JP7706296B2 - Carbon fiber reinforced thermoplastic resin strands including blended yarns of recycled carbon fiber and thermoplastic resin fibers - Google Patents

Description

The present disclosure relates to carbon fiber reinforced thermoplastic resin strands containing blended yarns of recycled carbon fiber and thermoplastic resin fibers, suitable for three-dimensional modeling. The present invention also relates to a method for producing the same.

Conventionally, there has been known a technique for creating a three-dimensional model of an object by sequentially layering resin on each cross section of an object cut along multiple parallel planes. This technique is called three-dimensional modeling, and can be used for prototyping parts and manufacturing products. Known three-dimensional modeling methods include the optical modeling method using photocurable resin, the powder lamination method using metal or resin powder, the molten deposition method in which resin is melted and deposited, the thin plate lamination method in which paper, resin sheets, or thin metal plates are layered, and the inkjet method in which liquid or powdered resin or metal is sprayed.

As an example of the melt deposition method, Patent Document 1 discloses a melt deposition method using a thermoplastic polymer material. In this technique, a solid thermoplastic polymer material is supplied to a discharge head, the polymer material is melted in the discharge head, and the molten polymer material is discharged from the discharge head and deposited in layers. Examples of thermoplastic polymers that can be used in this technique include polyethersulfone, polyetherimide, polyphenylsulfone, polyphenylene, polycarbonate, high impact polystyrene, polysulfone, polystyrene, acrylic resin, amorphous polyamide, polyester, nylon, PEEK, and ABS. In particular, polylactic acid, a thermoplastic polyester derived from plants, is preferably used in this method because it can be melted and discharged at a relatively low temperature and has the advantage of being environmentally friendly.

Furthermore, fiber-reinforced resin compositions (FRP molded products) in which fibers are incorporated into resin are lightweight and have excellent mechanical properties, and are therefore used in a wide range of fields, from aerospace applications to sports applications. In the aforementioned melt lamination method, a fiber resin material in which molten thermoplastic resin and fibers are integrated is laminated onto a mold material, and a fiber resin material is further laminated onto the laminated fiber resin material. (Patent Document 2)

Furthermore, in recent years, there has been an increasing demand for recycled carbon fibers recovered from used carbon fiber-containing products. Most recycled carbon fibers are discontinuously cut carbon fibers, and methods have been developed to produce spun yarn from recycled carbon fibers so that they can be handled in the same way as unused continuous carbon fibers and used as a raw material for carbon fiber-containing products such as carbon fiber reinforced resin composite materials.

Patent document 3 describes a method for producing spun yarn by blending a charcoal containing recycled carbon fibers, obtained by heat treating scraps of carbon fiber reinforced resin composite material at a temperature of 900°C or higher, with thermoplastic resin fibers.

Patent Document 4 describes a method of utilizing spun yarn made by blending cut unused discontinuous carbon fibers with synthetic fibers, imitating recycled carbon fibers that do not contain carbides, rather than recycled carbon fibers recovered by heat treatment of carbon fiber reinforced resin composite materials.

Patent document 5 describes a method for producing spun yarn using only carbon fiber or recycled carbon fiber without blending with thermoplastic resin fiber.

Special Publication No. 2005-531439 Special table 2016-520459 publication JP 2018-35492 A JP 2018-123438 A JP 2020-90738 A

There are several methods for decomposing used carbon fiber reinforced resin composite materials to obtain recycled carbon fiber, including decomposing the resin components through heat treatment, dissolving and removing them using a solvent, and electrolysis. The most widely used method is decomposing the resin components through heat treatment.

However, when recycled carbon fibers obtained by the heat treatment decomposition method are used as a raw material to produce blended yarns with thermoplastic resin fibers, there are problems such as the resin in the carbon fiber reinforced resin composite material not being completely decomposed, residual char being generated, making it difficult to produce blended yarns, and the mechanical properties of the fiber obtained after decomposition, especially the strength of the fiber, being reduced compared to the fiber before decomposition, and the strength of molded products of fiber reinforced composite materials using this fiber is also reduced.

The present disclosure aims to provide a carbon fiber reinforced thermoplastic resin strand that can be stably produced using recycled carbon fiber obtained by a method of decomposing the carbon fiber through heat treatment and a thermoplastic resin as raw materials, and that is particularly suitable for producing three-dimensional objects with excellent mechanical properties.

According to the following aspects of the present disclosure, the above problems can be solved:
<Aspect 1>
A carbon fiber reinforced thermoplastic resin strand including a blended yarn including recycled carbon fiber and thermoplastic resin fiber,
The recycled carbon fiber has a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more; and The recycled carbon fiber contains a residual carbon component, and the content of the residual carbon component is more than 0% by weight and 5.0% by weight or less with respect to the recycled carbon fiber.
A carbon fiber reinforced thermoplastic resin strand.
<Aspect 2>
The carbon fiber reinforced thermoplastic resin strand according to claim 1, wherein the content of the recycled carbon fiber is more than 50% by weight and not more than 98% by weight with respect to the blended yarn.
<Aspect 3>
The carbon fiber reinforced thermoplastic resin strand according to aspect 1 or 2, wherein the recycled carbon fibers and the thermoplastic resin fibers have average lengths of 20 mm or more and 80 mm or less, respectively.
<Aspect 4>
The carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 3, wherein the thermoplastic resin fiber is selected from polyolefin resin fiber, polyester resin fiber, polyamide resin fiber, polyether ketone resin fiber, polycarbonate resin fiber, phenoxy resin fiber, and polyphenylene sulfide resin fiber, and mixtures thereof.
<Aspect 5>
It has a core-sheath structure,
The blended yarn is a core component, and a thermoplastic resin is a sheath component.
The carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 4,
<Aspect 6>
The melting point T0 (°C) of the thermoplastic resin as the sheath component is lower than the melting point T1 (°C) of the thermoplastic resin fiber contained in the blended yarn, and satisfies T1-T0>10. The carbon fiber reinforced thermoplastic resin strand according to claim 5.
<Aspect 7>
A method for producing the carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 6, comprising: decomposing a plastic component contained in a carbon fiber-containing plastic product by a semiconductor thermal activation method to produce recycled carbon fiber; and blending the recycled carbon fiber with a thermoplastic resin fiber to produce a blended yarn.
<Aspect 8>
A method for producing a three-dimensional object by a fused deposition method using the carbon fiber reinforced thermoplastic resin strand according to any one of aspects 1 to 6.

According to the present invention, it is possible to provide a carbon fiber reinforced thermoplastic resin strand that can be stably produced using recycled carbon fiber obtained by a method of decomposing through heat treatment and thermoplastic resin fiber as raw materials, and that is particularly suitable for producing three-dimensional objects with excellent mechanical properties.

FIG. 1 is a schematic diagram of a carbon fiber reinforced thermoplastic resin strand according to the present disclosure. FIG. 2 is a conceptual cross-sectional view for explaining a disassembly method according to the present invention. FIG. 3 is a schematic cross-sectional view showing one embodiment of the disassembly method according to the present disclosure. FIG. 4 is a photograph of the CFRP plate before heat treatment. FIG. 5 is a photograph of the CFRP plate after the treatment according to Reference Example 4. FIG. 6 is a photograph of the CFRP plate after the treatment according to Reference Comparative Example 2.

The carbon fiber reinforced thermoplastic resin strand according to the present disclosure has a blended yarn containing recycled carbon fiber and thermoplastic resin fiber. The blended yarn of recycled carbon fiber and thermoplastic resin fiber is
The recycled carbon fiber has a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more, and the recycled carbon fiber contains a residual carbon component, and the content of the residual carbon component is more than 0% by weight and 5.0% by weight or less with respect to the recycled carbon fiber.
It is characterized by:

In one embodiment of the present disclosure, the carbon fiber reinforced thermoplastic resin strand is made of the above blended yarn.

The blended yarn according to the present disclosure may be produced by a general spinning method, and may include the steps of carding, drawing, and roving. Here, the carding step may be a step of separating and opening up a discontinuous fiber aggregate, and orienting the mixed single fibers in one direction to produce a thick sliver, the drawing step may be a step of combining several slivers and stretching the yarn to further improve the degree of fiber orientation, and the roving step may be a step of further stretching and twisting the sliver to wind up the blended yarn.

The tensile strength of a sliver is determined by the frictional force caused by contact or entanglement between single fibers, so the tensile strength improves when each fiber is opened well and the contact area or entanglement between single fibers increases. In addition, the decrease in tensile strength can be prevented by suppressing single fiber breakage and not increasing the number of connections between single fibers. Increasing the twist also increases the contact area between single fibers, improving the tensile strength. To compensate for the frictional force, an oil may be applied to each fiber or to the sliver as necessary.

Generally, carbon fibers are not prone to crimping and have a high surface smoothness, so the intertwining of single fibers is weak. Furthermore, because they are hard with a high elastic modulus and low elongation, they are relatively prone to breaking and cutting. Thermoplastic resin fibers that are blended with recycled carbon fibers have the effect of increasing the intertwining of single fibers, and even blending a small amount of thermoplastic resin fibers can greatly improve the tensile strength of the sliver.

Without intending to be limited by theory, it is believed that the blended yarn according to the present disclosure has a high Weibull shape coefficient for the single fiber tensile strength of the recycled carbon fiber, so the variation in tensile strength is small, and there are fewer single fibers with low tensile strength compared to general unused carbon fiber with the same tensile strength, so single fiber breakage of the recycled carbon fiber is relatively suppressed.

In addition, a general feature of recycled carbon fibers obtained by the method of decomposition through heat treatment is that the tensile strength is easily reduced due to the generation of oxidation defects in the recycled carbon fibers, and that carbonized resin components may be contained in the recycled carbon fibers as residual carbon components. The residual carbon components may strongly bond the single fibers of the recycled carbon fibers together. In the production of spun yarn, the residual carbon components are thought to hinder the opening during the carding process and the entanglement of the single fibers.

Without intending to be limited by theory, the blended yarn according to the present disclosure is made from recycled carbon fibers with a low content of residual carbon components, which makes it easier to obtain entanglement between the single fibers.

For the reasons described above, it is believed that the blended yarn disclosed herein is more likely to have sufficient tensile strength for stable production without yarn loss due to the tension of the spinning process, because frictional force is more likely to be achieved through the entanglement of single fibers.

In one embodiment according to the present disclosure, the carbon fiber reinforced thermoplastic resin strand has a core-sheath structure,
The blended yarn is a core component, and the thermoplastic resin is a sheath component.
It is characterized by:

The carbon fiber reinforced thermoplastic resin strand having a core-sheath structure according to one embodiment of the present disclosure can be produced by coating the blended yarn with a thermoplastic resin in the same manner as a general long fiber reinforced strand, except that the blended yarn according to the present disclosure is used as a substitute for continuous carbon fiber. Therefore, compared to a short fiber reinforced thermoplastic resin strand in which discontinuous carbon fibers are kneaded and dispersed in a thermoplastic resin, the carbon fiber reinforced thermoplastic resin strand having a relatively long fiber length can be included. In other words, the carbon fiber reinforced thermoplastic resin strand according to the present disclosure can be supplied to an injection molding machine or a three-dimensional modeling device to produce a carbon fiber-containing product having excellent mechanical properties.

