Recycled carbon fiber, carbon fiber roving, woven fabric, prepreg, and method for producing recycled carbon fiber

Abstract

Provided are a recycled carbon fiber, a carbon fiber roving, a woven fabric, a prepreg, and a method for producing a recycled carbon fiber. A recycled carbon fiber according to the present disclosure is separated from a carbon fiber-reinforced thermosetting resin composite. The fiber length is at least 1 m, and the value of m (shape parameter) in an expression (W1) which is obtained by transforming, by using the value of the single filament tensile strength of the recycled carbon fiber as evaluated in accordance with JIS R7606, an expression (W) indicating the failure probability in a Weibull distribution based on the weakest link theory is 2-5.

Description

Recycled carbon fiber, carbon fiber roving, fabric, prepreg, and method for producing recycled carbon fiber

[0001] The present invention relates to recycled carbon fiber, carbon fiber roving, fabric, prepreg, and a method for producing recycled carbon fiber.

[0002] Conventionally, carbon fiber reinforced thermosetting resin composites such as prepregs, UD tapes, or fiber reinforced plastics (FRP) have been studied. In recent years, recycling of various materials has been studied aiming at recycling limited resources. The recycling of carbon fiber in carbon fiber reinforced thermosetting resin composites is no exception and has been studied (Patent Document 1, Patent Document 2). In Patent Document 1 and Patent Document 2, a carbon fiber reinforced thermosetting resin composite is fired to obtain recycled carbon fiber.

[0003] Japanese Unexamined Patent Application Publication No. 2023-20639 Japanese Unexamined Patent Application Publication No. 2018-202810

[0004] However, with the techniques described in Patent Document 1 and Patent Document 2, it is possible to obtain recycled carbon fiber of short fibers, but it is difficult to obtain recycled carbon fiber having a long fiber length, particularly a fiber length of 1 m or more. In particular, when trying to separate long fibers from a carbon fiber reinforced thermosetting resin composite, variations occur in the strength of the obtained carbon fiber. The present invention aims to solve such problems, and provides recycled carbon fiber derived from a fiber reinforced curable resin composite, with suppressed variations in strength, as well as carbon fiber roving, fabric, prepreg, and a method for producing recycled carbon fiber.

[0005] Based on the above problems, the inventors conducted investigations and found that the above problems can be solved by adjusting the value of m (shape parameter) in the failure probability of the Weibull distribution to 2 to 5. Specifically, the above problems were solved by the following means: [1] Recycled carbon fiber separated from a carbon fiber reinforced thermosetting resin composite, having a fiber length of 1 m or more, and the value of m (shape parameter) in equation (W1), which is a modified equation (W) showing the failure probability of the Weibull distribution based on the weakest link theory, using the value obtained when the single filament tensile strength of the recycled carbon fiber was evaluated in accordance with JIS R7606, is 2 to 5. Equation (W) Formula (W1) [2] The recycled carbon fiber according to [1], wherein the content of amorphous carbon precursor in the recycled carbon fiber is 0.5 to 5% by mass. [3] The recycled carbon fiber according to [1] or [2], wherein the carbon fiber reinforced thermosetting resin composite is an unformed prepreg. [4] A carbon fiber roving comprising the recycled carbon fiber according to any one of [1] to [3]. [5] A fabric of recycled carbon fiber according to any one of [1] to [3]. [6] A recycled carbon fiber according to any one of [1] to [3], or a prepreg obtained by impregnating the fabric of recycled carbon fiber with resin. [7] A method for producing recycled carbon fiber according to any one of [1] to [3], comprising heating the carbon fiber reinforced thermosetting resin composite, and comprising performing thermal diffusion during the heating. [8] The method for producing recycled carbon fiber according to [7], wherein the carbon fiber reinforced thermosetting resin composite is in the form of a sheet. [9] A method for producing recycled carbon fibers according to [7] or [8], comprising setting the oxygen concentration in the heating atmosphere gas to 10% by volume or more when heating the carbon fiber reinforced thermosetting resin composite.

