JP6040786B2 - Carbon fiber bundle - Google Patents

Description

The present invention relates to a carbon fiber bundle having a specific range of stress transmission ability and entanglement between carbon fiber and matrix resin, which is suitable for obtaining a carbon fiber reinforced composite material excellent in shear mode interlayer toughness G _IIC. It is.

Carbon fibers having excellent specific strength and specific elastic modulus are attracting attention as one of the effective trump cards for solving global energy problems. In particular, in the aircraft field, the application of carbon fiber reinforced composite materials is accelerating globally for the purpose of drastically reducing the weight of aircraft by replacing metal materials. For reducing the weight of aircraft, it is most effective to improve the tensile modulus of carbon fiber that governs the rigidity of the carbon fiber reinforced composite material. In addition, the mechanical properties of carbon fibers have been further improved, the tensile and compressive strength of carbon fiber reinforced composite materials has been improved, and recently, interlaminar toughness, which is resistant to fracture in the thickness direction of carbon fiber reinforced composite materials. An excellent balance of physical properties is required. It is also known that the shear mode interlaminar toughness G _IIC , which is a kind of interlaminar toughness, has a high correlation with the post-impact compressive strength CAI, which is an index of the reliability of the carbon fiber reinforced composite material against physical impact. It can be said that it is a mechanical property.

As a technique for improving the shear mode interlaminar toughness G _IIC of the carbon fiber reinforced composite material, investigations focusing on the matrix resin have so far been mainly conducted, and high toughness thermoplastic resin particles are blended in the matrix resin (patents). Document 1) and increasing the thickness of the matrix resin layer have been reported (Non-patent Document 1). The shear mode interlaminar toughness G _IIC has reached a high level by the method focusing on the matrix resin, and further mechanical properties such as the rigidity of the carbon fiber reinforced composite material may be lowered for further improvement. Therefore, as an example that focuses on other than the matrix resin, a method of driving a thin rod-shaped reinforcing material called Z-pin in the thickness direction of the carbon fiber reinforced composite material (Non-patent Document 2), or carbon fiber partially by needle punching. A method called Z-anchor (Non-Patent Document 3) that breaks and penetrates in the thickness direction of the carbon fiber reinforced composite material has been reported. However, in these methods, the carbon fiber is locally bent or discontinuous. Therefore, there is a problem that the tensile properties in the in-plane direction of the carbon fiber reinforced composite material are deteriorated.

EP 0274899

“Composites Science and Technology” (Netherlands), 1996, 56, p. 1223. “Composites: Part A” (Netherlands), 2005, 36, p. 55. “Composites Science and Technology” (Netherlands), 2009, 69, p. 2323.

This invention is made | formed in view of said situation, and it aims at providing the carbon fiber bundle suitable for obtaining the carbon fiber reinforced composite material excellent in shear mode interlayer toughness _GIIC .

As a result of intensive studies, the inventors of the present invention consisted of an assembly of single fibers having a stress recovery length within a specific range evaluated by observation with a polarizing microscope at a strain of 3.0% of the single fiber composite. A carbon fiber bundle having an average tearable distance in the range, subjected to electrolytic surface treatment and coated with a sizing agent is effective in obtaining a carbon fiber reinforced composite material having excellent shear mode interlayer toughness G _IIC And found the present invention. That is, the present invention for achieving the above object is a carbon fiber bundle that has been subjected to electrolytic surface treatment and coated with a sizing agent, and was evaluated by observation with a polarizing microscope at a strain of 3.0% of a single fiber composite. The carbon fiber bundle is composed of a single fiber aggregate having a stress recovery length of 500 μm or less, and an average tearable distance of 300 to 600 mm.

By using the carbon fiber bundle of the present invention, a carbon fiber reinforced composite material excellent in shear mode interlayer toughness G _IIC can be obtained. This greatly contributes to weight reduction of the aircraft and can improve the fuel consumption rate of the aircraft.

FIG. 1 is a diagram illustrating a method of measuring a tearable distance. FIG. 2 is a polarization microscope image of a broken fiber end when the indicated value of the strain gauge of the single fiber composite is 3.0%.

The carbon fiber bundle of the present invention is a carbon fiber bundle that has been subjected to electrolytic surface treatment and is coated with a sizing agent, and is a stress recovery length evaluated by observation with a polarizing microscope at a strain of 3.0% of a single fiber composite. Is a bundle of single fibers having a thickness of 500 μm or less, and a carbon fiber bundle having an average tearable distance of 300 to 600 mm.

  In the present invention, a single fiber composite refers to a carbon fiber reinforced composite material produced by embedding a carbon fiber single fiber in a matrix resin. Unlike a normal unidirectional carbon fiber reinforced composite material, Since they are present independently in the matrix resin, it is easy to observe fiber breakage and local deformation behavior of the matrix resin. When tensile strain is applied in the fiber axis direction of the single fiber composite, fiber breakage occurs at random positions of the carbon fiber single fiber. The carbon fiber monofilament hardly bears stress at the end of the broken fiber generated at this time, but the stress is transmitted to the surface of the carbon fiber monofilament through shear deformation of the matrix resin as it moves away from the end of the broken fiber in the fiber axis direction. The stress gradually recovers, and when it is separated from the end of the broken fiber by a certain distance, it becomes possible to bear the same amount of stress as when there is no fiber break. The distance from the end of the broken fiber to the point where the stress is completely recovered is called the stress recovery length. The average tearable distance is a value reflecting the density and strength of the entanglement in which the carbon fiber bundles are inherent, and means that the shorter the distance, the denser and stronger the entangled network is formed.

