JPS61194271A - Surface treatment of carbon fiber - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for surface treatment of carbon fibers used as carbon fiber reinforced composite materials.

<Prior art and its problems> Carbon fiber, which is lightweight, strong, and has a high modulus of elasticity, is attracting attention as a material that enhances the performance of composite materials with matrix materials such as plastics, metals, carbon, or ceramics. However, since the activity of the fiber surface is low and the wettability is poor, in order to effectively utilize these characteristics and exhibit a higher reinforcing effect, it is necessary to take measures to strengthen the adhesion or bond between the matrix and the reinforcing material. It is essential to do so. Therefore, various improvement methods such as dry methods and wet methods have been proposed. An example is described below.

(1) Patent No. 45-1287, which is characterized by a dry oxidation method that causes slow oxidation in the air at temperatures below 200°C. Carbon fibers may self-combust at temperatures close to 200"C. There are many cases, and reproducibility is poor.

(2) The wet oxidation method of Japanese Patent Publication No. 47-27000 is characterized by oxidizing carbon fibers by immersing them in an acidic solution containing an oxidizing agent based on chromic acid or manganic acid.The zero reaction is easy to control and reproducible. Although this method is good, it takes a long time for one reaction, and the processing steps are complicated because washing with water and drying are successive after the reaction, and there are many problems in practical use.

Furthermore, if the treatment lasts for a long time, the strength of the fiber itself will be significantly reduced due to the formation of excessive corrosion pores in areas where the reaction progresses intensively. As mentioned above, each treatment method has its advantages and disadvantages, and the current situation is that none of them can reach a practical level.

<Object of the Invention> The present invention improves the drawbacks of such conventional methods, and provides high-performance carbon fibers whose surfaces are activated in order to improve the adhesion of reinforcing materials, particularly to the matrix, in a short time using a simple process. The purpose is to provide a method to obtain.

<Structure of the invention> The above objects are achieved by the present invention as described below.

That is, the first Q aspect of the present invention is a method for surface treatment of carbon fibers, characterized in that the carbon fibers are immersed in an alkaline hydrogen peroxide solution to undergo oxidation treatment.

A second invention of the present invention is a method for surface treatment of carbon fibers, characterized in that carbon fibers are immersed in an alkaline hydrogen peroxide solution and subjected to oxidation treatment while applying ultrasonic vibrations.

A third aspect of the present invention is a method for surface treatment of carbon fibers, characterized in that the carbon fibers are immersed in an alkaline hydrogen peroxide solution to undergo an oxidation treatment, and then subjected to a heat treatment at a relatively low temperature.

A fourth aspect of the present invention is that the surface of the carbon fiber is immersed in an alkaline hydrogen peroxide solution, subjected to an oxidation treatment while applying ultrasonic vibration, and then subjected to a heat treatment at a relatively low temperature. This is a processing method.

Hereinafter, the method for surface treatment of carbon a fibers of the present invention will be explained in more detail.

As the carbonization temperature of carbon fibers increases, the alignment of graphite crystals in the fiber axis direction is improved, and the mechanical properties of the carbon fibers are improved.

On the other hand, by applying high temperature treatment.

This reduces the number of active sites on the surface of Iam, which causes a decrease in adhesion to the matrix during composite formation.

In other words, carbon fiber has many excellent properties as a composite material, but the fiber surface has poor wettability.
It does not mix well with the matrix, and this property is the only drawback that weakens the adhesive strength.

Therefore, various treatment methods or treatment agents intended to improve these properties have been proposed. In view of this fact, the present inventors also place particular emphasis on simplifying the treatment process and shortening the treatment time. After careful consideration. In other words, either perform the oxidation treatment in a short time without using various oxidizing agents (especially metals and metal salt compounds) that adhere to and remain on the surface of the carbon tang, or perform the heat treatment and low-temperature treatment. We have carefully considered this issue. As a result, we have proposed a surface treatment method that can fully achieve this objective.

The treatment method of the present invention combines immersion treatment as a primary treatment and heat treatment as a secondary treatment. That is,
Forming oxides of acidic functional groups such as carboxyl groups, carbonyl groups, phenolic hydroxyl groups, etc. on the fiber surface in liquid phase oxidation as a primary treatment, and increasing the surface area of the fibers in a heat treatment as a secondary treatment. The aim is to modify the fiber surface to improve the adhesive effect.

