JP5961943B2 - Porous hollow fiber membrane mainly composed of polyacetal and method for producing the same - Google Patents

Description

  The present invention relates to a porous hollow fiber membrane mainly composed of polyacetal, which can be produced at low cost, has excellent durability and dimensional stability, and has a low formaldehyde residual concentration, and a method for producing the same.

  Polyacetal resin is a resin having an excellent balance of mechanical properties and moldability, and is widely used as an injection molded product. However, since the crystallization speed is high, it is difficult to process into a film or fiber, and its use is limited.

  On the other hand, mixing two polymers is widely known as a polymer blend or polymer alloy, and is widely used for the purpose of improving the defects of individual polymers or developing new functions. When two kinds of polymers are mixed, many of them are separated into two phases, and thus often exhibit low physical properties, but in rare cases, the two kinds of polymers may form a uniform amorphous phase. This type is generally described as compatible or miscible and is expected to exhibit excellent properties, but there are few examples of combinations thereof.

  Patent Document 1 describes the compatibility between polyacetal and other resins, and discloses the formation of a porous body by extraction of other resin components from the resin composition. An example of the application is a separation membrane.

  Patent Document 2 describes a polyolefin-based porous hollow fiber membrane, and mentions polyoxymethylene as an example of the polyolefin-based resin.

JP 2004-250555 A JP 2000-84368 A

  In Patent Document 1, a separation membrane is cited as an example of the use of a resin composition comprising polyacetal and other resins, but there is no mention of a specific technical idea relating to this. There is no description regarding the shape as a hollow fiber membrane in particular. In addition, the use of an organic solvent such as chloroform is cited as an example of a method for extracting other resins from the resin composition, but this has the disadvantage that formaldehyde, which is a polyacetal resin decomposition product, remains in the porous body. The inventors have found that there is.

  Patent Document 2 mentions polyoxymethylene as an example of a polyolefin-based resin, but there is no description that it is specifically implemented. The hollow fiber membrane disclosed in Patent Document 2 is a chemical degassing membrane having a composite structure in which a homogeneous thin film having no communication holes is sandwiched between polyolefin-based porous membranes, and the substance to be separated is physically removed by the pores. It is not intended for use. Moreover, since the hollow fiber membrane shown in Patent Document 2 is opened by stretching a polyolefin and does not undergo a dipping step in a solvent or alkali, formaldehyde remains in the porous body. The inventors have found that there are drawbacks.

  Therefore, the present invention has an object to provide a porous hollow fiber membrane mainly composed of polyacetal, which can be produced at low cost, is excellent in durability and dimensional stability, and particularly has a low formaldehyde residual concentration, and a method for producing the same. To do.

  The present invention for solving the above-described problems has the configurations described in the following (1) to (7).

  (1) A porous hollow fiber membrane comprising a polyacetal resin containing 60% by weight or more of acetal units and having a formaldehyde concentration of 100 ppm or less.

  (2) The porous hollow fiber membrane according to (1), wherein the film thickness is 1 to 100 μm and the average pore diameter on the outer surface is 0.001 to 500 μm.

  (3) A method for producing a porous hollow fiber membrane, the step of molding a resin composition comprising a polyacetal resin A and a resin B different from the polyacetal resin A so that the cross-sectional shape is hollow, and a resin molding molded into the hollow A method for producing a porous hollow fiber membrane, comprising the step of immersing a product in an alkaline aqueous solution and removing a part or all of the resin B.

  (4) A method for producing a porous hollow fiber membrane, wherein a resin composition comprising a polyacetal resin A and a resin B different from the polyacetal resin A is disposed in the sheath, and a resin C different from the polyacetal resin A is disposed in the core. Forming the core-sheath composite form into a cross-sectional shape, immersing the resin molding molded into the core-sheath composite form in an alkaline aqueous solution, and part or all of the resin B and part or all of the resin C A method for producing a porous hollow fiber membrane, comprising the step of removing

  (5) The method for producing a porous hollow fiber membrane according to (3) or (4), wherein the resin B is a polylactic acid resin.

