JPS61146522A - Preparation of porous body made of polytetrafluoroethylene resin - Google Patents

Abstract

PURPOSE:To obtain a porous film provided with uniform perforations each having an almost real circular shape and excellent in mechanical strength, by irradiating a baked body, which was obtained from a molding grade polytetrafluoroethylene resin (PTFE) powder prepared by a suspension polymerization method, with ionizable radioactive rays before stretching. CONSTITUTION:A molding grade PTFE powder with an average particle size of 25mu obtained by a suspension polymerization method is preparatorily molded in a mold and the molded one is baked at 350 deg.C for 1hr to form a molded body which is, in turn, cut into a cut film with a thickness of 0.1mm. Next, this PTFE cut film is irradiated with gamma rays from cobalt 60 in an irradiation dose of 10<4> rad at room temp. in air and stretched 6 times on the basis of the original length to obtain a PTFE porous film.

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a method for producing a porous body made of polytetrafluoroethylene resin, and more specifically, a porous body made of polytetrafluoroethylene resin, which has excellent mechanical strength,
Moreover, the present invention relates to a method for manufacturing a porous body having a nearly circular pore size and a relatively uniform pore diameter.

(Technical background of the invention and its problems) Polytetrafluoroethylene resin (hereinafter sometimes abbreviated as 8-FE) has excellent chemical resistance.

Because it has heat resistance and mechanical properties, it is used in various fields. For example, porous bodies made of PTFE are widely used as filters for corrosive substances or high-temperature substances by utilizing the above-mentioned characteristics, and are also used in electrolytic separators, fuel cells, and the like.

Conventionally, the method for manufacturing porous bodies from PTFE resin involves mixing PTFE fine particles called fine powder obtained by emulsion polymerization of tetrafluoroethylene with a liquid lubricant, preforming the mixture, and then extruding or rolling it. Alternatively, a well-known method is to form a PTFE membrane into a film by a method including both methods, then remove the liquid mtrtt agent from the obtained film-like PTFE membrane, and then stretch it in at least one axis under normal heating conditions. Alternatively, fired P
After heating TFEW to 327°C or higher, it is slowly cooled, heat-treated so that its crystallinity becomes 80% or higher, and then heated at 25°C.
Japanese Patent Publication No. 53-42974 discloses a method for producing a porous PTFE material, which is characterized by uniaxial stretching at a temperature of -260 DEG C. and a stretching ratio of 1.5 to 4 times.

However, the PTFE porous body manufactured by such a method has a problem in that it is difficult to manufacture the pores to have a nearly perfect circular shape and a predetermined pore diameter. Further, there was also the problem that the mechanical strength of the porous body was not sufficient.

In order to solve the above problems, the present inventors conducted extensive research and found that a molded article made from PTFE molding powder obtained by suspension polymerization was made from the above-mentioned fine powder obtained by emulsion polymerization. It was found that the produced porous body had excellent pore characteristics and strength, although it had a lower elongation rate and extremely poor workability than the molded body produced by the same method. Furthermore, the present inventors irradiated a PTFE molded body made from molding powder with ionizing radiation, and found that when the irradiation dose was 103 to 10 Brad, compared to a case where no ionizing radiation was irradiated at all. As a result, they discovered the fact that the stretching ratio of PTFE can be increased, and based on these facts, they further researched a method for producing a porous body made of polytetrafluoroethylene resin and completed the present invention.

(Objects of the Invention and Summary thereof) The present invention is intended to solve the above-mentioned problems, and has the following objects.

(a) To provide a method for producing a porous body made of PTFE, which has pores that are close to a perfect circle and whose pore diameter can be uniformly controlled to a predetermined size.

(b) To provide a method for producing a porous PTFE membrane that has superior mechanical strength compared to a porous PTFE membrane made from fine powder.

In the first aspect of the method for producing a porous PTFE body according to the present invention, a PTFE molding powder obtained by a suspension polymerization method is compression-molded to create a PTFE preform, and the preform is heated at 327°C. PTF obtained by firing at a temperature above
The E resin molded product is characterized by being uniaxially or biaxially stretched at a temperature of 327° C. or lower after being irradiated with ionizing radiation.

Further, in the second aspect of the method for producing a porous body made of PTFE according to the present invention, a PTFE molding powder obtained by a suspension polymerization method is compression molded to create a PTFE preform, and the obtained PTFE resin is After irradiating the molded body with ionizing radiation, this polytetrafluoroethylene resin molded body 327
It is characterized in that it is re-fired at a temperature of 327° C. or higher, and then uniaxially or biaxially stretched at a temperature of 327° C. or lower.

(Detailed Description of the Invention) The PTFE used in manufacturing the PTFE porous body according to the present invention is a PTFE obtained by a suspension polymerization method.
It is a powder for molding, and the average particle size thereof is preferably 1 to 900 microns, more preferably 10 to 50 microns.

