JP2013237808A - Micropore film made of modified polytetrafluoroethylene and method of manufacturing the same, porous resin film composite, and filter element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a method of manufacturing a micropore film made of modified polytetrafluoroethylene in which a difference between an average flow rate pore size and the maximum pore size is small and that is excellent in permeability; the micropore film manufactured by the manufacturing method; a porous resin film composite using the micropore film; and a filter element using the porous resin film composite.SOLUTION: A method of manufacturing a micropore film made of modified polytetrafluoroethylene comprises: heating a film obtained by forming particles of modified polytetrafluoroethylene to a given shape dimension, at higher temperature than a melting point thereof to be non-porous; and then stretching the resultant film to be porous, wherein an amount of heat of fusion of the modified polytetrafluoroethylene is less than 32 J/g, and the stretching step has a low temperature stretching process and a high temperature stretching step. In addition, the micropore film manufactured by the manufacturing method, a porous resin film composite obtained by using the micropore film, and a filter element obtained by using the porous resin film composite are disclosed.

Description

  The present invention relates to a microporous membrane (hereinafter referred to as “modified PTFE”) made of polytetrafluoroethylene (hereinafter referred to as “modified PTFE”) obtained by copolymerizing a small amount of hexafluoropropylene (HFP), perfluoroalkyl vinyl ether (PAVE) or the like. It is related with a manufacturing method of “made fine pore diameter membrane”. More specifically, the present invention relates to a method for producing a modified PTFE microporous membrane in which modified PTFE resin particles are molded into a membrane and then heated to make it nonporous, and the resulting nonporous membrane is stretched to make it porous. The present invention also relates to a modified PTFE fine pore diameter membrane obtained by the above-described production method, having a narrow pore size distribution and having excellent treatment liquid permeability when used as a filtration membrane. The present invention further relates to a porous resin membrane composite having the modified PTFE microporous membrane as a constituent element, and a filter element using the porous resin membrane composite as a filtration membrane.

  The microporous membrane made of fluororesin is a membrane-like porous body formed from fluororesin such as polytetrafluoroethylene (hereinafter referred to as “PTFE”) as a main raw material, and has a micropore size. . Among the fluororesin microporous membranes, PTFE microporous membranes (PTFE porous body) formed mainly of PTFE are excellent in chemical resistance and heat resistance, and therefore are filtration membranes for filtering fine particles ( Filter) and the like.

  As a method for producing a PTFE porous body, for example, in Patent Document 1, an extrusion aid is blended with PTFE fine powder, mixed and paste-extruded into a desired shape, and the extruded product is heated to 345 ° C. A method is disclosed in which a semi-fired state in which a high melting point is left in the vicinity is left, and then it is stretched to make it porous. The PTFE fine powder used here is a number obtained by drying PTFE particles (primary particles) having a particle size of 0.15 to 0.35 μm produced by emulsion polymerization of tetrafluoroethylene (emulsion polymerization product). It is a powder granulated to 100 μm to several thousand μm.

  In the above method, the molded product is made porous by stretching the extruded product. However, since this method is basically powder molding and does not completely melt the molded product, there are voids between the particles before stretching. Therefore, large pores are easily generated in the porous molded product (porous body), and it has been difficult to produce a microporous membrane having excellent filtration performance.

  Therefore, after forming a powder such as PTFE fine powder into a film shape, it is heated and sintered to a temperature higher than the melting point of PTFE, completely melted until it loses air permeability and alcohol permeability, and then made nonporous. A method of stretching to make it porous is disclosed in Patent Document 2 and the like. Further, in Patent Document 3, after applying a dispersion in which a fluororesin powder such as PTFE is dispersed in a dispersion medium, the dispersion medium is dried and the fluororesin powder is sintered to obtain a fluororesin powder. By completely melting, a non-porous fluororesin thin film with few defects such as voids and cracks can be obtained, and by making this non-porous fluororesin thin film porous, it has fine pores. It is disclosed that a fluororesin thin film (fluorine resin microporous membrane) having a high porosity and no defects can be obtained. Further, Patent Documents 2 and 3 describe that stretch workability is good when PTFE having a heat of fusion of 32 J / g or more, which is an index of molecular weight, is used.

  Furthermore, in Patent Document 4, a porous film having a smaller pore diameter, particularly a porous film having an average flow pore diameter of 45 nm or less, is manufactured in the method of making a porous film after it is completely melted and made nonporous. In order to do so, it is described that it should be stretched at 30 ° C. or lower.

JP 2007-332342 A JP 2007-7732 A Japanese Patent No. 4371176 JP 2010-94579 A

  In recent years, in order to obtain better fractionation performance as a filtration membrane used in a filter element, a fine pore size membrane having a smaller difference between the average flow pore size and the maximum pore size and a narrow pore size distribution is desired. Furthermore, in order to obtain a large treatment flow rate used as a filter membrane in a filter element, a fine pore diameter membrane having excellent permeability of a treatment liquid (liquid to be filtered through the filter element) is desired.

  Here, the average flow pore size is the relationship between the differential pressure applied to the membrane and the air flow rate through the membrane by the bubble point method (ASTM F316-86, JIS K3832) using a pore size distribution measuring device or the like. Measured when the film is dry and when the film is wet with a liquid, and the obtained graph is a dry curve and a wet curve, respectively. When the differential pressure is P (Pa), it is a value of d (μm) represented by the formula d = cγ / P, and is an index corresponding to the average pore diameter (c is a constant 2860, γ is Liquid surface tension (dynes / cm)).

  The maximum pore diameter is a value measured by the bubble point method using the above-mentioned pore diameter distribution measuring instrument or the like, and is an index corresponding to the upper limit of the pore diameter distribution. Specifically, when the air pressure on one side is increased while the entire membrane surface is wet with liquid, the pressure at which air permeation exceeds the capillary force of the membrane (minimum pressure) is measured. P1 is calculated from the above formula (maximum pore diameter = cγ / P1). Recently, PMI has developed a new bubble point method using a combination of two liquids, and this method can measure pore sizes from 1 nm to several tens of nm (Internet: pmiapp.com/products/liquid- liquid-porometer.html etc.).

