JP2001064304A - Production of aqueous dispersion containing polytetrafluoroethylene - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an aqueous dispersion that can give a stretched porous object having excellent mechanical properties by polymerizing tetrafluoroethylene in an aqueous medium containing a surfactant having a specified critical micelle concentration. SOLUTION: Tetrafluoroethylene is polymerized at 50-120 deg.C under a pressure of 6-40 kg/cm2 in an aqueous medium containing 0.01-0.2 wt.% surfactant represented by the formula: CnF2n+1X and having a critical micelle concentration of at most 0.02 mol/l to obtain an aqueous dispersion containing polytetrafluoroethylene in a concentration of 20-40 wt.%. This dispersion is diluted with water to a polytetrafluoroethylene concentration of 10-20 wt.% and vigorously agitated to agglomerate the polytetrafluoroethylene, and the agglomerates are dried at 10-250 deg.C to obtain a polytetrafluoroethylene powder. In the formula, (n) is 7-12; X is COOM or SO3M; and M is an H ion, an alkali metal ion, or an ammonium ion, provided that the case in which M is an ammonium ion is excluded when n=7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an aqueous dispersion containing polytetrafluoroethylene (hereinafter also referred to as PTFE), and PTFE obtained from the aqueous dispersion.
And a stretched porous body having excellent strength obtained by secondary processing of the fine powder.

[0002]

2. Description of the Related Art Fine powder of PTFE is generally prepared from an aqueous dispersion produced by a so-called emulsion polymerization method in which tetrafluoroethylene (hereinafter, also referred to as TFE) is polymerized using a surfactant in an aqueous medium. Obtained by aggregation. As the above-mentioned surfactant, ammonium perfluorooctanoate which is excellent in stability of the dispersion particles during polymerization is used in many cases.

[0003] Fine powder obtained by agglomerating dispersion particles is extruded with a suitable auxiliary agent, for example, liquid paraffin, naphtha, white oil or the like to form a molded product such as a tube, a rod or a film. Manufacturing,
It is known that an expanded porous body of PTFE can be obtained by stretching the molded body at a high temperature.

However, conventionally, there has been a problem that the expanded porous body obtained from the PTFE fine powder thus produced has low mechanical properties such as breaking strength.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing an aqueous dispersion containing PTFE for finally obtaining a stretched porous body having excellent mechanical properties.

[0006] Another object of the present invention is to provide the above-mentioned PTFE.
Provided is a fine powder of PTFE obtained from an aqueous dispersion containing: and a stretched porous body having excellent mechanical properties obtained from the fine powder.

[0007]

The present inventors have found that the critical micelle concentration of the surfactant present during the polymerization of TFE in an aqueous medium is determined by the secondary processing of the fine powder of PTFE produced by the polymerization. It has been newly found that it is related to the strength of the obtained expanded porous body. The mechanism is that the use of an emulsifier with a low critical micelle concentration, especially at the beginning of the polymerization, results in a more crystalline PTFE.
It is considered that polymerization for obtaining As a result, by using a surfactant having a critical micelle concentration in a specific range, a porous PTFE having excellent mechanical properties was successfully produced.

Thus, the present invention provides a method for producing an aqueous dispersion containing PTFE, comprising polymerizing TFE in an aqueous medium containing a surfactant having a critical micelle concentration of 0.02 mol / l or less. It is in.

The present invention also resides in a powdery PTFE fine powder obtained by coagulating PTFE in the aqueous dispersion obtained above.

Further, the present invention resides in a porous PTFE material obtained by extruding the fine powder obtained as described above and extruding the paste at a temperature of 250 ° C. or higher.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, it is important to use a surfactant having a critical micelle concentration (hereinafter, also referred to as CMC) in a specific range of 0.02 mol / l or less. Here, the critical micelle concentration is determined from a graph showing the relationship between the concentration of the surfactant and the surface tension of water. That is, the surface tension of water generally decreases as the concentration of the surfactant increases, but the concentration of the surfactant at which the latter no longer decreases even when the former increases is referred to as the critical micelle concentration. In the present invention, the critical micelle concentration refers to a value at 25 ° C.
For surfactants having a Kraft point of 25 ° C. or higher where the surfactant is the lowest temperature at which micelle formation starts, the value at the temperature of the Kraft point is used.