The reason why the carbon fiber bundle having the stress recovery length and the average tearable distance of the present invention in a specific range gives a carbon fiber reinforced composite material excellent in shear mode interlayer toughness G _IIC is not necessarily clear, but I think so. That is, the shear mode interlaminar toughness G _IIC is calculated from the energy required to cause a crack introduced in advance in the unidirectional carbon fiber reinforced composite material to progress by shear deformation. Since the crack often propagates near the boundary interface between the carbon fiber layer and the matrix resin layer, if a dense and strong entanglement network is formed, the fracture form near the boundary interface between the carbon fiber layer and the matrix resin layer It is presumed that the shear mode interlaminar toughness G _IIC is improved by complicating and consuming excess energy for crack _propagation . In addition, a short stress recovery length means that the shear yield stress of the matrix resin in the vicinity of the carbon fiber monofilament is high and that the interface between the carbon fiber monofilament and the matrix resin is difficult to peel off. It is estimated that the energy required for progress will increase.

The carbon fiber bundle of the present invention has an average tearable distance of 300 to 600 mm. The shorter average tearable distance is better, but if the entanglement is excessively introduced, fluff and yarn breakage increase, and the quality and physical properties of the carbon fiber decrease, so the lower limit is about 300 mm. On the other hand, if the average tearable distance is 600 mm or less, the entanglement is sufficiently dense and strong, so that the shear mode interlayer toughness G _IIC is improved. The average tearable distance is more preferably 400 to 600 mm, and further preferably 400 to 500 mm. In the carbon fiber bundle of the present invention, as a means for setting the average tearable distance within the above range, any method can be adopted, but for simplicity and ease of control, for the carbon fiber bundle, Alternatively, it is preferable to use an entanglement treatment with a fluid in any step of the production process of the carbon fiber bundle (specific means will be described later).

The carbon fiber bundle of the present invention is composed of an aggregate of single fibers having a stress recovery length of 500 μm or less evaluated by observation with a polarizing microscope at a strain of 3.0% of the single fiber composite. Although the detailed evaluation method will be described later, in the present invention, the stress recovery length causes the fiber breakage by stepwise straining the single fiber composite, and the state of light emission due to the stress birefringence around the broken fiber end by a polarizing microscope This is evaluated by analyzing the light emission pattern. An example of a polarization microscope image of the broken fiber end is shown in FIG. The stress recovery length is affected by several factors such as the type of matrix resin and the surface condition of the carbon fiber monofilament, but when the type of matrix resin is fixed, only the influence of the surface condition of the carbon fiber monofilament is extracted and evaluated. be able to. If the stress recovery length is 500 μm or less, practically sufficient stress transmission can be expected, and the shear mode interlayer toughness G _IIC is improved. The shorter the stress recovery length, the better. For example, in the case of a combination of an epoxy resin matrix resin and a sizing agent, the lower limit is often about 300 μm. The stress recovery length is preferably 350 to 500 μm, and more preferably 400 to 500 μm.

The carbon fiber bundle of the present invention has a peel length of 50 μm or less evaluated by observation with a polarizing microscope at a strain of 3.0% of the single fiber composite. Although the detailed evaluation method will be described later, in the present invention, the peel length is the same as the stress recovery length, and the single fiber composite is strained stepwise to cause fiber breakage. Evaluation is made by analyzing the light emission pattern by capturing the light emission caused by the stress birefringence around the periphery. When delamination occurs between the carbon fiber and the matrix resin, stress transfer to the carbon fiber monofilament surface through shear deformation of the matrix resin is not performed in the delaminated portion, or the matrix resin and the carbon fiber monofilament surface Since only a slight stress transmission due to the frictional force due to the contact is performed, the stress recovery length is increased, and the shear mode interlaminar toughness G _IIC may be lowered. The peeling length is more preferably 30 μm or less, and further preferably 15 μm or less.

  By using the preferable surface treatment and sizing agent coating method described later, the stress recovery length can be controlled to 500 μm or less, and the peel length can be controlled to 50 μm or less.

  A method for producing a carbon fiber precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention will be described. In the production of the carbon fiber precursor fiber bundle, the polyacrylonitrile-based polymer preferably has a weight average molecular weight of 200,000 to 900,000, more preferably 300,000 to 600,000. If the weight average molecular weight is less than 200,000, the viscosity of the spinning dope may be insufficient and the discharge from the die may become unstable. A polyacrylonitrile polymer having a high molecular weight exceeding 900,000 is not economical because the polymer concentration needs to be set low when spinning as a polymer solution, and the amount of solvent used increases. The weight average molecular weight of the polyacrylonitrile-based polymer can be controlled by changing the amounts of monomers, initiators, chain transfer agents and the like during polymerization. Specifically, the weight average molecular weight can be increased by increasing the monomer concentration at the start of polymerization, decreasing the initiator concentration, and decreasing the chain transfer agent concentration. In the present invention, the polyacrylonitrile-based polymer means that at least acrylonitrile is the main constituent of the polymer skeleton, and the main constituent usually occupies 90 to 100 mol% of the polymer skeleton. Say.

  In the production of the carbon fiber precursor fiber bundle, the polyacrylonitrile-based polymer contains a copolymer component from the viewpoint of improving the yarn-making property and efficiently performing the flameproofing treatment. Generally, when the amount of the copolymer component is small, the flameproofing reaction becomes non-uniform, and when the amount of the copolymer component is large, the site itself may be thermally decomposed and recognized as a carbon fiber defect. A preferable amount of the copolymer component is 0.1 to 0.8% by mass. Preferred examples of the copolymer component include those having one or more carboxyl groups or amide groups from the above viewpoint. In order to prevent a decrease in heat resistance, it is preferable to use a small amount of a monomer having a high flame resistance promoting effect, and it is preferable to use a copolymer component having a carboxyl group rather than an amide group. Further, the number of amide groups and carboxyl groups contained is more preferably two or more than one, and from this viewpoint, acrylic acid, methacrylic acid, itaconic acid, crotonic acid, citraconic acid, ethacrylic acid Maleic acid and mesaconic acid are preferred, itaconic acid, maleic acid and mesaconic acid are more preferred, and itaconic acid is most preferred.

