JPWO2017170770A1 - Discontinuous carbon fiber surface treatment method - Google Patents

Abstract

  Providing an effective surface treatment method for discontinuous carbon fibers that have a relatively small degree of diameter reduction (volume reduction or weight reduction), can be implemented with simple operations, and have a low environmental impact. It is characterized by treating discontinuous carbon fibers with a treatment liquid in which ammonium persulfate is substantially the only active ingredient. An additional feature is that the treatment liquid is an aqueous ammonium persulfate solution and that the discontinuous carbon fibers are immersed in the treatment liquid.

Description

  The present invention relates to a surface treatment technique for improving tensile strength and adhesive strength with a resin or the like for discontinuous carbon fibers such as short carbon fibers and recycled carbon fibers.

Since carbon fiber has a remarkably high strength such as tensile strength and is light, it has been widely used as a reinforcing material for resins and the like, while maintaining the required strength of various articles including aircraft bodies and automobile bodies. It has greatly contributed to weight saving, energy saving and CO ₂ reduction based on it.

  Since carbon fibers are inferior in adhesion to a matrix such as a resin in an industrially manufactured state, for continuous carbon fibers, carbon fibers to which an oxidation potential is added via a transfer roller or the like are used as an electrolytic solution. In general, surface treatment is performed by electrolytic oxidation to increase the number of functional groups that contribute to adhesion and use as a reinforcing material such as a resin.

However, for discontinuous carbon fibers such as carbon short fibers, the electrolytic oxidation method as described above cannot be used effectively, and therefore a simple and effective surface treatment method is desired.
Also, carbon fiber recycled from carbon fiber reinforced composite articles (CFRP) should be surface treated before reuse because the adhesive functional groups as described above are reduced in the process of removing the matrix such as resin. Is desired (see Non-Patent Document 1). However, since the continuous fibers in the composite article are cut into discontinuous fibers during the recycling process, and have a wide distribution from long fibers to short fibers, the above-mentioned disadvantages are also observed for recycled carbon fibers. As with continuous carbon fiber, the electrolytic oxidation method cannot be used effectively.

As a surface treatment method for carbon fibers other than electrolytic oxidation, a method of treating by immersing in an aqueous solution of nitric acid, sulfuric acid or the like (see Non-Patent Document 2), a gas phase oxidation treatment using air, ozone, or the like in a gas. A method (see Patent Document 1), a treatment method using a treatment liquid in which various chemical solutions are combined, a treatment method in which a plurality of treatment steps are combined, and the like are known (see Patent Documents 2 and 3).
Further, in the past cases of surface regeneration treatment of recycled carbon fiber, chemical treatment with nitric acid is common.

JP-A-5-77239 Chinese Patent No. 103044680 Specification US Pat. No. 5,472,742

G. Jiang et al., Characterization of carbon fibers recycled from carbon fiber / epoxy resincomposites using supercritical n-propanol, Composites Science and Technology, Volume 69, Issue 2, Pages 192-198, 2009.2 Nan Feng et al. Surface modification of recycled carbon fiber and its reinforcement effect on nylon 6 composites: Mechanical properties, morphology and crystallization behaviors.Current Applied Physics.Volume 13, Issue 9, Pages2038-2050, 2013.11

In the course of studying the use of discontinuous carbon fibers, the present inventor also examined the above-described conventional techniques, but the following problems (1) to (5) were recognized.
(1) Surface treatment using strong acid such as nitric acid and sulfuric acid requires waste liquid treatment and safety measures, and is not preferable for industrial use.
(2) In the surface treatment using a strong acid such as nitric acid or sulfuric acid, relatively expensive carbon fibers are thinned, and the overall volume reduction or weight reduction (reduction in yield) is relatively large.
(3) Of the gas phase oxidation treatment, the air oxidation requires a long heat treatment, and the mechanical strength of the carbon fiber tends to be lowered.
(4) Among the gas phase oxidation treatments, ozone oxidation is inefficient in use of ozone and requires safety measures against ozone.
(5) Surface treatment with a treatment liquid in which various chemical solutions are combined and surface treatment in which a plurality of treatment steps are combined are complicated in terms of processing operations and steps, and are not practical from the viewpoint of cost.

  The present invention has been made to solve the above-mentioned problems recognized with respect to the prior art, and the degree of diameter reduction (volume reduction or weight reduction) of the carbon fiber to be treated is relatively small and simple. It is an object of the present invention to provide an effective surface treatment method for discontinuous carbon fibers that can be carried out by simple operation and has a low environmental load.

