FLUOROPOLYMER AQUEOUS DISPERSION PRODUCTION METHOD AND FLUOROPOLYMER AQUEOUS DISPERSION

Abstract

A method for producing a purified fluoropolymer aqueous dispersion, which includes: (A) bringing a fluoropolymer aqueous dispersion obtained using a hydrocarbon surfactant into contact with an anion exchange resin A or a synthetic adsorbent. The anion exchange resin A has an ion-exchange group represented by the following general formula (A1):

—N+R.sup.1R.sup.2R.sup.3X—

wherein each of R1, R2, and R3 are the same or different, and are each a hydrogen atom or an organic group, and at least one of R.sup.1, R.sup.2, and R.sup.3 is an organic group having 3 or more carbon atoms; and X is a counter ion; or an ion-exchange group represented by the following general formula (A2):

—NR.sup.4R.sup.5

wherein each of R.sup.4 and R.sup.5 are the same or different, and are each a hydrogen atom or an organic group, and at least one of R.sup.4 and R.sup.5 is an organic group having 2 or more carbon atoms.

Claims

1. A method for producing a purified fluoropolymer aqueous dispersion, comprising: (A) bringing a fluoropolymer aqueous dispersion obtained using a hydrocarbon surfactant into contact with an anion exchange resin A or a synthetic adsorbent, wherein the anion exchange resin A has: an ion-exchange group represented by the following general formula (A1):
—N+R.sup.1R.sup.2R.sup.3X— wherein R.sup.1, R.sup.2, and R.sup.3 are the same as or different from each other, and are each a hydrogen atom or an organic group, and at least one of R.sup.1, R.sup.2, and R.sup.3 is an organic group having 3 or more carbon atoms; and X is a counter ion; or an ion-exchange group represented by the following general formula (A2):
—NR.sup.4R.sup.5 wherein R.sup.4 and R.sup.5 are the same as or different from each other, and are each a hydrogen atom or an organic group, and at least one of R.sup.4 and R.sup.5 is an organic group having 2 or more carbon atoms.

2. The method according to claim 1, wherein in the general formula (A1), at least one of R.sup.1, R.sup.2, and R.sup.3 is an organic group having 4 or more carbon atoms.

3. The method according to claim 1, wherein in the general formula (A1), R.sup.1, R.sup.2, and R.sup.3 are each an organic group having 2 or more carbon atoms.

4. The method according to claim 1, wherein the synthetic adsorbent has a pore volume of 0.6 to 2.5 cm3/g.

5. The method according to claim 1, wherein the step (A) is performed twice or more times.

6. The method according to claim 1, further comprising: (B) bringing the fluoropolymer aqueous dispersion into contact with an anion exchange resin B, wherein the anion exchange resin B is different from the anion exchange resin A.

7. The method according to claim 6, wherein the anion exchange resin B has: an ion-exchange group represented by the following general formula (B1):
—N.sup.+(CH.sub.3).sub.3X.sup.− wherein X represents a counter ion; or an ion-exchange group represented by the following general formula (B2):
—N.sup.+(CH.sub.3).sub.2(C.sub.2H.sub.4OH)X.sup.− wherein X represents a counter ion.

8. The method according to claim 6, wherein the step (B) is carried out before the step (A).

9. The method according to claim 1, further comprising: (C) adding a nonionic surfactant to the fluoropolymer aqueous dispersion that has undergone the step (A) for concentration by phase separation.

10. The method according to claim 9, wherein the step (C) is performed twice or more times.

11. The method according to claim 10, wherein in the first step (C), the concentration by phase separation is performed by heating the fluoropolymer aqueous dispersion at a temperature 5° C. lower than the cloud point of the nonionic surfactant or higher and then allowing it to stand still to separate it into a supernatant phase and a concentrated phase.

12. The method according to claim 10, wherein in the second step (C), the concentration by phase separation is performed by heating the fluoropolymer aqueous dispersion at a temperature 5□C lower than the cloud point of the nonionic surfactant or higher and then allowing it to stand still to separate it into a supernatant phase and a concentrated phase.

