JP2730691B2 - Surface electrolytic treatment method for carbon fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a surface electrolytic treatment method for carbon fibers. [Prior art] Carbon fibers that have recently attracted attention as structural materials include various matrices such as epoxy resins, unsaturated polyester resins, thermosetting resins such as phenolic resins, polyamide resins, polyacetal resins, and polysulfone resins. Used as a composite material with various resins such as thermoplastic resins. Usually, surface treatment of carbon fiber is required to enhance the adhesiveness between the carbon fiber and these resins, and various methods have been proposed. Of these, the so-called electrolytic surface treatment method of energizing carbon fibers in an aqueous solution of an electrolyte such as sodium hydroxide, potassium hydroxide, sulfuric acid, and phosphoric acid is considered to be useful from an economic viewpoint, for example, Japanese Patent Publication No. 47-40119. This is known in the official gazette and Japanese Patent Publication No. 55-20033. Among these electrolytes, sodium hydroxide is particularly widely used because it has high conductivity and therefore has the advantage that the electrolytic treatment can be performed at a low voltage in a short time. [Problems to be Solved by the Invention] However, sodium hydroxide tends to remain in the carbon fiber after the electrolytic treatment, and is usually removed by washing with water, but it is said that it is difficult to completely remove the sodium hydroxide. If the removal of the electrolyte is insufficient, the resulting carbon fiber has inferior thermal oxidation resistance, or is used to cure the resin matrix, the catalyst is deactivated and exhibits poor curing, or the resulting composite material Problems such as deterioration of physical properties have been pointed out. Furthermore, there is a problem of how to treat a large amount of wastewater coming out of the washing step. According to the prior art, various methods for removing the electrolyte remaining in the carbon fiber after such electrolytic treatment have been described, but they are not always convenient. For example, Japanese Patent Publication No. 49-29906 discloses a method in which carbon fibers containing an electrolyte solution are passed by a cathode and / or an anode in a cleaning solution to remove the electrolyte. However, such a method has a drawback in that, in addition to energizing for the electrolytic treatment, energizing needs to be further performed for removing the electrolyte, thereby increasing power consumption. Japanese Patent Application Laid-Open No. 50-40891 discloses a method in which carbon fibers are subjected to electrolytic surface treatment and then dried by hot air. A method of reducing the remaining adhesion of the electrolyte by doing so is disclosed. However, this method also requires a washing step, and requires treatment of washing wastewater and, in some cases, decomposition gas at the time of drying, and extra energy is required to decompose at a high temperature, which is economical. Not a target. Further, Japanese Patent Publication No. 58-24554 discloses that when graphite fibers are subjected to electrolytic surface treatment, ammonium bicarbonate or the like which can be decomposed to generate gaseous products when heated at a temperature of 250 ° C. or less as an electrolyte. Although a method of using an ammonium compound and heat treating electrolytically treated fibers at a temperature of 250 ° C. or less to remove residual electrolyte from the fibers is disclosed, as is well known, ammonium bicarbonate is 35 to
When heated to 60 ° C, it decomposes into ammonia, carbon dioxide, and water. In particular, ammonia is a highly corrosive and highly toxic substance. There is a problem in terms of equipment or cost such as having to take measures. From such a viewpoint, JP-A-59-100762 discloses that
A method has been proposed in which carbon fibers are energized in water substantially free of electrolytes, such as river water, well water, industrial water or tap water. However, this method does not require a washing step, and has the advantage that no harmful gas is emitted during drying.
