Method for producing recycled carbon fiber

Abstract

To provide a method for producing recycled carbon fiber capable of obtaining an uncoated carbon fiber whose tensile strength reduction is suppressed in comparison with the tensile strength of an uncoated carbon fiber used in the manufacture of carbon fiber reinforced plastics or carbon fibers to which a sizing agent is applied, when the carbon fiber reinforced plastics or carbon fibers to which a sizing agent is applied are used as raw materials.SOLUTION: A method for producing recycled carbon fiber of the present invention includes a mixed gas contact step of bringing a raw material containing at least one selected from a carbon fiber reinforced plastic and a carbon fiber to which a sizing agent is applied into contact with a mixed gas containing superheated steam and an ammonia gas. The content ratio of the ammonia gas contained in the mixed gas is preferably 0.1 to 2.4 volume%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing recycled carbon fibers, in which uncoated carbon fibers are obtained from carbon fiber reinforced plastics or carbon fibers to which a sizing agent has been applied. [Background technology]

[0002] Carbon fibers have excellent physical properties such as strength and elastic modulus, and are therefore used in combination with matrix resins (curable resins, thermoplastic resins, etc.) as a raw material for producing carbon fiber reinforced plastics. Molded products made of carbon fiber reinforced plastics are widely used in the fields of the aerospace industry, the automobile industry, power generation facilities, home appliances, sports, leisure, toys, etc. When producing molded products of carbon fiber reinforced plastics, sizing agent-attached carbon fibers (bundles) may be used, which are prepared in advance by subjecting untreated carbon fibers (bundles) to a sizing agent treatment in order to improve the performance of the composite material.

[0003] As the use of carbon fiber reinforced plastics increases, research is being conducted into technologies to recycle carbon fiber from waste materials generated during the manufacturing process of products made from carbon fiber reinforced plastics, and from waste components that contain carbon fiber reinforced plastics.

[0004] Patent Document 1 discloses a method for recovering carbon fibers, which comprises treating a workpiece containing carbon fiber reinforced plastic with superheated steam at 800°C or higher to remove the plastic from the carbon fiber reinforced plastic and recover the carbon fibers.

[0005] Patent document 2 discloses a method for recovering (recycling) carbon fibers from carbon fiber-containing resins, in particular from carbon fiber reinforced resins (CFPs or CFP materials), preferably from carbon fiber-containing and / or carbon fiber reinforced composites (composite materials), characterized in that an object based on a carbon fiber-containing resin comprising carbon fibers in a polymer matrix is ​​subjected to a multi-stage pyrolysis in the presence of oxygen, and the polymer of the polymer matrix is ​​decomposed during the pyrolysis to give carbon fibers.

[0006] Patent Document 3 describes a method for producing recycled carbon fibers from carbon fiber reinforced plastics containing carbon fibers and matrix resin components, which includes (a) a crushing process step of crushing carbon fiber reinforced plastic waste material to produce carbon fiber reinforced plastic crushed pieces having a predetermined fiber length, (b) a transport and storage process step of sending the carbon fiber reinforced plastic crushed pieces to a hopper and storing them, and (c) a powder removal process step of feeding a fixed amount of the carbon fiber reinforced plastic crushed pieces from the hopper to a powder removal device, removing powder contained in the carbon fiber reinforced plastic crushed pieces in the powder removal device, and generating powdered carbon fiber reinforced plastic crushed pieces. (d) a pyrolysis process in which the carbon fiber reinforced plastic powder scraps are supplied in a fixed amount to a pyrolysis furnace while being heated, and the matrix resin components contained in the carbon fiber reinforced plastic powder scraps are removed to obtain a recycled carbon fiber pyrolyzate; (e) a cooling and transport process in which the recycled carbon fiber pyrolyzate is cooled and sent to the next process; (f) a classification process in which the recycled carbon fiber pyrolyzate is classified to obtain a recycled carbon fiber fraction; and (g) an iron removal process in which metal powder is removed from the recycled carbon fiber fraction by magnetic force.

