JP2011052175A - Polytetrafluoroethylene porous film, porous fluorine resin film composite, and method for producing them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a PTFE porous film having ≤50 nm mean flow pore diameter, a porous fluorine resin film composite containing the PTFE porous film having ≤50 nm mean flow pore diameter and a method for producing the PTFE porous film having a smaller pore diameter of ≤50 nm mean flow pore diameter by stretching the PTFE under a condition in which the stretching was impossible until now. <P>SOLUTION: This polytetrafluoroethylene porous film having ≤50 nm mean flow pore diameter, and the porous fluorine resin film composite consisting of the polytetrafluoroethylene porous film and a porous material bonded/fixed to the same are provided. The method for producing the polytetrafluoroethylene porous film or the porous fluorine resin film composite is provided by comprising a process of fixing a non-porous film consisting of a polytetrafluoroethylene having ≥32 J/g amount of the heat of fusion and having ≤20 μm film thickness on a supporting body having a characteristic of elongating uniformly by stretching to obtain a laminate body and a process of stretching the laminate body. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a polytetrafluoroethylene porous membrane having a small average flow pore size that can be used for ultrafiltration and the like, and a porous fluororesin membrane composite comprising the polytetrafluoroethylene porous membrane and a porous support. The present invention also relates to a method for producing these polytetrafluoroethylene porous membrane and porous fluororesin membrane composite.

  A porous film made of polytetrafluoroethylene (PTFE) is excellent in chemical resistance and heat resistance, and thus is used in a filter for filtering fine particles. As a method for producing a porous membrane made of PTFE, for example, a method is known in which a nonporous PTFE membrane is produced and made porous by stretching. A non-porous PTFE membrane may include a method (casting method) in which a dispersion in which PTFE powder is dispersed in a liquid is coated on a substrate, and the liquid is removed and heated to a temperature equal to or higher than the melting point (casting method). For example, it is disclosed by Unexamined-Japanese-Patent No. 5-32810 (patent document 1) etc.

  When a porous membrane made of PTFE is used for a filter, excellent filtration efficiency (filterability, low flow resistance) and high strength are required. Further, in order to enable finer particles to be separated by filtration, it is desired to have finer and uniform pore diameters and no defects such as voids and cracks.

  Higher porosity membranes and thinner membranes are desired for superior filtration efficiency. Generally, increasing the stretch ratio of the nonporous PTFE membrane increases the porosity of the resulting porous membrane and also tends to decrease the film thickness. Efficiency can be improved. However, the increase in the stretch ratio also increases the pore size, and fine particles cannot be separated by filtration. In addition, the strength also decreases due to the decrease in film thickness.

  In Japanese Patent Publication No. 2009-501632 (Patent Document 2), such a problem is solved, and a small pore diameter (to enable filtration and separation of fine particles) and a low flow resistance (excellent filtration efficiency). As a thin and strong filtration membrane (paragraph 0011) that provides both, a novel PTFE porous membrane is disclosed, which has high strength, low flow resistance (excellent filtration efficiency) and fine particles (Paragraph 0012), which has a filtration performance that has been unattainable until now.

JP-A-5-32810 Japanese Patent Publication No. 2009-501632 (paragraph 0012)

  However, the smallest average flow pore size (average flow pore size) of the PTFE porous membrane disclosed in Patent Document 2 is about 55 nm (0.055 μm) (FIG. 6), and is disclosed in Patent Document 2. The average flow pore size of the porous fluororesin composite is 47 nm (0.047 μm), and a PTFE porous membrane having an average flow pore size of less than 47 nm is not disclosed. On the other hand, a PTFE porous membrane having a finer pore size, for example, an ultrafiltration membrane capable of removing polyethylene glycol having a molecular weight of about 50000 is also desired. In order to enable removal of polyethylene glycol having a molecular weight of about 50,000, the average flow pore size of the PTFE porous membrane needs to be about 50 nm or less.

  An object of the present invention is to first provide a PTFE porous membrane having an average flow pore size of 50 nm or less.

  Another object of the present invention is to provide a porous fluororesin membrane composite including a PTFE porous membrane having an average flow pore size of 50 nm or less.

  The present invention also provides a method for producing a PTFE porous membrane having a mean flow pore size of 50 nm or less and a small pore size by a technique of stretching PTFE under conditions that could not be stretched until now, and a porous material having this PTFE porous membrane. It is an object to provide a method for producing a porous fluororesin membrane composite.

  As a result of intensive studies to achieve the above-mentioned problems, the present inventor found the following facts and completed the present invention based on this finding.

1) A PTFE porous membrane having a lower average flow pore size is obtained as the stretching is performed at a lower temperature or PTFE having a higher heat of fusion is used.
2) When stretching is performed at a low temperature or PTFE having a high heat of fusion is used, stretching processability is lowered, and breakage, pinholes, and the like are likely to occur, and the stretching process is difficult.
3) However, when the PTFE membrane is stretched at a low temperature such that a PTFE porous membrane having an average flow pore diameter of 50 nm or less can be obtained by fixing and stretching the PTFE membrane on a support having a property of stretching uniformly by stretching, Even when PTFE having a high heat of fusion is used, the stretching process is facilitated.

