JP2008248424A - Method of multistage electrolytic treatment of carbon fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently carrying out electrolytic treatment in a method for surface electrolytic treatment for improving the surface state of a carbon fiber. <P>SOLUTION: In the method for continuously carrying out the surface electrolysis of carbon fiber by using multistage treatment baths, preferably treatment baths composed of 3-20 units, the method for surface electrolytic treatment of carbon fiber comprises varying electric quantities of the continuous treatment baths. A method for increasing electric quantities of continuous treatment baths in turn is preferable in terms of electric quantities. The coefficient of variation of electric quantities of the continuous treatment baths is preferably set at ≥10%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a multistage surface electrolytic treatment method for producing a carbon fiber excellent in interfacial adhesion with a matrix resin.

In recent years, composite materials using carbon fibers as reinforcing fibers are light and have excellent mechanical properties such as high strength, and thus are increasingly used as members of aircraft, automobiles and the like. These composite materials are molded, for example, from a prepreg, which is an intermediate product in which a reinforcing fiber is impregnated with a matrix resin, through molding and processing steps such as heating and pressing.

In order to pursue high performance in the composite of carbon fiber and matrix resin, in addition to the mechanical properties such as strength and elastic modulus of the carbon fiber itself, the carbon fiber involved in the adhesion to the matrix resin It is essential to improve surface properties. In other words, by combining materials with high adhesion between the carbon fiber surface and the matrix resin, and dispersing the matrix resin and the carbon fiber more uniformly, higher performance composite properties (high strength, high elasticity, high impact resistance) It is expected that a composite material having properties such as properties) can be obtained.

On the other hand, the relationship between the surface condition of carbon fiber and the strength of the composite material is that the carbon fiber with a flat surface generally has low adhesiveness with the matrix resin, so that the strength as a composite material is not fully expressed. Carbon fibers with large irregularities have high adhesion to the matrix resin, but it is said that irregularities on the surface that are too large become fiber defects leading to a decrease in strength of the composite material. Therefore, it is considered important to form an appropriate amount of appropriate irregularities on the carbon fiber surface by surface treatment.

As described above, the carbon fiber used as the reinforcing fiber for the composite material is usually a carbon fiber obtained by subjecting the precursor fiber to flame resistance treatment, carbonization treatment, or further carbonization / graphitization treatment. Processing and sizing processing are performed. Examples of surface treatment methods and means include liquid phase oxidation using a chemical solution, electrolytic oxidation in which a carbon fiber is treated as an anode in an electrolytic solution, and vapor phase oxidation by a plasma treatment in a gas phase. As the surface treatment method, an electrolytic oxidation treatment method that is relatively easy to handle and is advantageous in terms of cost is preferably employed. As the electrolytic solution, either an acidic aqueous solution or an alkaline aqueous solution can be used. However, it is preferable to use a solution having high electrical conductivity because the degree of treatment increases.

By surface treatment by electrolytic oxidation, functional groups such as carboxyl groups and hydroxyl groups are generated on the surface of the carbon fiber, which is considered to contribute to enhancing the adhesiveness with the matrix resin. Focusing on this point, a method for improving the surface state of the carbon fiber has been proposed. For example, a method has been proposed in which carbon fiber tow is pulsed and anodized to increase the efficiency of electrolyte diffusion into the tow (see, for example, Patent Documents 1 and 2). In addition, in order to improve the adhesion between the carbon fiber tow and the matrix resin, a method is also proposed in which anodization and cathodic reduction are periodically repeated on the carbon fiber tow to perform surface treatment (Patent Document). 3).
JP 7-189113 A JP-A-7-207573 Japanese Patent Laid-Open No. 10-266066

