remove the extrusion aid by heating and drying the film to obtain a crystalline polymer unheated film. The heating temperature at this time is not particularly limited and can be appropriately selected depending on the type of crystalline polymer used, but 40 ° C to 400 ° C is preferable, and 60 ° C to 350 ° C is more preferable. .. When polytetrafluoroethylene is used as the crystalline polymer, for example, 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 oven. The thickness of the crystalline polymer unheated film produced in this manner can be appropriately adjusted according to the thickness of the crystalline polymer microporous film to be finally produced, and is the thickness due to stretching in a subsequent step. It is necessary to make adjustments in consideration of the decrease in In the production of the crystalline polymer unheated film, the matters described in the "Polyflon Handbook" (published by Daikin Industries, Ltd., revised edition in 1983) can be appropriately adopted.

-Asymmetric heating process- The asymmetric heating step is a step of heating one surface of a film made of a crystalline polymer to form a semi-baked film having a temperature gradient formed in the thickness direction of the film. Here, the semi-firing means that the crystalline polymer is heat-treated at a temperature equal to or higher than the melting point of the heated body and not more than the melting point of the unheated body + 15 ° C. In the present invention, the unheated body of the crystalline polymer means one that has not been subjected to asymmetric heat treatment. Further, the heated body of the crystalline polymer means a body that has been heat-treated at a temperature equal to or higher than the melting point of the unheated body. The melting point of the crystalline polymer means the temperature of the peak of the endothermic curve that appears when the unheated body of the crystalline polymer is measured by a differential scanning calorimeter. The melting point of the heated body and the melting point of the unheated body vary depending on the type of crystalline polymer, the average molecular weight, and the like, but are preferably 50 ° C to 450 ° C, more preferably 80 ° C to 400 ° C. Such a temperature can be considered as follows. For example, in the case of polytetrafluoroethylene, the melting point of the heated body is about 324 ° C and the melting point of the unheated body is about 345 ° C. Therefore, in order to obtain a semi-baked product, in the case of a polytetrafluoroethylene film, 327 ° C to 360 ° C is preferable, 335 ° C to 350 ° C is more preferable, and the film is heated to a temperature of, for example, 345 ° C. The semi-fired body is in a state in which one having a melting point of about 324 ° C and one having a melting point of about 345 ° C are mixed.

The semi-firing is performed by heating one surface of a film made of a crystalline polymer. Thereby, the heating temperature can be controlled asymmetrically in the thickness direction, and the crystalline polymer microporous membrane of the present invention can be easily produced. As for the temperature gradient in the thickness direction of the film, the temperature difference between the front surface and the back surface is preferably 30 ° C. or more, more preferably 50 ° C. or more. The heating method is not particularly limited and may be appropriately selected depending on the intended purpose. (1) A method of blowing hot air on the crystalline polymer film, (2) a method of contacting the crystalline polymer film with a heat medium, Examples include (3) a method of bringing the crystalline polymer film into contact with the heating member, and (4) a method of irradiating the crystalline polymer film with an electromagnetic wave.

The method of blowing the hot air of (1) is not particularly limited as long as it uses a device capable of heating a gas, and can be appropriately selected depending on the intended purpose. Examples thereof include a heat gun and a duct heater. Among these, the duct heater is particularly preferable. The temperature of the hot air is preferably 350 ° C. or higher, and particularly preferably 360 ° C.

Examples of the method of contacting with the heat medium of (2) include a method of using heated steam, a molten salt, a molten metal, and the like as a heat medium. Among these, a method using heated steam as a heat medium is particularly preferable. The temperature of the heat medium is preferably 350 ° C. or higher, particularly preferably 360 ° C.

Examples of the heating member of (3) include a heating plate and a heating roll, and the heating roll is particularly preferable. With the heating roll, asymmetric heating can be continuously performed in an assembly line in an industrial manner, and temperature control and maintenance of the device are easy. The temperature of the heating roll can be set to the temperature at which the semi-fired body is formed. The time required for the film to come into contact with the heating roll is the time required for the desired asymmetric heating to proceed sufficiently, preferably 5 seconds to 120 seconds, more preferably 10 seconds to 90 seconds, and 15 seconds to 80 seconds. Is more preferable.

