JPH06128406A - Production of porous membrane of polytetrafluoroethylene-based resin - Google Patents

Abstract

PURPOSE:To produce a porous membrane of a polytetrafluoroethylene-based resin excellent in mechanical performances (strength and elongation and puncture pressure). CONSTITUTION:In this method for producing a porous membrane of polytetrafluoroethylene-based resin, comprising molding an uniform mixture obtained by mixing a dispersion of the polytetrafluoroethylene-based resin with a fiber forming polymer and then heat-treating the resultant moldings at a temperature not lower than melting point of the resin and removing the fiber- forming polymer, preliminary heat treatment is carried out at >=160 deg.C and a temperature lower than melting point of the resin prior to the heat treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a novel polytetrafluoroethylene-based resin porous membrane suitable for concentration such as reverse osmosis, ultrafiltration, microfiltration and separation of substances.

[0002]

2. Description of the Related Art Conventionally, porous membranes such as cellulose acetate type, polyethylene type, polypropylene type, polymethylmethacrylate type, polyacrylonitrile type and polysulfone type have been used for reverse osmosis, ultrafiltration and microfiltration. However, they have drawbacks in permeation performance, mechanical strength, heat resistance, alkali resistance, acid resistance, solvent resistance, chemical resistance and the like.

From this point of view, polytetrafluoroethylene resins having excellent properties such as mechanical strength, heat resistance, alkali resistance, acid resistance, solvent resistance, and chemical resistance have attracted attention, and formation of a porous film has been studied. It has been. For example, Shokoku Sho 4
2-135060, JP-A-46-7284,
A molded product from an unsintered polytetrafluoroethylene resin mixture containing a liquid lubricant or a coagulated mixture of a solid pore-forming agent and a resin dispersion as disclosed in JP-A-50-71759. Up to now, there is an example in which the unheated state is stretched in at least one direction and heated to about 327 ° C. or higher, but the control of the porous structure of the membrane is insufficient and the performance is low. However, the film-forming property was poor, and only a thick film could be produced.

Further, attempts have been made to improve the above-mentioned drawbacks, and for example, a production method described in Japanese Patent Application Laid-Open No. 1-129043 has been proposed.

However, although the manufacturing method described in Japanese Patent Application Laid-Open No. 1-129043 has a remarkable effect as compared with the prior art, a problem to be improved has been found.

The heat treatment of the formed product obtained from the homogeneous mixture of the polytetrafluoroethylene-based resin dispersion and the fiber-forming polymer is carried out under the temperature conditions above the melting point of the resin for the purpose of fusing the particles of the resin to each other. In air, nitrogen, sulfur gas,
Although it can be carried out in various atmospheres such as helium gas and silicon oil, it is a process at a high temperature, and the uniformity of temperature distribution, operability of work, safety of work environment, manufacturing cost, etc. From the viewpoint of the quality of the final product, heat treatment in air is the best method. However, although the heat treatment in air is most industrially adopted, the polymer in the formed product undergoes an oxidative thermal decomposition reaction in the heat treatment process, and the formed product is heated to a temperature equal to or higher than the set temperature of the heat treatment condition due to a rapid heat generation. However, there is a problem in that the temperature rises abnormally, which in turn causes thermal deterioration of the quality of the resin, and the mechanical performance (strength / elongation) of the finally obtained polytetrafluoroethylene-based resin porous membrane is significantly reduced. It was

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a method for producing a polytetrafluoroethylene resin porous membrane having excellent mechanical performance.

[0008]

The present invention has the following configuration.

That is, "a homogeneous mixture obtained by mixing a polytetrafluoroethylene-based resin dispersion and a fiber-forming polymer is molded, and the obtained molded product is heat-treated at a temperature equal to or higher than the melting point of the resin. In a method for producing a polytetrafluoroethylene-based resin porous membrane, which is characterized in that a fiber-forming polymer is removed, a preliminary heat treatment is performed at a temperature of 160 ° C. or higher and lower than the melting point of the resin before the heat treatment. And a method for producing a polytetrafluoroethylene-based resin porous membrane. "

The present invention will be described in detail below.