[0007] Patent Document 4 also discloses a method for producing recycled carbon fibers, which includes heat treating a raw material containing a resin material and carbon fibers, characterized in that (i) the heat treatment consists of a carbonization treatment, or (ii) the heat treatment consists of a carbonization treatment and an oxidation treatment, and the content of residual carbon derived from the resin material in the recycled carbon fibers is more than 10.0 mass% and not more than 40.0 mass% with respect to the carbon fibers contained in the recycled carbon fibers. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2011-122032 A [Patent Document 2] Special Publication No. 2016-521295 [Patent Document 3] JP 2019-127040 A [Patent Document 4] JP 2023-13706 A Summary of the Invention [Problem to be solved by the invention]

[0009] It has been found that when carbon fiber reinforced plastic or carbon fiber to which a sizing agent has been applied (hereinafter referred to as "carbon fiber with sizing agent") is contacted with superheated steam, the uncoated carbon fiber can be efficiently recovered as recycled carbon fiber, but the tensile strength of the resulting recycled carbon fiber is significantly reduced compared to the tensile strength of the uncoated carbon fiber (hereinafter referred to as "virgin carbon fiber") used in the production of carbon fiber reinforced plastic, etc. An object of the present invention is to provide a method for efficiently producing recycled carbon fiber from carbon fiber reinforced plastic or carbon fiber with a sizing agent attached, which is an uncoated carbon fiber and has reduced reduction in tensile strength compared to virgin carbon fiber. [Means for solving the problem]

[0010] The present invention is illustrated below. 1. A method for obtaining uncoated carbon fibers from a raw material containing at least one material selected from carbon fiber reinforced plastics and carbon fibers with a sizing agent, comprising the steps of: A method for producing recycled carbon fibers, comprising a mixed gas contacting step of contacting the raw material with a mixed gas containing superheated steam and ammonia gas. 2. The method for producing recycled carbon fibers according to item 1, wherein the mixed gas contains ammonia gas at a content of 0.1 to 2.4% by volume. 3. The method for producing recycled carbon fibers according to item 1 or 2, wherein in the mixed gas contacting step, the superheated steam and ammonia water are used to generate ammonia gas from the ammonia water by heat of the superheated steam, then a mixed gas containing the superheated steam and the ammonia gas is formed, and then the raw material is contacted with the mixed gas in a chamber in which the raw material is placed. Effect of the Invention

[0011] According to the present invention, recycled carbon fibers, which are uncoated carbon fibers and have reduced reduction in tensile strength compared to virgin carbon fibers, can be efficiently produced from carbon fiber reinforced plastics or carbon fibers having a sizing agent attached thereto. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic diagram showing a recycled carbon fiber manufacturing apparatus used in Example 1 etc. [Diagram 2] 1 is a graph showing the relative strength of the recycled carbon fibers obtained in Examples 1 to 3 and Comparative Example 1. [Diagram 3] 2 is a graph showing the relative strength of the recycled carbon fibers obtained in Examples 4 to 6 and Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The method for producing recycled carbon fibers of the present invention is a method for obtaining uncoated carbon fibers from a raw material containing at least one selected from carbon fiber reinforced plastics and carbon fibers with a sizing agent, and includes a mixed gas contacting step of contacting the raw material with a mixed gas containing superheated steam and ammonia gas. The method for producing recycled carbon fibers of the present invention can include other steps as necessary after the mixed gas contacting step. The superheated steam according to the present invention is a gas in which saturated steam has been further heated to a temperature exceeding 100°C.

[0014] The mixed gas contact step according to the present invention is a step of contacting a raw material containing at least one selected from carbon fiber reinforced plastics and carbon fibers having a sizing agent attached thereto with a mixed gas containing superheated steam and ammonia gas.

[0015] First, the raw material to be contacted with the mixed gas will be described. Carbon fiber reinforced plastic is a material in which at least one of virgin carbon fiber and carbon fiber with a sizing agent attached thereto is contained, for example, as a reinforcing material in a matrix containing a plastic material.

[0016] The virgin carbon fiber may be obtained by any method, for example, polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, rayon-based carbon fiber, etc. Furthermore, the size, such as fiber diameter and fiber length, is not particularly limited.

