WO2021149022A1

WO2021149022A1 - Process for polymerizing fluoromonomers using a combination of fluorinated and non-fluorinated surfactant

Info

Publication number: WO2021149022A1
Application number: PCT/IB2021/050545
Authority: WO
Inventors: Rajeev Chauhan; Navin SONI; Gaurav Kumar; Subbarao PEDDINTI; Biswajit Bhattacharya; Munish Mehta; Anamika DUTTA
Original assignee: Gujarat Fluorochemicals Limited
Priority date: 2020-01-24
Filing date: 2021-01-25
Publication date: 2021-07-29

Abstract

The present invention relates to a process for polymerizing a fluoromonomer in an aqueous dispersion medium, comprising: (a) forming an aqueous solution comprising a first surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated sulfonate type hydrocarbon surfactant in a polymerization reactor; (b) pressurizing the reactor with fluoromonomers to form an aqueous emulsion; (c) initiating polymerization reaction of said fluoromonomers, using suitable initiator combination; (d) propogation of polymerization reaction, wherein a second surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated sulfonate type hydrocarbon surfactant is metered into the reactor; and (e) termination of the reaction after consumption of desired quantity of fluoromonomers; wherein the proportions of the surfactants in the first surfactant combination is same or different from the second surfactant combination.

Description

PROCESS FOR POLYMERIZING FLUOROMONOMERS USING A COMBINATION OF FLUORINATED AND NON-FLUORINATED SURFACTANT

FIELD OF THE INVENTION

The present invention pertains to a method for polymerizing fluoromonomers using a combination of surfactants. More particularly, the present invention relates to a process for polymerizing fluoromonomers using a combination of a fluorinated and a non fluorinated surfactant.

BACKGROUND OF THE INVENTION

Fluoropolymers represent a class of materials exhibiting extreme chemical resistance and favorable dielectric properties. Consequently, there is an ever-increasing demand for these materials from industries engaged in manufacturing coatings, tapes and tubing, architectural fabric, nonstick and industrial coatings, fluroelastomer hoses for auto industry and sealing, gaskets and liners for chemical industry, insulation wires and cables, lubricants and so forth. This increasing demand in turn is driving a renewed interest in developing environmental friendly and more efficient routes for manufacturing fluoropolymers. Fluoropolymers are typically synthesized from alkenes in which one or more hydrogen atoms have been replaced by fluorine atom. These include, tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), hexa fluopropylene (HFP), polypropyl vinyl ether (PPVE), polymethyl vinyl ether (PMVE), vinylidene fluoride (VDF), vinylfluoride (VF), etc. Polymerization of the aforesaid monomers affords the corresponding polymers, viz., polytetrafluoroethylene (PTFE), per fluoro alkoxy ether (PFA) polymer, fluorinated ethylene propylene (FEP) polymer, polychlorotrifluoroethylene (PCTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), Fluroelastomers etc.

Fluoropolymers are primarily manufactured via heterogeneous polymerization reactions including aqueous systems. Generally, the reaction requires monomers and a radical initiator in a suitable aqueous reaction medium. Aqueous dispersion polymerization of fluorine containing monomers typically requires a surfactant capable of emulsifying both the reactants and the reaction products for the duration of the polymerization reaction. As discussed below, the surfactant of choice in the synthesis of fluoropolymers is generally a perfluorinated surfactant or a partially fluorinated surfactant. The most frequently used perfluorinated surfactant in the production of fluoropolymers and fluoroelastomers is a Perfluorooctanoic acid (PFOA) salt.

