JPWO2018199264A1 - Binary tetrafluoroethylene / ethylene copolymer and method for producing the same - Google Patents

Abstract

Provided is a method for producing a binary tetrafluoroethylene (TFE) / ethylene (E) copolymer having excellent mechanical strength and thermal stability. In a reaction system containing an aqueous medium, an emulsifier, and a radical polymerization initiator in an amount of 10 to 5000 ppm based on the aqueous medium, TFE and E are used. A method for producing a binary TFE / E copolymer by polymerizing E while maintaining a ratio of 40 to 15 mol%, wherein the radical polymerization initiator generates a radical when reduced in water. A polymerization temperature of 5 to 40 ° C., and an alkali for neutralizing an acid generated during the polymerization reaction and a reducing agent for reducing the radical polymerization initiator, respectively, are added to the aqueous medium.

Description

  The present invention relates to a binary tetrafluoroethylene / ethylene copolymer excellent in mechanical strength and thermal stability and a method for producing the same.

Fluororesin molded articles have excellent heat resistance, chemical resistance, and weather resistance, and are widely used in various applications. For example, a fluororesin porous material is widely used as a filter or a waterproof membrane having air permeability.
Of the fluororesins, polytetrafluoroethylene (hereinafter also referred to as PTFE) easily fibrillates. Therefore, a PTFE film (porous body) having fine pores can be obtained relatively easily by stretching a preform obtained by calendering PTFE fine powder obtained by emulsion polymerization.

In recent years, attempts have been made to mold a porous body of a tetrafluoroethylene / ethylene copolymer (hereinafter, also referred to as a TFE / E copolymer). As a method for producing a porous body of a TFE / E copolymer, for example, the following method has been proposed.
(1) A method of melt-molding a mixture of a TFE / E copolymer, a solvent-soluble resin, and an inorganic fine powder, and extracting the solvent-soluble resin and the inorganic fine powder from the obtained molded article (Patent Documents 1 and 2) .
(2) The TFE / E copolymer is dissolved in a specific solvent, the solution is molded at a temperature equal to or higher than the phase separation temperature, and the obtained molded product is cooled to a temperature equal to or lower than the phase separation temperature to obtain the TFE / E copolymer. A method of coagulating a copolymer (Patent Document 3).
(3) A method of lubricating a high crystalline high molecular weight TFE / E copolymer which can be fibrillated, calendering it, and pressurizing the TFE / E copolymer at a temperature lower than the melting temperature of the TFE / E copolymer (Patent Document 4).

Japanese Patent No. 3265678 Japanese Patent Application Laid-Open No. 2008-013615 WO 2010/044255 International Publication No. WO 2016/018970

  However, the TFE / E copolymer used in Patent Documents 1 to 3 has insufficient mechanical strength. Therefore, the porous bodies obtained by the methods described in these documents have a problem in that the mechanical strength at the time of molding and after the molding, particularly the mechanical strength at the time of trying to increase the porosity, becomes low.

  In the field of TFE / E copolymers, a method of copolymerizing a monomer (ethylene, tetrafluoroethylene) with another monomer (third component) has been studied in order to increase mechanical strength. However, usually, when the third component is copolymerized, the melting point of the obtained polymer decreases, and the thermal stability deteriorates.

The TFE / E copolymer used in Patent Document 4 is a high molecular weight binary TFE / E copolymer, and although its thermal stability is high to some extent, it cannot be said that it is sufficient for some applications.
In Patent Document 4, the TFE / E copolymer is obtained by emulsion polymerization using emulsion polymerization using a fluorine-based emulsifier and a fluorine-based solvent. Fluorinated emulsifiers and fluorinated solvents are suitable for obtaining high molecular weight compounds because radicals required for polymerization are unlikely to be chain-transferred to these fluorinated organic compounds, but they are expensive and have a high environmental load. . Therefore, a production method which can obtain a high-molecular-weight binary TFE / E copolymer in a system containing as little as possible of these materials is desired.

  An object of the present invention is to provide a binary TFE / E copolymer excellent in mechanical strength and thermal stability, and a method for producing the polymer.

The present invention has the following aspects.
[1] In a reaction system containing a mixed solution of water and a water-soluble organic solvent or a water-based aqueous medium, an emulsifier, and a radical polymerization initiator of 10 to 5000 ppm with respect to the aqueous medium, tetrafluoroethylene and ethylene are used. Polymerizing tetrafluoroethylene at a ratio of 60 to 85 mol% and ethylene at a ratio of 40 to 15 mol% with respect to the total amount of tetrafluoroethylene and ethylene to produce a binary TFE / E copolymer A way to
The radical polymerization initiator is to generate a radical when reduced in water,
The polymerization temperature is 5 to 40 ° C.,
An alkali for neutralizing an acid generated during the polymerization reaction is added to the reaction system at least one of before and during the polymerization, and a reducing agent for reducing the radical polymerization initiator is collectively or continuously added. A method for producing a binary TFE / E copolymer, which is added to the reaction system intermittently or intermittently.
[2] The production method of [1], wherein the pH of the reaction system is 3.5 to 12 during the period from the start of polymerization to the end of polymerization.
[3] The production method of [1] or [2], wherein at least a part of the alkali is continuously or intermittently added to the reaction system during polymerization.
[4] The amount of [1] to [3], wherein the amount of the emulsifier added (the total amount used in the polymerization step) is 1,500 to 20,000 ppm based on the yield of the binary TFE / E copolymer. Any manufacturing method.
[5] The method according to any one of [1] to [4], wherein the amount of the reducing agent (the total amount used in the polymerization step) is 1 to 200 parts by mass with respect to 100 parts by mass of the radical polymerization initiator. .
[6] The method of any one of [1] to [5], wherein the alkali is ammonia or an alkali metal hydroxide.
[7] The method according to any one of [1] to [6], wherein the polymerization is performed while maintaining the polymerization pressure at 1.8 to 5.0 MPa.
[8] the ratio of the tetrafluoroethylene unit to the total amount of the tetrafluoroethylene unit and the ethylene unit is 45 to 65 mol%,
A binary TFE / E copolymer having a volume flow rate of 10 mm ³ / sec or less measured at a temperature of 320 ° C. and a load of 588.4 N according to ASTM1238.
[9] A binary system in which the ratio of the tetrafluoroethylene unit to the total amount of the tetrafluoroethylene unit and the ethylene unit is 45 to 65 mol%, and the melting point 1st measured by the following measurement method is 294 to 300 ° C. TFE / E copolymer.
"Measurement method of melting point"
The endothermic peak due to melting is measured by differential scanning calorimetry. The temperature transition program during the measurement is -20.degree. C. → 310.degree. C. → -70.degree. C. → 310.degree. C., and the rate of temperature rise is 10.degree. The temperature at the endothermic peak due to the heat of fusion at the time of the first temperature rise is defined as “melting point 1st”, and the temperature at the endothermic peak due to the heat of fusion at the second temperature rise after the temperature is lowered to −70 ° C. is defined as “melting point 2nd”.
[10] The ratio of the tetrafluoroethylene unit to the total amount of the tetrafluoroethylene unit and the ethylene unit is 45 to 65 mol%,
A binary TFE / E copolymer having a melting point 2nd measured by the above-mentioned measuring method of 290 to 295 ° C.