Specifically, as a primary treatment, the carbon fibers are immersed for a short time in an alkaline pure hydrogen peroxide solution to undergo oxidation treatment, and then charged into a heating furnace adjusted to a predetermined temperature as necessary.
Heat and oxidize in an air atmosphere.

Due to the presence of impurities, hydrogen peroxide undergoes its own decomposition reaction, reducing its effectiveness. And a.

It is more effective if the alkaline compound to be added is easily thermally decomposed and preferably the decomposition product acts as an oxidizing agent. This is because, unlike alkali metals, it must not be easily decomposed and remain on the fiber surface, thereby interfering with the adhesion to the matrix during composite formation.

The alkaline solution may be prepared in any way as long as it can be made weakly alkaline (about pH 7 to 9) by injecting NH3 gas into pure hydrogen peroxide in advance or adding a dilute ammonia solution, or by other methods. The concentration of pure hydrogen peroxide is 50 parts or more, preferably 80 parts or more to obtain better results. The liquid temperature is preferably 50 to 90°C, preferably 70°C or higher.

The immersion treatment time is determined by the concentration of hydrogen peroxide, the liquid temperature, the bulk density of the carbon fiber, etc.
m″ or higher, the liquid temperature is set to 50°C regardless of the hydrogen peroxide concentration.
It is necessary to maintain the above level.

Furthermore, when the liquid temperature exceeds 90°C, the decomposition and volatilization of hydrogen peroxide and alkali components become more significant.

This is not preferable because it causes a decrease in the oxidation effect.

In the treatment method of the present invention, the total amount of functional groups formed on the surface of the oxidized fiber (according to the conductivity titration method) is 3
For example, if the liquid concentration, temperature, and fiber bulk density are fixed, the lower limit of the total amount of functional groups is 10' mol/g or more, and this value is the end point of the reaction. This can be achieved in a treatment time of 120 minutes, preferably 30 to 60 minutes.

In addition, the treatment method of the present invention further activates the oxidation reaction by contacting hydrogen peroxide with carbon fibers and applying vibrations from an ultrasonic generator or the like to the immersion liquid and the carbon fibers passing through the liquid. This effectively utilizes the properties of hydrogen oxide. Application of ultrasonic vibration is 0.2 to 0.4
It is preferable to set it to about W/cm2.

Therefore, the heat treatment in the secondary treatment process can be applied for both purposes, such as a treatment that also serves as drying of the carbon fibers when the target can be achieved by dipping treatment, and a replenishment treatment when the oxidation reaction is not carried out sufficiently or uniformly. , 100-35
It is sufficient to treat in a temperature range of 0°C for 10 to 120 minutes. The reason for using this temperature range and treatment time is that, as shown in the examples, it is possible to achieve the desired surface treatment effect.

Furthermore, it has been found that applying minute vibrations to the fiber bundle in the dipping solution promotes opening of the fiber bundle and penetration of the dipping fluid, which is highly effective in making the oxidation reaction uniform.

Ammonia adhering to the oxidized fiber is decomposed during the heat treatment and contributes to the secondary oxidation reaction of the fiber as an S-forming gas, so there is no need to remove it by washing with water or the like.

However, even if the method for surface treatment of carbon fibers according to the present invention is applied, it is possible to odor a composite material using the fibers without any adverse effect on the properties of the carbon fibers, and it is a highly practical method. .

Practical example The effects of the present invention will be explained by examples.

(Example 1) Pitch-based carbon fiber with an average fiber diameter of 1O- was cut into 100+am pieces, and 50g of the cut was added to pure hydrogen peroxide (a degree of 32 wlv
%) to ammonia alkaline (pH 8) 5
00all and subjected to ultrasonic vibration (0,3
1+l/c+s2) for 1°, 30 minutes, 9
After holding for 0 minutes, the fibers were washed with water and dried, and the total amount of oxides, that is, acidic functional groups formed on the fiber surface was determined using a conductivity titration method. The results are shown in Table 1.

By increasing the immersion time, the oxidation reaction tends to proceed not only on the fiber surface but also inside the fiber, but electron microscopic observation shows that no particular unevenness of the oxidation reaction is observed, and even when the immersion time is 10 It can be seen that the target concentration of functional groups can be easily secured by setting E from 1 to 5.

