JP2012006988A - Method of manufacturing fiber strengthening polyvinylidene fluoride porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a fiber strengthening polyvinylidene fluoride porous film using a heat inducing phase separation method which is a method of enabling to manufacture a fiber strengthening porous film in which there is not a clear spherulite, and which has resistance to contamination over a fouling material, chemical resistance, and high mechanical strength or the like.SOLUTION: The method of manufacturing a fiber strengthening polyvinylidene fluoride porous film is characterized as follows. A mixture of 25-35 wt.% of a polyvinylidene fluoride resin having 100,000-300,000 of a weight-average molecular weight Mw and 75-65 wt.% of a citric ester compound expressed by the general formula wherein R, Rand Rare the same or different and denote a Cto Calkyl group, Rdenotes an acyl group is melt-molded on an organic fiber substrate, then the citric ester compound is removed by a citric ester compound extraction solvent to make a molding porous.

Description

  The present invention relates to a method for producing a fiber-reinforced polyvinylidene fluoride porous membrane. More specifically, the present invention relates to a method for producing a fiber-reinforced polyvinylidene fluoride porous membrane by a thermally induced phase separation method.

  Compared with conventional filtration methods for coagulation and sedimentation, purification water treatment and membrane wastewater treatment by membrane filtration are easier to maintain and manage, and the quality of the treated water is good. It is used. In particular, these treatment methods are characterized by the ability to completely remove pathogenic microorganisms such as Cryptosporidium, which was insufficiently removed by conventional methods.

As materials used for these membrane filtration,
(1) Contamination resistance to fouling substances such as fine particles and organic matter
(2) Chemical resistance that does not deteriorate even by chemical cleaning of the membrane
(3) It is required to have characteristics such as high mechanical strength that can withstand the intense shaking of the membrane caused by aeration performed to remove clogged filter materials or to supply oxygen to activated sludge.

  As a filter membrane having such characteristics, a solution obtained by dissolving a polyvinylidene fluoride resin in a good solvent is applied to or impregnated into an organic fiber base material such as a woven fabric or a nonwoven fabric or a hollow fiber braid, and a polyvinylidene fluoride type Fiber reinforced polyvinylidene fluoride porous membranes such as flat membranes and hollow fiber membranes obtained by coagulation with a poor resin solvent have been proposed (Patent Documents 1 and 2).

  However, all of these membranes are based on non-solvent induced phase separation, and such separation methods require poor solvents, so it is difficult to control the membrane preparation process and the reproducibility is low. (Non-Patent Document 1). Furthermore, when the membrane material is a polyvinylidene fluoride resin, since a mixed solution of water and an organic solvent is usually used as a poor solvent, a waste liquid treatment is required and the membrane production process is complicated.

  On the other hand, a porous membrane obtained by the thermally induced phase separation method based on the basic principle of film formation does not require a poor solvent, and is easy to control the process and highly reproducible (Non-patent Document 1). In particular, the polyvinylidene fluoride porous membrane obtained using the thermally induced phase separation method as the basic principle of membrane formation is a mechanical problem that has been a problem of membranes obtained by the conventional non-solvent induced phase separation method (liquid / liquid separation method). In recent years, it has been actively developed because it solves the problems of strength and generation of macrovoids, and also has excellent chemical durability.

  However, when the thermally induced phase separation method is applied to a polyvinylidene fluoride resin material, the film forming process of the porous film is performed by melt-kneading the film forming mixture, and then forming into a hollow fiber shape by an extruder, and further drawing process Further, since a heat treatment step and an inorganic particle extraction step are required, the film forming step is complicated, and the process control is not easy (Patent Document 3). Further, because of such a film forming process, it has been difficult to increase the strength by applying or impregnating woven fabric, nonwoven fabric, braid and the like.

  Furthermore, in this production method, phthalic acid esters such as dibutyl phthalate and dioctyl diphthalate are used as solvents. However, phthalic acid esters are regarded as a kind of endocrine disrupting substance in the world, and even in Japan. In food utensils, containers and packaging, synthetic resin toys, etc., the prohibition on the use of dioctyl phthalate (DOP) is summarized. For this reason, when using a phthalate ester as a solvent, it is required to extract a very small amount of the solvent in the step after film formation.

