Process for manufacture of fluoropolymers

Abstract

The invention relates to a process for preparing a fluoropolymer in a liquid reaction medium, the process comprising the steps of:-providing a reactor containing a liquid reaction medium comprising water,-introducing into said reactor an ethylene monomer and a fluorinated monomer selected from tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) or mixtures thereof, the invention relates to a method for producing a fluorinated surfactant, comprising the steps of:-adding a reaction medium to a reactor,-pressurizing the reactor,-feeding a redox initiator into the reactor to initiate polymerization, the redox initiator comprising an oxidizing agent and a reducing agent, where the reducing agent in the redox initiator does not contain sulfur atoms having an oxidation number of 4 or less, and where the reaction medium does not contain a fluorinated surfactant.

Description

This case is a divisional application of the patent application with the application date of December 9, 2020, application number 202080083290.4, and invention name "Method for Manufacturing Fluoropolymers". Technical Field

[0001] This application claims priority to U.S. Provisional Application No. 62 / 947,318, filed December 12, 2019, and European Application 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 comprising repeating units derived from ethylene and repeating units derived from chlorotrifluoroethylene (CTFE) and / or tetrafluoroethylene (TFE) using a redox couple initiator. The method of the invention allows the preparation of ethylene-tetrafluoroethylene (ETFE) copolymers and / or ethylene-chlorotrifluoroethylene (ECTFE) copolymers in an aqueous environment without the use of 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 Art

[0003] The method for preparing ETFE and ECFTE multipolymer is well known in the art.This type of multipolymer is typically prepared in aqueous media using peroxide initiators (such as perhalogenated acyl peroxides) in industry, and bis(trichloroacetyl) peroxide (TCAP) is the most frequently used initiator in this area.Perhalogenated acyl peroxides can be used as polymerization initiators, particularly in the polymerization of halogenated monomers (such as chlorotrifluoroethylene and tetrafluoroethylene).Do not have any hydrogen atom in these peroxides to make the preparation have the halogenated polymer of fully halogenated chain end.

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

[0005] Halogenated hydrocarbons, such as those disclosed in US Pat. No. 2,816,147 (e.g., trichlorotrifluoroethane), have a high potential to destroy ozone in the stratosphere and their production and use have been strictly restricted by the Montreal Protocol. Potential alternatives, such as perfluorocarbons, are also greenhouse gases and may exacerbate global warming, so their use is restricted, making them unsuitable for replacing trichlorotrifluoroethane.

[0006] Co-pending European patent application 18214305.7 describes the use of isooctane as a solvent for bis(trichloroacetyl)peroxide. Isooctane is a good solvent for initiators but has the disadvantage of being a flammable solvent, which can be a problem in certain situations with regard to safe handling.

[0007] Another shortcoming of the prior art is the use of fluorinated surfactants in conventional polymerization of ETFE / ECTFE (see, for example, US 4482685). As is well known to the skilled person, fluorinated surfactants are also undesirable from an environmental point of view.

[0008] Alternative peroxides and persulfates are being considered as initiators for ETFE and ECTFE copolymers. However, many peroxide and persulfate initiators only function effectively as initiators at temperatures high enough to break the peroxide or persulfate bonds, thereby generating a sufficient number of free radicals that can initiate polymerization. Most peroxides and persulfates require relatively high temperatures to be effective.

