JP2005336331A - Method for producing recycled carbon fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a recycled carbon fiber achieving excellent modification of the carbon fiber surface by using a supercritical fluid or a subcritical fluid. <P>SOLUTION: A carbon fiber-reinforced plastic (CFRP) 5 is treated at a high temperature in a pressure vessel 4 with supercritical water or subcritical water by adding a fresh water to the pressure vessel 4 while removing the supercritical water or subcritical water used in the high-temperature treatment from the vessel 4 and the recycled carbon fiber is separated and extracted from the resin. The surface of the recycled carbon fiber is modified by keeping the temperature of the supercritical water or subcritical water at or about the critical temperature (374°C) during the high-temperature treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing recycled carbon fiber in which carbon fiber is taken out from a carbon fiber reinforced resin (hereinafter sometimes referred to as “CFRP”).

  In addition to being lightweight, CFRP has excellent strength and elastic modulus, so its application is being developed across a wide range of fields, including components for sports and leisure equipment and components for spacecraft. . Recently, the use of CFRP has been expanded from a special product to a general-purpose product, and thus it has become a problem how to treat CFRP waste.

  CFRP waste treatment methods include (1) use as a blast furnace raw material after pulverization, (2) use as a composite material after pulverization, and (3) recovering carbon fiber by decomposing resin. ing. Since carbon fiber is more expensive than glass fiber, (3) the method of decomposing the resin to recover the carbon fiber and using it as recycled carbon fiber is the most preferable method. As a method for decomposing a resin used for recovering carbon fiber from CFRP, methods such as thermal decomposition, chemical decomposition, chemical dissolution, and photodecomposition have been studied. Among them, thermal decomposition is often studied.

On the other hand, in recent years, research on the treatment of waste using supercritical and subcritical fluids has been conducted. For example, Japanese Patent Laid-Open No. 10-87872 discloses a fiber recovery and reuse method in which fibers are separated and recovered by reacting fiber reinforced plastics with supercritical water or subcritical water in a pressure-resistant processing vessel. Has been. Japanese Patent Laid-Open No. 2003-190759 discloses a method of continuously removing water from a pressure-resistant treatment container when CFRP is reacted with supercritical water or subcritical water in the pressure-resistant treatment container to separate and recover fibers. It is disclosed.
Japanese Patent Laid-Open No. 10-87872 JP 2003-190759 A

Although the technique disclosed in Japanese Patent Laid-Open No. 2003-190759 is a very effective method, there is still room for improvement in fine condition setting.
In addition, when producing CFRP from carbon fiber, in order to increase the bonding force between the carbon fiber and the resin, oxygen is bonded to the surface of the carbon fiber in advance to improve the oxygen / carbon concentration ratio (modified). Is done). Conventionally, carbon fiber oxidation treatment has been performed as one of the surface modifications. For this purpose, a large amount of chemicals must be used and a complicated process may be required.

  The present invention has been made in view of the above circumstances, and an object thereof is a method for producing recycled carbon fiber using a supercritical fluid or a subcritical fluid, which appropriately modifies the surface of the carbon fiber. Is to provide.

As a result of intensive investigations, the inventors of the present invention processed CFRP with a supercritical fluid or a subcritical fluid in a pressure-resistant treatment container, and separated and recovered the carbon fiber. By making the temperature of the fluid near the critical temperature, it was found that the surface of the carbon fiber from which the resin was separated and extracted was modified, and the present invention was basically completed.
In this way, the method for producing recycled carbon fiber according to the invention for achieving the above-described object involves high-temperature treatment of a carbon fiber reinforced resin with a supercritical fluid or a subcritical fluid in a pressure-resistant treatment container, and the treatment during the high-temperature treatment. A method of separating and extracting carbon fiber from a resin while removing a supercritical fluid or subcritical fluid from the pressure-resistant treatment vessel while adding a new fluid to the pressure-resistant treatment vessel, the method comprising: The temperature of the critical fluid is maintained in the vicinity of the critical temperature.