The invention disclosed herein is described in further detail below.

<Recycled carbon fiber>
Recycled carbon fibers (recycled carbon fibers) contain carbon fiber components and carbon components other than the carbon fiber components (particularly residual carbon components). In recycled carbon fibers, the carbon components other than the carbon fiber components are usually attached to the surface of the carbon fiber components.

The recycled carbon fiber according to the present disclosure is characterized in that it has a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more, and contains residual carbon components, the content of which is more than 0% and not more than 5.0% by weight of the recycled carbon fiber.

There are no particular limitations on the recycling method as long as the recycled carbon fiber has these characteristics, but it may be, for example, recycled carbon fiber obtained by heat treating a plastic product containing carbon fiber, such as carbon fiber reinforced plastic (CFRP).

Particularly preferably, the recycled carbon fiber is a recycled carbon fiber obtained by a semiconductor thermal activation method. That is, a particularly preferred embodiment of the present disclosure includes recycled carbon fiber produced by decomposing plastic components contained in a carbon fiber-containing plastic product by a semiconductor thermal activation method.

The "thermal activation of semiconductors" (TASC method) is a method that uses the thermal activation of semiconductors (TASC) to decompose compounds such as polymers.

The method for obtaining recycled carbon fibers having the properties disclosed herein using semiconductor thermal activation will be described later.

<Single fiber tensile strength>
The single fiber tensile strength is preferably 3.1 GPa or more, 3.2 GPa or more, 3.3 GPa or more, or 3.4 GPa or more. The upper limit of the single fiber tensile strength is not particularly limited, but may be 6.0 GPa or less.

The single fiber tensile strength can be measured in accordance with JIS R7606 as follows:
At least 30 single fibers are taken from the fiber bundle;
The diameter of the single fiber is measured in a side image of the single fiber taken by a digital microscope, and the cross-sectional area is calculated.
The sampled single fiber was fixed to a perforated mount using adhesive.
The mount on which the single fiber was fixed was attached to a tensile tester, and a tensile test was carried out at a test length of 10 mm and a strain rate of 1 mm/min to measure the tensile breaking stress.
The tensile strength is calculated from the cross-sectional area and tensile breaking stress of the single fiber.
The single fiber tensile strength is taken as the average of the tensile strengths of at least 30 single fibers.

<Weibull shape coefficient>
The Weibull shape coefficient is preferably 6.5 or more, 7.0 or more, 7.5 or more, 8.0 or more, or 8.5 or more. The upper limit of the Weibull shape coefficient is not particularly limited, but may be 15.0 or less. A high Weibull shape coefficient of single fiber tensile strength means that the variation in single fiber tensile strength is small.

The Weibull shape factor can be calculated according to the following formula:
lnln{1/(1-F)}=m×lnσ+C
In the formula, F is the fracture probability obtained by the symmetric sample cumulative distribution method, σ is the single fiber tensile strength (MPa), m is the Weibull shape coefficient, and C is a constant.

The Weibull shape coefficient m can be calculated by plotting lnln{1/(1-F)} and lnσ and using the linear approximation slope.

<Carbon fiber component>
The carbon fiber component in the recycled carbon fiber is usually derived from the carbon fiber contained in the carbon fiber-containing product that was the raw material for the recycled carbon fiber. The carbon fiber component in the recycled carbon fiber may be modified by being subjected to a heat treatment or the like during the production process of the recycled carbon fiber.

The carbon fiber component in the recycled carbon fiber may be, for example, PAN-based carbon fiber or pitch-based carbon fiber.

The form of the carbon fiber component in the recycled carbon fiber is not particularly limited, but may be in the form of a carbon fiber bundle composed of multiple single threads (single fibers, filaments). The number of filaments constituting the carbon fiber bundle may be in the range of 1,000 to 80,000, or 3,000 to 50,000. The diameter of the filaments constituting the carbon fiber component in the recycled carbon fiber may be 0.1 μm to 30 μm, 1 μm to 10 μm, or 3 μm to 8 μm.

<Residual carbon components>
The residual carbon component contained in the recycled carbon fibers is, in particular, residual carbon derived from the resin contained in the carbon fiber-containing plastic product used as a raw material in producing the recycled carbon fibers.

In the present disclosure, the residual carbon component is more than 0% and 5.0% by weight or less of the recycled carbon fiber. In this case, a blended yarn with improved resistance to thread pull-out due to the tension in the spinning process can be obtained.

In addition, when the residual carbon content is more than 0% by weight and 5.0% by weight or less, contamination caused by a relatively large amount of carbon components (especially charcoal) can be avoided, and the carbon components that can become foreign matter when manufacturing carbon fiber-containing products, etc. using the blended yarn as a material can be reduced.

Preferably, the residual carbon component is 4.0% by weight or less, 3.0% by weight or less, or 2.0% by weight or less relative to the recycled carbon fiber. It is preferable that the residual carbon component is as low as possible, but it may be 0.1% by weight or more, 0.2% by weight or more, 0.4% by weight or more, 0.6% by weight or more, 0.8% by weight or more, 1.0% by weight or more, or 1.2% by weight or more relative to the carbon fiber.

The residual carbon content in recycled carbon fiber can be measured using thermogravimetric analysis (TGA).

The residual carbon content by thermogravimetry can be measured by the following procedure:
(i) A sample piece of 1 to 4 mg obtained by pulverizing the recycled carbon fiber was subjected to a thermogravimetric analysis at an air supply rate of 0.2 L/min, a heating rate of 5°C/min, and a recording speed of 1/6 s.
Heating from room temperature to 100°C,
Hold at 100° C. for 30 minutes;
Rising the temperature from 100°C to 400°C, and
Thermogravimetric analysis was performed for a total of 300 minutes, including a step of holding at 400°C.
(ii) In a graph in which the weight loss rate is plotted against time, an inflection point in the slope is identified, and the amount of residual carbon is calculated by subtracting the weight loss rate during the holding period at 100°C from the weight loss rate value at the inflection point.

If the inflection point of the slope cannot be identified under the above conditions, instead of a total of 300 minutes of thermogravimetric analysis, a total of about 600 minutes of thermogravimetric analysis may be performed with a holding time of 400°C for 480 minutes, and further, instead of holding at 400°C for 480 minutes, a specific temperature in the range of more than 400°C and not more than 500°C may be held for 480 minutes.

In addition, if the recycled carbon fiber contains resin derived from a sizing agent or the like, the above measurements can be performed after removing the resin.

<Thermoplastic resin fiber>
Examples of thermoplastic resin fibers contained in the blended yarn according to the present disclosure include polyolefin resin fibers (e.g., polypropylene resin fibers and polyethylene resin fibers), polyester resin fibers (e.g., polyethylene terephthalate resin fibers, polybutylene terephthalate resin fibers, and polylactic acid resin fibers), polyamide resin fibers, polyether ketone resin fibers, polycarbonate resin fibers, phenoxy resin fibers, and polyphenylene sulfide resin fibers. The thermoplastic resin fibers may be of only one type, or may be a mixture of two or more types of thermoplastic resin fibers.

<Blended yarn>
The blended yarn is a spun yarn containing recycled carbon fiber and thermoplastic resin fiber. The blended yarn may also contain, for example, a binder applied to the recycled carbon fiber. In particular, the blended yarn is a spun yarn substantially composed of recycled carbon fiber and thermoplastic resin fiber.

<Recycled carbon fiber content>
The content of recycled carbon fibers is preferably more than 50% by weight and not more than 98% by weight, particularly preferably more than 55% by weight, more than 60% by weight, more than 65% by weight, or more than 70% by weight, and/or not more than 97% by weight, not more than 96% by weight, not more than 95% by weight, not more than 94% by weight, not more than 93% by weight, not more than 92% by weight, not more than 91% by weight, or not more than 90% by weight.

If the recycled carbon fiber content is more than 50% by weight, when using this recycled carbon fiber to manufacture the carbon fiber reinforced thermoplastic resin strand according to the present disclosure, there is no need to reduce the amount of coating with thermoplastic resin in order to increase the carbon fiber content, and the stability of the coating process is improved. If the recycled carbon fiber content is 98% by weight or less, there is sufficient entanglement with the blended thermoplastic resin fiber, making it less likely for the thread to come off due to the tension in the spinning process.

The content of thermoplastic resin fibers in the blended yarn may be 50% by weight or less, 40% by weight or less, or 30% by weight or less, and/or may be more than 3% by weight, more than 4% by weight, more than 5% by weight, more than 6% by weight, more than 7% by weight, more than 8% by weight, more than 9% by weight, or more than 10% by weight.

<Average fiber length>
The recycled carbon fibers contained in the blended yarn according to the present disclosure can have an average length of 20 mm or more and 80 mm or less. Fibers having a length in this range can be obtained, for example, by cutting fibers having a relatively long dimension. The average length of the recycled carbon fibers can be 20 mm or more, 30 mm or more, or 40 mm or more, and/or 80 mm or less, 70 mm or less, or 60 mm or less.

The thermoplastic resin fibers contained in the blended yarn according to the present disclosure may have an average length of 20 mm or more and 80 mm or less. Fibers having a length in this range may be obtained, for example, by cutting fibers having a relatively long dimension. The average length of the thermoplastic resin fibers may be 20 mm or more, 30 mm or more, or 40 mm or more, and/or 80 mm or less, 70 mm or less, or 60 mm or less.

In the production of blended yarn, when the average length of the recycled carbon fiber and the thermoplastic resin fiber is 20 mm or more, the resistance to yarn slippage due to the tension in the spinning process of the sliver can be improved.
When the average length of the recycled carbon fibers and thermoplastic resin fibers is 80 mm or less, the likelihood of the fibers wrapping around manufacturing equipment parts can be reduced.

The average length of the recycled carbon fibers and thermoplastic resin fibers can be calculated by measuring the length of 50 fibers visually using calipers or in images taken with a digital camera or optical microscope, and averaging the measured values.

<Method of manufacturing blended yarn>
As described above, the blended yarn of recycled carbon fiber and thermoplastic resin fiber according to the present disclosure can be manufactured by a general spinning method. That is, the manufacturing method of the blended yarn according to the present disclosure may include a carding process in which discontinuous fiber aggregates are separated and opened, and the mixed single fibers are oriented in one direction to manufacture a thick sliver, a drawing process in which several slivers are combined and stretched to further improve the degree of fiber orientation, and a roving process in which the sliver is further stretched and twisted to wind up the blended yarn.