[0009] In contrast, it is preferred to produce ETFE and ECTFE at relatively low temperatures because ETFE and ECTFE polymers are preferably formed from regular alternating sequences of ethylene monomer (ET) and fluorinated monomer. The properties of these polymers are related to the presence of these regular alternating sequences, so that for a given stoichiometry, the number of ET-ET, TFE-TFE, or CFTE-CTFE sequences should generally be minimized (although some is unavoidable, especially when ET is in excess or deficiency relative to the fluorinated monomer). As is well known in the art, increasing the polymerization temperature corresponds to an increase in irregular sequences, which leads to a deterioration in the polymer properties. Therefore, it is generally desirable to conduct the polymerization of ECTFE and ETFE in a temperature range below 30°C, and preferably around 15°C. This further limits the number of free radical initiators that can be used, as many free radical initiators require higher temperatures for free 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. Even at very low temperatures, redox reactions are typically very rapid and result in the formation of free radicals, which, in the presence of polymerizable monomers, initiate their polymerization. Continuous, controlled feeding of a redox initiator (typically in the form of two separate feeds of oxidizing agent and reducing agent) can maintain the polymerization reaction until it is completed. Methods for polymerizing ETFE and / or ECTFE using redox initiators have been reported in the prior art (e.g., as in WO97 / 17381). However, the methods of the prior art typically use reducing agents containing bisulfite or other compounds containing S(IV) atoms (sulfur atoms with an oxidation number of +4). The present application has found that the use of bisulfite may cause discoloration of the polymers thus obtained. For this same reason, the use of redox initiators has so far hardly been industrialized in the ETFE / ECTFE industry.

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

[0012] The present invention relates to a process for preparing a fluoropolymer in a liquid reaction medium, comprising the following steps: - providing a reactor containing a liquid reaction medium comprising water - introducing into the reactor an ethylene monomer and a fluorinated monomer selected from TFE, CTFE or a mixture thereof, - pressurizing the reactor, - feeding a redox initiator into the reactor to initiate polymerization, the redox initiator comprising an oxidizing agent and a reducing agent, wherein the reducing agent in the redox couple initiator does not contain a sulfur atom having an oxidation number of 4 or less, - wherein the reaction medium does not contain a fluorinated surfactant. DETAILED DESCRIPTION

[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 comprising water, preferably wherein water is greater than 50% or greater than 80% or greater than 95% by weight of the liquid reaction medium (the entire liquid contents of the reactor (excluding monomers, polymers, initiators and chain transfer agents) are considered as the reaction medium), and can be carried out by copolymerization of the monomers according to known techniques. Typically, TFE and / or CTFE monomers and ethylene monomer are fed into a sealed reactor and dispersed (typically kept under stirring) in a reaction medium comprising water and other optional additives. It has been found that the presence of a fluorinated surfactant is not necessary in the method of the present invention, and therefore, in order to minimize environmental impact, the method of the present invention is carried out in the absence of a fluorinated surfactant. In some embodiments, a non-fluorinated surfactant may be used to aid monomer dispersion, but the method of the present invention is preferably carried out in the absence of an added surfactant.

[0014] Monomer is typically fed into the sealed reactor in gaseous form (preferably from independent gas stream). The gas containing monomer can also optionally contain other gaseous components or is only formed by monomer. Typically, the gas containing monomer is used to pressurize the reactor to a pressure of from 2 to 60 bar, preferably from 5 to 40 bar, more preferably from 8 to 20 bar. Typically, during the polymerization reaction, one or more of the gas containing monomer is continuously added to the reactor to maintain the reactor to be pressurized in a desired scope. Typically, ethylene gas is used to maintain the reactor to be pressurized.

[0015] According to the present invention, polymerization is initiated by introducing a water-based redox initiator into the reactor. The redox initiator comprises an oxidizing agent and a reducing agent. The two reagents can be continuously introduced into the reactor together from two separate feeds so that when the oxidizing agent and the reducing agent come into contact with each other, they form free radicals capable of initiating polymerization. Alternatively, one of the oxidizing agent or the reducing agent can be present in the reaction mixture, while the other component of the redox initiator is gradually fed during the polymerization process.

[0016] When polymerization is complete, the redox initiator feed is interrupted. The reactor can now be drained and the polymer collected. Depending on the reaction conditions, the polymer can be collected in the form of a slurry, aqueous suspension, or latex. Conventional techniques can then be used to extract and further process the polymer for any purpose.

