METHOD FOR AQUEOUS POLYMERIZATION OF FLUOROMONOMERS USING PERFLUOROBUTANESULFONIC ACID OR SALT THEREOF

FIELD OF THE INVENTION

The present invention pertains to a method for polymerizing fluoromonomers using fluorinated surfactants. More particularly, the present invention relates to a method of aqueous polymerization using perfluorobutaesulfonic acid or salt thereof.

BACKGROUND OF THE INVENTION

Fluoropolymers have immense industrial utility due to their extreme chemical resistance and favorable dielectric properties. They are generally synthesized from alkenes in which one or more hydrogen atoms have been replaced by fluorine atom. The most important members of this class of polymers are polytetrafluoroethylene (PTFE), perfluoroalkoxyether (PFA), fluorinated ethylene propylene (FEP) polychlorotrifluoroethylene (PCTFE), polyvinyl fluoride (PVF), polyvinylidenefluoride (PVDF), Ethylene tetrafluoroethylene (ETFE) and fluoroelastomer (FKM), primarily manufactured via heterogeneous polymerization reactions including aqueous systems. Generally, the reaction requires a monomer and a radical initiator in a suitable aqueous reaction medium. Aqueous 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 perfluorosurfactant or a partially fluorinated surfactant. The most frequently used perfluoroalkylsurfactant in the production of fluoropolymers is ammonium perfluorooctanoate (AFPO).

United States Patent No. 4,864,006 discloses the polymerization of fluorinated monomers in the presence of a perfluoropolyether having neutral end groups, perfluoropolyether oil, which is used in the form of an aqueous microemulsion. The perfluoropolyether oil has molecular weight of at least about 500 and the aqueous microemulsion of the oil is prepared using a suitable surfactant which can be selected from known perfluorinated carboxylic or sulfonic acids or from perfluoropolyethers having one or two acid end groups. The suitable surfactants were the ones consisting of perfluorinated compounds, in particular those having: 6 to 1 1 carbon atoms, of the class of carboxylic and sulphonic acids.

United States Patent No. US 6,395,848 B1 discloses a process comprising polymerizing at least one fluorinated monomer in an aqueous medium containing initiator and dispersing agent to obtain an aqueous dispersion of particles of fluoropolymer, wherein said dispersing agent is a combination of at least two fluorosurfactants, at least one of said fluorosurfactants being perfluoropolyether carboxylic or sulfonic acid or salt thereof, and at least one of said fluorosurfactants being fluoroalkyl carboxylic or sulfonic acid or salt thereof, or fluoroalkoxy aryl sulfonic acid or salt thereof.

The aforestated fluorosurfactants are expensive, specialized material, difficult to synthesize and because of their high stability, persist in the environment for a long time. Their persistence in the environment leads to bio-accumulation in living organisms. Hence, they are now under the watch of environmental and regulatory authorities. Perfluorooctane sulfonate (PFOS) was added to list of chemicals under the Stockholm Convention on persistent organic pollutants in 2009, and almost all use of PFOS is banned in Europe, with some exemptions. Final regulations have not yet been promulgated for polyfluoroalkyl substances (PFAS); current criteria for PFAS are typically in the form of guidance or advisory levels. According to the advisory, any substance may not contain

PFOS above the limit of 0.001 % by weight, EU 757/2010. In the U.S., PFOS manufacturing was voluntarily phased out in 2002.

According to United States publication No. US 20170198070 A1 , because of environmental concerns with regard to perfluorooctanoic acid and salts and perfluorooctane sulfonate (PFOS), there is interest in reducing or eliminating the aforestated in fluoropolymer polymerization processes. For uses such as in stain repellent materials, fluorosurfactants with short hydrophobic chain lengths, e.g., perfluorobutane sulfonate, have been used to replace perfluorooctane sulfonate. Consequently, there is a need in the art for a process for polymerization of fluoromonomers which requires reduced quantity of fluorosurfactant, without affecting the particle size of the fluoropolymers. Further, the use of long chain fluorosurfactants in the polymerization of fluoromonomers suffers from drawbacks such as toxicity, bio-accumulation, cost, discolouration of resulting fluoropolymers, etc. Flence, there is a need in the art to explore the use of perfluorobutane sulfonic acid and salts thereof as a surfactant in the aqueous polymerization of fluoromonomers to overcome aforestated drawbacks.

