JPH0433907B2 - - Google Patents

Description

[Detailed description of the invention]

(a) Industrial Application Field The present invention relates to a surface treatment method for improving the wettability of carbon fibers to various liquids and the adhesion to adhesives. The present invention relates to a surface treatment method particularly suitable for pitch-based carbon fibers, which are more difficult to undergo surface modification by oxidation treatment than PAN-based carbon fibers. (b) Conventional Technology Carbon fiber has been widely used in various applications such as structural materials for aerospace and sporting goods due to its mechanical, chemical, electrical properties and light weight. In these applications, carbon fibers are generally used as reinforcing materials for composite materials made of carbon fibers and various resins, but in order to reflect the characteristics of carbon fibers, especially their mechanical properties, in composite materials, Adhesion and integration between the composite material matrix and carbon fiber are important. Unless carbon fiber is subjected to some kind of surface treatment in advance, it will not have sufficient adhesion to the matrix, will easily "sneak" from the matrix, and will not be able to fully exhibit its reinforcing effect. This phenomenon significantly affects the strength, especially in the direction perpendicular to the direction of reinforcing fibers in composite materials, and evaluation of adhesion between carbon fibers and matrices is performed using interlaminar shear strength as a representative characteristic. There are many. As surface treatment methods for carbon fibers, liquid phase oxidation using an oxidizing agent, heat cleaning, gas phase oxidation, whiskering, and electrolytic oxidation are known. Among these, the liquid phase oxidation method and the electrolytic oxidation method, especially the electrolytic oxidation method, are widely adopted due to their excellent operability. JP-A No. 56-128362 discloses a method of electrolytic treatment using a sulfate aqueous solution under conditions such as specific current density. As disclosed in these patents, electrolytic oxidation treatment of carbon fibers has conventionally been carried out by using the carbon fibers as anodes and passing a direct current through them. In this case, a metal roller is usually placed in place of the positive terminal, and the contact resistance is likely to fluctuate due to the low voltage used. There's a problem. For this reason, it is difficult to reduce the contact pressure and degree of bending, and a considerable decrease in strength is unavoidable. In order to solve this problem, we
No. 239521, shown in FIG. 1, discloses an apparatus that can carry out electrolytic treatment in a substantially linear manner by utilizing the flow resistance of the electrolytic solution or the swelling of the surface of the electrolytic solution due to surface tension. This device has the advantage of suppressing a decrease in strength due to contact between the carbon fiber and the processing device and bending of the fiber, and is effective in preventing a decrease in strength in the case of PAN-based carbon fiber. However, for recent composite materials with particularly high strength and elastic modulus, fibers such as mesophasic pitch-based ultra-high elastic modulus carbon fibers are becoming required. It has been pointed out that such carbon fiber composite materials having an ultra-high modulus of elasticity have poor adhesion and a low utilization rate of fiber strength. In the case of mesophatic pitch-based ultra-high modulus carbon fibers, graphite crystals have developed, so there is a problem that the surface treatment effect is poor under the conditions used for conventional PAN-based carbon fibers. I understand. In the case of mesophasic carbon fibers, like most graphite crystals, it is estimated that the rate of intercalation is faster than oxidation, and the interior of the fiber tends to deteriorate first, resulting in a decrease in strength and brittleness. It seems that it is. When intercalation occurs, graphite crystals expand in a direction perpendicular to the laminated plane, creating cracks where the crystals come into contact with each other.
There is a problem that the fibers themselves are susceptible to shear failure. (c) Problems to be Solved by the Invention The present invention provides a surface treatment method using electrolytic oxidation that solves the problem that it is difficult to improve the adhesion of pitch-based carbon fibers, especially ultra-high modulus carbon fibers, by electrolytic oxidation. It is. (d) Means for Solving the Problems The present invention, when electrolytically oxidizing carbon fibers, uses overflowing electrolyte from a liquid tank as negative and anode poles, and a large number of cathodes and anodes are arranged alternately. ,
The carbon fibers between the anode and the cathode are separated by running the carbon fibers through the swollen portion of the liquid level of the electrode so that the carbon fibers are not substantially bent by the rim of the bath. A carbon material characterized in that a direct current is passed through the electrolyte adhering to the carbon, the length of the negative and negative poles in the fiber running direction is 3 mm to 300 mm, and the number of negative and negative poles is 5 to 100. This is a method for surface treatment of fibers. In the present invention, since the electrolytic cells are arranged in multiple stages for processing, the electric energy used per one stage of processing is small. If a large amount of electrical energy is used in one stage of treatment, oxidation will progress not only to the surface of the fibers but also to deep layers, causing a decrease in strength. In addition, intercalation occurs in which electrolytic products enter the graphite crystal, distorting the graphite crystal and forming cracks in the fibers, resulting in a large decrease in strength and little increase in adhesive strength. Furthermore, electrolytic treatment produces graphite oxide on the surface of the carbon fibers, which tends to separate from the fibers, and this seems to impede improvements in adhesion. In the present invention, since a liquid tank is used for the anode, hydrogen is generated from the anode, and as hydrogen passes through the tank, oxidation stops each time, and the progress of oxidation to the deep part is suppressed, so that no strength loss occurs. It seems to be. In addition, since graphite oxide is reduced and efficiently removed from the fiber surface during this process, it is presumed that the adhesion is improved. In the present invention, each electrode is preferably short with respect to the running direction of the fibers. This is not only because the degree of electrolytic oxidation in one stage is small, but also with mesophasic pitch carbon fibers, most of the electrolytic oxidation of the fibers occurs near the edges of the liquid tank where the electrodes are formed, and the electrolytic oxidation occurs in the center of the liquid tank. This is presumed to be due to the fact that the existence value of the electrolyzed product is small, and the reduction in the anode tank is carried out before the intercalation of the electrolyzed product progresses. The length of the electrode is 3 mm to 300 mm, preferably 15 mm to 200 mm, with the fiber immersed in the liquid. The rim of the liquid tank that forms the electrode should preferably have a shape that allows the liquid to easily flow down, especially if the length of the electrode in the fiber running direction is short. It is preferably made of a material with a high friction coefficient and a low coefficient of friction. Electrically conductive materials can undergo electrolytic oxidation and corrosion due to leakage current, so electrical insulators such as ceramic materials seem to be better. If the electrode is too long, not only the adhesion improvement effect will be reduced, but also process passability will be lowered and loss will increase. The solution used for electrolytic oxidation is an aqueous solution containing alkali, acid, and salts to the extent that electrolysis is possible, but the concentration is preferably as low as possible in order to reduce intercalation into graphite crystals. In the present invention, the process as a whole is generally short, and therefore, compared to the case where a single large-sized electrolytic oxidation tank is used, the electrolyte can generally have a fairly fast flow as a process characteristic, and the flow rate can be increased to This has the advantage of reducing processing unevenness caused by air bubbles entrained by the electrolyte and uneven concentration of the electrolyte. Example 1 Mesophase pitch carbon fiber (diameter 10 μm,
Number of fibers: 2000, strength: 302Kg/ ^mm2 , modulus of elasticity: 60,000
Kg/mm ² ) was surface treated by electrolytic oxidation. The electrolyte was a 0.2% NaOH aqueous solution, and the temperature was 25℃.
The circulation rate was 2/min. The tank was made of PVC, and only the rim was made of alumina. Under these conditions, the length that the fibers passed through the liquid bath was 98 mm. 16 anode baths and 15 cathode baths were arranged alternately, and the fiber passing speed was 1.5 m/min. The results are shown in Table 1. For LSS measurements, the obtained fibers were treated with epoxy resin (a mixture of 100 parts of Epikote 828 manufactured by Ciel Chemical Co., Ltd. and 3 parts of boron trifluoride monoamine).
The mixture was impregnated into a unidirectionally reinforced composite (fiber content 60% by ^volume , fiber content 60% by volume, A thickness of 2 mm and a span length of 8 mm) were made and implemented.

