Electroconductive paste, electric circuit, flexible electric circuit object, and method for producing molded object

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

Conductive paste, electric circuit, flexible electric circuit body, and method for manufacturing molded body

[0001] The present invention relates to a conductive paste for forming electrodes and wiring etc. in electric circuits and electronic circuits etc. The present invention relates to an electric circuit and a flexible electric circuit body including a cured product of the conductive paste.

[0002] In recent years, conductive pastes have been developed for forming electrodes and wiring on stretchable and bendable substrates. Furthermore, attempts have been made to use the conductive pastes to form electrodes and wiring for thermoformable electric and electronic circuits.

[0003] Patent Literature 1 discloses a multilayer film adapted for use as a substrate for printed circuits, and describes that the multilayer film further includes a conductive ink printed on the surface of the substrate, and that suitable inks include Ag, carbon, Cu, CNT, graphene, PEDOT, AgNWs, and the like.

[0004] Special Publication No. 2020-523232

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

[0006] Furthermore, when molding using heat molding, pressure molding, vacuum molding, and other methods, various three-dimensional shapes may be formed. In some shapes, the conductive paste contains a conductive filler during three-dimensional molding, resulting in poor extensibility of the conductive paste. Therefore, it is difficult for the electrical circuit pattern (wiring pattern) of the conductive paste printed on the substrate to elongate in response to the elongation of the substrate during three-dimensional molding. Therefore, stress cannot be relieved, resulting in the problem of cracks occurring in the electrical circuit pattern (wiring pattern) during molding. The occurrence of these cracks can lead to disconnections in the electrical wiring. 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, an object of the present invention is to provide a conductive paste that can form electrical wiring that is less likely to break, even when electrical wiring for electric circuits and / or electronic circuits is formed using three-dimensional molding.

[0008] In order to solve the above problems, an embodiment of the present invention has the following configuration.

[0009] (Configuration 1) Configuration 1 is a conductive paste for forming a flexible electric circuit for forming an electric circuit on the surface of a flexible substrate, the conductive paste comprising: (A) conductive particles; (B) a thermoplastic resin; and (C) a solvent; the temperature Tm (°C) for measuring the storage modulus of the conductive paste is 178°C; and the conductive paste is cured under conditions of 120°C and 30 minutes, and the storage modulus of a cured product at the temperature Tm is 0.001 to 0.5 GPa.

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

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

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

[0013] (Configuration 5) Configuration 5 is the conductive paste of any one of configurations 1 to 4, in which the (A) conductive particles are silver particles.

[0014] (Configuration 6) Configuration 6 is the conductive paste of any one of Configurations 1 to 5, wherein the shape of the (A) conductive particles is amorphous or scaly.

[0015] (Configuration 7) Configuration 7 is the conductive paste of any one of Configurations 1 to 6, wherein the flexible substrate includes 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 the conductive paste of any one of Configurations 1 to 7.

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

[0018] (Configuration 10) Configuration 10 is a method for producing a molded body, the method including: forming an electric circuit on a surface of the flexible substrate using the conductive paste of any one of Configurations 1 to 7; and molding the flexible substrate on which the electric circuit has been formed to form a molded body.

[0019] The present invention makes it possible to provide a conductive paste that can form electrical wiring that is less likely to break, even when electrical wiring for electric circuits and / or electronic circuits is formed using three-dimensional molding.

[0020] 1 is an optical microscope photograph (magnification: 20x) showing an example of a case where no cracks occur in the cured product (wiring pattern) of the conductive paste after three-dimensional molding. FIG. 2 is an optical microscope photograph (magnification: 20x) showing an example of a case where cracks occur in the cured product after three-dimensional molding of the cured product (wiring pattern) of the conductive paste. FIG. 3 is a schematic diagram showing a wiring pattern of the conductive paste (printed pattern of the conductive paste) used for measuring the resistivity of the cured product of the conductive paste. FIG. 4 is a photograph of an example of the shape of a flexible substrate with a wiring pattern of an example and a comparative example after three-dimensional molding. FIG. 5 is a schematic diagram showing the shape of a flexible substrate with a wiring pattern of an example and a comparative example after three-dimensional molding.

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the following embodiments are forms for embodying the present invention and are not intended to limit the scope of the present invention.

[0022] This embodiment is a conductive paste for forming a flexible electrical circuit for forming an electrical circuit on the surface of a flexible substrate. The conductive paste of this embodiment includes (A) conductive particles, (B) a thermoplastic resin, and (C) a solvent. When the conductive paste of this embodiment is cured under predetermined conditions to produce a cured product, the cured product has a storage modulus of 0.001 to 0.5 GPa at a predetermined temperature Tm (°C). The temperature Tm (°C) is the temperature used to measure the storage modulus of the cured product of the 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 having a predetermined storage modulus at temperature Tm, it is possible to form electrical wiring that is less likely to break even when the electrical wiring of an electric circuit and / or electronic circuit (which may be collectively referred to simply as "electric circuit") is stretched and / or bent by utilizing three-dimensional molding. Specifically, when an electric circuit that 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 included in the electric circuit.

