JP2006218441A - Porous membrane and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous membrane with excellent mechanical strength which can be obtained in a practical method by using a mixed solvent having an appropriate solubility formed by mixing a non-solvent to cause a liquid-liquid phase separation, and a production method of the porous membrane. <P>SOLUTION: A production method of a porous membrane comprises cooling a membrane forming solution where a polyvinylidene fluoride resin is at least heated and melted in a solvent at a temperature higher than the phase separation temperature to a temperature not higher than the phase separation temperature to be solidified, wherein the solvent is a mixed solvent formed by mixing a poor solvent defined by the formula (2) and a non-solvent defined by the formula (3) using three-dimensional solubility parameters, and both the solvents satisfy the formula (4) and the mixed solvent satisfies the formula (1). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a porous membrane of polyvinylidene fluoride resin that is produced by a thermally induced phase separation method (TIPS method) and is suitable for water treatment as a microfiltration membrane or an ultrafiltration membrane, and a method for producing the same.

  Separation membranes such as microfiltration membranes and ultrafiltration membranes are used in various fields including water production, wastewater treatment, river and lake water purification, and the like. The demand and the market are increasing rapidly, especially in China and the Middle East where there are water problems.

  Conventionally, filtration membranes such as polysulfone, polyacrylonitrile, cellulose acetate, and polyvinylidene fluoride produced by a non-solvent phase separation method (NIPS) are known as separation membranes with excellent permeation performance. However, for the purpose of sterilizing permeated water and preventing biofouling of the membrane, durability of the membrane against the introduction of a chemical into the membrane module is required. Therefore, in recent years, development of a separation membrane made of polyvinylidene fluoride (PVDF) resin has attracted attention as a material excellent in chemical resistance.

  As a method for producing a PVDF separation membrane, a PVDF resin is dissolved in a good solvent to prepare a raw material solution, immersed in a liquid containing a non-solvent of PVDF resin such as water, and a porous membrane is obtained by a non-solvent phase separation method. There is a wet manufacturing method for forming. However, this method is liable to generate macrovoids and has a problem in terms of mechanical strength. In addition, there are many film forming condition factors in the film forming process, and it is difficult to control the film structure and film performance in the film forming process, and the reproducibility is poor.

  In addition, inorganic fine particles and organic liquid are melt-kneaded into polyvinylidene fluoride resin, solidified by cooling from a temperature above the melting point of polyvinylidene fluoride resin, and then the organic liquid and inorganic fine particles are extracted to form a porous structure. There is a melt extraction method to be formed (for example, see Patent Document 1). In the case of the melt extraction method, a macro-void is not formed and a relatively homogeneous and high-strength film can be obtained. However, if the dispersibility of the inorganic fine particles is poor, defects such as pinholes may occur. Furthermore, the melt extraction method is a production method having a drawback that the production cost is extremely high.

  Under these circumstances, a heat-induced phase separation method (TIPS method) that causes a phase separation mechanism by heat is currently attracting attention. The heat-induced phase separation method is a method in which a polymer and a poor solvent that does not dissolve at room temperature are mixed at a high temperature to prepare a uniform polymer solution, and the temperature of the solution is reduced to Binodal. Below the line, or below the crystallization line, the solution becomes cloudy and phase separation occurs. Eventually the polymer hardens, the polymer becomes a matrix and the poor solvent is extracted to form a porous structure.

  The structure generation mechanism in the TIPS method is roughly classified into solid-liquid phase separation and liquid-liquid phase separation. Polyvinylidene fluoride resin has high crystallinity, phase separation is likely to occur during crystallization in the phase separation process, crystal nuclei grow, and a structure in which coarse spherical crystals are partially connected (for example, Patent Document 2). reference). That is, the generation mechanism of the film structure is only solid-liquid phase separation, that is, Ts (structure formation temperature) = Tc (crystallization temperature). Therefore, it is difficult to suppress the coarsening of the structure, and there is a problem that the film manufactured by this method is inferior in mechanical strength. In addition, the film forming factor such as the cooling rate greatly affects the performance of the generated film, and thus the manufacturing conditions are severely limited.

