JP7565097B2 - Conductive paste and conductive film - Google Patents

Description

The present disclosure relates to a conductive paste and a conductive film, and more particularly to a conductive paste containing at least one of tannic acid and a tannic acid derivative (A), copper powder (B), a thermosetting resin (C), and a solvent (D), and a conductive film containing a cured product of this conductive paste.

A technology is used to form wiring by printing a conductive paste on various substrates. The main conductive pastes known so far are silver paste and copper paste. Silver paste has good conductivity, but is expensive and prone to migration in high humidity conditions, which can lead to problems with short circuits. For this reason, the use of copper paste instead of silver paste has been considered, but copper paste is more susceptible to oxidation than silver paste, which causes problems such as an increase in resistivity over time and a tendency for conductivity to decrease.

As methods for inhibiting the oxidation of such copper pastes, methods have been proposed in which the surface of the copper powder is preliminarily treated with an organic substance, or an additive that inhibits the oxidation of copper is mixed into a composition containing copper powder.

Patent Document 1 discloses a method of effectively removing the oxide film formed on the surface of easily oxidizable metal particles by pretreating metal fine particles with an aqueous surface treatment agent containing an organic acid, and then adding an organic carboxylic acid as an additive to suppress the oxidation of copper to a composition containing the treated copper powder. Patent Document 2 discloses a method of suppressing the oxidation of copper powder by adding an organic carboxylic acid compound to a conductive copper paste containing copper powder, thereby donating protons to the oxide film on the copper powder surface and dissolving it.

Treating copper powder with an organic acid has been shown to be effective in removing the copper oxide film already present on the copper powder used as raw material, and is effective in terms of developing initial conductivity, but is not sufficient to inhibit oxidation over time. Furthermore, the method of adding an organic carboxylic acid compound is not sufficient to inhibit the oxidation of copper powder. For this reason, there is a demand for a material that develops a more effective reducing power for copper powder, one that can develop copper's inherent conductivity at an early stage, and that can maintain that conductivity over a long period of time to ensure product reliability.

JP 2014-011006 A JP 2008-130301 A

The objective of the present disclosure is to provide a conductive paste and a conductive film that have excellent initial conductivity and can maintain this excellent conductivity over a long period of time.

The conductive paste according to one embodiment of the present disclosure contains at least one of tannic acid and a tannic acid derivative (A), copper powder (B), a thermosetting resin (C), and a solvent (D).

The conductive film according to one embodiment of the present disclosure includes a hardened product of the conductive paste.

<Conductive paste>
The conductive paste of this embodiment (hereinafter, also referred to as conductive paste (X)) contains at least one of tannic acid and a tannic acid derivative (A) (hereinafter, also referred to as component (A)), copper powder (B), a thermosetting resin (C), and a solvent (D). The conductive paste (X) can be suitably used for forming a conductive film by thermal curing.

The present inventors have found that the use of tannic acid or a derivative thereof in a conductive paste using copper powder as the metal powder makes it possible to achieve excellent initial conductivity and maintain excellent conductivity for a long period of time. That is, the conductive paste (X) can achieve a low initial resistivity and can suppress oxidation over time as evaluated by a moist heat test, etc., and as a result, can maintain conductivity for a long period of time. The reason for this is not necessarily clear, but it can be inferred that, for example, tannic acid and tannic acid derivatives can increase the dispersibility of copper powder (B) in the conductive paste (X), and can suppress the oxidation of copper powder (B) due to their reducing properties, and can reduce copper oxide generated by the oxidation of copper to the original copper to restore conductivity. In particular, it is believed that component (A) exerts a high reducing power when thermosetting resin (C) is thermally cured, and the conductive paste (X) can exhibit high initial conductivity, and in addition, it is believed that this high conductivity can be maintained for a long period of time due to the reducing power of component (A). Thus, the conductive paste (X) has excellent initial conductivity and can maintain this excellent conductivity for a long period of time.