  In the production of the carbon fiber precursor fiber bundle, the production method of the polyacrylonitrile-based polymer can be selected from known polymerization methods. In the production of a carbon fiber precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention, the spinning dope is composed of the above-described polyacrylonitrile-based polymer and soluble in polyacrylonitrile such as dimethyl sulfoxide, dimethylformamide and dimethylacetamide. Dissolved in an appropriate solvent. The concentration of the spinning dope is preferably 10 to 23% by mass. If the concentration of the spinning solution is less than 10% by mass, the amount of solvent used increases, which is not economical. If the concentration of the spinning solution exceeds 23% by mass, the stability of the stock solution for spinning decreases. In some cases, the weight average molecular weight of the polymer must be lowered for spinnability.

  In the production of the carbon fiber precursor fiber bundle, the spinning process includes a spinning process in which spinning is performed by discharging from a spinneret by a dry and wet spinning method, a water washing process in which fibers obtained in the spinning process are washed in a water bath, and the water washing. A fiber bath obtained by stretching a fiber obtained in the process in a water bath and a drying heat treatment process for drying and heat treating the fiber obtained in the water bath stretching process. It consists of a steam stretching process that stretches the steam.

  In the production of the carbon fiber precursor fiber bundle, the coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide used as a solvent for the spinning dope and a so-called coagulation promoting component. As the coagulation accelerating component, a component that does not dissolve the polyacrylonitrile polymer and is compatible with the solvent used in the spinning solution can be used. Specifically, it is preferable to use water as a coagulation promoting component.

  In the production of the carbon fiber precursor fiber bundle, the water bath temperature in the water washing step is preferably washed using a water bath having a plurality of stages of 20 to 90 ° C. Moreover, it is preferable that the draw ratio in a water bath extending process is 1.3-5 times, More preferably, it is 2-4 times. After the water bath stretching step, it is preferable to apply an oil agent made of silicone or the like to the yarn for the purpose of preventing adhesion between single fibers. As such a silicone oil agent, it is preferable to use a modified silicone, and it is preferable to use one containing an amino-modified silicone having high heat resistance.

  Carbon suitable for obtaining the carbon fiber bundle of the present invention by performing steam stretching as necessary after the water washing step, water bath stretching step, oil agent applying step, and drying heat treatment step performed by a known method. A fiber precursor fiber bundle is obtained. In the present invention, the steam stretching is preferably performed at least 3 times, more preferably 4 times or more, and still more preferably 5 times or more in the pressurized steam.

  Next, the manufacturing method of the carbon fiber bundle used suitably by this invention is demonstrated. The carbon fiber bundle of the present invention can be obtained by flameproofing, pre-carbonizing, and carbonizing the carbon fiber precursor fiber bundle described above.

  In the production of the carbon fiber bundle, the flame resistance of the carbon fiber precursor fiber bundle is preferably performed at a temperature as high as possible without causing a runaway reaction, and specifically, it is preferably performed in air at 200 to 300 ° C. The treatment time for flame resistance can be suitably selected within a range of 10 to 100 minutes, but for the purpose of improving the mechanical properties of the obtained carbon fiber, the specific gravity of the obtained flame resistant fiber is 1.3 to 1. It is preferable to set so as to be in the range of .4.

In the production of the carbon fiber bundle, preliminary carbonization is performed following the flame resistance. In the preliminary carbonization step, it is preferable that the obtained flame-resistant fiber is heat-treated in an inert atmosphere at a maximum temperature of 500 to 1200 ° C. until the specific gravity is 1.5 to 1.8 g / cm ³ .

In the production of the carbon fiber bundle, carbonization is performed following the preliminary carbonization. In the present invention, in the carbonization step, the pre-carbonized fiber bundle obtained is carbonized in an inert atmosphere at a maximum temperature of 1200 to 2000 ° C. so that the carbonization tension satisfies the formula (1). And it is preferable that it is a manufacturing method of the carbon fiber whose average tearable distance of a pre-carbonized fiber bundle is 500-700 mm.
4.9 ≦ carbonization tension (mN / dtex) ≦ −0.0225 × (average tearable distance of pre-carbonized fiber bundle (mm)) + 23.5 (1).

  In the production of carbon fiber bundles, the temperature of the carbonization step is preferably higher from the viewpoint of increasing the strand elastic modulus of the obtained carbon fiber, but if it is too high, the strength of the high strength region may be reduced. It is good to set in consideration. A more preferable temperature range is 1200 to 1800 ° C, and a more preferable temperature range is 1200 to 1600 ° C.

  In the production of carbon fiber bundles, the tension in the carbonization process (carbonization tension) was obtained by dividing the tension (mN) measured on the exit side of the carbonization furnace by the fineness (dtex) of the polyacrylonitrile-based precursor fiber when it was completely dry. It shall be indicated by value. When the tension is lower than 4.9 mN / dtex, the crystallite orientation of the carbon fiber cannot be increased, and a high strand elastic modulus is not exhibited. The tension is preferably higher from the viewpoint of increasing the strand elastic modulus of the carbon fiber to be obtained, but if it is too high, the process passability and the quality may be lowered, and the range satisfying the formula (1) is satisfied. It is preferable to set. The meaning of the first-order coefficient −0.0225 on the right-hand side of equation (1) is the tension gradient that can be set as the average tearable distance increases, and the constant term 23.5 represents the average tearable distance to the limit. This tension can be set when the length is shortened.