  This phthalate ester is suitable and widely used as a plasticizer for synthetic resins such as polyvinyl chloride, and is a substance that is difficult to extract due to its properties. For this reason, methylene chloride, which is excellent in solubility of various organic compounds, is generally used as the extraction solvent. Due to its chemical safety, methylene chloride is difficult to decompose once released into the environment, so its use and disposal is monitored as a PRTR-regulated substance, and the amount released into the atmosphere may be deleted. It has been demanded. In addition, it is a substance that is also of concern for human toxicity.

  On the other hand, although a method using adipic acid ester as a solvent instead of phthalic acid ester is disclosed (Patent Document 4), it is the same as in the case of phthalic acid ester in that methylene chloride is used as an extraction solvent.

  Furthermore, a method for producing a polyvinylidene fluoride porous membrane by a thermally induced phase separation method using a water-soluble organic solvent such as γ-butyrolactone or dimethyl sulfoxide is also disclosed (Patent Documents 5 to 6). Although extraction can be performed with water, the structure of the obtained film is a structure in which coarse spherulites are linked, so that a reduction in mechanical strength is inevitable.

  As a method for suppressing the formation of such spherulites, a method has been proposed in which inorganic particles are added to a film-forming stock solution (Patent Document 7). A strong alkali such as a sodium hydroxide aqueous solution is used for extraction of inorganic particles. Therefore, not only is the cost high, but if the dispersibility of the inorganic particles is poor, pinholes may be generated.

JP 2004-290830 A WO 2009/142279 Japanese Patent No. 2,899,903 WO 2007/032331 JP 2003-320228 A JP 2006-224051 A JP 2008-062227 A

MEMBRANE, 26 (3), 116-123 (2001) Report: Brussels, C7 / GF / csteeop / ATBC / 080104 (04)

  An object of the present invention is a method for producing a fiber-reinforced polyvinylidene fluoride porous membrane using a heat-induced phase separation method, which has no clear spherulites and is resistant to fouling, chemical resistance and An object of the present invention is to provide a method capable of producing a fiber-reinforced porous membrane having a high mechanical strength.

The object of the present invention is to provide 25 to 35% by weight of a polyvinylidene fluoride resin having a weight average molecular weight Mw of 100,000 to 300,000 and a general formula
(Wherein R ¹ , R ² and R ³ are the same or different C ₄ -C ₆ alkyl groups and R ⁴ is an acyl group) and a mixture thereof with 75 to 65% by weight of a citrate compound This is achieved by a method in which a fiber-reinforced polyvinylidene fluoride-based porous membrane is produced by melt molding on an organic fiber substrate, removing the citrate ester compound with a citrate ester compound extraction solvent, and making the molded body porous. Is done.

  In the method for producing a fiber-reinforced polyvinylidene fluoride porous membrane according to the present invention, the amount of the polyvinylidene fluoride resin in the membrane-forming solution is 25 to 35% by weight, whereby the viscosity of the membrane-forming solution is low and organic Application onto the fiber substrate can be easily performed. In addition, since there is no need for a stretching step, a heat treatment step, an inorganic particle extraction step, etc. after film formation, the fiber-reinforced polyvinylidene fluoride-based porous membrane is easily fiber-reinforced without causing damage or deterioration of the substrate. Can be manufactured.

  Furthermore, by using a specific citrate ester compound as a solvent for the polyvinylidene fluoride resin, phthalate ester having the above-mentioned problems, methylene chloride, and inorganic particles as additives are used by the thermally induced phase separation method. Thus, a fiber-reinforced polyvinylidene fluoride porous membrane using a membrane-forming solvent that is highly safe for the human body and has a low environmental load can be obtained. Such fiber reinforced polyvinylidene fluoride porous membranes have no clear spherulites, have fouling resistance, chemical resistance and high mechanical strength, and are suitable for membrane filtration for water purification or sewage treatment. ing.

It is a scanning electron micrograph of the cross section of the porous flat membrane obtained in the Example. It is a scanning electron micrograph of the surface of the porous flat membrane obtained in the Example.

  In the method of the present invention, a fiber-reinforced polyvinylidene fluoride porous membrane is produced by a thermally induced phase separation method.

  In the heat-induced phase separation method, a thermoplastic resin is intimately mixed with a solvent in a heated and melted state, molded in a heated and melted state, and then the molded body is converted into an insoluble liquid (such as water) of the molded body components. The polymer layer and the solvent layer are phase-separated by immersion or cooling in air, and this is immersed in a solution to extract the solvent in the film to obtain a porous material.

