JPH08283447A - Porous membrane made of hydrophilic tetrafluoroethylene resin and its production - Google Patents

Abstract

PURPOSE: To obtain a hydrophilic tetrafluoroethylene resin porous membrane which is excellent in durability, water flux maintenance and resistance to chemicals and useful as a membrane filer by forming a specific coating layer on the surface of the microporous structure of a resin porous membrane and satisfying the relation of the flux ratio between specific liquids. CONSTITUTION: A porous membrane made of tetrafluoroethylene resin (X) is coated with a crosslinked hydrophilic material (e.g. polyvinyl alcohol) (Y) to form a hydrophilic layer on the surface of the microporous structure and the coated membrane satisfies the formula: A1 /A0 <=0.90, wherein is defined as a ratio of the water flux (ml/cm<2> /min) to the isopropyl flux measured under 0.42kg/cm<2> differential pressure and A1 , is the A value after X is soaked in a isopropanol/water 6:4 mixed solution and dried, while A2 is the A value of X before soaking and drying treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrophilic tetrafluoroethylene resin porous membrane, and more specifically, it has a hydrophilic material layer having excellent durability and is capable of maintaining water permeation flow rate and chemical resistance. The present invention relates to an excellent hydrophilic tetrafluoroethylene resin porous membrane and a method for producing the same.

[0002]

2. Description of the Related Art A porous body (porous film) made of tetrafluoroethylene resin (PTFE) is a material having the original characteristics of PTFE such as chemical resistance and heat resistance, and a uniform fine porosity. Since it has structural features such as high resolution based on structure and high permeability based on high porosity, it is used as a highly functional material in a wide range of fields such as membrane filters, battery diaphragms, electric wires, clothing and artificial blood vessels. Has been done. by the way,
The water repellency inherent in PTFE is very effective for fields requiring water repellency such as application to ski wear, but it cannot be said to be advantageous in all applications. PT
The FE porous membrane has a high permeation flow rate of air and an organic solvent in the use of a separation membrane, for example. Is difficult to permeate, and a satisfactory separation function cannot be exerted. Although it is possible to make hydrophilic by impregnating a water-soluble organic solvent in the microporous structure of the PTFE porous membrane and then substituting with water, this method is very complicated, Even with the hydrophilic treatment, there is a problem that bubbles and dissolved gas in the aqueous solution are gradually attached and the water permeation flow rate is reduced in a short period of time.

Conventionally, as a method of semipermanently hydrophilizing a PTFE porous membrane, for example, energy irradiation of electron beam or the like and surface modification of the PTFE porous membrane with an alkali metal,
Alternatively, various methods have been proposed, such as fixing a hydrophilic resin to the surface of the microporous structure of the PTFE porous membrane.
Among these methods for hydrophilizing the PTFE porous membrane, the effect is certain and widely used is that the PTFE porous membrane is impregnated with polyvinyl alcohol (PVA),
This is a method in which PVA is applied to the surface of a fine porous structure and the PVA is crosslinked to make it insoluble (Japanese Patent Publication No. 53-21270).
Issue). This PVA coated PTFE porous membrane is made of PVA
Due to its hydrophilicity, it becomes wet when it comes into contact with water, which makes it easier for water to penetrate into the microporous structure.
The aqueous solution can be easily filtered even at a low pressure at which water hardly permeates with the FE porous membrane as it is.

The hydrophilic PTFE porous membrane in which a hydrophilic material is coated on the surface of such a fine porous structure is relatively easy to manufacture and is excellent in the reliability of the hydrophilization treatment. When it is used for a period of time, the permeation flow rate decreases with time, and finally the aqueous solution hardly permeates, which is a problem in durability. In addition, such conventional hydrophilic PTF
The E porous film also had insufficient resistance to organic solvents, acids and alkalis. It was presumed that the cause of the decrease in the water permeation flow rate with time was due to the fall-off of the hydrophilic material, but actually, when the weight of the PTFE porous membrane that became difficult to wet with water was measured, the hydrophilic treatment was performed. It is almost the same as the original weight, and for the total amount of hydrophilic material applied,
Only a few percent weight loss is observed at most. Therefore, P having a hydrophilic material applied to the surface of the microporous structure
The cause of the decrease in hydrophilicity of the TFE porous membrane has not been sufficiently understood so far, and no satisfactory method has been proposed as a measure for maintaining the water permeation flow rate. Especially after using a PTFE porous membrane coated with a hydrophilic material,
When dried, the volume shrinks. When this is immersed again in the aqueous solution, the volume returns to almost the original volume, but the water permeation flow rate decreases. When the number of such dipping-drying cycles with respect to the aqueous solution increases, the wettability of the hydrophilic PTFE porous membrane with respect to the aqueous solution is finally lost. However, the cause and measures have not been reported so far.

[0005]

An object of the present invention is to form a hydrophilic material layer having excellent durability on the surface of a fine porous structure, which is excellent in water permeation flow rate maintenance and chemical resistance. It is an object of the present invention to provide a porous fluorinated ethylene resin membrane and a method for producing the same. The present inventor has conducted earnest research on the problem of a decrease in the permeation flow rate of water into a hydrophilic PTFE porous membrane provided with a coating layer of a hydrophilic material such as PVA. As a result, the cause of the decrease in the flow rate is microporous before use. The hydrophilic material, which covered the surface of the textured structure relatively uniformly, partially aggregates to form the PT of the base film.
It was found that the FE surface was exposed.