Under the above-mentioned problems, the present inventor has obtained the following findings (A) to (C) in the course of various test studies.
(A) The surface treatment of carbon fiber using ammonium persulfate as the only active ingredient is similar to the case of using nitric acid, in which oxygen functional groups are added by oxidation reaction and the fiber diameter is reduced (fiber surface is oxidized by oxidation). Although it has an action to scrape off), the degree of diameter reduction (volume reduction or weight reduction) of the carbon fiber is smaller than when using nitric acid, and under the processing conditions that the product yield is good, compared with using nitric acid. In addition, oxygen functional groups can be added more efficiently (a large amount of oxygen functional groups can be introduced).
(B) Carbon fiber surface treatment using ammonium persulfate as the only active ingredient can be easily carried out by dipping carbon fiber, etc., and requires advanced waste liquid treatment as in the case of using nitric acid. And not.
(C) Recycled carbon fiber has a defect that easily breaks in the recycling process, resulting in a decrease in strength, but the strength of carbon fiber is improved by removing surface defects in the process of surface oxidation with ammonium persulfate (strength regeneration). ) The effect is obtained.

  Although Patent Document 2 describes that ammonium persulfate is used as one of a plurality of components of a chemical solution for treating carbon fiber, ammonium persulfate is not used as the only active ingredient. Patent Document 2 describes that ammonium persulfate is used as one of a plurality of selective components of a pretreatment chemical solution that uses a COO-forming agent such as nitric acid, but COO such as nitric acid. Regardless of the treatment with the forming agent, ammonium persulfate is not the only active ingredient. In addition, since these Patent Documents 2 and 3 do not disclose the above knowledge at all, the present invention cannot be derived from those Patent Documents.

The present invention has been completed based on the findings as described above, and the present invention provides the following invention.
<1> A surface treatment method for discontinuous carbon fibers, characterized in that discontinuous carbon fibers are treated with a treatment liquid in which ammonium persulfate is substantially the only active ingredient.
<2> The surface treatment method for discontinuous carbon fibers according to <1>, wherein the treatment liquid is an aqueous ammonium persulfate solution.
<3> The surface treatment method for discontinuous carbon fibers according to <1> or <2>, wherein the discontinuous carbon fibers are immersed in the treatment liquid.
<4> The surface treatment method for discontinuous carbon fibers according to any one of <1> to <3>, wherein the discontinuous carbon fibers are recovered from the carbon fiber reinforced composite article.
<5> The surface treatment method for discontinuous carbon fibers according to any one of <1> to <4>, wherein the discontinuous carbon fibers are short fibers.
<6> The surface treatment method for discontinuous carbon fibers according to any one of <1> to <5>, wherein the concentration of the ammonium persulfate aqueous solution is 0.1 to 5 mol / L.
<7> A method for producing a carbon fiber composite article, wherein the carbon fiber composite article is produced using the discontinuous carbon fiber treated by the surface treatment method according to any one of <1> to <6>.

The present invention can include the following embodiments.
<8> The surface treatment method for discontinuous carbon fibers according to any one of <1> to <6>, wherein the treatment liquid is an aqueous solution containing substantially only ammonium persulfate.
<9> The discontinuous carbon fiber according to any one of <1> to <6> and <8>, which does not include a treatment using a COO-forming agent different from the treatment with the treatment liquid. Surface treatment method.
<10> The surface treatment method for discontinuous carbon fibers according to any one of <1> to <6>, <8>, and <9>, wherein the temperature of the treatment liquid is 10 to 95 ° C.
<11> The surface treatment method for discontinuous carbon fibers according to any one of <1> to <6> and <8> to <10>, wherein the treatment time is from 1 minute to 10 days.
<12> A method for producing a carbon fiber composite article, wherein the carbon fiber composite article is produced using the discontinuous carbon fiber treated by the surface treatment method according to any one of <8> to <11>.

Since the surface treatment method for discontinuous carbon fibers of the present invention only uses ammonium persulfate as the only active ingredient of the chemical solution for treatment, there is an environmental load that does not require high-level waste liquid treatment as in the case of using nitric acid. It is small and can be easily implemented by dipping carbon fiber or the like.
The surface treatment method for non-continuous carbon fiber of the present invention has an action of adding an oxygen functional group by an oxidation reaction and reducing the diameter of the fiber (scraping the fiber surface by oxidation) as in the case of using nitric acid. However, under the processing conditions in which the degree of diameter reduction (volume reduction or weight reduction) of the carbon fiber is smaller and the product yield is better than in the case of using nitric acid, the oxygen functional group is less than in the case of using nitric acid. The addition can be performed more efficiently.
The carbon fiber treated by the surface treatment method of the present invention not only improves the adhesion with a resin or the like, but also improves the tensile strength.

Carbon fiber subjected to surface treatment of 1-day oxidation and 5-day oxidation of Examples of the present invention, commercially available carbon fiber of comparative example (STS40, after removal of sizing agent), single fiber resin of unsurface-treated carbon fiber and The drawing which shows the interfacial shear strength (adhesion strength).