  However, according to the above-described method, although a fluororesin microporous membrane having an average flow pore size of 50 nm or less and having few defects can be obtained, the permeability of the treatment liquid does not always satisfy the recent demand. There wasn't. In particular, as a result of intensive studies by the present inventors, it has been found that when the pore diameter is adjusted to be smaller, the permeability is poor and the treatment flow rate tends to be small. When the processing flow rate is reduced, in order to obtain a predetermined processing amount, it is necessary to increase the membrane area, increase the capacity of the pump, etc., and increase the size of the equipment. Furthermore, it has been found by the present inventor that the difference between the average flow pore diameter and the maximum pore diameter (pore diameter distribution) increases when the average flow pore diameter is 40 nm or less.

  When the pore size distribution becomes wide, fine foreign matters cannot be removed with high removal efficiency (sufficient fractionation performance). That is, the fractionation performance is reduced. Therefore, when the pore diameter is very small, for example, even when the average flow pore diameter is adjusted to 40 nm or less, the difference between the average flow pore diameter and the maximum pore diameter is small, and the fractionation performance is excellent, and the permeability is also excellent. Development of a resin microporous membrane is desired.

  The present invention provides a fluororesin microporous membrane having a small difference between the average flow pore size and the maximum pore size as compared with a fluororesin microporous membrane manufactured by a conventional method and having excellent fractionation performance and excellent permeability. It is an object of the present invention to provide a method that can be manufactured, and a fluororesin-made microporous membrane that is manufactured by this method and has excellent fractionation performance and permeability. Another object of the present invention is to provide a porous resin membrane composite using the fluororesin microporous membrane and a filter element using the porous resin membrane composite.

  As a result of diligent research to solve the above-mentioned problems, the inventor formed a fluororesin powder (particles) into a film shape, and heated the resulting molded product to completely melt the fluororesin powder and to make it nonporous. In the method of forming a porous material by subsequent stretching, a modified PTFE having a heat of fusion of less than 32 J / g is used as the raw material fluororesin particles, and the stretching is performed at a temperature of 80 ° C. or less and at a temperature exceeding 80 ° C. By performing the stretching in at least two steps, it is possible to obtain a fluororesin microporous membrane having an average flow pore size of 40 nm or less, a smaller difference between the average flow pore size and the maximum pore size, and excellent permeability. The headline and the present invention were completed.

  The invention according to claim 1 is a film forming step for obtaining a film-shaped molded product by forming a powder of modified PTFE into a film shape, and heating the film-shaped molded product to a temperature equal to or higher than the melting point of the modified PTFE. And a stretching step of stretching the nonporous membrane-shaped molded product to make it porous, the heat of fusion of the modified PTFE is less than 32 J / g, and the stretching The modified PTFE micropore diameter is characterized in that the process has a low temperature stretching step of stretching in at least one direction at 80 ° C. or less and a high temperature stretching step performed after the low temperature stretching and stretching in at least one direction at a temperature exceeding 80 ° C. It is a manufacturing method of a film | membrane.

  In the production method of the present invention, the raw material modified PTFE powder is a powder composed of fine particles of modified PTFE. A modified PTFE dispersion, which is an emulsion in which fine particles of modified PTFE (modified PTFE powder) are dispersed in a liquid (dispersion medium), can also be used as a raw material modified PTFE powder. Examples of the modified PTFE powder include modified PTFE fine powder produced by emulsion polymerization and modified PTFE molding powder produced by emulsion polymerization, which is a powder composed of fine particles of modified PTFE.

  A method of obtaining a film-shaped molded article having a predetermined shape by forming a powder of modified PTFE into a film shape is a known method for forming a film from a powder, for example, paste extrusion described in Patent Document 1, It can also be carried out by the method described in Patent Document 2. A film-shaped molded article can also be obtained by the method described in Patent Document 3 using a modified PTFE dispersion or the like. Specifically, a method (casting method) in which the modified PTFE dispersion is coated on a flat plate to form a film and the dispersion medium is dried and removed can be used. According to this method, the generation of voids and other defects in the modified PTFE non-porous membrane can be greatly suppressed, the membrane has no orientation and is isotropic and homogeneous, and it does not shrink or deform during stretching. A pore diameter membrane can be obtained. Since the modified PTFE usually has a high melt viscosity and is difficult to melt-extrusion and it is difficult to produce a solution thereof, the above-described method is generally employed.

  Modified PTFE is a PTFE copolymerized with a small amount of hexafluoropropylene (HFP), alkyl vinyl ether (PAVE), chlorotrifluoroethylene (CTFE) or the like, preferably 1/50 (molar ratio) or less with respect to tetrafluoroethylene. Say.

  The film-shaped molded article obtained in the film forming process is heated to a temperature equal to or higher than the melting point of the modified PTFE constituting the film to completely melt the modified PTFE powder (sintering process). As a result of completely melting the modified PTFE powder, a nonporous membrane-shaped molded article (modified PTFE nonporous membrane) is produced. The non-porous film-shaped molded article means a film having few holes penetrating the film, and specifically, a film having a Gurley second of 5000 seconds or more is preferable. In order to completely melt the modified PTFE powder and produce a nonporous membrane-shaped molded article having a large Gurley second, it is preferably heated at a temperature higher than the melting point of the raw material, and also suppresses decomposition and modification of the resin. Therefore, the heating temperature is preferably 450 ° C. or lower.

  The nonporous membrane-like molded article thus obtained is stretched and made porous. Before stretching, the temperature is raised above the melting point of the modified PTFE and then slowly cooled below the crystalline melting point, or the heating is performed at a temperature slightly lower than the melting point of the modified PTFE for a certain time (hereinafter referred to as “constant temperature treatment”). It is preferable to perform annealing. Annealing can be performed in the same manner as the method described in Patent Document 3.

  By annealing, the crystallinity of the modified PTFE resin can be saturated before stretching, and the reproducibility of the pore diameter can be further enhanced in the production of the porous membrane. In the crystallization process, the lower the slow cooling rate or the longer the constant temperature treatment time, the higher the crystallinity and the higher the heat of fusion. On the other hand, the higher the slow cooling rate or the shorter the constant temperature treatment time, the lower the crystallinity and the lower the heat of fusion.

