KR20100114808A - Method for asymmetric microporous hollow fiber membrane - Google Patents

Abstract

PURPOSE: A method for producing an asymmetric microporous hollow fiber membrane is provided to have stronger mechanical strength and superior chemical resistance. CONSTITUTION: A method for producing an asymmetric microporous hollow fiber membrane comprises the following steps. Radiation liquid including polyvinylidene difluoride 8~25 weight, organic solvent 50~90 weight, inorganic salt 0.5~10 weight, poor solvent 1~10 weight, and hydrophilic polymer 1~10 weight is produced. The radiation liquid is discharged through a pipe shaped radiation nozzle of a double structure which simultaneously discharging the radiation liquid and internal coagulant. The discharged liquid becomes solid. The solidified hollow fiber membrane is washed and dried.

Description

Method for manufacturing asymmetric porous hollow fiber separator {METHOD FOR ASYMMETRIC MICROPOROUS HOLLOW FIBER MEMBRANE}

The present invention relates to a method for producing an asymmetric porous hollow fiber membrane by wet spinning, and more particularly, to a polyvinylidene difluoride mixed with a solvent, a hydrophilic polymer, and an additive. And an internal coagulant is discharged through a tubular spinning nozzle having a double structure to form micropores on the outer surface of the hollow fiber membrane.

Membranes can be classified into reverse osmosis (RO) membranes, ultra filtration UF membranes, micro filtration membranes (MF) membranes, or sheet membranes, Hollow fiber membrane (hollow fiber membrnae), tubular membrane (tubular membrane) and the like can be classified.

The present invention relates to a separation membrane in the range of MF to UF in terms of separation performance and hollow fiber, which is mainly used for removing particulate matter in water treatment, wastewater treatment, pharmaceutical or food industry.

Hollow fiber membranes are easier to manufacture modules that have a relatively larger membrane area than flat membranes, and thus are often used for water purification. Especially, in the case of water treatment for tap water, chlorine disinfection is used for protozoa such as Cryptosporidium or Giardia. On the contrary, the separation membrane can be used to remove them completely, which is a recent trend in the manufacture of tap water.

Separation membranes should be used while periodically removing fouling, which can be removed by back washing to remove the fouling material by flowing the treated water in the reverse direction, and with acid, alkali and sodium hypochlorite (NaOCl). Chemical cleaning using the

However, if fouling is removed in this manner, the membrane may be damaged. The membrane may be broken due to pressure during backwashing or the membrane may be weakened due to chemical cleaning, that is, aging or degradation of the membrane.

Therefore, mechanical strength and chemical resistance of the polymer membrane are very important factors that determine the performance and lifetime of the membrane.

The polymer membrane is mainly manufactured by phase separation, which includes a non-solvent phase separation (NIPS) process that dissolves in an appropriate solvent and generates pores by phase transition between a solvent and a non-solvent, and dilutes and melt-kneaks the polymer. There is a thermally induced phase separation (TIPS) process that generates pores through liquid-liquid phase separation or solid-liquid phase separation.

In addition, a method of preparing a separator by dispersing an inorganic particle having a predetermined size in a molten polymer and then lowering the temperature to convert the polymer into a solid state and then removing the inorganic particle is known.

Membranes of membranes known to date vary widely, but polypropylene (PP), polysulfone (PSf), polyether sulfone (PES), polyacrylonitrile (PAN), polyvinyl Polyvinylidene difluoride (PVDF) is the most widely used material.

Among them, polyvinylidene difluoride (PVDF) is the most widely used material for water treatment and industrial separators because of its excellent chemical resistance and mechanical strength.

US Pat. No. 4,399,035, US Pat. No. 5,472,607, International Patent WO 02 / 070,115 and the like are known as methods for preparing MF or UF separators using polyvinylidenedifluoride.

U.S. Patent No. 4,399,035 discloses a process for preparing a spinning stock solution with dimethyl adipate, diethyl oxalate, dioxane, surfactant as a solvent and a dispersing solvent such as DMF, DMAC, NMP and the like, and then manufacturing the same by hollow fiber.