Although, Pefluorosurfactants are better in lowering the surface tension of water than comparable hydrocarbon surfactants, fluorinated surfactants persist in the environment for a longer duration and have been detected in humans and wildlife. Annexure-XVII to REACH, Entry 68, by the European Chemicals Agency, places restrictions on the manufacture, placing on the market and use of certain dangerous substances, mixtures and articles containing Perfluorooctanoic acid (PFOA) and its salts. Further, according to the document there are also restrictions on any related substance (including its salts and polymers) having a linear or branched perfluoroheptyl group with the formula C7F15- directly attached to another carbon atom, as one of the structural elements. Also, the use of any related substance (including its salts and polymers) having a linear or branched perfluorooctyl group with the formula CsFn- as one of the structural elements is restricted. According to the document, the aforementioned, shall not be manufactured, or placed on the market as substances on their own from 4 July 2020. Further, they shall not be used in the production of, or placed on the market in: (a) another substance, as a constituent; (b) a mixture; (c) an article, in a concentration equal to or above 25 ppb of PFOA including its salts or 1000 ppb of one or a combination of PFOA-related substances. Hence, in view of REACH 2020 guidelines of the European Chemicals Agency, there is a need for a process for polymerization of fluoromonomers, which does not involve the use of fluorinated surfactants.

In prior art US9255164B2, A process is provided for the polymerization of fluoromonomerto an dispersion of fluoropolymer particles in an aqueous medium in a polymerization reactor, by (a) providing theaqueous medium in the reactor, (b) adding the fluoromonomer to the reactor, (c) adding initiator to the aqueous medium, the combination of steps (b) and (c) being carried out essentially free of hydrocarbon- containing Surfactant and resulting in the kickoff of the polymerization of the fluoromonomer, and (d) metering hydrocarbon-containing Surfactant into the aqueous medium after the kickoff of polymerization, e.g. after the concentration of the fluoropolymer in the aqueous medium is at least 0.6 wt %, the metering being at a rate reducing the telogenic activity of said Surfactant while maintaining Surface activity.

Another patent WO201917238B2 discloses a method for producing a fluoropolymer, which is capable of reducing the content of impurities. The present invention is a method for producing a fluoropolymer, which is characterized by comprising a polymerization step wherein a fluoropolymer is obtained by carrying out polymerization of a fluoromonomer in an aqueous medium in the presence of a surfactant, and which is also characterized in that the surfactant is a carboxylic acid type hydrocarbon -containing surfactant.

In prior art there were carboxylic acid type surfactant and other type of surfactant, there was a need to establish use of a non fluorinated sulfonate type hydrocarbon containing surfactant along with fluorinated surfactant in polymerisation process.

A process for the polymerization of fluoromonomers using a non-fluorinated surfactant would solve the aforestated issues of persistence in the eco-system, bio- accumulation of fluorosurf actants. However, the exclusive use of non-fluorinated surfactants in polymerization reaction results in inhibition of the reaction and formation of fluoropolymers with low molecular weights. Moreover, the exclusive use of non- fluorinated surfactants might prevent kickoff of the polymerization reaction or inhibit the rate of the polymerization reaction after kickoff. Degradation of the surfactant prior to kickoff of the polymerization reaction might prevent inhibition of the polymerization reaction due to the exclusive use of non-fluorinated surfactant. Degradation of the surfactant using a suitable degradation agent, leads to reduction or elimination of telogenecity. Telogenecity in effect leads to inhibition of the polymerization reaction.

However, a facile process for polymerization, which does not involve the use of degradation agents for passivating the surfactants is highly desirable for reducing costs, time duration and complexity of the polymerization process, even if it requires addition of a small amount of fluorinated surfactant.

Consequently, there is a need to explore a process for polymerizing fluoromonomers to produce fluoropolymers having low to high molecular weights, using a combination of fluorinated and non-fluorinated surfactants, which is devoid of passivating the surfactant.

OBJECTIVES OF THE INVENTION: The main objective of the invention is to overcome the aforestated problems in the prior art.

The other objective of the present invention is to provide a process for the aqueous dispersion polymerization of fluoromonomers using a combination of fluorinated and non-fluorinated surfactants.

Yet another objective of the present invention is to provide a process for the aqueous dispersion polymerization of tetrafluoroethylene and other monomers using a combination of fluorinated perfluorobutanesulfonic acid and non-fluorinate Sulfonate type hydrocarbon containing surfactants.