Examples of the “fluid” include water, methanol, ethanol, carbon dioxide and the like, but the present invention is not limited to these. Of these fluids, water is preferably used.
“Supercritical fluid” means a critical point of temperature, pressure, and entropy diagram in a phase diagram (for example, 374.3 ° C. and 22.1 MPa for water, 239 ° C. and 8.1 MPa for methanol, and 243.0 ° C. for ethanol). , 6.14 MPa, 31 ° C. for carbon dioxide, 7.4 MPa) and a fluid in a supercritical state to which a temperature and pressure higher than those are applied. The “subcritical fluid” means a fluid having a temperature or pressure lower than the critical point, and the compressed gas and the compressed liquid coexist.

  For example, when the fluid is water, the oxygen / carbon ratio between 340 ° C. and 420 ° C. is expected when the surface element concentration of the separated and extracted carbon fiber is measured. It means a region protruding above the smooth curve. Specifically, when the fluid is water, a temperature range including 374 ° C. which is a critical temperature, that is, about 360 ° C. to about 395 ° C., preferably about 365 ° C. to about 390 ° C., more preferably about 370 ° C. To about 385 ° C. (or about 380 ° C.).

  According to the method of the present invention, since the oxygen / carbon concentration ratio is improved on the surface of the extracted carbon fiber, there is no need to use a conventionally used chemical, and the process is simplified while the resin and A carbon fiber having a strong bonding force between the two can be obtained.

  Next, embodiments of the present invention will be described in detail with reference to the drawings. However, the technical scope of the present invention is not limited by the following embodiments, and various changes can be made without changing the gist thereof. It can be carried out with modification.

  Any carbon fiber such as polyacrylonitrile (hereinafter referred to as PAN) carbon fiber, cellulosic carbon fiber, pitch carbon fiber, or gas-grown carbon fiber is used as the carbon fiber forming the CFRP treated in the present invention. You can also. Of these carbon fibers, the carbon fiber precursor is preferably flameproofed in an oxidizing atmosphere, and then the flameproofed fiber is fired at 800 ° C. to 2000 ° C. in an inert gas atmosphere. PAN-based carbon fiber obtained by firing in a higher temperature inert gas is used. Generally, carbon fibers are subjected to oxidation treatment such as electrolytic oxidation treatment and vapor phase oxidation treatment after firing for the purpose of surface modification in order to enhance the bonding strength with the resin, and sizing agents are added to the CFRP. Used for. However, according to the method of the present embodiment, surface modification can be performed without performing such oxidation treatment.

  As the resin forming the CFRP processed in the present embodiment, either a thermosetting resin or a thermoplastic resin can be used. Examples of the thermosetting resin include phenol resin, epoxy resin, unsaturated polyester resin, vinyl ester resin, polyimide, and cross-linked acrylic resin. Examples of thermoplastic resins include polyvinyl chloride, fluororesin, polyethylene, polypropylene, polyamide, polymethyl methacrylate, polycarbonate, polyacetal, polyester, modified PPE, polysulfone, polyethersulfone, polyphenylene sulfide, polyarylate, polyetheretherketone, Examples include wholly aromatic polyesters, polyamide imides, polyether imides, and liquid crystal polyesters.