The production method of the present invention comprises:
1) Use a raw material modified PTFE having a heat of fusion of less than 32 J / g,
2) The stretching step includes a low-temperature stretching step of stretching at least in one direction at 80 ° C. or less and a high-temperature stretching step of stretching in at least one direction at a temperature exceeding 80 ° C. performed after the low-temperature stretching. Due to these features, a modified PTFE microporous membrane having an average flow pore size of 40 nm, a smaller difference between the average flow pore size and the maximum pore size and excellent permeability is obtained.

The heat of fusion of the modified PTFE (fluororesin) is a value measured by the following method.
1) A sample to be measured (modified PTFE) is heated from room temperature to 100 ° C. at 50 ° C./min, and then heated to 365 ° C. at 10 ° C./min (first step).
2) Next, cool to 350 ° C. at a rate of −10 ° C./min and hold at 350 ° C. for 5 minutes. Further, cooling is performed at a rate of −10 ° C./min from 350 ° C. to 330 ° C., and cooling is performed at a rate of −1 ° C./min from 330 ° C. to 305 ° C. (second step).
3) Next, after cooling from 305 ° C. to 100 ° C. at a rate of −50 ° C./min, heating from 100 ° C. to 365 ° C. at a rate of 10 ° C./min (third step).
The amount of heat absorbed during heating in this third step is defined as the amount of heat of fusion.

  The heat of fusion of the modified PTFE can be adjusted by the crystallization tendency of the modified PTFE (fluororesin), and the heat of fusion increases as the crystallization tendency increases. The crystallinity of the modified PTFE resin can be adjusted by changing the molecular weight of the resin or the annealing conditions after forming the nonporous film.

  In the stretching process of the production method of the present invention, after stretching at 80 ° C. or lower, stretching is further performed at a temperature exceeding 80 ° C. That is, at least two stages of stretching processes are performed: a low-temperature stretching process in which stretching is performed at 80 ° C. or less and a high-temperature stretching process in which stretching is performed at a temperature exceeding 80 ° C. As a result of performing stretching at different temperatures, a modified PTFE microporous membrane having a small difference between the maximum pore size and the average flow rate pore size and good filtration performance and excellent permeability can be obtained.

  Stretching can be performed by uniaxial stretching or biaxial stretching for both low temperature stretching and high temperature stretching, but biaxial stretching is preferred. Further, stretching in different directions may be performed for stretching at a low temperature and stretching at a high temperature.

  By stretching, the non-porous film-shaped molded article (modified PTFE non-porous film) is made porous to form pores. As the size of stretching increases, the pore size increases and the average flow pore size also increases. Therefore, the degree of stretching at low temperature and stretching at high temperature is selected so that the desired average flow pore size is obtained. Preferably, the stretching is performed so that the stretching ratio is approximately the same in the low-temperature stretching step and the high-temperature stretching step.

  According to the production method of the present invention, a modified PTFE microporous membrane having a small average flow pore size can be produced. Even if the average flow pore size is 40 nm or less, the modified PTFE microporous membrane is not suitable. The difference between the mean flow pore size and the maximum pore size can be made small (pore size distribution is narrow and therefore excellent in filtration performance), and the permeability can be made high. Therefore, if the modified PTFE microporous membrane obtained by the production method of the present invention is used as a filtration membrane for removing fine particles, fine particles can be reliably removed and high removal efficiency can be achieved. In addition, the flow rate of filtration can be increased, and problems such as an increase in membrane area, an increase in pump capacity, and an increase in equipment size can be solved.

  The invention according to claim 2 is the method for producing a modified PTFE microporous membrane according to claim 1, wherein the low-temperature stretching step is performed at a temperature lower than 30 ° C.

  It is preferable to perform stretching at a temperature exceeding 80 ° C. after stretching at less than 30 ° C., because the permeability of the produced modified PTFE microporous membrane can be further improved. More preferably, it is a case where a low-temperature extending | stretching process is performed at less than 20 degreeC, and permeability can further be improved.

  According to a third aspect of the present invention, the modified PTFE powder is a PFA-modified PTFE or FEP-modified PTFE powder, wherein the modified PTFE-made microporous membrane according to the first or second aspect is produced. Is the method.

  By using PFA-modified PTFE or FEP-modified PTFE powder as the raw material modified PTFE powder, the modified PTFE has high heat resistance and chemical stability, and has a narrower pore size distribution and a smaller difference between the average flow pore size and the maximum diameter. It is preferable because a microporous membrane can be obtained.

  Invention of Claim 4 is manufactured by the manufacturing method of the microporous film | membrane made from the modified PTFE of any one of Claim 1 thru | or 3, The average flow hole diameter is 40 nm or less, It is characterized by the above-mentioned. This is a modified PTFE microporous membrane. Even if the modified PTFE microporous membrane produced by the production method of the present invention has a fine pore diameter of 40 nm or less, the average flow pore diameter and the maximum diameter are as described above. Difference is small (the pore size distribution is narrow, and thus the filtration performance is excellent), and the permeability is high. And it is used suitably as a filtration membrane for fine particle removal, a fine particle can be removed reliably, high removal efficiency can be achieved, and the processing flow rate of filtration can be increased.

  The invention according to claim 5 is the modified PTFE fine pore diameter membrane according to claim 4, wherein the difference between the average flow pore size and the maximum pore size is less than 15 nm.

  In order to reliably remove fine particles and achieve high removal efficiency, the average flow pore diameter of the fine pore diameter membrane is small and the difference between the average flow pore diameter and the maximum diameter is small (pore diameter distribution is narrow). It is desired that the dimensional accuracy is high). Among these, when the average flow pore size is 40 nm or less and the difference between the average flow pore size and the maximum pore size is less than 15 nm, fine particles can be more reliably removed, and high removal efficiency of fine particles is achieved. Since an effect becomes larger, it is preferable.

  A sixth aspect of the present invention is the modified PTFE microporous membrane according to the fifth aspect, wherein the value of (maximum pore diameter−average flow pore diameter) / average flow pore diameter is 0.5 or less. When the average flow pore size is 40 nm or less, the difference between the average flow pore size and the maximum pore size is less than 15 nm, and the difference between the average flow pore size and the maximum pore size is 50% or less of the average flow pore size, It is more preferable because the above-described effect becomes more remarkable and finer particles can be removed with high removal efficiency.