The polytetrafluoroethylene-based resin in the present invention means tetrafluoroethylene homopolymer,
Tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-
It is a copolymer mainly composed of tetrafluoroethylene such as an ethylene copolymer, or a mixture thereof.

The polytetrafluoroethylene resin in the present invention is used as an aqueous or organic dispersion liquid, and an aqueous dispersion liquid obtained by emulsion polymerization in an aqueous medium containing a surfactant or a concentrated liquid thereof is particularly preferable. A uniform dispersion liquid of polytetrafluoroethylene-based resin particles having a particle diameter of 1 μm or less, more preferably 0.8 μm or less is more preferable.

The fiber-forming polymer in the present invention may be any polymer as long as it is a polymer which can be made into fibers and can be mixed with a polytetrafluoroethylene-based resin dispersion to form a uniform mixture, but an aqueous dispersion. In this case, a sodium cellulose xanthate-based polymer, a polyvinyl alcohol-based polymer, a sodium alginate-based polymer alone or a mixture thereof is preferable.

The mixing ratio of the fiber-forming polymer to the polytetrafluoroethylene resin in the present invention varies depending on the kind of the fiber-forming polymer used, but is preferably 10 to 300% by weight, more preferably 30 to 200%.
Weight% is good. If the mixing ratio of the fiber-forming polymer is less than 10% by weight, a porous film having an average pore diameter of 0.01 μ or more cannot be obtained, and if it is more than 300% by weight, the mechanical strength of the film is low and not practical.

In the present invention, an additive may be mixed when the polytetrafluoroethylene resin dispersion and the fiber-forming polymer are mixed at a temperature of 100 ° C. or lower. The additive may be any as long as it can be removed by thermal decomposition, extraction, dissolution, radiolysis, etc., for example, causium silicate, silicates such as aluminum silicate, calcium carbonate, carbonates such as magnesium carbonate, Phosphates such as sodium phosphate and calcium phosphate, acetates, oxalates, ammonium chloride, sodium chloride and other hydrochlorides, sodium sulfate, barium sulfate and other sulfates, nitrates, perchlorates and other weak acids / strong acids Salts, metal powders such as iron powder, metal oxides such as alumina, zirconia, and magnesium oxide, finely divided silicic acid, kaolin clay, inorganic fine powders such as diatomaceous earth, polyamide-based, polyester-based, polyolefin-based, polysulfone-based, polyvinyl chloride System, polyvinylidene fluoride system,
Polyvinyl fluoride-based resin fine powder, silicone oil, hexafluoropropylenoxide oligomer, chlorotrifluoroethylenoligomer, phthalic acid esters, trimellitic acid esters, sebacic acid esters, adipic acid esters, Azelaic acid esters,
These can be used alone or in combination by selecting from heat resistant organic substances such as phosphoric acid esters. Further, a commercially available surfactant or antifoaming agent may be added for the purpose of improving the stability of the film-forming mixture or improving the film-forming property and the yarn-forming property.

The total amount of the additives in the present invention depends on the type of the additive, the polytetrafluoroethylene-based resin and the fiber-forming polymer used, and cannot be generally stated. 100
It is preferably 0% by weight or less, and if it exceeds 1000% by weight, the mechanical strength of the film is low and it is not practical. On the other hand, it is preferable that an additive can adjust the porous structure by adding it, but if the content is not more than 10% by weight, the effect of addition cannot be seen. Therefore, the range of 30 to 700 wt% is more preferable, and the range of 50 to 500 wt% is more preferable.

In the present invention, it is important to mix the polytetrafluoroethylene-based resin dispersion and the fiber-forming polymer at a temperature of 100 ° C. or lower. When mixing at a temperature higher than 100 ° C., resin particles in the dispersion or Additives that are mixed as needed may aggregate, causing filter clogging during molding, causing troubles, and uneven molded products.
There are problems such as defects in the molded product. Furthermore 80
It is preferable that the temperature is as low as 60 ° C or lower and 60 ° C or lower, more preferably 40 ° C or lower.