NH ₄ ⁺ , Na ⁺ , K ⁺ , Ca contained in trace amounts in water as described above
^{_{2+, Mg 2+, HCO 3 -}} , Cl -, there is temporal or seasonal variations in different concentrations and ions of an ion such as SO ₄ ^2-, this corresponds to change the electrical conductivity of the water Therefore, there is a problem that the surface electrolytic treatment effect of the carbon fiber varies, and accordingly, the adhesiveness between the carbon fiber and the matrix resin varies, which causes a quality problem. If the above-mentioned fluctuation of the electric conductivity is expressed, for example, for a certain industrial water, the fluctuation width with respect to the average value over time (24 hours) is ± 60 to 70%, and the fluctuation width with respect to the average value seasonally (1 year). Is reported to be ± 70-90%. The report also shows that not only the electrical conductivity but also the total hardness, iron content and oxygen consumption fluctuate greatly over time and seasonally [by Shizuo Suzuki; Industrial Water Treatment, July 1968. Published by Uchida Lao Tsuruho Shinsha on March 5th, see pages 4-28). [Means for Solving the Problems] Accordingly, the present inventors have studied the surface electrolytic treatment of carbon fibers in order to solve the problems of the prior art and find an industrially advantageous method. In particular, in the case of an aqueous electrolyte solution having a concentration on the order of several percent, which is generally used in the prior art, the electric conductivity of the aqueous electrolyte solution is much higher than that of the carbon fiber, while not only removing the electrolyte adhering to the carbon fibers. large. Therefore, the current flowing between the carbon fiber (anode) and the cathode running on the surface electrolytic treatment apparatus has an excessive value at the inlet of the apparatus, thereby considerably reducing the strength of the carbon fiber. On the other hand, in the case of a low-concentration aqueous solution of an electrolyte, it has been found that since the absolute value of the concentration is small, the fluctuation greatly affects the electrolytic surface treatment effect of carbon fibers. As a result of intensive studies focusing on the above points, the carbon fiber was surface-treated using an aqueous solution adjusted to a certain low concentration by adding an inorganic salt to water substantially free of electrolyte as an electrolyte solution. It has been found that by performing the electrolytic treatment, it is possible to obtain a carbon fiber having high strength and interlayer shear strength excellent in adhesiveness to a matrix resin very easily and economically, and having very little variation in quality. completed. That is, the object of the present invention is to adopt a simple and economical method in the surface electrolytic treatment of carbon fiber,
An object of the present invention is to provide a surface electrolytic treatment method for obtaining carbon fibers having high strength excellent in adhesiveness to a matrix resin and interlayer shear strength, and having very little variation in quality. And the purpose is that the electrical conductivity of the electrolyte solution is 10μs / cm
An inorganic salt selected from one or a mixture of two or more halides, sulfates or phosphates of an alkaline earth metal or an alkali metal is added to the following substantially electrolyte-free water. Using an aqueous solution with a set concentration,
This can be easily achieved by performing the surface electrolytic treatment while adjusting the fluctuation range with respect to the concentration set value to 20% or less. Hereinafter, the present invention will be described in detail. Carbon fiber used in the present invention can be produced from various known raw materials, for example, coal tar pitch, petroleum pitch, coal liquefaction, polyacrylonitrile, cellulose, produced from polyvinyl alcohol, etc., 1000 to 2000
Either carbon fiber which has been carbonized at about ℃ or graphitized fiber which has been graphitized at about 2,000 ° C. or higher can be used. Electrolyte solution used when performing surface electrolytic treatment of these carbon fibers, the inorganic salt is added to water substantially containing no electrolyte, the inorganic salt concentration is set from the range of 10 to 1000 ppm, and the variation with respect to the concentration set value. This is an aqueous solution whose width has been adjusted to 20% or less. Water that is substantially free of electrolytes is, specifically, substantially free of electrolytes such as acids, alkalis, inorganic salts, and organic salts, such as demineralized water, distilled water, condensed water, and ion exchange treatment. Such as water. Since this water serves as a basis for adjusting the concentration in the electrolyte solution, it is desirable that the electric conductivity be as low as possible. As the electrical conductivity of this water
Preferably, the concentration of the electrolyte is 10 μs / cm or less, and the concentration of the electrolyte is 10 ppm or less. When the electric conductivity of the water exceeds 10 μs / cm or the concentration of the electrolyte exceeds 10 ppm, it is difficult for the water to serve as a base for adjusting the inorganic salt concentration. Even if an attempt is made to prepare it, it will be difficult to keep the concentration low, or it will cause concentration fluctuation. As the inorganic salt to be added to water containing substantially no electrolyte, one or a mixture of two or more halides, sulfates or phosphates of alkaline earth metals or alkali metals is used. Calcium, magnesium chloride, sodium chloride, calcium chloride, calcium sulfate, magnesium sulfate, sodium sulfate, potassium sulfate, calcium phosphate, magnesium phosphate, sodium phosphate, potassium phosphate, and mixtures thereof are used. In the present invention, an electrolyte such as an acid, an alkali or an organic salt cannot be used in place of the inorganic salt. When an electrolyte such as an acid or an alkali is used, the strength of the single fiber of the carbon fiber is greatly reduced, and the electrolyte attached to the fiber is hardly dropped even when washed with water. It is likely to cause catalyst deactivation. Further, even when an organic salt is used as an electrolyte, the effect of the present invention cannot be expected in that the adhesiveness with the matrix resin is not sufficiently improved and only a low value of ILSS (interlaminar shear strength) is obtained. . It is not desirable that the concentration of the inorganic salt in the aqueous solution prepared by adding the inorganic salt to water containing substantially no electrolyte is too low or too high. In other words, if the concentration is too low, the electric conductivity of the aqueous solution is low, so that a high voltage is required to anodize the carbon fiber, resulting in operational danger, or the capacity of the surface treatment apparatus by anodization is unclear. I need to make it bigger. On the other hand, if the concentration is too high, the electric conductivity of the aqueous solution is large, so that a current distribution is generated in the carbon fiber direction in the surface treatment device, and the current density becomes abnormally high near the entrance of the surface treatment device,
This may cause deterioration of the strength of the carbon fiber and requires washing of the inorganic salt attached to the carbon fiber with water. From such a point, the concentration of the inorganic salt in the aqueous solution is usually 10 to 1000 ppm, preferably 100 to 50 ppm.