When producing the blended yarn, a binder can be applied. The binder has a role of promoting the bonding of the recycled carbon fiber to the thermoplastic resin fiber in particular. The timing of applying the binder is not particularly limited, but it may be applied directly to the recycled carbon fiber, or it may be applied to the blended yarn containing the recycled carbon fiber and the thermoplastic resin fiber. The binder may be, for example, an epoxy-based resin. The method of applying the binder is not particularly limited, and a known method can be used. For example, the binder can be applied by immersing the recycled carbon fiber or the blended yarn in a solution or dispersion of the binder and drying it. The amount of the binder may be 0.1% by weight to 25% by weight, or 1% by weight to 20% by weight, relative to the recycled carbon fiber.

<Carbon fiber reinforced thermoplastic resin strand with core-sheath structure>
According to one embodiment of the present disclosure, a carbon fiber-reinforced thermoplastic resin strand having a core-sheath structure, in which the blended yarn is a core component and the thermoplastic resin is a sheath component, is provided by covering the blended yarn according to the present disclosure with a thermoplastic resin. This strand contains recycled carbon fibers having a relatively long fiber length compared to short fiber reinforced strands obtained by kneading and dispersing recycled carbon fibers in a thermoplastic resin, and therefore, by supplying the strand to a three-dimensional modeling device or the like, a carbon fiber-containing product having particularly excellent mechanical properties can be produced.

<Method for producing carbon fiber reinforced thermoplastic resin strand with core-sheath structure>
The carbon fiber reinforced thermoplastic resin strand having a sheath-core structure according to one embodiment of the present disclosure can be produced by coating a blended yarn with a thermoplastic resin, and more specifically, as described above, can be produced by coating the blended yarn with a thermoplastic resin in the same manner as in the production method of a general long fiber reinforced strand, except that the blended yarn according to the present disclosure is used instead of continuous carbon fiber. That is, the strand can be produced by a method including a coating step in which the blended yarn, which is continuously transported while being rewound, is threaded through a die through which molten thermoplastic resin is continuously supplied from a supply port separate from the blended yarn, and a discharge step in which the resin is discharged from the die.

<Thermoplastic resin>
Examples of the thermoplastic resin used in the coating process in the production of the carbon fiber reinforced thermoplastic resin strand (i.e., the thermoplastic resin that becomes the sheath component of the carbon fiber reinforced thermoplastic resin strand having a core-sheath structure) include polyolefin resins (e.g., polypropylene resin and polyethylene resin), polyester resins (e.g., polyethylene terephthalate resin, polybutylene terephthalate resin, and polylactic acid resin), polyamide resins, polyether ketone resins, polycarbonate resins, phenoxy resins, and polyphenylene sulfide resins. The thermoplastic resin may be one type only, or may be a mixture of two or more types of thermoplastic resins.

The thermoplastic resin that forms the sheath component of the carbon fiber reinforced thermoplastic resin strand may be of the same type as the above-mentioned thermoplastic resin fiber contained in the blended yarn according to the present disclosure, or may be of a different type. In particular, when the melting point T0 (°C) of the thermoplastic resin that forms the sheath component of the carbon fiber reinforced thermoplastic resin strand is lower than the melting point T1 (°C) of the thermoplastic resin fiber contained in the blended yarn according to the present disclosure (particularly when T1-T0>10), when producing the carbon fiber reinforced thermoplastic resin strand according to the present disclosure, the blended yarn can be covered with a thermoplastic resin that is melted at a temperature lower than the melting point of the thermoplastic resin fiber contained in the blended yarn, and the blended yarn may be stably threaded through the die.

The above-mentioned thermoplastic resins used in the coating process may also be blended with various polymers, fillers, stabilizers, pigments, etc., to improve fluidity, external gloss, flame retardant properties, thermal stability, weather resistance, impact resistance, etc., without impairing mechanical strength.

<Strand core-sheath structure>
The carbon fiber reinforced thermoplastic resin strand according to one embodiment of the present disclosure is produced by coating the above-mentioned blended yarn according to the present disclosure with a thermoplastic resin, so that the blended yarn serves as the core component and the thermoplastic resin serves as the sheath component.

The carbon fiber reinforced thermoplastic resin strands according to the present disclosure are particularly suitable for the production of three-dimensional objects. From the viewpoint of the stability of the three-dimensional objects, the average diameter of the strands is preferably 0.7 mm or more and 2.2 mm or less, and more preferably 1.0 mm or more and 2.0 mm or less.

Figure 1 is a schematic diagram of a carbon fiber reinforced thermoplastic resin strand 100 according to the present disclosure. The drawing is a schematic diagram for illustrative purposes and is not to scale. The strand 100 in Figure 1 has a core-sheath structure, with a core component 110 made of blended yarn and a sheath component 120 made of thermoplastic resin.

Regarding the proportion of the thermoplastic resin as the sheath component in the carbon fiber reinforced thermoplastic resin strand with a core-sheath structure, the thermoplastic resin as the sheath component is preferably 50 to 1000 parts by weight, more preferably 100 to 750 parts by weight, and most preferably 250 to 500 parts by weight, per 100 parts by weight of recycled carbon fiber contained in the blended yarn.

≪3D object≫
The present disclosure includes a method for producing a three-dimensional object by a fused deposition modeling process using the carbon fiber reinforced thermoplastic resin strand according to the present disclosure.

In three-dimensional modeling, the object is cut into multiple parallel planes, and resin is layered on each cross section to produce a three-dimensional model of the object. Such three-dimensional objects can be used, for example, for prototyping parts and manufacturing products. As a method for three-dimensional modeling, in particular, the molten deposition method, in which resin is melted and deposited, can be used.

The method for producing a three-dimensional object according to the present disclosure can be carried out according to a known melt deposition method, except that the carbon fiber reinforced thermoplastic resin strands according to the present disclosure are used.

Specifically, for example, the carbon fiber reinforced thermoplastic resin strand according to the present disclosure can be supplied to the discharge head of a melt deposition three-dimensional modeling device, the carbon fiber reinforced thermoplastic resin strand can be melted in the discharge head, the molten carbon fiber reinforced thermoplastic resin strand can be discharged from the discharge head, and the molten carbon fiber reinforced thermoplastic resin strand can be repeatedly deposited in layers to produce a three-dimensional model.

<Production method of recycled carbon fiber by semiconductor thermal activation method>
A method for obtaining recycled carbon fibers having the properties according to the present disclosure using a semiconductor thermal activation method will be described below. First, a method for decomposing plastic-containing materials will be described below, and then a method for recovering inorganic materials (carbon fibers) from plastic-containing materials containing plastics and inorganic materials (carbon fibers) using the decomposition method will be described below.

<Method of decomposing plastic-containing materials>
The method for decomposing plastic-containing materials according to the present disclosure includes:
Heating the plastic-containing material to a first surface temperature in an atmosphere in a heating furnace into which a low-oxygen concentration gas having an oxygen concentration of less than 10% by volume has been introduced, in the presence of a semiconductor material, to decompose the plastic in the plastic-containing material;
Includes.

Figure 2 is a conceptual cross-sectional view for explaining the decomposition method according to the present invention. The decomposition mechanism of the plastic-containing material according to the present invention is explained below. Note that there is no intention to limit the present invention by theory.

It is believed that when the semiconductor material 11 is heated in an oxygen atmosphere, holes h ⁺ and electrons ^e- are generated in the semiconductor material 11, and these electrons ^e- react with oxygen _O2 in the atmosphere to generate _O2- ^radicals (active oxygen). Radical propagation then occurs in the plastic-containing material 12 disposed adjacent to the semiconductor material 11, and the plastic-containing material 12 is decomposed into low molecular weight components, and further oxidatively decomposed into decomposition gases such as _CO2 , _H2O , and _CH4 . It is believed that the radicals promote the oxidative decomposition of the plastic by abstracting hydrogen from the plastic.

By using semiconductor materials, it is possible to reduce the heat required for decomposing plastics compared to when semiconductor materials are not used, and as a result, energy consumption can be reduced.

Conventional plastic decomposition methods based on the thermal activation of semiconductors could sometimes generate excessive heat from oxidation during the heat treatment process. With conventional methods, heat treatment was carried out in air where the oxygen concentration was not controlled, which is thought to have generated excess radicals and resulted in excessive heat from oxidation.

In response to this, the inventors have discovered that by keeping the oxygen concentration at a relatively low value, even in the presence of semiconductor materials, it is possible to efficiently heat-treat plastics while suppressing excessive heat generation due to oxidation.

In particular, in the early stages of the thermal decomposition of plastics, there is a high risk of excessive heat generation due to the relatively large amount of plastic. In contrast, according to the method disclosed herein, the heat treatment is carried out efficiently at a relatively low oxygen concentration, so that the generation of excessive heat generation due to oxidation can be suppressed even in the early stages of decomposition.

Therefore, according to the method disclosed herein, since the heat treatment is performed in the presence of semiconductor material, good decomposition efficiency can be obtained even at a relatively low oxygen concentration, and by setting the oxygen concentration in the heating furnace to a relatively low value, it is possible to suppress excessive oxidation heat generation and achieve a decomposition process with improved stability.

The decomposition method according to the present invention will be described with reference to the drawings depicting an exemplary embodiment.

Figure 3 is a cross-sectional view showing a schematic diagram of one embodiment of the decomposition method according to the present disclosure. The heating furnace 20 shown in Figure 3 has a heat source (heater) 23, a gas supply section 24, an exhaust port 25, and an internal space 26. A porous carrier 21 is disposed in the internal space 26 of the heating furnace 20, and a semiconductor material is supported on the surface of the carrier 21. In the embodiment of Figure 3, a plastic-containing material 22 is disposed in contact with the carrier 21.

The temperature of the atmosphere in the internal space 26 of the furnace 20 can be controlled via the heat source (heater) 23 of the furnace 20, so that the surface temperature of the plastic-containing material 22 is at a specific temperature. The surface temperature of the plastic-containing material 22 can be measured via a temperature sensor 27 positioned within 5 mm of the surface of the plastic-containing material 22.

A low-oxygen gas with an oxygen concentration of less than 10% by volume is introduced into the internal space 26 of the heating furnace 20 through the gas supply unit 24. The oxygen concentration of the atmosphere in the heating furnace 20 can be controlled by setting the introduction speed of the low-oxygen gas according to the furnace volume. The low-oxygen gas can be forced into the heating furnace, for example, through a gas supply unit provided in the heating furnace, or can be sucked into the heating furnace by applying suction pressure to the exhaust port 25.

The low-oxygen gas is, for example, a mixture of air and nitrogen gas. By selecting the ratio of air to nitrogen gas, the oxygen concentration in the heating furnace can be controlled.

In an atmosphere in which the oxygen concentration is controlled to less than 10% by volume by introducing a low-oxygen concentration gas, the plastic-containing material is heated to a first surface temperature, for example, a first surface temperature of 300°C to 600°C. As a result, the plastic contained in the plastic-containing material 22 is decomposed into decomposition gases such as water vapor, carbon dioxide, and methane, and the decomposition gases are exhausted from the exhaust port 25 of the heating furnace 20.

After the above-mentioned heat treatment with an oxygen concentration of less than 10% by volume, a further heat treatment can be carried out in which the oxygen concentration is increased to 10% by volume or more.

