Production method of fluoropolymer

Abstract

To provide a preparation method of an ethylene-tetrafluoroethylene (ETFE) copolymer and / or an ethylene-chlorotrifluoroethylene (ECTFE) copolymer in an aquatic environment without requiring use of any organic solvent and / or any fluorinated surface active agent.SOLUTION: Provided is a preparation method of a fluoropolymer in a liquid reaction medium, in which the reaction medium does not contain any fluorinated surface active agent, and which comprises: a step to prepare a reactor having a water-containing liquid reaction medium placed therein; a step to introduce an ethylene monomer and a fluorinated monomer selected from tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) or a mixture thereof into the reactor; a step to pressurize the reactor; and a step to start polymerization by supplying a redox initiator containing an oxidizer and a reducer to the reactor in which the reducer in the redox initiator contains no sulfur atom with an oxidation number of 4 or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Patent Application No. 62 / 947318, filed December 12, 2019, and European Patent Application Publication No. 20153524.2, filed January 24, 2020, the entire contents of which are incorporated herein by reference for all purposes.

[0002] The present invention relates to a method for preparing copolymers containing repeat units derived from ethylene and repeat units derived from chlorotrifluoroethylene (CTFE) and / or tetrafluoroethylene (TFE) using a redox pair initiator. The method of the present invention allows for the preparation of ethylene-tetrafluoroethylene (ETFE) copolymers and / or ethylene-chlorotrifluoroethylene (ECTFE) copolymers in an aqueous environment without the need for organic solvents and / or fluorinated surfactants, thereby simplifying and reducing the environmental impact of producing such copolymers. The resulting copolymers have excellent physical and mechanical properties, and in particular, very low levels of yellowing (discoloration). [Background technology]

[0003] Methods for preparing ETFE and ECFTE copolymers are well known in the art. Such copolymers are typically prepared industrially in aqueous media using peroxide initiators, such as perhaloacyl peroxides, with bis(trichloroacetyl) peroxide (TCAP) being the most commonly used initiator in this field. Perhaloacyl peroxides are useful as polymerization initiators, particularly in the polymerization of halogenated monomers such as chlorotrifluoroethylene and tetrafluoroethylene. The absence of hydrogen atoms in these peroxides results in the preparation of halogenated polymers with fully halogenated chain ends.

[0004] Perhaloacyl peroxide initiators are typically introduced into the reaction medium as a solution in a halogenated hydrocarbon (as disclosed in U.S. Pat. No. 2,816,147). Such solutions have been used without further purification to initiate the polymerization of halogenated monomers. In particular, solutions of bis(trichloroacetyl) peroxide in trichlorotrifluoroethane, especially 1,1,2-trichloro-1,2,2-trifluoroethane, have been used to prepare polymers containing chlorotrifluoroethylene, as described, for example, in U.S. Pat. No. 3,847,881.

[0005] Halogenated hydrocarbons, such as trichlorotrifluoroethane, as disclosed in U.S. Patent No. 2,816,147, have a high potential for depleting stratospheric ozone, and their production and use are strictly restricted by the Montreal Protocol. Potential alternatives, such as perfluorocarbons, are also greenhouse gases and may contribute to global warming, so their use is restricted and they are therefore not suitable to replace trichlorotrifluoroethane.

[0006] Co-pending European Patent Application No. 18214305.7 describes the use of isooctane as a solvent for bis(trichloroacetyl) peroxide. Although isooctane is an excellent solvent for the initiator, it still has the disadvantage of being a flammable solvent, which can be problematic from the standpoint of safe handling in some circumstances.

[0007] Another drawback of the prior art is the use of fluorinated surfactants in conventional polymerization of ETFE / ECTFE (see, for example, U.S. Pat. No. 4,482,685). As is well known to those skilled in the art, fluorinated surfactants are also undesirable from an environmental standpoint.

[0008] While peroxides and persulfates have been considered as alternative initiators for ETFE and ECTFE copolymers, many peroxide and persulfate initiators function effectively as initiators only at temperatures high enough to break the peroxide or persulfate bond, thereby generating radicals in sufficient quantities to initiate polymerization. Most peroxides and persulfates require relatively high temperatures to be effective.

