1.  Field of the Invention
The present invention relates to a method of treating carbon fibers, and more specifically to a method of treating carbon fibers in which carbon fibers obtained by a usual method are supplied into an electrolyte solution containing a vinyl monomer and the carbon fibers are caused to act as the cathode to produce electrolytic reaction in said solution so that the surfaces of the carbon fibers are covered with a polymerization product whereby the ability of the carbon fibers to bundle themselves can be elevated and the adhesiveness between the carbon fibers and resin to be reinforced upon producing carbon fiber composite materials can be improved. 
2.  Description of the Prior Art
Carbon fibers are noted as a new, useful structural material, and in most cases they are used, embedded in various resins, as a reinforcing support fiber (structural material) for composite materials like the so-called fiber-reinforced plastics (FRP).  It is now widely acknowledged that, among many reinforcing fibers of this type, carbon fibers have an especially good reinforcing ability.  However, it is also true that several technical problems remain unsolved for carbon fiber composite materials.  As fundamental properties required for the composite materials, it is necessary that the carbon fibers and resin to be reinforced (matrix) should adhere intimately to each other and that the resulting structure should be of uniform, homogeneous structure containing no vacant spaces or voids.  But, it is difficult to say that, as regards conventional composite materials, the above-mentioned requirements have been always satisfied.  Namely, the poor adhesiveness between the two and the presence of voids induce separation of carbon fibers from the matrix, resulting in a severe drop of the physical properties (for example, interlaminar shear stress, flexural strength, etc.)  of the composite materials. 
Of course, several methods for surface treatment of carbon fibers have been proposed to eliminate such defects of the composite materials.  However, all of them still involve unsolved points in order to put them into practice on an industrial scale.  Namely, there have been attempted oxidation treatments of carbon fibers including wet oxidation treatment with concentrated nitric acid, and high-temperature gas phase oxidation treatment, etc.;  surface covering treatment in which carbon fibers are immersed in a solution containing a high molecular weight substance to cover the filaments with said high molecular weight substance; and whisker-raising method in which single crystals are developed on the surfaces of the carbon fibers.  However, in the oxidation methods, there is a disadvantage in that the oxidation reaction proceeds to the internal portions of the carbon fibers, thereby lowering the strength, although some degree of improvement in adhering power is ackowledged; in the surface covering method, a high molecular weight substance is merely adsorbed physically to the carbon fibers, with the covering being not only uneven but also its durability being not satisfactory; and in the whisker-raising method, it is difficult to increase the reaction efficiency and this makes it extremely difficult to bring this method into practice on an industrial scale.  In these conventional methods which cause such various disadvantages, it is also attempted to provide a sizing step after treatment in order to increase the ability to bundle the carbon fibers and to improve the adhesiveness between the carbon fibers and matrix.  However, since the defects which the carbon fibers have received in the surface treatment step are carried intact into the sizing step, it is technically difficult of course to improve the physical properties of the composite material, even though the sizing step is specially established. 
When thus studying the conventional methods as mentioned above, it will be found that a treating means for carbon fibers which can elevate the ability to bundle carbon fibers without providing any sizing step and which, when producing a carbon fiber composite material, can improve the adhesiveness between the carbon fibers and the resin to be reinforced, is a great future technical problem.