  Moreover, in the following patent document 3, a polymer / mixture component mixture is extruded, and a hollow fiber is formed through a quench tube (cooling tank) in the shape of a U-shaped tube. As a feature of the present invention, a cooling liquid is caused to flow in the direction of the extruded polymer solution, and the linear velocity of the polymer solution extrusion is 20% or more higher than the average linear velocity of the cooling liquid. A non-solvent is allowed to flow through the hollow fiber membrane to produce a hollow portion. It is a feature of this invention to avoid stress on the extruded polymer solution, and the rate of the polymer solution exiting the forming die and the stretch ratio of the formed and cooled film is in the range of 1.33.

  In addition, in the following Patent Document 4, in a method for producing a hollow fiber with a polymer solution composed of a polymer and a mixed solvent, the hollow fiber membrane formed product is formed into a cavity-forming fluid (hollow fiber-forming core solution), the outer surface of the hollow fiber. It is characterized by a coating solution and a film forming method in which the coating solution is extruded simultaneously with a cooling liquid having a lower temperature around the coating solution. In other words, there is no air gap (the distance of the dry part that existed from the nozzle to the coolant). Although the solvent is prevented from evaporating in the presence of the coating liquid, the coating liquid is simultaneously extruded at the same temperature as the dope liquid and the cavity liquid (core liquid), so the film is difficult to form and is considered to be mechanically weak. The film forming process is also complicated. Usually, the cavity fluid is more likely to form pores on the inner surface of the hollow fiber membrane when using a poor solvent or a mixed solvent containing a poor solvent than a gas or a non-solvent, but tends to be slightly weak in strength.

  Recently, Patent Document 5 listed below discloses a method for producing a polyvinylidene fluoride porous membrane using a solvent that satisfies a specific formula defined by a three-dimensional solubility parameter in the TIPS method. In this invention, in Example 1, a mixed solvent of γ-butyrolactone and diethylene glycol (weight ratio 1: 1) is used (in this solvent, the left side is 0.18), and a three-dimensional solubility parameter (dispersion) close to this is used. In the case of using a solvent having a force term, a dipole bond strength term, and a hydrogen bond term), a liquid-liquid phase separation occurs when the polymer solution is cooled, thereby obtaining a three-dimensional network structure having high elongation. It is disclosed. In the present invention, even a single solvent can be used as long as it satisfies the specific formula defined by the three-dimensional solubility parameter.

  However, in the TIPS method, when a poor solvent is used alone, liquid-liquid phase separation does not occur and solid-liquid phase separation easily occurs, and it is difficult to suppress the coarsening of the structure, and the mechanical strength of the membrane (From the viewpoint of the film forming method described in this publication, the elongation strength of the film remains at 39% or less).

Further, the solubility in polyvinylidene fluoride is that γ-butyrolactone is a poor solvent close to a non-solvent, and diethylene glycol is a non-solvent, so the mixed solvent of both is a non-solvent defined in the present invention. For this reason, it is difficult to prepare a uniform polymer solution even when heated, and it is difficult to control the temperature, which is difficult to say as a practical production method.
Japanese Patent No. 2899903 International Publication WO03 / 031038 Japanese Patent Publication No.7-91688 Japanese Patent No. 3442384 JP 2004-182919 A

  Therefore, an object of the present invention is to provide a practical method for producing a porous membrane having good mechanical strength and the like by causing liquid-liquid phase separation by mixing a non-solvent and using a moderately soluble mixed solvent. It is providing the porous membrane which can be obtained by this, and its manufacturing method.

  In order to achieve the above-mentioned object, the present inventors diligently studied a mixed solvent when producing a porous membrane of polyvinylidene fluoride resin by a thermally induced phase separation method (TIPS method), and mixed a non-solvent. The present inventors have found that the above object can be achieved by using a moderately soluble mixed solvent, and have completed the present invention.