U.S. Patent No. 5,472,607 also discloses a support hollow fiber membrane prepared by coating a solution in which polyvinylidenepyfluoride is dissolved in a knit woven of polyester or polyamide fibers.

The US Patent No. 4,399,035 is a conventional wet spinning method is a hollow fiber manufacturing method using the NIPS method to form micropores by the phase transition phenomenon by a solvent, PVDF hollow fiber produced in this way is easy to manufacture due to the simple equipment The mechanical strength of the NIPS method is weak.

The U.S. Patent No. 5,472,607 uses a support made of fiber, which is advantageous in that it exhibits very good mechanical strength, but a disadvantage in that the support and the coated polymer are separated during use.

In the membrane production method disclosed in WO 02 / 070,115, polyvinylidene difluoride is kneaded together with inorganic particles at a high temperature above the melting point of the polymer, and then spun together to form a hollow fiber, and then the inorganic particles are removed. As a method of forming fine pores, hollow fibers having very high mechanical strength can be obtained, but since inorganic particles are not dispersed, it is difficult to obtain a uniform pore size, and an anisotropic structure, that is, dense on an outer surface and pores inside. There is a disadvantage that cannot be manufactured in a large structure.

In the present invention, a fine dope is formed on the outer surface of the hollow fiber membrane by discharging a spinning dope and an internal coagulant mixed with a polyvinylidene difluoride and a solvent, a hydrophilic polymer and an additive through a tubular spinning nozzle having a double structure. Disclosed is a method for preparing asymmetric porous hollow fiber membranes.

The manufacturing method of the asymmetric porous hollow fiber membrane according to the present invention is a hollow fiber manufacturing method based on the conventional NIPS method because the hollow fiber is prepared by dissolving polyvinylidene difluoride in a solvent and wet spinning, The technical characteristics are that the mechanical strength is very strong and the chemical resistance is also excellent.

Therefore, the problem to be solved by the present invention is the use of the wet spinning method compared to the conventional membrane is very strong mechanical strength and excellent chemical resistance, the asymmetric structure of the membrane is excellent in water permeability and narrow pore size distribution is good exclusion It is to provide a method for producing a hollow fiber membrane showing the rate.

In order to solve the above problems in the present invention

To prepare a spinning solution containing 8 to 25 parts by weight of polyvinylidene difluoride, 50 to 90 parts by weight of an organic solvent, 0.5 to 10 parts by weight of an inorganic salt, 1 to 10 parts by weight of a poor solvent, and 1 to 10 parts by weight of a hydrophilic polymer. step;

Discharging the spinning solution using a tubular spinning nozzle having a double structure capable of simultaneously discharging the spinning solution and solidifying the phase shift in the external coagulation solution;

Washing and drying the solidified hollow fiber membrane;

It provides a method comprising the step of subjecting the washed and dried hollow fiber membranes to hydrophilic post-treatment to prevent deformation or shrinkage of the fine pores as a problem solving means.

Hereinafter, the present invention will be described in more detail.

The spinning stock solution contains 8 to 25 parts by weight of polyvinylidene difluoride, 50 to 90 parts by weight of organic solvent, 0.5 to 10 parts by weight of inorganic salt, 1 to 10 parts by weight of poor solvent, and 1 to 10 parts by weight of hydrophilic polymer. Manufacture.

At this time, the polyvinylidene di fluoride is preferably a molecular weight 100,000 ~ 800,000, more preferably 400,000 ~ 700,000 is used.

If the polyvinylidene difluoride is used in less than 8 parts by weight in the composition of the spinning solution, there is a problem that the viscosity of the spinning solution is lowered, the pore size becomes large, and the strength of the manufactured hollow fiber is weakened. Due to the difficulty in stirring the spinning solution and the porosity is small.

As used above, polyvinylidene difluoride having a high molecular weight and a high viscosity may use inorganic salts to solve this problem because of slow movement between coagulants and solvents and poor pore formation during phase transition. If the amount is less than 0.5 parts by weight, the porosity decreases, and if it is used more than 10 parts by weight, the porosity increases, but the pore size increases due to the aggregation of inorganic salts.

