In recent years, carbon fiber composite materials have been used in a wide field of applications including sports, aerospaces and industries and the consumption thereof is remarkably increasing in quantity. In correspondence to such conditions, the properties of carbon fibers used are also being improved by leaps and bounds.
In regard to the modulus of elasticity of carbon fibers, whereas it was about 20 ton/mm.sup.2 ten years ago, 23-24 ton/mm.sup.2 became its standard value several years ago. Further, recent efforts in development are being directed to attaining a modulus of elasticity of about 30 ton/mm.sup.2, and it is generally believed that such a value would become the mainstream of the moduli of elasticity of carbon fibers.
However, if such improvement of the modulus of elasticity of a carbon fiber is achieved while keeping the tenacity of the carbon fiber at a constant value, it will naturally cause the decrease of elongation of the carbon fiber, which will result in brittleness of carbon fiber composite materials produced by using such carbon fibers and in lowering the reliability of the properties of the composite materials.
Accordingly, there is a strong need at present for a carbon fiber having a high modulus of elasticity and a high elongation, in other words, a carbon fiber having a characteristic that it has a high elongation and at the same time has a high tenacity.
Conventional methods for improving the modulus of elasticity of a carbon fiber comprised increasing the carbonization temperature, namely the ultimate heat-treatment temperature, of the carbon fiber. However, though such a method is effective in improving the modulus of elasticity of carbon fibers, it has a defect in that the improvement is accompanied by the decrease in the tenacity of the carbon fibers and consequently results in the decrease in the elongation of the fibers. The attached drawing is a graph showing the correlation between the carbonization temperature of a carbon fiber and the physical properties of the resulting carbon fiber to illustrate such situations. As shown in the drawing, with the increase of the carbonization temperature of a carbon fiber, the modulus of elasticity of the fiber increases as indicated by curve A, whereas the tenacity and the density of the carbon fiber decrease as shown by curves B and C in the drawing in keeping with the above increase of the modulus.
For example, a temperature of about 1800.degree. C. is necessary for carbonization of a carbon fiber in order to produce a carbon fiber having a modulus of elasticity of 28 ton/mm.sup.2. As is shown from the drawing, a carbon fiber obtained by a heat treatment at the above-mentioned temperature has a tenacity of about 370 kg/mm.sup.2, which is 100 kg/mm.sup.2 or more lower than the tenacity of a carbon fiber obtained by treating at 1300.degree. C., 470 kg/mm.sup.2, and thus is far from being a high-tenacity carbon fiber. Further, the fiber has a decreased elongation of 1.3% or less. As is shown in the drawing, such lowering in tenacity accompanying the increase of carbonization temperature is in good correspondence to the decrease of the density of the fiber, and is assumed to be caused by generation of microscopic voids in the fiber during the course of elevating the carbonization temperature, which voids cause the lowering of the tenacity.
Thus, since the conventional techniques of elevating the treating-temperature of carbonized fibers to obtain carbon fibers having a high modulus of elasticity have the disadvantage that the tenacity of resulting carbon fibers is sharply lowered, a high performance carbon fiber having a characteristic that it has both a high tenacity and a high elongation cannot be obtained by such methods. For example, there have been disclosed in Japanese Patent Application Kokai (Laid-open) Nos. 94,924/74 and 42934/82 inventions for producing a carbon fiber which comprise subjecting a bundle of acrylonitrile-type fibers of fine size to a flame-resisting treatment followed by carbonization.
In the former invention, acrylonitrile-type fibers which have been formed from an acrylonitrile-type polymer having an intrinsic viscosity of 1.5 or more, particularly 1.5 to 1.87, and whose single yarn has a fineness of 0.3 to 0.6 denier and a coefficient of fineness variation of 15% or less are subjected to a flame-resisting treatment in the air at a temperature of 200.degree. to 300.degree. C., then to a carbonization treatment in an inert atmosphere at a temperature of 1200 to 1600.degree. C. to give carbon fibers having a single fiber tenacity of 260 to 360 kg/mm.sup.2 and a modulus of elasticity of 26 to 27.5 ton/mm.sup.2. However, since the tenacity and the Young's modulus of elasticity of each of the carbon fibers vary considerably with one another, the tenacity and the Young's modulus of a strand of the carbon fibers produced by such a method are usually 10% or more lower than the respective values mentioned above.
In the latter invention, acrylonitrile-type fibers having a single fiber fineness of 0.02 to 0.6 denier and a fiber tenacity of 6 g/denier are subjected to a heat treatment in the air at 240.degree. to 300.degree. C. under conditions such that a shrinkage of 4 to 10% is given to the fiber until the equilibrium moisture content of the heat-treated fiber reaches 5%, then further given a shrinkage of 2 to 8% to complete the flame-resisting treatment, and then subjected to a carbonization treatment in an inert atmosphere at a temperature of 1000.degree. to 1800.degree. C. to give carbon fibers having a single fiber diameter of 1 to 6 .mu.m and a knot strength of the strand of 7 kg or more. However, the strand of the carbon fibers obtained according to the above invention has a tenacity of 360 to 420 kg/mm.sup.2 and a modulus of elasticity of 24 ton/mm.sup.2, and is thus not yet satisfactory as a carbon fiber strand of high tenacity and high modulus of elasticity.