A carbon-fiber-reinforced composite material prepared by dispersing carbon fibers in a matrix resin is excellent in terms of its lightweight, mechanical properties, dimensional stability, electrical conductivity, and the like. Hence, such composite material is used in wide-ranging fields including those relating to automobiles, aircraft, electric or electronic devices, optical apparatuses, precision apparatuses, entertainment toys and games, home or office equipment and supplies, architectural materials, and the like. The demand for such composite material is increasing yearly.
As one of methods for obtaining a substrate for molding such carbon-fiber-reinforced composite material, a method (also referred to as a wet process) that involves processing carbon fiber in an aqueous process such as a paper-making process is employed. An example of such method involves dispersing chopped carbon fibers in an aqueous medium and then binding the chopped carbon fibers to a matrix resin through bonding, entangling, or suturing.
Furthermore in recent years, a paper-making process has been employed as a method for obtaining an electrode substrate that is used for a fuel cell or the like. For example, JP Patent Publication (Kokai) No. 5-205750 A (1993) (p. 1, lines 8 to 18) discloses a method that involves forming chopped carbon fibers into a sheet in a paper-making process and then baking the sheet.
Generally, in the case of a fiber-reinforced composite material, the longer the fiber length, the better the mechanical properties. Particularly in the case of such fiber-reinforced composite material prepared by dispersing reinforcing fibers in a matrix resin, the higher the degree of dispersion of reinforcing fibers in the form of single fibers, the greater the advantages in terms of uniformity of characteristics and qualities of surface appearance.
In paper-making processes, carbon fiber is cut into chopped carbon fibers in advance and then the chopped carbon fibers are dispersed in dispersion medium, followed by paper-making. Convergence properties are important in view of such cutting into chopped carbon fibers and in view of the handleability of the chopped carbon fibers. When convergence properties are poor, the carbon fiber may break at sites other than sites to be cut at the time of cutting into chopped carbon fibers because of poor handleability. Such cases result in fiber lengths shorter than a predetermined length. Furthermore, dispersion of fibers in the form of single fibers is important in paper-making processes, so that excellent dispersibility in an aqueous medium is required. Also for use in electrodes, the better the dispersibility, the greater the advantages in terms of electrical conductivity.
Accordingly, carbon fiber to be applied to a paper-making process is required to have both (1) convergence properties for maintaining the form of the carbon fiber upon handling prior to the paper-making process and (2) excellent dispersibility for rapid dispersion in the form of single fibers when they are introduced into an aqueous medium in the paper-making process and for suppressing additional cohesion.
In general, to enhance the handleability of carbon fiber, an epoxy resin or the like has adhered as a converging agent (sizing agent) to the carbon fiber in many cases. The resulting convergence properties are of a level that is too high for application of carbon fiber to a paper-making process, resulting in the inhibited dispersibility of the carbon fiber.
In view of such problem, JP Patent Publication (Kokai) No. 2003-293264 A (p. 2, line 2) proposes carbon fiber for paper making, to which a sizing agent comprising an aqueous compound has adhered such that the ratio of the number of oxygen atoms to the number of carbon atoms (O1s/C1s) in the vicinity of the carbon fiber surface ranges from 0.01 to 0.20.
However, application alone of a sizing agent comprising an aqueous compound results in insufficient dispersibility of the carbon fiber into an aqueous medium. Thus, a problem may arise such that an undispersed fiber bundle is formed on the surface upon substrate formation in a paper-making process.