Abstract: A method of producing a precursor fiber for carbon fiber includes extruding a polyacrylonitrile copolymer solution from a spinneret into the air, immersing it in a coagulation bath liquid stored in a coagulation bath, redirecting traveling of the coagulating fiber bundle by a first guide immersed in the coagulation bath disposed below the spinneret, and pulling it out of the coagulation bath liquid into the air to prepare a coagulated fiber bundle, which is then subjected to at least a washing step in water, stretching step, oil agent applying step, and drying step, wherein the depth-directional coagulation bath immersion length, which means a distance between the starting point of the immersion of the spinning dope solution in the coagulation bath liquid and the first guide immersed in the coagulation bath where traveling of the coagulating fiber bundle is redirected, is 3 to 40 cm.

Abstract: A method of producing a carbon fiber bundle suppresses penetration of a process oil agent into the fiber surface layer and suppresses adhesion between fibers and the generation of surface voids, and also provides a carbon fiber bundle. The carbon fiber bundle is characterized by having a crystallite size of 3.0 nm or less as measured by wide-angle X-ray diffraction, containing a point where the Si/C ratio is 10 or more as calculated by SIMS (secondary ion mass spectrometry) in the region ranging from 0 to 10 nm in depth from the fiber surface, and also showing a Si/C ratio of 1.0 or less as calculated by SIMS at a depth of 10 nm from the fiber surface.

Abstract: A method of producing a precursor fiber for carbon fiber includes extruding a polyacrylonitrile copolymer solution from a spinneret into the air, immersing it in a coagulation bath liquid stored in a coagulation bath, redirecting traveling of the coagulating fiber bundle by a first guide immersed in the coagulation bath disposed below the spinneret, and pulling it out of the coagulation bath liquid into the air to prepare a coagulated fiber bundle, which is then subjected to at least a washing step in water, stretching step, oil agent applying step, and drying step, wherein the depth-directional coagulation bath immersion length, which means a distance between the starting point of the immersion of the spinning dope solution in the coagulation bath liquid and the first guide immersed in the coagulation bath where traveling of the coagulating fiber bundle is redirected, is 3 to 40 cm.

Abstract: A sizing agent-coated carbon fiber bundle has a sizing agent containing an aliphatic epoxy compound (C) and an aromatic epoxy compound (D) coated on the carbon fiber bundle, wherein the carbon fiber in the carbon fiber bundle is the one which exhibits, when measured by single-fiber composite fragmentation method, a number of fiber breaks of at least 2.0/mm when apparent single-fiber stress is 15.3 GPa and the number of fiber breaks of up to 1.7/mm when the apparent single-fiber stress is 12.2 GPa.

Abstract: A sizing agent-coated carbon fiber bundle has a sizing agent containing an aliphatic epoxy compound (C) and an aromatic epoxy compound (D) coated on the carbon fiber bundle, wherein the carbon fiber in the carbon fiber bundle is the one which exhibits, when measured by single-fiber composite fragmentation method, a number of fiber breaks of at least 2.0/mm when apparent single-fiber stress is 15.3 GPa and the number of fiber breaks of up to 1.7/mm when the apparent single-fiber stress is 12.2 GPa.

Abstract: A polyacrylonitrile-based polymer which satisfies at least one of [a] to [d]: [a] Z-average molecular weight (Mz) determined by gel-permeation chromatograph is 800,000 to 6,000,000 and degree of polydispersity (Mz/Mw) (Mw denotes weight average molecular weight) is 3.0 to 10.0; [b] Z+1-average molecular weight (Mz+1) determined by GPC method is 3,000,000 to 10,000,000 and degree of polydispersity (Mz+1/Mw) is 6.0 to 25.0; [c] Mzm determined by gel-permeation chromatograph multi-angle laserlight scattering photometry is 400,000 to 1,000,000 and degree of polydispersity (Mzm/Mwm) is 3.0 to 10.0; and [d] Z-average radius of gyration (Rz) determined by gel-permeation chromatograph multi-angle laserlight scattering photometry is 25 to 45 nm and its ratio to weight average radius of gyration (Rz/Rw) is 1.3 to 2.5.

Abstract: A polyacrylonitrile-based polymer, containing a polymer of which main component is acrylonitrile, which satisfies at least one kind requirement selected from the following [a] to [d]. [a] Z-average molecular weight (Mz) determined by gel-permeation chromatograph is 800,000 to 6,000,000 and degree of polydispersity (Mz/Mw) (Mw denotes weight average molecular weight) is 3.0 to 10.0. [b] Z+1-average molecular weight (Mz+1) determined by GPC method is 3,000,000 to 10,000,000 and degree of polydispersity (Mz+1/Mw) is 6.0 to 25.0. [c] Mzm determined by gel-permeation chromatograph multi-angle laserlight scattering photometry is 400,000 to 1,000,000 and degree of polydispersity (Mzm/Mwm) is 3.0 to 10.0. [d] Z-average radius of gyration (Rz) determined by gel-permeation chromatograph multi-angle laserlight scattering photometry is 25 to 45 nm and its ratio to weight average radius of gyration (Rz/Rw) is 1.3 to 2.5.