FIELD OF THE INVENTION

The present invention pertains to a method for formation of a fluoropolymer using fluorinated and non-fluorinated surfactants. More particularly, the present invention relates to a process for polymerization of either a primary monomer or a primary monomer and a co-monomer, using a surfactant that comprises either a combination of a fluorinated and a non-fluorinated surfactant, or 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, fluoroelastomer hoses for auto industry, sealing gaskets and liners for chemical industry, insulation for wires and cables, lubricants and so forth. This increasing demand in turn is driving a renewed interest in developing environment 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 Fluoropolymers are typically synthesized from alkenes in which one or more hydrogen atoms have been replaced by fluorine atoms. These fluorinated monomers include, tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), poly(propyl vinyl ether) (PPVE), poly(methyl vinyl ether) (PMVE), vinylidene fluoride (VDF), vinyl fluoride (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), fluoroelastomers and their modified grades etc. Fluoropolymers based on CF2=CH2 (VDF), PVDF are known to offer excellent mechanical properties, chemical inertness and good resistance to ageing. These qualities are made use of in varied fields of applications for example, manufacturing of extruded or injection-molded components for the chemical or microelectronics industry, use in the form of sealing sheaths for the transportation of gases or of hydrocarbons, the production of films or coatings for architectural applications, binders for storage batteries and the production of protective components for electrical engineering uses.

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 require a surfactant capable of emulsifying both the reactants and the reaction products for the duration of the polymerization reaction. The surfactant of choice in the synthesis of fluoropolymers is generally a perfluorinated surfactant. The most frequently used perfluorinated surfactant in the production of fluoropolymers and fluoroelastomers is a Perfluorooctanoic acid (PFOA) salt.

Although perfluorosurfactants are better in lowering the surface tension of water than comparable hydrocarbon surfactants, they 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 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 CsFi7- 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 onwards. 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.

A process for the polymerization of fluoromonomers using a non-fluorinated surfactant would solve the aforestated issues of persistence in the eco-system and bioaccumulation of perfluorosurfactants. 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 or cause discoloration in the final product. 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 telogenicity. Telogenicity, 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 or non-fluorinated surfactants.

OBJECTIVES OF THE INVENTION: An objective of the present invention is to provide a process for aqueous dispersion polymerization of either a primary monomer or a primary monomer and a comonomer using a surfactant comprising a combination of a fluorinated and a nonfluorinated surfactant or a non-fluorinated surfactant.

It is an objective of the present invention to provide a process for aqueous dispersion polymerization of vinylidene fluoride monomer using a surfactant comprising a combination of a fluorinated and a non-fluorinated surfactant or a non-fluorinated surfactant.

It is yet another objective of the invention to provide a simplified one step process for the preparation of fluoropolymers, particularly based on vinylidene fluoride as the primary monomer, wherein the said fluoropolymer is a homopolymer of vinylidene fluoride or a copolymer formed by using other fluorinated or non-fluorinated comonomers in addition to vinylidene fluoride.

It is an objective of the invention to provide a simplified one step process including addition of the surfactant in one shot, preferably at a delayed stage after initiation or kickoff of the polymerization reaction for the preparation of the fluoropolymers.

It is another objective of the present invention to provide fluoropolymer resins obtained by aqueous polymerization using a combination of a fluorinated and a non- fluorinated surfactant particularly using one or more salts of alkyl sulfate or Lauryl- imino-di-acetic acid surfactants as the non-fluorinated surfactants.

It is yet another objective of the present invention to provide fluoropolymer resins obtained by aqueous polymerization using a non-fluorinated surfactant particularly using one or more salts of alkyl sulfate or Lauryl-imino-di-acetic acid surfactants as the non-fluorinated surfactants.

It is yet another objective of the present invention to provide fluoropolymer resins obtained by aqueous polymerization, where the surfactant comprising of a combination of a fluorinated and a non-fluorinated surfactant is added at a delayed stage after 5 - 10% of the total monomer has polymerized after initiation of the polymerization reaction.

SUMMARY OF THE INVENTION

V arious embodiments of the present invention relate to a process for polymerization in an aqueous medium for preparing fluoropolymers based on a primary monomer or a primary monomer and a co-monomer.

