JPH0544155A - Surface treatment of carbon fiber - Google Patents

Abstract

PURPOSE:To obtain a carbon fiber having improved adhesivity to a matrix resin and giving a composite material having excellent interlaminar shear strength by carrying out electrolytic oxidation of a carbon fiber under severe condition and heating the fiber in an oxygen-containing atmosphere. CONSTITUTION:A carbon fiber is subjected to electrolytic oxidation treatment by using the carbon fiber as an anode in an electrolytic solution preferably containing inorganic strong electrolyte such as nitric acid, sulfuric acid and phosphoric acid as an electrolyte at 10-90 deg.C for about 1-10min under a severe condition to pass an electricity of >=50 coulomb/g, especially 150-1,000 coulomb/g through the fiber. The objective carbon fiber having increased surface unevenness is produced by washing the treated fiber with water, drying the fiber and heating in an oxygen-containing atmosphere, thereby eliminating the formed anion or adsorbed gas molecule while essentially leaving the oxygen-containing functional groups on the surface of the carbon fiber. The heat-treatment is carried out at <=350 deg.C under reduced pressure and an oxygen partial pressure of >=1Torr or at <=350 deg.C under ordinary pressure at an oxygen concentration of >=20vol.%. The above treatment is especially effective for the treatment of highly oriented pitch-based carbon fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel surface treatment method for carbon fibers, and more particularly to a surface treatment method for increasing irregularities on the surface of carbon fibers.

[0002] In particular, the present invention relates to a surface treatment method for carbon fibers, which can improve the adhesiveness with a resin matrix or reinforcing fibers constituting a composite material.

Furthermore, the present invention relates to a surface treatment method which is particularly suitable for pitch-based carbon fibers which are difficult to be surface-modified by electrolytic oxidation treatment as compared with PAN-based carbon fibers. In addition, the present invention, after amorphizing the surface by electrolytic oxidation of carbon fibers,
The present invention relates to a carbon fiber surface treatment method capable of forming appropriate surface characteristics by heat treatment in an oxygen-containing atmosphere.

The present invention also relates to a surface treatment method for carbon fibers, which gives reinforcing carbon fibers having more excellent interlaminar shear strength of the composite material.

[0005]

2. Description of the Related Art Conventionally, carbon fiber has been used for various purposes due to its mechanical, chemical and electrical characteristics and lightness.
For example, it is widely used in structural materials for aviation and space, ships, automobiles, sports equipment and the like.

In these applications, carbon fibers are generally used as a reinforcing material for composite materials of the carbon fibers and various resin matrices. In order to reflect the mechanical properties of the composite materials, the composite materials should be used. The adhesiveness and integration of the constituent resin matrix and the carbon fiber are important.

However, if the carbon fiber is not subjected to any surface treatment in advance, the adhesiveness to the matrix is insufficient and the fiber is easily pulled out from the matrix.
The reinforcing effect cannot be fully exerted.

As the surface treatment, various methods are known, for example, a liquid phase oxidation method using an oxidizing agent, a heat cleaning method, a gas phase oxidation method, a whiskerizing method, an electrolytic oxidation method, etc. The method is widely adopted because of its excellent operability.

However, in the conventional electrolytic oxidation method, particularly in the case of mesophase pitch type high elastic modulus carbon fiber, there is a problem that the adhesiveness is poor and the interlaminar shear strength of the obtained composite material is inferior, and the crystal is crystallized. Since the structure is developed, there is a problem that the surface treatment effect is poor under the conditions used for the conventional PAN-based carbon fiber, and the surface treatment method which is sufficiently effective for such high elastic modulus carbon fiber, In particular, improvement of the electrolytic oxidation method is urgently required.

[0010]

DISCLOSURE OF THE INVENTION The present invention provides a carbon fiber,
In particular, the object is to solve the problem that the surface treatment effect of the pitch-based carbon fiber having a high elastic modulus by electrolytic oxidation is poor and the adhesiveness to the matrix is not sufficient.

[0011]

Means for Solving the Problems As a result of various studies on the above problems, the present inventors have found that in electrolytic oxidation treatment of carbon fiber,
(A) An electrolytic oxidation step performed under harsh conditions for changing the surface of the graphite-structured carbon fiber to an amorphous structure, and (b) after that,
By adopting multiple processes consisting of heat treatment in an oxygen-containing atmosphere to clear the surface layer, it is possible to efficiently introduce irregularities and functional groups such as OH groups on the surface of carbon fiber. The present invention has been completed and the present invention has been completed.