OBJECTIVES OF THE INVENTION:

The objective of the present invention is to provide a process for the aqueous polymerization of fluoromonomers using short chain fluorinated surfactant.

Yet another objective of the present invention is to provide a process for the aqueous polymerization of fluoromonomers using short chain sulfonic acid surfactant or salt thereof.

Yet another objective of the present invention is to provide a process for the aqueous polymerization of fluoromonomers using Perfluorobutanesulfonic acid or salt thereof in reduced quantity, without affecting the fluoropolymer particle size.

Yet another objective of the present invention is to provide a fluoropolymer dispersion comprising Perfluorobutanesulfonic acid or salt thereof.

It is another objective of the present invention to provide a fluoropolymer resin obtained by aqueous polymerization using Perfluorobutanesulfonic acid or salt thereof.

SUMMARY OF THE INVENTION

The present invention relates to a process for aqueous polymerization of fluoromonomers using short chain fluorinated surfactants, particularly Perfluorobutanesulfonic acid or salt thereof.

In accordance with an embodiment of the invention, there is provided a process for preparing a fluoropolymer in an aqueous medium, comprising the steps of:

(a) forming an aqueous emulsion comprising a fluoromonomer, and Perfluorobutanesulfonic acid of Formula I or a salt thereof in a reactor; and

Formula I

(b) initiating polymerization of said fluoromonomer using an initiator and agitating the reaction mixture;

wherein the process requires lower amount of fluorosurfactant.

The process optionally comprises coagulating, on completion of the polymerization reaction in the presence of at least one coagulating agent selected from the group comprising of inorganic salts, mineral acids, or organic compounds. The process also optionally comprises adding chain transfer agents in step (a).

Above step (a) comprises the steps of:

i. adding deionized water and optionally paraffin wax into the reactor; ii. adding Perfluorobutanesulfonic acid or salt thereof in one shot or in multiple steps into the reactor; and

iii. adding fluoromonomer into the reactor and agitating the reaction mixture.

In step (b) polymerization of said fluoromonomer is initiated by an initiator, and wherein the initiator is added in one shot or in multiple steps into the reactor.

In an embodiment, the cation of the salt form of perfluorobutanesulfonic acid is selected from the group comprising of potassium, sodium or ammonium.

In another embodiment, the initiator is selected from the group consisting of a water soluble radical initiator, a water soluble oxidation-reduction catalyst or an oil soluble radical polymerization initiator. Preferably, the initiator is selected from the group consisting of organic peroxides such as azo compounds, for example, azobisisobutyronitrile (AIBN), disuccinic acid peroxide; inorganic

peroxides, for example, Ammonium Persulphate (APS), potassium persulfate, and combinations thereof.

In yet another embodiment of the invention, the reaction temperature is in the range of 50 to 150 °C, preferably 60 to 120 °C, and more preferably 65 to 80 °C.

Further, the reaction pressure ranges from 10 to 80 bar, and preferably 10 to 60 bar. In a preferred embodiment the reaction pressure is 10 to 50 bar. The reaction mixture is preferably agitated at between 20 to 300 rpm.

In accordance with an embodiment of the invention, the concentration of the surfactant in the reaction mixture ranges from 2000 to 16000 ppm, and preferably 4000 to 1 1000 ppm based on the weight of the aqueous medium.

In accordance with another embodiment, the concentration of the initiator ranges from 2 to 2000 ppm, and preferably from 4 to 500 ppm, based on the weight of the aqueous medium.

In accordance with yet another embodiment of the invention, the solid content of the fluoropolymer ranges from 7 to 40%, and wherein the particle size of the fluoropolymer ranges from 80 nm to 500 nm.

Preferably, the fluoromonomer useful in the present invention is selected from the group comprising of tetrafluoroethylene, chlorotrifluoroethylene, vinyl fluoride, vinylidene fluoride, hexafluoropropylene, perfluoro (alkyl vinyl ether), perfluoropropylvinylether, perfluorobutylethylene and combinations thereof. Preferably, the fluoropolymer is any one of polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP), perfluorealkoxyalkane (PFA), Poly vinyl fluoride (PVF), polychlorotrifluoroethylene (PCTFE) and fluoroelastomer (FKM).

