JP2592294B2 - Post-processing method of carbon fiber - Google Patents

Description

The present invention relates to a novel post-treatment method for carbon fibers.

[Prior art] Composite materials using carbon fiber as a reinforcing material are lightweight and strong,
Due to its excellent elastic modulus, its use is being developed in a wide range of fields as a component of sports and leisure goods or as an aerospace equipment. Conventionally, carbon fibers that have been used as a reinforcing material for composite materials do not always have sufficient adhesiveness to the matrix resin, so to activate the surface, chemical oxidation treatment, gas phase oxidation treatment, electrolytic oxidation treatment, etc. Surface treatment methods have been employed. Among them, the electrolytic oxidation treatment method is a practical surface treatment method from the viewpoint of good operability, easy reaction control and the like.

Conventionally, various electrolytes have been used as the electrolytic oxidation treatment method. On the other hand, after the electrolytic oxidation treatment, oxidized impurities are present on the carbon fiber surface and need to be removed by washing. If the washing after the electrolytic oxidation treatment is performed with warm water, it is necessary to perform the treatment for a long time, and a method that can perform the treatment in a short time and does not impair the performance of the carbon fiber has not yet been found. .

In cleaning, the use of ultrasonic waves has been improved. However, if ultrasonic waves are used for objects having low elongation, such as carbon fibers, the carbon fibers themselves may be damaged. JP-A-62-149967 discloses that a carbon fiber is subjected to an electrolytic treatment in an aqueous solution containing an acidic electrolyte, followed by ultrasonic cleaning, followed by 400 to 9
Although it is shown that the inactivation treatment is performed at 00 ° C., it is insufficient from the viewpoint of the development of interfacial adhesion, and furthermore, the combination of electrolyte and ultrasonic cleaning in electrolytic oxidation, and ultrasonic cleaning It was necessary to consider appropriate conditions.

[Problems to be solved by the invention]

The purpose of the present invention is the composite properties (especially interfacial adhesion)
The object of the present invention is to produce a carbon fiber which has a good expression and a high tensile strength.

The present invention provides a novel surface treatment method for that purpose.

[Means for solving the problem]

The gist of the present invention is that, using carbon fiber as an anode, the ammonium ion concentration is 0.2 to 4.0 mol /,
Electrolytic oxidation treatment in an aqueous solution having a pH of 7 or more, followed by ultrasonic treatment in another water at a frequency of 20 kHz or more at a strength that satisfies the following formula to remove fragile portions on the carbon fiber surface. In the post-treatment method of the fiber.

The ammonium salt used in the present invention is not particularly limited,
For example, ammonium carbamate, ammonium carbonate,
Ammonium hydrogen carbonate or the like can be used alone or as a mixture of two or more. Further, an alkali metal hydroxide such as NaOH or KOH may be used in combination to increase the conductivity of the electrolytic oxidation treatment solution.

On the surface of the carbon dioxide fiber, tar
Mist components and their oxides oxidized in the surface treatment process adhere or deposit in microvoids between crystals to form a fragile layer or portion, which is generally weakly bonded to the fiber matrix, It is in a state of easy peeling.

In order to improve the residual compressive strength (CAI) of the composite, it is important to minimize the exfoliation inside the composite caused by the impact, so that the carbon fiber surface layer is oxidized and fragile at the same time. Based on the recognition that it was indispensable to remove the part, the result of the examination was that carbon fiber was used as the anode and the ammonium ion concentration was reduced to 0.
It was found that by performing electrolytic oxidation treatment in an aqueous solution having a pH of 2 to 4.0 mol / and a pH of 7 or more, the carbon fiber substrate was oxidized and the fragile portion of the surface layer was removed at the same time.

Before electrolytic oxidation treatment to remove the fragile part of this surface layer,
In order to introduce as much oxygen as possible into the carbon fiber surface, electrolytic oxidation treatment may be performed in an acidic electrolyte.