According to the production method of the present invention, a binary TFE / E copolymer excellent in mechanical strength and thermal stability can be produced without using a fluorine-based solvent.
The binary TFE / E copolymer of the present invention is novel and has excellent mechanical strength and thermal stability.

The meaning of the following terms in this specification, the measuring method, and the like are as follows.
The “binary TFE / E copolymer” is a binary copolymer of tetrafluoroethylene (hereinafter, also referred to as TFE) and ethylene, and is a unit derived from TFE (hereinafter, also referred to as a TFE unit). ) And a unit derived from ethylene (hereinafter, also referred to as E unit). The binary copolymer does not contain units derived from monomers other than TFE and ethylene. In the binary copolymer, the masses of components other than the TFE unit and the E unit are traces and can be ignored.
“The copolymer composition (unit: mol%) is calculated from the results of the total fluorine amount measurement and the melt ¹⁹ F-NMR measurement.
The “volume flow rate” (hereinafter, also referred to as Q value (unit: mm ³ / sec)) is a value measured at a temperature of 320 ° C. and a load of 588.4 N (= 60 kgf) in accordance with ASTM1238.

The “melting point (unit: ° C.)” is a temperature at an endothermic peak due to melting measured under a nitrogen atmosphere using a differential scanning calorimeter (DSC Q200 manufactured by TA Instruments). The temperature transition program during the measurement is -20.degree. C. → 310.degree. C. → -70.degree. C. → 310.degree. C., and the rate of temperature rise is 10.degree. The temperature at the endothermic peak due to the heat of fusion at the time of the first temperature rise is defined as “melting point 1st”, and the temperature at the endothermic peak due to the heat of fusion at the second temperature rise after the temperature is lowered to −70 ° C. is defined as “melting point 2nd”.
"Ppm" which is a unit of "content" is based on mass.
"Polymerization pressure" is represented by gauge pressure.
“%” Indicates “% by mass” unless otherwise specified.

[Method for producing binary TFE / E copolymer]
In the method for producing a binary TFE / E copolymer of the present invention, TFE and ethylene are polymerized in a reaction system containing an aqueous medium, an emulsifier, and a specific amount of a radical polymerization initiator while maintaining a specific ratio. (Polymerization step).
As the radical polymerization initiator, one that generates a radical when reduced in water is used.
In the production method of the present invention, the radical polymerization initiator is used as a redox polymerization initiator in combination with a reducing agent that reduces the radical polymerization initiator. Therefore, in the polymerization step, a reducing agent is added to the reaction system. In the polymerization step, an alkali is also added.
In the polymerization step, an aqueous emulsion (latex) in which the binary TFE / E copolymer is dispersed in an aqueous medium is obtained. After the polymerization step, if necessary, a binary TFE / E copolymer is recovered from the aqueous emulsion (recovery step).

(Aqueous medium)
As the aqueous medium, a mixed solution of water and a water-soluble organic solvent, or water is used.
Examples of the water include ion-exchanged water, pure water, ultrapure water, and the like.
Examples of the water-soluble organic solvent include alcohols, ketones, ethers, ethylene glycols, propylene glycols and the like. Of these, alcohols are preferred in terms of gas absorption.
Examples of alcohols include tert-butanol, n-butanol, ethanol, methanol and the like. In particular, tert-butanol is preferable since a chain transfer reaction hardly occurs and a high molecular weight TFE / E copolymer is easily formed.

When the aqueous medium is a mixture of water and a water-soluble organic solvent, the content of the water-soluble organic solvent in the aqueous medium is preferably 1 to 50 parts by mass, and more preferably 3 to 20 parts by mass with respect to 100 parts by mass of water. Parts are more preferred.
The content of the water-soluble organic solvent (alcohols, thiols, amines, etc.) that can function as a chain transfer agent is preferably 0 to 20 parts by mass, particularly preferably 0 to 5 parts by mass with respect to 100 parts by mass of water. preferable. The smaller the content of the water-soluble organic solvent that can function as a chain transfer agent, the higher the molecular weight of the obtained binary TFE / E copolymer, and the higher the thermal stability tends to be.

(emulsifier)
The emulsifier is not particularly limited, and a known fluorine-containing emulsifier can be used. One or more fluorine-containing emulsifiers selected from the group consisting of fluorine-containing carboxylic acids having 4 to 7 carbon atoms which may have an etheric oxygen atom and salts thereof, which have low persistence and low bioaccumulation. Are preferable, and among them, a fluorine-containing emulsifier having an etheric oxygen atom is more preferable. Here, the carbon number means the total carbon number in the fluorinated emulsifier.
The fluorine-containing carboxylic acid having an etheric oxygen atom is a compound having 4 to 7 carbon atoms, having an etheric oxygen atom in the middle of the carbon chain of the main chain, and having -COOH at a terminal. The terminal -COOH may form a salt. The number of etheric oxygen atoms present in the middle of the main chain is one or more, preferably one to four, more preferably one or two. The carbon number is preferably 5-7.