In accordance with an embodiment, the mineral acid used as a coagulating agent is any one of hydrochloric acid, nitric acid, sulphuric acid or combinations thereof. Alternatively, an organic compound selected from ethanol, methanol, acetone, succinic acid, chloromethanes or combinations thereof can be used as a coagulating agent. The coagulating agent may also be an inorganic salt, for example, ammonium carbonate, ammonium hydroxide, sodium chloride or combinations thereof. Preferably, the coagulating agent is used in an amount of less than or equal to 0.35 parts by 100 parts by mass of fluoropolymer.

In accordance with another embodiment, the chain transfer agent is one of halogen compounds, hydrocarbons, aromatic hydrocarbons, thiols (mercaptans), alcohols, esters of organic acids or combinations thereof. Preferably, the chain transfer agents are present in an amount in the range of 5 to 6000 ppm based on the weight of aqueous medium.

In accordance with another aspect of the invention the process for preparing fluoropolymers is carried out in the presence of an additional non-fluorinated monomer - ethylene, resulting in the fluoropolymer ethylene tetrafluoroethylene (ETFE).

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

Figure 1 is a flowchart of the process of polymerization of fluoromonomers using Perfluorobutanesulfonic acid or salt thereof.

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. Illustrative examples are 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.

All cited references are incorporated herein by reference in their entireties. Citation of any reference is not an admission regarding any determination as to its availability as prior art to the claimed invention.

The present invention, in all its aspects, is described in detail as follows:

The present invention relates to a process for preparing a fluoropolymer in an aqueous medium, comprising:

(a) forming an aqueous emulsion comprising a fluoromonomer and Perfluorobutanesulfonic acid of Formula I or salt thereof as surfactant in a reactor; and

(b) initiating polymerization of said fluoromonomer using an initiator and agitating the reaction mixture;

wherein the process requires lower amount of fluorosurfactant.

The process optionally comprises coagulating, on completion of the polymerization reaction in the presence of at least one coagulating agent selected from the group comprising of inorganic salts, mineral acids, or organic compounds. The process also optionally comprises adding chain transfer agents in step (a).

The above process is useful for preparing fluoropolymers such as polytetrafluoroethylene (PTFE), aqueous PTFE, fine particle PTFE, fluorinated ethylene polymer (FEP), perfluoroalkoxy (PFA) polymer, polychlorotrifluoroethylene (PCTFE), polyvinyl fluoride (PVF) and polyvinylidene fluoride (PVDF), Poly-perfluoroalkyvinylethers, Poly-perfluoropropylvinylethers (PPVE), Ethylene tetrafluoroethylene (ETFE) from their corresponding monomers. ETFE can be manufactured by using the non-fluorinated monomer -ethylene along with Tetrafluoroethylene during the polymerization reaction of the present invention.

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. The surfactant used in the present invention is perfluorobutane sulfonic acid (PFBS) or a salt thereof. PFBS is a straight four carbon fluorocarbon chain having a sulfonic acid functional group. Preferably, the surfactant PFBS is used in the form of potassium, sodium or ammonium salt. In a preferred embodiment PFBS is used in the form of Potassium perfluorobutane sulfonate (K-PFBS).

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. Examples of fluoromonomers useful in the present invention include tetrafluoroethylene (TFE), hexafluoropropene (HFP), vinyl fluoride, 1 ,2-difluoroethylene, vinylidene fluoride (VDF), trifluoroethylene (TrFE), pentafluoropropylene, hexafluoroisobutylene, chlorotrifluoroethylene, perfluoroalkylvinyl ethers (PAVE) and so forth, each of which can be used independently or in combination. The term“fluoropolymer” means a polymer formed by the polymerization of at least one fluoromonomer, and it is inclusive of homopolymers, copolymers, terpolymers and higher polymers. Preferably, the fluoromonomer is tetrafluoroethylene (TFE) and the fluoropolymer is polytetrafluoroethylene (PTFE).