Used in the range of 0 ppm. The concentration of the inorganic salt in the aqueous solution may be arbitrarily selected from the above range, depending on the properties of the carbon fibers to be subjected to the surface electrolytic treatment, or the required properties and uses of the carbon fibers. What is important with the selection of the inorganic salt concentration in the aqueous solution is
It is to control the concentration of the inorganic salt in the aqueous solution to a constant value of 20% or less without greatly changing the concentration of the inorganic salt in the aqueous solution during the operation of the electrolytic surface treatment of the carbon fiber. That is, in the prior art, the concentration of the electrolyte in the aqueous solution is performed at a relatively high value of several% or more when performing the electrolytic surface treatment of the carbon fiber,
As the absolute value of the electrolyte in the aqueous solution, that is, the inorganic salt concentration in the present invention is small, the change in the value has a sensitive effect on the surface electrolytic treatment effect of the carbon fiber. Therefore, the concentration of the inorganic salt in the aqueous solution should be kept constant as described above, preferably with a fluctuation range of 10%.
It is necessary to control to the following fixed value. This is, for example, a sensor that detects the concentration of the inorganic salt in the aqueous solution or the electric conductivity of the aqueous solution in the surface electrolytic treatment device, and, corresponding to the detected value, water substantially containing no electrolyte such as demineralized water, or a predetermined concentration. By providing a supply tank or the like for supplying an aqueous solution having a slightly higher inorganic salt concentration, the object can be easily achieved. In the present invention, as described above, it is important to once make substantially electrolyte-free water and base it on it. Industrial water, river water, well water, etc. can also be controlled to a predetermined electrical conductivity, but fluctuations in other physical properties (total hardness, iron content, oxygen consumption, etc.) cannot be avoided. However, it is not suitable as an electrolyte, and an effect without variation cannot be expected. Other conditions for anodization include voltage, current density,
There are the amount of electricity, time, temperature, etc., which also vary depending on the type of carbon fiber, the processing speed, the degree to be processed, etc., but usually, the voltage is 1 to 50 V and the current density is 1 cm of carbon fiber.
0.1-100mA per carbon fiber, 0.05-50 per g of carbon fiber
0 coulomb, several seconds to several minutes as time, room temperature to 8 as temperature
0 ° C. EXAMPLES Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to the examples unless it exceeds the gist of the present invention. (Example 1) 3,000 tows of 10 μm diameter carbon fiber obtained by firing pitch fiber at 2,300 ° C. were placed in an apparatus shown in FIG. 1 (materials of anode rotors 3, 3 ′ and cathode plate: graphite) , 1 m in length) under the following conditions. Electrolyte: Demineralized water (electric conductivity 2μs / cm, inorganic salt concentration 5pp
An aqueous solution prepared by adding CaCl ₂ .2H ₂ O to m) to a concentration of 250 ppm CaCl ₂ . The electric conductivity is 375μs / cm.