When such PTFE molding powder is preformed in a mold at a molding pressure of 100 to 300 kg/cs2, PT
An FE preform is obtained. Next, this preformed body was
By firing at a temperature of 27° C. or higher, preferably from 350 to 380° C. and cooling, a molded article having a desired shape, such as a film shape, can be obtained.

In order to obtain, for example, a film-like molded product from the preform, the preform may be made into a cylindrical shape and cut to an appropriate thickness. In the above, the PTFE film is P
Although the PTFE film is obtained by cutting after firing the TFE preform, in the present invention, a PTFE film obtained by a method other than cutting can also be used. Further, the shape of the molded body applicable to the present invention may be a tube shape.

In short, the PTFE molded body used in the present invention may be one obtained by compression molding a PTFE molding powder obtained by a suspension polymerization method, followed by firing.

The PTFE molded body thus obtained is irradiated with ionizing radiation such as gamma rays and electron beams.

Generally, when polytetrafluoroethylene is irradiated with ionizing radiation such as gamma rays or electron beams, a decay reaction occurs, resulting in a decrease in the degree of polymerization, a decrease in molecular weight, and a decrease in mechanical strength such as tensile strength.

However, when a PTFE molded body is irradiated with electronic radiation at a dose range of 1 x 103 to 1 x 106 rad, it is quite surprising that within that irradiation dose range, there is less stretching compared to when no radiation is irradiated at all. It was found that it was possible to increase the stretching ratio at the time, and there was no problem in terms of strength in practical use.

The dose of ionizing radiation is 1x103 to 1x106, preferably 1x104 to 1x106 rad. If the irradiation amount is less than 1 x 103 rad, the effect of improving the stretching ratio during stretching will hardly be observed, while if the irradiation exceeds 1 x 108 rad, the collapse reaction will proceed further and most of the intermolecular bonds will be severed. , tensile strength, elongation, and other mechanical properties are extremely reduced.

In addition, in the present invention, the PTFE molded body obtained by firing as described above is heated at a temperature of 327°C or higher, preferably 350°C or higher.
A PTFE porous body may be produced by firing again to a temperature of 390°C and cooling, followed by irradiation with ionizing radiation as described above, followed by uniaxial or biaxial stretching at a temperature of 327°C or less. . In this way, once fired PT
By further baking the molded body in FE at a temperature of 327°C or higher, for example, fine scratches that occurred during the production of the PTFE film can be fused again, and therefore,
Breakage caused by minute scratches during stretching can be prevented.

Further, cooling during re-firing may be gradual cooling or rapid cooling. For example, when the material is rapidly cooled at a cooling rate of 70'C/hrIJ, it is observed that the pore diameter of the PTFE porous material obtained later tends to become close to a perfect circle. Furthermore, if the material is cooled by gradual cooling, the porosity of the PTFE porous body obtained later tends to increase.

The uniaxial or biaxial stretching in the present invention is performed using PTF as described above.
This is done after irradiating the E molded body with ionizing radiation.
This stretching is 327°C or less, preferably 100°C to 320°C.
℃, more preferably at a temperature of 200-250℃
E The molded body is stretched 1.3 to 8.0 times while heating it. The temperature of the PTFE molded body during stretching was 320'C.
If it is more than that, pores having a uniform pore diameter will not be formed in the resulting PTFE porous body, which is not preferable.

PTFE molded product (7) [The stretching ratio is preferably 1.3 to 8.0 times; if this stretching ratio exceeds 8.0 times, pinholes may occur in the PTFE molded product or the molding will be damaged during stretching. This is not desirable as it may cause damage to the body.

On the other hand, the stretching ratio is less than 1.3 times. This is not preferable because fine pores with the desired pore diameter cannot be obtained.

Such stretching can be carried out in the -axial or biaxial direction, but
Preferably it is carried out biaxially. By subjecting the PTFE molded body to biaxial stretching, the pores of the resulting porous PTFE body become closer to a perfect circle.

Moreover, the production of the PTFE porous body according to the second aspect of the present invention!
In the method, PT′FE calcined as described above
After irradiating the molded body with ionizing radiation, the PTFE is roughly
The molded body is characterized by being refired at a temperature of 327°C or higher, and then uniaxially or biaxially stretched at a temperature of 327°C or lower as described above. In this case as well, by re-firing the PTFE molded body to a temperature of 327°C or higher before stretching, it is possible to re-fuse the fine scratches that occurred during the creation of the PTFE film. Destruction caused by scratches can be prevented.

In addition, the same effect as mentioned above was observed regarding the cooling after re-firing. In the case of rapid cooling, the pore shape becomes close to a perfect circle, while in the case of slow cooling, the porosity tends to increase. Is recognized.

