Prepreg and method for producing same

Abstract

To provide a prepreg containing carbon fibers and a thermoplastic resin, in which the contained carbon fibers have a high degree of orientation, and a method for producing the same. To provide a molded body having enhanced physical properties in the fiber orientation direction, which is obtained by molding a prepreg containing carbon fibers and a thermoplastic resin, and a method for producing the same. This prepreg contains carbon fibers having a number average fiber length of 10-100 mm and a thermoplastic resin. The fiber orientation tensor in direction x in which the orientation tensor of the carbon fibers is largest in a plane perpendicular to the thickness direction of the prepreg is 0.7 or more, and the fiber orientation tensor in the thickness direction z of the prepreg is 0.2 or less, as calculated by performing fiber orientation analysis using an image of the prepreg captured using three-dimensional X-ray CT. Also provided are a molded body obtained by pressurizing the prepreg under heat and a method for producing the prepreg.

Description

Prepreg and method for manufacturing the same

[0001] The present invention relates to prepregs and methods for producing them.

[0002] In recent years, carbon fiber reinforced polymer (CFRP), which is composed of carbon fibers and matrix resin, has attracted attention from the perspective of energy and environmental issues, as it offers high rigidity, high strength, and significant weight reduction benefits. Its use or application in various applications such as aircraft, automobiles, tanks, concrete reinforcement materials, and sports equipment is being considered.

[0003] In recent years, in order to produce CFRP with improved physical properties, it has been proposed, for example, to align discontinuous carbon fibers in roughly the same direction, as described in Patent Documents 1 and 2.

[0004] Japanese Patent Publication No. 2023-14995 Japanese Patent Publication No. 2017-155358

[0005] However, the prepreg described in Patent Document 1 and the carbon fiber sheet described in Patent Document 2 do not have sufficient physical properties in the fiber orientation direction, and further performance improvements are needed.

[0006] The object of the present invention is to provide a prepreg containing carbon fibers and a thermoplastic resin, wherein the carbon fibers have a high degree of orientation, and a method for producing the same; and to provide a molded article with improved physical properties in the fiber orientation direction obtained by molding a prepreg containing carbon fibers and a thermoplastic resin, and a method for producing the same.

[0007] As a result of diligent research, the inventors have found that the above problems can be solved by the means described below, and have arrived at the present invention. That is, the present invention is as follows: [1] A prepreg comprising carbon fibers with a number average fiber length of 10 to 100 mm and a thermoplastic resin, wherein, by performing fiber orientation analysis using an image of the prepreg captured using three-dimensional X-ray CT, the fiber orientation tensor in the direction x in which the carbon fiber orientation tensor is largest in a plane perpendicular to the thickness direction of the prepreg is 0.7 or more, and the fiber orientation tensor in the thickness direction z of the prepreg is 0.2 or less. [2] The prepreg according to [1], wherein at least a portion of the carbon fibers are recycled carbon fibers. [3] The prepreg according to [1] or [2], wherein the volume content of the carbon fibers is in the range of 30 to 60%. [4] The prepreg according to any one of [1] to [3], wherein the thickness is 0.1 to 0.3 mm. [5] A prepreg according to any one of [1] to [4], wherein the thermoplastic resin is one or more selected from polyamide resin, polyester resin, polyolefin resin, polyetherketone resin, polyphenylene sulfide resin, polyetherimide resin, and polycarbonate resin, and modified versions thereof. [6] A prepreg according to any one of [1] to [5], wherein the fiber orientation tensor in the x direction is less than 0.95. [7] A prepreg according to any one of [1] to [6], wherein the fiber orientation tensor in the z direction is 0.02 or more. [8] A method for manufacturing a molded article, comprising at least the steps of: (P) using a plurality of prepregs according to any one of [1] to [7] and arranging the plurality of prepregs so that the orientation direction of the carbon fibers is substantially the same; and (Q) molding the arranged prepregs by heating and applying pressure. [9] The method for manufacturing a molded article according to [8], wherein the arrangement in step (P) and step (Q) are carried out in a molding die.

[10] A method for producing a prepreg according to any one of [1] to [7], comprising the steps in this order: (A) preparing a precursor (X) comprising a bundle of carbon fibers (V) having orientation and a number average fiber length of 10 to 100 mm and a thermoplastic resin (TP), wherein the contained carbon fibers have orientation in a predetermined direction; (B) preheating the precursor (X) to obtain a preheated precursor (XB); (C) heating the preheated precursor (XB) to above the melting point of the thermoplastic resin (TP) while applying pressure to melt the thermoplastic resin (TP) and impregnate it into the gaps between the carbon fibers to obtain a molten resin-impregnated precursor (XC); and (D) cooling the molten resin-impregnated precursor (XC) while applying pressure to solidify it below the melting point of the thermoplastic resin to obtain a prepreg (Y).

[11] A method for producing a prepreg according to

[10] , wherein at least a portion of the carbon fibers constituting the bundle of carbon fibers (V) are recycled carbon fibers (rCFb).

[12] The method for producing a prepreg according to

[11] , wherein the number average fiber length of the recycled carbon fiber (rCFb) is 10 to 100 mm.

[13] The method for producing a prepreg according to any one of

[10] to

[12] , wherein the precursor (X) is a sliver (W) obtained by blending the bundle of carbon fibers (V) and the fibers (TPFb) made of the thermoplastic resin (TP), carding them, and then kneading them.

[14] The method for producing a prepreg according to

[13] , wherein the number average fiber length of the fibers (TPFb) made of the thermoplastic resin (TP) is 25 to 100 mm.

[15] The method for producing a prepreg according to

[13] or

[14] , wherein the fibers (TPFb) made of the thermoplastic resin (TP) are crimped fibers.

[16] The method for producing a prepreg according to

[15] , wherein the crimped fibers satisfy at least one of the following (1) to (3). (1) The crimp count is 5 to 25 crimps / 25 mm, (2) The crimp rate is 3 to 30%, (3) The single fiber fineness is 0.5 to 5 dtex

[17] A method for producing a prepreg according to any one of

[13] to

[16] , wherein the precursor (X) is a film (TPFm) made of the thermoplastic resin (TP) with the sliver (W) arranged on at least one surface.