In accordance with an aspect of the invention, a process for polymerizing monomers, particularly vinylidene fluoride and optionally a co-monomer in an aqueous medium to form a fluoropolymer has been provided. The said process involves pressurizing a polymerization reactor containing an aqueous medium with either a primary monomer or a primary monomer and a co-monomer followed by initiation. The primary monomer used in the invention is vinylidene fluoride. After initiation of the polymerization reaction, a surfactant comprising a combination of a fluorinated and a non-fluorinated surfactant is added into the polymerization reactor at a delayed stage. After the desired quantity of the primary monomer and/or the comonomer is consumed; the polymerization reaction is terminated.

Alternatively, the surfactant added is solely a non-fluorinated surfactant with fluorinated surfactants being absent instead of the combination of the fluorinated and the non-fluorinated surfactant.

In accordance with another aspect of the invention, the non-fluorinated surfactant comprises one or more salts of alkyl sulfate or one or more salts of Lauryl -imino-di- acetic acid surfactant.

In accordance with an embodiment, the surfactant is added in one-shot into the polymerization reactor.

In accordance with another embodiment, the one-shot dosing of the surfactant is a single feeding step which is preferably less that thirty minutes long and wherein the surfactant is not metered continuously for the entire duration of the polymerization reaction.

In accordance with another embodiment, the initiator is metered into the polymerization reactor.

Alternatively, the initiator is added in one-shot into the polymerization reactor.

The initiator for initiating the polymerization reaction is selected from the group consisting of Disuccinic Acid Peroxide (DSAP), Ammonium Persulfate (APS), Potassium Persulfate (KPS), Sodium Persulfate (NaPS), peroxides, peroxy dicarbonates or azo compounds.

In accordance with an embodiment, the amount of initiator ranges from 150 to 650 ppm based on the weight of the total primary monomer or of the total primary monomer and the co-monomer to be polymerized.

In accordance with an embodiment, the surfactant is added after 5 to 10 weight % of the total primary monomer or of the total primary monomer and the co-monomer to be polymerized are consumed.

Preferably, the surfactant is added in an amount of 20 to 100 ppm based on the weight of the aqueous medium. The aqueous medium further comprises of stabilizing agents such as paraffin wax.

The salt of alkyl sulfate surfactant is represented by the formula R-SOT M ⁺ , wherein R is an alkyl group and M ⁺ = Na ⁺ , NH4 ⁺ , triethanolamine cation or monoethanolamine cation. Preferably, the alkyl group is unbranched C12H25 (Lauryl Group). In a preferred embodiment, the alkyl sulfate surfactant is sodium lauryl sulfate (SLS), ammonium lauryl sulfate (ALS), triethanolamine lauryl sulfate (TEALS) or monoethanolamine lauryl sulfate (MEALS). The salt of Lauryl-imino-di- acetic acid (LIDA) surfactant is ammonium salt of Lauryl-imino-di-acetic acid surfactant.

The fluorinated surfactant has a structure represented by formula 1 , (formula 1) wherein:

R ¹ is a fully or partially fluorinated linear or branched alkyl group with or without ether functionality;

Y7 is an anionic head group selected from — SO ₂ (NR ^s R ^ff# )~ with being a hydrocarbon; and is a monovalent cation selected from the group of hydrogen, an alkali metal and NH

In accordance with an embodiment, the co-monomer is a non-fluorinated monomer.

The non-fluorinated monomer is selected from the group consisting of styrene, ethylene, propene, 2-hydroxyethyl allyl ether, 3 -allyloxypropanediol, hydrophilic (meth)acrylic monomers such as acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate, or hydroxyethylhexyl(meth)acrylates.

In accordance with another embodiment, the co-monomer is a fluorinated monomer.

The fluorinated monomer is selected from the group consisting of tetrafluoroethylene (TFE), trifluoroethylene, chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), vinyl fluoride, hexafluoroisobutylene, perfluorobutylethylene (PFBE), pentafluoropropene, 3,3,3-tri fluoro- 1 -propene, 2-trifhroromethyl-3,3,3- trifluoropropene, fluorinated vinyl ethers, fluorinated allyl ethers, fluorinated dioxoles, 1,2,3,3,3-pentafluoropropene, hexafluorocyclobutylene or 3, 3, 3,4,4- pentafluoro- 1 -butene.

