JPH0693983B2 - Ethylene-tetrafluoroethylene copolymer porous membrane - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention provides excellent chemical resistance consisting of ethylene-tetrafluoroethylene copolymer, excellent heat resistance, excellent overperformance, and excellent mechanical properties. The present invention relates to a porous membrane which has a uniform porous structure including fine pores. In particular, the present invention is a porous film suitable for a microfilter having excellent heat resistance and excellent overperformance, and a porous film suitable for a microfilter for chemical purification of strong acids, strong alkalis, etc. by utilizing its excellent chemical resistance. It is about.

(Prior art and its problems) Fluorocarbon resin is a resin with excellent chemical resistance and heat resistance,
Furthermore, an ethylene-tetrafluoroethylene copolymer is selected as a fluorine resin that also has excellent mechanical properties. This porous film made of fluorine resin has heat resistance,
Expected as a porous film with excellent chemical resistance and durability.

As a porous film made of an ethylene-tetrafluoroethylene copolymer, there have already been disclosed in JP-A-50-136354 and JP-A-54-1354.
JP-A-158465 and JP-A-59-147030 are known. Japanese Unexamined Patent Publication No. 50-136354 discloses a method in which a fine powder of an ethylene-tetrafluoroethylene copolymer is mixed with a styrene monomer to prepare a slurry mixture, and after styrene polymerization, a film is formed and the styrene polymer is eluted to form a porous film. The porous membrane obtained by this method has a large pore size of 10 μm and has a very low permeability and is not suitable for a microfilter. JP-A-54-158465 discloses a method of irradiating an ethylene-tetrafluoroethylene copolymer film with charged particles and then etching the film with a caustic soda aqueous solution to form a porous film. The obtained porous film has a three-dimensional network structure. Since it does not have such properties, there is a problem that it is not suitable for mass production because it is inferior in overperformance and inferior in mechanical properties, a uniform hollow fiber-like porous membrane cannot be obtained, and a reactor is used. In JP-A-59-147030, an ethylene-tetrafluoroethylene copolymer film is coated with a resist to form a perforated resist pattern, and then a sputtering process is performed to form a through hole corresponding to the resist pattern. Although it is a method of making a porous membrane, this porous membrane also has poor dimensional performance because it does not have a dimensional network structure, and also has poor mechanical properties, and it is difficult to obtain a uniform hollow fiber-like porous membrane, and it takes a long time. Therefore, there is a problem in productivity as well, since it requires a spatula etching process. As a method for improving the above problems, JP-A-55-79011, JP-A-56-159128, JP-A-57-28139, JP-A-58-93798,
JP-A-58-179297 and the like, a method of forming a porous film by mixing ethylene-tetrafluoroethylene copolymer, finely divided silicic acid and dioctyl phthalate and then melt-molding the mixture, and then extracting finely-divided silicic acid and dioctyl phthalate from the molded product. It has been known.

However, the porous membrane obtained by this method does not have sufficient uniformity in the porous structure, and has many voids that are orders of magnitude larger than the average pore size. Therefore, although this porous membrane is a membrane having a three-dimensional network structure, the number of fibers constituting the network is small in the thickness direction of the membrane, and when used as a microfilter, the removability of fine particles is poor. Furthermore, this porous membrane has a high frequency of pinholes (abnormally large communicating holes), and
There was also a problem that the quality of the film was unstable (the performance varied greatly) and the productivity (yield of good products) was poor.

As described above, there has been no satisfactory porous membrane made of an ethylene-tetrafluoroethylene copolymer.

(Means for Solving Problems) The inventors of the present invention have excellent chemical resistance composed of an ethylene-tetrafluoroethylene copolymer, excellent heat resistance, excellent over-characteristics, excellent mechanical properties, As a result of extensive studies on a porous membrane having a uniform porous structure composed of fine pores, the present invention has been completed.

That is, the present invention provides the porous membrane described in the claims.

The features of the porous membrane of the present invention will be described below.