However, the remaining oxidized impurities on the carbon fiber surface are still attached to the surface by the above treatment. The present invention further removes residual oxidized impurities by applying ultrasonic treatment.

If the ultrasonic intensity is low, the remaining oxidized impurities on the carbon fiber surface cannot be removed, and if the ultrasonic intensity is too high, the carbon fiber is damaged and fluff is generated. The present inventors have further found that this phenomenon is affected by the frequency of the ultrasonic waves and the processing time of the ultrasonic waves. In other words, the lower the frequency of the ultrasonic wave, the easier it is to remove the oxidized impurities, but the more easily the carbon fiber is damaged, and the ultrasonic intensity cannot be increased much. On the other hand, if the frequency of the ultrasonic wave is high, the carbon fiber is hardly damaged and fluff is unlikely to occur, but it is difficult to remove oxidized impurities, and the ultrasonic intensity cannot be reduced too much.

When the ultrasonic treatment time is increased, oxidized impurities are easily removed, but the carbon fibers are easily damaged.

That is, the ultrasonic intensity If it is smaller, the removal of residual oxide impurities is insufficient, If it is larger, a part of the carbon fiber is cut, and fluff is generated.

At this time, the higher the temperature of the water to be treated, the better the removal of residual oxidized impurities. Preferably, the treatment is performed at a temperature of 60 ° C. or higher.

It is necessary to perform ultrasonic treatment so that residual oxidized impurities on the carbon fiber surface have an absorbance at 230 nm of 0.2 or less. When the value is larger than 0.2, the remaining oxidized impurities are not sufficiently removed from the surface of the carbon fiber, and the desired performance cannot be obtained.

The treatment of the present invention greatly improves the tensile strength. Although the reason for this is not obvious, it is considered that the treatment of the present invention alleviates the defective portion of the surface layer, and that the strength of the carbon fiber is dramatically improved.

The tensile strength of carbon fiber not only improves the tensile strength of the composite, but also improves the CAI of the composite. As a result, the CAI of the composite is greatly improved by the removal of the fragile portion of the surface layer and the mitigation of the surface defect portion by the treatment of the present invention.

Further, CAI greatly depends on the crystal structure of the surface layer of carbon fiber. If the graphite surface greatly expands in the surface layer of the carbon fiber, surface oxidation hardly occurs in the post-treatment step. Even if the surface is oxidized to a certain level, the oxidized part is localized only on the periphery of the wide graphite surface, and the graphite surface, which has poor interaction with the matrix resin, will cover the surface a lot. However, the CAI does not improve because the surface oxidation effect of the post-treatment does not easily appear. In order to improve the CAI to a practical value by the treatment of the present invention with a small graphite surface, it is preferable that the target carbon fiber has an elastic modulus of 40 t / mm ² or less.

〔Example〕

 Hereinafter, the present invention will be described specifically with reference to examples.

Residual compressive strength after impact (CA
"I)" was measured by the following method in accordance with NASA RP1092.

50 parts of bis (4-maleimidophenyl) methane is added to 2,2-
450 parts of bis (4-cyana-tophenyl) propane and 120 ° C
For 20 minutes to obtain a pre-reacted product. Add Epicoat 834 (Yuka Kasei Co., Ltd., epoxy equivalent: 250) to this
-Diaminodiphenyl sulfone was reacted with an amino group / epoxy group at an equivalent ratio of 1/4 at 160 ° C for 4 hours, and epicoat 80
Add 2000 parts of the pre-reaction product diluted to 80% with 7 (Yuika Ciel, epoxy equivalent: 170), mix uniformly at 70 ° C for 30 minutes, and further add N- (3,4-dichlorophenyl) -N ′, N′−
100 parts of dimethyl urea, 1 part of dicumyl peroxide and fine powder of silicon oxide Aerosil 380 (manufactured by Nippon Aerosil Co., Ltd.) 2
5 parts were added, and the resin composition obtained by mixing uniformly at 70 ° C. for 1 hour was formed into a film by a hot melt method, and a unidirectional prepreg was prepared using carbon fibers. [+ 45 ° / 0 ° / −45 °
{/ + 90}] 4S quasi-isotropic lamination and curing at 180 ° C for 2 hours to prepare test specimens of dimensions 4 x 6 x 0.25 inches.
After fixing it on a steel table with a hole of × 5 inches,
A 4.9 kg weight having a nose of 0.5 inch R was dropped at the center thereof, and an impact of 1500 lb-in was applied per 1 inch of the plate thickness. Then, the plate was subjected to a compression test to obtain "CAI". "Strand strength, strand elastic modulus" is JIS-R76
Measured according to 01.

