JP2008013615A - Method of producing fluoroplastic porous body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a melt-moldable fluoroplastic porous body which is excellent in chemical resistance and has high mechanical strength. <P>SOLUTION: A composition obtained by mixing 20-60 mass% melt-moldable fluororesin, 20-60 mass% solvent-soluble resin and 5-30 mass% inorganic fine powder is subjected to melt molding to obtain a molded product, the solvent-soluble resin and the inorganic fine powder are extracted from the molded product to produce the melt-moldable fluororesin porous body, and the porous body is subjected to a heat-treatment at a temperature equal to or higher than the glass transition temperature (Tg) but lower than the melting point (Tm) of the fluororesin. Preferable examples of the melt-moldable fluororesin used are an ethylene-tetrafluoroethylene copolymer, a tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, polychlorotrifluoroethylene and an ethylene-chlorotrifluoroethylene copolymer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a melt-formable fluororesin porous body, and more specifically, a melt-formable fluororesin porous body useful for water treatment having excellent chemical resistance, filtration performance, and mechanical properties. It relates to the manufacturing method.

  Conventionally, porous molded bodies such as porous films and porous hollow fibers made of polyolefin resin have been widely used in various fields because they have desired fine pores and are inexpensive and lightweight. For example, separation of cleaning chemicals and fine particles in gases in semiconductor manufacturing processes, aseptic separation of brewed products, removal of viruses in blood products, blood dialysis, seawater desalination, etc. Is suitably used as a battery separator or the like.

  Among these, fluororesins are particularly excellent in properties such as chemical resistance, solvent resistance, and heat resistance, and therefore, many studies have been made on the porous molded body as a filter material. However, the fluororesins currently in practical use as porous molded bodies are substantially only polytetrafluoroethylene (hereinafter sometimes referred to as “PTFE”) and vinylidene fluoride resins.

  PTFE is a preformed product obtained by calendering the fine powder obtained by emulsion polymerization, and it is relatively easily made into a fiber (fibrillation) by stretching it. A highly porous PTFE film is obtained. The highly porous PTFE film is used in clinical medicine fields such as blood component analysis, serum and sterilization of injections, semiconductor industry fields such as removal of fine particles in LSI cleaning water and cleaning chemicals, and public health fields such as air pollution tests. And is suitably used as a filter membrane. In addition, since the highly porous PTFE has strong water and oil repellency, it is not only an industrial field but also a general waterproof as a breathable waterproof cloth whose micropores allow water vapor to pass through but blocks water droplets. It is also widely used in the clothing field.

  However, since the porous molded body of PTFE is relatively soft due to its material, it has a problem that its creep resistance is not sufficient, and when it is wound, it is crushed and its filterability is lowered. In addition, PTFE has a very high melt viscosity, and there is a problem that melt molding such as extrusion molding and injection molding used in general polyolefin-based resins is difficult. Therefore, the form of the molded body is limited to a film form or the like, and it is generally difficult to form an arbitrary form according to the use, for example, a form of a hollow fiber or the like.

  Moreover, the PTFE porous tube manufactured by the so-called stretching method has a strength in the tube axis direction because PTFE is fiberized in the tube axis direction, but has a low strength in the radial direction, and is used as a filter medium. There are problems such as the formation of tears by filtration pressure.

  On the other hand, a porous film made of a vinylidene fluoride resin, which is similarly produced by a stretching method, is also used as a water treatment membrane or a battery separator, although it has excellent chemical resistance compared to a general polyolefin resin. , It has the disadvantage of being easily attacked by some chemicals and has low mechanical strength.

  A hollow tube or a film-like molded body obtained by mixing a solvent-soluble resin (or heat-resistant organic liquid material) or a soluble inorganic fine powder with a fluororesin and melt-molding the porous body by these stretching methods, A method (elution method) in which the soluble resin or the like is eluted by treatment with a solvent or the like to make it porous is known.

  For example, in Patent Documents 1 and 2, a film obtained by melt-molding a chlorotrifluoroethylene oligomer, a vinylidene fluoride copolymer, and silica fine powder in an ethylene-tetrafluoroethylene copolymer, A hollow tube or the like is treated with 1,1,1-trichloroethane or acetone to extract chlorotrifluoroethylene oligomers, and then treated with an alkaline aqueous solution to extract silica fine powder by dissolution and extraction. A process for producing a porous molded body of an ethylene-tetrafluoroethylene copolymer is described. However, the porous bodies obtained by these elution methods do not necessarily have sufficient mechanical strength, and there is a problem that the mechanical strength is greatly lowered particularly when the porosity is increased.