[18] A method for manufacturing a prepreg according to any one of

[10] to

[17] , wherein the precursor (X) is made of a bundle of carbon fibers (V) and a film (TPFm) made of the thermoplastic resin (TP) arranged adjacent to each other.

[19] A method for manufacturing a prepreg according to any one of

[10] to

[18] , wherein at least step (C) and step (D) are carried out continuously using a double belt press device.

[0008] According to the present invention, since the carbon fibers contained in the prepreg have a high degree of orientation in a predetermined direction, a molded article containing carbon fibers with a high degree of orientation in a predetermined direction can be easily manufactured by aligning the orientation of the prepreg and then performing molding such as press molding. The molded article obtained in this way has excellent mechanical properties such as tensile properties and bending properties in the fiber orientation direction. Furthermore, in embodiments in which recycled carbon fiber (rCFb) is used as the carbon fiber, it is expected that this will be one of the effective ways to utilize recycled carbon fiber obtained from scraps during CFRP manufacturing or from used CFRP waste.

[0009] This diagram illustrates the reference plane used to determine the orientation tensor in the present invention.

[0010] The present invention will be described in detail below.

[0011] The prepreg of the present invention contains carbon fibers with a number-average fiber length of 10 to 100 mm and a thermoplastic resin, wherein the orientation tensor in the direction x in which the orientation tensor of the carbon fibers is largest in a plane perpendicular to the thickness direction z of the prepreg is 0.7 or higher, and the orientation tensor in the thickness direction z of the prepreg is 0.2 or lower. Here, the orientation tensor is calculated by performing fiber orientation analysis using an image of the prepreg acquired using three-dimensional X-ray CT. Note that "the direction in which the orientation tensor of the carbon fibers is largest in a plane perpendicular to the thickness direction z" may be referred to as "carbon fiber orientation direction" or "(orientation) direction x". The volume content of the carbon fibers is preferably in the range of 30 to 60%, and the thickness of the prepreg is preferably 0.1 to 0.3 mm. The method for manufacturing a molded article from the prepreg is not particularly limited, but for example, a molded article with aligned carbon fiber orientation directions can be manufactured by arranging the prepreg in a press die with the orientation directions aligned and press molding it under heating.

[0012] <Carbon Fibers> The carbon fibers are not particularly limited, but it is preferable that at least a portion of them be recycled carbon fibers (hereinafter sometimes abbreviated as rCFb), and the ratio of rCFb to the carbon fibers is preferably 10% by weight or more, more preferably 30% by weight or more, even more preferably 50% by weight or more, even more preferably 70% by weight or more, particularly preferably 90% by weight or more, and may be 95% by weight or more, or 98% by weight or more, or may be all rCFb. Using rCFb is preferable from an environmental standpoint because it requires less energy during manufacturing compared to newly manufacturing CFRP and leads to the reuse of carbon fiber resources. rCFb refers to, for example, carbon fibers recovered from scraps of prepregs or cloth materials made of carbon fiber reinforced resin generated in the CFRP manufacturing process, or carbon fibers recovered from used CFRP. Methods for recovering carbon fibers from used CFRP include a firing method, in which carbon fibers are recovered by firing the matrix resin of the CFRP, and a dissolution method, in which carbon fibers are recovered by decomposing the resin by immersing the CFRP in a solution such as sulfuric acid. Surface treatment may be applied to rCFb for purposes such as improving the adhesion between rCFb and thermoplastic resin. Surface treatment methods include, but are not limited to, electrolytic treatment, ozone treatment, and ultraviolet treatment.

[0013] The number-average fiber length of the carbon fibers is preferably 10 to 100 mm, more preferably 20 to 70 mm, and even more preferably 30 to 50 mm. By setting the number-average fiber length within the above range, a sufficient reinforcing effect from the carbon fibers can be obtained, and a prepreg with excellent impact properties and moldability can be obtained, which is therefore preferable.

[0014] The diameter of the carbon fiber single filament is not particularly limited, but considering the ease of the composite process, which will be explained separately, and the reinforcing effect of the carbon fiber, the average diameter is preferably 3 to 9 μm, and more preferably 5 to 8 μm.

[0015] <Fiber Orientation Tensor> The fiber orientation tensor is a probability distribution that represents the direction and degree to which fibers are oriented relative to a three-dimensional coordinate system. The sum of the three orthogonal axes of the fiber orientation tensor is 1, and the closer the value is to 1 for a given axis, the more aligned the fibers are in that axis direction.

[0016] The carbon fibers contained in the prepreg have orientation in a predetermined direction. By giving the carbon fibers orientation, the anisotropy of the physical properties of the carbon fibers can be efficiently utilized, and the mechanical strength of the prepreg in the orientation direction x of the carbon fibers can be increased. Therefore, a prepreg with high orientation and a molded article obtained by laminating this prepreg have desirable strength as a reinforcing material. The fiber orientation tensor in the orientation direction x of the carbon fibers is preferably 0.7 or more and 1.0 or less, more preferably 0.8 or more and 0.98 or less, and even more preferably 0.85 or more and less than 0.95. Furthermore, the fiber orientation tensor in the thickness direction z is preferably 0 or more and 0.2 or less, more preferably 0.01 or more and 0.15 or less, and even more preferably 0.02 or more and 0.1 or less. By keeping the fiber orientation tensor in the thickness direction z low, the orientation of the fibers in the thickness direction z of the prepreg is reduced. This tends to result in better moldability when molding with this prepreg, as it reduces the likelihood of carbon fiber springback, and also facilitates resin impregnation between fibers during prepreg manufacturing. The fiber orientation tensor in the thickness direction z can usually be kept low by reducing the thickness of the prepreg, but this becomes difficult if there is a lot of entanglement between the fibers used. By untangling the fibers and setting the thickness of the prepreg within a predetermined range, it is possible to further reduce the fiber orientation tensor in the thickness direction z.