[0006] The present invention makes it possible to provide recycled carbon fibers derived from fiber-reinforced curable resin composites, which have suppressed variations in strength, as well as carbon fiber rovings, fabrics, prepregs, and methods for manufacturing recycled carbon fibers.

[0007] Figure 1(a) is a schematic diagram showing a single filament of recycled continuous carbon fiber, and Figure 1(b) is a diagram showing a single filament of recycled continuous carbon fiber in a ring model. Figure 2 is a schematic diagram showing the state in which carbon fiber reinforced thermosetting resin composite (raw material FRP) is hung on a rack in the example.

[0008] Hereinafter, embodiments for carrying out the present invention (hereinafter simply referred to as "this embodiment") will be described in detail. Note that the following embodiments are illustrative examples for explaining the present invention, and the present invention is not limited to these embodiments. In this specification, "~" is used to mean that the numerical values ​​before and after it include the lower and upper limits. "A~B" means A or greater and B or less. Furthermore, the upper and lower limits of numerical values ​​in this specification are given as examples of this embodiment, regardless of the combination of the upper and lower limits. In this specification, various physical properties and characteristic values ​​are given at 23°C unless otherwise specified.

[0009] In this specification, each step may be performed in any preferred order, unless otherwise specified or unless it is clearly inconsistent with the context. If the measurement methods, etc., described in the standards shown in this specification differ from year to year, the standards as of January 1, 2024 shall apply unless otherwise specified. If the measurement methods, etc., described in the standards shown in this specification are obsolete as of January 1, 2024, the standards in effect at the time of obsolete shall apply. Figures 1 and 2 may not be consistent with reality in terms of scale, etc. Also, in this specification, continuous carbon fiber means carbon fiber with a fiber length of 1 m or more, unless otherwise specified.

[0010] The recycled carbon fiber of this embodiment is a recycled carbon fiber separated from a carbon fiber reinforced thermosetting resin composite, having a fiber length of 1 m or more, and the value of m (shape parameter) in equation (W1), which is a modified version of equation (W) showing the failure probability of the Weibull distribution based on the weakest link theory, using the value obtained when the single filament tensile strength of the recycled carbon fiber is evaluated in accordance with JIS R7606, is 2 to 5. Equation (W) Formula (W1)

[0011] This configuration allows for the production of recycled carbon fibers derived from carbon fiber-reinforced thermosetting resin composites, with reduced variations in strength. Previously, methods for recovering recycled carbon fibers separated from carbon fiber-reinforced thermosetting resin composites included recovering them as short-fiber recycled carbon fibers, or calcining the carbon fibers, finely crushing them, and reusing them. On the other hand, carbon fiber-reinforced thermosetting resin composites include recycled materials containing continuous carbon fibers, such as the ends of UD tape (Uni-directional Tape, a unidirectional continuous fiber reinforced material) and unused prepregs. It would be beneficial to separate such carbon fiber-reinforced thermosetting resin composites containing continuous carbon fibers while maintaining their continuous fiber state. However, the inventors' investigations revealed that continuous carbon fibers separated from carbon fiber-reinforced thermosetting resin composites are brittle and prone to fracture due to defects. This point will be explained using Figure 1. Figure 1(a) schematically shows a single filament of recycled continuous carbon fiber. As shown in Figure 1(a), defects occur in the recycled continuous carbon fiber at various points. Such defects can easily trigger fracture in continuous carbon fibers. Figure 1(b), on the other hand, shows a more schematic representation of this fracture initiation point as a ring model. That is, assuming that recycled continuous carbon fibers exist as a ring model as shown in Figure 1(b), when tensile stress is applied to the recycled continuous carbon fibers in one direction, equal stress is applied to each ring. If any of these rings fracture, the recycled continuous carbon fiber will fracture. Based on this concept, in this embodiment, the range in which recycled continuous carbon fibers are less likely to fracture is determined based on the fracture probability of the Weibull distribution according to the weakest link theory. Equation (W) Formula (W1)

