JP2008248427A - Method for surface electrolytic treatment of carbon fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an efficient method for the surface electrolytic treatment to improve the surface condition of a carbon fiber and to provide a carbon fiber having a high strength. <P>SOLUTION: The surface of a carbon fiber is electrolyzed by using a multi-stage electrolytic treatment bath having a plurality of continuously arranged electrolytic treatment baths having the combination of an anodic cell and a cathodic cell, carrying out the electrolytic treatment keeping the electric quantity of 20-300 C/g for each stage and 150-500 C/g in total, and, thereafter, reversing the potential by using an electrolytic treatment bath comprising a combination of a cathodic cell and an anodic cell to perform the electrolytic treatment at an electric quantity of 20-60 C/g at the reversed potential. The invention further provides a high-strength carbon fiber produced by the method and having a strength of resin-impregnated strand of ≥6,000 MPa, an elastic modulus of resin-impregnated strand of ≥340 GPa and a density of ≥1.76 g/cm<SP>3</SP>. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a surface electrolytic treatment method for producing carbon fibers having excellent mechanical properties and excellent 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.

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 graphitization treatment. The sizing process has been 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 electrolyte solution, and gas phase oxidation by 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 used for the electrolytic oxidation treatment, either an acidic aqueous solution or an alkaline aqueous solution can be used, but an aqueous solution having high electrical conductivity such as nitric acid is used in terms of treatment efficiency such as the amount of functional groups introduced to the fiber surface. Is advantageous.

By surface treatment by electrolytic oxidation, the fragile layer on the surface generated in the formation process of carbon fiber is removed,
It is considered that mechanical properties such as fiber strength are improved and functional groups such as a carboxyl group, a carbonyl group, and a hydroxyl group are generated on the surface of the carbon fiber, and this contributes to enhancing the adhesion to 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 of subjecting the carbon fiber tow to surface electrolytic treatment by periodically repeating anodization and cathodic reduction has been proposed (patent) Reference 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, as the amount of electricity increases, the roughness of the fiber surface increases and the mechanical properties of the fiber decrease. In addition, uneven processing tends to occur, resulting in a non-uniform surface state, resulting in deterioration of composite characteristics. In order to avoid this problem, a method of reducing the amount of electricity in one processing stage (anode tank and cathode tank) by performing surface electrolytic treatment in multiple stages is also known. For uniform surface electrolytic treatment, multi-stage electrolytic treatment in which the anode tank and the cathode tank are repeated about 2 to 12 times is preferable (for example, see Patent Document 4). However, when the number of processing stages increases, there is a problem that the equipment cost increases accordingly. Under the circumstances as described above, in order to efficiently produce a higher performance composite material, it is desired to develop a carbon fiber productivity, specifically, an efficient method of electrolytic treatment as a surface treatment. It was.
JP 2003-64577 A

An object of the present invention is to provide a surface electrolytic treatment method for improving the surface state of carbon fiber, to perform uniform treatment by electrolytic oxidation, removal of defects that cause deterioration in physical properties of the fiber, and appropriate functional groups by electrolytic reduction. An object of the present invention is to provide a method for efficiently forming a surface state having high strength carbon fibers obtained thereby.

The invention described in claim 1 of the present invention is a method for electrolytically treating the surface of a carbon fiber. First, a multi-stage electrolytic treatment in which a plurality of electrolytic treatment baths comprising a combination of an anode tank and a cathode tank are installed continuously. Using a bath, electrolytic treatment is performed in a range where the amount of electricity at each stage is in the range of 20 to 300 C / g and the total amount of electricity is in the range of 150 to 500 C / g, and then an electrolytic treatment comprising a combination of a cathode cell and an anode cell. This is a carbon fiber surface electrolytic treatment method characterized in that the electric potential is reversed using a bath and the amount of electricity at the reversed electric potential is in the range of 20 to 60 C / g.

In the present invention, the electrolytic 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 the electrolytic treatment bath is continuously installed. Is defined as a multi-stage electrolytic treatment bath. In the present invention, an electrolytic 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. In the case of reversing the electric potential, a unit arranged in the order of a cathode tank and an anode tank is defined as one unit of electrolytic treatment bath.