It is yet another objective of the invention to provide a simplified one step process for the preparation of fluoropolymers.

It is another objective of the invention to provide a process for preparing fluoropolymers, which is devoid of the step of passivating the surfactants. It is another objective of the invention to provide a process for preparing fluoropolymers, with optimum particle size.

Yet another objective of the present invention is to provide a fluoropolymer dispersion comprising a combination of fluorinated perfluorobutanesulfonic acid and non- fluorinated sulfonate type hydrocarbon containing surfactants. Yet another objective of the present invention is to produce low to high molecular weight fluoropolymers using a combination of fluorinated and non -fluorinated surfactants.

It is another objective of the present invention to provide a fluoropolymer resin obtained by aqueous polymerization using a combination of fluorinated perfluorobutanesulfonic acid and non-fluorinated sulfonate type hydrocarbon as surfactants.

SUMMARY OF THE INVENTION

The present invention relates to a process for preparing high molecular weight fluoropolymers using a combination of fluorinated Perfluorobutanesulfonic acid and non-fluorinated sulfonate type hydrocarbon containing surfactants.

In accordance with an embodiment of the invention, there is provided a process for polymerizing a fluoromonomer in an aqueous dispersion medium, comprising:

(a) forming an aqueous solution comprising a first surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non- fluorinated sulfonate type hydrocarbon surfactantin a polymerization reactor; (b) pressurizing the reactor with fluoromonomer to form an aqueous emulsion;

(c) initiating polymerization reaction of said fluoromonomer using suitable initiator combination;

(d) propogation of polymerization reaction, wherein a second surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated sulfonate type hydrocarbon containing surfactant is metered or one-shot dosing done into the reactor and

(e) termination of the reaction after consumption of desired quantity of fluoromonomers ; wherein the sulfonate type hydrocarbon comprises 18 to 36 carbon atoms, wherein the molecular weight of the fluoropolymer ranges from l x 10 to 9 x 10 , wherein said process is devoid of passivating the surfactant, and wherein the proportions of the surfactants in the first surfactants combination is same or different from the second surfactants combination. In accordance with an embodiment, the first surfactants combination is added in one shot into the reactor and the second surfactants combination is metered or one-shot dosing done into the polymerization reactor during the polymerization reaction at a pre determined rate.

Preferably, the sulfonate type hydrocarbon surfactant has a structure represented by Formula 1 ; and Perfluorobutanesulfonic acid or a salt thereof is represented by Formula 2;

Rl- [ArnXn-l] - (S03M₊)m

Formula 1

Formula 2

Wherein;

R being an alkyl group consisting of a number of 2 to 20 carbon atoms;

1 being an alkyl group varyimg from integer from 1 to 2

Ar being an aryl group; n being an integer ranging from 1 to 2;

X being a bridging between aryl groups Ar, wherein the bridging is CH2 or linkage by either an ether or an amine function or N (C2H5)3 or carbonyl group (C=0) M+ being a monovalent cation consisting of hydrogen, an alkali metal, NH4+ or combinations thereof; and m being an integer ranging from 1 to 2.

In accordance with another embodiment, the aqueous emulsion comprises an initiator, for initiating the polymerization process, selected from the group consisting of Disuccinic Acid Peroxide (DSAP), Ammonium Persulphate (APS), redox initiators and combinations thereof.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

Figure 1: Is a flow diagram illustrating process steps in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Discussed below are some representative embodiments of the present invention. The invention in its broader aspects is not limited to the specific details and representative methods. An illustrative example is described in this section in connection with the embodiments and methods provided.

It is to be noted that, as used in the specification, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a composition containing “a compound” includes a mixture of two or more compounds. It should also be noted that the term "‘or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.

