JP5564018B2 - Fluororesin porous membrane - Google Patents

Description

  The present invention relates to a fluororesin porous membrane used as a separation membrane for removing fine particles of submicron order, and more particularly to a fluororesin porous membrane made of polytetrafluoroethylene.

  Porous membranes (hereinafter referred to as “PTFE porous membranes”) made of fluororesins, particularly polytetrafluoroethylene (hereinafter referred to as “PTFE”), are originally made of fluororesins themselves such as chemical resistance and heat resistance. Since it has both structural features such as high material separation characteristics based on a uniform and fine porous structure and high permeability based on high porosity, it can be used for fuel cells, membrane filters and electric wires. It is suitably used for applications such as coating materials, analyzers, artificial blood vessels, stents, and catheters.

  However, the fluororesin porous membrane is inherently water-repellent, and when it is not subjected to any treatment, it is difficult for water to permeate through the fine porous structure and it is difficult to use it for aqueous separation. Therefore, a treatment for hydrophilizing the fluororesin porous membrane is performed.

  For example, Japanese Patent Publication No. 53-21270 (Patent Document 1) discloses a method of fixing a hydrophilic resin such as polyvinyl alcohol to the surface of a fine porous structure of a PTFE porous membrane by means such as adsorption or coating. Proposed. However, the heat resistance and chemical resistance of the film produced by this method depends on the coating material used for the hydrophilic treatment, and as a result, the excellent properties of PTFE cannot be fully utilized. In addition, there is a problem that the porosity and the pore diameter of the original PTFE porous membrane change due to the coating.

  Also known is a method of improving hydrophilicity (wetting) by introducing a hydrophilic group on the surface of the porous PTFE membrane by a plasma beam. However, this method also has a problem that chemical resistance and heat resistance are lowered.

  Without such treatment, water-soluble organic solvent, for example, alcohol such as ethanol or isopropanol (IPA) is impregnated in the microporous structure of the fluororesin porous membrane and then replaced with water. Also by the method, hydrophilicity is imparted and water easily permeates. Therefore, this method is widely performed when a fluororesin porous membrane is used for filtration of an aqueous solution or the like.

However, after this treatment, if the fluororesin porous membrane comes into contact with gas, or if bubbles or dissolved gases mixed in the aqueous solution adhere to the fluororesin porous membrane, the membrane will dry again. The permeate flow rate (that is, the processing capacity) gradually decreases, and it cannot be used at the end. In particular, this problem is likely to occur when used for filtering a liquid that generates decomposition gas or a foaming liquid such as a wafer cleaning liquid containing hydrochloric acid or hydrogen peroxide (Non-patent Document 1).
Japanese Patent Publication No.53-21270 Green Technology, June 1996, page 59

  When the porous membrane is dried and the processing capacity is lowered, it is necessary to perform an operation of again impregnating the porous body with alcohol or the like and then replacing it with water. During this operation, it is necessary to stop the operation, and a complicated operation is required to prevent alcohol or the like from entering the filtration system. Therefore, a porous membrane excellent in water retention (having a large effect of retaining water) that is difficult to dry even when bubbles are touched or bubbles or dissolved gas mixed in an aqueous solution adheres is desired.

  The present invention is a porous film made of a porous fluororesin such as a PTFE porous film, which has excellent water retention and can be dried even if bubbles or dissolved gas adhering to gas or mixed in an aqueous solution adhere to it. It is an object of the present invention to provide a fluororesin porous membrane that has difficult properties and also has excellent properties inherent to the porous fluororesin membrane.

  The present inventors diligently investigated the ease of drying of the fluororesin porous membrane, and obtained the knowledge that the size of the bubble point in the fluororesin porous membrane is related to the ease of drying of the membrane. . Moreover, the knowledge that it became difficult to dry by carrying out chemical etching or a roughening process on both surfaces of a fluororesin porous membrane was acquired. The present invention has been completed based on these findings.

