Forming material and formed product

Abstract

The present invention addresses the problem of providing a resin composition which offers high formability of a member with complicated geometry by injection molding and with which it is possible to obtain a molded article that exhibits excellent bending elastic modulus and impact characteristics.　A molding material comprising carbon fibers and a resin composition containing a thermoplastic resin, wherein: the lengths of the carbon fibers is substantially identical with the length of the molding material; the carbon fibers have a twisting angle of 2.0-30.5° at the surface thereof; a cross-sectional surface of the molding material perpendicular to the axial direction of the carbon fibers has a core ratio A / (A+B) of 0.1-0.5, where A represents the area ratio of a core portion including the carbon fibers, while B represents the area ratio of a sheath portion other than the core portion; and the area ratio of voids accounts for 5% or less in a cross-sectional surface of the molding material perpendicular to the axial direction of the carbon fibers.

Description

Technical Field

[0001] The present invention relates to a resin composition containing a thermoplastic resin and a molding material containing carbon fibers. More specifically, the present invention relates to a molding material in which carbon fibers are well dispersed in a molded article during injection molding.

Background Art

[0002] Since carbon fiber composite materials, particularly carbon fiber reinforced plastics, exhibit excellent mechanical properties, they have been widely used in recent years as lightweight materials to replace members to which light metals such as aluminum were conventionally applied. However, in order for carbon fiber reinforced plastics to exhibit excellent mechanical properties, they are often used in the form of fibers with a length of several millimeters or more, either as continuous fibers or discontinuous fibers. In that case, there has been a problem that it is difficult to form them into complex shapes. On the other hand, when injection molding, which is excellent in formability into complex shapes, is applied to a molding material of a thermoplastic resin containing carbon fibers, the flexural modulus of the molded article is generally low, and the mechanical properties are not satisfactory as a replacement for light metals, and the quality of the molded article is also not satisfactory.

[0003] Although the molding material processed into pellets is suitable for injection molding, in order to increase the flexural modulus of an injection molded article of a thermoplastic resin containing carbon fibers, a method of performing molding so as to leave the fiber length of the carbon fibers long, that is, making the length of the carbon fibers substantially the same as the length of the molding material without shortening the length of the carbon fibers at the molding material stage has been carried out (Patent Document 1). Further, attempts have been made to reduce voids in the molding material (Patent Document 2). In addition, attempts have been made to impart twist to reinforcing fibers in order to improve the handleability of the molding material (Patent Documents 3 and Non-Patent Document 1). Further, although the carbon fibers are short fibers, there is an example in which the physical properties of the molded article are improved by using carbon fibers having a tensile modulus of 390 to 450 GPa (Patent Document 4).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Document

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the prior art has the following problems.

[0007] In Patent Document 1, although the length of the carbon fiber is substantially the same as the length of the molding material and it is divided into a layer with a high content rate of reinforcing fiber and a layer with a low content rate, since the carbon fiber has no twist, the physical properties and dispersibility of the molded product are low. In Patent Document 2, although it is shown that it is better if there are fewer voids contained in the molding material, the void ratio of the examples is still as high as 40%, and the physical properties and dispersibility of the molded product are low. In Patent Document 3 and Non-Patent Document 1, although twist is added to the reinforcing fiber when manufacturing the molding material, since the molding material contains voids, the physical properties and dispersibility of the molded product are low. In Patent Document 4, since it is a molding material with carbon fiber as short fiber, the physical properties and dispersibility of the molded product are low.

[0008] As described above, in the prior art, there were ideas for resin compositions containing thermoplastic resins and molding materials containing carbon fibers, such as molding materials with substantially the same length of carbon fibers and molding materials, molding materials that reduce voids, and adding twists to carbon fibers. However, there was no suggestion at all regarding molding materials suitable for injection molding that achieved both of these at a high level. Also, using carbon fibers with a high tensile modulus in molding materials where the length of the carbon fibers and the molding material were substantially the same had not been able to improve the physical properties and dispersibility of the molded products.

Means for Solving the Problems

[0009] The present invention for solving the above problems is composed of the following configuration.

[0010] That is, the molding material of the present invention is a resin composition containing a thermoplastic resin and a molding material containing carbon fibers, wherein the length of the carbon fibers is substantially the same as the length of the molding material, the twist angle of the carbon fiber surface layer is 2.0 to 30.5°, and the core ratio A / (A + B) consisting of the area ratio A of the core portion containing carbon fibers and the area ratio B of the other sheath portion in the cross-section perpendicular to the carbon fiber axial direction of the molding material is 0.1 to 0.5, and the area ratio of voids in the cross-section perpendicular to the carbon fiber axial direction of the molding material is 5% or less.

[0011] Also, the molded product of the present invention is formed by molding the above molding material.

Effects of the Invention

[0012] The molding material of the present invention has high moldability for members with complex shapes by injection molding, and in addition, is excellent in the physical properties and dispersibility of the obtained molded products.

Brief Description of the Drawings

[0013]

Figure 1

Modes for Carrying Out the Invention

[0014] The molding material of the present invention contains a resin composition containing a thermoplastic resin and carbon fibers.

