JPH0521009B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a hydrophilic polytetrafluoroethylene membrane having excellent heat resistance and chemical resistance, and a method for producing the same.

[Conventional technology and problems]

Although polytetrafluoroethylene (hereinafter referred to as PTFE) has excellent heat resistance and chemical resistance, it is difficult to apply to water or strong acid/strong alkali aqueous solutions because it is water repellent. It is also connected. To impart hydrophilicity to a membrane made of PTFE, for example, as disclosed in Japanese Patent Publication No. 36-20742 or U.S. Pat. A method of grafting is known. However, hydrophilic treatment using these methods still lacks heat resistance and chemical resistance.
Furthermore, although graft polymerization progresses in the surface layer, it is difficult to progress in the interior, and in the end, homogeneous hydrophilicity cannot be imparted. On the other hand, in order to advance graft polymerization to the inside, for example, US Patent No. 3390067
In 3632387, it is also known that porous tetrafluoroethylene resin is impregnated with a surfactant, defluorinated and etched with sodium-naphthalene, etc., and then graft polymerized with a polymerizable monomer to impart hydrophilicity. These methods also use PTFE.
When the pore size of the membrane is large, such as felt, etching can be carried out uniformly; however, as the pore size becomes smaller, the etching process becomes uneven and the hydrophilicity becomes uneven.

On the other hand, a technology was developed in which a hydrocarbon surfactant was mixed into a film made of a water-insoluble polymer material and the surface was cross-linked by irradiating plasma.
8645, 57-31924, the membrane here is semi-permeable, specifically for application to reverse osmosis and ultrafiltration, and has strong acid, It is not intended for application to strong alkalis.

A PTFE material with excellent chemical resistance is used for the diaphragm of the electrolytic cell, and since chlorine, sodium hydroxide, etc. are generated in the electrolysis of alkali metal halides, a wetting method using a fluorinated surfactant was proposed in JP-A-Sho. 56−
130486. However, the fluorinated surfactant is brought into contact with the PTFE diaphragm, dried to inactivate it, and then reactivated by simply contacting it with an aqueous solution at the time of use. The fluorinated surfactant is in contact with the surfactant simply due to chemical affinity, and since the fluorinated surfactant has not been subjected to water-insolubilization treatment, it has the disadvantage of being easily desorbed.

[Means for solving problems]

As a result of intensive investigation into the shortcomings of the conventional technology, the present invention combines a fluorinated surfactant with excellent heat resistance and chemical resistance and a PTFE film, and combines a fluorinated surfactant with excellent heat resistance and chemical resistance. The purpose is to provide a PTFE membrane that has both heat resistance and chemical resistance by crosslinking the agent until it becomes insoluble in an aqueous solution. In particular, in acid etching baths used in the manufacturing process of semiconductor parts, solid foreign matter, Na, K, etc.
The contamination of external contaminant metal ions such as these impedes the shared functions of aluminum as a conductor, silicon oxide as an electrical insulator, and silicon as a semiconductor, causing product defects. The objective is to develop a hydrophilic porous membrane suitable for.

[Effect]

The PTFE membrane used in the present invention is
It is necessary that the pore diameter distribution is narrow.

Furthermore, the fluorine-based surfactant used in the present invention needs to be a non-metallic anionic type surfactant, and its structure is CF ₃ (CF ₂ ) _n COONH ₄ or CF ₃
( _CF2 ) _{nSO3NH4 ,} where m has a value of _3-19 , preferably 6-12. Fluorade FC-93 manufactured by 3M is famous as a commercially available product.

On the other hand, those commercially available as metal salts are
By adsorbing metal ions with an ion exchange resin column, they can be converted into a free acid form, and then converted into a non-metal ion form by neutralization with highly pure ammonia. In order to avoid contamination with extremely small amounts of metal ions, consider the exchange capacity of the ion exchange resin and use a large column with an exchange capacity of 3 to 4 times the amount, or use a large column that has an exchange capacity of 3 to 4 times the amount, or perform ion exchange once. Afterwards, it is necessary to replace it with a new resin column. As a fluorine-based surfactant containing metal ions, 3M's Fluorade
FC-95, FC-98, FC-129, Asahi Glass Surflon S-111, S-113, Daikin Industries Unidyne
There are DS-101, DS-102, etc.

The perfluoroalkyl group represented by CF ₃ (CF ₂ ) _n- is connected to the porous surface of the PTFE membrane.
It has a fairly strong bond due to the waals force,
When stored in water or a salt solution, it maintains its hydrophilic properties for a long time. However, if the fluorine-based surfactant is not cross-linked to the point where it becomes insoluble in an aqueous solution, most of the fluorine-based surfactant will diffuse into the water or aqueous solution due to long-term storage in water. Once the PTFE membrane is dried, its wettability and permeability will be drastically reduced.
The PTFE membrane itself exhibits water repellency.

If the fluorine-based surfactant is crosslinked until it becomes insoluble in an aqueous solution as in the present invention, it will not diffuse even when stored in water for a long time, and the same hydrophilicity as the initial one will be maintained. be able to.