Inorganic salts include lithium nitrate (LiNO ₃ ), lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl ₂ ), zinc chloride (ZnCl ₂ ), calcium chloride (CaCl ₂ ), magnesium sulfate ( MgSO ₄ ) may optionally be used in the group consisting of.

In addition, it is possible to use a poor solvent (poor solvent) and a hydrophilic polymer for controlling the size and porosity of the fine pores and impart hydrophilicity.

Alcohol, such as water, methanol, or ethanol, glycols, such as ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, glycerol, etc. are used for the said poor solvent.

In order to impart mechanical strength and hydrophilicity, polymers such as polyvinylpyrrolidone (PVP) and cellulose acetate (CA) are used alone or in the form of a mixture thereof. If the amount of is less than 3 parts by weight, the number of fine pores decreases and the hydrophilicity is low, so that the transmittance is low. When used in more than 15 parts by weight, the hydrophilicity is improved and the water permeability is improved, so that the size of the fine pores increases and the radiation source solution is used. This becomes unstable and premature phase transitions can occur before spinning.

Solvents for dissolving polyvinylidenedifluoride and additives include NMP (N-methyl-2-pyrrolidone), DMF (N, N'-dimethylformamide), DMAc (N, N'-dimethyl acetamide), chloroform, THF When using a solvent (tetrahydrofuran) alone or a mixture of these, when the amount of the solvent is less than 50 parts by weight, there is a problem in dissolution, if more than 90 parts by weight it may fail to control the pore size due to the low viscosity.

The spinning stock solution forms the hollow fiber membrane after spinning, and the viscosity according to the composition ratio affects the production speed and the shape, size and distribution of the micropores of the surface active layer on the outer surface of the hollow fiber. It will also affect.

The viscosity of the spinning stock solution thus prepared is about 5,000 to 50,000 cP at 25 ° C., preferably about 10,000 to 30,000 cP.

Since the high viscosity spinning stock solution is difficult to stir, it is usually stirred at a higher temperature than normal temperature, preferably at 40 to 100 ° C, more preferably at 50 to 80 ° C.

Internal coagulants (core solution) are NMP, DMF, DMAc, chloroform, THF, water, alcohols such as methanol or ethanol, glycols such as ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, glycerin, and polyvinylpi It is used in the form of any one or a mixture of two or more selected from the group consisting of lollidon (polyvinylpyrrolidone, PVP), dioxane and the like.

The role of the internal coagulant is to solidify the internal surface of the hollow fiber membrane, that is, inside the hollow fiber, until the solidification of the external surface of the hollow fiber membrane by the external coagulant is started in the air gap between the spinning nozzle and the external coagulant of the coagulation tank. As it proceeds, the structure of the cross section of the hollow fiber membrane serves to form an asymmetric structure in which the pore size increases from the outer surface to the inner surface.

In addition, the internal coagulant serves to form the pore structure of the inner surface of the hollow fiber membrane in a circular or similar form.

Particularly, in the present invention, the internal coagulant has a large hollow fiber internal pore size within 30 μm and has a shape close to a circle so that the permeability of the treated water is well maintained while maintaining the pressure resistance. It is prepared by mixing additives such as water and PVP.

In this case, the composition ratio of the main solvent and the additive is preferably 60 to 99% by weight of the main solvent and 1 to 40% by weight of the additive, and more preferably 70 to 90% by weight of the main solvent and 10 to 30% by weight of the additive. do.

If the content of the main solvent is more than 99% by weight, the concentration of the solvent is too high compared to the concentration of the solvent in the spinning solution, so that the solidification hardly occurs at the inner surface of the hollow fiber membrane while the hollow fiber passes through the air gap. It solidifies at a slow rate inside the tank. Therefore, in this case, since the structure of the hollow internal pores to be produced exhibits the shape of a relatively large macropore, the hollow fiber has a weak mechanical strength as a whole.

In addition, when the content of the main solvent is less than 60% by weight, solidification of the inner surface of the hollow fiber membrane occurs too quickly, and a dense skin layer having a thickness of 0.01 to 30 μm, similar to the outer surface, is formed on the inner surface of the hollow fiber membrane to form a symmetrical structure. As a result, it is impossible to achieve an asymmetric structure having excellent water permeability.