Japanese Patent Publication No. 63-11370 (Claims (Claims 1 to 2), Examples 1 to 11) Japanese Patent No. 3265678 (Claims (Claims 1 to 6), Examples 1 to 4)

  An object of the present invention may be abbreviated as a melt-formable fluororesin porous molded body (hereinafter referred to as “fluororesin porous body”) having excellent chemical resistance and excellent filtration performance and high mechanical strength. ) Manufacturing method.

  As a result of intensive studies from such a viewpoint, the inventors of the present invention can achieve the object of the present invention by making a melt-molded fluororesin molded body porous by an elution method and then heat-treating it under specific conditions. The present invention was completed. That is, according to the present invention, the following method for producing a melt-formable fluororesin porous body is provided.

[1]
In the method for producing a melt-formable fluororesin porous body, a composition comprising 20 to 60% by mass of the melt-formable fluororesin, 20 to 60% by mass of a solvent-soluble resin, and 5 to 30% by mass of an inorganic fine powder is melt-molded. The first step of obtaining the molded body, and then the second step of extracting the solvent-soluble resin and the inorganic fine powder from the molded body to obtain a porous body of a melt moldable fluororesin, and the porous body, A method for producing a porous melt-formable fluororesin comprising the third step of heat-treating at a temperature not lower than the glass transition temperature (Tg) and lower than the melting point (Tm) of the melt-formable fluororesin.

[2]
The melt moldable fluororesin is an ethylene-tetrafluoroethylene copolymer, a tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, or a polychlorotrifluoroethylene. And the method for producing a porous fluororesin according to [1], which is at least one fluororesin selected from the group consisting of ethylene-chlorotrifluoroethylene copolymers.

[3]
In addition to the melt-formable fluororesin, solvent-soluble resin, and inorganic fine powder, a heat-resistant organic liquid is further added in an amount of 5 to 100 parts by mass in total of 100 parts by mass of the melt-formable fluororesin, solvent-soluble resin, and inorganic fine powder. The method for producing a fluororesin porous body according to [1] or [2], wherein 45 parts by mass are mixed.

[4]
The method for producing a fluororesin porous body according to any one of [1] to [3], wherein the melt-formable fluororesin porous body is a hollow tube, film, or sheet selected from tubes, pipes, and hollow fibers.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the porous molded object of the melt-formability fluororesin provided with the outstanding chemical resistance and the outstanding filtration performance, and high mechanical strength is provided.

Hereinafter, the present invention will be described in detail.
In the present invention, basically, a composition comprising a melt-formable fluororesin, a solvent-soluble resin, and an inorganic fine powder is first melt-molded and then made porous.

(Melt moldable fluororesin)
The melt moldable fluororesin (hereinafter sometimes simply referred to as “fluororesin”) used in the present invention may be abbreviated as “ethylene-tetrafluoroethylene copolymer” (hereinafter “ETFE”). The same applies hereinafter), tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer (PFA) (wherein the perfluoroalkyl group has 1 to 18 carbon atoms), tetrafluoroethylene-hexafluoropropylene Tetrafluoroethylene fluororesin such as copolymer (FEP); chlorotrifluoroethylene fluororesin such as polychlorotrifluoroethylene (PCTFE) and ethylene-chlorotrifluoroethylene copolymer (ECTFE) . These fluororesins can be used alone or as a mixture thereof.

  Of the above, ETFE and FEP are preferred as the melt moldable fluororesin used in the present invention, and ETFE is more preferred.

(ETFE)
As ETFE, the molar ratio of repeating units based on tetrafluoroethylene (hereinafter referred to as “TFE”) / repeating units based on ethylene is preferably 70/30 to 30/70, more preferably 65/35 to 40. / 60, most preferably 60/40 to 40/60.

ETFE may contain repeating units based on other monomers in addition to TFE and ethylene. Other monomers include fluoroethylenes such as CF ₂ = CFCl and CF ₂ = CH ₂ (excluding TFE); fluoropropylenes such as CF ₂ = CFCF ₃ and CF ₂ = CHCF ₃ ; CF _{3 CF 2 CF 2 CF 2 CH} = CH 2, CF 3 CF 2 CF 2 CF 2 CF = carbon number, such as CH ₂ has a 4-12 fluoroalkyl group (perfluoroalkyl) ethylenes; R ^f (OCFXCF ₂ ) _m OCF = CF ₂ (wherein R ^f is a C 1-6 perfluoroalkyl group, X is a fluorine atom or a trifluoromethyl group, and m is an integer of 0-5). _{perfluorovinylethers; CH 3 OC (= O)} CF 2 CF 2 CF 2 OCF = CF 2 or _{FSO 2 CF 2 CF 2 OCF (} CF 3) CF 2 OCF = such as CF _2, easily carboxylic acid group and sul Perfluorovinyl ethers having a group convertible to an acid group; C3 olefins such as propylene, and olefins such as C4 olefins such as butylene and isobutylene (excluding ethylene), and the like. (Comonomer) may be contained alone or in combination of two or more.