[0024] As used herein, the term "flexible substrate" refers to a substrate that is the target of printing when an electrical circuit pattern included in an electrical circuit is printed using a conductive paste. The term "electrical circuit pattern" may also be referred to as a "wiring pattern." In this specification, the term "flexible substrate" is also simply referred to as 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." A preferred example of a flexible substrate is a flat sheet (film) made of polycarbonate resin, polyethylene terephthalate (PET) resin, and / or acrylic resin. A flexible substrate is a substrate that has flexibility, such as the ability to undergo deformation, such as extension and / or bending, when heated to at least a predetermined temperature. Therefore, the flexible substrate does not necessarily have flexibility at room temperature (e.g., 30°C or below). The predetermined temperature when heated is, for example, 140°C to 220°C.

[0025] In this specification, the term "flexible electrical circuit" refers to 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 to harden it. Flexible electrical circuits can be used for three-dimensional molding. Flexible electrical circuits that include a flexible substrate are flexible at least when heated to a predetermined temperature. Therefore, flexible electrical circuits do not need to be flexible at room temperature (for example, 30°C or lower). As with flexible substrates, the predetermined temperature when heated is, for example, 140°C to 220°C.

[0026] In this specification, the term "molded body" refers to a body obtained by molding a flexible electrical circuit body (a flat sheet or film having an electrical circuit pattern (wiring pattern)) together with other resins (for example, a flexible substrate and a molding resin) by three-dimensional molding, injection molding, or the like. In this specification, "molding" refers to a processing method for processing a material into a predetermined shape using a mold, such as heat molding, pressure molding, vacuum molding, or injection molding. In this specification, "three-dimensional molding" refers to a processing method for processing a two-dimensional flat sheet or film having an electrical circuit pattern (wiring pattern) into a three-dimensional (3-dimensional) shape by molding using a mold, such as heat molding, pressure molding, or vacuum molding. A flexible electrical circuit body that has been three-dimensionally molded can be referred to as a "three-dimensional molded body."

[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). FIG. 1 shows an example of an optical microscope photograph (magnification: 20x) of a case where no cracks have occurred in the cured conductive paste (wiring pattern) after three-dimensional molding. FIG. 2 shows an example of an optical microscope photograph (magnification: 20x) of a case where cracks have occurred in the cured conductive paste (wiring pattern) after three-dimensional molding. In the optical microscope photograph of the wiring pattern in FIG. 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 FIG. 2. In contrast, no striped pattern was observed in the optical microscope photograph of the wiring pattern in FIG. 1. Therefore, it can be determined that no cracks have occurred in the wiring pattern in FIG. 1.

[0028] The conductive paste of this embodiment is preferably a conductive paste for in-mold electronics. Generally, a technique for bonding a flat sheet-like or film-like substrate simultaneously with molding is called in-mold. There is also a technology called in-mold electronics that utilizes in-mold. In-mold electronics is generally a technique for integrating a substrate with an electrical circuit pattern on the surface of a molded body. Specifically, an electrical circuit pattern (flexible electrical circuit body) such as a film-type electronic component such as a touch sensor or a film antenna is formed on a flat sheet-like or film-like substrate, and the substrate with the electrical circuit pattern is three-dimensionally molded as needed, sandwiched between a mold, and molded, thereby integrating the substrate with the electrical circuit pattern on the surface of the molded body. By using the conductive paste of this embodiment, electrical wiring that is less likely to break can be formed 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, so that electrical wiring can be formed that is less likely to break even when the electrodes and wiring (also referred to as "electrical wiring" or simply "wiring") of an electric circuit and / or electronic circuit (sometimes simply referred to as "electric circuit") are stretched and / or bent. Furthermore, the conductive paste of this embodiment can form electrical wiring that is less likely to break, even when the electrical wiring is formed using three-dimensional molding.

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

[0032] In the present invention, the conductive particles have an average particle diameter of 0.01 to 100 μm and an electrical conductivity of 10 ６S / m or more. The conductive particles may be conductive materials formed into particles, or may be nuclei (core particles) coated with a conductive material. The nuclei (core particles) may be made of a non-conductive material as long as even a portion of them is coated with a conductive material. Examples of conductive particles include metal powders and coated powders.

[0033] The 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 (particularly, bismuth-tin alloys, solder, etc.), aluminum, indium tin oxide, silver-coated copper, silver-coated aluminum, metal-coated glass spheres, silver-coated fibers, silver-coated resin, antimony-doped tin, tin oxide, carbon fibers, graphite, carbon black, and mixtures thereof.

[0034] In consideration of 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, a silver alloy, a nickel alloy, a copper alloy, a tin alloy, and an aluminum alloy, 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 predetermined metal in the conductive particles, it is possible to form electrical wiring with low electrical resistance. The conductive particles may be particles (coated powder) in which the surfaces of insulating particles are coated with a conductive material such as the above-mentioned metal.

[0036] The conductive particles contained in the conductive paste of this embodiment are preferably silver particles. Silver (Ag) has a 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, electrical wiring with low electrical resistance can be formed.

[0037] The conductive particles contained in the conductive paste of this embodiment are preferably conductive particles 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 acceptable for the conductive particles to contain unavoidable impurities. The same applies to conductive particles made of metals other than silver and components other than conductive particles.

