JP2013104034A - Fiber-reinforced composite material and method for producing fiber-reinforced composite material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To create a fiber-reinforced composite material in which carbon fibers are randomly oriented so as to reduce unevenness in mechanical property, and which has excellent mechanical characteristics and a high fiber volume content, by forming a carbon fiber sheet comprising carbon fibers randomly oriented as single fibers and having high apparent density, and reliably impregnating a resin into the sheet.SOLUTION: The fiber-reinforced composite material comprises carbon fibers and a resin, wherein the carbon fibers are randomly oriented as single fibers and constitute a sheet having apparent density of 0.25-1.5 g/cm, the sum of the percentages of COO groups and C-O groups in C1s peak measured by ESCA in the carbon fiber surfaces of the sheet surface is ≥5%, and the fiber volume content of the carbon fibers is 20-80%.

Description

  The present invention relates to a fiber reinforced composite material, and more particularly, to a fiber reinforced composite material composed of a thermoplastic or thermosetting resin and a reinforced fiber, and a method for producing the same.

  Fiber reinforced composite materials consisting of reinforced fibers and matrix resins have high specific strength, specific elastic modulus, excellent mechanical properties, and high functional properties such as weather resistance and chemical resistance. It is also attracting attention for industrial use, and its demand is increasing year by year.

  As a fiber reinforced composite material, a unidirectionally aligned fiber sheet is produced using non-crimp continuous fibers as reinforcing fibers, and a prepreg is produced from this and a resin (for example, Patent Document 1). When the fibers are arranged in one direction, the fibers can be filled with a high density, so that a composite material having a high fiber volume content and excellent mechanical properties can be obtained. Furthermore, the required mechanical properties can be designed with high accuracy, and the variation in the mechanical properties is small, so that it is widely used in aircraft and the like. However, in order to arrange fibers in one direction with high accuracy, advanced control technology is required, and there are problems such as low production speed and high cost.

  Therefore, unlike continuous fibers, a fiber-reinforced resin sheet is known in which reinforcing fibers of about 5 to 50 mm are randomly oriented (for example, Patent Document 2). Since such a sheet is easy to mold and relatively low in cost, it is becoming popular especially for general industrial materials. However, the technique described in the document is a technique related to a so-called chopped strand mat, in which a continuous fiber bundle is first impregnated with a resin and then cut and then randomly oriented to obtain in-plane pseudoisotropy. Although the bundles are randomly oriented, they are not randomly oriented at the single fiber level.

  On the other hand, it is known that it is generally difficult to impregnate a resin after a carbon fiber is randomly oriented sheet. In order to improve the impregnation property of the resin into the carbon fiber sheet, the resin is used as a thermoplastic fiber. A technique of mixing with carbon fiber in advance is known (for example, Patent Document 3).

  In Patent Document 4, it is described that the affinity with a matrix as a reinforcing material is enhanced by surface-treating carbon fibers.

JP 2004-277955 A JP-A-9-155862 Japanese Patent Laid-Open No. 2002-212311 Japanese Patent Laid-Open No. 6-166953

  As described above, the technique described in Patent Document 2 is a chopped strand mat technique in which fiber bundles are randomly oriented, but are not randomly oriented in a single fiber state. In order to reduce the variation in mechanical properties, it is preferable that the fiber bundle is randomized in a single fiber state rather than in a random state. However, when randomizing in a single fiber state, unlike a method of randomizing a resin-impregnated fiber bundle after impregnating the fiber bundle as in Patent Document 2, generally a sheet randomized in a single fiber state, for example, a nonwoven fabric shape After that, the resin is impregnated. However, as described above, the impregnation property is extremely deteriorated due to the increase in the surface area of the fibers in contact with the resin and the bulkiness of the substrate.

  In addition, when the single fiber is randomized, it becomes bulky, so the fiber packing density is lower than when the fiber bundle is randomized, and it is difficult to achieve a high fiber volume content with excellent mechanical properties. . In particular, carbon fibers with high rigidity tend to become more bulky with the randomization of single fibers, and the apparent density becomes low, making it difficult to obtain a composite material with a high fiber volume content. The present inventors have continued diligent efforts to achieve a high fiber volume content, and in order to increase the fiber volume content, it is necessary to increase the apparent density of the carbon fiber sheet itself as a base material. As found, in addition to the difficulty in obtaining a carbon fiber sheet having a high apparent density, the gap between the carbon fibers, which are the resin flow paths, is narrowed.

  Furthermore, in Patent Document 2, glass fibers are specifically described as reinforcing fibers, and there is no specific description about carbon fibers. In the case of the carbon fiber which the present inventors tried, it turned out that the impregnation property of resin worsens compared with glass fiber.

