WO2024253099A1 - Method for producing fluoromonomer - Google Patents

Abstract

The present invention provides a method for producing a fluoromonomer, with which corrosion of the inner wall of a heating furnace is not likely to progress.　Provided is a method for producing a fluoromonomer, in which a fluorine-containing polymer is heated in a heating furnace so as to thermally decompose the fluorine-containing polymer, thereby obtaining a fluoromonomer. This method for producing a fluoromonomer is characterized in that the thermal decomposition of the fluorine-containing polymer is performed, while having a second member, which has a corrosion potential lower than the corrosion potential of a first member that constitutes the inner wall surface of the heating furnace, present in the heating furnace.

Description

Method for producing fluoromonomer

The present invention relates to a method for producing a fluoromonomer.

There has been a demand for recycling fluoropolymers such as polytetrafluoroethylene. For example, Patent Document 1 discloses a method for obtaining fluoroolefins by pyrolyzing a fluoropolymer in the presence of water vapor using a rotary kiln.

Patent No. 4010300

In producing a fluoromonomer from a fluoropolymer, it is desired that corrosion of the inner wall of a heating furnace is prevented from progressing.
The present inventors produced a fluoromonomer from a fluoropolymer by the method described in Patent Document 1, but found that corrosion of the inner wall of the heating furnace was likely to progress and that there was room for improvement.

The present invention was made in consideration of the above problems, and aims to provide a method for producing fluoromonomers that is less likely to cause corrosion of the inner walls of the heating furnace.

As a result of extensive research into the above-mentioned problems, the inventors discovered that if a second member exhibiting a lower corrosion potential than the corrosion potential of the first member constituting the inner wall surface of the heating furnace is present in the heating furnace, corrosion of the inner wall (first member) of the heating furnace is less likely to progress even when a fluoropolymer is thermally decomposed in the heating furnace, and thus arrived at the present invention.

That is, the inventors discovered that the above problems can be solved by the following configuration.
[1] A method for producing a fluoromonomer, comprising heating a fluoropolymer in a heating furnace to pyrolyze the fluoropolymer to obtain a fluoromonomer,
A method for producing a fluoromonomer, characterized in that a second member exhibiting a corrosion potential lower than the corrosion potential of a first member constituting the inner wall surface of the heating furnace is present in the heating furnace, and the fluoropolymer is thermally decomposed.
[2] The method for producing a fluoromonomer according to [1], wherein the first member contains at least one selected from the group consisting of stainless steel, nickel, and chromium.
[3] The method for producing a fluoromonomer according to [1] or [2], wherein the second member contains at least one selected from the group consisting of aluminum, iron, copper, zinc, and magnesium.
[4] The method for producing a fluoromonomer according to any one of [1] to [3], wherein the fluoropolymer comprises at least one selected from the group consisting of polytetrafluoroethylene, tetrafluoroethylene copolymers, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, vinylidene fluoride-hexafluoropropylene copolymers, and perfluoropolyether rubber.
[5] The method for producing a fluoromonomer according to any one of [1] to [4], wherein the heating furnace is a rotary kiln.
[6] The method for producing a fluoromonomer according to any one of [1] to [5], wherein the corrosion potential of the first member is greater than −0.3 V and not greater than 0.3 V, and the corrosion potential of the second member is −2.0 to −0.3 V.
[7] The method for producing a fluoromonomer according to any one of [1] to [6], wherein the ratio of the total surface area of the second member to the total surface area of the inner wall surface of the heating furnace is 1.0 to 20.0%.
[8] The method for producing a fluoromonomer according to any one of [1] to [7], wherein the heating temperature during heating of the fluoropolymer is 300° C. or higher and 800° C. or lower.

The present invention provides a method for producing fluoromonomers that is less susceptible to corrosion of the inner walls of the heating furnace.

FIG. 1 is a schematic side view showing an example of a thermal decomposition apparatus used in the method for producing a fluoromonomer of the present invention.