  When the repeating unit based on these comonomers is contained, the content is usually 30 mol% or less, more preferably 0.1 to 15 mol%, most preferably based on all repeating units of ETFE. Preferably it is 0.2-10 mol%.

(FEP)
Further, as FEP, the molar ratio of the repeating unit based on TFE / the repeating unit based on hexafluoropropylene is preferably 98/2 to 50/50, more preferably 95/15 to 60/40, most preferably 90 / 10-75 / 25.

FEP may contain repeating units based on other monomers in addition to TFE and hexafluoropropylene. Other monomers include fluoroethylenes such as CF ₂ = CFCl and CF ₂ = CH ₂ (excluding TFE); fluoropropylenes such as CF ₂ = CHCF ₃ (excluding hexafluoropropylene) _{.); CF 3 CF 2 CF} 2 CF 2 CH = CH 2, CF 3 CF 2 CF 2 CF 2 CF = carbon number, such as CH ₂ has a 4-12 fluoroalkyl group (perfluoroalkyl) ethylenes; ^{_{R f (OCFXCF 2) m OCF}} = CF 2 ( wherein R ^f is a perfluoroalkyl group having 1 to 6 carbon atoms, X represents a fluorine atom or a trifluoromethyl group, m represents an integer of 0 to 5 .) perfluoro vinyl ethers such _{as; CH 3 OC (= O)} CF 2 CF 2 CF 2 OCF = CF 2 or _{FSO 2 CF 2 CF 2 OCF (} CF 3) volumes, such CF ₂ OCF = CF ₂ Perfluorovinylethers having a group convertible to a carboxyl group or sulfonic acid group; C3 olefins such as propylene, butylene, olefins except ethylene C4 olefins such as such as isobutylene and the like. These comonomers may be included alone or in combination of two or more.

  When the repeating unit based on these comonomers is contained, the content is usually 30 mol% or less, more preferably 0.1 to 15 mol%, most preferably based on the entire repeating unit of FEP. Preferably it is 0.2-10 mol%.

(MI)
The melt-formable fluororesin in the present invention is made porous after being melt-molded into a form of a hollow body or a film. In order to have an appropriate melt-formability, its melt index value ( MI) is preferably 0.5 to 40, more preferably 1 to 30. MI is a measure of melt moldability of a resin. Generally, when MI is large, the polymer molecular weight is small, and when MI is small, the polymer molecular weight is large.

  In the present invention, if MI is too larger than the above range, the effect of improving the strength is small even if the obtained fluororesin porous body is subjected to heat treatment. Further, if MI is too smaller than the above range, the moldability of the resin is deteriorated, so that the processing temperature is inevitably increased, and thermal decomposition of components other than the melt moldable fluororesin tends to occur. It is not preferable.

(Solvent soluble resin)
In the present invention, the solvent-soluble resin used by blending with the melt-formable fluororesin is formed into a porous body by forming voids in the molded body by forming with a solvent after forming the molded body. Is. Therefore, it is required to have heat resistance because it can be finely and well dispersed in the fluororesin using the melt-formable fluororesin as a matrix and is kneaded and melt-molded with the fluororesin at a high temperature. The

  The solvent-soluble resin is not particularly limited, but a fluorine-based resin is preferable, and for example, polyvinylidene fluoride and a vinylidene fluoride copolymer are preferable. Examples of the vinylidene fluoride copolymer include a copolymer of vinylidene fluoride (VDF) and one or more selected from the group consisting of TFE, hexafluoropropylene, and ethylene. More preferably, it is polyvinylidene fluoride. In addition, as long as it has the said characteristic as a solvent soluble resin in the range which does not largely inhibit the effect of this invention, it is also possible to use well-known various resin.

(Inorganic fine powder)
In the present invention, together with the solvent-soluble resin, the inorganic fine powder used in the melt-formable fluororesin is also extracted with the solvent after forming the molded body, thereby forming pores in the molded body. Although it is made porous, it enhances the dispersibility of the solvent-soluble resin in the fluororesin, contributes to the formation of a more uniform porous structure, and further superimposes pore formation by extraction of the inorganic fine powder itself. By this, it has the function which contributes to the increase in the porosity as the whole porous body, and maintenance of mechanical strength.