[0017] Carbon fibers are usually bound with a sizing agent to improve handling, but when rCFb is recovered from CFRP, the sizing agent is removed along with the matrix resin, etc., making the fibers themselves bulky and prone to springback. To suppress this springback, it is preferable to carry out the prepreg manufacturing process under constant pressure from heating to cooling. By keeping the process under constant pressure from heating to cooling, it becomes easier to adjust the thickness of the prepreg, and the fiber orientation tensor in the z direction of the prepreg thickness can be kept low, thereby improving the quality of the prepreg. There are no particular limitations on the method of keeping the process under constant pressure from heating to cooling, but using a double belt press is one example of a preferred method.

[0018] <Thermoplastic Resin> The thermoplastic resin is not particularly limited in composition as long as it is a thermoplastic resin, and may be one or more selected from, for example, polyamide resins such as nylon 6 (PA6), nylon 11, nylon 66, and nylon 46; polyester resins such as polyethylene terephthalate and polybutylene terephthalate; polyolefin resins such as polyethylene and polypropylene (PP); polyether ketone resins; polyphenylene sulfide resins; polyetherimide resins; polycarbonate resins; and modified versions thereof. The thermoplastic resin may be a crystalline resin or an amorphous resin. The thermoplastic resin may be a homopolymer or a copolymer. The thermoplastic resin may be used alone or in combination of two or more types.

[0019] The modified thermoplastic resin may be, for example, an acid-modified resin. Acid-modified thermoplastic resins have acid-modifying groups introduced into them. The type of acid-modifying group is not particularly limited; there may be only one type of acid-modifying group, or two or more types may be included, but it is preferable that it be a carboxylic acid anhydride residue (-CO-O-OC-) or a carboxylic acid residue (-COOH). The acid-modifying group may be introduced by any compound. Specific examples of compounds used for acid modification include unsaturated carboxylic acid anhydrides such as maleic anhydride and itaconic anhydride as carboxylic acids, unsaturated polycarboxylic acids such as maleic acid, itaconic acid and fumaric acid; saturated polycarboxylic acids such as succinic acid, glutaric acid and adipic acid; and unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid, among which unsaturated carboxylic acid anhydrides are preferred. Modification with unsaturated carboxylic acids or unsaturated carboxylic acid anhydrides can impart radical polymerizability to thermoplastic resins. Modification with polycarboxylic acids can impart polycondensation properties to thermoplastic resins through ester bonds and amide bonds. In this specification, "thermoplastic resin" also includes modified thermoplastic resins such as acid-modified thermoplastic resins. These modified resins may be used as matrix resins, as compatibilizers added to matrix resins, or as surface treatment agents for carbon fibers.

[0020] The thermoplastic resin used in the present invention preferably contains at least one of polyamide resin, polyolefin resin, and acid-modified polyolefin resin, from the viewpoint of ease of handling and cost. Additives such as crystal nucleating agents, lubricants, antioxidants, and flame retardants may be added to the thermoplastic resin used in the present invention for the purpose of improving physical properties, moldability, and durability. The blending ratio of the additives is not particularly limited, but the total amount is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, and even more preferably 3 parts by weight or less, per 100 parts by weight of the thermoplastic resin.

[0021] The volume content of carbon fibers in the prepreg is preferably 30-60%, more preferably 35-55%, and even more preferably 40-55%. By setting the volume content of carbon fibers within the above range, it is possible to obtain a prepreg that has sufficient reinforcement effect from carbon fibers and good impregnation properties for thermoplastic resin, which is preferable.

[0022] The thickness of the prepreg is preferably 0.1 to 0.3 mm, and more preferably 0.15 to 0.25 mm. Setting the thickness within the above range is preferable because it can increase the production efficiency of the prepreg and also improve the impregnation of the thermoplastic resin. It is also preferable because it improves handling in subsequent processes such as laminate manufacturing. Furthermore, setting the thickness within the above range is preferable because it can keep the fiber orientation tensor in the thickness direction z low while increasing the orientation tensor in the orientation direction x of the carbon fibers.

[0023] The prepreg of the present invention is cut to a width of 15 mm and a length of 100 mm by cutting or the like, laminated and molded to a thickness of 2 mm, and a three-point bending test is performed in accordance with JIS K7074 using a load cell with a capacity of 1 kN, under the conditions of a support distance of 80 mm and a crosshead speed of 5 mm / min. The bending modulus of elasticity is preferably 67 GPa or higher, and more preferably 70 GPa or higher. The upper limit of the bending modulus of elasticity is not particularly limited, and for example, it may be 200 GPa or lower. Furthermore, the bending strength when the above three-point bending test is performed is preferably 350 MPa or higher, and more preferably 400 MPa or higher. The upper limit of the bending strength is not particularly limited, and for example, it may be 2000 MPa or lower.

[0024] <Method for Manufacturing a Molded Article> A molded article can be manufactured using the prepreg of the present invention. The method for manufacturing a molded article preferably comprises at least the steps of: (P) using a plurality of prepregs of the present invention and arranging the plurality of prepregs so that the orientation direction of the carbon fibers is substantially the same; and (Q) molding the arranged prepregs by heating and applying pressure. In the arrangement in step (P), it is more preferable to laminate the plurality of prepregs so that the orientation direction of the carbon fibers is substantially the same, and it is even more preferable to laminate the plurality of prepregs in a molding die so that the orientation direction of the carbon fibers is substantially the same. The arrangement in step (P) is preferably carried out inside a die, and it is more preferable to carry out the arrangement in step (P) and step (Q) inside a molding die. The heating temperature is preferably above the melting point of the thermoplastic resin contained in the prepreg, and more preferably melting point + 20°C to melting point + 50°C. The pressurizing pressure is preferably 2 MPa or more, and more preferably 5 MPa or more and 20 MPa or less. If the above arrangement is inside a mold, the mold temperature is cooled to below the melting point of the thermoplastic resin contained in the prepreg before removing the press-molded product from the mold.