Examples of the (4) 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. For the general definition of infrared rays, "Practical infrared rays" (Human and History, published in 1992) can be referred to. In the present invention, the infrared rays mean electromagnetic waves having a wavelength of 0.74 μm to 1,000 μm, of which a wavelength range of 0.74 μm to 3 μm is defined as near infrared rays and a wavelength range of 3 μm to 1,000 μm is defined as far infrared rays. In the present invention, since it is preferable that there is a temperature difference between the front surface and the back surface of the unheated film, far infrared rays, which are advantageous for heating the surface layer, are preferably used. The type of the infrared device is not particularly limited as long as it can irradiate infrared rays of a target wavelength, and can be appropriately selected according to the purpose. Generally, near infrared rays are light bulbs (halogen lamps), and far infrared rays are far infrared rays. A heating element such as a ceramic, quartz, or metal oxide surface can be used. Further, with infrared irradiation, semi-firing can be continuously performed in an assembly line industrially, and temperature control and maintenance of the device are easy. Moreover, since it is non-contact, it is clean and does not cause defects such as fluffing. The film surface temperature due to the infrared irradiation can be controlled by the output of the infrared irradiation device, the distance between the infrared irradiation device and the film surface, the irradiation time (conveyance speed), and the atmospheric temperature, and is set to the temperature at which the semi-baked body is formed. However, 327 ° C to 380 ° C is preferable, and 335 ° C to 360 ° C is more preferable. If the surface temperature is less than 327 ° C, the crystal state does not change and the pore size cannot be controlled. If the surface temperature exceeds 380 ° C, the entire film melts and the shape is excessively deformed. , Polymer thermal decomposition may occur. The infrared irradiation time is not particularly limited and is a time required for the target semi-firing to proceed sufficiently, preferably 5 seconds to 120 seconds, more preferably 10 seconds to 90 seconds, and 15 seconds to ~. 80 seconds is more preferred.

The infrared irradiation in the asymmetric heating step may be performed continuously, or may be divided into several parts and performed intermittently. When one surface of the film is continuously heated by infrared irradiation, it is preferable to cool the other surface at the same time as heating one surface in order to maintain the concentration gradient between one surface and the other surface of the film. The method for cooling the other surface is not particularly limited and may be appropriately selected depending on the intended purpose. For example, a method of blowing cold air, a method of contacting with a refrigerant, a method of contacting with a cooled material, or cooling is used. Various methods such as cooling can be used, and the method of bringing the cooled material into contact with the heated surface of the film is not preferable because the surface of the contacted material is heated by far infrared rays. Further, even when the asymmetric heating step is performed intermittently, it is preferable to intermittently heat and cool the other surface of the film to suppress the temperature rise of the one surface.

-Sintering process- The sintering step is a step of heating and sintering the first stretched film at a temperature equal to or higher than that in the asymmetric heating step. The sintering step preferably heats at a temperature of 350 ° C. or higher, more preferably 350 ° C. to 390 ° C. If the heating in the sintering step exceeds 390 ° C, fibril may be cut or fused. The sintering method is not particularly limited and may be appropriately selected depending on the intended purpose. (1) A method of blowing hot air on the crystalline polymer film, and (2) contacting the crystalline polymer film with a heat medium. Examples include a method, (3) a method of bringing the crystalline polymer film into contact with a heating member, and (4) a method of irradiating the crystalline polymer film with an electromagnetic wave. Among these, (3) a method of contacting with a heating member and (4) a method of irradiating electromagnetic waves are particularly preferable.