[0038] The shape of the conductive particles is not particularly limited, and may be any of spherical, irregular, scaly, filamentous (needle-like), and dendritic. Here, scaly refers to a shape in which the ratio of major axis to minor axis (aspect ratio) is 2 or more, including flat shapes such as plate and scale. 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 diameter of a line segment 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 diameter of a line segment passing through the approximate center of gravity of the particle in the particle image obtained by SEM.

[0039] The conductive particles such as silver particles preferably have an irregular or flaky shape. When the conductive particles contained in the conductive paste of this embodiment have an irregular or flaky shape, electrical wiring with low electrical resistance can be formed more stably. When the conductive particles are silver particles, the silver particles more preferably have an irregular or flaky shape.

[0040] The particle shapes may be a combination of particles having different shapes.

[0041] The particle size of the conductive particles can be defined as the particle size (D50) of the cumulative 50% of all particles. In this specification, D50 is also referred to as the average particle size. The average particle size (D50) can be determined from the results of particle size distribution measurement performed using the Microtrac method (laser diffraction scattering method).

[0042] The average particle diameter (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 viewpoints of resistance to elongation and / or bending and workability. If the average particle diameter (D50) is larger than the above range, problems such as clogging may occur during screen printing. Furthermore, if the average particle diameter is smaller than the above range, excessive sintering of the particles may occur during hardening of the conductive paste, making it difficult to form electrical wiring that is resistant to elongation and / or bending.

[0043] 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 m ２ / g, more preferably 0.2 to 5m ２ / g, more preferably 0.5 to 3 m ２ The BET value can be measured by a gas adsorption method (BET method) in accordance with, for example, JIS Z8830:2013 (ISO9277:2010).

[0044] If the specific surface area of ​​the conductive particles is too large, the viscosity increases when the conductive particles are made into a paste, making the paste less easy to handle, whereas if the specific surface area of ​​the conductive particles is too small, the contact area between the conductive particles decreases, resulting in a decrease in conductivity.

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

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

[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). Typical temperatures for in-mold electronics are 140 to 220°C during heat molding, 140 to 160°C during pressure molding, and 160 to 220°C during vacuum molding. By using a resin component having a glass transition temperature within the above range as the resin component of the conductive paste, the flat sheet (film) and the cured product of the conductive paste can be smoothly conformed during three-dimensional molding using heat molding or the like. Therefore, even if the electrical circuit pattern is deformed during three-dimensional molding, the possibility of electrical wiring breakage can be reduced. If the glass transition temperature is within the above range, cracks can be suppressed in the resulting cured product, resulting in a lower resistance and usable as wiring.

[0048] In the conductive paste of this embodiment, the thermoplastic resin (B) preferably contains at least one selected from the group consisting of a polycarbonate resin, a hydrogenated styrene-based thermoplastic elastomer, and a styrene-butadiene-styrene block copolymer. By using such a material as the thermoplastic resin (B), the possibility of disconnection of the electrical wiring can be further reduced.

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

[0050] During three-dimensional molding, a molded article having electrical wiring can be produced by printing predetermined electrical wiring on a flat sheet (film) of polycarbonate resin and using the flat sheet for three-dimensional molding. By including polycarbonate resin in the thermoplastic resin, the electrical wiring can be made to conform well to the deformation of the flat sheet during three-dimensional molding. Therefore, the possibility of electrical wiring breakage can be reduced.

[0051] In the conductive paste of this embodiment, the polycarbonate resin preferably contains at least one selected from poly(4,4'-cyclohexylidene diphenyl)carbonate and copoly[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane]carbonate. By using these materials as the polycarbonate resin, the possibility of disconnection of electrical wiring can be further reduced.

[0052] The thermoplastic resin may be two or more different thermoplastic resins. For example, two or more different polycarbonate resins may be used as the thermoplastic resin. Alternatively, two or more different resins other than polycarbonate resin may be used as the thermoplastic resin. Alternatively, the two or more different resins may be a combination of polycarbonate resin and a resin other than polycarbonate resin.

[0053] Furthermore, thermoplastic resins are generally produced using raw materials derived from petroleum resources. However, in recent years, concerns have arisen about the depletion of petroleum resources, and the development of thermoplastic resins using raw materials obtained from biomass resources such as plants has progressed. In addition, since there are concerns that global warming due to the increase and accumulation of 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] The thermoplastic resin contained in the conductive paste of this embodiment may be a hydrogenated styrene-based thermoplastic elastomer or a styrene-ethylene-butylene-styrene (SEBS) block copolymer. Commercially available SEBS block copolymers of hydrogenated styrene-based thermoplastic elastomers that can be used in the conductive paste of this embodiment include Tuftec H1041, H1043, H1051, and M1913 manufactured by Asahi Kasei Corporation, and Septon 4077 and Septon 4099 manufactured by Kuraray Co., Ltd. Furthermore, resins with a low elastic modulus such that the storage modulus of the cured product of the conductive paste is 0.001 to 0.5 GPa may 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 Tufprene (registered trademark) A manufactured by Asahi Kasei Corporation. An example of a commercially available SEEPS block copolymer is Septon 4033 manufactured by Kuraray Co., Ltd.