13. A fluoropolymer aqueous dispersion comprising a fluoropolymer and water, wherein the dispersion comprises a compound represented by the following general formula (1) and a total content of the compound represented by the following general formula (1) is 1,000 ppb or less based on the fluoropolymer:
(H—(CF.sub.2).sub.m—COO).sub.pM.sup.1  General Formula (1) wherein m is 3 to 19; M.sup.1 is H, a metal atom, NR.sup.5.sub.4 where R.sup.5 is the same as or different from each other and is H or an organic group having 1 to 10 carbon atoms, imidazolium optionally having a substituent, pyridinium optionally having a substituent, or phosphonium optionally having a substituent; and p is 1 or 2.

Description

EXAMPLES

[1219] Hereinafter, the present disclosure will be described with reference to experimental examples, but the present disclosure is not limited solely to such Examples.

[1220] In Examples, each physical property was measured by the following method.

[1221] Solid Content

[1222] In an air dryer, 1 g of PTFE aqueous dispersion was dried at a condition of 150° C. for 60 minutes, and the proportion of the mass of the non-volatile matter to the mass of the aqueous dispersion (1 g) was expressed by percentage and taken as the solid concentration thereof.

[1223] Average Primary Particle Size

[1224] The average primary particle size is determined by dynamic light scattering. A fluoropolymer aqueous dispersion with the fluoropolymer solid concentration being adjusted to about 1.0% by mass was prepared. The average primary particle size was determined from 70 measurement processes using ELSZ-10005 (manufactured by Otsuka Electronics Co., Ltd.) at 25° C. The refractive index of the solvent (water) was 1.3328 and the viscosity of the solvent (water) was 0.8878 mPa.Math.s.

[1225] Content of Specific Compound Containing Fluorine

[1226] The following describes the method of measuring the contents of compounds represented by the following general formulas (1) and (2), as specific compounds containing fluorine.

(H—(CF.sub.2).sub.m—COO).sub.pM.sup.1  General Formula (1)

[1227] wherein m is 3 to 19; M.sup.1 is H, a metal atom, NR.sup.5.sub.4, imidazolium optionally having a substituent, pyridinium optionally having a substituent, or phosphonium optionally having a substituent; and p is 1 or 2.

(H—(CF.sub.2).sub.n—SO.sub.3).sub.qM.sup.2  General Formula (2)

[1228] wherein n is 4 to 20; M.sup.2 is H, a metal atom, NR.sup.5.sub.4, imidazolium optionally having a substituent, pyridinium optionally having a substituent, or phosphonium optionally having a substituent; and q is 1 or 2.

[1229] The contents of specific compounds containing fluorine were measured under the following conditions using liquid chromatography-mass spectrometry.

[1230] The solid content in the aqueous dispersion was determined, and the aqueous dispersion in an amount equivalent to 0.5 g of the solid PTFE was put into a 100-mL screw tube. Thereafter, water and methanol were added thereto such that the extraction solvent was to be 40 g (43.14 mL) having a water/methanol ratio by vol % of 50/50 including the water originally contained in the aqueous dispersion. Thereafter, the mixture was well shaken until coagulation occurred. The solid was removed and the liquid phase was centrifuged at 4,000 rpm for one hour, and then the supernatant containing the compound represented by the general formula (1) and the compound represented by the general formula (2) was extracted as the extracted liquid.

[1231] (Method of Measuring Content of Compound Represented by General Formula (1))

[1232] The content of the compound represented by the general formula (1) contained in the extract was determined by conversion in terms of perfluorooctanoic acid equivalent.