[0015] First, the carbon fibers used in the present invention will be described.

[0016] The carbon fibers used in the present invention are in an aggregated form having a fiber bundle form and are present in the molding material. The number of filaments of the carbon fibers is not particularly limited, but is preferably 3,000 to 60,000. The twist angle of the surface layer of the carbon fibers in the molding material is 2.0 to 30.5°, preferably 4.8 to 30.5°, and more preferably 4.8 to 24.0°. The twist angle of the carbon fiber surface layer refers to the angle formed by the fiber axis direction of the single fiber existing on the outside of the carbon fibers present in the aggregated form in the molding material, that is, at the boundary between the core portion and the sheath portion described later, with respect to the long axis direction of the carbon fiber bundle. It may be observed directly, but more precisely, it can be calculated from the twist number of the carbon fiber bundle, the number of its filaments, and the single fiber diameter. The twist angle of the carbon fiber surface layer in the molding material can be evaluated by taking out the carbon fiber bundle by burning off or the like from the molding material and observing it. By controlling such a twist angle within the above range in the molding material of the present invention, the obtained molded product has high dispersibility of carbon fibers, and a molded product with high physical properties can be obtained. The number of filaments of the carbon fibers in the molding material and the twist angle of the carbon fiber surface layer can be adjusted by the number of filaments of the carbon fiber bundle, the number of carbon fiber bundles, and the twist conditions of the carbon fiber bundle (use of twisted yarns and twisting during the process) in the manufacturing process of the raw material carbon fibers and the molding material.

[0017] The number of filaments of carbon fiber contained in each molding material of the present invention is preferably from 3,000 to 60,000, more preferably from 10,000 to 60,000, and still more preferably from 20,000 to 60,000. The volume content of carbon fiber in the molding material can be adjusted according to the size of the molding material and the number of filaments of carbon fiber contained in the molding material. In order to adjust the number of filaments of carbon fiber contained in the molding material, the number of filaments contained in one carbon fiber bundle, which is a raw material when manufacturing the molding material, may be adjusted, or it may be adjusted by charging a plurality of carbon fiber bundles, which are raw materials when manufacturing the molding material.

[0018] The carbon fiber used in the present invention preferably has a tensile elastic modulus of 280 to 500 GPa, more preferably 330 GPa or more, and still more preferably 350 GPa or more. The higher the tensile elastic modulus of the carbon fiber, the higher the flexural elastic modulus of the injection molded product obtained by molding the molding material can be, and when the dispersibility of the carbon fiber in the molded product is increased, it becomes easier to obtain the effect that the flexural elastic modulus becomes higher. If the tensile elastic modulus is 280 GPa or more, the flexural elastic modulus of the injection molded product can be made significantly high, so the industrial value is great. From the viewpoint of increasing the flexural elastic modulus of the injection molded product, it is preferable that the tensile elastic modulus of the carbon fiber is high, but if it is too high, the breakage of the carbon fiber during molding increases, so the effect of making the flexural elastic modulus of the injection molded product high weakens. Therefore, the tensile elastic modulus is preferably 500 GPa or less. The tensile elastic modulus of the carbon fiber can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608:2004. Details of the evaluation method of the strand elastic modulus will be described later.

[0019] In the carbon fiber used in the present invention, the upper limit value of the crystallization parameter Iv / Ig by Raman spectroscopy is preferably 0.80, more preferably 0.70, and still more preferably 0.60. Further, the lower limit value of Iv / Ig is preferably 0.25, more preferably 0.30, and still more preferably 0.40. The Raman spectrum obtained from the single fiber cross section of the carbon fiber is 1580 cm -1Near the G band, 1360 cm -1 Near the D band, 1480 cm -1 A valley can form between these bands nearby. Taking the peak intensity of the G band as Ig and the part with the weakest spectral intensity near 1480 cm -1 as Iv, the ratio serves as an indicator showing the degree of progress of the crystallization of the internal structure of the carbon fiber. For commercially available carbon fibers, those with a tensile elastic modulus of around 380 GPa have Iv / Ig less than 0.2, and those with a tensile elastic modulus of 230 - 290 GPa have Iv / Ig of 0.70 or more. The fact that Iv / Ig is 0.80 or less fully indicates that the crystallization of the internal structure of the carbon fiber has progressed, and carbon fibers with such an internal structure often have a high tensile elastic modulus. The fact that Iv / Ig is 0.25 or more indicates that the crystallization inside the carbon fiber has not progressed too much. By using carbon fibers with such an internal structure, it becomes easy to make the physical properties of the obtained molded product high. The crystallization parameter Iv / Ig is evaluated by Raman spectroscopy. The detailed evaluation method will be described later. Such a parameter can be adjusted by the maximum carbonization temperature during carbon fiber production.