In addition, after the heat treatment, the inorganic materials contained in the plastic-containing material can be recovered.

The disassembly method disclosed herein is described in more detail below.

<Plastic-containing materials>
A plastic-containing material contains plastic. A plastic-containing material may be a plastic material or a plastic composite material.

The plastics contained in plastic-containing materials include thermoplastic resins and thermosetting resins.

Examples of thermoplastic resins contained in plastic-containing materials include polycarbonate (PC) resin, polyethylene (PE) resin, polypropylene (PP) resin, polyvinyl chloride (PVC) resin, polystyrene (PS) resin, polyethylene terephthalate (PET) resin, acrylonitrile-butadiene-styrene (ABS) resin, polyamide (PA) resin, polylactic acid (PLA) resin, polyimide (PI) resin, polymethyl methacrylate (PMMA) resin, methacrylic resin, polyvinyl alcohol (PVA) resin, polyacetal resin, petroleum resin, AS resin, modified polyphenylene ether resin, polybutylene terephthalate (PBT) resin, polybutene (PB) resin, fluororesin, polyacrylate resin, polyether ether ketone (PEEK) resin, and polyphenylene sulfide (PPS) resin.

Examples of thermosetting resins contained in plastic-containing materials include phenolic resins, urethane foam resins, polyurethane resins, urea resins, epoxy resins, unsaturated polyester resins, melamine resins, alkyd resins, vinyl ester resins, and cyanate resins.

The plastic-containing material may contain at least one resin selected from the group consisting of the thermoplastic resins and thermosetting resins described above.

(Plastic composite materials)
The plastic-containing material is particularly a plastic composite material. The plastic composite material is, for example, a fiber reinforced plastic (FRP). An example of the reinforcing fiber contained in the fiber reinforced plastic is carbon fiber.

(Carbon fiber reinforced plastic)
The plastic composite material is particularly a plastic product containing carbon fiber, such as carbon fiber reinforced plastic (CFRP). Carbon fiber reinforced plastic contains plastic and carbon fiber material. Carbon fiber reinforced plastic may contain other members and/or materials (e.g., reinforcing fibers other than carbon fiber, resin moldings, metals, ceramics, etc.).

The carbon fibers contained in carbon fiber reinforced plastics are not particularly limited, but examples include PAN-based carbon fibers and pitch-based carbon fibers. The carbon fibers may be one type or may be composed of two or more types.

The carbon fiber material contained in the carbon fiber reinforced plastic may be in any form, for example, a carbon fiber bundle, a fabric formed from carbon fiber bundles, or a nonwoven fabric of carbon fibers.

<Introduction of low oxygen concentration gas>
In the method according to the present disclosure,
A low-oxygen concentration gas having an oxygen concentration of less than 10% by volume is introduced into the atmosphere of the heating furnace.

Preferably, the oxygen concentration of the low-oxygen gas introduced into the atmosphere of the heating furnace is greater than 0 vol.%, 1 vol.% or more, 2 vol.% or more, 3 vol.% or more, or 4 vol.% or more, and/or 9 vol.% or less, 8 vol.% or less, or 7 vol.% or less.

The timing of introducing the low-oxygen gas into the heating furnace can be determined based on the type of plastic contained in the plastic-containing material and the surface temperature at which the decomposition of the plastic begins. In addition, the timing of introducing the low-oxygen gas into the heating furnace can be determined based on data previously obtained regarding the self-heating of the plastic-containing material.

In one preferred embodiment of the present disclosure, a low-oxygen gas is introduced into the atmosphere of the furnace while the surface temperature of the plastic-containing material held in the furnace is below 300°C.

More preferably, a low-oxygen gas having an oxygen concentration of less than 10% by volume is introduced into the atmosphere of the heating furnace while the surface temperature of the plastic-containing material held in the heating furnace is below 250°C, below 200°C, or below 150°C.

More preferably, a low-oxygen gas having an oxygen concentration of less than 10% by volume is introduced into the atmosphere of a heating furnace holding plastic-containing material having a surface temperature of less than 300°C, thereby making the oxygen concentration of the atmosphere in the heating furnace less than 10% by volume.

More preferably, a low-oxygen gas having an oxygen concentration of less than 10% by volume is introduced into the atmosphere of a heating furnace holding plastic-containing material with a surface temperature of less than 300°C, thereby controlling the oxygen concentration in the atmosphere in the heating furnace to greater than 0% by volume, 1% by volume or more, 2% by volume or more, 3% by volume or more, or 4% by volume or more, and/or 9% by volume or less, 8% by volume or less, or 7% by volume or less.

The oxygen concentration in the furnace can be measured directly by an oxygen monitor or can be determined based on the furnace volume and the amount of gas introduced into the furnace. The oxygen concentration in the furnace is preferably the average oxygen concentration over the course of the heat treatment.

The low-oxygen gas can be introduced into the furnace, for example, by forcing the low-oxygen gas into the furnace through a gas supply provided in the furnace, or by drawing in air from an intake (or exhaust) provided in the furnace, thereby allowing gas to flow into the furnace from a gas supply provided at a different location from the intake. The gas supply of the furnace may, for example, have an opening and/or may have a gas-permeable material.

The amount of gas introduced into the furnace can be set based on the unit amount of resin (e.g., epoxy resin) to be decomposed, according to the capacity of the heating furnace, the desired oxygen concentration, etc.

For example, the amount of gas introduced into the furnace per unit amount of resin can be determined within the range of 1 to 1000 (L/min)/kg or less, and preferably within the range of 2 to 700 (L/min)/kg or less. In addition, the time it takes for the atmosphere in the furnace to be replaced by the introduced gas can be determined based on the determined amount of gas introduced and the volume of the heating furnace used.

In particular, the amount of low-oxygen gas introduced relative to the furnace volume can be set so that the atmosphere in the furnace is replaced by the introduced low-oxygen gas while the surface temperature of the plastic-containing material held in the furnace is below 300°C.

The low-oxygen gas introduced into the heating furnace can contain a diluent gas, and in particular is a mixed gas of air and a diluent gas. Examples of diluent gas include nitrogen gas, carbon dioxide gas, water vapor, and superheated steam. Note that superheated steam is water vapor that has been heated to a temperature above its boiling point. Superheated steam has the advantage of having relatively high heat transfer to the object to be decomposed.

(heating furnace)
The furnace may be a combustion furnace or an electric furnace. The furnace may have, for example, an internal space for accommodating the plastic-containing material and the semiconductor material, a heat source (heater) for heating the atmosphere in the furnace, a gas supply unit for introducing a low-oxygen concentration gas into the furnace, an exhaust port for discharging the decomposition gas, and, optionally, a suction port for applying suction pressure inside the furnace. Note that one structure may be used as both the exhaust port and the suction port. As the exhaust port and/or the suction port, for example, one or more openings provided in the furnace may be used.

(Surface temperature)
The "skin temperature" of a plastic-containing material can be determined by measuring the temperature within 5 mm around the plastic-containing material during the heat treatment.

The surface temperature of the plastic-containing material can be controlled, for example, by controlling the temperature in the furnace via the furnace heat source and/or by introducing a cold gas into the furnace and/or by reducing the oxygen concentration to suppress oxidation heat generation.

In addition, the surface temperature of the plastic-containing material can be measured using a temperature sensor placed within 5 mm of the surface of the plastic-containing material, and the measured value can be fed back to the heat source of the heating furnace, allowing for even more precise control of the surface temperature.

Furthermore, for example, data regarding the correlation between the temperature inside the heating furnace and/or the output of the heat source and the surface temperature measured by a sensor may be obtained in advance, and the heating process may be performed based on this data.

<Heat Treatment>
The method according to the present disclosure comprises:
Heating the plastic-containing material to a first surface temperature in an atmosphere in a heating furnace into which a low-oxygen concentration gas having an oxygen concentration of less than 10% by volume has been introduced, in the presence of a semiconductor material, to decompose the plastic in the plastic-containing material;
Includes.

Preferably, this heat treatment is carried out in an atmosphere in which the oxygen concentration is less than 10% by volume by introducing a low-oxygen gas, and in particular, under an oxygen concentration of more than 0% by volume, 1% or more by volume, 2% or more by volume, 3% or more by volume, or 4% or more by volume, and/or under an oxygen concentration of 9% or less by volume, 8% or less by volume, or 7% or less by volume.

(First surface temperature)
The "first surface temperature" is the surface temperature of the plastic-containing material. The "first surface temperature" can be determined by measuring the temperature within 5 mm around the plastic-containing material during the heat treatment, similar to the "surface temperature" described above.

The first surface temperature may be 300°C or more, 325°C or more, or 350°C or more, and/or 600°C or less, 550°C or less, 500°C or less, or 450°C or less. The first surface temperature is, for example, 300°C to 600°C, 300°C to 550°C, 300°C to 500°C, or 300°C to 450°C.

If the first surface temperature is below the above range, decomposition of the plastic may not occur. If the first surface temperature exceeds the above range, when the plastic-containing material contains valuables such as carbon fibers, deterioration and/or burning of the carbon fibers etc. due to heating in the presence of oxygen may become significant. By setting the first surface temperature at a relatively low temperature, the effect of suppressing excessive oxidation heat generation is further enhanced.

In a preferred embodiment of the present disclosure, during the heat treatment at an oxygen concentration of less than 10% by volume, the first surface temperature is set to a temperature of 450°C or higher. When the heating temperature is relatively low, further decomposition of the plastic may be inhibited by char generated on the surface of the plastic-containing material, but by setting the surface temperature to a temperature of 450°C or higher, the char can be removed even at a relatively low oxygen concentration. This makes it possible to further improve the efficiency of decomposition of the plastic.

Preferably, the heat treatment under the introduction of low-oxygen gas is carried out for a predetermined time. This predetermined time may be 1 to 600 minutes, preferably 30 to 300 minutes, and more preferably 60 to 180 minutes. The start point of the "predetermined time" may be the time when the temperature of the heating furnace reaches 300°C or when the start of decomposition of the plastic is confirmed.

(Semiconductor Materials)
The semiconductor material may be placed in the furnace along with the plastic-containing material, or the semiconductor material may be placed in the furnace first and then the plastic-containing material may be placed adjacent to or in contact with the semiconductor material.

In one embodiment of the present disclosure, the plastic-containing material and the semiconductor material to be subjected to the heat treatment are spaced apart from each other by a distance of 50 mm or less. For this purpose, for example, a spacer can be used that is placed between the plastic-containing material and the semiconductor material.

The distance between the plastic-containing material and the semiconductor material can be measured at the point where they are closest to each other.

In another preferred embodiment of the present disclosure, the plastic-containing material to be subjected to the heat treatment and the semiconductor material are placed in contact with each other.

The manner of contact between the plastic-containing material and the semiconductor material is not particularly limited. For example, the two can be brought into contact by placing the plastic-containing material on the semiconductor material. The plastic-containing material may also be placed on the semiconductor material supported on the surface of a carrier. Furthermore, the two can be brought into contact by surrounding or covering at least a part or the whole of the plastic-containing material with the semiconductor material.