Examples of the fluorinated carboxylic acid include perfluoromonooxaalkanoic acid and perfluoropolyoxaalkane having 2 to 4 carbon atoms substituted with an etheric oxygen atom (that is, having 2 to 4 etheric oxygen atoms). Acids or carboxylic acids in which 1 to 3 fluorine atoms of these carboxylic acids have been replaced by hydrogen atoms are preferred. In addition, the carbon number of these carboxylic acids does not include the carbon atom substituted by the oxygen atom.
Preferred specific examples of the fluorinated carboxylic acid include C ₂ F ₅ OCF ₂ CF ₂ OCF ₂ COOH, C ₃ F ₇ OCF ₂ CF ₂ OCF ₂ COOH, CF ₃ OCF ₂ OCF ₂ OCF ₂ OCF ₂ COOH, CF _{3 O (CF 2 CF 2 O)} 2 CF 2 COOH, CF 3 CF 2 O (CF 2) 4 COOH, CF 3 CFHO (CF 2) 4 COOH, CF 3 OCF (CF 3) CF 2 OCF (CF 3) COOH _{, CF 3 O (CF 2)} 3 OCF (CF 3) COOH, CF 3 O (CF 2) 3 OCHFCF 2 COOH, C 4 F 9 OCF (CF 3) COOH, C 4 F 9 OCF 2 CF 2 COOH, CF _{3 O (CF 2) 3 OCF} 2 COOH, CF 3 O (CF 2) 3 OCHFCOOH, CF 3 OCF 2 O _{F 2 OCF 2 COOH, C 4} F 9 OCF 2 COOH, C 3 F 7 OCF 2 CF 2 COOH, C 3 F 7 OCHFCF 2 COOH, C 3 F 7 OCF (CF 3) COOH, CF 3 CFHO (CF 2) _{3 COOH, CF 3 OCF 2 CF} 2 OCF 2 COOH, C 2 F 5 OCF 2 CF 2 COOH, C 3 F 7 OCHFCOOH, CF 3 OCF 2 CF 2 COOH, CF 3 (CF 2) 4 COOH and the like.
More preferred _{examples, C 2 F 5 OCF 2 CF} 2 OCF 2 COOH, CF 3 O (CF 2) 3 OCF 2 COOH, CF 3 OCF (CF 3) CF 2 OCF (CF 3) COOH, CF 3 O _{(CF 2) 3 OCF 2 CF} 2 COOH, CF 3 O (CF 2) 3 OCHFCF 2 COOH, C 4 F 9 OCF (CF 3) COOH, C 3 F 7 OCF (CF 3) COOH can be mentioned.

Examples of the fluorinated carboxylate include alkali metal salts and ammonium salts thereof, and specific examples thereof include Li salts, Na salts, K salts, and NH ₄ salts. Particularly preferred is an ammonium salt (NH ₄ salt) of the above compound. An ammonium salt is excellent in solubility in an aqueous medium, and there is no possibility that a metal ion component remains as an impurity in the obtained TFE / E copolymer.

(Redox polymerization initiator)
The radical polymerization initiator that generates a radical when reduced in water can be appropriately selected from known radical polymerization initiators, and a water-soluble radical polymerization initiator is typically used. Examples of the water-soluble radical polymerization initiator include persulfates such as ammonium persulfate (hereinafter also referred to as “APS”), hydrogen peroxide, potassium permanganate, and the like.

  The reducing agent for reducing the radical polymerization initiator can be appropriately selected from known reducing agents, and includes, for example, Rongalite (sodium hydroxymethanesulfinate), sodium hydrogen sulfate, sodium thiosulfate, oxalic acid and the like.

  A small amount of iron, a ferrous salt (eg, ferrous sulfate), silver sulfate, and the like may coexist in the redox polymerization initiator. Further, a chelating agent (for example, disodium salt of ethylenediaminetetraacetic acid (EDTA) or the like) may be allowed to coexist with the redox polymerization initiator.

(alkali)
Examples of the alkali include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, and cesium hydroxide, and ammonia. As the alkali, ammonia or cesium hydroxide is preferred among the above, because the dispersion stability of the binary TFE / E copolymer in the aqueous emulsion is excellent. When residual metal ions in the obtained copolymer are concerned, ammonia is more preferable.
In the aqueous emulsion, the surface of the binary TFE / E copolymer particles is negatively charged. It is considered that the alkali-derived positive ions are distributed on the surface of the particles to form a diffusion electric double layer, suppress aggregation of the particles due to electrostatic repulsion, and enhance dispersion stability. In particular, since the cation radius (NH ₄ ⁺ or Cs ⁺ ) derived from ammonia or cesium hydroxide has a larger ionic radius than Na ⁺ , K ^+, etc., the electrostatic repulsion is easily extended to a long distance, and the dispersion stability is high. It is considered to be excellent in the effect of improving.

(Other ingredients)
The reaction system may contain components other than the aqueous medium, the emulsifier, the redox polymerization initiator, and the alkali. Other components include, for example, salts, acids, and the like, in which the mixed solution with the alkali becomes a buffer solution. Salts and acids whose mixed solution with the alkali becomes a buffer solution are used for pH adjustment.
Examples of the salt in which the mixed solution with the alkali becomes a buffer solution include phosphates, carbonates, borates and the like of the alkali. For example, when the alkali is sodium hydroxide, examples of the salt include disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen carbonate, sodium carbonate and the like.
Examples of the acid include hydrochloric acid, phosphoric acid, boric acid and the like.

(Polymerization step)
In the polymerization step, TFE and ethylene are polymerized in an aqueous medium in the presence of an emulsifier and a radical polymerization initiator.

In the polymerization reaction of TFE and ethylene, the ratio of TFE is 60 to 85 mol% and ethylene is 40 to 15 mol% based on the total amount of TFE and ethylene (TFE / ethylene = 60/40 to 85/15 ( (Molar ratio)) is maintained.
By maintaining the ratio of the monomer at the time of this polymerization reaction, the ratio of the TFE unit to the total amount of the TFE unit and the E unit in the binary TFE / E copolymer is 45 to 65 mol% (E unit Is 55 to 35 mol%), and a binary TFE / E copolymer is easily produced.
When the proportion of TFE units in the TFE / E copolymer is 45 to 65 mol%, the crystallinity of the copolymer increases. When the proportion of TFE units is 45 to 55 mol%, the crystallinity becomes higher. When the proportion of TFE units is 50 to 55 mol%, not only the degree of crystallinity is high, but also the chain of E units is reduced in the TFE / E copolymer, so that the heat resistance of the binary TFE / E copolymer is reduced. Is excellent.