[0017] The carbon fiber with a sizing agent attached thereto may be obtained by attaching, to at least a portion of the surface of a virgin carbon fiber, a sizing agent containing, for example, an aliphatic epoxy compound and an aromatic epoxy compound, a sizing agent containing a polymer having in its main chain bonds one kind of bond selected from ester bonds, urethane bonds, and carbonate bonds, or a sizing agent containing a phenol resin.

[0018] The plastic material may be either a thermoplastic resin or a cured resin derived from a curable resin (such as a thermosetting resin composition, a photocurable resin composition, or a room temperature curable resin composition).

[0019] Thermoplastic resins include styrene-based resins such as polystyrene, styrene-acrylonitrile copolymers, styrene-maleic anhydride copolymers, (meth)acrylic acid ester-styrene copolymers, and ABS resins; rubber-reinforced thermoplastic resins; olefin-based resins such as polyethylene, polypropylene, ionomers, ethylene-vinyl acetate copolymers, ethylene-vinyl alcohol copolymers, cyclic olefin copolymers, and chlorinated polyethylene; vinyl chloride-based resins such as polyvinyl chloride, ethylene-vinyl chloride polymers, and polyvinylidene chloride; acrylic resins such as (co)polymers using one or more types of (meth)acrylic acid esters, such as polymethyl methacrylate (PMMA); polyamide-based resins (PA) such as polyamide 6, polyamide 6,6, and polyamide 6,12; polyethylene terephthalate resins such as polyethylene terephthalate resins; Examples of suitable resins include polyester-based resins such as poly(ethylene terephthalate) (PET), polybutylene terephthalate (PBT), and polyethylene naphthalate; polyacetal resin (POM); polycarbonate resin (PC); polyarylate resin; polyphenylene ether; polyphenylene sulfide; fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride; liquid crystal polymers; imide-based resins such as polyimide, polyamideimide, and polyetherimide; ketone-based resins such as polyether ketone and polyether ether ketone; sulfone-based resins such as polysulfone and polyether sulfone; urethane-based resins; polyvinyl acetate; polyethylene oxide; polyvinyl alcohol; polyvinyl ether; polyvinyl butyral; phenoxy resins; and biodegradable plastics.

[0020] Examples of curable resins include epoxy resins, acrylic resins (including acrylic polymers having epoxy groups), phenolic resins, unsaturated polyester resins, alkyd resins, melamine resins, urethane resins, urea resins, silicone resins, polyimide resins, bismaleimide-triazine resins, furan resins, xylene resins, guanamine resins, and dicyclopentadiene resins.

[0021] When the raw material according to the present invention is carbon fiber reinforced plastic, waste materials and the like generated in the process of manufacturing products made of carbon fiber reinforced plastic can be subjected to the mixed gas contacting step, but a composite (waste material, etc.) containing a portion made of a material that does not react with superheated steam and ammonia and a portion made of carbon fiber reinforced plastic may also be subjected to the mixed gas contacting step.

[0022] The mixed gas to be contacted with the raw material preferably consists of superheated steam and ammonia gas, but may contain other gases as long as they do not react with the superheated steam and ammonia gas. Examples of other gases include air, oxygen, nitrogen, and carbon dioxide.

[0023] The content of ammonia gas in the mixed gas is preferably 0.1 to 2.4% by volume, more preferably 0.1 to 1.5% by volume, and even more preferably 0.5 to 1.5% by volume, in order to fully obtain the effects of the present invention.

[0024] The method of preparing the mixed gas is not particularly limited. If the mixed gas is produced in the mixed gas preparation chamber, for example, (a) a method of separately introducing superheated steam and ammonia gas into the mixed gas preparation chamber, (b) a method of introducing ammonia gas into the mixed gas preparation chamber containing superheated steam in advance, (c) a method of introducing superheated steam into the mixed gas preparation chamber containing ammonia gas in advance, (d) a method of supplying ammonia water into the airflow of superheated steam into the mixed gas preparation chamber, converting the ammonia water into ammonia gas by the heat of the superheated steam, and forming the mixed gas, etc. may be mentioned. Also, a method of heating the ammonia water using a superheated steam generating means via a steam boiler may be used.