The semiconductor material is not particularly limited as long as it is stable at the temperature and oxygen concentration of the present invention. The semiconductor material contains, for example, at least one selected from the group consisting of the following substances:
BeO, MgO, CaO, SrO, BaO, CeO ₂ , ThO ₂ , UO ₃ , U ₃ O ₈ , TiO ₂ , ZrO ₂ , V ₂ O ₅ , Y ₂ O ₃ , Y ₂ O ₂ S, Nb ₂ O ₅ , Ta ₂ O ₅ , MoO ₃ , WO ₃ , MnO ₂ , Fe ₂ O ₃ , MgFe ₂ O ₄ , NiFe ₂ O ₄ , ZnFe ₂ O ₄ , ZnCo ₂ O ₄ , ZnO, CdO, Al ₂ O ₃ , MgAl ₂ O ₄ , ZnAl ₂ O ₄ , Tl ₂ O ₃ , In ₂ O ₃ , SiO ₂ , SnO ₂ , PbO ₂ , UO ₂ , Cr ₂ O ₃ , MgCr ₂ O ₄ , FeCrO ₄ , CoCrO ₄ , ZnCr ₂ O ₄ , WO ₂ , MnO, Mn ₃ O ₄ , Mn ₂ O ₃ , FeO, NiO, CoO, Co ₃ O ₄ , PdO, CuO, Cu ₂ O, Ag 2 O, CoAl ₂ O ₄ , NiAl ₂ O ₄ , Tl _{2 O} , GeO, PbO, TiO, Ti ₂ O ₃ , VO, MoO ₂ , IrO ₂ , RuO ₂ , CdS, CdSe, CdTe, Cu _2O , _{Sb2O3 , MnO3} , and _CoCrO4 .

Preferably, the semiconductor material is an oxide semiconductor material. Preferred oxide semiconductor materials include chromium oxide, titanium oxide, zinc oxide, vanadium oxide, tungsten oxide, molybdenum oxide, cobalt oxide, iron oxide, and copper oxide.

The shape of the semiconductor material is not particularly limited, and may be, for example, plate-like, granular, or honeycomb-shaped. From the viewpoint of promoting the decomposition of plastics, it is preferable that the semiconductor material is supported on the surface of a breathable carrier. The breathable carrier may be a porous body made of ceramics or the like, a honeycomb-shaped support, or the like. The semiconductor material may be a sintered semiconductor.

<Two-stage heating>
A heat treatment method according to one preferred embodiment of the present disclosure includes the steps of:
Heating the plastic-containing material subjected to the heat treatment at the first surface temperature in an atmosphere having an oxygen concentration of 10% by volume or more in the presence of a semiconductor material;
Includes.

Heat treatment at an oxygen concentration of 10% by volume or more is preferably carried out under an oxygen concentration of more than 10% by volume, 12% by volume or more, 15% by volume or more, or 20% by volume or more, and/or at an oxygen concentration of 30% by volume or less, or 25% by volume or less.

In this embodiment, also referred to as two-stage heating, the plastic-containing material is heat-treated at a relatively low oxygen concentration and then further heat-treated under an increased oxygen concentration.

In the early stages of the thermal decomposition of plastics, there is a high risk of excessive heat generation due to the relatively large amount of plastic. In contrast, in the above embodiment involving two-stage heating, a heat treatment with a relatively low oxygen concentration is performed, followed by a heat treatment with an increased oxygen concentration. In other words, heating with an increased oxygen concentration is performed at a stage when the decomposition of plastics has progressed and the amount of plastic has been reduced, thereby suppressing the generation of excessive heat generation due to oxidation.

In addition, when the heat treatment is performed at a relatively low oxygen concentration, some of the plastic may not vaporize and may remain as char. In contrast, in the above embodiment involving two-stage heating, the oxygen concentration is increased and further heat treatment is performed, so that the remaining char can be efficiently decomposed and removed, and as a result, the decomposition efficiency can be further improved.

The heat treatment under the introduction of low oxygen concentration gas and the heat treatment at an oxygen concentration of 10% by volume or more can be carried out continuously in the same heating furnace. That is, for example, after the plastic-containing material is heated to a first surface temperature, the oxygen concentration can be increased to 10% by volume or more in the same heating furnace, and further heat treatment can be carried out. In conventional plastic decomposition methods, it was necessary to carry out processing such as shredding of the object to be treated following batch processing, but by carrying out the heat treatment under the introduction of low oxygen concentration gas and the heat treatment at an oxygen concentration of 10% by volume or more continuously in the same heating furnace, it is possible to reduce the number of steps.

An atmosphere with an oxygen concentration of 10% by volume or more can be formed, for example, by introducing a high-oxygen concentration gas with an oxygen concentration of 10% by volume or more into the heating furnace, and in particular by introducing air and/or oxygen gas into the heating furnace. The oxygen concentration of the high-oxygen concentration gas may be more than 10% by volume, 12% by volume or more, 15% by volume or more, or 20% by volume or more, and/or 30% by volume or less, or 25% by volume or less.

In a heat treatment with an oxygen concentration of 10% by volume or more, the semiconductor material used in the heat treatment at the first surface temperature can be used.

(Second surface temperature)
In particular, the plastic-containing material that has been subjected to a heat treatment at a first surface temperature is heated to a second surface temperature in the presence of a semiconductor material and in an atmosphere having an oxygen concentration of 10% by volume or more.

The "second surface temperature" is the surface temperature of the plastic-containing material being treated. As with the above-mentioned "surface temperature" and "first surface temperature," the "second surface temperature" can be determined by measuring the temperature within 5 mm around the plastic-containing material during heat treatment.

The second surface temperature may be in the range of 400°C to 600°C. More preferably, the second surface temperature is in the range of 425°C to 575°C, or 450°C to 550°C.

If the second surface temperature is below the above range, the decomposition efficiency may not improve. If the second surface temperature exceeds the above range, when the plastic-containing material contains valuables such as carbon fibers, the deterioration and/or burning of the carbon fibers and the like due to heating in the presence of oxygen may become significant.

The second surface temperature may be substantially the same as the first surface temperature.

The second surface temperature may also be equal to or greater than the first surface temperature, or may be at least 5°C, at least 10°C, at least 25°C, at least 50°C, or at least 75°C higher than the first surface temperature.

Heat treatment at an oxygen concentration of 10% by volume or more can be carried out for a predetermined time. This predetermined time can be, for example, 1 to 600 minutes, 60 to 360 minutes, or 90 to 300 minutes.

<Application>
The method for decomposing plastic-containing materials according to the present disclosure can efficiently vaporize and decompose a wide variety of plastics. The method for decomposing plastic-containing materials according to the present disclosure can also be applied to volatile organic compounds (VOCs), exhaust gas, particulate matter (PM), and the like, and can be used for exhaust gas treatment.

<How to recover inorganic materials>
A method according to the present disclosure for recovering inorganic materials contained in plastic composite materials is described below.

In one embodiment of the recovery method according to the present disclosure, the plastic-containing material is a plastic composite material, which contains plastic and inorganic materials, and the recovery method includes:
Decomposing plastics in the plastic-containing material by the above-mentioned decomposition method to recover inorganic materials;
Includes.

For the decomposition method according to the present disclosure that can be used in the recovery method according to the present disclosure and its details, please refer to the above description of the decomposition method for plastic-containing materials.

As described above, the decomposition method according to the present disclosure can efficiently decompose plastics. Therefore, the recovery method according to the present disclosure can efficiently and selectively decompose the plastics contained in the plastic composite material, and therefore can efficiently recover inorganic materials with good physical properties.

In addition, the recovery method according to the present disclosure allows inorganic materials to be recovered without crushing the plastic composite material. Crushing the plastic composite material is not specifically excluded, and crushing can be performed at will.

Furthermore, when recovering inorganic materials from plastic composite materials, it is necessary to minimize the effects of heat treatment on the inorganic materials in order to maintain the quality of the recovered inorganic materials. In this regard, in the decomposition method according to the present disclosure, excessive oxidation heat generation is suppressed by a relatively low oxygen concentration. Therefore, the recovery method according to the present disclosure makes it possible to recover inorganic materials with relatively good physical properties.

(Two-stage heating)
In another embodiment of the recovery method according to the present disclosure, the plastic-containing material as a plastic composite material is subjected to two-stage heating, that is, the plastic composite material is subjected to heating under the introduction of a gas with a low oxygen concentration, and then to a heat treatment with an oxygen concentration of 10% by volume or more, thereby decomposing the plastic and recovering the inorganic material.

When recovering inorganic materials, it is desirable to suppress deterioration of the inorganic material's physical properties due to excessive oxidation heat generation, and it is also preferable to suppress deterioration of the inorganic material's physical properties due to carbonization of plastics and remaining attached to the inorganic material. In this regard, in the embodiment in which the plastic composite material is subjected to two-stage heating, the oxygen concentration is kept relatively low in the initial stage where excessive oxidation heat generation is likely to occur, thereby suppressing deterioration of the inorganic material's physical properties due to excessive oxidation heat generation. Furthermore, since a heat treatment with an increased oxygen concentration is performed after that, the carbonized resin remaining on the inorganic material can be efficiently removed. As a result, the method for recovering inorganic materials via two-stage heating treatment can ensure even better physical properties of the recovered inorganic material.

(Inorganic material)
Plastic composite materials containing inorganic materials and plastics include carbon fiber reinforced plastics.

The inorganic material is, in particular, carbon fiber (carbon fiber material).

(Carbon fiber)
The carbon fiber contained in the carbon fiber reinforced plastic is not particularly limited, but may be a PAN-based carbon fiber or a pitch-based carbon fiber. The carbon fiber may be one type or may be composed of two or more types.

The carbon fibers contained in the carbon fiber reinforced plastic may be in any form, for example, a carbon fiber bundle, a fabric formed from carbon fiber bundles, or a nonwoven fabric of carbon fibers.

(Collected items)
According to one embodiment of the recovery method of the present disclosure, the residual carbon derived from the plastic recovered together with the inorganic material is reduced, and in particular, the residual carbon is 5% by weight or less of the inorganic material recovered. Preferably, the residual carbon is 4% by weight or less, 3% by weight or less, 2% by weight or less, 1% by weight or less, 0.9% by weight or less, 0.5% by weight or less, or 0.1% by weight or less of the inorganic material recovered. It is preferable that the residual carbon is reduced as much as possible, but the lower limit may be 0.001% by weight or more.

Furthermore, the recovery method according to the present disclosure makes it possible to recover carbon fiber material that has better physical properties than the carbon fiber material before it is used to produce carbon fiber reinforced plastic.

In particular, according to the recovery method disclosed herein, it is possible to obtain a carbon fiber material having a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more as an inorganic material from carbon fiber reinforced plastic.