  Here, in patent document 3, the means to impregnate resin to a carbon fiber sheet is examined. However, although the carbon fibers are bundled and the resin is also fiberized and mixed in the carbon fiber sheet, the carbon fiber weight content is 40% or more (carbon fiber density 1.75, nylon 6 density 1. If the volume content when converted to 14 was 30% or more), the impregnation was still poor, and it could not be improved greatly.

  As described above, it was difficult to impregnate the resin into a high apparent density carbon fiber sheet in which carbon fibers were randomly oriented.

  An object of the present invention is to create a fiber-reinforced composite material having a high fiber volume content, in which carbon fibers are randomly oriented in order to reduce variation in mechanical properties and have excellent mechanical properties. For this purpose, the resin is steadily impregnated with a carbon fiber sheet that is randomly oriented in a single fiber state and has a high apparent density.

  In order to obtain a carbon fiber sheet in which single fibers are randomly oriented and have a high apparent density and to steadily impregnate the resin, the present inventors have considered as follows based on the above findings. Since the impregnation property is changed between glass fiber and carbon fiber, and the surface area of carbon fiber and the gap between the fibers influence the impregnation property, attention is paid to the state of the carbon fiber surface.

  Here, Patent Document 4 describes that the hydrophilicity of the matrix is enhanced when the carbon fiber surface has a COO group. However, the carbon fiber sheet described in Patent Document 4 has a low apparent density, and there is no description that the resin is actually impregnated.

  It was thought that if the affinity between the carbon fiber surface and the resin is increased, it will adhere to the resin flow path and the impregnation property may deteriorate, but surprisingly, it was surprisingly found that the impregnation property is remarkably improved. It was.

  This is because, when applied to a carbon fiber sheet that is randomly oriented in a single fiber state and has a high apparent density, the flow path of the resin is very narrow, and the affinity of the resin to the flow path is narrow. It is thought that the introduction was promoted.

In order to solve the above problems, the present invention has the following configuration. That is, the fiber-reinforced composite material of the present invention is a fiber-reinforced composite material composed of carbon fibers and a resin, and the carbon fibers are randomly oriented in a single fiber state, and the carbon fibers have an apparent density of 0.25 to 1. A sheet shape of 0.5 g / cm ³ , the sum of the ratio of COO groups and C—O groups in the C1s peak measured by ESCA on the carbon fiber surface of the sheet surface is 5% or more, and carbon The fiber volume content of the fiber is 20 to 80%.

  Further, in the method for producing a fiber-reinforced composite material of the present invention, the sum of the ratios of COO groups and C—O groups in the C1s peak measured by ESCA on the surface of the carbon fiber on the surface of the carbon fiber sheet is 5% or more. The carbon fiber sheet is impregnated with a resin to have a fiber volume content of 20 to 80%.

  According to the present invention, it is possible to create a fiber-reinforced composite material having a high fiber volume content, in which carbon fibers are randomly oriented so as to reduce the variation in mechanical properties and have excellent mechanical properties.

  Examples of the carbon fiber referred to in the present invention include polyacrylonitrile (hereinafter abbreviated as PAN), pitch, rayon, phenol resin, and the like, but PAN-based carbon fibers are preferred in terms of excellent strength. . It is preferable that the carbon fiber is in an opened state because the resin is easily impregnated and the adhesiveness is excellent. In particular, when the fiber volume content is 20% or more, the impregnation property of the resin is greatly reduced. Therefore, the carbon fiber is preferably opened.

  In the present invention, the carbon fibers are not bundled as fiber bundles, but are randomly oriented in a single fiber state. Variations in mechanical properties can be suppressed by using single fibers instead of fiber bundles. On the other hand, although it is generally difficult to obtain a high fiber volume content and mechanical properties are lowered, it is possible to achieve both by achieving a high fiber volume content according to the present invention. Moreover, it becomes possible to manufacture a base material at lower cost by orienting fibers at random without controlling them in one direction. In addition, the moldability is good, and it is also preferable in that it has an effect that pseudo isotropic property is easily obtained.

  Whether or not it is randomly oriented in a single fiber state is determined by taking the surface of the carbon fiber composite material at random at five locations with a scanning electron microscope and observing it at 300 times to form a bundle of three or more fibers. It can be confirmed that the number of fiber bundles is 1 or less. In addition, being oriented randomly means an oriented state in which the fibers are not aligned in one direction.

  The carbon fibers preferably have at least one bend per one. By having at least one bend per piece, it is possible to further suppress variations in mechanical properties.