[0017] In the present invention, the selection of oxidant is not particularly limited, preferably, oxidant is water-soluble or water-dispersible, and this is advantageous because it does not need to use the organic solvent to be introduced into reactor.More preferably, oxidant comprises one or more compounds selected from following: inorganic persulfate (for example alkali metal persulfate, particularly sodium persulfate, lithium persulfate, potassium persulfate or ammonium persulfate), inorganic peroxide (for example hydrogen peroxide, sodium peroxide, potassium peroxide, lithium peroxide, ammonium peroxide), manganese-based oxidant (for example manganese triacetate, metal permanganate, particularly alkali metal permanganate such as sodium permanganate, lithium permanganate and potassium permanganate) and organic peroxide (for example tert-butyl hydroperoxide, di-tert-butyl peroxide, cumene hydroperoxide, tert-amyl hydroperoxide).Organic peroxide can preferably be selected as enough hydrophilicity to use in aqueous reaction medium without the need for additional solvent.Preferred oxidant comprises potassium permanganate.In addition, the mixture of these compounds listed above can be used as oxidant of the present invention.

[0018] In the present invention, the reducing agent must not contain a sulfur atom with an oxidation number of 4 or less. The reducing agent preferably does not contain a sulfur atom. Suitable compounds that can be used as the reducing agent of the present invention are 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. In addition, mixtures of the compounds listed above can be used as the reducing agent of the present invention.

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

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

[0021] The molecular weight of the fluoropolymer of the present invention can be controlled using techniques known to those skilled in the art, particularly by controlling the dosage of the initiator and / or by using a chain transfer agent. Optional chain transfer agents can be added to the polymerization reactor at any stage of the polymerization reaction. Conventionally used for polymerizing fluoropolymers and particularly for making any chain transfer agent of ETFE and ECTFE copolymers can be used in the present invention. Non-limiting examples of suitable chain transfer agents are: alcohols, ketones and carboxylic esters, halogenated hydrocarbons, mercaptans. Examples of suitable chain transfer agents are, for example, isopropyl alcohol, methanol, acetone, ethyl acetate, chloroform, 1,1,2,2-tetrachloroethane, bromotrichloromethane or butanethiol and other substances as described in US 3,069,401. Chain transfer agents are introduced into the reactor continuously or stepwise at the start of the reaction or during polymerization. Depending on the polymerization conditions (reaction temperature, monomer, molecular weight required for polymer, etc.), the amount of chain transfer agent can vary within a fairly wide range. Generally, this amount varies between 0.001 and 5% by weight, preferably between 0.05 and 1% by weight, relative to the total amount of monomers introduced into the reactor.

[0022] Preferably, the chain transfer agent used for the present invention comprises C1-C4 hydrocarbon or halogenated hydrocarbon. Particularly preferred chain transfer agent is chloroform. As is known in the art, in some cases, certain comonomers (except ethylene, TFE and CTFE) can have the effect of chain transfer agent. For example, acrylic acid comonomer can serve as chain transfer agent.

[0023] The method of the present invention can be applied to suspension polymerization or emulsion polymerization. However, it is preferred to use suspension polymerization.

[0024] The method is carried out in a reaction medium that does not contain a fluorinated surfactant. "Fluorinated surfactant" in the present invention means a compound that conforms to the following formula (I): R f§ (X - ) k (M + ) k (I) in: -R f§ Selected from C5-C 16 (per)fluoroalkyl chains (including linear, branched or ring-containing chains), which optionally contain one or more chained or non-chained oxygen atoms, and (per)fluoropolyoxyalkyl chains (including linear, branched or ring-containing chains), -X - Selected from -COO - 、-PO3 - and -SO3 - , -M + Selected from NH4 +and alkali metal ions, and -k is 1 or 2.