The invention described in claim 7 is characterized in that a permeability index expressed by film thickness (nm) / [(average flow pore size (nm)) ² × Gurley second] is 0.009 or more. The modified PTFE microporous membrane according to any one of claims 4 to 6. The modified PTFE microporous membrane of the present invention has an excellent porosity and can obtain a high processing speed when used as a filtration membrane. The permeability index represented by the above formula is an index indicating the size of the porosity. When a modified PTFE microporous membrane having a permeability index of 0.009 or more is used as a filtration membrane, the permeability that permeates the membrane. This is preferable because the bundle (flow rate) can be increased and a high processing speed can be obtained.

  According to an eighth aspect of the present invention, the modified PTFE fine pore diameter membrane according to any one of the fourth to seventh aspects has an average flow pore size larger than that of the modified PTFE fine pore diameter membrane and has a breaking load. It is a porous resin film composite characterized by being fixed on a high porous support.

  When the modified PTFE microporous membrane is used as a filtration membrane, the thickness of the membrane is preferably thin and usually 50 μm or less in order to reduce pressure loss and increase the filtration speed. However, with such a thickness, sufficient strength as a filtration membrane may not be obtained. Therefore, it is preferable to use the modified PTFE fine pore diameter membrane as a composite by fixing it on a porous support having a larger average flow pore size and a higher breaking load. Since the porous support has a large average flow pore size, it does not decrease the filtration speed even when combined with a modified PTFE fine pore membrane, and the porous resin membrane has excellent mechanical strength due to high breaking load. A complex is obtained.

  The porous support imparts mechanical strength to the composite. On the other hand, when the composite is used as a filtration membrane, it is desirable that the properties as a filtration membrane, such as processing ability and processing speed, are not impaired. It is. Therefore, as the porous support, a PTFE porous body excellent in mechanical strength, chemical resistance, and heat resistance is preferably used, and the pore diameter (average flow pore diameter) is combined with it. In addition to being larger than the pore diameter (average flow pore diameter) of the pore diameter membrane, it is also desired that the porosity be high. Specifically, a PTFE porous body produced by stretching a PTFE membrane to form pores of 100 nm or more, preferably 200 nm or more, having a thickness that gives sufficient mechanical strength. Preferably used.

  The porous resin membrane composite of the present invention can be obtained by pasting the modified PTFE non-porous membrane on a porous support to produce a composite, and then stretching the composite. By stretching, the modified PTFE non-porous membrane constituting the composite is also stretched to form the modified PTFE microporous membrane of the present invention.

  The porous resin membrane composite of the present invention is excellent in the effect of a modified PTFE microporous membrane that is a constituent element thereof, that is, when used as a filtration membrane for removing fine particles, fine particles are reliably removed. It is possible to achieve high removal efficiency, while maintaining the effect that the processing flow rate of filtration can be increased, and it has high mechanical strength. In addition, by using a porous resin membrane composite, handling at the time of use or processing becomes easier than in the case of using only a modified PTFE microporous membrane. Therefore, this porous resin membrane composite is suitably used as a filter membrane for filtering fine particles in a filter element or the like for filtering fine particles. Therefore, the present invention further provides, as claim 9, a filter element characterized by using the porous resin membrane composite according to claim 8 as a filtration membrane.

  By the method for producing a modified PTFE fine pore diameter membrane of the present invention, even if the average flow pore diameter is 40 nm or less, the difference between the average flow pore diameter and the maximum diameter is small (pore diameter distribution is narrow) and the permeability is high. Although the modified PTFE microporous membrane of the present invention obtained by this production method can produce a microporous membrane, the difference between the average flow pore size and the maximum diameter is small even though the average flow pore size is 40 nm or less. And because of its high permeability, when used as a filtration membrane for removing fine particles, fine particles can be reliably removed, high removal efficiency can be achieved, and the filtration processing flow rate can be increased. Can do.

  Furthermore, the porous resin membrane composite of the present invention formed by fixing this modified PTFE fine pore membrane on a porous support can be used as a filtration membrane for removing fine particles, Excellent effect can be achieved. Therefore, the filter element of the present invention using this porous resin membrane composite as a filtration membrane can surely remove fine particles, achieve high removal efficiency, and reduce the filtration treatment flow rate. Can be bigger.

2 is an electron micrograph of the surface of the intermediate (after the low temperature stretching step) obtained in Example 1. It is an electron micrograph of the surface of the product (after a high temperature extending process) obtained in Example 1. It is an electron micrograph of the cross section of the product (after a high temperature extending | stretching process) obtained in Example 1. FIG.

  Next, specific modes for carrying out the present invention will be described. In addition, this invention is not limited to the form described here.

  The modified PTFE, which is a copolymer of HFP and tetrafluoroethylene, or a copolymer of PAVE and tetrafluoroethylene, which is suitable as a raw material in the present invention, has the following structural formula (I) or the following structural formula (II), respectively. It is represented by

  M in the structural formulas (I) and (II) is preferably 50 or more. In this case, the monomer constituting the copolymer is mainly tetrafluoroethylene, and its melting point is relatively high at 320 ° C. or higher. The copolymer in this case (especially when m is 100 or more) has a high melt viscosity and is difficult to be melt-extruded, and is a melt-extrudable thermoplastic resin such as tetrafluoroethylene / hexafluoropropylene copolymer (FEP) A distinction is made from tetrafluoroethylene / perfluoroalkyl ether copolymers (PFA). In addition, when m exceeds 400, it becomes difficult to obtain the effect of copolymerizing HFP and PAVE, that is, the effect that the difference between the average flow pore size and the maximum diameter can be made smaller, so m is in the range of 50 to 400. preferable.

In Structural Formulas (I) and (II), n represents the degree of polymerization. The range of n is not particularly limited, but as the raw material of the present invention, a copolymer having a heat of fusion in the third step in the range of 18 to 32 J / g (a molecular weight such as to be) is preferably used. If the molecular weight is too high, the porosity (that is, the permeability index) tends to decrease, and if the molecular weight is too low, pinholes are generated, and the film tends to be broken during stretching. Further, the formula (II) Rf in represents a perfluoroalkyl group is preferably a perfluoro propyl -C ₃ F ₇ or perfluoromethyl -CF ₃ carbon atoms is 1 the carbon atoms 3.