For the above reason, the ratio of TFE units in the binary TFE / E copolymer is preferably 45 to 65 mol% (the unit ratio of E is 55 to 35 mol%), and is preferably 45 to 55 mol% (the unit ratio of E Is more preferably 55 to 45 mol%, and still more preferably 50 to 55 mol% (the ratio of the E unit is 50 to 45 mol%).
In order to obtain a binary TFE / E copolymer having a TFE unit ratio of 45 to 65 mol%, in the polymerization reaction, TFE is added in an amount of 60 to 85 mol% based on the total amount of TFE and ethylene. Is preferably maintained at a ratio of 40 to 15%. In order to obtain a binary TFE / E copolymer having a TFE unit ratio of 45 to 55 mol%, in the polymerization reaction, TFE is added in an amount of 60 to 78 mol% based on the total amount of TFE and ethylene. Is preferably maintained at a ratio of 40 to 22%. In order to obtain a binary TFE / E copolymer in which the proportion of TFE units is 50 to 55 mol%, in the polymerization reaction, 65 to 78 mol% of TFE and ethylene to TFE are added to the total amount of TFE and ethylene. Is preferably maintained at a ratio of 35 to 22%.
In the method for producing a binary TFE / E copolymer, the yield of the binary TFE / E copolymer is almost 100%, and the moles of TFE units and E units in the binary TFE / E copolymer The ratio can be estimated to be equal to the molar ratio of TFE and ethylene consumed in the polymerization reaction of TFE and ethylene.

The polymerization temperature is from 5 to 40C, preferably from 5 to 25C, more preferably from 5 to 15C. When the polymerization temperature is equal to or higher than the lower limit of the above range, a good polymerization rate is easily obtained. When the polymerization temperature is lower than the upper limit of the above range, the gaseous monomer (TFE, ethylene) is well dissolved in the reaction system, and the elongation reaction proceeds easily. Therefore, the molecular weight of the TFE / E copolymer increases, and a TFE / E copolymer having a high melting point, a low MFR, and a low Q value is easily obtained.
When the polymerization temperature is higher than 40 ° C., the radical polymerization initiator is reduced in water by heat even if there is no reducing agent, and the polymerization reaction proceeds.

  The polymerization pressure is preferably 1.8 to 5.0 MPa, more preferably 2.0 to 4.0 MPa, and particularly preferably 3.0 to 4.0 MPa. When the polymerization pressure is equal to or higher than the lower limit of the above range, the polymerization reaction is rapidly started, and a high molecular weight binary TFE / E copolymer is easily obtained. When the polymerization pressure is at most the upper limit of the above range, the operability will be excellent.

During the period from the start of the polymerization to the end of the polymerization, the pH of the reaction system is preferably from 3.5 to 12.0, more preferably from 7.0 to 10.5, and particularly preferably from 9.0 to 10.0. When the pH of the reaction system is within the above range, dispersion stability of the emulsion is easily obtained, and a good polymerization rate is easily obtained. The pH of the reaction system is the pH at 20 ° C.
Usually, if the pH of the reaction system at the start of polymerization and at the end of polymerization are within the above range, it can be determined that the pH of the reaction system during the period between them is also within the above range.

  In the period from the start of the polymerization to the end of the polymerization, the positive ion concentration of the reaction system is preferably 0.200 mol / L or less, more preferably 0.050 mol / L or less, and particularly preferably 0.010 mol / L or less. . When the temperature of the reaction system (polymerization temperature) decreases, the binary TFE / E copolymer generated by the polymerization reaction tends to aggregate. On the other hand, if the positive ion concentration in the reaction system is too high, the binary TFE / E copolymer tends to aggregate due to the salting out effect. When the positive ion concentration of the reaction system is equal to or lower than the above upper limit, good dispersion stability can be maintained and the polymerization reaction proceeds favorably even when the temperature of the reaction system is low.

The positive ion concentration is obtained by the following equation (1).
Positive ion concentration = total molar concentration of a substance which is an alkali or a salt of the alkali (mol / L) × the number of positive ions generated when the substance is completely dissociated (1)
However, the concentration of positive ions derived from a substance that does not completely ionize, such as ammonia, is preferably a value that takes into account the degree of dissociation from the base dissociation constant Kb. The positive ion concentration x in consideration of the degree of dissociation is obtained by substituting Kb at the polymerization temperature and the concentration c (mol / L) assuming complete dissociation into the following equation (2).
Kb = x2 / (c−x) (2)

Dissociation equilibrium reaction: MOH ⇔ M ^{+ +} OH ^-
Concentration before dissociation c 0 0
Concentration after dissociation c-xxx
Kb = [M ⁺ ] [OH ⁻ ] / [MOH]

For example, in the case of ammonia, since Kb at 25 ° C. is 1.81 × 10 ⁻⁵ , when the concentration of positive ions generated at the time of complete dissociation is 0.1 mol / L, the positive ion calculated from the above equation (2) is obtained. The ion concentration [NH ₄ ⁺ ] is 1.34 × 10 ⁻³ mol / L.
When a plurality of the above-mentioned substances are added to the reaction system, the above-mentioned positive ion concentrations are obtained for each substance, and the total value thereof is defined as the positive ion concentration of the reaction system.

In the polymerization step, an alkali for neutralizing an acid generated during the polymerization reaction is added to the reaction system at least one of before and during the polymerization. Thereby, the pH of the reaction system is prevented from being too low (for example, below 3.5) during the polymerization, and the pH of the reaction system is maintained within the above range from the start of the polymerization to the end of the polymerization. The polymerization reaction proceeds at the polymerization rate.
“Before the start of polymerization” refers to the time before the addition of the reducing agent (initial preparation).
The term “during polymerization” refers to a point in time when the reducing agent is first added (at the start of polymerization) and a point in time thereafter.