[0012] In the above formulas (W) and (W1), F(σ) is the probability of failure, and since the sample size is 30 or less, the median rank method was used. Each value used is the value obtained when the tensile strength of the single filament of recycled carbon fiber was evaluated in accordance with JIS R7606. σ ０ L is a scale parameter, and σ represents the tensile strength of the single filament. ０ L represents the tab-to-tab distance of the specimen used for Weibull parameter estimation. m is the shape parameter, and a larger value indicates a narrower strength distribution of the continuous carbon fibers.

[0013] In this embodiment, the recycled carbon fiber could have m set to 2 to 5. In particular, the same carbon fiber could have the shape parameter of virgin carbon fiber set to m. ０ In that case, m / m ０ This can be increased to 86% or more, and further to 87% or more, 88% or more, and 89% or more. This means of making the m of recycled carbon fiber 2 to 5 is achieved by performing thermal diffusion when heating the carbon fiber reinforced thermosetting resin composite (raw material).

[0014] In formula (W1), m is 2 or greater, preferably 2.5 or greater, more preferably 3.0 or greater, and even 4.5 or less is sufficient to meet the required performance.

[0015] On the other hand, the carbon fiber reinforced thermosetting resin composite (raw material) used in this embodiment is not particularly specified in terms of type, but is usually preferred to be an unformed prepreg. Examples of unformed prepregs include the ends of UD tapes and prepregs that have been stored in warehouses or other places in an unused state, which are carbon fiber reinforced thermosetting resin composites that have not undergone active heat processing. Depending on the storage conditions, such unformed prepregs may have undergone some hardening of the carbon fiber reinforced thermosetting resin composite. The carbon fiber reinforced thermosetting resin composite (raw material) used in this embodiment is not particularly specified in terms of shape, but is preferably in sheet form. More specifically, it is preferably 1 cm to 2 m wide, 1 m to 100 m long, and 0.1 mm to 1 cm thick, and more specifically, it is preferably 5 cm to 2 m wide, 5 m to 100 m long, and 0.1 mm to 1 cm thick.

[0016] The carbon fibers contained in the carbon fiber reinforced thermosetting resin composite (raw material) are usually 1 m or longer, preferably 3 m or longer, may be 5 m or longer, and preferably 1,000,000 m or less. In this embodiment, it is preferable that the carbon fibers contained in the carbon fiber reinforced thermosetting resin composite are recycled carbon fibers that have been separated while maintaining a length of 1 m or longer. It is preferable that the carbon fibers contained in the carbon fiber reinforced thermosetting resin composite are arranged in parallel in one direction.

[0017] On the other hand, there are no specific requirements regarding the type of thermosetting resin included in carbon fiber reinforced thermosetting resin composites (raw materials). Examples of thermosetting resins include epoxy resins, phenolic resins, melamine resins, urea resins, unsaturated polyester resins, alkyd resins, silicone resins, polyurethane resins, and polyimide resins.

[0018] Furthermore, the carbon fiber reinforced thermosetting resin composite (raw material) may contain other components besides the continuous carbon fibers and thermosetting resin mentioned above. These other components may include additives such as stabilizers, hydrolysis resistance improvers, weathering stabilizers, matting agents, UV absorbers, nucleating agents, plasticizers, dispersants, flame retardants, antistatic agents, color inhibitors, gelation inhibitors, colorants, mold release agents, polymerization initiators, polymerization inhibitors, etc. The content of these other components is preferably less than 10% by mass, more preferably less than 5% by mass, even more preferably less than 3% by mass, and most preferably less than 1% by mass, based on 100% by mass of the carbon fiber reinforced thermosetting resin composite.

