JP2012183522A - Hydrophilization method for polyvinylidene fluoride-based porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrophilization method for a polyvinylidene fluoride-based porous membrane by a thermally-induced phase separation method without causing the significant lowering of water permeability.SOLUTION: The polyvinylidene fluoride-based porous membrane manufactured by the thermally-induced phase separation method is immersed in a solution containing the perhydropolysilazane of a concentration of 0.01-3 wt.%, and then subjected to heat drying at 140-165°C. By immersing the polyvinylidene fluoride-based porous membrane into the solution containing the perhydropolysilazane of a predetermined concentration, the solution effectively soaks into membrane fine holes. By heating the membrane, -SiO- groups are formed on the membrane surface and inner surfaces of the membrane fine holes, and not only the membrane surface but also the entire membrane are hydrophilized. The membrane is hydrophilized without causing the significant lowering of the water permeability.

Description

  The present invention relates to a method for hydrophilizing a polyvinylidene fluoride porous membrane. More specifically, the present invention relates to a method for hydrophilizing a polyvinylidene fluoride porous membrane prepared by a thermally induced phase separation method.

  In recent years, deterioration of the global water environment has become a problem, and there is an increasing demand for low-cost and simple water treatment technology. As such a water treatment technique, there is a membrane reactor method (MBR) that combines activated sludge treatment and membrane separation treatment. Compared with conventional filtration methods for coagulation and precipitation, this water purification process using membrane filtration and wastewater treatment are easier to maintain and manage, and the quality of treated water is good. Widely 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 a material used for these membrane filtrations, a polyvinylidene fluoride resin that is excellent in contamination resistance against fouling substances such as fine particles and organic substances has attracted attention. 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). The problem of strength and generation of macrovoids is solved, and the chemical durability is also excellent (see Patent Documents 1 and 2).

  Since such a polyvinylidene fluoride porous membrane has high hydrophobicity, it needs to be hydrophilized when used for water treatment. As a hydrophilization treatment method, as disclosed in Patent Document 3, there is a method of immersing a membrane in ethanol and then replacing with water, and this method makes it possible to increase water permeability. Further, in order to eliminate the need for such pretreatment, a method of immersing the membrane in a glycerin aqueous solution (see Patent Document 4), a method of immersing in a surfactant aqueous solution (see Patent Document 5), a combination of these features, and HLB An aqueous solution containing a nonionic surfactant having a value of 8 to 15 and glycerin as a preservation solution for a membrane separation membrane has been proposed (see Patent Document 6).

  Further, as a substance to be mixed with a hydrophilic substance at the time of film production, a dispersion of titanium oxide (see Patent Document 7), a dispersion of organic clay (see Patent Documents 8 to 9), and an organosilicon compound are dispersed. A proposed one (see Patent Document 10) has been proposed.

  However, the method of immersing the membrane in an aqueous solution of glycerin, etc. requires a step of making the membrane in the dried membrane module hydrophilic after ethanol membrane immersing and then immersing the membrane in an aqueous solution of glycerin. In addition, it is necessary to package the module so that leakage of the glycerin aqueous solution from the module and drying do not occur. Furthermore, the wet membrane is highly likely to generate bacteria during storage, and the generation of bacteria is a factor that reduces the water permeability of the module, so that a suppression treatment is also required.

  On the other hand, in the method of dispersing titanium oxide or organic clay at the time of film production, it is necessary to increase the mixing amount of hydrophilic substances in order to increase the hydrophilicity of the film, but if these mixing amounts are too high, However, the phase stability of the film-forming stock solution is lowered, and the chemical resistance or strength of the obtained film is lowered, so that there is a limit to the amount added. Furthermore, in this method, even if the hydrophilic substance is miniaturized, since the polyvinylidene fluoride porous membrane itself is hydrophobic, the entire membrane cannot be hydrophilized, and there is still a limit to improving the performance. Be looked at.

Perhydropolysilazane manufactured by Clariant is known as a method for converting the membrane surface into silica glass among the methods for hydrophilizing the membrane. Commercial products containing this substance include the AZ Electromaterials product Aquamica series. AQUAMICA is mainly composed of perhydropolysilazane. By coating the diluted liquid on the membrane surface, it chemically bonds with functional groups such as hydroxyl and carboxyl groups on the coating surface, and with acrylic and polyurethane resins. It has a feature that high adhesion can be obtained because of compatibility. In addition, the surface of the film coating becomes silica (SiO ₂ ) by the following reaction, so that hydrophilicity and sludge property can be improved.
(SiH ₂ NH) + 2H ₂ O → (SiO ₂ ) + NH ₃ + 2H ₂

Japanese Patent Publication No. 4-33302 Japanese Patent No. 2,899,903 JP 2006-63095 A JP 2002-95939 A JP-A 63-277251 JP 2010-88996 A WO2006 / 006340 JP 2006-169498 A JP 2004-352824 A Japanese Patent Application Laid-Open No. 2005-296846

  An object of the present invention is to provide a method for hydrophilizing a polyvinylidene fluoride porous membrane by a heat-induced phase separation method and hydrophilizing the membrane without causing a significant decrease in water permeability.