(Single fiber tensile strength)
The single fiber tensile strength is preferably 3.1 GPa or more, 3.2 GPa or more, 3.3 GPa or more, or 3.4 GPa or more. The upper limit of the single fiber tensile strength is not particularly limited, but may be 6.0 GPa or less.

The single fiber tensile strength can be measured in accordance with JIS R7606 as follows:
At least 30 single fibers are taken from the fiber bundle;
The diameter of the single fiber is measured in a side image of the single fiber taken by a digital microscope, and the cross-sectional area is calculated.
The sampled single fiber was fixed to a perforated mount using adhesive.
The mount on which the single fiber was fixed was attached to a tensile tester, and a tensile test was carried out at a test length of 10 mm and a strain rate of 1 mm/min to measure the tensile breaking stress.
The tensile strength is calculated from the cross-sectional area and tensile breaking stress of the single fiber.
The single fiber tensile strength is taken as the average of the tensile strengths of at least 30 single fibers.

(Weibull shape factor)
The Weibull shape coefficient is preferably 6.5 or more, 7.0 or more, 7.5 or more, 8.0 or more, or 8.5 or more. The upper limit of the Weibull shape coefficient is not particularly limited, but may be 15.0 or less.

The Weibull shape factor can be calculated according to the following formula:
lnln{1/(1-F)}=m×lnσ+C
In the formula, F is the fracture probability obtained by the symmetric sample cumulative distribution method, σ is the single fiber tensile strength (MPa), m is the Weibull shape coefficient, and C is a constant.

The Weibull shape coefficient m can be calculated by plotting lnln{1/(1-F)} and lnσ and using the linear approximation slope.

<Application>
According to the recovery method of the present disclosure, for example, only the matrix resin of fiber reinforced plastics (FRP), solar cell panels, electronic circuits, etc. can be efficiently decomposed, and valuable materials such as reinforcing fibers and rare metals can be efficiently recovered.

<Recycled carbon fiber materials>
The recycled carbon fiber material (recycled carbon fiber) according to the present disclosure can be produced by recovering carbon fiber material from carbon fiber reinforced plastic. In one embodiment of the present disclosure, the recycled carbon fiber material can have better physical properties than the carbon fiber before the carbon fiber reinforced plastic is produced.

The method for producing the recycled carbon fiber material is not particularly limited. For example, the recycled carbon fiber material can be produced by recovering the recycled carbon fiber material from carbon fiber reinforced plastic using the recovery method according to the present disclosure.

In one embodiment, the recycled carbon fiber material has a single fiber tensile strength of 3.0 GPa or greater and a Weibull shape factor of 6.0 or greater. The single fiber tensile strength and Weibull shape factor can be measured and determined by the methods described above.

The single fiber tensile strength of the recycled carbon fiber material is preferably 3.1 GPa or more, 3.2 GPa or more, 3.3 GPa or more, or 3.4 GPa or more.

The Weibull shape coefficient of the recycled carbon fiber material is preferably 6.5 or more, 7.0 or more, 7.5 or more, 8.0 or more, or 8.5 or more.

(Method of producing recycled carbon fiber)
In particular, a method for producing recycled carbon fibers having a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more, and a residual carbon content of more than 0% by weight and not more than 5.0% by weight based on the recycled carbon fibers, comprising:
Heating the plastic-containing material to a first surface temperature in an atmosphere in a heating furnace into which a low-oxygen concentration gas having an oxygen concentration of less than 10% by volume has been introduced, in the presence of a semiconductor material, to decompose the plastic in the plastic-containing material;
Including,
Here, the plastic-containing material is a carbon fiber reinforced plastic containing carbon fiber material.

In particular, a method for producing a recycled carbon fiber material having a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more, and a residual carbon component content of more than 0 wt% and 5.0 wt% or less based on the recycled carbon fiber, comprising:
Heating the plastic-containing material to a first surface temperature in an atmosphere in a heating furnace into which a low-oxygen concentration gas having an oxygen concentration of less than 10% by volume has been introduced, in the presence of a semiconductor material, and heating the plastic-containing material subjected to the heat treatment at the first surface temperature in an atmosphere having an oxygen concentration of 10% by volume or more, in the presence of the semiconductor material, thereby decomposing the plastic in the plastic-containing material;
Including,
Here, the plastic-containing material is a carbon fiber reinforced plastic containing carbon fiber material.

In one preferred embodiment of the method for producing recycled carbon fiber material according to the present disclosure, a low-oxygen gas is introduced into the atmosphere of the heating furnace while the surface temperature of the plastic-containing material is below 300°C.

(Carbon Residual)
Preferably, the amount of residual carbon in the recycled carbon fiber material according to the present disclosure is reduced, and in particular, the amount of residual carbon is 5% by weight or less relative to the recycled carbon fiber material, and more preferably, the amount of residual carbon is 4% by weight or less, 3% by weight or less, 2% by weight or less, 1% by weight or less, 0.9% by weight or less, 0.5% by weight or less, or 0.1% by weight or less relative to the recycled carbon fiber material.

The "residual carbon" is a carbonized component derived from the plastic contained in the carbon fiber reinforced plastic, which is the raw material used to manufacture recycled carbon fiber materials.

The amount of residual carbon in recycled carbon fiber materials can be determined by thermogravimetric analysis (TGA).

The determination of the amount of residual carbon by thermogravimetry can be carried out by the following procedure:
(i) A sample piece of 1 to 4 mg obtained by pulverizing the recycled carbon fiber material was subjected to a thermogravimetric analysis at an air supply rate of 0.2 L/min, a heating rate of 5°C/min, and a recording speed of 1/6 s.
Heating from room temperature to 100°C,
Hold at 100° C. for 30 minutes;
Rising the temperature from 100°C to 400°C, and
Thermogravimetric analysis was performed for a total of 300 minutes, including a step of holding at 400°C.
(ii) In a graph in which the weight loss rate is plotted against time, an inflection point in the slope is identified, and the amount of residual carbon is calculated by subtracting the weight loss rate during the holding period at 100°C from the weight loss rate value at the inflection point.

If the inflection point of the slope cannot be identified under the above conditions, instead of a total of 300 minutes of thermogravimetric analysis, a total of about 600 minutes of thermogravimetric analysis may be performed with a hold at 400°C for 480 minutes, and further, instead of holding at 400°C for 480 minutes, a hold at a specific temperature in the range of more than 400°C and not more than 500°C may be performed for 480 minutes.

The present disclosure will be described in more detail below using examples. Note that the examples are merely illustrative and the present application is not limited thereto.

Example 1
<Material preparation>
(Recycled carbon fiber)
The recycled carbon fibers used were those having an average length of 50 mm, an average single fiber diameter of 6.7 μm, a single fiber tensile strength of 4.7 GPa, a Weibull shape coefficient of 7.0, and a residual carbon content of 0.6 wt%, which had been regenerated by a semiconductor thermal activation method using CFRP as the raw material according to the method disclosed herein.

(Single fiber tensile strength)
The single fiber tensile strength was measured according to JIS R7606 as follows:
At least 30 single fibers are taken from the fiber bundle;
The diameter of the single fiber is measured in a side image of the single fiber taken by a digital microscope, and the cross-sectional area is calculated.
The sampled single fiber was fixed to a perforated mount using adhesive.
The mount on which the single fiber was fixed was attached to a tensile tester, and a tensile test was carried out at a test length of 10 mm and a strain rate of 1 mm/min to measure the tensile breaking stress.
The tensile strength is calculated from the cross-sectional area and tensile breaking stress of the single fiber.
The single fiber tensile strength was determined as the average of the tensile strengths of at least 30 single fibers.

(Weibull shape factor)
The Weibull shape factor was calculated according to the following formula:
lnln{1/(1-F)}=m×lnσ+C
(In the formula, F is the fracture probability obtained by the symmetric sample cumulative distribution method, σ is the single fiber tensile strength (MPa), m is the Weibull shape coefficient, and C is a constant.)

A Weibull plot was made using lnln{1/(1-F)} and lnσ, and the Weibull shape coefficient m was calculated from the linear approximation slope.

(Residual carbon amount)
The amount of residual carbon in the recycled carbon fibers was determined by thermogravimetric analysis (TGA method) as follows:
(i) A 4 mg sample piece obtained by pulverizing the recycled carbon fiber was subjected to a thermogravimetric analysis for a total of about 600 minutes, the analysis comprising steps of heating from room temperature to 100°C, holding at 100°C for 30 minutes, heating from 100°C to 400°C, and holding at 400°C for 480 minutes, at an air supply rate of 0.2 L/min, a heating rate of 5°C/min, and a recording speed of 1/6 s, in a thermogravimetric analyzer;
(ii) In a graph plotting the weight loss rate against time, an inflection point in the slope was identified, and the amount of residual carbon was calculated by subtracting the weight loss rate during the holding period at 100°C from the weight loss rate value at the inflection point.

The recycled carbon fibers were immersed in an aqueous dispersion of epoxy resin, then removed and dried in a dryer. 2% by weight of epoxy resin was applied to the recycled carbon fibers as a binder for the thermoplastic resin.

(Thermoplastic resin fiber)
As the thermoplastic resin fiber, polyamide 66 resin fiber (PA66 resin fiber (manufactured by Toray Industries, Inc., 1401-1.3T-38 E9), single yarn fineness 1.3 dtex, number of crimps 17 crimps/25 mm, melting point 265° C.) having an average length of 38 mm was used.

<Production of blended yarn>
A continuous blended yarn of 0.86 g/m was produced by mixing 80% by weight of recycled carbon fiber with a binder and 20% by weight of PA66 resin fiber, and then spinning the mixture through carding, drawing, and roving processes. Note that the roving process gave a twist of 200 times/m.

<Production of carbon fiber reinforced thermoplastic resin strand>
Next, the blended yarn obtained above was coated with polyamide 6 (Akulon (registered trademark) F-X9182, manufactured by DSM, melting point 220°C) using a crosshead die for coating electric wire with an outlet diameter of 2.3 mm, and this was wound up with a winder to obtain a strand-shaped fiber-reinforced plastic (carbon fiber-reinforced thermoplastic resin strand according to Example 1) having a core-sheath structure with a carbon fiber content of 17% by mass (463 parts by mass of polyamide 6 per 100 parts by mass of carbon fiber) and an average diameter of 2.0 mm.

<Modeling using a 3D modeling device>
The above-mentioned strand-like fiber reinforced plastic having an average diameter of 2.0 mm was set in a fused deposition modeling device ("BS01" manufactured by Bonsai Lab Co., Ltd.), and a modeling test was performed under the following conditions at a discharge head temperature of 280°C.
Discharge head movement speed: 30 mm/min (when discharging resin), 130 mm/min (when discharging stops). Discharge amount (magnification): 100%
Layer pitch: 0.2 mm
Lamination bed temperature: 25°C
Ambient temperature: 25°C
Ambient relative humidity: 40%
During printing, no fans or blowers were used to cool the model.