Since the carbon fiber develops a graphite crystal structure and becomes difficult to be oxidized as the carbonization or graphitization temperature increases, it is necessary to increase the amount of electrolysis when a functional group is imparted by surface electrolytic treatment. However, when the amount of electricity increases, the composite characteristics tend to deteriorate due to a decrease in strength due to excessive treatment or uneven surface electrolytic treatment. As a method for improving this, there is known a method of reducing the amount of electricity in one processing stage (a pair of anode tank and cathode tank) by using electrolytic oxidation treatment in a multi-stage treatment system. For uniform and average surface electrolytic treatment, multi-stage electrolytic treatment (multi-stage electrolytic treatment in which the anode tank and the cathode tank are repeated about 2 to 12 times is preferred (for example, see Patent Document 4). If the number of processing stages increases, the process lengthens and the equipment cost increases.In such a situation, in order to efficiently produce a higher performance composite material, the productivity of carbon fiber, Specifically, it has been desired to develop an efficient method of electrolytic treatment that is a surface treatment.
JP 2003-64577 A

The subject of this invention is providing the method of performing an electrolytic treatment efficiently in the surface electrolytic treatment method for improving the surface state of carbon fiber.

The present inventor has optimized the surface condition of the carbon fiber, specifically, improved adhesion to the matrix resin, and eventually has high composite properties such as compressive strength after impact (CAI) and bending strength. In order to obtain a composite material, it is necessary to increase the surface oxygen concentration (O / C) and the interfacial shear strength (IPSS) of the carbon fiber. The present invention has been reached in the process of studying the conditions.

The invention described in claim 1 of the present invention is characterized in that, in the method of continuously performing the surface electrolytic treatment of carbon fibers using a multistage treatment bath, the electric quantity of each successive treatment bath is varied. The carbon fiber multistage surface electrolytic treatment method.

In the present invention, the treatment bath means a pair of treatment tanks composed of an anode tank and a cathode tank in which an anode and a cathode are installed in separate tanks, and a plurality of such treatment baths are continuously installed. Defined as treatment bath. In the present invention, a treatment bath comprising the pair of treatment tanks, that is, a pair of anode tank and cathode tank in which electrolytic oxidation and electrolytic reduction are performed is referred to as one unit.

The invention described in claim 2 is the carbon fiber multi-stage surface electrolytic treatment method according to claim 1, wherein the treatment bath of the multi-stage treatment bath comprises 3 to 20 units.

The invention described in claim 3 is the carbon fiber multi-stage surface electrolytic treatment method according to claim 1 or 2, wherein the amount of electricity in each successive treatment bath increases in order.

The invention described in claim 4 is the carbon fiber multi-stage surface electrolytic treatment method according to any one of claims 1 to 3, wherein the variation rate of the amount of electricity in each successive treatment bath is 10% or more. .

The invention described in claim 5 has a surface oxygen concentration (O / C) obtained by the multistage surface electrolytic treatment method of any one of claims 1 to 4 in a range of 20 to 35%. It is a certain carbon fiber.

According to the present invention, a carbon fiber having improved surface condition, specifically, a surface oxygen concentration (O / C) and a high IPSS value, can be efficiently produced by simple condition setting of a surface electrolytic treatment method. Can do. And the obtained carbon fiber can improve the adhesiveness with matrix resin, and can manufacture the composite material which has high composite characteristics, such as CAI, with high productivity. A high surface oxygen concentration indicates that the amount of functional groups on the carbon fiber surface is increasing, and a high IPSS value indicates that interfacial adhesion is improved.

Further, in the surface electrolytic treatment using the multistage treatment bath of the present invention, the amount of energy required can be reduced, and furthermore, the energy for each treatment bath can be reduced by one stage for the multistage treatment. The effect that it leads to the miniaturization and improvement of safety is also obtained.

The carbon fiber is usually subjected to further surface treatment and sizing treatment on the carbon fiber obtained by subjecting the precursor fiber to flame resistance treatment, carbonization treatment, or further carbonization / graphitization treatment. This invention relates to the specific method at the time of performing surface treatment by the electrolytic treatment method among these processes.