[0020] The carbon fiber used in the present invention preferably has a heat loss rate at 450 °C of 0.15% or less, more preferably 0.10% or less, and even more preferably 0.07% or less. In the present invention, the detailed measurement method of the heat loss rate at 450 °C will be described later, but it refers to the mass change rate before and after weighing a certain amount of the carbon fiber to be measured and heating it in an oven with an inert gas atmosphere set at a temperature of 450 °C for 15 minutes. Carbon fibers with a low heat loss rate under such conditions contain less components that decompose upon exposure to high temperatures, such as sizing agents. When the heat loss rate is 0.15% or less, the dispersibility of the carbon fiber in the resin composition is excellent, making it easy to obtain a high flexural elastic modulus for the injection molded product.

[0021] The carbon fiber used in the present invention preferably has a single fiber diameter of 6.0 μm or more, more preferably 6.5 μm or more, and even more preferably 6.9 μm or more. The larger the single fiber diameter, the easier it is for the fiber to remain long during injection molding, and as a result, a molded product with a higher flexural modulus can be obtained. Therefore, when the single fiber diameter is 6.0 μm or more, it becomes easier to increase the flexural modulus of the injection molded product. In the present invention, there is no particular limitation on the upper limit of the single fiber diameter. However, if it is too large, the tensile modulus of the carbon fiber may decrease, so about 15 μm may be considered as a provisional upper limit. The method for evaluating the single fiber diameter will be described later, but it may be calculated from the density, basis weight, and number of filaments of the carbon fiber, or it may be evaluated by scanning electron microscope observation. If the evaluation apparatus used is correctly calibrated, equivalent results can be obtained by any evaluation method. When evaluating by scanning electron microscope observation, if the cross-sectional shape of the single fiber is not a perfect circle, the equivalent circle diameter is used instead. The equivalent circle diameter refers to the diameter of a perfect circle having the same cross-sectional area as the actually measured cross-sectional area of the single fiber.

[0022] The resin composition in the present invention contains a thermoplastic resin and, if necessary, additives. Also, even when it consists of only a single thermoplastic resin, it is included in the resin composition. In the core part and sheath part described later, the composition of the resin composition may be the same or different.

[0023] The thermoplastic resin used in the present invention is preferably at least one thermoplastic resin selected from the group consisting of polyolefin, polyamide, polyester, polycarbonate, and polyarylene sulfide. From the viewpoint of the flexural modulus of the obtained molded product, polyamide and polyarylene sulfide are more preferable, and polyarylene sulfide is particularly preferable. By combining with the carbon fiber used in the present invention, the mechanical properties such as the flexural modulus of the molded product can be improved without being restricted by the type of thermoplastic resin, so a wide range of thermoplastic resins can be selected. However, by selecting a thermoplastic resin that easily obtains a molded product with high mechanical properties, specifically, a thermoplastic resin that exhibits a high tensile yield stress, the effects of the present invention can be easily obtained.

[0024] Examples of the polyolefin include a homopolymer of propylene or a copolymer of propylene and at least one α-olefin, conjugated diene, non-conjugated diene, or the like.

[0025] Examples of the polyamide include polymers having a main chain formed by repeating amide groups, such as aliphatic polyamides like polyamide 6, polyamide 66, polyamide 11, polyamide 610, polyamide 612, or aromatic polyamides like polyamide 6T. Mixtures thereof or multiple types of polyamide copolymers may also be used.

[0026] Examples of the polyarylene sulfide include those composed of p-phenylene sulfide units, m-phenylene sulfide units, o-phenylene sulfide units, phenylene sulfide sulfone units, phenylene sulfide ketone units, phenylene sulfide ether units, diphenylene sulfide units, substituent-containing phenylene sulfide units, or branched structure-containing phenylene sulfide units, and poly p-phenylene sulfide is particularly preferred.

[0027] As the polycarbonate, for example, known ones as described in JP-A-2018-059087 may be used.

[0028] In the molding material of the present invention, additives can be added as long as the effects of the present invention are not impaired. Specific examples of the additives include antioxidants, heat stabilizers, weathering agents, mold release agents, lubricants, pigments, dyes, plasticizers, antistatic agents, flame retardants, and resin A shown below. Resin A is preferably at least one selected from the group consisting of terpene resins, epoxy resins, phenolic resins, and cyclic polyphenylene sulfide. Resin A is appropriately selected according to the combination with the thermoplastic resin that is the matrix resin. For example, if the molding temperature is in the range of 150 to 270 °C, a terpene resin is preferably used. If the molding temperature is in the range of 270 to 320 °C, an epoxy resin is preferably used. Specifically, when the thermoplastic resin is a polypropylene resin, resin A is preferably a terpene resin. When the thermoplastic resin is a polycarbonate resin or a polyphenylene sulfide resin, resin A is preferably an epoxy resin. When the thermoplastic resin is a polyamide resin, resin A is preferably a terpene phenol resin. The epoxy resin preferably used as resin A is a compound having two or more epoxy groups, substantially free of a curing agent, and not undergoing curing by so-called three-dimensional crosslinking even when heated. Examples thereof include glycidyl ether type epoxy resins, glycidyl ester type epoxy resins, glycidyl amine type epoxy resins, and alicyclic epoxy resins. Two or more of these may be used. Among them, glycidyl ether type epoxy resins are preferred because of their excellent balance of viscosity and heat resistance, and examples thereof include bisphenol A type epoxy resins and bisphenol F type epoxy resins.