  That is, the method for producing a porous membrane of the present invention comprises producing a porous membrane in which a membrane-forming solution in which at least a polyvinylidene fluoride-based resin is heated and dissolved in a solvent at a temperature higher than or equal to the phase separation temperature is cooled to a temperature lower than the phase separation temperature and solidified In the method, the solvent is a mixed solvent obtained by mixing a poor solvent defined by the following formula (2) and a non-solvent defined by the following formula (3) using a three-dimensional solubility parameter. The two solvents satisfy the following formula (4), and the mixed solvent satisfies the following formula (1). Here, the following formulas (1) to (3) are relational expressions between the three-dimensional solubility parameter of each solvent and the three-dimensional solubility parameter of polyvinylidene fluoride, and define the solubility of each solvent. The following formula (4) specifies the relationship between the solubility parameters of the poor solvent and the non-solvent.

式（１）の不等号の左辺は、三次元的にＨａｎｓｅｎ溶解性パラメータの溶解範囲を表すもので、三次元溶質の中心点の（δd,p、δp,p、δh,p）から液体三次元パラメータ（δd,ms、δp,ms、δh,ms）までの距離を定量的に表す。Ｒpは溶質の相互作用半径である。左辺は溶解性の尺度を示すものであり、通常では、Ｒpより小さいことは溶解する見込みがあると考えられるが、本発明では、熱的に溶かすことを促進するため、Ｒpより大きく、２Ｒpより小さい範囲でも、溶かすことができ、かつ、液−液相分離機構も存在することを見出した。熱誘起相分離法では、高温でポリマーと常温で溶けない貧溶媒と混合させ、均一なポリマー溶液を調製し、温度を下げてゆくと、溶液の温度はバイノーダル（Ｂｉｎｏｄａｌ）線以下、或いは、結晶化ラインを超えると、溶液は濁り、相分離が起こる。最終的に固まり、ポリマーはマトリクスで、貧溶媒を抽出され多孔質構造を作る。式（１）の左辺の値は小さければなるほどポリマーと混合溶媒が混合しやすくなり、固−液相分離機構となり、一方、左辺の値は高ければ高いほど溶解しにくくなるということである。

The left-hand side of the inequality sign in equation (1) represents the solubility range of the Hansen solubility parameter in three dimensions. From the center point (δd, p, δp, p, δh, p) of the three-dimensional solute, The distance to the parameters (δd, ms, δp, ms, δh, ms) is expressed quantitatively. Rp is the solute interaction radius. The left side shows a measure of solubility. Normally, it is considered that a value smaller than Rp is likely to be dissolved. However, in the present invention, it is larger than Rp and more than 2Rp in order to promote thermal melting. It was found that even a small range can be dissolved and a liquid-liquid phase separation mechanism exists. In the heat-induced phase separation method, a polymer is mixed with a poor solvent that does not dissolve at room temperature at a high temperature to prepare a uniform polymer solution, and when the temperature is lowered, the temperature of the solution is less than the binodal line or a crystal. Beyond the conversion line, the solution becomes turbid and phase separation occurs. Finally, the polymer is a matrix and the poor solvent is extracted to create a porous structure. The smaller the value on the left side of the formula (1), the easier the polymer and the mixed solvent are mixed, and the solid-liquid phase separation mechanism is obtained. On the other hand, the higher the value on the left side, the harder it is to dissolve.

  In the present invention, by adding a non-solvent to moderately adjust the solubility of the mixed solvent in the polymer, the binodal line is shifted onto the crystallization line, and two phases of a polymer dilute liquid and a concentrated phase liquid are obtained. It is possible to cause liquid-liquid phase separation. This suppresses the growth of spherulites, so that a porous film having a cellular structure in which bubbles are connected without having an agglomerate structure is obtained, and mechanical strength such as tensile strength and elongation at break is obtained. Becomes a good porous membrane.

  Moreover, although Formula (4) specifies the relationship of the solubility parameter of a poor solvent and a non-solvent, about non-solvent about the sum of the square of the dipole binding force term of a solubility parameter, and a hydrogen bond term. If this is smaller, the polarity of the non-solvent becomes smaller and liquid-liquid phase separation does not occur even when other formulas (1) to (3) are satisfied. Further, since the non-solvent has a small contribution to the phase separation and the binodal line is below the crystallization line, the phase separation is caused by crystallization, so that liquid-liquid phase separation does not occur.