The homogeneous mixture in the present invention may be any one as long as a molded product can be obtained by a rolling process, an extrusion process or a combination process of both methods, but a liquid having a viscosity of 10 to 10,000 poise at the molding temperature is preferable. A liquid of 100 to 5000 poise is preferably used.

The concentration of the polytetrafluoroethylene resin in the homogeneous mixture according to the present invention varies depending on the type of the fiber-forming polymer and the additives used, the molding method, etc.
It is usually in the range of 1 to 50% by weight, preferably 5 to 30% by weight.

The molded product according to the present invention is obtained by a rolling process, an extrusion process, or a combination process of both methods, and a sheet form or a hollow fiber form is selected according to the intended shape of the porous membrane. The hollow fiber is preferable from the viewpoint that the effective area per unit volume can be large, the apparatus can be downsized and the cost can be reduced, and it is economical.

The term "molding" in the present invention means a material obtained by a rolling method, an extrusion molding method or a combination method of both methods, which is used for producing a sheet-like material or a hollow fiber according to the shape of a desired molded article. Although spinning is selected, spinning of a hollow fiber is preferred because various molding conditions can be taken and the structure of the molded product can be easily controlled.

For example, the molding mixture is cast on a flat plate such as a glass plate, a metal plate, or a continuous belt and then immersed in a coagulating liquid to coagulate, or the molding mixture is passed through a flat film slit mouthpiece. Extruded, either directly or once in air to lead to a coagulating liquid for coagulation, or from a hollow fiber die to extrude a non-coagulating or coagulating fluid into the core at the same time as the molding mixture, and then directly or once in air. It can be molded by a method in which it is introduced into a coagulating liquid through the core, or the coagulating liquid is extruded to the core simultaneously with the molding mixture and is introduced directly or once into the air into a non-coagulating fluid to be solidified. The term "non-coagulable fluid" as used herein may be any as long as it does not have a coagulating action, but it cannot be unequivocally stated because it varies depending on the type of the fiber-forming polymer used. Glycol, liquid paraffin, isopropyl alcohol, freon, and the like, a mixed liquid thereof, air, nitrogen, a gas such as an inert gas, and the like are appropriately selected and used.

The spinneret temperature cannot be specified because it has a great influence on the spinnability in relation to the viscosity of the stock solution, but it is usually in the range of 5 to 100 ° C.

When the extruded molding mixture is once introduced into the coagulating liquid through the air, the conditions under which the air travels vary depending on the size of the molded product, the molding speed, etc.
Although it cannot be generally specified, the distance from the die surface to the introduction into the coagulating liquid is preferably in the range of 0.2 to 200 cm from the viewpoint of molding stability. The ambient temperature is
Usually, the temperature is the atmospheric temperature or the room temperature, but in some cases, it can be performed by cooling.

Any coagulation liquid may be used as long as it is a non-solvent of the fiber-forming polymer of the present invention and has an affinity and compatibility with the solvent of the molding mixture. Depending on the type of polymer, for example, sodium sulfate, ammonium sulfate, zinc sulfate, potassium sulfate,
Aqueous inorganic salt solution such as zinc sulfate, copper sulfate, magnesium sulfate, aluminum sulfate, calcium chloride, magnesium chloride, zinc chloride, sulfuric acid, hydrochloric acid, nitric acid, acetic acid, oxalic acid,
It may be appropriately selected from acids such as boric acid or a mixture thereof. The temperature of the coagulating liquid has a great influence on the moldability, but it is usually carried out at around 0 to 98 ° C.