  In general, PTFE is characterized by low stretch workability such as being hard and having a low elongation at break when the temperature is below 30 ° C. Especially in the case of a thin film, especially when the film thickness is 20 μm or less, the film thickness unevenness becomes large, and the thin part exceeds the limit of elongation before other parts, so that it is more likely to break, and stretching at a low temperature. Is difficult. Therefore, stretching has been carried out at 30 ° C. or higher, particularly between 50 ° C. and 100 ° C. at which the elongation at break increases.

  In particular, in the case of low molecular weight PTFE having a large heat of fusion and exceeding 30 J / g, the elongation at break is further reduced, so that a pinhole is partially generated exceeding the elongation at break, and it is completely broken only by stretching several tens of percent. Stretching is difficult.

  The present inventor can prevent the occurrence of breakage and pinholes by fixing and stretching on a support having a property of extending uniformly by stretching, even under such difficult conditions. The inventors have found that a PTFE porous membrane having an average flow pore size of 50 nm or less can be obtained by this stretching.

  As a first aspect of the present invention, there is provided a PTFE porous membrane (Claim 1) having an average flow pore size of 50 nm or less. Since this PTFE porous membrane has an average flow pore size of 50 nm or less, it is possible to remove polyethylene glycol having a molecular weight of about 50,000 by ultrafiltration. This PTFE porous membrane can be obtained by the method described in claim 4 described later.

  A second aspect of the present invention provides the PTFE porous membrane according to the first aspect, wherein the film thickness is 20 μm or less. Since this PTFE porous membrane has a thickness of 20 μm or less, the flow resistance during filtration is low, and excellent filtration efficiency can be obtained.

  The second aspect of the present invention is characterized in that it comprises a PTFE porous membrane having an average flow pore size of 50 nm or less, and a porous body that is bonded and fixed to the PTFE porous membrane and has an average flow pore size greater than 50 nm. A porous fluororesin membrane composite is provided. This porous fluororesin membrane composite can be obtained by the method described in claim 7 described later.

  In this porous fluororesin membrane composite, a PTFE porous membrane having an average flow pore size of 50 nm or less is bonded and fixed to a porous body having an average flow pore size greater than 50 nm, and thus high strength is obtained. Therefore, high mechanical strength is desired for the porous body as the support. The average pore diameter and porosity of the support (porous body) are not particularly limited as long as the average flow pore diameter is larger than 50 nm and functions as a support, but from the viewpoint of obtaining excellent filtration efficiency. The average pore diameter and porosity are preferably large.

  The material used for the support is not particularly limited as long as it is a porous body having continuous pores. Specific examples include foams, non-woven fabrics, stretched porous bodies, and the like. Polyolefin resins such as polyethylene and polypropylene, fluorine resins such as PTFE and PFA, polyimide, polyamide Examples thereof include polyimide resins such as imide.

The third aspect of the present invention is a method for producing a PTFE porous membrane having an average flow pore size of 50 nm or less,
Fixing a non-porous film made of PTFE and having a film thickness of 20 μm or less to a support having the property of extending uniformly by stretching to obtain a laminate;
There is provided a method for producing a porous PTFE membrane (Claim 4) comprising a step of stretching the laminate and a step of removing the support from the laminate after stretching.

  This method is characterized in that a nonporous film made of PTFE is stretched, and a PTFE porous membrane having an average flow pore size of 50 nm or less can be obtained by this feature. The low molecular weight PTFE having a heat of fusion exceeding 30 J / g has a small elongation at break and is difficult to stretch, and this tendency is particularly noticeable in the case of a thin film having a thickness of 20 μm or less. Since the laminate is stretched by fixing it to a support having a property of stretching uniformly by stretching, the laminate is stretched, so that uniform stretching can be achieved while suppressing the formation of breaks and pinholes.

  The support used here is not particularly limited as long as it has a property of extending uniformly by stretching and can adhere and fix a nonporous film. However, a support having high mechanical strength and easy elongation is preferable. Specific examples include films made of rubber and other elastomers, polyolefin resins such as polyethylene and polypropylene, and fluorine resins such as PTFE and PFA.

  Examples of the method for fixing the nonporous film to the support include a method of bonding using an adhesive or a pressure-sensitive adhesive, a method of fusing by heating, and the like. Use of a solvent-soluble or thermoplastic fluororesin or fluororubber as the adhesive or pressure-sensitive adhesive is more preferable because it can be used for applications that make use of the heat resistance and chemical resistance of the fluororesin thin film itself.

  After stretching, the support is removed from the laminate to obtain a PTFE porous membrane having an average flow pore size of 50 nm or less and a film thickness of 20 μm or less and having no defects. The support is removed by a method in which the adhesive or pressure-sensitive adhesive for fixing the nonporous film and the support is softened or removed with an organic solvent or the like, or the support is peeled off, or a machine while performing heating or cooling if necessary. It can carry out by the method of peeling automatically.