[0019] Furthermore, the carbon fiber reinforced thermosetting resin composite (raw material) may contain fillers other than continuous carbon fibers, but the content of these fillers is preferably less than 10% by mass, more preferably less than 5% by mass, even more preferably less than 3% by mass, and most preferably less than 1% by mass, relative to 100% by mass of continuous carbon fibers.

[0020] On the other hand, the recycled carbon fiber of this embodiment has a length of 1 m or more, but may also be 5 m or more, and preferably 1,000,000 m or less.

[0021] In this embodiment, the recycled carbon fibers are preferably bundled in a tow shape with amorphous carbon precursors derived from the thermosetting resin contained in the carbon fiber reinforced thermosetting resin composite (raw material). Amorphous carbon precursors are carbon-rich solids that are produced by heating thermosetting resins and do not melt or soften; they are sometimes called char. That is, heating amorphous carbon precursors at a higher temperature can produce amorphous carbon (glassy carbon), resulting in a phase change different from the carbonization reaction that graphitizes via general mesophase carbon. In this embodiment, the carbon fiber reinforced thermosetting resin composite can also be heat-treated to produce amorphous carbon precursors instead of mesophase carbon, which does not offer sufficient strength, and these amorphous carbon precursors can be used to bind the carbon fibers together. The presence or absence of the amorphous carbon precursors can be confirmed using a scanning electron microscope.

[0022] In this embodiment, it is preferable that the amorphous carbon precursor content in the recycled carbon fibers is 0.5 to 5% by mass. With this configuration, the recycled carbon fibers can be made into a tow shape without using treatment agents such as surface treatment agents or sizing agents.

[0023] In this embodiment, if the recycled carbon fiber is a tow-shaped carbon fiber bundle made up of a large number of continuous carbon fibers, the number of carbon fibers constituting the tow-shaped carbon fiber bundle is, for example, 500 to 480,000, more preferably 1,000 or more, even more preferably 5,000 or more, even more preferably 400,000 or less, and even more preferably 300,000 or less. Such a tow-shaped carbon fiber bundle can be used as carbon fiber roving, as described later. In particular, it can be used as carbon fiber roving without any treatment agent.

[0024] Next, the method for manufacturing recycled carbon fibers according to this embodiment will be described. The method for manufacturing recycled carbon fibers according to this embodiment includes heating a carbon fiber-reinforced thermosetting resin composite (raw material), and includes performing thermal diffusion during the heating process. By performing thermal diffusion in this manner, the occurrence of defects in the resulting recycled carbon fibers can be effectively suppressed. In other words, unlike in the case of short fibers, when recycling continuous carbon fibers, it is necessary to suppress the occurrence of defects in the continuous carbon fibers. In this embodiment, by performing thermal diffusion, the carbon fiber-reinforced thermosetting resin composite can be heated uniformly, and continuous carbon fibers with fewer defects can be obtained.

[0025] Methods for heat diffusion include methods that promote heat diffusion and / or methods that ensure heat is evenly distributed to the carbon fiber reinforced thermosetting resin composite. Methods for promoting heat diffusion include installing a mesh at the heat source's supply port to adjust the heat so that it diffuses in multiple directions within the heating device. Methods for ensuring heat is evenly distributed to the carbon fiber reinforced thermosetting resin composite include arranging the carbon fiber reinforced thermosetting resin composites so that they do not come into contact with each other. Specifically, this involves heating the carbon fiber reinforced thermosetting resin composite while it is continuously hung on a rack.

[0026] In this embodiment, the heating temperature of the carbon fiber reinforced thermosetting resin composite (raw material) is preferably 300°C or higher, preferably 400°C or higher, preferably less than 600°C, and more preferably less than 480°C. By setting the temperature below the upper limit, recycled carbon fibers can be produced without damaging the continuous carbon fibers, and by setting the temperature above the lower limit, the resin in the composite material can be efficiently removed. When heating the carbon fiber reinforced thermosetting resin composite, the average heating rate is more preferably 25°C / min or less, even more preferably 20°C / min or less, and also preferably 0.1°C / min or more, more preferably 0.5°C / min or more, and even more preferably 1°C / min or more. By setting such a heating rate, recycled carbon fibers can be produced without damaging the continuous carbon fibers. The heating time is preferably 30 minutes to 5 hours.