  The object of the present invention is to immerse a polyvinylidene fluoride-based porous membrane produced by a thermally induced phase separation method in a perhydropolysilazane-containing solution having a concentration of 0.01 to 3% by weight, and then heat at a temperature of 140 to 165 ° C. This is achieved by performing drying.

According to the method for hydrophilizing a polyvinylidene fluoride porous membrane according to the present invention, the solution is effectively contained in the pores of the membrane by immersing the polyvinylidene fluoride porous membrane in a perhydropolysilazane-containing solution having a predetermined concentration. By soaking and heating, a —SiO ₂ — group is formed on the membrane surface and the membrane pore inner surface, and not only the membrane surface but also the entire membrane is hydrophilized. Therefore, the module prepared using the dried membrane does not require pretreatment such as ethanol soaking before water treatment, and can be used immediately, and the glycerin solution treatment or pretreatment before maintaining a wet state is possible. Since there is no need for packaging and sterilization to prevent leakage of the glycerin aqueous solution or drying of the membrane, there is an effect that the hydrophilization process is simplified.

  In addition, the polyvinylidene fluoride porous membrane hydrophilized by the method of the present invention has a feature that the membrane is hydrophilized without causing a significant decrease in water permeability.

  In the method of the present invention, the polyvinylidene fluoride porous membrane produced by the thermally induced phase separation method is hydrophilized by dipping in a perhydropolysilazane-containing solution and heat drying. The method of the present invention can be applied to a polyvinylidene fluoride porous membrane other than the heat-induced type, but is particularly effective for a heat-induced type polyvinylidene fluoride-based porous membrane.

  Thermally induced phase separation, which is used as a method for producing polyvinylidene fluoride porous membranes, is a solvent such as dioctyl phthalate and dibutyl phthalate in a state where a thermoplastic resin is heated and melted, and an additive such as hydrophobic silica (inorganic And then forming it into a hollow fiber shape or flat membrane shape in a heated and melted state, and then immersing the molded body in an insoluble liquid (such as water) of the molded body component or air The polymer layer and the solvent layer are phase-separated by cooling in the solution, and this is immersed in a solution to extract the solvent, the filler and the like in the film, thereby forming a porous film.

  Examples of the polyvinylidene fluoride resin to which the thermally induced phase separation method is applied include a homopolymer of vinylidene fluoride, at least one of vinylidene fluoride and tetrafluoroethylene, hexafluoropropylene, trifluorochloroethylene, ethylene, etc. A vinylidene fluoride homopolymer is preferably 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.

(ここでR¹、R²、R³は同一または互いに異なるC₄〜C₆のアルキル基であり、R⁴はアシル基である)で表わされるクエン酸エステル化合物75〜65重量％との混合物を溶融成形した後、クエン酸エステル化合物抽出溶媒でクエン酸エステル化合物を除去し、多孔質化してポリフッ化ビニリデン系多孔質膜を製造するといった方法により製造されたポリフッ化ビニリデン系多孔質膜を用いることもできる。かかる製造法について、以下詳述する。 As a method for producing a polyvinylidene fluoride porous membrane to which a thermally induced phase separation method is applied, a known method disclosed in Patent Documents 1 and 2 can be used as it is. In the present invention, the weight average molecular weight is further increased. Polyvinylidene fluoride resin with Mw of 100,000-300,000, 25-35% by weight and 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 After melt molding, a polyvinylidene fluoride-based porous membrane produced by a method of removing a citrate ester compound with a citrate ester compound extraction solvent and making it porous to produce a polyvinylidene fluoride-based porous membrane is used. You can also. This manufacturing method will be described in detail below.

  In the heat-induced phase separation method, generally, when the resin concentration in the film-forming stock solution, which is a mixture of a thermoplastic resin and a solvent, increases, the formation of spherulites is suppressed. However, film formation using a citrate ester compound is performed. In the undiluted solution, spherulites are generated when the resin concentration is increased, and fine spherulite structures are observed when the resin concentration is decreased. Therefore, the polyvinylidene fluoride resin is different from the citrate ester compound as a solvent. It must be used in a proportion of 25 to 35% by weight in the total amount.