  The present invention is a film formed of a porous fluororesin, having an outer layer on both surface sides of the film and an inner layer between the outer layers, the bubble point of the outer layer being 70 kPa or less, and A fluororesin porous membrane is provided in which the bubble point of the inner layer is 90 kPa or more.

  This invention is made by paying attention to the knowledge obtained as a result of the study, that is, it is difficult to dry when the bubble point is small in the fluororesin porous membrane, and conversely, it is easy to dry when the bubble point is large. The porous layer (outer layer) with a small bubble point is arranged on both surface sides of the fluororesin porous membrane to make the membrane difficult to dry, and inside the membrane (in the thickness direction) A porous layer (inner layer) having a large bubble point is disposed to maintain high separation ability.

  The porous membrane of the present invention has an inner layer (the inner portion in the thickness direction of the membrane) with a bubble point of 90 kPa or more inside, and is dried at a bubble point of 70 kPa or less on both surface sides of the inner layer. It is characterized by having a difficult outer layer (portion on the surface side of the film). That is, the porous layer that forms both surfaces and the porous layer that forms the inside are porous layers with different easiness of drying, and constitutes both surface portions (outer layers) The bubble point when the film is formed with only the porous layer is 70 kPa or less, and the bubble point when the film is formed with only the porous layer constituting the inner region (inner layer) of the porous film is 90 kPa. That's it.

  Here, the bubble point specifically means an IPA bubble point, and is a value measured by ASTM F316.

  When the porous membrane of the present invention does not have a layer having a bubble point of 90 kPa or more, the function as a separation membrane becomes insufficient and high separation ability is not maintained. On the other hand, when the bubble points on both surface sides (outer layers) exceed 70 kPa, the effect of suppressing drying, that is, sufficient water retention is not imparted. Since the bubble point becomes smaller as the pore diameter is larger, the porous pore diameter constituting the outer layer is considered to be larger than the porous pore diameter of the inner layer.

  Further, the layer having the bubble point of 70 kPa or less needs to be formed not only on one side of the film but also on the outer surfaces on both sides. When it is formed only on one side, the other surface side is easy to dry, so the effect of suppressing drying (water retention) is small, and the object of the present invention is not achieved.

  In this way, layers having a small bubble point that is excellent in maintaining water retention are arranged on both surface sides, and a layer having a large bubble point that functions as a separation membrane is arranged inside, so that it can touch gas or an aqueous solution Even if bubbles or dissolved gas mixed therein adheres, it becomes difficult to dry and high separation ability can be maintained.

  In the present invention, a film formed of a porous fluororesin and having a bubble point of 70 kPa or less is laminated on both surfaces of a film formed of a porous fluororesin and having a bubble point of 90 kPa or more. A fluororesin porous membrane is provided.

  This porous membrane is the above-mentioned fluororesin porous membrane and is manufactured by this specific method. More specifically, this porous membrane is formed by superposing a porous fluororesin film having a bubble point of 70 kPa or less on both surface sides of a porous fluororesin film having a bubble point of 90 kPa or more. Can be obtained by heating to above the melting point of these materials (in the case of PTFE, at least 327 ° C.) and integrating them. After the integration, the films superposed on both surface sides, that is, the film having a bubble point of 70 kPa or less becomes the outer layer in the fluororesin porous film, and the film having a bubble point of 90 kPa or more is the fluororesin porous film. It is the inner layer in the membrane.

  In addition, the film | membrane laminated | stacked on both surface side may have a mutually different bubble point within the restriction | limiting that each bubble point is 70 kPa or less. Each of the inner fluororesin porous film having a bubble point of 90 kPa or more and the fluororesin porous film on both surfaces having a bubble point of 70 kPa or less are films in which two or more films are stacked. Also good.

  Further, another porous film may be sandwiched between the inner film and the film laminated on both surface sides, and these may be overlapped and integrated. For example, a film having a structure in which a multilayer porous film is further laminated may be formed so that the bubble points are sequentially reduced from the inner side toward the both surface sides.