-First stretching step and second stretching step- The first stretching step is a step of stretching the semi-baked film in the uniaxial direction, and the second stretching step is a step of stretching the sintered film in a direction orthogonal to the uniaxial direction. The stretching is performed in the longitudinal direction or the width direction. The first stretching step is preferably performed in the longitudinal direction shown in A of FIGS. 1A and 2A, and the second stretching step is preferably performed in the width direction shown in B of FIGS. 1B and 2B. preferable. The stretching ratio in the longitudinal direction is preferably 4 to 100 times, more preferably 8 to 90 times, and even 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 draw ratio in the width direction is preferably 3 times to 100 times, more preferably 5 times to 90 times, further preferably 10 times to 70 times, and particularly preferably 20 times to 40 times. The stretching temperature in the width direction is preferably 100 ° C to 400 ° C, more preferably 200 ° C to 390 ° C, and particularly preferably 250 ° C to 380 ° C. The area stretching ratio is preferably 50 times to 250 times, more preferably 75 times to 200 times, still more preferably 100 times to 150 times. When stretching, the crystalline polymer film may be preheated to a temperature equal to or lower than the stretching temperature in advance. After stretching, heat fixing can be performed if necessary. The heat-fixing temperature is usually preferably higher 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 purposes, and in particular, it can be suitably used as a filter for filtration described below.

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

The filtration filter of the present invention is at least 50 L / m when filtered at a differential pressure of 100 kPa.²-It can be possible to filter for min or more. The shape of the filtration filter of the present invention includes a pleated type in which the filtration membrane is folded, a spiral type in which the filtration membrane is glued, a frame-and-plate type in which a disc-shaped filtration membrane is laminated, and a filtration membrane. There is a tube type that makes it tubular. Among these, the pleated type is particularly preferable because the effective surface area used for filtering the filter per cartridge can be increased. Further, it is classified into an element exchange type filter cartridge in which only the filter element is replaced when the deteriorated filtration membrane is replaced, and a capsule type filter cartridge in which the filter element is processed integrally with the filtration housing and the housing is made a disposable type.

Here, FIG. 7 is a developed view showing the structure of an element-replaceable pleated filter cartridge element. The microfiltration membrane 103 is folded in a sandwiched state by two membrane supports 102 and 104 and wrapped around a core 105 having many collection ports. There is an outer peripheral cover 101 on the outside to protect the microfiltration membrane. Microfiltration membranes are sealed at both ends of the cylinder by end plates 106a and 106b. The end plate contacts the seal portion of the filter housing (not shown) via the gasket 107. The filtered liquid is collected from the core's collection port and discharged from the fluid outlet 108.

Capsule-type pleated filter cartridges are shown in FIGS. 8 and 9. FIG. 8 is a developed view showing the overall structure of the microfiltration membrane filter element before being incorporated into the housing of the capsule filter cartridge. The microfiltration membrane 2 is folded in a sandwiched state by two supports 3 and 5, and is wrapped around a filter element core 9 having many collection ports. There is a filter element cover 8 on the outside to protect the microfiltration membrane. Microfiltration membranes are sealed at both ends of the cylinder by upper end plates 6 and lower end plates 7. FIG. 9 shows the structure of a capsule-type pleated filter cartridge in which the filter element is integrated into the housing. The filter element 12 is incorporated in a housing consisting of a housing base and a housing cover. The lower end plate is sealed to a water collection pipe (not shown) in the center of the housing base via an O-ring 10. The liquid enters the housing through the liquid inlet nozzle, passes through the filter media 11, is collected from the liquid collection port of the filter element core 9, and is discharged from the liquid outlet nozzle 16. The housing base and housing cover are usually liquid-tightly heat-welded at the welded portion 19.

FIG. 8 shows an example in which the lower end plate and the housing base are sealed via an O-ring, but the lower end plate and the housing base may be sealed by heat fusion or an adhesive. In addition to heat fusion, the seal between the housing base and the housing cover can also be made by using an adhesive. 7 to 9 are specific examples of the microfiltration filter cartridge, and the present invention is not limited to these figures.

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

The filtration filter of the present invention can be used in various situations where filtration is required, and is suitably used for microfiltration of gases, liquids, etc., for example, corrosive gas, various types used in the semiconductor industry. It is used for filtration of gas, etc., filtration of washing water for electronic industry, pharmaceutical water, water for pharmaceutical manufacturing process, food water, etc., and sterilization. In particular, since the filtration filter 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, which cannot be handled by conventional filtration filters.