  The copolymer constituting the modified PTFE microporous membrane of the present invention may be a copolymer obtained by copolymerizing other monomers in addition to HFP or PAVE as long as the gist of the present invention is not impaired. Moreover, what co-polymerized HFP and PAVE may be used.

  In the production method of the present invention, the film forming step (nonporous film forming step) can be performed, for example, by the following method.

(1) A method in which an extrusion aid is added to the fine powder of modified PTFE, and then paste extrusion molding into a sheet shape or a tube shape and rolling as necessary.

(2) A method in which a fine film of modified PTFE is formed into a cylindrical shape by compression molding, and after making the melting point or higher, a thin film of about 20 μm is produced by cutting while rotating.

(3) A method in which a dispersion of modified PTFE is applied (casting) onto a smooth foil, and then the dispersion medium is dried and further heated to a temperature equal to or higher than the melting point of the polymer (that is, the casting method described above). Method described in Document 3).

  The modified PTFE fine powder used in the method (1) is a particle latex (particle diameter: 150 to 350 nm) produced by emulsion polymerization of tetrafluoroethylene and a copolymerized monomer such as HFP or PAVE. This is a powder (diameter: 300 to 600 μm) which has been deposited, dried and granulated. In emulsion polymerization, particles (primary particles) close to true spheres are grown to about 150 to 350 nm while polymerizing a tetrafluoroethylene monomer and a copolymerized monomer (such as HFP or PAVE) in water.

  The modified PTFE fine powder used in the method (2) is the same powder as that used in the method (1).

  The dispersion of the modified PTFE used in the method (3) is produced by emulsion polymerization of tetrafluoroethylene and a monomer to be copolymerized (such as HFP or PAVE).

  The dispersion medium of the modified PTFE dispersion is usually an aqueous medium such as water. The content of the modified PTFE resin particles in the dispersion is preferably in the range of 20 to 50% by volume. When the dispersion further contains a nonionic, water-soluble polymer having a molecular weight of 10,000 or more, these do not affect the dispersion of the dispersion, and also gelate during moisture drying to form a film, and the modified PTFE has fewer defects. Since a thin film (non-porous film made of modified PTFE) is obtained, it is preferable. Examples of the nonionic water-soluble polymer having a molecular weight of 10,000 or more include polyethylene oxide and polyvinyl alcohol.

  The smooth foil used in this method is a smooth film in which no holes or irregularities are observed on the surface in contact with the dispersion. The smooth foil is preferably a metal foil that has flexibility and is easy to dissolve and remove with an acid or the like after the film is formed. Among the metal foils, the aluminum foil is particularly suitable from the viewpoint of flexibility and ease of dissolution and removal, and also from the viewpoint of availability.

  The range of the thickness of the smooth foil is not particularly limited, but a thickness having flexibility is desirable, and when the smooth foil is removed after the film is formed, a thickness that does not make removal difficult is desirable. For example, when a smooth foil is dissolved and removed, a thickness that can be easily dissolved and removed is desired.

  In this method, the dispersion medium is dried after casting by a method such as simply applying a dispersion on a smooth foil. Drying can be performed by heating to a temperature close to or higher than the boiling point of the dispersion medium. A film made of a modified PTFE resin is formed by drying, and this film is heated to a temperature equal to or higher than the melting point of the modified PTFE to sinter a nonporous modified PTFE thin film (modified PTFE nonporous film). Can be obtained. Drying and sintering heating may be performed in the same step.

  The modified PTFE nonporous film obtained in the film formation step is preferably annealed. The annealing is performed by a method in which the temperature is raised above the melting point of the modified PTFE constituting the nonporous film and then slowly cooled below the crystalline melting point, or by a constant temperature treatment. The slow cooling is performed at a cooling rate of 10 ° C./min or less, for example. The isothermal treatment is performed, for example, at a temperature 2 ° C. to 10 ° C. lower than the melting point peak of the modified PTFE and held for 30 minutes to 10 hours.

  The modified PTFE non-porous film obtained in the film forming step (or further annealed) is stretched at a temperature of 80 ° C. or less as described above, and further stretched at a temperature exceeding 80 ° C., The modified PTFE non-porous membrane of the present invention is obtained. The initial stretching temperature is preferably 80 ° C. or lower, more preferably 30 ° C. or lower, and even more preferably 19 ° C. or lower.

  By bonding the modified PTFE microporous membrane of the present invention to a porous support, a porous resin membrane composite having further excellent mechanical strength can be produced. As a manufacturing method of a porous resin film composite, the method which consists of the following processes 1-4 can be mentioned, for example.

Step 1: After applying a dispersion in which modified PTFE resin particles are dispersed in a dispersion medium on a smooth foil, the dispersion medium is dried, and further heated to a temperature equal to or higher than the melting point of the modified PTFE, and then sintered. A non-porous membrane made of PTFE is formed.
Step 2: After repeating Step 1 as necessary, a porous support is bonded onto the modified PTFE nonporous membrane. The bonding of the modified PTFE nonporous membrane and the porous support may be performed using a thermoplastic resin having a melting point lower than that of PTFE, for example, PFA as an adhesive.
Step 3: After Step 2, the smooth foil is removed to obtain a composite composed of the modified PTFE nonporous membrane and a porous support. The method of removing the smooth foil is not particularly limited, but when the smooth foil is a metal foil, a method of dissolving and removing with an acid or the like can be mentioned.
Step 4: The composite is stretched. As described above, by annealing before stretching, the crystallinity of the modified PTFE resin before stretching can be saturated, and as a result, a high porosity (large permeability index) can be obtained, and the pore diameter can be increased. This is preferable because it can be manufactured with high reproducibility.

  First, measurement methods of heat of fusion, air permeability (Gurley second), average flow pore size, maximum pore size, and permeability index in Examples and Comparative Examples will be described.