  Therefore, in the present invention, it is preferable to cause solidification of the polyvinylidene fluoride resin after liquid-liquid phase separation occurs when the film-forming solution is cooled.

  In the above, it is preferable that the produced porous membrane is a porous hollow fiber membrane. When producing a porous hollow fiber membrane, a mechanical strength of a certain level or more is required in the take-up or winding operation of the porous hollow fiber membrane, and therefore the present invention that provides the above-described effects is particularly effective. .

  Moreover, it is preferable to contain the clay organized with the alkylene oxide compound. This makes it possible to obtain a porous hollow fiber membrane that has been hydrophilized and to suppress the nucleation by using clay as a nucleus for phase separation, so that a cellular structure having larger pores can be obtained.

  The mixed solvent is preferably extractable with water. By using such a mixed solvent, the solvent extraction operation can be easily performed industrially.

  On the other hand, the porous membrane of the present invention is a porous membrane obtained by any one of the above-described porous membrane manufacturing methods, and the cross-sectional structure of the membrane does not have a spherulite aggregate structure, It has the cellular structure which communicated. Since the porous film having such a structure does not have a spherulite aggregate structure, mechanical strength such as tensile strength and elongation at break is increased, and there are few problems such as temperature control in the production method.

  Hereinafter, embodiments of the present invention will be described. In the method for producing a porous membrane of the present invention, a membrane-forming solution in which at least a polyvinylidene fluoride-based resin is heated and dissolved in a solvent at a temperature higher than the phase separation temperature is cooled to a temperature lower than the phase separation temperature to be solidified.

  In the present invention, in such a thermally induced phase separation method (TIPS method), the solvent is a poor solvent defined by the following formula (2) using the three-dimensional solubility parameter and the following formula (3): A mixed solvent obtained by mixing a non-solvent defined by the formula (1), wherein both solvents satisfy the following formula (4), and the mixed solvent satisfies the following formula (1).

好ましくは、式（１）の左辺の値が５．２〜８．０であり、式（２）の左辺の値が４．０〜１．５であり、式（３）の左辺の値が１０．０〜２０．１であり、式（４）の左辺と右辺の値の差が５を超える場合である。

Preferably, the value of the left side of Formula (1) is 5.2 to 8.0, the value of the left side of Formula (2) is 4.0 to 1.5, and the value of the left side of Formula (3) is 10.0 to 20.1, and the difference between the values of the left side and the right side of Equation (4) is greater than 5.

  First, a mixed solvent that plays an important role in the present invention will be described. In the present invention, the mixed solvent comprises at least one poor solvent and at least one non-solvent. A poor solvent generally indicates a solvent that can dissolve the resin only within a certain temperature range, unlike a good solvent or a normal solvent. On the other hand, a non-solvent generally refers to a solvent that cannot dissolve the resin in any temperature range below the polymer decomposition temperature.

  Reference is now made to a work detailing three-dimensional solubility parameters (Allan FM Barton, “CRC Handbook of solubility parameters and other cohesion parameters” CRCCorp. 1991). The solubility parameter used here is a Hansen parameter, but a Hoy parameter can be used for those not described in the Hansen parameter. What is not described in both can be estimated by Hansen's parameter formula (see above CRCCHbookbook of solubility parameters and other cohesion parameters).

  In addition, about the solubility parameter of a mixed solvent, the parameter calculated by the additive law based on weight ratio like Formula (6) is used. Expression (5) is an expression for calculating a one-dimensional solubility parameter. Table 1 shows the three-dimensional solubility parameters of typical solvents and polyvinylidene fluoride.

本発明では、貧溶媒としては、トリアセチンのようなグリセロールエステル、エチレンカーボネート、プロピレンカーボネート、シクロヘキサン、カプロラクタム、リン酸系エステル、マロン酸系エステル、アジピン酸系エステル、安息香酸系エステルなどが挙げられる。

In the present invention, examples of the poor solvent include glycerol esters such as triacetin, ethylene carbonate, propylene carbonate, cyclohexane, caprolactam, phosphoric acid esters, malonic acid esters, adipic acid esters, benzoic acid esters, and the like.