Initiators

The aqueous emulsion comprises an initiator for initiating the polymerization process. 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, peroxydicarbonates 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. Preferably, the addition of the initiator into the reaction vessel or reactor is carried out in one shot. Alternatively, the addition of the initiator into the reaction vessel is carried out in multiple steps. 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 alternate embodiments the initiators may be organic initiators, such as Azobisisobutyronitrile (AIBN) or Disuccinic acid peroxide (DSAP). In a preferred embodiment, the radical initiator is selected from the group consisting of organic Peroxides, Ammonium Persulphate (APS), or potassium persulfate 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 is not limited to, halogen compounds, hydrocarbons in general, aromatic hydrocarbons, thiols (mercaptans), alcohols, esters of organic acids and so forth; each of which can be used individually or in combination.

Coagulating agents

In addition to particle growth due to polymerization, coagulation is one of the vital processes that determine the particle size distribution of a product made by emulsion polymerization. Coagulation leads to an increase in the particle size distribution of the polymer from nanometer range to micrometers. Preferably coagulation is carried out till the particle size distribution of the fluoropolymer particles is in the range of 2 to 600 pm. In an embodiment of the invention the coagulation of polymer particles is achieved by using inorganic salts, mineral acids or organic compounds. Examples of mineral acids, that can be used in the present invention include, but is not limited to phosphoric acid, nitric acid, sulphuric acid, hydrochloric acid and so forth, each of which can be used alone or in combination. Alternatively, the coagulating agents are selected from the group of organic compounds, including ethanol, ammonia, urea, methanol, acetone, succinic acid, oxalic acid, chloromethanes, and so forth, whereas inorganic salts useful as coagulating agents include any one of sodium salts, ammonium salts, aluminium salts, ammonium hydroxide, ammonium carbonate, sodium chloride, aluminium sulphate.

Polymerization conditions

The process of the present invention is carried out according to the process flowchart (100) depicted in Figure 1 . The temperature used for polymerization may vary, for example, from 50 to 150 °C, depending on the initiator system chosen and the reactivity of the fluoromonomer(s) selected. Preferably, the polymerization is carried out at a temperature in the range from 60 to 120 °C, more preferably the temperature is 65 to 100 °C.

The pressure used for polymerization may vary from 10 to 80 bar, depending on the reaction equipment, the initiator system, and the monomer selection. Preferably, the pressure in the reaction vessel is maintained in the range of 10 to 60 bar. In a preferred embodiment the polymerization reaction is carried out at a pressure in the range of 10 to 50 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. In a preferred embodiment the agitation of the reaction mixture is carried out at 20 to 300 rotations per minute.

According to one embodiment of the process of the invention in step 104, a pressurized polymerization reactor equipped with a stirrer and heat control

means is charged with water, preferably deionized water, Perfluorobutanesulfonic acid or salt thereof, at least one chain transfer agent, and at least one fluoromonomer. Preferably, Perfluorobutanesulfonic acid or salt thereof is added in an amount in the range from 2000 to 16000 ppm, more preferably from 4000 to 1 1000 ppm, based on the weight of water. Preferably, the surfactant is added in one shot into the reaction vessel in step 106. The cation of the salt form of Perfluorobutanesulfonic acid is selected from the group comprising of potassium, sodium or ammonium. The mixture may optionally contain paraffin wax.

Preferably, the chain transfer agents are also added to the reaction mixture in an amount in the range of 5 to 6000 ppm on the weight of aqueous medium.

The reactor is then heated up to the reaction temperature and pressure is increased by adding the fluoromonomer in step 108. Thereafter, initiators are added into the reaction vessel to initiate the polymerization reaction in step 1 10. Preferably, the initiator is introduced into the reaction vessel in one shot or multiple steps. Preferably, the initiator is added in an amount in the range from 5 to 2000 ppm, more preferably from 10 to 500 ppm, based on the weight of de-ionized water. Prior to introduction of the surfactant and monomer or monomers into the reaction vessel, air is preferably removed from the reactor in order to obtain an essentially oxygen free environment for the polymerization reaction. Preferably, the oxygen is removed from the reaction vessel until its concentration is less than 20 ppm. The reactor may also be purged with a neutral gas such as, for example, nitrogen or argon.