  In view of the function as a separation membrane, the inner layer or the porous membrane serving as the inner layer is more preferably one having a bubble point of 100 kPa or more. In view of the effect of suppressing drying, the porous layer on both surface sides, that is, the outer layer or the porous film serving as the outer layer, more preferably has a bubble point of 50 kPa or less.

  The present invention is further characterized in that, in claim 1, the film is formed of a porous fluororesin and has a bubble point of 90 kPa or more, and both surfaces thereof are subjected to chemical etching treatment. A fluororesin porous membrane is provided. Further, the present invention is the fluororesin porous membrane according to claim 1, wherein the inside of the fluororesin porous membrane is further subjected to chemical etching treatment. A fluororesin porous membrane is provided.

  Instead of sticking a porous fluororesin film with a bubble point of 70 kPa or less on both surfaces of a porous fluororesin film with a bubble point of 90 kPa or more, a chemical is applied to a porous fluororesin film with a high bubble point. By performing the selective etching process, it is possible to form a membrane having excellent functions as a separation membrane and having excellent water retention that is difficult to dry. That is, the chemical etching treatment weakens the high hydrophobicity of the fluororesin porous membrane surface and increases the affinity with water.

  Examples of the chemical etching treatment include an oxidative decomposition treatment in which an alkali metal is used to modify the fluororesin constituting the porous film and remove the modified portion. A third aspect of the present invention corresponds to the fluororesin porous membrane according to the first or second aspect, wherein the chemical etching process is an oxidative decomposition process.

  The oxidative decomposition treatment is performed using, for example, an organic alkali metal solution. When the fluororesin porous membrane is subjected to a chemical etching treatment with an organic alkali metal solution, the surface is modified to impart hydrophilicity, and a browned layer (brown layer) is formed. This brown layer is composed of sodium fluoride, a decomposition product of a fluororesin having a carbon-carbon double bond, a polymer of these with naphthalene, anthracene, etc., but these are separated into the filtrate by dropping, decomposition, elution or the like. Since it may be mixed, it is desirable to remove it. These can be removed by oxidative decomposition with hydrogen peroxide, sodium hypochlorite, ozone, or the like.

  It is known that such a chemical etching process removes fluorine on the surface and introduces hydrophilic groups or the like (Japanese Patent Laid-Open No. 5-237141), but the present inventor has decomposed and removed the brown layer. It was also found that water retention is maintained in Therefore, by applying such a chemical etching treatment to a porous fluororesin membrane having a high bubble point, that is, a membrane having an excellent function as a separation membrane, the porous membrane has excellent separation performance and water retention. Can be obtained.

  As described above, the chemical etching treatment can be performed using an organic alkali metal solution or the like, and specifically, can be performed by immersing the fluororesin porous film in the organic alkali metal solution. In this case, since the chemical etching process is performed from the surface side of the porous film, it is also possible to perform the chemical etching process only in the vicinity of both surfaces of the film. However, in order to further increase the water retention of the film, it is preferable to form the film of claim 2 by performing chemical etching treatment not only in the vicinity of both surfaces but also inside the porous film. Even if the chemical etching treatment is applied to the inside of the porous membrane, the function as a separation membrane is not significantly lowered.

  A porous fluororesin film having a high bubble point is a porous film having a small pore diameter, but when the pore diameter is small, it becomes easy to dry, and thus higher water retention is required. As described above, in order to obtain higher water retention, it is preferable that chemical etching treatment is performed to the inside of the porous film. However, according to the study of the present inventor, the smaller the pore diameter, the less likely the chemical etching process is performed to the inside of the porous film. Therefore, conditions and methods are desired that allow chemical etching treatment to be performed further to the inside even when the pore diameter of the porous membrane is small.

