JPH0468010B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a novel polytetrafluoroethylene resin porous membrane suitable for concentration and substance separation such as reverse osmosis, ultrafiltration, and microfiltration, and a method for producing the same. It is.

(Conventional technology) Cellulose acetate, polyethylene, polypropylene, polymethyl methacrylate,
Porous membranes such as polyacrylonitrile and polysulfone have been used, but they have drawbacks in permeability, mechanical strength, heat resistance, alkali resistance, acid resistance, solvent resistance, chemical resistance, etc.

From this point of view, polytetrafluoroethylene resins, which have excellent properties such as mechanical strength, heat resistance, alkali resistance, acid resistance, solvent resistance, and chemical resistance, have attracted attention, and the creation of porous membranes has been studied. .

For example, JP 42-13560, JP 46-7284.
No. 50-71759, a molded product made from an unsintered polytetrafluoroethylene resin mixture containing a liquid lubricant, or an agglomerated mixture of a solid pore-forming agent and a resin dispersion. To date, there have been examples in which membranes have been obtained using a method characterized by heating the membrane in an unsintered state stretched in at least one direction to a temperature of approximately 327°C or higher, but the pore structure of the membrane was insufficiently controlled and the performance was poor. Either the thickness of the film was low, or the film forming properties were poor, and only thick films could be formed.

(Problems to be Solved by the Invention) The present inventors have intensively studied a polytetrafluoroethylene resin porous membrane that does not have the above-mentioned drawbacks, and as a result, have arrived at the present invention.

(Means for solving problems) The present invention has the following configuration.

(1) A porous polytetrafluoroethylene resin membrane with an average pore diameter of 0.01 to 2μ and consisting of a microporous network structure in which resin particles are fused together.

(2) Molding a homogeneous mixture of a polytetrafluoroethylene resin dispersion and a fiber-forming polymer, heat-treating the resulting molded product at a temperature equal to or higher than the melting point of the resin, and then removing the fiber-forming polymer. A method for producing a porous polytetrafluoroethylene resin membrane, characterized by:

The present invention will be explained in detail below.

In the present invention, a membrane consisting of a microporous network structure has a large number of micropores with an average pore diameter of 0.01 to 2μ, and the pores that are involved in the permeability of the membrane are connected to each other within the membrane. It refers to something that has a uniformly distributed structure.

In addition, there is no substantial orientation in the shape of the pores when viewed in a plane parallel to the membrane surface, and more specifically, the ratio of the maximum average pore diameter measured aligned in one direction to the parallel pore diameter in the direction perpendicular to that direction is It is preferably 5 or less, more preferably 3 or less. The porosity is preferably 20% or more.

The polytetrafluoroethylene resin in the present invention includes a tetrafluoroethylene homopolymer,
A copolymer mainly composed of tetrafluoroethylene, such as a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, a tetrafluoroethylene-ethylene copolymer, or a mixture thereof. .

The polytetrafluoroethylene resin in the present invention is used as an aqueous or organic dispersion, but particularly preferably an aqueous dispersion obtained by emulsion polymerization in an aqueous medium containing a surfactant or a concentrated solution thereof. , more specifically, the particle size
A uniform dispersion of polytetrafluoroethylene resin particles of 1 μm or less, more preferably 0.8 μm or less is preferred.

The fiber-forming polymer in the present invention may be any polymer that can be made into fibers and forms a homogeneous mixture that can be molded by mixing with a polytetrafluoroethylene resin dispersion, but in the case of an aqueous dispersion, is preferably a cellulose sodium xanthate-based, polyvinyl alcohol-based, or sodium alginate-based polymer alone or a mixture thereof.

The mixing ratio of the fiber-forming polymer to the polytetrafluoroethylene resin in the present invention varies depending on the type of fiber-forming polymer used, but
Preferably 10-200% by weight, more preferably 30%
~100% by weight is good. If the mixing ratio of the fiber-forming polymer is less than 10% by weight, a porous membrane with an average pore diameter of 0.01 μm or more cannot be obtained, and if it is more than 200% by weight, the mechanical strength of the membrane is low and is not practical.