When the critical micelle concentration of the surfactant exceeds 0.02 mol / l, PTFE obtained by polymerization is used.
However, the mechanical properties of the stretched porous body based on the fine powder are deteriorated. In particular, the critical micelle concentration is 0.
It has been found that a content of not more than 01 mol / l is particularly preferred.
The lower the critical micelle concentration, the higher the strength and the more preferable. On the other hand, from the viewpoint of the stability of polymerization, the lower limit is 0.1.
It is preferably at least 00001 mol / l.

As the surfactant having a critical micelle concentration within the above range, various surfactants can be used. For example, a cationic surfactant includes a carboxylic acid type, a sulfate ester type, a sulfonic acid type or a phosphate ester type fluorine-containing surfactant, and the like. And a quaternary ammonium type fluorinated surfactant. In addition, neutral or amphoteric fluorinated surfactants can also be used.

As a representative surfactant used in the present invention, preferably, a compound represented by the following formula 1 can be mentioned.

C _n F _{2n + 1} X Expression 1 In Expression 1, n is an integer of 7 to 12. When n is less than 7, the surfactant effect is low, while when n exceeds 12, the water solubility as a surfactant decreases,
The surfactant effect is low, and it is difficult to obtain an aqueous dispersion. Of these, n is preferably 8 to 10, and particularly preferably 9 is n. In addition, a perfluoroalkyl group (-C
_n F _{2n + 1)} may be a branched be linear.

X is a hydrophilic group portion of the surfactant (formula 1) and is —COOM or —SO ₃ M, and M is a hydrogen ion, an alkali metal ion such as sodium, potassium, lithium, or an ammonium ion. And preferably an ammonium ion. However, in Formula 1, when n of the perfluoroalkyl group is 7, the case where M is ammonium is excluded.

The alkali metal salt or ammonium salt of the surfactant (formula 1) is usually C _n F _{2n + 1} COOH (n is 7
(An integer of ~ 12) obtained by adding an alkali metal hydroxide or aqueous ammonia.

Another typical surfactant used in the present invention preferably includes the following formula 2.

C ₃ F ₇ O (CF (CF ₃ ) CF ₂ O) _m CF (CF ₃ ) Y Formula 2 In Formula 2, m is an integer of 0 to 2, but 1 is preferred. When m is 3 or more, the water solubility is reduced, the surfactant effect is low, and an aqueous dispersion cannot be obtained. Y is a hydrophilic group moiety and is -COOM; M is sodium, potassium,
It is an alkali metal ion such as lithium or an ammonium ion, preferably an ammonium ion. Among the surfactants of Formula 2, C ₃ F ₇ OCF (CF ₃ ) C
F ₂ OCF (CF ₃ ) COONH ₄ is particularly preferred.

The surfactant of the formula 2 is usually prepared by synthesizing an oligomer composed of HFPO dimer, HFPO trimer and HFPO tetramer from hexapropylene oxide (hereinafter referred to as HFPO) in the presence of a metal fluoride such as KF. Is further hydrolyzed to obtain a carboxylic acid, which is then obtained by adding aqueous ammonia or an alkali metal hydroxide.

In the present invention, one surfactant is often used, but two or more surfactants may be used in combination. In this case, in addition to the surfactant having a critical micelle concentration in the above range, a surfactant having no critical micelle concentration in the above range can also be used in combination, for example, for stabilizing polymer dispersion particles. , Ammonium perfluorooctanoate and the like may be used in combination.

In the present invention, the amount of the surfactant present in the aqueous medium is preferably not more than 0.2% by weight (outer numerical value). If the abundance is large, dispersion particles having a large aspect ratio are generated, and the stability of the dispersion particles is impaired.
Further, it is preferable that the surfactant is present in an amount of 0.01% by weight or more so that the polymerization rate is not extremely reduced. Among them, it is appropriate that the amount of the surfactant is 0.03 to 0.1% by weight.

In the present invention, when the particle size of the polymer increases as the polymerization proceeds, the dispersion becomes unstable. Therefore, from the viewpoint of compensating the stability of the dispersion, generation of particles having a large aspect ratio is not induced during the polymerization. A surfactant may be added to a certain extent. The additional amount may be such that the concentration of the surfactant exceeds 0.2% by weight with respect to water. If the aspect ratio is extremely large, entanglement between the dispersion particles occurs, and the aqueous dispersion becomes gel-like, making it difficult to obtain a stable aqueous dispersion.

