Method for manufacturing conductive paste, electrical circuit, flexible electrical circuit body, and molded body.

Abstract

Provided is an electroconductive paste capable of forming, in an electric circuit, an electric wiring line that is less likely to break even when the electric circuit and / or the electric wiring line is elongated and / or flexed.　This flexible electric circuit-forming electroconductive paste is for forming an electric circuit on the surface of a flexible substrate, and comprises (A) electroconductive particles, (B) a thermoplastic resin, and (C) a solvent. When the temperature Tm (°C) for measuring the storage modulus of the electroconductive paste is set to Tm=178°C, the storage modulus of a cured product obtained by curing the electroconductive paste at 120°C for 30 minutes is 0.001-0.5 GPa at said temperature Tm.

Description

[Technical Field]

[0001] The present invention relates to a conductive paste for forming electrodes and wiring in electrical circuits and electronic circuits. The present invention also relates to an electrical circuit including a cured product of a conductive paste, and a flexible electrical circuit body. [Background technology]

[0002] In recent years, conductive pastes have been developed for forming electrodes and wiring on stretchable and flexible substrates. Furthermore, attempts are being made to use conductive pastes to form electrodes and wiring for heat-molded electrical and electronic circuits.

[0003] Patent Document 1 discloses a multilayer film suitable for use as a substrate for printed circuits. Patent Document 1 states that the multilayer film further comprises a conductive ink printed on the surface of the substrate, and that suitable inks include Ag, carbon, Cu, CNT, graphene, PEDOT, AgNW, and the like. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Special Publication No. 2020-523232 [Disclosure of the Invention]

[0005] In recent years, attempts have been made to form electrodes and wiring for electrical and / or electronic circuits (sometimes collectively referred to as "electrical circuits") on the surface of stretchable and / or bendable substrates. Furthermore, attempts have been made to form electrical wiring for electrical circuits, etc., by molding using conductive paste. The technology of forming electrical wiring by molding using conductive paste is called "In-Mold Electronics (IME)". During three-dimensional molding such as heat molding, pressure molding, and vacuum molding, the electrical wiring deforms due to the stretching and / or bending of the substrate (e.g., a flat sheet or film), which may cause the electrical wiring of electrical circuits, etc., used in in-mold electronics to break.

[0006] Furthermore, when performing molding processes such as heat molding, pressure molding, and vacuum molding, various three-dimensional shapes may be formed. In certain shapes, the conductive paste contains conductive fillers during three-dimensional molding, resulting in poor extensibility of the conductive paste. Therefore, it is difficult for the electrical circuit pattern (wiring pattern) printed on the substrate to stretch in accordance with the stretching of the substrate during three-dimensional molding. Consequently, stress cannot be relieved, and a problem arises in which cracks occur in the electrical circuit pattern (wiring pattern) during molding. This cracking can lead to the electrical wiring breaking. The presence or absence of cracks can be determined by observing the electrical circuit pattern (wiring pattern) with an optical microscope after three-dimensional molding.

[0007] Therefore, the present invention aims to provide a conductive paste that can form electrical wiring with a low probability of disconnection, even when electrical wiring of an electrical circuit and / or electronic circuit is formed using three-dimensional molding.

[0008] To solve the above problems, embodiments of the present invention have the following configuration.

[0009] (Composition 1) Configuration 1 is a conductive paste for forming a flexible electrical circuit on the surface of a flexible substrate, comprising (A) conductive particles, (B) thermoplastic resin, and (C) solvent. The temperature Tm (°C) for measuring the storage modulus of the conductive paste is Tm = 178°C. The conductive paste is such that the storage modulus of the cured product obtained by curing the conductive paste at 120°C for 30 minutes is 0.001 to 0.5 GPa at the temperature Tm.

[0010] (Configuration 2) Configuration 2 is a conductive paste of Configuration 1, wherein the absolute difference between the storage modulus of the flexible substrate at temperature Tm and the storage modulus of the cured product obtained by heating and drying the conductive paste at 120°C for 30 minutes is 0 to 0.50 GPa.

[0011] (Composition 3) Configuration 3 is a conductive paste of Configuration 1 or 2, wherein the (B) thermoplastic resin comprises at least one selected from the group consisting of polycarbonate resin, hydrogenated styrene-based thermoplastic elastomer, and styrene-butadiene-styrene block copolymer.

[0012] (Composition 4) Configuration 4 is a conductive paste of Configuration 3, wherein the polycarbonate resin comprises at least one selected from poly(4,4'-cyclohexyllidenediphenyl)carbonate and copoli[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane]carbonate.

[0013] (Composition 5) Configuration 5 is a conductive paste according to any of Configurations 1 to 4, wherein the conductive particles (A) are silver particles.

[0014] (Composition 6) Configuration 6 is a conductive paste of any of Configurations 1 to 5, wherein the shape of the conductive particles (A) is irregular or flaky.

[0015] (Configuration 7) Configuration 7 is a conductive paste of any one of Configurations 1 to 6, wherein the flexible substrate contains at least one selected from the group consisting of polycarbonate, polyethylene terephthalate (PET), and acrylic resin.

[0016] (Configuration 8) Configuration 8 is an electric circuit including a cured product of a conductive paste of any one of Configurations 1 to 7.

[0017] (Configuration 9) Configuration 9 is a flexible electric circuit body including the flexible substrate and the electric circuit of Configuration 8 disposed on the surface of the flexible substrate.

[0018] (Configuration 10) Configuration 10 includes a step of forming an electric circuit on the surface of the flexible substrate using a conductive paste of any one of Configurations 1 to 7, and a step of forming a molded body by molding the flexible substrate on which the electric circuit is formed, and is a method for manufacturing a molded body.

[0019] According to the present invention, even when an electric wiring of an electric circuit and / or an electronic circuit is formed using three-dimensional molding, it is possible to provide a conductive paste capable of forming an electric wiring with a low possibility of disconnection.

Brief Description of the Drawings

[0020] [Figure 1] It is an optical micrograph (magnification: 20 times) showing an example when no crack has occurred in the cured product after three-dimensional molding of the cured product (wiring pattern) of the conductive paste. [Figure 2] It is an optical micrograph (magnification: 20 times) showing an example when a crack has occurred in the cured product after three-dimensional molding of the cured product (wiring pattern) of the conductive paste. [Figure 3]This is a schematic diagram showing the wiring pattern (printed pattern of the conductive paste) of the conductive paste used to measure the resistivity of the cured conductive paste. [Figure 4] This is a photograph showing an example of the shape of a flexible substrate with a wiring pattern from the examples and comparative examples after three-dimensional molding. [Figure 5] This is a schematic diagram showing the shape of the flexible substrate with wiring patterns of the examples and comparative examples after three-dimensional molding. [Modes for carrying out the invention]

[0021] The embodiments of the present invention will be described in detail below with reference to the drawings. Note that the following embodiments are examples of how the present invention can be implemented, and do not limit the present invention to their scope.

[0022] This embodiment is a conductive paste for forming flexible electrical circuits on the surface of a flexible substrate. The conductive paste of this embodiment comprises (A) conductive particles, (B) thermoplastic resin, and (C) solvent. When the conductive paste of this embodiment is cured under predetermined conditions to produce a cured product, the storage modulus of the cured product at a predetermined temperature Tm (°C) is 0.001 to 0.5 GPa. The temperature Tm (°C) is the temperature used to measure the storage modulus of the cured conductive paste. The temperature Tm (°C) can be 178°C. The predetermined conditions for curing the conductive paste can be 120°C and 30 minutes.