[0025] In the mixed gas contact step according to the present invention, the method of contacting the raw material with the mixed gas is not particularly limited. The raw material is usually contacted with the mixed gas under atmospheric control, and therefore, hereinafter, a chamber in which the raw material is contacted with the mixed gas to obtain recycled carbon fiber is referred to as a "recycled carbon fiber production chamber," and a specific method of the mixed gas contact step is exemplified.

[0026] (1) A method in which a recycled carbon fiber manufacturing chamber is configured to allow the introduction and exhaust of gas, and after the raw materials are placed in the chamber, a prepared mixed gas is introduced into the chamber and the mixed gas is brought into contact with the raw materials. (2) A method in which the raw materials are placed in a recycled carbon fiber manufacturing chamber that allows the introduction and exhaust of gases, and then superheated steam and ammonia gas are separately introduced into the chamber to form a mixed gas in the chamber, and the mixed gas is brought into contact with the raw materials. (3) A method in which superheated steam and ammonia water are separately introduced into a mixed gas preparation chamber which allows the introduction and exhaust of gases and liquids, and the heat of the superheated steam is used to convert the ammonia water into ammonia gas to form a mixed gas, and the mixed gas is supplied into a recycled carbon fiber production chamber in which raw materials are placed, and the mixed gas is brought into contact with the raw materials.

[0027] In the above methods (1), (2) and (3), the raw material may be contacted with the mixed gas while continuously introducing the mixed gas or the raw material of the mixed gas into the recycled carbon fiber production chamber, or the chamber may be filled with the mixed gas and the raw material may be contacted with the mixed gas. In addition, when the raw material comes into contact with the mixed gas, the resin attached to the carbon fiber in the raw material may be decomposed to generate decomposition gas, so in order to improve the contact rate between the raw material and the mixed gas, a method of continuously introducing the mixed gas into the chamber or the raw material of the mixed gas into the chamber while exhausting the decomposition gas is also a preferred embodiment. The speed of introducing and exhausting the gas is not particularly limited.

[0028] The temperature of the mixed gas contacted with the raw material is not particularly limited as long as it exceeds 100° C. at which superheated steam is maintained, and is preferably 300° C. or higher. This temperature is preferably 300° C. to 700° C., more preferably 350° C. to 650° C., and even more preferably 400° C. to 600° C., because the effects of the present invention can be fully obtained at this temperature. For example, when attempting to obtain recycled carbon fiber from a single raw material, the mixed gas temperature may be kept constant throughout as long as it is within the above range, or may be increased and decreased in temperature in combination.

[0029] The contact time between the raw material and the mixed gas may be appropriately set depending on the shape, mass, etc. of the raw material, but is usually 10 minutes or more.

[0030] The apparatus applied to the mixed gas contact step according to the present invention is preferably an apparatus capable of carrying out the above-mentioned method (1), (2) or (3), and its configuration is not particularly limited. In the present invention, it may be of a batch type or a continuous type. The "continuous type" means that a large amount of raw material is successively supplied and contacted with the mixed gas in order to mass-produce recycled carbon fibers.

[0031] According to the recycled carbon fiber production method of the present invention, the mixed gas contact step makes it possible to efficiently produce recycled carbon fibers that are uncoated carbon fibers and have reduced loss in tensile strength compared to virgin carbon fibers. Conventionally, when uncoated carbon fibers are obtained by contacting only superheated steam with a raw material, the loss in tensile strength is about 30%, but according to the recycled carbon fiber production method of the present invention, the loss in tensile strength can be kept to less than 30%, preferably 20% or less.

[0032] The recycled carbon fibers obtained by the mixed gas contact step are suitable for producing products containing carbon fibers by conventionally known methods, but in order to efficiently produce such products, other steps such as a surface treatment step and a resin impregnation step can be included after the mixed gas contact step, as necessary. EXAMPLES

[0033] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0034] 1. Test material A small piece of carbon fiber reinforced plastic, a carbon fiber bundle with a sizing agent attached, and carbon fiber (virgin carbon fiber) obtained by immersing the carbon fiber bundle with the sizing agent attached in acetone to remove the sizing agent were used. The carbon fiber reinforced plastic is obtained by impregnating a number of the above-mentioned carbon fiber bundles having a sizing agent thereon with an epoxy resin composition and then heating the same.