That is, the present invention is: In a method of surface-treating carbon fibers by electrolytic oxidation, the method comprises: (a) first using carbon fibers as an anode and
Surface of carbon fiber characterized in that electrolytic oxidation treatment is carried out under harsh conditions in which an electric quantity of 0 c / g or more is passed, (b) then, after being washed with water and dried, heat treatment is carried out in an oxygen-containing atmosphere. Processing method, and

It is also characterized in that the electrolyte used for electrolytic oxidation is a strong inorganic electrolyte such as nitric acid, sulfuric acid and phosphoric acid,
Also, the heat treatment is performed at 350 ° C or lower and the oxygen pressure is 1 under reduced pressure.
It is also characterized in that it is performed at a temperature of not less than Torr, or at a temperature of 350 ° C. or less and under atmospheric pressure and an oxygen concentration of about 20 wt% or more.

The present invention will be described in detail below.
The present invention basically performs electrolytic oxidation treatment under harsh conditions in which the surface structure of carbon fiber is changed from a highly crystallized state, which is a graphite, to an amorphous state, and then, under atmospheric pressure in an oxygen-containing atmosphere. Alternatively, it is important to perform heat treatment under reduced pressure, and by this heat treatment, unnecessary components such as anions remaining on the amorphous surface obtained by electrolytic oxidation treatment are treated under atmospheric pressure or under atmospheric pressure. It is characterized in that it is efficiently removed (cleared) by heat treatment under reduced pressure, and only the oxygen-containing functional groups on the surface are substantially left.

By such a specific multi-step treatment, the anchor effect is increased due to the increase in the unevenness of the carbon fiber surface, which is effective for improving the adhesiveness with the matrix resin, and the removal of unnecessary components mainly results in the oxygen-containing functional group. The introduction of the group can be efficiently performed.

The multi-step surface treatment method of the present invention can be advantageously applied to any type of electrolytic oxidation method of carbon fibers, but it is said that effective electrolytic oxidation is difficult due to the particularly developed crystal structure. It is effective for electrolytic oxidation treatment of highly oriented pitch-based carbon fiber.

Examples of pitch-based carbon fibers effective for the method of the present invention include highly-oriented pitch-based carbon fibers produced by using mesophase pitch purified from coal pitch, petroleum pitch, etc. as a precursor. ..

(A) Electrolytic oxidation treatment: The electrolytic oxidation method of the present invention can be applied to any type of electrolytic oxidation method such as a continuous method or a batch method. Specifically, in the batch method, carbon fibers are immersed in an electrolytic treatment tank filled with an electrolyte aqueous solution having a predetermined concentration, a positive voltage is applied to the carbon fibers, and a predetermined amount of current is passed to perform electrolytic oxidation treatment. ..

Further, in the continuous method, a representative method is JP-A-63 / 63.
The method disclosed in Japanese Patent Laid-Open No. 282364, that is, a method in which an electrolytic solution overflows from an electrolytic solution tank is used as a negative and positive electrode, and a large number of the cathode and the anode are alternately arranged, and
The carbon fibers and carbon between the cathode and the anode are run by running carbon fibers through the swollen portion of the liquid surface of the positive pole so that the carbon fibers are not substantially bent by the edging of the bath. A direct current is applied to the electrolytic solution adhering to the fibers for electrolytic oxidation.

In this case, the length of the negative and positive poles in the fiber running direction is 3 to 300 mm, and the number of the negative and positive poles is 5 to 10 mm.
It is preferable that the number is 0 or less. In the electrolytic oxidation treatment, the electrolytic solution is mainly composed of an inorganic acid such as nitric acid, sulfuric acid and phosphoric acid, and if necessary, a small amount of formic acid, oxalic acid, an organic acid such as tartaric acid, or their ammonium salts, calcium salts and aluminum. It is preferable to use an aqueous solution of a strong electrolyte in which salts such as salts are also combined, and it is more preferable to use an inorganic acid.

In this case, in particular, when an inorganic acid is used, it easily penetrates into the graphite structure layers on the surface of the carbon fiber and distorts the structure, and at the same time, it effectively destroys the graphite structure itself and changes it to an amorphous state. However, the rationale is not clear.

Furthermore, during this electrolytic oxidation, 50 c / g
It is necessary to carry out the above-mentioned amount of electricity passed, and by setting severe conditions such as the use of a strong electrolyte and the application of a high amount of electricity, it is possible to efficiently increase the unevenness on the carbon fiber surface. ..

The unit of the amount of electricity flowing represents the amount of electricity (coulomb; c) applied to 1 g of carbon fiber. here,
If the amount of electricity used for electrolytic oxidation is 50 c / g or more,
It is not possible to develop sufficient unevenness on the carbon fiber surface,
It is preferably 50 c / g to 1200 c / g, more preferably 150 to 1000 c / g in terms of operation convenience.