  (6) The method for producing a porous hollow fiber membrane according to (3) or (4), wherein the alkaline aqueous solution is a sodium hydroxide aqueous solution.

  ADVANTAGE OF THE INVENTION According to this invention, the porous hollow fiber membrane which can be manufactured at low cost and was excellent in durability and dimensional stability is provided.

  Hereinafter, the present invention will be described in detail.

  The polyacetal resin A used in the present invention is a polymer having an oxymethylene unit as a main repeating unit, and even a so-called polyacetal homopolymer obtained by polymerization reaction using formaldehyde or trioxane as a main raw material is 60% by weight or more. The copolymer may be any of a block copolymer, a terpolymer, and a crosslinked polymer. Examples of structural units other than the oxymethylene unit include oxyalkylene units having 2 to 8 adjacent carbon atoms in the main chain.

  Further examples of other structural units include ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, styrene oxide, oxetane, 3,3-bis (chloromethyl) oxetane, tetrahydrofuran, trioxepane, 1,3 -Dioxolane, propylene glycol formal, diethylene glycol formal, triethylene glycol formal, 1,4-butanediol formal, 1,5-pentanediol formal, 1,6-hexanediol formal, and the like can be mentioned.

  The upper limit of the weight fraction of the oxymethylene unit is not particularly defined, but the polyacetal copolymer has a lower crystallinity than the polyacetal homopolymer and improves moldability and toughness, so the oxymethylene unit is 98% by weight or less. More preferably, the oxymethylene unit is particularly preferably 70% by weight or more from the viewpoint of improving the film strength due to high crystallinity. There is no restriction | limiting in particular in the manufacturing method of the polyacetal resin in this invention, It can manufacture by a well-known method. As an example of a typical method for producing a polyacetal homopolymer, high-purity formaldehyde is introduced into an organic solvent containing a basic polymerization catalyst such as an organic amine, an organic or inorganic tin compound, or a metal hydroxide. Examples of the method include polymerizing and filtering the polymer, followed by heating in acetic anhydride in the presence of sodium acetate to acetylate the polymer terminal.

  In addition, as a preferable example of another production method, a high purity trioxane and a copolymer component such as ethylene oxide or 1,3-dioxolane are introduced into an organic solvent such as cyclohexane, and boron trifluoride diethyl ether complex After cationic polymerization using such Lewis acid catalyst, trioxane and copolymerization into a self-cleaning stirrer without using a solvent, or using a manufacturing method by deactivating the catalyst and stabilizing the end groups Examples thereof include a method in which a component and a catalyst are introduced and bulk polymerization is performed, and then an unstable terminal is further decomposed and removed.

  The viscosity of these polymers is not particularly limited as long as it can be used as a molding material, but the melt index (MI) can be measured by the ASTM D1238 method at a temperature of 190 ° C. and a load of 2160 g. Is preferably in the range of 1.0 to 70 g / 10 min, and 1.5 to 35 g / 10 min is melt-molding, ensuring fluidity in kneading and piping, or discharge stability during extrusion. This is particularly preferable from the viewpoint of securing.

  The polyacetal resin A used in the present invention can contain a matting agent, a plasticizer, a dye, a pigment, inorganic particles, a filler, a crystal nucleating agent and the like as long as its properties are not greatly changed. In particular, by including a plasticizer, the moldability and toughness are improved, so that it is preferably employed.

  The porous hollow fiber membrane of the present invention preferably has a thickness of 1 to 100 μm. In general, the thinner the separation functional layer, the lower the pressure loss during water permeation, and the better the water permeation performance is realized. From this viewpoint, the film thickness is preferably 100 μm or less, and from the viewpoint of maintaining strength, it is preferably 1 μm or more. Further, from the viewpoint of water permeability, the film thickness is more preferably 50 μm or less, and even more preferably 10 μm or less.