[Component (A)]
Component (A) is at least one of tannic acid and a tannic acid derivative. "Tannic acid" includes both tannic acid in the broad sense and m-galloyl gallic acid, which is tannic acid in the narrow sense. Tannic acid in the broad sense is a general term for aromatic compounds having a large number of phenolic hydroxyl groups, such as condensed tannins, which are derivatives of flavanols, and hydrolyzed tannins in which one or more gallic acids are ester-bonded to sugars (usually glucose). The number of phenolic hydroxyl groups in one molecule of tannic acid is usually 3 to 100, preferably 10 to 50, and more preferably 20 to 30. The molecular weight of tannic acid is usually 300 to 15,000, preferably 500 to 5,000, and more preferably 1,000 to 2,500.

"Tannic acid derivative" refers to, for example, tannic acid in which some or all of the hydrogen atoms in the phenolic hydroxyl group are replaced with a substituent (hereinafter also referred to as the substituent (S)). Tannic acid derivatives are tannic acid that has been hydrophobized.

Component (A) preferably contains a tannic acid derivative. By using a tannic acid derivative obtained by hydrophobizing tannic acid as component (A), the conductive paste (X) has a greater effect of suppressing oxidation over time, and in particular, in a moist heat resistance test (e.g., 85°C, 85% RH, 100 hours, etc.), the increase in the resistivity can be further suppressed.

The tannic acid derivative preferably includes a tannic acid derivative in which some or all of the phenolic hydroxyl groups of the tannic acid are reacted with a compound having an isocyanate to form a urethane bond. By introducing a urethane bond into the tannic acid derivative, the flexibility of the conductive film formed from the conductive paste (X) is increased, and it is expected that the conductive film will be applicable to flexible substrates such as film substrates.

In addition, the tannic acid derivative is preferably one in which some or all of the phenolic hydroxyl groups of tannic acid have formed -Si-O- bonds by reaction with a silane coupling agent. By introducing -Si-O- bonds into the tannic acid derivative, it is expected that the dispersibility of the copper powder (B) will be further improved, and the adhesion between the conductive film formed from the conductive paste (X) and the substrate will be improved.

Examples of the substituent (S) include hydrocarbon groups such as alkyl groups such as methyl groups, cycloalkyl groups such as cyclohexyl groups, aryl groups such as phenyl groups, and aralkyl groups such as benzyl groups; groups in which some or all of the hydrogen atoms of these hydrocarbon groups have been substituted with other substituents; -CONHR (wherein R is a monovalent organic group having 1 to 20 carbon atoms); -Si(OR') _2R " (wherein R' is a monovalent hydrocarbon group having 1 to 10 carbon atoms, and R" is a monovalent organic group having 1 to 20 carbon atoms). An "organic group" refers to a group containing at least one carbon atom.

It is preferable that the substituent (S) contains a group having a polymerizable group. In this case, the curing shrinkage during thermal curing of the conductive paste (X) can be made larger, and as a result, the conductivity can be further improved. Examples of the polymerizable group include a (meth)acryloyl group, a vinyl group, an epoxy group, a glycidyl group, an amino group, and a mercapto group.

The -CONHR substituent (S) is formed by reacting tannic acid with a compound having an isocyanate group. Examples of the organic group represented by R include monovalent hydrocarbon groups such as substituted or unsubstituted butyl groups, and groups containing polymerizable groups such as (meth)acryloyl groups, vinyl groups, and glycidyl groups. By making R a group containing a polymerizable group, the cure shrinkage of the conductive paste (X) during thermal curing can be increased, and the conductivity can be further improved.

The -Si(OR') _2R " as the substituent (S) is formed by reacting tannic acid with a silane coupling agent. Examples of the hydrocarbon group represented by R' include alkyl groups such as methyl and ethyl groups. Examples of the organic group represented by R" include substituted or unsubstituted monovalent hydrocarbon groups, and groups containing a polymerizable group such as a glycidyl group, epoxy group, (meth)acryloyl group, vinyl group, amino group, and mercapto group. By using a group containing a polymerizable group as R", the cure shrinkage during thermal curing of the conductive paste (X) can be made larger, and the conductivity can be further improved.

