Process for producing carbon fiber

Claim 1

A process for producing a carbon fiber, which comprises making a carbon fiber travel through about a 5% aqueous phosphoric acid solution, applying an electric potential in said solution between the carbon fiber serving as a working electrode and a platinum counter electrode provided in the solution, said potential being swept continuously at a +2 or -2 mV/sec constant scanning rate over the range of the standard electrode potentials of said electrolyte solution, said standard electrode potentials being in the range of about -0.2 to 0.8 V with respect to a Ag/AgCl reference electrode, wherein the resulting change in the current intensity, i, is detected continuously to obtain cyclic current-potential curves, in which i is read off at about +0.40 V selected as the standard voltage versus Ag/AgCl reference electrode, and the current intensity per unit area, ipa is calculated, defined by the equation, ipa (μA/cm 2 )=i/Total surface area of the carbon fiber tow, in which the total surface area of the tow is calculated from the length of the carbon fiber electrode, weight per unit length of the carbon fiber tow, number of monofilaments and density of the carbon fiber, so that a preceeding surface oxidation step of said carbon fiber is controlled by estimating said ipa and keeping it at a desired constant value to thereby result in a carbon fiber having a substantially constant surface condition.