  Bending as used in the present invention refers to a state in which a single fiber is bent or a state in which the single fiber is bent, and is a shape in which the entire fiber is curved as seen in a fiber obtained by a spunbond method or a melt blow method. Is different. Preferably, there are one or more locations per single fiber, more preferably 2 locations or more, and even more preferably 5 locations or more.

  The bent portion is obtained by randomly extracting 400 carbon fibers from a carbon fiber sheet obtained by cutting out a part of the fiber-reinforced composite material and removing the resin, and counting and averaging the bent portions per one.

  The number average fiber length of the carbon fibers is not particularly limited, and an optimal fiber length can be appropriately selected according to various randomizing means. However, when the number average fiber length is 5 mm or more, high strength can be obtained. It is preferable in that it can be performed. More preferably, it is 30 mm or more, More preferably, it is 55 mm or more. Moreover, when it is 200 mm or less, it is preferable at the point which is easy to randomize and is easy to open. More preferably, it is 100 mm or less. The number average fiber length of the carbon fibers is obtained by randomly extracting 400 carbon fibers from a carbon fiber sheet obtained by cutting out a part of the fiber reinforced composite material and removing the resin, and using an optical microscope or a scanning electron microscope. The fiber length can be measured up to 10 μm and the average can be obtained.

  In the present invention, the sum of the ratios of COO groups and C—O groups in the C1s peak of the substrate surface measured by ESCA on the carbon fiber surface is 5% or more. Preferably it is 10% or more, More preferably, it is 15% or more. By introducing COO groups and CO groups by oxidizing carbon fibers, the resin is easily impregnated into the carbon fiber sheet, and as described above, the fibers are randomly oriented and high in fiber while suppressing voids. A fiber-reinforced composite material having a volume content can be obtained. As a result, the adhesion between the fiber and the resin can be improved.

  Here, as the carbon fiber, carbon fiber present on the surface of the sheet is arbitrarily sampled at five points, and an average value obtained by the following method is used. The ratio of the COO group and the C—O group is a value obtained by measuring a carbon fiber base material with ESCA (Electron Spectroscopy for Chemical Analysis) and calculating by the following method. That is, by dividing the C1s peak into peaks, the ratio of COO groups, C—O groups, and C—C groups in the C1s peak is obtained, and the total ratio of COO groups and C—O groups is calculated from this value. To do. In the peak division, in order to correct the charge of the spectrum obtained by the C1s spectrum measurement, the main peak is 284.6 eV indicating the binding energy of C—C, C═C, and CHx, and the peak position of the C—O group is 286.6 eV, the peak position of the C═O group is 287.6 eV, the peak position of the COO group is 288.6 eV, the π-π * satellite components of the conjugated system such as a benzene ring are 285.9 eV and 290.8 eV, and C -The peak heights of C, C = C, and CHx are set to be the same as the height of the main peak of C1s. For example, the COO group ratio can be obtained by dividing the peak area of the COO group by the area of the entire peak of the C1s spectrum. Similarly, the ratio of each C═O group, C—O group, and C—C group can be determined.

  Moreover, the sizing material may be provided to the carbon fiber. Examples of sizing materials include epoxy resins, epoxy-modified polyurethane resins, polyester resins, phenol resins, polyamide resins, polyurethane resins, polycarbonate resins, polyetherimide resins, polyamideimide resins, polyimide resins, bismaleimide resins, urethane-modified epoxy resins. Examples thereof include one, two or more of polyvinyl alcohol resin, polyvinyl pyrrolidone resin, polyether sulfone resin, etc. in solution, emulsion, suspension and the like.

  The composite material of this invention consists of carbon fiber and resin, and the volume content rate of carbon fiber is 20 to 80%. When the volume content is 20% or more, preferably 35% or more, and more preferably 40% or more, high tensile strength and tensile elastic modulus can be exhibited. Moreover, it is 80% or less, Preferably it is 70%, More preferably, it can be set as the material which can suppress the fracture | rupture of a fiber, can exhibit the physical property of a reinforced fiber more, and is excellent in specific strength. The fiber volume content can be determined according to JIS K 7075 (1991).

In the composite material of the present invention, the apparent density of the carbon fiber sheet is 0.25 to 1.5 g / cm ³ . In general, a sheet in which carbon fibers are randomly oriented has bulkiness due to the rigidity of the carbon fibers, and when the carbon fibers are simply deposited to form a sheet, it is difficult to obtain the apparent density of the present invention. As described above, if the resin is forcibly impregnated into a bulky carbon fiber sheet in order to make the volume content of the carbon fiber 20 to 80%, the carbon fiber breaks or a high-pressure press is required. Various problems occur. On the other hand, such a problem can be suppressed by setting the apparent density of the carbon fiber sheet within the scope of the present invention by the production method of the present invention described later. The apparent density of the carbon fiber sheet in the fiber reinforced composite material of the present invention is a value measured by removing the resin from the fiber reinforced composite material made of carbon fiber and resin, or the carbon fiber sheet just before impregnating the resin. Can be used as long as at least one of them is within the scope of the present invention.