[0025] Non-limiting examples of fluorinated surfactants include the following: (a)CF3(CF2) n0 COOM', wherein 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", where T represents a Cl atom or a x F 2x+1-x’ Cl x’ O, wherein x is an integer ranging from 1 to 3 and 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 difunctional fluorinated surfactant, wherein A and B are the same or different from each other, having the formula -(O) p CFX"-COOM*, wherein 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 a divalent (per)fluoroalkyl chain or (per)fluoropolyether chain, such that AR bf - The number average molecular weight of B is in the range of from 300 to 1800. (e) cC6O4(perfluoro{acetic acid, 2-[(5-methoxy-1,3-dioxolan-4-yl)oxy]}), in the form of an acid or as an alkali metal or ammonium salt.

[0026] The process of the present invention can be used to prepare copolymers comprising repeating units derived from ethylene and repeating units derived from a fluoromonomer selected from CTFE and TFE. Other comonomers may be present, however, preferably greater than 70% by mole, more preferably greater than 80% by mole, even more preferably greater than 90% by mole, and most preferably greater than 95% by mole of repeating units of the polymer derived from ethylene, TFE or CTFE.

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

[0028] The CTFE or TFE copolymers of the present invention may contain from 0% to 10%, preferably from 0% to 5%, by mole, based on the total amount of monomers, of one or more fluorinated and / or hydrogenated comonomers different from CTFE, TFE, and ethylene. Preferably, when the optional comonomer is present, it comprises a hydrogenated comonomer selected from the group of (meth)acrylic acid 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 by the process of the invention are endowed with 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 related to the different chain terminations formed when following the method of the present invention.

[0031] In fact, conventional initiation of ECTF / ECTFE polymerization using TCAP as initiator leads to the formation of end groups that are mainly CCl3: this is because the initiation reaction with TCAP leads to Cl3C . The formation of free radicals, which ultimately lead to the chain termination reaction depicted below: Initiation: (CCl3COO)2→2CO2+2Cl3C Termination: Cl3C·+(chain)CFClCF2CH2CH2·→(chain)CFClCF2CH2CH2CCl3

[0032] In contrast, in the case of, for example, the oxalate / permanganate redox couple, the initiation leads to COO . The formation of free radicals, which lead to the termination reaction depicted below: Termination: COO . + (chain)CFClCF2CH2CH2=(chain)CFClCF2CH2CH2COO - For example, whenever the reducing agent in a redox couple is an organic acid, a similar termination comprising a carboxyl group is formed.

[0033] The process of the present invention requires that the reducing agent does not contain sulfur atoms having an oxidation number of 4 or less. Such reducing agents tend to form SO4 . and SO3H . Free radicals that form sulfur-containing chain terminals according to the following termination reactions: SO4 . + (chain)CFClCF2CH2CH2=(chain)CFClCF2CH2CH2SO4 SO3H . + (chain)CFClCF2CH2CH2=(chain)CFClCF2CH2CH2SO3H Without being bound by theory, it is believed that these chain terminations have a negative impact on 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 a yellowness index measured according to ASTM E313-05 of less than 25. "Predominantly terminated" means that when all the terminating groups of the polymer are considered, more than 50% of the chain terminations are carboxyl groups. In the polymers according to the present invention, the concentration of chain terminations containing carboxyl groups is typically higher than 5 mmoles / kg of polymer.

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

[0036] Should the disclosure of any patents, patent applications, and publications incorporated herein by reference conflict with the description of the present application to the extent that a term is unclear, the present description shall take precedence.

[0037] The present invention will now be described with reference to the following examples, which are intended to be illustrative only and not to limit the scope of the invention.

[0038] Yellowness Index Test 0.15 g of the polymer powder obtained from Examples 1-5 was flash compression molded into a 50 micron thick film (circular with a diameter of approximately 45 mm) at 270°C under a pressure of 160 bar. The yellowness index of the obtained film was directly measured using a Gardner Colorimeter according to ASTM E313-05, "Standard practice for calculating Yellowness and Whiteness indices from Instrumentally Measured Color Coordinates." The ratings range from 0 to 100, with smaller values indicating a lower (better) yellowness index.