  A fifth aspect of the present invention provides the method for producing a porous PTFE membrane according to the fourth aspect, wherein the heat of fusion of PTFE constituting the nonporous film is 32 J / g or more. The higher the heat of fusion of PTFE constituting the nonporous film, the smaller the average flow pore size of the PTFE porous membrane obtained by stretching. In particular, it is preferable to form a nonporous film from PTFE having a heat of fusion of 32 J / g or more.

  A sixth aspect of the present invention is the method for producing a porous PTFE membrane according to the fourth or fifth aspect, wherein the stretching is performed at a temperature of less than 30 ° C. The lower the stretching temperature, the smaller the average flow pore size of the PTFE porous membrane obtained by stretching. It is particularly preferable to stretch at a temperature of less than 30 ° C. Conventionally, stretching at a temperature of less than 30 ° C. has been difficult. However, in the method of claim 4, the film is fixed to a support having the property of being stretched uniformly by stretching, and this laminate is stretched. The film can be stretched even at such a low temperature, and the occurrence of breakage and pinholes is suppressed.

As a fourth aspect of the present invention, a porous fluororesin membrane composite comprising a PTFE porous membrane having an average flow pore size of 50 nm or less and a porous body bonded and fixed to the PTFE porous membrane having an average flow pore size greater than 50 nm. A method for manufacturing a body,
A non-porous film made of PTFE having a thickness of 20 μm or less is fixed to a porous support having a property of being uniformly stretched by stretching, and a laminate is obtained; and the laminate is stretched Provided is a method for producing a characteristic porous fluororesin membrane composite (claim 7).

  The support used here is not particularly limited as long as it is a porous material having a property of extending uniformly by stretching and can adhere and fix a nonporous film, but a material having high mechanical strength and easy elongation is preferable. Specific examples include porous films made of rubber and other elastomers, polyolefin resins such as polyethylene and polypropylene, and fluorine resins such as PTFE and PFA.

  As a method for fixing the nonporous film to the support, a method similar to the method according to claim 4 can be employed.

  An eighth aspect of the present invention provides the method for producing a porous fluororesin membrane composite according to the seventh aspect, wherein the heat of fusion of PTFE constituting the nonporous film is 32 J / g or more. A ninth aspect provides the method for producing a porous fluororesin membrane composite according to the seventh or eighth aspect, wherein the stretching is performed at a temperature of less than 30 ° C.

  As in the case of the inventions of claims 5 and 6, the higher the heat of fusion of PTFE and the lower the stretching temperature, the smaller the average flow pore size of the PTFE porous membrane obtained by stretching.

  Since the PTFE porous membrane and the porous fluororesin membrane composite of the present invention have pores having an average flow pore size of 50 nm or less, fine particles can be removed. Therefore, for example, it can be used as a filtration membrane for ultrafiltration of polyethylene glycol having a molecular weight of about 50,000.

  Moreover, the PTFE porous membrane and porous fluororesin membrane composite of the present invention exhibit excellent filterability as well as excellent chemical resistance and heat resistance. When the film thickness is thin, excellent filtration efficiency is also shown. Further, it is very flexible, and in the case of a porous fluororesin membrane composite, it is easy to handle because the porous support is excellent in mechanical strength and the like. Therefore, it can be used for production of various known separation membrane elements.

  The PTFE porous membrane and porous fluororesin membrane composite of the present invention enable the removal of fine particles that could not be removed by filtration with a conventional PTFE porous membrane. The porous fluororesin membrane composite of the present invention is also excellent in terms of mechanical strength. The PTFE porous membrane and the porous fluororesin membrane composite having such excellent characteristics are produced by the method for producing the PTFE porous membrane of the present invention and the method for producing the porous fluororesin membrane composite of the present invention, respectively. can do.

  Next, the form for implementing this invention is demonstrated concretely. In addition, this invention is not limited to this form and an Example, As long as the meaning of this invention is not impaired, it can change into another form.

  The nonporous film used in the method for producing a porous PTFE membrane of the present invention (Claim 4) and the method for producing a porous fluororesin membrane composite (Claim 7) is made of PTFE. PTFE is preferably one having a heat of fusion of 32 J / g or more, but is not limited thereto. Here, the heat of fusion means heating from room temperature to 245 ° C. at 50 ° C./min, heating at 10 ° C./min to 365 ° C., cooling to 350 ° C. at a rate of −10 ° C./min, holding at 350 ° C. for 5 minutes, Cool from 350 ° C to 330 ° C at a rate of -10 ° C / min, cool from 330 ° C to 305 ° C at a rate of -1 ° C / min, cool from 305 ° C to 245 ° C at a rate of -50 ° C / min, and 10 Defined as an endothermic amount between 296 and 343 ° C. when heating from 245 ° C. to 365 ° C. at a rate of 10 ° C./min when heating from 245 ° C. to 365 ° C. in this order at a rate of ° C./min. Is done.