In the present invention, an ethylene-tetrafluoroethylene copolymer is selected as the porous membrane material because it has excellent chemical resistance, heat resistance and mechanical properties.

In the porous membrane of the present invention, the average pore size is preferably 0.01 to 5 µ, more preferably 0.05 to 1 µ. If it is less than 0.01 μ, the permeability is too low, and if it exceeds 5 μ, the removability of fine particles is poor and neither is preferable. Porosity is preferably 40-90%, 40
If it is less than 100%, the permeability is too low, and if it exceeds 90%, the mechanical properties are remarkably deteriorated, and neither is preferable.

The porous membrane of the present invention has a three-dimensional network structure. The three-dimensional network structure is a porous structure in which a network structure made of resin is observed on the surface of the porous film and the cross section in each direction, and has a communicating hole. Refers to the so-called sponge structure. Such a three-dimensional network structure is excellent in strength as a porous film, and when this porous film is used for a filter, it is excellent in fine particle removability due to the same effect as that of laminating a large number of screens. Further, the fiber constituting the network structure is a network resin portion surrounding the hole, and is expressed as a fiber for convenience, but its shape is not particularly limited, and it may form a three-dimensional network structure. For example, in addition to fibrous materials, thin films, nodules, and irregular shapes can be used.

In the present invention, it is necessary that the number N of fibers constituting the network structure is N ≧ 2 P / D, preferably N ≧ 5 P / D per 1 mm in the thickness direction of the film. However, P is the porosity (%) and D is the average pore diameter (μ). When N <2P / D, it is not preferable because it is inferior in the effect of removing fine particles when used as a filter.

Further, the ratio of the maximum pore diameter to the average pore diameter is preferably 4 or less,
If it exceeds 4, the fine particle removing effect tends to be poor. The maximum pore size is measured by the bubble point method (ASTM F316-70).

The thickness of the porous film is preferably 0.025 to 2.5 mm, and if it is less than 0.025 mm, the mechanical properties are poor, and if it exceeds 2.5 mm, the permeability tends to be poor, which is not preferable in some cases.

The shape of the membrane may be a hollow fiber shape, a tube shape, a flat membrane shape or the like, but in a microfilter application, a hollow fiber shape is preferable because of the compactness of the device when it is modularized.

Next, the features of the method for producing the porous membrane of the present invention will be described.

In the present invention, ethylene-tetrafluoroethylene copolymer, inorganic fine powder, chlorotrifluoroethylene oligomer or sp value excluding chlorotrifluoroethylene oligomer and chlorotrifluoroethylene oligomer 5-11
A mixture of the above heat-resistant organic substances is used.

The inorganic fine powder is preferably fine particles having a specific surface area of 50 to 500 m ² / g and an average primary particle diameter of 0.005 to 0.5 μ, and the material is silicic acid, calcium silicate, aluminum silicate, magnesium oxide, alumina, calcium carbonate, kaolin. , Clay, silicate and the like are used. Of these, finely divided silicic acid is particularly preferable. The average primary particle diameter is the average value of the diameters of fine powder single particles, and is an aggregate of single particles (secondary particles).
Is not the diameter of. The average primary particle diameter can be measured by an electron microscope.

In the present invention, the use of a chlorotrifluoroethylene oligomer is essential, and by using this, it is the first time to produce a porous film having a uniform pore structure and excellent in fine particle removal with stable film quality and high productivity. It was possible. The chlorotrifluoroethylene oligomer is preferably a 4- to 20-mer of chlorotrifluoroethylene, and 8 to
The 15-mer is more preferable, and the 9- to 12-mer is most preferable.
If it is a trimer or less, it is not preferable because the heat resistance is low, the evaporation at the time of melt molding is large, and the permeability of the porous membrane is small, and if it is 21 or more, the mixing workability is poor and the extractability is poor. The number of chlorotrifluoroethylene oligomers in the present invention is an average number of chlorotrifluoroethylene oligomers composed of a mixture of various chlorotrifluoroethylene oligomers.