  Examples of the organic alkali metal solution used in the chemical etching treatment include an organic solvent solution such as methyllithium, a metal sodium-naphthalene complex, a metal sodium-anthracene complex in tetrahydrofuran, a metal sodium-liquid ammonia solution, and the like.

  Among them, a solution of a complex of sodium metal with naphthalene as an aromatic anion radical is generally widely used. However, in order to perform chemical etching treatment to the inside of the porous film, benzophenone, anthracene, and biphenyl are aromatic. It is preferable to use it as a group anion radical. As a result of the study, the present inventor has found that chemical etching treatment can be easily performed to the inside of the porous film when benzophenone, anthracene, and biphenyl are used as aromatic anion radicals rather than naphthalene. Claim 4 corresponds to this preferred embodiment.

  The present invention is further characterized in that, in claim 5, the film is formed of a porous fluororesin and has a bubble point of 90 kPa or more, and both surfaces thereof are roughened by physical means. A fluororesin porous membrane is provided.

  Instead of attaching a film with a bubble point of 70 kPa or less to both surfaces of a fluororesin porous film having a bubble point of 90 kPa or more, both surfaces can be dried by applying a roughening treatment by physical means. It is possible to form a film having excellent water retention that is difficult to perform at low cost. Examples of the roughening treatment by physical means include polishing with a file, sandblast, etc., surface etching with a plasma beam, etc., flame treatment, and the like.

  The fluororesin constituting the porous membrane of the present invention means a polymer containing fluorine, and includes an elastomer such as fluororubber in addition to a resin such as PTFE and polytrifluoride ethylene. Among such porous membranes made of a fluororesin, the PTFE porous membrane has excellent chemical resistance and heat resistance as properties inherent to PTFE, and further has a structural property such as high resolution and high permeability. Since it has the outstanding characteristic, it is preferable. Claim 6 corresponds to this preferable mode.

  The PTFE porous membrane is not particularly limited as long as it is a membrane having PTFE as a main component and a fine porous structure. Such a porous PTFE membrane can be produced, for example, by the method described in Japanese Patent Publication No. 42-13560.

  That is, first, a mixed paste of PTFE fine powder and a lubricant is formed into a predetermined shape such as a sheet. The molded body is stretched at least in a uniaxial direction after or without removing the lubricant. After stretching, it is usually heated to 327 ° C., which is the melting point of PTFE, and sintered.

  In the stretching process, fine fibers are formed in the stretching direction so as to draw a thread between crack-like holes formed such that the PTFE fine powders pressed in the extrusion process are torn apart by stretching. A porous structure is formed by such a fine fibrous structure. A PTFE porous membrane obtained by kneading particles soluble in a solvent into PTFE fine powder or PTFE dispersion liquid to form a predetermined shape, and then dissolving and removing the particles to make it porous, or a PTFE porous film obtained by stretching this A membrane or the like can also be used.

  The thickness of the fluororesin porous membrane of the present invention, the thickness of the inner layer and both outer layers constituting it, and the porous fluorine used for forming the membrane in the fluororesin porous membrane of the above embodiment The thickness of the resin film, the porous pore diameter, and the like are not particularly limited as long as the bubbling point satisfies the scope of the present invention, and are appropriately determined according to the required permeability and mechanical strength. For example, when a film having an average pore diameter of 0.2 μm or less and a bubble point of 100 kPa or more is used as the inner layer, high permeability can be obtained by setting the film thickness to 30 μm or less.

  Although the fluororesin porous membrane of the present invention does not have the property of absorbing water, it has a feature that the water retention after immersion in alcohol or the like and then replacement with water is very high. That is, even if the gas is touched or bubbles or dissolved gas mixed in the aqueous solution adheres, it is difficult to dry. As a result, the processing capacity gradually decreases during use, and problems such as the need to stop the operation for the return operation are small. In addition, excellent characteristics such as excellent properties inherent to the fluororesin porous body, high separation ability, and high permeability are also retained. In particular, in the case of a PTFE porous membrane using PTFE as a fluororesin, the membrane has excellent water retention without impairing the chemical properties and the porous structure inherent to PTFE, such as chemical resistance and heat resistance. Drying can be suppressed.