When the amount of electricity supplied exceeds 1200 c / g, no improvement in the surface treatment of the carbon fiber can be expected even if the amount of electricity supplied is further increased. May impair strength.

The electrolyte used for electrolytic oxidation is preferably present as a solute in an aqueous solution from the viewpoint of handling, but if necessary, a small amount of a hydrophilic organic solvent such as dimethylformamide, dimethylacetamide, ethyl cellosolve or the like may be used. You may mix. The concentration of the electrolyte aqueous solution is not particularly limited, but it is usually preferable to use an amount in the range of 0.1 to several tens% by weight.

The processing time is not particularly limited,
About 1 to 10 minutes is good. The treatment temperature is not particularly limited, but is preferably room temperature or higher, preferably 10 to 90 ° C. for reasons such as operation.

(B) Heat treatment in an oxygen-containing atmosphere: Following the electrolytic oxidation treatment (a), it is necessary to wash with water and dry, and then heat-treat in an oxygen-containing atmosphere. By this treatment, anions or CO, C remaining on the carbon fiber surface
Adsorbed gas molecules such as O ₂ and H ₂ O are removed.

The heat treatment temperature is 350 ° C. or lower, preferably 200 to 300 ° C. When the temperature is 350 ° C. or higher, the surface of the carbon fiber is excessively oxidized, and the tensile strength and elastic modulus of the carbon fiber decrease. The heat treatment time is not particularly limited, but is usually 30 minutes to 5 hours, preferably 1 to 2 hours.

The heat treatment is preferably performed under reduced pressure. When it is performed under reduced pressure, the remaining anions and adsorbed gas molecules on the surface can be efficiently removed. The pressure is preferably 1
The pressure is preferably about Torr or higher, and the pressure is preferably controlled by oxygen or an oxygen-containing gas such as air.

When the heat treatment is carried out at normal pressure, the oxygen-containing atmosphere must have an oxygen content of about 20% by volume or more. When the oxygen content is less than about 20% by volume, it is difficult to sufficiently remove unnecessary components such as impurities on the carbon fiber surface. Examples of the oxygen-containing atmosphere include oxygen, oxygen-rich air, and air.

Such a specific surface treatment causes oxygen-containing functional groups such as OH groups and COOH groups to substantially remain on the carbon fiber surface, but the anions formed on the carbon fiber surface or CO, CO ₂ , It removes adsorbed gas molecules such as H ₂ O. This makes it possible to minimize voids and non-uniform curing, which induce a decrease in strength at the interface with the carbon fibers, in the production of a composite material with a matrix resin such as an epoxy resin. In addition, this surface treatment increases irregularities on the surface of the carbon fiber, and the anchoring effect improves the adhesive strength with a matrix such as an epoxy resin.

[0032]

In the composite material of carbon fiber and epoxy resin matrix, in order to improve the adhesiveness or adhesiveness with the resin matrix, the surface structure of the carbon fiber is changed from crystalline to amorphous, and irregularities are formed on the surface. The present inventors have found that it is important to increase the number of OH groups and at the same time mainly introduce functional groups having a high affinity with the matrix resin such as OH groups.

Based on these assumptions, in the present invention, (a) an electrolytic oxidation step performed under severe conditions for changing the surface of the carbon fiber having a crystalline structure to an amorphous structure, and (b) after that, an oxygen-containing atmosphere In this process, heat treatment is performed to clear the surface layer and increase surface irregularities and oxygen functional groups.

The electrolytic oxidation (a) under severe conditions of passing a high electricity amount of 50 c / g or more in a strong electrolyte solution such as nitric acid is
The electrolytic solution easily enters between the layers of the graphite structure on the surface of the carbon fiber to distort the structure, and at the same time, destroys the graphite structure itself and changes the surface structure into an easily oxidizable amorphous structure.

(B) The heat treatment in an oxygen-containing atmosphere causes oxygen-containing functional groups such as OH groups and COOH groups to substantially remain on the carbon fiber surface, but the impurities formed on the carbon fiber surface, ie, remain. It removes anions (SO ₄ ²⁻ , NO ₃ ⁻ etc.) or adsorbed gas molecules such as CO, CO ₂ and H ₂ O. This makes it possible to minimize voids and non-uniform curing, which induce a decrease in strength at the interface with the carbon fibers, in the production of a composite material with a matrix resin such as an epoxy resin.