[0009] In contrast, ETFE and ECTFE polymers are preferably formed by regular alternating sequences of ethylene monomer (ET) and fluorinated monomers, and therefore ETFE and ECTFE are preferably produced at relatively low temperatures. Because the properties of these polymers correlate with the presence of these regular alternating sequences, the number of ET-ET, TFE-TFE, or CFTE-CTFE sequences typically needs to be minimized for a given stoichiometry (which is unavoidable to some extent, especially when ET is in excess or deficient relative to the fluorinated monomer). As is well known in the art, increasing the polymerization temperature corresponds to an increase in disordered sequences, which results in a deterioration in the polymer's properties. Therefore, it is generally desirable to carry out the polymerization of ECTFE and ETFE at a temperature below 30°C, preferably in the range of around 15°C. This further limits the number of free radical initiators that can be used, as many radical initiators require higher temperatures for the radicals to initiate and propagate the polymerization reaction.

[0010] In contrast, redox initiators are formed by introducing an oxidizing agent and a reducing agent into a reactor. The redox reaction is typically very fast, even at cryogenic temperatures, and results in the formation of radicals that, in the presence of polymerizable monomers, initiate their polymerization. Continuous, controlled feeding of the redox initiator (typically in the form of two separate feed streams, oxidizing and reducing) allows the polymerization reaction to continue to completion. ETFE and / or ECTFE polymerization processes using redox initiators have been reported in the prior art, such as WO 97 / 17381. However, prior art processes typically use reducing agents containing bisulfite or other compounds containing an S(IV) atom (a sulfur atom in the +4 oxidation state). Applicant has discovered that the use of bisulfite can result in discoloration of the resulting polymer. For this reason, the use of redox initiators has thus far found little commercial application in the ETFE / ECTFE industry.

[0011] Therefore, there is a need to develop new polymerization processes for forming ETFE and ECFTE polymers using redox initiators that can be carried out at low temperatures in an aqueous environment, do not require hazardous solvents, fluorinated surfactants, or flammable solvents, and at the same time form polymers that have desirable mechanical properties and exhibit minimal or no yellowing / discoloration. Summary of the Invention [Means for solving the problem]

[0012] The present invention provides - providing a reactor containing a liquid reaction medium comprising water; - introducing into a reactor ethylene monomer and a fluorinated monomer selected from TFE, CTFE, or a mixture thereof; - pressurizing the reactor; - feeding a redox initiator comprising an oxidizing agent and a reducing agent to the reactor to initiate polymerization, wherein the reducing agent in the redox initiator pair does not contain sulfur atoms with an oxidation number of 4 or less; Including, - the reaction medium is free of fluorinated surfactants, The present invention relates to a method for preparing fluoropolymers in a liquid reaction medium. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention relates to a method for preparing a fluoropolymer comprising repeating units derived from chlorotrifluoroethylene (CTFE) and / or tetrafluoroethylene (TFE) monomers and repeating units derived from ethylene monomers. The reaction is carried out in a liquid reaction medium containing water, preferably greater than 50% by weight, or greater than 80% by weight, or greater than 95% by weight of the liquid reaction medium (considering the entire liquid components of the reaction, excluding the monomer, polymer, initiator, and chain transfer agent, as the reaction medium), and the reaction can be carried out by copolymerization of the monomers according to known techniques. Typically, TFE and / or CTFE monomers and ethylene monomers are fed to a sealed reactor and dispersed in a reaction medium containing water and other optional additives, typically maintained under agitation. The presence of a fluorinated surfactant is not essential in the process of the present invention, and therefore, it has been found that the process of the present invention is carried out in the absence of a fluorinated surfactant to minimize environmental impact. While in some embodiments, a non-fluorinated surfactant can be used to aid in the dispersion of the monomers, preferably the process of the present invention is carried out in the absence of added surfactants.

[0014] The monomers are typically supplied in gaseous form, preferably from a separate gas stream, in a sealed reactor. The monomer-containing gas may optionally contain other gaseous components or may consist solely of the monomers. The reactor is typically pressurized to a pressure of 2 to 60 bar, preferably 5 to 40 bar, and more preferably 8 to 20 bar, using the monomer-containing gas. Typically, one or more monomer-containing gases are continuously added to the reactor during the polymerization reaction to maintain the reactor pressurized within the desired range. Typically, ethylene gas is used to maintain the reactor pressurization.