In the present invention, a formed product (hereinafter, formed product) obtained from a homogeneous mixture of a polytetrafluoroethylene resin (hereinafter, resin) dispersion and a fiber-forming polymer (hereinafter, polymer) is a resin. In order to fuse the particles of each other with each other, heat treatment is performed in air at a temperature equal to or higher than the melting point of the resin (hereinafter,
Before performing the fusion heat treatment, it is essential to perform the preliminary heat treatment in air (hereinafter, preliminary heat treatment) at a temperature lower than the melting point of the resin.

Under the conditions of the preliminary heat treatment, the oxidative thermal decomposition reaction of the polymer is allowed to proceed mildly and the generated heat is dissipated, and then the fusion heat treatment is carried out.

In the fusion heat treatment process, the purpose is to obtain a polytetrafluoroethylene-based resin porous film which is capable of suppressing sudden heat generation, preventing an abnormal rise in temperature, and preventing thermal deterioration of quality, and which has excellent mechanical performance. Can be achieved.

In order to achieve the above-mentioned object, it is preferred that 20% or more of the calorific value due to the oxidative thermal decomposition reaction of the fiber-forming polymer is generated by the preliminary heat treatment, more preferably 50% or more, further preferably 80%. % Or more.

The conditions of the preliminary heat treatment vary depending on the types of resin, polymer and additive, but the temperature must be 160 ° C. or higher and lower than the melting point of the resin, and preferably 1
The temperature is 80 ° C. or higher and 3 ° C. lower than the melting point of the resin, more preferably 200 ° C. or higher and 10 ° C. lower than the melting point of the resin. Here, the melting point of the resin varies depending on the molecular weight and the type of the resin. Approximate values of the various resins are shown in Table 1. Tetrafluoroethylene homopolymer shows 327 ° C and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer The combined temperature is 310 ° C., the tetrafluoroethylene-hexafluoropropylene copolymer is 275 ° C., and the tetrafluoroethylene-ethylene copolymer is 280 ° C.

The time for the preliminary heat treatment is from 0.5 hr.
A range of 1000 hr, preferably 1.0 hr to 500 hr is required. The conditions of the pre-heat treatment are limited so that the oxidative thermal decomposition reaction of the polymer is allowed to proceed mildly and sufficiently.

When the preheat treatment temperature is lower than 160 ° C., the oxidative thermal decomposition reaction of the polymer becomes insufficient, and the purpose of the preheat treatment of dissipating the generated heat before the fusion heat treatment cannot be achieved. On the other hand, at a temperature equal to or higher than the melting point of the resin, the heat treatment becomes the same as the fusion heat treatment, and the oxidative thermal decomposition reaction of the polymer rapidly progresses. The present invention cannot be achieved, because a local abnormal temperature rise occurs due to the heat buildup and the polytetrafluoroethylene-based resin porous film finally obtained shows a significant decrease in mechanical performance.

Even when the preheat treatment time is shorter than 0.5 hr, the oxidative thermal decomposition reaction of the polymer becomes insufficient, and when it exceeds 1000 hr, the resin is thermally deteriorated. Can not be achieved.

The preliminary heat treatment of the present invention may be a single treatment, or may be a discontinuous treatment in which heating and cooling are repeated many times. In the case of discontinuous processing, the cumulative time may be within the limited conditions of the above-mentioned preliminary heat treatment.

After the preliminary heat treatment is completed, the molded product is subsequently subjected to a heat treatment at a temperature equal to or higher than the melting point of the resin. The preliminary heat treatment and the fusion treatment may be continuous treatments, or after the preliminary heat treatment. After cooling once, the temperature may be raised again to perform a fusion heat treatment, which may be a discontinuous treatment. The fusion heat treatment may be performed under the condition that the polytetrafluoroethylene-based resin particles can be fused with each other. It can be carried out in various atmospheres such as vacuum, air, nitrogen, oxygen, sulfur gas, helium gas, and silicon oil at a temperature not lower than the melting point of the polytetrafluoroethylene-based resin. The molded product can be heat-treated under tension or without tension, and batch treatment or continuous treatment can be selected. In more detail, a method of processing in a free state without fixing, a method of stretching before heat treatment and fixing in a treatment frame, or a method of fixing treatment in a treatment frame under a condition with a fixed length or shrinkage ratio, or stretching, A method of continuous treatment under the condition of either constant length or shrinkage or a combination thereof can also be appropriately adopted.