  Any inorganic fine powder may be used as long as it has the above functions, and is not particularly limited. For example, anhydrous silica, talc, clay, kaolin, mica, zeolite, calcium carbonate, barium carbonate, magnesium carbonate, calcium sulfate, barium sulfate, magnesium sulfate, zinc oxide, calcium oxide, magnesium oxide, titanium oxide, aluminum hydroxide, water Suitable examples include magnesium oxide and calcium phosphate. Of these, anhydrous silica is particularly preferable.

  The particle diameter of the inorganic fine powder is preferably 30 to 0.01 μm, more preferably 20 to 0.02 μm, and most preferably 10 to 0.03 μm in order to form suitable fine pores.

(Composition ratio)
In the present invention, the composition comprising the above-described melt-formable fluororesin, solvent-soluble resin, and inorganic fine powder is melt-molded as a basic compound, but the composition ratio (mixing ratio) is melt-formable fluorine. The range of 20-60 mass% of resin, 20-60 mass% of solvent soluble resin, and 5-30 mass% of inorganic fine powder is preferable.

  The content of the melt moldable fluororesin in the composition is more preferably 25 to 55% by mass, and further preferably 30 to 50% by mass. If the content of the melt-formable fluororesin is too small and less than 20% by mass, it is difficult to obtain a porous body having sufficient strength even if heat treatment described later is performed, and if it exceeds 60% by mass, the porous body has a low porosity. Therefore, the water permeability tends to be low, which is not preferable. When the ratio of the melt moldable fluororesin in the composition is in the above range, a porous body having excellent mechanical strength and excellent water permeability can be obtained.

  On the other hand, the content of the solvent-soluble resin in the composition is more preferably 25 to 55% by mass, and further preferably 30 to 50% by mass. When the amount of the solvent-soluble resin is small and less than 20% by mass, the porosity of the resulting porous body is very low. On the other hand, when the solvent-soluble resin exceeds 60% by mass, a porous body having high mechanical strength is obtained. It is difficult and difficult to do. It is preferable that the ratio of the solvent-soluble resin in the composition is in the above range because a porous body having excellent mechanical strength and high porosity can be obtained.

  Furthermore, content of the inorganic fine powder in a composition becomes like this. More preferably, it is 8-28 mass%, More preferably, it is 10-25 mass%. If the amount of the inorganic fine powder is less than the above range, the dispersibility of the solvent-soluble resin in the melt-formable fluororesin cannot be sufficiently improved. Is not preferable. When the proportion of the inorganic fine powder in the composition is in the above range, the dispersibility of the solvent-soluble resin in the melt moldable fluororesin is excellent, and there is no adverse effect when forming a molded body such as a film.

(Heat resistant organic liquid)
In the present invention, it is also preferable to further mix a heat-resistant organic liquid, if desired, with a composition comprising a melt-formable fluororesin, a solvent-soluble resin and an inorganic fine powder. By mixing the heat-resistant organic liquid, the uniformity of the porous structure of the porous body obtained by making the molded body obtained by melt-molding the composition porous can be further improved. That is, it is presumed that these heat-resistant organic liquids are usually extracted together with the extraction of the solvent-soluble resin and contribute to more uniform porosity.

  The heat-resistant organic liquid is preferably a liquid that is liquid at the time of melt molding and inert to the inorganic fine powder. For example, chlorotrifluoroethylene oligomer, adipic acid isobutyl ester, trimellitic acid ester, silicone oil and the like are preferable. The mixing amount of the heat-resistant organic liquid is preferably 5 to 45 parts by mass, and more preferably 10 to 35 parts by mass with respect to 100 parts by mass in total of the melt moldable fluororesin, the solvent-soluble resin and the inorganic fine powder.

(Melting process (first process))
In the present invention, a melt moldable fluororesin composition comprising 20-60% by mass of melt moldable fluororesin, 20-60% by mass of solvent-soluble resin, and 5-30% by mass of inorganic fine powder obtained as described above. A product (hereinafter, also simply referred to as “fluororesin composition”) is melt-molded to obtain a molded product (first step).

  The fluororesin composition comprises a melt-formable fluororesin, a solvent-soluble resin, and an inorganic fine powder at a predetermined ratio in an appropriate powder mixer such as a V-type mixer, a double cone mixer, a ribbon-type mixer. Machine, short shaft rotor type mixer, turbine type mixer, Henschel mixer, high speed mixer, super mixer, tumbler mixer, etc. It is preferable to pelletize in advance.

  Alternatively, each component of the melt-formable fluororesin, solvent-soluble resin, and inorganic fine powder is directly fed from each independent feeder at a predetermined ratio into the single-screw or twin-screw extruder, kneaded, It may be pelletized.