Next, a method for treating CFRP with a supercritical fluid or a subcritical fluid will be specifically described. The conditions and method of treatment with the supercritical fluid or subcritical fluid can be arbitrarily set according to the type of fluid used, the type of CFRP and the resin content, the form, thickness, number, etc. of carbon fibers. For example, when water is used as the fluid, it can be performed as follows.
First, CFRP and water are put into a pressure-resistant processing vessel equipped with a pressure sensor and heated. Since the critical point of water is a temperature of 374.4 ° C. and a pressure of 22.1 MPa, it is about 360 ° C. to about 395 ° C., preferably about 365 ° C. to about 390 ° C., more preferably about 370 ° C. to about 385 ° C. (or about 380 ° C.), the pressure is set to about 20 MPa to about 30 MPa, and water is brought into a supercritical state or a subcritical state in the pressure-resistant treatment container. Next, this state is maintained for about 10 minutes to about 3 hours, and the CFRP in the pressure-resistant processing container is processed. When subcritical water is used, the high temperature treatment time is preferably about 30 minutes to about 3 hours, and when supercritical water is used, the high temperature treatment time is preferably about 10 minutes to about 2 hours. Furthermore, if the treatment time is shorter than the above range, removal of the resin tends to be insufficient, and if it exceeds the above range, the strength of the carbon fiber tends to decrease.

After the CFRP is treated with the supercritical fluid or subcritical fluid in this way, the processing is performed while removing the supercritical fluid or subcritical fluid subjected to the high temperature treatment from the pressure resistant processing vessel and adding a new fluid to the pressure resistant processing vessel. At this time, the flow rate of the supercritical fluid or subcritical fluid is preferably 2.5 ml / min or more with respect to 1 g of CFRP. The recycled carbon fiber separated and extracted from the resin by the high-temperature treatment is taken out after the treatment is completed, the pressure-resistant treatment container is cooled to normal pressure.
The form of the pressure-resistant treatment vessel is not particularly limited, but the portion in direct contact with the supercritical fluid or subcritical fluid is made of a corrosion-resistant material such as Hastelloy (an alloy made of Ni, Cr, Mo, etc.) or SUS. It is preferable to use the formed one.

  In the high-temperature treatment as described above, methanol, ethanol, carbon dioxide, or the like can be used as the fluid in addition to water. If an appropriate fluid is selected, the CFRP can be processed while maintaining the temperature and pressure near the critical state of the fluid. However, it is most preferable to select water as the fluid because it is easy to handle, low in cost, and rich in reactivity. In addition, there is no restriction | limiting in the form of the carbon fiber in CFRP processed, For example, it may be a short fiber or a continuous fiber. In addition, during high temperature processing, CFRP may be introduced into a pressure resistant processing vessel in which the fluid is preheated so as to be in a supercritical state or a subcritical state.

  By treating in this way, the resin forming CFRP is dissolved in the fluid, or is modified or decomposed by hydrolysis or oxidation reaction, whereby the resin can be almost removed from the carbon fiber. Further, by removing the supercritical fluid or subcritical fluid subjected to the high temperature treatment from the pressure resistant container, it is possible to recover the separated and extracted recycled carbon fiber from the pressure resistant container without post-washing. In addition, by maintaining the temperature of the supercritical fluid or subcritical fluid during the high temperature treatment close to the critical temperature, the surface of the separated and extracted recycled carbon fiber can be modified. There is no need to process.

  Next, the present invention will be specifically described with reference to examples. In FIG. 1, the manufacturing system of the recycled carbon fiber used for the Example was shown. This system includes a high-pressure pump 1 that sends water W as a fluid, a molten salt bath 2 that can be controlled to a predetermined temperature, a heat exchanger 3 that is immersed in the molten salt bath 2, and a pressure treatment vessel 4. Is provided. The CFRP 5 is accommodated inside the pressure-resistant processing container 4. Water W sent from the high-pressure pump 1 and brought to a predetermined temperature by the heat exchanger 3 is continuously flowed into and out of the pressure-resistant treatment container 4. In addition, the arrow in a figure has shown the direction through which the water W flows.

  The molten salt tank 2 can be made of Hastelloy. The heat exchanger 3 immersed in the molten salt tank 2 is obtained by spirally winding an elongated tube body. The pressure-resistant treatment container 4 is made of Hastelloy, and its volume is 100 ml. The pressure-resistant processing container 4 is provided with a thermometer 8 made of a thermocouple for measuring the internal temperature. An inflow pipe 10 that receives the water W flowing in from the heat exchanger 3 is provided below the pressure-resistant processing container 4, and an outflow pipe 11 that sends the outflowing water W to the trap 7 is provided above the pressure-resistant treatment container 4. A pressure gauge 9 is provided in the middle of the outflow pipe 11. In the outflow pipe 11, a valve 6 is provided on the downstream side of the pressure gauge 9.