  The nonporous film is preferably thin in order to obtain an excellent filtering ability, but is preferably one having few defects such as voids and cracks.

The feature that there are few defects such as voids and cracks can be expressed by Gurley seconds. Specifically, Gurley seconds are preferably 300 seconds or more, more preferably 1000 seconds or more, and even more preferably 5000 seconds. That's it. Here, the Gurley second is a numerical value representing the air permeability (air permeation amount) described in JIS-P8117 and the like. Specifically, the time (second) in which 100 ml of air passes through an area of 645 cm ² is shown. To express. When the thin film has a defect, air permeates through the defect and thus the Gurley second decreases. However, as the defect decreases, the air hardly permeates and the Gurley second increases.

  A nonporous film having a film thickness of 20 μm or less and having few defects such as voids and cracks is obtained by applying a fluororesin dispersion in which PTFE powder is dispersed in a dispersion medium on a smooth foil, and then applying the dispersion medium. It can be manufactured by a method of drying this, sintering PTFE, and then removing this smooth foil. As the dispersion medium for the fluororesin dispersion, an aqueous medium such as water is usually used.

  A smooth foil is a smooth film in which no holes or irregularities are observed on the surface in contact with the fluororesin dispersion in this production method. Although the thickness range of the smooth foil is not particularly limited, it is a thickness having flexibility such that an operation for covering the fluororesin dispersion coated on the substrate so as to prevent bubbles from entering can be easily performed, A thickness that does not make removal difficult is desirable. After the thin film is formed, the smooth foil is removed. Examples of the removing method include a method of dissolving and removing with an acid or the like when the smooth foil is a metal foil.

  The metal foil is flexible so that it can be easily covered so that air bubbles do not enter the fluororesin dispersion, and it is easy to dissolve and remove with acid after forming a thin film. preferable. Among metal foils, aluminum foil is particularly suitable in terms of flexibility, ease of dissolution and removal, and availability.

  The dispersion medium can be dried by heating to a temperature close to or higher than the boiling point of the dispersion medium. A film made of PTFE powder is formed by drying, and a non-porous film of PTFE can be obtained by heating this film to a temperature equal to or higher than the melting point of the fluororesin and sintering. Drying and sintering heating may be performed in the same step.

  The effect of reducing defects such as voids and cracks is improved by adding a water-soluble polymer that gels under high concentration conditions to the PTFE powder. The water-soluble polymer is preferably nonionic than anionic or cationic, and the molecular weight is preferably 10,000 or more. Specific examples of the water-soluble polymer include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, starch, and agarose.

  In order to suppress the occurrence of defects such as surface irregularities and pinholes, it is also preferable to add 0.5 mg / ml or more of an anionic surfactant. More preferably, it is 2.5 mg / ml or more. Examples of anionic surfactants include carboxylic acid types such as polyoxyethylene, alkyl ether, and carboxylate, sulfate ester types such as polyoxyethylene, alkyl ether, and sulfonate, polyoxyethylene, alkyl ether, and phosphorus. A surfactant such as a phosphate ester type such as an acid salt can be used.

  In the porous fluororesin membrane composite of the present invention, the number of porous supports may be one, but the average flow pore size is 50 nm or less between the two porous supports. A PTFE porous membrane may be sandwiched.

  The measuring method of each physical property value shown in Examples and Reference Examples is shown below.

[Measurement method of 0.055 particle collection rate]
A spherical polystyrene particle latex having an outer shape of 0.055 μm (latex for standard particles STADEX SC0055-D, solid content: 1%, manufactured by JSR) was diluted 100 times with pure water (solid content: 0.01%), and this solution was used as a test solution. And A sample (PTFE porous membrane) was punched into a disk having a diameter of 47 mm, impregnated with isopropanol, fixed to a filtration holder, and 32 ml of the test solution was filtered at a differential pressure of 0.42 kgf / cm ² . The standard particle concentrations of the test liquid and the filtrate were measured using a spectrophotometer (Shimadzu Corporation UV-160). The 0.055 particle collection rate is a value obtained from the following formula using this measured value.
Collection rate = {1− (standard particle concentration of filtrate) / (standard particle concentration of test liquid)} × 100 [%]

[Measurement method of average flow pore size]
GALWICK (propylene, 1,1,2,3,3,3 oxide hexafluoric acid (manufactured by Porous Materials, Inc.) was used as a liquid with a pore distribution measuring device (palm porometer CFP-1500A: manufactured by Porous Materials, Inc.). Specifically, it is obtained as follows: First, the relationship between the differential pressure applied to the membrane and the flow rate of air passing through the membrane is shown in the case where the membrane is dry and the membrane is liquid. The graph obtained was measured as a dry curve and a wet curve, respectively, and the differential pressure at the intersection of the dry curve flow rate ½ and the wet curve is P (Pa). The average flow pore size is determined by the following formula.
Average flow pore diameter d (μm) = cγ / P
Here, c is a constant of 2860, and γ is the surface tension (dynes / cm) of the liquid.