  Examples of the polyvinylidene fluoride resin include a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and tetrafluoroethylene, hexafluoropropylene, trifluorochloroethylene, and ethylene, and preferably vinylidene fluoride alone A polymer is used. The polyvinylidene fluoride resin preferably has a weight average molecular weight Mw (measured as a polystyrene-equivalent molecular weight by the GPS method) of about 100,000 to 300,000. When Mw is larger than this, the formation of spherulite structure becomes remarkable, while when Mw is smaller than this, the mechanical strength is lowered.

As a solvent for polyvinylidene fluoride resin, a general formula
A citrate ester compound represented by the formula: Here, the groups R ¹ , R ² , and R ³ are the same or different C _{4 to} C ₆ alkyl groups, the group R ⁴ is an acyl group such as an acetyl group and a benzoyl group, and such compounds include Examples thereof include tributyl acetyl citrate, tripentyl acetyl citrate, trihexyl acetyl citrate, and mixtures thereof, and preferably tributyl acetyl citrate is used.

In the groups R ¹ , R ² , and R ³ , alkyl citrate having an alkyl group with a carbon number of C ₃ or less, noticeable formation of spherulites in the cross-sectional structure of the resulting polyvinylidene fluoride porous membrane. The same applies to the case where the group R ⁴ is a hydrogen atom. For example, the US Food and Drug Administration (FDA) and the British Plastics Federation (BPF) are widely recognized as safe, and the European Commission's Scientific Checkout Committee (CSTEE) expressed their opinion that they are safe. When these citrate esters (Non-patent Document 2) are used as a film-forming solvent, the resulting cross-sectional structure of the polyvinylidene fluoride porous film is a structure in which minute spherulites are linked, The maximum pore diameter is a large value unsuitable for a peritoneum. On the other hand, in the groups R ¹ , R ² , and R ³ , in the citric acid alkyl ester in which the alkyl group has a carbon number of C ₇ or more, the compatibility with the polyvinylidene fluoride resin is deteriorated, or from the obtained molded body Extraction of the citric acid alkyl ester becomes difficult.

  In the heat-induced phase separation method, when the resin concentration in the film-forming stock solution, which is generally a mixture of a thermoplastic resin and a solvent, increases, the formation of spherulites is suppressed, but this is suitable for the film-forming solution of the present invention. As the resin concentration increases, spherulites are formed, and when the resin concentration decreases, fine spherulite structures are observed. Therefore, the polyvinylidene fluoride resin is included in the total amount of the citrate compound as a solvent. It must be used in a proportion of 25 to 35% by weight, and the citrate ester compound serving as the solvent is used in a proportion of 75 to 65% by weight in the total amount with the polyvinylidene fluoride resin.

  A mixture of a polyvinylidene fluoride resin and a citrate compound is melt-molded, and then the citrate compound is removed with a citrate compound extraction solvent to make the molded body porous. Phase separation methods are applied.

  The melt mixing temperature or melt molding temperature is a temperature above the temperature at which the polyvinylidene fluoride resin melts and mixes in one phase and below the boiling point of the citrate compound, generally about 150 to 200 ° C., preferably Is about 160-180 ° C.

  In melt molding, a melt as a film-forming solution is applied onto a substrate made of organic fibers by a gap coater such as a knife coater, a baker applicator and a doctor blade, or extruded into a sheet form from a die such as a T die, Subsequently, it is carried out by immersing in a cooling bath of water, organic liquid or the like to solidify. At this time, the coating thickness of the melt is generally set to 0.1 to 2.0 mm, preferably 0.2 to 1.0 mm.

Examples of the base material of the organic fiber base material include woven fabric, non-woven fabric, knitted fabric, paper or net, preferably woven fabric or non-woven fabric, the thickness is 0.1 to 2.0 mm, and the basis weight is 20 to 200 g. / m ² is used. Examples of the organic fiber include polyester, polypropylene, polyethylene, rayon, vinylon, nylon, polyamide, polyimide, and aramid, and polyester is preferably used.