Stretching can be carried out before, after, and during the heat treatment, or can be carried out in combination, but if the stretching ratio is too high, the shape of the holes will be the one seen in the plane parallel to the film surface. A film with substantially no orientation can be obtained, or only a film with low reliability of permeation performance because the pore size cannot be controlled. Usually, the draw ratio is 1.1 to 3 times, the draw temperature can be appropriately selected within the range of room temperature to the heat treatment temperature, and the draw can be performed in two directions.

The present invention is characterized in that the fiber-forming polymer is removed from the molded product after the heat treatment, but the fiber-forming polymer referred to here may be different from the original one by the heat treatment.

In the present invention, the method for removing the fiber-forming polymer and, if necessary, the additive to be added from the heat-treated molded article is a dissolution method, a decomposition method, a method using liquid, gas, heat or radiation. Alternatively, a method combining these methods can be adopted, and a batch method or a continuous method can be carried out. It cannot be said unconditionally because it varies depending on the type of the fiber-forming polymer used and the additive added if necessary, but usually, sulfuric acid, nitric acid, hydrochloric acid, perchloric acid or a mixture of acids such as hydrofluoric acid, or A method of immersing the molded product after the heat treatment in a liquid containing a single alkali or a mixture of alkalis such as sodium hydroxide and potassium hydroxide as a main component at a temperature in the range of room temperature to 200 ° C. is simply used.

Further, the membrane thus formed can be further stretched to change the permeation performance, mechanical strength, dimensional stability and the like of the membrane. Stretch ratio is 1.1-
About 3 times, the temperature is usually in the range of room temperature to the melting point of the polytetrafluoroethylene-based resin, but the temperature can be fixed after stretching by applying temperature.

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

The polytetrafluoroethylene-based resin porous membrane according to the present invention can be used for desalination of seawater, desalination, removal of bases and acids in industrial wastewater, ultrapure water for electronic industries, and high-purity chemicals. Manufacturing, recovery of defatted actual liquid, electrodeposition coating liquid, etc., industrial wastewater treatment such as paper pulp waste liquid treatment, oil water separation, oil emulsion separation, separation and purification of fermentation products, concentration of fruit juice and vegetable juice, soybean treatment, sugar industry It is widely used in the food industry such as concentration, separation, purification, artificial kidney, separation of blood components, microfilters for separating bacteria, medical applications such as separation and purification of pharmaceuticals, and biotechnology fields such as bioreactors.

Examples will be shown below, but the present invention is not limited thereto.

[0043]

【Example】

Example 1 and Comparative Example 1 Sodium alginate (manufactured by Hanai Chemical Co., Ltd., 300 cps) 30
Part of the solution was added to purified water with stirring to dissolve it into a uniform solution, and barium sulfate (X-ray shadow enhancer Varitop P,
A water slurry consisting of 320 parts of Sakai Chemical Industry Co., Ltd.) was added and stirred to form a uniform mixed solution, and an aqueous dispersion of polytetrafluoroethylene (Mitsui DuPont Fluorochemical Co., Teflon 30J, polymer concentration 60%) was added. Was added as an active ingredient and stirred to obtain a uniform mixed stock solution. The water content of the stock solution is 650 parts. The viscosity of this stock solution was 1950 poise at 10 ° C.

This stock solution was fed from the hollow fiber die to the die temperature of 10
Approximately 40 wt% after extruding with 10 wt% calcium chloride aqueous solution core liquid at 10 ° C and running in air for 10 cm
The hollow fiber was introduced into a coagulation solution of an aqueous solution of calcium chloride at 50 ° C. for coagulation, followed by washing with water and winding the hollow fiber at 10 m / min. This hollow fiber membrane was bundled into 500 pieces, cut, washed with water and dried.