The above characteristics make a noticeable difference when performing the operation of passing. In other words, when sulfuric acid or hydrofluoric acid is passed under pressure or reduced pressure through a PTFE membrane that is not cross-linked with a fluorine-based surfactant, gases such as air will pass through when the liquid to be passed is no longer in the membrane. It passes through the membrane, which greatly reduces hydrophilicity. Therefore, it is necessary to add the next liquid to be drained while the liquid to be drained remains in the overcontainer, making the entire operation cumbersome.

This tendency becomes even more pronounced when filtering a liquid that easily generates gas from the solution, such as hydrogen peroxide (H ₂ O ₂ ), and when a small amount of gas is filtered, the generated gas is absorbed by the porosity of the membrane. It occupies space and becomes water repellent.

In contrast, the permeate membrane of the present invention maintains its hydrophilicity even after the permeate liquid has completely flowed through the permeate container, and also retains H ₂ O ₂
Even in the case of liquids that easily generate gas, such as liquids, the generated gas never occupies the porous space, and as a result, the generated gas and H ₂ O ₂ can pass in their entirety at the same time.

Such an effect is solely due to the water-insolubilized fluorine surfactant.

Next, the manufacturing method will be explained in detail.

Manufactured by the method of Special Publication Showa 60-3842 etc.
Although it is preferable to use a PTFE membrane, a PTFE membrane with a somewhat wide pore size distribution is equally applicable. The porous surface of these PTFE membranes is first coated with a fluorosurfactant. For this purpose, the fluorosurfactant is diluted with a soluble liquid such as water or alcohol. In addition to diluting liquids, acetone, dimethylformamide,
Methyl cellosolve, carbon tetrachloride, toluene, etc. are also available, and it is also possible to use two types at the same time.
If the concentration after dilution is 5% by weight or more, the next step can be started immediately after immersion, but if it is less than 5% by weight, it is necessary to increase the immersion time to 10 hours or more.

As a way to shorten the soaking time, the PTFE membrane can be first placed under vacuum and then the diluted solution is injected. Most preferably, a reduced pressure of 100-150 mmHg is used for this purpose, but any pressure up to 760 mmHg can be used. Using reduced pressure, the dilution concentration can be further reduced, for example to 3% by weight.
Even at a concentration of 2 hours, complete immersion can be achieved. Furthermore, even at a low concentration of 0.5% by weight, complete immersion can be achieved by injecting under reduced pressure and setting the immersion time to 12 hours. Complete immersion here refers to a state where the PTFE membrane is dried after being immersed, and when it is brought into contact with water again, it becomes completely transparent or translucent over the entire surface and exhibits hydrophilic properties. Therefore, incomplete immersion means that some parts become transparent or translucent, but white spots that do not penetrate remain in other parts. In order to achieve complete immersion by increasing the immersion time, time is required for the fluorine-based surfactant in the diluted solution to diffuse to completely cover the porous surface of the PTFE membrane. It is presumed that this is because

After confirming that complete immersion has been completed, high-energy radiation is irradiated to make the fluorine-based surfactant insoluble in water. Suitable forms of radiation include gamma radiation (particularly Co ⁶⁰ ), electron beams, and high energy plasmas. In the case of irradiation with gamma rays or electron beams, the presence of oxygen causes deterioration, particularly a significant decrease in mechanical strength, so it is desirable to carry out the irradiation in a state where oxygen is substantially absent. One method for this purpose is to completely immerse the PTFE membrane in a solution of a fluorine-based surfactant diluted with water, remove the excess, and then pack it in a plastic film bag so that the PTFE membrane becomes wet. Irradiation as is can reduce the presence of oxygen in the PTFE membrane. For further completeness, the interior of the bag-like material may be replaced with nitrogen before irradiation.

The irradiation dose is preferably 0.5 Mrad or more and 3 Mrad. If it is less than 0.5 Mrad, the amount of reactions such as crosslinking will be insufficient and the amount of water-insolubilized products will be reduced. on the other hand
When it exceeds 3 Mrad, the mechanical strength decreases significantly.

On the other hand, water insolubilization treatment using plasma can also be performed. The energy of gamma rays and electron beams is as high as 15 eV, but the electrons in non-equilibrium plasma are
2 eV, but contains a small amount of high-energy electrons, ranging from 10 to 12 eV. It was confirmed that these high-energy electrons have sufficient energy to make the fluorine-based surfactant insoluble in water. When using this high-energy plasma, the PTFE membrane is completely immersed in the fluorine-based surfactant, and then the diluting liquid is completely removed and the membrane is irradiated in a dry state. The PTFE membrane is fixed to a support, but when it is a support that fixes only the periphery, it is convenient because the front and back sides can be irradiated at the same time, but when it is a flat support, one side is treated and then the opposite side is treated. It is desirable to further perform irradiation treatment. To perform irradiation with non-equilibrium plasma, the inside of the system must be
Reduce the pressure to below mmHg, preferably below 0.6 mmHg,
Further, it is preferable to use a carrier gas containing hydrogen, such as hydrogen, methane, and water, and gases such as helium, argon, and nitrogen, which are often used in general non-equilibrium plasma, are not preferable.
This is because a carrier gas containing hydrogen efficiently makes the fluorine-containing surfactant insoluble in water.