In general, when a carbon fiber reinforced composite material is used as a stampable sheet, when the resin is removed, the carbon fiber sheet has a property of recovering to a thickness before impregnation with the resin by so-called spring back. If the springback is large, unintended deformation is likely to occur and the thickness is likely to be non-uniform, which is not preferable. In this invention, the effect that this springback can be suppressed can be acquired by making the apparent density of a carbon fiber sheet into the range of this invention. When the apparent density of the carbon fiber sheet is 0.25 g / cm ³ or more, more preferably 0.7 g / cm ³ or more, the spring back is small, and the molding cost can be suppressed.

Since the apparent density is adjusted according to the fiber volume content, it is not simply a high density, and it is preferable that the apparent density is equal to or slightly smaller than the apparent density corresponding to the target volume content. . In addition, it is preferable to set it as 1.0 g / cm < ³ > or less since the deformation | transformation of the fiber by compression, the fall of intensity | strength, etc. can be suppressed.

  The means for removing the resin is not particularly limited, and a method of treating with a solution capable of dissolving and decomposing the resin, a method described in JIS K 7075 (1991), and the like can be applied.

  The apparent density of the carbon fiber sheet after removing the resin from the composite material or before impregnating the resin is determined according to JIS L 1913 6.1 (thickness (Method A)). Five samples were collected, and each test piece after 10 seconds under a pressure of 0.5 kPa using a fully automatic compression elasticity / thickness measuring instrument (model: CEH-400) manufactured by Daiei Kagaku Seisakusho Co., Ltd. Thickness is measured at 10 locations, and the average value is obtained as the thickness. Then, the thickness is obtained by rounding off to the third decimal place from the thickness, length (20 cm × 20 cm), and weight. In addition, the average value of the obtained five apparent density was made into the apparent density of the carbon fiber sheet as used in the field of this invention.

  Examples of the resin used in the present invention include thermosetting resins such as epoxy resins, unsaturated polyesters, melamine, phenol and polyimide, and thermoplastic resins such as polyether ether ketone, polyphenylene sulfide, polyamide, polypropylene and polyester. Can do. In the present invention, a thermoplastic resin is preferred because it is easy to mold and advantageous in terms of cost, and the viscosity at the time of molding becomes high and the fluidity of fibers can be improved. Of the thermoplastic resins, polyamide is particularly preferable because of its excellent impregnation property.

  Next, the manufacturing method of the fiber reinforced composite material of this invention is demonstrated.

  Examples of a method of randomly orienting carbon fibers in a single fiber state include, for example, a method of cutting a fiber bundle and then forming a sheet, and a method of forming a single fiber by pelletizing it, but the cost and strength are compatible. In the present invention, the former is preferable. Specifically, for example, various types of non-woven fabrics such as a method of forming a non-woven web by cutting a carbon fiber to make a short fiber and then forming a non-woven web by a dry method using a card or a cross wrapper, a wet method using a paper machine, etc. The method can be applied. At this time, it is preferable to use a cotton spreader in the dry method, a beating machine in the wet method, and the like to form a nonwoven fabric after sufficiently opening the fiber. Although it is generally difficult to open the fiber after making it into a non-woven fabric, it can also be performed by means such as ultrasonic wave vibration, sandblasting or the like.

  As a method for imparting bending to the fiber, for example, a crimping means for the fiber can be employed. Examples of a method for imparting crimp include shape imparting by mechanical crimping, shape imparting by heat treatment such as composite fiber having a core-sheath structure of an eccentric type, side-by-side, false twisted yarn, and the like.

  Shape imparting by mechanical crimping is a method of imparting crimp to a straight fiber by a crimping imparting device such as a push-in crimper, or a method of imparting a shape by introducing fibers between two or more gears. The number of crimps and the crimp rate can be controlled by the peripheral speed difference, heat, and pressurization of the line speed.

  Forming by heat treatment includes a method in which heat is applied to a composite fiber made of two or more resins having different melting points, and the fiber is crimped in three dimensions by a heat shrinkage rate. Examples of the cross section of the composite fiber include an eccentric type with a core-sheath structure and a side-by-side type in which the melting points of the left and right components are different.

  In the present invention, in order to suppress variations in mechanical properties, it is preferably a fiber having a linear portion, and mechanical crimping can be preferably employed because it can be easily obtained.