Two factors are involved in the phenomenon that the hydrophilic material, which covers the PTFE surface in the microporous structure relatively before use, partially aggregates to expose the PTFE surface. Presumed. The first factor is the strength of the hydrophilic material coating layer. Originally, hydrophobic PTFE and a hydrophilic material have a poor affinity, and the volume change when the hydrophilic material contains water and when it is dried is very large. If the strength is weak and the shape cannot be kept strong, the exposed portion of the PTFE surface increases even if the coating layer falls off from the surface of the microporous structure during repeated use, or even if it does not fall off, The hydrophilicity will decrease.

The second factor is the hydrophilic PT during drying.
This is the contraction of the volume of the FE porous membrane. When the hydrophilic PTFE porous membrane is dipped in an aqueous solution and then dried, the volume of the coating layer of the hydrophilic material decreases from several tenths to several hundreds of times when it is wet, and the shrinkage at this time occurs. The force causes the entire porous membrane to contract. In fact, when the porous membrane is shrunk in the aqueous solution and returned to the wet state, the contracted porous membrane returns to almost its original volume, and therefore the permeation resistance of the aqueous solution hardly increases. However, due to the large volume change of the porous film, peeling occurs between the coating layer of the hydrophilic material and the PTFE surface. The peeling of PTFE and the hydrophilic material caused by these two factors changes the surface of the microporous structure of the hydrophilic PTFE porous membrane from hydrophilic to original hydrophobic. Therefore, the permeation resistance of water increases and the flow rate gradually decreases.

[0008] The inventors of the present invention used the starting material for the hydrophilic PTFE porous membrane as a starting material when impregnated with a mixed solution of alcohol / water (wet state) and as a starting material after impregnation in a dried state. In addition, a PTFE porous membrane whose surface area and film thickness do not substantially change is used.
It was found that the above object can be achieved by devising the coating amount, coating method, crosslinking method, etc. of the hydrophilic material. The hydrophilic PTFE porous membrane of the present invention is immersed in a mixed solution of isopropyl alcohol (hereinafter, abbreviated as IPA) and water and dried to perform a treatment, followed by measuring the permeation flow rate of IPA and the permeation flow rate of water. The ratio holds a high ratio of the permeation flow rate of IPA and the permeation flow rate of water measured for the untreated one. That is, the hydrophilic PTFE of the present invention
The porous film has a small volume shrinkage and is excellent in durability of the coating layer of the hydrophilic material. The present invention has been completed based on these findings.

[0009]

According to the present invention, a hydrophilic tetrafluoroethylene resin having a coating layer of a crosslinked hydrophilic material formed on the surface of the microporous structure of a tetrafluoroethylene resin porous membrane is provided. Porous membrane with a differential pressure of 0.42 kg /
isopropyl alcohol permeation flow rate measured in cm ² (m
Water permeation flow rate (ml / cm ² / l / cm ² / min)
/ Min) as an A value, a hydrophilic tetrafluoroethylene resin porous membrane satisfying the following relational expression 1 is provided. A ₁ / A ₀ ≧ 0.90 (1) A ₁ : A treatment in which a hydrophilic tetrafluoroethylene resin porous membrane is immersed in a mixed solution of isopropyl alcohol and water at a volume ratio of 6: 4 and dried. A value after performing A ₀ : A value of the hydrophilic tetrafluoroethylene resin porous film before the immersion drying treatment

Further, according to the present invention, there is provided a hydrophilic tetrafluoroethylene resin porous film having a coating layer of a crosslinked hydrophilic material formed on the surface of the fine porous structure of the tetrafluoroethylene resin porous film. In the manufacturing method, (1) a step of impregnating alcohol into the fine porous structure of the tetrafluoroethylene resin porous membrane, (2) impregnating an aqueous solution of a hydrophilic material with a concentration of 0.4 to 1.0% by weight. The hydrophilic tetrafluoroethylene resin porous material is characterized by including the steps of substituting the hydrophilic material with the alcohol, (3) immersing in water, and (4) crosslinking the impregnated hydrophilic material. A method of manufacturing a quality membrane is provided.

The present invention will be described in detail below. The PTFE porous membrane used in the present invention is not particularly limited as long as it has a fine porous structure. Such a PTFE porous membrane can be produced, for example, by the method described in Japanese Patent Publication No. 42-13560. That is, first, a mixed paste of PTFE fine powder and a lubricant is molded into a predetermined shape. Preforming may be performed prior to paste extrusion. It can be molded into a predetermined shape by rolling with a calendar roll or the like, or rolling after extruding the paste. The shape of the molded body includes a sheet, a tube, a rod, a strip, a film and the like. This molded body is stretched at least uniaxially after or without removing the lubricant. After stretching, it is usually heated to 327 ° C. or higher, which is the melting point of PTFE, and sintered. In the stretching step, fine fibers are formed in the stretching direction between the crack-shaped holes formed by allowing the PTFE fine powders pressed in the extrusion step to be separated and split by stretching. Such a fine fibrous structure creates a porous structure. A particle which is soluble in a solvent or a solution is kneaded with a PTFE fine powder or a PTFE dispersion liquid to form a predetermined shape, and then the particle is dissolved and removed to make a porous PTFE membrane, or this is stretched. P
A TFE porous membrane or the like can also be used. PTFE
The shape of the porous body may be any of a sheet shape, a film shape, a tube shape and the like.