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

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

The invention described in claim 4 is the carbon fiber surface electrolytic treatment method according to claim 3, wherein the variation rate of the amount of electricity in each electrolytic treatment bath is 10% or more.

The invention described in claim 5 is the carbon fiber surface electrolytic treatment method according to any one of claims 1 to 4, wherein the electrolytic treatment bath in which the potential is reversed comprises 1 to 5 units.

The invention described in claim 6 is the resin impregnated strand strength obtained by the method according to any one of claims 1 to 5 is 6000 MPa or more, the resin impregnated strand elastic modulus is 340 GPa or more, and the density is It is a high-strength carbon fiber that is 1.76 g / cm ³ or more.

According to the present invention, carbon fibers having improved mechanical properties such as tensile strength and compressive strength can be obtained. In addition, carbon fibers having improved surface conditions, specifically, surface oxygen concentration (O / C) and surface roughness having appropriate values should be efficiently manufactured with simple conditions for surface electrolytic treatment. Can do. And the obtained carbon fiber can improve the adhesiveness with matrix resin, and can manufacture the composite material which has a high composite characteristic.

The carbon fiber obtained by the present invention has a high resin-impregnated strand strength, a resin-impregnated strand elastic modulus, and a high density, and when it is combined with a matrix resin to form a composite material, it has a good adhesion to the matrix resin. Function. Therefore, by using the carbon fiber of the present invention, it is possible to obtain a composite material with higher performance (high strength, high elasticity) than the conventional one, and these are highly safe in the aerospace field, the automobile field, etc. And can be used as a lightweight composite material.

The carbon fiber is usually further subjected to surface treatment and sizing treatment on the carbon fiber obtained by subjecting the precursor fiber to flame resistance treatment, carbonization treatment, or further 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. Such etching is considered to remove the brittle layer generated during the firing process of the carbon fiber and generate functional groups such as a carboxyl group, a carbonyl group, and a hydroxyl group on the surface of the carbon fiber by the oxidation reaction. 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 flaws such as cracks and voids and a fragile layer generated during the fiber firing process serve as a starting point for carbon fiber breakage. Therefore, in order to form an optimum surface state and improve the mechanical properties of the carbon fiber, appropriate etching is required.

In the present invention, when electrolytically treating the surface of the carbon fiber, first, electrolytic treatment is performed using a multi-stage electrolytic treatment bath in which a plurality of electrolytic treatment baths composed of a combination of an anode tank and a cathode tank are continuously installed, and electrolysis is performed. In the final stage of the treatment, a method is employed in which the electrolytic treatment is performed by reversing the potential using an electrolytic treatment bath in which the combination of the cathode and anode vessels is reversed. Surprisingly, by this method, the mechanical properties of carbon fibers represented by strand strength and single fiber compressive strength, and the surface state of carbon fibers expressed by surface oxygen concentration and surface roughness are made very favorable. It can be improved.

In the present invention, as described above, the multi-stage electrolytic treatment bath is a series of treatment tanks (one unit) composed of an anode tank and a cathode tank, preferably 2 to 20 units, more preferably 2 to 10 units. This is a surface electrolytic treatment apparatus. Using such an apparatus, carbon fiber, for example, carbon fiber tow is indirectly supplied with power through an electrolyte solution, and surface electrolytic treatment by anodization and surface electrolytic treatment by cathodic reduction are alternately and continuously performed. .

In the present invention, the electrolytic treatment bath for performing surface electrolytic treatment by reversing the potential is basically a pair of treatment tanks (one unit) composed of the same anode tank and cathode tank as described above. Is an electrolytic treatment bath consisting of a combination of a cathode cell and an anode cell, the order of combination being reversed. This electrolytic treatment bath may be one stage, that is, one unit or multiple stages. In the case of multiple stages, 2 to 5 units are sufficient.

In the present invention, the electrolytic 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. What penetrates is preferred.