[Measurement method of heat of fusion]
A heat flux differential scanning calorimeter (manufactured by Shimadzu Corporation; heat flux differential scanning calorimeter DSC-50) was used by the following method.

  10-20 mg of sample is heated from room temperature to 100 ° C. at 50 ° C./min, and then heated 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. Further, cooling is performed at a rate of −10 ° C./min from 350 ° C. to 330 ° C., and cooling is performed at a rate of −1 ° C./min from 330 ° C. to 305 ° C. (second step). Next, after cooling from 305 ° C. to 100 ° C. at a rate of −50 ° C./min, heating is performed from 100 ° C. to 365 ° C. at a rate of 10 ° C./min (third step). The sampling time is 0.5 seconds / time. The heat absorption amount of the first step is 303 to 353 ° C, the heat generation amount of the second step is 318 to 309 ° C, and the heat absorption amount of the third step is the integration of the 48 ° C interval from the end of the endothermic curve. The amount of heat absorbed in this third step is defined as the amount of heat of fusion.

[Measurement method of air permeability (Gurley second)]
It was measured using a Wangken type air permeability measuring device (manufactured by Asahi Seiko Co., Ltd.) having the same structure as the Gurley air permeability tester specified in JIS P 8117 (Paper and paperboard air permeability test method). The measurement result is expressed in Gurley seconds.

[Measurement method of average flow pore size]
GALWICK (propylene, 1,1,2,3,3,3 hexafluoric acid oxide; manufactured by Porous Materials, Inc.) as a liquid using a pore distribution measuring device (palm porometer CFP-1500A: manufactured by Porous Materials, Inc.) ) And the bubble point method (ASTM F316-86, JISK3832), and was obtained by the following formula as described above.
Average flow pore diameter d (μm) = cγ / P
(P is pressure (Pa), c is 2860, γ is surface tension of liquid (dynes / cm))

[Measurement method of maximum pore size]
GALWICK (propylene, 1,1,2,3,3,3 hexafluoric acid oxide; manufactured by Porous Materials, Inc.) as a liquid using a pore distribution measuring device (palm porometer CFP-1500A: manufactured by Porous Materials, Inc.) ) And the bubble point method (ASTM F316-86, JISK3832). Specifically, when the gas pressure on one side was increased while the entire membrane surface was wet with liquid, the pressure at which the gas pressure exceeded the capillary force of the membrane and permeation began (minimum pressure) was measured. This is a value calculated from the above formula (maximum pore diameter = cγ / P) where P is the pressure.

[Transparency index]
It is the value calculated | required by the following formula from the air permeability (Gurley second) measured by the said method, the average flow hole (nm), and the thickness (nm) of the collection layer measured by the following method.
Permeability index = collecting layer thickness (nm) / [(average flow pore size (nm)) ² × Gurley second]

[Measurement of collection layer thickness]
After impregnating isopropyl alcohol (IPA) into the resulting modified PTFE microporous membrane, the fluororesin microporous membrane is immersed in distilled water while keeping the IPA from drying, and the IPA is replaced with water. This is immersed in a bath of liquid nitrogen and cut with a razor knife in liquid nitrogen. The fracture surface is observed with an SEM, and the thickness of only the collection layer is measured to obtain the thickness of the collection layer.

Example 1
[Dispersion adjustment]
PFA-modified PTEE represented by the formula (II), wherein m in the formula (II) is 283, and Rf is a perfluoromethyl-CF ₃ group (there is absorption of PAVE in the IR spectrum, and the heat of fusion of the third step 23) 0.1 J / g, hereinafter simply referred to as “PTFE”) (dispersion medium: water, solid content concentration of about 55% by mass) and MFA latex (manufactured by Solvay Solexis, PFA dispersion D5010) MFA / (PTFE + MFA + PFA) and PFA / (PTFE + MFA + PFA) are 2% each (volume ratio of solid content of fluororesin, that is, PTFE, MFA, and PFA). The dispersion of the modified PTEE was adjusted. Furthermore, the dispersion of the modified PTEE was adjusted by adding polyethylene oxide having a molecular weight of 2 million to a concentration of 0.003 g / ml.

[Film Formation Process: Production of Modified PTFE (Fluororesin) Nonporous Film]
Next, an aluminum foil having a thickness of 50 μm is spread and fixed on a flat glass plate so that there are no chicks. After the dispersion adjusted as described above is dropped, a stainless steel slide shaft manufactured by Nippon Bearing Co., Ltd. The dispersion of the modified PTEE was stretched uniformly over the entire surface of the aluminum foil by sliding the shaft SNSF type (outer diameter 20 mm). The 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 modified PTEE thin film (modified PTEE nonporous material) fixed on an aluminum foil. Film). The average thickness of the non-porous membrane made of modified PTEE calculated from the weight difference per unit area of the aluminum foil before and after the modified PTEE thin film is formed and the true specific gravity (2.25 g / cm ³ ) of the fluororesin is about 2 .4 μm.

  Next, after diluting PFA dispersion 920HP (made by Mitsui DuPont Fluorochemical Co., Ltd.) to 4 times the volume with distilled water, a polyethylene oxide having a molecular weight of 2 million was added to a concentration of 0.003 g / ml, A 4-fold diluted PFA dispersion was prepared.

[Preparation of specimen (porous resin membrane composite)]
After the modified PTFE resin thin film fixed on the aluminum foil obtained above was spread and fixed on a glass plate so as not to be wrinkled, the 4-fold diluted PFA dispersion prepared above was dropped, While sliding the same stainless steel slide shaft made by Nippon Bearing Co., Ltd. as described above, the PFA dispersion diluted 4 times was uniformly spread over the entire surface of the aluminum foil, while the moisture did not dry. Expanded PTFE porous body having a thickness of 45 μm and a thickness of 80 μm (porous support, manufactured by Sumitomo Electric Fine Polymer Co., Ltd., trade name: Poreflon FP-045-80, average flow pore size: 0.173 μm, porosity: 74%, Gurley Second = 10.7 seconds).