The substitution rate in the tannic acid derivative (the ratio of the number of substituents (S) in one molecule of the tannic acid derivative to the number of phenolic hydroxyl groups in one molecule of tannic acid before substitution) is preferably 10% or more. In this case, the moist heat resistance of the conductive paste (X) can be more effectively improved. The substitution rate is more preferably 15% or more, even more preferably 20% or more, and particularly preferably 30% or more. The substitution rate in the tannic acid derivative is preferably 65% or less. In this case, the oxidation inhibition effect in the conductive paste (X) can be further enhanced. The substitution rate is more preferably 60% or less, even more preferably 55% or less, and particularly preferably 50% or less.

The ratio of component (A) is preferably 0.05 parts by mass or more and 5.0 parts by mass or less relative to 100 parts by mass of copper powder (B). In this case, the conductivity and continuity of the conductive paste (X) can be further improved. The ratio of component (A) is more preferably 0.1 parts by mass or more and 3.0 parts by mass or less relative to 100 parts by mass of copper powder (B), more preferably 0.4 parts by mass or more and 2.0 parts by mass or less, and particularly preferably 0.5 parts by mass or more and 1.2 parts by mass or less.

The proportion of component (A) in the conductive paste (X) is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.05% by mass or more and 5% by mass or less, even more preferably 0.1% by mass or more and 3% by mass or less, and particularly preferably 0.4% by mass or more and 1.5% by mass or less.

[Copper powder (B)]
The copper powder (B) is metal particles containing copper as a main component, with copper exposed on the particle surface.

The shape of the copper powder (B) may be, for example, spherical, flat (scale-shaped), dendritic, or amorphous. The copper powder (B) may be a combination of two or more of these shapes. The excellent conductivity and oxidation resistance of the conductive paste (X) are due to the excellent reducing power of component (A) against copper, and the shape and particle size of the copper powder (B) are not particularly limited.

From the viewpoint of printability, the average particle diameter of the copper powder (B) is preferably 0.1 μm or more and 30 μm or less, more preferably 0.5 μm or more and 20 μm or less, and even more preferably 1 μm or more and 10 μm or less. The average particle diameter is the median diameter, and indicates the particle diameter at 50 volume percent of the cumulative distribution obtained by measuring the particle size distribution (volume basis) of the copper powder (B).

The proportion of copper powder (B) in the conductive paste (X) is preferably 50% by mass or more and 99% by mass or less, more preferably 60% by mass or more and 98% by mass or less, even more preferably 70% by mass or more and 95% by mass or less, and particularly preferably 80% by mass or more and 90% by mass or less.

[Thermosetting resin (C)]
The conductive paste (X) contains a thermosetting resin (C), which allows the conductive paste (X) to be cured by heating to form a conductive film.

Examples of thermosetting resins (C) include amino resins, urethane resins, unsaturated polyester resins, epoxy resins, cyanate resins, acrylic resins, phenolic resins such as novolac-type phenolic resins and resol-type phenolic resins. Thermosetting resins (C) do not include tannic acid and tannic acid derivatives.

It is preferable that the thermosetting resin (C) contains one having a phenolic hydroxyl group. In this case, the cure shrinkage due to thermal curing of the thermosetting resin (C) can be made larger, and as a result, the conductivity of the conductive paste (X) can be further improved. Examples of the thermosetting resin (C) having a phenolic hydroxyl group include phenolic resins.

The proportion of thermosetting resin (C) relative to the conductive paste (X) is preferably 1% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 20% by mass or less, and even more preferably 5% by mass or more and 15% by mass or less.

In addition, the conductive paste (X) may contain, for example, a curing agent, a curing accelerator, etc. to promote the curing of the thermosetting resin (C).

As the curing agent, any agent capable of curing the thermosetting resin (C) can be used, and examples thereof include novolac resins; latent amine-based curing agents such as dicyandiamide, imidazole, BF ₃ -amine complexes, and guanidine derivatives; aromatic amines such as metaphenylenediamine, diaminodiphenylmethane, and diaminodiphenylsulfone; curing agents containing nitrogen atoms such as cyclophosphazene oligomers; and acid anhydride-based curing agents such as polyamide resins, maleic anhydride, phthalic anhydride, hexahydrophthalic anhydride, and pyromellitic anhydride. The proportion of the curing agent relative to the thermosetting resin (C) is usually 0.1% by mass or more and 10% by mass or less, and preferably 0.5% by mass or more and 5% by mass or less.