[0055] The present inventors discovered that when a cured product of a conductive paste 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, the occurrence of cracks in the cured product of the conductive paste (electrical circuit pattern) placed in a three-dimensionally molded body can be suppressed, and the possibility of electrical wiring breakage can be reduced, resulting in 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 within 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, adjusting the ratio of hard segments, which have a rigid structure, in the thermoplastic resin, and / or adjusting the ratio of soft segments, which have a flexible structure, in the thermoplastic resin. The thermoplastic resin may be at least one selected from polycarbonate resins, polystyrene resins, polyolefin resins, polyvinyl chloride resins, polyurethane resins, polyester resins, polyamide resins, polybutadiene resins, hydrogenated versions of these resins, and modified copolymer hydrogenated versions of these hydrogenated versions. Two or more may be used in combination. 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 below, the predetermined conditions for curing the cured product of the conductive paste are 120°C and 30 minutes. The temperature Tm (°C) for measuring the storage modulus of the cured product of the conductive paste may be 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. When the molecular weight of the thermoplastic resin is high, it may be necessary to increase the amount of solvent blended to achieve the desired viscosity. When the amount of solvent blended is large, the problem of the coating film becoming thin occurs. By having the molecular weight of the thermoplastic resin (e.g., polycarbonate resin) within the above range, it is possible to achieve a good balance of viscosity, elongation characteristics when heated, and electrical resistance value of the cured product.

[0057] When polycarbonate is used as a thermoplastic resin, some types of polycarbonate resins have high crystallinity. There is a problem in that highly crystalline polycarbonate resins crystallize when heated and dissolved in a solvent and then returned to room temperature. It is not easy to use such polycarbonate resins as resin components of conductive pastes. On the other hand, the above-mentioned specific polycarbonate resins are soluble in specific solvents, such as isophorone, which has a high boiling point, and the problem of crystallization after returning to room temperature can be avoided. Therefore, by using the above-mentioned polycarbonate resins as a thermoplastic resin, the possibility of electrical wiring breakage can be further reduced.

[0058] The conductive paste of this embodiment allows an electrical circuit pattern to be easily printed on the surface of a flexible substrate made of polycarbonate resin, and also allows an electrical circuit pattern to be easily printed on the surface of a substrate made of a material other than polycarbonate resin.

[0059] In the conductive paste of this embodiment, the blending amount of the (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 the (A) conductive particles. By setting the weight ratio of the conductive particles to the (B) thermoplastic resin within an appropriate range, the shape of the printed pattern of the electrical wiring can be maintained appropriately, 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, as long as the effect is not impaired.

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

[0062] The conductive paste of this embodiment preferably uses a solvent with a boiling point of 150°C or higher and 300°C or lower. 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. Furthermore, by using a solvent with a boiling point higher than a predetermined temperature, after screen-printing an electrical circuit pattern using the conductive paste, the drying time can be appropriately extended without being extremely short, improving workability. Therefore, it is easier to deform the electrical circuit pattern in response to deformation of the flat sheet or film during three-dimensional molding. Note that if the boiling point of the solvent exceeds 300°C, the solvent may not be sufficiently removed during heating for drying and molding.

[0063] Specifically, 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 tetraglyme. These solvents can reliably dissolve polycarbonate resin and hydrogenated styrene-based thermoplastic elastomer resin. Furthermore, the inclusion of a solvent in the conductive paste further reduces the likelihood of breakage in the resulting electrical wiring. In particular, the above-mentioned solvents are preferably used when dissolving poly(4,4'-cyclohexylidene diphenyl)carbonate and / or copoly[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane]carbonate as the thermoplastic resin. This facilitates the formation of a conductive paste, further reducing the likelihood of breakage in the resulting electrical wiring.

[0064] The solvent preferably contains isophorone. By using isophorone as the solvent, polycarbonate resin can be more easily dissolved in the solvent. Furthermore, the boiling point of isophorone is 215°C, which can be said to be a boiling point of a solvent suitable for printing conductive pastes. Furthermore, conductive pastes containing isophorone solvent have advantages such as being less likely to gel or solidify even when left standing and not undergoing solid-liquid separation during printing. Therefore, the solvent preferably contains isophorone. In particular, when poly(4,4'-cyclohexylidene diphenyl)carbonate and / or copoly[2,2-bis(4-hydroxyphenyl)propane / 2,2-bis(4-hydroxy-3-methylphenyl)propane]carbonate is dissolved in a solvent as the thermoplastic resin, 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 more easily dissolved in the solvent. Furthermore, the boiling point of diethylene glycol dibutyl ether is 255°C, which is considered to be a suitable boiling point for a solvent for printing conductive pastes. Furthermore, conductive pastes containing diethylene glycol dibutyl ether solvents have the advantage of being less likely to gel or solidify even when left standing, and of not undergoing solid-liquid separation during printing. Therefore, the solvent preferably contains diethylene glycol dibutyl ether, and more preferably consists solely of diethylene glycol dibutyl ether. In particular, when dissolving a maleic acid-modified styrene-butadiene elastomer resin as the thermoplastic resin in a 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, relative to 100 parts by weight of the thermoplastic resin. Usually, the thermoplastic resin can be appropriately dissolved by using a solvent in an amount about four times the weight of the thermoplastic resin.