In an embodiment, the process further comprises adding chain transfer agents selected from halogen compounds, aliphatic hydrocarbons, aromatic hydrocarbons, thiols (mercaptans), alcohols, and esters. In a preferred embodiment, the chain transfer agent is ethyl acetate. The solid content of the fluoropolymer obtained by the said process after termination of reaction ranges from 15 to 35%. The size of the fluoropolymer particles ranges from 200 to 500 nm.

Preferably, the vinylidene fluoride units comprise at least 45% of the total weight of all the monomer units in the co-polymer, preferably at least 75% of the total weight of all monomer units.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

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

Figure 2: Is a flow diagram illustrating process steps in accordance with another 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. 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.

The present invention, in all its aspects, is described in detail as follows: Described herein, is a novel process that minimizes the use of fluorinated surfactants in the polymerization of the primary monomer, i.e., vinylidene fluoride (VDF), and optionally a co-monomer, without adding complex reaction steps. The novel process for preparing fluoropolymers using a combination of a fluorinated and a nonfluorinated surfactant, comprises the steps of (1) pressurizing the polymerization reactor containing an aqueous medium with either a primary monomer or a primary monomer and a co-monomer, wherein the primary monomer is VDF; (2) initiating a polymerization reaction of the primary monomer or the primary monomer and the comonomer to form said fluoropolymer by adding an initiator followed by the (3) propagation of the polymerization reaction, wherein a combination of the fluorinated and the non-fluorinated surfactants is added at a delayed stage after initiation of the polymerization reaction; and (4) terminating of the polymerization reaction after consumption of a desired quantity of the primary monomer and/or the co-monomer.

Alternatively, the process for preparing fluoropolymer involves addition of a surfactant comprising only the non-fluorinated surfactant into the polymerization reactor with any fluorinated surfactant being absent instead of using the combination of the fluorinated and the non-fluorinated surfactant.

Preferably, oxygen is removed from the polymerization reactor, after adding the aqueous reaction medium but prior to pressurizing the polymerization reactor, to a concentration of less than or equal to 20 ppm. This is followed by pressurizing the polymerization reactor with VDF and optionally co-monomer, initiating the polymerization reaction and addition of either a surfactant comprising of a combination of a fluorinated and a non-fluorinated surfactant or only a non- fluorinated surfactant comprising one or more salts of alkyl sulfate or one or more salts of Lauryl-imino-di-acetic acid (LIDA) surfactant, which results in an aqueous dispersion of fluoropolymers.

The aqueous reaction mixture formed in the present invention comprises either a combination of a fluorinated surfactant and salts of alkyl sulfate or LIDA surfactant or solely salts of alkyl sulfate or LIDA surfactant, vinylidene fluoride as the primary monomer, initiators, chain transfer agents, and optionally one or more co-monomers and 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. The present invention uses either a combination of a fluorinated surfactant and a non-fluorinated surfactant or only a non-fluorinated surfactant comprising one or more salts of alkyl sulfate or one or more salts of LIDA surfactant. The term “surfactant” is, thus, used in the specification to refer either to a combination of a fluorinated and a non-fluorinated surfactant or to a non-fluorinated surfactant alone.

The fluorinated surfactant useful in the present invention has structure represented by formula 1: (formula 1) wherein:

R ¹ is a fully or partially fluorinated linear or branched alkyl group with or without ether functionality;

Y- is an anionic head group selected from —COO-, — SO3, — PO|-, — SO ₂ (NR ^# R ^ff# )“ with R ^s and being hydrocarbons; and is a monovalent cation selected from the group of hydrogen, an alkali metal and NH

Preferably, R ¹ is represented by formula 1 ’:

R ^; — (0 — R") _n — (formula 1’)

With n = 0 to 4 and wherein

R' is a fully or partially fluorinated linear or branched alkyl group and R" is a fully or partially fluorinated linear or branched alkyl group; and wherein R' and R" consist of a maximum number of 20 carbon atoms combined.