[0035] 2.Recycled carbon fiber manufacturing equipment The apparatus for producing recycled carbon fibers from the above test materials is shown in FIG. This apparatus is equipped with a recycled carbon fiber production chamber in which a mixed gas consisting of superheated steam and ammonia gas is brought into contact with the placed test material, a superheated steam production means for preparing superheated steam using water (ion-exchanged water) as a raw material, and a means for supplying ammonia water, which is the raw material for ammonia gas.The ion-exchanged water is heated to form saturated steam, and then the steam is passed through a ceramic tube equipped with a stack of pellet-type heaters that generate heat using an induction heating method to generate superheated steam.The generated superheated steam and a 38% by mass aqueous solution of ammonia (ammonia water) separately supplied from an ammonia water supply means are introduced into a mixed gas preparation section provided in the recycled carbon fiber production chamber, and in this mixed gas preparation section, ammonia gas is generated using the high-temperature superheated steam to form a mixed gas.This mixed gas is then continuously supplied from a mixed gas outlet connected from the mixed gas preparation section into the recycled carbon fiber production chamber in which the test material is placed, and resin and the like are removed from the test material to obtain uncoated carbon fiber.This is a batch-type production apparatus. In addition, in order to stabilize the temperature inside the recycled carbon fiber production chamber, i.e., the production temperature of the recycled carbon fiber, a plurality of assist heaters were installed inside the recycled carbon fiber production chamber. Furthermore, in order to set a predetermined ratio (volume ratio) of ammonia gas in the mixed gas, the amount of ammonia water supplied from the ammonia water supply means to the mixed gas preparation unit was controlled. In the following experimental example, the mixed gas was continuously supplied to the recycled carbon fiber production chamber, so that an exhaust port was provided in the recycled carbon fiber production chamber so that the gas after contacting the test material could be exhausted. In addition, the water contained in the ammonia water was converted into superheated steam by contacting with the supplied superheated steam, so the content ratio of ammonia gas contained in the mixed gas was calculated with respect to the total amount of the supplied superheated steam, the superheated steam derived from the ammonia water, and the ammonia gas purified from the ammonia water.

[0036] 3. Production and evaluation of recycled carbon fibers Each of the above test materials was contacted with various mixed gases consisting of superheated steam and ammonia gas, or with superheated steam alone, to recover uncoated carbon fibers. Then, 20 single fibers were extracted from each of the obtained carbon fibers using an Instron "Universal Material Tester 5942" and the maximum tensile load was measured by a method conforming to JIS R7606. For comparison, the maximum tensile load was also measured for 20 virgin carbon fibers. The plots in Figures 2 and 3 reflect the average value of 20 measurements (the ratio of the average value of the maximum tensile load of the recycled carbon fibers obtained in the examples and comparative examples to the average value of the measured maximum tensile load of the virgin carbon fibers: relative strength), and the error bars shown above and below the plots reflect the standard deviation σ of the normal distribution of the measured values.

[0037] Example 1 Using the apparatus shown in Fig. 1, ion-exchanged water and ammonia water were supplied into a recycled carbon fiber production chamber so that a mixed gas with a volumetric ratio of superheated steam and ammonia gas of 99.9 volume % and 0.1 volume %, respectively, was formed, and the mixed gas was brought into contact with a small piece (about 2 g) of carbon fiber reinforced plastic for 2 hours in the recycled carbon fiber production chamber adjusted to 500°C by an assist heater, to obtain uncoated carbon fiber. After that, the maximum tensile load was measured according to the above-mentioned method, and the relative strength (76%) was obtained (see Fig. 2).

[0038] Example 2 In the recycled carbon fiber production chamber, the same operation as in Example 1 was carried out, except that a mixed gas with the volumetric ratios of superheated steam and ammonia gas of 99.0 volume % and 1.0 volume %, respectively, was formed, to obtain an uncoated carbon fiber. Then, the maximum tensile load was measured according to the above-mentioned method, and the relative strength (83%) was obtained (see FIG. 2).