Examples of the curing accelerator include tertiary amines such as benzyldimethylamine, imidazole, organic acid metal salts, Lewis acids, amine complex salts, etc. The ratio of the curing accelerator to the thermosetting resin (C) is usually 0.01% by mass or more and 10% by mass or less, and preferably 0.1% by mass or more and 5% by mass or less.

[Solvent (D)]
The conductive paste (X) contains a solvent (D), which allows the viscosity of the conductive paste (X) to be adjusted more appropriately, making it suitable for use in screen printing and the like.

Examples of the solvent (D) include polyhydric alcohols such as diols such as ethylene glycol, propylene glycol, and dipropylene glycol, and triols such as glycerin; sugar alcohols; lower alcohols such as ethanol, methanol, butanol, propanol, and isopropanol; 1-methyl-1-methoxybutanol; cellosolves such as ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol mono-iso-propyl ether (iso-propyl cellosolve), ethylene glycol mono-n-butyl ether (n-butyl cellosolve), and ethylene glycol mono-t-butyl ether (t-butyl cellosolve); diethylene glycol monomethyl ether (methyl carbitol), diethylene glycol monoethyl ether (ethyl carbitol), diethylene glycol mono-n-propyl ether (n-propyl carbitol), diethylene glycol mono-iso-propyl ether (iso-propyl carbitol), ethylene glycol mono-n-butyl ether (n-butyl carbitol), and diethylene glycol mono carbitols such as triethylene glycol mono-t-butyl ether (t-butyl carbitol); triglycols such as triethylene glycol monoethyl ether (ethyl triglycol); propylene glycol monoethers such as propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, and propylene glycol mono-iso-propyl ether; dipropylene glycol monoethers such as dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, and dipropylene glycol mono-iso-propyl ether; glycol ethers such as tripropylene glycol monoethers such as tripropylene glycol monomethyl ether; glycol ether carboxylates such as cellosolve carboxylates such as ethylene glycol monomethyl ether acetate; and alkanolamines such as ethanolamine, diethanolamine, and triethanolamine.

It is preferable that the solvent (D) contains one having an alcoholic hydroxyl group. In this case, the solvent (D) can dissolve the component (A) well, and as a result, the oxidation inhibitory effect of the component (A) can be further improved. In addition, the solvent (D) having an alcoholic hydroxyl group exhibits reducing properties when cured by heating, and therefore the oxidation inhibitory effect of the component (A) can be further improved.

From the viewpoints of solubility of tannic acid, printability, etc., it is preferable that the solvent (D) contains at least one selected from the group consisting of lower alcohols and glycol ethers, and it is more preferable that the solvent (D) contains at least one selected from the group consisting of methanol, ethanol, and ethyl carbitol.

From the viewpoint of adjusting the viscosity of the conductive paste (X) more appropriately, the proportion of the solvent (D) is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less, and even more preferably 3% by mass or more and 7% by mass or less, relative to the conductive paste (X).

[Boric acid]
Boric acid includes not only orthoboric acid (H ₃ BO ₃ ) but also metaboric acid, tetraboric acid, and the like, which are condensates of orthoboric acid. The conductive paste (X) preferably contains boric acid. This allows the conductive paste (X) to have a further low resistivity. In addition, when the thermosetting resin (C) has a hydroxyl group, the hydroxyl group and boric acid form a hydrogen bond, thereby allowing the conductive paste (X) to have a further low resistivity. In addition, boric acid also forms a hydrogen bond with the phenolic hydroxyl group of the component (A), and thus a network of the thermosetting resin (C)-boric acid-tannic acid is formed, thereby allowing the conductive paste (X) to have a further good conductivity.