Examples of the hydrophilic material used in the present invention include polyvinyl alcohol (PVA), ethylene vinyl alcohol copolymer (EVOH), and acrylate resin. Among these, PVA is easily adsorbed on the PTFE surface in the fine porous structure when the aqueous solution of PTFE is impregnated into the porous PTFE membrane, and the PVA is applied uniformly and with high shape retention by controlling the concentration of the aqueous solution. It is particularly preferable because the layer can be easily formed.
PVA having an average degree of polymerization of 300 to 2500 and a degree of saponification of 85 to 99% can be preferably used.

In the present invention, a coating layer of a hydrophilic material is formed on the surface of the microporous structure of the PTFE porous membrane, and the coated hydrophilic material is crosslinked and immobilized. The microporous structure means, for example, a porous structure composed of fine fibers (fibrils) or a fiber-nodule structure, and the surface thereof means the PTFE surface constituting the fibers or nodules. Therefore, in order to form a coating layer of a hydrophilic material on the surface of the fine porous structure of the PTFE porous membrane,
The PTFE porous membrane is dipped in a solution of hydrophilic material to impregnate the hydrophilic material into the porous space.

When the ratio of the permeation flow rate of water to the permeation flow rate of IPA measured at a differential pressure of 0.42 kg / cm ² is taken as the A value, the hydrophilic PTFE porous membrane of the present invention has the following relational expression 1. I am satisfied. A ₁ / A ₀ ≧ 0.90 (1) A ₁ : A hydrophilic PTFE porous membrane composed of IPA and water 6: 4.
A value after dipping in a mixed solution (volume ratio) (60% IPA aqueous solution) and drying A ₀ : A value of hydrophilic tetrafluoroethylene resin porous membrane before dipping and drying treatment A = Permeate flow rate of water / Permeate flow rate of IPA (Differential pressure 0.42k
Measured in g / cm ² ) For each permeation flow rate, measure the permeation flow rate (unit: ml / cm ² / min) per minute of IPA or water under conditions of differential pressure 0.42 kg / cm ² and room temperature Or convert the measured value to the above unit. Also, assuming that A ₁ / A ₀ = B, this value may be hereinafter referred to as a B value. The larger the B value, the better the durability of the coating layer of the hydrophilic material. The B value of the hydrophilic PTFE porous membrane of the present invention is 0.90 or more.

In order to prevent the hydrophilic material from adhering to the surface of PTFE and preventing it from peeling off, it is first necessary to impart a strong shape-retaining property to the coating layer of the hydrophilic material. For that purpose, it is necessary to uniformly apply the hydrophilic material to the surface of the microporous structure of the PTFE porous membrane, and to crosslink and fix the applied hydrophilic material. In order to enhance the shape-retaining property, it is sufficient to apply the hydrophilic material thickly, but if the coating layer of the hydrophilic material becomes too thick, the pores of the PTFE porous membrane will be clogged. Therefore, it is necessary to control the thickness of the coating layer of the hydrophilic material within a certain range.

The preferred coating amount of the hydrophilic material is PTFE.
It depends on the surface area of the microporous structure of the porous membrane.
The surface area of this fine porous structure has a correlation with the porosity of the PTFE porous membrane. The present inventors prefer to use a PTFA porous membrane having a porosity of 60% or more, preferably 60 to 80%, in order to produce a hydrophilic PTFE porous membrane having excellent durability, and It was found that it is desirable to optimize the coating amount of the hydrophilic material in relation to the porosity. That is, when a PTFE porous film having a porosity of 60% or more is used and the coating amount of the hydrophilic material on the PTFE porous film is adjusted so as to satisfy the following relational expression 2, the hydrophilic material having excellent durability is obtained. A hydrophilic PTFE porous membrane having a layer and excellent in water permeation flow rate maintenance and chemical resistance can be obtained. (C / 5) -11.5 ≦ D ≦ (C / 5) -9.5 (2) C: Porosity (%) of porous tetrafluoroethylene resin membrane D: (Coating weight of hydrophilic material / Polytetrafluoroethylene resin porous membrane weight) x 100

The lower the porosity of the PTFE porous membrane, the smaller the coating amount of the hydrophilic material, and conversely, the higher the porosity, the larger the coating amount. The range is defined by the above relational expression 2. It should be within the range. FIG. 3 shows the first embodiment.
6 is a graph showing the relationship between the porosity of each hydrophilic PTFE porous membrane (hydrophilized sheet) obtained in Comparative Examples 1 to 6 and the PVA coating amount. Even if a PVA-coated PTFE porous membrane is prepared by the same method, if the PVA coating amount does not satisfy the above relational expression 2 (Comparative Example), hydrophilic P excellent in durability is obtained.
It is not possible to obtain a TFE porous membrane. If the coating amount of the hydrophilic material related to the porosity is too small, the hydrophilicity will be insufficient, and it will be difficult to form a strong coating layer. Conversely, if it is too large, the pores will be clogged or when immersed. This causes a change in the volume of the porous membrane during drying. As a method of adjusting the coating amount of the hydrophilic material so as to satisfy the relational expression 2, as will be described later, in the manufacturing process of the hydrophilic PTFE porous membrane, the concentration of the aqueous solution of the hydrophilic material used in the impregnation process is adjusted. There is a method of adjusting to a specific range and further immersing in water for washing between the impregnation step and the crosslinking step.