In the citrate compound represented by the above general formula, the groups R ¹ , R ² , and R ³ are the same or different C _{4 to} C ₆ alkyl groups, and the group R ⁴ is an acyl group such as an acetyl group or a benzoyl group 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. 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 method for producing a polyvinylidene fluoride porous membrane using a citrate ester compound, the citrate ester compound 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 ester compound in a predetermined ratio is melt-molded, and then the citric acid compound is removed with a citrate ester compound extraction solvent to make the molded body porous. 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. The shape of the molded porous body may be a flat membrane or a hollow fiber membrane.

  In the case of a flat film, the molten mixture is extruded from a die such as a T-die into a sheet and cooled and solidified via a cast roll, or the molten mixture is cooled and solidified in advance, and the solidified product is melted again by hot pressing. A method of forming and cooling and solidifying is applied. The film thickness is generally set to 0.1 to 1.0 mm, preferably 0.2 to 0.5 mm.

  In the case of a hollow fiber membrane, a method is generally employed in which it is melt-extruded from a double annular nozzle into a hollow fiber membrane, passed through a predetermined idle running section, and then immersed in a cooling bath to solidify. As the cooling bath, water is preferably used because it is inexpensive and has a large heat capacity. However, other solvents that do not dissolve the molded body components or air cooling may be used. The fluid for forming the hollow portion of the hollow fiber membrane may be an insoluble liquid having a boiling point equal to or higher than the extrusion temperature, or a gas such as air or nitrogen. The film thickness is generally 0.05 to 3.0 mm, preferably 0.2 to 2.0 mm, and the outer diameter is generally set to 0.1 to 5.0 mm, preferably 0.3 to 3.0 mm.

  Examples of the film forming solution (citrate ester 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- Examples thereof include hydrocarbon solvents such as hexane and cyclohexane.

The polyvinylidene fluoride-based porous membrane is immersed in a perhydropolysilazane-containing solution at a predetermined concentration and then heated and dried at a predetermined temperature to form -SiO ₂ -groups on the membrane surface and the inner surface of the membrane pores. Is hydrophilized.

  As the perhydropolysilazane-containing solution, a solution obtained by dissolving perhydropolysilazane in an organic solvent such as xylene, dibutyl ether, sorbents, and terpenes is used. be able to. The perhydropolysilazane concentration in this solution is 0.01 to 3% by weight, preferably 0.1 to 2.5% by weight. When used at a concentration higher than this, the water permeability decreases due to the small pores of the membrane. On the other hand, it becomes difficult to obtain a desired hydrophilizing effect used at a lower concentration. The perhydropolysilazane-containing solution may contain a catalyst such as a palladium catalyst or an amine catalyst.

  The film is dried by heating at a temperature of 140 to 165 ° C, preferably 145 to 155 ° C. If the drying process is performed at a temperature higher than this, the film shrinks, whereas at a temperature lower than this, the film is not sufficiently hydrophilized. In this case, the heating and drying time is generally about 10 to 90 minutes.

In the obtained hydrophilic polyvinylidene fluoride porous membrane, the water permeability X of the membrane measured after 1 minute of water passage represented by the following formula is 80% or more when ethanol pretreatment is further performed. Even if only the hydrophilization treatment according to the present invention is used, the water permeability is not significantly reduced.
X (%) = 100 x J ₁ / J ₀
J ₁ : Pure water permeability coefficient 1 minute after passing water with a differential pressure of 0.1 MPaG applied
J ₀ : Soaking the hydrophilic polyvinylidene fluoride porous membrane in ethanol for 5 minutes,
After water replacement for 5 minutes, water was passed with a differential pressure of 0.1 MPaG applied.
Pure water permeability coefficient after 1 minute

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

Example 1
Hydrophobic silica (Nippon Aerosil product Aerosil R-972) 24.0 wt%, dioctyl phthalate 29.7 wt% and dibutyl phthalate 5.8 wt% were mixed in a Henschel mixer, then polyvinylidene fluoride (Kureha product Kureha KF polymer # 1000) 40.5 The wt% was added and mixed again with a Henschel mixer. This mixture was heated and mixed at 220 ° C. using a 48 mm diameter twin screw extruder into pellets, and then a hollow fiber production apparatus provided with a hollow fiber spinneret in a 30 mm diameter twin screw extruder. After melt extrusion at 0 ° C., the mixture was cooled at 25 ° C. and formed into a hollow fiber shape. The obtained hollow fiber was immersed in methylene chloride at 30 ° C. for 1 hour to extract and degrease dioctyl phthalate and dibutyl phthalate, and then dried. A hydrophobic silica-containing polyvinylidene fluoride hollow fiber membrane having an outer diameter of 0.7 to 2.0 mm and an inner diameter of 0.4 to 1.2 mm was obtained.