[0027] In this embodiment, when heating the carbon fiber reinforced thermosetting resin composite, it is preferable to have an oxygen concentration of 10% by volume or more in the heating atmosphere gas, more preferably 15% by volume or more, preferably 25% by volume or less, and preferably 19% by volume or less. In this embodiment, in particular, it is preferable to have an oxygen concentration of 10% by volume or more in the atmosphere gas from the start of heating the carbon fiber reinforced thermosetting resin composite until the heating temperature reaches 300°C and the heating is completed.

[0028] The details of the carbon fiber-reinforced thermosetting resin composite and the obtained recycled carbon fibers in the method for producing recycled carbon fibers of this embodiment are the same as those described in the section on recycled carbon fibers above, and the preferred ranges are also the same. In the method for producing recycled carbon fibers of this embodiment, good recycled carbon fibers can be obtained without performing a treatment to dissolve the carbon fiber-reinforced thermosetting resin composite in a solution.

[0029] The recycled carbon fibers of this embodiment can be used in a variety of applications by taking advantage of their continuous carbon fiber structure. One example of the recycled carbon fibers of this embodiment is carbon fiber roving. Another example of the recycled carbon fibers of this embodiment is woven fabric.

[0030] The prepreg of this embodiment is obtained by impregnating recycled carbon fiber of this embodiment, or a fabric of recycled carbon fiber of this embodiment, with a resin (thermosetting resin and / or thermoplastic resin). Examples of thermosetting resins include epoxy resin, phenolic resin, melamine resin, urea resin, unsaturated polyester resin, alkyd resin, silicone resin, polyurethane resin, and polyimide resin. Examples of thermoplastic resins include polyamide resin, polyacetal resin, polycarbonate resin, modified polyphenylene ether resin, polyester resin, polyolefin resin, polystyrene resin, acrylic resin, polyurethane resin, and thermoplastic polyimide resin.

[0031] In addition to the above, additives such as stabilizers, hydrolysis resistance improvers, weather resistance stabilizers, matting agents, UV absorbers, nucleating agents, plasticizers, dispersants, flame retardants, antistatic agents, color inhibitors, gelation inhibitors, colorants, mold release agents, polymerization initiators, and polymerization inhibitors may be added to the prepreg. The content of these other components is preferably less than 10% by mass, more preferably less than 5% by mass, even more preferably less than 3% by mass, and most preferably less than 1% by mass, based on 100% by mass of the prepreg.

[0032] The present invention will be described in more detail below with reference to examples. The materials, amounts used, proportions, processing content, processing procedures, etc., shown in the following examples can be modified as appropriate, as long as they do not depart from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. If the measuring instruments, etc., used in the examples are difficult to obtain due to discontinuation or other reasons, measurements can be taken using other instruments with equivalent performance.

[0033] 1. Carbon fiber reinforced thermosetting resin composite (FRP raw material) ・UD tape: Continuous carbon fiber T800 impregnated in epoxy resin, cut to a length of 1m and used ・T800: Virgin continuous carbon fiber, manufactured by Toray Industries, Inc. ・Wide prepreg: Continuous carbon fiber T300 impregnated in epoxy resin, cut to a length of 1m and used ・T300: Virgin continuous carbon fiber, manufactured by Toray Industries, Inc. The raw materials were stored in a dark place at 23°C and 40% relative humidity for one month, after which the following procedures were carried out.

[0034] Comparative Example 1: The UD tape was placed directly into the pyrolysis furnace while still wound up. After loading the UD tape into the reflux furnace, the lid was closed, the fan was operated, and the atmospheric gas was circulated. The average flow velocity of the atmospheric gas in the reflux furnace was 5 m / min.