  It is important that the porous hollow fiber membrane of the present invention has fine pores inside. The fine pores herein are preferably pores having an outer surface average pore diameter of 0.001 μm or more, and preferably 500 μm or less. More preferred are pores of 0.01 μm or more, and still more preferred are pores of 100 μm or less. More preferred are pores of 10 μm or less, and particularly preferred are pores of 1 μm or less.

  The size of the pores can be appropriately selected depending on the use because it affects the separation object and the water permeability. When the separation target is pyrogen, albumin, protein or the like, the average pore diameter on the outer surface is 0.01 μm or more and 0.1 μm or less, which is a preferable aspect with good balance with water permeability. In addition, when the separation target is bacteria or the like, the average pore diameter on the outer surface is 0.1 μm or more and 1 μm or less, which is a preferable aspect with a good balance with water permeability.

  In addition to the above holes, the outer surface may have holes of 500 μm or more, but it is preferable that the number of holes of 500 μm or more is as small as possible depending on the application to be used. The average pore diameter on the outer surface can be calculated by measurement with a mercury porosimeter or observation with an electron microscope. Further, it is preferable that the fine pores communicate the outer surface and the inner surface.

  A porous separation membrane having communicating pores is excellent in water permeability and the separation function can be controlled by the pore size, and exhibits excellent performance as a separation membrane. When judging the pores communicated here, fill the liquid that does not change the membrane material on the outer membrane surface and the inner membrane surface, apply pressure to the liquid from either the inner membrane surface or the outer membrane surface, This is done because the liquid has passed through the inside of the membrane. Alternatively, it is assumed that a liquid is filled on the outer surface side of the membrane, a pressure of 1 MPa is applied by gas on the inner surface side of the membrane, and if bubbles are generated, a communication hole exists.

  The formaldehyde concentration specified in the present invention is measured by the following method. First, 2.5 g of a porous hollow fiber membrane is collected and immersed in 100 mL of water at 40 ° C. for 1 week. Thereafter, by reacting the immersion liquid with a 2,4-dinitrophenylhydrazine (DNPH) derivative, the eluted formaldehyde is derivatized to 2,4-DNPH, which is detected by a high performance liquid chromatograph (HPLC) method.

  In the porous hollow fiber membrane of the present invention, it is important that the formaldehyde concentration determined by the above method is 100 ppm or less. The concentration is not achieved by the extraction treatment of the resin B with an organic solvent such as chloroform, but can be achieved by performing an immersion treatment with an alkaline aqueous solution. The reason for this is not necessarily clear, but it is more advantageous that formaldehyde which is water-soluble is dissolved in water as a solvent rather than a porous hollow fiber membrane by immersion treatment with an alkaline aqueous solution which is a preferred form of the present invention. Therefore, it is presumed that the formaldehyde concentration could be kept very low as compared with the conventional solvent extraction method. The formaldehyde concentration is preferably 10 ppm or less because it is possible to suppress elution of formaldehyde into water, and is more preferably 1 ppm or less.

  The formaldehyde concentration of the present invention is not particularly limited, but it is less than the measurement limit of the analytical method and is so low that it is practically impossible to measure. This is a preferred embodiment.

  The porous hollow fiber membrane of the present invention is alkaline after, for example, discharging and molding a resin composition comprising a polyacetal resin A and a resin B different from the polyacetal resin A to take a core-sheath composite form so that the cross-sectional shape is hollow. It is obtained by immersing in an aqueous solution and removing part or all of the resin B.

  The porous hollow fiber membrane of the present invention has a core-sheath composite form in which the cross-sectional shape of the resin composition composed of, for example, polyacetal resin A and resin B different from polyacetal resin A is different from that of polyacetal resin A using resin C. After being discharged and molded in such a manner, it is obtained by immersing in an alkaline aqueous solution to remove part or all of the resin B and part or all of the resin C.