  Conventionally, the hook drop method has been generally used as a method for evaluating the entangled state. As defined in JIS L1013 (2010), the degree of entanglement of the fiber bundle by the hook drop method is fixed to the upper part of the drooping device, the weight is suspended at the lower end of the fiber bundle, and the sample is placed vertically. After that, a hook with a weight of 10 g with a smooth surface of 0.6 mm in diameter is inserted so as to divide the fiber bundle into two parts 1 cm below the upper fixed end of the sample, and the descent distance is measured. .

  As the degree of entanglement of the pre-carbonized fiber bundle in the carbonization process, it is not the conventional hook drop method, but by making the average tearable distance within a specific range, the carbonization process avoids the strength reduction of the carbon fiber high-strength region High stretching tension can be expressed. In order to apply a high drawing tension in the carbonization process, it is necessary to create a fiber bundle state in which the stress transmission ability between single fibers is high. For that purpose, it is important to form a fine entanglement network between single fibers. The conventional hook drop method is an evaluation at a “point” using a hook, whereas the tearable distance is an evaluation at a “surface” that looks at the entire bundle. Due to this difference, in the carbonization process, It is considered that a state for expressing a high stretching tension can be appropriately defined.

  In the production of carbon fiber bundles, the shorter the tearable distance of the pre-carbonized fiber bundle in the carbonization process, the higher the degree of entanglement, the higher the ability to transfer stress between single fibers, and the higher the drawing tension in the carbonization process. However, when the distance is less than 500 mm, the strength of the carbon fiber high strength region is likely to decrease. If the entangling process with a tearable distance exceeding 700 mm is not performed or weak, the stress of the single fiber that breaks with a certain probability, which is the carbonization process, is borne by the other single fiber that has not broken. It induces structural variation between fibers and accompanying strength variation.

  In the production of the carbon fiber bundle, any means can be adopted as the means for achieving the average tearable distance of the preliminary carbonized fiber as long as the above numerical range can be achieved. As a method for controlling the possible distance, an entanglement process using a fluid to the fiber bundle is preferably used. Among them, it is preferable to perform the fluid entanglement process in any of the manufacturing process and the flameproofing process of the polyacrylonitrile-based precursor fiber, that is, in a place where a fiber bundle having a fiber elongation of 5% or more is processed. In other steps, fiber strength may be reduced.

  As the fluid used for the fluid entanglement treatment, both gas and liquid can be used, but air or nitrogen is preferable because it is inexpensive. In the fluid entanglement process, the fluid is preferably sprayed onto the fiber bundle using a nozzle, and the shape of the nozzle that sprays the fluid is not particularly limited, but it is preferable to use one having 2 to 8 ejection holes. The arrangement of the ejection ports is not particularly limited, but an even number of ejection holes are arranged so as to surround the fiber bundle so that the angle formed by the fiber bundle longitudinal direction and the fluid blowing direction is in the range of 88 ° to 90 °. It is preferable to arrange the ejection holes at opposing positions so that there are two holes and one set. Other conditions such as fiber bundle tension and fluid discharge pressure during the fluid entanglement process may be examined so as to adjust the tearable distance appropriately.

  In the carbonization step, the fiber bundle is preferably substantially untwisted. Here, “substantially untwisted” means that it is 1 turn or less per 1 m of fiber bundle even if twist is present.

  The higher the strand strength and strand elastic modulus of the carbon fiber bundle, the better from the viewpoint of improving the properties of the carbon fiber reinforced composite material obtained using the same, but it is preferable that both 6200 MPa and 320 GPa are satisfied.

  The carbon fiber bundle obtained as described above is subjected to an oxidation treatment to introduce an oxygen-containing functional group in order to improve adhesion with the matrix resin. As the oxidation treatment method, vapor phase oxidation, liquid phase oxidation, and liquid phase electrolytic oxidation are used. From the viewpoint of high productivity and uniform treatment, liquid phase electrolytic oxidation is preferably used.

  In the present invention, examples of the electrolytic solution used in the liquid phase electrolytic oxidation include an acidic electrolytic solution and an alkaline electrolytic solution. From the viewpoint of adhesion, a liquid phase electrolytic oxidation is performed in an alkaline electrolytic solution, and then a sizing agent is applied. Is more preferable.

  In the present invention, examples of the acidic electrolyte include inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, boric acid, and carbonic acid, organic acids such as acetic acid, butyric acid, oxalic acid, acrylic acid, and maleic acid, or ammonium sulfate. And salts such as ammonium hydrogen sulfate. Of these, sulfuric acid and nitric acid exhibiting strong acidity are preferably used.

  In the present invention, specific examples of the alkaline electrolyte include aqueous solutions of hydroxides such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide and barium hydroxide, sodium carbonate, potassium carbonate, magnesium carbonate. , Aqueous solutions of carbonates such as calcium carbonate, barium carbonate and ammonium carbonate, aqueous solutions of bicarbonates such as sodium bicarbonate, potassium bicarbonate, magnesium bicarbonate, calcium bicarbonate, barium bicarbonate and ammonium bicarbonate, ammonia, water Examples include aqueous solutions of tetraalkylammonium oxide and hydrazine. Among these, from the viewpoint of not containing an alkali metal that causes curing inhibition of the matrix resin, an aqueous solution of ammonium carbonate and ammonium hydrogen carbonate or an aqueous solution of tetraalkylammonium hydroxide exhibiting strong alkalinity is preferably used.