[1233] Calibration Curve of Perfluorooctanoic Acid

[1234] Five methanol standard solutions of perfluorooctanoic acid having known concentrations within 1 ng/mL to 100 ng/mL were prepared, and subjected to analysis using a liquid chromatograph-mass spectrometer (Waters, LC-MS ACQUITY UPLC/TQD). Using the first order approximation from the respective sample concentrations and the peak integral values, the values a and b were determined by the following relational formula (1):

A=a×X+b(1)

[1235] A: peak area of perfluorooctanoic acid

[1236] X: concentration (ng/mL) of perfluorooctanoic acid

[1237] Measurement Equipment Configuration and LC/MS/MS Measurement Conditions

TABLE-US-00002 TABLE 1 LC unit Apparatus Acquity UPLC manufactured by Waters Column Acquity UPLC manufactured by Waters BEH C18 1.7 mm (2.1 × 50 mm) Mobile phase A CH.sub.3CN B 20 mM CH.sub.3COONH.sub.4/H.sub.2O   0 .fwdarw. 1.5 min A:B = 10:90 1.5 .fwdarw. 8.5 min A:B = 10:90 .fwdarw. A:B = 90:10 Linear gradient 8.5 .fwdarw. 10 min A:B = 90:10 Flow rate 0.4 mL/min Column 40° C. temperature Sample 5 μL injection volume MS unit Apparatus TQ Detecter Measurement MRM (Multiple Reaction Monitoring) mode Ionization Electrospray ionization method Negative mode

[1238] MRM Measurement Parameters

TABLE-US-00003 TABLE 2 Compound Precursor Product Perfluorooctanoic acid 413 369

[1239] Content of Compounds Represented by General Formula (1) Having 4 or More and 20 or Less Carbon Atoms Contained in Extract

[1240] Using a liquid chromatograph-mass spectrometer, compounds represented by the general formula (1) having 4 or more and 20 or less carbon atoms were subjected to analysis. For the extracted liquid phase, the peak areas of the compounds represented by the general formula (1) having the respective numbers of carbon atoms were determined by MRM.

[1241] MRM Measurement Parameters

TABLE-US-00004 TABLE 3 Number of Compound name carbon atoms Precursor Product (H—(CF.sub.2).sub.3—COO) M.sup.1 4 195 131 (H—(CF.sub.2).sub.4—COO) M.sup.1 5 245 181 (H—(CF.sub.2).sub.5—COO) M.sup.1 6 295 231 (H—(CF.sub.2).sub.6—COO) M.sup.1 7 345 281 (H—(CF.sub.2).sub.7—COO) M.sup.1 8 395 331 (H—(CF.sub.2).sub.8—COO) M.sup.1 9 445 381 (H—(CF.sub.2).sub.9—COO) M.sup.1 10 495 431 (H—(CF.sub.2).sub.10—COO) M.sup.1 11 545 481 (H—(CF.sub.2).sub.11—COO) M.sup.1 12 595 531 (H—(CF.sub.2).sub.12—COO) M.sup.1 13 645 581 (H—(CF.sub.2).sub.13—COO) M.sup.1 14 695 631 (H—(CF.sub.2).sub.14—COO) M.sup.1 15 745 681 (H—(CF.sub.2).sub.15—COO) M.sup.1 16 795 731 (H—(CF.sub.2).sub.16—COO) M.sup.1 17 845 781 (H—(CF.sub.2).sub.17—COO) M.sup.1 18 895 831 (H—(CF.sub.2).sub.18—COO) M.sup.1 19 945 881 (H—(CF.sub.2).sub.19—COO) M.sup.1 20 995 931

[1242] The content of the compound represented by the general formula (1) having (m+1) carbon atoms in the extract was calculated by the following formula (3). The values a and b in the formula (3) were determined by the formula (1):

XCm=((ACm−b)/a)×((50×m+45)/413)  (3)

[1243] XCm: content (ng/mL) of compound represented by general formula (1) having (m+1) carbon atoms in extract solution

[1244] ACm: peak area of compound represented by general formula (1) having (m+1) carbon atoms in extract solution

[1245] The quantification limit in this measurement is 1 ng/mL.

[1246] Content of Compound Represented by General Formula (1) Having (m+1) Carbon Atoms Contained in Aqueous Dispersion

[1247] The content of the compound represented by the general formula (1) having (m+1) carbon atoms contained in the aqueous dispersion was determined by the following formula (5):

ZCm=XCm×86.3  (5)

[1248] ZCm: content (ppb based on PTFE) of compound represented by general formula (1) having (m+1) carbon atoms contained in aqueous dispersion

[1249] (Method of Measuring Content of Compound Represented by General Formula (2))

[1250] Measurement of Content of Compound Represented by General Formula (2) Contained in Extract

[1251] The content of the compound represented by the general formula (2) contained in the extract was determined by conversion in terms of perfluorooctanesulfonic acid equivalent.