Examples of the acid generated during the polymerization reaction include a radical-derived acid generated from a radical polymerization initiator, and TFE-derived hydrofluoric acid.
For example, when persulfate is used as the radical polymerization initiator, the following reaction occurs.
⁻ O ₃ SOOSO ₃ ⁻ → 2 (OSO ₃ ⁻ )
_{^{- O 3 SO · + CF 2}} = CF 2 → - O 3 SOCF 2 CF 2 ·
_{^{- O 3 SOCF 2 CF 2 ·}} + n (CF 2 = CF 2) → - O 3 SO (CF 2 CF 2) n ·
_{^{- O 3 SO (CF 2 CF}} 2) n · + H 2 O → HO (CF 2 CF 2) n - + HSO 4 -
^{_{OH (CF 2 CF 2) n}} - + H 2 O → HOOCCF 2 (CF 2 CF 2) n-1 + 2HF
It is considered that the reaction actually occurring is not limited to this. For example, when an oxidation-reduction reaction in which one molecule of persulfate is decomposed per molecule of the reducing agent is used as a radical generation mechanism, and NaOH, CsOH, etc. In the case of adding an alkali that completely dissociates, adding about 10 equivalents of alkali per molecule of the reducing agent keeps the pH of the reaction system almost equal before and after the polymerization reaction, and improves the stability of the aqueous emulsion. When adding an alkali that does not completely dissociate, such as ammonia, the amount of addition is adjusted in consideration of the degree of ionization so that the pH of the reaction system is maintained within the above range before and after the polymerization reaction. Adjustment of the addition amount may be performed empirically or experimentally.

In the polymerization step, in addition to an emulsifier, a radical polymerization initiator, a reducing agent, and an alkali, if necessary, a component (coexisting component) that may coexist with the above-described redox polymerization initiator, and other components are added to the reaction system. May be.
Among the other components, when the mixed solution with the alkali contains a salt that becomes a buffer solution together with the alkali, the reaction system has a buffering action, and pH fluctuation due to an acid generated during the polymerization reaction is suppressed. On the other hand, when the salt is contained, the positive ion concentration of the reaction system increases, and the stability of the aqueous emulsion tends to decrease.

The emulsifier and the radical polymerization initiator may be added in their entirety before the start of the polymerization (that is, as an initial preparation), or a part thereof may be added before the start of the polymerization, and the rest may be added continuously or intermittently during the polymerization. May be. The same applies to the coexisting components added as needed.
The reducing agent may be added to the reaction system in its entirety at the beginning of the polymerization, or may be continuously or intermittently added to the reaction system during the polymerization. The lower the concentration of the reducing agent in the reaction system, the higher the molecular weight of the resulting binary TFE / E copolymer tends to be. Therefore, the reducing agent is preferably added to the reaction system continuously or intermittently.
The alkali may be added to the reaction system all at once at the start of the polymerization, as long as the positive ion concentration of the reaction system does not exceed the upper limit in the period from the start of the polymerization to the end of the polymerization, or during the polymerization. May be continuously or intermittently added to the reaction system. From the viewpoint that the pH at the start of polymerization is within the above range, and the positive ion concentration of the reaction system is equal to or lower than the upper limit during the period from the start of polymerization to the end of polymerization, at least a part of the alkali is continuously added during polymerization. Alternatively, it is preferable to add the reaction system intermittently. The same applies to a salt which is added as necessary and which becomes a buffer solution from the mixed solution with the alkali.

Specific procedures of the polymerization step include, for example, the following procedures.
The pressure-resistant reactor is degassed, and an aqueous medium, an emulsifier and a radical polymerization initiator are charged into the reactor. If necessary, an alkali may be charged so as to have a predetermined pH. If necessary, coexisting components and other components may be charged. Next, the temperature is raised to a predetermined polymerization temperature, monomers (TFE and ethylene) initially charged are introduced, and thereafter, a polymerization is started by adding a reducing agent. When adding the reducing agent, an alkali may be added.
When the polymerization reaction of the monomer is started, the pressure in the reactor starts to decrease. Therefore, the start of the polymerization reaction (the starting point of the polymerization reaction time) can be confirmed by the pressure drop in the reactor.
After confirming the pressure drop in the reactor, a monomer is additionally supplied, and a polymerization reaction is carried out while maintaining a predetermined polymerization temperature and a predetermined polymerization pressure to produce a binary TFE / E copolymer. . During the polymerization, a reducing agent, an alkali or the like may be additionally supplied continuously or intermittently.
When the total amount of monomers additionally supplied during the polymerization reaction period reaches a predetermined value, the reactor is cooled to stop the polymerization reaction (end point of the polymerization reaction time), and the binary TFE / E An aqueous emulsion of the polymer is obtained.

  The amount of the emulsifier added in the polymerization step (the total amount used in the polymerization step) is preferably 1,500 to 20,000 ppm, more preferably 2,000 to 20,000 ppm, based on the yield of the binary TFE / E copolymer. Is more preferable, and 3,000 to 20,000 ppm is further preferable. When it is not less than the lower limit of the above range and not more than the upper limit of the above range, good emulsion stability during polymerization is easily obtained.

The addition amount of the radical polymerization initiator (the total amount used in the polymerization step) is 10 to 5000 ppm, preferably 30 to 3000 ppm, and more preferably 50 to 2500 ppm based on the aqueous medium. When the amount is at least the lower limit of the above range, a good polymerization rate can be obtained. When the amount is less than or equal to the upper limit of the above range, a high molecular weight TFE / E copolymer is easily obtained.
The amount of the aqueous medium serving as a reference for the amount of the polymerization initiator is the total amount of water and the water-soluble organic solvent, and does not include the content of other additives such as the polymerization initiator.

  The addition amount of the reducing agent (the total amount used in the polymerization step) is preferably 1 to 200 parts by mass, more preferably 1 to 100 parts by mass, and further preferably 1 to 100 parts by mass with respect to 100 parts by mass of the radical polymerization initiator. preferable. When the amount of the reducing agent used is equal to or more than the lower limit of the above range, a good polymerization rate is easily obtained. When the amount of the reducing agent used is not more than the upper limit of the above range, a high molecular weight TFE / E copolymer is easily obtained.