  Here, the resin composition comprising the polyacetal resin A of the present invention and the resin B different from the polyacetal resin A is preferably kneaded by a conventionally known method, and examples thereof include a single screw kneading extruder, a twin screw kneading extruder, Examples include kneaders and static kneaders. The kneaded resin composition may be subjected to melt molding as it is, or may be subjected to melt molding after being cooled and pelletized once. By being subjected to melt molding as it is after kneading and extruding, a continuous and simplified process can be constructed, which enables production at a low cost and is preferable.

  Resin B and resin C used in the present invention may be the same or different. As the resin B, those that can be eluted by immersion in an alkaline aqueous solution can be used. Specific examples of the resin B or the resin C include polylactic acid, polyhydroxyalkanoate, polycaprolactone, polybutylene succinate, polyethylene succinate, polybutylene terephthalate, polyethylene terephthalate, and polyesters such as nylon 6 and nylon 66. Polyamide, polyethylene oxide, polyvinyl pyrrolidone, polyvinyl alcohol and the like can be mentioned. The resin B or the resin C may contain only one kind of these exemplified resins or may contain two or more kinds.

  The resin B or resin C used in the present invention can contain a matting agent, a plasticizer, a dye, a pigment, inorganic particles, a filler, a crystal nucleating agent, and the like as long as the properties are not greatly changed.

  Among these resins, polylactic acid is particularly preferable from the viewpoint of affinity with the polyacetal resin A. The polylactic acid used in the present invention is mainly composed of L-lactic acid and / or D-lactic acid, but may contain a copolymer component other than lactic acid.

  Other monomer units include ethylene glycol, propylene glycol, butanediol, heptanediol, hexanediol, octanediol, nonanediol, decanediol, 1,4-cyclohexanedimethanol, neopentylglycol, glycerin, pentaerythritol, bisphenol. A, glycol compounds such as polyethylene glycol, polypropylene glycol and polytetramethylene glycol, oxalic acid, adipic acid, sebacic acid, azelaic acid, dodecanedioic acid, malonic acid, glutaric acid, cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, phthalate Acid, naphthalene dicarboxylic acid, bis (p-carboxyphenyl) methane, anthracene dicarboxylic acid, 4,4'-diphenyl ether dicarbo Acids, dicarboxylic acids such as 5-sodiumsulfoisophthalic acid, 5-tetrabutylphosphonium isophthalic acid, glycolic acid, hydroxypropionic acid, hydroxybutyric acid, hydroxyvaleric acid, hydroxycaproic acid, hydroxybenzoic acid and other hydroxycarboxylic acids, caprolactone And lactones such as valerolactone, propiolactone, undecalactone, and 1,5-oxepan-2-one.

  Such a copolymer component is preferably contained in an amount of usually 30 mol% or less, preferably 10 mol% or less, in all monomer components. In the present invention, the optical purity of the lactic acid component as polylactic acid is not particularly limited.

  The ratio of the polyacetal resin A in the resin composition composed of the polyacetal resin A and the resin B greatly affects the structure and performance of the final film. If the ratio of the polyacetal resin A is too small, the film strength is not maintained. The pore diameter on the surface becomes excessively fine and water permeability cannot be obtained. From that viewpoint, the proportion of the polyacetal resin A is preferably 20% by weight or more, preferably 70% by weight or less, more preferably 30% by weight or more, more preferably 60% by weight or less, and particularly preferably 50% by weight or less.

  The kneading temperature is preferably 20 ° C. or higher with respect to the higher melting point of the polyacetal resin A and the resin B, because the kneading can be efficiently performed. Although there is no particular upper limit, if the kneading temperature is 350 ° C. or lower, thermal decomposition proceeds and a resin composition of excellent quality can be obtained without causing a decrease in molecular weight, coloring or fuming.