[Table] Comparative Example 1 The carbon fibers of Example 1 before electrolytic oxidation treatment were subjected to electrolytic oxidation treatment using one cathode bath with a length of 500 mm, with the carbon fibers as anodes. The electrolyte composition and processing speed were the same as in Example 1. The results of the treatment are shown in Table 2. As can be seen from the table, as the current density increases, the fibers deteriorate, and both strength and elastic modulus tend to decrease. Furthermore, the effect of improving adhesion was also slightly inferior to that of Example 1. In particular, the fluctuation rate of LSS was large, suggesting that processing was performed unevenly.

[Table] Example 2 The carbon fiber before the electrolytic oxidation treatment of Example 1 was treated by changing the number of anode cells and cathode cells, and increasing the amount of electricity during treatment by 4.
clones/g, current 32-34mA, processing speed 1.5
Electrolytic oxidation treatment was carried out at m/min. The tensile strength, tensile modulus, and interlaminar shear strength of the epoxy resin composite of the obtained carbon fibers were measured in the same manner as in Example 1. The results are shown in Table 3.

[Table] Example 3 The carbon fiber before the electrolytic oxidation treatment of Example 1 was modified by changing the length of the anode tank and cathode tank in the fiber running direction, the number of cathode tanks was set to 15, and the amount of electricity during treatment was changed to 4. Electrolytic oxidation treatment was performed at coulombs/g, current of 32 to 34 mA, and processing speed of 1.5 m/min. The tensile strength, tensile modulus, and interlaminar shear strength of the epoxy resin composite of the obtained carbon fibers were measured in the same manner as in Example 1. The results are shown in Table 4.

【table】

[Table] (e) Effects of the invention When the carbon fibers surface-treated according to the present invention are made into a fiber-reinforced composite, they have excellent adhesion to a matrix, and have high strength in the direction intersecting the fibers of the composite. Excellent fatigue resistance. The present invention is more effective on pitch-based carbon fibers, which have a smaller surface treatment effect than PAN-based carbon fibers.

Claims (1)

[Claims] 1 When electrolytically oxidizing carbon fiber, the electrolyte overflowed from the liquid tank is used as the positive and negative poles,
In addition to arranging a large number of cathodes and anodes alternately, carbon fibers are run through the swollen portions of the liquid level of the cathodes and anodes, so that the carbon fibers are not substantially bent by the edges of the liquid tank. Then, a direct current is applied to the carbon fibers between the anode and the cathode and the electrolyte attached to the carbon fibers, and the length of the cathode and anode poles in the fiber running direction is set to 3 mm to 300 mm. A method for surface treatment of carbon fibers, characterized in that the number of carbon fibers is 5 or more and 100 or less.