  Non-solvents include polyhydric alcohols such as diethylene glycol, glycerin, and triethylene glycol that exhibit polar and hydrogen bonding. A polyol such as low molecular weight polyethylene glycol is also used as a non-solvent, but since it is a polymer, its function as a non-solvent is low because of its low polarity and hydrogen bonding. In the polyethylene glycol 600, the formula (4) is not satisfied, and only a solid-liquid phase separation mechanism exists as a membrane structure formation mechanism.

  On the other hand, in the case of polyethylene glycol 200, by satisfying the formula (4), there is a liquid-liquid phase separation mechanism at a certain ratio with the poor solvent (in actual measurement, poor solvent / non-solvent = 4/3 or more). To do.

  Next, in the present invention, the polyvinylidene fluoride resin is a resin containing a vinylidene fluoride homopolymer and a vinylidene fluoride copolymer. A plurality of types of vinylidene fluoride copolymers may be contained. The weight average molecular weight of the polyvinylidene fluoride resin is preferably 50,000 to 1,000,000, more preferably 100,000 to 580,000. Resins having different weight average molecular weights may be used together.

  A fourth component can also be added. For example, a porous hollow fiber membrane of a polyvinylidene fluoride resin with improved hydrophilicity can be obtained by using an organized clay organized with a hydrophilic compound.

  The higher the temperature at which the resin and mixed solvent are dissolved, the higher the solubility. However, the solvent temperature and the polymer decomposition temperature may approach the solvent boiling point and the polymer decomposition temperature, so the solution temperature is the boiling point of the solvent. And lower than the polymer decomposition temperature.

  In the present invention, a sufficiently uniform polymer solution can be obtained at a temperature around 200 ° C. to 170 ° C. That is, there is no need to make it 200 ° C or higher. In the present invention, when the polymer sap composed of the above-mentioned polyvinylidene fluoride resin and mixed solvent is heated to around 180 ° C. to make a uniform polymer solution and cooled with a cooling liquid or a cooling controller, the temperature is lowered. When a certain temperature is reached, the solution begins to become cloudy, which is the structure formation temperature Ts. If the crystallization temperature and the structure formation temperature are the same, solid-liquid phase separation is achieved. On the other hand, if the structure formation temperature is higher than the crystallization temperature, it is liquid-liquid phase separation. The observed temperature at which the solution began to become turbid is liquid-liquid phase separation because the structure formation temperature is higher than the crystallization temperature unless there is an exothermic or endothermic peak at that temperature with a differential scanning calorimeter (DSC). You can prove that.

  In the present invention, when the amount of the non-solvent added to the poor solvent is increased, the structure formation temperature Ts of the polymer solution and the crystallization temperature Tc shift to Ts> Tc from the relationship of Ts = Tc. The structure of the formed film also changes from a spherulite structure to a structure having a cellular structure. In the present invention, it is preferable to cause the polyvinylidene fluoride resin to coagulate after the liquid-liquid phase separation has occurred.

  In the present invention, the cooling rate is preferably 10,000 to 100 ° C./min from the viewpoint of controlling the structure generation by cooling. Here, the cooling rate of 10,000 ° C./min substantially corresponds to the condition of cooling with ice water.

  After cooling and solidification, the solvent is usually extracted with alcohols or acetone. In the present invention, alcohol can be used, but because of the polarity described above, the mixed solvent can be removed with water. And reduction of production cost can be expected.

  Thereafter, the porous membrane is dried as necessary. Examples of the drying method include methods other than natural drying, such as heat drying, drying with hot air, and contact with a heating roll.

  The composition of the cooling tank is not particularly limited, and water, a solvent, a non-solvent, or a mixed solvent composed of these may be used. Usually, a mixed solvent tends to have a higher water flux than water.