The PTFE obtained according to the present invention is produced by aqueous dispersion polymerization. The polymerization temperature is 50-120
C., the polymerization pressure is 6-40 k, which is the pressure of TFE itself.
g / cm ² G is preferred. Initiators include:
It is preferable to use peroxides such as disuccinic acid peroxide and diglutaric acid peroxide or persulfates such as ammonium persulfate and potassium persulfate alone or in combination. Further, the initiator may be used in a redox system in common with a reducing agent such as sodium sulfite. Furthermore, during the polymerization, the radical concentration during the polymerization can be adjusted by adding a radical scavenger such as hydroquinone or catechol, or by adding a peroxide decomposer such as ammonium sulfite.

In the course of the above polymerization, when the concentration of the produced PTFE in the dispersion reaches preferably 20 to 40% by weight, TFE is purged out of the system, stirring is stopped, and the polymerization is terminated. Next, after extracting the aqueous dispersion of PTFE, the dispersion is aggregated to separate and collect the PTFE particles.

Aggregation is preferably performed as follows. For example, after diluting with water to adjust the concentration of the PTFE dispersion to 10 to 20% by weight, the mixture is vigorously stirred. In some cases, the pH may be adjusted, and an aggregation aid such as an electrolyte or a water-soluble organic solvent may be added. By performing appropriate stirring, the PTFE dispersion is separated from water, and the PTFE is appropriately granulated and sized according to the above stirring conditions.
Fine powder is obtained.

Drying of the PTFE fine powder is performed by vacuum, high frequency, hot air or the like while keeping the wet powder obtained by agglomeration in a state where it does not flow much, preferably in a still state. Fine powder has a property of easily fibrillating even with a small shearing force and losing the original crystal structure after the polymerization. In particular, contact or friction between powders at a high temperature is not preferable for fine powders in order to prevent a reduction in workability in stretching applications. Thus, the drying temperature is preferably from 10 to 250C, and particularly preferably from 100 to 250C.

The stretched porous body is preferably manufactured by the following method. That is, PTFE fine powder 10
Preferably, 10 to 50 ml of a lubricant is added to 0 g, mixed, and then matured sufficiently in a closed container. Examples of the lubricant include petroleum solvents such as solvent naphtha and white oil, hydrocarbon solvents such as toluene, ketones and esters, silicone oil, fluorine oil, and fluorine-containing compounds. The lubricant is preferably one that wets the fine powder and that can be easily removed by evaporation after extrusion.

The fine powder to which the lubricant is added is preferably preformed at a pressure of about 1 to 50 kg / cm ² , and then the paste is extruded. In some cases, the extruded product can be formed into a sheet by calendering or the like.

It is preferable to promote the orientation of the fine powder particles at the time of extrusion, and it is preferable that the reduction ratio of the extruder, that is, R / R (the ratio of the area of the barrel to the area of the extrusion die) is sufficient. R / R is 50-80
0 is preferable, and particularly, R / R is suitably from 80 to 200. At this time, the pressure applied to the paste,
The extrusion pressure is preferably from 100 to 1000 kg / cm ² . The lower the extrusion pressure, the better the moldability. However, if the extrusion pressure is too small, the orientation of the fine powder particles is not sufficient, and the stretchability decreases.

Thereafter, the lubricant contained in the extruded product is removed by evaporation and stretched 2 to 50 times at a temperature of 250 ° C. or more to obtain a preferable porous material. Stretching temperature is 250 ° C
When it is less than 350 ° C., a porous body having a small stretching ratio is easily obtained. On the other hand, when it exceeds 350 ° C., it is difficult to obtain a porous body because PTFE melts.

The stretching of the extruded product is preferably performed uniformly. The term “uniform” as used herein means that the entire molded body is uniformly stretched, and specifically, that the weight distribution, the pore size distribution, and the like in the stretching direction of the porous body are uniform.

The stretching ratio is preferably about 2 to 50 times. If the stretching ratio is less than 2, the porous structure is not substantially obtained. The stretching speed is usually preferably 50 to 1000% / sec.