[Measurement method of heat of fusion]
10 mg to 20 mg of PTFE sample is taken and sealed in an aluminum cell if necessary. Here, since it is important to keep PTFE free so that it can contract and deform as much as possible, the cells should not be crushed or crushed.

This sample is heated and cooled under the following conditions.
Heat from room temperature to 245 ° C. at 50 ° C./min, and then heat to 365 ° C. at 10 ° C./min (first step).
Next, it is cooled to 350 ° C. at a rate of −10 ° C./min and held at 350 ° C. for 5 minutes. Next, cooling is performed from 350 ° C. to 330 ° C. at a rate of −10 ° C./min, and from 330 ° C. to 305 ° C. at a rate of −1 ° C./min (second step). The smaller the molecular weight of PTFE, the easier the crystallization is promoted, and the calorific value in the second step tends to increase. Next, it is cooled from 305 ° C. to 245 ° C. at a rate of −50 ° C./min.
Next, it is heated from 245 ° C. to 365 ° C. at a rate of 10 ° C./min (third step).

  A sampling time is performed at 0.5 sec / time, and an endothermic curve and an exothermic curve are obtained using a heat flux differential scanning calorimeter DSC-50 manufactured by Shimadzu Corporation. The endothermic amount and the exothermic amount can be obtained from the endothermic and exothermic curves, and the heat of fusion is a value obtained by integrating the endothermic amount in the section from 296 ° C to 343 ° C.

[Measurement method of IPA bubbling point]
When a sample (PTFE porous membrane or porous fluororesin membrane composite) is impregnated with isopropyl alcohol and the hole in the tube wall is filled with isopropyl alcohol, air pressure is gradually applied from one side for the first time. IPA bubbling point was defined as the pressure at which the water came out from the opposite surface.

Example 1
A PTFE film (No. 920UL, film thickness 20 μm) manufactured by Nitto Denko Corporation was irradiated with an electron beam to adjust the heat of fusion to 42 J / g. The film was heated at 370 ° C. for 5 minutes and then heated at 315 ° C. for 8 hours. This film contracted in the length direction before heating, and the film thickness increased to about 50 μm. This film was processed to a thickness of 13 μm with a rolling roll. This film was fixed by being sandwiched between PTFE tapes (Scotch 5490) manufactured by Sumitomo 3M Co., Ltd. having a width of 50 mm.

  Next, it was stretched 3 times in the width direction of the tape. This was attached to a solvent (MEK) to separate and remove the membrane from the PTFE tape. Next, this film was fixed by being sandwiched between PTFE tapes similar to those described above so that the stretching direction and the length direction of the PTFE tape coincided. This was stretched twice in the width direction of the tape, and then attached to a solvent (MEK) to separate the membrane (PTFE porous membrane) from the PTFE tape. The thickness of the PTFE porous membrane after stretching was 9 μm. The PTFE porous membrane had a Gurley second of 120 seconds, an IPA bubbling point of 549 kPa, and a 0.055 particle collection rate of 75%.

Example 2
[Adjustment of fluororesin dispersion]
Using PTFE dispersion 30J (made by Mitsui DuPont Fluorochemical Co., Ltd.) having a heat of fusion of 50 J / g, MFA latex, and PFA dispersion 920HP, MFA / (PTFE + MFA + PFA) (volume ratio) and PFA / (PTFE + MFA + PFA) (volume) The ratio of the fluororesin dispersion is 2% each, and the concentration of polyethylene oxide having a molecular weight of 2 million is 3 mg / ml, and the polyoxyethylene alkyl ether sulfate triethanolamine (Kao 20T) is 10 mg / ml. The fluororesin dispersion was adjusted by adding as described above.

[Preparation of specimen]
An aluminum foil with a thickness of 50 μm is spread and fixed on a flat glass plate so that there is no wrinkle, and after a fluororesin dispersion is dropped, a stainless steel slide shaft made by Nihon Bearing Co., Ltd. (trade name: stainless fine shaft SNSF) The fluororesin dispersion was stretched uniformly over the entire surface of the aluminum foil by sliding the mold (outer diameter 20 mm).

This foil was dried at 80 ° C. for 60 minutes, heated at 250 ° C. for 1 hour, heated at 340 ° C. for 1 hour, then naturally cooled, and a fluororesin thin film (mainly PTFE) fixed on the aluminum foil. Nonporous fluororesin thin film) was formed. The average thickness of the fluororesin thin film calculated from the weight difference per unit area of the aluminum foil before and after the fluororesin thin film was formed and the true specific gravity (2.25 g / cm ³ ) of the fluororesin was about 3 μm.

  Next, a polyethylene oxide with a molecular weight of 2 million was added to a PFA dispersion obtained by diluting 920HP to 4 times with distilled water, and a polyoxyethylene alkyl ether sulfate ester triethanolamine (20T manufactured by Kao Corporation). It added so that it might become 10 mg / ml, and adjusted the PFA dispersion of 4-fold dilution.