"Remaining oxidized impurities" on the carbon fiber surface, which are indicated by the measured values of absorbance, were measured by the following method. 1 g of carbon fiber is immersed in 10 g of distilled water, and heated to 80 ° C with 45 kHz ultrasonic waves.
0.2 W / cm ² , treated for 10 minutes, oxidized impurities were dispersed or dissolved in distilled water from the carbon fiber surface, the supernatant was placed in a 1 cm quartz cell, and the absorbance was measured at 230 nm with a UV spectrum meter. . The control solution was distilled water.

Example 1 98% by weight of acrylonitrile, 1% by weight of methyl acrylate,
A polymer having a composition of 1% by weight of methacrylic acid is dissolved in dimethylformamide so as to have a solid concentration of 26% by weight, and a dove is prepared. The film was stretched, washed and dried, and further stretched 1.3 times at 170 ° C. to obtain a precursor having a filament number of 9000 and a fineness of 0.8 denier.

The precursor was passed through a hot-air circulation type flame stabilization furnace at 220 to 260 ° C. for 60 minutes, and subjected to a flame stabilization treatment while elongating by 15%.

Next, the oxidized fiber was passed through a first carbonization furnace having a temperature gradient of 300 to 600 ° C. in a pure N ₂ gas stream while elongating 8%, and further a second carbon having a maximum temperature of 1300 ° C. in the same atmosphere. Heat treatment was performed for 2 minutes under a tension of 400 mg / d in a gasifier to obtain carbon fibers.

Then, it is run in a 5 wt% ammonium bicarbonate aqueous solution (ammonium ion concentration: 0.6 mol /), and the carbon fiber is used as the anode, and the electricity is passed between the counter electrode so that the amount of electricity becomes 100 coulombs per gram of the carbon fiber to be treated. Then, sonication was performed in water at a temperature of 90 ° C. at a frequency of 38 kHz and an intensity of 0.46 W / cm ² for 2 minutes.

The strand strength of this carbon fiber is 650 kg / mm ² and the elastic modulus is 3
The CAI of the composite was ² kg / mm ² and ² t / mm ² . The residual oxidized impurity indicated by the absorbance was 0.17.

Example 2 A carbon fiber obtained in the same manner as in Example 1 was mixed with a 5 wt% aqueous solution of ammonium heavy carbon (ammonium ion concentration: 0.6 mol
Electrolytic oxidation was performed at 150 coulombs / g in l /), and then sonication was performed in water at a temperature of 90 ° C. for 1 minute at a frequency of 38 kHz and an intensity of 1.0 W / cm ² .

The strand strength of this carbon fiber is 640 kg / mm ² and the elastic modulus is 3
The CAI of the composite was 37 kg / mm ² , ² t / mm ² . The residual oxidized impurity indicated by the absorbance was 0.15.

Example 3 The same processing as in Example 1 was performed by changing the frequency of the ultrasonic treatment to 27 kHz.

The strand strength of this carbon fiber is 650 kg / mm ² and the elastic modulus is 3
2.4t / mm ^2, CAI of Konpojitsuto was found to be 38kg / mm ^2. The residual oxidized impurity indicated by the absorbance was 0.10.

Comparative Example 1 A carbon fiber obtained in the same manner as in Example 1 was phosphoric acid 5%
Perform electrolytic oxidation at 20 coulomb / g in aqueous solution,
The washing process was performed for 15 minutes.