In the present invention, the use of a mixture of a chlorotrifluoroethylene oligomer and a heat-resistant organic substance having an sp value of 5 to 11 excluding the chlorotrifluoroethylene oligomer makes it easy to control the pore diameter of the porous membrane. That is, the pore diameter can be easily controlled to a desired pore diameter by selecting the heat-resistant organic material and / or adjusting the mixing ratio of the chlorotrifluoroethylene oligomer and the heat-resistant organic material.

In addition, chlorotrifluoroethylene oligomer and sp value 11
When a mixture with a heat-resistant organic substance having a sp value of more than 11 is used (no heat-resistant organic substance having an sp value of less than 5 is presently found), the compatibility between the chlorotrifluoroethylene oligomer and the heat-resistant organic substance having an sp value of more than 11 is poor and The pore size of the obtained membrane is too large and has a non-uniform pore structure, which is not preferable.

The heat-resistant organic substance in the present invention is an organic substance having a heat resistance such that the boiling point at 1 atm is at least 200 ° C. or higher, preferably 250 ° C. or higher, and is a liquid when melt-molding the porous membrane of the present invention. Silicone oil, perfluoropolyether oligomer, as a heat-resistant organic substance with an sp value of 5 to 11,
Examples thereof include phthalic acid ester, trimellitic acid ester, sebacic acid ester, adipic acid ester, azelaic acid ester, and phosphoric acid ester.

Of these, silicone oil, perfluoropolyether oligomer and trimellitate ester are particularly preferable.

In particular, silicone oil is more preferable from the viewpoints of thermal stability during melt molding, price, and the like. Silicon oil is a heat-resistant organic substance with a siloxane structure, dimethyl silicone,
Examples include methyl phenyl silicon and diphenyl silicon.

The mixing ratio of the chlorotrifluoroethylene oligomer and the heat-resistant organic substance having an sp value of 5 to 11 excluding the chlorotrifluoroethylene oligomer varies depending on the kind of the heat-resistant organic substance.
The volume of the heat-resistant organic substance is preferably 10 volumes or less, more preferably 4 volumes or less. If it exceeds 10 volumes, the pore size of the obtained film tends to be large, and the structure tends to have a non-uniform pore structure, and pin holes (abnormally large continuous pores) tend to occur. When the heat-resistant organic substance is silicone oil, it is preferably 2 volumes or less.

In producing the porous membrane of the present invention, first of all, an ethylene-tetrafluoroethylene copolymer inorganic fine powder, a chlorotrifluoroethylene oligomer or a sp value of 5 to 11 excluding a chlorotrifluoroethylene oligomer and a chlorotrifluoroethylene oligomer is used. Mix the mixture of refractory organic materials. The mixing ratio is ethylene-tetrafluoroethylene copolymer 10 to 60% by volume, preferably 15 to 40% by volume,
The inorganic fine powder is 7 to 42% by volume, preferably 10 to 20% by volume, and the mixture with the chlorotrifluoroethylene oligomer or the heat-resistant organic substance is 30 to 75% by volume, preferably 50 to 70% by volume.

10% by volume of ethylene-tetrafluoroethylene copolymer
If it is less than, the resin is too small and the strength is small and the moldability is poor,
If it exceeds 60% by volume, a porous film having a large porosity cannot be obtained, which is not preferable. If the inorganic fine powder is less than 7% by volume, molding becomes difficult, and if it exceeds 42% by volume, the fluidity at the time of melting is poor.
Moreover, the obtained molded product is brittle and cannot be put to practical use. If the content of the chlorotrifluoroethylene oligomer or a mixture of the chlorotrifluoroethylene oligomer and the heat-resistant organic substance is less than 30% by volume, the porosity of the obtained porous membrane is less than 40%, and a porous membrane having excellent permeability cannot be obtained. If it exceeds the capacity%, molding becomes difficult and a porous film having high mechanical strength cannot be obtained.