  In this invention, it is preferable that the density | concentration of the electrolyte solution used exists in the range of 0.01-5 mol / liter, More preferably, it exists in the range of 0.1-1 mol / liter. When the concentration of the electrolytic solution is 0.01 mol / liter or more, the electrolytic treatment voltage is lowered, which is advantageous for the operating cost. On the other hand, when the concentration of the electrolytic solution is 5 mol / liter or less, it is advantageous from the viewpoint of safety.

  In the present invention, the temperature of the electrolytic solution used is preferably in the range of 10 to 100 ° C, more preferably in the range of 10 to 40 ° C. When the temperature of the electrolytic solution is 10 ° C. or higher, the efficiency of the electrolytic treatment is improved, which is advantageous for the operating cost. On the other hand, when the temperature of the electrolytic solution is 100 ° C. or lower, it is advantageous from the viewpoint of safety.

  In the present invention, the amount of electricity in the liquid phase electrolytic oxidation is preferably optimized in accordance with the carbonization degree of the carbon fiber, and a larger amount of electricity is required when processing the carbon fiber having a high elastic modulus.

In the present invention, the current density in the liquid phase electrolytic oxidation is preferably in the range of 1.5 to 1000 amperes / m ² per 1 m ² of the surface area of the carbon fiber in the electrolytic treatment solution, more preferably 3 to 500 amperes. / M ² within the range. When the current density is 1.5 amperes / m ² or more, the efficiency of the electrolytic treatment is improved, which is advantageous for the operating cost. On the other hand, when the current density is 1000 amperes / m ² or less, it is advantageous from the viewpoint of safety.

  In the present invention, the total amount of electrolytic electricity employed in the electrolytic treatment is preferably 3 to 300 coulomb / g per gram of carbon fiber. If the total amount of electrolytic electricity is less than 3 coulombs / g, there may be cases where sufficient functional groups cannot be imparted to the carbon fiber surface, and the peel length becomes long, and the effects of the present invention may not be obtained. On the other hand, if the total amount of electrolytic electricity exceeds 300 coulombs / g, the tensile strength of the carbon fiber may decrease, and the tensile strength of the carbon fiber reinforced composite material may decrease.

  In the present invention, it is preferable to wash and dry the carbon fiber after the electrolytic treatment. As a cleaning method, for example, a dip method and a spray method can be used. Especially, it is preferable to use a dip method from a viewpoint that washing | cleaning is easy, Furthermore, it is a preferable aspect to use a dip method, vibrating a carbon fiber with an ultrasonic wave. Also, if the drying temperature is too high, the functional groups present on the outermost surface of the carbon fiber are likely to disappear due to thermal decomposition, so it is desirable to dry at the lowest possible temperature. Specifically, the drying temperature is preferably 250 ° C. or lower. More preferably, drying is performed at 210 ° C. or lower.

After the electrolytic treatment, the carbon fiber bundle is subjected to sizing treatment in order to impart convergence to the obtained carbon fiber bundle . As the sizing agent, a sizing agent having good compatibility with the matrix resin can be appropriately selected according to the type of the matrix resin used in the composite material.

  EXAMPLES Next, although an Example demonstrates this invention concretely, this invention is not restrict | limited by these Examples.

<Average tearable distance>
The carbon fiber bundle was cut to a length of 1160 mm, and one end of the carbon fiber bundle was fixed on a horizontal base so as not to move with an adhesive tape (this point is called a fixing point A). One end of the fiber bundle that was not fixed was divided into two with a finger, and one of the ends was tensioned and fixed on the table so as not to move with an adhesive tape (this point is called a fixing point B). Move the other of the two halves along the table so that the fixed point A becomes a fulcrum and does not come loose, rest at a position where the linear distance from the fixed point B is 500 mm, and fix it on the table so that it does not move with adhesive tape. (This point is called a fixed point C). The area surrounded by the fixed points A, B, and C is visually observed, the entanglement point farthest from the fixed point A is found, and the distance projected on the straight line connecting the fixed point A and the fixed point B is 1 mm at the minimum scale. It was read with the ruler of No., and it was set as the tearable distance. The arithmetic average value of the measurement repeated 30 times was defined as the average tearable distance. A method of measuring the tearable distance is shown in FIG. In this measurement method, the entanglement point farthest from the fixed point A is the point where the linear distance from the fixed point A is the longest and three or more single fibers having no slack are entangled.

<Shear Mode Interlaminar Toughness G _{IIC of} Carbon Fiber Reinforced Composite Material>
Thirty unidirectional prepregs were cut into a size of 200 × 250 mm and laminated so that the fiber directions were the same. Further, at the time of lamination, a polyimide film (thickness: 0.3 mm) subjected to mold release treatment was inserted into the central surface of the lamination so that the edge was perpendicular to the fiber direction in order to introduce an initial crack. The tip of the film was disposed at a position 40 mm from the edge of the laminate. This laminate was cured by heating and pressing for 2 hours in an autoclave under conditions of 180 ° C. and an internal pressure of 0.6 MPa to form a unidirectional fiber-reinforced composite material. The unidirectional fiber reinforced composite material was cut into 20 × 195 mm to obtain a test piece. This test piece was subjected to an ENF test in accordance with JIS K7086 (1993) appendix 2.

<Strand strength and strand elastic modulus of carbon fiber>
The strand strength and strand elastic modulus of the carbon fiber bundle were determined according to the following procedure in accordance with the resin impregnated strand test method of JIS-R-7608 (2004). As the resin formulation, “Celoxide (registered trademark)” 2021P (manufactured by Daicel Chemical Industries) / 3 boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.) / Acetone = 100/3/4 (part by mass) is used. As curing conditions, normal pressure, temperature of 125 ° C., and time of 30 minutes were used. Ten resin-impregnated strands of carbon fiber bundles were measured, and the average values were defined as strand tensile strength and strand elastic modulus.