(1) Drooping evaluation:
Two cubes with sides of 10 mm were used as bridge girders, which were placed parallel to each other on a horizontal plane with a center-to-center distance of 50 mm, and a plate with a length of 60 mm, width of 10 mm, and thickness of 0.2 mm was placed on top of the bridge girders. A three-dimensional bridge-shaped model was designed using three-dimensional CAD, and this model was molded under the above conditions to produce a three-dimensional object. The distance Z between the part of the plate-shaped part of the molded three-dimensional model closest to the stacked bed and the stacked bed was measured, and the amount of sagging was calculated using the following formula.
Droop amount (mm) = 10 - Z
The shaping was repeated three times to calculate the average amount of sagging, and an average value of 0 mm or more and less than 2 mm was rated as excellent (◎), 2 mm or more and less than 4 mm was rated as pass (○), and 4 mm or more was rated as fail (×). When the obtained strands were evaluated for sagging, the average amount of sagging was 3.4 mm.

(2) Evaluation of discharge/stop response:
A three-dimensional model was designed using three-dimensional CAD, in which 25 rectangular columns with a cross section of 2 mm on each side and a height of 20 mm were arranged on a 5 × 5 square lattice point drawn at 10 mm intervals on a horizontal plane, and this model was molded under the above conditions to produce a three-dimensional object. The dimensions of the molded three-dimensional model were measured with a vernier caliper, and the number of rectangular columns with a cross section of 3 mm or more on two sides was determined as the number of molding failures. The molding was repeated three times to calculate the average number of molding failures, and an average of 0 to less than 2 was determined as excellent pass (◎), 2 to less than 5 was determined as pass (○), and 5 or more was determined as fail (×). When the discharge/stop responsiveness of the obtained strand was evaluated, the average number of molding failures was 3.7.

(3) Evaluation of the stability of the model:
A three-dimensional model was designed using three-dimensional CAD, in which 25 rectangular columns, each with a 10 mm square cross section and a height of 100 mm, were arranged upright on a 5 x 5 square lattice drawn at 20 mm intervals on a horizontal plane, and this model was molded under the above conditions to produce a three-dimensional object. The strands used for molding were those that weighed well over 300 g (volume 250 cm ³ , continuous length approximately 104 m). If molding was completed without any abnormalities, it was evaluated as passed (○), and if the discharge amount decreased or discharge stopped midway, it was evaluated as failed (×). When the molding stability of the obtained strands was evaluated, molding was completed without any abnormalities.

The results for Example 1 are shown in Table 1 below.

Comparative Example 1
Except for using recycled carbon fiber with a residual carbon content of 7.1% by weight, a blended yarn was produced in the same manner as in Example 1. It was difficult to measure the single fiber tensile strength and Weibull shape coefficient of the recycled carbon fiber because the single fibers were strongly bonded to each other by the residual carbon.

In Comparative Example 1, it was observed that the bonds between the single fibers of the recycled carbon fiber were not sufficiently opened during the carding process, and the single fibers were not entangled sufficiently, resulting in thread loss, and it was not possible to produce a continuous blended yarn. The results are shown in Table 1 below.

<Reference Examples 1 to 3 and Reference Comparative Example 1>
In Reference Examples 1 to 3 and Reference Comparative Example 1, a carbon fiber reinforced plastic (CFRP) plate as a plastic-containing material was heat-treated, and the plastic decomposition efficiency was evaluated.

<Reference Example 1>
Reference Example 1 was carried out as follows.

(Provision and arrangement of materials)
The semiconductor material used was a carrier having a honeycomb structure with chromium oxide (Cr ₂ O ₃ , purity 99% or more, manufactured by Junsei Chemical Co., Ltd.) applied to the surface thereof. The carrier having a honeycomb structure had 13 cells/25 mm.

A CFRP plate with an epoxy resin content of 41% by weight was used as the plastic-containing material.

The furnace volume was 9 L. Inside the furnace, a CFRP plate was placed on top of a carrier carrying chromium oxide as a semiconductor material. The carrier and the CFRP plate were placed in contact with each other.

The surface temperature of the CFRP plate was measured using a sensor placed within 5 mm of the surface of the CFRP plate.

(Heat Treatment)
The inner temperature of the furnace was increased by controlling the inner temperature of the furnace via the heater output of the furnace.

Before the surface temperature of the CFRP plate reached 300°C, a mixed gas of air and nitrogen gas with an oxygen concentration of 6% by volume was introduced into the heating furnace. The mixed gas was introduced into the heating furnace by sucking in the gas at a gas introduction rate of 70 L/min from a suction port installed in the upper part of the heating furnace, and by allowing the mixed gas to flow in from a gas supply port installed in the lower part of the heating furnace. The oxygen concentration inside the heating furnace was measured by an oxygen monitor.

The heat treatment was carried out for 30 minutes in an atmosphere where the oxygen concentration was controlled to 6% by volume by introducing the above mixed gas. During the heat treatment, the heater output of the heating furnace was adjusted to raise the surface temperature of the CFRP plate to a first surface temperature of 376°C.

(evaluation)
The plastic decomposition efficiency in the method according to Reference Example 1 was evaluated by calculating the weight reduction rate (wt%) from the difference between the weight of the CFRP plate before and after the heat treatment. The results are shown in Table 2.

<Reference Example 2>
The heat treatment and evaluation of Reference Example 2 was carried out in the same manner as Reference Example 1, except that the carrier and the CFRP plate were placed 30 mm apart in the heating furnace, and the surface temperature of the CFRP plate was raised to 377° C. during the heat treatment. The results are shown in Table 2.

<Reference Example 3>
The heat treatment and evaluation of Reference Example 3 were carried out in the same manner as Reference Example 1, except that the heat treatment time was 60 minutes and the surface temperature of the CFRP plate was raised to 371° C. during the heat treatment. The results are shown in Table 2.

<Reference Comparative Example 1>
The heat treatment and evaluation of Reference Comparative Example 1 were carried out in the same manner as Reference Example 1, except that no semiconductor material was used and that the surface temperature of the CFRP plate was raised to 370° C. during the heat treatment. The results are shown in Table 2.

As can be seen in Table 2, Reference Examples 1 to 3, which were heated to a surface temperature of 371°C to 377°C in the presence of semiconductor material and with the introduction of a low-oxygen gas with an oxygen concentration of 6% by volume, had a higher decomposition efficiency for plastic-containing materials than Reference Comparative Example 1, which did not use semiconductor material.

In addition, in Reference Example 3, the treatment time was extended to 60 minutes, but the increase in the weight loss rate was limited compared to Reference Example 1, in which the treatment time was 30 minutes. This is thought to be because the sample surface was covered with charcoal during the heat treatment, resulting in a decrease in the decomposition efficiency.

Reference Example 4 and Reference Comparative Example 2
<Reference Example 4>
The treatment and evaluation of Reference Example 4 were carried out in the same manner as Reference Example 1, except that heating was carried out for 60 minutes at a surface temperature of 500° C. The results are shown in Table 3 below.

<Reference Comparative Example 2>
Except for not using a semiconductor material, the treatment and evaluation of Reference Comparative Example 2 were carried out in the same manner as Reference Example 4. The results are shown in Table 3 below.

Photographs of the samples after the treatments of Reference Example 4 and Reference Comparative Example 2 are shown in Figures 5 and 6, respectively. A photograph of the sample before treatment is shown in Figure 4.

As can be seen in Table 3, Reference Example 4, in which heat treatment was performed under semiconductor material with the introduction of low-oxygen concentration gas with an oxygen concentration of 6% by volume, showed a higher plastic decomposition efficiency than Reference Comparative Example 2, in which heat treatment was performed without semiconductor material with the introduction of low-oxygen concentration gas with an oxygen concentration of 6% by volume.

As can be seen in Figure 6, residual carbon derived from plastic was found to adhere in clumps to the surface of the sample after treatment in Reference Comparative Example 2, whereas, as can be seen in Figure 5, no such residual carbon was found to adhere to the sample after treatment in Reference Example 4, and the sample had a relatively smooth and highly uniform surface, similar to the surface of the sample before treatment shown in Figure 4.

Reference Examples 5 to 9
In Reference Examples 5 to 9, a two-stage heat treatment was performed on a CFRP plate or a pressure vessel as a plastic-containing material. Then, the decomposition efficiency was evaluated, and the physical properties of the carbon fiber material obtained by the heat treatment were evaluated.

<Reference Example 5>
Reference Example 5 was carried out as follows.

(Provision and arrangement of materials)
The semiconductor material used was a carrier having a honeycomb structure with chromium oxide (Cr ₂ O ₃ , purity 99% or more, manufactured by Junsei Chemical Co., Ltd.) applied to the surface thereof. The carrier having a honeycomb structure had 13 cells/25 mm.

A CFRP plate with an epoxy resin content of 41% by weight was used as the plastic-containing material. The physical properties of the carbon fiber material contained in the CFRP plate before treatment are shown in Table 4 as Reference Example 1.

The furnace volume was 0.0525 ^m3 . Inside the furnace, a CFRP plate was placed on the support carrying chromium oxide as a semiconductor material. The support and the CFRP plate were placed in contact with each other.

The surface temperature of the CFRP plate was measured using a sensor placed within 5 mm of the surface of the CFRP plate.

(Heat Treatment)
The internal temperature of the heating furnace was increased by controlling the heater output of the heating furnace. Then, before the surface temperature of the CFRP plate reached 300° C., a mixed gas of air with an oxygen concentration of 8% by volume and nitrogen gas was introduced into the heating furnace. The mixed gas was introduced into the heating furnace by suction at a gas introduction rate of 190 L/min and by flowing the mixed gas from a gas supply unit provided in the heating furnace.

In addition, in Reference Example 5, as well as Reference Example 6 and Reference Comparative Example 4 described below, the oxygen concentration in the heating furnace was measured using an oxygen monitor installed in the heating furnace. For Reference Examples 7 to 9, the oxygen concentration in the furnace was determined based on the furnace volume and the amount of gas introduced.

The generation of decomposition gas was confirmed when the surface temperature of the CFRP plate reached 300°C.

The heat treatment was carried out for 120 minutes in an atmosphere where the oxygen concentration was controlled to 8% by volume by introducing the above mixed gas. During the heat treatment, the heater output of the heating furnace was adjusted to raise the surface temperature of the CFRP plate to a first surface temperature of 450°C.

(Secondary Heat Treatment)
Then, while maintaining the gas suction pressure, the supply of nitrogen gas was stopped and only air was supplied, and after confirming that the oxygen concentration in the heating furnace had reached 10% by volume or more, further heat treatment was performed. During the 260-minute heat treatment, the surface temperature of the CFRP plate was raised to 500°C. The average oxygen concentration over the 260-minute period was 14% by volume, and the maximum oxygen concentration was 18% by volume.

In Reference Example 5, 0.8 kg of a CFRP plate was treated. The treatment amount relative to the furnace volume was 15.2 kg/ ^m3 .

(Residual carbon amount)
The amount of residual carbon derived from plastics in the carbon fiber material recovered after the heat treatment was determined by thermogravimetric analysis.