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

(Example 1) &lt;Preparation of polytetrafluoroethylene microporous membrane&gt; -Preparation of preformed body- 100 parts by mass of polytetrafluoroethylene fine powder (manufactured by Asahi Glass Co., Ltd., "Fluon PTFE CD123") having a number average molecular weight of 10 million as a crystalline polymer, and hydrocarbon oil (manufactured by Esso Petroleum Co., Ltd., "" Isopar H ") 22 parts by mass was added. This is spread as shown in Fig. 10, pressurized, and has a density of 1.33 kg / m.³(See FIG. 11).

-Making unfired film- The prepared preformed body 110 was inserted into a cylinder of a paste extrusion die as shown in FIG. 12, and paste was extruded in the form of a sheet. This was calendared with a calendar roll heated to 60 ° C. to prepare a polytetrafluoroethylene film. The obtained polytetrafluoroethylene film was passed through a hot air drying oven at 250 ° C to dry and remove the extrusion aid, and the average thickness was 150 μm, the average width was 150 mm, and the density was 1.55 kg / m.³Polytetrafluoroethylene unfired film was prepared.

-Preparation of semi-baked film 1 (roll heating)- An induction heating metal roll mounted on a steel roll (Yuri Roll Co., Ltd. "Induction heating method high temperature high speed calender machine (installed in Yuri Roll Co., Ltd.)" in which the obtained polytetrafluoroethylene unfired film is heated to 345 ° C. ) For 1 minute to prepare a semi-baked film 1. When the temperature distribution in the width direction in the steady state of the roll used at this time (the temperature variation for 10 seconds is within 1 ° C in the temperature measurement at 0.1 second intervals) was measured by infrared thermography, the maximum temperature part and the minimum temperature part were measured. The temperature difference from the temperature part was 1.0 ° C.

-Preparation of polytetrafluoroethylene microporous membrane- The obtained semi-baked film 1 was stretched 13 times between rolls in the longitudinal direction (direction A in FIG. 2A) at 270 ° C (first stretching step), and was once wound onto a take-up roll. The film was then heated on a steel roll heated to 345 ° C for 1 minute. FIG. 2A is a schematic view showing the first stretching step. FIG. 5A is a laser micrograph (Laser microscope VK8700 manufactured by KEYENCE CORPORATION) showing the unheated surface of the film after the first stretching step. The film was then heated at 380 ° C for 1 minute using a heating zone with a duct heater (sintering step). Then, both ends of the film were clipped and stretched 5 times in the width direction (B direction in FIG. 2B) at 375 ° C (second stretching step). FIG. 2B is a schematic view showing the second stretching step. After that, heat fixation was performed at 380 ° C. From the above, the polytetrafluoroethylene microporous membrane of Example 1 was prepared. FIG. 5B is a laser micrograph showing the unheated surface of the membrane. FIG. 6 is a laser micrograph showing the heated surface of the film. From Fig. 5B, it was found that there was no new fibrillation, the nodules were extended, and the aspect ratio was increased. From the laser micrograph of the cross section of the obtained microporous film, from one surface (non-heated surface) to 70% of the total thickness in the thickness direction was a fine structure.

(Example 2) &lt;Preparation of polytetrafluoroethylene microporous membrane&gt; A polytetrafluoroethylene unfired film was prepared in the same manner as in Example 1.

-Preparation of semi-baked film 2 (infrared heating)- One surface of the obtained polytetrafluoroethylene unfired film was heated by near infrared rays with a halogen heater having a built-in tungsten filament at a film surface temperature of 345 ° C. for 1 minute to prepare a semi-baked film 2.

-Preparation of polytetrafluoroethylene microporous membrane- The obtained semi-baked film 2 was stretched 13 times between rolls in the longitudinal direction (direction A in FIG. 2A) at 270 ° C (first stretching step), and was once wound on a take-up roll. The film was then heated at 345 ° C for 1 minute. After that, the film was heated at 380 ° C for 1 minute using a halogen heater (sintering step), both ends were clipped, and the film was stretched 5 times in the width direction (B direction in FIG. 2B) at 375 ° C (B direction in FIG. 2B). Second stretching step). After that, heat fixation was performed at 380 ° C. From the above, the polytetrafluoroethylene microporous membrane of Example 2 was prepared. From the laser micrograph of the cross section of the obtained microporous film, from one surface (non-heated surface) to 70% of the total thickness in the thickness direction was a fine structure.