<Average stress recovery length>
The average stress recovery length was evaluated by the following procedures (a) to (f).
(A) Preparation of resin 190 parts by mass of bisphenol A type epoxy resin compound “Epototo YD-128” (manufactured by Nippon Steel Chemical Co., Ltd.) and 20.7 parts by mass of diethylenetriamine (manufactured by Wako Pure Chemical Industries, Ltd.) The mixture was stirred with a spatula and defoamed using an automatic vacuum defoamer.
(B) Sampling of carbon fiber single fiber and fixing to mold A carbon fiber bundle having a length of about 20 cm was divided into approximately four equal parts, and single fibers were sampled in order from the four bundles. At this time, the entire bundle was sampled as evenly as possible. Next, double-sided tape was applied to both ends of the perforated mount, and the single fibers were fixed to the perforated mount in a state where a constant tension was applied to the sampled single fibers. Next, a glass plate on which a polyester film “Lumirror” (registered trademark) (manufactured by Toray Industries, Inc.) was attached was prepared, and a 2 mm thick spacer for adjusting the thickness of the test piece was fixed on the film. . A perforated mount with single fibers fixed thereon was placed on the spacer, and a glass plate on which a film was similarly attached was further set on the spacer with the surface on which the film was attached facing downward. At this time, in order to control the fiber embedding depth, a tape having a thickness of about 70 μm was attached to both ends of the film.
(C) From resin casting to curing The resin prepared in the procedure (a) was poured into the mold (the space surrounded by the spacer and the film) in the procedure (b). The mold into which the resin was poured was heated for 5 hours using an oven preliminarily heated to 50 ° C., and then cooled to a temperature of 30 ° C. at a temperature lowering rate of 2.5 ° C./min. Thereafter, the mold was removed and cut to obtain a test piece of 2 cm × 7.5 cm × 0.2 cm. At this time, the test piece was cut so that the single fiber was located within the center 0.5 cm width of the test piece width.
(D) Fiber Embedding Depth Measurement Fiber embedding depth for the test piece obtained in the procedure (c) above using a laser Raman spectrophotometer (JASCO NRS-3200) laser and a 532 nm notch filter. Measurements were made. First, a laser is applied to the surface of the single fiber, the stage height is adjusted so that the beam diameter of the laser becomes the smallest, and the height at that time is defined as A (μm). Next, a laser is applied to the surface of the test piece, the stage height is adjusted so that the beam diameter of the laser becomes the smallest, and the height at that time is defined as B (μm). The fiber embedding depth d (μm) was calculated by the following formula using the refractive index of the resin 1.732 measured using the above laser.
Embedding depth d (μm) = (A−B) × 1.732 (2)
(E) Four-point bending test Tensile strain was applied by four-point bending to the test piece obtained in the above procedure (c) using a jig having an outer indenter interval of 50 mm and an inner indenter interval of 20 mm. The value when the indicated value of the strain gauge affixed at a position about 5 mm away from the center of the test piece in the width direction is 3.0% for the strain of the single fiber composite obtained in advance by equation (3). The strain was loaded until the following conditions were obtained, and a polarizing microscope image was obtained under the following conditions (for reference, FIG. 2 shows a polarizing microscope image of a broken fiber end when the indicated value of the strain gauge is 3.0%).
ε _c = ε × (2 / κ) × (1000−d) /1000−0.14 (3)
Here, ε _c (%) is the strain of the single fiber composite, ε (%) is the indicated value of the strain gauge, κ is the gauge factor of the strain gauge used, and d (μm) is measured according to the above procedure (d). The fiber embedding depth, 0.14 (%) at the end is the residual strain.
Polarization microscope: Olympus BX51
Objective lens: 50 times Image size: Vertical 1024 pixels, Horizontal 1360 pixels Observation range: Range of the central part 10 mm in the longitudinal direction of the specimen Polarization condition: Stress birefringence pattern is mirror-symmetrical with respect to the fiber axis and antiphase Adjust the direction of the polarizing plate so as to obtain an image: Select a field of view so that only one end of the broken fiber per image is captured, and acquire 30 images (f) Evaluation of stress recovery length Above (e) Each polarization microscope image obtained in (1) was analyzed using image processing software imageJ (Image J), and the stress recovery length was determined. First, the inclination was finely adjusted so that the fiber axis coincided with the long axis direction of the image. The image was then converted to 16 bit grayscale. Subsequently, one of the vertices coincides with the broken fiber end (vertex A), one of the long sides including the vertex coincides with the interface between the carbon fiber and the matrix resin, and the other long side exists on the matrix resin side. A rectangular region of 10 pixels in the vertical direction and 600 pixels in the horizontal direction was selected, and the luminance information in the vertical direction was averaged to calculate the luminance change state over 600 pixels. The maximum and minimum absolute values of the luminance data are obtained, a point B where the luminance is 0.01 × (maximum luminance value−minimum luminance value) is obtained, and the distance from the vertex A to the point B is defined as the stress recovery length. did. Since there are four ways of selecting the rectangular region, the same method was applied to the remaining three ways, and an average value of these four values was calculated as the stress recovery length of the focused broken fiber end. However, when another broken fiber end exists within 1.2 mm in the fiber axis direction of the focused broken fiber end, the region sandwiched between these two broken fiber ends was not analyzed. This was done for each of the 30 broken fiber ends. After plotting the stress recovery length of each end of the broken fiber thus obtained against the strain of the single fiber composite, the stress recovery length when the strain of the single fiber composite is 3.0% is linearly approximated by the least square method. This was calculated as the average stress recovery length.