In addition, the temperature of the spinning solution and the internal coagulant plays an important role in forming the film. Preferably, the spinning solution should be discharged while maintaining the temperature in the range of 25 to 140 ℃, more preferably in the range of 50 to 80 ℃ Let's do it.

If the temperature of the spinning solution is less than 25 ℃, the viscosity of the spinning solution is high, causing problems in the discharge and difficult to obtain the desired pore size, and if the temperature above 140 ℃, the phase transition speed is slowed down and the size of the desired micropores Difficulties in forming numbers.

The internal coagulant is preferably maintained at a temperature in the range of 10 to 60 ° C., more preferably in the range of 25 to 50 ° C.

If the temperature of the internal coagulant is less than 10 ℃ can generate a finger structure rather than the desired sponge structure, and if more than 60 ℃ is the phase transition rate is slow, the size of the fine pores, as well as difficult to obtain the desired porosity.

In the case of the external coagulant of the coagulation bath, the internal coagulant and the components are usually used in the same manner, but the composition ratio is different.

In the present invention, the external coagulant is used by mixing an additive in water, which is the main solvent, preferably 100% of water, or 70 to 99.9% by weight of the additive is used in an amount of 0.01 to 30% by weight.

As the rate of water decreases, the coagulation rate at the outer surface of the hollow fiber membrane decreases as the proportion of water decreases, so that the size of micropores increases and the number increases on the outer surface of the hollow fiber membrane. As this occurs rapidly, the porosity inside the hollow fiber increases, improving water permeability, but a finger structure with low mechanical strength may be formed.

In addition, the higher the temperature of the coagulation bath with respect to the spinning stock solution, the faster the solvent exchange rate between the solvent and additives in the spinning stock solution and the nonsolvent (water, alcohols) included in the external coagulant, that is, the phase separation rate, so that the size of the micropores in the formation of hollow fibers It is preferable to keep the temperature of the coagulation bath at 40 to 100 ° C. because the porosity increases and thus a high water permeability can be obtained.

Along with the temperature of the coagulation bath, the length and humidity of the air gap also influence the formation of the microstructure of the hollow fiber surface. Usually, the higher the humidity in the air gap, the higher the porosity and the distribution of pores.

In particular, as the temperature of the coagulation bath increases, the humidity of the air gap increases, so that the coagulation of the outer surface of the hollow fiber membrane is gradually coagulated by water vapor, and then it is impregnated into the external coagulant of the coagulation tank, thereby solidifying the entire hollow fiber membrane. The porosity of the micropores and the distribution of the pores on the outer surface of the separator are increased, thereby improving water permeability.

Therefore, in the present invention, the humidity of the air gap is preferably 50 to 99% relative humidity when measured by a hygrometer.

Along with the coagulation bath conditions, an important factor is the air gap, which is the distance from the spinning nozzle to the external coagulant in the coagulation bath.The larger the air gap, the longer the coagulation effect of the internal coagulant on the spinning coagulation solution. This is because the external coagulant can fully function prior to the coagulation effect, and the outer surface of the hollow fiber membrane is sufficiently affected by humidity, which is advantageous for the formation of an asymmetric structure of the cross section of the hollow fiber membrane that becomes the flow path of the hollow fiber membrane.

However, if the air gap is too large, single yarns may occur in the spinning process or the hollow yarn may be out of a circle.

Therefore, in the present invention, it is preferable to generally set the air gap to 1 to 20 cm according to the conditions of the spinning solution and the coagulation bath.

When washing the solidified polyvinylidene difluoride hollow fiber membrane in the above step is washed at a high temperature to facilitate the movement of the solvent and additives, preferably at 40 ~ 100 ℃, more preferably at 50 ~ 80 ℃ do.

After the washing, the dried hollow fiber membrane is subjected to hydrophilization post-treatment to prevent deformation and shrinkage of the micropores and to impart hydrophilicity.

At this time, the hydrophilizing agent used is a mixture of alcohols such as methyl alcohol, ethyl alcohol isopropanol and glycols such as ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, glycerin and the like. It is preferable to use 50 weight% and water in the ratio of 50 to 90 weight%.