[0025] <Method for Manufacturing Prepregs> The prepreg of the present invention is not particularly limited, but can be manufactured by a manufacturing method including the following steps (A) to (D). Step (A): Prepare a precursor (X) which includes a bundle (V) of carbon fibers (V) having orientation and a number average fiber length of 10 to 100 mm, and a thermoplastic resin (TP), wherein the included carbon fibers have orientation in a predetermined direction. Step (B): Preheat the precursor (X) to obtain a preheated precursor (XB). Step (C): Heat the preheated precursor (XB) to above the melting point of the thermoplastic resin (TP) while applying pressure to melt the thermoplastic resin (TP) and impregnate the gaps between the carbon fibers to obtain a molten resin-impregnated precursor (XC). Step (D): Cool the molten resin-impregnated precursor (XC) while applying pressure to solidify it below the melting point of the thermoplastic resin to obtain a prepreg (Y). Furthermore, if the thermoplastic resin (TP) is an amorphous resin, it is preferable to heat and solidify it within the preferred temperature range described later, instead of "heating above the melting point" in step (C) and "solidifying below the melting point" in step (D).

[0026] <Process (A)> Process (A) is a process of preparing a precursor (X) which includes a bundle (V) of carbon fibers (V) having orientation and a number average fiber length of 10 to 100 mm, and a thermoplastic resin (TP), wherein the included carbon fibers have orientation in a predetermined direction.

[0027] Here, the carbon fiber bundle (V) is an aggregate of oriented carbon fibers, and its number-average fiber length is preferably 10 to 100 mm, more preferably 20 to 70 mm, and even more preferably 30 to 50 mm. A number-average fiber length within the above range is preferable because it allows for sufficient reinforcement of the reinforcing fibers, resulting in a prepreg with excellent impact properties and moldability. The number-average fiber length of the carbon fiber bundle (V) and the number-average fiber length of the carbon fibers contained in the prepreg (Y) are approximately the same length. The average number of individual carbon fibers constituting the bundle is not particularly limited, but is preferably 1000 or less, more preferably 700 or less, and even more preferably 500 or less. An average number within the above range is preferable because it allows for excellent dispersion of carbon fibers in the prepreg.

[0028] Preferably, at least a portion of the fibers constituting the carbon fiber bundle (V) are recycled carbon fibers (rCFb), and the ratio of rCFb to the carbon fiber bundle (V) is preferably 10% by weight or more, more preferably 30% by weight or more, even more preferably 50% by weight or more, even more preferably 70% by weight or more, particularly preferably 90% by weight or more, and may also be 95% by weight or more, or 98% by weight or more, or may be all rCFb. The number average fiber length of the recycled carbon fibers (rCFb) is preferably 10 to 100 mm, more preferably 20 to 70 mm, and even more preferably 30 to 50 mm. A number average fiber length within the above range is preferable because it allows for sufficient reinforcement effect from the carbon fibers, and a prepreg with excellent impact properties and moldability can be obtained.

[0029] The composition of the thermoplastic resin (TP) is as described above.

[0030] The form of the thermoplastic resin (TP) is not particularly limited and may be, for example, powder, particulate, fibrous, or film, or a mixture of two or more of these. A fibrous form is preferable because it can be expected to improve the impregnation effect in the prepreg. A film form is also preferable because it can be expected to improve the smoothness of the surface of the prepreg and improve its appearance quality.

[0031] <Precursor (X)> The prepreg of the present invention is preferably manufactured by processing a precursor (X) which contains a bundle of carbon fibers (V) having orientation and a number average fiber length of 10 to 100 mm, and a thermoplastic resin (TP), wherein the contained carbon fibers have orientation in a predetermined direction. At least a portion of the carbon fibers constituting the bundle of carbon fibers (V) may be recycled carbon fibers (rCFb). The number average fiber length of the recycled carbon fibers (rCFb) is preferably 10 to 100 mm. The bundle of carbon fibers (V) contained in the precursor (X) is preferably aligned in a uniform manner, but the method for achieving such a state is not particularly limited. The precursor (X) preferably contains a sliver (W) obtained by blending carbon fibers and fibers (TPFb) made of the thermoplastic resin (TP), and passing through a carding process and a blending process. The precursor (X) may also be a form in which the bundle of carbon fibers (V) and a film (TPFm) made of the thermoplastic resin (TP) are arranged adjacent to each other. In some cases, the surface smoothness of the surface on which the film is placed can be improved by arranging at least one of the carbon fiber bundle (V) or the sliver (W) on at least one surface of a film (TPFm) made of the thermoplastic resin (TP), and then producing a prepreg through steps (B) to (D). It is preferable to arrange at least one of the carbon fiber bundle (V) or the sliver (W) between two films (TPFm), and then producing a prepreg through steps (B) to (D).

[0032] Carding refers to the operation of aligning the direction of discontinuous fibers and / or opening the fibers by applying force in roughly the same direction to a collection of discontinuous fibers using a comb-like object. Carding is generally performed using a carding device that has a roll with numerous needle-like protrusions on its surface and / or a roll around which a metallic wire with saw-tooth-like protrusions is wound. Because carbon fibers are rigid, damage to the fibers may occur during the carding process. It is expected that damage to the fibers can be prevented or reduced by adding an appropriate amount of spinning oil to the carbon fibers before and / or during carding.

[0033] The number-average fiber length of the fiber (TPFb) is not particularly limited as long as it is within a range that allows for the preservation of the shape of the precursor (X) and the prevention of the detachment of the rCFb. Generally, fibers made of thermoplastic resin with a number-average fiber length of about 10 to 100 mm can be used, but it is preferable that the number-average fiber length of the fiber (TPFb) be 25 to 100 mm.