The mixing of the above components is carried out by a mixer such as a Henschel mixer, a V-blender, a ribbon blender, or the like. The mixing order is as follows. First, the mixture of the inorganic fine powder and the chlorotrifluoroethylene oligomer or the heat-resistant organic substance is mixed, and then the ethylene-tetrafluoroethylene copolymer is mixed and mixed, rather than mixing the respective components at the same time. Is preferred. It is preferable that this mixture is further kneaded by a melt-kneading device such as an extruder. The obtained kneaded product is crushed by a crusher as necessary, and then flat film-shaped by an extruder or the like.
It is melt-molded into a hollow film shape or the like. It is also possible to directly mold the mixture with a device having both functions of kneading and extrusion, such as a kneader ruder.

Then, the mixture of the chlorotrifluoroethylene oligomer or the heat-resistant organic substance is extracted from the formed film-like substance with a solvent. The extraction is performed by a general method for extracting a membranous material such as a batch method or a countercurrent multistage method. The solvent used for the extraction is preferably a halogenated hydrocarbon such as 1,1,1-trichloroethane or tetrachloroethylene.

The semi-extracted porous membrane on which the extraction of the mixture with the chlorotrifluoroethylene oligomer or the heat-resistant organic substance has been completed is then subjected to extraction of the inorganic fine powder with a solvent for the inorganic fine powder. The extraction is completed within a few seconds to a few tens of hours by a general extraction method such as a batch method or a countercurrent multistage method. Solvents used for extracting inorganic fine powder include calcium carbonate, magnesium carbonate, magnesium oxide, calcium silicate, magnesium silicate, and other acids such as hydrochloric acid, sulfuric acid, hydrofluoric acid, and silicic acid such as caustic soda and caustic potash. An aqueous alkaline solution is used. There is no particular limitation as long as it does not substantially dissolve the ethylene-tetrafluoroethylene copolymer and dissolves the inorganic fine powder.

In the present invention, when a porous film having a high degree of heat resistance is desired, the inorganic fine powder is annealed in the state of a semi-extracting porous film present in the film and then the inorganic fine powder is extracted and removed as described below. It is valid.

When the porous membrane is exposed to a high temperature when it is subjected to a module, or when it is subjected to a high temperature, a change in the pore diameter of the porous membrane and a decrease in the permeability are often observed.

As a cause of the performance deterioration of the porous membrane at high temperature, the inventors have found that "strain" generated during the porous membrane molding process exists inside the resin forming the porous membrane, and the "strain" is eliminated when heated. It was thought that this was the main factor, and we studied earnestly a method for producing a porous membrane that minimizes the "strain" and has little performance degradation at high temperatures. Usually, an annealing treatment is performed to eliminate the above-mentioned “strain”, but when the annealing treatment is performed on a porous film made only of a resin, which is a usual method, the physical properties change greatly, and The physical properties do not change uniformly at each part of the film, but the film shape changes, and the resulting film becomes non-uniform.
In many cases, reproducibility cannot be obtained. In order to prevent such non-uniform changes in physical properties, it is sufficient to restrain the shape of the membrane so that it does not change, but it is generally difficult to restrain the membrane from the outside, and even if it is a flat membrane. Although it is possible to restrain in the vertical and horizontal directions, it is difficult to restrain in the thickness direction of the film. Furthermore, in the hollow fiber porous membrane, it is difficult to constrain it in a direction other than the length direction, and it is difficult to perform an annealing treatment to obtain a uniform membrane.

The present inventors annealed the porous film filled with the inorganic fine powder, whereby the inorganic fine powder itself restrains the shape of the porous film from the inside, and as a result, a uniform film can be obtained with good reproducibility. I found a thing. The annealing temperature may be a temperature equal to or higher than the glass transition point of the resin, but it is preferably in the range of the melting point of the resin to the melting point minus 100 ° C. in consideration of productivity such as the time required for the annealing treatment. In addition, it is better to process at a temperature higher than the expected operating temperature (including heating conditions in the module assembly process, etc.)
It is more effective and preferable. The treatment time depends on the treatment temperature, but is usually in the range of several minutes to several days.