Inside the molten salt tank 2, a heater 12 and a rotary blade 13 are provided. With the heater 12 attached, the rotary blade 13 is rotated to mix the liquid in the molten salt tank 2, A uniform temperature can be obtained. Note that a computer (not shown) is connected to the heater 12 so that the temperature of the liquid in the molten salt tank 2 can be controlled within a predetermined range. In this system, the temperature of the internal liquid of the molten salt bath 2 can be controlled within a range of about 150 ° C to about 500 ° C.
First, CFRP 5 and a fluid were put into the pressure-resistant treatment container 4 and heated. The reaction temperature was maintained at a constant temperature in the range of 330 ° C. to 420 ° C., and the pressure was 30 MPa. This state was maintained for 30 minutes, and the CFRP 5 in the pressure-resistant processing container 4 was processed.

During the high-temperature treatment, the high-pressure pump 1 is driven to send water into the pressure-resistant treatment vessel 4 and remove the supercritical water or subcritical water subjected to the high-temperature treatment from the pressure-resistant treatment vessel 4 while supplying a new fluid. The pressure-resistant treatment container 4 was supplied. At this time, water: CFRP was adjusted to be approximately 1:25. The recycled carbon fiber separated and extracted from the resin by the high-temperature treatment was cooled to the normal pressure by cooling the pressure-resistant treatment container 4 and removed from the pressure-resistant treatment container 4.
Using carbon fiber separated and extracted from CFRP using X-ray photoelectron spectroscopy-ESCALAB220iXL [X-ray source: Al-MONOkα200W, lens mode; LargeAreaXL, Pass Energy; 100 eV (survey), 20 eV (narrow)] manufactured by VG The concentration of surface elements (oxygen, carbon, nitrogen) was measured.

  (Example 1) 1 g of CFRP composed of polyacrylonitrile-based carbon fiber pyrofil TR50S (manufactured by Mitsubishi Rayon Co., Ltd.) and an epoxy resin was charged into a pressure-resistant treatment container together with pure water and sealed. After sealing the pressure-resistant treatment container, it was put into a molten salt bath that had been heated to 330 ° C. in advance, and the pressure was maintained at 30 MPa while water was poured into the pressure-resistant treatment container at a flow rate of 5 ml / min. At this time, the drain side valve was adjusted so that it could drain at a flow rate of 5 ml / min. The time when the pressure reached 30 MPa was set as 0 minutes, and the high temperature treatment was started. After 30 minutes, the processing container was pulled up from the molten salt tank, and then the entire pressure-resistant processing container was immersed in water and cooled to room temperature. Thereafter, a recycled carbon fiber sample in the pressure resistant container was collected.

(Example 2) The treatment was performed in the same manner as in Example 1 except that the temperature was 340 ° C.
(Example 3) The process was performed in the same manner as in Example 1 except that the temperature was 350 ° C.
(Example 4) The treatment was performed in the same manner as in Example 1 except that the temperature was 360 ° C.
(Example 5) The same treatment as in Example 1 was conducted except that the temperature was 370 ° C.
(Example 6) The process was performed in the same manner as in Example 1 except that the temperature was 375 ° C.
(Example 7) The treatment was performed in the same manner as in Example 1 except that the temperature was 380 ° C.
(Example 8) The treatment was performed in the same manner as in Example 1 except that the temperature was 390 ° C.
(Example 9) The treatment was performed in the same manner as in Example 1 except that the temperature was 400 ° C.
(Example 10) The same treatment as in Example 1 was conducted except that the temperature was 420 ° C.