Further, the pores formed in the PTFE porous body obtained according to the present invention have a shape similar to that of a stretched pore, as confirmed by electron microscopy.

11 to 11 The method for producing a PTFE porous body according to the present invention is performed at a temperature of 327°C.
A PTFE molded body fired at a temperature above 327°C is uniaxially or biaxially stretched at a temperature of 327°C or lower after being irradiated with ionizing radiation, or a PTFE molded body fired at a temperature of 327°C or higher is
After irradiating the TFE molded body with ionizing radiation, it is refired at a temperature of 327°C or higher, and then vAr! Since the film is uniaxially or biaxially stretched at I, it is possible to increase the stretching ratio compared to the case where no ionizing radiation is irradiated, and the following effects are obtained.

(d) A porous PTFE body having pores that are nearly perfectly circular in shape and having uniform pore diameters can be obtained.

(b) Control of pore diameter and pore shape is easier.

(C) The PTFE porous body produced by fF has excellent mechanical strength compared to a PTFE porous membrane made from fine powder.

EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples.

fi PTF with an average particle size of 25μ obtained by suspension polymerization method
E molded body powder (Polyflon M12. Manufactured by Daikin Corporation)
was preformed in a mold at a molding pressure of 150 ko/Q10, and then fired at 350°C for 1 hour to create a PTFE molded body. This PTFE molded body was cut to a thickness of 0.1ml.
A PTFE cutting film was created. The elongation of this cut film was 410% as measured by Instron.

Next, this PTFE cutting film was irradiated with cobalt-60 gamma rays at a dose of 104 rad in air at room temperature. The elongation of this cut film was measured by Instron and was 550%. Next, the irradiated cut PTFE film was stretched at a temperature of 200° C. to six times its original length to obtain a porous PTFE film.

The PTFE cut film obtained in Example 1 was re-baked in a 350° C. furnace for 1 hour and rapidly cooled, and then irradiated with cobalt-60 gamma rays of 104 rad at room temperature in air.

Next, the film after irradiation was biaxially stretched at 250°C with
A PTFE porous membrane was produced by stretching each film up to 3.0 times in the Y direction.

The obtained porous PTFE membrane had many pores having a shape close to a perfect circle and having a pore diameter of 0.5 μm or less.

Note that the elongation of the PTFE cut film after irradiation with gamma rays was 460%.

Friend mf! 4 3 The PTFE cut film obtained in Example 1 was re-baked in a 360°C furnace for 1 hour, and after being slowly cooled at a cooling rate of 70°C/hr or less, it was coated with cobalt-60 gamma in air at room temperature. The beam was irradiated with 104 rad. Next, the film after irradiation was
A PTFE porous membrane was produced by stretching the film in the X and Y directions up to 3.0 times in each direction using a biaxial stretching machine at 0°C.

The obtained porous PTFE membrane had many pores having a shape close to a perfect circle and having a pore diameter of 0.5 μm or less.

Note that the elongation of the P-cho FE cutting film after irradiation with gamma rays was 600%.

The PTFE cut film obtained in Example 1 was irradiated with cobalt-60 gamma rays at 104 rad at room temperature in air. Next, this film was refired in a 360'C furnace for 1 hour, and then rapidly cooled in Trichlorene. Next, the film was stretched at 250°C with a biaxial stretching machine of 3.0% in each of the X and Y directions.
A porous PTFE membrane was produced by stretching the membrane to double the original length.

Note that the elongation of the PTFE cutting film after irradiation with gamma rays was 550%.

L19-2 When the PTFE cut film obtained in Example 1 was stretched to 1.7 times in the X and Y directions using a biaxial stretching machine without irradiating it with ionizing radiation, a large number of pins appeared. A hole was occurring.

Claims (1)

[Claims] 1. After irradiating a polytetrafluoroethylene resin molded product fired at a temperature of 327° C. or higher with ionizing radiation, 3.
A method for producing a porous body made of polytetrafluoroethylene resin, which comprises uniaxially or biaxially stretching at a temperature of 27°C or lower. 2. The method for producing a porous body made of polytetrafluoroethylene resin according to claim 1, wherein the ionizing radiation dose is 10^3 to 10^6 rad. 3. After irradiating a polytetrafluoroethylene resin molded body fired at a temperature of 327°C or higher with ionizing radiation,
The polytetrafluoroethylene resin molded body was heated to 327°C.
A method for producing a porous body made of polytetrafluoroethylene resin, which comprises re-firing at a temperature above and then uniaxially or biaxially stretching at a temperature below 327°C. 4. The method for producing a porous body made of polytetrafluoroethylene resin according to claim 3, wherein the ionizing radiation dose is 10^3 to 10^6 rad.