Automatic control was performed by using a sensor for measuring the electric conductivity in the electrolytic solution, and a yarn consisting of a supply line for deionized water and a supply line for a 500 ppm aqueous solution of CaCl ₂ (an aqueous solution prepared by adding CaCl ₂ .2H ₂ O to demineralized water). Voltage: 8V Temperature: 30 ° C Current: 15mA Time: 60sec Electricity: 1.8C / g-CF Yarn speed: 1m / min Resin-impregnated strand tensile strength of carbon fiber obtained JI
According to SR-7601 and ILSS (interlaminar shear strength)
The measurement was performed according to D-2344, and the results are shown in Table 1. (Example 2) The same operation as in Example 1 was carried out except that an aqueous solution adjusted to a CaCl ₂ concentration of 100 ppm was used as an electrolyte and the voltage was set to 20 V. The tensile strength of the resin-impregnated strand and the ILSS of the obtained carbon fiber were measured in the same manner as in Example 1, and the results are shown in Table 1. (Example 3) MgSO ₄ · H ₂ O was added to demineralized water as an electrolyte to form MgSO ₄ 2
Using an aqueous solution prepared to a concentration of 50 ppm, voltage 8 V, CaC
Example 1 was carried out in the same manner as in Example 1 except that a MgSO ₄ 500 ppm aqueous solution supply yarn was used for automatic control instead of the l ₂ 500 ppm aqueous solution supply yarn. The tensile strength of the resin-impregnated strand and the ILSS of the obtained carbon fiber were measured in the same manner as in Example 1, and the results are shown in Table 1. (Comparative Example 1) Tensile strength of resin-impregnated strand and ILSS of carbon fiber not subjected to surface electrolytic treatment were measured in the same manner as in Example 1, and the results are also shown in Table 1. Comparative Example 1 shows a blank for each example. As is apparent from Table 1, although the tensile strength is slightly large, the ILSS is quite small, and it can be seen that the adhesion of the carbon fiber to the matrix resin is not good. (Comparative Examples 2 to 7) The same operation as in Example 1 was carried out except that superhydrogen (summer and winter), industrial water, river water (summer and winter), and well water were used for one month as electrolytes. . The electric conductivity of each electrolytic solution, the tensile strength of the resin-impregnated strand of the obtained carbon fiber, and the ILSS were measured for one month in the same manner as in Example 1, and the minimum and maximum values were used as fluctuation ranges in Table 1. It is described together. As is evident from Table 1, these waters have different values of electric conductivity and different types of dissolved ions depending on the type and season, so that the tensile strength and ILSS are generally low. Even at low values, even at relatively high ILSS values, carbon fibers with very low tensile strength and well-balanced properties have not been obtained. In Comparative Examples 2 to 7, it can be seen that the fluctuation width of the electric conductivity is large, but the fluctuation width of the carbon fiber characteristics is also large. [Effects of the Invention] According to the present invention, it is possible to produce a stable high-performance carbon fiber of a quality that requires little water washing of the carbon fiber after the surface electrolytic treatment, has a small decrease in strength, and has a large interlayer shear strength. This high-performance carbon fiber is very useful for various fiber-reinforced composite materials.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an example of a surface electrolytic treatment apparatus used in the present invention. 1: Carbon fiber 2: Electrolyte tank 3, 3 ': Anode roller 4: Cathode plate

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Keisuke Nakano               Kitakyushu City                 Inside the Kurosaki Plant of Mitsubishi Chemical Industry Co., Ltd.                (56) References JP-A-61-282470 (JP, A)                 JP-A-59-100762 (JP, A)                 Tokiko 47-40119 (JP, B2)

Claims (1)

(57) [Claims] In a method of immersing carbon fibers in an electrolyte solution and performing surface treatment by electrolysis using the carbon fibers as an anode, as an electrolyte solution, the electrical conductivity is 10 μs / cm or less in water substantially containing no electrolyte, Add an inorganic salt selected from one or a mixture of two or more of alkaline earth metal or alkali metal halides, sulfates or phosphates, and use an aqueous solution in which the inorganic salt concentration is set. A method for electrolytically treating carbon fibers, comprising performing electrolysis while adjusting the fluctuation range to 20% or less. 2. 10 pp concentration of water electrolyte substantially free of electrolyte
2. The surface electrolytic treatment method according to claim 1, wherein the diameter is not more than m. 3. 2. The method according to claim 1, wherein the inorganic salt concentration is set in the range of 10 to 1000 ppm. 4. 2. The method according to claim 1, wherein the variation range with respect to the concentration set value is 10% or less.