  The core-sheath composite form can be formed using a conventionally known method. An example of a preferred embodiment is the use of a double-tube base. The double-tube type base can be molded by injecting resin C or air into the inner tube, thereby forming a core-sheath composite form including a core part formed of air. In particular, when the core portion is formed of air, the cross-sectional shape can be made close to a perfect circle, and the pressure resistance of the resulting porous hollow fiber membrane can be increased, which is a preferable mode. It is.

  As another example, it is also a preferable aspect to use a confluence type base. In particular, when the core portion is made of resin C, it is possible to easily obtain a stable core-sheath composite form, and it is easy to increase the number of spindles, which is difficult with the double pipe type, and to increase productivity. Is possible.

  The merging base here refers to a resin composition comprising separately melted and weighed resins in the base, resin B or resin C in the core, and polyacetal resin A and resin B in the sheath. The structure can be a conventionally known structure. In particular, when the polyacetal resin A and the resin B or the resin C are merged immediately before discharge, it is a preferable mode because the mixed resin is less mixed and the core-sheath composite form is stable.

  The outer shape of the core-sheath composite form is not particularly limited as long as the core-sheath composite form is not destroyed, and can be arbitrarily selected from circular, elliptical, multi-leaf cross sections such as polygonal and trilobe cross sections. Among these, a multi-leaf cross section is a preferred embodiment because fouling is suppressed during operation of the separation membrane. The number of convex portions of the multilobal cross section is not particularly limited, but is preferably about 3 to 10 from the viewpoint of accurately maintaining the cross-sectional form and maintaining the strength.

  Although cooling after discharge is not particularly limited, air or liquid may be used as the refrigerant. From the viewpoint of controlling the structure of the resin composition, cooling efficiency using a liquid is preferable because of high efficiency. Since the polyacetal resin A used in the present invention is insoluble in water, it is preferable to use water as a refrigerant for convenience of facility management. However, when cooling and operating below the freezing point of water, It is also a preferred embodiment to add an alcohol such as ethylene glycol to lower the freezing point.

  The water bath temperature is preferably −20 ° C. or more and 50 ° C. or less, from the viewpoints of energy efficiency for cooling and operation and cooling efficiency, more preferably 5 ° C. or more, and more preferably 30 ° C. or less.

  The form of the water bath is not particularly limited as long as the cooling water and the discharge are in sufficient contact with each other and can be cooled, and may be in the form of a liquid tank in which the cooling water is stored, Further, if necessary, the liquid tank may be circulated and updated with a liquid whose temperature and composition are adjusted. The liquid tank form is most preferable, but depending on the case, the form in which the cooling water flows in the pipe may be used, or the form in which the cooling water is jetted onto the film running in the air may be used. .

  When the refrigerant is air, it is preferable to cool the molten resin discharged in the air by blowing air whose temperature is adjusted from the periphery or from one direction. Particularly preferred is a method in which air whose temperature is evenly adjusted from the periphery to the molten resin is blown to uniformize the structure in the cross section of the porous hollow fiber membrane and obtain an excellent product. It is. Further, when air is used as the refrigerant when the diameter of the porous hollow fiber membrane is 200 μm or less, the frictional resistance with the refrigerant accompanying the passage of the process is small, the process speed can be increased, and the productivity is excellent. This is also a preferred embodiment.

  In the elution step of the resin B or the resin C, the method is not particularly limited as long as the resin B or the resin C can be immersed in an alkaline aqueous solution and a part or all of the resin B or the resin C can be removed, but through a solution bath filled with the alkaline aqueous solution. Examples of preferred embodiments include immersing and removing, and immersing in a packaged state once wound in a core-sheath composite form. In general, by using a stronger alkaline aqueous solution, the elution rate of the resin increases, but from the viewpoint of minimizing the structural destruction of the polyacetal resin A and reducing the risk of handling, the type of resin to be eluted Further, it is preferable that the composition of the solution is appropriately changed depending on the elution rate to be realized.

  When removing by immersion through a solution bath filled with an alkaline aqueous solution, it is preferable from the viewpoint of elution efficiency to stir the solution in the solution bath.