[0034] Furthermore, it is preferable that the fibers (TPFb) are crimped in order to maintain the shape of the precursor (X) and to prevent the rCFb from falling off. There are no particular limitations on the degree of crimping, but it is preferable that the number of crimps is about 5 to 25 crimps / 25 mm and the crimping rate is about 3 to 30%. In addition, the single fiber fineness of the fibers (TPFb) is preferably 0.5 to 5 dtex. In particular, a number of crimps of 10 to 20 crimps / 25 mm, a crimping rate of 10 to 20%, and a fiber diameter of 1 to 5 dtex are preferred in terms of the uniformity of the precursor (X).

[0035] <Process (B)> Process (B) is a process of preheating the precursor (X) obtained in process (A) to obtain a preheated precursor (XB). From the viewpoint of production efficiency, the preheating temperature is preferably Tm-100°C to Tm, and more preferably Tm-50°C to Tm, relative to the melting point Tm of the thermoplastic resin (TP).

[0036] <Step (C)> Step (C) is a step in which the preheated precursor (XB) obtained in step (B) is heated to a temperature above the melting point Tm of the thermoplastic resin (TP) while being pressurized, the thermoplastic resin (TP) is melted and impregnated into the gaps between the carbon fibers to obtain a molten resin-impregnated precursor (XC). The heating temperature is preferably Tm to Tm + 50°C, and more preferably Tm + 20°C to Tm + 50°C, from the viewpoint of suppressing thermal degradation of the thermoplastic resin (TP). If the thermoplastic resin (TP) is an amorphous resin, the heating temperature is preferably Tg to Tg + 50°C, and more preferably Tg + 20°C to Tg + 50°C, relative to the glass transition temperature Tg of the thermoplastic resin. The pressurizing pressure is preferably 0.2 to 10 MPa, and more preferably 1 to 5 MPa, from the viewpoint of improving the uniformity of the resin impregnation state of the prepreg to be produced while reducing the load on the machine.

[0037] <Step (D)> Step (D) is a step in which the molten resin-impregnated precursor (XC) obtained in step (C) is cooled and pressurized to solidify below the melting point Tm of the thermoplastic resin (TP) to obtain a prepreg (Y). From the viewpoint of production efficiency, the cooling temperature is preferably Tm-20°C or lower, and more preferably Tm-200°C or higher and Tm-50°C or lower. If the thermoplastic resin (TP) is an amorphous resin, the cooling temperature is preferably Tg-20°C or lower, and more preferably Tg-200°C or higher and Tg-50°C or lower. From the viewpoint of improving the uniformity of resin impregnation of the prepreg to be produced while reducing the load on the machine, the pressurizing pressure is preferably 0.2 to 10 MPa, and more preferably 1 to 5 MPa.

[0038] The resulting prepreg (Y) may be subjected to slitting to a predetermined width and / or cutting to a predetermined length, as needed.

[0039] <Double Belt Press Device> A double belt press device typically comprises an upper endless belt and a lower endless belt, and the upper and lower endless belts are wrapped around rollers provided at one end and the other end of the double belt press device and travel in a circular motion. The upper and lower endless belts are positioned to be approximately horizontal, and the clearance between them can be set arbitrarily. In addition, a heating section and a cooling section are provided between the rollers provided at one end and the other end of the upper and lower belts, allowing for continuous heating and cooling of the workpiece. Some or all of steps (B), (C), and (D) in the method for manufacturing prepregs may be performed using a double belt press device.

[0040] In the heating section of the double belt press device, when the thermoplastic resin contained in the precursor (X) which is the object to be processed is a crystalline resin, the temperature reached on the surface of the endless belt is preferably set to be Tm to Tm + 50°C, more preferably Tm + 20°C to Tm + 50°C, with respect to the melting point Tm of the thermoplastic resin. When the thermoplastic resin is an amorphous resin, it is preferably set to be Tg to Tg + 50°C, more preferably Tg + 20°C to Tg + 50°C, with respect to the glass transition temperature Tg of the thermoplastic resin. By setting the temperature reached on the surface of the endless belt within the above range, it is preferable in that the uniformity of the resin impregnation state of the prepreg can be enhanced while suppressing the thermal deterioration of the thermoplastic resin.

[0041] As the heating means for the heating section in the double belt press device, there is no particular limitation, and for example, an IR heater, a hot air heater, or a heat medium heater using oil can be used. The adjustment of the temperature reached on the surface of the endless belt can be performed by adjusting the output of the heating means or by adjusting the running speed of the endless belt.

[0042] In the cooling section, when the thermoplastic resin contained in the precursor (X) is a crystalline resin, the temperature reached on the surface of the endless belt is preferably Tm - 20°C or lower, more preferably Tm - 200°C or higher and Tm - 50°C or lower. When it is an amorphous resin, the temperature reached is preferably set to be Tg - 20°C or lower, more preferably Tg - 200°C or higher and Tg - 50°C or lower. By setting the temperature reached on the surface of the endless belt within the above range, it is preferable in that it has excellent production efficiency, problems such as the adhesion of the prepreg formed on the surface of the endless belt are suppressed, and the warping of the prepreg can be reduced.

[0043] As the cooling means for the cooling section in the double belt press device, there is no particular limitation, and cold air, cooling water circulation, refrigerant circulation, etc. can be used. The adjustment of the temperature reached on the surface of the endless belt can be performed by adjusting the set temperature of the cooling device, adjusting the air volume, adjusting the water volume, etc., and also by adjusting the conveyance speed, etc.

[0044] A pressurizing mechanism may be introduced into the double belt press device as needed, and the method is not particularly limited. For example, a press using press rolls, hydraulic plates, or vibrating pressurizing plates, or a combination of multiple pressurizing mechanisms can be used, with a press roll system being more preferable as it offers good responsiveness to large thickness changes.