More preferably, R ¹ consists of 2 to 6 carbon atoms. Alternatively, the fluorinated surfactant is devoid of ether functions. An example, but not limited thereto, of a partially fluorinated surfactant is l,5-dioxo-l,5-bis[l-(2,2,3,3,3- pentafluoropropoxy)butan-2-yloxy] -3 - [ 1 -(2,2, 3 ,3 , 3-pentafluoropropoxy)butan-2- yloxycarbonyl]pentane-2-sulfonate.

Further examples of fluorinated surfactants useful for the polymerization process according to the embodiments of the present invention include, but not limited to, sodium or potassium salt of perfluorobutane sulfonic acid, branched C2 or C3 short chain, dimer or trimer, fluorinated surfactants. Non-limiting examples of such surfactants are hexafluoropropylene oxide-dimer acid or hexafluoropropylene oxidetrimer acid.

Non-fluorinated surfactants useful for the present invention belong to a class of alkyl sulfate salt surfactants represented by the general formula R-SO4’ M ⁺ , wherein R is an alkyl group or a hydrocarbon group preferably C12H25 (Lauryl Group), and M ⁺ = Na ⁺ , NH ₄ ⁺ , monoethanolamine or triethanolamine cation. In a preferred embodiment, the alkyl sulfate salt is triethanolamine lauryl sulfate, shown below: Non-fluorinated surfactant may also comprise of one or more salts of LIDA surfactant, as shown below:

Preferably the surfactant, which may either be a combination of a fluorinated surfactant and a non-fluorinated surfactant or only a non-fluorinated surfactant comprising one or more salts of alkyl sulphate surfactant or one or more salts of LIDA surfactant, is added into the polymerization reactor at a delayed stage after the initiation or kickoff of the polymerization. Preferably, the surfactant is added in one shot into the polymerization reactor. Preferably the one-shot dosing of the surfactant is a single feeding step which is preferably less than thirty minutes long and the surfactant is not metered continuously for the entire duration of the polymerization reaction. Ideally, the surfactant is added after 5 to 10% of the total primary monomer and optionally co-monomer to be polymerized are consumed. Preferably, the surfactant is added in an amount of 20 to 150 ppm based on the weight of the aqueous medium.

Monomers

The term “primary monomer” refers to particularly vinylidene fluoride monomer. The optional “co-monomer” refers to a monomer which is not vinylidene fluoride and may be a fluorinated or a non-fluorinated monomer. The term “fluoropolymer” means a polymer or elastomer formed by the polymerization of the primary monomer with or without the co-monomer, and it is inclusive of homopolymers, copolymers, terpolymers and higher polymers. The process of the present invention results in either polyvinylidene fluoride (PVDF) or co-polymers of VDF with other comonomers.

In certain embodiments, the VDF units may comprise at least 45% of the total weight of all the monomer units in the co-polymer, and more preferably, comprise at least 75% of the total weight of all the monomer units.

Non-limiting examples of fluorinated co-monomers that can be used in the present invention include, tetrafluoroethylene (TFE), trifluoroethylene, chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), vinyl fluoride, hexafluoroisobutylene, perfluorobutylethylene (PFBE), pentafluoropropene, 3,3,3-tri fluoro- 1 -propene, 2-trifluoromethyl-3,3,3-trifluoropropene, fluorinated vinyl ethers, fluorinated allyl ethers, fluorinated dioxoles, 1,2,3,3,3-pentafluoropropene, hexafluorocyclobutylene and 3,3,3,4,4-pentafluoro-l-butene, perfluorinated vinyl ethers, perfluoromethyl vinyl ether, perfluoroethyl vinyl ether, perfluoro -n-propyl vinyl ether, perfluoro-2-propoxypropyl vinyl ether, fluorinated dioxoles, perfluoro( 1,3 -dioxole) and perfluoro(2,2-dimethyl-l,3-dioxole), partly fluorinated allylic monomers, fluorinated allylic monomers, and so forth, each of which can be used individually or in combination.

Examples of non-fluorinated co-monomers, include but not limited to, ethylene, styrene, propene, 2-hydroxyethyl allyl ether, 3-allyloxypropanediol, hydrophilic (meth)acrylic monomers such as acrylic acid, methacrylic acid, hydroxyethyl (meth)acrylate, hydroxypropyl(meth)acrylate or hydroxyethylhexyl(meth)acrylates.