[0039] Example 3 In the recycled carbon fiber production chamber, the same operation as in Example 1 was carried out, except that a mixed gas with the volumetric ratios of superheated steam and ammonia gas of 97.6 volume % and 2.4 volume %, respectively, was formed, to obtain an uncoated carbon fiber. Then, the maximum tensile load was measured according to the above-mentioned method, and the relative strength (79%) was obtained (see FIG. 2).

[0040] Comparative Example 1 Except for supplying only superheated steam into the recycled carbon fiber production chamber instead of the mixed gas containing superheated steam and ammonia gas, the same operation as in Example 1 was carried out to obtain an uncoated carbon fiber. Then, the maximum tensile load was measured according to the above-mentioned method, and the relative strength (69%) was obtained (see FIG. 2).

[0041] Example 4 Using the device shown in Fig. 1, ion-exchanged water and ammonia water were supplied into a recycled carbon fiber production chamber so that a mixed gas with a volumetric ratio of superheated steam and ammonia gas of 99.9 volume % and 0.1 volume %, respectively, was formed, and the mixed gas was contacted with a small piece (about 0.5 g) of a carbon fiber bundle with a sizing agent for 2 hours in the recycled carbon fiber production chamber adjusted to 500°C by an assist heater, to obtain an uncoated carbon fiber. After that, the maximum tensile load was measured according to the above method, and the relative strength (93%) was obtained (see Fig. 3).

[0042] Example 5 In the recycled carbon fiber production chamber, the same operation as in Example 4 was carried out except that a mixed gas with the volumetric ratios of superheated steam and ammonia gas of 99.0 volume % and 1.0 volume %, respectively, was formed to obtain uncoated carbon fibers. Then, the maximum tensile load was measured according to the above-mentioned method to obtain the relative strength (94%) (see FIG. 3).

[0043] Example 6 In the recycled carbon fiber production chamber, the same operation as in Example 4 was carried out, except that a mixed gas with the volumetric ratios of superheated steam and ammonia gas of 97.6 volume % and 2.4 volume %, respectively, was formed, to obtain an uncoated carbon fiber. Then, the maximum tensile load was measured according to the above-mentioned method, and the relative strength (79%) was obtained (see FIG. 3).

[0044] Comparative Example 2 The same operation as in Example 4 was carried out, except that only superheated steam was supplied into the recycled carbon fiber production chamber instead of the mixed gas containing superheated steam and ammonia gas, to obtain uncoated carbon fibers. Then, the maximum tensile load was measured according to the above-mentioned method, and the relative strength (72%) was obtained (see FIG. 3).

[0045] As is clear from Figures 2 and 3, when carbon fiber reinforced plastic and carbon fiber with a sizing agent attached were contacted with a mixed gas containing superheated steam and ammonia gas, recycled carbon fiber was obtained in which the decrease in maximum tensile load, i.e., tensile strength, was suppressed compared to the case where only superheated steam was contacted. [Industrial Applicability]

[0046] According to the present invention, recycled carbon fibers with reduced deterioration in physical properties can be efficiently produced from waste materials generated in the process of manufacturing products made of carbon fiber reinforced plastics and waste materials of components containing carbon fiber reinforced plastics in the fields of aerospace industry, automobile industry, power generation equipment, home appliances, sports, leisure, toys, etc. Furthermore, environmental pollution caused by the above waste materials can be suppressed.

Claims

1. A method for obtaining uncoated carbon fibers from a raw material containing at least one selected from carbon fiber reinforced plastics and carbon fibers to which a sizing agent is applied, comprising the steps of: A method for producing recycled carbon fibers, comprising a mixed gas contacting step of contacting the raw material with a mixed gas containing superheated steam and ammonia gas.

2. 2. The method for producing recycled carbon fibers according to claim 1, wherein the content of the ammonia gas in the mixed gas is 0.1 to 2.4 volume %.

3. 3. The method for producing recycled carbon fibers according to claim 1 or 2, wherein in the mixed gas contacting step, the superheated steam and ammonia water are used to generate ammonia gas from the ammonia water by heat of the superheated steam, then a mixed gas containing the superheated steam and the ammonia gas is formed, and thereafter, the raw material is contacted with the mixed gas in a chamber in which the raw material is placed.

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