The hydrophilic PTFE porous membrane of the present invention comprises 60
% Surface area (S ₀ ) and film thickness (d ₀ ) measured in a wet state with an IPA aqueous solution, and surface area (S ₁ ) and film thickness (d ₁ ) measured after drying after impregnation with a 60% IPA aqueous solution It is also characterized in that the following relational expressions 3 and 4 are established between and. | [(S ₁ −S ₀ ) / S ₀ ] × 100 | ≦ 2% (3) | [(d ₁ −d ₀ ) / d ₀ ] × 100 | ≦ 2% (4) The above relational expression is When measuring the surface area (S ₀ ) and film thickness (d ₀ ) when wet, instead of the 60% IPA aqueous solution,
This is also true when wetted with an organic solvent such as water or IPA. Further, each surface area (S ₀ , S ₁ ) and film thickness (d ₀ , d ₁ )
In general, it also holds true when water or an organic solvent such as IPA is used alone instead of the 60% IPA aqueous solution. PVA is almost insoluble in solvents other than water, while IPA is one of the solvents with the highest affinity for PTFE. The mixed solution of water and IPA is IPA
If the ratio is less than 60%, the hydrophobic PTFE cannot be wetted. Therefore, 60% IPA aqueous solution
The A-coated PTFE porous film has the highest affinity, and performing the test using this aqueous solution corresponds to the test under the most severe conditions from the viewpoint of delamination between PVA and PTFE. The same applies to the case of using a hydrophilic material other than PVA. Since the volume of the conventional PVA-coated PTFE porous film shrinks during drying, the surface area and the film thickness of the porous film greatly differ between when wet and when dry. On the other hand, the product of the present invention has little volume change between wet and dry, and peeling of the coating layer of the hydrophilic material is prevented.

FIG. 1 shows the PVA obtained by the prior art.
Immerse the coated PTFE porous membrane in a 60% IPA aqueous solution,
FIG. 3 is a graph showing changes in surface area and film thickness of a PTFE porous membrane and changes in water permeation flow rate measured at a differential pressure of 0.42 kg / cm ² in the case where a non-constrained drying operation (immersion-drying cycle) is repeated. is there. FIG. 2 is a graph showing changes in surface area and film thickness of the PTFE porous membrane and changes in water permeation flow rate when the immersion-drying cycle was similarly repeated for the product of the present invention. First, hydrophilic PT
Using a FE porous membrane, the water permeation flow rate was measured, the surface area and the film thickness were measured in a wet state with water, and the sample was immersed in a 60% IPA aqueous solution and then dried, and the surface area and the film thickness were measured in a dried state. Then, the cycle of returning to the measurement of the water permeation flow rate was repeated. The measurement of the surface area and the film thickness in the above-mentioned wet state was carried out by immersing in 60% IPA aqueous solution and
It may be measured in a 0% IPA aqueous solution in a wet state.

As is apparent from FIG. 1, the conventional product has a large change in surface area and film thickness between wet and dry, and the water permeation flow rate greatly decreases as the dipping-drying cycle is repeated. On the other hand, as is apparent from FIG. 2, the product of the present invention has a very small change in surface area and film thickness during immersion and drying, and has a large water permeation flow rate even after repeated immersion-drying cycles. It remains high, unchanged. The conventional product is a Pore Clon sheet WP-020-40 manufactured by Sumitomo Electric Industries, Ltd.
(Porosity of 75%) is impregnated with IPA, then impregnated with a 2.5 wt% PVA aqueous solution to replace IPA with PVA, and then PVA is electron beam crosslinked.

The hydrophilic PTFE porous membrane of the present invention can be manufactured by the following steps (1) to (4). (1) a step of impregnating alcohol into the fine porous structure of the tetrafluoroethylene resin porous membrane, (2) 0.4 to 1.0
A step of impregnating an aqueous solution of a hydrophilic material with a concentration of wt% to replace the alcohol with the hydrophilic material, (3) a step of immersing in water, and (4) a step of crosslinking the impregnated hydrophilic material.

Since PTFE has a high hydrophobicity, even if it is directly immersed in an aqueous solution of a hydrophilic material, the hydrophilic material cannot be impregnated into the inside of the fine porous structure. Even if the aqueous solution of the hydrophilic material is impregnated in a vacuum state, it does not penetrate into the fine porous structure. Therefore, once the microporous structure is filled with an alcohol compatible with water (eg, isopropyl alcohol), it is immersed in a large amount of an aqueous solution of a hydrophilic material, and the impregnated alcohol is replaced with the hydrophilic material. To apply the hydrophilic material in a controlled coating amount on the surface of the microporous structure of the PTFE porous membrane that needs to be impregnated, the aqueous concentration of the hydrophilic material is 0.4 to 1.0.
It is necessary to adjust to a relatively low concentration by weight. If this concentration is too low, the degree of hydrophilicity and shape retention of the coating layer will be reduced, and if it is too high, pores will be clogged and the volume change of the porous membrane during dipping and drying will be large.

By the way, the thickness of the coating layer of the hydrophilic material is
It is presumed that it is also related to the pore size of the PTFE porous membrane. This is because the microporous structure of the PTFE porous membrane and the hydrophilic material are fixed not only partially by physical adsorption, but also by entanglement of complicated porous structures. That is, the coating layer of the hydrophilic material formed along the surface of the finer porous structure having a smaller pore diameter has a complicated and fine shape and is complicatedly entangled with the PTFE porous membrane, whereas Is large, the porous structure also becomes rough, and the thickness and length of each PTFE fiber become large, and it is presumed that the effect of entanglement due to the shape is reduced. According to the results of the study by the inventors, the pore size is 0. In the case of a membrane equivalent to several μ, the optimum concentration range of the hydrophilic material is relatively wide from 0.4 to 1.0% by weight. It was found that clogging is likely to occur and it is preferable to use an aqueous solution having a concentration near 0.75% by weight.