The terms used in the present invention have the following meanings.
A numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
The term "unit" refers collectively to an atomic group derived from one molecule of a monomer that is formed directly by polymerization of the monomer, and an atomic group obtained by chemically converting a part of the atomic group. Hereinafter, a "unit based on a monomer" will also be simply referred to as a "unit".

[Fluoromonomer production method]
The method for producing a fluoromonomer of the present invention (hereinafter also referred to as "the present production method") is a method for producing a fluoromonomer, which comprises heating a fluoropolymer in a heating furnace and pyrolyzing the fluoropolymer to obtain a fluoromonomer, and in which a second member having a corrosion potential lower than the corrosion potential of a first member constituting the inner wall surface of the heating furnace is present in the heating furnace to pyrolyze the fluoropolymer. The present production method is suitable for recycling the fluoropolymer.
According to this production method, when the fluoropolymer is pyrolyzed, corrosion of the inner wall of the heating furnace does not progress easily. Although the details of the reason for this are unclear, it is presumed that in the presence of a second member exhibiting a corrosion potential lower than that of the first member, the second member corrodes preferentially over the first member, and as a result, corrosion of the first member constituting the inner wall surface of the heating furnace can be suppressed.

<Fluorine-containing polymer>
The fluorine-containing polymer is not particularly limited as long as it is a polymer containing fluorine atoms, and may be a resin or a rubber.
The fluoropolymer may be one obtained from waste materials containing a fluoropolymer.

In order to obtain a more excellent effect of the present invention, the fluoropolymer preferably contains at least one selected from the group consisting of polytetrafluoroethylene (hereinafter also referred to as "PTFE"), tetrafluoroethylene copolymer, polychlorotrifluoroethylene (hereinafter also referred to as "PCTFE"), polyvinylidene fluoride (hereinafter also referred to as "PVdF"), polyvinyl fluoride (hereinafter also referred to as "PVF"), vinylidene fluoride-hexafluoropropylene copolymer (hereinafter also referred to as "FKM"), and perfluoropolyether rubber. The fluoropolymer may be used alone or in combination of two or more types.

Specific examples of tetrafluoroethylene copolymers include tetrafluoroethylene-ethylene copolymer (hereinafter also referred to as "ETFE"), tetrafluoroethylene-hexafluoropropylene copolymer (hereinafter also referred to as "FEP"), tetrafluoroethylene-propylene copolymer (hereinafter also referred to as "FEPM"), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer.

As the tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a copolymer in which the ratio of tetrafluoroethylene units (hereinafter also referred to as "TFE units") and perfluoroalkyl vinyl ether units (hereinafter also referred to as "PAVE units") to the total of PAVE units is 0.1 mol % or more and less than 20 mol % (hereinafter also referred to as "PFA"), and a copolymer in which the ratio of PAVE units is 20 to 70 mol % (hereinafter also referred to as "FFKM") are preferred.
PFA may be a copolymer consisting of only TFE units and PAVE units, or may contain one or more units based on other monomers other than these. Specific examples of other monomers include other fluorine monomers (e.g., hexafluoropropylene) and monomers having an oxygen-containing polar group (e.g., 5-norbornene-2,3-dicarboxylic anhydride).
FFKM may be a copolymer consisting of only TFE units and PAVE units, or may contain one or more units based on other monomers other than these.Specific examples of other monomers include units based on a monomer having a fluorine atom and a nitrile group, and units based on a monomer having a fluorine atom and multiple vinyl groups.

The form of the fluoropolymer is not particularly limited, but is preferably in the form of a sheet or granules in order to further improve the pyrolysis efficiency. In this case, the size of the fluoropolymer is preferably 1 to 10 mm, more preferably 2 to 5 mm, in order to further improve the effects of the present invention. Here, the size of the fluoropolymer means the thickness when the fluoropolymer is in the form of a sheet, and means the average particle size based on a sieve when the fluoropolymer is in the form of granules. The thickness can be measured with a thickness gauge or the like, and the particle size can be measured with a sieve having various openings.
The fluoropolymer may be previously subjected to a pulverization treatment, cutting treatment or the like so as to have the above-mentioned size.