[0045] In a double-belt press device, the clearance between the upper endless belt and the lower endless belt is preferably set to a range greater than 0 mm and within 2 mm. Setting the clearance within the above range is preferable because it improves the efficiency of applying pressure to the prepreg. Within the above range, the clearance may be sloped from the inlet of the heating section to the outlet of the cooling section. For example, in a double-belt press, the clearance of the heating section corresponding to process (B) may be set to 2 mm, and the clearance from the heating section corresponding to process (C) to the cooling section corresponding to process (D) may be set to 0.5 mm, so that the clearance of process (C) is lower than the clearance of process (B).

[0046] The speed setting of the endless belt depends on the length of the heating and cooling sections, and is therefore not particularly limited as long as the speed allows the temperature reached on the endless belt surface in the heating and cooling sections to be within the above range.

[0047] In prepregs produced using a double-belt press, the total width of the precursors (X) fed into the belt is preferably 50% or less, and more preferably 40% or less. Setting the total width of the precursors (X) to the above level is preferable because it makes it less likely for the press load received by the precursors (X) when passing through the double-belt press to be dispersed, thus making it easier to improve the quality of the resulting prepreg. It is also preferable from the standpoint of equipment maintenance because it eliminates the need to apply excessive load. There is no particular lower limit to the total width of the precursors (X) fed into the belt relative to the belt width; for example, it may be 10% or more. Since the above setting range is the total width of the precursors fed into the belt, one wide precursor may be used, or multiple narrow precursors may be arranged in a row.

[0048] This application claims the benefit of priority based on Japanese Patent Application No. 2024-220987, filed on 17 December 2024. The entire specification of Japanese Patent Application No. 2024-220987, filed on 17 December 2024, is incorporated herein by reference.

[0049] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. The evaluation methods used in each example and comparative example are as follows.

[0050] <Number-average fiber length of carbon fibers> From a precursor (X) cut to a length of 500 mm, 100 carbon fibers were randomly selected, and the length of each fiber was measured using calipers. The number-average fiber length was calculated by taking the average of the 100 fibers.

[0051] <Volume Content of Carbon Fiber> After cutting a sample from the prepreg, the weight of the prepreg was measured. Then, the sample was placed in an electric furnace and heated to 500°C over one hour, and maintained at 500°C for four hours to burn off the resin components. The weight of the remaining carbon fibers was then measured to determine the weight content of the carbon fibers. Using the measured weight content of the carbon fibers, the volume content of the carbon fibers contained in the sample was calculated using the following formula. Note that the fiber density and resin density are values ​​at 25°C, and the unit is g / cm³. ３ Let's assume that.

[0052]

[0053] <Analysis of Orientation Tensor> A sample was cut from the prepreg, and using a Shimadzu 3D X-ray CT scanner (inspXio SMX-225CT FPD HR), the x-axis direction shown in Figure 1 was defined as the rotation axis, and the sample was magnified to a voxel size of 0.004 mm. Measurements were performed with an acceleration voltage of 120 kV and a tube current of 70 μA, and the obtained images were reconstructed to obtain 3D image data with an effective field of view of 7 mmφ × 5 mm. Note that the direction in which the carbon fibers are oriented visually can be defined as the x-axis direction, and it does not need to perfectly coincide with the direction x described later.

[0054] Using this 3D image data, the average orientation tensor was evaluated using the fiber orientation analysis function of Volume Graphics' analysis software (VGSTUDIO MAX, version 2024.4). First, the 3D image data was trimmed so that the analysis region was limited to the area containing the prepreg in the acquired 3D image. Image binarization for analysis was performed by defining the plane so that the volume ratio of carbon fibers in the image was within ±5% of the volume ratio of carbon fibers in the prepreg, as determined above. The orientation tensor was calculated based on the x-y plane shown in Figure 1, and the orientation tensor for the orientation direction of the carbon fibers (direction x) and the orientation tensor for the thickness direction (direction z) were obtained. The direction with the largest orientation tensor (the direction with the largest eigenvalue) within the plane perpendicular to the thickness direction z was defined as direction x. The above analysis was performed for any three points in the prepreg, and the average value was taken as the orientation tensor.

[0055] <Bending Test> In accordance with JIS K7074, a Shimadzu Corporation universal testing machine (AUTOGRAPH AG-X plus 5kN) was used to conduct a three-point bending test using a 1kN load cell under the conditions of a support distance of 80mm and a crosshead speed of 5mm / min. The test specimens used were 15mm wide, 100mm long, and 2mm thick, and the average of the results of five tests was used as the measured value.

[0056] (Example 1) Carbon fiber reinforced resin prepregs and cloth scraps, etc., generated during the CFRP manufacturing process, were recovered with an average diameter of 7 μm and a density of 1.8 g / cm³. ３ Five slivers (manufactured by Tatsuta Spinning Co., Ltd.) were prepared by blending recycled carbon fiber and thermoplastic resin crimped fibers (T201 2.2T x 51mm, manufactured by Toray Industries, Inc.) made of PA6, with the volume content of recycled carbon fiber being approximately 50%. These slivers were cut to a length of 500 mm, arranged with the fiber direction aligned, and used as a precursor (X). The total width of these five slivers was 25% of the endless belt width of the double belt press. The melting point of PA6 is 225°C and its density is 1.14 g / cm³. ３The precursor (X) was sandwiched between 0.2 mm thick Teflon® sheets placed above and below it, and passed through a double belt press device set to a belt surface temperature of 200°C in the preheating section, 260°C in the heating section, and 40°C in the cooling section, thereby obtaining a prepreg with a thickness of 0.25 mm and a recycled carbon fiber volume content of 49.3%. The number-average fiber length of the recycled carbon fibers contained in the prepreg was 45.4 mm, the orientation tensor in the carbon fiber orientation direction x of this prepreg was 0.72, and the orientation tensor in the thickness direction z was 0.10.