  Examples of the citrate compound extraction solvent used to make the molded body porous include alcohol solvents such as methanol, ethanol and isopropanol, ketone solvents such as methyl ethyl ketone and methyl isobutyl ketone, and n-hexane and cyclohexane. A hydrocarbon solvent is mentioned. However, depending on the type of the base material, there are those that dissolve or swell depending on the type of the extraction solvent. Therefore, it is necessary to select according to the type of the organic fiber.

  As for the internal structure of the fiber reinforced polyvinylidene fluoride porous membrane thus made porous, it is desirable that a clear spherulite structure is not confirmed, and macrovoids of 10 μm or more are not seen, and the pore diameter is From the viewpoint of preventing Cryptosporidium of 3 to 5 μm, the maximum pore size required by the bubble point method should be 2.0 μm or less, preferably 1.0 μm or less, but the fiber reinforcement obtained by the production method according to the present invention The polyvinylidene fluoride-based porous membrane sufficiently satisfies these requirements.

  Next, the present invention will be described with reference to examples.

Example 30% by weight of polyvinylidene fluoride resin (manufactured by Sigma-Aldrich; Mw275,000) and 70% by weight of tributyl acetylcitrate were melt-mixed at a temperature of 170 ° C. using a three-one motor. A polyester woven fabric having a basis weight of 130 g / m ² and a thickness of 0.5 mm was fixed on a SUS steel plate heated to 170 ° C., and the molten mixture was applied onto the woven fabric by a baker applicator. Next, a SUS steel sheet heated to 170 ° C. is placed on the molten mixture-coated woven fabric, and the molten mixture-coated woven fabric is held for 1 minute with two SUS steel plates sandwiched therebetween, and then in a 20 ° C. water cooling bath. The film-like molded object was obtained by making it solidify with. The obtained molded body was immersed in ethanol, and tributyl acetyl citrate was extracted and removed, followed by drying to obtain a flat membrane-like polyvinylidene fluoride porous membrane having increased strength by a woven fabric.

  When the cross-sectional structure of the obtained porous flat membrane was observed with a scanning electron microscope, a clear spherulite structure did not occur as shown in FIG. 1, and no macrovoids of 10 μm or more were present. FIG. 2 is a scanning electron micrograph of the film surface.

Furthermore, when the maximum pore size, average pore size and water permeability were measured for this porous flat membrane, they were 1.0 μm, 0.3 μm and 3200 L / m ² / hour, respectively. The measurement of each of these items was performed according to the following conditions.
Maximum pore size: measured by the bubble point method based on ASTM F316-86
Using ethanol as the liquid, the bubble point at 25 ° C
Calculated from
Maximum pore size (μm) = (2860 × T) / P ₁
T: Surface tension (Unit: mN / m)
P ₁ : Air pressure at which bubbles are first detected (unit: Pa)
Average pore size: Half dry average pore size measured based on ASTM F316-03
Using ethanol as the liquid, with the flat membrane wet at 25 ° C
Air pressure (Ha
Calculated from the following equation:
Average pore diameter (μm) = (2860 × T) / P ₂
T: Surface tension (Unit: mN / m)
P ₂ : Half dry pressure (unit: Pa)
Permeability: punched flat membrane into a circular shape with a diameter of 25mm,
Set in the stirring type ultra holder UHP-25K manufactured by Ku-ku (effective effective area 3.5 cm ² )
, 25 ° C, 0.1MPa under water pressure test

Claims (4)

Polyvinylidene fluoride resin having a weight average molecular weight Mw of 100,000 to 300,000, 25 to 35% by weight and a general formula
(Wherein R ¹ , R ² and R ³ are the same or different C ₄ -C ₆ alkyl groups and R ⁴ is an acyl group) and a mixture thereof with 75 to 65% by weight of a citrate compound Is melt-molded on an organic fiber base material, and then the citrate ester compound is removed with a citrate ester compound extraction solvent to make the molded body porous. Law.   The method for producing a fiber-reinforced polyvinylidene fluoride porous membrane according to claim 1, wherein the citrate ester compound is tributyl acetylcitrate.   A fiber-reinforced polyvinylidene fluoride porous membrane produced by the method according to claim 1.   The fiber reinforced polyvinylidene fluoride porous membrane according to claim 3, wherein the maximum pore size determined by the bubble point method (according to ASTM F316-86) is 2.0 µm or less, and no clear spherulites are confirmed in the membrane cross-sectional structure.