Process for producing carbon fiber fabrics

Abstract: A carbon fiber fabric is produced by making carbon fibers having a tensile strength exceeding 250 kgf/mm² and an elongation to break of 0.4% to 10% into a two- or three-dimensional fabric, said carbon fibers having been obtained by melt-spinning a carbonaceous pitch, making the resulting fibers infusible and then subjecting the thus-infusiblized fibers to a primary heat treatment at a temperature exceeding 650 o C and not higher than 2,500 o C, and subjecting said two- or three-dimensional fabric to a secondary heat treatment at a temperature which is not lower than 700 o C and is higher than the temperature used in said primary heat treatment.

Description: The present invention relates to a process for producing carbon fiber fabrics. As methods for producing carbon fiber fabrics there are known a method of weaving carbon fibers as a finished product and a method in which an intermediate product is subjected to weaving and the resulting fabric is carbonized or graphitized. As an example of the latter there is disclosed in Japanese Patent Laid-Open No.120136/1988 (corresponding to EP-A-0266788) a three-dimensional fabric containing a pitch based carbon fiber as one component thereof, the carbon fiber having, before heat treatment in a relaxed state, a strength of 15 to 250 kgf/mm², an elongation to failure of 0.5% to 8.0% and an elastic modulus of 400 to 40,000 kgf/mm², but, after the said heat treatment, capable of increasing in both strength and elastic modulus to 1.1 times as high as the strength and elastic modulus before the heat treatment and capable of a strength of 150 kgf/mm² or higher and an elastic modulus 40,000 kgf/mm² or higher. In Japanese Publication No.20281/1987 there is disclosed a process for producing a carbon fiber product in which as-spun pitch fibers are subjected to an initial carbonization treatment, then a bundle of the fibers is subjected to weaving and the resulting fabric is carbonized or graphitized. Further, in Carbon-Fiber-Reinforced Plastics, Bamberg, West Germany, May 11-12, 1977, there is a description to the effect that infusibilzed fibers can be woven if they have a strength of about 40 kgf/mm² and elongation to failure of 5%, and can be carbonized into a carbon fiber fabric. As an additional example of the processes of the prior art JP-A-01 455,819 discloses a process for producing a carbon fiber which comprises making carbon fiber filaments having an elongation of 1.8 - 4.0% and a tensile strength of 20 - 40 kg/mm², said carbon fibers having been obtained by melt-spinning a 100% mesophase pitch, making the resulting fibers infusible, and then subjecting the infusibilized fibers to a heat treatment at 550-1000°C, collecting the fiber filaments into multifilament bundles composed of 100 to 1,000 filaments followed by twisting, knitting or weaving the multifilaments and then heat treating the fabric at a higher temperature. However,in producing fabrics according to the aforementioned conventional methods, breaking of fibers or fuzzing occurs because the strength of the fibers in the weaving stage is not sufficiently high, and therefore even if the resulting fabric is carbonized or graphitized, it is impossible to obtain a high fiber volume fabric. Also a permanent strain may remain after carbonization, so the inherent strength cannot be developed when used as a composite material. Summary of the Invention It is the object of the present invention to overcome the above mentioned problems, particularly to provide a process capable of efficiently producing carbon fiber fabrics with little fuzzing and free of permanent strain. The present invention is concerned with a process for oroducing a carbon fiber fabric, which process comprises making carbon fibers having a tensile strength exceeding 250 kgf/mm² and an elongation to break of 0.4% to 10% into a two- or three-dimensional fabric, the said carbon fibers having been obtained by melt-spinning a carbonaceous pitch, making the resulting fibers infusible and then subjecting the thus-infusiblized fibers to a primary heat treatment at a temperature of at least 1500°C and not higher than 2,500°C, and subjecting the two- or three-dimensional fabric to a secondary heat treatment at a temperature which is higher than the temperature used in the primary heat treatment . Detailed Description of the Invention The process for producing carbon fiber fabrics according to the present invention will be described in detail hereinunder. As the carbonaceous pitch there is used a coal or petroleum pitch having a softening point of 100°C to 400°C, preferably 150° to 350°C. Both optically isotropic and anisotropic pitches are employable examples of the carbonaceous pitch, but particularly preferred is an optically anisotropic pitch having an optically anisotropic phase content of 60% to 100%. The melt spinning may be carried out by any suitable known method. The resulting pitch fibers are then rendered infusible. The infusiblization treatment may be perforred at a temperature of 50°C to 400°C, preferably 100° to 350°C, in an oxidizing gas atmosphere. As the oxidizing gas there may be used air, oxygen, nitrogen oxide, sulfur oxide, halogen, and mixtures thereof. The primary heat treatment is conducted in an inert gas at a temperature of at least 1500°C and not higher than 2,500°C, preferably in the range of 1500° to 2000°C. The treatment time is selected suitably so as to obtain primary heat-treated fibers having later-described tensile strength and elongation to failure, but usually it is in the range of 1 second to 10 hours. According to a method wherein fibers which have been treated at a temperature Icer than the above range are subjected to weaving, followed by heat treatment at a high temperature, there remain so-called permanent strain or permanent deformation of the fibers because of a low carbonized state, and thus since the fibers are woven in a bent state, breakage will result at inflection points if pulled. The primary heat-treated fibers, obtained by going through the above primary heat treatment, have a tensile strength exceeding 250 kgf/mm² and a breaking elongation of 0.4% to 10%. It is an essential condition that the tensile strength of the primary heat-treated fibers should be in excess of 250 kgf/mm². Preferably, the said tensile strength is not lower than 285 kgf/mm², more preferably not lower than 300 kgf/mm², and most preferably not lower than 330 kgf/mm². Although there is no upper limit, the tensile strength in question is usually not higher than 1000 kgf/mm². If the tensile strength is outside the range just specified, there will occur breakage of fibers and fuzz during weaving, with the result that a high fiber volume fabric cannot be obtained. The elongation to break is in the range of 0.4% to 10% preferably 0.4% to 1.3%, more preferably 0.6% to 10%, still more preferably 0.6% to 5% preferably, 0.4% to 1.3%. In the case of a three-dimensional fabric it is preferable that the elongation to failure be not lower than 0.6% because of small radius of curvature of bundle in a fabric. The value of elastic modulus is determined optionally according to the combination of the above tensile strength and elongation to failure, but usually it is in the range of 5 to 100 x10³ kgf/mm². The fiber diameter is in the range of 3 to 100 µm, preferably 5 to 30 µm. In the present invention, the foregoing primary heat-treated fibers are made into a two- or three-dimensional fabric. Examples of the "fabric" as referred to herein are fabrics obtained using 100 to 25,000 continuous filaments. More concrete examples include two-dimensional fabrics such as plain weave, satin weave, twill weave, bias weave fabrics braid, and stitch knit, three-dimensional fabrics such as three-dinensional orthogonal fabric, leno, interlock and braid, as well as fabrics reinforced in three or more directions such as special shape fabrics, mat-like fabric and felt-like fabric. The fabric of the primary heat-treated fibers is subjected to a secondary heat treatment. The secondary heat treatment is performed at a temperature which is higher than the temperature in the primary heat treatment. Usually, the secondary heat treatment temperature is higher by 50°C or more preferably by 100-2,000°C, more preferably by 200-1,000°C, than the primary heat treatment temperature. The treatment time in the secondary heat treatment is selected optionally for obtaining the secondary heat-treated fabric falling under the scope. of the present invention, but usually it is in the range of 1 second to 10 hours. According to the process of the present invention there can be obtained a carbon fiber fabric with little fuzzing and free of permanent strain. <Examples> The following examples are given to illustrate the present invention more concretely. Examples 1 - 6 A carbonaceous pitch was melt-spun and the resultant fibers were rendered infusible. The fibers thus infusiblized were subjected to a primary heat treatment at temperatures ranging from 1,700°C to 2,450°C. Using the fibers thus heat treated, three-dimensional orthogonal fabrics were produced. Then, the fabrics were each subjected to a secondary heat treatment at 2,500°C. The three-dimensional fabrics thus heat treated were evaluated, the results of which are as shown in Table 1. Comparative Examples 1 - 3 Three-dimensional orthogonal fabrics were produced using the primary heat-treated fibers shown in Table 1, and then subjected to a secondary heat treatment at 2,500°C. The three-dimensional fabrics thus heat treated were evaluated, the results of which are as set forth in Table 1. Primary Heat-Treated Fibers Evaluation of three-dimensional fabrics after sencondary heat treatment at 2,500°C Tensile Strength kgf/mm² Elongation to break % Elastic Modulus 10³kgf/mm² Treating Temp °C Ex. 1 280 0.7 40 2100 Good 2 300 0.75 40 2100 Good 3 330 0.83 40 2100 Good 4 370 1.2 30 2000 Good 5 430 1.3 33 2000 Good 6 350 0.7 50 2450 Good Comp.Ex.1 205 0.7 29.3 1700 Fuzz 2 245 0.5 48.8 2450 fuzz 3 10 3.0 0.3 600 Permanent strain P.S.: To convert from : kgf/mm² to : GPa, multiply by : 0.0098.