  In addition, when a heat-resistant organic liquid is further mixed with the fluororesin composition, is a feeder mixed with a mixture of inorganic fine powder and heat-resistant organic liquid previously supplied to a single-screw or twin-screw extruder? Alternatively, pelletization can be similarly performed by supplying the heat-resistant organic liquid material independently from the middle of the extruder. Of course, a heat-resistant organic liquid may be added to a mixer such as a Henschel mixer together with the main component such as a melt-formable fluororesin and mixed, and further kneaded and pelletized using a single-screw or twin-screw extruder.

  Next, the pellet is melt-extruded by an extrusion machine combined with a suitable die at a temperature of at least the melting point of the fluororesin, preferably a melting point + 20 ° C. or more, and a temperature below the decomposition temperature, A molded body of the fluororesin composition comprising at least a melt moldable fluororesin, a solvent-soluble resin, and an inorganic fine powder is obtained. That is, the pellets supplied to the extruder are plasticized, kneaded and melted by a single screw or twin screw part of the extruder, and the fluororesin composition is supplied to a die directly connected to the extruder. The molded body is formed into a desired shape.

  The die can be appropriately changed according to the shape of the molded body. When the molded body is a hollow tube such as a tube, pipe, or hollow fiber, the inner diameter, outer diameter, and tube of the hollow tube It is preferable to use an annular die (hollow die or pipe die) having a die diameter corresponding to the size such as thickness. As the annular die, a straight die, a cross head die, an offset die or the like is used. The hollow tube extruded from the annular die is usually immersed in a water bath and cooled.

  Moreover, when a molded object is flat films, such as a film and a sheet | seat, it is preferable to use T die (or flat die) according to the thickness. The fluororesin composition plasticized and melted from pellets by an extruder is supplied to a T-die, widened, extruded from a gap at the tip, taken off, and cooled and solidified by a cast roll or the like to form a film or sheet Etc. are obtained.

  In addition, when a molded object is a comparatively thick sheet | seat, it can also form by calendar molding or press molding other than extrusion molding.

  Moreover, when forming a small diameter hollow tube, you may apply the coating process currently performed with respect to the electric wire and the cable. That is, a core wire such as copper is supplied to an ordinary extrusion molding machine, and the molten fluororesin composition is extruded and coated on the surface of the core wire from an annular die. By pulling out the core wire from the obtained coated core wire, a hollow tube such as a thin tube is formed.

(Extraction process (second process))
The molded body of the fluororesin composition formed in the first step is treated with a solvent, and the solvent-soluble resin and the inorganic fine powder are extracted and made porous, so that the porous molded body of the fluororesin (as defined above) In some cases, the extraction step (second step), that is, the porosification step, is obtained.

  The solvent used for extraction of the solvent-soluble resin is not particularly limited as long as it dissolves the solvent-soluble resin and does not substantially dissolve the fluororesin. For example, acetone, methyl ethyl ketone, diethyl ketone, methyl Examples thereof include ketones such as isobutyl ketone and acetophenone; amides such as N, N-dimethylformamide and N, N-dimethylacetamide, and halogenated hydrocarbons such as 1,1,1-trichloroethane, trichloroethylene and tetrachloroethylene.

  The apparatus for performing the extraction is not particularly limited as long as it is an apparatus that can generally perform solid extraction (leaching), and is a tank-type extractor provided with stirring means such as mechanical or air lift. An immersion type extractor, a once-through type extractor, a tower type extractor, a rotary chamber type extractor and the like are preferably used. The extraction operation can be carried out by any of batch operation, semi-batch operation, and continuous operation. In the case of continuous operation, it is preferable to use a countercurrent multistage extraction operation according to a conventional method.

  The extraction temperature is usually from room temperature to less than the boiling point of the solvent. The extraction time varies depending on the type of extraction solvent, the amount of extraction solvent used in the molded body, the shape and size of the molded body, the content of solvent-soluble resin in the molded body, the form of extraction operation, the type of extraction device, etc. However, it is usually 0.1 to 50 hours, preferably 1 to 30 hours, more preferably about 5 to 20 hours.

  Subsequent to the extraction of the solvent-soluble resin, the inorganic fine powder is extracted. The solvent used for extraction of the inorganic fine powder is not particularly limited as long as it dissolves the inorganic fine powder and does not dissolve the melt-formable fluororesin. When the inorganic fine powder is a powder soluble in acid such as calcium carbonate or barium carbonate, hydrochloric acid or sulfuric acid is used as the solvent, and when the inorganic fine powder is soluble in alkali such as anhydrous silica. An alkaline aqueous solution such as sodium hydroxide or potassium hydroxide is used. In the case of anhydrous silica, it is possible to extract with hydrofluoric acid.