In the present invention, the electrolytic treatment of the carbon fiber surface is performed by first using a multi-stage electrolytic treatment bath in which a plurality of electrolytic treatment baths comprising a combination of an anode tank and a cathode tank are continuously installed, Electrolytic treatment is performed at 300 C / g (the number of coulombs per gram of carbon fiber), preferably in the range of 30 to 200 C / g, and the total amount of electricity in the range of 150 to 500 C / g, preferably 150 to 350 C / g. (Hereinafter, this quantity of electricity is referred to as the quantity of oxidized electricity). Usually, direct current is used as the electricity to be energized, but the amount of electricity may be constant, but a method of sequentially increasing the amount of electricity in each successive treatment bath is preferable. At that time, the variation rate of the amount of electricity of each successive electrolytic treatment bath is 10% or more, preferably 10 to 500%, and more preferably 20 to 100%.

If the amount of electricity is larger than the above range, 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, the treatment becomes excessive, the surface roughness increases, and the fiber strength decreases, which is not preferable. On the other hand, if the amount of electricity is too small, a sufficient etching effect cannot be obtained.

The amount of electricity in the electrolytic treatment bath with the potential reversed is 20 to 60 C / g, preferably 20 to 40 C / g (hereinafter, this amount of electricity is referred to as reducing electricity). Here again, in the case of performing multi-stage processing, it is sufficient that the total amount of electricity in 2 to 5 units satisfies the above conditions. When the amount of electricity is smaller than the above range, the effect of reduction is insufficient.

The number of coulombs per gram of carbon fiber (C / g) is a value calculated by the following formula.
C / g = 0.36 × A / (E × S × Y × F)
Here, A (current value: A), E (number of carbon fiber strands), S (speed: m / hr), Y (fineness: dtex), and F (number of filaments).

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. In addition, a functional group such as a carboxyl group, a carbonyl 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 adhesion between the carbon fiber and the matrix resin is improved, and the composite properties of the obtained composite material are improved.

In the present invention, the surface oxygen concentration (O / C) means the abundance ratio of oxygen atoms to carbon atoms on the surface of the carbon fiber measured by an X-ray photoelectron spectrometer, and the O / C value is in the range of 20 to 30%. It is necessary to be in Those having an O / C value of 21 to 29% are more preferred. 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, if the O / C value exceeds 30%, the adhesion between the carbon fiber and the matrix resin is too strong, and stress concentration occurs on the contrary, and composite properties such as impact resistance deteriorate, which is not preferable.

In the present invention, the surface roughness means an average surface roughness (Ra) of 0.5 μm square of the carbon fiber surface measured by AFM (Atomic Force Microscope), and this value indicates a very small unevenness on the fiber surface. It is desirable that it is 7.0 nm or less. The microscopic irregularities on the surface of the carbon fiber become the starting point of breakage of the carbon fiber as microdefects on the fiber surface. When the surface roughness is larger than 7.0 nm, the physical properties of the carbon fiber are lowered due to the microdefects. Inappropriate.

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, barium hydroxide 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. Among these, it is particularly preferable to use an aqueous nitric acid solution.

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, an epoxy resin, an epoxy-modified polyurethane resin, a polyester resin, a phenol resin, a polyamide resin, a polyurethane resin, a polyimide resin, a ponovinyl alcohol resin, Polyvinyl pyrrolidone resin, polyether sulfone resin, etc. 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 in which the sizing agent is dissolved or a dispersion liquid (sizing solution) and then dried.

The carbon fiber of the present invention preferably has a resin-impregnated strand strength of 6000 MPa or more, a resin-impregnated strand elastic modulus of 340 GPa or more, and a density of 1.76 g / cm ³ or more. . More preferably, the resin-impregnated strand strength is 6100 MPa or more, the resin-impregnated strand elastic modulus is 340 to 370 GPa, and the density is 1.76 to 1.80 g / cm ³ . With the above configuration, the carbon fiber of the present invention functions as a reinforcing material having good adhesion to the matrix resin when it is combined with the matrix resin to form a composite material. Moreover, this carbon fiber has less fuzz and yarn breakage.