The aqueous dispersion of PTFE obtained in the present invention can be used as a coating material, and can be used for coating on rolls, cooking utensils,
It can be used for impregnating glass cloth. Further, the fine powder of the present invention can be used as a raw material for paste extrusion other than the production of a stretched porous body. In particular, the processed stretched porous body is excellent in strength, water-resistant clothing and industrial goods, such as bag filters, packing, gaskets,
It is useful for other coating applications.

[0035]

EXAMPLES The present invention will be described below with reference to Examples (Examples 1 to 6) and Comparative Examples (Example 7), but is not limited thereto. In addition,
Examples 1 to 7 are examples of the present invention, and Example 8 is a comparative example. Further, each property of the obtained polymer was measured by the following method.

(1) Standard Specific Gravity: The standard specific gravity of a powdery polymer was measured by the amount of water replaced by a standard molding test sample according to ASTM D1457-69. This standard molding test sample weighed 12.0 g of the molding powder with a diameter of 2.8.
Under a pressure of 352 kg / cm ^{2 in} a 6 cm mold,
Hold for a minute and preform. Then, the molded body was heated in an oven from 300 ° C. to 380 ° C. at 2 ° C./min.
After holding at 0 ° C. for 30 minutes and cooling at a rate of 1 ° C./min to 294 ° C., the sample was taken out of the oven and held at 23 ° C. for 3 hours or more to obtain a measurement sample.

(2) Preparation of stretch processability evaluation sample: 50 g of fine powder and an extruded lubricant as a hydrocarbon oil (Super Sol FP25, trade name of Idemitsu Petrochemical Co., Ltd.)
Mix 8 ml and ripen at 25 ° C. for 1 hour or more. Next, the mixture is filled into an extrusion die (having an orifice with an inner diameter of 1 mm and a throttle angle of 30 degrees) equipped with a cylinder (with an inner diameter of 9.95 mm), and a load of 20 kg is applied to the piston inserted into the cylinder and held for 10 minutes. After this ram speed 100
An extruded rod is obtained at mm / min. The extrudate at the part where the pressure is in the equilibrium state in the latter half of the extrusion is placed in an oven, and the lubricant is evaporated off at 180 ° C. This was cut into about 50 mm to obtain a sample for stretching.

(3) Preparation of a stretched porous body: The above-mentioned sample was subjected to a tensile tester equipped with a thermostat, with a distance between chucks of 10 m.
m, a temperature of 250 ° C., and a stretching speed of 200% / sec.

(4) Measurement of strength of stretched porous body: The breaking strength of four stretched porous bodies was measured, and the average value was taken as the breaking strength (kg) per stretched porous body. Example 1 49 g of perfluorodecanoic acid (C ₉ F ₁₉ COOH) purified by recrystallization using chloroform was added to 140 ml of ion-exchanged water and suspended. After adding 7.1 g of 29% aqueous ammonia to ammonium chloride, water and excess ammonia were distilled off to obtain a white solid of ammonium perfluorodecanoate (C ₉ F ₁₉ COON).
H ₄₎ was obtained 50g. The critical micelle concentration of this surfactant was 0.0018 mol / l.

634 g of deionized water, 0.435 g of ammonium perfluorodecanoate, and 7.9 g of paraffin wax were charged into a 1000 ml autoclave equipped with a stirrer, and the system was replaced with TFE. The internal temperature was adjusted to 65 ° C., and the pressure was increased to 16.0 kg / cm ² G with TFE.
10 ml of a 0.5% by weight aqueous solution of disuccinic peroxide was injected under pressure to initiate polymerization. The polymerization was continued while introducing TFE to maintain the internal pressure. When TFE was added in an amount of 70 g, 0.791 g of ammonium perfluorodecanoate was dissolved in 15 ml of water and pressurized to continue polymerization. When 250 g of TFE was added, TFE was purged, the autoclave was cooled, and the polymerization was stopped. The solid concentration of the obtained aqueous dispersion was 26.0% by weight.