[0067] The solvent can be added to the conductive paste as needed to adjust the viscosity of the conductive paste.

[0068] <Other Components> 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, interface treatment agents such as coupling agents, pigments, dyes, plasticizers, antifoaming agents, foam breakers, antioxidants, leveling agents, thixotropic agents, and the like, within a range that does not impair the effects of the present invention or in order to improve the effects of the present invention.

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

[0070] The conductive paste of this embodiment may be a conductive paste consisting only of the above-described components including the conductive particles (A), the thermoplastic resin (B), and the solvent (C). However, to the extent that the effects of the present invention are not impaired or to improve the effects of the present invention, the conductive paste of this embodiment may contain other components in addition to the above-described components (A) to (C). 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, interface treatment agents such as coupling agents, pigments, dyes, plasticizers, defoamers, foam breakers, antioxidants, leveling agents, and thixotropic agents.

[0071] The conductive paste of this embodiment can be produced by mixing the above-mentioned components ((A) conductive particles, (B) thermoplastic resin, and (C) solvent) and other optional components in a mixer such as a meteoric mixer, dissolver, bead mill, Raikai mixer, 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 of this embodiment can be adjusted to a viscosity that can be appropriately used in a predetermined coating film or electrical wiring formation method such as screen printing, etc. The viscosity can be adjusted by appropriately controlling the amount of solvent.

[0073] The viscosity of the conductive paste of 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 (B-type) 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, it is possible to form electrical circuit wiring that is less likely to break on the surface of a flexible substrate by means of known coating methods (dip coating, spray coating, bar coater coating, gravure coating, reverse gravure coating, spin coater coating, etc.), known printing methods (lithographic printing, carton printing, metal printing, offset printing, screen printing, gravure printing, flexographic printing, inkjet printing, etc.), jet dispensers, air dispensers, etc.

[0075] <Flexible Substrate> 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 substrates 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, vacuum molding, etc. 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 Including Cured Product of Conductive Paste> This embodiment is an electrical circuit including a cured product of a predetermined conductive paste. In this specification, the "cured product" refers to the product obtained by printing the conductive paste of this embodiment in a predetermined pattern and curing it by heating and drying under predetermined heating and drying conditions (e.g., 120°C, 30 minutes). The cured product can be used as electrical wiring for an electrical circuit or the like. Therefore, the conductive paste of this embodiment can be used to form an electrical circuit with a low possibility of disconnection. Note that the heating of the conductive paste when producing 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, measured at a predetermined temperature Tm for measuring the storage modulus after curing the cured product of the conductive paste under predetermined conditions. In the conductive paste of this embodiment, the predetermined conditions for curing the cured product of 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 present inventors discovered that when the storage modulus of the cured product (electrical circuit pattern) at 178°C, which is close to the temperature during three-dimensional molding using a flexible substrate, is 0.001 to 0.5 GPa, it is possible to suppress the occurrence of cracks in the cured product (electrical circuit pattern) of the conductive paste placed on the molded body, and to reduce the possibility of disconnection of the electrical wiring, and thus arrived at the present invention.

[0079] Examples of three-dimensional molding methods include heat molding, pressure molding, and vacuum molding. It is possible that the molding temperature varies depending on the three-dimensional molding method. Furthermore, the molding temperature may vary 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. Meanwhile, the inventors measured the storage modulus at a temperature Tm (°C) of 178°C and found that when the storage modulus of the cured conductive paste is 0.001 to 0.5 GPa, the occurrence of cracks in the cured conductive paste can be suppressed, thereby reducing the possibility of electrical wiring breakage. This suggests that although the molding temperature varies depending on the molding method and the type of flexible substrate, the occurrence of cracks in the cured conductive paste can be suppressed 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 glass transition point of the flexible substrate material, Tg (°C), can be set to a temperature in the range of Tg to Tg + 60°C as the temperature Tm (°C) for measuring the storage modulus of the cured product. On the other hand, determining which temperature to select as Tm (°C) from 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 prevent cracks from occurring 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), a storage modulus of 0.001 to 0.5 GPa can prevent cracks from occurring 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 a flat sheet or film made of polycarbonate resin is used as the flexible substrate, the temperature Tm (°C) is preferably Tm = 178°C.

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

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

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

[0084] The flexible substrate on which the conductive paste of the present 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 of an electric circuit in a molded article that is less likely to break even when the electrical wiring of the electric 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 of the conductive paste is heated and dried to harden. The temperature and time for hardening the electrical circuit pattern of the conductive paste can be appropriately selected depending on the type of thermoplastic resin contained in the conductive paste. The temperature and time for hardening the conductive paste can be appropriately adjusted and determined taking into account the heat resistance of the substrate. For example, the temperature and time for hardening the conductive paste can be set to 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. A cured product of the electrical circuit pattern can be obtained by hardening the electrical circuit pattern at a predetermined temperature and time. Specific examples of hardening conditions include 120°C and 30 minutes.