[0023] By using the conductive paste of this embodiment, which has a predetermined storage modulus at temperature Tm, it is possible to use three-dimensional molding to form electrical wiring that is less likely to break even when the electrical wiring of an electrical circuit and / or electronic circuit (these are sometimes collectively referred to simply as "electrical circuit") is stretched and / or bent. Specifically, when an electrical circuit, which is a cured product of the conductive paste of this embodiment, is three-dimensionally molded, it is possible to suppress the occurrence of cracks in the electrical wiring contained in the electrical circuit.

[0024] In this specification, "flexible substrate" means the material to be printed when printing an electrical circuit pattern included in an electrical circuit using a conductive paste. Note that "electrical circuit pattern" may also be referred to as "wiring pattern." In this specification, a flexible substrate is also simply called a "substrate." Furthermore, since flexible substrates are generally in the form of a flat sheet or film, they may also be referred to as a "flat sheet" or "film." Preferred examples of flexible substrates are flat sheets (films) made from polycarbonate resin, polyethylene terephthalate (PET) resin, and / or acrylic resin. A flexible substrate is a substrate that has flexibility, meaning it can be deformed, such as stretching and / or bending, when heated to at least a predetermined temperature. Therefore, a flexible substrate does not necessarily have flexibility at room temperature (e.g., below 30°C). The predetermined temperature when heated is, for example, 140°C to 220°C.

[0025] In this specification, "flexible electrical circuit body" includes a flexible substrate (flat sheet or film) and a cured product obtained by printing an electrical circuit pattern (wiring pattern) on the surface of the flexible substrate using a conductive paste, and then heating and drying the printed conductive paste. The flexible electrical circuit body can be used for three-dimensional molding. The flexible electrical circuit body, including the flexible substrate, is flexible when heated to at least a predetermined temperature. Therefore, the flexible electrical circuit body does not need to be flexible at room temperature (e.g., 30°C or below). Similar to the flexible substrate, the predetermined temperature when heated is, for example, 140°C to 220°C.

[0026] In this specification, "molded article" means a flexible electrical circuit body (a flat sheet or film having an electrical circuit pattern (wiring pattern)) molded together with other resins (for example, a flexible substrate and a molding resin) by three-dimensional molding or injection molding. In this specification, "molding" means a processing method for shaping a material into a predetermined form using a mold, such as heat molding, pressure molding, vacuum molding, and injection molding. In this specification, "three-dimensional molding" means a processing method for shaping a two-dimensional flat sheet or film having an electrical circuit pattern (wiring pattern) into a three-dimensional form by molding using a mold, such as heat molding, pressure molding, and vacuum molding. A flexible electrical circuit body that has been three-dimensionally molded can be called a "three-dimensional molded article."

[0027] In this specification, the presence or absence of "cracks" can be determined by observing the surface of the cured conductive paste (wiring pattern) after three-dimensional molding using an optical microscope (magnification: 20x). Figure 1 shows an example of an optical microscope image (magnification: 20x) of a cured conductive paste (wiring pattern) after three-dimensional molding in which no cracks have occurred in the cured product. Figure 2 shows an example of an optical microscope image (magnification: 20x) of a cured conductive paste (wiring pattern) after three-dimensional molding in which cracks have occurred in the cured product. In the optical microscope image of the wiring pattern in Figure 2, a striped pattern can be seen. This striped pattern is a pattern caused by the occurrence of cracks. Therefore, it can be determined that cracks have occurred in the wiring pattern in Figure 2. In contrast, no striped pattern was observed in the optical microscope image of the wiring pattern in Figure 1. Therefore, it can be determined that no cracks have occurred in the wiring pattern in Figure 1.

[0028] The conductive paste of this embodiment is preferably a conductive paste for in-mold electronics. Generally, the technique of bonding a planar sheet or film-shaped substrate simultaneously with molding is called in-mold molding. There is also a technique called in-mold electronics that utilizes in-mold molding. In-mold electronics is generally a technique for integrating a substrate with an electrical circuit pattern onto the surface of a molded body. Specifically, an electrical circuit pattern (flexible electrical circuit body), such as a film-type electronic component like a touch sensor or film antenna, is formed on a planar sheet or film-shaped substrate. The substrate with the electrical circuit pattern is then three-dimensionally molded as needed, sandwiched in a mold, and molded to integrate the substrate with the electrical circuit pattern onto the surface of the molded body. By using the conductive paste of this embodiment, it is possible to form electrical wiring that has a low possibility of disconnection even when the substrate is stretched and / or bent. Therefore, the conductive paste of this embodiment can be preferably used as a conductive paste for in-mold electronics.

[0029] Next, the conductive paste of this embodiment will be described.

[0030] The conductive paste of this embodiment contains predetermined components, which makes it possible to form electrical wiring that has a low probability of breakage even when electrodes and wiring (also called "electrical wiring" or simply "wiring") of an electrical circuit and / or electronic circuit (sometimes simply referred to as "electrical circuit") are stretched and / or bent. Furthermore, the conductive paste of this embodiment makes it possible to form electrical wiring that has a low probability of breakage even when three-dimensional molding is used to form the electrical wiring.

[0031] <(A) Conductive particles> The conductive paste of this embodiment contains conductive particles as component (A).

[0032] In this invention, conductive particles have an average particle diameter of 0.01 to 100 μm and an electrical conductivity of 10 6These are particles with a density of S / m or greater. Conductive particles may be made by forming a conductive material into particulates, or by coating a core particle with a conductive material. The core particle may be made of a non-conductive material as long as at least a part of it is coated with a conductive material. Examples of conductive particles include metal powders and coating powders.

[0033] Conductive particles are used to impart thermal conductivity and / or electrical conductivity to the cured product of the conductive paste of this embodiment. There are no particular limitations on the conductive material. Examples of such conductive materials include gold, silver, nickel, copper, palladium, platinum, bismuth, tin, alloys thereof (especially bismuth-tin alloys, solder, etc.), aluminum, indium tin oxide, silver-plated copper, silver-plated aluminum, metal-plated glass spheres, silver-plated fibers, silver-plated resins, antimond-doped tin, tin oxide, carbon fibers, graphite, carbon black, and mixtures thereof.

[0034] Considering thermal conductivity and electrical conductivity, the conductive material component preferably contains at least one metal selected from the group consisting of silver, nickel, copper, tin, aluminum, silver alloys, nickel alloys, copper alloys, tin alloys, and aluminum alloys; more preferably, at least one metal selected from the group consisting of silver, copper, and nickel; even more preferably, silver or copper; and most preferably, silver.

[0035] By including a specific metal in the conductive particles, electrical wiring with low electrical resistance can be formed. The conductive particles can be particles (coating powder) in which the surface of insulating particles is coated with a conductive material such as the aforementioned metal.

[0036] The conductive particles contained in the conductive paste of this embodiment are preferably silver particles. Silver (Ag) has relatively high electrical conductivity. Therefore, it is preferable to use silver (Ag) particles (i.e., conductive particles made of silver) as the conductive particles. By using silver particles as the conductive particles, it is possible to form electrical wiring with low electrical resistance.

[0037] The conductive particles contained in the conductive paste of this embodiment are more preferably made of silver. In this specification, for example, "conductive particles made of silver" means that no components other than silver are intentionally added, and it is permissible that impurities that are inevitably present are included in the conductive particles. The same applies to conductive particles made of metals other than silver, and to components other than conductive particles.

