JPH0651945B2 - Carbon fiber sizing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for sizing carbon fiber. More specifically, it is a sizing method capable of improving the sizing properties of the carbon fibers, improving the physical properties of the composite material, particularly the interlaminar shear strength thereof, and improving the uniformity of the physical properties.

[Conventional technology]

Due to its excellent properties (specific strength, specific elastic modulus), carbon fiber is used as various composite materials in aviation, space materials, industrial materials, sports, leisure goods and the like. However, since this carbon fiber is rigid and brittle, and has poor scratch resistance and bending resistance, fluff and yarn breakage are likely to occur in the manufacturing and processing steps. Therefore, a sizing agent that alleviates such characteristics and further improves the adhesion to the matrix resin is generally used. Conventionally, carbon fibers are immersed, dropped,
Alternatively, the sizing treatment is performed under atmospheric pressure under atmospheric pressure by a contact method using a roller or the like. These conventional sizing methods are focused on the prevention of fluff generation, yarn breakage, etc., as disclosed in, for example, JP-A-59-36763. Moreover, since the conventional sizing method is performed under atmospheric pressure under atmospheric pressure, it is said that the adhesion state of the sizing agent changes due to environmental changes such as temperature and humidity changes, resulting in uneven adhesion between the matrix resin and the carbon fibers. Conceivable. This is judged from the magnitude of the fluctuation of the interfacial shear strength of the commercialized carbon fiber shown in Table 4, and from this fact, the carbon fiber composite material is still unsatisfactory in terms of quality.

[Problems to be solved by the invention]

The present inventors have improved various physical properties of the carbon fiber composite material,
In addition, with a focus on improving the quality, the sizing method of the carbon fiber was examined by focusing on the adhesiveness between the carbon fiber and the matrix resin and the interfacial shear strength.

According to the present invention, the sizing property of the carbon fiber is improved, the interfacial shear strength is improved, and the fluctuation of the strength is reduced, so that the carbon fiber having high adhesion and high quality can be obtained. As a result of further diligent examination, generally, a remarkable effect is exhibited even on carbon fibers having surface characteristics with poor adhesion to a matrix resin, for example, carbon fibers having a low surface polarity and carbon fibers having a large surface area having pores on the surface. The inventors have found that this is a sizing method and have reached the present invention.

[Means for solving problems]

The carbon fiber used in the present invention is a carbonized yarn obtained by firing polyacrylonitrile-based fiber flame-proofed by a known method at about 1000 to 1800 ° C. in an inert gas atmosphere,
The carbonized yarn is further heated in an inert atmosphere at 1800-3000.
The term refers to graphitized yarn obtained by heating to ℃, carbonized yarn produced from petroleum or coal yarn pitch as a raw material, graphitized yarn, and the like. Further included are carbonized yarns and graphitized yarns which have been surface-treated for the purpose of improving the adhesiveness with the matrix resin. Here, the surface-treated yarn is, for example, a liquid-phase oxidized surface-treated yarn that has been subjected to a surface treatment by heating the carbon fiber in an aqueous solution of an oxidant, or directly to the carbon fiber through an electrode roller in an electrolytic solution. The term refers to an electrolytically oxidized surface-treated yarn that has been electrically energized and electrolytically oxidized, or a vapor-phase oxidized surface-treated yarn that has been heated and oxidized in an oxidizing gas atmosphere such as air.

The gist of the present invention resides in a method for sizing carbon fibers, which is characterized in that these carbon fibers are allowed to stay under reduced pressure for at least 2 seconds and then subjected to a sizing treatment without being substantially exposed to the atmosphere.

Hereinafter, the present invention will be described in more detail.

According to the method of the present invention, that is, the carbon fiber is introduced from normal pressure to reduced pressure at room temperature or at elevated temperature, and the carbon fiber is allowed to stay under reduced pressure for at least 2 seconds, and then subjected to sizing treatment without being substantially exposed to the atmosphere. It is a thing. In this case, the decompression degree is 700 to
It is preferably rr or less, and particularly preferably 500 torr or less. Also,
In order to obtain a further effect within a short time, it is preferable to degas the carbon fiber under as high a vacuum as possible and under heating, and then perform the sizing treatment without substantially contacting the atmosphere. In the method of the present invention, for example, carbon fiber is placed in a box, sealed, led from normal pressure to reduced pressure using a vacuum pump, allowed to stay in the atmosphere for at least 2 seconds, and a sizing agent introduction valve for the box is introduced. Open and suddenly send the sizing agent into the depressurized box,
The carbon fiber surface is subjected to a sizing treatment without being substantially exposed to the atmosphere.

When the pressure is normal pressure or the residence time under reduced pressure is shorter than 2 seconds, the interfacial shear strength becomes insufficient. Also, after this decompression treatment and before applying the sizing agent,
When it comes into contact with the atmosphere, the gas and moisture in the atmosphere are again adsorbed by the carbon fibers and the adhesion of the sizing agent is hindered.

Examples of sizing agents useful in the method of the present invention include:
A treatment agent prepared by dissolving a bisphenol A glycidyl ether type epoxy resin (Epicote 828 or Epicoat 1001 manufactured by Yuka Shell Epoxy Co., Ltd.) and a polyfunctional glycidyl amine type epoxy resin (TETRAD-X manufactured by Mitsubishi Tetsu Chemical Co., Ltd.) in methyl ethyl ketone. However, it is not limited to these. Generally, the sizing treatment agent should be adjusted to a concentration such that the active ingredient adheres to the carbon fiber in an amount of 0.2 to 6%.