Of the obtained dry hollow fibers, half of the hollow fibers were first subjected to a preliminary heat treatment at 260 ° C. for 20 hours and then subjected to a fusion heat treatment at 340 ° C. for 1 hour (Example 1).

Of the dried hollow fibers, the remaining half of the hollow fibers were subjected to the fusion treatment at 340 ° C. × 1 hr without the preliminary heat treatment (Comparative Example 1).

The preliminary heat treatment and the fusion heat treatment were all carried out in a hot air dryer (air). DS before fusion heat treatment
As a result of thermal analysis with C, the hollow fiber of Example 1 generated heat of 3 calories / g and the hollow fiber of Comparative Example 1 generated heat of oxidative pyrolysis of 70 calories / g. That is, in Example 1 in which the preliminary heat treatment was performed, it was confirmed that 95% or more of the calorific value (70 calories / g) due to calcium alginate was generated during the preliminary heat treatment. In Comparative Example 1, since the preliminary heat treatment was not performed, it was almost 0%.

The melting point (endothermic peak temperature) of polytetrafluoroethylene by DSC analysis was 327.degree.

Then, the hollow fiber bundles of Example 1 and Comparative Example 1 after the fusion treatment were immersed in concentrated sulfuric acid by a conventional method to remove calcium alginate or its heat-denatured product, barium sulfate and the like, and further washed with water. Then, post-treatment after spinning was performed to obtain a polytetrafluoroethylene-based resin porous film. The evaluation results are shown in Table 1. In Example 1, in particular, remarkable improvement effects were observed in mechanical performance (strength and elongation, puncture pressure), and
The effect that the color tone of the hollow fiber membrane was white was also recognized.
Permeability (calculating the permeability from the amount of water that permeates the membrane and the effective membrane area and the pressure difference between the membranes by applying pressure to the inside of the hollow fiber while keeping the hollow fiber membrane as a small module at 37 ° C) Were equivalent. {Water permeability 1000
(ml / m ² · hr · mmHg, at 37 ° C), rejection rate 99.9 (%,
0.1 μm polystyrene latex 200ppm)}

[Table 1] Examples 2 to 8 and Comparative Examples 2 and 3 A spinning dope having the same composition as that of Example 1 and Comparative Example 1 was prepared, spun, coagulated, washed with water and cut to obtain a bundle of 500 yarns. After pre-heat treatment in a hot air dryer (air) under various treatment conditions, fusion heat treatment at 340 ° C. × 1 hr was performed, and then post treatment was performed.

Table 2 shows the evaluation results of the obtained polytetrafluoroethylene resin porous membrane. The hollow fiber membranes of Examples 2 to 8 were found to have remarkable effects of improving strength, elongation and puncture pressure, and exhibited excellent mechanical performance.

[0051]

[Table 2]

[0052]

Industrial Applicability According to the present invention, it becomes possible to produce a polytetrafluoroethylene-based resin porous membrane having excellent mechanical performance (strength and elongation, puncture pressure), and separation with remarkably improved durability and pressure resistance. It is now possible to provide a membrane.

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Claims (2)

[Claims] 1. A homogeneous mixture obtained by mixing a polytetrafluoroethylene-based resin dispersion and a fiber-forming polymer is molded, and the molded product is heat-treated at a temperature not lower than the melting point of the resin to form a fiber. In the method for producing a polytetrafluoroethylene-based resin porous membrane, which is characterized by removing a forming polymer, a preliminary heat treatment is performed at a temperature of 160 ° C. or higher and lower than the melting point of the resin before the heat treatment. A method for producing a polytetrafluoroethylene-based resin porous membrane. 2. A preheat treatment which is performed at a temperature lower than the melting point of the polytetrafluoroethylene-based resin to generate 20% or more of the heat generated by the oxidative thermal decomposition reaction of the fiber-forming polymer. 1. A method for producing a polytetrafluoroethylene-based resin porous membrane.