  Next, it was immersed in 50% ethyl alcohol aqueous solution for 30 minutes, and further transferred to water and immersed for 30 minutes to make the hollow fiber hydrophilic. Next, a treatment of immersing in an aqueous solution of caustic soda at 40 ° C. and 5% for 1 hour was performed twice, followed by 10 times of immersion in warm water at 40 ° C. to extract hydrophobic silica.

  The hollow fiber membrane that has been washed with water is removed by blowing air, then immersed in a 2% by weight terpene solution of perhydropolysilazane (Aquamica NP140, containing amine catalyst), and then blown to remove excess air. After removing the perhydropolysilazane soaking solution, dry heat treatment was performed at 150 ° C. for 1 hour.

The obtained hydrophilized polyvinylidene fluoride porous membrane was measured for water permeability and bubble point.
[Permeability]
A hollow fiber membrane is inserted into a glass tube having openings in three T-shapes, and both ends of the hollow fiber membrane and the glass tube are bonded with an epoxy resin, and the effective length of the hollow fiber membrane is 160 mm. A module for evaluating water permeability was prepared. By supplying pure water at 25 ° C from one end of the hollow fiber membrane and adjusting the pressure at the other end of the hollow fiber membrane, the water flow from the inside of the hollow fiber membrane to the outside of the hollow fiber membrane at an average overpressure of 0.1 MPaG I went over. Measure the water permeation amount 1 minute after the start of the test and divide the water permeation amount for 10 seconds by the inner surface area of the hollow fiber membrane and the average pressure (water pressure difference 0.1 MPa between the inner side and the outer side of the hollow fiber) J ₁ (ml / hour · cm ² · 0.1 MPa) was calculated. Next, after removing the water outside the hollow fiber membrane of the module, after immersing in ethanol for 5 minutes, and then replacing with water for 5 minutes, water was passed with a differential pressure of 0.1 MPaG applied, 1 The permeability coefficient J ₀ (ml / hour · cm ² · 0.1 MPa) after a minute was measured. From these results, transmittance X (%) = 100 × J ₁ / J ₀ was calculated.
Incidentally, the hydrophilic polyvinylidene fluoride porous membrane obtained by the method of the present invention can be used in an initial state in a dry state, i.e., without performing a hydrophilization treatment with ethanol. For the samples obtained in each comparative example, the permeation coefficient was calculated in a sufficiently wet state, and ethanol immersion treatment was performed in order to compare with each example.
[Bubble Point]
Insert a loop-shaped hollow fiber membrane (height up to 30-40 mm to the top of the loop) into the upper part of the SUS tube, and attach the insertion point with epoxy resin. Pressure was applied from the end where the hollow fiber membrane was not bonded, and the pressure at which bubbles emerged from the surface of the membrane (bubble point) was measured.

Example 2
In Example 1, a perhydropolysilazane solution further diluted with a terpene solvent to a concentration of 0.1% by weight was used.

Comparative Example 1
In Example 1, the hollow fiber membrane was not immersed in the perhydropolysilazane solution, and the dry heat treatment temperature was changed to 50 ° C. and the dry heat treatment was performed.

Comparative Example 2
In Example 1, a 20% by weight (Aquamica NP110; amine-based catalyst, xylene solvent) was used as a perhydropolysilazane solution.

Comparative Example 3
In Example 1, a perhydropolysilazane solution further diluted with a terpene solvent to 0.005% by weight was used.

Comparative Example 4
A polyphenylsulfone membrane with a permeability coefficient (ml / hour · cm ² · 0.1 MPa) of 10 and a molecular weight cut off of 20,000 is immersed in a 2% by weight solution of perhydropolysilazane (Aquamica NP110), and then air is blown. After removing the excess perhydropolysilazane immersion solution, a dry heat treatment was performed at 150 ° C. for 1 hour. As a result, the treated film was partially transparent, and the permeability coefficient was 0.001 or less.

The water permeability and bubble points for the examples and comparative examples 1 to 3 are shown in the table below.
table
Measurement Item real 1 real 2 ratio 1 ratio 2 ratio 3
Permeation coefficient (ml / hour ・ cm ²・ 0.1MPa)
Initial water flow J _{1 4} 173 4050 0.7 790 1000
After ethanol hydrophilization treatment J ₀ 4320 4500 4988 800 4700
Transmittance X (%) 96.5 90.0 0.01 98.8 21.3
Bubble point (kPaG) 210 205 200 300 200

Claims (2)

  It is characterized in that a polyvinylidene fluoride porous membrane produced by a thermally induced phase separation method is immersed in a perhydropolysilazane-containing solution having a concentration of 0.01 to 3% by weight, and then heated and dried at a temperature of 140 to 165 ° C. A method for hydrophilizing a polyvinylidene fluoride porous membrane.   A polyvinylidene fluoride-based porous membrane that has been subjected to a hydrophilization treatment by the hydrophilization method according to claim 1.