The surface treatment method for discontinuous carbon fibers of the present invention is characterized in that the discontinuous carbon fibers are treated with ammonium persulfate. According to the surface treatment method of the present invention, the treated carbon fiber has improved adhesion with a resin and the like, and also has improved tensile strength.
The discontinuous carbon fiber targeted by the present invention cannot be continuously processed using a transfer roll or the like, and is recovered from wastes of various short fibers and carbon fiber reinforced composite articles (CFRP). Recycled carbon fiber is included.

The surface treatment of the present invention is performed by bringing ammonium persulfate into contact with the surface of the discontinuous carbon fiber. The treatment liquid containing ammonium persulfate is preferably an aqueous solution containing only ammonium persulfate as an oxidizing agent, that is, an aqueous solution containing only ammonium persulfate as the only oxidizing agent active ingredient, but is inevitably contained. Impurities are allowed to be contained, and it is also allowed to contain other components as long as the action of ammonium persulfate is not significantly reduced. The surface treatment method of the present invention is a treatment using a COO-forming agent such as nitric acid in a separate step from the treatment with a treatment liquid in which ammonium persulfate is substantially the only active ingredient [the nitric acid described in Patent Document 3]. And other treatments using a COO treatment agent (CO ₂ former).

Although the concentration of ammonium persulfate in the treatment liquid is not limited, it is usually 0.1 to 5 mol / L, preferably 0.2 to 3 mol / L, more preferably 0.3 to 2 mol / L in view of economy and treatment efficiency.
The contact of the ammonium persulfate with the surface of the discontinuous carbon fiber is preferably performed by immersing the discontinuous carbon fiber in a treatment liquid containing ammonium persulfate, but is not limited thereto, and may be performed by spraying or coating. .

A higher temperature of the treatment liquid is preferable from the viewpoint of speeding up the treatment, but considering economy, a normal temperature or a temperature slightly higher than normal temperature is desirable. Therefore, it is usually 10 to 95 ° C, preferably 15 to 60 ° C, more preferably about 20 to 50 ° C.
The treatment time is appropriately determined in consideration of the concentration of ammonium persulfate, the treatment solution temperature, etc., but is usually 1 minute to 10 days, preferably 3 minutes to 5 days, more preferably 5 minutes to 2 days.

  EXAMPLES Hereinafter, although an Example etc. demonstrate this invention further more concretely, this invention is not limited at all by these Examples.

<Preparation of carbon fiber for surface treatment>
Commercially available carbon fiber (STS40 manufactured by Toho Tenax Co., Ltd.) was already surface-treated and had a sizing agent attached, so acetone cleaning for sizing removal and heat treatment at 800 ° C under a nitrogen atmosphere for functional group removal To obtain discontinuous carbon fibers to be surface-treated (unsurface-treated discontinuous carbon fibers).

<Surface treatment of carbon fiber>
Oxidation treatment by immersing the discontinuous carbon fiber prepared for surface treatment as described above in an aqueous solution of ammonium persulfate [(NH ₄ ) ₂ S ₂ O ₈ ] (20 ° C) with a concentration of 1 mol / L for 1 day or 5 days Went.
For comparison, the discontinuous carbon fibers to be surface-treated prepared as described above were subjected to an oxidation treatment by immersing them in concentrated nitric acid (20 ° C.) having a concentration of 60% by mass for 1 day or 5 days.

<Surface functional group analysis by XPS>
In order to confirm the treatment effect of the surface-treated carbon fiber as described above, X-ray photoelectron spectroscopy (XPS) analysis of oxygen, nitrogen, and sulfur was carried out as elements that could be introduced on the surface. . The analysis results are shown in Table 1 together with the analysis results of unsurface-treated discontinuous carbon fibers. O / C, N / C, and S / C indicate the number of oxygen (O), nitrogen (N), and sulfur (S) atoms as a ratio to the number of carbon (C) atoms.
As is clear from the analysis results in Table 1, the carbon concentration treated with ammonium persulfate in the examples significantly increased the oxygen concentration compared to the untreated surface. Moreover, in the case of 5-day oxidation with a longer treatment time than 1-day oxidation, the rate of oxygen increase was also greater. Although the same tendency was observed when treated with nitric acid of the comparative example, both the 1-day oxidation and the 5-day oxidation greatly exceeded the case of treatment with the ammonium persulfate of the example, and the addition of oxygen functional groups It can be said that it was more efficient than the case of treating with nitric acid in the comparative example. On the other hand, the nitrogen concentration was oxidized once a day and increased once, but decreased as the oxidation proceeded further. The change in nitrogen concentration was slight compared with the change in oxygen concentration. Further, since only a trace amount of elemental sulfur was detected, it was found that the processing agent ammonium persulfate hardly remained on the carbon surface.