(Comparative example 1) &lt;Preparation of polytetrafluoroethylene microporous membrane&gt; A polytetrafluoroethylene unfired film was prepared in the same manner as in Example 1.

-Preparation of semi-baked film 2 (infrared heating)- One surface of the obtained polytetrafluoroethylene unfired film was heated by near infrared rays with a halogen heater having a built-in tungsten filament at a film surface temperature of 345 ° C. for 1 minute to prepare a semi-baked film 2.

-Preparation of polytetrafluoroethylene microporous membrane- The obtained semi-baked film 2 was stretched 13 times between rolls in the longitudinal direction (direction A in FIG. 1A) at 270 ° C (first stretching step), and was once wound on a take-up roll. The film was then heated at 304 ° C. for 1 minute. FIG. 1A is a schematic view showing a first stretching step. FIG. 3A is a laser micrograph showing the unheated surface of the film after the first stretching step. Next, both ends of the film were clipped and stretched 5 times in the width direction (B direction in FIG. 1B) at 270 ° C (second stretching step). FIG. 1B is a schematic view showing a second stretching step. After that, heat fixation was performed at 380 ° C. From the above, the polytetrafluoroethylene microporous membrane of Comparative Example 1 was prepared. FIG. 3B is a laser micrograph showing the unheated surface of the membrane. FIG. 4 is a laser micrograph showing the heated surface of the film. From Fig. 3B, it was found that a new fibrillation occurred and the aspect ratio became smaller. From the laser micrograph of the cross section of the obtained microporous film, it was found that from one surface (non-heated surface) to 0% of the total thickness in the thickness direction was a fine structure.

(Comparative example 2) &lt;Preparation of polytetrafluoroethylene microporous membrane&gt; A polytetrafluoroethylene microporous film of Comparative Example 2 was prepared in the same manner as in Example 1 except that the unfired film was not subjected to a semi-baking treatment (asymmetric heat treatment) in Example 1. From the laser micrograph of the cross section of the obtained microporous film, 100% of the total thickness in the thickness direction from one surface (non-heated surface) was a fine structure.

(Comparative example 3) &lt;Preparation of polytetrafluoroethylene microporous membrane&gt; A polytetrafluoroethylene microporous film of Comparative Example 3 was prepared in the same manner as in Example 1 except that the sintering step between the first stretching and the second stretching was not performed in Example 1. .. From the laser micrograph of the cross section of the obtained microporous film, 0% of the total thickness in the thickness direction from one surface (non-heated surface) was a fine structure.

Next, for each of the prepared polytetrafluoroethylene microporous membranes of Examples 1 to 2 and Comparative Examples 1 to 3, the aspect ratio of the nodule and the area ratio of the fibril / nodule were determined as follows. The results are shown in Table 1.

&lt;Aspect ratio of nodules&gt; Using a laser microscope (Keyence, Inc., laser microscope VK8700), the length and width of the nodules of each of the polytetrafluoroethylene microporous membranes of Examples 1 to 2 and Comparative Examples 1 to 3 were measured in any five visual fields. The length / width of was calculated and the average value was used as the aspect ratio. The results are shown in Table 1.

&lt;Fibril / nodule area ratio&gt; The fibrils and nodules of each of the polytetrafluoroethylene microporous membranes of Examples 1 to 2 and Comparative Examples 1 to 3 were photographed with a scanning electron microscope (Hitachi S-4000 type or Hitachi E1030 type) (SEM photograph, magnification 1000). Double to 5000 times). This photo was taken into an image processor (main unit name: Nippon Avionics Co., Ltd., TV image processor TVIP-4100II, control software name: Ratoc System Engineering Co., Ltd., TV image processor image command 4198) and separated into knots and fibrils. , An image consisting only of nodules and an image consisting only of fibrils were obtained. Next, the maximum nodule area was calculated by arithmetically processing the image consisting of only nodules, and the average diameter of the fibrils was calculated by arithmetically processing the image consisting of only fibrils (dividing the total area by 1/2 of the total circumference). ). The area ratio between the fibril and the nodule was calculated from the ratio of the total area of the fibril image to the total area of the nodule image. The results are shown in Table 1.