In the present invention, the carbon fiber sizing property and the adhesive property were evaluated by the methods described below.

(1) Consistency: Carbon fiber was cut into a length of about 1 m, and one end was fixed and hung, and after the openness between single yarns and the overall openability were evaluated, it was hung again and after 8 hours. The focusing ability of each of the samples was visually evaluated. The contents of the evaluation ranks A, B, C and D are shown below.

(A): Single yarn and whole fiber can be opened easily. Further, the focusing property is uniform.

(B): Single yarn and whole fiber can be opened easily, but 1-2
There is uneven focusing.

(C): There is unevenness in the openability of the single yarn and the whole, and there is unevenness in the focusing property.

(D): Partially splitting has already occurred when initially suspended.

(2) Adhesiveness: A single fiber of carbon fiber was embedded in a matrix resin (a resin obtained by mixing epoxy resin Epicoat 828 with a curing agent metaphenylenediamine at a weight ratio of 6.9 to 1), and heated at 75 ° C. for 2 hours. Then, after curing at 125 ° C. for 2 hours, it is gradually cooled to near room temperature. The sample taken out is left to stand in a desiccator for a certain period of time, and after confirming that there are no air bubbles or defects, it is fixed to a tension machine. Observe a sample added with 4% or more elongation by a tension machine using a polarization plus sensitive color inspection plate under 100 times magnification, and measure the distance from the break point of the single yarn to the break point, that is, the break length. Measure with a measuring device. In this measurement, 10 samples having a sample length of 30 mm were measured, and the average value and the variation rate of the average value were obtained. It can be said that the shorter the breaking length is, the higher the adhesiveness between the carbon fiber and the matrix resin is, and if the strength in the lengthwise direction is constant, the smaller the variation rate of the average value is, the higher the adhesion uniformity is. On the other hand, the adhesiveness of carbon fibers having different physical properties can be compared from the interfacial shear strength (τ) calculated by the following equation. A difference of 0.5 kg / mm ² or more in the strength was considered as a significant difference.

τ = σf / 2 × d / l _c d: Fiber diameter l _c : Breaking length σ f: Tensile strength at l _c Tensile strengths of samples with different single yarn lengths of 4, 10 and 25 mm were measured and extrapolated. The strength at the critical rupture length (l _c ) was obtained by the method and used as the σ f value.

〔Example〕

Examples are shown below, but the invention is not limited thereto.

The carbon fiber used in the present invention is obtained by carbonizing polyacrylonitrile-based synthetic fiber at a maximum temperature of 1300 ° C. and further surface-oxidizing it, and its surface characteristics are as shown in Table-1. The surface area in Table-1 is the value obtained by the Kr gas adsorption method, and O / C is the atomic ratio of oxygen and carbon determined by XPS.

[Example 1] The vapor-phase oxidized surface-treated yarns (Sample 1 and Sample 2) shown in Table 1 were enclosed in a glass ampoule and led from normal pressure to reduced pressure at 12 ° C to reach a vacuum degree of 500 torr and 1 torr. After reaching 2 seconds and a vacuum degree of 10 ⁻¹ , the glass ampoule was broken in a sizing agent prepared by dissolving the epoxy resin (Epicoat 828) in methyl ethyl ketone for 2, 5, 60 seconds. Sized.

Ranking evaluation of carbon fiber bundles obtained by this method,
Table 2 shows the amount of sizing agent attached, the critical rupture length, the fluctuation rate of the critical rupture length, and the interfacial shear strength. In addition, it was immersed in the same sizing agent as in Example 1 at room temperature and atmospheric pressure and was 1.5 ± 0.5%.
The physical properties of the carbon fiber sized in the range of the attached amount of are shown as Comparative Example 1.

[Example 2] Untreated surface yarns and vapor-phase oxidized surface treated yarns (Sample 1, Sample 2) and electrolytically oxidized surface treated yarns (Sample 3, Sample 4) shown in Table-1 were enclosed in a glass ampoule, and 120 The sample was heated at ℃ for 18 hours and exhausted for 18 hours, then cooled to room temperature (15 ℃), and then allowed to stand for 2 seconds at a vacuum of 10 ^-1 torr.
The glass ampoule was sized by breaking the glass ampoule in the sizing agent prepared in Example 1 and a sizing agent prepared by dissolving a polyfunctional glycidyl amine type epoxy resin (TETRAD-X) in methyl ethyl ketone.

Table 3 shows the ranking of the sizing properties of the carbon fibers sized by this method, the amount of sizing agent attached, the critical breaking length, the variation rate of the critical breaking length, and the interfacial shear strength. Immerse in the same sizing agent as in Example 2 at room temperature and atmospheric pressure 1.
The physical properties of the carbon fiber sized in the range of the adhesion amount of 5 ± 0.2% are shown as Comparative Example 2.

[Comparative Example 3] With respect to commercialized carbon fibers, the critical breaking length, the variation rate of the critical breaking length, and the interfacial shear strength were determined according to the evaluation method used in the present invention, and shown in Table-4.

EFFECTS OF THE INVENTION The sizing method of the present invention can improve the sizing property of the carbon fibers and also improve the interlaminar shear strength and the uniformity of the adhesion between the carbon fibers and the matrix resin.

The sizing method of the present invention shows a remarkable effect even on carbon fibers having a small surface polarity and carbon fibers having a large surface area having pores.

Claims (1)

[Claims] 1. A method for sizing carbon fibers, which comprises allowing carbon fibers to stay under reduced pressure for at least 2 seconds and then subjecting them to sizing treatment without substantially exposing them to the atmosphere.