[0086] In the conductive paste of this embodiment, the absolute value of the difference between the storage modulus at temperature Tm of the flexible substrate and the storage modulus at temperature Tm of a cured product obtained by curing the conductive paste under conditions of 120°C for 30 minutes is preferably 0 to 0.50 GPa. Since the absolute value of the difference between the storage modulus at temperature Tm of the cured conductive paste of this embodiment and the flexible substrate is within a predetermined range, deformation of the flexible substrate and the cured conductive paste will be similar even if the electrical wiring of the electric circuit and / or electronic circuit is stretched and / or bent when forming a molded product. Therefore, electrical wiring with a low possibility of breakage can be formed.

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

[0088] <Molded body>

[0089] The molded article of this embodiment can be produced by molding a flexible substrate (flexible electrical circuit body) on which an electrical circuit is formed. Specifically, for example, the molded article of this embodiment can be produced by molding the flexible electrical circuit body together with other resins (e.g., a flexible substrate and a molding resin) by three-dimensional molding, injection molding, or the like. That is, the molded article of this embodiment includes an electrical circuit. Note that the technology for producing molded articles including 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 producing the molded article of this embodiment is not particularly limited. Examples of molding methods for producing the molded article of this embodiment include the following molding methods. For example, the molded article of this embodiment can be produced by three-dimensionally molding a flexible substrate (flexible electrical circuit body) on which an electric circuit is formed. Alternatively, the molded article of this embodiment can be produced by injection molding a flexible substrate (flexible electrical circuit body) on which an electric circuit is formed and a molding resin. Alternatively, the molded article of this embodiment can be produced by three-dimensionally molding a flexible substrate (flexible electrical circuit body) on which an electric circuit is formed, and then injection molding the flexible substrate (flexible electrical circuit body) with a molding resin.

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

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

[0093] Specifically, an example of a manufacturing process for a molded body by in-mold electronics is as follows.

[0094] First, the conductive paste of the present embodiment is produced.

[0095] Next, the conductive paste of this embodiment is printed on 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, the printed pattern (electrical circuit pattern) of the conductive paste printed on the flexible substrate is heated and dried to harden it, thereby producing a flexible electrical circuit body. The hardening conditions can be, for example, a heating temperature of 120°C and a heating time of 30 minutes.

[0097] Next, the flexible electric circuit body with the electric circuit formed thereon is formed into a three-dimensional shape by a molding method such as heat molding, pressure molding, vacuum molding, etc. In molding methods such as heat molding, the heating temperature of the flexible electric circuit body is preferably 140°C to 220°C.

[0098] If necessary, the three-dimensional flexible electrical circuit body and molding resin are integrally molded by injection molding, for example.

[0099] According to the method for manufacturing a molded body of this embodiment, a molded body having an electric circuit with a low possibility of breakage can be manufactured by three-dimensional molding.

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

[0101] <Materials and Preparation Ratios of Conductive Paste> Tables 1 and 2 show the compositions of the conductive pastes of Examples 1 to 13 and Comparative Examples 1 to 4. The composition of each component listed in Tables 1 and 2 is the composition (parts by weight) 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), a thermoplastic resin, and a solvent.

[0102] <(A) Conductive Particles> In the examples and comparative examples, silver particles were used as the conductive particles. Table 3 shows the manufacturers and model numbers, particle shapes, average particle diameters (D50), tapped (TAP) densities, and BET specific surface areas of silver particles A to D (conductive particles A to E) used in the examples and comparative examples. Tapped density is the "bulk density" obtained after mechanically tapping a container containing a powder sample.

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

[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 resin was dissolved in the solvent and mixed with the conductive particles.

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

[0106] <Viscosity Measurement Method> The viscosity of the conductive pastes of the examples and comparative examples was measured using a Brookfield (B-type) viscometer with an SC4-14 spindle (utility cup and spindle (UC / S) = #14) at two rotation speeds of 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 also shows the ratio of the viscosity at a rotation speed of 10 rpm to the viscosity at a rotation speed of 1 rpm (viscosity at a rotation speed of 1 rpm / viscosity at a rotation speed of 10 rpm).

[0107] <Method for Measuring Storage Modulus> The storage modulus at a temperature of 178° C. (Tm) of the cured products of the conductive pastes of the Examples and Comparative Examples was measured as follows.

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

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

[0110] Next, only the cured product of the test pattern (cured product of the coating film) was peeled off from the Teflon tape and removed. This cured product was cut into a size of 40 mm in length and 5 mm in width to prepare a test specimen. The thickness of the cured product was 200 μm ± 50 μm.

[0111] Next, the storage modulus of a cured product having a length of 40 mm, a width of 5 mm, and a thickness of 200 μm±50 μm was measured. The measurement was performed using a HITACHI DMS7100 under the following conditions: The measurement temperature (Tm) was 178°C. Measurement conditions: Amplitude: 5 μm, Tension: 1.2, Force: 0.05 N, Force Amplitude Default Value: 0.05 N

[0112] Tables 1 and 2 show the results of measuring the storage modulus at a temperature of 178° C. (Tm) of the cured products of the conductive pastes of the Examples and Comparative Examples.

