Process for the Preparation of Carbon Fiber

Abstract

A process for the preparation of carbon fibers from polyacrylonitrile type fibers, comprising oxidizing the polyacrylonitrile type fibers to form oxidized fibers therefrom and carbonizing said oxidized fibers in an acidic atmosphere containing a small amount of hydrogen chloride and, if desired, a small amount of moisture with the remainder being an inert gas, thereby obtaining the carbon fibers. This invention relates to a process for the production of high-strength carbon fibers which are excellent as reinforcing material for use in a composite material or article. Carbon fibers prepared by heating polyacrylonitrile type or other type fibers to carbonize or further graphitize the fibers have heretofore been used as reinforcing material for composing a composite material or article together with a plastic, metallic, ceramic or other material since such carbon fibers have high strength and elastic modulus as their features. The term "carbon" used herein is intended to mean non-crystalline carbon and crystalline carbon, that is graphite. However, it is necessary to effect a secure bond or adhesion between the carbon fibers and a matrix in which they are to be incorporated in order to make effective use of the fibers as the reinforcement for the matrix. Thus it has usually been indispensable to subject the fibers to surface treatment such as wet-type oxidation, dry-type oxidation or surface coating (with a polymer, silane, amine or the like) so as to further enhance the surface activity of the fibers. As an example of a process for the production of carbon fibers having high strength, Japanese Patent Gazette No. 40575/72 discloses such process comprising preliminarily heat-treating polyacrylonitrile type fibers under such tension that the original length of said fibers is not shrunk by less than 10% thereof, in an oxidizing atmosphere in a short time thereby obtaining intermediate fibers each containing from 4 to 20% of oxygen, and further heating the thus-obtained less oxidized fibers to their carbonizing temperature in an atmosphere of a hydrogen halide during a part or the whole of their subsequent treatments. The carbon fibers so obtained have a strength of from 320 to 386 kg/mm.sup.2, never exceeding 400 kg/mm.sup.2. In addition, they are disadvantageous in that they will need the surface treatment as previously mentioned if they are to be used as reinforcing material. As a result of their own study made on a process for the production of carbon fibers, the present inventors have found that suitable organic fibers in previously oxygen permeated or oxidized form may be subjected to carbonization treatment in an atmosphere containing hydrogen chloride, an inert gas and, if desired, a suitable amount of moisture thereby obtaining carbon fibers having higher strength and great surface activity entirely without surface treatment. More particularly, this process found by the inventors comprises heating polyacrylonitrile type fibers in an oxidizing atmosphere to obtain oxidized fibers thereof and further heating the thus-obtained oxidized fibers in an atmosphere containing from 0.2 to less than 10% by volume of hydrogen chloride and, if desired, from 0.5 to 6% by volume of moisture (or steam) with the remainder being an inert gas to carbonize the oxidized fibers thereby producing carbon fibers. The polyacrylonitrile from which the fibers used in the process of the present invention are made, may preferably be an acrylonitrile homopolymer or copolymer containing 85% by weight of acrylonitrile units, these polymers being producible by the use of usual known techniques. The oxidized polyacrylonitrile type fibers used in the process of this invention are obtained by heating polyacrylonitrile type fibers without or after the immersion thereof in a solution of potassium permanganate or bichromate to treat them with this oxidizing agent to the extent that they are changed in color to from yellow to yellowish-brown, to not higher than 350.degree.C for at least not less than 10 minutes in an oxidizing atmosphere of air, a halogen gas, oxygen, ozone, nitrate gas or the like. As previously mentioned, the oxidized fibers so obtained have then been found by the present inventors to be converted to carbon fibers by heating to the carbonizing temperature of the oxidized fibers in an atmosphere containing less than 10% by volume of hydrogen chloride and, if desired, a suitable amount of moisture (H.sub.2 O) with the remainder being an inert gas. The atmosphere in which the carbonization is carried out according to this invention should consist of, by volume, less than 10%, preferably 0.2 to less than 10% of hydrogen chloride and, if desired, 0.5 to 6%, preferably 1.5 to 4.5% of moisture with the balance being an inert gas such as nitrogen or argon. The presence of hydrogen chloride in the atmosphere used in the carbonizing step is helpful in enhancement of carbon yield, decrease of carbonizing time and increase of strength of the resulting carbon fibers. However, the presence of less than 0.2% by volume of hydrogen chloride will result in the production of carbon fibers having insufficiently increased strength, while the presence of not less than 10% by volume of hydrogen chloride will result in the production of carbon fibers having little further increased strength and having not a further enhanced surface activity for such high concentration of the hydrogen chloride used. The presence of moisture or water in suitable amounts in the atmosphere of the carbonizing step will be helpful in remarkably increasing the resulting carbon fibers in strength and surface activity. On the other hand, the presence of moisture in amounts of less than 0.5% by volume will be little helpful in increasing the resulting carbon fibers in strength, and the presence thereof in amounts exceding 6% by volume will result in a lower yield of carbon fibers with remarkably non-uniform or indefinite properties, this proving that the use of moisture in such high concentrations is undesirable. It is thus necessary to limit the moisture in amount to within 0.5 to 6%, preferably, 1.5 to 4.5% by volume. There are a few processes for preparing the oxidized fibers according to this invention; one is a process wherein the carbonization is effected in the atmosphere containing hydrogen chloride, an inert gas and, if desired, moisture in the desired ratios in accordance with this invention, another is a process comprising immersing the oxidized fibers in hydrochloric acid to impregnate them with the acid in a suitable amount before the carbonization and then heat treating the acid-impregnated fibers in the specific acidic atmosphere according to this invention. In this latter process, when heated the hydrochloric acid impregnated into the oxidized fibers is vaporized to provide a suitable concentration of hydrogen chloride in the atmosphere of the carbonizing step. The former process is more advantageous from the view-point of much smaller variation in properties of the products obtained. The carbonizing treatment is effected at temperatures of not lower than 350.degree.C. If high strength is required in the resulting products then the carbonizing temperatures should be from 1100.degree. to 1700.degree.C, preferably 1300.degree. - 1500.degree.C, while if high modulus of elasticity is required then the temperatures should be not lower than 2000.degree.C, preferably from 2400.degree. to 3000.degree.C. Furthermore, in the oxidizing step and, if desired, in the carbonizing step the material to be oxidized or carbonized is passed via the step or steps under tension at a drawing ratio of from -15 to +80%. The heating time for the carbonization according to this invention ranges from about 1 second to about 10 minutes, this indicating that the heating time according to this invention is much shortened as compared with that of conventional carbonization. The products, that is carbon fibers, so obtained are not found at all to be fluffy on the surface.

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