[0015] According to the present invention, polymerization is initiated by introducing an aqueous redox initiator into a reactor. The redox initiator includes an oxidizing agent and a reducing agent. The two reagents can be continuously introduced together into the reactor from two separate feed streams, so that when the oxidizing agent and reducing agent come into contact with each other, they form free radicals that can initiate the polymerization reaction. Alternatively, one of the oxidizing agent or reducing agent can be present in the reaction mixture, while the other component of the redox initiator is gradually added during the course of the polymerization.

[0016] Once polymerization is complete, the redox initiator feed is discontinued. At that point, the reactor can be vented and the polymer recovered. Depending on the reaction conditions, the polymer can be recovered in the form of a slurry, an aqueous suspension, or a latex. The polymer can then be extracted and further processed for any purpose using conventional techniques.

[0017] In the present invention, the selection of the oxidizing agent is not particularly limited. Preferably, the oxidizing agent is water-soluble or water-dispersible, which is advantageous because it does not require the use of an organic solvent introduced into the reactor. More preferably, the oxidizing agent comprises one or more compounds selected from inorganic persulfates (e.g., alkali metal persulfates, particularly sodium persulfate, lithium persulfate, potassium persulfate, or ammonium persulfate), inorganic peroxides (e.g., hydrogen peroxide, sodium peroxide, potassium peroxide, lithium peroxide, and ammonium peroxide), manganese-based oxidizing agents (e.g., manganese triacetate, metal permanganates, particularly alkali metal permanganates such as sodium, lithium, and potassium permanganates), and organic peroxides (e.g., t-butyl hydroperoxide, di-t-butyl peroxide, cumene hydroperoxide, and t-amyl hydroperoxide). The organic peroxide can preferably be selected so as to be sufficiently hydrophilic for use in an aqueous reaction medium without the need for an additional solvent. Preferred oxidizing agents include potassium permanganate. Mixtures of the above-listed compounds can also be used as the oxidizing agent of the present invention.

[0018] In the present invention, the reducing agent must not contain sulfur atoms with an oxidation number of 4 or less. Preferably, the reducing agent does not contain sulfur atoms. Suitable compounds that can be used as the reducing agent in the present invention include hydroxylamine, hydrazine, ferrous iron, and organic acids. Preferred reducing agents are organic acids, particularly oxalic acid, malonic acid, and citric acid. A particularly preferred reducing agent is oxalic acid. A mixture of the compounds listed above can also be used as the reducing agent in the present invention.

[0019] In a particularly preferred embodiment, the redox initiator comprises potassium permanganate as the oxidizing agent and oxalic acid as the reducing agent.

[0020] As mentioned above, the entire polymerization process is preferably carried out at a relatively low temperature of 0 to 30°C, preferably 5 to 20°C.

[0021] The molecular weight of the fluoropolymers of the present invention can be controlled using techniques known to those skilled in the art, particularly by controlling the amount of initiator added and / or by using chain transfer agents. The optional chain transfer agent can be added to the polymerization reactor at any stage of the polymerization reaction. Any chain transfer agent conventionally used in the polymerization of fluoropolymers, particularly in the production of ETFE and ECTFE copolymers, can be used in the present invention. Non-limiting examples of suitable chain transfer agents include alcohols, ketones, and carboxylic acid esters, halogenated hydrocarbons, and mercaptans. Examples of suitable chain transfer agents include isopropanol, methanol, acetone, ethyl acetate, chloroform, 1,12,2-tetrachloroethane, bromotrichloromethane, or butyl mercaptan, as well as others such as those disclosed in U.S. Pat. No. 3,069,401. The chain transfer agent is introduced into the reactor at the beginning of the reaction or continuously or stepwise during the polymerization. The amount of chain transfer agent can vary within a fairly wide range, depending on the polymerization conditions (reaction temperature, monomers, desired molecular weight of the polymer, etc.). Usually, such an amount is in the range of 0.001 to 5% by weight, preferably 0.05 to 1% by weight, based on the total amount of monomers introduced into the reactor.