  The concentration in the solution bath is preferably from 0.1% by weight to 30% by weight from the viewpoint of productivity. In particular, since the process can be simplified, the concentration is more preferably 0.1% by weight or more, and more preferably 10% by weight or less. The temperature of the solution bath is preferably 60 ° C. or higher and 130 ° C. or lower from the viewpoint of the dissolution elution rate of Resin B or Resin C. Decomposition while achieving stable treatment while suppressing evaporation of water from the solution and keeping the concentration constant. From the viewpoint of maintaining the elution rate, it is more preferably 90 ° C. or lower.

  In the case where the resin B or the resin C is polyester, the solution bath is more preferably an alkaline aqueous solution from the viewpoint of the alkali solubility of the polyester.

  The solution bath in the case where the resin B or the resin C is polyethylene oxide, polyvinyl pyrrolidone, or polyvinyl alcohol is preferably water from the viewpoint of the water solubility of the resin group and the ease of handling.

  Moreover, in the method of immersing in a packaged state in which the core-sheath composite form is wound up, the elution conditions are not particularly limited, but the method of immersing the package in a heated container is preferable from the viewpoint of increasing the elution efficiency. It is. In particular, since the reaction rate can be increased and productivity can be improved, it is also a preferable aspect that a package and an alkaline aqueous solution are placed in a sealed container and subjected to high temperature treatment under pressure. The concentration in the solution bath at this time is not particularly limited, but 0.1 wt% or more and 30 wt% or less is a preferable embodiment from the viewpoint of productivity. In particular, since the process can be simplified, the concentration is more preferably 10% by weight or less.

The winding density of the package is preferably 1.0 g / cm ³ or less because the solution can easily penetrate into the inside and elution efficiency can be increased. The lower limit of the winding density of the package is not particularly set, but the lower limit guideline is 0.20 g / cm ³ or more from the viewpoint of production efficiency.

For example, when the porous hollow fiber membrane defined in the present invention has a film thickness of 100 μm, the water permeability under the conditions of 25 ° C. and 50 kPa is 8.0 × 10 ^{−5 to} 2.5 × 10 ⁶ m ³ / From the viewpoint of the operation performance of the separation membrane, m ² · hour is preferable, and 7.5 × 10 ^{-3 to} 8.0 × 10 ⁵ m ³ / m ² · hour is further preferable, and 7.5 × 10 more preferably from ^{10 -3 ~6.8 × 10 2 m 3} / m 2 · hr, and particularly preferably ^{7.5 × 10 -3 ~7.2m 3 / m} 2 · h.

In the porous hollow fiber membrane defined in the present invention, the average pore diameter of the outer surface can be appropriately changed depending on the separation target. In particular, if the average pore diameter of the outer surface is 0.01 to 0.1 μm, the water permeability is 7. is preferably 5 ^{× 10 -3 ~2.2 × 10 -1 m} 3 / m 2, if the average pore diameter of the outer surface of 0.1~1μm water permeability 2.2 × ¹⁰ -3 to 7- It is preferably ² m ³ / m ² .

For example, when the average pore diameter of the outer surface is 1 μm, the water permeability under the conditions of 25 ° C. and 50 kPa is preferably 260 m ³ / m ² · hour or more from the viewpoint of the operation performance of the separation membrane, and 780 m ³ / m more preferably at least ^2-hours, and particularly preferably 800 m ³ / ^{m 2} · hr or more.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. The physical properties described in this specification, including the following description, are based on measured values by the following method.

(Average pore diameter on the outer surface)
The outer surface of the porous hollow fiber membrane was observed with a scanning electron microscope (SEM), and the pore diameters in the longitudinal direction and the transverse direction were obtained for all the pores in the fixed compartment, and the average was calculated.

(Film thickness)
A porous hollow fiber membrane is cut with a single-blade razor in the direction perpendicular to the fiber axis to produce a cross-sectional sample, and the cross-section is observed with a scanning electron microscope (SEM). From the relationship between the observed image and magnification The film thickness was measured.