In the present invention, a step of immersing the PTFE porous membrane in an aqueous solution of a hydrophilic material to impregnate the hydrophilic material, and then immersing it in water to wash it is adopted. If this washing step is omitted, it will be difficult to obtain a high water permeation flow rate even if the other steps and materials used are the same. By this water washing step, the coating amount of the hydrophilic material can be optimized, and the excess residual alcohol can be removed. As a method of immersing the PTFE porous membrane impregnated with a hydrophilic material in water, a method of immersing each cut sheet in water, a method of rewinding the porous membrane wound on a roll and successively immersing in water, Examples thereof include a method of rewinding on another roll and a method of winding the porous film wound on a roll on another roll in water. The time for immersing the PTFE porous membrane impregnated with the hydrophilic material in water is usually 30 seconds or more, and about 1 minute is sufficient. If desired, it may be dipped for 1 minute or more, for example, for about 1 hour or 24 hours, but if it is dipped for a long time, the effect is saturated and it is not efficient.

There are various methods for crosslinking the hydrophilic material in the present invention, such as irradiation crosslinking by ionizing radiation such as electron beam, thermal crosslinking, or chemical crosslinking using a crosslinking agent. Among these crosslinking methods, chemical crosslinking using a crosslinking agent is suitable from the viewpoint of reliability of crosslinking. When PVA is used as the hydrophilic material, the state in which the PTFE porous membrane is impregnated and coated is very stable in an aqueous solution at room temperature. However, in the case of heat crosslinking or irradiation crosslinking performed anaerobically, PVA is used.
However, the chemical cross-linking is possible in an aqueous solution.

In particular, the cross-linking method using glutaraldehyde or terephthalaldehyde as a cross-linking agent in the presence of an acid catalyst is a reaction which is highly reactive at room temperature and stabilizes the cross-linking amount to a certain amount, and is the acetal which is the cross-linking point. Bonding is also a method particularly suitable for the purpose of the present invention because it has relatively high chemical resistance. Furthermore, the advantage that the crosslinking with these aldehydes is particularly advantageous for the production of the hydrophilic PTFE porous membrane is that the crosslinking is not affected by alcohol. The alcohol impregnated in the step (1) is replaced by the hydrophilic material in the step (2) and further removed by washing in the water washing step (3). However, the alcohol is completely removed from the fine porous structure of the PTFE porous membrane. It's not easy to remove.

According to the results of the examination by the present inventors, for example, PT
When IPA that is generally used for displacement impregnation of the FE porous membrane and PVA as the hydrophilic material are used, the concentration of residual IPA is 0.1% by weight when crosslinked by electron beam.
Unless below, crosslinking is affected. In order to stably keep the IPA concentration in the fine porous structure at 0.1% by weight or less, it is necessary to perform water washing for 1 hour per membrane area of 50 m ² three times or more. However, in the case of cross-linking using glutaraldehyde or terephthalaldehyde, the cross-linking is hardly affected even if several wt% of IPA is present. In the present invention, it is at least necessary that the four steps (1) to (4) be performed in order, but among these steps, the steps (2) and (3) ensure the effect. You may repeat to do this. However, as mentioned above, when PVA is crosslinked with aldehydes, it is sufficient to perform each step once in order.

As described above, the hydrophilic PTFE porous membrane of the present invention is suppressed in shrinkage of the volume (membrane surface area and membrane thickness) due to drying. The shrinkage of the porous film during drying is PT
The contraction phenomenon of the porous membrane due to drying, which is one of the causes of separation between the FE and the hydrophilic material, does not bring about an increase in liquid permeation resistance due to a decrease in porosity and pore diameter. Figure 1 shows 60% of a general PVA coated PTFE porous membrane.
An example of changes in the apparent surface area and film thickness when the film is unconstrained and dried after being immersed in the IPA aqueous solution is shown.
Even if the porous membrane shrinks during drying, it cannot be said that it impedes the permeation flow rate because it is almost restored to its original volume when immersed in water again.

The filtration membrane is usually used in a supported state so that its area does not change, whether it is folded into a cartridge by pleating it or used as a flat membrane. Cannot happen. Therefore,
Conventionally, the shrinkage of the membrane area has been considered to be a problem in the assembling process. Furthermore, this contraction phenomenon is caused by the usual hydrophobic P
The TFE film is not so large. On the other hand, in the porous film coated with the hydrophilic material, the hydrophilic material that has been swollen by being wetted with water shrinks to about several tenths to several hundredths when it is dried. A considerably large contraction force works as compared with the case of using the porous film alone. Therefore, it can be said that the above-described shrinkage phenomenon during drying is a phenomenon peculiar to the hydrophilic PTFE porous membrane using the hydrophilic material.

The present inventors have earnestly studied the relationship between the shrinkage of the porous membrane due to drying and the decrease in the water permeation flow rate, which seemed to be unrelated. It was considered that the coating layer of the hydrophilic material was displaced and the PTFE was exposed on the surface. Then, the surface area (S ₀ ) and film thickness (d ₀ ) of the film in the state of being impregnated with the aqueous solution or the organic solvent, and the surface area (S ₁ ) and film thickness (d ₁ ) of the film when dried. When a hydrophilic PTFE porous membrane having substantially no change was developed and this porous membrane was evaluated in the same manner as in FIG. 1, it was found that the decrease in water permeation flow rate was suppressed as shown in FIG. .