When carbon fiber is subjected to surface electrolytic treatment using a pair of anode tank and cathode tank in which electrolytic oxidation and electrolytic reduction are performed, etching of the carbon fiber surface by electrolytic oxidation is performed in the anode tank, and carboxyl groups, hydroxyl groups, etc. It is thought that a functional group is generated. In the cathode chamber, it is considered that a part of the purified functional group is reduced and removed by electrolytic reduction. The degree of etching by electrolytic oxidation depends on the amount of electricity used, and the higher the amount of electricity, the more strongly the fiber surface is etched. However, if excessive treatment is performed, the excessively shaved part becomes a new defect. Therefore, it is not preferable. Physical defects such as cracks and voids (structural portions with high crystallinity and low orientation) serve as starting points for breaking carbon fibers. Accordingly, in order to form an optimum surface state, appropriate etching is necessary.

In the present invention, when the surface electrolytic treatment of the carbon fiber is carried out using a multistage treatment bath, a method of changing the amount of electricity of each successive treatment bath is adopted. Surprisingly, the surface state of the carbon fiber can be improved efficiently and stably by such a method.

In the present invention, the multistage treatment bath is an electrolytic surface treatment apparatus in which a pair of treatment tanks (one unit) composed of an anode tank and a cathode tank are continuously installed as described above. Using such an apparatus, carbon fiber, for example, carbon fiber tow, is indirectly fed through an electrolyte solution, and electrolytic surface treatment by anodization and electrolytic surface treatment by cathodic reduction are alternately performed continuously. .

In the present invention, the multi-stage treatment bath is composed of an anode tank and a cathode tank separately, and any apparatus can be used as long as it is applied via an electrolytic solution. For example, it is preferable to supply by ejecting from below the tow.

The present invention is a method for sequentially increasing the amount of electricity in each successive treatment bath when performing electrolytic treatment of the carbon fiber surface in a multistage treatment bath. When increasing the amount of electricity in each successive treatment bath, the variation rate of the amount of electricity in each successive treatment bath is 10% or more, preferably 10 to 500%, more preferably 20 to 100%. The amount of electricity in each treatment bath is 5 to 500 coulombs / g (the number of coulombs per gram of carbon fiber), preferably 5 to 200 coulombs / g.

In addition, the number of coulombs per 1 g of carbon fibers is a value calculated by the following formula.
Number of coulombs per gram of carbon fiber (coulomb / g) = 0.36 × A / ((number of carbon fiber strands) × S × Y × (number of filaments))
Here, A (current value: A), S (speed: m / hr), and Y (fineness: dtex).

In the present invention, the number of treatment baths in the multistage treatment bath is 3 to 20 units, preferably 3 to 10 units, and more preferably 3 to 6 units. The amount of electricity used is in the range of 10 to 1000 coulomb / g, preferably 10 to 500 coulomb / g in terms of total electricity. If it exceeds 1000 coulombs / g, the strength of the carbon fiber itself may decrease, which is not preferable. If the amount of electricity is increased, the amount of etching increases, but if the amount of electricity is too large, surface irregularities become surface defects and fiber strength decreases, which is not preferable. Also, if the amount of electricity is too small, etching is not sufficient, which is not preferable.

When the surface of the carbon fiber is subjected to electrolytic treatment, the weakened portion generated in the firing process, which becomes a surface defect of the carbon fiber, is removed by etching by etching by electrolytic oxidation, and the strength of the carbon fiber itself is improved. Further, with the removal of the fragile portion, fine irregularities are generated on the fiber surface, the surface area of the carbon fiber is increased, and sufficient contact can be obtained between the carbon fiber and the matrix resin. Furthermore, a functional group such as a carboxyl group or a hydroxyl group having an effect of improving the affinity with the matrix resin is introduced. As a result, it is estimated that the anchor effect improves the adhesion between the carbon fiber and the matrix resin and improves the composite properties of the obtained composite material.

In the present invention, the surface oxygen concentration means an O / C value of carbon fiber measured by an X-ray photoelectron spectrometer, and the O / C value needs to be in a range of 20 to 35%. When the O / C value is less than 20%, the adhesion between the carbon fiber and the matrix resin is inferior, causing a decrease in physical properties of the resulting composite material. On the other hand, when the O / C value exceeds 35%, the adhesiveness with the resin is too high,
Since the flexibility at the interface is lost, the physical properties of the composite material tend to deteriorate, which is inappropriate.