The amount of the alkali added (the total amount used in the polymerization step) is an amount that neutralizes the acid generated during the polymerization reaction. It is preferable that the pH of the reaction system at the start of polymerization and at the end of polymerization be such an amount that the pH is maintained within the above preferred range.
The amount of alkali added is preferably such that the equivalent ratio represented by alkali / reducing agent is more than 0 and not more than 5000. The equivalent ratio represented by alkali / reducing agent is more preferably more than 0 and 3000 or less, and further preferably more than 0 and 1000 or less. When the equivalent ratio is within the above range, the pH of the reaction system during polymerization tends to be within the above preferable range. When the equivalent ratio is equal to or less than the upper limit, the positive ion concentration of the reaction system does not become too high, and the stability of the aqueous emulsion is more excellent.

From the viewpoint of the stability of the aqueous emulsion, the smaller the amount of the salt (the total amount used in the polymerization step) in which the mixed solution with the alkali becomes a buffer solution, the more preferable.
The reaction system preferably does not contain a chain transfer agent. If a chain transfer agent is present during polymerization, a low molecular weight TFE / E copolymer is likely to be produced.

The aqueous emulsion thus obtained contains a granular binary TFE / E copolymer and an emulsifier in an aqueous medium.
The average particle size of the binary TFE / E copolymer is typically 30 to 400 nm. The average particle diameter is “D50: median diameter” measured by a dynamic light scattering method using a product name SZ100 manufactured by Horiba, Ltd.
The concentration of the binary TFE / E copolymer in the aqueous emulsion is preferably 1 to 50% by mass, more preferably 5 to 40% by mass.

(Recovery process)
The binary TFE / E copolymer can be recovered from the aqueous emulsion obtained in the polymerization step by a known method.
For example, a coagulant can be added to the aqueous emulsion to coagulate the binary TFE / E copolymer. Alternatively, the aqueous emulsion may be frozen and aggregated.
Examples of the coagulant include water-soluble salts such as calcium chloride, magnesium chloride, aluminum chloride, and aluminum nitrate; acids such as nitric acid, hydrochloric acid, and sulfuric acid; and water-soluble organic solvents such as alcohol and acetone.
The amount of the coagulant to be added is preferably 0.001 to 20 parts by mass, particularly preferably 0.01 to 10 parts by mass, per 100 parts by mass of the aqueous emulsion.

The coagulated binary TFE / E copolymer is preferably separated by filtration and washed with washing water. Examples of the washing water include ion exchange water, pure water, ultrapure water, and the like.
After washing and drying, a binary TFE / E copolymer powder is obtained. Examples of the drying method include vacuum drying, high frequency drying, and hot air drying. The drying temperature is preferably from 60 to 110C, more preferably from 75 to 90C.

According to the method for producing a binary TFE / E copolymer of the present invention, a binary TFE / E copolymer excellent in mechanical strength and thermal stability can be obtained.
By using an emulsion polymerization method, which is a copolymerization method using an emulsifier, for the polymerization of TFE and ethylene and setting the polymerization temperature to 40 ° C. or lower, a high molecular weight TFE / E copolymer is easily obtained. In particular, when the reducing agent is continuously or intermittently added to the reaction system, the concentration of the reducing agent in the reaction system does not become too high, and a high molecular weight (low MFR, low Q value) TFE / E copolymer is obtained. Easy to be.
Further, in the polymerization reaction of TFE and ethylene, the ratio of each monomer of TFE and ethylene with respect to the total amount of TFE and ethylene falls within a specific range, whereby the binary TFE / E copolymer has A binary TFE / E copolymer is produced in which the proportion of TFE units to the total amount of TFE units and E units is 45 to 65 mol%, more preferably 45 to 55 mol%, and more preferably 50 to 55 mol%. It's easy to do. Such a binary TFE / E copolymer has excellent heat resistance (high melting point) by containing TFE units at a certain ratio or more, and has high crystallinity by containing E units at a certain ratio or more. .
Therefore, the obtained binary TFE / E copolymer has high molecular weight, high crystallinity, and high melting point. High mechanical strength due to high molecular weight and high crystallinity. In addition, because of its high melting point, it has excellent thermal stability.

In the method for producing a binary TFE / E copolymer of the present invention, the pH of the reaction system during the polymerization is adjusted by adding an alkali that neutralizes an acid generated during the polymerization reaction. And a good polymerization rate can be obtained. In particular, the lower the positive ion concentration of the reaction system at the start of polymerization, the better the dispersion stability (stability of the aqueous emulsion) of the resulting binary TFE / E copolymer, and the better the polymerization reaction proceeds. Tends to be easy.
Further, in the method for producing a binary TFE / E copolymer of the present invention, a high molecular weight binary TFE / E copolymer is used without using a fluorine-based solvent as used in Patent Document 4. A copolymer is obtained. Therefore, cost can be reduced. Further, not using a fluorinated solvent realizes a reduction in environmental load in waste liquid treatment and the like.
Therefore, according to the method for producing a binary TFE / E copolymer of the present invention, the binary TFE / E copolymer as described above can be produced with excellent productivity.

[Binary TFE / E copolymer]
The first embodiment of the binary TFE / E copolymer of the present invention (hereinafter also referred to as a binary TFE / E copolymer (1)) is based on the total amount of TFE units and E units. Is 45 to 65 mol%, and the Q value is 10 mm ³ / sec or less.
The second embodiment of the binary TFE / E copolymer of the present invention (hereinafter also referred to as a binary TFE / E copolymer (2)) includes a TFE unit based on the total amount of the TFE unit and the E unit. Is 45 to 65 mol%, and the melting point 1st is 294 to 300 ° C.
The third embodiment of the binary TFE / E copolymer of the present invention (hereinafter also referred to as a binary TFE / E copolymer (3)) is based on the total amount of TFE units and E units. Is 45 to 65 mol%, and the melting point 2nd is 290 to 295 ° C.

In each of the binary TFE / E copolymers (1) to (3), the ratio of TFE units to the total amount of TFE units and E units is preferably 45 to 55 mol%, more preferably 50 to 55 mol%. preferable.
When the proportion of TFE units is at least the lower limit of the above range, the binary TFE / E copolymer will have excellent heat resistance. When the proportion of TFE units is at most the upper limit of the above range, the binary TFE / E copolymer will have high crystallinity and excellent mechanical strength.

Q values of binary system TFE / E copolymer (1) is 10 mm ³ / sec or less, more preferably 1 mm ³ / sec. When the Q value is equal to or less than the upper limit of the above range, the mechanical strength is excellent. The lower limit of the Q value is not particularly limited, but may be, for example, 0.1 mm ³ / sec.