  In the production method of the present invention, a porous film having high strength and elongation characteristics can be obtained. Therefore, when high water permeability is required, the film can be stretched in the range of 0.5 to 4 times as necessary. I can do it.

  In the polymer solution prepared from the polyvinylidene fluoride resin and the mixed solvent, the polymer is preferably in the range of 10 to 70% by weight, and more preferably in the range of 15 to 40% by weight.

  The production method of the present invention is effective for producing a porous hollow fiber membrane. When the porous hollow fiber membrane is produced by the TIPS method, any of the conventionally known TIPS method conditions (conditions other than the solvent) can be employed. Specifically, for example, at least a polyvinylidene fluoride resin is heated and dissolved in the above mixed solvent at a phase separation temperature or higher to prepare a film-forming solution, and this film-forming solution is discharged from the outside of the double tube exit □. At the same time, the hollow fiber-forming core liquid, which is a poor solvent or a mixed solvent thereof, was allowed to flow inside the double tube outlet, passed through a predetermined air gap, and then solidified in a cooling bath and wound up. The mixed solvent is extracted from the hollow fiber membrane with water.

  In that case, it is preferable to hold | maintain an air gap at 1 mm-10 mm from a viewpoint of preventing evaporation of a solvent.

  The porous membrane of the present invention is a porous membrane obtained by the production method as described above, and has a cellular structure in which bubbles are communicated in the cross-sectional structure of the membrane without having a spherulite aggregate structure. It is. Preferably, it does not have a non-uniform structure such as a macrovoid.

Further, the porous membrane of the present invention preferably has an average pore diameter measured by observation with a scanning electron microscope (SEM) of 0.01 to 5 μm, particularly 0.02 to 2 μm. Moreover, it is preferable that the porosity calculated | required from a density is 40 to 90%, especially 60 to 80%. The film strength is preferably 60 kgf / cm ² or more and the breaking elongation is preferably 120% or more.

  The porous membrane of the present invention is used for sterilization, turbidity and protein removal of alcoholic beverages and fruit juice beverages in the food industry, production of ultrapure water in the semiconductor manufacturing industry, production of sterile water in the pharmaceutical industry, various industrial wastewaters, buildings Porous hollow for microfiltration or ultrafiltration with excellent mechanical strength, which can be used for pretreatment of desalination by reverse osmosis of river water, brine, seawater, etc. A yarn separation membrane can be provided.

  Examples and the like specifically showing the configuration and effects of the present invention will be described below. In addition, the evaluation item in an Example etc. measured as follows.

(Structure formation temperature)
The structure formation temperature (phase separation temperature) was measured with a hot stage (LINKAM, LK600) capable of controlling the cooling rate.

(Crystallization temperature)
The crystallization temperature was measured with a differential scanning calorimeter (DSC) (PERKIN ELMER, DSC7).

(Structure observation)
It measured from the scanning electron microscope (SEM) (HITACHI, S-800) photograph of the cross section of a porous membrane.

(Tensile strength / elongation rate of hollow fiber membrane)
Using a Shimadzu autograph (SHIMADZU, AGS-J), a 50 mm long film in a wet state was stretched under conditions of a tensile strength of 100 mm / min, and the strength and elongation when it was broken were measured.

Example 1
Polyvinylidene fluoride (Solvey 6020, Solef 6020) was used as the polymer, triacetin was used as the poor solvent for the polymer solution, and diethylene glycol was used as the non-solvent. The polyvinylidene fluoride resin was kept constant at 30% by weight, and the triacetin parts by weight / diethylene glycol parts by weight were 70/0, 60/10, 50/20, 40/30, and 35/35. These mixed systems were put into a kneader and kneaded at a temperature of 180 ° C. to obtain a uniform polymer sample (film forming solution). Take an appropriate amount of the sample obtained, put it between thin glasses, place it on a hot stage, heat it to 180 ° C, hold it for 5 minutes, then cool the sample at a cooling temperature of 10 ° C / min and make it cloudy The starting temperature was recorded as the structure formation temperature Ts. The crystallization temperature Tc was measured by DSC using the sample obtained with the same kneader. The influence of non-solvent addition on structure formation temperature and crystallization temperature is shown in FIG. 1-1. FIG. 1-2 shows a cross-sectional structure of a film in which triacetin parts by weight / diethylene glycol is 40/30 (A) and 50/20 (B).