The present invention will be further explained below with reference to Examples.

Example 1 Fluoropore FP-022 (PTFE membrane manufactured by Sumitomo Electric)
was immersed in a 5% by weight acetone solution of Fluorade FC93 (ammonium perfluoroalkyl sulfonate manufactured by 3M) for 20 minutes. After air drying, set it in a bell jar type plasma device and adjust the system to 0.1mmHg.
The pressure was reduced to Hydrogen gas was supplied at a flow rate of 1 c.c. (STP)/min and adjusted to a pressure of 0.5 mmHg, 13.56 MHz.
Plasma was generated with a radio wave output of 50W.
After 10 minutes of discharge, the Fluoropore was taken out and immersed in water, sulfuric acid (95%), hydrochloric acid (37%), and nitric acid (70%), which penetrated the entire surface.

It was punched into a circular shape with a diameter of 47 mm, and placed in a filtering device to carry out the strong acid filtration described above. When strong acid was divided into 100 c.c. units and the time required for the first 100 c.c. to pass was compared with the time required for the fifth 100 c.c. It was found that they matched.

Comparative Example 1 A film was formed under the same conditions as in Example 1 except that nitrogen was used as the carrier gas supplied to the plasma apparatus. This membrane was only partially penetrated by hydrochloric acid and nitric acid, but completely penetrated by water and sulfuric acid.

When 500 c.c. of sulfuric acid was passed through the membrane 5 times at 100 c.c. intervals, the elapsed time became longer with each repetition, and the permeability of sulfuric acid was significantly reduced in the membrane after the fifth pass. Ta.

Example 2 Surflon S-111 (perfluoroalkyl carboxylic acid salt manufactured by Asahi Glass) was made into a 1% by weight solution in a mixed solvent of 70% water and 30% isopropyl alcohol.
It was passed through an ion exchange resin column to form a free acid. In order to completely remove residual potassium salts, the column was connected to another ion exchange column for a second exchange, and then neutralized with aqueous ammonia. The obtained aqueous solution was concentrated under reduced pressure to remove volatile ammonia components, and a pH 8 solution was obtained. Place Fluoropore FP-010 in a container with a reduced pressure of 150 mmHgr.
Surflon solution ion-exchanged to ammonia type was injected from the top. Immediately after the injection was completed, Fluoropore FP-010 was removed from the solution and dried at a temperature of 90°C.

It was suspended and fixed in a low-temperature plasma device so that both surfaces could be treated at the same time, and piping was installed so that water vapor at 40°C in equilibrium with nitrogen gas could be supplied at the same time. Adjust the inside of the low temperature plasma device to 0.3mmHg.
The plasma was excited at 50W for 15 minutes.

Fluoropore subjected to this treatment showed the same strong acid resistance as in Example 1.

Example 3 Ion exchange was carried out in the same manner as in Example 2 using a 2% by weight aqueous solution of Unidyne DS-101 (perfluoroalkyl carboxylate, manufactured by Daikin Industries, Ltd.). When Fluoropore FP-010 was impregnated under a reduced pressure of 100 mmHg and the excess solution was removed, the same electron beam irradiation as in Example 2 was performed, and the same strong acid permeability and excess characteristics as in Example 1 were obtained.

Claims (1)

[Claims] 1. A nonmetallic ion type fluorinated surfactant is laminated on the porous surface of a polytetrafluoroethylene membrane, and the fluorinated surfactant is crosslinked until it becomes insoluble in an aqueous solution. A hydrophilic polytetrafluoroethylene membrane characterized by: 2. The hydrophilic polytetrafluoroethylene membrane according to claim 1, wherein the fluorinated surfactant is an ammonium salt of perfluoroalkyl acid. 3. A hydrophilic method characterized by applying a non-metal ion type fluorinated surfactant solution to the porous surface of a polytetrafluoroethylene membrane and irradiating it with an electron beam or non-equilibrium plasma using a hydrogen-containing gas as a carrier gas. A method for producing a polytetrafluoroethylene membrane. 4. The method for producing a hydrophilic polytetrafluoroethylene membrane according to claim 3, wherein the fluorinated surfactant is an ammonium salt of perfluoroalkyl acid. 5. The method for producing a hydrophilic polytetrafluoroethylene membrane according to claim 3, characterized in that the nonmetallic ion type fluorinated surfactant is applied to the polytetrafluoroethylene membrane under reduced pressure. . 6 The fluorinated surfactant is CF ₃ (CF ₂ ) _n COONH ₄ ,
or CF ₃ (CF ₂ ) _n SO ₃ NH ₄ (in the above formula, m is 3 to
19) Claim 4 is characterized in that
A method for producing a hydrophilic polytetrafluoroethylene membrane as described in 2.