  The melting point 1st of the binary TFE / E copolymer (2) is 294 to 300 ° C, preferably 296 to 300 ° C, and more preferably 298 to 300 ° C. When the melting point 1st is equal to or higher than the lower limit of the above range, thermal stability is excellent.

  The melting point 2nd of the binary TFE / E copolymer (3) is from 290 to 295 ° C, preferably from 293 to 295 ° C. When the melting point is 2nd or more, the thermal stability is excellent.

The Q value of the binary TFE / E copolymer (3) is preferably 10 mm ³ / sec or less. The more preferable Q value is the same as that of the binary TFE / E copolymer (1).
The melting point 1st of the binary TFE / E copolymer (3) is preferably from 294 to 300 ° C. The more preferable melting point 1st is the same as that of the binary TFE / E copolymer (2).

  The binary TFE / E copolymers (1) to (3) can be produced by the aforementioned method for producing a binary TFE / E copolymer of the present invention.

The binary TFE / E copolymer of the present invention has a TFE unit ratio of 45 to 65 mol% with respect to the total amount of TFE units and E units, a Q value of 10 mm ³ / sec or less, and a melting point of 1st of 294. Since it has a melting point of -300 ° C or a melting point of 2nd of 290-295 ° C, it has excellent mechanical strength and thermal stability.

  The binary TFE / E copolymer of the present invention does not show melt fluidity or has low melt fluidity. Therefore, as a molding method when the binary TFE / E copolymer of the present invention is formed into a molded body, a known PTFE molding method is suitably used. For example, the binary TFE / E copolymer of the present invention can be formed into a desired shape by using a compression molding method. For example, a method of filling a binary TFE / E copolymer powder in a mold, compressing (preforming) by pressing at room temperature, taking out the preformed product, heating and sintering, and then cooling is used. Can be used. There is also a method in which the preform is heated and sintered, and then cooled while applying pressure. Alternatively, a method may be used in which the powder of the binary TFE / E copolymer is filled in a mold, heated, and then compressed under pressure and cooled.

  Further, the paste extruded product of the powder of the binary TFE / E copolymer of the present invention can be dried and stretched to obtain a porous body. As the stretching conditions, an appropriate speed, for example, a speed of 10% / sec to 50,000% / sec, and an appropriate stretching ratio, for example, a stretching ratio of 100% or more are adopted. Examples of the shape of the article made of the porous body include various shapes such as a tube, a sheet, a film, and a fiber.

  Hereinafter, the present invention will be described in detail with reference to examples. However, the present invention is not construed as being limited by the following examples.

[Test Example 1]
NaCl or CsCl was added to the binary TFE / E copolymer latex obtained in Example 1 to be described later so as to have a positive ion concentration shown in Table 1, and allowed to stand for 4 days. Thereafter, the presence or absence of aggregation in the latex was visually observed and evaluated according to the following criteria. Table 1 shows the results.
：: no aggregation. X: There is aggregation.

  From the above results, it was confirmed that when the positive ion concentration was 0.050 mol / L or less, the dispersion stability was good.

[Example 1]
In a beaker, 566 g of ion-exchanged water, 2.98 g of C ₂ F ₅ OC ₂ F ₄ OCF ₂ COONH _{4 as} an emulsifier, 66.0 g of tert-butanol (t-BuOH), 1.40 g of APS, and 0 of NaOH were used. .35 g was added, and this was designated as solution A.
Another beaker deionized water 33.3 g, and the disodium EDTA 0.12 g, and FeSO ₄ · 7H ₂ O was added to the above order 0.09g of B solution. Liquid A and liquid B were mixed to obtain liquid C. The pH of the solution C was 9.5 (room temperature). After degassing the inside of a 0.97 L stainless steel pressure-resistant reactor equipped with a stirring anchor blade and a baffle plate, liquid C was charged into the reactor.
Further, in another container, 180 g of ion-exchanged water, 1.51 g of NaOH, and 0.59 g of Rongalite were mixed to obtain a liquid D. The NaOH / Rongalit (molar ratio) in the solution D is 10.

Next, at 25 ° C., an initial monomer mixed gas of TFE / ethylene (molar ratio) = 70/30 was injected so that the internal pressure of the reactor became 1.92 MPa. The anchor blade was rotated at 200 rpm, and 9.0 mL of solution D was added into the reactor. While maintaining the polymerization temperature at 25 ° C. and the internal pressure in the reactor at 1.92 MPa, a monomer mixed gas of TFE / ethylene (molar ratio) = 50/50 was continuously injected, and TFE was 65 to 78 mol%. The polymerization was carried out so that ethylene maintained a ratio of 35 to 22 mol%. During the polymerization, when the injection rate of the monomer mixed gas became lower than 100 mL / min, 0.5 mL of Solution D was added. When the injection amount of the monomer mixed gas reached 60 g, the reactor was cooled to 5 ° C., the monomer mixed gas was removed, the reaction was stopped, and a latex of a binary TFE / E copolymer was obtained. I got The polymerization time was 205 minutes. The pH of the latex at the end of the polymerization was 9.7.
The latex was freeze-aggregated to precipitate a binary TFE / E copolymer. The precipitated binary TFE / E copolymer was collected by filtration. Next, the obtained copolymer was washed with ion-exchanged water and dried in an oven at 100 ° C. to obtain a powder of a binary TFE / E copolymer. The yield was 94.0% by mass based on the mass of the injected monomer gas.

[Examples 2 to 5, Comparative Examples 1 and 2]
A powder of a binary TFE / E copolymer was obtained in the same manner as in Example 1 except that the compositions (g) of the solutions A, B, and D were as shown in Table 2. In Table 2, “NH ₃ (28) aq.” Indicates a 28% by mass aqueous ammonium solution.