  The porous film of the present invention is made of a fluororesin, but a porous film made of a water-repellent polymer material such as polyacetal is also used on both outer surface sides of the film instead of the fluororesin. By arranging a film that has a small bubble point and is difficult to dry, the same effect as in the present invention can be obtained.

  Next, the best mode for carrying out the present invention will be specifically described by way of examples. In addition, this invention is not limited to this Example, It can change into another form, unless the meaning of this invention is impaired.

  On both surface sides of a PTFE porous membrane having a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore diameter of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) In a state where a PTFE porous membrane having a bubble point of 15 kPa and a thickness of 100 μm (a porous membrane made of expanded PTFE, a pore diameter of 5 μm, trade name: Poeflon WP-500-100, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) is overlaid and fixed. A PTFE porous membrane was obtained by heating and integrating at 340 ° C. for 20 minutes.

  On both surface sides of a PTFE porous membrane having a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore diameter of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) In a state in which a PTFE porous membrane having a bubble point of 40 kPa and a thickness of 100 μm (a porous membrane made of expanded PTFE, a pore diameter of 1 μm, trade name: Poeflon WP-100-100, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) is stacked and fixed. The porous PTFE membrane was obtained by heating and integration at 340 ° C. for 20 minutes.

  Both sides of a PTFE porous membrane having a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore diameter of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) A PTFE porous membrane was obtained by polishing with a paper file.

  PTFE porous membrane with a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, pore diameter of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) is made of polypropylene so as not to shrink. The sample was fixed on a jig and immersed in a metal sodium-naphthalene complex solution (trade name: Tetra Etch, manufactured by Junkosha Co., Ltd.) for 0.5 seconds, and then washed with IPA and then distilled water. At this time, since the surface of the porous membrane was browned, it was further immersed in hydrogen peroxide solution having a concentration of 30% at 80 ° C. for 24 hours to decompose and remove the brown portion to obtain a PTFE porous membrane.

  A PTFE porous membrane was obtained in the same manner as in Example 4 except that the time for immersion in the metal sodium-naphthalene complex solution was changed from 0.5 second to 2 seconds.

(Comparative Example 1)
A PTFE porous membrane having a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore diameter of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) itself was used.

(Comparative Example 2)
Bubble points on both sides of a PTFE porous membrane with a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore size of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) In a state where 80 kPa, 30 μm thick PTFE porous membrane (a porous membrane made of expanded PTFE, pore diameter 0.45 μm, trade name: Poeflon HP-045-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) is overlaid and fixed. A PTFE porous membrane was obtained by heating and integrating at 340 ° C. for 20 minutes.

(Comparative Example 3)
Bubble point 120 kPa, 30 μm thick PTFE porous membrane (Porous membrane made of expanded PTFE, pore diameter 0.2 μm, trade name: Poaflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) In a state in which a PTFE porous membrane having a point of 15 kPa and a thickness of 100 μm (a porous membrane made of expanded PTFE, a pore diameter of 5 μm, a trade name: Poeflon WP-500-100, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) The porous PTFE membrane was obtained by heating and integration at 340 ° C. for 20 minutes.

(Comparative Example 4)
Only one side of a PTFE porous membrane having a bubble point of 120 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore size of 0.2 μm, trade name: Poeflon HP-020-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) A PTFE porous membrane was obtained by polishing with a numbered paper file.

  The PTFE porous membranes obtained in the above Examples and Comparative Examples all repelled water even when immersed in water as they were, and did not absorb water.

  The PTFE porous membranes obtained in the above Examples and Comparative Examples are punched into a 47 mm-diameter disk to obtain test specimens. The specimen is fixed to the filter holder, and the IPA flow rate is measured through IPA at a pressure of 50 kPa, and then the specimen is immersed in pure water stored in a beaker to replace IPA with pure water.