After diluting with water so that the solid concentration of the aqueous dispersion becomes about 10% by weight, the above aqueous dispersion polymer is aggregated by mechanical stirring, the water is filtered off, and dried at 180 ° C. for 7 hours. Thus, PTFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.170. This PT
Using a fine powder of FE, it was molded according to the above-mentioned method, and the breaking strength of the stretched porous body was evaluated. As a result, the breaking strength was 0.553 kg. [Example 2] 634 g of deionized water, 0.436 g of ammonium perfluorodecanoate, and 7.9 g of paraffin wax were charged into a 1000 ml autoclave equipped with a stirrer, and the system was replaced with TFE. Set the internal temperature to 65 ° C,
The pressure was increased with TFE to 16.5 kg / cm ² G. 0.5
A 10% by weight aqueous solution of disuccinic peroxide was injected under pressure to initiate polymerization. The polymerization was continued while introducing TFE to maintain the internal pressure. When 70 g of TFE had been added, 0.644 g of ammonium perfluorooctanoate (C ₇ F ₁₅ COONH ₄ ) was dissolved in 15 ml of water and pressurized to continue polymerization. When 250 g of TFE was added, TFE was purged, the autoclave was cooled, and the polymerization was stopped. The solid concentration of the obtained aqueous dispersion was 26.9% by weight.

After diluting with water so that the solid concentration of the aqueous dispersion becomes about 10% by weight, the above aqueous dispersion polymer is aggregated by mechanical stirring, the water is filtered off, and dried at 180 ° C. for 7 hours. Thus, PTFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.165. This PT
Using a fine powder of FE, it was molded based on the above-mentioned method, and the breaking strength of the stretched porous body was evaluated. As a result, the breaking strength was 0.533 kg. Example 3 To 10 g of perfluorodecanoic acid (C ₉ F ₁₉ COOH) purified by recrystallization using chloroform, 30 ml of ion-exchanged water was added and suspended. 11 g of a 2.3N aqueous sodium hydroxide solution was added thereto, and the mixture was sodium-chlorinated, filtered, and purified to obtain 9.9 g of white solid sodium perfluorodecanoate (C ₉ F ₁₉ COONa). The critical micelle concentration of this surfactant is 0.003
It was 6 mol / l.

The surfactant used for the initial preparation was sodium perfluorodecanoate (C ₉ F ₁₉ COONa) 0.4
Polymerization was carried out in the same manner as in Example 2 except that the amount was changed to 40 g. The solid concentration of the obtained aqueous dispersion is 26.7% by weight.
Met.

From the above aqueous dispersion, P
A TFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.184. Using this PTFE fine powder, the molded body was molded based on the above-mentioned method, and the breaking strength of the stretched porous body was evaluated.
It was 31 kg. [Example 4] Perfluorooctanoic acid (C7 F15COO
H) was purified by recrystallization using chloroform. The critical micelle concentration of this surfactant is 0.0090 mol / l
Met.

The surfactant used for the initial preparation is
Perfluorooctanoic acid (C ₇F_FifteenCOOH)
Polymerization was carried out in the same manner as in Example 2 except that the amount was changed to 0.340 g.
went. The solid concentration of the obtained aqueous dispersion was 27.3 plies.
%.

From the above aqueous dispersion, P
A TFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.170. Using this PTFE fine powder, the molded body was molded based on the above-mentioned method, and the breaking strength of the stretched porous body was evaluated.
It was 23 kg. [Example 5] Ammonium perfluorononanoate (C ₈ F ₁₇
The critical micelle concentrations of COONH ₄ ) and ammonium perfluoroundecanoate (C ₁₀ F ₂₁ COONH ₄ ) were measured to be 0.0067 mol / l and 0. 67 mol / l, respectively.
0,048 mol / l.

The surfactant used for the initial preparation was ammonium perfluorononanoate (C ₈ F ₁₇ COONH ₄ ) and ammonium perfluoroundecanoate (C ₁₀ F ₂₁ C).
0.436 mixture of OONH ₄ ) = 1/1 (molar ratio)
Polymerization was carried out in the same manner as in Example 2 except that g was changed. The solid concentration of the obtained aqueous dispersion was 25.9% by weight.