[0029] A preferred embodiment of the method for manufacturing the molding material of the present invention is a method in which the above resin A is first adhered to a carbon fiber bundle, and then a thermoplastic resin is adhered. For the adhesion step of resin A, known manufacturing methods such as applying an oil agent, a sizing agent, and a matrix resin to the fiber bundle can be used. As a more specific example, a coating of a certain thickness of molten resin A is coated on the surface of a heated rotating roll, and the carbon fiber bundle is run while being in close contact with or rubbed against the roll surface, so that a predetermined amount of resin A is adhered per unit length of the carbon fiber bundle. Regarding the coating of resin A on the roll surface, it can be realized by applying the mechanisms of known coating devices such as reverse rolls, direct rotation rolls, kiss rolls, sprays, curtains, and extrusion. In the step of adhering resin A to the carbon fiber bundle, at the temperature at which resin A melts, operations such as applying tension with a roll or a bar, repeating widening and bunching, and applying pressure or vibration are performed on the carbon fiber bundle with resin A adhered thereto, so that resin A is impregnated between the single fibers constituting the carbon fiber bundle. As a more specific example, a method of passing the carbon fiber bundle in contact with the surfaces of a plurality of heated rolls or bars can be mentioned.

[0030] Furthermore, a resin A - adhered carbon fiber bundle composed of a carbon fiber bundle and resin A is brought into contact with the above - mentioned thermoplastic resin to form a molding material. As the arrangement step of the thermoplastic resin, the molten thermoplastic resin is arranged in contact with the resin A - adhered carbon fiber bundle. More specifically, a method of continuously arranging and coating the thermoplastic resin around the resin A - adhered carbon fiber bundle using an extruder and a coating die for an electric wire coating method, or a method of arranging a molten film - shaped thermoplastic resin from one side or both sides of the resin A - adhered carbon fiber bundle flattened by a roll or the like using an extruder and a T - die and integrating them with a roll or the like can be mentioned.

[0031] After the carbon fiber bundle is integrated with the thermoplastic resin in the above preferred embodiment, it may be cut into a fixed length of, for example, 1 to 50 mm using a device such as a pelletizer or a strand cutter and then used. This cutting process may be continuously installed after the placement process of the thermoplastic resin. When the molding material is flat or sheet-shaped, it may be slit to make it slender and then cut. A sheet pelletizer that performs slitting and cutting simultaneously may also be used.

[0032] In the molding material of the present invention, the length of the carbon fiber is substantially the same as the length of the molding material. The molding material of the present invention preferably has a columnar shape such as a pellet. Substantially the same length means that, for example, in a pellet-shaped molding material, the carbon fiber bundle or the carbon fiber contained in the carbon fiber bundle is not cut in the middle of the pellet, or there is substantially no carbon fiber significantly shorter than the total length of the pellet. In particular, although the amount of carbon fiber shorter than the total length of the molding material is not defined, when the content of carbon fiber having a length of 50% or less of the total length of the molding material is 30% by mass or less, it is considered that there is substantially no carbon fiber significantly shorter than the total length of the molding material. In the present invention, even when the length of the carbon fiber contained in the molding material is longer than the length of the molding material, it is regarded as having substantially the same length. The total length of the molding material is the length of the molding material in the direction of the synthetic vector obtained by adding up all the vectors connecting the two ends of the single fiber of each individual carbon fiber in the molding material (in the present invention, defined as the carbon fiber axial direction of the molding material). Since the carbon fiber has substantially the same length as the molding material, the carbon fiber length in the obtained molded product can be made long, and thus a molded product having excellent physical properties can be obtained.

[0033] As illustrated in FIG. 1, the molding material of the present invention has a core-sheath structure in which a "core portion" containing carbon fibers is surrounded by a "sheath portion" substantially free of carbon fibers. However, in the present invention, even if a part of the core portion is exposed on the surface of the molding material, it is regarded as a core-sheath structure when the length of the exposed portion is 10% or less of the total length of the contour of the core portion. A detailed definition of the core portion in the present invention will be described. First, regarding the molding material, a cross-section perpendicular to the carbon fiber axial direction of the molding material (hereinafter sometimes abbreviated as a perpendicular cross-section) is exposed and observed with an optical microscope. At this time, the smallest convex polygon that includes all the single fibers of each carbon fiber, that is, the so-called convex hull, is defined as the core portion in the present invention. The method of obtaining the convex hull is generally known. Therefore, in the same perpendicular cross-section, the region other than the core portion is defined as the sheath portion in the present invention. Note that the molding material of the present invention may contain voids, so-called voids, in either or both of the core portion and the sheath portion. Voids existing inside the boundary line of the convex hull are counted as the area of the core portion, and voids existing outside are counted as the area of the sheath portion. For voids straddling the boundary line of the convex hull, the portion inside the boundary line is counted as the area of the core portion, and the portion outside is counted as the area of the sheath portion. The molding material of the present invention has a core ratio A / (A + B) composed of the area ratio A of the core portion and the area ratio B of the sheath portion, which is 0.1 to 0.5, preferably 0.1 to 0.4, and more preferably 0.2 to 0.4. Such a core ratio is determined by the volume fraction of carbon fibers and voids in the core portion, and it is important that the gaps between individual single fibers of carbon fibers are completely impregnated with resin. If such a core ratio is 0.1 or more, the physical properties of the molded product can be improved, and if it is 0.5 or less, the dispersibility can be maintained at a satisfactory level.