(Example 2)
Polyvinylidene fluoride (Solvay, Solef 6020) was used as the polymer, triacetin was used as the poor solvent for the polymer solution, and glycerin was used as the non-solvent. The polyvinylidene fluoride resin was kept constant at 30% by weight, and the triacetin parts by weight / glycerin parts by weight were 69/1, 67/3, 65/5, and 60/10. The method of film formation and evaluation was the same as that used in Example 1. The effect of non-solvent addition on the temperature and crystallization temperature of structure formation in a triacetin / glycerin mixed system is shown in FIG.

  As shown in FIGS. 1-1 and 2, when the addition amount of the non-solvent was increased, the structure formation temperature Ts and the crystallization temperature Tc tended to shift from Ts = Tc to Ts> Tc. The parameters of the mixed solvent used in Example 1 and Example 2 were calculated by Equation (6) using the solubility parameter shown in Table 1, and were obtained by substituting into Equation (1). This mixed solvent satisfies the formula (1). These values are shown in Table 2. The solubility parameters of the poor solvent and the non-solvent were substituted into formulas (2) and (3), the values on the left side of each formula were calculated, and the values on both sides were also calculated for formula (4). These poor solvents and non-solvents satisfy the formulas (2) to (4). These values are shown in Table 3.

（比較例１）
ポリマーとしてポリフッ化ビニリデン（ソルベイ社製、ＳＯＬＥＦ６０２０）、ポリマー溶液用の貧溶媒としてトリアセチン、非溶媒としてはポリエチレングリコール６００を用いた。ポリフッ化ビニリデン系樹脂を３０重量％に一定にして、トリアセチン重量部／ポリエチレングリコール６００重量部は６０／１０、５０／２０、４０／３０、３５／３５にした。製膜および評価の仕方は実施例１に用いた手法と同じであった。トリアセチン／ポリエチレングリコール６００混合系における構造形成温度及び結晶化温度に及ぼす非溶媒添加の影響を図３に示す。非溶媒ポリエチレングリコール６００は式（３）を満たすものの、式（４）を満たさなかったため、Ｔｓ＞Ｔｃの関係は存在しない（即ち、液−液相分離が生じない）。

(Comparative Example 1)
Polyvinylidene fluoride (SOLEF 6020, manufactured by Solvay) was used as the polymer, triacetin was used as the poor solvent for the polymer solution, and polyethylene glycol 600 was used as the non-solvent. The polyvinylidene fluoride resin was kept constant at 30% by weight, and triacetin parts by weight / polyethylene glycol 600 parts by weight were adjusted to 60/10, 50/20, 40/30, and 35/35. The method of film formation and evaluation was the same as that used in Example 1. FIG. 3 shows the influence of non-solvent addition on the structure formation temperature and the crystallization temperature in the triacetin / polyethylene glycol 600 mixed system. Although the non-solvent polyethylene glycol 600 satisfies the formula (3) but does not satisfy the formula (4), there is no relationship of Ts> Tc (that is, no liquid-liquid phase separation occurs).

(Example 3)
A mixed solution was prepared such that the polyvinylidene fluoride was 30 parts by weight, triacetin was 60 parts by weight as a poor solvent, and glycerin was 10 parts by weight as a non-solvent, and dissolved at 180 ° C. This polymer solution was discharged from the outside of the double tube □, and at the same time, a hollow fiber-forming core solution, which was a poor solvent (triacetin), was allowed to flow inside the double tube outlet. It passed through a 5 mm air gap, solidified in a cooling bath made of 5 ° C. water and wound up. The mixed solvent was extracted from the obtained hollow fiber membrane with water. Regarding the physical properties of the obtained hollow fiber membrane, the tensile strength was 67 kgf / cm ² and the elongation was 125%.

