Method of making fluoropolymer dispersion

Abstract

The invention pertains to a method for making a fluoropolymer dispersion, said method comprising an aqueous emulsion polymerization of one or more fluorinated monomers wherein said aqueous emulsion polymerization is carried out in an aqueous medium comprising a surfactant mixture [mixture (S)] comprising: at least one perfluorohexanoic acid or salt thereof [surfactant (C6)], in an amount of 1 to 5 g/I, with respect to said aqueous medium; and at least one linear bifunctional perfluoropolyether surfactant [surfactant (PFPE)] complying with formula (I) here below: XpOOC—CF.sub.2-0-(CF.sub.20) .sub.n(CF.sub.2CF.sub.20).sub.m-CF.sub.2—COOXp (I) wherein: Xp, equal to or different from each other, is a hydrogen atom, a monovalent metal, preferably an alkaline metal, or an ammonium group of formula —N(R′.sub.n).sub.4, wherein R′.sub.n, equal or different at each occurrence, is a hydrogen atom or a C.sub.1-C.sub.6hydrocarbon group, preferably an alkyl group; n′ and m′ are independently integers >0 such that the number average molecular weight of the surfactant (PFPE) is of 500 to 2500, said surfactant (PFPE) being used in an amount of 0.005 to 0.5 g/I, with respect to said aqueous medium.

Claims

1. A method for making a fluoropolymer dispersion, said method comprising an aqueous emulsion polymerization of tetrafluoroethylene, optionally in combination with one or more perfluoromonomers selected from the group consisting of perfluoroolefins having 3 to 8 carbon atoms and perfluoroalkyl(oxy)vinylethers, wherein said aqueous emulsion polymerization is carried out in an aqueous medium comprising a surfactant mixture (S), wherein mixture (S) consists of: at least one surfactant (C6), wherein surfactant (C6) is at least one perfluorohexanoate acid or salt, in an amount of 1 to 5 g/l, with respect to said aqueous medium; and at least one surfactant (PFPE), wherein surfactant (PFPE) is at least one linear bifunctional perfluoropolyether surfactant complying with formula (I) here below:
X.sub.pOOC—CF.sub.2—O—(CF.sub.2O).sub.n′(CF.sub.2CF.sub.2O).sub.m′—CF.sub.2—COOX.sub.p   (I) wherein: X.sub.p, equal to or different from each other, is a hydrogen atom, a monovalent metal, or an ammonium group of formula —N(R′.sub.n).sub.4, wherein R′.sub.n, equal or different at each occurrence, is a hydrogen atom or a C.sub.1-C.sub.6 hydrocarbon group; n′ and m′ are independently integers >0 such that the number average molecular weight of the surfactant (PFPE) is of 500 to 2500, said surfactant (PFPE) being used in an amount of at least 0.01 g/l and/or of at most 0.25 g/l, with respect to said aqueous medium.

2. The method of claim 1, wherein the mixture (S) comprises at least one perfluorohexanoic acid salt selected from the group consisting of sodium salt, potassium salt, ammonium salt.

3. The method of claim 1, wherein the amount of surfactant (C6) is of at least 1.5 g/l, and/or of at most 5 g/l, with respect to the aqueous medium.

4. The method of claim 1, wherein the aqueous emulsion polymerization is carried out at a temperature between 10 to 150° C. and/or at a pressure of between 2 and 50 bar.

5. The method of claim 1, wherein the amount of solids in the fluoropolymer dispersion directly resulting from the polymerization is between 3% by weight and about 40% by weight.

6. The method of claim 1, wherein the particle size (volume average diameter) of the fluoropolymer is between 40 nm and 400 nm.

7. The method of claim 1, wherein X.sub.p, equal to or differnt from each other, is a hydrogen atom, an alkaline metal, a monovalent metal, or an ammonium group of formula —N(R′.sub.n).sub.4, wherein R′.sub.n, equal or different at each occurrence, is a hydrogen atom or an alkyl group.

8. The method of claim 2, wherein mixture (S) comprises the ammonium salt of perfluorohexanoic acid.

9. The method of claim 3, wherein the amount of surfactant (C6) is of at least 2 g/l and/or of at most 4.8 g/l, with respect to the aqueous medium.

10. The method of claim 3, wherein the amount of surfactant (C6) is of at least 2.5 g/l and/or of at most 4.5 g/l, with respect to the aqueous medium.

11. The method of claim 1, wherein the amount of surfactant (PFPE) is of at least 0.02 g/l and/or of at most 0.1 g/l, with respect to the aqueous medium.

12. The method of claim 4, wherein the aqueous emulsion polymerization is carried out at a temperature of between 20° C. and 130° C. and/or at a pressure of between 5 and 35 bar.