  After spreading and fixing the fluororesin thin film fixed on the aluminum foil on the glass plate so as not to be wrinkled, and dropping this 4-fold diluted PFA dispersion, the same stainless steel made by Nihon Bearing Co., Ltd. as described above. Stretched PTFE porous material with a pore diameter of 0.45 μm and a thickness of 80 μm while the steel slide shaft is slid and the PFA dispersion diluted four times is uniformly spread over the entire surface of the aluminum foil while the moisture does not dry. A body (manufactured by Sumitomo Electric Fine Polymer Co., Ltd., trade name: BoaFLON FP-045-80) (IPA-BP: 150 kPa, porosity: 70%, Gurley second: 9.1 seconds) was covered. Then, after passing through the steps of drying at 80 ° C. for 60 minutes, heating at 250 ° C. for 1 hour, heating at 320 ° C. for 1 hour, and heating at 317.5 ° C. for 8 hours, then naturally cooling to the expanded PTFE porous body A composite in which a non-porous fluororesin thin film mainly composed of PTFE was bonded with thermoplastic PFA having a melting point lower than that of PTFE and an aluminum foil was fixed thereon was obtained. Next, the aluminum foil was dissolved and removed with hydrochloric acid to obtain a test body (a laminate of nonporous PTFE membranes).

  The test specimen had a Gurley second of 5000 seconds or more and was contacted with ethanol at room temperature from the PTFE thin film side, but there was no permeation hole, and this test specimen was a non-porous PTFE membrane (PTFE mainly containing no ethanol penetration). And a non-porous fluororesin thin film).

[Stretching]
Next, this test specimen was stretched in the width direction at a temperature of 25 ° C., a chuck interval of 55 mm, and a stroke of 165 mm (stretching rate: 200%) using a tensile tester, and further at a temperature of 25 ° C. between chucks. The film was stretched in the direction perpendicular to the width direction at 55 mm and a stroke of 88 mm (stretching rate 60%) to obtain a porous fluororesin membrane composite. The Gurley second of this porous fluororesin membrane composite was 80 seconds. The IPA bubbling point was 1180 kPa. The average flow pore size was 0.027 μm.

Example 3
The fluororesin dispersion was adjusted and the test specimen was prepared under the same conditions as in Example 2 to obtain a test specimen (laminate of nonporous PTFE membrane). The Gurley second of this test body was 5000 seconds or more, and ethanol was contacted from the PTFE thin film side at room temperature. However, there was no hole that could penetrate, and this test body was made of a nonporous PTFE membrane (PTFE membrane that did not penetrate ethanol). It was shown to be a laminate including a non-porous fluororesin thin film as a main component.

  Next, the specimen was stretched in the width direction at a temperature of 15 ° C., a chuck interval of 55 mm, and a stroke of 165 mm (stretching rate of 200%) using a tensile tester, and further at a temperature of 15 ° C. between chucks. The film was stretched in the direction perpendicular to the width direction at 55 mm and a stroke of 88 mm (stretching rate 60%) to obtain a porous fluororesin membrane composite. The Gurley second of this porous fluororesin membrane composite was 360 seconds. The IPA bubbling point was the measurement limit of 3000 kPa. The average flow pore size was 0.015 μm or less, which is the measurement limit.

Reference example 1
A PTFE film thickness 20 μm (No. 920UL) manufactured by Nitto Denko Corporation was irradiated with an electron beam to adjust the heat of fusion to 42 J / g. The film was heated at 370 ° C. for 5 minutes and then heated at 315 ° C. for 8 hours. This film contracted in the length direction before heating, and the film thickness increased to about 50 μm. This film was processed to a thickness of 13 μm with a rolling roll. Next, when this film was stretched at 60 ° C., it was broken at a draw ratio of less than 2 times.

  In Example 1, the same film as in Reference Example 1 was stretched after being sandwiched between PTFE tapes (Scotch 5490). In this case, stretching was performed 3 times in length and 2 times in width. From the comparison of the results of Example 1 and Reference Example 1, it was found that a PTFE film that was difficult to stretch and a support (PTFE tape: Scotch 5490) having the property of stretching the PTFE film uniformly by stretching were obtained. It has been shown that stretching is possible if fixed and stretched.

Reference example 2
In the same manner as in Example 2, a fluororesin dispersion was prepared.

[Preparation of specimen]
An aluminum foil with a thickness of 50 μm is spread and fixed on a flat glass plate so that there is no wrinkle, and after a fluororesin dispersion is dropped, a stainless steel slide shaft made by Nihon Bearing Co., Ltd. (trade name: stainless fine shaft SNSF) The fluororesin dispersion was stretched uniformly over the entire surface of the aluminum foil by rolling the mold (outer diameter 20 mm).