[0034] The volume content Vf of carbon fibers contained in the molding material is preferably 5 to 25%, more preferably 10 to 25%, and even more preferably 15 to 25%. The higher the volume content, the higher the physical properties of the obtained molded product, and such an effect is likely to be exhibited if it is 5% or more. If the volume content is 25% or less, it becomes easy to obtain a molded product with good dispersibility of carbon fibers. The volume content Vf of carbon fibers may be measured by a known method of burning out components other than carbon fibers or dissolving them in a solvent. When the heat resistance temperature of components other than carbon fibers is high, the volume content Vf of carbon fibers can also be evaluated from the area ratio of carbon fibers in a cross section perpendicular to the axial direction of carbon fibers in the molding material. In order to adjust such a volume content, it can be adjusted with the respective mass contents in consideration of the density of the thermoplastic resin used and the density of carbon fibers.

[0035] In addition, the molding material of the present invention preferably satisfies the following relational expression between the volume content Vf of carbon fibers and the core ratio A / (A + B) while the volume content Vf of carbon fibers satisfies the above range. 1.5×Vf / 100 ≦ A / (A + B) ≦ 3×Vf / 100 If Vf / 100 and the core ratio match, the volume content of carbon fibers in the core part is 100%, and if the core ratio is 2×Vf / 100, the volume content of carbon fibers in the core part is 50%. Therefore, the above relational expression means that the volume content of carbon fibers in the core part is 33 to 67%. If the core ratio is adjusted within the above range, it is an appropriate impregnation degree of the core part, and the core-sheath ratio is also appropriate from the viewpoint of the dispersibility of the molded product. In order to adjust the core ratio within the above range, it can be controlled by the selection of resin A and the impregnation degree.

[0036] The molding material of the present invention has a void area ratio in a cross-section perpendicular to the carbon fiber axial direction of the molding material of 5% or less, preferably 4% or less, and more preferably 3% or less. When voids are included in the molding material, kneading during injection molding becomes non-uniform, and the dispersibility of carbon fibers in the molded product deteriorates. If the void area ratio is 5% or less, the dispersibility of carbon fibers in the molded product is excellent. The void area ratio in a cross-section perpendicular to the carbon fiber axial direction of the molding material is observed by cross-section observation in the same manner as checking the core ratio of the molding material, and the void portion containing neither the resin composition nor the carbon fiber is observed. The void portion can be easily discriminated by an optical microscope. In order to reduce voids, when manufacturing the molding material, it is preferable to use carbon fibers with twist rather than manufacturing while adding twist, and it is more preferable to enhance the impregnation property by using Resin A.

[0037] The maximum area of voids contained in a cross-section perpendicular to the carbon fiber axial direction of the molding material of the present invention is preferably 3000 μm 2 or less, more preferably 1000 μm 2 or less, and even more preferably 500 μm 2 or less. Not only the total amount of voids but also the inclusion of large voids deteriorates the dispersibility of carbon fibers in the molded product. Therefore, when the maximum area of voids is 3000 μm 2 or less, the dispersibility of carbon fibers in the molded product is likely to be at a good level. In order to reduce the maximum area of voids, when manufacturing the molding material, it is preferable to use carbon fibers with twist rather than manufacturing while adding twist, and it is more preferable to enhance the impregnation property by using Resin A.

[0038] Hereinafter, the evaluation method of the parameters used in the present invention will be described in detail.

[0039] <Tensile modulus of carbon fiber> The tensile modulus of carbon fiber is determined according to the following procedure in accordance with the resin-impregnated strand test method of JIS R7608:2004. However, when the fiber bundle of carbon fiber has twist, it is untwisted by applying the same number of reverse twists as the number of twists and then evaluated. As the resin formulation, "Celloxide (registered trademark)" 2021P (manufactured by Daicel Chemical Industries, Ltd.) / boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.) / acetone = 100 / 3 / 4 (parts by mass) is used, and as the curing conditions, normal pressure, a temperature of 125°C, and a time of 30 minutes are used. Ten strands of the carbon fiber bundle are measured, and the average value is taken as the strand strength and the strand modulus. Note that the strain range for calculating the strand modulus is 0.1 to 0.6%.