  In general, in the case of carbon fiber, since there is a high possibility that the fiber breaks due to the bending, it is preferable that the carbon fiber is fired after being applied to the precursor fiber. For example, in the case of a PAN-based carbon fiber, a PAN-based carbon fiber having a bend can be obtained by imparting a bend to the flameproof yarn before carbonization and then firing it. Similarly, in the case of pitch-based carbon fibers, it is preferable to fire after applying bending in the state of, for example, pitch precursor fibers. Among carbon fibers, PAN-based carbon fibers, in particular, have high strength and elongation at break of the flame-resistant yarn of the precursor, and are easy to impart a clear bend. Therefore, a method of obtaining a PAN-based carbon fiber using a PAN-based flame resistant yarn Is a preferred production method of the present invention.

  In addition, the flame resistance of PAN for producing the PAN-based flameproof yarn is usually 0.8 to 1.2 in an atmosphere containing 4 to 25 mol% or more of air, a treatment time of 10 to 100 minutes, The temperature can be in the range of 150 to 350 ° C. It is preferable to set so that the specific gravity of the PAN-based flameproof yarn is in the range of 1.3 to 1.38.

It is preferable that the flame resistant yarn is formed into a sheet, and then the apparent density is adjusted using a calendar or a press, so that the apparent density of the carbon fiber sheet is 0.25 to 1.5 g / cm ³ . If the apparent density is adjusted with a press or the like after the flame resistant yarn sheet is fired, the fiber already breaks and the fiber is broken, making it difficult to obtain a sheet having the expected apparent density.

  It is generally difficult to uniquely determine the apparent density of the carbon fiber sheet from the apparent density of the flame resistant yarn sheet because it is affected by the flame resistant yarn conditions. For example, by firing with a load in the thickness direction, the flame resistant yarn A carbon fiber sheet having an apparent density of about ± 20% of the apparent density of the sheet can be obtained.

  The apparent density is preferably set in consideration of the target fiber volume content. It is preferable that at least the porosity of the sheet is the same as the target resin volume content, or 1.5 times or less. If the apparent density of the carbon fiber sheet is too small, the resin can only be impregnated into the voids, making it difficult to obtain the target fiber volume content. It is preferable to control the apparent density assuming that the resin is impregnated.

  Since the sum of the ratios of COO groups and C—O groups in the C1s peak measured by ESCA on the surface of the carbon fiber is 5% or more, the surface treatment of the carbon fiber is performed. The surface treatment may be performed in the fiber state before forming the sheet shape or may be performed after forming the sheet shape, but the latter is preferable in terms of efficiency of processing and cost.

  As the surface treatment method, in gas phase treatment such as oxidation with ozone gas, corona treatment, plasma treatment, etc., there is a high possibility that treatment variation will occur between the carbon fiber substrate surface and internal fibers, so oxidation treatment by liquid phase should be performed Is preferred.

  When the surface treatment is performed in the form of a sheet, it is preferable to positively remove the processing spots due to the presence of bubbles in the sheet. The removal of the bubbles can be performed by vibration, stirring, flow, or the like of the processing liquid. Moreover, a defoaming agent can also be provided.

  In the present invention, it is one of preferred embodiments that the surface treatment is performed by electrolytic oxidation. There are no particular restrictions on the electrolyte used in the electrolytic oxidation treatment, but acids such as sulfuric acid, nitric acid, hydrochloric acid, carbonic acid, ammonium nitrate, ammonium hydrogen nitrate, ammonium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium hydroxide, potassium hydroxide , Hydroxides such as barium hydroxide, inorganic salts such as sodium carbonate, sodium bicarbonate, sodium phosphate and potassium phosphate, organic salts such as sodium maleate, sodium acetate, potassium acetate and sodium benzoate, or ammonia In addition, alkalis such as ammonium carbonate and ammonium hydrogen carbonate can be used alone or in a mixture of two or more.

  The density | concentration of electrolyte solution should just be a range which does not impair processing efficiency, and can be performed at about 0.1-2 mol / liter.

  The amount of electricity oxidized in the electrolytic cell is preferably optimized in accordance with the carbonization degree of the surface-treated carbon fiber tow, and the electrolytic treatment is preferably performed in a plurality of electrolytic cells. Moreover, it is excellent in the impregnation property of the matrix resin, and the total amount of electricity is 5 to 1000 coulombs / g (the number of coulombs per gram of carbon fibers) in that the decrease in strength of the carbon fibers due to excessive oxidation of the fibers can be suppressed. Preferably, the range is 10 to 500 coulomb / g.