Description

EXAMPLES

(1) Raw materials:

(2) Perfluorohexanoic acid ammonium salt, commercially available as BAOFLON 6A from Shanghai Shenglei (C6 surfactant, herein below) was used as received.

(3) Krytox® 157 FSL commercially available from DuPont™ is a monofunctional branched PFPE based on a chain comprising recurring units derived from hexfluoropropylene oxide combined with one carboxylic acid end group (Krytox®, herein below).

(4) A difunctional linear PFPE of formula HOOC—CF.sub.2—O—(CF.sub.2O).sub.n′(CF.sub.2CF.sub.2O).sub.m′—CF.sub.2—COOH, having averaged molecular weight of 1600 (Z-DIAC, herein after) was used.

Example 1C

C6 Surfactant Alone

(5) A polymerization reactor with a total volume of 90 l equipped with an impeller agitator was charged with 52 l deionized water. The oxygen free reactor was heated up to 69° C. and the agitation system was set to 48 rpm. The reactor was charged with 1 kg of paraffin wax, a water solution containing 235 g of C6 surfactant and with TFE to a pressure of 20 barg. The polymerization was initiated by 130 mg of ammonium peroxodisulfate (NH.sub.4).sub.2S.sub.2O.sub.8 (APS) and 2600 mg of disuccinic acid peroxide (DSAP) in water solutions. As the reaction started, the reaction pressure of 20 barg was maintained by the feeding of TFE into the gas phase. The reaction temperature was increased until 85° C. with a rate of 0.25° C./min. After the feeding of 12 kg of TFE the monomer inlet valves were closed and the stirring stopped. The reactor was depressurized, vented and cooled. The so obtained polymer dispersion was instable and high amount of coagulum was detected, totalling about 49% of converted TFE. The measured polymer in latex was found to be 9.4% w/w whereas the theoretical polymer in latex should have been 18% w/w. The latex particle diameter was 280 nm according to the Laser Light Scattering (LLS) and using DSC analysis the heat of second fusion was 27.3 J/g.

Examples 2C and 3C

(6) Same polymerization procedure as above detailed for Example 1 was repeated, except that varying the amount of TFE fed and converted, and the reaction times. Details are summarized in Table 1.

Examples 4 to 6

C6 Surfactant+ZDiac

(7) Polymerization procedure similar to Example 1 was repeated but using instead of the water solution containing C6 surfactant alone, a water solution including both C6 surfactant and ZDiac, in concentrations and with the amounts of TFE and reaction times as described in Table 1.

Examples 7C to 9C

C6 Surfactant+Krytox®

(8) Polymerization procedure similar to Example 1 was repeated but using instead of the water solution containing C6 surfactant alone, a water solution including both C6 surfactant and Krytox®, in concentrations and with the amounts of TFE and reaction times as described in Table 1.

(9) TABLE-US-00001 TABLE 1 Reaction C6 Z-DIAC Krytox TFE time Run (g) (g/l) (g) (g/l) (g) (g/l) (kg) (min) 1C 4.50 — — — — 12 90 2C 4.50 — — — — 11 120 3C 4.50 — — — — 15 90 4 231.4 4.45 2.6 0.05 — — 22.7 70 5 179.4 3.45 2.6 0.05 — — 19 85 6 226.2 4.35 1.3  0.025 — — 20 79 7C 231.4 4.45 — — 2.6 0.05 22.5 85 8C 179.4 3.45 — — 2.6 0.05 19 100 9C 226.2 4.35 — — 1.3 0.025 20 105

(10) TABLE-US-00002 TABLE 2 Polymer content ΔH 2.sup.nd P* in latex (% wt) Coagulum APS fusion Run (kg/min) measured theoretical (% wt) (nm) (J/g) 1C 0.13 9 18 49 280 27.3 2C 0.09 9 17 48 n.d. n.d. 3C 0.17 10 22 56 296 26.3 4 0.32 30 30 0 187 33.9 5 0.22 20 27 25 183 29.9 6 0.25 22 28 22 213 29.0 7C 0.26 30 30 0 192 33.2 8C 0.19 4 27 86 330 34.4 9C 0.19 16 28 41 199 30.0 P*: average polymerization rate expressed as ratio among the converted TFE and overall reaction time.

(11) Data provided in above table well demonstrate that perfluorohexanoate surfactant alone is unable to effectively stabilize fluoropolymers during polymerization, hence leading to a substantial amount of coagulum and very poor solids content in latex.

(12) The addition of a linear difunctional PFPE compound is such to enable maximizing polymerization rate while avoiding coagulation and/or build-up on reactor walls at higher concentration of surfactant (C6). When decreasing the amount of surfactant (C6), the combination with linear difunctional PFPE is more effective in minimizing coagulum formation and achieving a stable dispersion.