[0113] <Method for Measuring Resistivity Before Three-Dimensional Molding> The resistivity of the cured products of the conductive pastes of the Examples and Comparative Examples before three-dimensional molding was measured as follows. A flexible, flat sheet of polycarbonate resin (film, Makrofol (registered trademark) DE 1-1 000000 manufactured by Covestro) was used as the flexible substrate. The substrate measured A4 size (210 mm x 297 mm) and had a thickness of 250 μm. This substrate was deformable (stretchable) at a temperature of approximately 160°C. A wiring pattern (length: 120 mm, width: 2 mm) of the conductive paste of the Examples and Comparative Examples was printed on the surface of this substrate. A screen printer was used for printing. After printing, the pattern 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 the "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 a section 80 mm apart near the center in the longitudinal direction and passing a current through them. The electrical resistance values ​​of the wiring patterns were measured using a 7461A digital multimeter manufactured by ADC Corporation using the four-terminal method. The specific resistance was calculated from the electrical resistance values ​​and the dimensions of the wiring patterns.

[0115] The "Before molding" column of "Specific Resistivity" in Tables 1 and 2 shows the specific resistance values ​​before three-dimensional molding calculated from the initial resistance values ​​of the Examples and Comparative Examples in exponential form. For example, the specific resistance of "1.3E-4" before three-dimensional molding in Example 1 in Table 1 means that the specific resistance is 1.3 × 10 －４ In Comparative Example 4, the measured value was below the measurement limit (specific resistance: 5.0 × 10 ＋３ Since the resistivity was higher than the specified value (Ω cm) and measurement was not possible, it is shown as ">5E+3" in Table 1. The same applies to the resistivity after molding shown in the "After molding" column of "Resistivity" in Tables 1 and 2.

[0116] <Method for Measuring Resistivity After Three-Dimensional Molding> The resistivity of the cured products (wiring patterns) of the conductive pastes of the Examples and Comparative Examples after three-dimensional molding was measured as follows. First, as described above, a flexible, flat sheet of polycarbonate resin (film, Makrofol (registered trademark) DE 1-1 000000 manufactured by Covestro) was used as the flexible substrate. The substrate had an A4 size (210 mm x 297 mm) and a thickness of 250 μm. This substrate was deformable (stretchable) at a temperature of approximately 160°C. The wiring pattern shown in FIG. 3 was printed on the surface of this substrate using the conductive pastes of the Examples and Comparative Examples. The wiring pattern shown in FIG. 3 had two patterns, each 2 mm wide and 120 mm long, spaced 56 mm apart. A screen printer was used for printing. After printing, the printed product was heated and dried in a constant-temperature dryer at 120°C for 30 minutes to obtain a cured product. The film thickness of the resulting cured wiring pattern (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.

[0117] Next, the planar sheet (flexible substrate) on which the cured wiring patterns of the examples and comparative examples were formed was heated and molded into a three-dimensional shape. Heat molding was performed using a vacuum molding machine (model number: Formech 450T). The conditions for heat molding were a heater output of 100% and a heating time of 11 seconds. After heating the planar sheet (flexible substrate) with a heater, the pattern was pressed against it and a vacuum was applied to form a three-dimensional shape. 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 on the top of Figure 5 is a planar schematic diagram (top schematic diagram), and the diagram shown on the bottom of Figure 5 is a side schematic diagram. The wiring after three-dimensional molding is omitted in Figure 5.

[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 a section 80 mm away from the center of the length and passing a current through them. The electrical resistance values ​​of the wiring patterns were measured using a four-terminal method using a 7461A digital multimeter manufactured by ADC Corporation. The resistivity was calculated from the electrical resistance value and the dimensions of the wiring pattern. The "After Molding" column of "Resistivity" in Tables 1 and 2 shows the resistivity values ​​after three-dimensional molding calculated from the resistance values ​​after three-dimensional molding of the examples and comparative examples in exponential form.

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

[0120] <Crack Measurement Method> The occurrence of cracks was measured for the wiring patterns of the Examples and Comparative Examples after three-dimensional molding, which were manufactured to measure the resistivity after three-dimensional molding. Figures 4 and 5 show photographs and shapes after three-dimensional molding. As shown in the photograph after three-dimensional molding in Figure 4, the wiring pattern was deformed by three-dimensional molding. The deformation was different between the center and end of the wiring. The deformation due to three-dimensional molding in the center of the wiring was a deformation in which the width in the width direction of the wiring expanded by approximately 60% (a deformation from a width of 2 mm before molding to approximately 3.2 mm after molding). Compared to the center of the wiring, the end was stretched in an arc shape, and the deformation was complex and large. Therefore, it was found that cracks occurred more frequently at the end than at the center of the wiring pattern. Therefore, the center and end of the wiring patterns of the Examples and Comparative Examples after three-dimensional molding were each observed using an optical microscope at 20x magnification to determine whether or not cracks occurred.