The decrease in the water permeation flow rate does not occur in a porous membrane which does not substantially change in volume between a dry state and a wet state so that the relations of the equations 3 and 4 are established, but it is shown in FIG. So it looks like it has almost no shrinkage 1
It was found that a decrease in the water permeation flow rate became apparent when the surface area or the thickness of the membrane changed by about 0%. If the surface area of the membrane or the membrane thickness changes by several tens of percent, it may become almost impermeable to water. A film that shrinks in the area also causes shrinkage in the thickness direction, so it is impossible to constrain it in the thickness direction even if it is made into a cartridge to prevent area changes, so a film that shrinks during drying will result in a decrease in the flow rate. .

In order to produce the hydrophilic PTFE porous membrane of the present invention, which does not have a change in volume during dry and wet, PTFE is used.
As the porous membrane, the surface area (S _A ) and the membrane thickness (d _A ) measured in a state of being wet with a 60% IPA aqueous solution, and the surface area (S _B ) of the membrane measured after the aqueous solution was impregnated and dried.
And the film thickness (d _B ), the following relational expressions 5 and 6 are preferably satisfied. | _{[(S B -S A) / S} A ] × 100 | ≦ 2% (5 ) | _{[(d B -d A) / d} A ] × 100 | ≦ 2% (6 ) such porous PTFE The membrane can be obtained by heat-treating a PTFE porous membrane obtained by an ordinary stretching method under restraint to relax strain due to stretching. Specifically, it can be obtained by heating the PTFE porous film to a metal plate (including a metal belt, a sheet, etc.) in a pressure-bonded state to a temperature equal to or higher than the melting point. In the manufacturing process of the PTFE porous membrane, it is preferable to arrange such a heating process after the stretching process. The heat treatment is carried out, for example, by applying a PTFE porous film obtained by a stretching method to a stainless belt with a surface pressure of 1 to 30.
After pressure bonding with kg / cm ² , preferably 5-15 kg / cm ² , the belt is 290-380 ° C., preferably 300-
It can be performed by heating at 360 ° C. for about 30 seconds to 5 minutes.

In the PTFE porous membrane obtained by such a manufacturing method, the strain due to stretching is relaxed in the heating step. Therefore, even in the state of only the PTFE porous membrane, the area when wet with a solvent or the like, There is almost no change from the dry area.
Furthermore, since the heat-treated PTFE porous film is compressed during pressure bonding to a metal belt or the like, it tends to expand in the film thickness direction, and as a result, a shape that can withstand the shrinking force of the hydrophilic material. Holding strength is given. This shape-retaining property is particularly necessary because when the coating layer of the hydrophilic material described above is formed as a layer having high strength, the shrinkage force during drying also becomes strong. The product of the present invention has almost no decrease in the water permeation flow rate with time as compared with the similar hydrophilic PTFE porous body of the related art, and has excellent chemical resistance to organic solvents, alkalis, acids and the like. It can be suitably used as a separation membrane application.

[0034]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Examples 1 to 7, Comparative Examples 1 to 9] Example 1
7 to 7 and the hydrophilic PTFE porous membranes of Comparative Examples 1 to 9 are all obtained by impregnating a PTFE porous substrate (PTFE base membrane) with PVA and then cross-linking PVA. Any one of the methods was selected and produced. The materials and methods selected for each example are shown in Table 1.

(1) PTFE base film and belt of the base film
As a heating PTFE base film, Sumitomo Electric Co., Ltd. Pore Clon sheet UP-010-40RS (unsintered product), UP-020
Three types of sheets, -80 (unsintered product) and WP-020-40 (sintered product) were used, and each sheet was pressure-bonded to a stainless belt at a surface pressure of 10 kg / cm ² , and then the belt was placed at 340 ° C. for 1 hour. It heated for the conditions of 1 minute. However, Comparative Example 7
Then, this belt heating treatment was not performed. When the porosity of each sheet subjected to the belt heat treatment was measured, it was 75%, 70%, and 60% in the stated order. The B value defined by the formula 1 is 0.98 at the stage of the PTFE base membrane.
That was all. The value of the left term of the formula 5 of each sheet subjected to the belt heat treatment is -1.7%, -0.6%, and 1.3% in order.
And the values of the left-hand side of Equation 6 were all 1.5% or less. However, in Comparative Example 7 in which the belt heat treatment was not performed, the value of the left term of Expression 5 was 4%, and the value of the left term of Expression 6 was 5%.

(2) Impregnation of PVA into the PTFE base film Each sheet had a concentration of 0.3% by weight, 0.4% by weight, 0.9% by weight, and 1. after being immersed in IPA for 30 minutes. 1% to any concentration of PVA aqueous solution of 2% by weight
Soak for hours. Next, the liquid was changed, and it was again immersed in the PVA aqueous solution having the same concentration as the first time for 15 hours. Then, except for Comparative Example 9, after immersing in pure water for 1 minute, crosslinking was carried out by the following method.

(3) Cross-linking of PVA As a cross-linking method of PVA, one of three types of glutaraldehyde cross-linking (GA), terephthalaldehyde cross-linking (TPA), and electron beam cross-linking (EB) is used in each example.
Made a kind. In the glutaraldehyde cross-linking, the sheet after soaking in pure water in the PVA impregnation step immediately contains 2.5N of 0.1N hydrochloric acid.
It was immersed in a wt% glutaraldehyde aqueous solution, reacted at room temperature for 1 hour, and then washed with stirring in a large amount of pure water for 15 hours. In the terephthalaldehyde cross-linking, the sheet after being immersed in pure water in the PVA impregnation step is immediately immersed in a tetrahydrofuran solution of 5% by weight terephthalaldehyde, and after 15 minutes,
The sheet was transferred from this solution into 0.2 N hydrochloric acid and reacted at room temperature for 30 minutes. Then, it was washed with stirring in a large amount of pure water for 15 hours. Electron beam cross-linking should be carried out by dipping pure water in the PVA impregnation step between two 20 μm polyethylene sheets so as not to contain air bubbles, and removing pure water between the polyethylene sheet and the PTFE sheet as much as possible. , One side 3Mr
Electron irradiation of ad was performed on both sides, and a total of 6 Mrad of electron irradiation was performed. Then, the polyethylene sheet was removed, and washed with stirring in a large amount of pure water for 15 hours.