(Formaldehyde residual concentration)
A porous hollow fiber membrane (2.5 g) was collected and immersed in 100 mL of water at 40 ° C. for 1 week. Then, by reacting the immersion liquid from which the porous hollow fiber membrane was taken out with a 2,4-dinitrophenylhydrazine (DNPH) derivative, the eluted formaldehyde was derivatized to 2,4-DNPH, which was then analyzed by high performance liquid chromatography (HPLC). The formaldehyde residual concentration was calculated by detecting by the method.

(Alkaline elution bath immersion time)
After the sample was subdivided and weighed in advance, dipping was started, and the dry weight of the subdivided sample was measured one by one every hour. The time when it was confirmed that the elution components (components other than the polyacetal resin A) were completely removed from the change in weight was defined as “alkaline elution bath immersion time”.

Example 1
A barrel with a jacket with a cross-section of two circles, and a jacket with a heat (cold) medium on the outside and two rotating shafts with a number of paddles for stirring and propulsion inside. Using the provided continuous mixing reactor, hot water of 700 ° C. was passed through the jacket, the two rotating shafts were rotated at a constant speed, and at one end, 5% by weight of ethylene oxide and 700 ppm of methylal as a chain transfer agent were added. The trioxane contained is continuously supplied, and at the same time, the heteropolyacid catalyst shown in Table 1 (solution dissolved in di-n-butyl ether) is continuously added in the amount shown in Table 1 to all monomers. Copolymerization was performed. Next, the reaction product discharged from the discharge port of the polymerization machine was further polymerized by another apparatus (partially collected and the amount of residual monomer was measured), and then added with the quencher solution shown in Table 1 At the same time, the mixture was pulverized through a pulverizer and stirred at 60 ° C. for 30 minutes (90% or more had a particle size of 2 mm or less).

  Then tetrakis- [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane 0.5% and melamine 0.2% are added as stabilizers and 5 minutes in a Henschel mixer. After stirring and mixing, the mixture was melt kneaded and extruded at a temperature of 210 ° C. and a degree of vacuum of 5 mmHg using a vented twin-screw extruder to prepare polyacetal resin pellets having 95% acetal units.

  The polyacetal resin pellets and the poly-L lactic acid resin having a D-form content of 1.2% and a PMMA-converted weight average molecular weight of 160,000 are biaxially kneaded from the feeder so that the weight ratio is 40:60. Machine (“KZW-15TWIN-30MG” manufactured by Technobel, screw diameter 15 mm, L / D = 30) and melt-kneaded under the conditions of a temperature of 200 ° C. and a screw rotation speed of 200 rpm to obtain kneaded pellets.

  The kneaded pellets obtained by the above method are the sheath component, and the same poly-L lactic acid resin as that used above is used as the core component. After melt extrusion at 200 ° C., each is led to a spinning pack and cut with a 15 μm cut metal nonwoven fabric filter. It is filtered and introduced into a core-sheath type composite die (nozzle hole diameter (D) 0.5 mm × nozzle length (L) 1.0 mm, number of holes 8 holes), and the weight ratio of core component: sheath component is 50:50. And the total discharge amount was 10 g / min.

  The extruded hollow fiber-shaped molded body is maintained in a molten state at a temperature of 20 ° C., and is led to a water bath having a water surface at a position 100 mm away from the nozzle (that is, an air gap is 100 mm) and cooled and solidified (water bath The inner residence time was about 0.2 seconds), and after taking up at a take-up speed of 1000 m / min, this was wound up to obtain a core-sheath composite yarn in a package state.

  Next, the core-sheath composite yarn was immersed in an 8 wt% aqueous sodium hydroxide solution at 80 ° C. in the packaged state, and the poly-L lactic acid resin in the core-sheath composite yarn was hydrolyzed and eluted.