The polyvinylidene fluoride hollow fiber separator prepared as described above has an average pore size of the outer surface pores in the range of 0.02 to 0.08 μm, the outer diameter of the separator is 500 to 1,500 μm, and the inner diameter is 300 to 1,000 μm.

The asymmetric porous hollow fiber membrane according to the present invention has a very high mechanical strength and chemical resistance of the hollow fiber compared to the conventional technology, and the cross-sectional structure of the membrane is an asymmetric structure with excellent water permeability and narrow distribution of pore size, thereby providing good exclusion rate. Indicates.

Hereinafter, the present invention will be described in more detail with reference to Examples.

The spinning solution for preparing the hollow fiber separator was wet-spun under the compositions and conditions shown in Table 1 below to prepare a hollow fiber separator.

Composition and spinning conditions of spinning stock, internal coagulants and external coagulants Example PVDF
(g) NMP
(g) LiCl
(g) ethanol
(g) CA
(g) Pore size
(Μm) Breaking strength
(MPa) Water permeability
(LMH) One 15 40 One 5 3 0.03 10 150 2 20 40 One 5 3 0.02 11 120 3 25 40 One 5 3 0.01 12 100 4 15 50 8 5 3 0.2 5 1300 5 20 50 8 5 3 0.1 6 1000 6 25 50 8 5 3 0.09 7 990

At this time, 5% NMP was used for 95% water and 100% water was used for external coagulant, and the air gap was maintained at 15 cm.

Referring to the results of Table 1, it can be seen that the pore size decreases as the content of PVDF increases. In particular, it can be seen that the properties of the membrane changes from UF to MF as the content of LiCl, an inorganic additive that forms the size of micropores, increases.

Membrane according to the present invention showed a very good water permeability and excellent breaking strength, it can be seen that the flux reaches 1300LMH in Example 4 having a pore size of 0.2㎛.

In addition, it can be seen that the breaking strength also shows a very good value of 5 MPa or more as a whole.

Electron micrographs of the separator according to the present invention are shown in Figs.

1 is an electron micrograph showing the outer surface of the separator according to the present invention.

2 is an electron micrograph showing the inner surface of the separator according to the present invention.

3 is an electron micrograph showing a cross section of the separator according to the present invention.

Claims (5)

In the manufacturing method of the hollow fiber membrane using a wet spinning method, To prepare a spinning solution containing 8 to 25 parts by weight of polyvinylidene difluoride, 50 to 90 parts by weight of an organic solvent, 0.5 to 10 parts by weight of an inorganic salt, 1 to 10 parts by weight of a poor solvent, and 1 to 10 parts by weight of a hydrophilic polymer step; Discharging the spinning solution using a tubular spinning nozzle having a double structure capable of simultaneously discharging the spinning solution and solidifying the phase shift in the external coagulation solution; Washing and drying the solidified hollow fiber membrane; Method for producing an asymmetric porous hollow fiber membrane characterized in that the hydrophilized post-treatment step to prevent the deformation or shrinkage of the fine pores of the washed dried hollow fiber membrane. The method of claim 1, The organic solvent is any one selected from the group consisting of NMP (N-methyl-2-pyrrolidone), DMF (N, N'-dimethylformamide), DMAc (N, N'-dimethyl acetamide), chloroform, THF (tetrahydrofuran) Method for producing an asymmetric porous hollow fiber membrane, characterized in that one or two or more. The method of claim 1, The inorganic salt may be lithium nitrate (LiNO ₃ ), lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl ₂ ), zinc chloride (ZnCl ₂ ), calcium chloride (CaCl ₂ ), magnesium sulfate ( MgSO ₄ ) A method for producing an asymmetric porous hollow fiber membrane, characterized in that any one or two or more selected from the group consisting of. The method of claim 1, The poor solvent is any one or two or more selected from the group consisting of water, methanol, ethanol, ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, glycerin, the method of producing an asymmetric porous hollow fiber membrane . The method of claim 1, The hydrophilic polymer is a method for producing an asymmetric porous hollow fiber membrane, characterized in that using any one or two selected from the group consisting of polyvinylpylidone (PVP) and cellulose acetate (CA).

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