Next, for each of the prepared polytetrafluoroethylene microporous films of Examples 1 to 2 and Comparative Examples 1 to 3, the average pore diameter of the unheated surface of the microporous film is the average of the heated back surface. In order to confirm whether the average pore diameter is larger than the pore diameter and continuously changes from the front surface to the back surface, the film thickness (film thickness) and P1 / P2 are measured as follows. It was. The results are shown in Table 1.

&lt;Film thickness (film thickness)&gt; The thickness (thickness) of each of the polytetrafluoroethylene microporous membranes of Examples 1 to 2 and Comparative Examples 1 to 3 was measured with a 1/1000 mm dial thickness gauge (SM1201 manufactured by Teclock Co., Ltd.). Arbitrary 5 points were measured, and the average value was calculated and used as the film thickness.

&lt;Measurement of P1 / P2&gt; For each of the polytetrafluoroethylene microporous membranes of Examples 1 to 2 and Comparative Examples 1 to 3, the film thickness of the microporous film was set to "10", and the average pore size in the thickness portion in the depth direction "1" from the surface. Was P1, and P1 / P2 was obtained when the average pore diameter of the thick portion of "9" was P2. Here, the average pore size of the microporous film is a photograph of the film surface (SEM photograph, magnification 1,000 to 5,000) with a scanning electron microscope (Hitachi S-4000 type, vapor deposition is Hitachi E1030 type, both manufactured by Hitachi, Ltd.). Take a double) and import the obtained photo into an image processing device (main unit name: Nippon Avionics Co., Ltd., TV image processor TVIP-4100II, control software name: Ratoc System Engineering Co., Ltd., TV image processor image command 4198). An image consisting of only polytetrafluoroethylene fibers was obtained, and the average pore size was obtained by arithmetically processing the image.

<tables num="1"><img file="JP2010058024A_D0001.tif" /></tables>

&lt;Filtration life test&gt; The filtration life was measured using a latex dispersion with a multi-dispersion particle size, and the amount of filtration (L / m) until it was substantially clogged.²). In the present invention, "substantially clogged" is defined as a time when the flow rate drops to 1/2 of the initial flow rate at a constant filtration pressure. The type of latex used in the latex dispersion used in this measurement is appropriately selected depending on the pore size of the film. The conditions for selection were that the particles contained in the filtered liquid were 1 ppm or less, and the ratio of the average particle size of the latex to the pore size of the film was 1/5 to 5. Isopropanol was used as the dispersion medium, and the concentration was 100 ppm. The results are shown in Table 2.

<tables num="2"><img file="JP2010058024A_D0002.tif" /></tables>

&lt;Flow test&gt; The flow rate was measured under the following conditions in accordance with JIS K3831. The type of test method used was the "pressurized filtration test method", and the sample was cut into a circle with a diameter of 13 mm and set in a stainless steel holder for measurement. Isopropanol was used as the test solution, and the time required to filter 100 mL of the test solution at a pressure of 100 kPa was measured, and the flow rate (L / min · m) was measured.²) Was calculated. The results are shown in Table 3.