  Here are some points to note. That is, when the stress recovery length is long, it is considered that there is no broken fiber end that satisfies the image acquisition condition of the item (e) in the observation range. A polarizing microscope image is obtained with a strain of 2.5%, and the average stress recovery length when the strain of the single fiber composite is 2.5% is evaluated according to (f), and this is multiplied by a factor of 1.2. Thus, the average stress recovery length when the strain of the single fiber composite is 3.0% is preferable. Even if the strain of the single fiber composite is 2.5%, if there is no broken fiber end that satisfies the image acquisition condition in (e) above in the observation range, the stress recovery length is too long and the evaluation is terminated. .

<Average peel length>
The average peel length between the carbon fiber and the matrix resin was determined according to the following procedure (G) after performing the procedures (A) to (E) in the same manner as the evaluation of the average stress recovery length.
(G) Evaluation of peel length For each polarization microscope image obtained in (e) above, the distance from the end of the broken fiber to the point C where light emission due to stress birefringence starts in the fiber axis direction is determined, The distance from the point C was taken as the peel length at the end of the broken fiber. Only one direction was performed on each broken fiber end. This was done for each of the 30 broken fiber ends. After plotting the peel length of each end of the broken fiber thus obtained against the strain of the single fiber composite, linear approximation was performed by the least square method, and the peel length when the strain of the single fiber composite was 3.0% was calculated. This was calculated and this was taken as the average peel length.

[Example 1]
A copolymer composed of 99.5 mol% of acrylonitrile and 0.5 mol% of itaconic acid was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent and 2,2′-azobisisobutyronitrile as an initiator, and a weight average molecular weight was obtained. A polyacrylonitrile copolymer having 400,000 and Mz / Mw of 3.5 was produced. Ammonia gas was blown into the manufactured polyacrylonitrile-based polymer until the pH reached 8.5, and the polymer concentration was adjusted to 19% by mass to obtain a spinning solution. The obtained spinning solution was ejected into the air once at 40 ° C. using a spinneret having a diameter of 0.15 mm and a hole number of 6,000, passed through a space of about 4 mm, and then controlled at 5 ° C. 70 A coagulated yarn was obtained by a dry and wet spinning method introduced into a coagulation bath comprising an aqueous solution of% dimethyl sulfoxide. The coagulated yarn was washed with water by a conventional method, and then stretched 3.5 times in two warm water baths. Subsequently, an amino-modified silicone-based silicone oil agent was applied to the fiber bundle after stretching in the water bath, and a drying densification treatment was performed using a 160 ° C. heating roller to perform an entanglement treatment. In the entanglement process performed in Example 1, the angle formed by the longitudinal direction of the fiber bundle and the fluid blowing direction is 90 °, and eight ejection holes are arranged so as to surround the fiber bundle. Using fluid spray nozzles arranged at opposing positions so as to form a pair with two holes, the tension of the fiber bundle is adjusted to 2 mN / dtex, and the fluid discharge pressure is set to 0.25 MPa. It was a process of spraying. Two yarns are combined and the number of single fibers is 12,000, and the resultant is stretched 3.7 times in pressurized steam, so that the total draw ratio is 13 times, the single fiber fineness is 0.7 dtex, and the number of single fibers is 12000. A polyacrylonitrile-based precursor fiber bundle was obtained. Next, in the air of 240-260 degreeC, it flame-proofed, extending | stretching by the draw ratio 1, and obtained the flame-resistant fiber bundle of specific gravity 1.35-1.36. The obtained flame-resistant fiber bundle was subjected to a preliminary carbonization treatment while being stretched at a stretch ratio of 1.15 in a nitrogen atmosphere at a temperature of 300 to 800 ° C. to obtain a preliminary carbonized fiber bundle. The obtained preliminary carbonized fiber bundle was carbonized at a maximum temperature of 1500 ° C. and a tension of 5.5 mN / dtex in a nitrogen atmosphere to obtain a carbon fiber bundle.

  The obtained carbon fiber bundle was subjected to electrolytic surface treatment using an aqueous ammonium hydrogen carbonate solution having a concentration of 0.1 mol / l as an electrolytic solution at an electric charge of 80 coulomb per 1 g of carbon fiber. The carbon fiber subjected to the electrolytic surface treatment was subsequently washed with water and dried in heated air at a temperature of 150 ° C. to obtain an electrolytically treated carbon fiber bundle. At this time, the surface oxygen concentration O / C was 0.15. Next, 10 parts by mass of bisphenol A diglycidyl ether “jER (registered trademark)” 828 (manufactured by Mitsubishi Chemical Corporation) and bisphenol A diglycidyl ether “jER (registered trademark)” 1001 (manufactured by Mitsubishi Chemical Corporation) ) 10 parts by mass, bisphenol A EO 2 mol adduct 2 mol, maleic acid 1.5 mol, sebacic acid 0.5 mol condensate 20 parts by mass and emulsifier polyoxyethylene (70 mol) styrenation (5 Mol) After preparing an aqueous dispersion emulsion consisting of 10 parts by mass of cumylphenol, 50 parts by mass of polyglycerol polyglycidyl ether “Denacol (registered trademark)” EX-521 (manufactured by Nagase ChemteX Corporation) was mixed. A sizing solution was prepared. This sizing agent was applied to a carbon fiber bundle that was electrolytically treated by an immersion method, and then heat treated at a temperature of 210 ° C. for 75 seconds to obtain a carbon fiber bundle to which the sizing agent was applied. The adhesion amount of the sizing agent was adjusted to be 1.0 part by mass with respect to 100 parts by mass of the carbon fiber bundle subjected to electrolytic treatment. About the carbon fiber bundle which apply | coated this sizing agent, average tearable distance, strand strength, and strand elastic modulus were evaluated. Moreover, single fiber was extracted and average stress recovery length and average peeling length were evaluated.