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 7 to 40%, and the particle size of the fluoropolymer particles ranges from 80 to 500 nm.

Thereafter, an optional step, 1 12, of coagulation may be carried out. The coagulating agent may be present in an amount of less than or equal to 0.35 parts by mass per 100 parts by mass of fluoropolymer. Coagulation of the fluoropolymer dispersion may be carried out using any of the coagulation agents listed above. Preferably, coagulation is carried out using any one of nitric acid, hydrochloric acid, sulfuric acid, ammonium carbonate, sodium chloride, and aluminum sulfate.

The present invention is more particularly described in the following examples that are intended as illustrations 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. Potassium salt of Perfluorobutane sulfonic acid used in Examples 1 to 5 was prepared according to the process disclosed in Indian Patent No. 299008.

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

Example No 1

This polymerization reaction was carried out in a 150 liters horizontal batch reactor comprising a six blade impeller. 96 kg of de-ionized water, and 4 kg of paraffin wax were added into a 150 liters capacity reactor. Oxygen was removed from the reactor until its concentration was less than 20 ppm. After that, the surfactant, Potassium salt of Perfluorobutane Sulfonic acid, 4270 ppm, was added in one shot into the reactor. Further, 30 g of Butanedioic acid was added into the reactor vessel. Thereafter, the addition of Tetrafluoroethylene (TFE) into the reactor led to an increase in the pressure to 20 bar and the temperature was increased to 65 to 80 °C. After attaining the above pressure and temperature, a solution comprising an initiator ammonium persulfate (APS), 4.2 ppm, was added into the reactor in one shot for initiating the polymerization process. The reaction mixture was agitated at 50 rpm. The reaction was complete in 165 min, with a solid content of 23.75% and a latex particle size (LPS) of 223 nm. Product characteristics are as follows: Standard specific gravity (SSG) - 2.181 ; Bulk density (BD) (g/L) - 517; Particle Size (Sieve Method) pm - 693.68 (Particle size increases from 223 nm for latex to 693.69 microns in powder due to coagulation process) ; Tensile strength (MPa) -32.06; Elongation - 340.3%; Extrusion Pressure Mpa (400:1 ) -29.67. All the above properties were derived as per guidelines in ASTM D 4895.

Example No 2

This polymerization reaction was carried out in a 150 liters horizontal batch reactor comprising a six blade impeller. 96 kg of de-ionized water and 4 kg of paraffin wax were added into a 150 liters capacity reactor. Oxygen was removed from the reactor until its concentration was less than 20 ppm. After that, the surfactant, Potassium salt of Perfluorobutane Sulfonic acid, 8333 ppm, was added in one shot into the reactor. Further, 32.32 gm of Butanedioic acid was added into the reactor vessel. Thereafter, the addition of Tetrafluoroethylene (TFE) into the reactor led to an increase in the pressure to 22 bar and the temperature was increased to 65-75 °C. After attaining the operating pressure and temperature a solution comprising initiators, [Disuccinic Acid Peroxide (DSAP), 156 ppm and ammonium persulfate (APS) 52 ppm] , was added into the reactor in one shot for initiating the polymerization process. The reaction mixture was agitated at 50 rpm. The reaction was complete in 161 min, with a solid content of 20.51 % and a latex particle size (LPS) of 214 nm. The latex particle size of the polymer was determined by -Dynamic Laser light scattering for analysis of particle size using a Nano particle Analyzer - HORIBA SZ-100. Other product characteristics are as follows: Standard specific gravity (SSG) - 2.179; Bulk density (BD) (g/l) - 450; Particle Size (Sieve Method) pm - 795 (Fine powder particle size has increased due to coagulation); Tensile strength (MPa) -22.51 ; Elongation - 174.3%; Extrusion Pressure Mpa (400:1 ) -25.15. All the above properties were derived as per guidelines in ASTM D 4895.