[0039] Example 1-Comparison An enameled autoclave equipped with baffles and a Hastelloy stirrer was freed of oxygen by alternately evacuating and purging with nitrogen, and then 7.6 1 of demineralized water, 1.3 1 of methanol, 25 g of chloroform and 3.3 kg of chlorotrifluoroethylene were introduced. The stirrer was set to 600 rpm and the autoclave was heated to 15°C; ethylene gas was then fed until the pressure reached 14.5 bara. The polymerization was started by continuously feeding a solution of trichloroacetyl peroxide (TCAP) in isooctane at -15°C at a rate of 86 ml / h, with a titer equal to 0.14 g TCAP / ml. During the polymerization, the pressure was kept constant 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 discharged from the autoclave, centrifuged and dried at 120°C for about 16 hours.

[0040] Example 2 According to the present invention The same method used in Example 1 was used, except that a redox initiator was used instead of TCAP. The redox initiator consisted of an oxidizing agent (a fresh aqueous solution of potassium permanganate having a concentration of 23.7 g / l) and a reducing agent (a fresh aqueous solution of oxalic acid (C2H2O4) having a concentration of 33.75 g / l). The two solutions were fed simultaneously and separately to 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 comparison The same method of Example 2 was followed, except that the oxalic acid solution was replaced by a sodium bisulfite solution having a concentration of 23.4 g / l. As in Example 2, the two solutions were fed separately and simultaneously 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 were used. The ECTFE polymer was dried as in Example 1.

[0042] Example 4 comparison The same procedure of Example 2 was followed, except that chloroform was not introduced into the aqueous mixture and the redox initiator was formed from: - an aqueous solution of TBHP (tert-butyl hydroperoxide) having 34.44 g / l of TBHP -Sodium Hydroxymethanesulfinate ( E28) An aqueous solution having 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. Experimental results Examples Y Index 1 (comparison) 10 2 (the present invention) 10 3 (Comparison) 50 4 (Comparison) 70

Claims

1. A method for preparing a fluoropolymer in an aqueous reaction medium, the method comprising the steps of: - providing a reactor containing a liquid reaction medium comprising water - introducing into the reactor an ethylene monomer and a fluorinated monomer, the fluorinated monomer being selected from tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) or a mixture thereof, - pressurizing the reactor, - feeding a redox initiator into the reactor to initiate polymerization, the redox initiator comprising an oxidizing agent and a reducing agent, wherein the reducing agent in the redox couple initiator does not contain a sulfur atom having an oxidation number of 4 or less, - wherein the reaction medium does not contain a fluorinated surfactant, The polymerization is carried out at a temperature of from 0°C to 20°C.

2. The method according to claim 1, wherein The reactor is pressurized at a pressure of 2 to 60 bar.

3. A method according to any preceding claim, wherein: The oxidizing agent and the reducing agent are introduced simultaneously and continuously into the pressurized reactor from two separate feeds.

4. The method according to any one of claims 1 to 2, wherein One of the oxidizing agent or the reducing agent is at least partially contained in the liquid reaction medium, while the other is continuously introduced into the pressurized reactor.

5. A method according to any preceding claim, wherein: The oxidizing agent includes one or more of an inorganic persulfate, an inorganic peroxide, an organic peroxide, and a manganese-based oxidizing agent.

6. A method according to any preceding claim, wherein: The oxidizing agent includes one or more of manganese acetate and metal permanganate.

7. A method according to any preceding claim, wherein: The oxidizing agent includes potassium permanganate.

8. A method according to any preceding claim, wherein: The reducing agent does not contain sulfur atoms.

9. A method according to any preceding claim, wherein: The reducing agent includes one or more organic acids.

10. A method according to any preceding claim, wherein: The reducing agent comprises oxalic acid.

11. The method according to claim 1, wherein The liquid reaction medium also contains a chain transfer agent.

12. The method according to claim 1, wherein The chain transfer agent is preferably selected from C1-C4 halogenated hydrocarbons, preferably chloroform.