With respect to the binary TFE / E copolymer obtained in each example, the copolymer composition, Q value, melting point 1st, and melting point 2nd were measured by the methods described above. The results are shown in Tables 3 and 4. Tables 3 and 4 also show the main production conditions.
In Tables 3 and 4, the charged liquid is a liquid (liquid C) charged to the reactor before the start of the injection of the initial monomer mixed gas. The post-addition liquid is a liquid (liquid D) added to the reactor after the start of injection of the initial monomer mixed gas.
As for the composition of the charged liquid, the content ratio of each component in the charged liquid was represented by a mass ratio to ion-exchanged water. Note that the EDTA · 2Na and FeSO ₄ · 7H ₂ O is a radical polymerization initiator (APS) as well as coexisting components that were omitted in the charge liquid composition.

In Comparative Example 1, polymerization was not performed because the polymerization temperature was low. The Krafft point of the emulsifier is around 4 ° C. At a temperature lower than that, micelles do not form and polymerization does not proceed.
In Comparative Example 2, since no alkali was added, the pH of the reaction system was lowered, and the latex became unstable and aggregated.
In contrast, in Examples 1 to 5, binary TFE / E copolymers could be produced without problems such as aggregation. Above all, in Examples 2, 3, and 5, binary systems having a higher melting point than Examples 1 and 4 described in Patent Literature 4 using a fluorine-based solvent, despite not using a fluorine-based solvent. A TFE / E copolymer was obtained. Since it has a high melting point, it can be determined that it has excellent mechanical properties and thermal stability.
It is considered that the reason why the Q value was low and the melting point was high in Examples 2, 3, and 5 was that the polymerization temperature was low. It is considered that the result of Example 3 was particularly excellent because the amount of t-BuOH was reduced, thereby suppressing the chain transfer.

  The binary TFE / E copolymer obtained by the method for producing a binary TFE / E copolymer of the present invention can be suitably used especially for applications requiring mechanical strength and thermal stability. . More specifically, porous materials used for apparel and the like, insulators used for electric equipment and high-voltage wiring, chemical-resistant materials such as fuel hoses, antifouling agents used for cooking utensils, etc., 3D printing It can be suitably used in a wide variety of applications, such as molded articles, binders used for molding inorganic fine particles, protective films for solar cells and the like, slidable composite materials, powder coatings, and the like.

  The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2017-089846 filed on April 28, 2017 are cited here as the disclosure of the specification of the present invention. , Is to take in.

Claims (10)

In a reaction system containing a mixed solution of water and a water-soluble organic solvent or an aqueous medium that is water, an emulsifier, and a radical polymerization initiator in an amount of 10 to 5000 ppm with respect to the aqueous medium, tetrafluoroethylene and ethylene are converted to tetrafluoroethylene. The binary tetrafluoroethylene / ethylene copolymer is produced by polymerizing tetrafluoroethylene at a ratio of 60 to 85 mol% and ethylene at a ratio of 40 to 15 mol% with respect to the total amount of ethylene and ethylene. The method
The radical polymerization initiator is to generate a radical when reduced in water,
The polymerization temperature is 5 to 40 ° C.,
An alkali for neutralizing an acid generated during the polymerization reaction is added to the reaction system at least one of before and during the polymerization, and a reducing agent for reducing the radical polymerization initiator is collectively or continuously added. A method for producing a binary tetrafluoroethylene / ethylene copolymer, which is added to the reaction system intermittently or intermittently.   The method for producing a binary tetrafluoroethylene / ethylene copolymer according to claim 1, wherein the pH of the reaction system is 3.5 to 12 during a period from the start of the polymerization to the end of the polymerization.   The method for producing a binary tetrafluoroethylene / ethylene copolymer according to claim 1 or 2, wherein at least a part of the alkali is continuously or intermittently added to the reaction system during the polymerization.   The amount of the emulsifier added (the total amount used in the polymerization step) is 1,500 to 20,000 ppm with respect to the yield of the binary TFE / E copolymer. 3. The method for producing a binary tetrafluoroethylene / ethylene copolymer according to item 1.   The binary according to any one of claims 1 to 4, wherein the amount of the reducing agent added (the total amount used in the polymerization step) is 1 to 200 parts by mass with respect to 100 parts by mass of the radical polymerization initiator. A method for producing a tetrafluoroethylene / ethylene copolymer.   The method for producing a binary tetrafluoroethylene / ethylene copolymer according to any one of claims 1 to 5, wherein the alkali is ammonia or an alkali metal hydroxide.   The method for producing a binary tetrafluoroethylene / ethylene copolymer according to any one of claims 1 to 6, wherein the polymerization is performed while maintaining the polymerization pressure at 1.8 to 5.0 MPa. The ratio of the tetrafluoroethylene unit to the total amount of the tetrafluoroethylene unit and the ethylene unit is 45 to 65 mol%,
A binary tetrafluoroethylene / ethylene copolymer having a volumetric flow rate of 10 mm ³ / sec or less measured at a temperature of 320 ° C. and a load of 588.4 N according to ASTM1238. The ratio of the tetrafluoroethylene unit to the total amount of the tetrafluoroethylene unit and the ethylene unit is 45 to 65 mol%,
A binary tetrafluoroethylene / ethylene copolymer having a melting point 1st of 294 to 300 ° C. measured by the following measurement method.
"Measurement method of melting point"
The endothermic peak due to melting is measured by differential scanning calorimetry. The temperature transition program during the measurement is -20.degree. C. → 310.degree. C. → -70.degree. C. → 310.degree. C., and the rate of temperature rise is 10.degree. The temperature at the endothermic peak due to the heat of fusion at the time of the first temperature rise is defined as “melting point 1st”, and the temperature at the endothermic peak due to the heat of fusion at the second temperature rise after the temperature is lowered to −70 ° C. is defined as “melting point 2nd”. The ratio of the tetrafluoroethylene unit to the total amount of the tetrafluoroethylene unit and the ethylene unit is 45 to 65 mol%,
A binary tetrafluoroethylene / ethylene copolymer having a melting point 2nd measured by the following measurement method of 290 to 295 ° C.
"Measurement method of melting point"
The endothermic peak due to melting is measured by differential scanning calorimetry. The temperature transition program during the measurement is -20.degree. C. → 310.degree. C. → -70.degree. C. → 310.degree. C., and the rate of temperature rise is 10.degree. The temperature at the endothermic peak due to the heat of fusion at the time of the first temperature rise is defined as “melting point 1st”, and the temperature at the endothermic peak due to the heat of fusion at the second temperature rise after the temperature is lowered to −70 ° C. is defined as “melting point 2nd”.