(4) Preparation of Sample Each hydrophilic sheet was naturally dried for 6 hours and then vacuum dried for 6 hours while being fixed to a frame and restrained in area change, and then weighed. From the difference in PTFE weight of each PT
The fixed weight of PVA on the FE base membrane was determined. The results are collectively shown in Table 1.

[0040]

[Table 1]

(Footnote) (* 1) Cross-linking method GA: Glutaraldehyde cross-linking TPA: Terephthalaldehyde cross-linking EB: Electron beam irradiation cross-linking (* 2) PVA amount: PV per weight of PTFE porous membrane
It is the ratio of the fixed weight of A. (* 3) B value of Expression 1 The B value is a value defined by the following Expression 1. B = A ₁ / A ₀ (1) A ₀ : A value before immersion drying treatment in 60% IPA aqueous solution A ₁ : A value after immersion drying treatment in 60% IPA aqueous solution A = water permeation flow rate / IPA Permeation flow rate (differential pressure 0.42k
(measured in g / cm ² ) (* 4) Range of Formula 2 The range of Formula 2 indicates whether or not the D value is within the range defined by Formula 2 below. C / 5-11.5 ≦ D ≦ C / 5-9.5 (2) C: Porosity (%) of the PTFE porous membrane that is the coated object D: (Coating weight of hydrophilic material / PTFE porous) Body weight)
× 100

(5) Evaluation of initial flow rate The water permeation flow rate of each hydrophilized sheet of each Example and Comparative Example was measured. Under the conditions of a differential pressure of 0.42 kg / cm ² and room temperature, the PTFE base membrane is UP-010-40RS with a rate of 4.7 to 5.0 ml / cm ² , and the UP-020-80 is with a rate of 0.4 minute. 7.8-8.2 ml / cm ² , and WP-
020-80, 5.1-5.3 ml / cm ² per minute
There was no significant difference between the hydrophilized sheets of Examples 1 to 7 and Comparative Examples 1 to 3, 7 and 8. Comparative Examples 4, 5,
The water permeation flow rates of 6 and 9 were specifically low at 1.5 to 3.0 ml / cm ² . As a result of observation with a scanning electron microscope, it was found that these hydrophilic sheets had a clogging portion of pores, which were considered as PVA lumps, on the entire surface. This clogged portion was especially seen in Comparative Example 9.

(6) Evaluation of flow rate retention Examples 1 to 7 and comparative examples 1 to 10 showing sufficiently high initial flow rates
For each of the hydrophilized sheets of Nos. 3, 7 and 8, as an accelerated evaluation of the reduction in the flow rate, the hydrophilized sheets were immersed in a 6: 4 mixed solution of isopropyl alcohol and water (60% IPA aqueous solution), and then dried without binding to permeate IPA and water. Flow rate differential pressure 0.42kg / c
A cycle test was conducted with the operation of measuring at m ² as one cycle. As a result, in the hydrophilized sheets of Comparative Examples 1 to 3, 7 and 8, the water permeation flow rates after 20 cycles were all reduced to 50% or less of the initial value, and especially the hydrophilized sheets of Comparative Examples 7 and 8 were made. The sheet does not get wet with water at all, and the water permeation flow rate is almost zero.
Became. On the other hand, in all of the hydrophilic sheets of Examples 1 to 7, the water permeation flow rate after 20 cycles was 8 which is the initial value.
It was found that the water permeation flow rate was maintained at 0% or more, and the decrease in water permeation flow rate hardly occurred. The IPA permeation flow rate of the hydrophilic sheets of Examples 1 to 7 was maintained at 80% or more of the initial value after 20 cycles in all cases. Similarly, the wet surface area and film thickness and the dry surface area and film thickness of the film before and after each cycle were measured, and the values in the left-hand side of formulas 3 and 4 were determined. Was 10%, and the others were within the ranges of Equations 3 and 4.

(7) Evaluation of chemical resistance Examples 1 to 7 showing comparatively high initial flow rates and Comparative Examples 1 to 1
For each of the hydrophilic sheets 3, 7, and 8, 30% hydrochloric acid,
Each was immersed in 30% sulfuric acid and 10% sodium hydroxide at 60 ° C. for 1 month, and the water permeation flow rates before and after the immersion were compared. As a result, in the hydrophilic sheets of Comparative Examples 1 to 3, 7 and 8, the water permeation flow rate after immersion was 5 which was the initial flow rate.
While it was 0% or less, the hydrophilic sheets of Examples 1 to 7 maintained 90% or more of the initial flow rate. In particular, in the case of the 10% sodium hydroxide treatment, none of the hydrophilization sheets of Comparative Examples became wet with water and the water permeation flow rate became 0, whereas in the hydrophilization sheets of Examples 1 to 7,
Almost no decrease in the water permeation flow rate occurred.