[0057] The obtained prepreg was cut to a size of 15 x 100 mm, and multiple cut prepregs were placed in a mold so that the orientation direction of the recycled carbon fibers was approximately the same. Lamination was performed to a predetermined weight, and press molding was carried out by heating on a 260°C faceplate for 15 minutes, followed by cooling on a water-cooled faceplate for 10 minutes. A molded body for bending tests with a width of 15 mm, a length of 100 mm, and a thickness of 2 mm was obtained by cutting the laminate obtained by press molding. Press molding was carried out by applying a load of 10 kN during both heating and cooling. When the bending properties of this molded body were evaluated, the bending modulus was 73.9 GPa and the bending strength was 702 MPa.

[0058] (Example 2) Samples recovered by solvent method with an average diameter of 7 μm and a density of 1.8 g / cm³ ３ Except for using a sliver (manufactured by Tatsuta Spinning Co., Ltd.) made by blending recycled carbon fibers and the above-mentioned thermoplastic resin crimped fibers made of PA6 so that the volume content of recycled carbon fibers was approximately 50%, a prepreg with a thickness of 0.23 mm and a volume content of recycled carbon fibers of 50.1% was obtained in the same manner as in Example 1. The number-average fiber length of the recycled carbon fibers contained in the prepreg was 47.8 mm, the orientation tensor in the orientation direction x of the carbon fibers of this prepreg was 0.87, and the orientation tensor in the thickness direction z was 0.05. Using the obtained prepreg, a molded body for bending tests was obtained in the same manner as in Example 1. When the bending properties of this molded body were evaluated, the bending modulus was 90.2 GPa and the bending strength was 984 MPa.

[0059] (Example 3) Carbon fiber reinforced resin prepregs and scraps of cloth materials, etc., generated during the CFRP manufacturing process, were recovered with an average diameter of 7 μm and a density of 1.8 g / cm³. ３ Five slivers (manufactured by Tatsuta Spinning Co., Ltd.) were prepared by blending recycled carbon fibers with the above-mentioned thermoplastic resin crimped fibers made of PA6, so that the volume content of recycled carbon fibers was approximately 85%. These slivers were cut to a length of 500 mm, arranged with the fiber direction aligned, and used as a precursor (X). The total width of these five slivers was 25% of the endless belt width of the double belt press. A 40 μm thick PA6 film was placed above and below the precursor (X), sandwiching the precursor (X). By adjusting the number and size of the PA6 film layers, the volume content of recycled carbon fibers was adjusted to approximately 50%. Subsequently, by passing the material through a double belt press device set to a belt surface temperature of 200°C in the preheating section, 260°C in the heating section, and 40°C in the cooling section, a prepreg with a thickness of 0.24 mm and a volume content of 52.8% recycled carbon fibers was obtained. The number-average fiber length of the recycled carbon fibers contained in the prepreg was 46.4 mm, the orientation tensor in the orientation direction x of the carbon fibers was 0.80, and the orientation tensor in the thickness direction z was 0.08. Using the obtained prepreg, a molded body for bending tests was obtained in the same manner as in Example 1. When the bending properties of this molded body were evaluated, the bending modulus was 83.2 GPa and the bending strength was 750 MPa.

[0060] (Example 4) Average diameter 7 μm, density 1.8 g / cm³ recovered by solvent method. ３Five slivers (manufactured by Tatsuta Spinning Co., Ltd.) made by blending recycled carbon fiber and modified PP thermoplastic resin crimped fibers (PZ-AD, manufactured by Yamato Spinning Co., Ltd.) so that the volume content of recycled carbon fiber was approximately 50%, were cut to a length of 500 mm, and arranged with the fiber direction aligned, and used as a precursor (X). The total width of these five slivers was 25% of the endless belt width of the double belt press. A prepreg with a thickness of 0.21 mm and a recycled carbon fiber volume content of 47.3% was obtained by coating the precursor (X) with a compatibilizer containing Toyo Tack® PMA-KH (manufactured by Toyobo MC), an acid-modified polyolefin resin, dispersed in water to a solid content of 2% by weight. Then, 0.2 mm thick Teflon® sheets were placed above and below the precursor (X), sandwiching the precursor (X). The prepreg was then passed through a double belt press device set to a belt surface temperature of 160°C in the preheating section, 230°C in the heating section, and 40°C in the cooling section. The number-average fiber length of the recycled carbon fibers contained in the prepreg was 45.4 mm, the orientation tensor in the carbon fiber orientation direction x of the prepreg was 0.82, and the orientation tensor in the thickness direction z was 0.07. Using the obtained prepreg, a molded body for bending tests was obtained in the same manner as in Example 1, except that the molding temperature was set to 220°C. When the bending properties of this molded article were evaluated, the bending modulus was found to be 85.6 GPa and the bending strength was 413 MPa. According to Japanese Patent Publication No. 2020-37710, PZ-AD manufactured by Yamato Spinning Co., Ltd. is made of acid-modified polypropylene fibers and has a melting point of 160°C. The density of acid-modified polypropylene is 0.9 g / cm³. ３ That is the case.

[0061] (Comparative Example 1) The same precursor (X) as that used in Example 1 was cut into lengths of 200 mm, aligned in the fiber direction, and arranged so as to cover the surface of a hand press, and an iron plate and a Teflon (registered trademark) sheet with a thickness of 0.2 mm were placed on top and bottom to sandwich the precursor (X). Thereafter, heating was performed with a hand press set at 260°C under a surface pressure of 0.2 MPa until the resin melted, and then the load was removed and it was allowed to cool while the iron plate was loaded, thereby obtaining a prepreg with a thickness of 0.42 mm and a volume content of recycled carbon fibers of 48.1%. The number average fiber length of the recycled carbon fibers contained in the obtained prepreg was 48.1 mm, and the orientation tensor in the carbon fiber orientation direction x of this prepreg was 0.68, and the orientation tensor in the thickness direction z was 0.13. Using the obtained prepreg, a molded body for a bending test was obtained in the same manner as in Example 1. When the bending physical properties of this molded body were evaluated, the bending elastic modulus was 66.7 GPa and the bending strength was 488 MPa.