[0022] Preferably, the chain transfer agent for the present invention comprises a C1-C4 hydrocarbon or a halogenated hydrocarbon. A particularly preferred chain transfer agent is chloroform. As is known in the art, in some cases, certain comonomers (other than ethylene, TFE, and CTFE) may have the effect of a chain transfer agent. For example, acrylic comonomers can function as chain transfer agents.

[0023] The method of the present invention can be applied to both suspension or emulsion polymerization processes, however it is preferred to use a suspension polymerization process.

[0024] The process is carried out in a reaction medium free of fluorinated surfactants. By "fluorinated surfactant" in the present invention is meant a compound according to the following formula (I): R f§(X - ) k (M + ) k (I) (In the formula, -R f§ is a C5-C alkyl group which may optionally contain one or more catenary or non-catenary oxygen atoms. 16 selected from (per)fluoroalkyl chains (which may be linear or branched, including chains containing rings), and (per)fluoropolyoxyalkyl chains (which may be linear or branched, including chains containing rings); - X - -COO - , -PO3 - , and -SO3 - is selected from -M + is NH4 + and alkali metal ions, - k is 1 or 2).

[0025] Non-limiting examples of fluorinated surfactants include: (a)CF3(CF2) n0 COOM', where n0 is an integer ranging from 4 to 10, preferably from 5 to 7, typically n1 is equal to 6, and M' represents NH4, Na, Li or K; (b) T-(C3F6O) n1 (CFXO) m1 CF2COOM" [wherein T is a Cl atom or a group of formula C x F 2x+1-x’ Cl x’ O (wherein x is an integer ranging from 1 to 3, x' is 0 or 1), n1 is an integer ranging from 1 to 6, m1 is an integer ranging from 0 to 6, M'' represents NH4, Na, Li, or K, and X represents F or -CF3; (c)F-(CF2CF2) n2 -CH2-CH2-RO3M''' [wherein R is a phosphorus or sulfur atom, M''' represents NH4, Na, Li or K, and n2 is an integer ranging from 2 to 5]; (d)AR bf-B bifunctional fluorinated surfactant, wherein A and B, which may be equal to or different from each other, are represented by the formula -(O) p CFX''-COOM * (In the formula, M * represents NH4, Na, Li or K, X'' is F or -CF3, and p is an integer equal to 0 or 1, and R bf is AR bf -B is a divalent (per)fluoroalkyl or (per)fluoropolyether chain such that the number average molecular weight of B is in the range of 300 to 1800; (e) cC6O4 (perfluoro{acetic acid, 2-[(5-methoxy-1,3-dioxolan-4-yl)oxy]}) in acid form or as an alkali metal or ammonium salt is.

[0026] The process of the present invention can be used to prepare copolymers comprising repeat units derived from ethylene and repeat units derived from a fluoromonomer selected from CTFE and TFE. Although other comonomers may be present, it is preferred that greater than 70 mol %, more preferably greater than 80 mol %, even more preferably greater than 90 mol %, and most preferably greater than 95 mol % of the repeat units of the polymer are derived from either ethylene, TFE, or CTFE.

[0027] In a preferred embodiment, the CTFE or TFE copolymer of the present invention comprises (a) 35 to 65 mol %, preferably 45 to 55 mol %, more preferably 48 to 52 mol % of ethylene (E) and (b) 65 to 35 mol %, preferably 55 to 45 mol %, more preferably 52 to 48 mol % of chlorotrifluoroethylene (CTFE) and / or tetrafluoroethylene (TFE).

[0028] The CTFE or TFE copolymer of the present invention may contain 0 to 10 mol %, preferably 0 to 5 mol %, based on the total amount of monomers, of one or more fluorinated and / or hydrogenated comonomers different from CTFE, TFE, or ethylene. Preferably, when present, the optional comonomer comprises a hydrogenated comonomer selected from the group of (meth)acrylic monomers. More preferably, the hydrogenated comonomer is selected from the group of hydroxyalkyl acrylate comonomers, such as hydroxyethyl acrylate, hydroxypropyl acrylate, and (hydroxy)ethylhexyl acrylate, and alkyl acrylate comonomers, such as n-butyl acrylate.