[1252] Calibration Curve of Perfluorooctanesulfonic Acid

[1253] Five methanol standard solutions of perfluorooctanesulfonic acid having known concentrations within 1 ng/mL to 100 ng/mL were prepared, and subjected to analysis using a liquid chromatograph-mass spectrometer (Waters, LC-MS ACQUITY UPLC/TQD). Using the first order approximation from the respective sample concentrations and the peak integral values, the values a and b were determined by the following relational formula (1):

A=a×X+b  (1)

[1254] A: peak area of perfluorooctanesulfonic acid

[1255] X: concentration (ng/mL) of perfluorooctanesulfonic acid

[1256] Measurement Equipment Configuration and LC/MS/MS Measurement Conditions

TABLE-US-00005 TABLE 4 LC unit Apparatus Acquity UPLC manufactured by Waters Column Acquity UPLC manufactured by Waters BEH C18 1.7 mm (2.1 × 50 mm) Mobile phase A CH.sub.3CN B 20 mM CH.sub.3COONH.sub.4/H.sub.2O   0 .fwdarw. 1.5 min A:B = 10:90 1.5 .fwdarw. 8.5 min A:B = 10:90 .fwdarw. A:B = 90:10 Linear gradient 8.5 .fwdarw. 10 min A:B = 90:10 Flow rate 0.4 mL/min Column 40° C. temperature Sample 5 μL injection volume MS unit Apparatus TQ Detecter Measurement MRM (Multiple Reaction Monitoring) mode Ionization Electrospray ionization method Negative mode

[1257] MRM Measurement Parameters

TABLE-US-00006 TABLE 5 Compound Precursor Product Perfluorooctanoic acid 499 99

[1258] Content of Compounds Represented by General Formula (2) Having 4 or More and 20 or Less Carbon Atoms Contained in Extract

[1259] Using a liquid chromatograph-mass spectrometer, compounds represented by the general formula (2) having 4 or more and 20 or less carbon atoms were subjected to analysis. For the extracted liquid phase, the peak areas of the compounds represented by the general formula (2) having the respective numbers of carbon atoms were determined by MRM.

[1260] MRM Measurement Parameters

TABLE-US-00007 TABLE 6 Number of Compound name carbon atoms Precursor Product (H—(CF.sub.2).sub.4—SO.sub.3) M.sup.2 4 281 99 (H—(CF.sub.2).sub.5—SO.sub.3) M.sup.2 5 331 99 (H—(CF.sub.2).sub.6—SO.sub.3) M.sup.2 6 381 99 (H—(CF.sub.2).sub.7—SO.sub.3) M.sup.2 7 431 99 (H—(CF.sub.2).sub.8—SO.sub.3) M.sup.2 8 481 99 (H—(CF.sub.2).sub.9—SO.sub.3) M.sup.2 9 531 99 (H—(CF.sub.2).sub.10—SO.sub.3) M.sup.2 10 581 99 (H—(CF.sub.2).sub.11—SO.sub.3) M.sup.2 11 631 99 (H—(CF.sub.2).sub.12—SO.sub.3) M.sup.2 12 681 99 (H—(CF.sub.2).sub.13—SO.sub.3) M.sup.2 13 731 99 (H—(CF.sub.2).sub.14—SO.sub.3) M.sup.2 14 781 99 (H—(CF.sub.2).sub.15—SO.sub.3) M.sup.2 15 831 99 (H—(CF.sub.2).sub.16—SO.sub.3) M.sup.2 16 881 99 (H—(CF.sub.2).sub.17—SO.sub.3) M.sup.2 17 931 99 (H—(CF.sub.2).sub.18—SO.sub.3) M.sup.2 18 981 99 (H—(CF.sub.2).sub.19—SO.sub.3) M.sup.2 19 1031 99 (H—(CF.sub.2).sub.20—SO.sub.3) M.sup.2 20 1081 99

[1261] The content of the compound represented by the general formula (2) having n carbon atoms in the extracted liquid was calculated by the following formula (3). The values a and b in the formula (3) were determined by the formula (1):

XSn=((ASn−b)/a)×((50×n+81)/499)  (3)

[1262] XSn: content (ng/mL) of compound represented by general formula (2) having n carbon atoms in extract solution

[1263] ASn: peak area of compound represented by general formula (2) having n carbon atoms in extract solution

[1264] The quantification limit in this measurement is 1 ng/mL.