[0035] The above-mentioned atmospheric gas was heated and supplied to a reflux furnace. Inside the reflux furnace, the UD tape was heated to a temperature range of 400°C or higher but not exceeding 480°C. Specifically, the target temperature was set to 300°C, and from the time 300°C was reached until the end of heating, the oxygen concentration in the atmospheric gas was set to 18% by volume. The holding time from 300°C to 350°C was set to 1 hour. The average heating rate from 300°C to 350°C was set to 50°C / min. After that, it was heated to a target temperature of 420°C. The temperature range of 400°C or higher and 420°C or lower was held for 30 minutes. After heating was completed, it was cooled to obtain recycled carbon fibers. The obtained recycled carbon fibers were bundled in a tow shape with amorphous carbon precursors derived from thermosetting resin (approximately 2.5% by mass in the recycled carbon fibers). The tensile strength of the obtained carbon fibers was measured in accordance with JIS R7606, and the value of m (shape parameter) in equation (W1) was calculated.

[0036] Example 1: In Comparative Example 1, a rack was used during heating to promote heat dissipation. That is, as shown in the schematic diagram of FIG. 2, the UD tape 2 was placed on the rack 1 in a way that it was hooked, and after loading the UD tape into the reflow furnace in this state, the lid was closed, and the rest was carried out in the same manner.

[0037] Comparative Example 2: In Comparative Example 1, the UD tape was changed to a wide prepreg, and the rest was carried out in the same manner.

[0038] Example 2: In Example 1, the UD tape was changed to a wide prepreg, and the rest was carried out in the same manner.

[0039]

[0040] In Table 1 above, for Reference Example 1 and Reference Example 2, after storing virgin continuous carbon fibers in the dark at 23°C and a relative humidity of 40% for one month, the single-filament tensile strength was measured, and the shape parameter m was calculated. As is clear from the above results, the recycled continuous carbon fibers of the present invention had a shape parameter and a single-filament tensile strength comparable to those of virgin carbon fibers.

[0041] Although the present invention has been described in detail using specific embodiments, it is obvious to those skilled in the art that various changes can be made without departing from the intention and scope of the present invention.

[0042] 1. Rack 2. Carbon fiber reinforced thermosetting resin composite (raw material)

Claims

1. Recycled carbon fibers separated from a carbon fiber reinforced thermosetting resin composite, wherein the fiber length is 1 m or more, and the value of m (shape parameter) in equation (W1), which is a modified version of equation (W) showing the failure probability of the Weibull distribution based on the weakest link theory, using the value obtained when the single filament tensile strength of the recycled carbon fibers is evaluated in accordance with JIS R7606, is 2 to 5. Equation (W) Formula (W1) 2. The recycled carbon fiber according to claim 1, wherein the content of amorphous carbon precursor in the recycled carbon fiber is 0.5 to 5% by mass.

3. The recycled carbon fiber according to claim 1 or 2, wherein the carbon fiber reinforced thermosetting resin composite is an unmolded prepreg.

4. Carbon fiber roving comprising recycled carbon fibers as described in any one of claims 1 to 3.

5. A recycled carbon fiber fabric according to any one of claims 1 to 3.

6. Recycled carbon fiber according to any one of claims 1 to 3, or a prepreg obtained by impregnating a fabric of recycled carbon fiber with resin.

7. A method for producing recycled carbon fibers according to any one of claims 1 to 3, comprising heating the carbon fiber-reinforced thermosetting resin composite, and comprising performing thermal diffusion during the heating.

8. The method for producing recycled carbon fibers according to claim 7, wherein the carbon fiber reinforced thermosetting resin composite is in the form of a sheet.

9. A method for producing recycled carbon fibers according to claim 7 or 8, comprising setting the oxygen concentration in the heating atmosphere gas to 10% by volume or more when heating the carbon fiber reinforced thermosetting resin composite.

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