[0029] The ETFE and ECTFE copolymers obtained from the process of the present invention have the typical properties of ETFE and ECTFE known in the art and are characterized by a particularly low yellowness index of less than 25 (measured according to ASTM E313-05).

[0030] Without being bound by theory, it is believed that the reduced yellowness index is associated with the different chain ends that are formed when following the method of the present invention.

[0031] In fact, conventional initiation of ECTF / ECTFE polymerization reactions using TCAP as initiator leads to the formation of end groups that are primarily CCl3: this is because the initiation reaction of TCAP is Cl3C . This is due to the formation of radicals that ultimately lead to the following chain termination reaction: Start: (CCl3COO)2→2CO2+2Cl3C· Stop: Cl3C + (chain) CFClCF2CH2CH2 → (chain) CFClCF2CH2CH2CCl3

[0032] In contrast, for example, in the case of the oxalate / permanganate redox couple, initiation leads to COO . A radical is formed, which undergoes the following termination reaction: Stop:COO .+(chain)CFClCF2CH2CH2=(chain)CFClCF2CH2CH2COO - For example, whenever the reducing agent of the redox couple is an organic acid, a similar terminus containing a carboxylic acid group is formed.

[0033] The method of the present invention requires that the reducing agent does not contain sulfur atoms with an oxidation number of 4 or less. This type of reducing agent is SO4 . and SO3H . They have a tendency to form radicals, which form sulfur-containing chain ends according to the following termination reaction: SO4 . +(chain)CFClCF2CH2CH2=(chain)CFClCF2CH2CH2SO4 SO3H . +(chain)CFClCF2CH2CH2=(chain)CFClCF2CH2CH2SO3H Without wishing to be bound by theory, it is believed that these chain ends adversely affect the yellowness index of the resulting material.

[0034] For these reasons, the fluoropolymers obtained by the process of the present invention are characterized by having chain ends terminated predominantly with carboxyl groups in the form of acids, salts, or esters, and by having a yellowness index measured according to ASTM E313-05 of less than 25. "Predominantly terminated" means that, considering all end groups of the polymer, more than 50% of the chain ends are carboxyl groups. In the polymers according to the present invention, the concentration of carboxyl groups, including the chain ends, is typically greater than 5 mmol / kg of polymer.

[0035] Fluoropolymer chain ends can be determined by NMR using known methods as described by PIANCA, M., et al., "End groups in fluoropolymers." Journal of Fluorine Chemistry. 1999, vol. 95, pp. 71-84. The chain end concentration is expressed as millimoles per kg of polymer.

[0036] If the disclosure of any patent, patent application, or publication incorporated herein by reference contradicts the statement of this application to the extent that it may render a term unclear, the statement shall control.

[0037] The present invention will now be described with reference to the following examples, the purpose of which is merely to illustrate and not to limit the scope of the invention.

[0038] Yellowness Index Test 0.15 grams of the polymer powders obtained from Examples 1-5 were flash-compressed into 50 micrometer thick (circular, approximately 45 mm diameter) films at 270°C and 160 bar pressure. The yellowness index of the resulting films was measured directly using a Gardner colorimeter according to ASTM E313-05, "Standard practice for calculating Yellowness and Whiteness indices from Instrumentally Measured Color Coordinates." Ratings range from 0 to 100, with lower values ​​indicating lower (better) yellowness indexes.

[0039] Example 1 - Comparative An enamel-lined autoclave equipped with Hastelloy C baffles and a stirrer was alternately evacuated and purged with nitrogen to remove oxygen, and then charged with 7.6 liters of demineralized water, 1.3 liters of methyl alcohol, 25 g of chloroform, and 3.3 kg of chlorotrifluoroethylene. The stirrer was set to 600 rpm, and the autoclave was heated to 15°C. Ethylene gas was then fed to a pressure of 14.5 bar (absolute). The polymerization was initiated at -15°C by continuously feeding a solution of trichloroacetyl peroxide (TCAP) in isooctane at a rate of 86 ml / h to a concentration equivalent to 0.14 g of TCAP / ml. The pressure was maintained constant throughout the polymerization by continuously feeding ethylene into the reactor. After 300 minutes, the polymerization was stopped by interrupting the initiator feed. At the end of the polymerization, 200 g of ethylene and 430 ml of initiator solution were used. The resulting ECTFE polymer was removed from the autoclave, centrifuged, and dried at 120° C. for approximately 16 hours.