In the present invention, by combining the treatments of a plurality of steps in this manner, carbon fibers, particularly pitch-based carbon fibers in which it is difficult to improve the surface modification effect by electrolytic oxidation, have OH groups on the carbon fiber surface. In addition to preferentially introducing oxygen-containing functional groups such as, it is possible to form a large number of irregularities on the amorphous surface, and the effect of improving the adhesiveness with the matrix resin that constitutes the composite material is extremely excellent, and a composite with high interlaminar shear strength. We can provide wood.

[0037]

The present invention is illustrated by the following examples, which do not limit the scope of the invention. The measurement of each experimental value used in the examples was performed according to the following.

Interlaminar Shear Strength of Composite Material: A value obtained by measuring the obtained composite material according to ASTM D2344. O / C ratio of carbon fiber: The ratio of the number of carbon atoms and the number of oxygen atoms obtained by analyzing the surface of the obtained carbon fiber by photoelectron spectroscopy (ESCA).

I ₁₃₅₀ / I ₁₅₈₀ : The peak intensity ratio of the band around ₁₃₅₀ cm ⁻ (turbulent structure carbon) and the band around 1580 cm ⁻ (graphite structure carbon) in the spectrum measured by the laser Raman spectroscopy. It is a scale showing the degree of graphitization and the degree of orientation.

Concavity / convexity: B by specific surface area (smooth surface area) calculated using measured fiber diameter and krypton gas adsorption
It is represented by the ratio of the measured surface area measured using the ET method. It is a scale showing the degree of unevenness of the carbon fiber surface.

[0041]

Examples 1 to 3 and Comparative Examples 1 to 3 Commercially available mesophase carbon fiber carbonic HM-50 (diameter 10 μ, strength 302 kg / mm ² , elastic modulus 50,000 kgf / mm
² ) Carbon fiber fiber bundle is used as an anode and 50c / g in an electrolytic cell containing 0.05N sulfuric acid aqueous solution (4% concentration).
While conducting the electricity amount of, the electrolytic oxidation treatment was performed at 40 ° C. for 5 minutes.

Next, after washing with water and drying with an infrared heater, the bobbin was wound on an aluminum bobbin, and the bobbin was sealed in a stainless steel container and depressurized to about 10 ^-3 torr. Then, the temperature was raised to 300 ° C., oxygen was introduced so that the pressure was 2 torr, and the treatment was performed for about 1 hour while controlling the pressure inside the vessel.

By this reduced-pressure heat treatment, a reduction in unit weight of about 1% by weight was observed. This surface-treated yarn is impregnated with a 50 wt% acetone solution of an epoxy resin (Epicoat 828 / BF ₃ · methanolamine complex manufactured by Shell Chemical Co., Ltd.), dried, laminated in a mold, and pressure-cured to strengthen it. A laminated body was obtained.

Furthermore, the amount of electricity for electrolytic oxidation was variously changed, and an alkaline electrolyte was used as the electrolytic solution instead of the acidic electrolyte.
The experiments of Comparative Examples 1 to 4 were performed, and the results are summarized in Table 1 together with Example 1.

[0045]

[Table 1]

From the above results, it is understood that the interlaminar shear strength (ILSS) does not exceed 9 kg / mm ² when the amount of electricity supplied is less than 50 c / g and when an alkaline electrolyte is used.

[0047]

According to the specific multi-step treatment method of the present invention, the anchor effect is increased and the oxygen-containing functional group is left on the surface of the carbon fiber, which is effective for improving the adhesiveness with the matrix resin. By mainly removing unnecessary components, the introduction of the oxygen-containing functional group can be efficiently performed.

Therefore, by using the carbon fiber obtained by the surface treatment method of the present invention, a composite material having excellent interlaminar shear strength can be provided.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location D06M 11/34 // D06M 101: 40

Claims (3)

[Claims] 1. A method of surface-treating carbon fibers by electrolytic oxidation, comprising the steps of: (a) first using carbon fibers as an anode and applying 50 c in an electrolytic solution.
Surface treatment of carbon fiber, characterized in that electrolytic oxidation treatment is carried out under severe conditions of passing an electricity amount of / g or more, and (b) then, after washing with water and drying, heat treatment is carried out in an oxygen-containing atmosphere. Method. 2. The method for surface treatment of carbon fiber according to claim 1, wherein the electrolyte used for electrolytic oxidation is a strong inorganic electrolyte such as nitric acid, sulfuric acid and phosphoric acid. 3. The heat treatment is carried out at 350 ° C. or lower and under reduced pressure and oxygen pressure of 1 Torr or higher, or 350 ° C.
The method for surface treatment of carbon fiber according to claim 1, wherein the method is performed below and under normal pressure in an atmosphere having an oxygen concentration of about 20% by volume or more.