<Process>
As an example of a pyrolysis apparatus including a heating furnace used in the present production method, a pyrolysis apparatus 1 shown in Fig. 1 will be described in detail. Fig. 1 is a schematic side view showing an example of a pyrolysis apparatus usable in the present production method, and shows a part of its internal structure.
The pyrolysis device 1 has a hopper 10 capable of storing raw material (such as a fluoropolymer), a cylindrical rotary kiln 20 into which the raw material is supplied, a recovery container 30 connected to the rotary kiln 20 and for recovering pyrolysis products of the raw material (such as a fluoromonomer) and residues, a first member 40 that forms the inner wall surface of the rotary kiln 20, and a second member 50 that is arranged inside the rotary kiln 20.

The rotary kiln 20 has a rotating means (not shown) for rotating the rotary kiln 20, and a heating means (not shown) for heating the raw materials supplied inside the rotary kiln 20. In addition, a stirring means may be provided inside the rotary kiln 20 to improve the efficiency of pyrolysis of the raw materials.

The first member 40 is a member that constitutes the inner wall surface of the rotary kiln 20, and may constitute all or part of the inner wall surface.
The corrosion potential of the first member 40 is preferably greater than −0.3 V and equal to or less than 0.3 V, and more preferably greater than −0.3 V and equal to or less than 0.2 V.
In this specification, the corrosion potential is a value (V vs. SCE) measured in seawater using an electrochemical measuring device (for example, "Electrochemical Measuring System HZ-5000" manufactured by Hokuto Denko Corporation). The measurement conditions are a saturated calomel electrode (SEC) as the reference electrode, a flow rate of 2.4 to 4.0 m/s, and a temperature of 11 to 27° C.
The first member 40 constituting the inner wall surface of the rotary kiln 20 is not particularly limited, but preferably contains at least one selected from the group consisting of nickel and chromium, and may be an alloy of these.
Examples of the member containing at least one selected from the group consisting of nickel and chromium include stainless steel and heat-resistant steel.
Stainless steels include SUS302, 304, 316, 317, 321, 347, 410, 416 and 430, as well as Alloy 20.
Examples of heat-resistant steels include SUH309, 310, 330, 660, 661, SUH21, 409, 409L, and 446.
Alloys include nickel-chromium alloys such as K-500, Alloy B, C, 825, 20, 400 and 600.

The second member 50 is a member that exhibits a corrosion potential lower than the corrosion potential of the first member 40, and the second member 50 makes the first member 40 less susceptible to corrosion even when the fluoropolymer is thermally decomposed.
The corrosion potential of the second member 50 is preferably −2.0 to −0.3V, and more preferably −1.1 to −0.5V.
The corrosion potential of the second member 50 is measured in the same manner as the corrosion potential of the first member 40 described above.
The second member 50 is not particularly limited, but preferably contains at least one selected from the group consisting of aluminum, iron, copper, zinc, and magnesium, and may be an alloy of these.
Iron and iron alloys include cast iron and mild steel.
The shape of the second member 50 may be any of a granule, a polyhedron, a pyramid, a double pyramid, a frustum, a cylinder, and an ellipsoid, and second members 50 of two or more different shapes may be used in combination.
The second member 50 may be present in the heating furnace, and there is no particular limitation on the arrangement of the second member 50. For example, the second member may be placed in contact with the inner wall surface of the heating furnace as shown in Fig. 1, or may be fixed to the first member 40 via a fixing member (not shown).
Specific examples of methods for disposing the second member 50 on the inner wall surface (first member 40) of the heating furnace include a method of directly welding the second member 50 to the inner wall surface of the heating furnace, a method of screwing the second member 50, a method of fixing the second member 50 to the inner wall surface of the heating furnace via a mounting hook, a method of providing a protrusion on part of the inner wall surface of the heating furnace and fixing the second member 50 to the protrusion, and combinations of these.
The ratio of the total surface area of the second member 50 to the total surface area of the inner wall surface of the heating furnace is preferably 1.0 to 20.0%, and more preferably 5.0 to 15.0%, in terms of being able to suppress corrosion of the first member 40.
The corrosion potential difference, which represents the absolute value of the difference between the corrosion potential of the first member 40 and the corrosion potential of the second member 50, is preferably 0.1 to 2.3 V, and more preferably 0.2 to 1.5 V, in order to suppress corrosion of the first member 40.