The proportion of boric acid is preferably 1.0% by mass or more and 40% by mass or less, and more preferably 2% by mass or more and 20% by mass or less, based on the total of component (A) and thermosetting resin (C) (including the curing agent and curing accelerator).

The proportion of boric acid in the conductive paste (X) is preferably 0.1% by mass or more and 4% by mass or less, and more preferably 0.2% by mass or more and 2% by mass or less.

[Other ingredients]
The conductive paste (X) may contain, as other components, for example, a rust inhibitor, an antioxidant, an adhesion imparting agent, a dispersant, a chelating agent, a leveling agent, a thixotropic agent, an antifoaming agent, etc. The ratio of the other components to the conductive paste (X) is, for example, 2 mass% or less.

(viscosity)
The viscosity of the conductive paste (X) at 25°C is preferably 5.0 Pa·s or more and 200 Pa·s or less. In this case, the conductive paste (X) is easy to print, does not impair the workability of screen printing, and is easy to form wiring having a good pattern. The thixotropic ratio (Ti value) of the conductive paste (X) is preferably 1.0 or more and 3.0 or less. In this case, the conductive paste (X) does not impair the workability of screen printing, and is easy to form wiring having a good pattern. The thixotropic ratio is expressed as the ratio of the viscosity at 25°C and 0.5 rpm to the viscosity at 25°C and 5 rpm (thixotropic ratio = (viscosity at 25°C and 0.5 rpm) / (viscosity at 25°C and 5 rpm)).

<Conductive film>
The conductive film of this embodiment includes a cured product of the conductive paste (X). Since the conductive film of this embodiment is formed from the conductive paste (X), it has excellent initial conductivity and can maintain excellent conductivity for a long period of time. In addition, when the conductive paste (X) contains boric acid and the tannic acid derivative has a urethane bond, the conductive film also has excellent bending resistance.

The conductive film of this embodiment is formed by applying the conductive film to a substrate such as a glass plate or a PET film by, for example, a screen printing method, and then heating and curing the conductive film. The heating temperature and heating time are appropriately selected depending on the type of thermosetting resin (C), etc., but the heating temperature is usually 100°C to 250°C, and preferably 130°C to 200°C. The heating time is usually 1 minute to 5 hours, and preferably 10 minutes to 1 hour. The shape of the conductive film is not particularly limited, and examples include a planar line-of-sight shape or band shape such as a circuit pattern, as well as a planar view shape such as a circle or a rectangle. The thickness of the conductive film is, for example, 1 μm to 1 mm, preferably 5 μm to 100 μm, and more preferably 10 μm to 50 μm.

Below, this disclosure is explained in detail using examples.

<Synthesis of tannic acid derivatives>
In a four-neck flask equipped with a reflux condenser, a thermometer, a nitrogen inlet tube, and a stirrer, 100 g of tannic acid (manufactured by Fuji Chemical Industry Co., Ltd., product name: "Tannic Acid", the total number of hydroxyl groups in one molecule of tannic acid is 25) and 100 g of methyl ethyl ketone were added and mixed to dissolve. 82 g of ethyl acrylate isocyanate (manufactured by Showa Denko KK, product name: "Kalends AOI") as an isocyanate compound was mixed into the resulting solution, and the mixture was reacted at 60°C for 5 hours. Methyl ethyl ketone was evaporated from the resulting solution, followed by drying to obtain a tannic acid derivative (1).

Similarly, tannic acid derivative (2) was obtained by reacting 141 g of an isocyanate compound, tannic acid derivative (3) was obtained by reacting 24 g of an isocyanate compound, tannic acid derivative (4) was obtained by reacting 160 g of an isocyanate compound, tannic acid derivative (5) was obtained by reacting 65 g of butyl isocyanate as an isocyanate compound, and tannic acid derivative (6) was obtained by reacting 36 g of a silane coupling agent (Toray Dow Co., Ltd., product name "Xiameter OFS-6040 Silane", glycidoxypropyltrimethoxysilane). The weight average molecular weight of the obtained tannic acid derivative was measured by the GPC measurement method shown below.