The expression of various quantities in terms of “%” or “% w/w” means the percentage by weight of the total solution or composition unless otherwise specified. The present invention, in all its aspects, is described in detail as follows:

As discussed above, Perfluoroalkylated substances (PFAS), such as Perfluorinated surfactants, for example PFOA, are banned under REACH 2020 guidelines of the European Chemicals Agency. Non-fluorinated surfactants on the other hand inhibit the polymerization reaction and afford low molecular weight fluoropolymers, necessitating passivation of the surfactants using degradation agents. Processes that minimize the use of perfluorinated surfactants in the polymerization of fluoromonomers, without adding complex reaction steps are therefore desirable. In view of the aforesaid challenges, the inventors of the present invention have developed a novel process for preparing fluoromonomers of low to high molecular weight, utilizing a combination of fluorinated and non-fluorinated surfactants, said process comprising the steps of:

(a) forming an aqueous solution comprising a first surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non- fluorinated hydrocarbon containing sulfonate type surfactant in a polymerization reactor; (b) pressurizing the reactor with fluoromonomer to form an aqueous emulsion;

(c) initiating polymerization reaction of said fluoromonomer, using suitable initiator combination; (e) propogation of polymerization reaction, wherein a second surfactants combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated Alkyldiphenyloxide disulfonate salt is metered or one- shot dosing done into the reactor; and (f) termination of the reaction after consumption of desired quantity of fluoromonomers; wherein the non fluorinated, hydrocarbon containing sulfonate type surfactant comprises 18 to 36 carbon atoms, wherein the molecular weight of the fluoropolymer ranges from 1 x 10 to 9 x 10 , wherein said process is devoid of passivating the surfactant, and wherein the proportions of the surfactants in the first surfactants combination is same or different from the second surfactant combination.

The aqueous emulsion formed in the present invention comprises surfactants, fluoromonomers, initiators, sodium sulfite and optionally paraffin wax.

Surfactant

The term “surfactant” means a type of molecule which has both hydrophobic and hydrophilic portions, which allows it to stabilize and disperse hydrophobic molecules and aggregates of hydrophobic molecules in aqueous systems. A preferred group of surfactants for the polymerization of fluoromonomers according to the embodiments of the present invention include a first surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated hydrocarbon containing sulfonate type surfactant and a second surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated hydrocarbon containing sulfonate type surfactant, wherein the non fluorinated, hydrocarbon containing sulfonate type surfactant comprises 18 to 36 carbon atoms. Preferably, the first surfactant combination is added in one shot into the reactor and the second surfactant combination is metered into the polymerization reactor during the polymerization reaction at a pre- determined rate. Preferably, the first surfactant combination is added into the polymerization reactor in one shot prior to kickoff of the polymerization reaction, and the second surfactant combination is metered into the polymerization reactor after consumption of predetermined amount of fluoromonomer. More preferably, the non fluorinated, hydrocarbon containing sulfonate type surfactant has a structure represented by Formula 1; and Perfluorobutanesulfonic acid or a salt thereof is represented by Formula 2;

Ri- [ArnXn-1] - (S03M₊)m

Formula 1

Formula 2

Wherein; R being an alkyl group consisting of a number of 2 to 20 carbon atoms; 1 being an alkyl group varyimg from integer from 1 to 2

Ar being an aryl group; n being an integer ranging from 1 to 2;

X being a bridging between aryl groups Ar, wherein the bridging is CH2 or linkage by either an ether or an amine function or N (C2H5)3 or carbonyl group (C=0)

M+ being a monovalent cation consisting of hydrogen, an alkali metal, NH4+ or combinations thereof; and rn being an integer ranging from 1 to 2.

In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (STR1) represented by Formula 1;

#STR1

Wherein R is an alkyl group and M is a monovalent cation selected from the group consisting of hydrogen ions, alkali metal ions and ammonium ions. Preferably, R may be a branched alkyl group or a linear alkyl group. Preferably, M is selected from the group consisting of potassium, sodium and ammonium.

In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (#STR 2) represented by Formula 1 ;

#STR2

Wherein R being an alkyl group consisting of a number of 2 to 20 carbon atoms;

In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (#STR 3) represented by Formula 1

#STR3

Wherein R being an alkyl group consisting of a number of 2 to 20 carbon atoms.