Copolymers made by the process of the invention include, but not limited to, copolymers of VDF with TFE, HFP or trifluoroethylene. Examples of terpolymers made by the process of the present invention include, but not limited to, terpolymer of VDF, HFP and TFE; terpolymer of VDF, trifluoroethylene, and TFE, and so forth.

Although, the embodiments of the present invention are described in terms of polymerization of VDF and/or co-monomer, 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” refers 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 Potassium persulfate (KPS), Disuccinic Acid Peroxide (DSAP), Ammonium Persulphate (APS), redox initiator and combinations thereof. These radical initiators may also function as oxidizing agents and may form redox systems with reducing agents such as sodium sulfite and sodium bisulfite.

The initiator is added in one shot into the polymerization reactor initially. Alternately the initiator can be metered into the polymerization reactor. Preferably, prior to adding the initiator into the polymerization reactor, it is dissolved in a suitable solvent such as water.

The amount of initiator added ranges from 150 to 650 ppm based on the weight of the total primary monomer and/or co-monomer to be polymerized.

Chain transfer agents

Chain transfer agents, also referred to as modifiers or regulators, comprise 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, esters, ethyl acetate, thiols (mercaptans), alcohols and so forth; each of which can be used individually or in combination. In a preferred embodiment, the chain transfer agent is ethyl acetate.

The amount of chain-transfer agents added to the polymerization reaction is preferably from about 0 to about 3 weight percent, more preferably from about 0 to about 2.5 weight percent based on the total weight of primary monomer and/or comonomer consumed in the polymerization reaction.

Polymerization conditions

The temperature for the polymerization reaction may vary, for example, from 15 to 135 °C, depending on the initiator system chosen and the reactivity of the monomer(s) selected. In a preferred embodiment, the polymerization is carried out at a temperature in the range of 65 to 100 °C. Preferably, the non-fluorinated surfactants used in the present invention are not passivated prior to addition of the same to the polymerization reactor.

The pressure of the polymerization reactor may vary from 2 - 100 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 invention, a pressurized polymerization reactor equipped with a stirrer and exotherm control is charged with water, preferably deionized water. Optionally, paraffin wax may be added to the reactor. 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 20 ppm. The reactor may also be purged with a neutral gas such as, for example, nitrogen or argon. Preferably, the concentration of O2 in the reactor is reduced to less than 20 ppm by applying nitrogen-vacuum cycles.

Optionally, the surfactant which may either be a combination of a fluorinated and a non-fluorinated surfactant or only a non-fluorinated surfactant comprising of one or more salts of alkyl sulfate or one or more salts of LIDA is added in one shot into the polymerization reactor to form an aqueous solution of surfactants. The one-shot dosing of the surfactant is a single feeding step which is less than thirty minutes long. The surfactant is not metered continuously for the entire duration of the polymerization reaction.

The reactor is then heated up to the reaction temperature followed by addition of monomers.

Figure 1 depicts an exemplary process 100 in accordance with an exemplary embodiment of the invention. As depicted in Figure 1, at step S101 the polymerization reactor containing an aqueous medium is pressurized with the primary monomer, i.e., VDF, and optionally a co-monomer. On reaching a pressure of 10 to 60 bar, preferably 15 to 35 bar, a chain transfer agent may be optionally added to the reactor. Heating of the reactor is continued until reactor temperature reaches a range of 65 to 100 °C, preferably 70 to 100 °C. Simultaneously, pressurization with the primary monomer, i.e., VDF, and optionally co-monomer is continued until the reactor pressure reaches a range of 10 to 60 bar, preferably 20 to 50 bar.

Thereafter, at step S102, initiators are added into the reaction vessel to initiate the polymerization reaction of VDF or VDF and co-monomers. The amount of initiator added initially is 150- 650 ppm, based on the weight of the total monomer (VDF and optionally co-monomer) to be polymerized and more preferably from 20- 150 ppm. The start/ initiation of the reaction or the kickoff of the reaction is indicated by a drop in the reactor pressure.