[0038] The shape of conductive particles is not particularly limited, but may be spherical, irregular, flaky, filamentous (needle-shaped), or dendritic. Here, flaky refers to a shape with a ratio of major axis to minor axis (aspect ratio) of 2 or more, and includes plate-like shapes such as plate-like and flaky shapes. The major and minor axes of the particles constituting the conductive particles can be determined based on images obtained from a scanning electron microscope (SEM) (n=20). The major axis refers to the longest distance of the line segments passing through the approximate center of gravity of the particle in the particle image obtained by SEM, and the minor axis refers to the shortest distance of the line segments passing through the approximate center of gravity of the particle in the particle image obtained by SEM.

[0039] The shape of conductive particles, such as silver particles, is preferably irregular or flaky. The irregular or flaky shape of the conductive particles in the conductive paste of this embodiment allows for the formation of more stable electrical wiring with lower electrical resistance. When the conductive particles are silver particles, the irregular or flaky shape of the silver particles is even more preferable.

[0040] Furthermore, the particle shapes may be combinations of particles with different shapes.

[0041] The particle size of conductive particles can be defined by the particle size (D50) of 50% of the cumulative value of all particles. In this specification, D50 is also referred to as the average particle diameter. The average particle diameter (D50) can be determined by measuring the particle size distribution using the microtrac method (laser diffraction scattering method).

[0042] The average particle size (D50) of the conductive particles is preferably 0.1 to 30 μm, more preferably 0.2 to 20 μm, even more preferably 0.5 to 15 μm, and particularly preferably 0.8 to 10 μm, from the standpoint of resistance to stretching and / or bending and workability. If the average particle size (D50) is larger than the above range, problems such as clogging may occur during screen printing. Also, if the average particle size is smaller than the above range, excessive sintering of the particles may occur during the curing of the conductive paste, making it difficult to form electrical wiring that is resistant to stretching and / or bending.

[0043] Furthermore, the specific surface area of ​​the conductive particles can be expressed as a BET value (BET specific surface area). The BET value of the conductive particles is preferably 0.1 to 10 m². 2 / g, more preferably 0.2~5m 2 / g, more preferably 0.5~3m 2 The value is / g. The BET value can be measured by gas adsorption method (BET method) according to, for example, JIS Z8830:2013 (ISO9277:2010).

[0044] If the specific surface area of ​​conductive particles is too large, the viscosity will increase when forming a paste, making it difficult to handle. On the other hand, if the specific surface area of ​​conductive particles is too small, the contact area between conductive particles will decrease, resulting in reduced conductivity.

[0045] <(B) Thermoplastic resin> The conductive paste of this embodiment contains a thermoplastic resin as component (B).

[0046] The thermoplastic resin included in the conductive paste of this embodiment is preferably such that the storage modulus of the cured product obtained by heating and drying the conductive paste containing the thermoplastic resin at 120°C for 30 minutes has a storage modulus of 0.001 to 0.5 GPa at temperature Tm. The temperature Tm can be 178°C. By including such a thermoplastic resin in the conductive paste of this embodiment, it becomes possible to form electrical wiring with a low possibility of breakage when the cured product of the conductive paste is three-dimensionally molded as electrical wiring.

[0047] The glass transition temperature of the thermoplastic resin contained in the conductive paste of this embodiment is 100 to 200°C, preferably 120 to 200°C, and more preferably 120 to 190°C. The glass transition temperature of the thermoplastic resin can be measured by differential scanning calorimetry (DSC). The typical temperature for heat molding of in-mold electronics is 140 to 220°C, the temperature for pressure molding is 140 to 160°C, and the temperature for vacuum molding is 160 to 220°C. By using a resin component having a glass transition temperature within the above range as the resin component of the conductive paste, the cured product of the conductive paste and the flat sheet (film) can follow each other well during three-dimensional molding using heat molding, etc. Therefore, even if the electrical circuit pattern deforms during three-dimensional molding, the possibility of electrical wiring breakage can be reduced. If the glass transition temperature is within the above range, the occurrence of cracks in the resulting cured product can be suppressed, resulting in a lower resistance value and making it usable as wiring.

[0048] The conductive paste of this embodiment preferably contains (B) a thermoplastic resin, which is selected from the group consisting of polycarbonate resin, hydrogenated styrene-based thermoplastic elastomer, and styrene-butadiene-styrene block copolymer. By using these materials as (B) thermoplastic resin, the possibility of electrical wiring breakage can be further reduced.

[0049] The conductive paste of this embodiment preferably contains a polycarbonate resin. The thermoplastic resin is more preferably made of a polycarbonate resin.

[0050] During three-dimensional molding, a molded body with electrical wiring can be manufactured by printing predetermined electrical wiring onto a flat sheet (film) of polycarbonate resin and then using the flat sheet for three-dimensional molding. Because the thermoplastic resin includes polycarbonate resin, the electrical wiring can better conform to the deformation of the flat sheet during three-dimensional molding. Therefore, the possibility of electrical wiring breakage can be reduced.

[0051] In this embodiment, the conductive paste preferably contains at least one polycarbonate resin selected from poly(4,4'-cyclohexyllidenediphenyl)carbonate and copoli[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane]carbonate. The polycarbonate resin being one of these materials further reduces the possibility of electrical wiring breakage.

[0052] Furthermore, two or more different thermoplastic resins can be used as the thermoplastic resin. For example, two or more different polycarbonate resins can be used as the thermoplastic resin. In addition, two or more different resins other than polycarbonate resin can be used as the thermoplastic resin. Moreover, polycarbonate resin and resins other than polycarbonate resin can be used in combination as two or more different resins.

[0053] Furthermore, thermoplastic resins are generally manufactured using raw materials derived from petroleum resources. However, in recent years, concerns have been raised about the depletion of petroleum resources, and the development of thermoplastic resins using raw materials obtained from biomass resources such as plants is progressing. In addition, given concerns that global warming caused by increased and accumulated carbon dioxide emissions will lead to climate change, it is also possible to use plant-derived thermoplastic resins that are carbon neutral even when disposed of after use.

[0054] Furthermore, the thermoplastic resin contained in the conductive paste of this embodiment can be a hydrogenated styrene-based thermoplastic elastomer or a styrene-ethylene-butylene-styrene (SEBS) block copolymer. Examples of commercially available hydrogenated styrene-based thermoplastic elastomer SEBS block copolymers that can be used in the conductive paste of this embodiment include ToughTec H1041, H1043, H1051, and M1913 from Asahi Kasei Corporation, and Septon 4077 and Septon 4099 from Kuraray Co., Ltd. In addition, any resin with a low modulus of elasticity such that the storage modulus of the cured conductive paste is 0.001 to 0.5 GPa can have a structure such as a styrene-butadiene-styrene (SBS) block copolymer or a styrene-ethylene-ethylene-propylene-styrene (SEEPS) block copolymer. An example of a commercially available SBS block copolymer is ToughPrene® A from Asahi Kasei Corporation. An example of a commercially available SEEPS block copolymer is Septon 4033 from Kuraray Co., Ltd.

[0055] The inventors of the present invention have discovered that when a conductive paste cured product is cured under predetermined conditions and the storage modulus measured at a predetermined temperature Tm for measuring the storage modulus is 0.001 to 0.5 GPa, it is possible to suppress the occurrence of cracks in the cured conductive paste (electrical circuit pattern) placed on a three-dimensionally molded body, thereby reducing the possibility of electrical wiring breakage, and thus arrived at the present invention. By appropriately controlling the type and amount of thermoplastic resin contained in the conductive paste, the storage modulus of a predetermined cured product of the conductive paste can be controlled to an appropriate range. For example, the storage modulus at the temperature Tm (°C) for measuring the storage modulus can be controlled by increasing the molecular weight, or by adjusting the ratio of hard segments, which are rigid structures in the thermoplastic resin, and / or by adjusting the ratio of soft segments, which are flexible structures in the thermoplastic resin. As the thermoplastic resin mentioned above, at least one can be selected from polycarbonate resins, polystyrene resins, polyolefin resins, polyvinyl chloride resins, polyurethane resins, polyester resins, polyamide resins, polybutadiene resins, and their hydrides, as well as modified copolymer hydrides obtained by modifying these hydrides. Two or more types may be used in combination. Furthermore, since increasing the amount of thermoplastic resin tends to lower the storage modulus, the storage modulus can be adjusted by adjusting the content of the thermoplastic resin. As described later, the predetermined conditions for curing the conductive paste are 120°C and 30 minutes. The temperature Tm (°C) for measuring the storage modulus of the conductive paste can be Tm = 178°C.

