Carbon fiber bundle and method for producing the same

Abstract

In order to provide a carbon fiber bundle excellent in strength, elastic modulus, and process stability when subjected to further processing in spite of having a high total fineness, and a method for producing the carbon fiber bundle, the carbon fiber bundle allows a relationship between a coefficient A obtained from an approximation formula (1) of nonlinearity in a stress σ-strain ε curve in a resin-impregnated strand tensile test in a range of stress of 0 to 3 GPa and an orientation parameter of crystallites Π (%) in wide-angle X-ray diffraction measurement to satisfy a formula (2), an initial elastic modulus of the carbon fiber bundle is 240 to 279 GPa, the number of filaments of the carbon fiber bundle is 24,000 to 72,000, and the carbon fiber bundle is substantially untwisted, in which ε = A ⁢ σ 2 + B ⁢ σ + C ( 1 ) - 410 ≤ ( 0 .   0 ⁢ 0 ⁢ 00832 ⁢ π 2   -   0 .0184 π   +   1 .   0 ⁢ 0 ) / A ≤ - 310 ( 2 ) where, A, B, and C are coefficients of a quadratic function of the stress σ and the strain ε, and Π is an orientation parameter of crystallites.

1 . A carbon fiber bundle that allows a relationship between a coefficient A obtained from an approximation formula (1) of nonlinearity in a stress σ-strain ε curve in a resin-impregnated strand tensile test in a range of stress of 0 to 3 GPa and an orientation parameter of crystallites Π (%) in wide-angle X-ray diffraction measurement to satisfy a formula (2), an initial elastic modulus of the carbon fiber bundle being 245 to 260 GPa, a number of filaments of the carbon fiber bundle being 24,000 to 72,000, the carbon fiber bundle being substantially untwisted, in which
               ε   =       A   ⁢     σ   2       +     B   ⁢   σ     +   C             (   1   )               −386≤(0.0000832Π 2 −0.0184Π+1.00)/A≤−352  (2)
 
 where, A, B, and C are coefficients of a quadratic function of stress σ and strain ε, and Π is an orientation parameter of crystallites. 
 
     
     
         2 . The carbon fiber bundle according to  claim 1 , a single fiber fineness of the carbon fiber bundle being 0.63 to 1.35 dtex. 
     
     
         3 . The carbon fiber bundle according to  claim 1 , a circularity of a single fiber cross section of the carbon fiber bundle being 0.86 to 0.98. 
     
     
         4 . A method for producing the carbon fiber bundle according to  claim 1 , the method comprising:
 a stabilization process of heat-treating a substantially untwisted polyacrylonitrile-based precursor fiber bundle having 24,000 to 72,000 filaments at a temperature of 220 to 280° C. in an oxidizing atmosphere;   a pre-carbonization process of heat-treating the stabilized fiber bundle obtained in the stabilization process in an inert gas at a maximum temperature of 300 to 1,000° C.; and   a carbonization process of heat-treating the pre-carbonized fiber bundle obtained from the pre-carbonized process in an inert gas at a maximum temperature of 1,000 to 1,600° C.,   wherein a stretching ratio in the pre-carbonization process is 1.10 to 1.20, a stretching ratio in the carbonization process is 0.975 to 0.990, and a product of the stretching ratios of the pre-carbonization process and the carbonization process is 1.070 to 1.130,   in the stabilization process, the polyacrylonitrile-based precursor fiber bundle is subjected to a stepwise heat treatment in a plurality of heat treatment ovens set to different temperatures from each other or in a plurality of heat treatment sections provided in a heat treatment oven and set to different temperatures from each other; in the stabilization process, the temperature of the heat treatment oven or the heat treatment section having the lowest temperature is set to less than 230° C. and the temperature of the heat treatment oven or the heat treatment section having the highest temperature is set to 280° C. or less.   
     
     
         5 . The method for producing a carbon fiber bundle according to  claim 4 , wherein a single fiber fineness of the polyacrylonitrile-based precursor fiber bundle is 1.20 to 2.40 dtex. 
     
     
         6 . The method for producing a carbon fiber bundle according to  claim 4 , wherein a circularity of a single fiber cross section of the polyacrylonitrile-based precursor fiber bundle is 0.86 to 0.98.