The annealing treatment is usually a batch treatment or a continuous treatment in a hot air oven.

Although the heat resistance of the porous film is improved by the above-mentioned method, when this is still insufficient, it is further improved by extracting and removing the inorganic fine powder after annealing and reannealing it.

In the present invention, in order to increase the pore size of the porous membrane, increase the porosity, and improve the mechanical properties, a chlorotrifluoroethylene oligomer or a mixture of this and a heat-resistant organic substance, an inorganic fine powder The porous membrane obtained by extracting one or both of them can be uniaxially or biaxially stretched under known conditions.

The various physical properties shown in the present specification are based on the following measuring methods.

-Average pore diameter (μ) The pore diameters of the surface and cross section of the sample were measured by an electron microscope and averaged (number average). Averaging is carried out by taking electron micrographs of the film surface, the film front and back, and the film cross section for 10 or more samples randomly sampled from the porous film manufactured under the same conditions, and randomly selecting from each electron micrograph. It is carried out by actually measuring the average diameter of the 10 or more holes, and averaging the diameters of the measurement points of 300 or more. It is necessary to increase the number of measurement points for a porous film having a non-uniform pore structure. The average pore size can be calculated from the electron microscope photograph by subjecting the photograph to an image processing device and subjecting it to a computer process.

-Porosity (%) Obtained by the following formula.

However, the pore volume was obtained by subtracting the weight of only the porous body from the weight of the porous body in which the pores were filled with water.

・ Number of fibers that make up the network structure N (pieces / mm) Randomly sampled from porous membranes manufactured under the same conditions 10
Electron micrographs of the cross-section of the film were taken for the samples above the points, and were randomly selected from each electron micrograph.
The number of fibers in the thickness direction of the film is counted at 30 or more points, converted into the number of fibers in a film thickness of 1 mm, and the number of fibers of 300 or more measurement points is averaged. The average value of the number of fibers from the electron micrograph can be calculated by subjecting the photograph to an image processing device and subjecting it to a computer process.

・ Frequency of pinholes (ke / m) One of the uniformity of the porous structure, which evaluates the number of unusually coarse holes.
It is one evaluation item. A continuous hollow fiber porous membrane of 150 m was immersed in ethyl alcohol and a pressure lower than the bubble point pressure of the porous membrane by 0.5 kg / cm ² was applied to one side of the hollow fiber (the other side was closed). Check the number of bubbles generated and calculate from the following ・ Three-dimensional network structure Determined by electron microscope observation ・ Particle removal rate (%) Dow Chemical Company UNIFORM LATEX PARTIC LES solid concentration
The solution diluted to 0.01% by weight is passed through a porous membrane and LATEX PART
Calculated from the percentage of ICLES removed.

・ Water permeability (/ m ² , hr atm. 25 ℃) Measured at 25 ℃, differential pressure 1kg / cm ² .

-Sp value (dissolution parameter) Calculated by the following formula (Small formula).

d: Specific gravity, G: Molar traction constant, M: Molecular weight. J ・ BRANDRUP
et al, Polymer Handbook, IV-344, 1966, INTERSCIENCE
PUBLISHERS · a division of John Wiley & Sons) ”.

(Examples) Next, examples will be shown to clarify the present invention, but the present invention is not limited to these examples.

Example 1 Finely powdered silicic acid [Aerosil R-972 (trade name), specific surface area 120
m ² / g, average primary particle size 16 mμ] 11.1% by volume, chlorotrifluoroethylene oligomer [Daifloyl # 20 (trade name), about 8 mer] 62.2% by volume were mixed with a Henschel mixer, and ethylene-tetra 26.7% by volume of a fluoroethylene copolymer [Aflon COP Z-8820 (trade name)] was added and mixed again with a Henschel mixer.