[0040] <Single fiber diameter of carbon fiber> The cross-section of a single fiber of the carbon fiber to be evaluated is observed with a scanning electron microscope, and the cross-sectional area is evaluated. The diameter of a perfect circle having the same cross-sectional area as such a cross-sectional area is calculated and taken as the single fiber diameter. The number N for calculating the single fiber diameter is 50, and the average value is adopted. Note that the acceleration voltage is 5 keV.

[0041] In the present invention, a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High-Technologies Corporation can be used as the scanning electron microscope.

[0042] <Twist angle of the carbon fiber bundle surface layer> · When evaluating the carbon fiber bundle used as a raw material A guide bar is installed at a position 60 cm above the horizontal plane, and an arbitrary position of the carbon fiber bundle is fixed as a fixed end by pasting it to the guide bar with tape. Then, the carbon fiber is cut at a position 50 cm away from the fixed end to form a free end. The free end is enclosed so as to be sandwiched by the tape and treated so as not to come loose in single fiber units. In order to eliminate temporary twists other than semi-permanent twists or twists that return with time, after standing in this state for 5 minutes, the free end is rotated while counting the number of rotations, and the number of rotations n (turns) rotated until it is completely untwisted is recorded. The remaining number of twists is calculated by the following formula. The average of performing the above measurement 3 times is taken as the remaining number of twists in the present invention.

[0043] The remaining number of twists (turns / m) = n (turns) / 0.5 (m).

[0044] After calculating the diameter (μm) of the entire carbon fiber from the single fiber diameter (μm) and the number of filaments using the following formula, the twist angle (°) of the carbon fiber bundle surface layer is calculated using the following formula with the number of twists (turns / m).

[0045] Diameter of the entire carbon fiber (μm) = {(single fiber diameter) 2 × number of filaments} 0.5 Twist angle (°) of the carbon fiber bundle surface layer = atan (diameter of the entire fiber × 10 -6 × π × number of twists).

[0046] · When evaluating the carbon fibers contained in the molding material The molding material is heated at a temperature of 500 °C for 30 minutes using an electric furnace in an air atmosphere to incinerate and remove the resin composition and separate the carbon fibers. The separated carbon fibers are gently taken out from the electric furnace so as not to loosen and allowed to cool, and a side view photograph is taken with a stereomicroscope at a magnification of 5 to 15 times. From such a photograph, the angle (°) formed between the fiber progression direction as a bundle of carbon fibers (which corresponds to the main axis direction of the molding material described above) and the fiber axis progression direction of the outermost single fiber of the taken-out carbon fiber bundle is read, and it is taken as the twist angle (°) of the carbon fiber surface layer. The measurement is performed on at least 3 grains of the molding material, and the average value is adopted.

[0047] <Weight loss rate of carbon fiber at 450 °C> The carbon fiber to be evaluated, cut to a mass of 2.5 g, is wound around a cassette with a diameter of about 3 cm, and the mass w 0 (g) is weighed. Next, it is heated in an oven in a nitrogen atmosphere at a temperature of 450 °C for 15 minutes, allowed to cool to room temperature in a desiccator, and then the mass w 1 (g) after heating is weighed. The weight loss rate at 450 °C is calculated using the following formula. The evaluation is performed 3 times, and the average value is adopted. Weight loss rate at 450 °C (%) = (w 0 - w1 ) / w 0 × 100 (%)。

[0048] <Crystallization parameter Iv / Ig by Raman spectroscopy> Embed the molding material in resin and polish it to expose the single-fiber cross-section of the carbon fiber contained in the molding material. To avoid the influence of polishing damage on the Raman spectrum, finish polishing is performed using an abrasive with a diameter of about 0.05 μm in the final stage of polishing. Randomly select 5 points on the single-fiber cross-section of the carbon fiber, and measure the Raman spectrum for each point using a microscopic Raman spectrometer. The measurement points are near the center of each single-fiber cross-section. The excitation wavelength is 532 nm, the laser intensity is 1 mW, and the measurement range is 900 - 2000 cm -1 , narrow the laser beam to a diameter of 2 μm, and perform the measurement for 60 seconds × 3 times integration. The baseline of the obtained spectrum is offset using a linear function so that the scattering intensities at 1000 cm -1 and 1800 cm -1 become 0, and calculate the crystallization parameter Iv / Ig with the height of the G band as Ig and the height of the valley bottom near 1480 cm -1 as Iv. To minimize the influence of errors, when obtaining Ig, a quadratic function is used to perform least-squares approximation on the range of ±10 cm -1 from the visually observed peak of the G band, and the peak top intensity of the fitting function is taken as Ig. For Iv, a quadratic function is used to perform least-squares approximation in the same way for the range of ±10 cm -1 from the valley near 1480 cm -1 to obtain Iv. In the present invention, the average value of Iv / Ig at each of the above 5 positions is used.