The raw materials supplied to and temporarily stored inside the hopper 10 are transported by a conveyor 12 provided at a position adjacent to the hopper 10, and are supplied to the inside of the rotary kiln 20 via a feed chute 14 connected to the rotary kiln 20.
Here, in addition to the raw material, steam (preferably superheated steam) may be supplied to the charging chute 14 through the steam supply pipe 16. The steam supplied into the charging chute is blown into the rotary kiln 20 through the charging chute. In FIG. 1, an example in which the steam supply pipe 16 is connected to the charging chute 14 is shown, but the present invention is not limited to this, and the steam supply pipe 16 may be directly connected to the rotary kiln 20.
The raw materials supplied to the inside of the rotary kiln 20 are heated by the heating means and stirred by the rotation of the rotary kiln 20. As a result, the raw materials are pyrolyzed inside the rotary kiln 20, and the pyrolyzed products of the raw materials are sent to a recovery container 30. The gas components of the pyrolyzed products of the raw materials are recovered via a gas recovery pipe 32, and the solid components of the pyrolyzed products of the raw materials are recovered from inside the recovery container 30.

The heating temperature of the fluoropolymer is preferably 300° C. or higher, more preferably 350° C. or higher, and even more preferably 400° C. or higher, from the viewpoint of improving the efficiency of thermal decomposition of the fluoropolymer.
The heating temperature of the fluoropolymer is preferably lower from the viewpoint of the design temperature of the equipment, and is preferably 800° C. or lower, more preferably 700° C. or lower, and even more preferably 600° C. or lower.

The pyrolysis of the fluoropolymer can be carried out by using a known heating device, but it is preferably carried out by using a rotary kiln in view of the superior effect of the present invention.
The rotary kiln is a rotary furnace, and in this production method, for example, the apparatus described in Japanese Patent No. 4,010,300 or the apparatus described in Japanese Patent No. 4,357,999 can be used.

<Fluoromonomer>
Specific examples of fluoromonomers obtainable by this production method include tetrafluoroethylene (hereinafter also referred to as "TFE"), hexafluoropropylene (hereinafter also referred to as "HFP"), octafluorocyclobutane (hereinafter also referred to as "c-318"), vinylidene fluoride, vinyl fluoride, chlorotrifluoroethylene, perfluoroalkyl vinyl ether, and mixtures of two or more of these.

The present invention will be described in detail below with reference to examples. Examples 1 to 4 are working examples, and Example 5 is a comparative example. However, the present invention is not limited to these examples.

[Raw materials]
Details of the raw materials used in each example are given below.
PTFE (polytetrafluoroethylene): A PTFE sheet (manufactured by AS ONE Corporation, thickness 1 mm) was cut into 5 mm squares and used.
PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer): A PFA sheet (manufactured by AS ONE Corporation, thickness 1 mm) was cut into 5 mm squares and used.

[Pyrolysis device]
For the thermal decomposition of the fluoropolymer in each example, the thermal decomposition apparatus shown in Fig. 1 was used. Details of the rotary kiln of the thermal decomposition apparatus are shown in Table 1.

[Corrosion potential]
The corrosion potentials of the first member and the second member were measured by the method described above. As a result, the corrosion potential of the second member was lower than the corrosion potential of the first member. Specifically, the corrosion potential of SUS304 was −0.1 V, the corrosion potential of Alloy C was 0 V, and the corrosion potential of cast iron was −0.7 V.