The mixture was mixed with a 30 mmφ twin-screw extruder at 260 ° C. and pelletized. This pellet was molded into a hollow fiber at 260 ° C. using a hollow fiber manufacturing apparatus in which a hollow spinneret was attached to a 30 mmφ twin-screw extruder. The molded hollow fiber was immersed in 1,1,1-trichloroethane at 50 ° C. for 1 hour to extract a chlorotrifluoroethylene oligomer, and then dried.

Then, the powdery silica was extracted by immersing it in a 40% caustic soda aqueous solution at 70 ° C. for 1 hour, washed with water, and dried.

The obtained ethylene-tetrafluoroethylene copolymer porous film has a three-dimensional network structure, and its performance is shown in Table 1.

Examples 2 to 4 Instead of chlorotrifluoroethylene oligomer, chlorotrifluoroethylene oligomer [Daifloyl # 20
(Trade name)] An ethylene-tetrafluoroethylene copolymer was prepared in the same manner as in Example 1 except that a mixture consisting of 1 volume and the following volume of dimethyl silicone [Shin-Etsu Silicone KF96 (trade name), sp value 6.3] was used. A porous film was obtained.

(Dimethyl Silicon Capacity) Example 2 0.17 Example 3 0.20 Example 4 0.25 The obtained porous membranes all have a three-dimensional network structure, and the performance is shown in Table 1.

Example 5 13.3% by volume of finely divided silicic acid [Aerosil R-972 (trade name)] and 60.0% by volume of a chlorotrifluoroethylene oligomer [Daifloyl # 100 (trade name), about 11-mer] were mixed with a Henschel mixer, and mixed. 26.7% by volume of ethylene-tetrafluoroethylene copolymer [Aflon COP Z-8820 (trade name)] was added to and mixed again with the Henschel mixer.

Thereafter, an ethylene-tetrafluoroethylene porous film was obtained in the same manner as in Example 1.

The obtained porous film has a three-dimensional network structure, and its performance is shown in Table 1.

Example 6 In Example 5, the chlorotrifluoroethylene oligomer was extracted, dried, and then annealed at 200 ° C. for 1 hour. After that, it was immersed in a 40% caustic soda aqueous solution at 70 ° C for 1 hour to extract fine silicic acid, and then washed with water,
Dried.

The obtained ethylene-tetrafluoroethylene copolymer porous film has a three-dimensional network structure, and its performance is shown in Table 1.

When this film was left in an atmosphere of 180 ° C. for 4 hours and evaluated for its physical properties, the rate of change with respect to the original physical properties was as small as water permeability 7%, porosity 3% and average pore size 0%. .

For comparison, the film of Example 5 which was not annealed was similarly left in an atmosphere of 180 ° C. for 4 hours, and then the physical properties were evaluated. The rate of change from the original physical properties was 47%.
%, Porosity 13%, average pore size 10%.

From the comparison of Examples 5 and 6 and the above facts, it can be seen that the annealing treatment of the porous film filled with the inorganic fine powder can improve the heat resistance without largely changing the physical properties of the film.

Example 7 14.4% by volume of finely divided silicic acid [Aerosil R-972 (trade name)] and 58.9% by volume of a chlorotrifluoroethylene oligomer [Daifloyl # 100 (trade name)] were mixed with a Henschel mixer, and ethylene-tetrafluoro was added thereto. 26.7% by volume of an ethylene copolymer [NEOFLON ETFE EP-540 (trade name)] was added and mixed again with a Henschel mixer.

The mixture was mixed with a 30 mmφ twin-screw extruder at 260 ° C. to form a pellet. This pellet was molded into a hollow fiber at 250 ° C. by a hollow fiber manufacturing apparatus in which a hollow spinneret was attached to a 30 mmφ twin-screw extruder. The molded hollow fiber was immersed in 1,1,1-trichloroethane at 50 ° C. for 1 hour to extract a chlorotrifluoroethylene oligomer, and then dried.

After that, an annealing treatment was performed at 200 ° C. for 1 hour, and then the powder was immersed in a 40% caustic soda aqueous solution at 70 ° C. for 1 hour to extract fine silicic acid, washed with water and dried.