(Example 2) Carbon fibers treated in the same manner as in Example 1 were charged into a tube furnace without washing with water, and air was blown into the tube furnace for 2 hours. Oxidation treatment was performed at 300° C. for 10 minutes, 60 minutes, and 120 minutes while supplying at a flow rate of 1/win.

The specific surface area of the obtained fibers was measured by a nitrogen adsorption method. The results are shown in Table 2.

As a result of electron microscopic observation of the sample treated for 120 minutes, it was observed that etch pits (corrosion holes) had occurred in several places. It was found that there is a risk of damage to the

However, in a treatment lasting less than 120 minutes, there is no need to be concerned about this problem, and the effect on uniformity of the oxidation reaction is clear, especially compared to static immersion.

(Example 3) Using carbon fibers prepared in the same manner as in Examples 1 and 2, an epoxy resin (Epicoat 82B manufactured by Shell Petrochemical Co., Ltd.) was added as a matrix to a resin solution in methyl ethyl ketone at a volume fraction. The test piece (width 12 x length 20 x thickness 2.5 m/m) was obtained by impregnating it uniformly at a ratio of 0.5, drying it at 100°C for 30 minutes, and then pressing it while heating it to 180°C. Ta.

Using this composite, the interlaminar shear strength in the fiber axis direction was measured according to the ASTM D-2344 method, and Table 3
The results shown in Table 4 (for Example 1) and Table 4 (for Example 2) were obtained.

In addition, similarly carbon fiber was dissolved in nitric acid aqueous solution (68% concentration) for 10 minutes.
The test results of a composite treated in the same manner as in the example, which was immersed at 0° C. for 4 hours, are shown as a comparative example.

However, by using carbon fibers subjected to the surface treatment method according to the present invention as reinforcing materials, the adhesion between the reinforcing materials and the matrix is improved, and the reinforcing effect, that is, the performance of composite materials using carbon fibers as reinforcing materials is improved. It is clear that it can be measured.

<Effects of the Invention> According to the treatment method of the present invention, the surface of carbon fiber is activated and at the same time treated to increase the specific surface area, thereby improving the surface characteristics, which is the only drawback of carbon fiber. This can significantly improve the adhesion with the matrix during composite formation.

Improved adhesive strength will improve the performance of composite materials and expand the use of carbon fiber as a reinforcing base material.

Table 1 Functional group concentration of surface treatment m Miscellaneous (1) Primary treatment time by the method of the present invention 10 minutes (2)
Same as above 30 minutes (3) Same as above
90 minutes (4) Comparative example using nitric acid oxidation method Table 2 Specific surface area of surface-treated fibers (1) Secondary treatment time of fibers primarily treated by the method of the present invention 10 minutes (2)
Same as above 30 minutes (3)
Same as above 90
(0 Comparative example by nitric acid oxidation method Table 3 Interlaminar shear strength of composite (1) Example 1 by the method of the present invention (1) Equivalent (2)
Same as above (2) equivalent (3)
Same h (3) equivalent (4) Comparative example by nitric acid oxidation method Table 4 Interlaminar shear strength of composite (1) Example 2 (1) equivalent (2) by the method of the present invention
Same as above (2) equivalent (3)
Same as above (3) equivalent (4)
Comparative example using nitric acid oxidation method

Claims (4)

[Claims] (1) A method for surface treatment of carbon fibers, which comprises immersing carbon fibers in an alkaline hydrogen peroxide solution to perform oxidation treatment. (2) A method for surface treatment of carbon fibers, which comprises immersing carbon fibers in an alkaline hydrogen peroxide solution and subjecting them to oxidation treatment while applying ultrasonic vibrations. (3) A method for surface treatment of carbon fibers, which comprises immersing the carbon fibers in an alkaline hydrogen peroxide solution to perform an oxidation treatment, and then subjecting the carbon fibers to a relatively low-temperature heat treatment. (4) A method for surface treatment of carbon fibers, which comprises immersing carbon fibers in an alkaline hydrogen peroxide solution, subjecting them to oxidation treatment while applying ultrasonic vibrations, and then subjecting them to heat treatment at a relatively low temperature.