  As an apparatus for carrying out the extraction, basically, those used in the extraction of the solvent-soluble resin are used as they are, and the extraction operation is similarly performed in any of batch operation, semi-batch operation, and continuous operation. be able to. In addition, in the case of continuous operation, the same can be applied to countercurrent multistage extraction operation.

  The extraction temperature is usually from room temperature to a temperature lower than the boiling point of the sodium hydroxide aqueous solution or the like. The extraction time may vary depending on the type of extraction solvent, the extraction temperature, etc. as described above, but is usually 0.1 to 30 hours, preferably 1.5 to 10 hours, and more preferably about 1 to 5 hours. is there. In addition, in the said extraction process, the order is not specifically limited, However, It is preferable to extract inorganic fine powder after extraction of solvent-soluble resin.

  The porous molded body after the extraction treatment is preferably pre-dried at a temperature of less than 100 ° C. after washing with water or a suitable solvent.

(Heat treatment process (3rd process))
In the present invention, the fluororesin porous molded body made porous by the extraction process in the second step is heat-treated at a temperature not lower than the glass transition temperature (Tg) and lower than the melting point (Tm) of the melt-formable fluororesin. The mechanical strength can be improved. In the present invention, the melting point is a value measured at a heating rate of 10 ° C./min using a differential scanning calorimeter (DSC220C, manufactured by Seiko Instruments Inc.) in accordance with JISK7121.

  The heating device or heating means (heat source) used for the heat treatment is not particularly limited as long as it contains the fluororesin porous molded body and can heat and hold it at the above temperature, For example, arbitrary things, such as a hot air dryer, an electric furnace, a vacuum dryer, a continuous drying furnace, a microwave dryer, an infrared heater, a high frequency heating apparatus, are employ | adopted. Specifically, there is no problem with the general method of charging the porous body to be heat-treated into an electric furnace set to a glass transition temperature or higher, a hot-air gear oven, etc., and storing in the oven. It can implement suitably.

  In the present invention, since the fluororesin porous body is heat-treated at a temperature higher than the glass transition temperature, stress distortion during melt molding is reduced and recrystallization is performed. It is assumed that the strength is greatly improved. Here, when the heat treatment is performed at a temperature lower than the glass transition temperature (Tg), stress strain is relaxed, but the above-described recrystallization is not achieved, and thus the effects of the present invention cannot be achieved. On the other hand, when heat treatment is performed at a melting point (Tm) or higher, the porous body itself melts and deforms, and the shape as the porous body cannot be maintained. More preferably, the temperature range is (Tg + 20 ° C.) or more (Tm−20 ° C.).

  In particular, when the fluororesin porous body is ETFE, the heat treatment temperature is preferably 100 to 250 ° C, more preferably 110 to 240 ° C.

  The heat treatment time of the porous molded body may vary depending on the heat treatment temperature, the treatment apparatus, the shape of the porous body, the type of fluororesin forming the porous body, and the like, but usually 15 minutes to 50 hours, preferably 30 minutes to 30 hours. More preferably, it is 30 minutes to 25 hours.

  As described above, the melt-formable fluororesin porous body is heat-treated at a temperature not lower than the melting point of the glass transition temperature defined in the present invention, and thus has excellent chemical resistance and water permeability, and has mechanical strength. A significantly improved fluororesin porous body is obtained.

  Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto. In these examples, the mixing ratio is expressed as a mass ratio. The melt index (MI), glass transition temperature (Tg), porosity (ε), and tensile strength (TS) were measured as follows.

(A) [Measurement of melt index (MI) (g / 10 min)]
Based on ASTM D3159-98, it measured at 297 degreeC using the Takara Kogyo make melt indexer.
(B) [Measurement of glass transition temperature (Tg (° C))]
In accordance with JIS K 7244-4, the measurement was performed at a measurement frequency of 10 Hz using a dynamic viscoelasticity measuring device (“Rheograph Solid” manufactured by Toyo Seiki Seisakusho).

(C) [Measurement of porosity (ε (%))]
The porosity (also referred to as porosity) ε (%) of the fluororesin porous body was determined using the following formula (1).
ε = [(d−d ′) / d] × 100 (1)
Here, d = true specific gravity of melt moldable fluororesin, d ′ = apparent specific gravity of the solvent-soluble resin and the porous material after extraction with inorganic fine powder.