Further, the surface treatment can be an oxidation treatment with ozone water. Oxidation with ozone water is preferable because it does not use an electrolytic solution and therefore does not affect the physical properties due to residual electrolyte.
In the oxidation treatment with ozone water, the carbon fiber structure is immersed in a bath in which ozone gas is dissolved in pure water. The ozone concentration in the ozone water is preferably 10 mg / L to 110 mg / L, more preferably 30 to 100 mg / L, and even more preferably 40 to 90 mg / L in terms of excellent cost performance of the oxidation treatment with ozone water. is there.

  For the same reason, the treatment time in the ozone water bath is preferably 1 to 10 minutes, more preferably 2 to 7 minutes, and further preferably 3 to 5 minutes.

  By treating in this way, the sum of the ratios of COO groups and C—O groups in the C1s peak measured by ESCA on the surface of the carbon fiber on the surface of the carbon fiber sheet is 5% or more, preferably 10% or more. More preferably, the carbon fiber sheet is 15% or more.

  Subsequently, the obtained carbon fiber sheet is impregnated with a resin, and the fiber volume content is set to 20 to 80%. As the impregnation method, a method suitable for the molding method to be employed can be appropriately employed.

  The physical property values in the examples were measured by the following methods.

A. Measurement of Apparent Density of Carbon Fiber Nonwoven Fabric After immersing the composite material in formic acid dissolving nylon 6 at 25 ° C. for 1 hour, the composite material was taken out, washed with water, and dried at room temperature. Next, in accordance with JIS L 1913 6.1 (thickness (Method A)), five 20 cm × 20 cm test pieces were collected, and a fully automatic compression elasticity / thickness measuring instrument manufactured by Daiei Kagaku Seiki Seisakusho Co., Ltd. Using (type: CEH-400), the thickness of each test piece after 10 seconds was measured under a pressure of 0.5 kPa, and the average value thereof was defined as the thickness. From this thickness, length (20 cm × 20 cm), and weight, the apparent density was determined by rounding off to the third decimal place. The average value of the apparent density of the five obtained sheets was taken as the apparent density of the sheet.

B. Carbon Fiber Orientation Five points were randomly sampled from the composite material, and the surface was observed with a scanning electron microscope at a magnification of 300 times. At this time, when the number of the bundles of three or more fibers is one or less, it is assumed that there is no fiber bundle (single fiber state).

C. COO group, CO group ratio ESCA (X-ray photoelectron spectroscopy) With carbon fiber existing on the surface of the carbon fiber substrate, the carbon fiber substrate is cut in an arbitrary surface, the carbon fiber present in the interior The C1s spectrum was measured under the following conditions, and peak splitting was performed.

In the peak division, the C1s main peak is adjusted to 284.6 eV indicating the binding energy of C—C, C═C, and CHx, and the peak position of the C—O group is set to 286.6 eV, C, in order to perform charge correction of the spectrum. The peak position of the O group is 287.6 eV, the peak position of the COO group is 288.6 eV, π-π * satellite components of the conjugated system such as a benzene ring are 285.9 eV and 290.8 eV, and C—C, C = C , CHx peak height is the same as the C1s main peak height, and peak division is performed, and the respective peak areas of the COO group and CO group are divided by the total area of the C1s peak. The obtained values were taken as the COO group ratio and the CO group ratio, and the sum was obtained.
・ Device: Quantera SXM (manufactured by PHI)
Excitation X-ray: monochromatic Al Kα1,2 line (1486.6 eV)
・ X-ray diameter: 200 μm
-Photoemission angle: 45 degrees (inclination of the detector with respect to the sample surface)
Data processing: Spectrum (narrow scan) smoothing: 9-point smoothing
-Number of measurement samples: The surface and the inner surface were measured at five points each with a sampling interval of 1 cm or more.
-Peak splitting: Peak splitting of the C1s peak was performed using an Avantage data system (manufactured by Thermo Fisher Scientific).

D. Tensile strength Type 1BA type in each direction of 0 °, 15 °, 30 °, 45 °, 60 °, 75 °, 90 ° within the sample surface according to the method described in JIS K 7161-7164 (1994) A small test piece was prepared and the tensile fracture stress was measured. The average of the tensile fracture stress in all directions was defined as the tensile strength.