[0049] In the examples, "EpoKwick" (registered trademark) FC (manufactured by Buehler) was used as the embedding resin, and "AutoMet" (registered trademark) 250Pro (manufactured by Buehler) was used as the polishing device. For polishing, rough polishing was performed using polishing pads of #320, #500, and #700, and then finish polishing was performed using "MasterTex" (manufactured by Buehler) as the polishing cloth and an alumina suspension with a diameter of 0.05 μm as the polishing agent. In order to confirm the presence or absence of polishing damage, when embedding the molding material in resin, "TORAYCA" (registered trademark) M40J-12000-50E manufactured by Toray Industries, Inc. was used as the verification level and embedded simultaneously in the direction perpendicular to the fiber axis with respect to the polished surface. If the Iv / Ig evaluated by the above method for such M40J is 0.18 ± 0.02, it is determined that the damage caused by polishing can be minimized. Otherwise, it is determined to be inappropriate as a sample and polishing is redone.

[0050] <Core ratio, void area ratio, and maximum void area in the molding material> The molding material to be evaluated is resin-embedded and polished so that a cross-section perpendicular to the carbon fiber axis direction can be observed, and is subjected to optical microscope observation. The observation magnification is set so that the entire molding material (pellet) can be observed, for example, to a size of 2 mm square, and the molding material is observed for 3 cross-sections. According to the above-described calculation methods for the core part, sheath part, and void part, the areas of the core part, sheath part, and void part are measured by image analysis, respectively, and the core ratio, void area ratio, and maximum void area are calculated from these values. The values also use the average value of 3 cross-sections.

[0051] <Bending test of the molded product> For ISO type dumbbell test pieces, in accordance with ISO 178 (2010), using a three-point bending test jig (roller radius 5 mm), the fulcrum distance is set to 64 mm, and the bending strength is measured under test conditions of a test speed of 2 mm / min. The test pieces are subjected to the characteristic evaluation test after being left in a thermo-hygrostat adjusted to a temperature of 23°C and 50% RH for 24 hours. The bending strength is determined as the average value by measuring n = 6 molded products.

[0052] In the following Examples and Comparative Examples, an Instron (registered trademark) universal testing machine, Model 4201 (manufactured by Instron Corporation), was used as the testing machine.

[0053] <Dispersibility of carbon fibers in molded products> When the molding material is processed into a molded product by injection molding, the state in which the carbon fibers are dispersed in the molded product in the form of single fibers is evaluated as the dispersibility. The term "dispersed in the form of single fibers" means that the single fibers of a plurality of adjacent carbon fibers are not parallel to each other in their length directions, or even if they are parallel, they are not in contact with each other. When a plurality of adjacent single fibers are parallel to each other and in contact with each other, they are in an undispersed state. The dispersibility when observing the molded product in a field of view of 80 mm × 80 mm is determined based on the average when evaluated by the following criteria. S: The number of aggregates of single fibers parallel to each other is 0 A: The number of aggregates of single fibers parallel to each other is 1 to 2 B: The number of aggregates of single fibers parallel to each other is 3 to 9 C: The number of aggregates of single fibers parallel to each other is 10 or more

Examples

[0054] Hereinafter, the present invention will be described in detail based on Examples, but the present invention is not limited thereto. In particular, although the evaluation is performed when a specific carbon fiber is applied by representing only one type of thermoplastic resin, the present invention does not limit the type of thermoplastic resin.

[0055] [Example 1] A spinning solution containing a polyacrylonitrile copolymer composed of acrylonitrile and itaconic acid was obtained. The obtained spinning solution was once discharged into the air from a spinneret and introduced into a coagulation bath composed of an aqueous solution of dimethyl sulfoxide to obtain a coagulated yarn by a dry-wet spinning method. After washing the coagulated yarn with water, it was stretched in warm water at 90°C at a draw ratio in the bath of 3 times, and further treated with a silicone oil agent, and dried using a roller heated to 160°C, and then stretched with pressurized steam at a draw ratio of 4 times to obtain a carbon fiber precursor fiber bundle having a single fiber fineness of 1.1 dtex.

[0056] Next, four of the obtained precursor fiber bundles were combined and sized to have 12,000 single fibers, and heat-treated in an air atmosphere at 230 to 280 °C with a draw ratio of 1 to convert them into flame-resistant fiber bundles. The obtained flame-resistant fiber bundles were subjected to a twisting process to impart a twist of 45 turns / m, and pre-carbonization treatment was performed in a nitrogen atmosphere at a temperature of 300 to 800 °C with a draw ratio of 1.0 to obtain pre-carbonized fiber bundles. Next, such pre-carbonized fiber bundles were carbonized under the conditions of a draw ratio of 1.02 and a carbonization temperature of 1900 °C, and then carbon fiber bundles were obtained without applying a sizing agent.