This foil was dried at 80 ° C. for 60 minutes, heated at 250 ° C. for 1 hour, heated at 340 ° C. for 1 hour, and heated at 317.5 ° C. for 8 hours, then naturally cooled and fixed on the aluminum foil. A fluororesin thin film (nonporous fluororesin thin film mainly composed of PTFE) was formed. The average thickness of the fluororesin thin film calculated from the weight difference per unit area of the aluminum foil before and after the fluororesin thin film was formed and the true specific gravity (2.25 g / cm ³ ) of the fluororesin was about 3 μm. Next, the aluminum foil was dissolved and removed with hydrochloric acid to obtain a test body (nonporous PTFE membrane).

[Stretching]
This specimen was stretched using a tensile tester, but it was difficult to handle because it was too thin and easily wrinkled, and could not be stretched homogeneously, such as torn with a chuck.

  The specimen of Reference Example 2 was prepared under the same conditions as the fluororesin thin film (non-porous fluororesin thin film mainly composed of PTFE) of Example 2, but in Reference Example 2, the expanded PTFE porous body was used. In other words, it was attempted to stretch only the nonporous PTFE membrane without forming a laminate with the above, and it was not possible to uniformly stretch as described above. On the other hand, in Example 2, a post-stretching attempt was made as a laminate with a stretched PTFE porous body, and uniform stretching was achieved. From this result, it is difficult to stretch the PTFE thin film as it is, but if it is stretched while being fixed to a porous support (stretched PTFE porous body, etc.) having the property of being uniformly stretched by stretching, stretching becomes possible. It is shown.

Comparative Example 1
The fluororesin dispersion was adjusted and the test specimen was prepared under the same conditions as in Example 2 to obtain a test specimen (laminate of nonporous PTFE membrane). The Gurley second of this test body was 5000 seconds or more, and ethanol was contacted from the PTFE thin film side at room temperature, but there was no hole that could penetrate, and this test body was a laminate containing a nonporous PTFE membrane that did not penetrate ethanol It was shown that.

  Next, this test body was stretched in the width direction using a tensile tester at a temperature of 60 ° C., a chuck distance of 55 mm, and a stroke of 165 mm (stretching rate of 200%), and then further the same tensile tester at a temperature of 60 ° C. The film was stretched in the direction orthogonal to the width direction at 55 mm and a stroke of 88 mm (stretching rate 60%) to obtain a porous fluororesin membrane composite. The Gurley second of this porous fluororesin membrane composite was 21 seconds. The IPA bubbling point was 745 kPa. The average flow pore size was 0.055 μm or less.

  From the comparison of the results of Example 2, Example 3 and Comparative Example 1, it is shown that the lower the stretching temperature, the smaller the PTFE porous membrane having a smaller average flow pore size. In Example 2 (25 ° C.) and Example 3 (15 ° C.) in which the temperature during stretching is less than 30 ° C., the PTFE porous membrane having an average flow pore size of 50 nm or less (that is, the PTFE porous membrane of the present invention). In Comparative Example 1 in which the temperature during stretching is 60 ° C., the average flow pore size is 55 nm, indicating that the PTFE porous membrane of the present invention cannot be obtained.

Example 4
In a drying and heating process for obtaining a composite in which a fluororesin thin film is bonded with PFA and an aluminum foil is fixed on the expanded PTFE porous body, drying is performed at 80 ° C. for 60 minutes, and at 250 ° C. for 1 Preparation of a fluororesin dispersion and preparation of a specimen under the same conditions as in Example 2 except that the heating time at 317.5 ° C. performed after heating at 320 ° C. for 1 hour was 0.5 hours. A test specimen was obtained. The Gurley second of this test body was 5000 seconds or more, and when ethanol was contacted from the PTFE thin film side at room temperature, there was no hole that could permeate, and there was a nonporous PTFE film (fluororesin thin film) that did not permeate ethanol. It was shown to be a body.

  Next, using a tenter type horizontal axis stretching machine (stretching zone length 1.5 m), stretching is performed at a temperature of 35 ° C., 230 mm between the inlet chucks, 552 mm at the outlet, and a line speed of 6.3 / min. Got the body. This porous fluororesin membrane composite had a Gurley second of 48 seconds, an IPA bubbling point of 1180 kPa, and an average flow diameter of 0.0485 μm.

Example 5
Adjustment of the fluororesin dispersion and preparation of a test specimen were performed under the same conditions as in Example 4 to obtain a test specimen. The Gurley second of this test body was 5000 seconds or more, and when ethanol was contacted from the PTFE thin film side at room temperature, it was shown that it was a laminate including a nonporous PTFE film that did not have a hole to penetrate and did not penetrate ethanol. It was done.

  Next, using a tenter-type horizontal axis stretching machine (stretching zone length 1.5 m), stretching is performed at a temperature of 24 ° C., 230 mm between the inlet chucks, 552 mm at the outlet, and a line speed of 6.3 / min. Got the body. This porous fluororesin membrane composite had a Gurley second of 378 seconds and an average flow diameter of 0.015 μm or less, which was the limit of measurement.