E. Fiber volume content (Vf)
It was measured according to JIS K 7075 (1991).

F. Number average fiber length Resin is removed from the composite material to obtain a carbon fiber sheet. Next, 400 carbon fibers were randomly extracted (excluding fibers that were broken during extraction), and the length was measured to the unit of 10 μm with an optical microscope or a scanning electron microscope to obtain the fiber length. The fiber length was obtained.
Number average fiber length (Ln) = (ΣLi) / 400
Li: measured fiber length (i = 1, 2, 3,... 400)
G. Number of bends Obtained by randomly extracting 400 carbon fibers from a carbon fiber sheet obtained by cutting out a portion of the fiber-reinforced composite material and removing the resin, and counting and averaging the number of bends per piece.

Example 1
Using a copolymer composed of 99.4 mol% of acrylonitrile (AN) and 0.6 mol% of methacrylic acid, an AN fiber bundle having a single fiber denier 1d and a filament number of 12,000 was obtained by a dry and wet spinning method. The obtained PAN-based fiber bundle was heated at a draw ratio of 1.05 in air at a temperature of 240 to 280 ° C. to obtain a PAN-based flame resistant yarn (density 1.38 g / cm ³ ).

Next, the PAN flame resistant yarn was crimped by a push-in crimper. The number of crimps of the obtained crimped yarn was 7.1 / 25 mm, and the crimp rate was 12.7%. This flame resistant yarn was cut into a number average fiber length of 76 mm, and then formed into a web sheet using a card and a cross wrapper, and then made into a PAN flame resistant yarn nonwoven fabric with an apparent density of 0.05 g / cm ^{3 by} needle punching.

The obtained PAN-based flameproof nonwoven fabric was compressed with a press machine heated to 200 ° C. to give an apparent density of 0.78 g / cm ³ .

Next, after heating to a temperature of 1500 ° C. in a nitrogen atmosphere and firing (carbon fiber density: 1.80 g / cm ³ ), the carbon fiber nonwoven fabric is immersed in an aqueous ammonium hydrogen carbonate solution (0.1 mol / liter). Then, an electrolytic oxidation treatment was performed so that the amount of electricity was 76 coulombs / g while performing ultrasonic treatment at 26 KHz in an aqueous solution, followed by washing with water and drying. The sum of the COO group ratio and the C—O group ratio in the C1s peak on the surface of the carbon fiber nonwoven fabric after electrolytic oxidation treatment was measured by ESCA. The apparent density of the carbon fiber nonwoven fabric at this time was 0.72 g / cm ³ .

This PAN-based carbon fiber nonwoven fabric was melt impregnated with nylon 6 having a density of 1.14 g / cm ³ to obtain a fiber-reinforced composite material having a fiber volume content (Vf) of 40%. The evaluation results of the obtained fiber reinforced composite material are as shown in the table, and it was found that the resin impregnation property is good and high physical properties can be obtained. Further, when the surface was observed with this scanning electron microscope, the carbon fibers were randomly oriented in a single fiber state. As a result of measuring the apparent density of the carbon fiber nonwoven fabric by removing nylon 6 with formic acid, 0.70 g / cm ³ . Moreover, the number of bendings was 17.5.

Example 2
The treatment was performed in the same manner as in Example 2 except that the amount of electricity was 100 coulomb / g. The impregnation property of the resin was better than that of Example 1, and the strength was slightly improved.

Example 3
The apparent density of the carbon fiber nonwoven fabric before impregnation with the resin is 0.52 g / cm ³ (the apparent density of the carbon fiber nonwoven fabric after removing the resin is 0.50 g / cm ³ ), and the fiber volume content (Vf) is 20%. The process was the same as in Example 1 except that. The obtained results showed a slight decrease in strength corresponding to the decrease in Vf compared to Example 1.

Example 4
The same treatment as in Example 3 was conducted except that the apparent density of the carbon fiber nonwoven fabric before impregnation with the resin was 0.26 g / cm ³ (the apparent density of the carbon fiber nonwoven fabric after removing the resin was 0.25 g / cm ³ ). did. As a result, the strength was slightly reduced as compared with Example 3. Moreover, after removing the resin, a springback occurred and a broken fiber piece was observed.

Example 5
The same treatment as in Example 1, except that the apparent density of the carbon fiber nonwoven fabric before resin impregnation was 0.26 g / cm ³ (the apparent density of the carbon fiber nonwoven fabric after removing the resin was 0.25 g / cm ³ ). did. As a result, the strength increased as Vf increased compared to Example 4, but decreased slightly compared to Example 1 of the same Vf. Moreover, after removing the resin, a springback occurred, and more broken fiber pieces than in Example 4 were observed.