<tables num="3"><img file="JP2010058024A_D0003.tif" /></tables>

(Example 3) -Filter cartridge- The PTFE microporous membrane of Example 1 is laminated as shown below, pleated to a fold width of 12.5 mm (pleated width = 220 mm), and 230 folds are taken and rolled into a cylindrical shape, and the seams thereof are formed. Is welded with an impulse sealer. 15 mm at each end of the cylinder was cut off, and the cut surface was heat-welded to a polypropylene end plate to complete an element-replaceable filter cartridge. -Constitution- Primary side net AET DELNET (RC-0707-20P) Thickness: 0.13mm, Basis weight: 31g / m², Area used: Approximately 1.3m² Primary side non-woven fabric Syntex (PK-404N) manufactured by Mitsui Chemicals, Inc. Thickness: 0.15mm, Area used: Approximately 1.3m² Filter Material PTFE Microporous Membrane of Example 1 Thickness: Approximately 0.05mm, Area used: Approximately 1.3m² Secondary side net AET DELNET (RC-0707-20P) Thickness: 0.13mm, Basis weight: 31g / m², Area used: Approximately 1.3m² Since the filter cartridge of Example 3 of the present invention uses the crystalline polymer microporous membrane of Example 1 of the present invention, it has excellent solvent resistance. Further, since the pores of the crystalline polymer microporous membrane have an asymmetric structure, the flow rate is large, clogging is unlikely to occur, and the life is long.

The crystalline polymer microporous film of the present invention and the filter for filtration using the same can efficiently capture fine particles over a long period of time, improve the scratch resistance of the particle trapping ability, and have heat resistance and chemical resistance. Because it is excellent in filtration, it can be used in various situations where filtration is required, and it is suitably used for precision filtration of gases, liquids, etc., for example, corrosive gas, various gases used in the semiconductor industry, etc. It can be widely used for filtration of gas, washing water for electronic industry, pharmaceutical water, water for pharmaceutical manufacturing process, filtration of food water, sterilization, high temperature filtration, filtration of reactive chemicals and the like.

<figref num="1A">FIG. 1A is a diagram showing a first stretching step.</figref><figref num="1B">FIG. 1B is a diagram showing a second stretching step.</figref><figref num="2A">FIG. 2A is a diagram showing a first stretching step.</figref><figref num="2B">FIG. 2B is a diagram showing a second stretching step after sintering.</figref><figref num="3A">FIG. 3A is an electron micrograph showing the unheated surface of the film after the first stretching step of Comparative Example 1.</figref><figref num="3B">FIG. 3B is an electron micrograph showing the unheated surface of the crystalline polymer microporous film obtained in Comparative Example 1.</figref><figref num="4">FIG. 4 is an electron micrograph showing the heated surface of the crystalline polymer microporous film obtained in Comparative Example 1.</figref><figref num="5A">FIG. 5A is an electron micrograph showing the unheated surface of the film after the first stretching step of Example 1.</figref><figref num="5B">FIG. 5B is an electron micrograph showing the unheated surface of the crystalline polymer microporous film obtained in Example 1.</figref><figref num="6">FIG. 6 is an electron micrograph showing the heated surface of the crystalline polymer microporous film obtained in Example 1.</figref><figref num="7">FIG. 7 is a diagram showing the structure of a general pleated filter element before being incorporated into the housing.</figref><figref num="8">FIG. 8 is a diagram showing the structure of a general filter element before being incorporated into the housing of the capsule type filter cartridge.</figref><figref num="9">FIG. 9 is a diagram showing the structure of a general capsule type filter cartridge integrated with a housing.</figref><figref num="10">FIG. 10 is a diagram showing an example of a manufacturing process of a crystalline polymer microporous membrane.</figref><figref num="11">FIG. 11 is a diagram showing an example of a preformed body.</figref><figref num="12">FIG. 12 is a diagram showing another example of the manufacturing process of the crystalline polymer microporous membrane.</figref>

Code description

1 nodule 2 fibril 3 Primary support 4 Microfiltration membrane 5 Secondary support 6 Top end plate 7 Lower end plate 8 Filter element cover 9 Filter element core 10 O-ring 11 Filter media 12 Filter element 13 Housing cover 14 Housing base 15 Liquid inlet nozzle 16 Liquid outlet nozzle 17 Air vent 18 drain 19 Welding part 101 Outer cover 102 Membrane support 103 Microfiltration membrane 104 Membrane Support 105 core 106a, 106b end plate 107 gasket 108 Liquid outlet 109 Lower mold 110 Preformed body 111 Polytetrafluoroethylene unheated film