Thermogravimetric analysis was carried out as follows:
(i) performing a thermogravimetric analysis on a sample piece of 1 to 4 mg obtained by pulverizing the recovered carbon fiber material for a total of 300 minutes in a thermogravimetric analyzer, the thermogravimetric analysis including steps of heating from room temperature to 100° C., holding at 100° C. for 30 minutes, heating from 100° C. to 400° C., and holding at 400° C., at an air supply rate of 0.2 L/min, a heating rate of 5° C./min, and a recording rate of 1/6 s;
(ii) In a graph plotting the weight loss rate against time, an inflection point in the slope was identified, and the amount of residual carbon was calculated by subtracting the weight loss rate during the holding period at 100°C from the weight loss rate value at the inflection point.

In addition, the fiber single yarn diameter and single fiber tensile strength of the carbon fiber material recovered after the heat treatment were measured, and the Weibull shape coefficient was calculated.

(Single fiber tensile strength)
The single fiber tensile strength was measured according to JIS R7606 as follows:
At least 30 single fibers are taken from the fiber bundle;
The diameter of the single fiber is measured in a side image of the single fiber taken by a digital microscope, and the cross-sectional area is calculated.
The sampled single fiber was fixed to a perforated mount using adhesive.
The mount on which the single fiber was fixed was attached to a tensile tester, and a tensile test was carried out at a test length of 10 mm and a strain rate of 1 mm/min to measure the tensile breaking stress.
The tensile strength is calculated from the cross-sectional area and tensile breaking stress of the single fiber.
The single fiber tensile strength was determined as the average of the tensile strengths of at least 30 single fibers.

(Weibull shape factor)
The Weibull shape factor was calculated according to the following formula:
lnln{1/(1-F)}=m×lnσ+C
(In the formula, F is the fracture probability obtained by the symmetric sample cumulative distribution method, σ is the single fiber tensile strength (MPa), m is the Weibull shape coefficient, and C is a constant.)

A Weibull plot was made using lnln{1/(1-F)} and lnσ, and the Weibull shape coefficient m was calculated from the linear approximation slope.

The evaluation results are shown in Table 4. The fiber single yarn diameter is the average of the diameters of single fibers measured as described above for at least 30 single fibers.

<Reference Example 6>

The treatment was carried out in the same manner as in Reference Example 5, except that a pressure vessel was used as the plastic-containing material, the mixed gas was aspirated at a gas introduction rate of 127 L/min, and the oxygen concentration, surface temperature, and heat treatment time were as shown in Table 4 below.

The pressure vessel treated in Reference Example 6 had an aluminum liner and 44% by weight of FRP (fiber reinforced plastic), which contained 31% by weight of reinforcing fibers and 13% epoxy resin. The reinforcing fibers were composed primarily of carbon fibers with a small amount of glass fibers. The volume of the pressure vessel was 2.0 L.

The amount of FRP treated in Reference Example 6 was 0.47 kg. The amount of FRP treated relative to the furnace volume was 9.0 kg/ ^m3 .

The evaluation results of the plastic decomposition efficiency for Reference Example 6 and the evaluation results of the physical properties of the recovered carbon fiber material are shown in Table 4. The average oxygen concentration over 180 minutes of secondary heating treatment was 18% by volume, and the maximum oxygen concentration was 20% by volume.

The physical properties of the carbon fiber material contained in the pressure vessel before treatment are shown in Table 4 as Reference Example 2.

<Reference Example 7>
The heating treatment was performed in the same manner as in Reference Example 5, except that the furnace volume of the heating furnace was 0.1435 ^m3 , a mixed gas of superheated steam and air was introduced into the heating furnace at a gas introduction rate of 29 L/min, and the surface temperature and the heating treatment time were as shown in Table 4 below.

The internal temperature of the heating furnace and the surface temperature of the sample were controlled via the heater output of the heating furnace and the temperature of the superheated steam.

In Reference Example 7, 1.0 kg of a CFRP plate was treated. The treatment amount relative to the furnace inner volume was 7.0 kg/ ^m3 .

The evaluation results of the plastic decomposition efficiency for Reference Example 7 and the evaluation results of the physical properties of the recovered carbon fiber material are shown in Table 4. In the secondary heating process, only air was forced into the furnace at 29 L/min.

<Reference Example 8>
The treatment was carried out in the same manner as in Reference Example 7, except that a pressure vessel was used as the plastic-containing material, and the surface temperature and the heat treatment time were as shown in Table 4 below.

The pressure vessel used in Reference Example 8 is the same as the pressure vessel used in Reference Example 6.

The amount of FRP treated in Reference Example 8 was 0.47 kg. The amount of FRP treated relative to the furnace volume was 3.3 kg/ ^m3 .

The evaluation results of the plastic decomposition efficiency for Reference Example 8 and the evaluation results of the physical properties of the recovered carbon fiber material are shown in Table 4. In the secondary heating process, only air was forced into the furnace at 29 L/min.

<Reference Example 9>
The treatment was carried out in the same manner as in Reference Example 7, except that the furnace volume of the heating furnace was 0.049 ^m3 , a mixed gas of superheated steam and air was introduced into the heating furnace at a gas introduction rate of 25 L/min, and the surface temperature and the heat treatment time were as shown in Table 4 below.

In Reference Example 9, a CFRP plate with an epoxy resin ratio of 38% was treated. The treatment amount relative to the furnace volume was 1.6 kg/ ^m3 .

The evaluation results of the plastic decomposition efficiency for Reference Example 9 and the evaluation results of the physical properties of the recovered carbon fiber material are shown in Table 4. In the secondary heating process, the supply of superheated steam was continued while the amount of air was increased, and gas with an oxygen concentration of 11% by volume was forced into the furnace at 38 L/min.

The physical properties of the carbon fiber material contained in the above CFRP plate of Reference Example 9 before treatment are shown in Table 4 as Reference Example 3.

<Reference Comparative Example 3>
In Reference Comparative Example 3, the materials were provided and arranged in the same manner as in Reference Example 5, and then the internal temperature of the heating furnace was increased without adjusting the oxygen concentration of the atmosphere in the heating furnace.

As a result, with the heater output remaining at the time when the surface temperature of the CFRP plate reached 300°C, excessive self-heating occurred, and the surface temperature of the CFRP plate rose to 485°C. The results are shown in Table 4.

In Comparative Example 3, 0.8 kg of CFRP plate was treated. The treatment amount relative to the furnace volume was 15.2 kg/ ^m3 .

≪Reference Comparative Example 4≫

The treatment was carried out in the same manner as in Reference Example 6, except that no semiconductor material was used.

The amount of FRP treated in Reference Comparative Example 4 was 0.47 kg. The amount of FRP treated relative to the furnace volume was 9.0 kg/ ^m3 .

Regarding Reference Comparative Example 4, the evaluation results of the physical properties of the recovered carbon fiber material are shown in Table 4. The average oxygen concentration over 180 minutes of secondary heating treatment was 18 vol.%, and the maximum oxygen concentration was 20 vol.%.

As can be seen in Table 4, Reference Examples 5 to 9, in which a low-oxygen gas with an oxygen concentration of 6 to 8% by volume was introduced, a primary heat treatment was performed in the presence of semiconductor material, and a secondary heat treatment was further performed under an elevated oxygen concentration, showed low residual carbon levels and excellent plastic decomposition efficiency.

In addition, in Reference Comparative Example 3, where oxygen concentration was not controlled, excessive self-heating occurred, whereas in Reference Examples 5 to 9, in which low-oxygen concentration gas with an oxygen concentration of 6 to 8% by volume was introduced into the heating furnace, excessive self-heating was not observed. In Reference Comparative Example 3, the oxygen concentration was excessive because the decomposition process was started without lowering the oxygen concentration beforehand, and as a result, it is believed that the decomposition temperature could not be appropriately controlled in the presence of semiconductor material.

Furthermore, the carbon fiber materials recovered in Reference Examples 5 to 9 maintained similar fiber single yarn diameters and single fiber tensile strengths compared to the carbon fiber materials before treatment (Reference Examples 1 to 3), and had higher Weibull shape coefficients of single fiber tensile strengths. In other words, in Reference Examples 5 to 9, carbon fiber materials with superior physical properties were recovered compared to the carbon fiber materials before carbon fiber reinforced plastics were produced.

Furthermore, the carbon fiber material recovered in Reference Example 6, which involved a heat treatment under a semiconductor material, had superior quality, particularly in terms of single fiber tensile strength and Weibull shape factor, to the carbon fiber material recovered in Reference Comparative Example 4, which involved a heat treatment without a semiconductor material.

100 Carbon fiber reinforced thermoplastic strand according to the present disclosure 110 Core component 120 Sheath component 11 Semiconductor material 12 Plastic-containing material 20 Heating furnace 21 Carrier having semiconductor material 22 Plastic-containing material 23 Heat source (heater)
24 Gas supply section 25 Exhaust port 26 Inner space of heating furnace 27 Temperature sensor

Claims (8)

A carbon fiber reinforced thermoplastic resin strand including a blended yarn including recycled carbon fiber and thermoplastic resin fiber,
The recycled carbon fiber has a single fiber tensile strength of 3.0 GPa or more and a Weibull shape coefficient of 6.0 or more ;
The recycled carbon fiber contains a residual carbon component, and the content of the residual carbon component is more than 0% by weight and 5.0% by weight or less with respect to the recycled carbon fiber ; and
The content of the recycled carbon fiber is more than 60% by weight and not more than 98% by weight based on the blended yarn.
A carbon fiber reinforced thermoplastic resin strand. The carbon fiber reinforced thermoplastic resin strand according to claim 1 , wherein the content of the recycled carbon fiber is more than 70 % by weight and not more than 98% by weight based on the blended yarn. The carbon fiber reinforced thermoplastic resin strand according to claim 1 or 2, wherein the average length of the recycled carbon fibers and the average length of the thermoplastic resin fibers are 20 mm or more and 80 mm or less, respectively. The carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 3, wherein the thermoplastic resin fibers are selected from polyolefin resin fibers, polyester resin fibers, polyamide resin fibers, polyether ketone resin fibers, polycarbonate resin fibers, phenoxy resin fibers, and polyphenylene sulfide resin fibers, and mixtures thereof. It has a core-sheath structure,
The blended yarn is a core component, and a thermoplastic resin is a sheath component.
The carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 4. The carbon fiber reinforced thermoplastic resin strand according to claim 5, characterized in that the melting point T0 (°C) of the thermoplastic resin as the sheath component is lower than the melting point T1 (°C) of the thermoplastic resin fiber contained in the blended yarn, and T1-T0>10 is satisfied. A method for producing the carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 6, comprising: decomposing a plastic component contained in a carbon fiber-containing plastic product by a semiconductor thermal activation method to produce recycled carbon fiber; and blending the recycled carbon fiber with a thermoplastic resin fiber to produce a blended yarn. A method for manufacturing a three-dimensional object by a fusion deposition method using the carbon fiber reinforced thermoplastic resin strand according to any one of claims 1 to 6.

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