(Measurement of weight average molecular weight)
The weight average molecular weight was determined by a GPC method using standard polystyrene as the standard. The measurement conditions are shown below.
Equipment: Shimadzu Corporation, "Prominence LC-20AD"
Column: Showa Denko K.K., "GPC KF-801, GPC KF-803, GPC KF-805" (total of 3) Guard column: Showa Denko K.K., "GPC-KF-G 4A"
Sample concentration: The sample was diluted with tetrahydrofuran so that the concentration of tannic acid or a tannic acid derivative was 0.5% by mass.
Mobile phase solvent: tetrahydrofuran Flow rate: 1.0 mL/min Column temperature: 40° C.

Table 1 below shows the weight average molecular weight of the synthesized tannic acid derivatives and tannic acid, as well as the substitution rate of hydrogen atoms of the phenolic hydroxyl groups in the tannic acid derivatives.

<Preparation of Conductive Paste>
Example 1
0.40 g of tannic acid derivative (1), 4.6 g of epoxy resin (EPICLON EXA4816, manufactured by DIC Corporation), 0.02 g of curing accelerator (Curesol 2PHZ-PW, manufactured by Shikoku Kasei Corporation), and 0.5 g of boric acid were mixed and dissolved in 3.0 g of ethyl carbitol solvent. 48.0 g of copper particles (manufactured by Fukuda Metal Foil and Powder Co., Ltd., product name "Cu-HWF-4") were mixed with the obtained resin solution and mixed with a hybrid mixer, and then kneaded with a roll mill to obtain a conductive paste 1 (DP-1).

Example 2
A conductive paste 2 (DP-2) was obtained in the same manner as in Example 1, except that boric acid was not added.

Example 3
Conductive paste 3 (DP-3) was obtained in the same manner as in Example 1, except that tannic acid was used instead of the tannic acid derivative (1).

Example 4
A conductive paste 4 (DP-4) was obtained in the same manner as in Example 1, except that the tannic acid derivative (6) was used.

Example 5
A conductive paste 5 (DP-5) was obtained in the same manner as in Example 2, except that a resol-type phenolic resin (manufactured by Meiwa Kasei Co., Ltd., product name: "MWF-2620", solid content 70% by mass) was used as the thermosetting resin instead of the epoxy resin and the curing accelerator.

Example 6
A conductive paste 6 (DP-6) was obtained in the same manner as in Example 1, except that the tannic acid derivative (2) was used.

(Example 7)
Conductive paste 7 (DP-7) was obtained in the same manner as in Example 1, except that the solvent was changed to ethyl carbitol acetate.

(Example 8)
A conductive paste 8 (DP-8) was obtained in the same manner as in Example 1, except that the tannic acid derivative (3) was used.

Example 9
A conductive paste 9 (DP-9) was obtained in the same manner as in Example 1, except that the tannic acid derivative (4) was used.

Example 10
A conductive paste 10 (DP-10) was obtained in the same manner as in Example 1, except that the tannic acid derivative (5) was used.

(Comparative Example 1)
Conductive paste 11 (DP-11) was obtained in the same manner as in Example 1, except that tannic acid and tannic acid derivatives were not added.

[Evaluation of Conductive Paste]
(Measurement of Viscosity and Ti Value)
The viscosity (5 rpm and 0.5 rpm) at 25° C. of each of the conductive pastes obtained in Examples 1 to 9 and Comparative Example 1 was measured using a cone-plate type viscometer (manufactured by Toki Sangyo Co., Ltd.) In addition, the Ti value (=viscosity at 0.5 rpm/viscosity at 5 rpm) was calculated from these measured values.

(Evaluation of copper powder dispersibility)
In order to evaluate the dispersibility of the copper powder in the obtained conductive paste, a grind gauge (manufactured by Taiyu Kizai Co., Ltd., "GM-7470", 0 to 25 μm) was used to check the coarse particles with reference to JIS K5600-2-5 (degree of dispersion).