Example 4
Film forming conditions were set so that 30 parts by weight of polyvinylidene fluoride, 40 parts by weight of triacetin as a poor solvent, and 30 parts by weight of a mixed solution of diethylene glycol and glycerin (mixing weight ratio = 28/2) as a non-solvent. A hollow fiber membrane was obtained in the same manner as in Example 3 except that it was passed through a 5 mm air gap, solidified in a cooling bath made of 50 ° C. water and wound up. Regarding the physical properties of the obtained hollow fiber membrane, the tensile strength was 112 Kgf / cm ² , the elongation was 235%, and the water permeability was 50 L / m ² / hr / atm.

(Example 5)
As an additive, an organic clay (SEN manufactured by Co-op Chemical Co., Ltd.) obtained by organizing an inorganic layered silicate with an alkylene oxide compound is added (100% by weight of a polyvinylidene fluoride resin, and clay 5 is organicized with a hydrophilic compound) Part by weight) A hollow fiber membrane was obtained in the same manner as in Example 4 except that it was passed through a 5 mm air gap and solidified in a cooling bath made of 25 ° C. water and wound. The resulting hollow fiber membrane has a tensile strength of 110 kgf / cm ² and an elongation of 223%. The cross-sectional structure of the porous membrane is shown in FIG. It was found that by adding clay, there was a cellular structure with larger pores.

(Comparative Example 2)
In a non-solvent-free system, the polymer solution composed of polyvinylidene fluoride and the poor solvent triacetin was made into a polymer / poor solvent weight ratio of 30/70, and the air gap was passed through 5 mm in a cooling bath made of 40 ° C. water. A hollow fiber membrane was obtained in the same manner as in Example 4 except that it was solidified and wound. Regarding the physical properties of the obtained hollow fiber membrane, the tensile strength was 31 kgf / cm ² , the elongation was 65%, and the water permeability was 188 L / m ² / hr / atm.

  Here, the poor solvent triacetin satisfies the relationship of the formula (1), but the left side is as small as 3.2, and no non-solvent is added. Therefore, solid-liquid phase separation occurs, and the aggregate structure of spherulites is reduced. Produced and the mechanical strength decreased.

Graph showing the effect of non-solvent addition on structure formation temperature and crystallization temperature in triacetin / diethylene glycol mixed system Scanning electron microscope (SEM) photograph of the cross section of the porous membrane obtained in Example 1 Graph showing the effect of non-solvent addition on structure formation temperature and crystallization temperature in triacetin / glycerin mixed system Graph showing the effect of non-solvent addition on structure formation temperature and crystallization temperature in triacetin / polyethylene glycol 600 mixed system Scanning electron microscope (SEM) photograph of the cross section of the porous hollow fiber membrane obtained in Example 4 Scanning electron microscope (SEM) copy of the cross section of the porous hollow fiber membrane obtained in Example 5

Claims (6)

In a method for producing a porous membrane in which at least a polyvinylidene fluoride resin is heated and dissolved in a solvent at a phase separation temperature or higher, and solidified by cooling to a phase separation temperature or lower,
The solvent is a mixed solvent in which a poor solvent defined by the following formula (2) and a non-solvent defined by the following formula (3) are mixed using a three-dimensional solubility parameter, A method for producing a porous membrane, wherein the solvent satisfies the following formula (4) and the mixed solvent satisfies the following formula (1).

  The method for producing a porous membrane according to claim 1, wherein, when the membrane-forming solution is cooled, the polyvinylidene fluoride resin is solidified after liquid-liquid phase separation occurs.   The method for producing a porous membrane according to claim 1 or 2, wherein the porous membrane is a porous hollow fiber membrane.   The manufacturing method of the porous membrane in any one of Claims 1-3 containing the clay organized by the alkylene oxide compound.   The method for producing a porous membrane according to claim 1, wherein the mixed solvent is extractable with water. A porous film obtained by the method for producing a porous film according to any one of claims 1 to 5, wherein the cross-sectional structure of the film has a cellular structure in which bubbles are communicated without having an agglomerated structure of spherulites. A porous membrane having.

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