  Next, the test body is taken out from pure water and exposed to the outside air for 1 minute. When the porous membrane dries, the membrane changes from a wet translucent state to a dry whitish color. Again, fix to the dry filter holder and immediately pass pure water at a pressure of 50 kPa and measure the water flow rate.

  The ease of drying can be quantitatively expressed by the ratio of the IPA flow rate and the water flow rate obtained in this way. That is, in the membrane that is easy to dry, the value of the ratio of the water flow rate to the IPA flow rate is extremely low, whereas in the membrane that is difficult to dry, the value of the ratio of the water flow rate to the IPA flow rate is high. The measurement results are shown in Table 1. Table 1 also shows the IPA bubble point of each PTFE porous membrane measured by the following method (indicated as IPA-BP).

  IPA bubble point measurement method: Measured by ASTM F316. That is, the pressure when air bubbles emerge from the opposite surface for the first time when the air pressure is gradually applied from one surface after impregnating with isopropanol and filling the pores of the tube wall with isopropanol.

From the results of Examples 1 to 5, the lowering of the water flow rate is suppressed by applying a film with a low bubble point, chemical etching, or roughening on both surfaces of the PTFE porous film. It is clear that water retention is maintained (drying is suppressed).

  On the other hand, when such a treatment was not performed (Comparative Example 1), when the bubble point of the film attached to both surfaces exceeded the range of the present invention (70 kPa or less) (Comparative Example 2), the above treatment was performed. In the case where it is applied only on one side (Comparative Examples 3 and 4), it is shown that excellent water retention cannot be obtained.

  Reference Examples 1 to 4 are described below.

(Reference Example 1)
A PTFE porous membrane having a bubble point of 180 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore diameter of 0.1 μm, trade name: Poreflon HP-010-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) itself was used.

(Reference Example 2)
A PTFE porous membrane having a bubble point of 80 kPa and a thickness of 30 μm (a porous membrane made of expanded PTFE, a pore diameter of 0.45 μm, trade name: Poeflon HP-045-30, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) itself was used.

(Reference Example 3)
A PTFE porous membrane having a bubble point of 40 kPa and a thickness of 100 μm (a porous membrane made of expanded PTFE, a pore diameter of 1.0 μm, trade name: Poreflon WP-100-100, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) itself was used.

(Reference Example 4)
A PTFE porous membrane having a bubble point of 15 kPa and a thickness of 100 μm (a porous membrane made of expanded PTFE, a pore diameter of 5.0 μm, trade name: Poeflon WP-500-100, manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) itself was used.

  For each of the reference examples, the same measurement as in the examples and comparative examples was performed, and the results are shown in Table 2. In Reference Examples 2 to 4, the water flow rate / IPA flow rate is excellent, but these are inferior to the fluororesin porous membrane of the above-described embodiment in terms of the fine particle capturing function of the separation membrane.

Claims (5)

It is formed of a porous fluororesin, and the fluororesin is polytetrafluoroethylene, and is a film having an IPA bubble point of 90 kPa or more, and both surfaces thereof are subjected to chemical etching treatment. Fluororesin porous membrane.   2. The fluororesin porous membrane according to claim 1, wherein the inside of the fluororesin porous membrane is also chemically etched.   3. The fluororesin porous membrane according to claim 1, wherein the chemical etching process is an oxidative decomposition process.   The fluororesin porous membrane according to claim 3, wherein the oxidative decomposition treatment is a treatment with an organic alkali metal solution using benzophenone, anthracene or biphenyl as an aromatic anion radical. It is formed of a porous fluororesin, and the fluororesin is polytetrafluoroethylene and is a film having an IPA bubble point of 90 kPa or more, and both surfaces thereof are roughened by physical means. A porous fluororesin membrane characterized by the above.