Claims: A process for producing a carbon fiber fabric, which process comprises making carbon fibers having a tensile strength exceeding 2.45 GPa (250 kgf/mm²) and an elongation to break of 0.4% to 10% into a two- or three-dimensional fabric, said carbon fibers having been obtained by melt-spinning a carbonaceous pitch, making the resulting fibers infusible and then subjecting the thus-infusibilized fibers to a primary heat treatment at a temperature of at least 1500°C and not higher than 2,500°C, and subjecting said two- or three-dimensional fabric to a secondary heat treatment at a temperature which is higher than the temperature used in said primary heat treatment. A process as set forth in Claim 1, wherein said carbonaceous pitch is an optically anisotropic pitch having an optically anisotropic phase content of 60% to 100%. A process as set forth in Claim 1 or Claim 2 wherein the fibers after said primary heat treatment have a tensile strength of not lower than 2.94 GPa (300 kgf/mm²). A process as set forth in any one of the preceding claims wherein the fibers after said primary heat treatment have an elongation to break in the range of 0.6% to 10%. A process as set forth in any one of the preceding claims wherein the temperature used in said secondary heat treatment is higher by at least 50°C than the temperature used in said primary heat treatment. A process as set forth in any one of the preceding claims wherein the temperature used in said secondary heat treatment is higher by 100° to 2,000°C than the temperature used in said primary heat treatment. A process as set forth in any one of claims 1 to 10 wherein the carbon fibres have an elongation to break of 0.4% to 1.3%.

Inventors: IDO YASUZI, SOHDA YOSHIO, UEMURA SEIICHI, IDO,YASUZI, SOHDA, YOSHIO, UEMURA, SEIICHI
Applicants: NIPPON OIL CO LTD, NIPPON OIL COMPANY, LIMITED
Application year: 1990
Main classification: D01F9
IPC codes: D03D15, D03D25, D01F9