Example No 3

This polymerization reaction was carried out in a 150 liters horizontal batch reactor comprising a six blade impeller. 83 kg of de-ionized water was added into the 150 liters capacity horizontal reactor. Oxygen was removed from the reactor until its concentration was less than 20 ppm. After that, the surfactant, Potassium salt of Perfluorobutane Sulfonic acid, 8333 ppm, was added in one shot into the reactor. Thereafter, the addition of Tetrafluoroethylene (TFE), Hexafluoroproylene (HFP) and perfluoro(propyl vinyl ether) (PPVE) resulted in an increase in the pressure to 24 bar and the temperature was increased to 80-90 °C. After attaining the above pressure and temperature a solution comprising initiators, [Potassium persulfate (KPS), 392 ppm and ammonium persulfate (APS) 392 ppm], was added into the reactor at high metering rate upto kick-off of polymerization and in low metering rate during propagation phase in polymerization process. The reaction mixture was agitated at 50 rpm. The reaction was complete in 545 min, with a solid content of 27.3% and a latex particle size (LPS) of 318 nm. HFP and PPVE % content in final product were 14.01 and 2.4 respectively. The latex particle size of the polymer was determined by -Dynamic Laser light scattering for analysis of particle size using a Nano particle Analyzer - HORIBA SZ-100.

Example No 4

This polymerization reaction was carried out in a 150 liters horizontal batch reactor comprising a six blade impeller. 100 kg of de-ionized water was added into the 150 liters capacity horizontal reactor. Oxygen was removed from the reactor until its concentration was less than 20 ppm. After that, the surfactant, Potassium salt of Perfluorobutane Sulfonic acid, 6000 ppm, was added in one shot into the reactor. Chain transfer agent, 160 ppm, was added into reactor. Thereafter, the addition of Tetrafluoroethylene (TFE), resulted in an increase in the pressure to 15 bar and the temperature was increased to 70-75 °C. After attaining the above pressure and temperature, a solution comprising an initiator, ammonium persulfate (APS) 130 ppm, was added into the reactor in one shot for initiating the polymerization process. The reaction mixture was agitated at 38 rpm. The reaction was complete in 231 min, with a solid content of 14.88% and a latex particle size (LPS) of 198 nm. The latex particle size of the polymer was determined by -Dynamic Laser light scattering for analysis of particle size using a Nano particle Analyzer - HORIBA SZ-100. Other product properties are as follows: Melt flow index (MFI) (372°C/ 2.16 Kg)- 4.35; Bulk Density - 267.8; Avg. Particle size - 6.12 pm. All the above properties were derived in accordance with ASTM D 4895.

Example No. 5

103 kg of de-ionized water was added into the reactor. Oxygen was removed from the reactor until its concentration was less than 20 ppm. After that, the surfactant, Potassium salt of Perfluorobutane Sulfonic acid, 2360 ppm, was added in one shot into the reactor. Thereafter, the addition of Vinylidene fluororide (VDF) resulted in an increase in the pressure to 45 bar and the temperature was increased to 78-83 °C. After attaining the operating pressure and temperature, a solution comprising an initiator potassium persulfate (KPS), 2912 ppm, was added into the reactor in one shot for initiating the polymerization process. The reaction mixture was agitated at 50 rpm. The

reaction was complete in 240 min, with a solid content of 27.3% and a latex particle size (LPS) of 310 nm. Other product properties are as follows: Melt flow index (MFI) (239°C/ 5 Kg): 25 - 40 g/10min; Melting point: 165 - 172 °C, Bulk Density : 250 - 300 g/L; Elongation %: 140.3; Tensile strength (MPa): 31 .17; Melting (Digital Scanning Calorimetry): 166.7 °C; Enthalpy (J/g): 45.87; Avg. Particle size: 5 - 10 pm. All properties were derived in accordance with ASTM D3222.

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

1 . A process for preparing fluoropolymers in an aqueous medium,

(a) forming an aqueous emulsion comprising a fluoromonomer, and Perfluorobutanesulfonic acid of Formula I or a salt thereof; and

(b) initiating polymerization of said fluoromonomer using an initiator and agitating the reaction mixture;

wherein the process requires lower amount of fluorosurfactant.

2. The process as claimed in claim 1 , optionally comprising coagulating, on completion of the polymerization reaction in the presence of at least one coagulating agent selected from the group comprising of inorganic salts, mineral acids, or organic compounds.