[0062] (Comparative Example 2) Recycled carbon fibers with an average diameter of 7 μm and a density of 1.8 g / cm ３ recovered by the solvent method were sandwiched using a PA6 film with a thickness of 40 μm and a density of 1.14 g / cm ３ without any particular orientation, and heating was performed with a hand press set at 260°C under a surface pressure of 0.2 MPa until the resin melted, and then the load was removed. Thereafter, it was transferred to a water-cooled press, and pressure was applied so that the surface pressure became 1 MPa and cooling was performed to obtain a prepreg. The number of laminated sheets and the size of the PA6 film were adjusted so that the volume content of the recycled carbon fibers became about 50%. The thickness of this prepreg was 0.28 mm and the volume content of the recycled carbon fibers was 51.2%. The number average fiber length of the recycled carbon fibers contained in the prepreg was 50.2 mm, and the orientation tensor in the carbon fiber orientation direction x was 0.45, and the orientation tensor in the thickness direction z was 0.17. Using the obtained prepreg, a molded body for a bending test was obtained in the same manner as in Example 1. When the bending physical properties of this molded body were evaluated, the bending elastic modulus was 31.4 GPa and the bending strength was 307 MPa.

[0063] Table 1 below shows the various physical properties of the prepregs and molded articles for bending tests obtained in Examples 1 to 4 and Comparative Examples 1 to 2.

[0064]

[0065] The prepreg of the present invention has a high degree of orientation of the contained carbon fibers, and molded articles obtained using this prepreg exhibit excellent mechanical properties such as tensile and bending properties in the fiber orientation direction. Therefore, in embodiments where recycled carbon fibers are used as part or all of the carbon fibers, it is expected to greatly contribute to expanding the fields in which recycled carbon fibers can be applied. Examples of specific applications include automobiles, sporting goods, nursing care products, conductive electrical and electronic components, and electromagnetic shielding materials.

[0066] 1. Prepreg 2. Bundle of carbon fibers

Claims

1. A prepreg comprising carbon fibers with a number-average fiber length of 10 to 100 mm and a thermoplastic resin, wherein, by performing fiber orientation analysis using an image of the prepreg acquired using three-dimensional X-ray CT, the fiber orientation tensor in the direction x where the carbon fiber orientation tensor is largest in a plane perpendicular to the thickness direction of the prepreg is 0.7 or greater, and the fiber orientation tensor in the thickness direction z of the prepreg is 0.2 or less.

2. The prepreg according to claim 1, wherein at least a portion of the carbon fibers is recycled carbon fiber.

3. The prepreg according to claim 1, wherein the volume content of the carbon fibers is in the range of 30 to 60%.

4. The prepreg according to claim 1, wherein the thickness is 0.1 to 0.3 mm.

5. The prepreg according to claim 1, wherein the thermoplastic resin is one or more selected from polyamide resins, polyester resins, polyolefin resins, polyetherketone resins, polyphenylene sulfide resins, polyetherimide resins, and polycarbonate resins, and modified versions thereof.

6. The prepreg according to claim 1, wherein the fiber orientation tensor in the direction x is less than 0.

95.

7. The prepreg according to claim 1, wherein the fiber orientation tensor in the direction z is 0.02 or greater.

8. A method for manufacturing a molded article, comprising at least the steps of: (P) using a plurality of prepregs described in any one of claims 1 to 7, aligning and arranging the plurality of prepregs so that the orientation direction of the carbon fibers is substantially the same; and (Q) molding the arranged prepregs by heating and applying pressure.

9. The method for manufacturing a molded article according to claim 8, wherein the arrangement in step (P) and step (Q) are performed in a molding die.

10. A method for producing a prepreg according to any one of claims 1 to 7, comprising the steps in this order: (A) preparing a precursor (X) comprising a bundle (V) of carbon fibers (V) having orientation and a number average fiber length of 10 to 100 mm and a thermoplastic resin (TP), wherein the contained carbon fibers have orientation in a predetermined direction; (B) preheating the precursor (X) to obtain a preheated precursor (XB); (C) heating the preheated precursor (XB) to a temperature above the melting point of the thermoplastic resin (TP) while applying pressure to melt the thermoplastic resin (TP) and impregnate it into the gaps between the carbon fibers to obtain a molten resin-impregnated precursor (XC); and (D) cooling the molten resin-impregnated precursor (XC) while applying pressure to solidify it to a temperature below the melting point of the thermoplastic resin (TP) to obtain a prepreg (Y).

11. The method for producing a prepreg according to claim 10, wherein at least a portion of the carbon fibers constituting the bundle of carbon fibers (V) are recycled carbon fibers (rCFb).

12. The method for producing a prepreg according to claim 11, wherein the number-average fiber length of the recycled carbon fiber (rCFb) is 10 to 100 mm.

13. The method for producing a prepreg according to claim 10, wherein the precursor (X) comprises a sliver (W) obtained by blending, carding, and further kneading a bundle of carbon fibers (V) and a fiber (TPFb) made of the thermoplastic resin (TP).

14. The method for producing a prepreg according to claim 13, wherein the number average fiber length of the fibers (TPFb) made of the thermoplastic resin (TP) is 25 to 100 mm.

15. The method for producing a prepreg according to claim 13, wherein the fiber (TPFb) made of the thermoplastic resin (TP) is a crimped fiber.

16. The method for producing a prepreg according to claim 15, wherein the crimped fiber satisfies at least one of the following (1) to (3). (1) Number of crimps is 5 to 25 crimps / 25 mm, (2) Crimp ratio is 3 to 30%, (3) Single fiber fineness is 0.5 to 5 dtex 17. The method for producing a prepreg according to claim 13, wherein the precursor (X) is a film (TPFm) made of the thermoplastic resin (TP) with the sliver (W) arranged on at least one surface.

18. The method for producing a prepreg according to claim 10, wherein the precursor (X) is a film (TPFm) made of a bundle of carbon fibers (V) and a thermoplastic resin (TP) arranged adjacent to each other.

19. A method for manufacturing a prepreg according to claim 10, wherein at least step (C) and step (D) are carried out continuously using a double belt press device.

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