The strand strength of this carbon fiber was 581 kg / mm ² , the elastic modulus was 31 t / mm ² , and the CAI of the composite was 24.5 kg / mm ² .
The residual oxidized impurity indicated by the absorbance was 0.43.

Comparative Example 2 The same treatment as in Example 1 was performed except for the treatment with ultrasonic waves, and then a washing treatment was performed at 90 ° C. for 15 minutes with hot water.

The strand strength of this carbon fiber was 590 kg / mm ² , the elastic modulus was 31.2 t / mm ² , and the CAI of the composite was 35 kg / mm ² .
The residual oxidized impurity indicated by the absorbance was 0.21.

Examples 4 to 7 and Comparative Examples 3 and 4 The same procedure as in Example 1 was carried out except that the ultrasonic treatment was carried out at a frequency of 38 kHz, a treatment time of 1 minute, and a temperature of 90 ° C. as shown in Table 1. The results shown in Table 1 were obtained.

Examples 8 to 12 and Comparative Examples 5 and 6 In the same manner as in Example 1, except that the ultrasonic treatment was performed at a frequency of 27 kHz, a treatment time of 1 minute, a temperature of 90 ° C., and changes as shown in Table 2. The results shown in Table 2 were obtained.

Examples 13 to 18 and Comparative Examples 7 and 8 In the same manner as in Example 1, except that the ultrasonic treatment was performed at a frequency of 45 kHz, a processing time of 1 minute, and a temperature of 90 ° C., as shown in Table 3. The results shown in Table 3 were obtained.

Examples 19 and 20 In the same manner as in Example 1 except that the ultrasonic treatment was performed at a frequency of 100 kHz, a temperature of 90 ° C., and the treatment was performed as shown in Table 4, the results shown in Table 4 were obtained. Obtained.

Examples 21 to 25 and Comparative Examples 9 and 10 In the same manner as in Example 1, except that the ultrasonic treatment was performed at a frequency of 38 kHz, an ultrasonic intensity of 0.5 W / cm ² and a temperature of 90 ° C., as shown in Table 5. Then, the results shown in Table 5 were obtained. However, in Comparative Example 10, a carbon fiber was wound around a plastic bobbin, and the bobbin was subjected to ultrasonic treatment.

Examples 26 and 27 and Comparative Example 11 The carbon fibers obtained in the same manner as in Example 1
Heat treatment was performed for 2 minutes under a tension of 400 mg / d in a third carbonization furnace having a maximum temperature of 00 ° C. The carbon fiber thus obtained is subjected to electrolytic oxidation treatment in a 5% aqueous solution of phosphoric acid so that the amount of electricity becomes 25 coulombs per 1 g of carbon fiber, and then 100 coulombs per 1 g of carbon fiber in a 5% aqueous solution of ammonium bicarbonate. The amount was subjected to electrolytic oxidation treatment. This carbon fiber was subjected to an ultrasonic treatment as shown in Table 6 to obtain a carbon fiber as shown in Table 6.

Claims (4)

(57) [Claims] 1. An electrolytic oxidation treatment in an aqueous solution having an ammonium ion concentration of 0.2 to 4.0 mol / and a pH of 7 or more using a carbon fiber as an anode, and further in another water at a frequency of 20 kHz.
As described above, a post-treatment method for carbon fiber, which comprises removing the brittle portion on the surface of the carbon fiber by ultrasonic treatment at a strength satisfying the following formula. (F is frequency (kHz), T is processing time (min), provided that T>
0.1) 2. The method according to claim 1, wherein the sonication is performed at a temperature of 60 ° C. or higher. 3. The residual oxidizing impurities of the treated carbon fiber are:
The method according to claim 1, wherein the absorbance is 0.2 or less. 4. The carbon fiber subjected to the treatment has an elastic modulus of 40 t / mm.
^2. The method according to claim 1, wherein the number is ² or less.