In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (#STR 4) represented by Formula 1 ;

Wherein R being an alkyl group consisting of a number of 2 to 20 carbon ato s.

In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (#STR 5) represented by Formula 1 ;

#STR 5

In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (#STR 6) represented by Formula 1 ;

#STR 6

Wherein R being an alkyl group consisting of a number of 2 to 20 carbon atoms In an embodiment, a non-fluorinated, hydrocarbon containing, sulfonate type surfactant in accordance with an embodiment has a structure (#STR 7) represented by formulal

Wherein R being an alkyl group consisting of a number of 2 to 20 carbon atoms.

Fluoromonomers

The term “fluoromonomer” or the expression “fluorinated monomer” means a polymerizable alkene which contains at least one fluorine atom, fluoroalkyl group, or fluoroalkoxy group attached to the double bond of the alkene that undergoes polymerization. The term “fluoropolymer” and fluoroelastomers means a polymer or elastomer formed by the polymerization of at least one fluoromomer, and it is inclusive of homopolymers, copolymers, terpolymers and higher polymers. Examples of fluoromonomers that can be used in the present invention include but is not limited to tetrafluoroethylene (TFE), Hexa Fluopropylene (HFP), Polypropyl vinyl Ether (PPVE), Polymethyl Vinyl Ether (PMVE), chlorotrifluoroethylene (CTFE), vinylidene fluoride (VDF), vinylfluoride (VF), and so forth, each of which can be used individually or in combination. Preferably, the fluoromonomer is tetrafluoroethylene (TFE) and the fluoropolymer is polytetrafluoroethylene (PTFE). Although, the embodiments of the present invention are described in terms of polymerization of TFE, the process described herein can be applied to the polymerization of any fluoromonomer. The aqueous emulsion further comprises an initiator for initiating the polymerization process.

Initiators

The term “initiator” and the expressions “radical initiator” and “free radical initiator” refer to a chemical that is capable of providing a source of free radicals, either induced spontaneously, or by exposure to heat or light. Examples of suitable initiators include peroxides, peroxy dicarbonates and azo compounds. Initiators may also include reduction-oxidation systems which provide a source of free radicals. The term “radical” and the expression “free radical” refer to a chemical species that contains at least one unpaired electron. The radical initiator is added to the reaction mixture in an amount sufficient to initiate and maintain the polymerization reaction rate. The radical initiator may comprise a persulfate salt, such as sodium persulfate, potassium persulfate, or ammonium persulfate. Alternatively, the radical initiator may comprise a redox system. “Redox system” is understood by a person skilled in the art to mean a system comprising an oxidizing agent, a reducing agent and optionally, a promoter as an electron transfer medium. In a preferred embodiment, the radical initiator is selected from the group consisting of Disuccinic Acid Peroxide (DSAP), Ammonium Persulphate (APS), redox initiator and combinations thereof. Chain transfer agents

Chain transfer agents, also referred to as modifiers or regulators, comprises of at least one chemically weak bond. A chain transfer agent reacts with the free -radical site of a growing polymer chain and halts an increase in chain length. Chain transfer agents are often added during emulsion polymerization to regulate chain length of a polymer to achieve the desired properties in the polymer. Examples of chain transfer agents that can be used in the present invention include, but not limited to, halogen compounds, hydrocarbons in general, aromatic hydrocarbons, thiols (mercaptans), alcohols and so forth; each of which can be used individually or in combination.

Polymerization conditions The temperature for the polymerization reaction may vary, for example, from 15 to

110°C, depending on the initiator system chosen and the reactivity of the fluoromonomer(s) selected. In a preferred embodiment, the polymerization is carried out at a temperature in the range of 65 to 100°C.

The pressure used for polymerization may vary from 2-200 bar, depending on the reaction equipment, the initiator system, and the monomer selection. In a preferred embodiment the reaction is carried out at a pressure in the range of 10 to 60 bar.