Furthermore, commercially available additives such as surfactants and anti-packaging agents can be added to the film-forming stock solution for the purpose of improving the stability of the stock solution or improving film-forming and thread-spinning properties, but the total amount It is preferably 50% by weight or less, more preferably 30% by weight or less, based on the tetrafluoroethylene resin.

The homogeneous mixture in the present invention may be any mixture that can be formed into a molded product by rolling, extrusion, or a combination of the two, but a liquid having a viscosity of 10 to 10,000 poise at the molding temperature is preferably used. , more preferably a liquid of 100 to 5000 poise.

The concentration of the polytetrafluoroethylene resin in the homogeneous mixture in the present invention varies depending on the type of fiber-forming polymer used, the molding method, etc.
It is usually in the range of 2 to 50% by weight, preferably 5 to 30% by weight.

The molded product in the present invention is obtained by rolling molding, extrusion molding, or a molding method that combines both, and the shape of a sheet or hollow fiber is selected depending on the shape of the intended porous membrane. Hollow fibers are preferable because they have a large effective area and are economical because they can reduce the size and cost of the device.

Molding in the present invention refers to rolling molding, extrusion molding, or a combination of both, and manufacturing of sheet-like products or spinning of hollow fibers is selected depending on the shape of the desired molded product, but various molding methods can be used. Hollow fiber spinning is preferable because the conditions can be met and the structure of the molded product can be easily controlled.

For example, after casting the molding mixture onto a flat plate such as a glass plate or metal plate, or a continuous belt, the molding mixture may be immersed in a coagulating liquid to solidify, or the molding mixture may be extruded from a flat film slit die. Directly or once through the air to a coagulating liquid to solidify it, or extrude a non-coagulable or coagulable fluid through the core simultaneously with the molding mixture from the hollow fiber mouthpiece, and then directly or once through the air. Molding can be carried out either by extruding the coagulating liquid through the core at the same time as the molding mixture, and then directly or by passing through air and introducing it into a non-coagulating fluid for solidification. The non-coagulable fluid mentioned here may be any fluid as long as it does not have a coagulating effect, but it cannot be generalized because it depends on the type of fiber-forming polymer used, but examples include water, glycerin, and ethylene glycol. , polyethylene glycol, liquid paraffin, isopropyl myristate, freon, etc., liquid mixtures thereof, air,
The gas may be appropriately selected from gases such as nitrogen and inert gases.

The temperature of the spinneret cannot be determined because it greatly affects the spinning properties due to its relationship with the viscosity of the raw solution.
Temperatures typically range from 20 to 120°C. Furthermore, the temperature is preferably at least 20°C lower than the coagulating liquid temperature, which can prevent deterioration in spinning properties due to steam condensation on the die surface, which becomes noticeable when the distance between the die surface and the coagulating liquid surface is short. .

When introducing the extruded molding mixture through the air into the coagulation liquid, the conditions during air travel are as follows:
It depends on the dimensions of the molded product, molding speed, etc., and cannot be generally specified, but the distance from the mouth surface to the point where it is introduced into the coagulating liquid is usually
A range of 0.2 to 200 cm is preferable from the viewpoint of molding stability. The ambient temperature is usually atmospheric temperature or room temperature, but depending on the case, it may be cooled.

Any coagulating liquid may be used as long as it is a non-solvent for the fiber-forming polymer of the present invention and has an affinity with and is compatible with the solvent of the molding mixture.
Depending on the type of fiber-forming polymer used,
For example, aqueous solutions of inorganic salts such as sodium sulfate, ammonium sulfate, zinc sulfate, potassium sulfate, zinc sulfate, copper sulfate, magnesium sulfate, aluminum sulfate, calcium chloride, magnesium chloride, zinc chloride, sulfuric acid, hydrochloric acid, nitric acid, acetic acid, oxalic acid. , an acid such as boric acid, or a mixture thereof. In addition, the temperature of the coagulating liquid is
Although it has a large effect on moldability, it is usually between 0 and 98℃.
It will be held nearby.