Hereinafter, although an example explains the present invention in detail, the present invention is not limited to this. The resin-impregnated strand strength and elastic modulus of carbon fiber were measured by the method defined in JIS R7601. The density was measured by the Archimedes method, and the sample fiber was measured after degassing in acetone.

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 spreading them on a stainless steel sample support table, the photoelectron escape angle was set to 90 degrees, 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.

The surface roughness was determined according to the following procedure using AFM (Atomic Force Microscope). The carbon fiber to be used for measurement was fixed to a sample stage, and measurement was performed in Tapping Mode using a Digital Scope Nanoscope III as an atomic force microscope. NCH (Si, cantilever length 125 μm) was used as a probe, and measurement was performed such that the axial direction of the carbon fiber was the scan direction, and measurement was performed with a measurement range of 0.5 μm square.

The compressive strength of a single fiber means the compressive strength (measured at n = 5) in a direction perpendicular to the fiber direction of the single fiber of carbon fiber. In the measurement, a sample in which a single fiber of carbon fiber was fixed on a slide glass was prepared, and a flat compression 50 μm indenter was used with a micro compression tester “MCTM-200” manufactured by Shimadzu Corporation, with a load speed of 7.25 mgf / Measurement was performed in sec.

[Examples 1 to 6]
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 carbons are heated within a temperature range of 300 to 2000 ° C. An unelectrolyzed carbon fiber was obtained.

Using the unelectrolyzed carbon fiber, a multi-stage electrolytic treatment bath having 2 to 5 units of a pair of treatment tanks (one unit) composed of an anode tank and a cathode tank, and one unit of an electrolytic treatment bath with the potential reversed. Then, non-contact electrolytic treatment was performed. A 6.3 mass% nitric acid aqueous solution having a liquid temperature of 20 to 40 ° C. was used as the electrolytic solution, and samples were collected by changing the amount of oxidation electricity, the total amount of oxidation electricity, and the amount of reduction electricity in each unit. 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. Table 1 shows the relationship between the strand strength, strand elastic modulus, single fiber compressive strength, surface oxygen concentration, carbon fiber surface roughness, and electric quantity of the obtained carbon fiber.

[Comparative Examples 1-5]
Experiments were conducted in the same manner as in Example 1 except that the total amount of electricity oxidized, the amount of electricity oxidized and the amount of electricity reduced were outside the scope of the present invention. Table 1 shows the relationship between the strand strength, strand elastic modulus, single fiber compressive strength, surface oxygen concentration, carbon fiber surface roughness, and electric quantity of the carbon fiber thus obtained. When any of the conditions is out of the range of the present invention, it was confirmed that an appropriate surface state cannot be obtained, so that the mechanical properties of the carbon fiber are lowered.

Claims (6)

In the method of electrolytically treating the surface of carbon fiber, first, using a multistage electrolytic treatment bath in which a plurality of electrolytic treatment baths composed of a combination of an anode tank and a cathode tank are installed, the amount of electricity in each stage is 20 to 300C. / G and the total amount of electricity is 150 to 500 C / g in the electrolytic treatment,
Thereafter, the carbon fiber is characterized in that the potential is reversed using an electrolytic treatment bath comprising a combination of a cathode tank and an anode tank, and the electrolytic treatment is performed in the range of 20 to 60 C / g of electricity at the reversed potential. Surface electrolytic treatment method. 2. The carbon fiber surface electrolytic treatment method according to claim 1, wherein the treatment bath of the multistage electrolytic treatment bath comprises 2 to 20 units. The carbon fiber surface electrolytic treatment method according to claim 2, wherein the amount of electricity in the multistage electrolytic treatment bath increases in order. The carbon fiber surface electrolytic treatment method according to claim 3, wherein the variation rate of the amount of electricity in each electrolytic treatment bath is 10% or more. The carbon fiber surface electrolytic treatment method according to any one of claims 1 to 4, wherein the electrolytic treatment bath with the potential reversed is composed of 1 to 5 units. A high-strength carbon fiber obtained by the method according to any one of claims 1 to 5, having a resin-impregnated strand strength of 6000 MPa or more, a resin-impregnated strand elastic modulus of 340 GPa or more, and a density of 1.76 g / cm ³ or more. .