[0040] Example 2 (present invention) The same process as in Example 1 was used, except that a redox initiator was used instead of TCAP. The redox initiator was formed from an oxidizing agent (fresh aqueous potassium permanganate solution at a concentration of 23.7 g / L) and a reducing agent (fresh aqueous oxalic acid (C2H2O4) solution at a concentration of 33.75 g / L). The two solutions were simultaneously fed into the reactor through two different inlets at a rate of 230 ml / h each. The polymerization was interrupted after 400 minutes. At the end of the polymerization, 200 g of ethylene and 1530 ml of each initiator solution were used. The ECTFE polymer was dried as in Example 1.

[0041] Example 3 (Comparative) The same process as in Example 2 was followed, except that the oxalic acid solution was replaced with a sodium bisulfite solution having a concentration of 23.4 g / l. As in Example 2, the two solutions were simultaneously fed separately into the reactor through two different inlets at a rate of 230 ml / h each. The polymerization was interrupted after 173 minutes. At the end of the polymerization, 200 g of ethylene was used. The ECTFE polymer was dried as in Example 1.

[0042] Example 4 (Comparative) The same process as in Example 2 was followed, except that no chloroform was introduced into the aqueous mixture and the redox initiator was formed by: - TBHP (tert-butyl hydroperoxide) aqueous solution containing 34.44 g / l of TBHP - an aqueous solution of sodium hydroxymethanesulfinate (Bruggolite® E28) at a concentration of 46.2 g / l. The initiator solutions were each fed at 300 ml / h. The polymerization was interrupted after 400 minutes. At the end of the polymerization, 200 g of ethylene and 2000 ml of each initiator solution were used. The ECTFE polymer was dried as in Example 1.

[0043] [Table 1]

Claims

1. - providing a reactor containing a liquid reaction medium comprising water; - introducing into said reactor ethylene monomer and a fluorinated monomer selected from tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), or a mixture thereof; - pressurizing the reactor; - feeding a redox initiator comprising an oxidizing agent and a reducing agent to the reactor to initiate polymerization, wherein the reducing agent in the redox initiator pair does not contain sulfur atoms with an oxidation number of 4 or less; Including, the reaction medium is free of fluorinated surfactants, A method for preparing a fluoropolymer in an aqueous reaction medium.

2. 10. The process of claim 1, wherein the reactor is pressurized at a pressure of from 2 to 60 bar.

3. 3. The process of claim 1 or 2, wherein the oxidizing agent and the reducing agent are simultaneously and continuously introduced into the pressurized reactor from two independent feed streams.

4. 3. The process of claim 1 or 2, wherein one of the oxidizing agent or the reducing agent is at least partially contained in the liquid reaction medium, and the other is continuously introduced into the pressurized reactor.

5. The method of any one of claims 1 to 4, wherein the oxidizing agent comprises one or more of inorganic persulfates, inorganic peroxides, organic peroxides, and manganese-based oxidizing agents.

6. The method of any one of claims 1 to 5, wherein the oxidizing agent comprises one or more of manganese acetate and metal permanganate salts.

7. The method of any one of claims 1 to 6, wherein the oxidizing agent comprises potassium permanganate.

8. The method of any one of claims 1 to 7, wherein the reducing agent does not contain sulfur atoms.

9. The method of any one of claims 1 to 8, wherein the reducing agent comprises one or more organic acids.

10. The method of any one of claims 1 to 9, wherein the reducing agent comprises oxalic acid.

11. The process according to any one of claims 1 to 10, wherein the polymerization is carried out at a T of from 0°C to 30°C.

12. The method of claim 1 wherein the liquid reaction medium also comprises a chain transfer agent.

13. The process according to claim 1, wherein the chain transfer agent is preferably selected from C1 to C4 halogenated hydrocarbons, preferably chloroform.

14. - its chain ends containing mainly carboxylic acid groups - Yellowness index according to ASTM E313-05 of less than 25 Fluoropolymer obtainable by the method according to any one of claims 1 to 13, characterized in that

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