The obtained ethylene-tetrafluoroethylene copolymer porous film has a three-dimensional network structure, and its performance is shown in Table 1.

Example 8 The ethylene-tetrafluoroethylene copolymer porous film obtained in Example 7 was reannealed at 200 ° C. for 2 hours.

The obtained porous film has a three-dimensional network structure, and its performance is shown in Table 1.

When the physical properties of this film were evaluated after leaving it in an atmosphere of 200 ° C for 2 hours, the rate of change from the original physical properties was 4
%, The porosity was 0%, and the average pore size was 0%.

For comparison, when the film of Example 7 which had not been re-annealed was similarly left in an atmosphere of 200 ° C. for 2 hours and then subjected to physical property evaluation, the rate of change from the original physical property was 24% of water permeability.
The decrease was large, the porosity was 4%, and the average pore size was 5%.

From the above facts, it is understood that the reannealing treatment of the porous film is effective for improving the heat resistance of the film.

Comparative Example 1 Finely powdered silicic acid [Aerosil 200 (trade name), specific surface area 200 m ² /
g, average primary particle size 16 mμ] 13.3% by volume, dioctyl phthalate 60.0% by volume was mixed with a Henschel mixer, and ethylene-tetrafluoroethylene copolymer [Aflon COP Z-8820 (trade name)] 26.7% by volume Add and mix again with the Henschel mixer.

The mixture was mixed with a 30 mmφ twin-screw extruder at 300 ° C. to form a pellet. This pellet was molded into a hollow fiber at 290 ° C. by a hollow fiber manufacturing apparatus in which a hollow spinneret was attached to a 30 mmφ twin-screw extruder. The molded hollow fiber was immersed in 1,1,1-trichloroethane at 50 ° C. for 1 hour to extract dioctyl phthalate, and then dried.

Then, the powdery silica was extracted by immersing it in a 40% caustic soda aqueous solution at 70 ° C. for 1 hour, washed with water, and dried.

The performance of the obtained ethylene-tetrafluoroethylene copolymer porous membrane is shown in Table 1.

This porous membrane is an inhomogeneous membrane with many large pores and has a small number of fibers constituting the net, and the removal rate of 0.085μ fine particles was measured to be as small as 95%. It was inferior to 100% of the film of the present invention.
Moreover, pinholes were frequently generated in the porous film, and the yield of non-defective products was very low. Furthermore, when the porous membrane was manufactured 5 times under the same conditions, the average pore diameter of the obtained porous membrane was 0.3 to 0.8 μ,
The permeation rate was 650-3000 / m ² · hr · atm · 25 ° C, and the membrane performance fluctuated greatly, resulting in unstable membrane quality.

Examples 9 to 11 Finely powdered silicic acid [Aerosil R-972 (trade name)], chlorotrifluoroethylene oligomer [Daifloyl # 100]
(Commercial name)], ethylene-tetrafluoroethylene copolymer [Neoflon ETFE EP-540 (commercial name)] having the composition shown in the table below is the same as in Example 1 except that ethylene-tetrafluoroethylene copolymer is used. A porous film was obtained.

Each of the obtained porous membranes has a three-dimensional network structure, and the performance is shown in Table 1.

(Effects of the Invention) According to the present invention, an ethylene-tetrafluoroethylene copolymer porous membrane having a uniform porous structure having excellent chemical resistance, excellent heat resistance, excellent overperformance, and excellent durability can be obtained. As a result, by using this porous membrane, it is possible to carry out highly precise overpurification under severe heat and chemical resistance conditions such as hot concentrated sulfuric acid.

Claims (1)

[Claims] 1. An ethylene-tetrafluoroethylene copolymer, having an average pore diameter of 0.01 to 5 μm and a porosity of 40 to 90%, wherein the number N of fibers constituting the network structure is the thickness direction of the membrane.
An ethylene-tetrafluoroethylene copolymer porous membrane having a three-dimensional network structure with N ≧ 2 P / D per 1 mm. However, P is the porosity (%) and D is the average pore diameter (μ).