[0056] The viscosity-average molecular weight of the thermoplastic resin (e.g., polycarbonate resin) contained in the conductive paste of this embodiment is preferably 10,000 to 100,000, more preferably 10,000 to 80,000, and even more preferably 10,000 to 60,000. If the molecular weight of the thermoplastic resin is high, it may be necessary to increase the amount of solvent added to achieve the desired viscosity. If the amount of solvent added is high, the problem of a thin coating film may arise. By having the molecular weight of the thermoplastic resin (e.g., polycarbonate resin) within the above range, the viscosity, elongation characteristics upon heating, and electrical resistance of the cured product can be appropriately balanced.

[0057] When using polycarbonate as a thermoplastic resin, some types of polycarbonate resins have high crystallinity. A problem arose when highly crystalline polycarbonate resins were heated and dissolved in a solvent and then returned to room temperature, causing them to crystallize. It is not easy to use such polycarbonate resins as a resin component in conductive pastes. On the other hand, the aforementioned specified polycarbonate resins can be dissolved in specified solvents, such as isophorone with a high boiling point, and the problem of crystallization after returning to room temperature can be avoided. Therefore, by using the aforementioned polycarbonate resins as thermoplastic resins, the possibility of electrical wiring breakage can be further reduced.

[0058] In this embodiment, the conductive paste allows for easy printing of electrical circuit patterns on the surface of a flexible substrate made of polycarbonate resin. Furthermore, it is also easy to print electrical circuit patterns on the surface of substrates made of materials other than polycarbonate resin.

[0059] In the conductive paste of this embodiment, the amount of (B) thermoplastic resin is preferably 1 to 50 parts by weight, more preferably 5 to 45 parts by weight, even more preferably 8 to 43 parts by weight, and particularly preferably 10 to 40 parts by weight, based on 100 parts by weight of (A) conductive particles. By setting the weight ratio of conductive particles to (B) thermoplastic resin within an appropriate range, the shape of the printed pattern of the electrical wiring can be appropriately maintained and the electrical resistance of the electrical wiring can be reduced.

[0060] The conductive paste of this embodiment may contain other resins, such as thermoplastic resins, thermosetting resins, and / or photocurable resins, to the extent that it does not impede the effect.

[0061] <(C) Solvent> The conductive paste of this embodiment contains a solvent as component (C).

[0062] In this embodiment, the conductive paste preferably has a solvent with a boiling point of 150°C to 300°C. By using a high-boiling-point solvent, such as isophorone, polycarbonate resin can be dissolved. Furthermore, by using a solvent such as diethylene glycol dibutyl ether, hydrogenated styrene-based thermoplastic elastomer resin can be dissolved. In addition, by having a solvent boiling point above a predetermined temperature, after screen printing an electrical circuit pattern using the conductive paste, the drying time can be set to an appropriate length without drastically shortening it, resulting in good workability. Therefore, it becomes easier to deform the electrical circuit pattern to follow the deformation of a flat sheet or film during three-dimensional molding. However, if the boiling point of the solvent exceeds 300°C, it may not be possible to sufficiently remove the solvent during heating for drying and molding.

[0063] The solvent preferably contains at least one selected from isophorone, 3-methoxy-N,N-dimethylpropanamide, benzyl alcohol, butyl carbitol (diethylene glycol monobutyl ether), diethylene glycol dibutyl ether, ethyl carbitol acetate (diethylene glycol monoethyl ether acetate), and tetraglyceride. These predetermined solvents can reliably dissolve polycarbonate resins and hydrogenated styrene-based thermoplastic elastomer resins. Furthermore, the presence of a predetermined solvent in the conductive paste can further reduce the possibility of disconnection of the resulting electrical wiring. In particular, when dissolving poly(4,4'-cyclohexyllidene diphenyl) carbonate and / or coply[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane] carbonate as the thermoplastic resin in the solvent, the above-mentioned solvents can be preferably used. This makes it easier to form the conductive paste and further reduces the possibility of disconnection of the resulting electrical wiring.

[0064] The solvent preferably contains isophorone. By using isophorone as the solvent, the polycarbonate resin can be dissolved in the solvent more easily. Furthermore, since the boiling point of isophorone is 215°C, it can be said that this is a suitable boiling point for a solvent suitable for printing conductive pastes. In addition, conductive pastes containing isophorone as a solvent have advantages such as being less likely to gel or solidify even when the conductive paste is left standing, and not separating solid and liquid during printing. For this reason, the solvent preferably contains isophorone. In particular, when dissolving poly(4,4'-cyclohexyllidenediphenyl) carbonate and / or coply[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane] carbonate as a thermoplastic resin in a solvent, a solvent containing isophorone can be preferably used.

[0065] The solvent preferably contains diethylene glycol dibutyl ether. By using diethylene glycol dibutyl ether as the solvent, hydrogenated styrene-based thermoplastic elastomer resins (e.g., maleic acid-modified styrene-butadiene elastomers) can be dissolved more easily in the solvent. Furthermore, since the boiling point of diethylene glycol dibutyl ether is 255°C, it can be said that this is a suitable boiling point for a solvent used in printing conductive pastes. Conductive pastes containing diethylene glycol dibutyl ether as a solvent also have advantages such as being less prone to gelation or solidification even when left standing, and not separating solid and liquid during printing. For this reason, the solvent preferably contains diethylene glycol dibutyl ether, and more preferably consists solely of diethylene glycol dibutyl ether. In particular, when dissolving maleic acid-modified styrene-butadiene elastomer resins as thermoplastic resins in the solvent, a solvent containing diethylene glycol dibutyl ether can be preferably used.

[0066] The amount of solvent added is preferably 50 to 1000 parts by weight, more preferably 80 to 900 parts by weight, even more preferably 100 to 800 parts by weight, and particularly preferably 100 to 500 parts by weight, per 100 parts by weight of thermoplastic resin. Typically, the thermoplastic resin can be properly dissolved by using a solvent in an amount approximately four times the weight of the thermoplastic resin.

[0067] Furthermore, solvents can be added to the conductive paste as needed to adjust its viscosity.

[0068] <Other ingredients> The conductive paste of the present invention may contain at least one selected from inorganic pigments, organic pigments, silane coupling agents, leveling agents, thixotropic agents, insulating particles, interfacial treatment agents such as coupling agents, pigments, dyes, plasticizers, defoaming agents, antifoaming agents, antioxidants, leveling agents, and thixotropic agents, to the extent that it does not impede the effects of the present invention, or to improve the effects of the present invention.

[0069] <Conductive paste> This embodiment is a conductive paste containing the above-mentioned components.

[0070] The conductive paste of this embodiment may consist only of the components (A) conductive particles, (B) thermoplastic resin, and (C) solvent described above. However, to the extent that it does not hinder the effects of the present invention, or to improve the effects of the present invention, the conductive paste of this embodiment may contain other components other than the components (A) to (C) described above. That is, the conductive paste of this embodiment may further contain at least one selected from inorganic pigments, organic pigments, silane coupling agents, leveling agents, thixotropic agents, insulating particles, interfacial treatment agents such as coupling agents, pigments, dyes, plasticizers, defoaming agents, antifoaming agents, antioxidants, leveling agents, and thixotropic agents.