[Fluoromonomer composition analysis]
The composition of the fluoromonomer obtained in each example was analyzed by gas chromatography using the following analytical device and conditions:
Apparatus: Agilent 7890B (manufactured by Agilent)
Column: Poraplot-Q (inner diameter 0.32 mm, length 25 m, film thickness 10 μm)
Detector: FID
Column temperature: 100 ° C.
Inlet temperature: 200℃
Detector temperature: 250°C

[Example 1]
In the rotary kiln of the pyrolysis device, a columnar cast iron (15 mm x 15 mm x 250 mm, 9.8% of the total surface area of the inner wall of the rotary kiln) was used as the second member, and one side of the second member (15 mm x 250 mm) was placed in contact with the surface of the first member (Figure 1). After that, 300 g of PTFE was introduced into the rotary kiln, and the inside of the rotary kiln was heated to 600 ° C. Then, 300 g / h of PTFE and 1000 g / h of steam were supplied to the inside of the rotary kiln to perform pyrolysis of PTFE. The rotation speed of the rotary kiln during pyrolysis was 5 rpm.
After the operation was continued for 100 hours, the amount of solid matter recovered from the rotary kiln and the recovery vessel was weighed, and the conversion rate of PTFE was calculated from the input amount. The results are shown in Table 1.
In addition, the gas generated during the operation was measured by gas chromatography, and TFE, HFP, and c-318 were detected. The selectivity of each component is shown in Table 1.
After the above operation, the first member and the second member were collected to check for the presence or absence of corrosion and the amount of reduction in the second member (the amount of mass reduction before and after the above operation). The results are shown in Table 1.

[Examples 2 to 4]
As shown in Table 1, pyrolysis and analysis were carried out in the same manner as in Example 1, except that the shapes and materials of the raw materials, the first member, and the second member were changed.

[Example 5]
Except for not using the second member, the pyrolysis and analysis of PTFE were carried out in the same manner as in Example 1. As a result, discoloration (corrosion) was confirmed in about 30% of the total surface area of the inner wall surface (first member) of the rotary kiln.

In the table, "surface area ratio" indicates the ratio of the total surface area of the second member to the total surface area of the inner wall surface of the heating furnace (surface area of the first member).

The evaluation results for Examples 1 to 4 and 5 showed that when the thermal decomposition of the fluoropolymer was carried out in the presence of the second component (Examples 1 to 4), corrosion of the first component was less likely to progress.

The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2023-095624, filed on June 9, 2023, are hereby incorporated by reference as the disclosure of the present invention.

Reference Signs List 1 Pyrolysis device 10 Hopper 12 Conveyor 14 Input chute 16 Steam supply pipe 20 Rotary kiln 30 Recovery container 32 Gas recovery pipe 40 First member 50 Second member

Claims (8)

A method for producing a fluoromonomer, comprising heating a fluoropolymer in a heating furnace to pyrolyze the fluoropolymer to obtain a fluoromonomer, comprising the steps of:
A method for producing a fluoromonomer, characterized in that a second member exhibiting a corrosion potential lower than a corrosion potential of a first member constituting an inner wall surface of the heating furnace is present in the heating furnace, and the fluoropolymer is thermally decomposed. The method for producing a fluoromonomer according to claim 1, wherein the first member includes at least one selected from the group consisting of nickel and chromium. The method for producing a fluoromonomer according to claim 1 or 2, wherein the second member includes at least one selected from the group consisting of aluminum, iron, copper, zinc, and magnesium. The method for producing a fluoromonomer according to claim 1 or 2, wherein the fluoropolymer comprises at least one selected from the group consisting of polytetrafluoroethylene, tetrafluoroethylene copolymers, polychlorotrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, vinylidene fluoride-hexafluoropropylene copolymers, and perfluoropolyether rubber. The method for producing a fluoromonomer according to claim 1 or 2, wherein the heating furnace is a rotary kiln. The method for producing a fluoromonomer according to claim 1 or 2, wherein the corrosion potential of the first member is greater than -0.3 V and less than or equal to 0.3 V, and the corrosion potential of the second member is -2.0 to -0.3 V. The method for producing a fluoromonomer according to claim 1 or 2, wherein the ratio of the total surface area of the second member to the total surface area of the inner wall surface of the heating furnace is 1.0 to 20.0%. The method for producing a fluoromonomer according to claim 1 or 2, wherein the heating temperature of the fluoropolymer is 300°C or higher and 800°C or lower.

Images