The heat treatment in the present invention may be performed under any conditions as long as the polytetrafluoroethylene resin particles can be fused together, such as in a vacuum, in the air,
In nitrogen, oxygen, sulfur gas, helium gas,
It can be carried out in various atmospheres such as in silicone oil at a temperature higher than the melting point of the polytetrafluoroethylene resin. Furthermore, the molded product can be heat treated under tension or without tension, and batch treatment or continuous treatment can also be selected. In more detail, there are two methods: processing in a free state without fixing, stretching before heat treatment and fixing to a processing frame, fixing to a processing frame under conditions with a fixed length or shrinkage rate, or stretching. A method of continuous processing under the conditions of , constant length, shrinkage, or a combination thereof can also be appropriately adopted.

In addition, stretching can be performed before, after, or during heat treatment, or can be performed in combination, but if the stretching ratio is too high, the shape of the pores as seen in a plane parallel to the membrane surface will be affected. Either it is impossible to obtain a membrane without orientation, or it is impossible to control the pore size, resulting in a membrane with low reliability in permeation performance. Normal stretching ratio is 1.1
~3 times. The stretching temperature can be selected as appropriate within the range from room temperature to the heat treatment temperature, and stretching can also be carried out in two directions.

The present invention is characterized in that the fiber-forming polymer is removed from the molded article after heat treatment, but the fiber-forming polymer referred to here may differ from the original polymer depending on the heat treatment.

The method for removing the fiber-forming polymer from the heat-treated molded article in the present invention can be a dissolution method, a decomposition method, or a combination of these using liquid, gas, heat, radiation, etc. formula, can be performed continuously. Although it cannot be generalized because it varies depending on the type of fiber-forming polymer used, it is usually a liquid whose main component is an acid such as sulfuric acid, nitric acid, hydrochloric acid, perchloric acid, or hydrofluoric acid, or a mixture thereof. A convenient method is to immerse the heat-treated molded product in a solution heated to a temperature ranging from room temperature to 200°C.

Further, the membrane thus formed can be further subjected to a stretching treatment to change its permeability, mechanical strength, dimensional stability, etc. The stretching ratio is
The temperature is usually in the range from room temperature to the melting point of the polytetrafluoroethylene resin, but it is also possible to heat set by applying a temperature after stretching.

The membrane of the present invention can be used in a dry state, but due to the hydrophobicity of the polytetrafluoroethylene resin, when used in an aqueous system, it is necessary to hydrophilize the pores of the membrane. It can also be stored in a moist state. In order to maintain the wet state, it is sufficient to attach a suitable wetting agent such as hydrous glycerin, ethylene glycol, polyethylene glycol, various surfactants, etc.

The polytetrafluoroethylene resin porous membrane according to the present invention can be used for desalination of seawater, desalination, removal of bases, acids, etc. from industrial wastewater, ultrapure water for use in the electronics industry, production of high purity chemicals, degreasing, etc. Collection of actual liquids, electrodeposition coating liquids, etc., paper pulp waste liquid treatment, oil/water separation, industrial wastewater treatment such as oil emulsion separation, separation and purification of fermentation products, concentration of fruit juice and vegetable juice, soybean processing, food products such as sugar manufacturing industry. It is widely used in industrial applications such as concentration, separation, and purification, artificial kidneys, blood component separation, microfilters for bacterial isolation, medical applications such as pharmaceutical separation and purification, and biotechnology fields such as bioreactors.

Examples are shown below, but the invention is not limited thereto.

(1) Membrane dimensions Measured using an optical microscope.

(2) Pore diameter of membrane This was determined by photographic observation using a scanning electron microscope (Akashi Seisakusho α-9).

(3) Porosity The weight W, absolute dry weight Wo, and volume V of a membrane whose pores were filled with pure water using the ethanol substitution method were measured, and calculated using the following formula.

(W-Wo) x 100/V (%) (3) Water permeability The flat membrane is assembled into a commercially available cartridge and water pressure is applied while keeping it at 37℃, and the amount of water that permeates through the membrane in a certain period of time and the effective membrane area are measured. and the water permeability was calculated from the transmembrane pressure difference.