The amount of carbon fiber treated during the surface treatment is not particularly limited. For example, in the case of tow, the number of filaments per unit width is 5000 filaments / mm or less, preferably 2000 filaments / mm or less. Exceeding 5000 filaments / mm is not preferable because the diffusion of the electrolyte solution becomes insufficient and the variation in the adhesive strength between the single fibers may increase.

There are no particular restrictions on the electrolyte used in the surface treatment, but inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, boric acid, and carbonic acid, organic acids such as acetic acid, butyric acid, oxalic acid, acrylic acid, and maleic acid Acid, ammonium sulfate, ammonium hydrogen sulfate, ammonium nitrate, ammonium hydrogen nitrate, ammonium dihydrogen phosphate, ammonium dihydrogen phosphate, ammonium carbonate, ammonium hydrogen carbonate ammonium salt or ammonia, sodium hydroxide, potassium hydroxide, marine or barium Inorganic salts such as alkali hydroxides such as sodium carbonate, sodium bicarbonate, sodium phosphate and potassium phosphate, and organic salts such as sodium maleate, sodium acetate, potassium acetate and sodium benzoate alone or in combination of two or more It can be used as a mixture.

The carbon fiber that has been surface-treated as described above is usually provided with a sizing agent. The sizing agent is preferably selected according to the matrix resin used for the composite material. For example, epoxy resin, epoxy-modified polyurethane resin, polyester resin, phenol resin, polyamide resin, polyurethane resin, polyimide resin, polyvinyl alcohol resin, polyvinyl pyrrolidone Resins, polyethersulfone resins and the like can be used alone or in admixture of two or more. When the sizing agent is applied to the carbon fiber, the carbon fiber is generally immersed in a solution obtained by dissolving the sizing agent in the solvent or a dispersion liquid (sizing solution) dispersed in the solvent, and then dried. Is.

The carbon fiber of the present invention is used as a reinforcing fiber, and a composite material can be obtained from this and a matrix resin by various known means and methods.

Carbon fiber is usually used as a sheet-like reinforcing fiber material. The sheet-like material is a material in which fiber materials are arranged in a sheet shape in one direction, these are laminated in an orthogonal manner, a fiber material is formed into a fabric such as a woven or knitted fabric or a non-woven fabric, or a strand-like material. All things, including multi-axis fabrics. As the fiber form, long fiber monofilaments or bundles of these are preferably used.

As the matrix resin, a thermosetting resin or a thermoplastic resin is used. Specific examples of thermosetting matrix resins include epoxy resins, unsaturated polyester resins, phenol resins, vinyl ester resins, cyanate ester resins, urethane acrylate resins, phenoxy resins, alkyd resins, urethane resins, maleimide resins and cyanate ester resins. And a prepolymerized resin, bismaleimide resin, polyimide resin and polyisoimide resin having acetylene terminal, and polyimide resin having nadic acid terminal. These can also be used as one type or a mixture of two or more types. Of these, epoxy resins and vinyl ester resins excellent in heat resistance, elastic modulus, and chemical resistance are particularly preferable. These thermosetting resins may contain commonly used colorants and various additives in addition to the curing agent and the curing accelerator.

Examples of the thermoplastic resin include polypropylene, polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, aromatic polyamide, aromatic polyester, aromatic polycarbonate, polyetherimide, polyarylene oxide, thermoplastic polyimide, polyamide , Polyamideimide, polyacetal, polyphenylene oxide, polyphenylene sulfide, polyarylate, polyacrylonitrile, polyaramid, polybenzimidazole and the like. The content of the resin composition in the composite material is 10 to 90% by weight, preferably 20 to 60% by weight, and more preferably 25 to 45% by weight.