[0071] The conductive paste of this embodiment can be manufactured by mixing the above-mentioned components ((A) conductive particles, (B) thermoplastic resin, and (C) solvent) and other components that may be added, in a mixer such as a meteorite stirrer, dissolver, bead mill, raika mill, three-roll mill, rotary mixer, or twin-screw mixer. In this way, a conductive paste suitable for screen printing, dipping, or other desired coating or electrical wiring formation methods can be prepared.

[0072] The viscosity of the conductive paste in this embodiment can be adjusted to a viscosity suitable for use in a predetermined coating film or electrical wiring formation method such as screen printing. The viscosity can be adjusted by appropriately controlling the amount of solvent.

[0073] The viscosity of the conductive paste in this embodiment is preferably 1 to 1000 Pa·sec, more preferably 2 to 700 Pa·sec, even more preferably 5 to 600 Pa·sec, and particularly preferably 7 to 300 Pa·sec. The viscosity can be measured using a Brookfield (Type B) viscometer with an SC4-14 spindle (utility cup and spindle (UC / S)=#14) at a rotation speed of 10 rpm and a measurement temperature of 25°C.

[0074] By using the conductive paste of this embodiment, electrical circuit wiring with a low probability of breakage can be formed on the surface of a flexible substrate by known coating methods (dip coating, spray coating, bar coater coating, gravure coating, reverse gravure coating, and spin coater coating, etc.), known printing methods (lithographic printing, cardboard printing, metal printing, offset printing, screen printing, gravure printing, flexographic printing, and inkjet printing, etc.), jet dispensers, and air dispensers.

[0075] <Flexible base material> In in-mold electronics, a flexible substrate (flat sheet or film) on which an electrical circuit pattern is printed using a conductive paste is used for three-dimensional molding. The material of the substrate is not particularly limited as long as it is a substrate that can be three-dimensionally molded. Preferred flexible substrates include those made of polycarbonate resin, polyethylene terephthalate (PET), and acrylic resin. In particular, by making the glass transition point and softening point of the material of the flexible substrate (flat sheet or film) and the resin component of the conductive paste similar, the elongation of the flat sheet or film during three-dimensional molding using heat molding, pressure molding, and vacuum molding can appropriately follow the elongation of the resin component of the conductive paste. Therefore, the possibility of electrical wiring breakage can be further reduced.

[0076] <Electrical circuit containing hardened conductive paste> This embodiment is an electrical circuit including a cured product of a predetermined conductive paste. In this specification, the conductive paste of this embodiment is printed in a predetermined pattern and cured by heating and drying under predetermined heating and drying conditions (for example, 120°C for 30 minutes), and the resulting product is referred to as a "cured product." The cured product can be used as electrical wiring for electrical circuits, etc. Therefore, the conductive paste of this embodiment can be used to form an electrical circuit with a low possibility of disconnection. Note that heating of the conductive paste during the production of the cured product does not have to be the final heating.

[0077] The conductive paste of this embodiment has a storage modulus of 0.001 to 0.5 GPa, which is measured at a predetermined temperature Tm after curing the conductive paste under predetermined conditions. In the conductive paste of this embodiment, the predetermined conditions for curing the conductive paste are 120°C and 30 minutes. The conductive paste can be cured in an air atmosphere. The temperature Tm (°C) for measuring the storage modulus of the conductive paste can be Tm = 178°C.

[0078] The inventors of the present invention have discovered that by having a storage modulus of 0.001 to 0.5 GPa for the cured product (electrical circuit pattern) at 178°C, which is close to the temperature used for three-dimensional molding using a flexible substrate, it is possible to suppress the occurrence of cracks in the cured product (electrical circuit pattern) of conductive paste placed on the molded body, thereby reducing the possibility of electrical wiring breakage, and thus arrived at the present invention.

[0079] Methods for three-dimensional molding include, for example, heat molding, pressure molding, and vacuum molding. Depending on the three-dimensional molding method, the molding temperature may differ. Also, the molding temperature may differ depending on the type of flexible substrate. Therefore, strictly speaking, the temperature Tm (°C) for measuring the storage modulus must be selected appropriately depending on the molding method and the type of flexible substrate. However, selecting an appropriate temperature depending on the molding method and the type of flexible substrate is complicated. On the other hand, the inventors measured the storage modulus with a temperature Tm (°C) of 178°C and found that when the storage modulus of the cured conductive paste was 0.001 to 0.5 GPa, it was possible to suppress the occurrence of cracks in the cured conductive paste, thereby reducing the possibility of electrical wiring breakage. This suggests that although the molding temperature differs depending on the molding method and the type of flexible substrate, it is possible to suppress the occurrence of cracks in the cured conductive paste by selecting an appropriate representative temperature Tm (°C).

[0080] It is also possible to select a temperature other than 178°C as the temperature Tm(°C). For example, the temperature Tm(°C) can be set to the glass transition point Tg(°C) of the flexible substrate material, and a temperature in the range of Tg to Tg+60°C can be used as the temperature Tm(°C) for measuring the storage modulus of the cured product. On the other hand, deciding which temperature to select as Tm(°C) within the range of Tg to Tg+60°C is complicated. Therefore, it is preferable to use 178°C as the temperature Tm(°C). The inventors have found that by using 178°C as an appropriate representative temperature Tm(°C), it is possible to suppress the occurrence of cracks in the cured product of the conductive paste during three-dimensional molding. Considering the type of molding method and the type of flexible substrate, when 178°C is selected as the temperature Tm(°C), the storage modulus is 0.001 to 0.5 GPa, which suppresses the occurrence of cracks in the cured product (electrical circuit pattern) of the conductive paste placed on the molded body, thereby reducing the possibility of electrical wiring breakage. In particular, when using a flat sheet or film made of polycarbonate resin as the flexible substrate, it is preferable that the temperature Tm (°C) is Tm = 178°C.

[0081] As described above, when the storage modulus of the cured conductive paste of this embodiment is cured under predetermined conditions and measured at a predetermined temperature Tm for measuring the storage modulus, the storage modulus of the cured product is 0.001 to 0.5 GPa, which suppresses the occurrence of cracks in the cured conductive paste (electrical circuit pattern) during three-dimensional molding. The reason for this is thought to be as follows: The storage modulus of a general flexible substrate used for three-dimensional molding (for example, a flat sheet or film made of polycarbonate resin) decreases sharply from near the molding temperature (for example, around 140°C) when the temperature is raised from room temperature to the molding temperature (for example, 140 to 220°C). When the storage modulus of the flexible substrate at the molding temperature and the storage modulus of the cured conductive paste (electrical circuit pattern) are close, the deformation of the flexible substrate and the cured conductive paste will be similar, which is thought to suppress the occurrence of cracks in the cured product. However, the present invention is not bound by this reasoning.

[0082] In this specification, a flexible electrical circuit body may refer to a structure in which an electrical circuit pattern is printed on the surface of a flexible substrate (for example, a flat sheet or film made of polycarbonate resin), and a cured product (electrical circuit) is formed by curing the printed conductive paste. The flexible electrical circuit body of this embodiment preferably includes a flexible substrate and an electrical circuit arranged on the flexible substrate.

[0083] The conductive paste of this embodiment can be preferably used as a conductive paste for forming flexible electrical circuits. By using the flexible electrical circuit of this embodiment in three-dimensional molding, a molded body containing a desired electrical circuit can be manufactured.