[0057] A long fiber reinforced resin pellet manufacturing apparatus in which a coating die for an electric wire coating method was installed at the tip of a TEX-30α type twin-screw extruder (screw diameter 30 mm, L / D = 32) manufactured by Nippon Steel Works, Ltd. was used. The extruder cylinder temperature was set to 330 °C, and polyphenylene sulfide resin (Torelina (registered trademark) M2888 manufactured by Toray Industries, Inc.), which is a thermoplastic resin, was supplied from the main hopper and melt-kneaded at a screw rotation speed of 200 rpm. After adding 8.7 parts by mass of solid bisphenol A type epoxy resin (jER (registered trademark) 1004AF (E-2) manufactured by Mitsubishi Chemical Corporation, softening point 97 °C) heated and melted at 200 °C to 100 parts by mass of carbon fiber, the composition containing the molten thermoplastic resin was supplied to a die orifice (diameter 3 mm) that discharges the composition, and the thermoplastic resin was continuously arranged so as to coat the periphery of the carbon fiber. After cooling the obtained strand, it was cut into pellets with a length of 7 mm using a cutter to obtain long fiber pellets. At this time, the take-up speed was adjusted so that the carbon fiber content was 30% by mass. The length of the carbon fiber was substantially the same as the length of the long fiber pellet.

[0058] The obtained long fiber pellets were injection molded using an injection molding machine (J110AD manufactured by Japan Steel Works, Ltd.) under the conditions of injection time: 5 seconds, back pressure: 5 MPa, holding pressure: 20 MPa, holding time: 10 seconds, cylinder temperature: 330 °C, and mold temperature: 130 °C to produce ISO type dumbbell test pieces as molded products. Here, the cylinder temperature indicates the temperature of the part of the injection molding machine that heats and melts the molding material, and the mold temperature indicates the temperature of the mold into which the resin is injected to form a predetermined shape. The obtained test pieces (molded products) were subjected to characteristic evaluation. The evaluation results evaluated by the above-described method are summarized in Table 1.

[0059] [Example 2] Evaluation was performed in the same manner as in Example 1, except that the carbon fiber obtained in Example 1 was further heat-treated at 2350 °C under a nitrogen atmosphere with a draw ratio of 1.00, and the resulting carbon fiber was used.

[0060] [Example 3] Evaluation was performed in the same manner as in Example 2, except that the draw ratio in the additional heat treatment at 2350 °C was changed to 1.02.

[0061] [Example 4] Evaluation was performed in the same manner as in Example 1, except that the mass content ratio of the carbon fiber contained in the resin composition was changed to 20% by mass.

[0062] [Comparative Example 1] Evaluation was performed in the same manner as in Example 1, except that the carbon fiber was changed to "TORAYCA (registered trademark)" T700S - 24000 - 50E manufactured by Toray Industries, Inc.

[0063] [Comparative Example 2] Evaluation was performed in the same manner as in Example 1, except that the carbon fiber was changed to "TORAYCA (registered trademark)" M40J - 12000 - 50E manufactured by Toray Industries, Inc.

[0064] [Comparative Example 3] Evaluation was performed in the same manner as in Example 1, except that the carbon fiber was changed to "TORAYCA (registered trademark)" M50J - 6000 - 50E manufactured by Toray Industries, Inc.

[0065] [Comparative Example 4] Evaluation was conducted in the same manner as in Comparative Example 1, except that long fiber pellets were obtained while adding a twist of 1.3° following the production method of JP-A-5-169445.

[0066] [Comparative Example 5] Evaluation was conducted in the same manner as in Comparative Example 4, except that the mass content of carbon fiber was changed from 30% by mass to 20% by mass.

[0067]

Table 1

Explanation of Signs

[0068] 1 Core portion densely containing carbon fiber 2 Sheath portion made of thermoplastic resin

Claims

Claim 1 A resin composition containing a thermoplastic resin and a molding material containing carbon fibers, wherein the length of the carbon fibers is substantially the same as the length of the molding material, the twist angle of the carbon fiber surface layer is 2.0 to 30.5°, and in a cross-section perpendicular to the carbon fiber axial direction of the molding material, it has a cross-sectional structure composed of a core portion containing carbon fibers and a sheath portion other than that, and the core ratio A / (A + B) obtained from the area ratio A of the core portion and the area ratio B of the sheath portion other than that is 0.1 to 0.5, the volume content rate Vf of the carbon fibers is 5 to 25%, the volume content rate Vf of the carbon fibers and the core ratio A / (A + B) satisfy the following relational expression, and the area ratio of voids in a cross-section perpendicular to the carbon fiber axial direction of the molding material is 5% or less. 1.5×Vf / 100 ≦ A / (A + B) ≦ 3×Vf / 100 Claim 2 The molding material according to claim 1, wherein the tensile elastic modulus of the carbon fibers is 280 to 500 GPa. Claim 3 The molding material according to claim 1 or 2, wherein the heating weight loss rate of the carbon fibers at 450°C is 0.15% or less. Claim 4 The molding material according to any one of claims 1 to 3, wherein the single fiber diameter of the carbon fibers is 6.0 μm or more. Claim 5 The molding material according to any one of claims 1 to 4, wherein the crystallization parameter Iv / Ig of the carbon fibers by Raman spectroscopy is 0.25 to 0.

80. Claim 6 The maximum area of the voids contained in a cross-section perpendicular to the carbon fiber axial direction of the molding material is 3000 μm ２ The molding material according to any one of claims 1 to 5, wherein the maximum area is 3000 μm or less. Claim 7 A molded article formed by molding the molding material according to any one of claims 1 to 6.

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