From the above aqueous dispersion, P
A TFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.173. Using this PTFE fine powder, the molded body was molded based on the above-mentioned method, and the breaking strength of the stretched porous body was evaluated.
It was 66 kg. Example 6 The critical micelle concentration of ammonium perfluorooctanesulfonate (C ₈ F ₁₇ SO ₃ NH ₄ ) was 0.0055 mol / l. As the surfactant, such an ammonium perfluorooctanesulfonate (C ₈ F ₁₇ SO ₃ NH ₄ ) was used, and the initial charge was 0.1 μm.
To 425 g, add 0.773 g when TFE is 70 g.
Polymerization was carried out in the same manner as in Example 1 except that the polymerization was changed to. The solid concentration of the obtained aqueous dispersion was 26.5% by weight.

From the above aqueous dispersion, P
A TFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.187. Using this PTFE fine powder, a molded article was molded based on the above-described method, and the breaking strength of the stretched porous body was evaluated.
It was 89 kg.

[Example 7] HFPO in the presence of potassium fluoride
After obtaining an oligomer consisting of a trimer, it is hydrolyzed to a carboxylic acid, and 16.8 g of 29% aqueous ammonia is added to 95 g of the carboxylic acid, ammonium chloride is added, and water and excess ammonia are distilled off to give a wax. C ₃ F ₇ OCF (CF ₃ ) CF ₂ OCF (CF ₃ ) COON
96 g of H ₄ were obtained. The surfactant micelle concentration of this surfactant was 0.018 mol / l.

In Example 1, the surfactant used for the initial preparation and the surfactant added when 70 g of TFE were added were both C ₃ F ₇ OCF (CF ₃ ) CF ₂ OCF (CF ₃ ) from ammonium perfluorodecanoate. )
Polymerization of TFE was carried out in the same manner as in Example 1, except that the amount of COONH ₄ was changed to 0.421 g and 0.771 g. The solid concentration of the obtained aqueous dispersion was 26.7% by weight.

From the above aqueous dispersion, P
A TFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.167, and the PTFE fine powder was molded based on the above-mentioned method, and the breaking strength of the expanded porous body was evaluated.
It was 1 kg.

Example 8 (Comparative Example) Ammonium perfluorooctanoate (C ₇ F ₁₅ COO)
The critical micelle concentration of NH ₄ ) was determined to be 0.03.
It was 3 mol / l. As a surfactant, such an ammonium perfluorooctanoate (C ₇ F ₁₅ COONH ₄ )
To an initial charge of 0.354 g and a TFE of 70
Polymerization was carried out in the same manner as in Example 1 except that the addition amount in g was changed to 0.644 g. The solid concentration of the obtained aqueous dispersion was 27.2% by weight.

From the above aqueous dispersion, P
A TFE fine powder was obtained. The standard specific gravity of the obtained PTFE was 2.172. Using this PTFE fine powder, a molded article was molded based on the above-described method, and the breaking strength of the stretched porous body was evaluated.
77 kg.

[0055]

The fine powder of PTFE obtained by agglomerating the aqueous dispersion containing PTFE according to the production method of the present invention is obtained by extruding the fine powder into a PTFE having excellent mechanical properties such as breaking strength. Give a stretched porous body.

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Claims (5)

[Claims] An aqueous dispersion containing polytetrafluoroethylene, characterized by polymerizing tetrafluoroethylene in an aqueous medium in which a surfactant having a critical micelle concentration of 0.02 mol / l or less is present. Method. 2. The method according to claim 1, wherein the surfactant is a compound represented by the formula 1 wherein n is an integer of 7 to 12, and X is -COOM or-
SO ₃ M. M is a hydrogen ion, an alkali metal ion or an ammonium ion. However, when n is 7,
Except when M is ammonium. The production method according to claim 1, which is selected from the group consisting of: C _n F _{2n + 1} X Formula 1 3. The method according to claim 1, wherein the surfactant is a compound represented by the formula 2 wherein m is an integer of 0 to 2, Y is -COOM, and M is a hydrogen ion, an alkali metal ion or an ammonium ion. The method according to claim 1, wherein the method is selected from the group consisting of: _{C 3 F 7 O (CF (} CF 3) CF 2 O) m CF (CF 3) Y ... Equation 2 4. A fine powder of polytetrafluoroethylene obtained by agglomerating polytetrafluoroethylene in an aqueous dispersion produced by the production method according to claim 1, 2 or 3. 5. A polytetrafluoroethylene porous body obtained by molding the fine powder according to claim 4 by paste extrusion, and stretching the molded body at a temperature of 250 ° C. or higher.