[0084] The flexible substrate on which the conductive paste of this embodiment is printed preferably contains at least one selected from the group consisting of polycarbonate, polyethylene terephthalate (PET), and acrylic resin. By using a predetermined flexible substrate, it is possible to form electrical wiring for an electrical circuit and / or electronic circuit in a molded body that has a low probability of breakage even when the electrical wiring of the electrical circuit and / or electronic circuit is stretched and / or bent.

[0085] To form a flexible electrical circuit, first, an electrical circuit pattern is printed on the surface of a flexible substrate using the conductive paste of this embodiment, and then the electrical circuit pattern in the conductive paste is heated and dried to cure it. The temperature and time for curing the electrical circuit pattern in the conductive paste can be appropriately selected depending on the type of thermoplastic resin contained in the conductive paste. The temperature and time for curing the conductive paste can be adjusted and determined as appropriate, taking into account the heat resistance of the substrate. For example, the temperature and time for curing the conductive paste can be 60°C to 160°C for 3 to 60 minutes, preferably 80°C to 150°C for 3 to 60 minutes, and more preferably 100°C to 130°C for 3 to 30 minutes. By curing the electrical circuit pattern at the predetermined temperature and time, a cured electrical circuit pattern can be obtained. A specific example of curing conditions is 120°C and 30 minutes.

[0086] In this embodiment, it is preferable that the absolute value of the difference between the storage modulus of the flexible substrate at temperature Tm and the storage modulus of the cured product obtained by curing the conductive paste at 120°C for 30 minutes is 0 to 0.50 GPa. By having the absolute value of the difference between the storage modulus of the cured conductive paste and the flexible substrate at temperature Tm be within a predetermined range, even if the electrical wiring of the electrical circuit and / or electronic circuit is stretched and / or bent during the formation of the molded body, the deformation of the flexible substrate and the cured conductive paste will be similar. Therefore, it is possible to form electrical wiring with a low possibility of breakage.

[0087] Furthermore, since the conductive paste of this embodiment is used for forming electrical circuits such as electrodes and wiring for electrical and / or electronic circuits, it is necessary that the electrical wiring obtained using the conductive paste of this embodiment has low resistance and excellent conductivity. By using the conductive paste of this embodiment with a predetermined composition, it is possible to form electrical wiring with appropriate resistivity even when the electrical wiring of an electrical and / or electronic circuit is stretched and / or bent by utilizing three-dimensional molding.

[0088] <Molded body>

[0089] The molded body of this embodiment can be manufactured by molding a flexible substrate (flexible electrical circuit body) on which an electrical circuit is formed. Specifically, for example, the molded body of this embodiment can be manufactured by molding the flexible electrical circuit body together with other resins (for example, a flexible substrate and a molding resin) using methods such as three-dimensional molding or injection molding. In other words, the molded body of this embodiment includes an electrical circuit. The technology for manufacturing molded bodies that include electrical circuits is called in-mold electronics. Therefore, the conductive paste of this embodiment can be preferably used as a conductive paste for in-mold electronics.

[0090] The molding method for manufacturing the molded article of this embodiment is not particularly limited. Examples of molding methods for manufacturing the molded article of this embodiment are as follows: For example, the molded article of this embodiment can be manufactured by three-dimensional molding of a flexible substrate (flexible electrical circuit body) on which an electrical circuit is formed. Alternatively, the molded article of this embodiment can be manufactured by injection molding using a flexible substrate (flexible electrical circuit body) on which an electrical circuit is formed and a molding resin. Furthermore, the molded article of this embodiment can be manufactured by three-dimensional molding of a flexible substrate (flexible electrical circuit body) on which an electrical circuit is formed, and then injection molding using a molding resin.

[0091] The molded body of this embodiment is preferably manufactured by three-dimensional molding using heat molding. By heating the wiring pattern formed on the flexible electrical circuit body using the conductive paste of this embodiment, it becomes possible to stretch and / or bend it. The flexible electrical circuit body of this embodiment is flexible, including the electrical circuit. Therefore, the molded body of this embodiment can be manufactured by forming it into a desired three-dimensional shape using heat molding. Furthermore, if necessary, a molded body of the desired shape and design can be obtained by integrally molding it with the three-dimensional molded body using methods such as injection molding.

[0092] In the method for manufacturing a molded article using in-mold electronics according to this embodiment, first, an electrical circuit is formed on the surface of a flexible substrate using the conductive paste of this embodiment described above. Next, the molded article can be formed by three-dimensional molding or injection molding of the flexible substrate (flexible electrical circuit body) on which the electrical circuit has been formed.

[0093] Specifically, an example of the manufacturing process for molded parts using in-mold electronics is as follows:

[0094] First, the conductive paste of this embodiment described above is manufactured.

[0095] Next, the conductive paste of this embodiment is printed onto the surface of a flexible substrate (for example, a flat sheet or film made of polycarbonate resin) to form a printed pattern (electrical circuit pattern).

[0096] Next, a flexible electrical circuit body is manufactured by heating and drying a printed pattern (electrical circuit pattern) of conductive paste printed on a flexible substrate to cure it. The curing conditions can be, for example, a heating temperature of 120°C and a heating time of 30 minutes.

[0097] Next, the flexible electrical circuit body, on which the initial electrical circuit has been formed, is molded into a three-dimensional shape by a molding method such as heat molding, pressure molding, or vacuum molding. In molding methods such as heat molding, the heating temperature of the flexible electrical circuit body is preferably 140°C to 220°C.

[0098] If necessary, a flexible electrical circuit body with a three-dimensional shape and a molding resin are integrally molded together. For example, injection molding can be used as the molding method.

[0099] Through the above process, a molded body can be manufactured using in-mold electronics. According to the manufacturing method of this embodiment, a molded body having an electrical circuit with a low probability of disconnection can be manufactured by three-dimensional molding. [Examples]

[0100] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0101] <Materials and preparation ratios for conductive paste> Tables 1 and 2 show the compositions of the conductive pastes for Examples 1-13 and Comparative Examples 1-4. The compositions of each component listed in Tables 1 and 2 are given when the weight of the conductive particles is 100 parts by weight. The conductive pastes of the Examples and Comparative Examples are conductive pastes consisting of silver particles (conductive particles), thermoplastic resin, and solvent.

[0102] <(A) Conductive particles> In the examples and comparative examples, silver particles were used as conductive particles. Table 3 shows the manufacturer and model number, particle shape, average particle diameter (D50), tap density, and BET specific surface area of ​​the silver particles A to D (conductive particles A to E) used in the examples and comparative examples. Tap density is the "bulk density" obtained after mechanically tapping a container containing the powder sample.

[0103] <(B) Thermoplastic resin> Table 4 shows the manufacturers, model numbers, resin types, molecular weights, and glass transition temperatures of resins A to F used as thermoplastic resins in the examples and comparative examples.

[0104] <(C) Solvent> Table 5 shows the types and boiling points of solvents A to C used in the examples and comparative examples. The thermoplastic resins mentioned above were mixed with conductive particles while dissolved in the solvent.

[0105] <Preparation of conductive paste> Next, the materials in the predetermined preparation ratios described above were mixed in a planetary mixer, and then dispersed in a three-roll mill to form a paste, thereby preparing a conductive paste.

[0106] <Method for measuring viscosity> The viscosity of the conductive pastes in the examples and comparative examples was measured using a Brookfield (Type B) viscometer with an SC4-14 spindle (utility cup and spindle (UC / S)=#14) at two rotation speeds, 10 rpm and 1 rpm, and at a measurement temperature of 25°C. Tables 1 and 2 show the measurement results of the viscosity of the conductive paste at rotation speeds of 10 rpm and 1 rpm. The "Paste Viscosity Ratio" column in Tables 1 and 2 shows the ratio of the viscosity at 10 rpm to the viscosity at 1 rpm (viscosity at 1 rpm / viscosity at 10 rpm).