Hereinafter, although an example explains the present invention in detail, the present invention is not limited to this. The measuring method of various physical property values in the examples is as follows.

The resin impregnated strand strength of carbon fiber is JIS R
It was measured by the method defined in 7601.

The interfacial shear strength (IPSS) is an index for measuring the adhesion between carbon fiber and resin, and is determined according to the following procedure. The IPSS measurement method conforms to the ASTM method. For the sample of IPSS, carbon fiber after sizing and epoxy resin (No. 133) resin manufactured by Toho Tenax Co., Ltd. are used, and a unidirectional prepreg with a carbon fiber basis weight of 270 g / m ² and a resin content of 33% is used. It was fabricated and laminated in a [+ 45 ° / −45 °] _4S pseudo-iso method. After the laminated specimen (sample) was cured at 180 ° C. for 2 hours, a specimen (sample) of 25 (width) × 250 (length) × 2.5 (thickness) mm was produced. After measuring the dimensions of each test piece, the sample was subjected to a tensile test using a tester (Autograph AG-10TD, manufactured by Shimadzu Corporation) until the test piece was broken.

The surface oxygen concentration (O / C) of the carbon fiber can be determined by XPS (ESCA) according to the following procedure. After cutting the carbon fibers and arranging them on a stainless steel sample support table, the photoelectron escape angle was set to 90 ° C., MgKα was used as the X-ray source, and the inside of the sample chamber was vacuumed at 1 × 10 ⁻⁶ Pa. Keep it up. As correction of the peak accompanying charging during measurement, first, the binding energy value B. of the main peak of C1s. E. Is adjusted to 284.6 eV. The O1s peak area is obtained by drawing a straight base line in the range of 528 to 540 eV, and the C1s peak area is obtained by drawing a straight base line in the range of 282 to 292 eV. The surface oxygen concentration O / C on the surface of the carbon fiber is determined by calculating the ratio of the O1s peak area to the C1s peak area.

[Example 1]
A copolymer spinning stock consisting of 95% by mass of acrylonitrile / 4% by mass of methyl acrylate / 1% by mass of itaconic acid is wet-spun by a conventional method, washed with water and dried, and then steam-stretched so that the total stretching ratio is 14 times. And a precursor fiber having a filament number of 12,000 having a fineness of 0.65 denier was obtained.

While the obtained precursor fiber is stretched in heated air, flameproofing treatment is performed within a temperature range of 240 to 250 ° C., and then, in a nitrogen atmosphere, primary and second carbon are within a temperature range of 350 to 2000 ° C. An unelectrolyzed carbon fiber was obtained.

The non-electrolytically treated carbon fiber was subjected to non-contact electrolytic treatment using a 3-unit multistage treatment bath. A 6.3% by mass nitric acid aqueous solution was used as the electrolyte solution, the total amount of electricity was 250 C / g, and the amount of electricity in each unit was increased to 1: 2: 3. The variation rate of the amount of electricity of each successive treatment bath was 100% and 50%. Thereafter, sizing treatment was performed by a conventional method and dried to obtain carbon fibers having a density of 1.77 g / cm ³ and a fiber diameter of 5.1 μm. The measured values of surface oxygen concentration O / C, IPSS and strand strength of the obtained carbon fiber were as shown in Table 1.

[Comparative Example 1]
Using the same multistage treatment bath as in Example 1, each unit was treated with an equal amount of electricity at a total electricity of 250 C / g. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

In the multistage electrolytic treatment, a multistage treatment bath in which the repetition (unit) of the anode tank and the cathode tank is about 3 to 12 times is preferable. However, when the number of treatment baths increases, the equipment cost increases accordingly. Therefore, it is preferable that carbon fibers having high O / C and IPSS values can be obtained more efficiently when the same number of treatment baths are used. Comparing Example 1 and Comparative Example 1, the number of units is 3 and the total amount of electricity is the same. It can be seen that carbon fibers having a higher IPSS value of / C are obtained.