(Examples 2-6, Comparative Examples 1 and 2)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the ratio of the acetal unit of the polyacetal copolymer was changed by changing the amount of ethylene oxide charged in the production of the polyacetal resin. The porous hollow fiber membrane obtained in Comparative Example 1 was partially destroyed in the continuous structure and could not be used as a separation membrane.

  The outline | summary of Examples 1-6 and Comparative Examples 1 and 2 and the physical-property evaluation result of the obtained porous hollow fiber membrane are collectively described in following Table 1.

(Examples 7 to 12)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the weight ratio of the polyacetal copolymer and polylactic acid in the resin composition used for the sheath portion was changed. In the porous hollow fiber membrane obtained in Example 11, the membrane structure was partially broken and the outer surface pore diameter was enlarged, but there was no problem in practical use.

(Comparative Example 3)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the weight ratio of the polyacetal copolymer and the polylactic acid in the resin composition used for the sheath was changed to 100/0% by weight. In addition, since the weight reduction by alkali elution was not seen here, the alkali elution bath immersion time was 1 hour.

  The outlines of Examples 7 to 12 and Comparative Example 3 and the evaluation results of physical properties of the obtained porous hollow fiber membranes are summarized in Table 2 below.

(Examples 13 and 14)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the temperature of the alkaline elution bath was changed.

(Examples 15 to 18)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the discharge amount or spinning speed was changed.

(Comparative Examples 4 and 5)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the elution bath was changed to chloroform or water. Since the porous hollow fiber membrane obtained in Comparative Example 4 did not permeate, it could not be used as a separation membrane.

  The outlines of Examples 13 to 18 and Comparative Examples 4 to 5 and the physical property evaluation results of the obtained porous hollow fiber membranes are summarized in Table 3 below.

(Example 19)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the cooling method of the core-sheath composite yarn after discharge was changed from a water bath to air cooling.

(Example 20)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the core was air.

(Example 21)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the shape of the sheath portion of the core-sheath composite die was a multileaf cross section (the number of convex portions was 6).

(Example 22)
A porous hollow fiber membrane was produced under the same conditions as in Example 1 except that the discharge rate, spinning speed, and temperature of the alkaline elution bath were changed.

  The summary of Examples 19 to 22 and the physical property evaluation results of the obtained porous hollow fiber membranes are summarized in Table 4 below.

Claims (6)

It consists of a polyacetal resin containing acetal units of 60% by weight or more, the average pore diameter of the outer surface is 0.001 to 500 μm, the formaldehyde concentration is 100 ppm or less, the liquid is filled on the outer membrane surface side, the inner membrane surface side A porous hollow fiber membrane characterized in that bubbles are generated when a gas pressure of 1 MPa is applied. Porous hollow fiber membrane having a thickness of claim 1 wherein Ru 1~100μm der.   A method for producing a porous hollow fiber membrane according to claim 1 or 2, wherein a step of molding a resin composition comprising a polyacetal resin A and a resin B different from the polyacetal resin A so that the cross-sectional shape is hollow, A method for producing a porous hollow fiber membrane, comprising a step of immersing a molded resin molded product in an alkaline aqueous solution to remove a part or all of the resin B.   A method for producing a porous hollow fiber membrane according to claim 1 or 2, wherein a resin composition comprising a polyacetal resin A and a resin B different from the polyacetal resin A is disposed in a sheath portion, and a resin C different from the polyacetal resin A A step of forming a core-sheath composite form by disposing the resin in the core part, immersing the resin molding molded into the core-sheath composite form in an alkaline aqueous solution, and a part or all of the resin B and the resin C A method for producing a porous hollow fiber membrane, comprising a step of removing a part or all of the membrane.   The method for producing a porous hollow fiber membrane according to claim 3 or 4, wherein the resin B is a polylactic acid resin. The method for producing a porous hollow fiber membrane according to claim 3 or 4, wherein the alkaline aqueous solution is a sodium hydroxide aqueous solution.