The hollow fiber membrane was made into a small module, and water pressure was applied to the inside of the hollow fiber while maintaining the temperature at 37°C, and the water permeability was calculated from the amount of water permeating through the membrane in a certain period of time, the effective membrane area, and the pressure difference between the membranes.

(4) Filtration performance of 5% albumin aqueous solution Using a commercially available aqueous solution of bovine serum albumin (Fraction V), water permeability was measured by the method described in (3) above.

The albumin rejection rate was calculated by measuring the concentration Co of the stock solution and the concentration C of the permeated solution using the following formula.

(Co-C) x 100/Co (%) (Example) Example 1 Sodium alginate (manufactured by Hanui Chemical Co., Ltd., 300 cps) 50
1 part was dissolved in 950 parts of purified water at 70°C to obtain a homogeneous stock solution. To this stock solution, 200 parts of an aqueous dispersion of polytetrafluoroethylene (D-2 manufactured by Daikin, solid content 61% by weight, surfactant 5.7% by weight) and 13 parts of silicone oil (SH-200 manufactured by Toray Silicone) were added. and stirred at 70°C to obtain a homogeneous stock solution. The viscosity of this stock solution was approximately 290 poise at 40°C. This stock solution was cast onto a glass plate at 40°C, and immediately poured at 40°C.
A flat membrane was prepared by immersing the membrane in a 40% by weight aqueous calcium chloride solution heated to 100 mL for 5 minutes, and then transferring it to water at room temperature. This film is placed in a hot air dryer and the temperature is raised.
After heat treatment at 340℃ for 30 minutes, about 70% by weight of sulfuric acid,
The membrane was immersed overnight in a room temperature mixture containing about 30% by weight of nitric acid to remove sodium alginate and silicone oil or their heat-treated modified products, and then washed with water to obtain a porous membrane. The structure of this film is
2 and 3. As shown in the figure, the membrane consists of a network structure with uniform pores of 0.5 to 1.0μ on the surface and inside, and no dense layer is observed on the membrane surface.

The porosity of this membrane is approximately 20%, the thickness of the obtained wet membrane is approximately 110μ, and the permeability of pure water is 310ml/ ^m2 .
The permeation performance with a 5% albumin aqueous solution was 24 ml/m ² ·hr·mmHg, and albumin rejection rate was 66%.

Example 2 The stock solution of Example 1 was transferred from the hollow fiber nozzle to a nozzle temperature of 40°C.
℃, extruded with a core liquid of approximately 10% by weight calcium chloride aqueous solution, and after traveling 5cm in the air, approximately 40%
After coagulating in a coagulating solution of approximately 50°C consisting of a wt% calcium chloride aqueous solution, the
The hollow fiber was wound at min. This hollow fiber membrane was heated in a hot air dryer, heat-treated at 340°C for 30 minutes, and then immersed overnight in a room temperature mixture of about 70% by weight sulfuric acid and about 30% by weight nitric acid, followed by sodium alginate. and silicone oil or heat-treated modified products thereof were removed, and then washed with water to obtain a porous hollow fiber membrane. The structure of this film is shown in Figures 4, 5, and 6.
As shown in the figure. As shown in the figure, the membrane consists of a network structure with uniform pores of 0.5 to 1.0μ on the surface and inside, and no dense layer is observed on the membrane surface.

The porosity of this hollow fiber membrane is approximately 22%, and the dimensions of the obtained wet hollow fiber membrane are: inner diameter: approximately 360μ membrane thickness: approximately 70μ
So, pure water permeability: 36ml/ ^m2・hr・mmHg, 5%
Permeation performance in albumin aqueous solution: Water permeability: 7.4
ml/m ²・hr・mmHg, albumin inhibition rate is approximately 46%
It was hot.

Example 3 The porous hollow fiber membrane of Example 2 after washing with water was stretched 1.5 times in a wet state. The structure of this film is shown in Figures 7 and 8.
As shown in Fig. 9. As seen in the figure, a dense layer consisting of micropores with a pore size of 0.5 to 1.0 μm was observed on the outer surface of the membrane, and the inner surface and inside of the membrane had a pore size of 0.3 to 2.0 μm.
It consists of a network of micropores.