The polymerization occurs under stirring or agitation. The stirring may be constant, or may be varied to optimize process conditions during the course of the polymerization. In one embodiment, both multiple stirring speeds and multiple temperatures are used for controlling the reaction. According to an embodiment of the process of the invention, a pressurized polymerization reactor equipped with a stirrer and heat control means is charged with water, preferably deionized water, a first surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated disulfonate type hydrocarbon surfactant in accordance with the invention, and at least one fluoromonomer. Preferably, the first surfactant combination is added in an amount greater than 50 ppm, based on the weight of the aqueous medium, in one shot into the polymerization reactor. The mixture may optionally contain paraffin wax. The reactor is then heated up to the reaction temperature and pressure. Thereafter initiators are added into the reaction vessel to initiate the polymerization reaction. Preferably, the initiator is added in an amount in the range from 70 to 400 ppm, based on the weight of de-ionized water. Prior to introduction of the surfactant, and monomer or monomers into the reaction vessel and commencement of the reaction, air is preferably removed from the reactor in order to obtain an oxygen-free environment for the polymerization reaction. Preferably, the oxygen is removed from the reaction vessel until its concentration is less than 10 ppm. The reactor may also be purged with a neutral gas such as, for example, nitrogen or argon. After initiating the polymerization reaction, the second surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated sulfonate type hydrocarbon surfactant is metered into the polymerization reactor at a rate ranging from 0.02 g/Lh to

0.06 g/Lh. Preferably, the second surfactant combination is added in an amount in the range of 100 to 500 ppm, more preferably in an amount of 250 to 350 ppm, based on the weight of the aqueous medium.

Upon completion of the polymerization reaction, the reactor is brought to ambient temperature and the residual unreacted monomer is vented to atmospheric pressure. The aqueous reaction medium containing the fluoropolymer is then recovered from the reaction vessel. Preferably, the solid content ranges from 10 to 65%, more preferably from 20 to 30% and the particle size of the fluoropolymer particles ranges from 100 to 350 nm.

The present invention is more particularly described in the following example that is intended as illustration only, since numerous modifications and variations within the scope of the present invention will be apparent to those of skill in the art. Unless otherwise noted, all parts, percentages, and ratios reported in the following examples are on a weight basis, and all reagents used in the examples were obtained or are available from the chemical suppliers.

The following examples illustrates the basic methodology and versatility of the present invention.

Example

A 150 L polymerization reactor equipped with a stirrer and heat control means was charged with 70 L of water, preferably deionized water, 3 g of paraffin was, sodium sulfite (0.5 g) and a first surfactant combination of Potassium salt of Perfluorobutanesulfonic acid, and Alkyldiphenyloxide disulfonate salt in a ratio of 95:5

(5 g). Prior to introduction of the surfactant, and monomer or monomers into the reaction vessel and commencement of the reaction, air was removed from the reactor in order to obtain an oxygen-free, oxygen concentration of less than 10 ppm, environment for the polymerization reaction. The reactor was then heated up to the reaction temperature of 80 to 90 ^°C and pressurized to 24 Kg/cm² with tetrafluoroethylene (TFE) monomer. Thereafter, initiators Ammonium persulfate (0.27 g) and disuccinic acid peroxide (90 g) were added into the reaction vessel to initiate the polymerization reaction. After initiating the polymerization reaction, and consumption of 1.4 kg of TFE, the second surfactant combination comprising Potassium salt of Perfluorobutanesulfonic acid and Alkyldiphenyloxide disulfonate salt in a ratio of 10:90 (18.7 g) was metered into the polymerization reactor at a rate ranging from 0.02 g/Lh to 0.06 g/Lh. Upon consumption of 24 kg of TFE in the polymerization reaction, the reactor was brought to ambient temperature and the residual unreacted monomer was vented to atmospheric pressure. The aqueous reaction medium containing the fluoropolymer is then recovered from the reaction vessel. The solid content of the aqueous medium was 25.41% and the latex particle size of the fluoropolymer particles was 181.4 nm.