Based on the results of confirming the coarse particles, the dispersibility of the copper powder was evaluated according to the following criteria.
A: The grind gauge result was 7.5 μm or less.
B: The grind gauge result was 10.0 μm.
C: The grind gauge judgment result was 12.5 μm or more.

Table 2 below shows the measured paste viscosity (Pa·s) (5 rpm, 0.5 rpm), Ti value, and evaluation results of copper powder dispersibility.

<Formation of Conductive Film>
The obtained conductive pastes 1 to 11 (DP-1 to DP-11) were each applied to a PET film by a screen printing method in the form of a strip-shaped wiring having a width of 1 mm, a length of 50 mm, and a thickness of 20 μm, and then heated at 150° C. for 30 minutes to be cured, thereby obtaining substrates 1 to 11 with conductive films having conductive films 1 to 11 (DM-1 to 11).

[Evaluation of Conductive Film]
(Measurement of the resistance value of the conductive film)
The resistance values (Ω) of the obtained conductive films 1 to 11 (DM-1 to DM-11) were measured using a four-probe resistance measuring meter (RESISTANCE METER RM3544-01, manufactured by Hioki Corporation).

(Durability test)
A durability test was carried out under a high temperature and high humidity environment for the substrates with conductive film 1 to 11. That is, the substrates with conductive films 1 to 11 (DM-1 to DM-11) were held in a high temperature and high humidity chamber at 85° C. and 85% RH for 100 hours, and then the resistance values of the conductive films 1 to 11 (DM-1 to DM-11) were measured and the specific resistance values after the durability test were calculated.

(Evaluation of bending resistance)
The obtained conductive films 1 to 11 (DM-1 to DM-11) were wrapped around a 2 mmΦ iron core using a bending tester, bent, and then the resistivity was measured after unfolding. The bending resistance was evaluated according to the following criteria.
S: The increase in resistivity before and after bending is 20% or less.
A: The rate of increase in resistivity before and after bending is more than 20% and 40% or less.
B: The rate of increase in resistivity before and after bending is more than 40% and 100% or less.
C: The increase in resistivity before and after bending is more than 100%.

Table 3 below shows the resistance value (Ω) of the conductive film, the film thickness (μm), the resistivity (volume resistivity) (μΩ·cm) calculated from the resistance value and film thickness, the resistivity value after the durability test (μΩ·cm), the rate of change after the durability test (= (resistivity value after durability test/resistivity value before durability test-1)×100(%)), and the evaluation results of bending resistance.

As can be seen from the results of Tables 2 and 3, the conductive pastes of the examples form conductive films that are excellent in initial conductivity and can maintain excellent conductivity for a long period of time. In particular, the conductive pastes of Examples 1 to 5 have excellent copper powder dispersibility, and Examples 1, 3, 4, and 5 in particular have low resistivity values of about 100 μΩ·cm, and excellent conductive films can be obtained. Furthermore, in Examples 1, 2, 4, 5, 6, 7, 8, and 10, which use tannic acid derivatives with substitution rates of 10 to 65%, conductive films with excellent durability can be obtained. The conductive paste of Comparative Example 1 has poor copper powder dispersibility, and the resistivity value of the conductive film is high from the beginning, which is thought to be due to oxidation during the conductive paste preparation process, and the resistivity value also increases in the moisture resistance test.

Furthermore, as can be seen from the results in Table 3, the conductive films of Examples 1, 6, 7, 9 and 10 formed from a conductive paste combining a tannic acid derivative having a urethane bond and boric acid have good bending resistance.

The conductive paste of Example 9, which uses a tannic acid derivative in which the substitution rate of hydrogen atoms in phenolic hydroxyl groups exceeds a certain value, shows an increase in the initial resistivity, a decrease in the dispersibility of copper powder, and a high rate of increase in resistivity after durability testing, which suggests that the effect of the tannic acid derivative is reduced. The conductive paste of Example 7 does not use a solvent with a hydroxyl group, which suggests that the rust-preventive effect of the tannic acid derivative is reduced when the paste is made.

(summary)
As is apparent from the above, the conductive paste of the first aspect according to the present disclosure contains at least one of tannic acid and a tannic acid derivative (A), copper powder (B), a thermosetting resin (C), and a solvent (D).