  Then, after 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 10 hours, it is naturally cooled, and PTFE is applied onto the expanded PTFE porous body. Also, a non-porous membrane made of modified PTEE was bonded with thermoplastic PFA having a low melting point, and a composite having an aluminum foil fixed thereon was obtained. Next, the aluminum foil was dissolved and removed with hydrochloric acid to obtain a test specimen. The Gurley second of this test body was 5000 seconds or more, and ethanol was contacted at room temperature from the modified non-porous membrane side made of PTEE. It was shown to be a fluororesin membrane composite (porous resin membrane composite) containing a substantially nonporous membrane that does not penetrate ethanol.

[Stretching]
1. Low-temperature stretching step Next, stretching was performed 3 times at 25 ° C. at a chuck width of 230 mm, an outlet of 690 mm, a length of the stretching zone of 1 m, a line speed of 6 m / min, and a specially made horizontal axis stretching machine. The average flow pore diameter and the maximum pore diameter of the stretched test specimen with the reagent GALWICK (propylene, 1,1,2,3,3,3 hexafluoric acid: manufactured by Porous Materials) are both from the measurement limit of 20 nm. It was small and the Gurley second (transmission coefficient) was 4500 seconds.
2. High-temperature stretching step Next, with a special horizontal axis stretching machine, the inlet chuck width is 300 mm, the outlet is 750 mm, the length of the stretching zone is 1 m, the line speed is 6 m / min, and the stretching is 2.5 times at 140 ° C., A modified PTEE microporous membrane was obtained. This modified PTEE microporous membrane has an average flow pore size of 32.7 nm and a maximum pore size of 45 with the reagent GALWICK (propylene, 1,1,2,3,3,3 hexafluoric acid: manufactured by Porous Materials). .6 nm. Therefore, the difference between the maximum pore diameter and the average flow pore diameter was 12.9 nm, (maximum pore diameter−average flow pore diameter) / average flow pore diameter = 38.2%. The Gurley second (permeability coefficient) was 88 seconds, the thickness of the trapping layer was 1.0 μm, and the permeability index was 0.0106. The measurement results are shown in Table 1. Moreover, the electron micrograph of the test body after a low-temperature extending process and a high-temperature extending process is shown to FIGS.

Example 2
Instead of PFA-modified PTEE, FEP-modified PTEE represented by the above formula (I) and m in the formula (I) is 148 (having absorption of HFP in IR spectrum, heat of fusion of 31.0 J / g, hereinafter referred to simply as “PTFE”), except that an aqueous dispersion (dispersion medium: water, solid content concentration of about 55% by mass) was used. The film formation step (preparation of a modified PTEE nonporous membrane), the preparation of a test body (porous resin membrane composite), and stretching were performed. Further, in the same manner as in Example 1, the maximum pore size, average flow pore size, and Gurley second after the low temperature stretching step and after the high temperature stretching step were measured, and the modified PTEE microporous membrane obtained after the high temperature stretching step was obtained. The thickness of the collection layer and the permeability index were obtained. The obtained results are shown in Table 1.

Example 3
Dispersion adjustment, film formation process (production of modified PTEE non-porous film), test specimen, except that the temperature of stretching in the low temperature stretching process was changed from 25 ° C. to 15 ° C. (Porous resin membrane composite) was prepared and stretched. Further, in the same manner as in Example 1, the maximum pore size, average flow pore size, and Gurley second after the low temperature stretching step and after the high temperature stretching step were measured, and the modified PTEE microporous membrane obtained after the high temperature stretching step was obtained. The thickness of the collection layer and the permeability index were obtained. The obtained results are shown in Table 1.

Comparative Example 1
Instead of PFA-modified PTEE, an aqueous dispersion 34JR made of PTFE homopolymer (manufactured by Mitsui DuPont Fluorochemical Co., Ltd., primary particle diameter of particles: 250 nm, heat of fusion of the third step 53.4 J / g) was used at a high temperature. Except that the stretching step was not performed, in the same manner as in Example 1, the adjustment of the dispersion, the film forming step (preparation of a nonporous film made of fluororesin), the preparation of a test body (porous resin film composite), And stretching. Further, in the same manner as in Example 1, the maximum pore size, average flow pore size, Gurley second, collection layer thickness, and permeability index after the stretching step were determined. The obtained results are shown in Table 1.

Comparative Example 2
Instead of PFA-modified PTEE, an aqueous dispersion composed of a PTFE homopolymer (manufactured by Mitsui DuPont Fluoro Chemical Co., Ltd., primary particle diameter of particles: 240 nm, heat of fusion 34.9 J / g in the third step) is used for high-temperature stretching. Except that the process was not performed, in the same manner as in Example 1, adjustment of dispersion, film formation process (preparation of a non-porous film made of fluororesin), preparation of a test body (porous resin film composite), and Stretching was performed. Further, in the same manner as in Example 1, the maximum pore size, average flow pore size, Gurley second, collection layer thickness, and permeability index after the stretching step were determined. The obtained results are shown in Table 1.

Comparative Example 3
Instead of PFA-modified PTEE, an aqueous dispersion (dispersion medium: water, solid content concentration of about 55% by mass) made of PTFE homopolymer (Asahi Glass Co., Ltd .: AD911: third step heat of fusion 30.0 J / g) Used, except that the high-temperature stretching step was not performed, and in the same manner as in Example 1, the dispersion adjustment, the film-forming step (preparation of a non-porous membrane made of fluororesin), and the test specimen (porous resin membrane composite) And stretching. Further, in the same manner as in Example 1, the maximum pore size, average flow pore size, Gurley second, collection layer thickness, and permeability index after the stretching step were determined. The obtained results are shown in Table 1.

Comparative Example 4
In the same manner as in Example 2 except that only the high temperature stretching step was performed without performing the low temperature stretching step in the stretching step, the adjustment of the dispersion, the film forming step (production of a modified PTEE nonporous film), and the test specimen ( (Porous resin membrane composite) was prepared and stretched. Further, in the same manner as in Example 1, the maximum pore size, average flow pore size, Gurley second, collection layer thickness, and permeability index after the stretching step were determined. The obtained results are shown in Table 2.

Comparative Example 5
Except for changing the temperature of the high-temperature stretching step from 140 ° C. to 60 ° C., in the same manner as in Example 2, the adjustment of the dispersion, the film formation step (preparation of a non-porous film made of modified PTEE), and the specimen (porous) Resin membrane composite) was produced and stretched, and tearing occurred in the specimen during the high-temperature stretching process.