Example-2

A 150 L polymerization reactor equipped with a stirrer and heat control means was charged with 70 L of water, preferably deionized water, 3 kg of paraffin was, sodium sulfite (0.5 g) , ammonium peroxide sulfate (0.5 g), discussnic peroxide (5g) and a surfactant Potassium salt of Perfluorobutanesulfonic acid (5 g). Prior to introduction of the surfactant, and monomer or monomers into the reaction vessel and commencement of the reaction, air was removed from the reactor in order to obtain an oxygen-free, oxygen concentration of less than 10 ppm, environment for the polymerization reaction. The reactor was then heated up to the reaction temperature of 80 to 90 ^°C and pressurized to 24 Kg/cm² with tetrafluoroethylene (TFE) monomer. Thereafter, initiators Ammonium persulfate (2 g) and disuccinic acid peroxide (110 g) were added into the reaction vessel to initiate the polymerization reaction. After initiating the polymerization reaction, and consumption of 1.3 kg of TFE, the second surfactant combination comprising Potassium salt of Perfluorobutanesulfonic acid and Alkyldiphenyloxide disulfonate salt in a ratio of 05:95 (18 g) was one-shot addition into reactor.

Upon consumption of 24 kg of TFE in the polymerization reaction, the reactor was brought to ambient temperature and the residual unreacted monomer was vented to atmospheric pressure. The aqueous reaction medium containing the fluoropolymer is then recovered from the reaction vessel. The solid content of the aqueous medium was 24.49% and the latex particle size of the fluoropolymer particles was 170 nm.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive.

Claims

We Claims

1. A process for polymerizing a fluoromonomer in an aqueous dispersion medium, comprising:

(a) forming an aqueous solution comprising a first surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non- fluorinated disulfonate type hydrocarbon containing surfactant in a polymerization reactor; (b) pressurizing the reactor with fluoromonomer to form an aqueous emulsion;

(c) initiating polymerization reaction of said fluoromonomer, using suitable initiator combination;

(d) propogation of polymerization reaction, wherein a second surfactant combination of fluorinated Perfluorobutanesulfonic acid or a salt thereof, and non-fluorinated disulfonate type hydrocarbon containing surfactant is metered or one shot dosing done into the reactor; and

(e) termination of the reaction after consumption of desired quantity of fluoromonomers ; wherein the non fluorinated disulfonate type hydrocarbon containing surfactant comprises 18 to 36 carbon atoms, wherein the molecular weight of the fluoropolymer ranges froml x lO to 9 x 10 , wherein said process is devoid of passivating the surfactant, and wherein the proportions of the surfactants in the first surfactant combination is same or different from the second surfactant combination.

2. The process as claimed in claim 1 , wherein the surfactant is added in one shot into the polymerization reactor.

3. The process as claimed in claim 1, wherein the surfactant is metered into the polymerization reactor during the polymerization reaction at a pre-determined rate.

4. The process as claimed in claim 3, wherein the surfactant is metered at a rate of 0.008 g/(L*h) to 0.6 g/(L*h).

5. The process as claimed in claim 1, wherein the non fluorinated disulfonate type hydrocarbon containing surfactant has a structure represented by Formula 1 ; and wherein Perfluorobutanesulfonic acid or a salt thereof is represented by Formula 2;

R1 - [ArnXn-1] - (S03M+)m

Formula 1

Wherein;

R being an alkyl group consisting of a number of 2 to 20 carbon atoms;

1 being an alkyl group varyimg from integer from 1 to 2;

Ar being an aryl group; n being an integer ranging from 1 to 2;

M+ being a monovalent cation consisting of hydrogen, an alkali metal, NH4+ or combinations thereof; and m being an integer ranging from 1 to 2.

5. The process as claimed in claim 1, wherein the aqueous emulsion comprises an initiator, for initiating the polymerization process, selected from the group consisting of Disuccinic Acid Peroxide (DSAP), Ammonium Persulphate (APS), redox initiators and combinations thereof.

6. The process as claimed in cla im 1 , wherein the aqueous emulsion comprises stabilizing agents such as paraffin wax.