(D) [Measurement of tensile strength (TS) (MPa)]
When the compact is a porous hollow tube, the tensile strength is the breaking strength when a sample of the porous hollow tube is pulled at a rate of 25 mm between chucks and at a speed of 50 mm / min. The value divided by was taken as the tensile strength.
The tensile strength when the molded body is a porous sheet is a break when a 1 mm thick porous sheet is pulled at a dumbbell shape of 5, a distance between chucks of 25.4 mm and a speed of 50 mm / min according to ASTM D638-00. The value obtained by dividing the strength by the apparent cross-sectional area before tension was defined as tensile strength.

[Example 1]
(1) As a melt moldable fluororesin, an ethylene / tetrafluoroethylene copolymer (hereinafter also referred to as “ETFE”) having a melt index value (MI) at 300 ° C. of 3.8 (Fluon C-88AX, Asahi Glass) 1,800 g of Tg = 93 ° C., Tm = 260 ° C., as a solvent-soluble resin, 1800 g of polyvinylidene fluoride (KF polymer T- # 1100, manufactured by Kureha), anhydrous silica (AEROSIL OX50, Nippon Aerosil) 750 g (manufactured by Kogyo Co., Ltd., primary particle average particle diameter: 40 nm) was melt-kneaded at a molding temperature of 280 ° C. using a twin screw extruder to obtain pellets. The mass ratio of ETFE / polyvinylidene fluoride / anhydrous silica was 32/48/20.

  The pellets were melt-kneaded at a resin temperature of 280 ° C. using a 30 mm single screw extruder, melt-extruded in an electric wire coating extruder equipped with a die having an outer diameter of 2.9 mmφ and an inner diameter of 2.0 mmφ, And cooled. Thereafter, the core wire was drawn out to obtain a hollow tube (tube) as a molded body having an outer diameter of 1.5 mmφ and an inner diameter of 0.9 mmφ.

(2) The obtained hollow tube is cut to a predetermined size and then immersed in N, N-dimethylformamide (a solvent of polyvinylidene fluoride) heated to 65 ° C. for 10 hours to extract the polyvinylidene fluoride. did. Next, the obtained hollow tube was further immersed in a 15% by mass KOH aqueous solution heated to 80 ° C. for 2 hours to extract anhydrous silica. The extracted hollow tube was washed with water and preliminarily dried (preliminary heat treatment) at 80 ° C. for 24 hours to obtain a porous hollow tube of an ethylene / tetrafluoroethylene copolymer.

(3) This porous hollow tube was heat-treated at 230 ° C. for 24 hours using a hot air gear oven to obtain a product porous hollow tube. The heat treatment temperature of 230 ° C. is selected as a temperature that is not lower than the glass transition temperature Tg (90 ° C.) of ETFE and lower than the melting point Tm (260 ° C.). Table 1 shows the results of measuring the physical properties (porosity and tensile strength) of the porous hollow tube thus obtained.

[Comparative Example 1]
In Example 1, the porous hollow tube of the ethylene / tetrafluoroethylene copolymer obtained by predrying (preliminary heat treatment) at 80 ° C. for 24 hours was subjected to heat treatment at 230 ° C. without performing heat treatment at 230 ° C. The results of measuring the physical properties are shown in Table 1.

[Example 2]
(1) 10.9 g of the same ethylene / tetrafluoroethylene copolymer (Fluon C-88AX, manufactured by Asahi Glass Co., Ltd.) used in Example 1 as a melt moldable resin, and polyvinylidene fluoride ( 7.9 g of KF polymer T- # 1100, manufactured by Kureha), 3.3 g of anhydrous silica (AEROSIL OX50, manufactured by Nippon Aerosil Co., Ltd., primary particle average particle size 40 nm) as inorganic fine powder, and chloro as heat-resistant organic liquid 5.2 g of trifluoroethylene oligomer (Daifloil # 1, manufactured by Daikin) was melt-kneaded for 10 minutes at a processing temperature of 300 ° C. using a Labo Plus Mill manufactured by Toyo Seiki, and pressed at 300 ° C. for 5 minutes. Processing was performed to prepare a 1 mm thick sheet as a molded body. The mass ratio of ETFE / polyvinylidene fluoride / anhydrous silica / chlorotrifluoroethylene oligomer was 49.4 / 35.8 / 14.8 / 23.5.

(2) The polyvinylidene fluoride was extracted by immersing the sheet in N, N-dimethylformamide, which is a polyvinylidene fluoride solvent, at 65 ° C. for 10 hours. Subsequently, the obtained sheet was immersed in a 15% by mass KOH aqueous solution at 80 ° C. for 2 hours to extract anhydrous silica.