<Tensile strength test and thinning (yield) investigation of surface-treated carbon fiber>
About 100 single fibers randomly sampled from the surface-treated carbon fiber bundle as described above, the diameter was measured using an optical microscope, and a tensile test was performed according to JISR760 using an autograph AG-Xplus manufactured by Shimadzu Corporation. . For comparison, the same diameter measurement and tensile test were performed on 100 single fibers collected from unsurface-treated carbon fibers. Each single fiber was fixed to a test mount, and it was attached along the load axis of the testing machine, and a diameter measurement and a tensile test were performed. The diameter was measured at three locations in the length direction of the single fiber, and the average was taken as the diameter of the single fiber, and the diameter (average) was calculated by averaging the diameters of 100 short fibers. Further, based on the diameter (average), the volume ratio V / V ₀ (%) with respect to the unsurface-treated single fiber was also calculated. The tensile speed was 1 mm / min. Table 2 shows the average tensile strength, coefficient of variation (CV), average diameter, and volume ratio V / V ₀ (%) as a result.

As is apparent from Table 2, the average tensile strength was greatly increased by the surface treatment using the ammonium persulfate of the example and the nitric acid of the comparative example as compared to the unsurface-treated product. Further, the value of the coefficient of variation CV representing the intensity variation is also reduced by the surface treatment. This is considered to be because the defects on the surface of the single fiber were removed by the surface treatment, and it was difficult to break.
On the other hand, by the surface treatment using ammonium persulfate of the example and nitric acid of the comparative example, the carbon fiber is reduced in diameter (volume reduction or weight reduction), but the degree of the reduction in the diameter (volume reduction or weight reduction). Is much smaller when the ammonium persulfate of the example is used than when the nitric acid of the comparative example is used. In consideration of the results of Table 1 above, the surface treatment of the carbon fiber using ammonium persulfate in the example adds an oxygen functional group as in the case of using nitric acid in the comparative example, and the diameter of the fiber is reduced. (The surface is removed by etching by oxidation), but the degree of diameter reduction or volume reduction (weight reduction) is smaller than when using the nitric acid of the comparative example, and the product yield is good It can be said that more oxygen functional groups can be added than when nitric acid is used.

<Interfacial bond strength test of surface treated carbon fiber>
A pinhole pull-out test was conducted on 10 daily oxidized single fibers and 10 5-day oxidized single fibers randomly collected from the surface-treated carbon fiber bundle as described above in order to examine the interfacial adhesion strength. For comparison, similar pinhole extraction was performed on 10 single fibers collected after removing the sizing agent from commercially available carbon fibers (STS40 manufactured by Toho Tenax Co., Ltd.) and 10 single fibers collected from unsurface-treated carbon fibers. A test was conducted. In the pinhole pull-out test, a resin is melted into a pinhole opened in a thin metal plate, a single fiber is passed through the pinhole, the single fiber and the resin are bonded, and then the interfacial shear strength when pulling the single fiber ( (Adhesive strength) was measured. The result is shown in FIG.
As is clear from FIG. 1, the surface-treated carbon fiber has an improved interfacial shear strength compared to the untreated surface, and is equal to or higher than the commercially available surface-treated carbon fiber. Interfacial shear strength was indicated.

According to the carbon fiber surface treatment method of the present invention, discontinuous fibers such as short fibers, which could not be treated by the conventional continuous carbon fiber surface treatment method, can be easily treated with good product yield to obtain a tensile strength or a resin. Adhesive strength can be improved. Therefore, it provides surface treatment technology that is indispensable in the recycling of carbon fibers recycled from carbon fiber reinforced composite materials that will be disposed of in large quantities in the future, and societal needs are extremely large.

Claims (7)

  A surface treatment method for discontinuous carbon fibers, characterized in that discontinuous carbon fibers are treated with a treatment liquid in which ammonium persulfate is substantially the only active ingredient.   The surface treatment method for discontinuous carbon fibers according to claim 1, wherein the treatment liquid is an aqueous ammonium persulfate solution.   The surface treatment method for discontinuous carbon fibers according to claim 1 or 2, wherein discontinuous carbon fibers are immersed in the treatment liquid.   The surface treatment method for discontinuous carbon fibers according to any one of claims 1 to 3, wherein the discontinuous carbon fibers are recovered from the carbon fiber reinforced composite article.   The surface treatment method for discontinuous carbon fibers according to any one of claims 1 to 4, wherein the discontinuous carbon fibers are short fibers.   The surface treatment method for discontinuous carbon fibers according to any one of claims 1 to 5, wherein the concentration of the ammonium persulfate aqueous solution is 0.1 to 5 mol / L. The manufacturing method of the carbon fiber composite article which manufactures a carbon fiber composite article using the discontinuous carbon fiber processed with the surface treatment method of any one of Claims 1-6.

Images