[0045]

EFFECT OF THE INVENTION The hydrophilic PTFE porous membrane of the present invention has a stable hydrophilic material layer, is extremely excellent in flow rate maintenance and chemical resistance, and is suitably used for various separation membrane applications. be able to. In particular, it is suitable for a separation solvent using a mixed solvent of an organic solvent and water, which is a weak point of the conventional hydrophilic PTFE porous membrane, or a chemical such as acid or alkali as a solvent. Since these chemicals are also used for cleaning when the separation membrane system is stopped, the hydrophilic PTFE porous membrane of the present invention can be used as a long-life separation membrane even in simple water-based separation. Becomes

[Brief description of drawings]

FIG. 1 PVA coated PTFE obtained by prior art
A change in surface area and film thickness of the PTFE porous membrane and a differential pressure 0.42 kg / cm when the operation of immersing the porous membrane in a 60% IPA aqueous solution and drying without restraint (immersion-drying cycle) is repeated. ³ is a graph showing the change in water permeation flow rate measured in ² .

FIG. 2 shows a change in surface area and film thickness of a PTFE porous membrane when an operation of immersing the product of the present invention in a 60% IPA aqueous solution and drying without restraint (immersion-drying cycle),
3 is a graph showing a change in water permeation flow rate measured at a differential pressure of 0.42 kg / cm ² .

FIG. 3 is a graph showing the relationship between the porosity of the PTFE porous film and the optimum coating amount of the hydrophilic material.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C08J 7/04 CEW C08J 7/04 CEWT // C08L 27:18 29:04

Claims (8)

[Claims] 1. A hydrophilic tetrafluoroethylene resin porous membrane having a coating layer of a crosslinked hydrophilic material formed on the surface of a fine porous structure of the tetrafluoroethylene resin porous membrane, each of which has a differential pressure. When the ratio of the water permeation flow rate (ml / cm ² / min) to the isopropyl alcohol permeation flow rate (ml / cm ² / min) measured at 0.42 kg / cm ² is A value, the following relational expression 1 is satisfied. What is claimed is: 1. A hydrophilic tetrafluoroethylene resin porous membrane, characterized in that A ₁ / A ₀ ≧ 0.90 (1) A ₁ : A treatment in which a hydrophilic tetrafluoroethylene resin porous membrane is immersed in a mixed solution of isopropyl alcohol and water at a volume ratio of 6: 4 and dried. A value after performing A ₀ : A value of the hydrophilic tetrafluoroethylene resin porous film before the immersion drying treatment 2. A porous tetrafluoroethylene resin membrane having a porosity of 60% or more, and a coating amount of a hydrophilic material on the porous tetrafluoroethylene resin membrane satisfies the following relational expression 2. The hydrophilic tetrafluoroethylene resin porous membrane according to claim 1, which is (C / 5) -11.5 ≦ D ≦ (C / 5) -9.5 (2) C: Porosity (%) of porous tetrafluoroethylene resin membrane D: (Coating weight of hydrophilic material / Polytetrafluoroethylene resin porous membrane weight) x 100 3. 6: 4 of isopropyl alcohol and water
The surface area (S ₀ ) and film thickness (d ₀ ) of the hydrophilic tetrafluoroethylene resin porous membrane measured in a wet state with the mixed solution (volume ratio), and after impregnation with the mixed solution and drying. The hydrophilic tetrafluoroethylene resin porous film according to claim 1 or 2, wherein the following relational expressions 3 and 4 are established between the measured surface area (S ₁ ) and film thickness (d ₁ ) of the film. . | [(S ₁ -S ₀ ) / S ₀ ] × 100 | ≦ 2% (3) | [(d ₁ −d ₀ ) / d ₀ ] × 100 | ≦ 2% (4) 4. The hydrophilic tetrafluoroethylene resin porous membrane according to any one of claims 1 to 3, wherein the hydrophilic material is polyvinyl alcohol. 5. A method for producing a hydrophilic tetrafluoroethylene resin porous membrane, wherein a coating layer of a crosslinked hydrophilic material is formed on the surface of a microporous structure of the tetrafluoroethylene resin porous membrane, comprising: (1) ) A step of impregnating alcohol into the fine porous structure of the tetrafluoroethylene resin porous membrane, (2) 0.4-
A step of impregnating an aqueous solution of a hydrophilic material with a concentration of 1.0% by weight to replace the alcohol with the hydrophilic material; (3) a step of immersing in water; and (4) a step of crosslinking the impregnated hydrophilic material. A method for producing a hydrophilic tetrafluoroethylene resin porous membrane, comprising: 6. _A surface area (S _A ) and a film thickness (d _A ) measured in a wet state with a mixed solution of isopropyl alcohol and water at a volume ratio of 6: 4 as a porous tetrafluoroethylene resin film. And the surface area (S _B ) and the film thickness (d _B ) of the film, which are measured after impregnation of the mixed solution and drying, are used so that the following relational expressions 5 and 6 hold, respectively. The method for producing the hydrophilic tetrafluoroethylene resin porous membrane according to claim 5. │ [(S _B -S _A ) / S _A ) × 100 | ≦ 2% (5) │ [(d _B −d _A ) / d _A ] × 100 | ≦ 2% (6) 7. The method for producing a hydrophilic tetrafluoroethylene resin porous membrane according to claim 6, wherein the porous tetrafluoroethylene resin membrane is heat-treated at a temperature equal to or higher than the melting point in a state of being pressed onto a metal plate. 8. A polyvinyl alcohol is used as the hydrophilic material, and in the crosslinking step (4), the chemical crosslinking is carried out in the presence of at least one compound selected from the group consisting of glutaraldehyde and terephthalaldehyde. The manufacturing method according to any one of 1.