The extracted porous sheet was washed with water and pre-dried (pre-heat treatment) at 80 ° C. for 24 hours to obtain a porous sheet of an ethylene / tetrafluoroethylene copolymer.
(3) This porous sheet was heat-treated at 120 ° C. for 24 hours using a hot air gear oven to obtain a product porous sheet. The heat treatment temperature of 120 ° C. is selected as a temperature not lower than the glass transition temperature Tg (90 ° C.) of ETFE and lower than the melting point Tm (260 ° C.). Table 2 shows the results of measuring the physical properties (porosity and tensile strength) of the porous sheet.

[Comparative Example 2]
In Example 2, the physical properties of the porous sheet of ethylene / tetrafluoroethylene copolymer obtained by pre-drying (pre-heat treatment) for 24 hours at 80 ° C. were not subjected to heat treatment at 120 ° C. The measurement results are shown in Table 2.

Example 3
(1) 12.6 g of the same ethylene / tetrafluoroethylene copolymer (Fluon C-88AX, manufactured by Asahi Glass Co., Ltd.) used in Example 2 as a melt-moldable resin, and polyvinylidene fluoride ( KF polymer T- # 1100, manufactured by Kureha Co., Ltd. A sheet having a thickness of 1 mm was prepared in the same manner as in Example 2 except that 10.5 g of trifluoroethylene oligomer (Daifloyl # 1, manufactured by Daikin) was used. The mass ratio of ETFE / polyvinylidene fluoride / anhydrous silica / chlorotrifluoroethylene oligomer was 40.0 / 46.7 / 13.3 / 33.3.

(2) The sheet was extracted with N, N-dimethylformamide and an aqueous KOH solution in the same manner as in Example 2.
The extracted porous sheet was washed with water and pre-dried (pre-heat treatment) for 24 hours at 80 ° C. in the same manner as in Example 2 to obtain a porous sheet of an ethylene / tetrafluoroethylene copolymer. .

(3) This porous sheet was heat-treated at 120 ° C. for 24 hours in the same manner as in Example 2 using a hot air gear oven to obtain a product porous sheet. Table 2 shows the results of measuring the physical properties (porosity and tensile strength) of the porous sheet.

[Comparative Example 3]
In Example 3, the porous sheet of the ethylene / tetrafluoroethylene copolymer obtained by predrying (preliminary heat treatment) at 80 ° C. for 24 hours was subjected to the physical properties without performing the heat treatment at 120 ° C. The results of measuring are shown in Table 2.

  As is clear from Comparative Examples 1 to 3 (and Tables 1 and 2 showing the results) corresponding to Examples 1 to 3, a porous body of a melt-formable fluororesin such as ETFE is used at a glass transition temperature Tg or higher. It can be seen that the heat treatment at a temperature lower than the melting point Tm greatly increases the mechanical strength (tensile strength) without substantially changing the porosity (porosity).

  The porous hollow tube (tube), film, or sheet, which is a fluororesin porous body, obtained by the production method of the present invention has excellent chemical resistance and excellent filtration performance, as well as its mechanical strength. Suitable for water treatment membranes, separation hollow fibers, water treatment hollow fibers, etc. in water treatment applications such as drinking water, purified water, sewage treatment, human waste treatment, membrane separation activated sludge treatment, wastewater treatment, waste liquid treatment. Can be used.

Claims (4)

  In the method for producing a melt-formable fluororesin porous body, a composition comprising 20 to 60% by mass of the melt-formable fluororesin, 20 to 60% by mass of a solvent-soluble resin, and 5 to 30% by mass of inorganic fine powder is melt-molded. The first step of obtaining the molded body, and then the second step of extracting the solvent-soluble resin and the inorganic fine powder from the molded body to obtain a porous body of a melt moldable fluororesin, and the porous body, A method for producing a porous melt-formable fluororesin comprising the third step of heat-treating at a temperature not lower than the glass transition temperature (Tg) and lower than the melting point (Tm) of the melt-formable fluororesin.   The melt moldable fluororesin is an ethylene-tetrafluoroethylene copolymer, a tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, or a polychlorotrifluoroethylene. The method for producing a fluororesin porous body according to claim 1, wherein the fluororesin porous body is at least one fluororesin selected from the group consisting of ethylene-chlorotrifluoroethylene-based copolymers.   In addition to the melt-formable fluororesin, solvent-soluble resin, and inorganic fine powder, a heat-resistant organic liquid is further added in an amount of 5 to 100 parts by mass with respect to a total of 100 parts by mass of the melt-formable fluororesin, solvent-soluble resin, and inorganic fine powder. The manufacturing method of the fluororesin porous body of Claim 1 or 2 which mixes 45 mass parts.   The method for producing a fluororesin porous body according to any one of claims 1 to 3, wherein the melt-moldable fluororesin porous body is a hollow tube, a film, or a sheet selected from a tube, a pipe, and a hollow fiber.