[0121] FIG. 1 shows an example of an optical microscope photograph (magnification: 20x) of a cured product (wiring pattern) of a conductive paste after three-dimensional molding, in which no cracks have occurred in the cured product. FIG. 2 shows an example of an optical microscope photograph (magnification: 20x) of a cured product (wiring pattern) of a conductive paste after three-dimensional molding, in which cracks have occurred in the cured product. In the optical microscope photograph of the wiring pattern in FIG. 2, stripes can be seen. These stripes are caused by cracks. In contrast, no stripes were observed in the optical microscope photograph of the wiring pattern in FIG. 1. Therefore, it can be determined that no cracks have occurred in the wiring pattern in FIG. 1. In this way, the surfaces of the wiring patterns of the examples and comparative examples were observed using an optical microscope (magnification: 20x) to determine whether cracks had occurred.

[0122] Tables 1 and 2 show the results of determining whether or not cracks occurred in the central and end portions of the wiring patterns of the examples and comparative examples, as described above. Tables 1 and 2 also show the results of determining the formability 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 end portions of the wiring patterns observed under an optical microscope. A wiring pattern was rated A for which no cracks occurred in the central portion but cracks occurred at the end portions. A wiring pattern was rated B for which no cracks occurred in the central portion but cracks occurred at the end portions. A wiring pattern was rated C for which cracks occurred in part of the central portion and at the end portions. A wiring pattern was rated D for which cracks occurred in both the central portion and the end portions. When the formability is A, the wiring pattern can be preferably used as wiring after three-dimensional molding. When the formability is B, the wiring pattern can be used as wiring after three-dimensional molding for specific applications. When the formability is C or D, it is difficult to use the wiring after three-dimensional molding, even for specific applications. Here, the specific use is an use in which, during three-dimensional molding, the wire width elongates by about 40 to 70%, preferably 55 to 65%, and more preferably 50 to 60%, and the wire lengthwise elongates by 20% or less, preferably 10% or less, and more preferably 5% or less. As described above, formability of B means that even if the deformation due to three-dimensional molding at the center of the wire is such that the width elongates by about 60% in the width direction of the wire, no cracks occur, and the wire can be used for such uses.

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

[0124] As shown in Tables 1 and 2, the absolute value of the 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 at the temperature Tm (Tm = 178°C) of the conductive paste of Examples 1 to 13 cured at 120°C for 30 minutes ranged from 0.001 GPa (Example 5) to 0.488 GPa (Example 11), that is, 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) obtained by curing the conductive pastes of Comparative Examples 1 to 3 at 120°C for 30 minutes after three-dimensional molding. Furthermore, cracks occurred in the center of the cured products (wiring patterns) of Comparative Examples 1 and 2, and cracks occurred in part of the center of the cured product (wiring pattern) of Comparative Example 3. Therefore, the moldability of Comparative Examples 1 and 2 was evaluated as D, and that of Comparative Example 3 was evaluated as C. Therefore, it became clear that it was 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, the cured product (wiring pattern) obtained by curing the conductive paste of Comparative Example 4 at 120°C for 30 minutes did not develop cracks after three-dimensional molding. However, the resistivity of the cured product (wiring pattern) of Comparative Example 4 was below the measurement limit (5.0 × 10 ＋３Since the resistivity is higher than the resistivity (Ω·cm), it has become clear that it is difficult to use the material as wiring after three-dimensional molding.

[0127] In contrast, no cracks occurred in the center of the cured products (wiring patterns) obtained by curing the conductive pastes of Examples 1 to 13 at 120°C for 30 minutes after three-dimensional molding. Therefore, the moldability of the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 was rated A or B. Therefore, it was revealed that the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 can form electrical wiring with a low probability of breakage, even when electrical wiring for electric circuits and / or electronic circuits is formed using three-dimensional molding.

[0128] The resistivity of the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 before three-dimensional molding was 7.0 × 10 －５ Ω cm (Example 12) to 1.1 × 10 ＋１ The resistivity of the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 after three-dimensional molding was in the range of 8.8×10 －５ Ω cm (Example 12) to 1.0 × 10 ＋１ Ω cm (Example 13). Therefore, the cured products (wiring patterns) of the conductive pastes of Examples 1 to 13 had resistivities that allowed them to be appropriately used as wiring both before and after three-dimensional molding.

[0129]

[0130]

[0131]

[0132]

[0133]

Claims

1. A conductive paste for forming a flexible electrical circuit for forming an electrical circuit on the surface of a flexible substrate, (A) conductive particles; (B) a thermoplastic resin; (C) a solvent; Including, The temperature Tm (°C) for measuring the storage modulus of the conductive paste is Tm = 178°C, The conductive paste, wherein the storage modulus of a cured product obtained by curing the conductive paste under conditions of 120°C and 30 minutes at the temperature Tm is 0.001 to 0.5 GPa.

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

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

4. 4. The conductive paste according to claim 3, wherein the polycarbonate resin comprises at least one selected from poly(4,4'-cyclohexylidene diphenyl) carbonate and copoly[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 conductive particles (A) have an amorphous or scaly shape.

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

9. A flexible electrical circuit comprising: the flexible substrate; and the electrical circuit of claim 8 disposed on a surface of the flexible substrate.

10. forming an electric circuit on the surface of the flexible substrate using the conductive paste according to claim 1 or 2; and forming a molded body by molding the flexible substrate on which the electric circuit is formed.