[0107] <Method for measuring the storage modulus> The storage modulus of the cured conductive pastes of the examples and comparative examples at a temperature of 178°C (Tm) was measured as follows.

[0108] First, a substrate was prepared by attaching Teflon® tape to a glass slide. The conductive pastes from the above-described examples and comparative examples were printed onto the surface of the Teflon® tape on the glass slide, in a size that included a test pattern measuring 40 mm in length and 5 mm in width. The thickness of the conductive paste was adjusted so that the thickness after curing was 200 ± 50 μm.

[0109] Next, the test pattern was placed on a Teflon® tape on a glass slide, and the conductive paste test pattern was cured. The curing conditions were 120°C for 30 minutes. That is, the conductive paste was dried by leaving the substrate with the test pattern in an oven with an air atmosphere at 120°C for 30 minutes, thereby obtaining a cured product of the test pattern.

[0110] Next, only the cured material (cured coating) of the test pattern was peeled off the Teflon® tape and removed. This cured material was cut into pieces measuring 40 mm in length and 5 mm in width to form test specimens. The thickness of the cured material was 200 μm ± 50 μm.

[0111] Next, the storage modulus of a cured material measuring 40 mm in length, 5 mm in width, and 200 μm ± 50 μm in thickness was measured. A HITACHI DMS7100 was used as the measuring device, and the storage modulus was measured under the following conditions. The measurement temperature (Tm) was set to 178°C. Measurement conditions: Amplitude: 5μm, Tension: 1.2, Force: 0.05N, Force Amplitude Default Value: 0.05N

[0112] Tables 1 and 2 show the measurement results of the storage modulus of the cured conductive pastes of the examples and comparative examples at a temperature of 178°C (Tm).

[0113] <Method for measuring resistivity before three-dimensional molding> The resistivity of the cured conductive pastes of the examples and comparative examples before three-dimensional molding was measured as follows. As the flexible substrate, a flexible flat sheet (film, Makrofol® DE 1-1 000000, manufactured by Covestro) made of polycarbonate resin was used. The size of the substrate was A4 size (210 mm x 297 mm) and the thickness was 250 μm. This substrate can be deformed (stretched) at a temperature of about 160°C. The wiring patterns (length: 120 mm, width: 2 mm) of the conductive pastes of the examples and comparative examples were printed on the surface of this substrate. A screen printing machine was used for printing. After printing, the substrate was cured by heating and drying in a constant-temperature dryer at 120°C for 30 minutes. The film thickness of the resulting cured wiring pattern (hereinafter simply referred to as "wiring pattern") was 10 to 20 μm. The film thickness was measured using a surface roughness and shape measuring instrument (model number: Surfcom 1500SD-2) manufactured by Tokyo Seimitsu Co., Ltd.

[0114] The electrical resistance values ​​(electrical resistance values ​​before three-dimensional molding) of the wiring patterns of the examples and comparative examples were measured by placing electrodes at both ends of an 80 mm section near the center in the longitudinal direction and applying current. The electrical resistance values ​​of the wiring patterns were measured using the four-terminal method with an ADC 7461A digital multimeter. The resistivity was calculated from the electrical resistance values ​​and the dimensions of the wiring patterns.

[0115] In Tables 1 and 2, the "Resistivity Before Molding" column shows the resistivity before three-dimensional molding, calculated from the initial resistance values ​​of the examples and comparative examples, in exponential form. For example, the resistivity before three-dimensional molding of Example 1 in Table 1, "1.3E-4," means that the resistivity is 1.3 × 10⁻⁶. -4 This indicates that the value was Ω·cm. Note that for Comparative Example 4, the measured value was within the measurement limit (resistivity: 5.0 × 10⁻⁶). +3 Since the value was higher than Ω·cm and could not be measured, it is recorded as ">5E+3" in Table 1. The same applies to the resistivity after molding listed in the "Resistivity After Molding" column of "Resistivity" in Tables 1 and 2.

[0116] <Method for measuring resistivity after three-dimensional molding> The resistivity of the cured conductive pastes (wiring patterns) of the examples and comparative examples after three-dimensional molding was measured as follows. First, as described above, a flexible sheet (film, Makrofol® DE 1-1 000000, manufactured by Covestro) of polycarbonate resin was used as the flexible substrate. The size of the substrate was A4 size (210 mm × 297 mm) and the thickness was 250 μm. This substrate can be deformed (stretched) at a temperature of about 160°C. The wiring pattern shown in Figure 3 was printed on the surface of this substrate using the conductive pastes of the examples and comparative examples. The wiring pattern shown in Figure 3 has a shape in which two patterns, each 2 mm wide and 120 mm long, are spaced 56 mm apart. A screen printing machine was used for printing. After printing, the cured product was obtained by heating and drying in a constant-temperature dryer at 120°C for 30 minutes. The film thickness of the cured wiring pattern (simply referred to as "wiring pattern") was 10-20 μm. The film thickness was measured using a surface roughness and shape measuring instrument (model number: Surfcom 1500SD-2) manufactured by Tokyo Seimitsu Co., Ltd.

[0117] Next, the planar sheets (flexible substrates) on which the cured wiring patterns of the examples and comparative examples were formed were heat-molded to create three-dimensional shapes. The heat molding was performed using a vacuum forming machine (model number: Formech 450T). The heat molding conditions were 100% heater output and a heating time of 11 seconds. After heating the planar sheet (flexible substrate) with the heater, the pattern was pressed onto it, and three-dimensional molding was performed by vacuum evacuation. Figure 4 shows a photograph of an example of the shape after three-dimensional molding. The wiring pattern of the cured conductive paste was deformed by three-dimensional molding, resulting in the shape shown in the photograph in Figure 4. Figure 5 shows a schematic diagram of the shape after three-dimensional molding. The diagram shown at the top of Figure 5 is a planar schematic diagram (top schematic diagram), and the diagram shown at the bottom of Figure 5 is a side schematic diagram. In Figure 5, the wiring after three-dimensional molding is omitted.

[0118] Next, the electrical resistance values ​​of the wiring patterns of the examples and comparative examples after three-dimensional molding (electrical resistance values ​​after three-dimensional molding) were measured by placing electrodes at both ends of an 80 mm section near the center in the longitudinal direction and applying current. The electrical resistance values ​​of the wiring patterns were measured using the four-terminal method with an ADC 7461A digital multimeter. The resistivity was calculated from the electrical resistance values ​​and the dimensions of the wiring patterns. In the "Resistivity" column of Tables 1 and 2, the resistivity values ​​after three-dimensional molding, calculated from the resistance values ​​after three-dimensional molding for the examples and comparative examples, are shown in exponential form.

[0119] Tables 1 and 2 also show the resistivity change rate before and after three-dimensional molding ((resistivity after three-dimensional molding / resistivity before three-dimensional molding) × 100 - 100).

[0120] <Method for measuring cracks> Crack occurrence was measured in the wiring patterns of the three-dimensionally molded examples and comparative examples, which were manufactured for the purpose of measuring resistivity after three-dimensional molding. Figures 4 and 5 show photographs and shapes after three-dimensional molding. As shown in the photograph of the three-dimensionally molded examples in Figure 4, the wiring patterns were deformed by three-dimensional molding. The deformation differed between the center and the ends of the wiring. The deformation in the center of the wiring due to three-dimensional molding was a deformation in which the width of the wiring was stretched by approximately 60% in the width direction (a deformation in which the width became approximately 3.2 mm after molding from 2 mm before molding). Compared to the center of the wiring, the ends were stretched in an arc shape, and the deformation was more complex and larger. Therefore, it was found that crack occurrence was more frequent at the ends of the wiring pattern than in the center. Accordingly, the presence or absence of cracks was determined by observing the center and ends of the wiring patterns of the three-dimensionally molded examples and comparative examples at 20x magnification using an optical microscope.