[1265] Content of Compound Represented by General Formula (2) Having n Carbon Atoms Contained in Aqueous Dispersion

[1266] The content of the compound represented by the general formula (2) having n carbon atoms contained in the aqueous dispersion was determined by the following formula (5):

ZSn=XSn×86.3  (5)

[1267] ZSn: content (ppb based on PTFE) of compound represented by general formula (2) having n carbon atoms contained in aqueous dispersion

[1268] In Synthesis Examples, polymerization was carried out using sodium 10-oxoundecyl sulfate (hereinafter, referred to as a surfactant A).

Synthesis Example 1

[1269] To a glass reactor with an internal volume of 1 L and equipped with a stirrer, 588.6 g of deionized water and 70.0 g of the surfactant A were added. The reactor was sealed, and the system was purged with nitrogen, so that oxygen was removed. The reactor was heated up to 90° C. and pressurized to 0.4 MPaG with nitrogen. Then, 41.4 g of ammonium persulfate (APS) was charged thereinto and stirred for 3 hours. The stirring was stopped, the pressure was released until the reactor was adjusted to the atmospheric pressure, and the reactor was cooled to obtain an aqueous surfactant solution B.

Synthesis Example 2

[1270] To a reactor made of SUS with an internal volume of 6 L and equipped with a stirrer, 3,600 g of deionized degassed water, 180 g of paraffin wax, and 0.540 g of the surfactant A were added. The reactor was sealed and the system was purged with nitrogen, so that oxygen was removed. The reactor was heated up to 90° C. and TFE was filled into the reactor such that the reactor was adjusted to 2.70 MPaG. Then, 0.031 g of ammonium persulfate (APS) and 1.488 g of disuccinic acid peroxide (DSP) serving as polymerization initiators were charged thereinto. TFE was charged so as to keep the reaction pressure constant at 2.70 MPaG. At the same time as TFE was started to be charged, the aqueous surfactant solution B was started to be continuously charged. When 1,650 g of TFE was charged, the stirring was stopped and the pressure was released until the reactor was adjusted to the atmospheric pressure. By the end of the reaction, 139 g of the aqueous surfactant solution B was charged. The contents were collected from the reactor and cooled so that the paraffin wax was separated, whereby a PTFE aqueous dispersion C was obtained.

[1271] The solid content in the resulting PTFE aqueous dispersion C was 31.7% by mass, and the average primary particle size was 357 nm.

Synthesis Example 3: Preparation Example of PTFE Aqueous Dispersion Containing Nonionic Surfactant

[1272] To the PTFE aqueous dispersion C obtained in Synthesis Example 2, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc., cloud point 44° C.) was added in an amount equivalent to 10.0% by mass based on PTFE, and gently dispersed with a resin rod to obtain a PTFE aqueous dispersion D containing the nonionic surfactant.

Synthesis Example 4

[1273] In a 200 mL beaker, 100 g of the PTFE aqueous dispersion D obtained in Synthesis Example 3 was placed, 18 g of an anion exchange resin (Amberjet IRA40020H, manufactured by DuPont de Nemours, Inc.) was added thereto, and the mixture was stirred using a stirrer for 30 min at an intensity sufficient to prevent agglomeration. After allowing it to stand still for 3 hours, the anion exchange resin was removed using a mesh to obtain a PTFE aqueous dispersion E.

Example 1

[1274] To the purified PTFE aqueous dispersion E obtained in Synthesis Example 4, an anion exchange resin (PFA694E, manufactured by Purolite Corporation) was further added at the same proportion (18 g) as in Synthesis Example 4, and the same operations were performed to obtain a PTFE aqueous dispersion F.