After that, additional 130 to 500 ppm initiator, based on the weight of the total monomer (VDF and optionally co-monomer) to be converted, may optionally be continuously metered into the reactor throughout the reaction which typically takes 200 to 400 minutes for complete conversion of the said monomers. After the initiation of the polymerization reaction, at step SI 03, during the propagation phase, a combination of a fluorinated surfactant and a non-fluorinated surfactant is added into the polymerization reactor. The surfactant is added, at a delayed stage, in one shot into the polymerization reactor after 5 - 10% of the total monomer (VDF and optionally co-monomer) is polymerized. Preferably, 20 to 100 ppm of surfactant is added based on the weight of the aqueous fluoropolymer emulsion. The one-shot dosing of surfactant refers to the addition of the required quantity of surfactant in a single feeding step which takes less than thirty minutes. The surfactants are not metered continuously for the entire duration of the polymerization reaction.

At step S104, upon consumption of the desired quantity of the primary monomer, i.e., VDF, and optionally co-monomers, addition of the primary monomer, i.e., VDF and optionally co-monomer is stopped and the reaction is terminated. However, initiator dosing is optionally continued till the pressure reduces to a range of 10 to 40 bar, preferably 25 to 30 bar. The total amount of initiator used in the polymerization reaction preferably ranges from 150 ppm to 650 ppm.

The aqueous reaction medium containing the fluoropolymer is then recovered from the reaction vessel. Preferably, the solid content ranges from 15 to 35 %, more preferably from 20 to 30 % and the particle size of the fluoropolymer particles ranges from 200 to 500 nm.

Figure 2 depicts another exemplary process 200 in accordance with another exemplary embodiment of the invention. As depicted in Figure 2, at step S201, a polymerization reactor containing an aqueous medium is pressurized with VDF and optionally co-monomer until the reactor pressure reaches a range of 10 to 60 bar, preferably 20 to 50 bar. Simultaneously, the reactor is heated until reactor temperature reaches a range of 65 to 100 °C, preferably 70 to 100 °C.

Once a pressure of 10 to 60 bar, preferably 15 to 35 bar, is attained, a chain transfer agent may be optionally added to the reactor.

At step S202, polymerization reaction of VDF and optionally co-monomers is initiated by adding initiators. A drop in the reaction pressure is indicative of the initiation of the reaction. The amount of the initiator added initially is 150-650 ppm, more preferably 20-150 ppm, based on the total weight of the monomer (VDF and/or co-monomer) to be polymerized. After that, additional 130 to 500 ppm initiator, based on the weight of the total monomer (VDF and optionally co-monomer) to be converted, may be continuously metered into the reactor throughout the reaction which typically takes 200 to 400 minutes for complete conversion of the said monomers.

At step S203, after initiation of the polymerization reaction, a non-fluorinated surfactant is added into the polymerization reactor during propagation of the polymerization reaction. The surfactant is added, at a delayed stage, in one-shot into the polymerization reactor after 5 - 10% of the total monomer (VDF and optionally co-monomer) is polymerized. The one-shot dosing of the surfactant is a single feeding step which is less than thirty minutes long. The surfactant is not metered continuously for the entire duration of the polymerization reaction. Preferably, 20 to 100 ppm of surfactant is added based on the weight of the aqueous fluoropolymer emulsion.

At Step S204, after the desired quantity of VDF and optionally co-monomer is consumed, the polymerization reaction is terminated. Initiator dosing may be optionally continued till the pressure reduces to a range of 10 to 40 bar, preferably 25 to 30 bar.

The present invention is more particularly described in the following examples that are intended as illustration only, since numerous modifications and variations within the scope of the present invention will be apparent to those skilled 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 illustrate the basic methodology and versatility of the present invention.

Examples 1 - 5

135 L of de-ionized water was charged in a 200 L horizontal reactor. The temperature in the reactor was maintained at 55 - 60 °C. Reactor was kept under agitation at 3 - 5 RPM while charging with de-ionized water. Thereafter, 37 g wax is added to the reactor. Nitrogen-vacuum cycles were applied to the reactor to bring O2 content to < 20 ppm.