The porosity of this hollow fiber membrane is approximately 40%, and the dimensions of the obtained wet hollow fiber membrane are: inner diameter: approximately 330μ membrane thickness: approximately 67μ
So, pure water permeability: 259ml/ ^m2・hr・mmHg, 5%
Permeation performance in albumin aqueous solution: Water permeability: 50
ml/m ²・hr・mmHg, albumin inhibition rate is approximately 38%
It was hot.

Comparative Example 1 Flat membranes and hollow fiber membranes were made in the same manner as in Examples 1 and 2 except that the heat treatment temperature was 310°C.
When sodium alginate and silicone oil or their heat-treated modified products were removed by immersing the membrane overnight in a mixed solution containing % by weight at room temperature, the membrane collapsed and could not retain its flat or hollow fiber membrane shape.

(Effects of the invention) The polytetrafluoroethylene resin porous membrane of the present invention consists of micropores with an average pore diameter of 0.01 to 2μ, and the shape of the pores when viewed in a plane parallel to the membrane surface has no substantial orientation. Therefore, high permeability and sharp fractionation characteristics can be obtained. The membrane also has excellent backwashing properties, high mechanical strength, and excellent heat resistance and chemical resistance. Furthermore, a homogeneous mixture of the polytetrafluoroethylene resin dispersion and the fiber-forming polymer is molded, and the molded product is heat-treated at a temperature higher than the melting point of the polytetrafluoroethylene resin, and then the fiber-forming polymer is removed. Since membranes are formed using the same method, even thin membranes can be easily formed, and separation membranes with a wide range of pore sizes can be easily obtained by changing the composition of the homogeneous mixture.

[Brief explanation of the drawing]

Figure 1 is a scanning electron micrograph (1600x magnification) showing the shape of the fibers on the surface that was in contact with the glass surface of the polytetrafluoroethylene resin porous membrane obtained in Example 1 of the present invention. show. Figure 2 is a scanning electron micrograph (560x magnification) showing the shape of fibers in the cross section of the polytetrafluoroethylene resin porous membrane obtained in Example 1 of the present invention.
shows. FIG. 3 is a scanning electron micrograph showing the shape of fibers on the surface of the polytetrafluoroethylene resin porous membrane obtained in Example 1 of the present invention that was in direct contact with the coagulation liquid (magnification: 160x). ) is shown. FIG. 4 shows a scanning electron micrograph (4000x magnification) showing the shape of the fibers on the outer surface of the polytetrafluoroethylene resin porous hollow fiber membrane obtained in Example 2 of the present invention. FIG. 5 shows a scanning electron micrograph (magnification: 800 times) showing the shape of the fibers in the cross section of the polytetrafluoroethylene resin porous hollow fiber membrane obtained in Example 2 of the present invention. FIG. 6 shows the results obtained in Example 2 of the present invention.
A scanning electron micrograph (1600x magnification) showing the shape of the fibers on the inner surface of a polytetrafluoroethylene resin porous hollow fiber membrane is shown. FIG. 7 is a scanning electron micrograph (magnification: 1600) showing the shape of the fibers on the outer surface of the polytetrafluoroethylene resin porous hollow fiber membrane obtained in Example 3 of the present invention.
times). FIG. 8 shows a scanning electron micrograph (magnification: 800 times) showing the shape of the fibers in the cross section of the polytetrafluoroethylene resin porous hollow fiber membrane obtained in Example 3 of the present invention. Figure 9 shows
A scanning electron micrograph (magnification:
1600x).

Claims (1)

[Scope of Claims] 1. A porous polytetrafluoroethylene resin membrane having an average pore diameter of 0.01 to 2μ and consisting of a microporous network structure in which resin particles are fused together. 2 A homogeneous mixture of a polytetrafluoroethylene resin dispersion and a fiber-forming polymer is molded, and the resulting molded product is heat-treated at a temperature equal to or higher than the melting point of the resin, and then the fiber-forming polymer is removed. A method for producing a polytetrafluoroethylene resin porous membrane.