Example 6
Example 4 except that PTFE dispersion AD911 (Asahi Glass Co., Ltd.) having a heat of fusion of 29.5 J / g was used instead of PTFE dispersion 30J (Mitsui DuPont Fluorochemical Co., Ltd.) having a heat of fusion of 50 J / g. The specimen was prepared by adjusting the fluororesin dispersion and preparing a specimen under the same conditions as in Example 1. The Gurley second of this test body was 5000 seconds or more, and when ethanol was contacted from the PTFE thin film side at room temperature, there was no hole that could penetrate, and it was a laminate including a nonporous PTFE membrane that did not allow ethanol to penetrate. Indicated.

  Next, using a tenter-type horizontal axis stretching machine (stretching zone length 1.5 m), stretching is performed at a temperature of 24 ° C., 230 mm between the inlet chucks, 552 mm at the outlet, and a line speed of 6.3 / min. Got the body. This porous fluororesin membrane composite had a Gurley second of 688 seconds and an average flow diameter of 0.015 μm or less, which was the limit of measurement.

[Evaluation of polyethylene glycol filterability]
Evaluation method Polyethylene glycol (polyethylene glycol powder manufactured by Wako Pure Chemical Industries, Ltd .; average molecular weight 50,000) is dissolved in distilled water to a concentration of 2%, and this is used as a test solution.

  The porous fluororesin membrane composite obtained in Example 4 above, the porous fluororesin membrane composite obtained in Example 6, and a commercially available PTFE membrane (Poreflon HP-010-30 manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) A nominal pore diameter of 100 nm, an average flow pore diameter of 121 nm, a bubble point of 185 kPa, and a Gurley second of 23 seconds measured in the same manner as the nanomembrane were used as separation membrane samples. About each separation membrane sample, polyethylene glycol filterability evaluation was performed by the procedure shown below using the said test liquid.

1) Each separation membrane sample is punched into a disk having a diameter of 47 mm and impregnated with isopropanol.
2) Next, the fluorosurfactant DSN403N manufactured by Daikin Industries, Ltd. was dissolved in distilled water to a concentration of 0.1% (hereinafter referred to as “rinse solution”) for 3 minutes, and then immersed in distilled water for 1 minute. And wash.
3) After the washed membrane was attached to the filter holder, 20 ml of the test solution was filtered.
4) About the test liquid and filtrate, the light absorbency of wavelength 220nm was measured, and the removal rate of PEG was computed by the following formula.
PEG removal rate = {1− (absorbance of filtrate / absorbance of test solution)} × 100 [%]

Evaluation Results The removal rate when using HP-010-30 is 24%, the removal rate when using the porous fluororesin membrane composite of Example 4 is 80%, and the porous fluororesin membrane composite of Example 6 is used. The removal rate when the body was used was 100%. From this result, it was shown that the porous fluororesin membrane composite of the present invention is more excellent in removing fine particles such as polyethylene glycol having a molecular weight of about 50000 than the known PTFE membrane. Further, among the porous fluororesin membrane composites of the present invention, Example 6, which was stretched at a lower temperature than Example 4, was superior in removing fine particles such as polyethylene glycol having a molecular weight of about 50,000. It was shown that.

Claims (9)

  A polytetrafluoroethylene porous membrane having an average flow pore size of 50 nm or less.   The polytetrafluoroethylene porous film according to claim 1, wherein the film thickness is 20 μm or less.   3. A porous fluorine film comprising: the polytetrafluoroethylene porous membrane according to claim 1 or 2; and a porous body bonded and fixed to the polytetrafluoroethylene porous membrane and having an average flow pore size larger than 50 nm. Resin membrane composite. An average flow pore size is 50 nm or less, a method for producing a polytetrafluoroethylene porous membrane,
Fixing a nonporous film made of polytetrafluoroethylene and having a film thickness of 20 μm or less to a support having a property of extending uniformly by stretching to obtain a laminate;
A method for producing a polytetrafluoroethylene porous membrane, comprising: a step of stretching the laminate; and a step of removing the support from the laminate after stretching.   The method for producing a porous polytetrafluoroethylene film according to claim 4, wherein the heat of fusion of polytetrafluoroethylene constituting the nonporous film is 32 J / g or more.   The method for producing a porous polytetrafluoroethylene film according to claim 4 or 5, wherein the stretching is performed at a temperature of less than 30 ° C. Method for producing a polytetrafluoroethylene porous membrane having an average flow pore size of 50 nm or less, and a porous fluororesin membrane composite comprising a porous body bonded and fixed to the polytetrafluoroethylene porous membrane and having an average flow pore size greater than 50 nm Because
From a step of fixing a non-porous film made of polytetrafluoroethylene and having a thickness of 20 μm or less to a porous support having a property of extending uniformly by stretching to obtain a laminate, and a step of stretching the laminate A method for producing a porous fluororesin membrane composite comprising:   8. The method for producing a porous fluororesin membrane composite according to claim 7, wherein the heat of fusion of polytetrafluoroethylene constituting the nonporous film is 32 J / g or more.   The method for producing a porous fluororesin membrane composite according to claim 7 or 8, wherein the stretching is performed at a temperature of less than 30 ° C.