An embodiment is a method of recycling carbon fibers that includes: preparing a carbon fiber reinforced plastic formed product that includes a carbon fiber reinforced plastic containing a carbon fiber and a resin; thermally decomposing or dissolving the resin in the carbon fiber reinforced plastic formed product by a first heating process or a first dissolving process; and winding while drawing the carbon fiber from the carbon fiber reinforced plastic formed product after the first heating process or the first dissolving process. The winding further includes thermally decomposing or dissolving a residue of the resin attached to the carbon fiber by a second heating process or a second dissolving process and adding a sizing agent to the carbon fiber after the second heating process or the second dissolving process.

An activated carbon fiber is obtained by activating: a polyphenylene ether fiber that contains a polyphenylene ether component having a rearrangement structure connected by a bond at an ortho-position in a repeating unit continuously bonded at a para-position; an infusibilized polyphenylene ether fiber obtained by infusibilizing the polyphenylene ether fiber; a flameproofed polyphenylene ether fiber obtained by flameproofing the polyphenylene ether fiber or the infusibilized polyphenylene ether fiber; or a carbon fiber obtained by carbonizing any of the polyphenylene ether fibers.

An activated carbon fiber is obtained by activating: a polyphenylene ether fiber that contains a polyphenylene ether component having a rearrangement structure connected by a bond at an ortho-position in a repeating unit continuously bonded at a para-position; an infusibilized polyphenylene ether fiber obtained by infusibilizing the polyphenylene ether fiber; a flameproofed polyphenylene ether fiber obtained by flameproofing the polyphenylene ether fiber or the infusibilized polyphenylene ether fiber; or a carbon fiber obtained by carbonizing any of the polyphenylene ether fibers.

Disclosed is a chemically decomposable thermosetting resin composition for recycling a fiber-reinforced composite, including an epoxy resin, a multifunctional glycidyl-ester-based compound, an additive having a hydroxyl group on at least one of terminals of a main chain thereof, and an acid-anhydride-based curing agent, thus exhibiting excellent chemical decomposition performance in a basic solution and a high glass transition temperature. A method of dissolving the thermosetting resin composition is also provided. Thereby, a thermosetting composite material containing a thermosetting resin of the invention enables recycling of carbon fiber through hydrolysis, and thus the carbon fiber recycling industry and the recycled resin industry can be newly expanded. The thermosetting resin can be applied to fields using not only carbon composite materials but also general thermosetting resins, and can be hydrolyzed in a basic solution, which can significantly reduce landfill or disposal costs.

Disclosed is a chemically decomposable thermosetting resin composition for recycling a fiber-reinforced composite, including an epoxy resin, a multifunctional glycidyl-ester-based compound, an additive having a hydroxyl group on at least one of terminals of a main chain thereof, and an acid-anhydride-based curing agent, thus exhibiting excellent chemical decomposition performance in a basic solution and a high glass transition temperature. A method of dissolving the thermosetting resin composition is also provided. Thereby, a thermosetting composite material containing a thermosetting resin of the invention enables recycling of carbon fiber through hydrolysis, and thus the carbon fiber recycling industry and the recycled resin industry can be newly expanded. The thermosetting resin can be applied to fields using not only carbon composite materials but also general thermosetting resins, and can be hydrolyzed in a basic solution, which can significantly reduce landfill or disposal costs.

An oil agent for a carbon fiber precursor is provided that contains a base component, a cationic surfactant, and a nonionic surfactant, wherein the cationic surfactant is a specific nitrogen-containing compound.

An oil agent for a carbon fiber precursor is provided that contains a base component, a cationic surfactant, and a nonionic surfactant, wherein the cationic surfactant is a specific nitrogen-containing compound.

A cured epoxy resin material is depolymerized by using a composition including a compound represented by the chemical formula of XO.sub.mY.sub.n (wherein X is hydrogen, alkali metal or alkaline earth metal, Y is halogen, m is a number satisfying 1m8 and n is a number satisfying 1n6), and a reaction solvent, wherein X is capable of being dissociated from XO.sub.mY.sub.n and Y radical is capable of being produced from XO.sub.mY.sub.n in the reaction solvent. It is possible to carry out depolymerization of a cured epoxy resin material, for example, at a temperature of 200 C., specifically 100 C. or lower, and to reduce processing cost and energy requirement. It is also possible to substitute for a reaction system using an organic solvent as main solvent, so that the contamination problems caused by the organic solvent functioning as separate contamination source may be solved and environmental contamination or pollution may be minimized.

A cured epoxy resin material is depolymerized by using a composition including a compound represented by the chemical formula of XO.sub.mY.sub.n (wherein X is hydrogen, alkali metal or alkaline earth metal, Y is halogen, m is a number satisfying 1m8 and n is a number satisfying 1n6), and a reaction solvent, wherein X is capable of being dissociated from XO.sub.mY.sub.n and Y radical is capable of being produced from XO.sub.mY.sub.n in the reaction solvent. It is possible to carry out depolymerization of a cured epoxy resin material, for example, at a temperature of 200 C., specifically 100 C. or lower, and to reduce processing cost and energy requirement. It is also possible to substitute for a reaction system using an organic solvent as main solvent, so that the contamination problems caused by the organic solvent functioning as separate contamination source may be solved and environmental contamination or pollution may be minimized.

There is provided a method of making a hollow fiber having improved resistance to microfracture formation at a fiber-matrix interface. The method includes mixing in a first solvent a plurality of nanostructures, one or more first polymers, and a fugitive polymer which is dissociable from the nanostructures and the one or more first polymers, to form an inner-volume portion mixture. The method further includes mixing in a second solvent one or more second polymers to form an outer-volume portion mixture, spinning the inner-volume portion mixture and the outer-volume portion mixture and extracting the fugitive polymer from the inner-volume portion mixture to form a precursor fiber, heating the precursor fiber to oxidize the precursor fiber and to change a molecular-bond structure of the precursor fiber, and obtaining a hollow fiber with the inner-volume portion having the nanostructures and the first polymers, and with the outer-volume portion having the second polymers.