[1275] To the PTFE aqueous dispersion F, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) was added to 15% by mass/PTFE and the mixture was allowed to stand still for 4 hours at 48° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion G).

[1276] To the concentrated phase (PTFE aqueous dispersion G), a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) and water were added such that the PTFE content was 25% by mass and the nonionic surfactant content was 15% by mass/PTFE, and the mixture was allowed to stand still for 4 hours at 44° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion H).

Example 2

[1277] To the concentrated phase (PTFE aqueous dispersion H) obtained in Example 1, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) and water were added such that the PTFE content was 25% by mass and the nonionic surfactant content was 15% by mass/PTFE, and the mixture was allowed to stand still for 4 hours at 44° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion I).

Example 3

[1278] To the PTFE aqueous dispersion F of Example 1, an anion exchange resin (PFA694E, manufactured by Purolite Corporation) was further added at the same proportion (18 g) as in Synthesis Example 4, and the same operations were performed to obtain a PTFE aqueous dispersion J.

[1279] To the PTFE aqueous dispersion J, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) was added to 15% by mass/PTFE and the mixture was allowed to stand still for 4 hours at 48° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion K).

[1280] To the concentrated phase (PTFE aqueous dispersion K), a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) and water were added such that the PTFE content was 25% by mass and the nonionic surfactant content was 15% by mass/PTFE, and the mixture was allowed to stand still for 4 hours at 44° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion L).

Example 4

[1281] To the concentrated phase (PTFE aqueous dispersion L) obtained in Example 3, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) and water were added such that the PTFE content was 25% by mass and the nonionic surfactant content was 15% by mass/PTFE, and the mixture was allowed to stand still for 4 hours at 44° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion M).

Example 5

[1282] To the PTFE aqueous dispersion F of Example 1, a synthetic adsorbent (Amberlite FPX66, manufactured by DuPont de Nemours, Inc., pore size: 243 A, pore volume: 1.9 cm.sup.3/g, specific surface area: 914 m.sup.2/g) was further added at the same proportion (18 g) as in Synthesis Example 4, and the same operations were performed to obtain a PTFE aqueous dispersion N.

[1283] To the PTFE aqueous dispersion N, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) was added to 15% by mass/PTFE and the mixture was allowed to stand still for 4 hours at 48° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion 0).

[1284] To the PTFE aqueous dispersion 0, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) was added to 15% by mass/PTFE and the mixture was allowed to stand still for 4 hours at 44° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion P).

Example 6

[1285] To the PTFE aqueous dispersion P obtained in Example 5, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) was added to 15% by mass/PTFE and the mixture was allowed to stand still for 4 hours at 44° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion R).

Comparative Example 1

[1286] To the PTFE aqueous dispersion E obtained in Synthesis Example 4, an anion exchange resin (Amberjet IRA4002OH, manufactured by DuPont de Nemours, Inc.) was further added at the same proportion (18 g) as in Synthesis Example 4, and the same operations were performed to obtain a PTFE aqueous dispersion S.

[1287] To the PTFE aqueous dispersion S, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) was added to 15% by mass/PTFE and the mixture was allowed to stand still for 4 hours at 48° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion T).

Comparative Example 2

[1288] To the PTFE aqueous dispersion T of Comparative Example 1, a nonionic surfactant (T-Det A138, manufactured by Hacros Chemicals Inc.) and water were added such that the PTFE content was 25% by mass and the nonionic surfactant content was 15% by mass/PTFE, and the mixture was allowed to stand still for 4 hours at 48° C. Then, the mixture was separated into two phases: a supernatant phase substantially free from PTFE and a concentrated phase. The supernatant phase was removed to obtain the concentrated phase (PTFE aqueous dispersion U)

TABLE-US-00008 TABLE 7 indicates data missing or illegible when filed

[1289] The quantification limit was 86 ppb for the aqueous dispersion. In the present disclosure, “E” in the table represents an exponent. For example, the description of “2.8E+03” means 2.8×10.sup.3.