Comparative Example 6
Except for changing the temperature of the high temperature drawing step from 140 ° C. to 80 ° C., in the same manner as in Example 2, the adjustment of the dispersion, the film forming step (preparation of a non-porous film made of modified PTEE), and the specimen (porous) Resin membrane composite) was produced and stretched, and tearing occurred in the specimen during the high-temperature stretching process.

Example 4
Except for changing the temperature of the high-temperature stretching step from 140 ° C. to 100 ° C., in the same manner as in Example 2, the adjustment of the dispersion, the film formation step (preparation of a non-porous film made of modified PTEE), and the specimen (porous) When the resin membrane composite) was produced and stretched, a modified PTFE microporous membrane (porous membrane) was obtained without tearing the test specimen in the high-temperature stretching step. However, the obtained modified PTEE microporous membrane had uneven light transmission and uneven stretching.

Example 5
Except for changing the temperature of the high-temperature stretching step from 140 ° C. to 100 ° C., in the same manner as in Example 2, the adjustment of the dispersion, the film formation step (preparation of a non-porous film made of modified PTEE), and the specimen (porous) When the resin membrane composite) was produced and stretched, a modified PTEE microporous membrane (porous membrane) was obtained without tearing the test specimen in the high-temperature stretching step. However, the obtained modified PTEE microporous membrane had uneven light transmission and uneven stretching.

From the above results, the following is shown.
In Examples 1 to 3, which were stretched at 25 ° C. and then stretched at 140 ° C., the difference between the maximum pore size and the average flow pore size was less than 15 nm despite the average flow pore size being 40 nm or less, And (maximum pore diameter-average flow pore diameter) / average flow pore diameter is 0.5 or less, and a modified PTFE fine pore diameter membrane having a narrow pore diameter distribution is obtained. Further, the modified PTEE microporous membranes obtained in Examples 1 to 3 have a permeability index of 0.009 or more and excellent permeability.

  On the other hand, in Comparative Examples 1 to 4 in which the low temperature stretching step or the high temperature stretching step was not performed, the difference between the maximum pore diameter and the average flow pore diameter exceeded 15 nm, and (maximum pore diameter−average flow pore diameter) / average flow pore diameter was 0. It is shown that the pore size distribution of the obtained fluororesin microporous membrane is wide. Further, in Comparative Examples 1, 2, and 4, it is also shown that the permeability index is smaller than 0.009 and the permeability is inferior.

  Further, in Comparative Example 3, a fluoropolymer having a homopolymerization PTFE and a heat of fusion of less than 32 J / g is used instead of the modified PTFE of the Example. The difference between the maximum hole diameter, the average flow hole diameter, and the maximum hole diameter and the average flow hole diameter is larger. From these results, it is shown that when homopolymerized PTFE is used, it is difficult to produce a fluororesin microporous membrane having excellent fractionation performance and permeability equivalent to those of the examples.

  Comparative Examples 5 and 6 and Examples 3 and 4 are examples in which a modified PTEE-made microporous membrane was prepared under the same conditions as in Example 2 except that the temperature of the high-temperature stretching step was changed. In Comparative Example 5 performed at 60 ° C. and Comparative Example 6 performed at 80 ° C., the test specimen was cracked during the high-temperature stretching process. On the other hand, in Example 4 in which the high-temperature stretching step was performed at 100 ° C. and in Example 5 in which the high-temperature stretching step was performed at 120 ° C., the test specimen could be stretched without causing tearing. From this result, it is shown that the high temperature stretching step (second stretching) needs to be performed at a temperature exceeding 80 ° C. In Examples 4 and 5, although the stretching could be performed, the obtained modified PTEE microporous membrane showed uneven light transmission, and the stretching was uneven. From this result, it is shown that the preferable temperature of the high-temperature stretching step is a temperature exceeding 120 ° C.

Claims (9)

  A film forming step for forming a film-shaped molded product by molding a powder of modified polytetrafluoroethylene into a film shape, and heating the film-shaped molded product to a temperature higher than the melting point of the modified polytetrafluoroethylene to form a nonporous film A sintering step for obtaining a molded product, and a stretching step for stretching the nonporous membrane-shaped molded product to make it porous, and the heat of fusion of the modified polytetrafluoroethylene is less than 32 J / g, And the said extending process has the low-temperature extending process extended | stretched to at least 1 direction at 80 degrees C or less, and the high temperature extending process extended | stretched to at least 1 direction at the temperature exceeding 80 degreeC performed after the said low-temperature extending | stretching, A method for producing a tetrafluoroethylene microporous membrane.   The method for producing a modified polytetrafluoroethylene microporous membrane according to claim 1, wherein the low-temperature stretching step is performed at a temperature lower than 30 ° C.   3. The powder according to claim 1, wherein the modified polytetrafluoroethylene powder is a hexafluoropropylene-modified polytetrafluoroethylene or perfluoroalkyl vinyl ether-modified polytetrafluoroethylene powder. 4. A method for producing a modified polytetrafluoroethylene microporous membrane.   A modified polytetrafluoroethylene produced by the method of producing a modified polytetrafluoroethylene microporous membrane according to any one of claims 1 to 3, wherein the average flow pore size is 40 nm or less. Ethylene microporous membrane.   5. A modified polytetrafluoroethylene microporous membrane according to claim 4, wherein the difference between the average flow pore size and the maximum pore size is less than 15 nm.   The value of (maximum pore diameter-average flow pore diameter) / average flow pore diameter is 0.5 or less, The modified polytetrafluoroethylene microporous membrane according to claim 5. The permeability index represented by film thickness (nm) / [(average flow pore diameter (nm)) ² × Gurley second] is 0.009 or more, and any one of claims 4 to 6 The microporous membrane made of modified polytetrafluoroethylene as described in the item.   The modified polytetrafluoroethylene microporous membrane according to any one of claims 4 to 7 has a larger average flow pore size and a higher breaking load than the modified polytetrafluoroethylene microporous membrane. A porous resin film composite which is fixed on a porous support.   A filter element comprising the porous resin membrane composite according to claim 8 as a filtration membrane.