Vinylidene fluoride (VDF) was charged into the reactor under pressure through a compressor and temperature of the reactor was increased by heating. When the reactor pressure rose to 30 bar, a first dose of Ethyl acetate (9 to 500 g) was added into the reactor. Heating of reactor and pressurization was continued till the temperature reached 83± 0.5 °C and pressure reached 44.5±0.2 bar.

When the above temperature and pressure conditions were achieved, initial dose of Potassium persulfate (KPS) initiator solution (10 g/L; 110 -370g solution) was fed into the reactor. Commencement of the reaction was indicated by a drop in the pressure of reactor. Thereafter, VDF monomer was continuously fed to maintain a reactor pressure in the range of 44.75±0.25 bar.

After 185 g of VDF was fed into the reactor, continuous dosing of initiator solution (10 g/E) was commenced. Further, after consumption of 2.6 kg VDF, 5 - 20 g of nonfluorinated surfactant and 0.3 - 1 g of fluorinated surfactant [l,5-dioxo-l,5-bis[l- (2,2,3,3,3-pentafluoropropoxy)butan-2-yloxy]-3-[l-(2,2,3,3,3 -pentafluoropropoxy) butan-2-yloxycarbonyl]pentane-2-sulfonate] was added to the reactor in a single shot in less than 30 minutes. After every 5.185 kg consumption of VDF, 12 -40 g of Ethyl Acetate was charged into the reactor within 3-4 minutes. Temperature inside the reactor was maintained in a range of 85 - 91 °C throughout the reaction.

Addition of VDF was stopped after 33.33 kg of VDF conversion. Initiator dosing was continued till reactor pressure dropped to 30 bar. Once the reactor pressure dropped to 30 bar, initiator dosing was stopped and reactor depressurization and cooling were initiated. Agitator speed was reduced to 3 - 5 rpm. After the temperature reached below 50 °C, a mild vacuum was applied to remove the residual VDF.

The particle size of the polymer obtained ranged from 349.5 - 385.1 nm. Total solid content obtained was between 22 - 22.3%.

Examples 6-9:

135 L of de-ionized water was charged in a 200 L horizontal reactor. The temperature in the reactor was maintained at 55 - 60 °C. Reactor was kept under agitation at 3 - 5 RPM while charging of de-ionized water. Thereafter, 37 g wax was added to the reactor. Nitrogen-vacuum cycles were applied to the reactor to bring O2 content to < 20 ppm.

Vinylidene fluoride (VDF) was charged into the reactor under pressure through a compressor and temperature of the reactor was increased by heating. When the reactor pressure reached 30 bar, a first dose of Ethyl acetate (9 to 500 g) was added into the reactor. Heating of reactor and pressurization was continued till the temperature reached 83± 0.5 °C and pressure reached 44.5±0.2 bar.

When the above temperature and pressure conditions were achieved, initial dose of Potassium persulfate (KPS) initiator solution (10 g/L; 110 -370g solution) was fed into the reactor. Commencement of the reaction was indicated by a drop in the pressure of reactor. Thereafter, VDF monomer was continuously fed to maintain a reactor pressure in the range of 44.75±0.25 bar.

After 185 g of VDF was fed into the reactor, continuous dosing of initiator solution (10 g/L) was commenced. Further, after consumption of 2.6 kg VDF, 5 - 20 g of non-fluorinated surfactant was added to the reactor in one shot in less than 30 minutes. After every 5.185 kg consumption of VDF, 12 -40 g of Ethyl Acetate was charged into the reactor within 3-4 mins. Temperature inside the reactor was maintained in a range of 85 - 91 °C throughout the reaction. Addition of VDF was stopped after 33.33kg of VDF conversion. Initiator dosing was continued till reactor pressure dropped to 30 bar. Once the reactor pressure dropped to 30 bar, initiator dosing was stopped and reactor depressurization and cooling were initiated. Agitator speed was reduced to 3-5 rpm. After the temperature reached less than 50 °C, a mild vacuum was applied to remove the residual VDF.

The particle size of the polymer obtained ranged from 421.9 - 448.6 nm. Total solid content obtained was between 21.7 - 23%.

Table 1: Examples 1 - 9

# LIDA refers to ammonium salt of Lauryl-imino-di-acetic acid surfactant.

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.