Comparative Example 1
In Example 1, the PAN-based flame resistant yarn was continuously fired as it was without crimping and cutting to obtain a PAN-based carbon fiber having no bending. This carbon fiber is immersed in an aqueous solution of ammonium hydrogencarbonate (0.1 mol / liter), subjected to electrolytic oxidation treatment so that the amount of electricity becomes 76 coulombs / g while being ultrasonically treated at 26 KHz in the aqueous solution, and washed with water. And drying. Next, after cutting to 3 mm to make carbon fiber short fibers without crimping, a web sheet having an apparent density of 0.05 g / cm ³ was formed by a papermaking method. Subsequently, nylon 6 having a density of 1.14 g / cm ³ was melt-impregnated to obtain a fiber-reinforced composite material having a fiber volume content (Vf) of 40%. The evaluation results of the obtained fiber reinforced composite material are as shown in the table, and the strength was greatly reduced as compared with Example 1.

  When the surface was observed with a scanning electron microscope, the carbon fibers were randomly oriented in a single fiber state, and when nylon 6 was removed with formic acid, the fibers were broken and the original shape could not be maintained. .

Comparative Example 2
In Example 1, the PAN-based flame resistant yarn was continuously fired as it was without crimping and cutting to obtain a PAN-based carbon fiber having no bending. Then, 2.0% by weight of epoxy resin was applied to focus the fibers, and then cut to 64 mm. The rest was processed in the same manner as in Example 1 to obtain a composite material. The fiber volume content was 40%, and the fiber could be normally impregnated, but the strength was lower than that of Example 1. The apparent density was not measured in the same manner as in Comparative Example 1, and the carbon fibers were not separated when the resin was removed.

  Further, when the surface was observed with a scanning electron microscope, two or more carbon fibers converged were confirmed.

Comparative Example 3
The treatment was performed in the same manner as in Example 1 except that the electrolytic treatment was not performed. Apparently, the target fiber volume content could be achieved, but fine voids were present at the interface, and the strength of the resulting composite material was very low.

Comparative Example 4
No electrolytic treatment, the apparent density of the carbon fiber sheet before impregnating the resin is 0.52 g / cm ³ (the apparent density of the carbon fiber substrate after removing the resin is 0.5 g / cm ³ ), and the fiber volume The same treatment as in Comparative Example 1 was performed except that the content rate was 20%. With this fiber volume content, the resin could be impregnated, but there were many voids and the strength was low.

Comparative Example 5
The PAN-based carbon fiber nonwoven fabric with an apparent density of 0.05 g / cm ³ obtained in Example 1 was compressed with a press machine heated to 200 ° C. to make the apparent density 0.10 g / cm ³ . Next, firing and electrolytic oxidation treatment were performed in the same manner as in Example 1. The apparent density of the carbon fiber nonwoven fabric at this time was 0.07 g / cm ³ .

This PAN-based carbon fiber nonwoven fabric was melt impregnated with nylon 6 having a density of 1.14 g / cm ³ to obtain a fiber-reinforced composite material having a fiber volume content (Vf) of 40%. The evaluation results of the obtained fiber reinforced composite material are as shown in the table, and the strength was greatly reduced as compared with Example 1.

  Further, when the surface was observed with this scanning electron microscope, the carbon fibers were randomly oriented in a single fiber state. After removing the resin, there were many broken fibers, the sheet shape was not maintained, and it was difficult to measure the apparent density and the number of bendings.

Claims (7)

A fiber-reinforced composite material composed of carbon fibers and a resin, wherein the carbon fibers are randomly oriented in a single fiber state, and the carbon fibers constitute a sheet shape having an apparent density of 0.25 to 1.5 g / cm ^3. The sum of the ratios of COO groups and CO groups in the C1s peak measured by ESCA on the carbon fiber surface of the sheet surface is 5% or more, and the fiber volume content of the carbon fibers is 20 to 80% Is a fiber reinforced composite material. The fiber-reinforced composite material according to claim 1, wherein the carbon fiber is a polyacrylonitrile-based carbon fiber. The fiber-reinforced composite material according to claim 1 or 2, wherein each single fiber of carbon fiber has at least one bend. The fiber-reinforced composite material according to claim 1, wherein the sheet is a nonwoven fabric. The number average fiber length of carbon fiber is 5-200 mm, The fiber reinforced composite material in any one of Claims 1-4. The fiber-reinforced composite material according to any one of claims 1 to 5, wherein the resin is a thermoplastic resin. A method for producing a fiber reinforced composite material in which a carbon fiber sheet is impregnated with a resin, wherein the carbon fiber sheet is oriented randomly in a single fiber state and has an apparent density of 0.25 to 1.5 g / cm ³ . The sum of the ratios of COO groups and CO groups in the C1s peak measured by ESCA on the surface of the carbon fiber on the surface is 5% or more, and the fiber volume content of the carbon fiber is 20 to 80% A method for producing a fiber-reinforced composite material.