3. The process as claimed in claim 1 , optionally comprising adding chain transfer agents in step (a).

4. The process as claimed in claim 1 , wherein step (a) comprises the steps of

i. adding deionized water and optionally paraffin wax into the reactor;

ii. adding Perfluorobutanesulfonic acid or salt thereof in one shot or in multiple steps into the reactor; and

iii. adding fluoromonomer into the reactor and agitating the reaction mixture.

5. The process as claimed in claim 1 , wherein step (b) comprises initiating the polymerization of said fluoromonomer by an initiator, and wherein the initiator is added in one shot and/or multiple steps into the reactor.

6. The process as claimed in claim 1 , wherein the cation of the salt form of perfluorobutanesulfonic acid is selected from the group comprising of potassium, sodium or ammonium.

7. The process as claimed in claim 1 , wherein the initiator is selected from the group comprising of a water soluble radical initiator, a water soluble oxidation-reduction catalyst or an oil soluble radical polymerization initiator.

8. The process as claimed in claim 7, wherein the initiator is selected from the group comprising of organic Peroxides, Ammonium Persulphate (APS), potassium persulfate (KPS), Azo-compounds, redox initiators or combinations thereof.

9. The process as claimed in claim 1 , wherein the reaction temperature is in the range of 50 to 150 °C, preferably 60 to 120 °C, and more preferably 65 to 100 °C.

10. The process as claimed in claim 1 , wherein the reaction pressure ranges from 10 to 80 bar, and preferably 10 to 60 bar.

1 1 .The process as claimed in claim 10, wherein the reaction pressure is 10-50 bar.

12. The process as claimed in claim 1 , wherein the reaction mixture is agitated at 20 to 300 rpm.

13. The process as claimed in claim 1 , wherein the concentration of the surfactant in the reaction mixture ranges from 2000 to 16000 ppm, and preferably 4000 to 1 1000 ppm based on the weight of the aqueous medium.

14. The process as claimed in claim 1 , wherein the concentration of the initiator ranges from 2 to 2000 ppm, and preferably from 4 to 500 ppm, based on the weight of the aqueous medium.

15. The process as claimed in claim 1 , wherein the solid content of the fluoropolymer ranges from 7 to 40%.

16. The process as claimed in claim 1 , wherein the particle size of the fluoropolymer ranges from 80 nm to 500 nm.

17. The process as claimed in claim 1 , wherein the fluoromonomer is selected from the group comprising of tetrafluoroethylene, chlorotrifluoroethylene, vinyl fluoride, vinylidene fluoride, hexafluoropropylene, perfluoro (alkyl vinyl ether), perfluorobutylethylene or combinations thereof.

18. The process as claimed in claim 1 , wherein the mineral acid is any one of hydrochloric acid, nitric acid, sulphuric acid or combinations thereof.

19. The process as claimed in claim 1 , wherein the organic compound is any one of ethanol, methanol, acetone, succinic acid, chloromethanes, ammonia, urea or combinations thereof.

20. The process as claimed in claim 1 , wherein inorganic salt is any one of ammonium salts, sodium salts, aluminum salts, ammonium carbonate, ammonium hydroxide, sodium chloride or combinations thereof.

21 .The process as claimed in claim 2, wherein the coagulating agent is present in an amount less than or equal to 0.35 parts by mass per 100 parts by mass of fluoropolymer.

22. The process as claimed in claim 17, wherein the fluoropolymer is any one of polytetrafluoroethylene (PTFE), Polyvinylidene fluoride (PVDF), fluorinated ethylene propylene (FEP), perfluorealkoxyalkane (PFA), Poly vinyl fluoride (PVF), polychlorotrifluoroethylene (PCTFE) and fluoroelastomer (FKM).

23. The process as claimed in claim 3, wherein the chain transfer agent is one of halogen compounds, hydrocarbons, aromatic hydrocarbons, thiols (mercaptans), alcohols, esters of organic acid or combinations thereof.

24. The process as claimed in claim 23, wherein the chain transfer agents are present in an amount in the range of 5 to 6000 ppm based on the weight of aqueous medium.

25. The process as claimed in claim 1 , wherein the process for preparing fluoropolymers is carried out in the presence of additional non-fluorinated monomer - ethylene, resulting in the fluoropolymer ethylene tetrafluoroethylene (ETFE).