<Results and discussion>
FIG. 2 shows surface element concentration ratios (nitrogen / carbon and oxygen / carbon) of recycled carbon fibers separated and recovered after treating CFRP at 330 ° C. to 420 ° C.
The nitrogen / carbon concentration ratio was almost constant (about 0.025) at the processing temperature of 330 ° C. to 420 ° C., and no change was observed (P region in FIG. 2). In carbon fiber, it is known that the nitrogen / carbon concentration ratio increases from the outer surface toward the inner center. For this reason, when a crack enters the surface of the carbon fiber, the nitrogen / carbon concentration ratio increases. In the above examples, even when the temperature was raised to 420 ° C., the nitrogen / carbon concentration ratio did not change, and thus it was found that the surface of the recycled carbon fiber was not damaged.

  The oxygen / carbon concentration ratio tended to decrease as the temperature increased as a whole at a processing temperature of 330 ° C. to 420 ° C. (Q region in FIG. 2). Resin (epoxy resin) contained in CFRP has an oxygen atom in the molecule. On the other hand, since the carbon fiber itself is substantially composed only of carbon atoms, the surface thereof does not have oxygen atoms. It is considered that oxygen atoms derived from the resin are attached to the surface of the carbon fiber in the CFRP. For this reason, in the low temperature range of 330 ° C. to 340 ° C., oxygen atoms found on the surface of the recycled carbon fiber originate from the CFRP resin. Since the oxygen / carbon concentration ratio decreased as the treatment temperature increased, it was considered that the resin on the carbon fiber surface was being removed.

  However, in the temperature range (360 ° C. to 390 ° C.) in the vicinity of the critical temperature of water (374 ° C.), the oxygen / carbon concentration ratio is other temperature ranges (330 ° C. to 350 ° C. and 400 ° C. to 420 ° C.). ) Protruded above the smooth curve expected from the data (region including the R region in FIG. 2). This was considered that high-temperature high-pressure water reacted with the surface of the carbon fiber and some oxygen atoms (and hydrogen atoms) were attached. In general, it is known that the higher the oxygen / carbon concentration ratio on the surface of the carbon fiber, the better the bondability with the resin. Actually, conventional carbon fibers have been treated with chemicals in order to modify the surface. According to the present Example, it turned out that the surface modification | reformation of a recycled carbon fiber is performed in the vicinity of critical temperature, and a chemical | medical treatment after that is unnecessary.

<Conclusion>
As described above, according to this example, CFRP can be subjected to high-temperature treatment in the vicinity of the critical temperature of water while replacing water, thereby removing the resin and separating and extracting the recycled carbon fiber. In this treatment, the surface modification of the recycled carbon fiber can be performed at the same time, so that it is possible to save the trouble of the acid treatment when reusing.

It is a figure which shows the outline | summary of the manufacturing system of the recycled carbon fiber in a present Example. It is a graph which shows the relationship between the surface element density | concentration ratio of the recycled carbon fiber separated and collect | recovered from CFRP, and reaction temperature.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... High pressure pump, 2 ... Molten salt tank, 3 ... Heat exchanger, 4 ... Pressure-resistant processing container, 5 ... CFRP, 6 ... Valve, 7 ... Trap, 8 ... Thermometer, 9 ... Pressure gauge

Claims (3)

The carbon fiber reinforced resin is subjected to high-temperature treatment with a supercritical fluid or subcritical fluid in a pressure-resistant treatment vessel, and the supercritical fluid or subcritical fluid subjected to the treatment during the high-temperature treatment is removed from the pressure-resistant treatment vessel, A method for separating and extracting carbon fibers from a resin while adding a fluid to the pressure-resistant treatment vessel, wherein the temperature of the supercritical fluid or subcritical fluid during high-temperature treatment is maintained near the critical temperature. A method for producing fibers. The method for producing recycled carbon fiber according to claim 1, wherein the fluid is water. The method for producing recycled carbon fiber according to claim 2, wherein the vicinity of the critical temperature is about 365 ° C to about 390 ° C.

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