[Example 2]
The same non-electrolytically treated carbon fiber as used in Example 1 was used, and the same electrolytic solution as that of Example 1 was used for the electrolytic solution, and non-contact electrolytic treatment was performed using six units. At this time, the total electric quantity is 250 C / g, and the electric quantity ratio in each unit is 1: 2: 3: 4.
: 5: 6. The variation rate of the amount of electricity of each successive treatment bath was 100%, 50%, 33%, 25%, and 20%. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

[Comparative Example 2]
Using the same multi-stage treatment bath of 6 units as in Example 2, the treatment in each unit was performed with an equal amount of electricity at a total electricity amount of 250 C / g. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.
[Example 3]
Using the same 6 unit multi-stage treatment bath as in Example 2, the electric quantity ratio in each unit was 2: 2: 2: 3 with a total electric quantity of 250 C / g.
: 3: The electrolytic treatment was performed once in the middle, varying at 3: 3. The variation rate of the amount of electricity of each successive treatment bath was 0%, 0%, 50%, 0%, and 0%. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

In the case of Example 2, which is a preferred example of the present invention, the O / C of the obtained carbon fiber was in an appropriate range, and the measured value of IPSS was a high value. On the other hand, although not a preferred example, in the case of Example 3 included in the scope of the present invention, an IPSS value higher than that of Comparative Example 2 was obtained although it was lower than that of Example 2.

[Example 4]
The same non-electrolytically treated carbon fiber as used in Example 1 was used, and the same electrolytic solution as that of Example 1 was used for the electrolytic solution, and non-contact electrolytic treatment was performed using nine units. At this time, the total electric quantity is 250 C / g, and the electric quantity ratio in each unit is 1: 2: 3: 4.
: 5: 6: 7: 8: 9. The variation rate of the amount of electricity of each successive treatment bath was 100%, 50%, 33%, 25%, 20%, 17%, 14%, and 13%. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

[Comparative Example 3]
Using the same multi-stage treatment bath of nine units as in Example 4, the treatment in each unit was carried out with an equal amount of electricity at a total electricity of 250 C / g. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

[Comparative Example 4]
The same unelectrolyzed carbon fiber as used in Example 1 was used, and the same nitric acid aqueous solution as that used in Example 1 was used as the electrolytic solution. The non-contact electrolytic treatment was performed using two units. At this time, the total amount of electricity was 250 C / g, and the amount of electricity in each unit was increased to 2: 3. The variation rate of the amount of electricity in each successive treatment bath was 50%. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

[Comparative Example 5]
Using the same multi-stage treatment bath of two units as in Comparative Example 4, the treatment in each unit was performed with an equal amount of electricity at a total electricity of 250 C / g. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

[Comparative Example 6]
The same non-electrolytically treated carbon fiber as used in Example 1 was used, and the same electrolytic solution as that of Example 1 was used for the electrolytic solution, and non-contact electrolytic treatment was performed using six units. At this time, the total amount of electricity was 350 C / g, and the amount of electricity in each unit was increased to 1: 2: 3: 4: 5: 6. The variation rate of the amount of electricity of each successive treatment bath was 100%, 50%, 33%, 25%, and 20%. The measured values of O / C, IPSS, and strand strength of the obtained carbon fiber were as shown in Table 1.

Claims (5)

A method for continuously performing surface electrolysis of carbon fibers using a multistage treatment bath, wherein the quantity of electricity in each successive treatment bath is varied. 2. The carbon fiber multi-stage surface electrolytic treatment method according to claim 1, wherein the treatment bath of the multi-stage treatment bath comprises 3 to 20 units. The carbon fiber multi-stage surface electrolytic treatment method according to claim 1 or 2, wherein the amount of electricity in each successive treatment bath increases in order. The multistage surface electrolytic treatment method for carbon fibers according to any one of claims 1 to 4, wherein the variation rate of the amount of electricity in each successive treatment bath is 10% or more. Carbon fiber having a surface oxygen concentration (O / C) in the range of 20 to 35%, obtained by the multistage surface electrolytic treatment method according to any one of claims 1 to 4.