A prepreg was produced from the carbon fiber bundle coated with the sizing agent by the following procedure. First, 35 parts by mass of tetraglycidyldiaminodiphenylmethane “Sumiepoxy (registered trademark)” ELM434 (manufactured by Sumitomo Chemical Co., Ltd.) and diglycidyl ether of bisphenol A “jER (registered trademark)” 828 as a component (A) in a kneading apparatus. (Mitsubishi Chemical Co., Ltd.) 35 parts by mass and N-diglycidylaniline GAN (Nippon Kayaku Co., Ltd.) 30 parts by mass, 14 parts by mass of Sumika Excel 5003P were dissolved, 40 parts by mass of 4,4′-diaminodiphenylsulfone as component B) was kneaded to prepare an epoxy resin composition for a carbon fiber reinforced composite material. The obtained epoxy resin composition was coated on a release paper with a resin basis weight of 52 g / m ² using a knife coater to prepare a resin film. This resin film is overlapped on both sides of a carbon fiber bundle coated with a sizing agent that is widened so that the basis weight is 190 g / m ² and aligned in one direction, and using a heat roll, the temperature is 100 ° C. and the atmospheric pressure is 1 atm. The sizing agent-coated carbon fiber was impregnated with the epoxy resin composition while being heated and pressurized at 1 to obtain a unidirectional prepreg. G _IIC was evaluated using such a unidirectional prepreg.

[Example 2]
Evaluation was performed in the same manner as in Example 1 except that the tension of the fiber bundle in the entanglement treatment was changed to 3 mN / dtex.

[Example 3]
Evaluation was carried out in the same manner as in Example 2 except that spinning was performed using a coagulation bath composed of an aqueous solution of 30% dimethyl sulfoxide controlled at 3 ° C.

[Example 4]
Except that a spinning solution having a weight average molecular weight of 600,000, Mz / Mw of 1.8, and a polymer concentration of 15% was obtained by adjusting the amount of initiator and charging timing at the time of solution polymerization. Evaluation was carried out in the same manner as in Example 3.

[Comparative Example 1]
Evaluation was performed in the same manner as in Example 4 except that the tension of the fiber bundle in the entanglement treatment was 2 mN / dtex and the discharge pressure of the fluid was 0.20 MPa.

[Comparative Example 2]
Evaluation was performed in the same manner as in Example 1 except that the electrolytic surface treatment was not performed.

[Comparative Example 3]
Evaluation was performed in the same manner as in Example 3 except that the electrolytic surface treatment was not performed.

[Example 5]
The entangling process was carried out except that the tension of the fiber bundle was adjusted to 2 mN / dtex and the fluid discharge pressure was set to 0.35 MPa before applying the amino-modified silicone-based silicone fluid. Evaluation was carried out in the same manner as in Example 3.

[Example 6]
Evaluation was performed in the same manner as in Example 1 except that the amount of the sizing agent adhered was adjusted to 0.4 parts by mass with respect to 100 parts by mass of the carbon fiber bundle subjected to electrolytic treatment.

[Comparative Example 4]
Evaluation was performed in the same manner as in Example 1 except that the amount of the sizing agent attached was adjusted to 0.2 parts by mass with respect to 100 parts by mass of the carbon fiber bundle subjected to electrolytic treatment.

[Comparative Example 5]
The sizing solution used was “jER (registered trademark)” 828, “jER (registered trademark)” 1001, 2 mol of EO2 mol adduct of bisphenol A, 1.5 mol of maleic acid, and 0.5 mol of sebacic acid, Evaluation was performed in the same manner as in Example 1 except that “Denacol (registered trademark)” EX-521 was changed from 10: 10: 20: 50 to 22.5: 22.5: 45: 0.

[Example 7]
Evaluation was performed in the same manner as in Example 1 except that the electrolytic solution at the time of electrolytic surface treatment was a sulfuric acid aqueous solution having a concentration of 0.1 mol / l and the amount of electricity was 10 coulomb per gram of carbon fiber.

[Example 8]
Evaluation was performed in the same manner as in Example 1 except that the amount of electricity at the electrolytic surface treatment was 500 coulomb per gram of carbon fiber.

1: Fiber bundle 2: Fixed point A
3: Fixed point B
4: Fixed point C
5: Entanglement point 6: Tearable distance

When the carbon fiber bundle of the present invention is used, a carbon fiber reinforced composite material having excellent shear mode interlayer toughness G _IIC can be obtained. This greatly contributes to weight reduction of the aircraft and can improve the fuel consumption rate of the aircraft.

Claims (3)

A carbon fiber bundle that has been subjected to electrolytic surface treatment and coated with a sizing agent, and has an average stress recovery length of 500 μm or less evaluated by observation with a polarizing microscope at a strain of 3.0% of the single fiber composite. The carbon fiber bundle which consists of an aggregate | assembly of this, and the average tearable distance is 300-600 mm. The carbon fiber bundle according to claim 1, comprising an aggregate of single fibers having an average peel length of 50 µm or less evaluated by observation with a polarizing microscope at a strain of 3.0% of the single fiber composite. The carbon fiber bundle according to claim 1 or 2, wherein the strand strength is 6200 MPa or more and the strand elastic modulus is 320 GPa or more.