According to the first aspect, the conductive paste has excellent initial conductivity and can maintain this excellent conductivity for a long period of time.

The conductive paste of the second embodiment is the first embodiment, in which the (A) component contains a tannic acid derivative, and the tannic acid derivative is a tannic acid derivative in which hydrogen atoms in some of the phenolic hydroxyl groups of tannic acid are replaced with substituents, and the replacement rate is 10% or more and 65% or less.

According to the second aspect, the effect of suppressing oxidation of the conductive paste over time can be further enhanced, and in particular, in a moist heat resistance test, the increase in resistivity can be further suppressed, and moist heat resistance can be more effectively improved.

The third aspect of the conductive paste is the first aspect, in which the component (A) contains a tannic acid derivative, and the tannic acid derivative forms a urethane bond by reacting some or all of the phenolic hydroxyl groups of the tannic acid with a compound having an isocyanate group.

According to the third aspect, the effect of inhibiting oxidation over time is greater, and in particular, the increase in resistivity can be further inhibited in moist heat resistance tests. Furthermore, by introducing urethane bonds into the tannic acid derivative, the flexibility of the conductive film formed from the conductive paste is increased, which is expected to enable application to flexible substrates such as film substrates.

The conductive paste of the fourth aspect, in any one of the first to third aspects, further contains boric acid.

According to the fourth aspect, it is possible to achieve a further reduction in the resistivity of the conductive paste. Furthermore, when the thermosetting resin (C) has a hydroxyl group, the hydroxyl group forms a hydrogen bond with boric acid, thereby achieving a further reduction in the resistivity. Furthermore, since boric acid also forms a hydrogen bond with the phenolic hydroxyl group of component (A), a network of thermosetting resin (C)-boric acid-tannic acid is formed, thereby achieving even better conductivity.

The fifth aspect of the conductive paste is any one of the first to fourth aspects, in which the solvent (D) has an alcoholic hydroxyl group.

According to the fifth aspect, the solvent (D) can dissolve the component (A) well, so that the oxidation inhibitory effect of the component (A) can be further improved. In addition, the solvent (D) having an alcoholic hydroxyl group exhibits reducing properties when cured by heating, so that the oxidation inhibitory effect of the component (A) can be further improved.

The sixth aspect of the conductive paste is any one of the first to fifth aspects, in which the thermosetting resin (C) has a phenolic hydroxyl group.

According to the sixth aspect, the cure shrinkage due to thermal curing of the thermosetting resin (C) can be made larger, and as a result, the conductivity of the conductive paste can be further improved.

The conductive film of the seventh aspect includes a hardened product of a conductive paste of any one of the first to sixth aspects.

According to the seventh aspect, the conductive film has excellent initial conductivity and can maintain excellent conductivity for a long period of time.

The conductive paste disclosed herein makes it possible to form a conductive film that has excellent initial conductivity, excellent moisture resistance, and is capable of maintaining this excellent conductivity for a long period of time through thermal curing, without the need for special equipment that requires inert gas or the like.

Claims (6)

The composition contains a tannic acid derivative ( A), copper powder (B), a thermosetting resin (C), and a solvent (D) ,
The conductive paste is characterized in that the tannic acid derivative (A) is a tannic acid in which some of the hydrogen atoms in the phenolic hydroxyl groups are substituted with substituents, and the substitution rate is 10% or more and 65% or less . The composition contains a tannic acid derivative (A), copper powder (B), a thermosetting resin (C), and a solvent (D),
The conductive paste is one in which the tannic acid derivative (A) is obtained by reacting a part or all of the phenolic hydroxyl groups of tannic acid with a compound having an isocyanate group to form a urethane bond. The conductive paste according to claim 1 or 2, further comprising boric acid. The conductive paste according to claim 1 , wherein the solvent (D) has an alcoholic hydroxyl group. The conductive paste according to claim 1 , wherein the thermosetting resin (C) has a phenolic hydroxyl group. A conductive film comprising a cured product of the conductive paste according to claim 1 .