[0121] Figure 1 shows an example of an optical microscope image (magnification: 20x) of a cured conductive paste (wiring pattern) after three-dimensional molding, where no cracks have occurred in the cured product. Figure 2 shows an example of an optical microscope image (magnification: 20x) of a cured conductive paste (wiring pattern) after three-dimensional molding, where cracks have occurred in the cured product. In the optical microscope image of the wiring pattern in Figure 2, a striped pattern can be seen. This striped pattern is a result of cracking. In contrast, no striped pattern was observed in the optical microscope image of the wiring pattern in Figure 1. Therefore, it can be concluded that no cracks occurred in the wiring pattern in Figure 1. In this way, the presence or absence of cracks was determined by observing the surface of the wiring patterns of the examples and comparative examples using an optical microscope (magnification: 20x).

[0122] Tables 1 and 2 show the results of determining the presence or absence of cracks in the central and peripheral parts of the wiring patterns of the examples and comparative examples, as described above. Tables 1 and 2 also show the results of determining the moldability of the wiring patterns of the examples and comparative examples during three-dimensional molding, based on the presence or absence of cracks in the central and peripheral parts of the wiring patterns observed using an optical microscope. Wiring patterns judged to have no cracks in either the central or peripheral parts were classified as A; wiring patterns judged to have no cracks in the central part but cracks in the peripheral parts were classified as B; wiring patterns judged to have cracks in part of the central part and cracks in the peripheral parts were classified as C; and wiring patterns judged to have cracks in either the central or peripheral parts were classified as D. If the moldability is A, it is preferable to use it as wiring after three-dimensional molding. If the moldability is B, it is possible to use it as wiring after three-dimensional molding in specific applications. If the moldability is C or D, it is difficult to use it as wiring after three-dimensional molding, even in specific applications. Here, the specific application refers to a three-dimensional molding process in which the width of the wiring is elongated by approximately 40-70%, preferably 55-65%, more preferably 50-60%, and the elongation in the longitudinal direction of the wiring is 20% or less, preferably 10% or less, more preferably 5% or less. As mentioned above, a moldability rating of B means that even if the deformation caused by three-dimensional molding in the central part of the wiring results in an elongation of approximately 60% in the width direction of the wiring, no cracks will occur, making it suitable for such applications.

[0123] <Measurement results of the examples and comparative examples> As shown in Tables 1 and 2, the storage modulus of the cured products of the conductive pastes of Examples 1 to 13, cured at 120°C for 30 minutes, was in the range of 0.001 to 0.5 GPa at the aforementioned temperature Tm (Tm = 178°C). On the other hand, the storage modulus of the cured products of the conductive pastes of Comparative Examples 1 to 4, cured at 120°C for 30 minutes, was either 0.51 GPa or higher (Comparative Examples 1 to 3) or 0.00025 GPa (Comparative Example 4). Therefore, the storage modulus of the cured products of the conductive pastes of Comparative Examples 1 to 4, cured under the specified conditions, was not in the range of 0.001 to 0.5 GPa.

[0124] As shown in Tables 1 and 2, the absolute difference between the storage modulus (0.002 GPa) of the flexible substrate at a temperature Tm (Tm = 178°C) and the storage modulus of the cured product obtained by curing the conductive pastes of Examples 1 to 13 at 120°C for 30 minutes at the same temperature Tm (Tm = 178°C) ranged from 0.001 GPa (Example 5) to 0.488 GPa (Example 11), and was in the range of 0.001 to 0.5 GPa.

[0125] As shown in Table 1, cracks occurred at the edges of the cured products (wiring patterns) of the conductive pastes of Comparative Examples 1 to 3 after three-dimensional molding, when cured at 120°C for 30 minutes. Furthermore, cracks occurred in the central part of the cured products (wiring patterns) of Comparative Examples 1 and 2, and cracks occurred in a portion of the central part of the cured product (wiring pattern) of Comparative Example 3. Therefore, the moldability evaluation of Comparative Examples 1 and 2 was D, and the moldability evaluation of Comparative Example 3 was C. Thus, it became clear that it is difficult to use the cured products (wiring patterns) of the conductive pastes of Comparative Examples 1 to 3 as wiring after three-dimensional molding.

[0126] As shown in Table 1, no cracks occurred in the cured product (wiring pattern) of the conductive paste of Comparative Example 4 after curing at 120°C for 30 minutes. However, the resistivity of the cured product (wiring pattern) of Comparative Example 4 was at the measurement limit (5.0 × 10⁻⁶). +3Since it is higher than Ω·cm), it has been clarified that it is difficult to use it as wiring after three-dimensional molding.

[0127] In contrast, no cracks occurred in the central part of the cured product (wiring pattern) obtained by curing the conductive pastes of Examples 1 to 13 under the conditions of 120°C for 30 minutes. Therefore, the moldability of the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 is A or B. Therefore, it has been clarified that the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 can form electrical wirings with a low possibility of disconnection even when forming electrical wirings of electrical circuits and / or electronic circuits using three-dimensional molding.

[0128] Also, the resistivity of the cured product (wiring pattern) of the conductive pastes of Examples 1 to 13 before three-dimensional molding was in the range of 7.0×10<​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​Table 4

[0133] Table 5

Claims

1. A conductive paste for forming flexible electrical circuits on the surface of a flexible substrate, (A) Conductive particles and (B) Thermoplastic resin and (C) Solvent and Includes, The temperature Tm (°C) for measuring the storage modulus of the conductive paste is Tm = 178°C. A conductive paste wherein the storage modulus of the cured product obtained by curing the conductive paste at 120°C for 30 minutes is 0.001 to 0.5 GPa at the temperature Tm.

2. The conductive paste according to claim 1, wherein the absolute difference between the storage modulus of the flexible substrate at temperature Tm and the storage modulus of a cured product obtained by heating and drying the conductive paste at 120°C for 30 minutes at temperature Tm is 0 to 0.50 GPa.

3. The conductive paste according to claim 1 or 2, wherein the (B) thermoplastic resin comprises at least one selected from the group consisting of polycarbonate resin, hydrogenated styrene-based thermoplastic elastomer, and styrene-butadiene styrene block copolymer.

4. The conductive paste according to claim 3, wherein the polycarbonate resin comprises at least one selected from poly(4,4'-cyclohexyllidenediphenyl) carbonate and copoli[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane] carbonate.

5. The conductive paste according to claim 1 or 2, wherein the conductive particles (A) are silver particles.

6. The conductive paste according to claim 1 or 2, wherein the shape of the conductive particles (A) is irregular or flaky.

7. The conductive paste according to claim 1 or 2, wherein the flexible substrate comprises at least one selected from the group consisting of polycarbonate, polyethylene terephthalate (PET), and acrylic resin.

8. An electrical circuit comprising a cured product of the conductive paste according to claim 1 or 2.

9. A flexible electrical circuit body comprising the flexible substrate and the electrical circuit according to claim 8 disposed on the surface of the flexible substrate.

10. A step of forming an electrical circuit on the surface of the flexible substrate using the conductive paste described in claim 1 or 2, A method for manufacturing a molded article, comprising the step of forming a molded article by molding the flexible substrate on which the electrical circuit is formed.

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